The InteractiveFly: Drosophila as a Model for Human Diseases


Alzheimer's disease
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Drosophila genes associated with
Alzheimer's disease
β amyloid protein precursor-like
forkhead box, sub-group O
Neurotrophin 1
NMNAT
Presenilin
refractory to sigma P
tau
Related terms


Autophagy
Axonogenesis
Circadian clock
Relevant studies of Alzheimer's disease

Fernandez-Funez, P., Zhang, Y., Sanchez-Garcia, J., de Mena, L., Khare, S., Golde, T.E., Levites, Y. and Rincon-Limas, D.E. (2015). Anti-Aβ single-chain variable fragment antibodies exert synergistic neuroprotective activities in Drosophila models of Alzheimer's disease. Hum Mol Genet 24: 6093-6105. PubMed ID: 26253732

Abstract
Both active and passive immunotherapy protocols decrease insoluble amyloid-ß42 (Aß42) peptide in animal models, suggesting potential therapeutic applications against the main pathological trigger in Alzheimer's disease (AD). However, recent clinical trials have reported no significant benefits from humanized anti-Aß42 antibodies. Engineered single-chain variable fragment antibodies (scFv) are much smaller and can easily penetrate the brain, but identifying the most effective scFvs in murine AD models is slow and costly. This study shows that scFvs against the N- and C-terminus of Aß42 (scFv9 and scFV42.2, respectively) that decrease insoluble Aß42 in CRND mice are neuroprotective in Drosophila models of Aß42 and amyloid precursor protein neurotoxicity. Both scFv9 and scFv42.2 suppress eye toxicity, reduce cell death in brain neurons, protect the structural integrity of dendritic terminals in brain neurons and delay locomotor dysfunction. Additionally, co-expression of both anti-Aß scFvs display synergistic neuroprotective activities, suggesting that combined therapies targeting distinct Aß42 epitopes can be more effective than targeting a single epitope. Overall, the study demonstrates the feasibility of using Drosophila as a first step for characterizing neuroprotective anti-Aß scFvs in vivo and identifying scFv combinations with synergistic neuroprotective activities (Fernandez-Funez, 2015).

Highlights

  • Two anti-Aß scFvs independently and synergistically suppress Aß42 neurotoxicity in the eye.
  • Anti-Aß scFvs suppress APP neurotoxicity in the eye.
  • Anti-Aß scFvs are secreted in Drosophila.
  • Anti-Aß scFvs co-localize with Aß42 in Drosophila brain neurons.
  • Anti-Aß scFvs protect neuronal architecture against Aß42 neurotoxicity.
  • Anti-Aß scFvs delay the progressive locomotor dysfunction induced by Aß42.
  • Anti-Aβ scFvs suppress neuronal loss in the brain but do not reduce Aß42 accumulation.
  • Anti-Aß scFvs do not reduce steady-state Aß42 levels.

Discussion
ScFvs are a class of engineered antibodies with promising applications for AD immunotherapy. Many anti-Aß scFvs prevent Aß42 aggregation and reduce its neurotoxicity in vitro. Some anti-Aß scFvs also show benefits in mouse models of AD either by direct administration in the brain, nose or muscle or by AAV-mediated expression in brain or muscle. These approaches report reduction of Aß deposition in the brain, supporting the amyloid attenuating effects observed with complete antibodies. Although anti-Aß scFvs have shown promising results in mouse models of AD, these are expensive and time-consuming experiments that mainly focus on Aß burden one scFv at a time. Furthermore, the physiological benefits of the anti-Aß scFvs are unclear because most AD mouse models overexpressing wild-type or mutant APP show no overt neurodegeneration. Thus, a new platform enabling efficient in vivo preliminary screening of the neuroprotective activity of new scFvs may be needed. This study demonstrates the feasibility of testing the neuroprotective activity of anti-Aß scFvs in Drosophila. As proof-of-principle, two scFvs known to reduce Aß42 burden in CRND8 mice were used and compelling evidence of their neuroprotective activity was obtained in several assays. ScFvs have been expressed in flies previously, although modified for intracellular retention against Huntingtin (intrabodies). A camelid heavy-chain antibody against oligomeric Aß has also been tested recently in flies with the surprising result that it promotes the toxicity of Aß40. This study reports for the first time that scFvs are expressed in flies with their original signal peptide, allowing them to interact with secreted Aß42. ScFv9 and scFv42.2 are properly secreted and folded in Drosophila based on their ability to suppress Aß42 and APP neurotoxicity. Overall, the results presented in this study support the use of Drosophila as a platform for screening multiple anti-Aß scFvs in the same experimental conditions to find the most effective in neurotoxicity assays (Fernandez-Funez, 2015).

In the original report, expression of scFv9 and scFv42.2 in CRND8 mice by somatic brain transgenesis shows significant reduction of insoluble Aß42 in brain homogenates and fewer amyloid plaques by histological analysis. But, the neuroprotective activity of the anti-Aß scFvs could not be determined at the time because APP mice do not show overt neurodegeneration. This study demonstrates that both scFv9 and scFv42.2 prevent neuronal loss, protect the structural integrity of the eye and the dendritic fields of mushroom body neurons and increase the activity of motor neurons. Although the protective activity of scFv42.2 was stronger than scFv9 in both the mushroom bodies and the locomotor assay, this difference may be attributed to its higher expression level. In fact, two copies of scFv9 show robust rescue of the eye phenotype, supporting the superior performance of N-terminal antibodies described in other models. Future efforts will take advantage of a common docking site (attP) to ensure similar expression levels, enabling the direct comparison of the relative effectiveness of anti-Aß scFv (Fernandez-Funez, 2015).

The flexibility of Drosophila genetics allows combining both anti-Aß scFvs and to determine that they exert synergistic neuroprotective activities. The combined anti-Aß scFvs drastically improve eye organization and protect axonal and dendritic terminals in the mushroom body neurons. This is the first time that two scFvs have been used in combination in vivo, revealing a novel application for full antibodies or antibody fragments. N-terminal antibodies (e.g. scFv9) are known to bind soluble and insoluble forms of Aß42 because this domain is always exposed. Central and C-terminal antibodies (e.g. scFv42.2) can mostly bind monomeric Aß42, but their mechanism of action is not well understood. It is possible that N- and C-terminal anti-Aß antibodies act by different mechanisms, thus explaining the advantages of combining them. These observations suggest that treatments based on combinations of antibodies against different epitopes may provide significant benefits by exploiting their advantages and minimizing their limitations. As the challenges for delivering AD immunotherapy and reducing negative events have been known for the last 15 years, introducing two antibodies represents a minor modification to the current procedures with potential advantages (Fernandez-Funez, 2015).

The benefits of anti-Aß antibodies are explained by at least three non-exclusive hypotheses. The ‘peripheral sink’ hypothesis poses that antibodies sequester circulating Aß42 in the serum, thus increasing efflux from the brain and reducing Aß42 load in the brain, but subsequent studies have largely excluded this as a primarily mechanism of action. A second hypothesis suggests that Aß42 clearance is mediated by the activation of the immune response, particularly activated microglia and Fc-mediated phagocytosis. The third hypothesis proposes that anti-Aß scFvs directly prevent Aß42 aggregation and/or promote Aß42 disaggregation. Experiments in Drosophila demonstrate the neuroprotective activity of two anti-Aß scFvs in animals lacking an adaptive immune response. This simplified experimental set-up indicates that anti-Aß scFvs are neuroprotective without implicating the first two mechanisms. Therefore, the direct binding of anti-Aß scFvs to Aß42 is the only mechanism that can explain their protective activity in Drosophila. According to some reports, optimal anti-Aß antibodies should remove soluble and insoluble Aß42 without increasing the levels of highly toxic, oligomeric Aß42. But results from this study suggest that anti-Aß scFvs can be protective without clearing or disaggregating Aß42. The study, thus, proposes an alternative mechanism, ‘Aß42 masking’, similar to the action of chaperones on misfolded substrates: binding of anti-Aß scFvs to Aß42 masks neurotoxic epitopes and prevents the interaction of Aß42 with cellular substrates, including neuronal membranes and synaptic proteins. According to this mechanism, the most effective anti-Aß antibodies would be those that stably bind Aß42 monomers, oligomers and other assemblies and mask their neurotoxicity (Fernandez-Funez, 2015).

Although the beneficial effect of the immune system on Aß42 clearance in rodent models and humans could not be discounted, there seems to be serious risks associated with the stimulation of the cellular response. Hence, engineered antibodies that avoid the immune response such as scFvs seem promising alternatives to full antibodies. New anti-Aß scFvs against linear or conformational epitopes can be efficiently selected in flies based on their neuroprotective activity and, then, modified to increase their brain penetration in mice or humans by adding targeting sequences. More complex approaches can include the dual targeting of Aß and tau, the two triggers of neurotoxicity in AD, following the same principle of binding and masking. Thus, changing the way we think about the role of aggregates in the toxicity of amyloids may contribute to the development of more efficient therapies for AD and other incurable proteinopathies (Fernandez-Funez, 2015).

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Ojelade, S. A., Lee, T. V., Giagtzoglou, N., Yu, L., Ugur, B., Li, Y., Duraine, L., Zuo, Z., Petyuk, V., De Jager, P. L., Bennett, D. A., Arenkiel, B. R., Bellen, H. J. and Shulman, J. M. (2019). cindr, the Drosophila homolog of the CD2AP Alzheimer's disease risk gene, is required for synaptic transmission and proteostasis. Cell Rep 28(7): 1799-1813. PubMed ID: 31412248

Abstract

The Alzheimer's disease (AD) susceptibility gene, CD2-associated protein (CD2AP), encodes an actin binding adaptor protein, but its function in the nervous system is largely unknown. Loss of the Drosophila ortholog cindr enhances neurotoxicity of human Tau, which forms neurofibrillary tangle pathology in AD. Cindr is expressed in neurons and present at synaptic terminals. cindr mutants show impairments in synapse maturation and both synaptic vesicle recycling and release. Cindr associates and genetically interacts with 14-3-3zeta, regulates the ubiquitin-proteasome system, and affects turnover of Synapsin and the plasma membrane calcium ATPase (PMCA). Loss of cindr elevates PMCA levels and reduces cytosolic calcium. Studies of Cd2ap null mice support a conserved role in synaptic proteostasis, and CD2AP protein levels are inversely related to Synapsin abundance in human postmortem brains. These results reveal CD2AP neuronal requirements with relevance to AD susceptibility, including for proteostasis, calcium handling, and synaptic structure and function (Ojelade, 2019).

Alzheimer's disease (AD) is a progressive and incurable neurodegenerative disorder that is estimated to affect 13 million people in the United States by 2050 (Alzheimer's Association). At autopsy, AD is characterized by extracellular neuritic plaques and intraneuronal neurofibrillary tangles, predominantly composed of misfolded and aggregated β-amyloid (Aβ) peptide and the microtubule-associated protein tau (MAPT or Tau), respectively. Substantial evidence indicates that AD pathology disrupts synaptic function at an early stage, and synapse loss is strongly associated with clinical progression. Human genome-wide association studies (GWASs) have discovered nearly 30 common variants associated with AD risk; however, most of these newly implicated genes remain poorly studied, especially in the nervous system. Prior work leveraged a human MAPT transgenic model in the fruit fly, Drosophila melanogaster, to identify AD susceptibility gene candidates that interact with Tau-mediated mechanisms. Among the findings, knockdown of cindr, the ortholog of the AD risk gene CD2-associated protein (CD2AP), enhanced Tau-induced neurodegeneration (Ojelade, 2019).

Before its identification from AD GWASs, loss-of-function mutations in CD2AP were identified as a rare cause of familial, autosomal dominant, focal segmental glomerulosclerosis, and progressive renal dysfunction has been recapitulated in mice hetero- or homozygous for a Cd2ap knockout allele. CD2AP encodes an adaptor protein that contains three SH3 motifs, a proline-rich region, and an actin binding domain. In the kidney, CD2AP is required to maintain structural integrity of the slit diaphragm, the specialized junctions between the foot processes of glomerular podocytes. Additional studies support a role for CD2AP in cytoskeletal anchoring of adhesion complexes and regulation of membrane localization for cell surface receptors via endocytosis. Several other AD susceptibility loci harbor genes with links to cell adhesion (CASS4, FERMT2, and PTK2B) and endocytosis (BIN1, PICALM, and RIN3). Nevertheless, the function of CD2AP in the adult nervous system, or its mechanism in neurological disorders, remains largely unknown. Studies in mice have connected CD2AP to both axonal growth and maintenance of blood-brain barrier integrity. CD2AP has also been implicated in amyloid precursor protein trafficking and processing in cultured neurons, although Cd2ap loss of function did not affect total Aβ levels or amyloid plaque burden in AD mouse models (Ojelade, 2019).

Besides suggesting potential interactions with Tau, investigations in the fruit fly have implicated cindr with conserved roles in nephrocytes, retinal development, and oogenesis. This study has used Drosophila to elucidate the role of cindr in the nervous system, confirming a requirement for synaptic vesicle endocytosis and revealing requirements for ubiquitin-proteasome-mediated protein turnover and presynaptic Ca2+ homeostasis. Moreover, evidence is presented that the role of CD2AP in synaptic proteostasis is conserved in the mammalian brain and may contribute to vulnerability for Tau-mediated neurotoxic mechanisms in AD (Ojelade, 2019).

CD2AP is an AD susceptibility gene, but to date, its function in the adult nervous system is poorly defined. A cross-species experimental strategy implicates Cindr in synaptic biology and defines potential links to AD pathogenic mechanisms. First, Cindr was shown to be present at the synapse and interacts with other presynaptic proteins. Second, cindr loss of function causes impairments in synapse maturation and synaptic vesicle recycling and release. Third, the synaptic structural and functional defects are directly linked to requirements for Cindr in UPS-mediated protein turnover and Ca2+ homeostasis. Lastly, this study shows that Cindr functions with 14-3-3ζ to regulate proteasome activity, and this role is likely conserved in mouse and human brains. The results therefore enhance understanding of cindr/CD2AP and suggest potential mechanisms for its contribution to AD susceptibility (Ojelade, 2019).

Synaptic loss is strongly associated with cognitive decline in AD, and both Aβ and Tau have been linked to synaptic dysfunction. In Drosophila, loss of cindr causes a combination of endocytic and exocytic synaptic defects. Prior studies have implicated cindr/CD2AP in endocytosis in other contexts. Consistent with this, loss of cindr caused a reduced synaptic vesicle density at the NMJ and depression of synaptic responses during high-frequency stimulation, reminiscent of other genes implicated in endocytosis. Notable among these is like-AP180, which is homologous to another AD susceptibility gene PICALM. Loss of cindr caused reduced evoked potentials and enhanced facilitation, which both result from diminished presynaptic Ca2+, possibly because of increased PMCA levels. Altered Ca2+ handling has also been linked to AD pathogenesis, in which PMCA has been implicated. PMCA is a Ca2+ efflux pump with an established requirement for neuronal Ca2+ homeostasis. It was possible to rescue cindr phenotypes either directly through increasing extracellular [Ca2+] or indirectly by manipulating buffer pH to suppress PMCA activity; however, the possibility cannot be excluded that other Ca2+ channels or transporters may participate. Overall, this study confirms a role for Cindr in synaptic vesicle endocytosis and recycling and highlight an unexpected requirement for presynaptic Ca2+ homeostasis and resulting vesicle exocytosis and release (Ojelade, 2019).

Actin cytoskeletal dynamics have been implicated in synaptic vesicle recycling and plasticity, including at the Drosophila NMJ. Cindr is an actin binding protein, and this study found that cindr loss of function stabilizes F-actin in the Drosophila nervous system, consistent with prior studies in both flies and human cells. Changes in actin could potentially influence many functions and phenotypes addressed in this study. Pharmacological manipulations that disrupt actin have been reported to attenuate ghost bouton formation and impair synaptic vesicle endocytosis. Based on biochemical studies, actin may also bind and regulate human PMCA to modulate Ca2+ homeostasis. In mammals, Synapsins have been shown to tether synaptic vesicles to the actin cytoskeleton, thereby regulating the vesicle reserve pool and activity-dependent, presynaptic plasticity. In Drosophila, mutations in synapsin attenuate the formation of NMJ ghost boutons following stimulation. While loss of cindr leads to an increase in Synapsin, this is unlikely to explain the accompanying ghost bouton formation, because Synapsin overexpression does not cause this phenotype. Although elevations were identified in Synapsin and other presynaptic proteins in Cd2ap null mice, it will be important in future work to directly confirm a conserved role for Cd2ap in the function of mammalian central synapses. In the rat peripheral nervous system, Cd2ap was implicated to participate in signal transduction during axonal outgrowth accompanying collateral sprouting, an important contributor to neuronal plasticity (Ojelade, 2019).

dhat reduced CD2AP expression is protective for AD; however, this interpretation contradicts experimental findings that cindr/CD2AP (1) is required for neuronal homeostasis and synaptic function and (2) knockdown enhances Tau-induced neurodegeneration. Moreover, the true causal variant or variants at the CD2AP locus remain unknown. A sequencing study identified several AD cases harboring a rare, CD2AP coding variant (K633R) predicted to be damaging. Although this finding requires replication, it underscores the importance of fine mapping to definitively identify the responsible allelic variants and their mechanisms. Furthermore, because available brain RNA sequencing data are from bulk tissue, it was not possible to differentiate the impact of CD2AP risk alleles on gene expression in neurons versus other cell types (e.g., astrocytes, endothelial cells, or microglia). CD2AP is broadly expressed, and although the studies define important neuronal requirements, prior work highlights complementary functions in other cell types and tissues. Finally, it is possible that susceptibility genes and the implicated biological pathways have dynamic and pleiotropic relationships with AD phenotypes, even flipping from protection to risk during distinct temporal or cell-type-specific disease phases. For example, while reduced neuronal [Ca2+] may attenuate synaptic strength and thereby increase vulnerability to early AD synaptic dysfunction, the identical perturbation may protect against late excitotoxic mechanisms. Thus, it is possible, if not likely, that CD2AP impinges on multiple targets and pathways to affect AD pathogenesis. In conclusion, although substantial challenges remain, functional dissection of susceptibility genes like CD2AP holds enormous promise for refinement of AD mechanistic models and development of improved therapeutics (Ojelade, 2019).

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Oyarce-Pezoa, S., Rucatti, G. G., Muñoz-Carvajal, F., Sanhueza, N., Gomez, W., Espinoza, S., Leiva, M., García, N., Ponce, D. P., SanMartín, C. D., Rojas-Rivera, D., Salvadores, N., Behrens, M. I., Woehlbier, U., Calegaro-Nassif, M., Sanhueza, M. (2023). The autophagy protein Def8 is altered in Alzheimer's disease and Aβ42-expressing Drosophila brains. G3 (Bethesda). PubMed ID: 37311212

Abstract

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized by protein accumulation in the brain as a main neuropathological hallmark. Among them, Aβ42 peptides (see Drosophila APPL) tend to aggregate and create oligomers and plaques. Macroautophagy, a form of autophagy characterized by a double-membrane vesicle, plays a crucial role in maintaining neuronal homeostasis by degrading protein aggregates and dysfunctional organelles as a quality control process. Recently, DEF8, a relatively uncharacterized protein, has been proposed as a participant in vesicular traffic and autophagy pathways. We have reported increased DEF8 levels in lymphocytes from mild cognitive impairment (MCI) and early-stage AD patients and a neuronal profile in a murine transgenic AD model. This study analyzed DEF8 localization and levels in the postmortem frontal cortex of AD patients, finding increased levels compared to healthy controls. To evaluate the potential function of DEF8 in the nervous system, an in silico assessment wzs performed of its expression and network profiles, followed by an in vivo evaluation of a neuronal Def8 deficient model using a Drosophila melanogaster model of AD based on Aβ42 expression. These findings show that DEF8 is an essential protein for maintaining cellular homeostasis in the nervous system, and it is upregulated under stress conditions generated by Aβ42 aggregation. This study suggests DEF8 as a novel actor in the physiopathology of AD, and its exploration may lead to new treatment avenues (Oyarce-Pezoa, 2023).

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Yang, M., Zinkgraf, M., Fitzgerald-Cook, C., Harrison, B. R., Putzier, A., Promislow, D. E. L. and Wang, A. M. (2023). Using Drosophila to identify naturally occurring genetic modifiers of Aβ42- and tau-induced toxicity. G3 (Bethesda). PubMed ID: 37311212

Abstract

Alzheimer's disease (AD) is characterized by two pathological proteins, amyloid beta 42 (Aβ42) and tau. The majority of AD cases in the population are sporadic and late-onset AD (LOAD), which exhibits high levels of heritability. While several genetic risk factors for LOAD have been identified and replicated in independent studies, including the ApoE ε4 allele, the great majority of the heritability of LOAD remains unexplained, likely due to the aggregate effects of a very large number of genes with small effect size, as well as to biases in sample collection and statistical approaches. This study presents an unbiased forward genetic screen in Drosophila looking for naturally occurring modifiers of Aβ42- and tau-induced ommatidial degeneration. The results identify 14 significant SNPs, which map to 12 potential genes in 8 unique genomic regions. These hits that are significant after genome-wide correction identify genes involved in neuronal development, signal transduction and organismal development. Looking more broadly at suggestive hits (P < 10-5), significant enrichment was seen in genes associated with neurogenesis, development and growth as well as significant enrichment in genes whose orthologs have been identified as significantly or suggestively associated with AD in human GWAS studies. These latter genes include ones whose orthologs are in close proximity to regions in the human genome that are associated with AD, but where a causal gene has not been identified. Together, these results illustrate the potential for complementary and convergent evidence provided through multi-trait GWAS in Drosophila to supplement and inform human studies, helping to identify the remaining heritability and novel modifiers of complex diseases.

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Upadhyay, A., Chhangani, D., Rao, N. R., Kofler, J., Vassar, R., Rincon-Limas, D. E., Savas, J. N. (2023). Amyloid fibril proteomics of AD brains reveals modifiers of aggregation and toxicity. . Molecular neurodegeneration18(1):61 PubMed ID: 37710351

Abstract
The accumulation of amyloid beta (β) peptides in fibrils is prerequisite for Alzheimer's disease (AD). understanding of the proteins that promote Aβ (see Drosophila ../hjmuller/appl1.htm) fibril formation and mediate neurotoxicity has been limited due to technical challenges in isolating pure amyloid fibrils from brain extracts. To investigate how amyloid fibrils form and cause neurotoxicity in AD brain, this study developed a robust biochemical strategy. The success of these purifications was benchmarked using electron microscopy, amyloid dyes, and a large panel of Aβ immunoassays. Tandem mass-spectrometry based proteomic analysis workflows provided quantitative measures of the amyloid fibril proteome. These methods allowed comparison oc amyloid fibril composition from human AD brains, three amyloid mouse models, transgenic Aβ42 flies, and Aβ42 seeded cultured neurons. Amyloid fibrils were found to be primarily composed by Aβ42 and unexpectedly harbor Aβ38 but generally lacked Aβ40 peptides. Multidimensional quantitative proteomics allowed redefinition of the fibril proteome by identifying 20 new amyloid-associated proteins. Notably, 57 previously reported plaque-associated proteins were identified. A panel of these proteins was validated as bona fide amyloid-interacting proteins using antibodies and orthogonal proteomic analysis. One metal-binding chaperone metallothionein-3 was found to be tightly associated with amyloid fibrils and modulates fibril formation in vitro. Lastly, a transgenic Aβ42 fly model was used to test if knock down or over-expression of fibril-interacting gene homologues modifies neurotoxicity. Here, it was possible to functionally validate 20 genes as modifiers of Aβ42 toxicity in vivo. These discoveries and subsequent confirmation indicate that fibril-associated proteins play a key role in amyloid formation and AD pathology (Upadhyay, 2023)

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Pragati, Sarkar, S. (2023). Protein retention in the endoplasmic reticulum rescues A β toxicity in Drosophila
. Neurobiology of aging, 132:154-174 PubMed ID: 37837732

Abstract

Reinstated Activity of Human Tau-induced Enhanced Insulin Signaling Restricts Disease Pathogenesis by Regulating the Functioning of Kinases/Phosphatases and Tau Hyperphosphorylation in Drosophila

Tauopathies such as Alzheimer's disease (AD), Frontotemporal dementia, and parkinsonism linked to chromosome 17 (FTDP-17), etc. are characterized by tau hyperphosphorylation and distinguished accumulation of paired helical filaments (PHFs)/or neurofibrillary tangles (NFTs) in a specific-neuronal subset of the brain. Among different reported risk factors, type 2 diabetes (T2D) has gained attention due to its correlation with tau pathogenesis. However, mechanistic details and the precise contribution of insulin pathway in tau etiology is still debatable. This study demonstrated that expression of human tau causes overactivation of insulin pathway in Drosophila disease models. It was subsequently noted that tissue-specific downregulation of insulin signaling or even exclusive reduction of its growth-promoting sub-branch effectively reinstates the overactivated insulin signaling pathway in human tau expressing cells, which in turn restricts pathogenic tau hyperphosphorylation and aggregate formation. It was further noted that restored tau phosphorylation was achieved due to a reestablished balance between the levels of different kinase(s) (GSK3β and ERK/P38 MAP kinase) and phosphatase (PP2A). Taken together, this study demonstrates a precise involvement of the insulin pathway and associated molecular events in the pathogenesis of human tauopathies in Drosophila, which will be immensely helpful in developing novel therapeutic options against these devastating human brain disorders. Moreover, this study reveals an interesting link between tau etiology and aberrant insulin signaling, which is a characteristic feature of Type 2 Diabetes (Pragati, 2023).

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Catterson, J. H., Minkley, L., Aspe, S., Judd-Mole, S., Moura, S., Dyson, M. C., Rajasingam, A., Woodling, N. S., Atilano, M. L., Ahmad, M., Durrant, C. S., Spires-Jones, T. L., Partridge, L. (2023). Protein retention in the endoplasmic reticulum rescues A β toxicity in Drosophila. Neurobiology of aging, 132:154-174 PubMed ID: 37837732

Abstract

Amyloid β (A β) accumulation is a hallmark of Alzheimer's disease. In adult Drosophila brains, human A β overexpression harms climbing and lifespan. It's uncertain whether A β is intrinsically toxic or activates downstream neurodegeneration pathways. This study uncovers a novel protective role against A β toxicity: intra-endoplasmic reticulum (ER) protein accumulation with a focus on laminin and collagen subunits. Despite high A β, laminin B1 (LanB1) overexpression robustly counters toxicity, suggesting a potential A β resistance mechanism. Other laminin subunits and collagen IV also alleviate A β toxicity; combining them with LanB1 augments the effect. Imaging reveals ER retention of LanB1 without altering A β secretion. LanB1's rescue function operates independently of the IRE1 α/XBP1 ER stress response. ER-targeted GFP overexpression also mitigates A β toxicity, highlighting broader ER protein retention advantages. Proof-of-principle tests in murine hippocampal slices using mouse Lamb1 demonstrate ER retention in transduced cells, indicating a conserved mechanism. Though ER protein retention generally harms, it could paradoxically counter neuronal A β toxicity, offering a new therapeutic avenue for Alzheimer's disease (Catterson, 2023).

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Hsieh, T. C., Chiang, H. C. (2023). IMD signaling in the gut and the brain modulates Amyloid-beta-induced deficits in Drosophila. Life sciences, 332:122118 PubMed ID: 37741318

Abstract

Evidence indicates accumulating Aβ peptides (see Drosophila Appl) in brain activates immune responses in neuronal and peripheral system, which may collaboratively influence pathogenesis of Alzheimer's disease (AD). This study aimed to investigate whether regulating intestinal innate immune signaling ameliorates Aβ-induced impairments in Drosophila melanogaster. Quantitative polymerase chain reaction (qPCR) was used to observe expression changes of innate immune responses related genes in brain and in gut under the circumstance of Aβ overexpressing in nerve system. Aversive olfactory conditioning and survival assay were used to investigate effects of modulating Attacin-A (AttA) and Dpitercin-A (DptA). Fluorometric assays of respiratory burst activity was introduced to explore whether reducing oxidative stress enables overexpressing intestinal AttA and DptA to reverse Aβ-induced deficits. In vivo genetic analysis revealed that accumulating Aβ42 in neurons modulates innate immune signaling of the IMD pathway both in the brain and in the gut. Increased expression levels of the intestinal AttA and DptA improved learning performance and extended the lifespan of Aβ42 flies. The administration of apramycin led to alleviations of Aβ-induced behavioral changes, indicating that gram-negative bacteria are associated with the development of Aβ-induced pathologies. Further analysis showed that the neural expression of Aβ42 increased oxidative stress in the gut, which disrupted intestinal integrity and decreased learning performance. In addition, increased levels of AMPs targeting gram-negative bacteria and antioxidants reduced oxidative stress in the gut and reversed Aβ-induced behavioral damage. These findings suggest that innate immune responses in the gut play a pivotal role in modulating Aβ-induced pathologies (Hsieh, 2023).

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Catterson, J. H., Minkley, L., Aspe, S., Judd-Mole, S., Moura, S., Dyson, M. C., Rajasingam, A., Woodling, N. S., Atilano, M. L., Ahmad, M., Durrant, C. S., Spires-Jones, T. L., Partridge, L. (2023). Protein retention in the endoplasmic reticulum rescues A β toxicity in Drosophila. ANeurobiology of aging, 132:154-174 PubMed ID: 37837732

Abstract

Amyloid β (A β; see Drosophila Appl) accumulation is a hallmark of Alzheimer's disease. In adult Drosophila brains, human A β overexpression harms climbing and lifespan. It's uncertain whether A β is intrinsically toxic or activates downstream neurodegeneration pathways. This study uncovers a novel protective role against A β toxicity: intra-endoplasmic reticulum (ER) protein accumulation with a focus on laminin and collagen subunits. Despite high A β, laminin B1 (LanB1) overexpression robustly counters toxicity, suggesting a potential A β resistance mechanism. Other laminin subunits and collagen IV also alleviate A β toxicity; combining them with LanB1 augments the effect. Imaging reveals ER retention of LanB1 without altering A β secretion. LanB1's rescue function operates independently of the IRE1α/XBP1 ER stress response. ER-targeted GFP overexpression also mitigates A β toxicity, highlighting broader ER protein retention advantages. Proof-of-principle tests in murine hippocampal slices using mouse Lamb1 demonstrate ER retention in transduced cells, indicating a conserved mechanism. Though ER protein retention generally harms, it could paradoxically counter neuronal A β toxicity, offering a new therapeutic avenue for Alzheimer's disease.

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Cai, Y., Cui, T., Yin, P., Paganelli, P., Vicini, S., Wang, T. (2023). Dysregulated glial genes in Alzheimer's disease are essential for homeostatic plasticity: Evidence from integrative epigenetic and single cell analyses. Aging Cell, 22(11):e13989 PubMed ID: 37712202

Abstract

Synaptic homeostatic plasticity is a foundational regulatory mechanism that maintains the stability of synaptic and neural functions within the nervous system. Impairment of homeostatic regulation has been linked to synapse destabilization during the progression of Alzheimer's disease (AD). Various glial cell types play critical roles in modulating synaptic functions both during the aging process and in the context of AD. This study investigated the impact of glial dysregulation of histone acetylation and transcriptome in AD on synaptic homeostatic plasticity, using computational analysis combined with electrophysiological methods in Drosophila. By integrating snRNA-seq and H3K9ac ChIP-seq data from the same AD patient cohort, this study pinpointed cell type-specific signature genes that were transcriptionally altered by histone acetylation. The role of these glial genes in regulating presynaptic homeostatic potentiation was subsequently investigated in Drosophila. Remarkably, nine glial-specific genes, which were identified through computational methods as targets of H3K9ac and transcriptional dysregulation, were found to be crucial for the regulation of synaptic homeostatic plasticity at the NMJ in Drosophila. This genetic evidence connects abnormal glial transcriptomic changes in AD with the impairment of homeostatic plasticity in the nervous system. In summary, these integrative computational and genetic studies highlight specific glial genes as potential key players in the homeostatic imbalance observed in AD (Cai, 2023).

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Tan, F. H. P., Ting, A. C. J., Najimudin, N., Watanabe, N., Shamsuddin, S., Zainuddin, A., Osada, H. and Azzam, G. (2023). 3-[[(3S)-1,2,3,4-Tetrahydroisoquinoline-3-carbonyl]amino]propanoic acid (THICAPA) is protective against Aβ42-induced toxicity in vitro and in an Alzheimer's Disease Drosophila. J Gerontol A Biol Sci Med Sci. PubMed ID: 37453137

Abstract

Alzheimer's disease (AD) is the most prevalent type of dementia globally. The accumulation of amyloid-beta (Aβ) extracellular senile plaques in the brain is one of the hallmark mechanisms found in AD. Aβ42 is the most damaging and aggressively aggregating Aβ isomer produced in the brain. Although Aβ42 has been extensively researched as a crucial peptide connected to the development of the characteristic amyloid fibrils in AD, the specifics of its pathophysiology are still unknown. Therefore, the main objective was to identify novel compounds that could potentially mitigate the negative effects of Aβ42. 3-[[(3S)-1,2,3,4-Tetrahydroisoquinoline-3-carbonyl]amino]propanoic acid (THICAPA) was identified as a ligand for Aβ42 and for reducing fibrillary Aβ42 aggregation. THICAPA also improved cell viability when administered to PC12 neuronal cells that were exposed to Aβ42. Additionally, this compound diminished Aβ42 toxicity in the current AD Drosophila model by rescuing the rough eye phenotype, prolonging the lifespan and enhancing motor functions. Through Next-generation RNA-sequencing, immune response pathways were downregulated in response to THICAPA treatment. Thus, this study suggests THICAPA as a possible disease-modifying treatment for AD.

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Wu, T., Deger, J. M., Ye, H., Guo, C., Dhindsa, J., Pekarek, B. T., Al-Ouran, R., Liu, Z., Al-Ramahi, I., Botas, J. and Shulman, J. M. (2023). Tau polarizes an aging transcriptional signature to excitatory neurons and glia. Elife 12. PubMed ID: 37219079

Abstract

Aging is a major risk factor for Alzheimer's disease (AD), and cell-type vulnerability underlies its characteristic clinical manifestations. Longitudinal, single-cell RNA-sequencing was performed in Drosophila with pan-neuronal expression of human tau, which forms AD neurofibrillary tangle pathology. Whereas tau- and aging-induced gene expression strongly overlap (93%), they differ in the affected cell types. In contrast to the broad impact of aging, tau-triggered changes are strongly polarized to excitatory neurons and glia. Further, tau can either activate or suppress innate immune gene expression signatures in a cell-type-specific manner. Integration of cellular abundance and gene expression pinpoints nuclear factor kappa B signaling in neurons as a marker for cellular vulnerability. This study also highlights the conservation of cell-type-specific transcriptional patterns between Drosophila and human postmortem brain tissue. Overall, these results create a resource for dissection of dynamic, age-dependent gene expression changes at cellular resolution in a genetically tractable model of tauopathy.

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Krupp, S., Tam, O., Gale Hammell, M. and Dubnau, J. (2023). TDP-43 pathology in Drosophila induces glial-cell type specific toxicity that can be ameliorated by knock-down of SF2/SRSF1. bioRxiv. PubMed ID: 37205372

Abstract

Accumulation of cytoplasmic inclusions of TAR-DNA binding protein 43 (TDP-43) is seen in both neurons and glia in a range of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and Alzheimer's disease (AD). Disease progression involves non-cell autonomous interactions among multiple cell types, including neurons, microglia and astrocytes. This study investigated the effects in Drosophila of inducible, glial cell type-specific TDP-43 overexpression, a model that causes TDP-43 protein pathology including loss of nuclear TDP-43 and accumulation of cytoplasmic inclusions. TDP-43 pathology in Drosophila is sufficient to cause progressive loss of each of the 5 glial sub-types. But the effects on organismal survival were most pronounced when TDP-43 pathology was induced in the perineural glia (PNG) or astrocytes. In the case of PNG, this effect is not attributable to loss of the glial population, because ablation of these glia by expression of pro-apoptotic reaper expression has relatively little impact on survival. To uncover underlying mechanisms, cell-type-specific nuclear RNA sequencing was used to characterize the transcriptional changes were identified that were induced by pathological TDP-43 expression. Numerous glial cell-type specific transcriptional changes. Notably,SF2/SRSF1 levels were found to be decreased in both PNG and in astrocytes. Further knockdown of SF2/SRSF1 in either PNG or astrocytes lessens the detrimental effects of TDP-43 pathology on lifespan, but extends survival of the glial cells. Thus TDP-43 pathology in astrocytes or PNG causes systemic effects that shorten lifespan and SF2/SRSF1 knockdown rescues the loss of these glia, and also reduces their systemic toxicity to the organism (Krupp, 2023).

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Coelho, D. S., Schwartz, S., Merino, M. M., Hauert, B., Topfel, B., Tieche, C., Rhiner, C. and Moreno, E. (2018). Culling less fit neurons protects against Amyloid-beta-induced brain damage and cognitive and motor decline. Cell Rep 25(13): 3661-3673. PubMed ID: 30590040

Abstract

Alzheimer's disease (AD) is the most common form of dementia, impairing cognitive and motor functions. One of the pathological hallmarks of AD is neuronal loss, which is not reflected in mouse models of AD. Therefore, the role of neuronal death is still uncertain. This study used a Drosophila AD model expressing a secreted form of human amyloid-beta42 peptide and showed that it recapitulates key aspects of AD pathology, including neuronal death and impaired long-term memory. Neuronal apoptosis is mediated by cell fitness-driven neuronal culling, which selectively eliminates impaired neurons from brain circuits. Removal of less fit neurons delays beta-amyloid-induced brain damage and protects against cognitive and motor decline, suggesting that contrary to common knowledge, neuronal death may have a beneficial effect in AD (Coelho, 2018).

This study reports that expression of misfolding-prone toxic peptides linked to AD and Huntington's disease affects neuronal fitness and triggers competition between neurons, leading to increased activation of the FlowerLoseB isoform and Azot in Drosophila neural tissues. The results demonstrate that fitness fingerprints are important physiological mediators of neuronal death occurring in the course of neurodegenerative diseases (Coelho, 2018).

This mechanism is associated with specific toxic peptides or with particular stages of the neurodegenerative disease, because competition is not elicited by expression of Parkinson-related α-Synuclein, for instance. The results suggest that the toxic effects of a given peptide correlate directly with the level of neuronal competition and death it induces (Coelho, 2018).

Surprisingly, neuronal death was found to have a beneficial effect against β-amyloid-dependent cognitive and motor decline. This finding challenges the commonly accepted idea that neuronal death is detrimental at all stages of the disease progression. Most amyloid-induced neuronal apoptosis is beneficial and likely acts to remove damaged and/ or dysfunctional neurons in an attempt to protect neural circuits from aberrant neuronal activation and impaired synaptic transmission (Coelho, 2018).

One curious observation in this study is that Ab42 induces cell death both autonomously and non-autonomously in clones of the eye disc. Dying cells co-localize with FlowerLoseB reporter both inside and outside of GFP-marked clones of the larva. It was observed that Ab42 peptide is secreted to regions outside of clone borders and accumulates at the basal side of the eye disc. The neurons of the eye disc that project their axons into the optic stalk through the basal side of the disc are likely affected by the accumulation of the toxic peptide, explaining the induction of cell death outside of clones (Coelho, 2018).

Blocking apoptosis in Ab42 expressing flies by either azot silencing or overexpression of dIAP1 increases the number of vacuoles in the brains of these flies. This seems to be a counterintuitive observation, because one would expect that a reduction in apoptosis would result in fewer cells being lost and a reduction of neurodegenerative vacuoles. However, this observation can be conciliated with the current model: it is suspected that less fit neurons have impaired dendritic growth and inhibit the expansion of neighboring neurons. This inhibition would disappear once the unfit neuron is culled, allowing compensatory dendritic growth and neuropil extension (Coelho, 2018).

Introduction of a single extra copy of azot was sufficient to prevent Ab42-induced motor and cognitive decline, which may suggest new venues for AD treatment that aim to support elimination of dysfunctional neurons at early stages of AD pathology. For example, in patients at early symptomatic stages, when cognitive impairment is first detected, enhancing physiological apoptotic pathways using Bcl-2 or Bcl-xL inhibitors, or promoting the cell competition pathway described in this study, may have strikingly beneficial effects (Coelho, 2018).

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Rieche, F., Carmine-Simmen, K., Poeck, B., Kretzschmar, D. and Strauss, R. (2019). Drosophila full-length Amyloid precursor protein is required for visual working memory and prevents age-related memory impairment. Curr Biol 28(5): 817-823. PubMed ID: 29478851

Abstract

The β-amyloid precursor protein (APP) plays a central role in the etiology of Alzheimer's disease (AD). APP is cleaved by various secretases whereby sequential processing by the β- and γ-secretases produces the β-amyloid peptide that is accumulating in plaques that typify AD. In addition, this produces secreted N-terminal sAPPβ fragments and the APP intracellular domain (AICD). Alternative cleavage by α-secretase results in slightly longer secreted sAPPalpha fragments and the identical AICD. Whereas the AICD has been connected with transcriptional regulation, sAPPalpha fragments have been suggested to have a neurotrophic and neuroprotective role.Loss of the Drosophila APP-like (APPL) protein impairs associative olfactory memory formation and middle-term memory that can be rescued with a secreted APPL fragment. This study show that APPL is also essential for visual working memory. Interestingly, this short-term memory declines rapidly with age, and this is accompanied by enhanced processing of APPL in aged flies. Furthermore, reducing secretase-mediated proteolytic processing of APPL can prevent the age-related memory loss, whereas overexpression of the secretases aggravates the aging effect. Rescue experiments confirmed that this memory requires signaling of full-length APPL and that APPL negatively regulates the neuronal-adhesion molecule Fasciclin 2. Overexpression of APPL or one of its secreted N termini results in a dominant-negative interaction with the FASII receptor. Therefore, these results show that specific memory processes require distinct APPL products (Rieche, 2018).

Age-related memory impairment (AMI) affects all animals, and cognitive decline is one of the devastating features of Alzheimer's disease (AD). Although APP, and more specifically the β-amyloid peptide, has been connected with memory deficits in AD, the role of full-length APP and its various other fragments in AMI is unknown. Wild-type Drosophila flies display AMI at middle age (30-40 days) when tested for middle-term or long-term olfactory memory. Furthermore, Drosophila not only encodes an ortholog for APP, called amyloid precursor protein-like (APPL), but also homologs for all three types of secretases; kuzbanian (kuz) corresponds to ADAM10 considered to be an α-secretase, dBace, the fly β-secretase, and Presenilin (Psn), the catalytic subunit of γ-secretase. APPL is processed in a similar way as human APP; however, the cleavage sites of the α- and β-secretase are reversed. Therefore, cleavage by KUZ produces a shorter secreted N-terminal fragment (NTF) than processing by dBACE. Nevertheless, subsequent γ-processing of the β-cleaved C-terminal fragment (βCTF) results in a neurotoxic dAβ peptide, whereas cleavage by KUZ does not. This study asked whether the very short-term (~4 s) visual working memory in flies is also affected by AMI and whether it requires APPL or one of its proteolytic fragments. Therefore, wild-type flies and heterozygous mutants for the three secretases were aged, and their visual orientation memory was assessed (Rieche, 2018).

This working memory is tested in the detour paradigm where walking flies navigate between two inaccessible landmarks. During an approach, the targeted landmark disappears and the fly is lured toward a novel distracting landmark. This distracter disappears one second after reorientation so that the fly is now left without any landmarks. Nevertheless, wild-type Canton-S (CS) flies can recall the position of the initial landmark and try to approach it although still invisible ('positive choices'). Whereas young CS males make about 80% positive choices, aged flies showed a reduced memory when tested at 4 weeks of age and a complete memory loss when 6 weeks old. Interestingly, heterozygosity for any of the three secretases prevented AMI, with 4- and 6-week-old Psn143/+ and kuze29-4/+ flies being indistinguishable from young CS flies. When using heterozygous dBace5243 flies, the improvement compared to age-matched CS controls did not reach significance; however, they made significantly more positive choices than chance level at 6 weeks, whereas CS did not. These findings show that visual working memory is deteriorating with age, and they suggest that reducing APPL processing can suppress AMI (Rieche, 2018).

To address whether increased processing of APPL disrupts this memory, the secretases were overexpressed in the R3 ring neurons of the ellipsoid body (using 189Y-GAL4) the seat of visual working memory. Expression of any of the secretases reduced the performance already in 3-day-old flies compared to controls, supporting a requirement of full-length flAPPL for this type of memory. On the other hand, western blot analyses using an antiserum directed against the NTFs of APPL (Ab952M) established that heterozygous secretase mutants have an overall increase especially in flAPPL which supported the hypothesis on the role of secretases and APPL processing in AMI. Furthermore, a quantitative analysis of the levels of APPL in head extracts from different ages revealed that flAPPL declines with age, whereas the NTF/flAPPL ratio increases, also suggesting that reduced levels of flAPPL are involved in AMI. To verify that APPL is indeed required for visual working memory, homozygous Appld-null mutants and transheterozygous combinations of hypomorphic Appl alleles were tested. All these mutants performed at chance level already when young, confirming that APPL is necessary for this short-termed memory. This function is dose sensitive because even young heterozygous Appld/+ showed a reduced memory that declined faster with age than in CS females (Rieche, 2018).

Next, an RNAi-mediated knockdown of Appl was introduced in the R3 neurons, which resulted in severe memory deficits already in 3- to 5-day-old flies, showing that APPL is required in these neurons for visual working memory. To identify domains in APPL that mediate this function, rescue experiments were performed expressing different APPL constructs via 189Y-GAL4 in young Appld mutant flies. This included full-length flAPPL, secretion-defective sdAPPL, specific deletion constructs, and secreted fragments. Because the exact cleavage sites in APPL are unknown, the secreted fragments are referred to as sAPPLLong (L), which comprises the N-terminal 788 amino acids and should represent the β-cleaved fragment, whereas the 758-amino-acid (aa)-long sAPPLShort (S) should represent the α-cleaved form of Drosophila APPL. In contrast to full-length wild-type APPL, neither of the secreted forms could rescue the memory deficit of Appld when induced in R3 neurons. Notably, sdAPPL very effectively rescued the memory phenotype, confirming that unprocessed flAPPL is crucial for visual working memory (Rieche, 2018).

Next, whether APPL functions as a receptor or ligand in R3 neurons was investigaged. Expression of sdAPPL that in addition lacks the intracellular C terminus (sdAPPL-ΔC) did rescue the Appld memory phenotype, which suggests that intracellular signaling is not required and that APPL does not act as a receptor. Rescue experiments with sdAPPL forms that, in addition, lack one of the two ectodomains (sdAPPL-ΔE1 and sdAPPL-ΔE2) revealed a requirement for E2 for the rescue but not for E1. To confirm this, the rescue experiments were repeated with the hypomorphic Appl4460 allele and APPL, sdAPPL, and sdAPPL-ΔE1 rescued to full extent, whereas sdAPPL-ΔE2 did not. These results suggest that membrane-bound APPL functions as a ligand in the ring neurons. This function was conserved in human APP because expression of APP695 via 189Y-GAL4 in Appld also resulted in a rescue. Using conditional expression of APPL (by combining 189Y-GAL4 with the temperature-sensitive GAL4 repressor Tub>GAL80ts) resulted in the same rescue as constitutive expression, revealing that expression in adult R3 neurons is sufficient to restore the memory in 3- to 5-day-old flies. Notably, using the same expression system to induce moderate overexpression of sdAPPL (at 25°C), it was possible to rescue the AMI in a wild-type background, demonstrating that the secretion-deficient unprocessed APPL can prevent the decline of visual working memory of aged flies. Together with the finding that the levels of endogenous flAPPL decrease with age, this suggests that a loss of flAPPL underlies the visual working memory deficits that occur during normal aging. Interestingly, an age-related increase in BACE1 activity has been described in vertebrates that could reduce levels of full-length APP (Rieche, 2018).

Most of APPL functions in synaptogenesis, neurite outgrowth, and guidance described so far required signaling via the C-terminal domain. Analyzing heterozygous Appld/+ mutant flies or inducing an adult-specific knockdown of Appl in the relevant mushroom body neurons, Previous work showed that APPL function is not required for olfactory learning, but for a 2-hr associative memory and long-term memory formation. Similar to findings in mice, overexpression of secreted sAPPLL (as well as APPL and sdAPPL) could restore the 2-hr memory in heterozygous Appld/+ flies, whereas only wild-type APPL could rescue the long-term memory deficit. Because endogenous APPL was still expressed in this rescue experiments, sAPPLL and sdAPPL might act as ligands that bind to flAPPL in mushroom body neurons. The authors therefore suggested that distinct memory phases require different forms of APPL and maybe different intracellular signaling pathways. Notably, overexpression of KUZ in the mushroom body did not affect the 2-hr olfactory memory, whereas KUZ in this study significantly reduced visual working memory. It should also be noted that unprocessed APPL can be deleterious, because expression of sdAPPL in photoreceptor cells caused cell death of lamina glia via an unknown receptor, further emphasizing that individual neuronal networks may require different APPL fragments and signaling pathways (Rieche, 2018).

Due to recent studies suggesting that output from the R3 neurons into the ellipsoid body is instrumental for visual working memory, it was hypothesized that full-length APPL is present at the axonal terminals of R3 neurons in the ellipsoid body. To analyze the sub-cellular localization of APPL and its fragments, a double-tagged version of APPL (dtAPPL) was used that carries an EGFP tag near the N terminus and RFP tag at the C terminus, resulting in yellow fluorescence of full-length dtAPPL (or co-localized fragments that have not been separated yet). Using 189Y-GAL4 to induce dtAPPL, it was observed that dtAPPL is processed differently in individual R3 neurons, whereby full-length as well as fragments of dtAPPL are found in the cell bodies and axonal/dendritic projections. That significant amounts of unprocessed APPL can be found in the R3 axons in the ellipsoid body supports the model, that full-length APPL is needed at the R3 output sites. Note that dtAPPL was able to rescue the memory deficit, as did a dtAPP695, which showed a similar distribution pattern as dtAPPL (Rieche, 2018).

Having established that full-length APPL can be found at the relevant output sides, it was asked whether proteolytic processing of APPL also changes in the R3 neurons with age. Comparing the pattern of dtAPP695 expressed with the 189Y-GAL4 driver in 3-day-old and 6-week-old flies indicated less full-length dtAPP695 in aged flies, but, when quantifying this, it did not reach significance. Therefore, another R3-neuron-specific GAL4 line (VT42759) was used that results in reduced levels of dtAPP695 with increasing age but could nevertheless be used to rescue the memory phenotype of Appld. Compared to 3-day-old flies, there was little co-localization of GFP-tagged N termini and RFP-tagged C termini in 6-week-old VT42759>dtAPP flies, revealing enhanced proteolytic processing of APP. This suggests that AMI of the visual orientation memory is caused by increased ectodomain shedding of APPL and western blot analysis of aged flies supports this notion because during aging the ratio of NTFs to flAPPL increases (Rieche, 2018).

To identify a possible receptor for APPL in visual working memory, focus was placed on the neural cell adhesion molecule Fasciclin 2 (FASII). FASII is enriched in most, if not all types of ring neurons in the ellipsoid body, and it has been shown to interact with APPL in synaptic bouton formation at the neuromuscular junction (NMJ) (Ashley, 2005). Moreover, FASII is the insect homolog of neural cell adhesion molecule (NCAM)-140, which has been demonstrated to bind APP in an E2-depending fashion. When young hemizygous mutants were tested for the strong hypomorphic FasIIe76 allele in the detour paradigm, they showed no working memory, and the same phenotype was observed when an RNAi against FasII was introduced in R3 neurons of 3- to 5-day-old flies. This phenotype could be rescued by expression of FASII in R3 neurons, which reveals that FASII is required in the same ring neuron subtype as APPL, providing a possible binding partner for APPL. At the NMJ, loss of APPL suppressed the increase in bouton number in heterozygous i>FasIIe76/+ larvae. This study therefore investigated whether removing one copy of Appl could rescue the memory deficits of homozygous i>FasIIe76 mutants. This resulted in a significant improvement in performance, as did one copy of the i>FasIIe76 mutant allele in homozygous Appld-null mutant flies. Moreover, reducing FASII expression in R3 neurons by RNAi also ameliorated the memory deficit of Appld-null mutants. Together, this suggests that APPL negatively regulates FASII in R3 neurons and that FASII acts downstream of APPL. That this negative interaction is essential to prevent AMI is demonstrated by the observation that heterozygosity for v suppresses the memory loss of 4-week-old heterozygous Appld/+ flies (Rieche, 2018).

To further investigate interactions between APPL and FASII, overexpression studies were performed using young flies, and elevated levels of flAPPL or sdAPPL were found to ameliorate the memory deficit induced by FASII overexpression. However, only wild-type APPL induced memory deficits when overexpressed without FASII. This suggested that a secreted form of APPL can induce a gain-of-function phenotype, and this study therefore overexpressed sAPPLL and sAPPLS in R3 neurons. Whereas sAPPLL had no effect, sAPPLS caused a severe impairment of visual working memory. Moreover, overexpression of sAPPLS suppressed the effects of elevated FASII levels, whereas sAPPLL did not. This shows that sAPPLS, which corresponds to the α-cleaved (KUZ) fragment, has deleterious effects when overexpressed and that these are also mediated by an interaction with FASII. Whether the lack of an effect of sAPPLL overexpression is due to a less efficient interaction with FASII or an inability to induce downstream pathways causing this gain of function remains to be determined (Rieche, 2018).

Interestingly, overexpression of the dAICD in R3 neurons also disrupted visual working memory of young flies, suggesting that α- and γ-cleavage of APPL has deleterious effects on visual working memory. This is in agreement with the finding that heterozygosity for kuz and Psn ameliorated the age-related decline in visual working memory, whereas heterozygosity for dBACE had only a modest effect. This could be explained by assuming that most of the ectodomain shedding in flies is done by KUZ activity. Therefore, reducing dBACE levels might result in a small increase of flAPPL. In addition, competitive KUZ cleavage in dBace/+ flies could result in more detrimental sAPPLS. Whereas in the case of kuz/+, the effects on memory are mediated by the interaction with FASII, in the case of Psn/+ this may be mediated by a transcriptional function of the AICD, affecting a so far unknown target (Rieche, 2018).

In summary, the results show that full-length APPL acts as a membrane-bound ligand that inhibits the FASII receptor (both acting in R3 neurons), thereby promoting visual working memory. Increased proteolytic APPL processing and therefore reduced suppression of FASII signaling then seems to cause AMI in flies. A similar interaction might also be required for working memories in vertebrates. Aging mice show reduced expression of NCAM-140 in the medial prefrontal cortex and a conditional knockout of NCAM in the forebrain promotes AMI in a delayed matching-to-place test in the Morris water maze and in a delayed reinforced alternation test in the T-maze (Rieche, 2018).

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Tao, X., Liu, J., Diaz-Perez, Z., Foley, J. R., Stewart, T. M., Casero, R. A. and Zhai, R. G. (2023). Reduction of Spermine Synthase Suppresses Tau Accumulation Through Autophagy Modulation in Tauopathy. bioRxiv. PubMed ID: 36993333

Tauopathy, including Alzheimer Disease (AD), is characterized by Tau protein accumulation and autophagy dysregulation. Emerging evidence connects polyamine metabolism with the autophagy pathway, however the role of polyamines in Tauopathy remains unclear. The present study investigated the role of spermine synthase (SMS) in autophagy regulation and tau protein processing in Drosophila and human cellular models of Tauopathy. A previous study showed that Drosophila spermine synthase (dSms) deficiency impairs lysosomal function and blocks autophagy flux. Interestingly, partial loss-of-function of SMS in heterozygous dSms flies extends lifespan and improves the climbing performance of flies with human Tau (hTau) overexpression. Mechanistic analysis showed that heterozygous loss-of-function mutation of dSms reduces hTau protein accumulation through enhancing autophagic flux. Measurement of polyamine levels detected a mild elevation of spermidine in flies with heterozygous loss of dSms . SMS knock-down in human neuronal or glial cells also upregulates autophagic flux and reduces Tau protein accumulation. Proteomics analysis of postmortem brain tissue from AD patients showed a significant albeit modest elevation of SMS protein level in AD-relevant brain regions compared to that of control brains consistently across several datasets. Taken together, this study uncovers a correlation between SMS protein level and AD pathogenesis and reveals that SMS reduction upregulates autophagy, promotes Tau clearance, and reduces Tau protein accumulation. These findings provide a new potential therapeutic target of Tauopathy (Tao, 2023).

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Praschberger, R., Kuenen, S., Schoovaerts, N., Kaempf, N., Singh, J., Janssens, J., Swerts, J., Nachman, E., Calatayud, C., Aerts, S., Poovathingal, S. and Verstreken, P. (2023). Neuronal identity defines α-synuclein and tau toxicity. Neuron. PubMed ID: 36948206

Pathogenic α-synuclein and tau are critical drivers of neurodegeneration, and their mutations cause neuronal loss in patients. Whether the underlying preferential neuronal vulnerability is a cell-type-intrinsic property or a consequence of increased expression levels remains elusive. This study explored cell-type-specific α-synuclein and tau expression in human brain datasets and use deep phenotyping as well as brain-wide single-cell RNA sequencing of >200 live neuron types in fruit flies to determine which cellular environments react most to α-synuclein or tau toxicity. Phenotypic and transcriptomic evidence was detectedof differential neuronal vulnerability independent of α-synuclein or tau expression levels. Comparing vulnerable with resilient neurons in Drosophila enabled prediction of numerous human neuron subtypes with increased intrinsic susceptibility to pathogenic α-synuclein or tau. By uncovering synapse- and Ca(2+) homeostasis-related genes as tau toxicity modifiers, this work paves the way to leverage neuronal identity to uncover modifiers of neurodegeneration-associated toxic proteins (Praschberger, 2023).

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Deshpande, P., Chimata, A. V., Snider, E., Singh, A., Kango-Singh, M. and Singh, A. (2023). N-Acetyltransferase 9 ameliorates Aβ42-mediated neurodegeneration in the Drosophila eye. Cell Death Dis 14(7): 478. PubMed ID: 37507384
Summary:

Alzheimer's disease (AD), a progressive neurodegenerative disorder, manifests as accumulation of amyloid-beta-42 (Aβ42) plaques and intracellular accumulation of neurofibrillary tangles (NFTs) that results in microtubule destabilization. Targeted expression of human Aβ42 (GMR > Aβ42) in developing Drosophila eye retinal neurons results in Aβ42 plaque(s) and mimics AD-like extensive neurodegeneration. However, there remains a gap in understanding of the underlying mechanism(s) for Aβ42-mediated neurodegeneration. To address this gap in information, a forward genetic screen was conducted, and N-acetyltransferase 9 (Mnat9) was identified as a genetic modifier of GMR > Aβ42 neurodegenerative phenotype. Mnat9 is known to stabilize microtubules by inhibiting c-Jun-N- terminal kinase (JNK) signaling. This study found that gain-of-function of Mnat9 rescues GMR > Aβ42 mediated neurodegenerative phenotype whereas loss-of-function of Mnat9 exhibits the converse phenotype of enhanced neurodegeneration. A new neuroprotective function of Mnat9 is proposed in downregulating the JNK signaling pathway to ameliorate Aβ42-mediated neurodegeneration, which is independent of its acetylation activity. Transgenic flies expressing human NAT9 (hNAT9), also suppresses Aβ42-mediated neurodegeneration thereby suggesting functional conservation in the interaction of fly Mnat9 or hNAT9 with JNK-mediated neurodegeneration. These studies add to the repertoire of molecular mechanisms that mediate cell death response following accumulation of Aβ42 and may provide new avenues for targeting neurodegeneration (Deshpande, 2023).

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Vourkou, E., Rouiz Ortega, E. D., Mahajan, S., Mudher, A. and Skoulakis, E. M. C. (2023). Human Tau Aggregates Are Permissive to Protein Synthesis-Dependent Memory in Drosophila Tauopathy Models. J Neurosci 43(16): 2988-3006. PubMed ID: 36868851
Summary:
Tauopathies including Alzheimer's disease, are characterized by progressive cognitive decline, neurodegeneration, and intraneuronal aggregates comprised largely of the axonal protein Tau. It has been unclear whether cognitive deficits are a consequence of aggregate accumulation thought to compromise neuronal health and eventually lead to neurodegeneration. This study use the Drosophila tauopathy model and mixed-sex populations to reveal an adult onset pan-neuronal Tau accumulation-dependent decline in learning efficacy and a specific defect in protein synthesis-dependent memory (PSD-M), but not in its protein synthesis-independent variant. It was demonstrated that these neuroplasticity defects are reversible on suppression of new transgenic human Tau expression and surprisingly correlate with an increase in Tau aggregates. Inhibition of aggregate formation via acute oral administration of methylene blue results in re-emergence of deficient memory in animals with suppressed human Tau (hTau)(0N4R) expression. Significantly, aggregate inhibition results in PSD-M deficits in hTau(0N3R)-expressing animals, which present elevated aggregates and normal memory if untreated with methylene blue. Moreover, methylene blue-dependent hTau(0N4R) aggregate suppression within adult mushroom body neurons also resulted in emergence of memory deficits. Therefore, deficient PSD-M on human Tau expression in the Drosophila CNS is not a consequence of toxicity and neuronal loss because it is reversible. Furthermore, PSD-M deficits do not result from aggregate accumulation, which appears permissive, if not protective of processes underlying this memory variant.

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Adedayo, B. C., Akinniyi, S. T., Ogunsuyi, O. B. and Oboh, G. (2023). In the quest for the ideal sweetener: Aspartame exacerbates selected biomarkers in the fruit fly (Drosophila melanogaster) model of Alzheimer's disease more than sucrose Aging Brain 4: 100090. PubMed ID: 37559954

Abstract
This study evaluated the effect of dietary inclusions of aspartame and sucrose on some selected behavioral and biochemical indices linked with Alzheimer's disease in a transgenic fruit fly (Drosophila melanogaster) model expressing human amyloid precursor protein and secretase. Flies were raised on a diet supplemented with sucrose and aspartame for 14 days. Thereafter, the flies were assessed for their survival rate, learning and memory, as well as locomotor performance, 14 days post-treatment. This was followed by homogenising the fly heads, and the homogenates were assayed for acetylcholinesterase and monoamine oxidase activities, as well as levels of lipid peroxidation, reactive oxygen species, and total thiol. The results showed aspartame at all levels of dietary intake and a high proportion of sucrose significantly aggravated the mortality rate, locomotor deficiency, and impaired biomarkers of oxidative stress and antioxidant status in the transgenic flies, while no significant effect was found on acetylcholinesterase activity or memory function. These findings therefore suggest that while low dietary inclusions of sucrose are tolerable under AD-like phenotypes in the flies, high inclusions of sucrose and all proportions of aspartame tested aggravated mortality rate, locomotion and oxidative stress in the flies.

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Miguel, L., Frebourg, T., Campion, D. and Lecourtois, M. (2023). Reduction of Spermine Synthase Suppresses Tau Accumulation Through Autophagy Modulation in Tauopathy. bioRxiv. PubMed ID: 36993333

Abstract
Tauopathy, including Alzheimer Disease (AD), is characterized by Tau protein accumulation and autophagy dysregulation. Emerging evidence connects polyamine metabolism with the autophagy pathway, however the role of polyamines in Tauopathy remains unclear. The present study investigated the role of spermine synthase (SMS) in autophagy regulation and tau protein processing in Drosophila and human cellular models of Tauopathy. A previous study showed that Drosophila spermine synthase (dSms) deficiency impairs lysosomal function and blocks autophagy flux. Interestingly, partial loss-of-function of SMS in heterozygous dSms flies extends lifespan and improves the climbing performance of flies with human Tau (hTau) overexpression. Mechanistic analysis showed that heterozygous loss-of-function mutation of dSms reduces hTau protein accumulation through enhancing autophagic flux. Measurement of polyamine levels detected a mild elevation of spermidine in flies with heterozygous loss of dSms. SMS knock-down in human neuronal or glial cells also upregulates autophagic flux and reduces Tau protein accumulation. Proteomics analysis of postmortem brain tissue from AD patients showed a significant albeit modest elevation of SMS protein level in AD-relevant brain regions compared to that of control brains consistently across several datasets. Taken together, this study uncovers a correlation between SMS protein level and AD pathogenesis and reveals that SMS reduction upregulates autophagy, promotes Tau clearance, and reduces Tau protein accumulation. These findings provide a new potential therapeutic target of Tauopathy.

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Praschberger, R., Kuenen, S., Schoovaerts, N., Kaempf, N., Singh, J., Janssens, J., Swerts, J., Nachman, E., Calatayud, C., Aerts, S., Poovathingal, S. and Verstreken, P. (2023). Neuronal identity defines α-synuclein and tau toxicity. Neuron. PubMed ID: 36948206

Abstract
Pathogenic α-synuclein and tau are critical drivers of neurodegeneration, and their mutations cause neuronal loss in patients. Whether the underlying preferential neuronal vulnerability is a cell-type-intrinsic property or a consequence of increased expression levels remains elusive. This study explored cell-type-specific α-synuclein and tau expression in human brain datasets and use deep phenotyping as well as brain-wide single-cell RNA sequencing of >200 live neuron types in fruit flies to determine which cellular environments react most to α-synuclein or tau toxicity. Phenotypic and transcriptomic evidence was detectedof differential neuronal vulnerability independent of α-synuclein or tau expression levels. Comparing vulnerable with resilient neurons in Drosophila enabled prediction of numerous human neuron subtypes with increased intrinsic susceptibility to pathogenic α-synuclein or tau. By uncovering synapse- and Ca(2+) homeostasis-related genes as tau toxicity modifiers, this work paves the way to leverage neuronal identity to uncover modifiers of neurodegeneration-associated toxic proteins.

Stubbs, K., Batchelor, B., Sivanantharajah, L., Sealey, M., Ramirez-Moreno, M., Ruiz, E., Richardson, B., Perry, V. H., Newman, T. A. and Mudher, A. (2023). Tau-mediated axonal degeneration is prevented by activation of the Wld(S) pathway. Brain Commun 5(2): fcad052. PubMed ID: 37013175
Summary:
Tauopathy is characterized by neuronal dysfunction and degeneration occurring as a result of changes to the microtubule-associated protein tau (see Drosophila Tau). The neuronal changes evident in tauopathy bear striking morphological resemblance to those reported in models of Wallerian degeneration (see Drosophila Wallenda). The mechanisms underpinning Wallerian degeneration are not fully understood although it can be delayed by the expression of the slow Wallerian degeneration (Wld(S)) protein, which has also been demonstrated to delay axonal degeneration in some models of neurodegenerative disease. Given the morphological similarities between tauopathy and Wallerian degeneration, this study investigated whether tau-mediated phenotypes can be modulated by co-expression of Wld(S). In a Drosophila model of tauopathy in which expression of human 0N3R tau protein leads to progressive age-dependent phenotypes, Wld(S) was expressed with and without activation of the downstream pathway. The olfactory receptor neuron circuit OR47b was used for these studies in adults, and the larval motor neuron system was employed in larvae. Tau phenotypes studied included neurodegeneration, axonal transport, synaptic deficits and locomotor behaviour. Impact on total tau was ascertained by assessing total, phosphorylated and misfolded tau levels by immunohistochemistry. Activation of the pathway downstream of Wld(S) completely suppressed tau-mediated degeneration. This protective effect was evident even if the pathway downstream of Wld(S) was activated several weeks after tau-mediated degeneration had become established. Though total tau levels were not altered, the protected neurons displayed significantly reduced MC1 immunoreactivity suggestive of clearance of misfolded tau, as well as a trend for a decline in tau species phosphorylated at the AT8 and PHF1 epitopes. In contrast, Wld(S) expression without activation of the downstream protective pathway did not rescue tau-mediated degeneration in adults or improve tau-mediated neuronal dysfunction including deficits in axonal transport, synaptic alterations and locomotor behaviour in tau-expressing larvae. This collectively implies that the pathway mediating the protective effect of Wld(S) intersects with the mechanism(s) of degeneration initiated by tau and can effectively halt tau-mediated degeneration at both early and late stages. Understanding the mechanisms underpinning this protection could identify much-needed disease-modifying targets for tauopathies.

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Thomas, E. O., Ramirez, P., Hyman, B. T., Ray, W. J. and Frost, B. (2022). Testing the neuroinflammatory role of tau-induced transposable elements in tauopathy. Alzheimers Dement 17 Suppl 2: e058664. Link to article: Thomas et al.
Summary:
Novel mechanisms through which pathogenic tau leads to neuronal toxicity remain largely unexplored as therapeutic targets for tauopathies. Activation of retrotransposons has been identified as a mechanism by which pathogenic tau contributes to neurotoxicity. This study has tested the hypothesis that transposable element-derived dsRNAs stimulate the immune response in tauopathy. Bioinformatic analysis of bidirectional transcription was completed on RNA-sequencing data from control and tau transgenic Drosophila. dsRNAs were quantified by J2 anti-dsRNA antibody via immunofluorescence and ELISA. Analysis of inflammatory markers in Drosophila was completed by RNA-sequencing and with a custom NanoString Gene Expression codeset. dsRNAs were found to be elevated in the brains of tau transgenic Drosophila, in the cortex of a mouse model of tauopathy, and in post-mortem brain tissue from human tauopathy. Retrotransposons were identified that are bidirectionally transcribed in tau transgenic Drosophila. RNA sequencing and NanoString analysis of tau transgenic flies reveals elevated levels of innate immune response gene transcripts. Loss of transposon transcriptional silencing results in similar dsRNA production and immune system activation found in tau transgenic Drosophila. The goal of this study was to determine if RNA intermediates from tau-induced transposable elements are a therapeutic target for tauopathies. Overall, the findings support a model where dsRNA production induced by loss of transcriptional transposon silencing contributes to inflammation in tauopathy.

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Hao, Y., Shao, L., Hou, J., Zhang, Y., Ma, Y., Liu, J., Xu, C., Chen, F., Cao, L. H. and Ping, Y. (2023). Resveratrol and Sir2 Reverse Sleep and Memory Defects Induced by Amyloid Precursor Protein. Neurosci Bull. PubMed ID: 37041405
Summary:
Resveratrol (RES), a natural polyphenolic phytochemical, has been suggested as a putative anti-aging molecule for the prevention and treatment of Alzheimer's disease (AD) by the activation of sirtuin 1 (Sirt1/Sir2). This study, tested the effects of RES and Sirt1/Sir2 on sleep and courtship memory in a Drosophila model by overexpression of amyloid precursor protein (APP), whose duplications and mutations cause familial AD. A mild but significant transcriptional increase of Drosophila Sir2 (dSir2) was found by RES supplementation for up to 17 days in APP flies, but not for 7 days. RES and dSir2 almost completely reversed the sleep and memory deficits in APP flies. It was further demonstrated that dSir2 acts as a sleep promotor in Drosophila neurons. Interestingly, RES increased sleep in the absence of dSir2 in dSir2-null mutants, and RES further enhanced sleep when dSir2 was either overexpressed or knocked down in APP flies. Finally, this study showed that A&betal aggregates in APP flies were reduced by RES and dSir2, probably via inhibiting Drosophila β-secretase (dBACE). These data suggest that RES rescues the APP-induced behavioral deficits and Aβ burden largely, but not exclusively, via dSir2.

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Periasamy, A., Mitchell, N., Zaytseva, O., Chahal, A. S., Zhao, J., Colman, P. M., Quinn, L. M. and Gulbis, J. M. (2022). An increase in mitochondrial TOM activates apoptosis to drive retinal neurodegeneration. Sci Rep 12(1): 21634. PubMed ID: 36517509
Summary:
Intronic polymorphic TOMM40 variants increasing TOMM40 mRNA expression are strongly correlated to late onset Alzheimer's Disease. The gene product, hTomm40, encoded in the APOE gene cluster, is a core component of TOM, the translocase that imports nascent proteins across the mitochondrial outer membrane. This study used Drosophila melanogaster eyes as an in vivo model to investigate the relationship between elevated Tom40 (the Drosophila homologue of hTomm40) expression and neurodegeneration. Evidence is provided that an overabundance of Tom40 in mitochondria invokes caspase-dependent cell death in a dose-dependent manner, leading to degeneration of the primarily neuronal eye tissue. Degeneration is contingent on the availability of co-assembling TOM components, indicating that an increase in assembled TOM is the factor that triggers apoptosis and degeneration in a neural setting. Eye death is not contingent on inner membrane translocase components, suggesting it is unlikely to be a direct consequence of impaired import. Another effect of heightened Tom40 expression is upregulation and co-association of a mitochondrial oxidative stress biomarker, DmHsp22, implicated in extension of lifespan, providing new insight into the balance between cell survival and death. Activation of regulated death pathways, culminating in eye degeneration, suggests a possible causal route from TOMM40 polymorphisms to neurodegenerative disease.

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Hao, Y., Shao, L., Hou, J., Zhang, Y., Ma, Y., Liu, J., Xu, C., Chen, F., Cao, L. H. and Ping, Y. (2023). Resveratrol and Sir2 Reverse Sleep and Memory Defects Induced by Amyloid Precursor Protein. Neurosci Bull. PubMed ID: 37041405
Summary:
Resveratrol (RES), a natural polyphenolic phytochemical, has been suggested as a putative anti-aging molecule for the prevention and treatment of Alzheimer's disease (AD) by the activation of sirtuin 1 (Sirt1/Sir2). This study tested the effects of RES and Sirt1/Sir2 on sleep and courtship memory in a Drosophila model by overexpression of amyloid precursor protein (APP), whose duplications and mutations cause familial AD. A mild but significant transcriptional increase was found of Drosophila Sir2 (dSir2) by RES supplementation for up to 17 days in APP flies, but not for 7 days. RES and dSir2 almost completely reversed the sleep and memory deficits in APP flies. It was further demonstrated that dSir2 acts as a sleep promotor in Drosophila neurons. Interestingly, RES increased sleep in the absence of dSir2 in dSir2-null mutants, and RES further enhanced sleep when dSir2 was either overexpressed or knocked down in APP flies. Finally, it was shown that Aβ aggregates in APP flies were reduced by RES and dSir2, probably via inhibiting Drosophila β-secretase (dBACE). The data suggest that RES rescues the APP-induced behavioral deficits and Aβ burden largely, but not exclusively, via dSir2.

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Ochoa, E., Ramirez, P., Gonzalez, E., De Mange, J., Ray, W. J., Bieniek, K. F. and Frost, B. (2023). Pathogenic tau-induced transposable element-derived dsRNA drives neuroinflammation. Sci Adv 9(1): eabq5423. PubMed ID: 36608133
Summary:
Deposition of tau protein aggregates in the brain of affected individuals is a defining feature of "tauopathies," including Alzheimer's disease. Studies of human brain tissue and various model systems of tauopathy report that toxic forms of tau negatively affect nuclear and genomic architecture, identifying pathogenic tau-induced heterochromatin decondensation and consequent retrotransposon activation as a causal mediator of neurodegeneration. On the basis of their similarity to retroviruses, retrotransposons drive neuroinflammation via toxic intermediates, including double-stranded RNA (dsRNA). dsRNA and dsRNA sensing machinery were found to be elevated in astrocytes of postmortem brain tissue from patients with Alzheimer's disease and progressive supranuclear palsy and in brains of tau transgenic mice. Using a Drosophila model of tauopathy, this study identified specific tau-induced retrotransposons that form dsRNA and find that pathogenic tau and heterochromatin decondensation causally drive dsRNA-mediated neurodegeneration and neuroinflammation. This study suggests that pathogenic tau-induced heterochromatin decondensation and retrotransposon activation cause elevation of inflammatory, transposable element-derived dsRNA in the adult brain.

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Bhatnagar, A., Krick, K., Karisetty, B. C., Armour, E. M., Heller, E. A. and Elefant, F. (2023). Tip60's Novel RNA-Binding Function Modulates Alternative Splicing of Pre-mRNA Targets Implicated in Alzheimer's Disease. J Neurosci 43(13): 2398-2423. PubMed ID: 36849418
Summary:
The severity of Alzheimer's disease (AD) progression involves a complex interplay of genetics, age, and environmental factors orchestrated by histone acetyltransferase (HAT)-mediated neuroepigenetic mechanisms. While disruption of Tip60 HAT action in neural gene control is implicated in AD, alternative mechanisms underlying Tip60 function remain unexplored. This study reports a novel RNA binding function for Tip60 in addition to its HAT function. Tip60 preferentially interacts with pre-mRNAs emanating from its chromatin neural gene targets in the Drosophila brain and this RNA binding function is conserved in human hippocampus and disrupted in Drosophila brains that model AD pathology and in AD patient hippocampus of either sex. Since RNA splicing occurs co-transcriptionally and alternative splicing (AS) defects are implicated in AD, this study investigated whether Tip60-RNA targeting modulates splicing decisions and whether this function is altered in AD. Replicate multivariate analysis of transcript splicing (rMATS) analysis of RNA-Seq datasets from wild-type and AD fly brains revealed a multitude of mammalian-like AS defects. Strikingly, over half of these altered RNAs are identified as bona-fide Tip60-RNA targets that are enriched for in the AD-gene curated database, with some of these AS alterations prevented against by increasing Tip60 in the fly brain. Further, human orthologs of several Tip60-modulated splicing genes in Drosophila are well characterized aberrantly spliced genes in human AD brains, implicating disruption of Tip60's splicing function in AD pathogenesis. These results support a novel RNA interaction and splicing regulatory function for Tip60 that may underly AS impairments that hallmark AD etiology.

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Hao, Y., Shao, L., Hou, J., Zhang, Y., Ma, Y., Liu, J., Xu, C., Chen, F., Cao, L. H. and Ping, Y. (2023). Resveratrol and Sir2 Reverse Sleep and Memory Defects Induced by Amyloid Precursor Protein. Neurosci Bull. PubMed ID: 37041405
Summary:
Resveratrol (RES), a natural polyphenolic phytochemical, has been suggested as a putative anti-aging molecule for the prevention and treatment of Alzheimer's disease (AD) by the activation of sirtuin 1 (Sirt1/Sir2). This study, tested the effects of RES and Sirt1/Sir2 on sleep and courtship memory in a Drosophila model by overexpression of amyloid precursor protein (APP), whose duplications and mutations cause familial AD. A mild but significant transcriptional increase of Drosophila Sir2 (dSir2) was found by RES supplementation for up to 17 days in APP flies, but not for 7 days. RES and dSir2 almost completely reversed the sleep and memory deficits in APP flies. It was further demonstrated that dSir2 acts as a sleep promotor in Drosophila neurons. Interestingly, RES increased sleep in the absence of dSir2 in dSir2-null mutants, and RES further enhanced sleep when dSir2 was either overexpressed or knocked down in APP flies. Finally, this study showed that A&betal aggregates in APP flies were reduced by RES and dSir2, probably via inhibiting Drosophila β-secretase (dBACE). These data suggest that RES rescues the APP-induced behavioral deficits and Aβ burden largely, but not exclusively, via dSir2.

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Periasamy, A., Mitchell, N., Zaytseva, O., Chahal, A. S., Zhao, J., Colman, P. M., Quinn, L. M. and Gulbis, J. M. (2022). An increase in mitochondrial TOM activates apoptosis to drive retinal neurodegeneration. Sci Rep 12(1): 21634. PubMed ID: 36517509
Summary:
xf Intronic polymorphic TOMM40 variants increasing TOMM40 mRNA expression are strongly correlated to late onset Alzheimer's Disease. The gene product, hTomm40, encoded in the APOE gene cluster, is a core component of TOM, the translocase that imports nascent proteins across the mitochondrial outer membrane. This study used Drosophila melanogaster eyes as an in vivo model to investigate the relationship between elevated Tom40 (the Drosophila homologue of hTomm40) expression and neurodegeneration. Evidence is provided that an overabundance of Tom40 in mitochondria invokes caspase-dependent cell death in a dose-dependent manner, leading to degeneration of the primarily neuronal eye tissue. Degeneration is contingent on the availability of co-assembling TOM components, indicating that an increase in assembled TOM is the factor that triggers apoptosis and degeneration in a neural setting. Eye death is not contingent on inner membrane translocase components, suggesting it is unlikely to be a direct consequence of impaired import. Another effect of heightened Tom40 expression is upregulation and co-association of a mitochondrial oxidative stress biomarker, DmHsp22, implicated in extension of lifespan, providing new insight into the balance between cell survival and death. Activation of regulated death pathways, culminating in eye degeneration, suggests a possible causal route from TOMM40 polymorphisms to neurodegenerative disease.

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Beckmann, A., Ramirez, P., Gamez, M., Gonzalez, E., De Mange, J., Bieniek, K. F., Ray, W. J. and Frost, B. (2023). Moesin is an effector of tau-induced actin overstabilization, cell cycle activation, and neurotoxicity in Alzheimer's disease. iScience 26(3): 106152. PubMed ID: 36879821
Summary:
In Alzheimer's disease, neurons acquire phenotypes that are also present in various cancers, including aberrant activation of the cell cycle. Unlike cancer, cell cycle activation in post-mitotic neurons is sufficient to induce cell death. Multiple lines of evidence suggest that abortive cell cycle activation is a consequence of pathogenic forms of tau, a protein that drives neurodegeneration in Alzheimer's disease and related "tauopathies." This study combined network analyses of human Alzheimer's disease and mouse models of Alzheimer's disease and primary tauopathy with studies in Drosophila to discover that pathogenic forms of tau drive cell cycle activation by disrupting a cellular program involved in cancer and the epithelial-mesenchymal transition (EMT). Moesin, an EMT driver, is elevated in cells harboring disease-associated phosphotau, over-stabilized actin, and ectopic cell cycle activation. It was further found that genetic manipulation of Moesin mediates tau-induced neurodegeneration. Taken together, this study identifies novel parallels between tauopathy and cancer.

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Gandini, A., Goncalves, A. E., Strocchi, S., Albertini, C., Janockova, J., Tramarin, A., Grifoni, D., Poeta, E., Soukup, O., Munoz-Torrero, D., Monti, B., Sabate, R., Bartolini, M., Legname, G. and Bolognesi, M. L. (2022). Discovery of Dual Abeta/Tau Inhibitors and Evaluation of Their Therapeutic Effect on a Drosophila Model of Alzheimer's Disease. ACS Chem Neurosci 13(23): 3314-3329. PubMed ID: 36445009
Summary:
Alzheimer's disease (AD), the most common type of dementia, currently represents an extremely challenging and unmet medical need worldwide. Amyloid-β (Aβ) and Tau proteins are prototypical AD hallmarks, as well as validated drug targets. Accumulating evidence now suggests that they synergistically contribute to disease pathogenesis. This could not only help explain negative results from anti-Aβ clinical trials but also indicate that therapies solely directed at one of them may have to be reconsidered. This study describes the development of a focused library of 2,4-thiazolidinedione (TZD)-based bivalent derivatives as dual Aβ and Tau aggregation inhibitors. The aggregating activity of the 24 synthesized derivatives was tested in intact Escherichia coli cells overexpressing Aβ(42) and Tau proteins. Their neuronal toxicity and ability to cross the blood-brain barrier (BBB) were evaluated, together with the in vitro interaction with the two isolated proteins. Finally, the most promising (most active, nontoxic, and BBB-permeable) compounds 22 and 23 were tested in vivo, in a Drosophila melanogaster model of AD. The carbazole derivative 22 (20 &mi;M) showed extremely encouraging results, being able to improve both the lifespan and the climbing abilities of Aβ(42) expressing flies and generating a better outcome than doxycycline (50 &mi;M). Moreover, 22 proved to be able to decrease Aβ(42) aggregates in the brains of the flies. It is concluded that bivalent small molecules based on 22 deserve further attention as hits for dual Aβ/Tau aggregation inhibition in AD.

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Kim, J., de Haro, M., Al-Ramahi, I., Garaicoechea, L. L., Jeong, H. H., Sonn, J. Y., Tadros, B., Liu, Z., Botas, J. and Zoghbi, H. Y. (2022). Evolutionarily conserved regulators of tau identify targets for new therapies. Neuron. PubMed ID: 36610398
Summary:
Tauopathies are neurodegenerative diseases that involve the pathological accumulation of tau proteins; in this family are Alzheimer disease, corticobasal degeneration, and chronic traumatic encephalopathy, among others. Hypothesizing that reducing this accumulation could mitigate pathogenesis, a cross-species genetic screen was performed targeting 6,600 potentially druggable genes in human cells and Drosophila. 83 hits were found and validated in cells and 11 hits were further validated in the mouse brain. Three of these hits (USP7, RNF130, and RNF149) converge on the C terminus of Hsc70-interacting protein (CHIP) to regulate tau levels, highlighting the role of CHIP in maintaining tau proteostasis in the brain. Knockdown of each of these three genes in adult tauopathy mice reduced tau levels and rescued the disease phenotypes. This study thus identifies several points of intervention to reduce tau levels and demonstrates that reduction of tau levels via regulation of this pathway is a viable therapeutic strategy for Alzheimer disease and other tauopathies.

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Ring, J., Tadic, J., Ristic, S., Poglitsch, M., Bergmann, M., ..., Hansen, N., Sommer, C., Ninkovic, M., Seba, S., Rockenfeller, P., Vogtle, F. N., Dengjel, J., Meisinger, C., Keller, A., Sigrist, S. J., Eisenberg, T. and Madeo, F. (2022). The HSP40 chaperone Ydj1 drives amyloid beta 42 toxicity.EMBO Mol Med: e13952. PubMed ID: 35373908
Summary:
Amyloid beta 42 (Abeta42; see Drosophila Appl) is the principal trigger of neurodegeneration during Alzheimer's disease (AD). However, the etiology of its noxious cellular effects remains elusive. In a combinatory genetic and proteomic approach using a yeast model to study aspects of intracellular Abeta42 toxicity, this study identified the HSP40 family member Ydj1, the yeast orthologue of human DnaJA1, as a crucial factor in Abeta42-mediated cell death. It was demonstrate that Ydj1/DnaJA1 physically interacts with Abeta42 (in yeast and mouse), stabilizes Abeta42 oligomers, and mediates their translocation to mitochondria. Consequently, deletion of YDJ1 strongly reduces co-purification of Abeta42 with mitochondria and prevents Abeta42-induced mitochondria-dependent cell death. Consistently, purified DnaJ chaperone delays Abeta42 fibrillization in vitro, and heterologous expression of human DnaJA1 induces formation of Abeta42 oligomers and their deleterious translocation to mitochondria in vivo. Finally, downregulation of the Ydj1 fly homologue, Droj2, improves stress resistance, mitochondrial morphology, and memory performance in a Drosophila melanogaster AD model. These data reveal an unexpected and detrimental role for specific HSP40s in promoting hallmarks of Abeta42 toxicity.

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Moulton, M. J., Barish, S., Ralhan, I., Chang, J., Goodman, L. D., Harland, J. G., Marcogliese, P. C., Johansson, J. O., Ioannou, M. S. and Bellen, H. J. (2021). Neuronal ROS-induced glial lipid droplet formation is altered by loss of Alzheimer's disease-associated genes. Proc Natl Acad Sci U S A 118(52). PubMed ID: 34949639
Summary:
A growing list of Alzheimer's disease (AD) genetic risk factors is being identified, but the contribution of each variant to disease mechanism remains largely unknown. Previous work has shown that elevated levels of reactive oxygen species (ROS) induces lipid synthesis in neurons leading to the sequestration of peroxidated lipids in glial lipid droplets (LD), delaying neurotoxicity. This neuron-to-glia lipid transport is APOD/E-dependent. To identify proteins that modulate these neuroprotective effects, the role was tested of AD risk genes in ROS-induced LD formation, and several genes were demonstrated to impact neuroprotective LD formation, including homologs of human ABCA1 (Drosophila ATP binding cassette subfamily A member 3), ABCA7, VLDLR, VPS26, VPS35, AP2A, PICALM, and CD2AP. These data also show that ROS enhances A&betal42 phenotypes in flies and mice. Finally, a peptide agonist of ABCA1 restores glial LD formation in a humanized APOE4 fly model, highlighting a potentially therapeutic avenue to prevent ROS-induced neurotoxicity. This study places many AD genetic risk factors in a ROS-induced neuron-to-glia lipid transfer pathway with a critical role in protecting against neurotoxicity.

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Hayne, M. and DiAntonio, A. (2022).Protein phosphatase 2A restrains DLK signaling to promote proper Drosophila synaptic development and mammalian cortical neuron survival. Neurobiol Dis 163: 105586. PubMed ID: 34923110
Summary:
Protein phosphatase 2A (PP2A) is a major cellular phosphatase with many protein substrates. In post-mitotic neurons, the microtubule associated protein Tau is a particularly well-studied PP2A substrate as hyperphosphorylation of Tau is a hallmark of Alzheimer's disease. This study finds that activation of a single pathway can explain important aspects of the PP2A loss-of-function phenotype in neurons. PP2A inhibits activation of the neuronal stress kinase DLK and its Drosophila ortholog Wallenda. In the fly, PP2A inhibition activates a DLK/Wallenda-regulated transcriptional program that induces synaptic terminal overgrowth at the neuromuscular junction. In cultured mammalian neurons, PP2A inhibition activates a DLK-dependent apoptotic program that induces cell death. Contrary to expectations, in the absence of Tau PP2A inhibition still activates DLK and induces neuronal cell death, demonstrating that hyperphosphorylated Tau is not required for cell death in this model. Moreover, hyperphosphorylation of Tau following PP2A inhibition does not require DLK. Hence, loss of PP2A function in cortical neurons triggers two independent neuropathologies: 1) Tau hyperphosphorylation and 2) DLK activation and subsequent neuronal cell death. These findings demonstrate that inhibition of the DLK pathway is an essential function of PP2A required for normal Drosophila synaptic terminal development and mammalian cortical neuron survival.

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Deolankar, S. C., Najar, M. A., Raghu, S. V. and Prasad, T. S. K.(2022). Abeta42 Expressing Drosophila melanogaster Model for Alzheimer's Disease: Quantitative Proteomics Identifies Altered Protein Dynamics of Relevance to Neurodegeneration. Omics 26(1): 51-63. PubMed ID: 35006003
Summary:
Production and deposition of β-amyloid peptides (Aβ) are among the major hallmarks of the pathogenesis of Alzheimer's disease (AD). Mapping the altered protein dynamics associated with Aβ accumulation and neuronal damage may open up new avenues to innovation for drug target discovery in AD. Using quantitative proteomics, new findings are reported from the amyloid beta-peptide with 42 amino acids (Aβ42) expressing Drosophila melanogaster model for AD compared to that of the wild-type flies. 302,241 peptide-spectrum matches were identified with 25,641 nonredundant peptides corresponding to 7959 D. melanogaster proteins. Furthermore, 538 significantly altered proteins were unraveled in Aβ42 expressing flies. These differentially expressed proteins were enriched for biological processes associated with neuronal damage leading to AD progression. 463 unique post-translational modification events mapping to 202 proteins were identified from the same dataset. Among these, 303 modified peptides corresponding to 246 proteins were also altered in the AD model. These modified proteins are known to be involved in the disruption of molecular functions maintaining neuronal plasticity. This study provides new molecular leads on altered protein dynamics relevant to neurodegeneration, neuroplasticity, and AD progression induced by Aβ42 toxicity. These proteins may prove useful to discover new drugs in an AD model of D. melanogaster and evaluate their efficacy and mode of molecular action in the future.

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Lambert, E., Saha, O., Soares Landeira, B., Melo de Farias, A. R., Hermant, X., Carrier, A., Pelletier, A., Gadaut, J., Davoine, L., Dupont, C., Amouyel, P., Bonnefond, A., Lafont, F., Abdelfettah, F., Verstreken, P., Chapuis, J., Barois, N., Delahaye, F., Dermaut, B., Lambert, J. C., Costa, M. R. and Dourlen, P. (2022). The Alzheimer susceptibility gene BIN1 induces isoform-dependent neurotoxicity through early endosome defects. Acta Neuropathol Commun 10(1): 4. PubMed ID: 34998435
Summary:
The Bridging Integrator 1 (BIN1) gene is a major susceptibility gene for Alzheimer's disease (AD). Deciphering its pathophysiological role is challenging due to its numerous isoforms. This study observed in Drosophila that human BIN1 isoform1 (BIN1iso1) overexpression, contrary to human BIN1 isoform8 (BIN1iso8) and human BIN1 isoform9 (BIN1iso9), induced an accumulation of endosomal vesicles and neurodegeneration. Systematic search for endosome regulators able to prevent BIN1iso1-induced neurodegeneration indicated that a defect at the early endosome level is responsible for the neurodegeneration. In human induced neurons (hiNs) and cerebral organoids, BIN1 knock-out resulted in the narrowing of early endosomes. This phenotype was rescued by BIN1iso1 but not BIN1iso9 expression. Finally, BIN1iso1 overexpression also led to an increase in the size of early endosomes and neurodegeneration in hiNs. Altogether, thee data demonstrate that the AD susceptibility gene BIN1, and especially BIN1iso1, contributes to early-endosome size deregulation, which is an early pathophysiological hallmark of AD pathology.

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Sun, Z. D., Hu, J. X., Wu, J. R., Zhou, B. and Huang, Y. P. (2022). Tau-induced deficits in nonsense-mediated mRNA decay contribute to neurodegeneration. Toxicities of amyloid-beta and tau protein are reciprocally enhanced in the Drosophila model. Neural Regen Res 17(10): 2286-2292. PubMed ID: 35259851
Summary:
Extracellular aggregation of amyloid-beta (A&betaa;) and intracellular tau tangles are two major pathogenic hallmarks and critical factors of Alzheimer's disease. A linear interaction between Aβ and tau protein has been characterized in several models. Aβ induces tau hyperphosphorylation through a complex mechanism; however, the master regulators involved in this linear process are still unclear. In this study with Drosophila melanogaster, it was found that Aβ regulated tau hyperphosphorylation and toxicity by activating c-Jun N-terminal kinase. Importantly, Aβ toxicity was dependent on tau hyperphosphorylation, and flies with hypophosphorylated tau were insulated against Aβ-induced toxicity. Strikingly, tau accumulation reciprocally interfered with Aβ degradation and correlated with the reduction in mRNA expression of genes encoding Aβ-degrading enzymes, including dNep1, dNep3, dMmp2, dNep4, and dIDE. These results indicate that AAβ and tau protein work synergistically to further accelerate Alzheimer's disease progression and may be considered as a combined target for future development of Alzheimer's disease therapeutics.

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Zuniga, G., Levy, S., Ramirez, P., De Mange, J., Gonzalez, E., Gamez, M. and Frost, B. (2022). Tau-induced deficits in nonsense-mediated mRNA decay contribute to neurodegeneration. Alzheimers Dement. PubMed ID: 35416419
Summary:
While brains of patients with Alzheimer's disease and related tauopathies have evidence of altered RNA processing, a mechanistic understanding is not available of how altered RNA processing arises in these disorders and if such changes are causally linked to neurodegeneration. Using Drosophila melanogaster models of tauopathy, this study found that overall activity of nonsense-mediated mRNA decay (NMD), a key RNA quality-control mechanism, is reduced. Genetic manipulation of NMD machinery significantly modifies tau-induced neurotoxicity, suggesting that deficits in NMD are causally linked to neurodegeneration. Mechanistically, this study found that deficits in NMD are a consequence of aberrant RNA export and RNA accumulation within nuclear envelope invaginations in tauopathy. A pharmacological activator of NMD was identified that suppresses neurodegeneration in tau transgenic Drosophila, indicating that tau-induced deficits in RNA quality control are druggable. These studies suggest that NMD activators should be explored for their potential therapeutic value to patients with tauopathies.

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da Costa Silva, J. R., Fujimura, P. T., Batista, L. L., Malta, S. M., Filho, R. M., Silva, M. H., de Souza, A. G., Silva, A. P. M., Borges, L. D. F., Bastos, V. A. F., Cossolin, J. F. S., Serrao, J. E., Bonetti, A. M., Junior, L. C. O. and Ueira-Vieira, C. (2022). Differential gene expression by RNA-seq during Alzheimer's disease-like progression in the Drosophila melanogaster model. Neurosci Res. PubMed ID: 35219723
Summary:
Alzheimer's disease (AD) is characterized by progressive, irreversible loss of memory and cognitive function. Drosophila melanogaster and other animal models are used to study several diseases, in order to elucidate unknown mechanisms and develop potential therapies. Molecular studies require biological samples and, for neuropathologies such as AD biopsy of the human brain, are invasive and potentially damaging. The solution is to use animal models, such as D. melanogaster, which is a model organism that can replace mammalian organisms in such studies. This study evaluated the climbing ability and differential gene expression during AD progression due to the amylodoigenic pathway using RNA-seq, and an in silico analysis of a fruit fly AD-like GFP (Green Fluorescent Protein) model was performed with GFP expression in the pan-neural elav driver. A total of 1388 genes were differentially expressed in all analyzed groups. The main pathways related to those Differentially Expressed Genes (DEGs) during aging and AD progression were evaluated using the fly genes and human orthologs, in order to link genomic information to higher-order functional information with gene pathway mapping. Pathways were identified as present in all analyzed groups, such as metabolic pathways, ribosomal pathways, proteasome pathways and immune system pathways. Some of the genes were validated by qPCR. Knockdown of CG17754 gene by RNAi promoted degeneration in the fly eye, validating these findings in vivo. The identification of similarities in molecular pathways between the transgenic fly AD-like GFP model and mammals related to AD provides new insights into the use of this fly in screening novel anti-AD drugs.

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Sun, Z. D., Hu, J. X., Wu, J. R., Zhou, B. and Huang, Y. P. (2022). Toxicities of amyloid-beta and tau protein are reciprocally enhanced in the Drosophila model. Neural Regen Res 17(10): 2286-2292. PubMed ID: 35259851
Summary:
Extracellular aggregation of amyloid-beta (Aβ) and intracellular tau tangles are two major pathogenic hallmarks and critical factors of Alzheimer's disease. A linear interaction between Aβ and tau protein has been characterized in several models. Aβ induces tau hyperphosphorylation through a complex mechanism; however, the master regulators involved in this linear process are still unclear. In a study with Drosophila melanogaster, was found to regulate tau hyperphosphorylation and toxicity by activating c-Jun N-terminal kinase. Importantly, Aβ toxicity was dependent on tau hyperphosphorylation, and flies with hypophosphorylated tau were insulated against Aβ-induced toxicity. Strikingly, tau accumulation reciprocally interfered with Aβ degradation and correlated with the reduction in mRNA expression of genes encoding Aβ-degrading enzymes, including dNep1, dNep3, dMmp2, dNep4, and dIDE. These results indicate that Aβ and tau protein work synergistically to further accelerate Alzheimer's disease progression and may be considered as a combined target for future development of Alzheimer's disease therapeutics.

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Prifti, E., Tsakiri, E. N., Vourkou, E., Stamatakis, G., Samiotaki, M., Skoulakis, E. M. C. and Papanikolopoulou, K. (2022). Systems genetic dissection of Alzheimer's disease brain gene expression networks. Mical modulates Tau toxicity via cysteine oxidation in vivo. Acta Neuropathol Commun 10(1): 44. PubMed ID: 35379354
Summary:
Tau accumulation is clearly linked to pathogenesis in Alzheimer's disease and other Tauopathies. However, processes leading to Tau fibrillization and reasons for its pathogenicity remain largely elusive. Mical emerged as a novel interacting protein of human Tau expressed in Drosophila brains. Mical is characterized by the presence of a flavoprotein monooxygenase domain that generates redox potential with which it can oxidize target proteins. In the well-established Drosophila Tauopathy model, genetic interactions were used to show that Mical alters Tau interactions with microtubules and the Actin cytoskeleton and greatly affects Tau aggregation propensity and Tau-associated toxicity and dysfunction. Exploration of the mechanism was pursued using a Mical inhibitor, a mutation in Mical that selectively disrupts its monooxygenase domain, Tau transgenes mutated at cysteine residues targeted by Mical and mass spectrometry analysis to quantify cysteine oxidation. The collective evidence strongly indicates that Mical's redox activity mediates the effects on Tau via oxidation of Cys322. Importantly, results from the fly model in human Tauopathy samples were validated by showing that MICAL1 is up-regulated in patient brains and co-localizes with Tau in Pick bodies. This work provides mechanistic insights into the role of the Tau cysteine residues as redox-switches regulating the process of Tau self-assembly into inclusions in vivo, its function as a cytoskeletal protein and its effect on neuronal toxicity and dysfunction.

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Srivastav, A. and Mohideen, S. S. (2021). Intestinal microbial dysbiosis induced tau accumulation establishes a gut-brain correlation for pathology Alzheimer's disease in Drosophila melanogaster. Alzheimers Dement 17 Suppl 2: e058708. PubMed ID: 34971141
Summary:
To analyze the concept of whether enteric dysbiosis contributes to the aggravation of tau protein, Drosophila melanogaster was used as a transgenic model organism. To study how the exposure of Pseudomonas sp., a gram-negative bacteria, influences tau accumulation and neurodegeneration thereby understanding the gut-brain correlation for the pathology of Alzheimer's Disease. The versatile Gal4-UAS gene expression system was used to express wild-type tau with the help of pan-eye specific GMR-Gal4 in the flies. All the flies were exposed to Pseudomonas sp. to initiate the gut microbial dysbiosis. The extent of neurodegeneration was quantified by automated analysis of the degenerated region in the eye, the number of aggregates, and immunofluorescence quantification in the medulla. To understand the response of glial cells upon bacterial infection in tauopathy-related dementia, tau was driven by glia-specific Repo-GAL4. Similarly, neuroinflammation which is found in tauopathies was studied using an eye-specific driver line GMR-GAL4, UAS- eiger/CyO. Upon exposure of Pseudomonas sp. on the wild type tau (tauwt) around the medulla, the degeneration was faster and, there was an increase in the levels of tau as compared to the control. Downregulation of Atg 1 and Atg 18 gene along with significant upregulation of Atg 12 is observed in tau expressing infected flies compared to tau expressing control flies. There was a significant degeneration as well as an increase in the number of aggregates in the medulla of the optic lobe. Similar degeneration was observed in flies expressing tauwt and egr along with an increased accumulation of tau in both retina and medulla. When the flies expressing tau are subjected to the pathogen, and aggravation in tau accumulation is observed along with neuroinflammation and changes in the homeostasis. Due to its role in maintaining of body's homeostasis, the gut-brain correlation can have both detrimental and beneficial effects on the brain and the survival of neurons.

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Smith, C. A., Smith, H., Roberts, L., Coward, L., Gorman, G., Verma, A., Li, Q., Buford, T. W., Carter, C. S. and Jumbo-Lucioni, P. (2021). Probiotic Releasing Angiotensin (1-7) in a Drosophila Model of Alzheimer's Disease Produces Sex-Specific Effects on Cognitive Function. J Alzheimers Dis. PubMed ID: 34924372
Summary:
While extensive research on the brain has failed to identify effective therapies, using probiotics to target the gut microbiome has shown therapeutic potential in Alzheimer's disease (AD). Genetically modified probiotics (GMP) are a promising strategy to deliver key therapeutic peptides with high efficacy and tissue specificity. Angiotensin (Ang)-(1-7) levels inversely correlate to AD severity, but its administration is challenging. This lab has successfully established a GMP-based method of Ang-(1-7) delivery. Since Drosophila represents an excellent model to study the effect of probiotics on complex disorders in a high throughput manner, this study tested whether oral supplementation with Lactobacillus paracasei releasing Ang-(1-7) (LP-A) delays memory loss in a Drosophila AD model. Flies overexpressing the human amyloid-β protein precursor and its β-site cleaving enzyme in neurons were randomized to receive four 24-h doses of Lactobacillus paracasei alone (LP), LP-A or sucrose over 14 days. Memory was assessed via an aversive phototaxic suppression assay. Optimal dilution,1:2, was determined based on palatability. LP-A improved memory in trained AD males but worsened cognition in AD females. LP-supplementation experiments confirmed that Ang-(1-7) conferred additional cognitive benefits in males and was responsible for the deleterious cognitive effects in females. Sex-specific differences in the levels of angiotensin peptides and differential activation of the kynurenine pathway of tryptophan metabolism in response to supplementation may underlie this male-only therapeutic response. In summary, LP-A ameliorated the memory deficits of a Drosophila AD model, but effects were sex-specific. Dosage optimization may be required to address this differential response.

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Atrian, F., Ramirez, P. and Frost, B. (2021). Investigating circular RNA-mediated neurotoxicity in tauopathies. Alzheimers Dement 17 Suppl 2: e058483. Article URL
Summary:
Circular RNAs (circRNA) are a subclass of non-coding RNAs with a covalently closed loop structure that are formed via non-canonical splicing. CircRNA biogenesis is regulated by N6-methyladenosine (m(6)A), a post transcriptional RNA modification. Elevated levels of circRNAs and abnormal m(6) A modification significantly correlate with development of Alzheimer's disease. Previous studies have shown that tau-induced dysfunction of lamin causes nuclear polymorphisms including invaginations and blebs. Polyadenylated RNA accumulates within nuclear invaginations in the context of tauopathy, and genetic and pharmacologic reduction of RNA export reduce RNA accumulation within invaginations and suppresses tau neurotoxicity. The role of m(6) A in circRNA accumulation, and their relationship with aging and tauopathy are currently unknown. Based on the association between nuclear polymorphism and RNA export, alongside the global enrichment of circRNAs and disrupted RNA methylation in Alzheimer's disease brains, it was hypothesized that m(6)A-dependent accumulation of circRNAs in tauopathies sequester complementary RNAs and RNA binding proteins into large inclusions that trigger RNA export via nuclear blebbing. It was found that circRNAs accumulate in brains of a Drosophila model of tauopathy and that RNAi-mediated reduction of mbl, which is particularly enriched in its circular form in the brain, significantly suppresses tau-induced neurotoxicity. RNAi-mediated reduction of an m(6)A writer and reader significantly reduces circMbl biogenesis and neuronal death in tau transgenic Drosophila. circMbl lines the nuclear blebs that contain large inclusions in brains of tau transgenic Drosophila. It is concluded that Tau-induced accumulation of circRNA is mediated by aberrant m(6) A modification. Presence of circMbl-lined nuclear buds suggests a potential role of nuclear blebs in circRNA nuclear export.

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Vourkou, E., Paspaliaris, V., Bourouliti, A., Zerva, M. C., Prifti, E., Papanikolopoulou, K. and Skoulakis, E. M. C. (2022). Differential Effects of Human Tau Isoforms to Neuronal Dysfunction and Toxicity in the Drosophila CNS. Int J Mol Sci 23(21). PubMed ID: 36361774

Abstract

Accumulation of highly post-translationally modified tau proteins is a hallmark of neurodegenerative disorders known as tauopathies, the most common of which is Alzheimer's disease. Although six tau isoforms are found in the human brain, the majority of animal and cellular tauopathy models utilize a representative single isoform. However, the six human tau isoforms present overlapping but distinct distributions in the brain and are differentially involved in particular tauopathies. These observations support the notion that tau isoforms possess distinct functional properties important for both physiology and pathology. To address this hypothesis, the six human brain tau isoforms were expressed singly in the Drosophila brain and their effects in an established battery of assays measuring neuronal dysfunction, vulnerability to oxidative stress and life span were systematically assessed comparatively. The results reveal isoform-specific effects clearly not attributed to differences in expression levels but correlated with the number of microtubule-binding repeats and the accumulation of a particular isoform in support of the functional differentiation of these tau isoforms. Delineation of isoform-specific effects is essential to understand the apparent differential involvement of each tau isoform in tauopathies and their contribution to neuronal dysfunction and toxicity (Vourkou, 2022).

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Lee, B., Choi, B., Park, Y., Jang, S., Yuan, C., Lim, C., Lee, J. H., Song, G. J. and Cho, K. S. (2022). Lee, B., Choi, B., Park, Y., Jang, S., Yuan, C., Lim, C., Lee, J. H., Song, G. J. and Cho, K. S.. ACS Chem Neurosci 13(1): 27-42. PubMed ID: 34931800

Abstract

Zinc is a fundamental trace element essential for numerous biological processes, and zinc homeostasis is regulated by the Zrt-/Irt-like protein (ZIP) and zinc transporter (ZnT) families. ZnT7 is mainly localized in the Golgi apparatus and endoplasmic reticulum (ER) and transports zinc into these organelles. Although previous studies have reported the role of zinc in animal physiology, little is known about the importance of zinc in the Golgi apparatus and ER in animal development and neurodegenerative diseases. This study demonstrate that ZnT86D, a Drosophila ortholog of ZnT7, plays a pivotal role in the neurodevelopment and pathogenesis of Alzheimer disease (AD). When ZnT86D was silenced in neurons, the embryo-to-adult survival rate, locomotor activity, and lifespan were dramatically reduced. The toxic phenotypes were accompanied by abnormal neurogenesis and neuronal cell death. Furthermore, knockdown of ZnT86D in the neurons of a Drosophila AD model increased apoptosis and exacerbated neurodegeneration without significant changes in the deposition of amyloid beta plaques and susceptibility to oxidative stress. Taken together, these results suggest that an appropriate distribution of zinc in the Golgi apparatus and ER is important for neuronal development and neuroprotection and that ZnT7 is a potential protective factor against AD (Lee, 2022).

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Singh, Y. P., Kumar, N., Priya, K., Chauhan, B. S., Shankar, G., Kumar, S., Singh, G. K., Srikrishna, S., Garg, P., Singh, G., Rai, G. and Modi, G. (2022). Exploration of Neuroprotective Properties of a Naturally Inspired Multifunctional Molecule (F24) against Oxidative Stress and Amyloid β Induced Neurotoxicity in Alzheimer's Disease Models. ACS Chem Neurosci 13(1): 27-42. PubMed ID: 34931800

Abstract
The pathological hallmarks of Alzheimer's disease (AD) are manifested as an increase in the level of oxidative stress and aggregation of the amyloid-β protein. In vitro, in vivo, and in silico experiments were designed and carried out with multifunctional cholinergic inhibitor, F24 (EJMC-7a) to explore its neuroprotective effects in AD models. The neuroprotection ability of F24 was tested in SH-SY5Y cells, a widely used neuronal cell line. The pretreatment and subsequent co-treatment of SH-SY5Y cells with different doses of F24 was effective in rescuing the cells from H2O2 induced neurotoxicity. F24 treated cells were found to be effective in the reduction of cellular reactive oxygen species, DNA damage, and Aβ(1-42) induced neurotoxicity, which validated its neuroprotective effectiveness. F24 exhibited efficacy in an in vivo Drosophila model by rescuing eye phenotypes from degeneration caused by Aβ toxicity. Further, computational studies were carried out to monitor the interaction between F24 and Aβ(1-42) aggregates. The computational studies corroborated in vitro and in vivo studies suggesting Aβ(1-42) aggregation modulation ability of F24. The brain entry ability of F24 was studied in the parallel artificial membrane permeability assay. Finally, F24 was tested at doses of 1 and 2.5 mg/kg in the Morris water maze AD model. The neuroprotective properties shown by F24 strongly suggest that multifunctional features of this molecule provide symptomatic relief and act as a disease-modifying agent in the treatment of AD. The results from these experiments strongly indicated that natural template-based F24 could serve as a lead molecule for further investigation to explore multifunctional therapeutic agents for AD management.

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Buhl, E., Kim, Y. A., Parsons, T., Zhu, B., Santa-Maria, I., Lefort, R. and Hodge, J. J. L. (2022). Effects of Eph/ephrin signalling and human Alzheimer's disease-associated EphA1 on Drosophila behaviour and neurophysiology. Neurobiol Dis 170: 105752. PubMed ID: 35569721

Abstract

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease placing a great burden on people living with it, careers and society. Yet, the underlying patho-mechanisms remain unknown and treatments limited. To better understand the molecular changes associated with AD, genome-wide association studies (GWAS) have identified hundreds of candidate genes linked to the disease, like the receptor tyrosine kinase EphA1. However, demonstration of whether and how these genes cause pathology is largely lacking. Utilising fly genetics, this study generated the first Drosophila model of human wild-type and P460L mutant EphA1 and tested the effects of Eph/ephrin signalling on AD-relevant behaviour and neurophysiology. EphA1 mis-expression did not cause neurodegeneration, shorten lifespan or affect memory but flies mis-expressing the wild-type or mutant receptor were hyper-aroused, had reduced sleep, a stronger circadian rhythm and increased clock neuron activity and excitability. Over-expression of endogenous fly Eph and RNAi-mediated knock-down of Eph and its ligand ephrin affected sleep architecture and neurophysiology. Eph over-expression led to stronger circadian morning anticipation while ephrin knock-down impaired memory. A dominant negative form of the GTPase Rho1, a potential intracellular effector of Eph, led to hyper-aroused flies, memory impairment, less anticipatory behaviour and neurophysiological changes. These results demonstrate a role of Eph/ephrin signalling in a range of behaviours affected in AD. This presents a starting point for studies into the underlying mechanisms of AD including interactions with other AD-associated genes, like Rho1, Ankyrin, Tau and APP with the potential to identify new targets for treatment.

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Larsson, J. N. K., Nystrom, S. and Hammarstrom, P. (2022). HSP10 as a Chaperone for Neurodegenerative Amyloid Fibrils. Front Neurosci 16: 902600. PubMed ID: 35769706

Abstract

Neurodegenerative diseases (NDs) are associated with accumulated misfolded proteins (MPs). MPs oligomerize and form multiple forms of amyloid fibril polymorphs that dictate fibril propagation and cellular dysfunction. Protein misfolding processes that impair protein homeostasis are implicated in onset and progression of NDs. A wide variety of molecular chaperones safeguard the cell from MP accumulation. A rather overlooked molecular chaperone is HSP10, known as a co-chaperone for HSP60. Due to the ubiquitous presence in human tissues and protein overabundance compared with HSP60, this work shows how HSP10 alone influences fibril formation in vitro of Alzheimer's disease-associated Aβ1-42 (see Appl). At sub-stoichiometric concentrations, eukaryotic HSP10s (human and Drosophila) significantly influenced the fibril formation process and the fibril structure of Aβ1-42, more so than the prokaryotic HSP10 GroES. Similar effects were observed for prion disease-associated prion protein HuPrP90-231. Paradoxically, for a chaperone, low concentrations of HSP10 appeared to promote fibril nucleation by shortened lag-phases, which were chaperone and substrate dependent. Higher concentrations of chaperone while still sub-stoichiometric extended the nucleation and/or the elongation phase. It is hypothesized that HSP10 by means of its seven mobile loops provides the chaperone with high avidity binding to amyloid fibril ends. The preserved sequence of the edge of the mobile loop GGIM(V)L (29-33 human numbering) normally dock to the HSP60 apical domain. Interestingly, this segment shows sequence similarity to amyloidogenic core segments of Aβ1-42, GGVVI (37-41), and HuPrP90-231 GGYML (126-130) likely allowing efficient competitive binding to fibrillar conformations of these MPs. Thee results propose that HSP10 can function as an important molecular chaperone in human proteostasis in NDs.

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Chocron, E. S., Munkacsy, E., Kim, H. S., Karpowicz, P., Jiang, N., Van Skike, C. E., DeRosa, N., Banh, A. Q., Palavicini, J. P., Wityk, P., Kalinowski, L., Galvan, V., Osmulski, P. A., Jankowska, E., Gaczynska, M. and Pickering, A. M. (2022). Genetic and pharmacologic proteasome augmentation ameliorates Alzheimer's-like pathology in mouse and fly APP overexpression models. Sci Adv 8(23): eabk2252. PubMed ID: 35675410

Abstract

The proteasome has key roles in neuronal proteostasis, including the removal of misfolded and oxidized proteins, presynaptic protein turnover, and synaptic efficacy and plasticity. Proteasome dysfunction is a prominent feature of Alzheimer's disease (AD). Prevention of proteasome dysfunction by genetic manipulation delays mortality, cell death, and cognitive deficits in fly and cell culture AD models. This study developed a transgenic mouse with neuronal-specific proteasome overexpression that, when crossed with an AD mouse model, showed reduced mortality and cognitive deficits. To establish translational relevance, a set of TAT-based proteasome-activating peptidomimetics was developed that stably penetrated the blood-brain barrier and enhanced 20S/26S proteasome activity. These agonists protected against cell death, cognitive decline, and mortality in cell culture, fly, and mouse AD models. The protective effects of proteasome overexpression appear to be driven, at least in part, by the proteasome's increased turnover of the amyloid precursor protein along with the prevention of overall proteostatic dysfunction.

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Schulz, L., Ramirez, P., Lemieux, A., Gonzalez, E., Thomson, T. and Frost, B. (2022). Tau-Induced Elevation of the Activity-Regulated Cytoskeleton Associated Protein Arc1 Causally Mediates Neurodegeneration in the Adult Drosophila Brain. Neuroscience. PubMed ID: 35487302

Abstract

Alzheimer's disease and other tauopathies are neurodegenerative disorders pathologically defined by aggregated forms of tau protein in the brain. While synaptic degradation is a well-established feature of tau-induced neurotoxicity, the underlying mechanisms of how pathogenic forms of tau drive synaptic dysfunction are incompletely understood. Synaptic function and subsequent memory consolidation are dependent upon synaptic plasticity, the ability of synapses to adjust their structure and strength in response to changes in activity. The activity regulated cytoskeleton associated protein ARC acts in the nucleus and at postsynaptic densities to regulate various forms of synaptic plasticity. ARC harbors a retrovirus-like Gag domain that facilitates ARC multimerization and capsid formation. Trans-synaptic transfer of RNA-containing ARC capsids is required for synaptic plasticity. While ARC is elevated in brains of patients with Alzheimer's disease and genetic variants in ARC increase susceptibility to Alzheimer's disease, mechanistic insight into the role of ARC in Alzheimer's disease is lacking. Using a Drosophila model of tauopathy, this study found that pathogenic tau significantly increases multimeric species of the protein encoded by the Drosophila homolog of ARC, Arc1, in the adult fly brain. Arc1 is elevated within nuclei and the neuropil of tau transgenic Drosophila, but does not localize to synaptic vesicles or presynaptic terminals. Lastly, this study found that genetic manipulation of Arc1 modifies tau-induced neurotoxicity, suggesting that tau-induced Arc1 elevation mediates neurodegeneration. Taken together, these results suggest that ARC elevation in human Alzheimer's disease is a consequence of tau pathology and is a causal factor contributing to neuronal death.

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Shafik, A. M., Zhou, H., Lim, J., Dickinson, B. and Jin, P. (2022). Dysregulated mitochondrial and cytosolic tRNA m1A methylation in Alzheimer's disease. Hum Mol Genet 31(10): 1673-1680. PubMed ID: 34897434

Abstract

RNA modifications affect many aspects of RNA metabolism and are involved in the regulation of many different biological processes. Mono-methylation of adenosine in the N1 position, N1-methyladensoine (m1A), is a reversible modification that is known to target rRNAs and tRNAs. m1A has been shown to increase tRNA structural stability and induce correct tRNA folding. Recent studies have begun to associate the dysregulation of epitranscriptomic control with age-related disorders such as Alzheimer's disease. This study applied the newly developed m1A-quant-seq approach to map the brain abundant m1A RNA modification in the cortex of an Alzheimer's disease mouse model, 5XFAD. Hypomethylation was observed in both mitochondrial and cytosolic tRNAs in 5XFAD mice compared with wild type. Furthermore, the main enzymes responsible for the addition of m1A in mitochondrial (TRMT10C, HSD17B10) and cytosolic tRNAs (TRMT61A) displayed decreased expression in 5XFAD compared with wild-type mice. Knockdown of these enzymes results in a more severe phenotype in a Drosophila tau model, and differential m1A methylation is correlated with differences in mature mitochondrial tRNA expression. Collectively, this work suggests that hypo m1A modification in tRNAs may play a role in Alzheimer's disease pathogenesis.

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Ogunsuyi, O. B., Aro, O. P., Oboh, G. and Olagoke, O. C. (2022). Curcumin improves the ability of donepezil to ameliorate memory impairment in Drosophila melanogaster: involvement of cholinergic and cnc/Nrf2-redox systems. Drug Chem Toxicol: 1-9. PubMed ID: 36069210

Abstract

One of the well-established models for examining neurodegeneration and neurotoxicity is the Drosophila melanogaster model of aluminum-induced toxicity. Anti-cholinesterase drugs have been combined with other neuroprotective agents to improve Alzheimer's disease management, but there is not much information on the combination of anti-cholinesterases with dietary polyphenols to combat memory impairment. This study assessed how curcumin influences some of the critical therapeutic effects of donepezil (a cholinesterase inhibitor) in AlCl(3)-treated Drosophila melanogaster. Harwich strain flies were exposed to 40 mM AlCl(3) - alone or in combination with curcumin (1 mg/g) and/or donepezil (12.5 &mi;g/g and 25 &mi;g/g) - for seven days. The flies' behavioral evaluations (memory index and locomotor performance) were analyzed. Thereafter, the flies were processed into homogenates for the quantification of acetylcholinesterase (AChE), catalase, total thiol, and rate of lipid peroxidation, as well as the mRNA levels of acetylcholinesterase (ACE1) and cnc/NRF2. Results showed that AlCl(3)-treated flies presented impaired memory and increased activities of acetylcholinesterase and lipid peroxidation, while there were decrease in total thiol levels and catalase activity when compared to the control. Also, the expression of ACE1 was significantly increased while that of cnc/NRF2 was significantly decreased. However, combinations of curcumin and donepezil, especially at lower dose of donepezil, significantly improved the memory index and biochemical parameters compared to donepezil alone. Thus, curcumin plus donepezil offers unique therapeutic effects during memory impairment in the D. melanogaster model of neurotoxicity.

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Martinez, P., Patel, H., You, Y., Jury, N., Perkins, A., Lee-Gosselin, A., Taylor, X., You, Y., Viana Di Prisco, G., Huang, X., Dutta, S., Wijeratne, A. B., Redding-Ochoa, J., Shahid, S. S., Codocedo, J. F., Min, S., Landreth, G. E., Mosley, A. L., Wu, Y. C., McKinzie, D. L., Rochet, J. C., Zhang, J., Atwood, B. K., Troncoso, J. and Lasagna-Reeves, C. A. (2022). Bassoon contributes to tau-seed propagation and neurotoxicity. Nat Neurosci. PubMed ID: 36344699

Abstract
Tau aggregation is a defining histopathological feature of Alzheimer's disease and other tauopathies. However, the cellular mechanisms involved in tau propagation remain unclear. This study performed an unbiased quantitative proteomic analysis to identify proteins that specifically interact with this tau seed. Bassoon (BSN), a presynaptic scaffolding protein, was identified as an interactor of the tau seed isolated from a mouse model of tauopathy, and from Alzheimer's disease and progressive supranuclear palsy postmortem samples. BSN was shown to exacerbate tau seeding and toxicity in both mouse and Drosophila models for tauopathy, and that BSN downregulation decreases tau spreading and overall disease pathology, rescuing synaptic and behavioral impairments and reducing brain atrophy. These findings improve the understanding of how tau seeds can be stabilized by interactors such as BSN. Inhibiting tau-seed interactions is a potential new therapeutic approach for neurodegenerative tauopathies.

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Chen, Y., Krishnan, G., Parsi, S., Pons, M., Nikolaki, V., Cao, L., Xu, Z. and Gao, F. B. (2022). The enhanced association between mutant CHMP2B and spastin is a novel pathological link between frontotemporal dementia and hereditary spastic paraplegias. Acta Neuropathol Commun 10(1): 169. PubMed ID: 36414997

Abstract
Chromosome 3-linked frontotemporal dementia (FTD3) is caused by a gain-of-function mutation in CHMP2B, resulting in the production of a truncated toxic protein, CHMP2B(Intron5). Loss-of-function mutations in spastin are the most common genetic cause of hereditary spastic paraplegias (HSP). How these proteins might interact with each other to drive pathology remains to be explored. This study found that Spastin binds with greater affinity to CHMP2B(Intron5) than to CHMP2B(WT) and colocalizes with CHMP2B(Intron5) in p62-positive aggregates. In cultured cells expressing CHMP2B(Intron5), spastin level in the cytoplasmic soluble fraction is decreased while insoluble spastin level is increased. These pathological features of spastin are validated in brain neurons of a mouse model of FTD3. Moreover, genetic knockdown of spastin enhances CHMP2B(Intron5) toxicity in a Drosophila model of FTD3, indicating the functional significance of their association. Thus, this study reveals that the enhanced association between mutant CHMP2B and spastin represents a novel potential pathological link between FTD3 and HSP.

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Choi, B., Lim, C., Lee, H., Lee, J. E., Kim, J., Chung, C. and Cho, K. S. (2022). Neuroprotective effects of linear ubiquitin E3 ligase against aging-induced DNA damage and amyloid β neurotoxicity in the brain of Drosophila melanogaster. Biochem Biophys Res Commun 637: 196-202. PubMed ID: 36403483

Abstract
E3 ubiquitin ligase, HOIL1-interacting protein (HOIP), forms the linear ubiquitin chain assembly complex (LUBAC) with HOIL and SHANK-associated RH domain interactor and catalyzes linear ubiquitination, directly linking the N- and C-termini of ubiquitin. Recently, several studies have implicated linear ubiquitination in aging and Alzheimer disease (AD). However, little is currently known about the roles of HOIP in brain aging and AD pathology. This studyinvestigated the role of linear ubiquitin E3 ligase (LUBEL), a Drosophila HOIP ortholog, in brain aging and amyloid β (A:beta;) pathology in a Drosophila AD model. DNA double-strand breaks (DSBs) were increased in the aged brains of neuron-specific LUBEL-knockdown flies compared to the age-matched controls. Silencing of LUBEL in the neuron of AD model flies increased the neuronal apoptosis and neurodegeneration, whereas silencing in glial cells had no such effect. A&bet; aggregation levels and DSBs were also increased in the LUBEL-silenced AD model fly brains, but autophagy and proteostasis were not affected by LUBEL silencing. Collectively, these results suggest that LUBEL protects neurons from aging-induced DNA damage and Aβ neurotoxicity.

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Ataellahi, F., Masoudi, R. and Haddadi, M. (2022). Differential dysregulation of CREB and synaptic genes in transgenic Drosophila melanogaster expressing shaggy (GSK3), Tau(WT), or Amyloid-beta. Mol Biol Rep. PubMed ID: 36399243

Abstract
Tau, Amyloid-beta (Aβ42), and Glycogen synthase kinase 3 (GSK3) contribute to synaptic dysfunction observed in Alzheimer's disease (AD), the most common form of dementia. In the current study, the effect of pan-neuronal expression of TauWT, Aβ42, or shaggy (orthologue of GSK3) in Drosophila melanogaster was assessed on the locomotor function, ethanol sensitivity, synaptic genes and CREB expression. The effect of TauWT and Aβ42 on the expression of shaggy was also determined. Gene expression analysis was performed using quantitative real-time RT-PCR method. While syt1, SNAP25 and CREB (upstream transcription factor of syt1 and SNAP25) were upregulated in flies expressing Tau(WT) or Aβ42, a prominent decline was observed in those genes in shaggy expressing flies. Although all transgenic flies showed climbing disability and higher sensitivity to ethanol, abnormality in these features was significantly more prominent in transgenic flies expressing shaggy compared to TauWT or Aβ42. Despite a significant upregulation of shaggy transcription in TauWT expressing flies, Aβ42 transgenic flies witnessed no significant changes. TauWT, A&beta42, and shaggy may affect synaptic plasticity through dysregulation of synaptic genes and CREB, independently. However shaggy has more detrimental effect on synaptic genes expression, locomotor ability and sensitivity to ethanol. It is important when it comes to drug discovery. It appears that CREB is a direct effector of changes in synaptic genes expression as they showed similar pattern of alteration and it is likely to be a part of compensatory mechanisms independent of the GSK3/CREB pathway in TauWT or Aβ(42) expressing flies.

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Choi, B., Lim, C., Lee, H., Lee, J. E., Kim, J., Chung, C. and Cho, K. S. (2022). Neuroprotective effects of linear ubiquitin E3 ligase against aging-induced DNA damage and amyloid β neurotoxicity in the brain of Drosophila melanogaster. Biochem Biophys Res Commun 637: 196-202. PubMed ID: 36403483

Abstract
E3 ubiquitin ligase, HOIL1-interacting protein (HOIP), forms the linear ubiquitin chain assembly complex (LUBAC) with HOIL and SHANK-associated RH domain interactor and catalyzes linear ubiquitination, directly linking the N- and C-termini of ubiquitin. Recently, several studies have implicated linear ubiquitination in aging and Alzheimer disease (AD). However, little is currently known about the roles of HOIP in brain aging and AD pathology. This studyinvestigated the role of linear ubiquitin E3 ligase (LUBEL), a Drosophila HOIP ortholog, in brain aging and amyloid β (A:beta;) pathology in a Drosophila AD model. DNA double-strand breaks (DSBs) were increased in the aged brains of neuron-specific LUBEL-knockdown flies compared to the age-matched controls. Silencing of LUBEL in the neuron of AD model flies increased the neuronal apoptosis and neurodegeneration, whereas silencing in glial cells had no such effect. A&bet; aggregation levels and DSBs were also increased in the LUBEL-silenced AD model fly brains, but autophagy and proteostasis were not affected by LUBEL silencing. Collectively, these results suggest that LUBEL protects neurons from aging-induced DNA damage and Aβ neurotoxicity.

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Zhang, S., Zhu, Y., Lu, J., Liu, Z., Lobato, A. G., Zeng, W., Liu, J., Qiang, J., Zeng, S., Zhang, Y., Liu, C., Liu, J., He, Z., Zhai, R. G. and Li, D. (2022). Specific binding of Hsp27 and phosphorylated Tau mitigates abnormal Tau aggregation-induced pathology. Elife 11. PubMed ID: 36048712

Abstract
Amyloid aggregation of phosphorylated Tau (pTau) into neurofibrillary tangles is closely associated with Alzheimer's disease (AD). Several molecular chaperones have been reported to bind Tau and impede its pathological aggregation. Recent findings of elevated levels of Hsp27 in the brains of patients with AD suggested its important role in pTau pathology. However, the molecular mechanism of Hsp27 in pTau aggregation remains poorly understood. This study, shows that Hsp27 partially co-localizes with pTau tangles in the brains of patients with AD. Notably, phosphorylation of Tau by microtubule affinity regulating kinase 2 (MARK2), dramatically enhances the binding affinity of Hsp27 to Tau. Moreover, Hsp27 efficiently prevents pTau fibrillation in vitro and mitigates neuropathology of pTau aggregation in a Drosophila tauopathy model. Further mechanistic study reveals that Hsp27 employs its N-terminal domain to directly interact with multiple phosphorylation sites of pTau for specific binding. This work provides the structural basis for the specific recognition of Hsp27 to pathogenic pTau, and highlights the important role of Hsp27 in preventing abnormal aggregation and pathology of pTau in AD.

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Kaur, P., Chua, E. H. Z., Lim, W. K., Liu, J., Harmston, N. and Tolwinski, N. S. (2022). Wnt Signaling Rescues Amyloid Beta-Induced Gut Stem Cell Loss. Cells 11(2). PubMed ID: 35053396

Abstract
Patients with Alzheimer's disease suffer from a decrease in brain mass and a prevalence of amyloid-β plaques. These plaques are thought to play a role in disease progression, but their exact role is not entirely established. This study developed an optogenetic model to induce amyloid-β intracellular oligomerization to model distinct disease etiologies. This study examined the effect of Wnt signaling on amyloid in an optogenetic, Drosophila gut stem cell model. It was observed that Wnt activation rescues the detrimental effects of amyloid expression and oligomerization. The gene expression changes downstream of Wnt that contribute to this rescue was analyzed, and changes were found in aging related genes, protein misfolding, metabolism, and ineflammation. It is proposed that Wnt expression reduces inflammation through repression of Toll activating factors. It was confirmed that chronic Toll activation reduces lifespan, but a decrease in the upstream activator Persephone extends it. It is proposed that the protective effect observed for lithium treatment functions, at least in part, through Wnt activation and the inhibition of inflammation.

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Deshpande, P., Chen, C. Y., Yeates, C., Chen, C. H., Kango-Singh, M. and Singh, A. (2021). miR-277 targets hid to ameliorate Aβ42-mediated neurodegeneration in Drosophila eye model of Alzheimer's Disease. Alzheimers Dement 17 Suppl 2: e058678. Article URL
Summary:
Alzheimer's disease (AD), an age-related progressive neurodegenerative disorder, exhibits reduced cognitive functions with no cure to date. One of the reasons for AD is the extracellular accumulation of Amyloid-beta 42 (Aβ42) plaques. Misexpression of human Aβ42 in the developing retina of Drosophila exhibits AD-like neuropathology. Accumulation of Aβ42 plaque(s) triggers aberrant signaling resulting in neuronal cell death by unknown mechanism(s). Alzheimer's disease (AD), an age-related progressive neurodegenerative disorder, exhibits reduced cognitive functions with no cure to date. One of the reasons for AD is the extracellular accumulation of Amyloid-beta 42 (Aβ42) plaques. Misexpression of human Aβ42 in the developing retina of Drosophila exhibits AD-like neuropathology. Accumulation of Aβ42 plaque(s) triggers aberrant signaling resulting in neuronal cell death by unknown mechanism(s). Gain-of-function of mir-277 rescues Aβ42 mediated neurodegeneration whereas loss-of-function of mir-277 enhances Aβ42 mediated neurodegeneration. Moreover, misexpression of higher levels of mir-277 in the GMR>Aβ42 background restores the retinal axonal targeting indicating functional rescue. Furthermore, this study has identified head involution defective (hid) as one of the targets of mir-277. The hid transcript levels are decreased by one third when mir-277 is misexpressed in the GMR>Aβ42 background in comparison to the GMR>Aβ42 fly model. This study provides a mechanism of how mir-277 modulates Aβ42 mediated neurodegeneration by regulating hid transcript levels and demonstrate its neuroprotective role in Aβ42-mediated neuropathology.

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Cheng, K. C., Hwang, Y. L. and Chiang, H. C. (2022). The double-edged sword effect of HDAC6 in Abeta toxicities. Faseb j 36(1): e22072. PubMed ID: 34907598
Summary:
Alzheimer's disease (AD) is marked by cognitive impairment, massive cell death, and reduced life expectancy. Pathologically, accumulated beta-amyloid (Aβ) aggregates and hyperphosphorylated tau protein is the hallmark of the disease. Although changes in cellular function and protein accumulates have been demonstrated in many different AD animal models, the molecular mechanism involved in different cellular functions and the correlation and causative relation between different protein accumulations remain elusive. In vivo genetic studies revealed that the molecular mechanisms involved in memory loss and lifespan shortening are different and that tau plays an essential role in mediating Aβ-induced early death. When the first deacetylase (DAC) domain of histone deacetylase 6 (HDAC6) activity was increased, it regulated cortactin deacetylation to reverse Aβ-induced learning and memory deficit, but with no effect on the lifespan of the Aβ flies. On the other hand, an increased amount of the second DAC domain of HDAC6 promoted tau phosphorylation to facilitate Aβ-induced lifespan shortening without affecting learning performance in the Aβ flies.These data also confirmed decreased acetylation in two major HDAC6 downstream proteins, suggesting increased HDAC6 activity in Aβ flies. These data established the double-edged sword effect of HDAC6 activity in Aβ-induced pathologies. Not only did memory loss and lifespan shortening in Aβ flies segregated, but also evidence is provided to link the Aβ with tau signaling.

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Kaur, P., Kibat, C., Teo, E., Gruber, J., Mathuru, A. and Tolwinski, A. N. S. (2020). Use of Optogenetic Amyloid-beta to Monitor Protein Aggregation in Drosophila melanogaster, Danio rerio and Caenorhabditis elegans. Bio Protoc 10(23): e3856. PubMed ID: 33659494

Abstract

Alzheimer's Disease (AD) has long been associated with accumulation of extracellular amyloid plaques (Aβ) originating from the Amyloid Precursor Protein. Plaques have, however, been discovered in healthy individuals and not all AD brains show plaques, suggesting that extracellular Aβ aggregates may play a smaller role than anticipated. One limitation to studying Aβ peptide in vivo during disease progression is the inability to induce aggregation in a controlled manner. This study developed an optogenetic method to induce Aβ aggregation, and its biological influence was tested in three model organisms-D. melanogaster, C. elegans and D. rerio. A fluorescently labeled, optogenetic Aβ peptide was generated that oligomerizes rapidly in vivo in the presence of blue light in all organisms. This paper details the procedures for expressing this fusion protein in animal models, investigating the effects on the nervous system using time lapse light-sheet microscopy, and metabolic assays were perfomred to measure changes due to intracellular Aβ aggregation. This method, employing optogenetics to study the pathology of AD, allows spatial and temporal control in vivo that cannot be achieved by any other method at present (Kaur, 2020).

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Wang, Y. and Westermark, G. T. (2021). The Amyloid Forming Peptides Islet Amyloid Polypeptide and Amyloid beta Interact at the Molecular Level. Int J Mol Sci 22(20). PubMed ID: 34681811
Summary:
Epidemiological studies support a connection between the two common disorders, type-2 diabetes and Alzheimer's disease. Both conditions have local amyloid formation in their pathogenesis, and cross-seeding between islet amyloid polypeptide (IAPP) and amyloid β (Aβ) could constitute the link. The bimolecular fluorescence complementation (BiFC) assay was used to investigate the occurrence of heterologous interactions between IAPP and Aβ and to compare the potential toxic effects of IAPP/Aβ, IAPP/IAPP, and Aβ/Aβ expression in living cells. Microscopy was used to confirm the fluorescence and determine the lysosomal, mitochondrial areas and mitochondrial membrane potential, and a FACS analysis was used to determine ROS production and the role for autophagy. Drosophila melanogaster expressing IAPP and Aβ was used to study their co-deposition and effects on longevity. Co-expression of IAPP and Aβ resulted in fluorophore reconstitution to the same extent as determined for homologous IAPP/IAPP or Aβ/Aβ expression. The BiFC(+)/BiFC(-) ratio of lysosomal area calculations increased in transfected cells independent of the vector combinations, while only Aβ/Aβ expression increased mitochondrial membrane potential. Expression combinations containing Aβ were necessary for the formation of a congophilic amyloid. In Drosophila melanogaster expressing IAPP/Aβ, co-deposition of the amyloid-forming peptides caused reduced longevity. The BiFC results confirmed a heterologous interaction between IAPP and Aβ, while co-deposits in the brain of Drosophila suggest mixed amyloid aggregates.

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Kim, Y. W., Al-Ramahi, I., Koire, A., Wilson, S. J., Konecki, D. M., Mota, S., Soleimani, S., Botas, J. and Lichtarge, O. (2020). Harnessing the paradoxical phenotypes of APOE epsilon2 and APOE epsilon4 to identify genetic modifiers in Alzheimer's disease. Alzheimers Dement. PubMed ID: 33576571

Abstract

The strongest genetic risk factor for idiopathic late-onset Alzheimer's disease (LOAD) is apolipoprotein E (APOE) ε4, while the APOE ε2 allele is protective. However, there are paradoxical APOE ε4 carriers who remain disease-free and APOE ε2 carriers with LOAD. Exomes of healthy APOE ε4 carriers and APOE ε2 Alzheimer's disease (AD) patients were compared, prioritizing coding variants based on their predicted functional impact; 216 genes were identified with differential mutational load between these two populations. These candidate genes were significantly dysregulated in LOAD brains, and many modulated tau- or β42-induced neurodegeneration in Drosophila. Variants in these genes were associated with AD risk, even in APOE ε3 homozygotes, showing robust predictive power for risk stratification. Network analyses revealed involvement of candidate genes in brain cell type-specific pathways including synaptic biology, dendritic spine pruning and inflammation. These potential modifiers of LOAD may constitute novel biomarkers, provide potential therapeutic intervention avenues, and support applying this approach as larger whole exome sequencing cohorts become available (Kim, 2020).

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Haghi, M., Masoudi, R. and Najibi, S. M. (2020). Distinctive alteration in the expression of autophagy genes in Drosophila models of amyloidopathy and tauopathy. Ups J Med Sci: 1-9. PubMed ID: 32657227

Abstract
Alzheimer's disease (AD) is one the most common types of dementia. Plaques of amyloid beta and neurofibrillary tangles of tau are two major hallmarks of AD. Metabolism of these two proteins, in part, depends on autophagy pathways. Autophagy dysfunction and protein aggregation in AD may be involved in a vicious circle. The aim of this study was to investigate whether tau or amyloid beta 42 (Aβ42) could affect expression of autophagy genes, and whether they exert their effects in the same way or not. Misxpression levels of some autophagy genes, Hook, Atg6, Atg8, and Cathepsin D, were measured using quantitative PCR in transgenic Drosophila melanogaster expressing either Aβ42 or Tau R406W. Hook mRNA levels were downregulated in Aβ42-expressing flies both 5 and 25 days old, while they were increased in 25-day-old flies expressing Tau R406W. Both Atg6 and Atg8 were upregulated at day 5 and then downregulated in 25-day-old flies expressing either Aβ42 or Tau R406W. Cathepsin D expression levels were significantly increased in 5-day-old flies expressing Tau R406W, while there was no significant change in the expression levels of this gene in 5-day-old flies expressing Aβ42. Expression levels of Cathepsin D were significantly decreased in 25-day-old transgenic flies expressing Tau R406W or Aβ42. It is concluded that both Aβ42 and Tau R406W may affect autophagy through dysregulation of autophagy genes. Interestingly, it seems that these pathological proteins exert their toxic effects on autophagy through different pathways and independently (Haghi, 2020).

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Nikookar, H., Haddadi, M., Haghi, M. and Masoudi, R. (2021). DNT1 Downregulation and Increased Ethanol Sensitivity in Transgenic Drosophila Models of Alzheimer's Disease. Arch Gerontol Geriatr 94: 104355. PubMed ID: 33550108

Abstract

Two major pathological hallmarks of Alzheimer's disease (AD) are amyloid plaques and neurofibrillary tangles of hyperphosphorylated tau. Aggregation of amyloid-β (Aβ) is considered as the primary insult in AD. However, failure in treatments based on targeting Aβ without considering the pathologic tau and close correlation between pathological tau and cognitive decline highlighted the crucial role of tau in AD. Loss of synaptic plasticity and cognitive decline, partly due to decrease in Brain Derived Neurotrophic Factor (BDNF), are other hallmarks of AD. Aβ and tau downregulate BDNF at both transcriptional and translational levels. The aim of this research was to study the expression levels of Drosophila Neurotrophin 1 (DNT1), as an orthologue of BDNF, in flies expressing Aβ(42) or tau(R406W). Levels of DNT1 were determined using quantitative real time PCR. Behavioral and Biochemical investigations were also performed in parallel. The results showed that there is a significant decrease in the levels of DNT1 expression in Aβ(42) or tau(R406W) expressing flies. Interestingly, a significant increase was observed in sensitivity to ethanol in both transgenic flies. Rise in Reactive Oxygen Species (ROS) levels was also detected. It is concluded that both Aβ and pathological tau exert their toxic effect on DNT1 expression, ROS production, and response to ethanol, independently. Interestingly, pathological tau showed higher impact on the ROS production compared to Aβ. It seems that Aβ(42) and tau(R406W) transgenic flies are proper models to investigate the interplay between BDNF and oxidative stress, and also to assess the mechanism underlying behavioral response to ethanol (Nikookar, 2021).

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Cheng, K. C., Chen, Y. H., Wu, C. L., Lee, W. P., Cheung, C. H. A. and Chiang, H. C. (2021). Rac1 and Akt Exhibit Distinct Roles in Mediating Abeta-Induced Memory Damage and Learning Impairment. Mol Neurobiol. PubMed ID: 34273104

Abstract

Accumulated β-amyloid (Aβ) in the brain is the hallmark of Alzheimer's disease (AD). Despite Aβ accumulation is known to trigger cellular dysfunctions and learning and memory damage, the detailed molecular mechanism remains elusive. Recent studies have shown that the onset of memory impairment and learning damage in the AD animal is different, suggesting that the underlying mechanism of the development of memory impairment and learning damage may not be the same. In the current study, with the use of Aβ42 transgenic flies as models, this study found that Aβ induces memory damage and learning impairment via differential molecular signaling pathways. In early stage, Aβ activates both Ras and PI3K to regulate Rac1 activity, which affects mostly on memory performance. In later stage, PI3K-Akt is strongly activated by Aβ, which leads to learning damage. Moreover, reduced Akt, but not Rac1, activity promotes cell viability in the Aβ42 transgenic flies, indicating that Akt and Rac1 exhibit differential roles in Aβ regulating toxicity. Taken together, different molecular and cellular mechanisms are involved in Aβ-induced learning damage and memory decline; thus, caution should be taken during the development of therapeutic intervention in the future (Cheng, 2021).

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Niccoli, T., Kerr, F., Snoeren, I., Fabian, D., Aleyakpo, B., Ivanov, D., Sofola-Adesakin, O., Cryar, A., Adcott, J., Thornton, J. and Partridge, L. (2021). Activating transcription factor 4-dependent lactate dehydrogenase activation as a protective response to amyloid beta toxicity. Brain Commun 3(2): fcab053. PubMed ID: 33977265

Abstract

Accumulation of amyloid beta peptides is thought to initiate the pathogenesis of Alzheimer's disease. Microarray analyses have shown that, in Drosophila models of amyloid beta 42 toxicity, genes involved in the unfolded protein response and metabolic processes are upregulated in brain. Comparison with the brain transcriptome of early-stage Alzheimer's patients revealed a common transcriptional signature, but with generally opposing directions of gene expression changes between flies and humans. Among these differentially regulated genes, lactate dehydrogenase (Ldh) was up-regulated by the greatest degree in amyloid beta 42 flies and the human orthologues (LDHA and LDHB) were down-regulated in patients. Functional analyses revealed that either over-expression or inhibition of Ldh by RNA interference (RNAi) slightly exacerbated climbing defects in both healthy and amyloid beta 42-induced Drosophila. This suggests that metabolic responses to lactate dehydrogenase must be finely-tuned, and that its observed upregulation following amyloid beta 42 production could potentially represent a compensatory protection to maintain pathway homeostasis in this model, with further manipulation leading to detrimental effects. The increased Ldh expression in amyloid beta 42 flies was regulated partially by unfolded protein response signalling, as ATF4 RNAi diminished the transcriptional response and enhanced amyloid beta 42-induced climbing phenotypes. This study thus reveals dysregulation of lactate dehydrogenase signalling in Drosophila models and patients with Alzheimer's disease, which may lead to a detrimental loss of metabolic homeostasis. Importantly, it was observed that down-regulation of ATF4-dependent endoplasmic reticulum-stress signalling in this context appears to prevent Ldh compensation and to exacerbate amyloid beta 42-dependent neuronal toxicity (Niccoli, 2021).

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Barati, A., Masoudi, R., Yousefi, R., Monsefi, M. and Mirshafiey, A. (2021). Tau and amyloid beta differentially affect the innate immune genes expression in Drosophila models of Alzheimer's disease and beta- D Mannuronic acid (M2000) modulates the dysregulation. Gene 808: 145972. PubMed ID: 34600048

Abstract

Alzheimer's disease (AD) is the most common cause of dementia and neuroinflammation is considered as one of the main culprits. The aim of this study was to evaluate the independent role of Aβ42 and tau on the inflammatory pathway in the Drosophila models of AD and investigating the potential modulating effect of M2000 (β-D-mannuronic acid) as a novel NSAIDs in those flies. The expression levels of relish, orthologs of NF-κB, antimicrobial peptide (AMP) including attacin A, diptericin B and a dual oxidase (Duox) as a ROS mediator, were evaluated in both M2000 treated and untreated groups followed by brain histology analysis to assess the extent of neurodegeneration. The potential inhibitory role of M2000 on the aggregation of tau protein was also investigated in vitro. According to the result, there was a significant induction of Duox, AMPs and its transcription factor expression in both aged and Drosophila models of AD which was in accordance with the increase in the number of vacuoles in the brain section of Drosophila models of AD. Interestingly M2000 treatment revealed a significant reduction in all neurodegeneration indexes in vivo and anti-aggregating property in vitro. Findings suggest that M2000 has potential to be an AD therapeutic agent (Barati, 2021).

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Lin, X., Wen, X., Wei, Z., Guo, K., Shi, F., Huang, T., Wang, W. and Zheng, J. (2021). Vitamin K2 protects against Abeta42-induced neurotoxicity by activating autophagy and improving mitochondrial function in Drosophila. Neuroreport 32(6): 431-437. PubMed ID: 33788812

Abstract

Alzheimer disease is characterized by progressive decline in cognitive function due to neurodegeneration induced by accumulation of Aβ and hyperphosphorylated tau protein. This study was conducted to explore the protective effect of vitamin K2 against Aβ42-induced neurotoxicity. Alzheimer disease transgenic Drosophila model used in this study was amyloid beta with the arctic mutation expressed in neurons. Alzheimer disease flies were treated with vitamin K2 for 28 days after eclosion. Aβ42 level in brain was detected by ELISA. Autophagy-related genes and NDUFS3, the core subunit of mitochondrial complex I, were examined using real-Time PCR (RT-PCR) and western blot analysis. Vitamin K2 improved climbing ability, prolonged lifespan and decreased Aβ42 levels, upregulated the expression of LC3 and Beclin1, increased the conversion of LC3I to LC3II and decreased p62 level. in Alzheimer disease flies. In addition, vitamin K2 upregulated the expression of NDUFS3 and increased ATP production in Alzheimer disease flies. It seems that vitamin K2 protect against Aβ42-induced neurotoxicity by activation of autophagy and rescue mitochondrial dysfunction, which suggests that it may be a potential valuable therapeutic approach for Alzheimer disease (Lin, 2021).

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Beaver, M., Karisetty, B. C., Zhang, H., Bhatnagar, A., Armour, E., Parmar, V., Brown, R., Xiang, M. and Elefant, F. (2021). Chromatin and transcriptomic profiling uncover dysregulation of the Tip60 HAT/HDAC2 epigenomic landscape in the neurodegenerative brain. Epigenetics: 1-22. PubMed ID: 34369292

Abstract
Disruption of histone acetylation-mediated gene control is a critical step in Alzheimer's Disease (AD), yet chromatin analysis of antagonistic histone acetyltransferases (HATs) and histone deacetylases (HDACs) causing these alterations remains uncharacterized. This study reports the first Tip60 HAT versus HDAC2 chromatin (ChIP-seq) and transcriptional (RNA-seq) profiling study in Drosophila melanogaster brains that model early human AD. Tip60 and HDAC2 were found to be predominantly recruited to identical neuronal genes. Moreover, AD brains exhibit robust genome-wide early alterations that include enhanced HDAC2 and reduced Tip60 binding and transcriptional dysregulation. Orthologous human genes to co-Tip60/HDAC2 D. melanogaster neural targets exhibit conserved disruption patterns in AD patient hippocampi. Notably, this study discovered distinct transcription factor binding sites close or within Tip60/HDAC2 co-peaks in neuronal genes, implicating them in coenzyme recruitment. Increased Tip60 protects against transcriptional dysregulation and enhanced HDAC2 enrichment genome-wide. Tip60 HAT/HDAC2 mediated epigenetic neuronal gene disruption is advocated as a genome-wide initial causal event in AD.

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Ismael, S., Sindi, G., Colvin, R. A. and Lee, D. (2021). Activity-dependent release of phosphorylated human tau from Drosophila neurons in primary culture. J Biol Chem: 101108. PubMed ID: 34473990

Abstract
Neuronal activity can enhance tau release and, thus accelerate tauopathies. This activity-dependent tau release can be used to study the progression of tau (see Drosophila Tau) pathology in Alzheimer's disease (AD), as hyper-phosphorylated tau is implicated in AD pathogenesis and related tauopathies. However, understanding of the mechanisms that regulate activity-dependent tau release from neurons and the role that tau phosphorylation plays in modulating activity-dependent tau release is still rudimentary. In this study, Drosophila neurons in primary culture expressing human tau (hTau) were used to study activity-dependent tau release. hTau release was markedly increased by 50 mM KCl treatment for 1 hour. A similar level of release was observed using optogenetic techniques where genetically targeted neurons were stimulated for 30 min using blue light (470 nm). These results showed that activity-dependent release of phospho-resistant hTau(S11A) was reduced when compared with wild type hTau. In contrast, release of phospho-mimetic hTau(E14) was increased upon activation. Released hTau was phosphorylated in its proline-rich and C-terminal domains using phosphorylation site-specific tau antibodies (e.g., AT8). Fold changes in detectable levels of total or phosphorylated hTau in cell lysates or following immunopurification from conditioned media (IP-CM) were consistent with preferential release of phosphorylated hTau after light stimulation. This study establishes an excellent model to investigate the mechanism of activity-dependent hTau release and to better understand the role of phosphorylated tau release in the pathogenesis of AD since it relates to alterations in the early stage of neurodegeneration associated with increased neuronal activity.

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Katsinelos, T., McEwan, W. A., Jahn, T. R. and Nickel, W. (2021). Identification of cis-acting determinants mediating the unconventional secretion of tau. Sci Rep 11(1): 12946. PubMed ID: 34155306

Abstract
The deposition of tau aggregates throughout the brain is a pathological characteristic within a group of neurodegenerative diseases collectively termed tauopathies, which includes Alzheimer's disease. While recent findings suggest the involvement of unconventional secretory pathways driving tau into the extracellular space and mediating the propagation of the disease-associated pathology, many of the mechanistic details governing this process remain elusive. Using Drosophila models of tauopathy, the hyperphosphorylation and aggregation state of tau was correlated with the disease-related neurotoxicity. These newly established systems recapitulate all the previously identified hallmarks of tau secretion, including the contribution of tau hyperphosphorylation as well as the requirement for PI(4,5)P(2) triggering the direct translocation of tau. Using a series of cellular assays, this study demonstrated that both the sulfated proteoglycans on the cell surface and the correct orientation of the protein at the inner plasma membrane leaflet are critical determinants of this process. Finally, two cysteine residues within the microtubule binding repeat domain were identified as novel cis-elements that are important for both unconventional secretion and trans-cellular propagation of tau.

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Kaldun, J. C., Lone, S. R., Humbert Camps, A. M., Fritsch, C., Widmer, Y. F., Stein, J. V., Tomchik, S. M. and Sprecher, S. G. (2021). Dopamine, sleep, and neuronal excitability modulate amyloid-beta-mediated forgetting in Drosophila. PLoS Biol 19(10): e3001412. PubMed ID: 34613972

Alzheimer disease (AD) is one of the main causes of age-related dementia and neurodegeneration. However, the onset of the disease and the mechanisms causing cognitive defects are not well understood. Aggregation of amyloidogenic peptides is a pathological hallmark of AD and is assumed to be a central component of the molecular disease pathways. Pan-neuronal expression of Aβ42 Arctic peptides in Drosophila melanogaster results in learning and memory defects. Surprisingly, targeted expression to the mushroom bodies, a center for olfactory memories in the fly brain, does not interfere with learning but accelerates forgetting. This study shows that reducing neuronal excitability either by feeding Levetiracetam or silencing of neurons in the involved circuitry ameliorates the phenotype. Furthermore, inhibition of the Rac-regulated forgetting pathway could rescue the Aβ42Arctic-mediated accelerated forgetting phenotype. Similar effects are achieved by increasing sleep, a critical regulator of neuronal homeostasis. Results provide a functional framework connecting forgetting signaling and sleep, which are critical for regulating neuronal excitability and homeostasis and are therefore a promising mechanism to modulate forgetting caused by toxic Aβ peptides (Kaldun, 2021).

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Azpurua, J., El-Karim, E. G., Tranquille, M. and Dubnau, J. (2021). A behavioral screen for mediators of age-dependent TDP-43 neurodegeneration identifies SF2/SRSF1 among a group of potent suppressors in both neurons and glia. PLoS Genet 17(11): e1009882. PubMed ID: 34723963

Cytoplasmic aggregation of Tar-DNA/RNA binding protein 43 (TDP-43) occurs in 97 percent of amyotrophic lateral sclerosis (ALS), ~40% of frontotemporal dementia (FTD) and in many cases of Alzheimer's disease (AD). Cytoplasmic TDP-43 inclusions are seen in both sporadic and familial forms of these disorders, including those cases that are caused by repeat expansion mutations in the C9orf72 gene. To identify downstream mediators of TDP-43 toxicity, This study expressed human TDP-43 in a subset of Drosophila motor neurons. Such expression causes age-dependent deficits in negative geotaxis behavior. Using this behavioral readout of locomotion, this study conducted an shRNA suppressor screen and identified 32 transcripts whose knockdown was sufficient to ameliorate the neurological phenotype. The majority of these suppressors also substantially suppressed the negative effects on lifespan seen with glial TDP-43 expression. In addition to identification of a number of genes whose roles in neurodegeneration were not previously known, this screen also yielded genes involved in chromatin regulation and nuclear/import export - pathways that were previously identified in the context of cell based or neurodevelopmental suppressor screens. A notable example is SF2, a conserved orthologue of mammalian SRSF1, an RNA binding protein with roles in splicing and nuclear export. The identification SF2/SRSF1 as a potent suppressor of both neuronal and glial TDP-43 toxicity also provides a convergence with C9orf72 expansion repeat mediated neurodegeneration, where this gene also acts as a downstream mediator (Azpurua, 2021).

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Rimal, S., Li, Y., Vartak, R., Geng, J., Tantray, I., Li, S., Huh, S., Vogel, H., Glabe, C., Grinberg, L. T., Spina, S., Seeley, W. W., Guo, S. and Lu, B. (2021). Inefficient quality control of ribosome stalling during APP synthesis generates CAT-tailed species that precipitate hallmarks of Alzheimer's disease. Acta Neuropathol Commun 9(1): 169. PubMed ID: 34663454

Abstract

Amyloid precursor protein (APP) metabolism is central to Alzheimer's disease (AD) pathogenesis, but the key etiological driver remains elusive. Recent failures of clinical trials targeting amyloid-β (Aβ) peptides, the proteolytic fragments of amyloid precursor protein (APP) that are the main component of amyloid plaques, suggest that the proteostasis-disrupting, key pathogenic species remain to be identified. Previous studies suggest that APP C-terminal fragment (APP.C99) can cause disease in an Aβ-independent manner. The mechanism of APP.C99 pathogenesis is incompletely understood. Drosophila models were used to expressing APP.C99 with the native ER-targeting signal of human APP, expressing full-length human APP only, or co-expressing full-length human APP and β-secretase (BACE), to investigate mechanisms of APP.C99 pathogenesis. Key findings are validated in mammalian cell culture models, mouse 5xFAD model, and postmortem AD patient brain materials. This study found that ribosomes stall at the ER membrane during co-translational translocation of APP.C99, activating ribosome-associated quality control (RQC) to resolve ribosome collision and stalled translation. Stalled APP.C99 species with C-terminal extensions (CAT-tails) resulting from inadequate RQC are prone to aggregation, causing endolysosomal and autophagy defects and seeding the aggregation of amyloid β peptides, the main component of amyloid plaques. Genetically removing stalled and CAT-tailed APP.C99 rescued proteostasis failure, endolysosomal/autophagy dysfunction, neuromuscular degeneration, and cognitive deficits in AD models. The finding of RQC factor deposition at the core of amyloid plaques from AD brains further supports the central role of defective RQC of ribosome collision and stalled translation in AD pathogenesis. These findings demonstrate that amyloid plaque formation is the consequence and manifestation of a deeper level proteostasis failure caused by inadequate RQC of translational stalling and the resultant aberrantly modified APP.C99 species, previously unrecognized etiological drivers of AD and newly discovered therapeutic targets (Rimal, 2021).

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Li, W. H., Gan, L. H., Ma, F. F., Feng, R. L., Wang, J., Li, Y. H., Sun, Y. Y., Wang, Y. J., Diao, X., Qian, F. Y. and Wen, T. Q. (2021). Deletion of Dcf1 Reduces Amyloid-beta Aggregation and Mitigates Memory Deficits. J Alzheimers Dis. PubMed ID: 33896839

Abstract

Alzheimer's disease (AD). is a progressive neurodegenerative disease. One of the pathologies of AD is the accumulation of amyloid-β (Aβ) to form senile plaques, leading to a decline in cognitive ability and a lack of learning and memory. However, the cause leading to Aβ aggregation is not well understood. Dendritic cell factor 1 (Dcf1) shows a high expression in the entorhinal cortex neurons and neurofibrillary tangles in AD patients. This study investigated the effect of Dcf1 on Aβ aggregation and memory deficits in AD development. The mouse and Drosophila AD model were used to test the expression and aggregation of Aβ, senile plaque formation, and pathological changes in cognitive behavior during dcf1 knockout and expression. Possible drug target effects were explored through intracerebroventricular delivery of Dcf1 antibodies. Deletion of Dcf1 resulted in decreased Aβ42 level and deposition, and rescued AMPA Receptor (GluA2) levels in the hippocampus of APP-PS1-AD mice. In Aβ42 AD Drosophila, the expression of Dcf1 in Aβ42 AD flies aggravated the formation and accumulation of senile plaques, significantly reduced its climbing ability and learning-memory. Data analysis from all 20 donors with and without AD patients aged between 80 and 90 indicated a high-level expression of Dcf1 in the temporal neocortex. Dcf1 contributed to Aβ aggregation by UV spectroscopy assay. Intracerebroventricular delivery of Dcf1 antibodies in the hippocampus reduced the area of senile plaques and reversed learning and memory deficits in APP-PS1-AD mice. Dcf1 causes Aβ-plaque accumulation, inhibiting dcf1 expression could potentially offer therapeutic avenues.

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Abreha, M. H., Ojelade, S., Dammer, E. B., McEachin, Z. T., Duong, D. M., Gearing, M., Bassell, G. J., Lah, J. J., Levey, A. I., Shulman, J. M. and Seyfried, N. T. (2021). TBK1 interacts with tau and enhances neurodegeneration in tauopathy. J Biol Chem: 100760. PubMed ID: 33965374

Abstract

One of the defining pathological features of Alzheimer's Disease (AD) is the deposition of neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau (see Drosophila Tau) in the brain. Aberrant activation of kinases in AD has been suggested to enhance phosphorylation and toxicity of tau, making the responsible tau kinases attractive therapeutic targets. The full complement of tau interacting kinases in AD brain and their activity in disease remains incompletely defined. In this study, immunoaffinity enrichment coupled with mass spectrometry (MS) identified TANK-binding kinase 1 (TBK1) as a tau-interacting partner in human AD cortical brain tissues. This interaction was validated in human AD, familial frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) caused by mutations in MAPT (R406W & P301L) and corticobasal degeneration (CBD) postmortem brain tissues as well as human cell lines. Further, this study documented increased TBK1 activation in both AD and FTDP-17 and map TBK1 phosphorylation sites on tau based on in vitro kinase assays coupled to MS. Lastly, in a Drosophila tauopathy model, activating expression of a conserved TBK1 ortholog (I-kappaB kinase ε) triggers tau hyperphosphorylation and enhanced neurodegeneration, whereas knockdown had the reciprocal effect, suppressing tau toxicity. Collectively, these findings suggest that increased TBK1 activation may promote tau hyperphosphorylation and neuronal loss in AD and related tauopathies (Abreha, 2021).

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Cowan, C. M., Sealey, M. A. and Mudher, A. (2020). Suppression of tau-induced phenotypes by vitamin E demonstrates the dissociation of oxidative stress and phosphorylation in mechanisms of tau toxicity. J Neurochem. PubMed ID: 33251603

Abstract

Various lines of evidence implicate oxidative stress in the pathogenic mechanism(s) underpinning tauopathies. Consequently, antioxidant therapies have been considered in clinical practice for the treatment of tauopathies such as Alzheimer's disease (AD), but with mixed results. Previous studies have reported increased protein oxidation upon expression of both human 0N3R (hTau(0N3R)) and 0N4R (hTau(0N4R)) tau (see Drosophila Tau) in vivo. Building on these studies, this study demonstrates here the suppression of hTau(0N3R) associated phenotypes in Drosophila melanogaster after treatment with vitamin C or vitamin E. Curiously the rescue of phenotype was seen without alteration in total tau level or alteration in phosphorylation at a number of disease-associated sites. Moreover, treatment with paraquat, a pro-oxidant drug, did not exacerbate the hTau(0N3R) phenotypes. This result following paraquat treatment is reminiscent of previous findings with hTau(0N4R) which also causes greater oxidative stress when compared to hTau(0N3R) but has a milder phenotype. Collectively these data imply that the role of oxidative stress in tau-mediated toxicity is not straight forward and there may be isoform-specific effects as well as contribution of other factors. This may explain the ambiguous effects of anti-oxidant treatments on clinical outcome in dementia patients (Cowan, 2020).

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Kessissoglou, I. A., Langui, D., Hasan, A., Maral, M., Dutta, S. B., Hiesinger, P. R. and Hassan, B. A. (2020). The Drosophila amyloid precursor protein homologue mediates neuronal survival and neuroglial interactions. PLoS Biol 18(12): e3000703. PubMed ID: 33290404

Abstract

The amyloid precursor protein (APP) is a structurally and functionally conserved transmembrane protein whose physiological role in adult brain function and health is still unclear. Because mutations in APP cause familial Alzheimer's disease (fAD), most research focuses on this aspect of APP biology. This study investigated the physiological function of APP in the adult brain using the fruit fly Drosophila melanogaster, which harbors a single APP homologue called APP Like (APPL). Previous studies have provided evidence for the implication of APPL in neuronal wiring and axonal growth through the Wnt signaling pathway during development. However, like APP, APPL continues to be expressed in all neurons of the adult brain where its functions and their molecular and cellular underpinnings are unknown. This study reports that APPL loss of function (LOF) results in the dysregulation of endolysosomal function in neurons, with a notable enlargement of early endosomal compartments followed by neuronal cell death and the accumulation of dead neurons in the brain during a critical period at a young age. These defects can be rescued by reduction in the levels of the early endosomal regulator Rab5, indicating a causal role of endosomal function for cell death. Finally, this study shows that the secreted extracellular domain of APPL interacts with glia and regulates the size of their endosomes, the expression of the Draper engulfment receptor, and the clearance of neuronal debris in an axotomy model. It is proposes that APP proteins represent a novel family of neuroglial signaling factors required for adult brain homeostasis (Kessissoglou, 2020).

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Neuman, S. D., Terry, E. L., Selegue, J. E., Cavanagh, A. T. and Bashirullah, A. (2020). Mistargeting of secretory cargo in retromer-deficient cells. Dis Model Mech. 14(1):dmm046417 PubMed ID: 33380435

Abstract

Intracellular trafficking is a basic and essential cellular function required for delivery of proteins to the appropriate subcellular destination; this process is especially demanding in professional secretory cells, which synthesize and secrete massive quantities of cargo proteins via regulated exocytosis. The Drosophila larval salivary glands are professional secretory cells that synthesize and secrete mucin proteins at the onset of metamorphosis. Using the larval salivary glands as a model system, a role was identified for the highly conserved retromer complex in trafficking of secretory granule membrane proteins. This study demonstrates that retromer-dependent trafficking via endosomal tubules is induced at the onset of secretory granule biogenesis, and that recycling via endosomal tubules is required for delivery of essential secretory granule membrane proteins to nascent granules. Without retromer function, nascent granules do not contain the proper membrane proteins; as a result, cargo from these defective granules is mistargeted to Rab7-positive endosomes, where it progressively accumulates to generate dramatically enlarged endosomes. Retromer complex dysfunction is strongly associated with neurodegenerative diseases, including Alzheimer's disease, characterized by accumulation of amyloid β (Aβ). Ectopically expressed amyloid precursor protein (APP) undergoes regulated exocytosis in salivary glands and accumulates within enlarged endosomes in retromer-deficient cells. These results highlight recycling of secretory granule membrane proteins as a critical step during secretory granule maturation and provide new insights into our understanding of retromer complex function in secretory cells. These findings also suggest that missorting of secretory cargo, including APP, may contribute to the progressive nature of neurodegenerative disease (Neuman, 2020).

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Lohr, K. M., Frost, B., Scherzer, C. and Feany, M. B. (2020). Biotin rescues mitochondrial dysfunction and neurotoxicity in a tauopathy model. Proc Natl Acad Sci U S A 117(52): 33608-33618. PubMed ID: 33318181

Abstract

Mitochondrial and metabolic dysfunction are often implicated in neurological disease, but effective mechanism-based therapies remain elusive. A genome-scale forward genetic screen was performed in a Drosophila model of tauopathy, a class of neurodegenerative disorders characterized by the accumulation of the protein tau, and identified manipulation of the B-vitamin biotin as a potential therapeutic approach in tauopathy. Tau transgenic flies have an innate biotin deficiency due to tau-mediated relaxation of chromatin and consequent aberrant expression of multiple biotin-related genes, disrupting both carboxylase and mitochondrial function. Biotin depletion alone causes mitochondrial pathology and neurodegeneration in both flies and human neurons, implicating mitochondrial dysfunction as a mechanism in biotin deficiency. Finally, carboxylase biotin levels are reduced in mammalian tauopathies, including brains of human Alzheimer's disease patients. These results provide insight into pathogenic mechanisms of human biotin deficiency, the resulting effects on neuronal health, and a potential therapeutic pathway in the treatment of tau-mediated neurotoxicity (Lohr, 2020).

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Konar, A., Kalra, R. S., Chaudhary, A., Nayak, A., Guruprasad, K. P., Satyamoorthy, K., Ishida, Y., Terao, K., Kaul, S. C. and Wadhwa, R. (2020). Identification of Caffeic Acid Phenethyl Ester (CAPE) as a Potent Neurodifferentiating Natural Compound That Improves Cognitive and Physiological Functions in Animal Models of Neurodegenerative Diseases. Front Aging Neurosci 12: 561925. PubMed ID: 33244299

Abstract

Using human neuroblastoma cells, this study identified caffeic acid phenethyl ester (CAPE) as a potent neurodifferentiating natural compound. Analyses of control and CAPE-induced neurodifferentiated cells revealed: (i) modulation of several key proteins (NF200, MAP-2, NeuN, PSD95, Tuj1, GAP43, and GFAP) involved in neurodifferentiation process; and (ii) attenuation of neuronal stemness (HOXD13, WNT3, and Msh-2) and proliferation-promoting (CDC-20, CDK-7, and BubR1) proteins. It is anticipated that the neurodifferentiation potential of CAPE was tested using the Drosophila model of Alzheimer's disease (AD) and mice model of amnesia/loss of memory. In both models, CAPE exhibited improved disease symptoms and activation of physiological functions. Remarkably, CAPE-treated mice showed increased levels of neurotrophin-BDNF, neural progenitor marker-Nestin, and differentiation marker-NeuN, both in the cerebral cortex and hippocampus. Taken together, this study demonstrated the differentiation-inducing and therapeutic potential of CAPE for neurodegenerative diseases (Konar, 2020).

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Gomes, L., Uytterhoeven, V., Lopez-Sanmartin, D., Tome, S. O., Tousseyn, T., Vandenberghe, R., Vandenbulcke, M., von Arnim, C. A. F., Verstreken, P. and Thal, D. R. (2021). Maturation of neuronal AD-tau pathology involves site-specific phosphorylation of cytoplasmic and synaptic tau preceding conformational change and fibril formation. Acta Neuropathol. PubMed ID: 33427938

Abstract

In Alzheimer's disease (AD), tau-protein (see Drosophila Tau) undergoes a multi-step process involving the transition from a natively unfolded monomer to large, aggregated structures such as neurofibrillary tangles (NFTs). However, it is not yet clear which events initiate the early preclinical phase of AD tauopathy and whether they have impact on the propagation of tau pathology in later disease stages. To address this question, the distribution was analyzed of tau species phosphorylated at T231, S396/S404 and S202/T205, conformationally modified at the MC1 epitope, and fibrillary tau was detected by the Gallyas method (Gallyas-tau), in the brains of 15 symptomatic and 20 asymptomatic cases with AD pathology as well as of 19 nonAD cases. As initial tau lesions, phosphorylated-T231-tau was identified diffusely distributed within the somatodendritic compartment (IC-tau) and phosphorylated-S396/pS404-tau in axonal lesions of the white matter and in the neuropil (IN-tau). The subcellular localization of pT231-tau in the cell body and pS396/pS404-tau in the presynapse was confirmed in hP301L mutant Drosophila larvae. Phosphorylated-S202/T205-tau, MC1-tau and Gallyas-tau were negative for these lesions. IC- and IN-tau were observed in all analyzed regions of the human brain, including early affected regions in nonAD cases (entorhinal cortex) and late affected regions in symptomatic AD cases (cerebellum), indicating that tau pathology initiation follows similar processes when propagating into previously unaffected regions. Furthermore, a sequence of AD-related maturation of tau-aggregates was observed, initiated by the appearance of IC- and IN-tau, followed by the formation of pretangles exhibiting pT231-tau, pS396/pS404-tau and pS202/pT205-tau, then by MC1-conformational tau, and, finally, by the formation of Gallyas-positive NFTs. Since cases classified as nonAD already showed IC- and IN-tau, these findings suggest that these lesions are a prerequisite for the development of AD (Gomez, 2021).

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Lee, S. H., Gomes, S. M., Ghalayini, J., Iliadi, K. G. and Boulianne, G. L. (2020). Angiotensin Converting Enzyme Inhibitors and Angiotensin Receptor Blockers rescue memory defects in Drosophila expressing Alzheimer's disease-related transgenes independently of the canonical Renin Angiotensin System. eNeuro. PubMed ID: 33060184

Abstract

Alzheimer's disease (AD) is a degenerative disorder that causes progressive memory and cognitive decline. Recently, studies have reported that inhibitors of the mammalian renin angiotensin system (RAS) result in a significant reduction in the incidence and progression of AD by unknown mechanisms. This study used a genetic and pharmacological approach to evaluate the beneficial effects of Angiotensin Converting Enzyme Inhibitors (ACE-Is) and Angiotensin Receptor Blockers (ARBs) in Drosophila expressing AD-related transgenes. Importantly, while ACE orthologs have been identified in Drosophila, other RAS components are not conserved. This study shows that captopril, an ACE-I, and losartan, an ARB, can suppress a rough eye phenotype and brain cell death in flies expressing a mutant human C99 (C-terminal fragment of APP) transgene. Captopril also significantly rescues memory defects in these flies. Similarly, both drugs reduce cell death in Drosophila expressing human Aβ42, and losartan significantly rescues memory deficits. However, neither drug affects production, accumulation or clearance of Aβ42. Importantly, neither drug rescued brain cell death in Drosophila expressing human Tau suggesting that RAS inhibitors specifically target the amyloid pathway. Of note, reduced cell death and a complete rescue of memory deficits were observed when a null mutation in Drosophila Acer was crossed into each transgenic line demonstrating that the target of captopril in Drosophila is Acer. Altogether, these studies demonstrate that captopril and losartan are able to modulate AD related phenotypes in the absence of the canonical RAS pathway and suggest that both drugs have additional targets that can be identified in Drosophila (Lee, 2020)

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Shaposhnikov, M. V., Zemskaya, N. V., Koval, L., Minnikhanova, N. R., Kechko, O. I., Mitkevich, V. A., Makarov, A. A. and Moskalev, A. (2020). Amyloid-beta peptides slightly affect lifespan or antimicrobial peptide gene expression in Drosophila melanogaster. BMC Genet 21(Suppl 1): 65. PubMed ID: 33092519

Abstract

Beta-amyloid peptide (Aβ) is the key protein in the pathogenesis of Alzheimer's disease, the most common age-related neurodegenerative disorder in humans. Aβ peptide induced pathological phenotypes in different model organisms include neurodegeneration and lifespan decrease. However, recent experimental evidence suggests that Aβ may utilize oligomerization and fibrillization to function as an antimicrobial peptide (AMP), and protect the host from infections. This study used the power of Drosophila model to study mechanisms underlying a dual role for Aβ peptides. The effects were investigated of Drosophila treatment with three Aβ42 peptide isoforms, which differ in their ability to form oligomers and aggregates on the lifespan, locomotor activity and AMP genes expression. Aβ42 slightly decreased female's median lifespan (by 4.5%), but the effect was not related to the toxicity of peptide isoform. The lifespan and relative levels of AMP gene expression in male flies as well as locomotor activity in both sexes were largely unaffected by Aβ42 peptide treatment. Regardless of the effects on lifespan, Aβ42 peptide treatment induced decrease in AMP genes expression in females, but the effects were not robust. The results demonstrate that chronic treatment with Aβ42 peptides does not drastically affect fly aging or immunity (Shaposhnikov, 2020).

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Keramidis, I., Vourkou, E., Papanikolopoulou, K. and Skoulakis, E. M. C. (2020). Functional Interactions of Tau Phosphorylation Sites That Mediate Toxicity and Deficient Learning in Drosophila melanogaster. Front Mol Neurosci 13: 569520. PubMed ID: 33192295

Abstract

Hyperphosphorylated Tau protein is the main component of the neurofibrillary tangles, characterizing degenerating neurons in Alzheimer's disease and other Tauopathies. Expression of human Tau protein (see Drosophila Tau) in Drosophila CNS results in increased toxicity, premature mortality and learning and memory deficits. This study used novel transgenic lines to investigate the contribution of specific phosphorylation sites previously implicated in Tau toxicity. These three different sites, Ser(238), Thr(245), and Ser(262) were tested either by blocking their phosphorylation, by Ser/Thr to Ala substitution, or pseudophosphorylation, by changing Ser/Thr to Glu. The hypothesis was validated that phosphorylation at Ser(262) is necessary for Tau-dependent learning deficits and a "facilitatory gatekeeper" to Ser(238) occupation, which is linked to Tau toxicity. Importantly this study reveals that phosphorylation at Thr(245) acts as a "suppressive gatekeeper", preventing phosphorylation of many sites including Ser(262) and consequently of Ser(238). Therefore, this study elucidates novel interactions among phosphosites central to Tau mediated neuronal dysfunction and toxicity, likely driven by phosphorylation-dependent conformational plasticity (Keramidis, 2020).

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Shaposhnikov, M. V., Zemskaya, N. V., Koval, L., Minnikhanova, N. R., Kechko, O. I., Mitkevich, V. A., Makarov, A. A. and Moskalev, A. (2020). Amyloid-beta peptides slightly affect lifespan or antimicrobial peptide gene expression in Drosophila melanogaster. BMC Genet 21(Suppl 1): 65. PubMed ID: 33092519

Abstract

Beta-amyloid peptide (Aβ) is the key protein in the pathogenesis of Alzheimer's disease, the most common age-related neurodegenerative disorder in humans. Aβ peptide induced pathological phenotypes in different model organisms include neurodegeneration and lifespan decrease. However, recent experimental evidence suggests that Aβ may utilize oligomerization and fibrillization to function as an antimicrobial peptide (AMP), and protect the host from infections. This study used the power of Drosophila model to study mechanisms underlying a dual role for Aβ peptides. The effects were investigated of Drosophila treatment with three Aβ42 peptide isoforms, which differ in their ability to form oligomers and aggregates on the lifespan, locomotor activity and AMP genes expression. Aβ42 slightly decreased female's median lifespan (by 4.5%), but the effect was not related to the toxicity of peptide isoform. The lifespan and relative levels of AMP gene expression in male flies as well as locomotor activity in both sexes were largely unaffected by Aβ42 peptide treatment. Regardless of the effects on lifespan, Aβ42 peptide treatment induced decrease in AMP genes expression in females, but the effects were not robust. The results demonstrate that chronic treatment with Aβ42 peptides does not drastically affect fly aging or immunity (Shaposhnikov, 2020).

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Aqsa, Sarkar, S. (2020). Age dependent trans-cellular propagation of human tau aggregates in Drosophila disease models. Brain Res: 147207. PubMed ID: 33212022

Abstract
Tauopathies is a class of neurodegenerative disorders which involves the transformation of physiological tau (see Drosophila Tau) into pathogenic tau. One of the prime causes reported to drive this conversion is tau hyperphosphorylation and the subsequent propagation of pathogenic protein aggregates across the nervous system. Although past attempts have been made to deduce the details of tau propagation, yet not much is known about its mechanism. A better understanding of this aspect of disease pathology can prove to be beneficial for the development of diagnostic and therapeutic approaches. For the first time, this study demonstrates that the human tau possesses an intrinsic property to spread trans-cellularly in the fly nervous system irrespective of the tau allele or the neuronal tissue type. Aggregate migration restricted by targeted down-regulation of a specific kinase, elucidates the role of hyper-phosphorylation in its movement. Contrary to the previous models, this study delivers an easy and rapid in-vivo model for comprehensive examination of tau migration pathology. Henceforth, the developed model would not only be immensely helpful in uncovering the mechanistic in-depths of tau propagation pathology but also aid in modifier and/or drug screening for amelioration of tauopathies (Aqsa, 2020).

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Mangleburg, C. G., Wu, T., Yalamanchili, H. K., Guo, C., Hsieh, Y. C., Duong, D. M., Dammer, E. B., De Jager, P. L., Seyfried, N. T., Liu, Z. and Shulman, J. M. (2020). Integrated analysis of the aging brain transcriptome and proteome in tauopathy. Mol Neurodegener 15(1): 56. PubMed ID: 32993812

Abstract
Tau (see Drosophila Tau) neurofibrillary tangle pathology characterizes Alzheimer's disease and other neurodegenerative tauopathies. This study performed paired, longitudinal RNA-sequencing and mass-spectrometry were performed in a Drosophila model of tauopathy, based on pan-neuronal expression of human wildtype Tau (Tau(WT)) or a mutant form causing frontotemporal dementia (Tau(R406W)). Tau-induced, differentially expressed transcripts and proteins were examined cross-sectionally or using linear regression and adjusting for age. Overlaps with human brain gene expression profiles in tauopathy were examined. Tau(WT) induced 1514 and 213 differentially expressed transcripts and proteins, respectively. Tau(R406W) had a substantially greater impact, causing changes in 5494 transcripts and 697 proteins. There was a ~ 70% overlap between age- and Tau-induced changes and the analyses reveal pervasive bi-directional interactions. Strikingly, 42% of Tau-induced transcripts were discordant in the proteome, showing opposite direction of change. Tau-responsive gene expression networks strongly implicate innate immune activation. Cross-species analyses pinpoint human brain gene perturbations specifically triggered by Tau pathology and/or aging, and further differentiate between disease amplifying and protective changes. These results comprise a powerful, cross-species functional genomics resource for tauopathy, revealing Tau-mediated disruption of gene expression, including dynamic, age-dependent interactions between the brain transcriptome and proteome (Mangleburg, 2020).

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Losev, Y., Frenkel-Pinter, M., Abu-Hussien, M., Viswanathan, G. K., Elyashiv-Revivo, D., Geries, R., Khalaila, I., Gazit, E. and Segal, D. (2020). Differential effects of putative N-glycosylation sites in human Tau on Alzheimer's disease-related neurodegeneration. Cell Mol Life Sci. PubMed ID: 32926180

Abstract

Amyloid assemblies of Tau (see Drosophila Tau) are associated with Alzheimer's disease (AD). In AD Tau undergoes several abnormal post-translational modifications, including hyperphosphorylation and glycosylation, which impact disease progression. N-glycosylated Tau was reported to be found in AD brain tissues but not in healthy counterparts. This is surprising since Tau is a cytosolic protein whereas N-glycosylation occurs in the ER-Golgi. Previous in vitro studies indicated that N-glycosylation of Tau facilitated its phosphorylation and contributed to maintenance of its Paired Helical Filament structure. However, the specific Tau residue(s) that undergo N-glycosylation and their effect on Tau-engendered pathology are unknown. High-performance liquid chromatography and mass spectrometry (LC-MS) analysis indicated that both N359 and N410 were N-glycosylated in wild-type (WT) human Tau (hTau) expressed in human SH-SY5Y cells. Asparagine to glutamine mutants, which cannot undergo N-glycosylation, at each of three putative N-glycosylation sites in hTau (N167Q, N359Q, and N410Q) were generated and expressed in SH-SY5Y cells and in transgenic Drosophila. The mutants modulated the levels of hTau phosphorylation in a site-dependent manner in both cell and fly models. Additionally, N359Q ameliorated, whereas N410Q exacerbated various aspects of hTau-engendered neurodegeneration in transgenic flies (Losev, 2020).

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Naito, Y., Tanabe, Y., Lee, A. K., Hamel, E. and Takahashi, H. (2017). Amyloid-beta Oligomers Interact with Neurexin and Diminish Neurexin-mediated Excitatory Presynaptic Organization. Sci Rep 7: 42548. PubMed ID: 28211900

Abstract
Alzheimer's disease (AD) is characterized by excessive production and deposition of amyloid-beta (Abeta) proteins (see Drosophila Appl) as well as synapse dysfunction and loss. While soluble Abeta oligomers (AbetaOs) have deleterious effects on synapse function and reduce synapse number, the underlying molecular mechanisms are not well understood. This study screened synaptic organizer proteins for cell-surface interaction with AbetaOs and identified a novel interaction between neurexins (NRXs; see Drosophila Neurexin) and AbetaOs. AbetaOs bind to NRXs via the N-terminal histidine-rich domain (HRD) of beta-NRX1/2/3 and alternatively-spliced inserts at splicing site 4 of NRX1/2. In artificial synapse-formation assays, AbetaOs diminish excitatory presynaptic differentiation induced by NRX-interacting proteins including neuroligin1/2 (NLG1/2; see Drosophila Neuroligin) and the leucine-rich repeat transmembrane protein LRRTM2. Although AbetaOs do not interfere with the binding of NRX1beta to NLG1 or LRRTM2, time-lapse imaging revealed that AbetaO treatment reduces surface expression of NRX1beta on axons and that this reduction depends on the NRX1beta HRD. In transgenic mice expressing mutated human amyloid precursor protein, synaptic expression of beta-NRXs, but not alpha-NRXs, decreases. Thus these data indicate that AbetaOs interact with NRXs and that this interaction inhibits NRX-mediated presynaptic differentiation by reducing surface expression of axonal beta-NRXs, providing molecular and mechanistic insights into how AbetaOs lead to synaptic pathology in AD (Naito, 2017).

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Tanaka, N., Okuda, M., Nishigaki, T., Tsuchiya, N., Kobayashi, Y., Uemura, T., Kumo, S., Sugimoto, H., Miyata, S. and Waku, T. (2020). Development of a brain-permeable peptide nanofiber that prevents aggregation of Alzheimer pathogenic proteins. PLoS One 15(7): e0235979. PubMed ID: 32706773

Abstract

Alzheimer's disease (AD) is proposed to be induced by abnormal aggregation of amyloidβ in the brain. This study designed a brain-permeable peptide nanofiber drug from a fragment of heat shock protein to suppress aggregation of the pathogenic proteins. To facilitate delivery of the nanofiber into the brain, a protein transduction domain from Drosophila Antennapedia was incorporated into the peptide sequence. The resulting nanofiber efficiently suppressed the cytotoxicity of amyloid βby trapping amyloid β onto its hydrophobic nanofiber surface. Moreover, the intravenously or intranasally injected nanofiber was delivered into the mouse brain, and improved the cognitive function of an Alzheimer transgenic mouse model. These results demonstrate the potential therapeutic utility of nanofibers for the treatment of AD (Tanaka, 2020).

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Wang, X. and Davis, R. L. (2020). Early Mitochondrial Fragmentation and Dysfunction in a Drosophila Model for Alzheimer's Disease. Mol Neurobiol. PubMed ID: 32909149

Abstract

Many different cellular systems and molecular processes become compromised in Alzheimer's disease (AD) including proteostasis, autophagy, inflammatory responses, synapse and neuronal circuitry, and mitochondrial function. This study focused on mitochondrial dysfunction owing to the toxic neuronal environment produced by expression of Aβ42, and its relationship to other pathologies found in AD including increased neuronal apoptosis, plaque deposition, and memory impairment. Using super-resolution microscopy, mitochondrial status was assayed in the three distinct neuronal compartments (somatic, dendritic, axonal) of mushroom body neurons of Drosophila expressing Aβ42. The mushroom body neurons comprise a major center for olfactory memory formation in insects. Calcium imaging was employed to measure mitochondrial function, immunohistochemical and staining techniques to measure apoptosis and plaque formation, and olfactory classical conditioning to measure learning. Mitochondria become fragmented at a very early age along with decreased function measured by mitochondrial calcium entry. Increased apoptosis and plaque deposition also occur early, yet interestingly, a learning impairment was found only after a much longer period of time-10 days, which is a large fraction of the fly's lifespan. This is similar to the pronounced delay between cellular pathologies and the emergence of a memory dysfunction in humans. These studies are consistent with the model that mitochondrial dysfunction and/or other cellular pathologies emerge at an early age and lead to much later learning impairments. The results obtained further develop this Drosophila model as a useful in vivo system for probing the mechanisms by which Aβ42 produces mitochondrial and other cellular toxicities that produce memory dysfunction (Wang, 2020).

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Coelho, D. S. and Moreno, E. (2020). Neuronal Selection Based on Relative Fitness Comparison Detects and Eliminates Amyloid-beta-Induced Hyperactive Neurons in Drosophila. iScience 23(9): 101468. PubMed ID: 32866827

Abstract

During adult life, damaged but viable neurons can accumulate in the organism, creating increasingly heterogeneous and dysfunctional neural circuits. One intriguing example is the aberrant increased activity of cerebral networks detected in vulnerable brain regions during preclinical stages of Alzheimer's disease. The pathophysiological contribution of these early functional alterations to the progression of Alzheimer's disease is uncertain. A unique cell selection mechanism based on relative fitness comparison between neurons is able to target and remove aberrantly active neurons generated by heterologous human amyloid-β in Drosophila. Sustained neuronal activity is sufficient to compromise neuronal fitness and upregulate the expression of the low fitness indicators Flower(LoseB) and Azot in the fly. Conversely, forced silencing of neurons restores brain fitness and reduces amyloid-β-induced cell death. The manipulation of this cell selection process, which was already proved to be conserved in humans, might be a promising new avenue to treat Alzheimer's (Coelho, 2020).

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Krittika, S. and Yadav, P. (2020). Dietary protein restriction deciphers new relationships between lifespan, fecundity and activity levels in fruit flies Drosophila melanogaster. Sci Rep 10(1): 10019. PubMed ID: 32572062

Abstract
Drosophila melanogaster has been used in Diet Restriction (DR) studies for a few decades now, due to easy diet implementation and its short lifespan. Since the concentration of protein determines the trade-offs between lifespan and fecundity, it is important to understand the level of protein and the extent of its influence on lifespan, fecundity and activity of fruit flies. This study intended to assess the effect of a series of protein restricted diets from age 1 day of the adult fly on these traits to understand the possible variations in trade-off across tested diets. Lifespan under different protein concentrations remains unaltered, even though protein restricted diets exerted an age-specific influence on fecundity. Interestingly, there was no difference in lifetime activity of the flies in most of the tested protein restricted (PR) diets, even though a sex-dependent influence of protein concentrations was observed. Additionally, not all concentrations of PR diet increase activity, thereby suggesting that the correlation between lifespan and the lifetime activity can be challenged under protein-restricted condition. Therefore, the PR does not need to exert its effect on lifespan and fecundity only but can also influence activity levels of the flies, thereby emphasizing the role of nutrient allotment between lifespan, fecundity and activity.

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Leila Abtahi, S., Masoudi, R. and Haddadi, M. (2020). The Distinctive Role of Tau and Amyloid beta in Mitochondrial Dysfunction through Alteration in Mfn2 and Drp1 mRNA Levels: A Comparative Study in Drosophila melanogaster. Gene: 144854. PubMed ID: 32525045

Abstract
Alzheimer's disease (AD) is one of the most common forms of neurodegenerative diseases. Aggregation of Aβ42 and hyperphosphorylated tau are two major hallmarks of AD. Whether different forms of tau (soluble or hyperphosphorylated) or are the main culprit in the events observed in AD is still under investigation. This study examined the effect of wild-type, prone to hyperphosphorylation and hyperphosphorylated tau, and also Aβ42 peptide on the brain antioxidant defense system and two mitochondrial genes, Marf (homologous to human MFN2) and Drp1 involved in mitochondrial dynamics in transgenic Drosophila melanogaster. AD is an age associated disease. Therefore, the activity of 3 antioxidant agents, CAT, SOD, and GSH levels and the mRNA levels of Marf and Drp1 was assessed in different time points of flies' life cycle. Reduction in cognitive function and antioxidant activity was observed in all lines and time points. The most and the least effect on the eye phenotype was exerted by hyperphosphorylated tau and Aβ42, respectively. In addition, the most remarkable alteration in Marf and Drp1 mRNA level was observed in transgenic flies expressing hyperphosphorylated tau when pan neuronal expression of transgenes was applied. However, when the disease causing gene expression was confined to the mushroom body, Marf and Drp1 mRNA level alteration was more prominent in tau(WT) and tau(E14) transgenic flies, respectively. This may suggest a role for propagation of tau(WT) compared to hyperphosphorylated tau or Aβ42. In conclusion, in spite of antioxidant deficiency caused by different types of tau and Aβ42, it seems that tau exerts more toxic effect on the eye phenotype and mitochondrial genes regulation (Marf and Drp1). Moreover, different mechanisms seem to be involved in mitochondrial gene dysregulation when various forms of tau are expressed (Leila Abtahi, 2020).

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Scholes, H. M., Cryar, A., Kerr, F., Sutherland, D., Gethings, L. A., Vissers, J. P. C., Lees, J. G., Orengo, C. A., Partridge, L. and Thalassinos, K. (2020). Dynamic changes in the brain protein interaction network correlates with progression of Abeta42 pathology in Drosophila. Sci Rep 10(1): 18517. PubMed ID: 33116184

Abstract

Alzheimer's disease (AD), the most prevalent form of dementia, is a progressive and devastating neurodegenerative condition for which there are no effective treatments. Understanding the molecular pathology of AD during disease progression may identify new ways to reduce neuronal damage. This paper presents a longitudinal study tracking dynamic proteomic alterations in the brains of an inducible Drosophila melanogaster model of AD expressing the Arctic mutant Aβ42 gene. 3093 proteins from flies that were induced to express Aβ42 and age-matched healthy controls were identified using label-free quantitative ion-mobility data independent analysis mass spectrometry. Of these, 228 proteins were significantly altered by Aβ42 accumulation and were enriched for AD-associated processes. Network analyses further revealed that these proteins have distinct hub and bottleneck properties in the brain protein interaction network, suggesting that several may have significant effects on brain function. This unbiased analysis provides useful insights into the key processes governing the progression of amyloid toxicity and forms a basis for further functional analyses in model organisms and translation to mammalian systems (Scholes, 2020).

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Zhang, H., Karisetty, B. C., Bhatnagar, A., Armour, E. M., Beaver, M., Roach, T. V., Mortazavi, S., Mandloi, S. and Elefant, F. (2020). Tip60 protects against amyloid-beta-induced transcriptomic alterations via different modes of action in early versus late stages of neurodegeneration. Mol Cell Neurosci 109: 103570. PubMed ID: 33160016

Abstract
Alzheimer's disease (AD) is an age-related neurodegenerative disorder hallmarked by amyloid-β (Aβ) plaque accumulation, neuronal cell death, and cognitive deficits that worsen during disease progression. Histone acetylation dysregulation, caused by an imbalance between reduced histone acetyltransferases (HAT) Tip60 and increased histone deacetylase 2 (HDAC2) levels, can directly contribute to AD pathology. However, whether such AD-associated neuroepigenetic alterations occur in response to Aβ peptide production and can be protected against by increasing Tip60 levels over the course of neurodegenerative progression remains unknown. This study profiled Tip60 HAT/HDAC2 dynamics and transcriptome-wide changes across early and late stage AD pathology in the Drosophila brain produced solely by human amyloid-β(42). Early Aβ(42) induction leads to disruption of Tip60 HAT/HDAC2 balance during early neurodegenerative stages preceding Aβ plaque accumulation that persists into late AD stages. Correlative transcriptome-wide studies reveal alterations in biological processes were classified as transient (early-stage only), late-onset (late-stage only), and constant (both). Increasing Tip60 HAT levels in the Aβ(42) fly brain protects against AD functional pathologies that include Aβ plaque accumulation, neural cell death, cognitive deficits, and shorter life-span. Strikingly, Tip60 protects against Aβ(42)-induced transcriptomic alterations via distinct mechanisms during early and late stages of neurodegeneration. These findings reveal distinct modes of neuroepigenetic gene changes and Tip60 neuroprotection in early versus late stages in AD that can serve as early biomarkers for AD, and support the therapeutic potential of Tip60 over the course of AD progression (Zhang, 2020).

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King, L. B., Boto, T., Botero, V., Aviles, A. M., Jomsky, B. M., Joseph, C., Walker, J. A. and Tomchik, S. M. (2020). Developmental loss of neurofibromin across distributed neuronal circuits drives excessive grooming in Drosophila. PLoS Genet 16(7): e1008920. PubMed ID: 32697780

Abstract
Neurofibromatosis type 1 is a monogenetic disorder that predisposes individuals to tumor formation and cognitive and behavioral symptoms. The neuronal circuitry and developmental events underlying these neurological symptoms are unknown. To better understand how mutations of the underlying gene (NF1) drive behavioral alterations, grooming was examined in the Drosophila neurofibromatosis 1 model. Mutations of the fly NF1 ortholog drive excessive grooming, and increased grooming was observed in adults when Nf1 was knocked down during development. Furthermore, intact Nf1 Ras GAP-related domain signaling was required to maintain normal grooming. The requirement for Nf1 was distributed across neuronal circuits, which were additive when targeted in parallel, rather than mapping to discrete microcircuits. Overall, these data suggest that broadly-distributed alterations in neuronal function during development, requiring intact Ras signaling, drive key Nf1-mediated behavioral alterations. Thus, global developmental alterations in brain circuits/systems function may contribute to behavioral phenotypes in neurofibromatosis type 1.

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Park, J., Choi, H., Kim, Y. D., Kim, S. H., Kim, Y., Gwon, Y., Lee, D. Y., Park, S. H., Heo, W. D. and Jung, Y. K. (2021). Developmental loss of neurofibromin across distributed neuronal circuits drives excessive grooming in Drosophila. Aberrant role of ALK in tau proteinopathy through autophagosomal dysregulation. Mol Psychiatry. PubMed ID: 33452442

Abstract

Proteinopathy in neurodegenerative diseases is typically characterized by deteriorating activity of specific protein aggregates. In tauopathies, including Alzheimer's disease (AD), tau protein abnormally accumulates and induces dysfunction of the affected neurons. This study reports that anaplastic lymphoma kinase (ALK), a receptor tyrosine kinase, is crucial for the tau-mediated AD pathology. ALK caused abnormal accumulation of highly phosphorylated tau in the somatodendritic region of neurons through its tyrosine kinase activity. ALK-induced LC3-positive axon swelling and loss of spine density, leading to tau-dependent neuronal degeneration. Notably, ALK activation in neurons impaired Stx17-dependent autophagosome maturation and this defect was reversed by a dominant-negative Grb2. In a Drosophila melanogaster model, transgenic flies neuronally expressing active Drosophila Alk exhibited the aggravated tau rough eye phenotype with retinal degeneration and shortened lifespan. In contrast, expression of kinase-dead Alk blocked these phenotypes. ALK levels were significantly elevated in the brains of AD patients showing autophagosomal defects. Injection of an ALK.Fc-lentivirus exacerbated memory impairment in 3xTg-AD mice. Conversely, pharmacologic inhibition of ALK activity with inhibitors reversed the memory impairment and tau accumulation in both 3xTg-AD and tauC3 (caspase-cleaved tau) transgenic mice. Together, it is proposed that aberrantly activated ALK is a bona fide mediator of tau proteinopathy that disrupts autophagosome maturation and causes tau accumulation and aggregation, leading to neuronal dysfunction in AD (Park, 2021).

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Prifti, E., Tsakiri, E. N., Vourkou, E., Stamatakis, G., Samiotaki, M. and Papanikolopoulou, K. (2020). The two Cysteines of Tau protein are functionally distinct and contribute differentially to its pathogenicity in vivo. J Neurosci. PubMed ID: 33334867

Abstract
Although Tau accumulation is clearly linked to pathogenesis in Alzheimer's disease (AD) and other Tauopathies, the mechanism that initiates the aggregation of this highly soluble protein in vivo remains largely unanswered. Interestingly, in vitro Tau can be induced to form fibrillar filaments by oxidation of its two cysteine residues, generating an intermolecular disulfide bond that promotes dimerization and fibrillization. The recently solved structures of Tau filaments revealed that the two cysteine residues are not structurally equivalent since Cys-322 is incorporated into the core of the fibril whereas Cys-291 projects away from the core to form the fuzzy coat. This study examined whether mutation of these cysteines to alanine affects differentially Tau mediated toxicity and dysfunction in the well-established Drosophila Tauopathy model. Experiments were conducted with both sexes, or with either sex. Each cysteine residue contributes differentially to Tau stability, phosphorylation status, aggregation propensity, resistance to stress, learning and memory. Importantly, this work uncovers a critical role of Cys-322 in determining Tau toxicity and dysfunction (Prifti, 2020).

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Dubey, T., Gorantla, N. V., Chandrashekara, K. T. and Chinnathambi, S. (2020). Photodynamic exposure of Rose-Bengal inhibits Tau aggregation and modulates cytoskeletal network in neuronal cells. Sci Rep 10(1): 12380. PubMed ID: 32704015

Abstract

The intracellular Tau aggregates are known to be associated with Alzheimer's disease. The inhibition of Tau aggregation is an important strategy for screening of therapeutic molecules in Alzheimer's disease. Several classes of dyes possess a unique property of photo-excitation, which is applied as a therapeutic measure against numerous neurological dysfunctions. Rose Bengal is a Xanthene dye, which has been widely used as a photosensitizer in photodynamic therapy. The aggregation inhibition and disaggregation potency of Rose Bengal and photo-excited Rose Bengal were observed by in-vitro fluorescence, circular dichroism, and electron microscopy. Rose Bengal and photo-excited Rose Bengal induce minimal cytotoxicity in neuronal cells. In these studies, it was observed that Rose Bengal and photo-excited Rose Bengal modulate the cytoskeleton network of actin and tubulin. The immunofluorescence studies showed the increased filopodia structures after photo-excited Rose Bengal treatment. Furthermore, Rose Bengal treatment increases the connections between the cells. Rose Bengal and photo-excited Rose Bengal treatment-induced actin-rich podosome-like structures associated with cell membranes. The in-vivo studies on UAS E-14 Tau mutant Drosophila suggested that exposure to Rose Bengal and photo-excited Rose Bengal efficiency rescues the behavioural and memory deficit in flies. Thus, the overall results suggest that Rose Bengal could have a therapeutic potency against Tau aggregation (Dubey, 2020).

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Yu, Y., Niccoli, T., Ren, Z., Woodling, N. S., Aleyakpo, B., Szabadkai, G. and Partridge, L. (2020). PICALM rescues glutamatergic neurotransmission, behavioural function, and survival in a Drosophila model of Aβ42 toxicity. Hum Mol Genet. PubMed ID: 32592479

Abstract

Alzheimer's disease (AD) is the most common form of dementia and the most prevalent neurodegenerative disease. Genome Wide Association Studies have linked PICALM to AD risk. PICALM has been implicated in Aβ42 production and turn-over, but whether it plays a direct role in modulating Aβ42 toxicity remains unclear. This study found that increased expression of the Drosophila PICALM orthologue like-AP180 (lap) could rescue Aβ42 toxicity in an adult-onset model of AD, without affecting Aβ42 level. Imbalances in the glutamatergic system, leading to excessive, toxic stimulation have been associated with AD. This study found that Aβ42 caused accumulation of pre-synaptic vesicular glutamate transporter (VGlut) and increased spontaneous glutamate release. Increased lap expression reversed these phenotypes back to control levels, suggesting that lap may modulate glutamatergic transmission. This study also found that lap modulated the localisation of Amph, the homologue of another AD risk factor BIN1, and that Amph itself modulated post-synaptic glutamate receptor (GluRII) localisation. A model is proposed where PICALM modulates glutamatergic transmission, together with BIN1, to ameliorate synaptic dysfunction and disease progression (Yu, 2020).

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Lim, C. H., Kaur, P., Teo, E., Lam, V. Y. M., Zhu, F., Kibat, C., Gruber, J., Mathuru, A. S. and Tolwinski, N. S. (2020). Application of optogenetic Amyloid-beta distinguishes between metabolic and physical damages in neurodegeneration. Elife 9. PubMed ID: 32228858

Abstract

The brains of Alzheimer's disease patients show a decrease in brain mass and a preponderance of extracellular Amyloid-beta plaques. These plaques are formed by aggregation of polypeptides that are derived from the Amyloid Precursor Protein (APP). Amyloid-beta plaques are thought to play either a direct or an indirect role in disease progression, however the exact role of aggregation and plaque formation in the aetiology of Alzheimer's disease (AD) is subject to debate as the biological effects of soluble and aggregated Amyloid-beta peptides are difficult to separate in vivo. To investigate the consequences of formation of Amyloid-beta oligomers in living tissues, a fluorescently tagged, optogenetic Amyloid-beta peptide was developed that oligomerizes rapidly in the presence of blue light. This system was applied to the crucial question of how intracellular Amyloid-beta oligomers underlie the pathologies of A. Drosophila, C. elegans and D. rerio were used to show that, although both expression and induced oligomerization of Amyloid-beta were detrimental to lifespan and healthspan, it was possible to separate the metabolic and physical damage caused by light-induced Amyloid-beta oligomerization from Amyloid-beta expression alone. The physical damage caused by Amyloid-beta oligomers also recapitulated the catastrophic tissue loss that is a hallmark of late AD. The lifespan deficit induced by Amyloid-beta oligomers was reduced with Li(+) treatment. These results present the first model to separate different aspects of disease progression (Lim, 2020).

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Morotz, G. M., Glennon, E. B., Greig, J., Lau, D. H. W., Bhembre, N., Mattedi, F., Muschalik, N., Noble, W., Vagnoni, A. and Miller, C. C. J. (2019). Kinesin light chain-1 serine-460 phosphorylation is altered in Alzheimer's disease and regulates axonal transport and processing of the amyloid precursor protein. Acta Neuropathol Commun 7(1): 200. PubMed ID: 31806024

Abstract

Damage to axonal transport is an early pathogenic event in Alzheimer's disease. The amyloid precursor protein (APP) is a key axonal transport cargo since disruption to APP transport promotes amyloidogenic processing of APP. Moreover, altered APP processing itself disrupts axonal transport. The mechanisms that regulate axonal transport of APP are therefore directly relevant to Alzheimer's disease pathogenesis. APP is transported anterogradely through axons on kinesin-1 motors and one route for this transport involves calsyntenin-1, a type-1 membrane spanning protein that acts as a direct ligand for kinesin-1 light chains (KLCs). Thus, loss of calsyntenin-1 disrupts APP axonal transport and promotes amyloidogenic processing of APP. Phosphorylation of KLC1 on serine-460 has been shown to reduce anterograde axonal transport of calsyntenin-1 by inhibiting the KLC1-calsyntenin-1 interaction. This study demonstrates that in Alzheimer's disease frontal cortex, KLC1 levels are reduced and the relative levels of KLC1 serine-460 phosphorylation are increased; these changes occur relatively early in the disease process. It was also shown that a KLC1 serine-460 phosphomimetic mutant inhibits axonal transport of APP in both mammalian neurons in culture and in Drosophila neurons in vivo. Finally, it was demonstrated that expression of the KLC1 serine-460 phosphomimetic mutant promotes amyloidogenic processing of APP. Together, these results suggest that increased KLC1 serine-460 phosphorylation contributes to Alzheimer's disease (Morotz, 2019).

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Bergkvist, L., Du, Z., Elovsson, G., Appelqvist, H., Itzhaki, L. S., Kumita, J. R., Kagedal, K. and Brorsson, A. C. (2019). Mapping pathogenic processes contributing to neurodegeneration in Drosophila models of Alzheimer's disease. FEBS Open Bio. PubMed ID: 31823504

Abstract

Drosophila models of Alzheimer's disease (AD) include the Abeta fly model and the AbetaPP-BACE1 fly model. In the Abeta fly model, the Abeta peptide is fused to a secretion sequence and directly over-expressed. In the AbetaPP-BACE1 model, human AbetaPP and human BACE1 are expressed in the fly, resulting in in vivo production of Abeta peptides and other AbetaPP cleavage products. Although these two models have been used for almost two decades, the underlying mechanisms resulting in neurodegeneration are not yet clearly understood. This study has characterised toxic mechanisms in these two AD fly models. Neuronal cell death and increased protein carbonylation (indicative of oxidative stress) were detected in both AD fly models. In the Abeta fly model, this correlates with high Abeta1-42 levels and down-regulation of the levels of mRNA encoding lysosomal associated membrane protein 1, lamp1 (a lysosomal marker), while in the AbetaPP-BACE1 fly model, neuronal cell death correlates with low Abeta1-42 levels, up-regulation of lamp1 mRNA levels and increased levels of C-terminal fragments (CTFs). In addition, a significant amount of AbetaPP/Abeta antibody (4G8) positive species, located close to the endosomal marker rab5, were detected in the AbetaPP-BACE1 model. Taken together, this study highlights the similarities and differences in the toxic mechanisms which result in neuronal death in two different AD fly models. Such information is important to consider when utilising these models to study AD pathogenesis or screening for potential treatments.

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Miguel, L., Frebourg, T., Campion, D. and Lecourtois, M. (2020). Moderate Overexpression of Tau in Drosophila Exacerbates Amyloid-beta-Induced Neuronal Phenotypes and Correlates with Tau Oligomerization. J Alzheimers Dis. PubMed ID: 32065789

Abstract

Alzheimer's disease (AD) is neuropathologically defined by two key hallmarks: extracellular senile plaques composed primarily of amyloid-beta (Abeta) peptide and intraneuronal neurofibrillary tangles, containing abnormally hyperphosphorylated tau protein. The tau protein is encoded by the MAPT gene. Recently, the H1 and H2 haplotypes of the MAPT gene were associated with AD risk. The minor MAPT H2 haplotype has been linked with a decreased risk of developing late-onset AD (LOAD). MAPT haplotypes show different levels of MAPT/Tau expression with H1 being approximately 1.5-fold more expressed than H2, suggesting that MAPT expression level could be related to LOAD risk. This study investigated whether this moderate difference in MAPT/Tau expression could influence Abeta-induced toxicity in vivo. It was shown that modest overexpression of tau protein in Drosophila exacerbates neuronal phenotypes in AbetaPP/BACE1 (amyloid-beta protein precursor/Beta-Secretase 1) flies. The exacerbation of neuronal defects correlates with the accumulation of insoluble dTau oligomers, suggesting that the moderate difference in level of tau expression observed between H1 and H2 haplotypes could influence Abeta toxicity through the production of oligomeric Tau insoluble species (Miguel, 2020).

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Feuillette, S., Charbonnier, C., Frebourg, T., Campion, D. and Lecourtois, M. (2020). A Connected Network of Interacting Proteins Is Involved in Human-Tau Toxicity in Drosophila. Front Neurosci 14: 68. PubMed ID: 32116515

Abstract

Tauopathies are neurodegenerative diseases characterized by the presence of aggregates of abnormally phosphorylated Tau. Deciphering the pathophysiological mechanisms that lead from the alteration of Tau biology to neuronal death depends on the identification of Tau cellular partners. Combining genetic and transcriptomic analyses in Drosophila, this study identified 77 new modulators of human Tau-induced toxicity, bringing to 301 the number of Tau genetic interactors identified so far in flies. Network analysis showed that 229 of these genetic modulators constitute a connected network. The addition of 77 new genes strengthened the network structure, increased the intergenic connectivity and brought up key hubs with high connectivities, namely Src64B/FYN, Src42A/FRK, kuz/ADAM10, heph/PTBP1, scrib/SCRIB, and Cam/CALM3. Interestingly, this study established a genetic link between Tau-induced toxicity and ADAM10, a recognized Alzheimer Disease protective factor. In addition, these data support the importance of the presynaptic compartment in mediating Tau toxicity (Feuillette, 2020).

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Krishnaswamy, S., Huang, H. W., Marchal, I. S., Ryoo, H. D. and Sigurdsson, E. M. (2020). Neuronally expressed anti-tau scFv prevents tauopathy-induced phenotypes in Drosophila models. Neurobiol Dis 137: 104770. PubMed ID: 31982516

Abstract

Single-chain variable fragments (scFv) have been derived from tau antibody hybridomas, and their promise as imaging diagnostic agents has been shown. This study examined the therapeutic potential of anti-tau scFv in transgenic Drosophila models that express in neurons wild-type (WT) human tau (htau) or the human tauopathy mutation R406W. scFv expressing flies were crossed with the tauopathy flies and analyzed. Overall, the survival curves differed significantly. Control flies not expressing htau survived the longest, whereas R406W expressing flies had the shortest lifespan, which was greatly prolonged by co-expressing the anti-tau scFv. Likewise, htau WT expressing flies had a moderately short lifespan, which was prolonged by co-expressing the anti-tau scFv. In addition, the htau expression impaired wing expansion after eclosion, and caused progressive abdomen expansion These features were more severe in htau R406W flies than in htau WT flies. Importantly, both phenotypes were prevented by co-expression of the anti-tau scFv. Lastly, brain analyses revealed scFv-mediated tau clearance, and its prevention of tau-mediated neurotoxicity. In summary, these findings support the therapeutic potential of an anti-tau scFv, including as gene therapies, and the use of Drosophila models for such screening (Krishnaswamy, 2020).

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Arnes, M., Romero, N., Casas-Tinto, S., Acebes, A. and Ferrus, A. (2019). PI3K activation prevents Abeta42-induced synapse loss and favors insoluble amyloid deposits formation. Mol Biol Cell: mbcE19050303. PubMed ID: 31877058

Abstract

Excess of Abeta42 peptide is considered a hallmark of the disease. This study expressed the human Abeta42 peptide to assay the neuroprotective effects of PI3K in adult Drosophila melanogaster. The neuronal expression of the human peptide elicits progressive toxicity in the adult fly. The pathological traits include reduced axonal transport, synapse loss, defective climbing ability and olfactory perception, as well as lifespan reduction. The Abeta42-dependent synapse decay does not involve transcriptional changes in the core synaptic protein encoding genes: bruchpilot, liprin and synaptobrevin. All toxicity features, however, are suppressed by the co-expression of PI3K. Moreover, PI3K activation induces a significant increase of 6E10 and Thioflavin-positive amyloid deposits. Mechanistically, it is suggested that Abeta42-Ser26 could be a candidate residue for direct or indirect phosphorylation by PI3K. Along with these in vivo experiments this study further analyzed Abeta42 toxicity and its suppression by PI3K activation in in vitro assays with SH-SY5Y human neuroblastoma cell cultures, where Abeta42 aggregation into large insoluble deposits is reproduced. Finally, it was shown that the Abeta42 toxicity syndrome includes the transcriptional shut down of PI3K expression. Taken together, these results uncover a potential novel pharmacological strategy against this disease through the restoration of PI3K activity.

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Dubey, T., Gorantla, N. V., Chandrashekara, K. T. and Chinnathambi, S. (2019). Photoexcited Toluidine Blue Inhibits Tau Aggregation in Alzheimer's Disease. ACS Omega 4(20): 18793-18802. PubMed ID: 31737841

Abstract

The aggregates of microtubule-associated protein Tau are considered as a major hallmark of Alzheimer's disease. Tau aggregates accumulate intracellularly leading to neuronal toxicity. Numerous approaches have been targeted against Tau protein aggregation, which include application of synthetic and natural compounds. Toluidine blue is a basic dye of phenothiazine family, which on irradiation with a 630 nm light gets converted into a photoexcited form, leading to generation of singlet oxygen species. Methylene blue is the parent compound of toluidine blue, which has been reported to be potent against tauopathy. The present work studied the potency of toluidine blue and photoexcited toluidine blue against Tau aggregation. Biochemical and biophysical analyses using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, ThS fluorescence, circular dichroism spectroscopy, and electron microscopy suggested that toluidine blue inhibited the aggregation of Tau in vitro. The photoexcited toluidine blue potentially dissolved the matured Tau fibrils, which indicated the disaggregation property of toluidine blue. The cell biology studies including the cytotoxicity assay and reactive oxygen species (ROS) production assay suggested toluidine blue to be a biocompatible dye as it reduced ROS levels and cell death. The photoexcited toluidine blue modulates the cytoskeleton network in cells, which was supported by immunofluorescence studies of neuronal cells. The studies in a UAS Tau E14 transgenic Drosophila model suggested that photoexcited toluidine blue was potent to restore the survival and memory deficits of Drosophila. The overall finding of these studies suggested toluidine blue to be a potent molecule in rescuing the Tau-mediated pathology by inhibiting its aggregation, reducing the cell death, and modulating the tubulin levels and behavioral characteristics of Drosophila. Thus, toluidine blue can be addressed as a potent molecule against Alzheimer's disease (Dubey, 2019)

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Garrido-Maraver, J., Loh, S. H. Y. and Martins, L. M. . (2019). Forcing contacts between mitochondria and the endoplasmic reticulum extends lifespan in a Drosophila model of Alzheimer's disease. Biol Open. PubMed ID: 31822473

Abstract

Eukaryotic cells are complex systems containing internal compartments with specialised functions. Among these compartments, the endoplasmic reticulum (ER) plays a major role in processing proteins for modification and delivery to other organelles, whereas mitochondria generate energy in the form of ATP. Mitochondria and the ER form physical interactions, defined as mitochondria-ER contact sites (MERCs) to exchange metabolites such as calcium ions (Ca(2+)) and lipids. Sites of contact between mitochondria and the ER can regulate biological processes such as ATP generation and mitochondrial division. The interactions between mitochondria and the ER are dynamic and respond to the metabolic state of cells. Changes in MERCs have been linked to metabolic pathologies such as diabetes, neurodegenerative diseases and sleep disruption. This study explored the consequences of increasing contacts between mitochondria and the ER in flies using a synthetic linker. Enhancing MERCs was shown to increase locomotion and extend lifespan. In a Drosophila model of Alzheimer's disease linked to toxic amyloid beta (Abeta), linker expression can suppress motor impairment and extend lifespan. It is concluded that strategies for increasing contacts between mitochondria and the ER may improve symptoms of diseases associated with mitochondria dysfunction.

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Pragati, Chanu, S. I. and Sarkar, S. (2019). Reduced expression of dMyc mitigates Tau(V337M) mediated neurotoxicity by preventing the Tau hyperphosphorylation and inducing autophagy in Drosophila. Neurosci Lett: 134622. PubMed ID: 31715291

Abstract

Tauopathies such as Alzheimer's disease (AD), Pick's disease (PiD), Frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) etc. represent a group of age-related neurodegenerative disorders in which tau protein loses its normal conformation mostly due to hyperphosphorylation and subsequent formation of the aggregates of defined shapes, known as Neurofibrillary Tangles (NFTs). Earlier work has demonstrated that reduced dosage of dmyc (Drosophila homolog of human cmyc proto-oncogene) restricts tau(WT) mediated disease pathogenesis by regulating the phosphorylation status of tau. This study demonstrates further that the downregulation of dmyc also alleviates the mutant human-tau [tau(V337M)] mediated neurotoxicity in Drosophila by improving disease defects. Moreover, tissue-specific downregulation of dmyc also induces cellular autophagy which facilitates the disposal of misfolded proteins via lysosome-mediated proteostasis. These findings demonstrate the capability of dmyc in the suppression of different forms of human tauopathies in Drosophila disease models. Interestingly, due to the conserved characteristics of dmyc/cmyc across the animal kingdom, this study strengthens the possibility of utilizing this gene as an effective drug target against tauopathies (Pragati, 2019).

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Helmfors, L., Boman, A., Civitelli, L., Nath, S., Sandin, L., Janefjord, C., McCann, H., Zetterberg, H., Blennow, K., Halliday, G., Brorsson, A.C. and Kågedal, K. (2015). Protective properties of lysozyme on β-amyloid pathology: implications for Alzheimer disease. Neurobiol Dis PubMed ID: 26334479

Abstract
The hallmarks of Alzheimer disease are amyloid-β plaques and neurofibrillary tangles accompanied by signs of neuroinflammation. Lysozyme is a major player in the innate immune system and has recently been shown to prevent the aggregation of amyloid-β1-40in vitro. This study found that patients with Alzheimer disease have increased lysozyme levels in the cerebrospinal fluid and lysozyme co-localizes with amyloid-β in plaques. In Drosophila, neuronal co-expression of lysozyme and amyloid-β1-42 reduces the formation of soluble and insoluble amyloid-β species, prolongs survival and improves the activity of amyloid-β1-42 transgenic flies. This suggests that lysozyme levels rise in Alzheimer disease as a compensatory response to amyloid-β increases and aggregation. In support of this, in vitro aggregation assays reveal that lysozyme associates with amyloid-β1-42 and alters its aggregation pathway to counteract the formation of toxic amyloid-β species. Overall, these studies establish a protective role for lysozyme against amyloid-β associated toxicities and identify increased lysozyme in patients with Alzheimer disease. Therefore, lysozyme has potential as a new biomarker as well as a therapeutic target for Alzheimer disease (Helmfors, 2015).

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Hwang, S., Jeong, H., Hong, E. H., Joo, H. M., Cho, K. S. and Nam, S. Y. (2019). Low-dose ionizing radiation alleviates Abeta42-induced cell death via regulating AKT and p38 pathways in Drosophila Alzheimer's disease models. Biol Open 8(2). PubMed ID: 30670376

Abstract

Ionizing radiation is widely used in medicine and is valuable in both the diagnosis and treatment of many diseases. However, its health effects are ambiguous. This paper reports that low-dose ionizing radiation has beneficial effects in human amyloid-beta42 (Abeta42)-expressing Drosophila Alzheimer's disease (AD) models. Ionizing radiation at a dose of 0.05 Gy suppressed AD-like phenotypes, including developmental defects and locomotive dysfunction, but did not alter the decreased survival rates and longevity of Abeta42-expressing flies. The same dose of gamma-irradiation reduced Abeta42-induced cell death in Drosophila AD models through downregulation of head involution defective (hid), which encodes a protein that activates caspases. However, 4 Gy of gamma-irradiation increased Abeta42-induced cell death without modulating pro-apoptotic genes grim, reaper and hid. The AKT signaling pathway, which was suppressed in Drosophila AD models, was activated by either 0.05 or 4 Gy gamma-irradiation. Interestingly, p38 mitogen-activated protein-kinase (MAPK) activity was inhibited by exposure to 0.05 Gy gamma-irradiation but enhanced by exposure to 4 Gy in Abeta42-expressing flies. In addition, overexpression of phosphatase and tensin homolog (PTEN), a negative regulator of the AKT signaling pathway, or a null mutant of AKT strongly suppressed the beneficial effects of low-dose ionizing radiation in Abeta42-expressing flies. These results indicate that low-dose ionizing radiation suppresses Abeta42-induced cell death through regulation of the AKT and p38 MAPK signaling pathways, suggesting that low-dose ionizing radiation has hormetic effects on the pathogenesis of Abeta42-associated AD (Hwang, 2019).

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Viswanathan, G. K., Shwartz, D., Losev, Y., Arad, E., Shemesh, C., Pichinuk, E., Engel, H., Raveh, A., Jelinek, R., Cooper, I., Gosselet, F., Gazit, E. and Segal, D. (2019). Purpurin modulates Tau-derived VQIVYK fibrillization and ameliorates Alzheimer's disease-like symptoms in animal model. Cell Mol Life Sci. PubMed ID: 31562564

Abstract

Neurofibrillary tangles of the Tau protein and plaques of the amyloid beta peptide are hallmarks of Alzheimer's disease (AD), which is characterized by the conversion of monomeric proteins/peptides into misfolded beta-sheet rich fibrils. Halting the fibrillation process and disrupting the existing aggregates are key challenges for AD drug development. In a previous study in vitro high-throughput screening was performed for the identification of potent inhibitors of Tau aggregation using a proxy model, a highly aggregation-prone hexapeptide fragment (306)VQIVYK(311) (termed PHF6) derived from Tau. This study has characterized a hit molecule from that screen as a modulator of Tau aggregation using in vitro, in silico, and in vivo techniques. This molecule, an anthraquinone derivative named Purpurin, inhibited ~ 50% of PHF6 fibrillization in vitro at equimolar concentration and disassembled pre-formed PHF6 fibrils. In silico studies showed that Purpurin interacted with key residues of PHF6, which are responsible for maintaining its beta-sheets conformation. Isothermal titration calorimetry and surface plasmon resonance experiments with PHF6 and full-length Tau (FL-Tau), respectively, indicated that Purpurin interacted with PHF6 predominantly via hydrophobic contacts and displayed a dose-dependent complexation with FL-Tau. Purpurin was non-toxic when fed to Drosophila and it significantly ameliorated the AD-related neurotoxic symptoms of transgenic flies expressing WT-FL human Tau (hTau) plausibly by inhibiting Tau accumulation and reducing Tau phosphorylation. Purpurin also reduced hTau accumulation in cell culture overexpressing hTau. Importantly, Purpurin efficiently crossed an in vitro human blood-brain barrier model. These findings suggest that Purpurin could be a potential lead molecule for AD therapeutics ( , 2019).

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Higham, J. P., Hidalgo, S., Buhl, E. and Hodge, J. J. L. (2019). Restoration of olfactory memory in Drosophila overexpressing human Alzheimer's disease associated Tau by manipulation of L-type Ca(2+) channels. Front Cell Neurosci 13: 409. PubMed ID: 31551716

Abstract

The cellular underpinnings of memory deficits in Alzheimer's disease (AD) are poorly understood. This study utilized the tractable neural circuits sub-serving memory in Drosophila to investigate the role of impaired Ca(2+) handling in memory deficits caused by expression of human 0N4R isoform of tau which is associated with AD. Expression of tau in mushroom body neuropils, or a subset of mushroom body output neurons, led to impaired memory. By using the Ca(2+) reporter GCaMP6f, changes were pbserved in Ca(2+) signaling when tau was expressed in these neurons, an effect that could be blocked by the L-type Ca(2+) channel antagonist nimodipine or reversed by RNAi knock-down of the L-type channel gene. The L-type Ca(2+) channel itself is required for memory formation, however, RNAi knock-down of the L-type Ca(2+) channel in neurons overexpressing human tau resulted in flies whose memory is restored to levels equivalent to wild-type. Expression data suggest that Drosophila L-type Ca(2+) channel mRNA levels are increased upon tau expression in neurons, thus contributing to the effects observed on memory and intracellular Ca(2+) homeostasis. Together, these Ca(2+) imaging and memory experiments suggest that expression of the 0N4R isoform of human tau increases the number of L-type Ca(2+) channels in the membrane resulting in changes in neuronal excitability that can be ameliorated by RNAi knockdown or pharmacological blockade of L-type Ca(2+) channels. This highlights a role for L-type Ca(2+) channels in tauopathy and their potential as a therapeutic target for AD (Higham, 2019).

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Hsieh, Y. C., Guo, C., Yalamanchili, H. K., Abreha, M., Al-Ouran, R., Li, Y., Dammer, E. B., Lah, J. J., Levey, A. I., Bennett, D. A., De Jager, P. L., Seyfried, N. T., Liu, Z. and Shulman, J. M. (2019). Tau-mediated disruption of the spliceosome triggers cryptic RNA splicing and neurodegeneration in Alzheimer's disease. Cell Rep 29(2): 301-316.e310. PubMed ID: 31597093

Abstract

In Alzheimer's disease (AD), spliceosomal proteins with critical roles in RNA processing aberrantly aggregate and mislocalize to Tau neurofibrillary tangles. This study tested the hypothesis that Tau-spliceosome interactions disrupt pre-mRNA splicing in AD. In human postmortem brain with AD pathology, Tau coimmunoprecipitates with spliceosomal components. In Drosophila, pan-neuronal Tau expression triggers reductions in multiple core and U1-specific spliceosomal proteins, and genetic disruption of these factors, including SmB, U1-70K, and U1A, enhances Tau-mediated neurodegeneration. It was further shown that loss of function in SmB, encoding a core spliceosomal protein, causes decreased survival, progressive locomotor impairment, and neuronal loss, independent of Tau toxicity. Lastly, RNA sequencing reveals a similar profile of mRNA splicing errors in SmB mutant and Tau transgenic flies, including intron retention and non-annotated cryptic splice junctions. In human brains, cryptic splicing errors were confirmed in association with neurofibrillary tangle burden. These results implicate spliceosome disruption and the resulting transcriptome perturbation in Tau-mediated neurodegeneration in AD (Hsieh, 2019).

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Xie, L., Gu, X., Okamoto, K., Westermark, G. T. and Leifer, K. (2019). 3D analysis of human islet amyloid polypeptide crystalline structures in Drosophila melanogaster. PLoS One 14(10): e0223456. PubMed ID: 31600260

Abstract

Expression of the Alzheimer's disease associated polypeptide Abeta42 and the human polypeptide hormone islet amyloid polypeptide (hIAPP) and the prohormone precursor (hproIAPP) in neurons of Drosophila melanogaster leads to the formation of protein aggregates in the fat body tissue surrounding the brain. This study determined the structure of these membrane-encircled protein aggregates using transmission electron microscopy (TEM) and observed the dissolution of protein aggregates after starvation. Electron tomography (ET) as an extension of transmission electron microscopy revealed that these aggregates were comprised of granular subunits having a diameter of 20 nm aligned into highly ordered structures in all three dimensions. This 3D structural analysis provides novel insight into the aggregation process of hIAPP in the fat body tissue of Drosophila melanogaster (Xie, 2019).

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Tower, J., Agrawal, S., Alagappan, M. P., Bell, H. S., Demeter, M., Havanoor, N., Hegde, V. S., Jia, Y., Kothawade, S., Lin, X., Nadig, C., Rajashekharappa, N. S., Rao, D., Rao, S. S., Sancheti, P., Saria, A., Shantharamu, N. H., Sharma, V., Tadepalli, K. and Varma, A. (2019). Behavioral and molecular markers of death in Drosophila melanogaster. Exp Gerontol 126: 110707. PubMed ID: 31445108

Abstract

Fly movement was tracked through 3-dimensional (3D) space as the fly died, using either reflected visible light, reflected infrared (IR) light, or fly GFP fluorescence. Behaviors measured included centrophobism, negative geotaxis, velocity, and total activity. In addition, frequency of directional heading changes (FDHC) was calculated as a measure of erratic movement. Nine middle-aged flies were tracked as they died during normal aging, and fifteen young flies were tracked as they died from dehydration/starvation stress. Episodes of increased FDHC were observed 0-8h prior to death for the majority of the flies. FDHC was also increased with age in flies with neuronal expression of a human Abeta42 protein fragment associated with Alzheimer's disease. Finally, green autofluorescence appeared in the eye and body immediately prior to and coincident with death, and fluorescence of GFP targeted to the retina increased immediately prior to and coincident with death. The results suggest the potential utility of FDHC, green autofluorescence, and retinal GFP as markers of neuronal malfunction and imminent death (Tower, 2019).

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Park, S. Y., Seo, J. and Chun, Y. S. (2019). Targeted downregulation of kdm4a ameliorates Tau-engendered defects in Drosophila melanogaster. J Korean Med Sci 34(33): e225. PubMed ID: 31436053

Abstract

Tauopathies, a class of neurodegenerative diseases that includes Alzheimer's disease (AD), are characterized by the deposition of neurofibrillary tangles composed of hyperphosphorylated tau protein in the human brain. As abnormal alterations in histone acetylation and methylation show a cause and effect relationship with AD, this study investigated the role of several Jumonji domain-containing histone demethylase (JHDM) genes, which have yet to be studied in AD pathology. To examine alterations of several JHDM genes in AD pathology, bioinformatics analyses were performed of JHDM gene expression profiles in brain tissue samples from deceased AD patients. Furthermore, to investigate the possible relationship between alterations in JHDM gene expression profiles and AD pathology in vivo, whether tissue-specific downregulation of JHDM Drosophila homologs (kdm) can affect tau(R406W)-induced neurotoxicity using transgenic flies containing the UAS-Gal4 binary system. The expression levels of JHDM1A, JHDM2A/2B, and JHDM3A/3B were significantly higher in postmortem brain tissue from patients with AD than from non-demented controls, whereas JHDM1B mRNA levels were downregulated in the brains of patients with AD. Using transgenic flies, it was revealed that knockdown of kdm2 (homolog to human JHDM1), kdm3 (homolog to human JHDM2), kdm4a (homolog to human JHDM3A), or kdm4b (homolog to human JHDM3B) genes in the eye ameliorated the tau(R406W)-engendered defects, resulting in less severe phenotypes. However, kdm4a knockdown in the central nervous system uniquely ameliorated tau(R406W)-induced locomotion defects by restoring heterochromatin. These results suggest that downregulation of kdm4a expression may be a potential therapeutic target in AD (Park, 2019).

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Westfall, S., Lomis, N. and Prakash, S. (2019). A novel synbiotic delays Alzheimer's disease onset via combinatorial gut-brain-axis signaling in Drosophila melanogaster. PLoS One 14(4): e0214985. PubMed ID: 31009489

Abstract

The gut-brain-axis (GBA) describing the bidirectional communication between the gut microbiota and brain was recently implicated in Alzheimer's disease (AD). The current study describes a novel synbiotic containing three metabolically active probiotics and a novel polyphenol-rich prebiotic which has beneficial impacts on the onset and progression of AD. In a transgenic humanized Drosophila melanogaster model of AD, the synbiotic increased survivability and motility and rescued amyloid beta deposition and acetylcholinesterase activity. Such drastic effects were due to the synbiotic's combinatorial action on GBA signaling pathways including metabolic stability, immune signaling, oxidative and mitochondrial stress possibly through pathways implicating PPARgamma. Overall, this study shows that the therapeutic potential of GBA signaling is best harnessed in a synbiotic that simultaneously targets multiple risk factors of AD (Westfall, 2019).

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Chatterjee, S., Ambegaokar, S. S., Jackson, G. R. and Mudher, A. (2019). Insulin-mediated changes in Tau hyperphosphorylation and autophagy in a Drosophila model of tauopathy and neuroblastoma cells. Front Neurosci 13: 801. PubMed ID: 31427921

Abstract

Almost 50 million people in the world are affected by dementia; the most prevalent form of which is Alzheimer's disease (AD). Although aging is considered to be the main risk factor for AD, growing evidence from epidemiological studies suggests that type 2 diabetes mellitus (T2DM) increases the risk of dementia including AD. Defective brain insulin signaling has been suggested as an early event in AD and other tauopathies but the mechanisms that link these diseases are largely unknown. Tau hyperphosphorylation is a hallmark of neurofibrillary pathology and insulin resistance increases the number of neuritic plaques particularly in AD. Utilizing a combination of Drosophila models of tauopathy (expressing the 2N4R-Tau) and neuroblastoma cells, this study has attempted to decipher the pathways downstream of the insulin signaling cascade that lead to tau hyperphosphorylation, aggregation and autophagic defects. Using cell-based, genetic, and biochemical approaches this study demonstrated that tau phosphorylation at AT8 and PHF1 residues is enhanced in an insulin-resistant environment. It was also shown that insulin-induced changes in total and phospho-tau are mediated by the crosstalk of AKT, glycogen synthase kinase-3beta, and extracellular regulating kinase located downstream of the insulin receptor pathway. Finally, this study demonstrated a significant change in the levels of the key proteins in the mammalian target of rapamycin/autophagy pathway, implying an increased impairment of aggregated protein clearance in the transgenic Drosophila models and cultured neuroblastoma cells (Chatterjee, 2019).

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Higham, J. P., Malik, B. R., Buhl, E., Dawson, J. M., Ogier, A. S., Lunnon, K. and Hodge, J. J. L. (2019). Alzheimer's disease associated genes Ankyrin and Tau cause shortened lifespan and memory loss in Drosophila. Front Cell Neurosci 13: 260. PubMed ID: 31244615

Abstract

Alzheimer's disease (AD) is the most common form of dementia and is characterized by intracellular neurofibrillary tangles of hyperphosphorylated Tau, including the 0N4R isoform and accumulation of extracellular amyloid beta (Abeta) plaques. However, less than 5% of AD cases are familial, with many additional risk factors contributing to AD including aging, lifestyle, the environment and epigenetics. Recent epigenome-wide association studies (EWAS) of AD have identified a number of loci that are differentially methylated in the AD cortex. Indeed, hypermethylation and reduced expression of the Ankyrin 1 (ANK1) gene in AD has been reported in the cortex in numerous different post-mortem brain cohorts. Little is known about the normal function of ANK1 in the healthy brain, nor the role it may play in AD. This study has generated Drosophila models to allow us to functionally characterize Drosophila Ank2, the ortholog of human ANK1 and to determine its interaction with human Tau and Abeta. Expression of human Tau 0N4R or the oligomerizing Abeta 42 amino acid peptide caused shortened lifespan, degeneration, disrupted movement, memory loss, and decreased excitability of memory neurons with co-expression tending to make the pathology worse. Drosophila with reduced neuronal Ank2 expression have shortened lifespan, reduced locomotion, reduced memory and reduced neuronal excitability similar to flies overexpressing either human Tau 0N4R or Abeta42. Therefore, this study shows that the mis-expression of Ank2 can drive disease relevant processes and phenocopy some features of AD. Therefore, it is proposed targeting human ANK1 may have therapeutic potential. This represents the first study to characterize an AD-relevant gene nominated from EWAS (Higham, 2019).

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Wu, W., Du, S., Shi, W., Liu, Y., Hu, Y., Xie, Z., Yao, X., Liu, Z., Ma, W., Xu, L., Ma, C. and Zhong, Y. (2019). Inhibition of Rac1-dependent forgetting alleviates memory deficits in animal models of Alzheimer's disease. Protein Cell. PubMed ID: 31321704

Abstract

Accelerated forgetting has been identified as a feature of Alzheimer's disease (AD), but the therapeutic efficacy of the manipulation of biological mechanisms of forgetting has not been assessed in AD animal models. Ras-related C3 botulinum toxin substrate 1 (Rac1), a small GTPase, has been shown to regulate active forgetting in Drosophila and mice. This study has shown that Rac1 activity is aberrantly elevated in the hippocampal tissues of AD patients and AD animal models. Moreover, amyloid-beta 42 could induce Rac1 activation in cultured cells. The elevation of Rac1 activity not only accelerated 6-hour spatial memory decay in 3-month-old APP/PS1 mice, but also significantly contributed to severe memory loss in aged APP/PS1 mice. A similar age-dependent Rac1 activity-based memory loss was also observed in an AD fly model. Moreover, inhibition of Rac1 activity could ameliorate cognitive defects and synaptic plasticity in AD animal models. Finally, two novel compounds, identified through behavioral screening of a randomly selected pool of brain permeable small molecules for their positive effect in rescuing memory loss in both fly and mouse models, were found to be capable of inhibiting Rac1 activity. Thus, multiple lines of evidence corroborate in supporting the idea that inhibition of Rac1 activity is effective for treating AD-related memory loss (Wu, 2019).

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Adusumalli, S., Ngian, Z. K., Lin, W. Q., Benoukraf, T. and Ong, C. T. (2019). Increased intron retention is a post-transcriptional signature associated with progressive aging and Alzheimer's disease. Aging Cell: e12928. PubMed ID: 30868713

Abstract

Intron retention (IR) by alternative splicing is a conserved regulatory mechanism that can affect gene expression and protein function during adult development and age-onset diseases. However, it remains unclear whether IR undergoes spatial or temporal changes during different stages of aging or neurodegeneration like Alzheimer's disease (AD). By profiling the transcriptome of Drosophila head cells at different ages, a significant increase was observed in IR events for many genes during aging. Differential IR affects distinct biological functions at different ages and occurs at several AD-associated genes in older adults. The increased nucleosome occupancy at the differentially retained introns in young animals suggests that it may regulate the level of IR during aging. Notably, an increase in the number of IR events was also observed in healthy older mouse and human brain tissues, as well as in the cerebellum and frontal cortex from independent AD cohorts. Genes with differential IR shared many common features, including shorter intron length, no perturbation in their mRNA level, and enrichment for biological functions that are associated with mRNA processing and proteostasis. The differentially retained introns identified in AD frontal cortex have higher GC content, with many of their mRNA transcripts showing an altered level of protein expression compared to control samples. Taken together, these results suggest that an increased IR is an conserved signature that is associated with aging. By affecting pathways involved in mRNA and protein homeostasis, changes of IR pattern during aging may regulate the transition from healthy to pathological state in late-onset sporadic AD (Adusumalli, 2019).

Alzheimer's disease (AD) is the most common cause of dementia, which is associated with an enormous personal, social and economic burden worldwide. However, there are few current treatments with none of them targeting the underlying causes of the disease. Expression of the 0N4R isoform of tau has been associated with AD pathology and this study showa that expressing it in the Drosophila clock network gives rise to circadian and sleep phenotypes which closely match the behavioural changes seen in human AD patients. Tauopathic flies exhibited greater locomotor activity throughout the day and night and displayed a loss of sleep, particularly at night. Under constant darkness, the locomotor behaviour of tau-expressing flies was less rhythmic than controls indicating a defect in their intrinsic circadian rhythm. Current clamp recordings from wake-promoting, pigment dispersing factor (PDF)-positive large lateral ventral clock neurons (l-LNvs) revealed elevated spontaneous firing throughout the day and night which likely underlies the observed hyperactive circadian phenotype. Interestingly, expression of tau in only the PDF-positive pacemaker neurons, which are thought to be the most important for behaviour under constant conditions, was not sufficient or even necessary to affect circadian rhythmicity. This work establishes Drosophila as a model to investigate interactions between human pathological versions of tau and the machinery that controls neuronal excitability, allowing the identification of underlying mechanisms of disease that may reveal new therapeutic targets (Buhl, 2019).

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Saito, T., Oba, T., Shimizu, S., Asada, A., Iijima, K. M. and Ando, K. (2019). Cdk5 increases MARK4 activity and augments pathological tau accumulation and toxicity through tau phosphorylation at Ser262. Hum Mol Genet. PubMed ID: 31174206

Abstract

Hyperphosphorylation of the microtubule-associated protein tau is associated with many neurodegenerative diseases, including Alzheimer's disease. Microtubule affinity-regulating kinases (MARK) 1-4 and cyclin-dependent kinase 5 (Cdk5) are tau kinases under physiological and pathological conditions. However, their functional relationship remains elusive. This study reports a novel mechanism by which Cdk5 activates MARK4 and augments tau phosphorylation, accumulation, and toxicity. MARK4 is highly phosphorylated at multiple sites in the brain and in cultured neurons, and inhibition of Cdk5 activity reduces phosphorylation levels of MARK4. MARK4 is known to be activated by phosphorylation at its activation loop by liver kinase B1 (LKB1). In contrast, Cdk5 increased phosphorylation of MARK4 in the spacer domain, but not in the activation loop, and enhanced its kinase activity, suggesting a novel mechanism by which Cdk5 regulates MARK4 activity. It was also demonstrated that co-expression of Cdk5 and MARK4 in mammalian cultured cells significantly increased the levels of tau phosphorylation at both Cdk5 target sites (SP/TP sites) and MARK target sites (Ser262), as well as the levels of total tau. Furthermore, using a Drosophila model of tau toxicity, it was demonstrated that Cdk5 promoted tau accumulation and tau-induced neurodegeneration via increasing tau phosphorylation levels at Ser262 by a fly ortholog of MARK, Par-1. This study suggests a novel mechanism by which Cdk5 and MARK4 synergistically increase tau phosphorylation and accumulation, consequently promoting neurodegeneration in disease pathogenesis (Saito, 2019).

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Galasso, A., Cameron, C. S., Frenguelli, B. G. and Moffat, K. G. (2017). An AMPK-dependent regulatory pathway in tau-mediated toxicity. Biol Open [Epub ahead of print]. PubMed ID: 28808138

Abstract

Neurodegenerative tauopathies are characterized by accumulation of hyperphosphorylated tau aggregates primarily degraded by autophagy. The 5'AMP-activated protein kinase (AMPK) is expressed in most cells, including neurons. Alongside its metabolic functions, it is also known to be activated in Alzheimer's brains, phosphorylate tau, and be a critical autophagy activator. While stress conditions can result in AMPK activation enhancing tau-mediated toxicity, AMPK activation is not always concomitant with autophagic induction. Using a Drosophila in vivo quantitative approach, this study has analysed the impact of AMPK and autophagy on tau-mediated toxicity, recapitulating the AMPK-mediated tauopathy condition: increased tau phosphorylation, without corresponding autophagy activation. It was demonstrated that AMPK, binding to and phosphorylating tau at Ser-262, a site reported to facilitate soluble tau accumulation, affects its degradation. This phosphorylation results in exacerbation of tau toxicity and is ameliorated via rapamycin-induced autophagy stimulation. These findings support the development of combinatorial therapies effective at reducing tau toxicity targeting tau phosphorylation and AMPK-independent autophagic induction. The proposed in vivo tool represents an ideal readout to perform preliminary screening for drugs promoting this process (Galasso, 2017).

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Lee, B. I., Suh, Y. S., Chung, Y. J., Yu, K. and Park, C. B. (2017). Shedding light on Alzheimer's beta-amyloidosis: Photosensitized methylene blue inhibits self-assembly of beta-amyloid peptides and disintegrates their aggregates. Sci Rep 7(1): 7523. PubMed ID: 28790398

Abstract

Abnormal aggregation of β-amyloid (Aβ) peptides is a major hallmark of Alzheimer's disease (AD). In spite of numerous attempts to prevent the β-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. With the aid of multiple photochemical analyses, this study first reports that photoexcited MB molecules can block Aβ42 aggregation in vitro. Furthermore, an in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to Aβ42 oxidation by singlet oxygen generated from photoexcited MB. Finally, this study shows that photoexcited MB possess a capability to disaggregate the pre-existing Aβ42 aggregates and reduce Aβ-induced cytotoxicity. This work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future (Lee, 2017).

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Kadas, D., Papanikolopoulou, K., Xirou, S., Consoulas, C. and Skoulakis, E. M. C. (2018). Human Tau isoform-specific presynaptic deficits in a Drosophila central nervous system circuit. Neurobiol Dis 124: 311-321. PubMed ID: 30529489

Abstract

Accumulation of normal or mutant human Tau isoforms (see Drosophila Tau) in Central Nervous System (CNS) neurons of vertebrate and invertebrate models underlies pathologies ranging from behavioral deficits to neurodegeneration that broadly recapitulate human Tauopathies. Although some functional differences have begun to emerge, it is still largely unclear whether normal and mutant Tau isoforms induce differential effects on the synaptic physiology of CNS neurons. This study used the oligosynaptic Giant Fiber System in the adult Drosophila CNS to address this question and revealed that 3R and 4R isoforms affect distinct synaptic parameters. Whereas 0N3R increased failure rate upon high frequency stimulation, 0N4R compromised stimulus conduction and response speed at a specific cholinergic synapse in an age-dependent manner. In contrast, accumulation of the R406W mutant of 0N4R induced mild, age-dependent conduction velocity defects. Because 0N4R and its mutant isoform are expressed equivalently, this demonstrates that the defects are not merely consequent of exogenous human Tau accumulation and suggests distinct functional properties of 3R and 4R isoforms in cholinergic presynapses (Kadas, 2018).

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Coelho, D. S., Schwartz, S., Merino, M. M., Hauert, B., Topfel, B., Tieche, C., Rhiner, C. and Moreno, E. (2018). Culling less fit neurons protects against Amyloid-beta-induced brain damage and cognitive and motor decline. Cell Rep 25(13): 3661-3673. PubMed ID: 30590040

Abstract

Alzheimer's disease (AD) is the most common form of dementia, impairing cognitive and motor functions. One of the pathological hallmarks of AD is neuronal loss, which is not reflected in mouse models of AD. Therefore, the role of neuronal death is still uncertain. This study used a Drosophila AD model expressing a secreted form of human amyloid-beta42 peptide and showed that it recapitulates key aspects of AD pathology, including neuronal death and impaired long-term memory. Neuronal apoptosis is mediated by cell fitness-driven neuronal culling, which selectively eliminates impaired neurons from brain circuits. Removal of less fit neurons delays beta-amyloid-induced brain damage and protects against cognitive and motor decline, suggesting that contrary to common knowledge, neuronal death may have a beneficial effect in AD (Coelho. 2018).

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Chiku, T., Hayashishita, M., Saito, T., Oka, M., Shinno, K., Ohtake, Y., Shimizu, S., Asada, A., Hisanaga, S. I., Iijima, K. M. and Ando, K. (2018). S6K/p70S6K1 protects against tau-mediated neurodegeneration by decreasing the level of tau phosphorylated at Ser262 in a Drosophila model of tauopathy. Neurobiol Aging 71: 255-264. PubMed ID: 30172839

Abstract

Abnormal accumulation of the microtubule-associated protein tau is thought to cause neuronal cell death in a group of age-associated neurodegenerative disorders. Tau is phosphorylated at multiple sites in diseased brains, and phosphorylation of tau at Ser262 initiates tau accumulation and toxicity. This study sought to identify novel factors that affect the metabolism and toxicity of tau phosphorylated at Ser262 (pSer262-tau). A biased screen using a Drosophila model of tau toxicity revealed that knockdown of S6K, the Drosophila homolog of p70S6K1, increased the level of pSer262-tau and enhanced tau toxicity. S6K can be activated by the insulin signaling, however, unlike knockdown of S6K, knockdown of insulin receptor or insulin receptor substrate nonselectively decreased total tau levels via autophagy. Importantly, activation of S6K significantly suppressed tau-mediated axon degeneration, whereas manipulation of either the insulin signaling pathway or autophagy did not. These results suggest that activation of S6K may be an effective therapeutic strategy for selectively decreasing the levels of toxic tau species and suppressing neurodegeneration (Chiku, 2018).

Stapper, Z. A. and Jahn, T. R. (2018) (2018). Changes in glutathione redox potential are linked to Abeta42-induced neurotoxicity. Cell Rep 24(7): 1696-1703. PubMed ID: 30110626

Abstract

Glutathione is the major low-molecular weight thiol of eukaryotic cells. It is central to one of the two major NADPH-dependent reducing systems and is likely to play a role in combating oxidative stress, a process suggested to play a key role in Alzheimer's disease (AD). However, the nature and relevance of redox changes in the onset and progression of AD are still uncertain. This study combined genetically encoded redox sensors with a Drosophila models of amyloid-beta (Abeta) aggregation. Changes in glutathione redox potential (EGSH) closely correlate with disease onset and progression. This redox imbalance was found to be specifically in neurons, but not in glia cells. EGSH changes and Abeta42 deposition are also accompanied by increased JNK stress signaling. Furthermore, pharmacologic and genetic manipulation of glutathione synthesis modulates Abeta42-mediated neurotoxicity, suggesting a causal relationship between disturbed glutathione redox homeostasis and early AD pathology (Strapper, 2018).

Furotani, K., Kamimura, K., Yajima, T., Nakayama, M., Enomoto, R., Tamura, T., Okazawa, H. and Sone, M. (2018). Suppression of the synaptic localization of a subset of proteins including APP partially ameliorates phenotypes of the Drosophila Alzheimer's disease model. PLoS One 13(9): e0204048. PubMed ID: 30226901

Abstract

APP (amyloid precursor protein), the causative molecule of Alzheimer's disease, is synthesized in neuronal cell bodies and subsequently transported to synapses. It has been show that the yata gene is required for the synaptic transport of the APP orthologue in Drosophila melanogaster. This study examined the effect of a reduction in yata expression in the Drosophila Alzheimer's disease model, in which expression of human mutant APP was induced. The synaptic localization of APP and other synaptic proteins was differentially inhibited by yata knockdown and null mutation. Expression of APP resulted in abnormal synaptic morphology and the premature death of animals. These phenotypes were partially but significantly rescued by yata knockdown, whereas yata knockdown itself caused no abnormality. Moreover, synaptic transmission accuracy was impaired in this model, and this phenotype was improved by yata knockdown. Thus, these data suggested that the phenotypes caused by APP can be partially prevented by inhibition of the synaptic localization of a subset of synaptic proteins including APP (Furotani, 2018).

This study examined whether the inhibition of the synaptic localization of APP affects the phenotypes of the Drosophila model of Alzheimer's disease. For this purpose, the yata gene. The yata gene is required for the intracellular transport of the APPL protein, which is the Drosophila orthologue of mammalian APP. This study induced the expression of the human Swedish mutant APP in larval motor neurons. Knockdown and null mutation of yata resulted in decreased synaptic localization of APP. In this study, immunostaining was performed using the anti-APP antibody MAb 4G8 that recognizes amino acid residues 17–24 of the amyloid β region. Therefore, the observed localization was the expression for full-length APP or the processed fragment that contains the amyloid β region. Notably, yata mutation resulted in the differential suppression of the synaptic localization of a subset of proteins including APP without uniformly affecting all of the synaptic molecules. The data showed that synaptic localization of APP was impaired whereas localization of Fasciclin II was not significantly affected by yata knockdown. Localization of Synaptotagmin and Cysteine string protein was also not significantly affected even in yata null mutants. Such selectivity is a desired property of a tool that therapeutically targets APP. On the other hand, localization of Fasciclin II was significantly decreased in the yata null mutants. yata has a mammalian orthologue, SCYL1. Although both yata and SCYL1 are suggested to be involved in the trafficking of the coated secretory vesicles, the data suggested that the impact of yata loss-of-function is differential among proteins that are transported by vesicular trafficking. The data suggested that APP and Fasciclin II are affected by yata mutation. As a possibility, specific secretory vesicles that contain a subset of proteins such as APP and Fasciclin II as cargos are relatively severely affected by yata mutation. Another molecule whose synaptic localization was found to be affected in yata mutants was the Bruchpilot protein, which is a cytoplasmic protein and is a component of the electron-dense T-bar structures of the presynaptic active zone. This finding was unexpected, because yata/SCYL1 is suggested to play a role in the trafficking of secretory vesicles. Therefore, synaptic dysfunction caused by impaired synaptic development may affect the assembly of presynaptic components such as Bruchpilot. Alternatively, yata may also be involved in the expression or trafficking of synaptic cytoplasmic proteins (Furotani, 2018).

Knockdown of yata partially ameliorated abnormalities in the Alzheimer's disease model, such as increased number of satellite boutons, lethality during development, lethality in the first 10 days after eclosion and impaired synaptic transmission accuracy. In addition, heterozygous introduction of the yata null allele also partially rescued the developmental lethality and lethality within 10 days after eclosion, although the effect was weaker and developmental lethality was not rescued in males. yata knockdown and heterozygosity of the yata null allele themselves caused no apparent abnormalities. The only phenotype observed to be caused by yata knockdown was the elevated variance in the amplitudes of eEJPs. While homozygotes of the yata null allele show phenotypes including developmental abnormalities, progressive brain volume reduction and shortened lifespan, heterozygotes of the yata null allele are viable and fertile and show no apparent abnormalities like many other recessive mutations. The reason why the knockdown of yata causes no apparent abnormalities may be because the effect of knockdown is partial and decreased expression of yata is still sufficient for most of molecular functions of yata. On the other hand, most of the observed rescuing effects were only partial. This finding may possibly be because the synaptic localization of induced APP is still sufficient for the expression of some phenotypes, although the synaptic localization was decreased. Alternatively, synapse-independent mechanisms controlled by induced APP may exist (Furotani, 2018).

Lifespan data showed that the lethality caused by the expression of APP is restricted to the developmental stage from the embryo to the the first 10 days of the adult stage after eclosion. The animals that survived past this stage showed an almost normal lifespan, suggesting that the cause of death is a defect in neural development. Because lethality can be suppressed by yata knockdown and possibly by the decreased synaptic localization of a subset of proteins including APP, this finding suggests that the cause of death is a defect in synapses. In addition, electrophysiological analysis revealed that the expression of APP resulted in impaired synaptic transmission accuracy, because expression of APP caused a significantly elevated variance in the amplitudes of eEJPs, while the amplitudes and frequencies of mEJPs were not significantly affected. The phenotype of the variance of eEJPs was suppressed by yata knockdown and possibly by the decreased synaptic localization of a subset of proteins including APP. Such instability of synaptic transmission may be a result of impaired synaptic development. Notably, knockdown of yata also caused a impairment in the accuracy of synaptic transmission similar to the phenotype caused by the expression of APP, potentially reflecting the physiological function of yata in synaptic physiology. While synaptic localization of ectopically induced APP is decreased by yata knockdown, synaptic localization of other endogenous synaptic proteins may also be affected, and this might contribute to the elevated variance in eEJPs by yata knockdown. Knockdown of yata caused elevated eEJP variance but did not cause premature lethality of animals. These data are against the possibility that the instability of synaptic transmission directly causes the death of animals. The identity of synapses that contribute significantly to the death of animals is also unknown. In this model, the excessive formation of satellite boutons was observed in the neuromuscular synapses on the body wall muscles of larvae. This means that the expression of APP resulted in the loss of proper control of synapse formation. Such a loss of control may cause fatal results in the synapses that are essential for the survival of animals (Furotani, 2018).

In this study, the expression of human Swedish mutant APP associated with familial Alzheimer's disease was induced. Among the phenotypes observed in this study, excessive formation of satellite boutons is known to be caused by the overexpression of human wild-type APP and Drosophila Appl. Therefore, these phenotypes are suggested to be caused by the elevated gene dosage of Appl/APP. It is necessary to examine the contribution of APP mutation to other phenotypes including observed electrophysiological phenotypes, especially because it has been previously shown that the overexpression of Drosophila Appl caused different electrophysiological phenotypes including a decrease in eEJP amplitude, an increase in mEJP amplitude and a decrease in quantal content, although experimental conditions including the selection of Gal4 were different (Furotani, 2018).

Synaptic loss is observed in patients with Alzheimer's disease. On the other hand, previous studies have shown that mammalian APP-family genes are involved in synaptic development. Double knock-out mice of several combinations of APP family genes show embryonic lethality, possibly caused by defects in synaptic morphology and function. Although the APP protein is transported and localized in synapses, whether the role of APP in synaptic morphology and function is attributed to the function of the APP protein localized in the synapses is still unclear. In this study, the data suggested that the suppression of synaptic localization of a subset of proteins including APP partially rescued the synaptic phenotypes caused by APP, suggesting the importance of the function of APP localized in synapses. On the other hand, while Alzheimer's disease is a late-onset disease, this model and double APP knock-out mice show developmental defects in synapses. In fact, loss-of-function mutants of the Drosophila Appl gene show not only defects in synaptic development but also late-onset shortened lifespan. The shortened lifespan of the Appl mutants may reflect the physiological function of Appl in the maintenance of the nervous system against aging. Alternatively, developmental defects may also cause late-onset phenotypes. It remains to be elucidated if the molecular function of Appl in synapses is related to the late-onset premature death of Appl mutant flies, although the relevance of the physiological functions of APP in the pathogenesis of Alzheimer's disease is still unclear and the current model did not show the late-onset premature death of animals (Furotani, 2018).

Because Drosophila yata mutants and SCYL1 null mutant mice share neurodegeneration phenotypes, and the human SCYL1 gene has been identified as a causative gene of a genetic disease causing peripheral neuropathy and cerebellar atrophy, the molecular function of these orthologues seems to be evolutionarily conserved. The synaptic pathology of Alzheimer's disease may be able to be modified if one could control the synaptic localization of APP. Although the mammalian orthologue of yata, SCYL1, is a candidate target molecule to affect the synaptic localization of APP, complete ablation of both Drosophila yata and mammalian SCYL1 result in fatal phenotypes including neurodegeneration. Moreover, knockdown of yata itself causes impaired synaptic transmission accuracy. Therefore, it is necessary to examine if there is a way to control the synaptic localization of APP without fatal side-effects, if it is possible to control the functional expression of SCYL1 strictly and if SCYL1 can be used as a target molecule in a therapeutic approach for the treatment of Alzheimer's disease (Furotani, 2018).

Sun, W, Samimi, H., Gamez, M., Zare, H. and Frost, B. (2018). Pathogenic tau-induced piRNA depletion promotes neuronal death through transposable element dysregulation in neurodegenerative tauopathies. Nat Neurosci 21(8): 1038-1048. PubMed ID: 30038280

Abstract

Transposable elements, known colloquially as 'jumping genes', constitute approximately 45% of the human genome. Cells utilize epigenetic defenses to limit transposable element jumping, including formation of silencing heterochromatin and generation of piwi-interacting RNAs (piRNAs), small RNAs that facilitate clearance of transposable element transcripts. This study utilize Drosophila melanogaster and postmortem human brain samples to identify transposable element dysregulation as a key mediator of neuronal death in tauopathies, a group of neurodegenerative disorders that are pathologically characterized by deposits of tau protein in the brain. Mechanistically, it was found that heterochromatin decondensation and reduction of piwi and piRNAs drive transposable element dysregulation in tauopathy. A significant increase is reported in transcripts of the endogenous retrovirus class of transposable elements in human Alzheimer's disease and progressive supranuclear palsy, suggesting that transposable element dysregulation is conserved in human tauopathy. Taken together, these data identify heterochromatin decondensation, piwi and piRNA depletion and consequent transposable element dysregulation as a pharmacologically targetable, mechanistic driver of neurodegeneration in tauopathy (Sun, 2018).

Sun, W, Samimi, H., Gamez, M., Zare, H. and Frost, B. (2018). Pathogenic tau-induced piRNA depletion promotes neuronal death through transposable element dysregulation in neurodegenerative tauopathies. Nat Neurosci 21(8): 1038-1048. PubMed ID: 30038280

Abstract

Transposable elements, known colloquially as 'jumping genes', constitute approximately 45% of the human genome. Cells utilize epigenetic defenses to limit transposable element jumping, including formation of silencing heterochromatin and generation of piwi-interacting RNAs (piRNAs), small RNAs that facilitate clearance of transposable element transcripts. This study utilize Drosophila melanogaster and postmortem human brain samples to identify transposable element dysregulation as a key mediator of neuronal death in tauopathies, a group of neurodegenerative disorders that are pathologically characterized by deposits of tau protein in the brain. Mechanistically, it was found that heterochromatin decondensation and reduction of piwi and piRNAs drive transposable element dysregulation in tauopathy. A significant increase is reported in transcripts of the endogenous retrovirus class of transposable elements in human Alzheimer's disease and progressive supranuclear palsy, suggesting that transposable element dysregulation is conserved in human tauopathy. Taken together, these data identify heterochromatin decondensation, piwi and piRNA depletion and consequent transposable element dysregulation as a pharmacologically targetable, mechanistic driver of neurodegeneration in tauopathy (Sun, 2018).

This study uncovered a therapeutically targetable mechanism whereby pathogenic tau drives neuronal death. Specifically, these studies identified dysregulation of transposable elements as a consequence of pathogenic tau and a driver of aberrant cell cycle activation in neurons and subsequent neuronal death. Genetic, dietary and pharmacological approaches were taken to reduce transposable element dysregulation and suppress tau-induced neurotoxicity in Drosophila. An unbiased transcriptomic approach was taken to extend these findings to postmortem human brains, and differentially expressed transposable elements were identified in Alzheimer's disease and progressive supranuclear palsy (Sun, 2018).

Because the complexity and repetitive nature of transposable elements present challenges to RNA-seq analysis, which is associated with a greater frequency of false positives and negatives compared to analysis of canonical messenger RNAs, this study performed secondary validation of differentially expressed transposable element transcripts in tau transgenic Drosophila by NanoString. While NanoString data showed a similar expression trend for most of the transposable elements that were identified as differentially expressed in tau transgenic Drosophila by RNA-seq, some of the elements were not confirmed as differentially expressed by NanoString. These data reveal the limitations of each assay when analyzing transposable element transcripts and stress the importance of rigorous secondary validation. Since many members of the copia family are increased at the transcript level in both RNA-seq and NanoString analyses, it is speculated that gypsy-TRAP reporter activation is a result of copia insertion into the <i>ovo locus, rather than gypsy. Attempts to sequence de novo copia insertions within the <i>ovo locus in homogenates prepared from tau transgenic Drosophila heads resulted in a high frequency of mismatches, which is likely a result of the stochastic nature of transposable element insertion (Sun, 2018).

According to current understanding, cells have two layers of defense against potentially deleterious transposable element activation: transposable element transcription is limited by heterochromatin-mediated silencing, and transposable element transcripts are cleared from the cell by piRNA-mediated degradation. Both mechanisms of transposable element suppression were compromised in tauopathy. It is speculated that tau-induced heterochromatin decondensation facilitates active transcription of transposable elements and that tau-induced piwi and piRNA reduction allows those transcripts to persist. While these results are consistent with the effects of heterochromatin decondensation and piwi reduction on transposable element expression that have been reported in the Drosophila germline, these studies reveal a previously undocumented role for heterochromatin- and piRNA-mediated transposable element silencing in the brain. On the basis of studies in the germline reporting a direct interaction between piwi and HP140 and a requirement for Rhino, a member of the HP1 subfamily, for piRNA production, it is possible that a direct interaction between piwi and HP1 is also required to silence transposable elements in the brain (Sun, 2018).

Among upregulated transposable elements in human tauopathy, the human endogenous retrovirus (HERV) family, including HERV-K, was significantly over-represented. Elevated HERV-K transcripts are associated with amyotrophic lateral sclerosis (ALS)8 and many human cancers, including melanoma, breast cancer, germ cell tumors, renal cancer and ovarian cancer. A causal association between HERV-K and neuronal dysfunction has previously been established, as ectopic expression of HERV-K or the retroviral envelope protein that it encodes decreases synaptic activity and induces progressive motor dysfunction in mice. Antiretroviral reverse transcriptase inhibitors inhibit HERV-K activation in cultured cells and are now in clinical trials for the treatment of ALS. On the basis of the data presented in this study, reverse transcriptase inhibitors have significant potential as a therapeutic strategy for the treatment of neurodegenerative tauopathies, including Alzheimer's disease (Sun, 2018).

The ability of flamenco loss-of-function mutations to enhance tau-induced neurotoxicity and the ability of piwi overexpression, dietary restriction and inhibition of reverse transcriptase to reduce transposable element dysregulation and suppress tau-induced neurotoxicity suggest that tau-induced transposable element dysregulation is deleterious to neuronal survival. In addition to the detrimental effects of transposable element jumping, double-stranded RNAs produced by transposable element transcripts, including HERVs, can trigger a type I interferon response through the innate immune system. In light of the HERV increase in human tauopathy and the involvement of the innate immune response as a disease-promoting mechanism in Alzheimer's disease, it is tempting to speculate that expression of endogenous retroviruses in human tauopathy contributes to neuroinflammation, in addition to promoting genomic instability. In future studies, it will be important to investigate a potential effect of transposable element activation on the innate immune response in the context of tauopathy (Sun, 2018).

Younan, N. D., Chen, K. F., Rose, R. S., Crowther, D. C. and Viles, J. H. (2018). Prion protein stabilizes amyloid-beta (Abeta) oligomers and enhances Abeta neurotoxicity in a Drosophila model of Alzheimer disease. J Biol Chem. PubMed ID: 29887525

Abstract

The cellular prion protein (PrPC) can act as a cell-surface receptor for amyloid-beta (Abeta) peptide; however, a role for PrPC in the pathogenesis of Alzheimer's disease (AD) is contested. This study has expressed a range of Abeta isoforms and PrPC in the Drosophila brain. Co-expression of Abeta and PrPC significantly reduces the lifespan, disrupts circadian rhythms, and increases Abeta deposition in the fly brain. In contrast, under the same conditions, expression of Abeta or PrPC individually did not lead to these phenotypic changes. In vitro studies revealed that substoichiometric amounts of PrPC trap Abeta as oligomeric assemblies and fragment-preformed Abeta fibers. The ability of membrane-anchored PrPC to trap Abeta as cytotoxic oligomers at the membrane surface and fragment inert Abeta fibers, suggests a mechanism by which PrPC exacerbates Abeta deposition and pathogenic phenotypes in the fly, supporting a role for PrPC in AD. This study provides a second animal model linking PrPC expression with Abeta toxicity and supports a role for PrPC in AD pathogenesis. Blocking the interaction of Abeta and PrPC represents a potential therapeutic strategy (Younan, 2018).

Martin-Pena, A., Rincon-Limas, D. E. and Fernandez-Funez, P. (2018). Engineered Hsp70 chaperones prevent Abeta42-induced memory impairments in a Drosophila model of Alzheimer's disease. Sci Rep 8(1): 9915. PubMed ID: 29967544

Abstract

Proteinopathies constitute a group of diseases in which certain proteins are abnormally folded leading to aggregation and eventual cell failure. Most neurodegenerative diseases belong to protein misfolding disorders and, among them, Alzheimer's disease (AD) is the most prevalent. AD is characterized by accumulation of the amyloid-beta42 (Abeta42) peptide in the extracellular space. Hence, this study genetically engineered a molecular chaperone that was selectively delivered to this cellular location. It has been reported that the heat shock protein 70 (Hsp70) binds Abeta42 (see Drosophila Appl) preventing self-aggregation. This study employed two isoforms of the Hsp70, cytosolic and extracellular, to evaluate their potential protective effect against the memory decline triggered by extracellular deposition of Abeta42. Both Hsp70 isoforms significantly improved memory performance of flies expressing Abeta42, irrespective of their age or the level of Abeta42 load. Using olfactory classical conditioning, a Drosophila model of AD was established based on Abeta42 neurotoxicity and memory decline was monitored through aging. The onset of the memory impairment observed was proportional to the cumulative level of Abeta42 in the Drosophila brain. These data support the use of this Drosophila model of AD to further investigate molecules with a protective activity against Abeta42-induced memory loss, contributing to the development of palliative therapies for AD (Martin-Pena, 2018).

Talmat-Amar, Y., Arribat, Y. and Parmentier, M. L. (2018). Vesicular Axonal Transport is Modified In Vivo by Tau Deletion or Overexpression in Drosophila. Int J Mol Sci 19(3). PubMed ID: 29509687

Abstract

Structural microtubule associated protein Tau is found in high amount in axons and is involved in several neurodegenerative diseases. Although many studies have highlighted the toxicity of an excess of Tau in neurons, the in vivo understanding of the endogenous role of Tau in axon morphology and physiology is poor. Indeed, knock-out mice display no strong cytoskeleton or axonal transport phenotype, probably because of some important functional redundancy with other microtubule-associated proteins (MAPs). This study took advantage of the model organism Drosophila, which genome contains only one homologue of the Tau/MAP2/MAP4 family to decipher (endogenous) Tau functions. Tau depletion was found to lead to a decrease in microtubule number and microtubule density within axons, while Tau excess leads to the opposite phenotypes. Analysis of vesicular transport in tau mutants showed altered mobility of vesicles, but no change in the total amount of putatively mobile vesicles, whereas both aspects were affected when Tau was overexpressed. In conclusion, this study shows that loss of Tau in tau mutants not only leads to a decrease in axonal microtubule density, but also impairs axonal vesicular transport, albeit to a lesser extent compared to the effects of an excess of Tau (Talmat-Amar, 2018).

Guo, C., Jeong, H. H., Hsieh, Y. C., Klein, H. U., Bennett, D. A., De Jager, P. L., Liu, Z. and Shulman, J. M. (2018). Tau activates transposable elements in Alzheimer's disease. Cell Rep 23(10): 2874-2880. PubMed ID: 29874575

Abstract

Aging and neurodegenerative disease are characterized by genomic instability in neurons, including aberrant activation and mobilization of transposable elements (TEs). Integrating studies of human postmortem brain tissue and Drosophila melanogaster models, TE activation was investigated in association with Tau pathology in Alzheimer's disease (AD). Leveraging RNA sequencing from 636 human brains, differential expression was discovered for several retrotransposons in association with neurofibrillary tangle burden and highlight evidence for global TE transcriptional activation among the long interspersed nuclear element 1 and endogenous retrovirus clades. In addition, Tau-associated, active chromatin signatures were detected at multiple HERV-Fc1 genomic loci. To determine whether Tau is sufficient to induce TE activation, retrotransposons were profiled in Drosophila expressing human wild-type or mutant Tau throughout the brain. Heterogeneous response profiles were discovered, including both age- and genotype-dependent activation of TE expression by Tau. The results implicate TE activation and associated genomic instability in Tau-mediated AD mechanisms (Guo, 2018).

Panikker, P., Xu, S. J., Zhang, H., Sarthi, J., Beaver, M., Sheth, A., Akhter, S. and Elefant, F. (2018). Restoring Tip60 HAT/HDAC2 balance in the neurodegenerative brain relieves epigenetic transcriptional repression and reinstates cognition. J Neurosci. PubMed ID: 29654189

Abstract

Cognitive decline is a debilitating hallmark during pre-clinical stages of AD yet causes remain unclear. As histone acetylation homeostasis is critical for mediating epigenetic gene control throughout neuronal development, it was postulated that its misregulation contributes to cognitive impairment preceding AD pathology. This study shows that disruption of Tip60 HAT/HDAC2 homeostasis occurs early in the brain of an AD associated amyloid precursor protein (APP) Drosophila model and triggers epigenetic repression of neuroplasticity genes well before Abeta plaques form in male and female larvae. Repressed genes display enhanced HDAC2 binding and reduced Tip60 and histone acetylation enrichment. Increasing Tip60 in the AD associated APP brain restores Tip60 HAT/HDAC2 balance by decreasing HDAC2 levels, reverses neuroepigenetic alterations to activate synaptic plasticity genes, and reinstates brain morphology and cognition. Such Drosophila neuroplasticity gene epigenetic signatures are conserved in male and female mouse hippocampus and their expression and Tip60 function is compromised in hippocampus from AD patients. It is advocated that Tip60 HAT/HDAC2 mediated epigenetic gene disruption is a critical initial step in AD that is reversed by restoring Tip60 in the brain (Panikker, 2018).

Sarkar, A., Gogia, N., Glenn, N., Singh, A., Jones, G., Powers, N., Srivastava, A., Kango-Singh, M. and Singh, A. (2018). A soy protein Lunasin can ameliorate amyloid-beta 42 mediated neurodegeneration in Drosophila eye. Sci Rep 8(1): 13545. PubMed ID: 30202077

Abstract

Alzheimer's disease (AD), a fatal progressive neurodegenerative disorder, also results from accumulation of amyloid-beta 42 (Abeta42) plaques. These Abeta42 plaques trigger oxidative stress, abnormal signaling, which results in neuronal death by unknown mechanism(s). This study misexpressed high levels of human Abeta42 in the differentiating retinal neurons of the Drosophila eye, which results in the Alzheimer's like neuropathology. Using s transgenic model, a soy-derived protein Lunasin (Lun) was tested for a possible role in rescuing neurodegeneration in retinal neurons. Lunasin is known to have anti-cancer effect and reduces stress and inflammation. Misexpression of Lunasin by transgenic approach can rescue Abeta42 mediated neurodegeneration by blocking cell death in retinal neurons, and results in restoration of axonal targeting from retina to brain. Misexpression of Lunasin downregulates the highly conserved cJun-N-terminal Kinase (JNK) signaling pathway. Activation of JNK signaling can prevent neuroprotective role of Lunasin in Abeta42 mediated neurodegeneration. This neuroprotective function of Lunasin is not dependent on retinal determination gene cascade in the Drosophila eye, and is independent of Wingless and Decapentaplegic signaling pathways. Furthermore, Lunasin can significantly reduce mortality rate caused by misexpression of human Abeta42 in flies. These studies identified the novel neuroprotective role of Lunasin peptide, a potential therapeutic agent that can ameliorate Abeta42 mediated neurodegeneration by downregulating JNK signaling (Sarkar, 2018).

Jonson, M., Nystrom, S., Sandberg, A., Carlback, M., Michno, W., Hanrieder, J., Starkenberg, A., Nilsson, K. P. R., Thor, S. and Hammarstrom, P. (2018). Aggregated Abeta1-42 is selectively toxic for neurons, whereas glial cells produce mature fibrils with low toxicity in Drosophila. Cell Chem Biol 25(5):595-610. PubMed ID: 29657084

Abstract

The basis for selective vulnerability of certain cell types for misfolded proteins (MPs) in neurodegenerative diseases is largely unknown. This knowledge is crucial for understanding disease progression in relation to MPs spreading in the CNS. This issue was addressed in Drosophila by cell-specific expression of human Abeta1-42 associated with Alzheimer's disease. Expression of Abeta1-42 in various neurons resulted in concentration-dependent severe neurodegenerative phenotypes, and intraneuronal ring-tangle-like aggregates with immature fibril properties when analyzed by aggregate-specific ligands. Unexpectedly, expression of Abeta1-42 from a pan-glial driver produced a mild phenotype despite massive brain load of Abeta1-42 aggregates, even higher than in the strongest neuronal driver. Glial cells formed more mature fibrous aggregates, morphologically distinct from aggregates found in neurons, and was mainly extracellular. Theses findings implicate that Abeta1-42 cytotoxicity is both cell and aggregate morphotype dependent (Jonson, 2018).

Martin-Pena, A., Rincon-Limas, D. E. and Fernandez-Funez, P. (2017). Anti-Abeta single-chain variable fragment antibodies restore memory acquisition in a Drosophila model of Alzheimer's disease. Sci Rep 7(1): 11268. PubMed ID: 28900185

Abstract

Alzheimer's disease (AD) is a prevalent neurodegenerative disorder triggered by the accumulation of soluble assemblies of the amyloid-beta42 (Abeta42) peptide. Despite remarkable advances in understanding the pathogenesis of AD, the development of palliative therapies is still lacking. Engineered anti-Abeta42 antibodies are a promising strategy to stall the progression of the disease. Single-chain variable fragment (scFv) antibodies increase brain penetration and offer flexible options for delivery while maintaining the epitope targeting of full antibodies. This study examined the ability of two anti-Abeta scFv antibodies targeting the N-terminal (scFv9) and C-terminal (scFv42.2) regions of Abeta42 to suppress the progressive memory decline induced by extracellular deposition of Abeta42 in Drosophila. Using olfactory classical conditioning, both scFv antibodies were observed to significantly improve memory performance in flies expressing Abeta42 in the mushroom body neurons, which are intimately involved in the coding and storage of olfactory memories. The scFvs effectively restore memory at all ages, from one-day post-eclosion to thirty-day-old flies, proving their ability to prevent the toxicity of different pathogenic assemblies. These data support the application of this paradigm of Abeta42-induced memory loss in Drosophila to investigate the protective activity of Abeta42-binding agents in an AD-relevant functional assay (Martin-Pena, 2017).

Galasso, A., Cameron, C. S., Frenguelli, B. G. and Moffat, K. G. (2017). An AMPK-dependent regulatory pathway in tau-mediated toxicity. Biol Open [Epub ahead of print]. PubMed ID: 28808138

Abstract

Neurodegenerative tauopathies are characterized by accumulation of hyperphosphorylated tau aggregates primarily degraded by autophagy. The 5'AMP-activated protein kinase (AMPK) is expressed in most cells, including neurons. Alongside its metabolic functions, it is also known to be activated in Alzheimer's brains, phosphorylate tau, and be a critical autophagy activator. While stress conditions can result in AMPK activation enhancing tau-mediated toxicity, AMPK activation is not always concomitant with autophagic induction. This study analysed in Drosophila the impact of AMPK and autophagy on tau-mediated toxicity, recapitulating the AMPK-mediated tauopathy condition: increased tau phosphorylation, without corresponding autophagy activation. It was demonstrated that AMPK, binding to and phosphorylating tau at Ser-262, a site reported to facilitate soluble tau accumulation, affects its degradation. This phosphorylation results in exacerbation of tau toxicity and is ameliorated via rapamycin-induced autophagy stimulation. These findings support the development of combinatorial therapies effective at reducing tau toxicity targeting tau phosphorylation and AMPK-independent autophagic induction. The proposed in vivo tool represents an ideal readout to perform preliminary screening for drugs promoting this process (Galasso, 2017).

Zhang, X., Wang, W.A., Jiang, L.X., Liu, H.Y., Zhang, B.Z., Lim, N., Li, Q.Y. and Huang, F.D.(2017). Down-regulation of RBO-PI4KIIIα facilitates Aβ42 secretion and ameliorates neural deficits in Aβ42-expressing Drosophila. J Neurosci [Epub ahead of print]. PubMed ID: 28424219

Abstract

Phosphoinositides and their metabolizing enzymes are involved in Aβ42 metabolism and Alzheimer's disease (AD) pathogenesis. In yeast and mammals, Eighty-five requiring 3 (EFR3), whose Drosophila homolog is Rolling Blackout (RBO), forms a plasma membrane-localized protein complex with phosphatidylinositol-4-kinase type IIIα (PI4KIIIα) and a scaffold protein to tightly control the level of plasmalemmal phosphatidylinositol-4-phosphate (PI4P). This study reports that RBO binds to Drosophila PI4KIIIα, and that in an Aβ42-expressing Drosophila model, separate genetic reduction of PI4KIIIα and RBO, or pharmacological inhibition of PI4KIIIα ameliorates synaptic transmission deficit, climbing ability decline, and premature death, and reduces neuronal accumulation of Aβ42. Moreover, RBO-PI4KIIIa downregulation increases neuronal Aβ42 release, and PI4P facilitates the assembly or oligomerization of Aβ42 in/on liposomes. These results indicate that RBO-PI4KIIIa downregulation facilitates neuronal Aβ42 release and consequently reduces neuronal Aβ42 accumulation likely via decreasing Aβ42 assembly in/on plasma membrane. This study suggests the RBO-PI4KIIIα complex as a potential therapeutic target and PI4KIIIα inhibitors as drug candidates for AD treatment (Zhang, 2017).

Lopez-Arias, B., Turiegano, E., Monedero, I., Canal, I. and Torroja, L. (2017). Presynaptic Abeta40 prevents synapse addition in the adult Drosophila neuromuscular junction. PLoS One 12(5): e0177541. PubMed ID: 28520784

Abstract

Complexity in the processing of the Amyloid Precursor Protein, which generates a mixture of βamyloid peptides, lies beneath the difficulty in understanding the etiology of Alzheimer's disease. Moreover, whether Aβ peptides have any physiological role in neurons is an unresolved question. By expressing single, defined Aβ peptides in Drosophila, specific effects can be discriminated in vivo. This study shows that in the adult neuromuscular junction (NMJ), presynaptic expression of Aβ40 hinders the synaptic addition that normally occurs in adults, yielding NMJs with an invariable number of active zones at all ages tested. A similar trend is observed for Aβ42 at young ages, but net synaptic loss occurs at older ages in NMJs expressing this amyloid species. In contrast, Aβ42arc produces net synaptic loss at all ages tested, although age-dependent synaptic variations are maintained. Inhibition of the PI3K synaptogenic pathway may mediate some of these effects, because western analyses show that Aβ peptides block activation of this pathway, and Aβ species-specific synaptotoxic effects persists in NMJs overgrown by over-expression of PI3K. Finally, individual Aβ effects are also observed when toxicity is examined by quantifying neurodegeneration and survival. These results suggest a physiological effect of Aβ40 in synaptic plasticity, and imply different toxic mechanisms for each peptide species (Lopez-Arias, 2017).

Frenkel-Pinter, M., Stempler, S., Tal-Mazaki, S., Losev, Y., Singh-Anand, A., Escobar-Alvarez, D., Lezmy, J., Gazit, E., Ruppin, E. and Segal, D. S.Altered protein glycosylation predicts Alzheimer's disease and modulates its pathology in disease model Drosophila. Neurobiol Aging. PubMed ID: 28552182

Abstract

The pathological hallmarks of Alzheimer's disease (AD) are pathogenic oligomers and fibrils of misfolded amyloidogenic proteins (e.g., beta-amyloid and hyper-phosphorylated tau in AD), which cause progressive loss of neurons in the brain and nervous system. In an analysis of available expression data sets this study indicates that many glycosylation-related genes are differentially expressed in brains of AD patients compared with healthy controls. The robust differences found enabled prediction of the occurrence of AD with remarkable accuracy in a test cohort and identification of a set of key genes whose expression determines this classification. Then the effect of reducing expression of homologs of 6 of these genes in was studied in transgenic Drosophila overexpressing human tau, a well-established invertebrate AD model. These experiments have led to the identification of glycosylation genes that may augment or ameliorate tauopathy phenotypes. These results indicate that OstDelta, l(2)not and beta4GalT7 are tauopathy suppressors, whereas pgnat5 and CG33303 are enhancers, of tauopathy. These results suggest that specific alterations in protein glycosylation may play a causal role in AD etiology (Frenkel-Pinter, 2017).

Feng, G., Pang, J., Yi, X., Song, Q., Zhang, J., Li, C., He, G. and Ping, Y. (2017). Down-regulation of KV4 channel in Drosophila mushroom body neurons contributes to Abeta42-induced courtship memory deficits. Neuroscience [Epub ahead of print]. PubMed ID: 28627422

Abstract

Accumulation of amyloid-β (Aβ) is widely believed to be an early event in the pathogenesis of Alzheimer's disease (AD). Kv4 is an A-type K+ channel, and previous work has shown that degradation of Kv4, induced by the β42 accumulation, may be a critical contributor to the hyperexcitability of neurons in a Drosophila AD model. This study used well-established courtship memory assay to investigate the contribution of the Kv4 channel to short-term memory (STM) deficits in the Aβ42-expressing AD model. Aβ42 over-expression in Drosophila leads to age-dependent courtship STM loss, which can be also induced by driving acute Aβ42 expression post-developmentally. Interestingly, mutants with eliminated Kv4-mediated A-type K+ currents (IA) by transgenically expressing dominant-negative subunit (DNKv4) phenocopied Aβ42 flies in defective courtship STM. Kv4 channels in mushroom body (MB) and projection neurons (PNs) were found to be required for courtship STM. Furthermore, the STM phenotypes can be rescued, at least partially, by restoration of Kv4 expression in Aβ42 flies, indicating the STM deficits could be partially caused by Kv4 degradation. In addition, IA is significantly decreased in MB neurons (MBNs) but not in PNs, suggesting Kv4 degradation in MBNs, in particular, plays a critical role in courtship STM loss in Aβ42 flies. These data highlight causal relationship between region-specific Kv4 degradation and age-dependent learning decline in the AD model, and provide a mechanism for the disturbed cognitive function in AD (Feng, 2017).

Wu, S. C., Cao, Z. S., Chang, K. M. and Juang, J. L. (2017). Intestinal microbial dysbiosis aggravates the progression of Alzheimer's disease in Drosophila. Nat Commun 8(1): 24. PubMed ID: 28634323

Abstract

Neuroinflammation caused by local deposits of Abeta42 in the brain is key for the pathogenesis and progression of Alzheimer's disease. However, inflammation in the brain is not always a response to local primary insults. Gut microbiota dysbiosis, which is recently emerging as a risk factor for psychiatric disorders, can also initiate a brain inflammatory response. It still remains unclear however, whether enteric dysbiosis also contributes to Alzheimer's disease. This study shows that in a Drosophila Alzheimer's disease model, enterobacteria infection exacerbated progression of Alzheimer's disease by promoting immune hemocyte recruitment to the brain, thereby provoking TNF-JNK mediated neurodegeneration. Genetic depletion of hemocytes attenuates neuroinflammation and alleviated neurodegeneration. It was further found that enteric infection increases the motility of the hemocytes, making them more readily attracted to the brain with an elevated oxidative stress status. This work highlights the importance of gut-brain crosstalk as a fundamental regulatory system in modulating Alzheimer's disease neurodegeneration. Emerging evidence suggests that gut microbiota influences immune function in the brain and may play a role in neurological diseases. This study offers in vivo evidence from a Drosophila model that supports a role for gut microbiota in modulating the progression of Alzheimer's disease (Wu, 2017).

Singh, S. K., Srivastav, S., Yadav, A. K. and Srikrishna, S. (2017). Knockdown of APPL mimics transgenic Abeta induced neurodegenerative phenotypes in Drosophila. Neurosci Lett 648: 8-13. PubMed ID: 28336338

Abstract

A variety of Drosophila mutant lines have been established as potential disease-models to study various disease mechanisms including human neurodegenerative diseases like Alzheimer's disease (AD), Huntington's disease (HD) and Parkinson's disease (PD). The evolutionary conservation of APP (Amyloid Precursor Protein) and APPL (Amyloid Precursor Protein-Like) and the comparable detrimental effects caused by their metabolic products strongly implies the conservation of their normal physiological functions. In view of this milieu, a comparative analysis on the pattern of neurodegenerative phenotypes between Drosophila APPL-RNAi line and transgenic Drosophila line expressing eye tissue specific human Aβ (Amyloid β) was undertaken. The results clearly show that Drosophila APPL-RNAi largely mimics transgenic Abeta in various phenotypes which include eye degeneration, reduced longevity and motor neuron deficit functions, etc. The ultra-structural morphological pattern of eye degeneration was confirmed by scanning electron microscopy. Further, a comparative study on longevity and motor behaviour between Abeta expressing and APPL knockdown lines revealed similar kind of behavioural deficit and longevity phenotypes. Therefore, it is suggested that APPL-knockdown can be used as an alternative approach to study neurodegenerative diseases in the fly model. This is the first report showing comparable phenotypes between APPL and Abeta in AD model of Drosophila (Singh, 2017).

Bergkvist, L., Sandin, L., Kagedal, K. and Brorsson, A. C. (2016). AβPP processing results in greater toxicity per amount of Aβ1-42 than individually expressed and secreted Aβ1-42 in Drosophila melanogaster. Biol Open [Epub ahead of print]. PubMed ID: 27387531

Abstract
The aggregation of the amyloid-β (Aβ; see Drosophila Appl) peptide into fibrillar deposits has long been considered the key neuropathological hallmark of Alzheimer's disease (AD). Aβ peptides are generated from proteolytic processing of the transmembrane Aβ precursor protein (AβPP) via sequential proteolysis through the β-secretase activity of β-site AβPP-cleaving enzyme (BACE1) and by the intramembranous enzyme γ-secretase. For over a decade, Drosophila melanogaster has been used as a model organism to study AD, and two different approaches have been developed to investigate the toxicity caused by AD-associated gene products in vivo. In one model, the Aβ peptide is directly over-expressed fused to a signal peptide, allowing secretion of the peptide into the extracellular space. In the other model, human AβPP is co-expressed with human BACE1, resulting in production of the Aβ peptide through the processing of AβPP by BACE1 and by endogenous fly γ-secretase. This study consisted of a parallel study of flies that expressed the Aβ1-42 peptide alone or that co-expressed AβPP and beta-secretase 1 (BACE1). Toxic effects (assessed by eye phenotype, longevity and locomotor assays) and levels of the Aβ1-42, Aβ1-40 and Aβ1-38 peptides were examined. The data reveal that the toxic effect per amount of detected Aβ1-42 peptide was higher in the flies co-expressing AβPP and BACE1 than in the Aβ1-42-expressing flies, and that the co-existence of Aβ1-42 and Aβ1-40 in the flies co-expressing AβPP and BACE1 could be of significant importance to the neurotoxic effect detected in these flies. Thus, the toxicity detected in these two fly models seems to have different modes of action and is highly dependent on how and where the peptide is generated rather than on the actual level of the Aβ1-42 peptide in the flies. This is important knowledge that needs to be taken into consideration when using Drosophila models to investigate disease mechanisms or therapeutic strategies in AD research (Bergkvist, 2016).

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Arnes, M., Casas-Tinto, S., Malmendal, A. and Ferrus, A. (2017)(2017). Amyloid beta42 peptide is toxic to non-neural cells in Drosophila yielding a characteristic metabolite profile and the effect can be suppressed by PI3K. Biol Open 6(11): 1664-1671. PubMed ID: 29141953

Abstract

The human Abeta42 peptide is associated with Alzheimer's disease through its deleterious effects in neurons. Expressing the human peptide in adult Drosophila in a tissue- and time-controlled manner, this study shows that Abeta42 is also toxic in non-neural cells, neurosecretory and epithelial cell types in particular. This form of toxicity includes the aberrant signaling by Wingless morphogen leading to the eventual activation of Caspase 3. Preventing Caspase 3 activation by means of p53 keeps epithelial cells from elimination but maintains the Abeta42 toxicity yielding more severe deleterious effects to the organism. Metabolic profiling by nuclear magnetic resonance (NMR) of adult flies at selected ages post Abeta42 expression onset reveals characteristic changes in metabolites as early markers of the pathological process. All morphological and most metabolic features of Abeta42 toxicity can be suppressed by the joint overexpression of PI3K (Arnes, 2017).

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Malmanche, N., Dourlen, P., Gistelinck, M., Demiautte, F., Link, N., Dupont, C., Vanden Broeck, L., Werkmeister, E., Amouyel, P., Bongiovanni, A., Bauderlique, H., Moechars, D., Royou, A., Bellen, H. J., Lafont, F., Callaerts, P., Lambert, J. C. and Dermaut, B. (2017). Developmental expression of 4-Repeat-Tau induces neuronal aneuploidy in Drosophila tauopathy models . Sci Rep 7: 40764. PubMed ID: 28112163

Abstract
Tau-mediated neurodegeneration in Alzheimer's disease and tauopathies is generally assumed to start in a normally developed brain. However, several lines of evidence suggest that impaired Tau isoform expression during development could affect mitosis and ploidy in post-mitotic differentiated tissue. Interestingly, the relative expression levels of Tau isoforms containing either 3 (3R-Tau) or 4 repeats (4R-Tau) play an important role both during brain development and neurodegeneration. This study used genetic and cellular tools to study the link between 3R and 4R-Tau isoform expression, mitotic progression in neuronal progenitors and post-mitotic neuronal survival. The results illustrated that the severity of Tau-induced adult phenotypes depends on 4R-Tau isoform expression during development. As recently described, a mitotic delay was observed in 4R-Tau expressing cells of larval eye discs and brains. Live imaging revealed that the spindle undergoes a cycle of collapse and recovery before proceeding to anaphase. Furthermore, a high level of aneuploidy was found in post-mitotic differentiated tissue. Finally, it was shown that overexpression of wild type and mutant 4R-Tau isoform in neuroblastoma SH-SY5Y cell lines is sufficient to induce monopolar spindles. Taken together, these results suggested that neurodegeneration could be in part linked to neuronal aneuploidy caused by 4R-Tau expression during brain development (Malmanche, 2017).

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Zhang, B., Li, Q., Chu, X., Sun, S. and Chen, S. (2016). Salidroside reduces tau hyperphosphorylation via up-regulating GSK-3β phosphorylation in a tau transgenic Drosophila model of Alzheimer's disease. Transl Neurodegener 5: 21. PubMed ID: 27933142

Abstract
Alzheimer's disease (AD) is an age-related and progressive neurodegenerative disease that causes substantial public health care burdens. Intensive efforts have been made to find effective and safe treatment against AD. The plant product Salidroside (Sal) is the main effective component of Rhodiola rosea L., which has several pharmacological activities. The objective of this study was to investigate the efficacy of Sal in the treatment of AD transgenic Drosophila and the associated mechanisms. Microtubule associated protein tau transgenic Drosophila line (TAU) was used in which tau protein is expressed in the central nervous system and eyes by the Gal4/UAS system. After feeding flies with Sal, the lifespan and locomotor activity were recorded. The appearance of vacuoles in the mushroom body was examined using immunohistochemistry, and the levels of total glycogen synthase kinase 3β (t-GSK-3β), phosphorylated GSK-3β (p-GSK-3β), t-tau and p-tau was detected in the brain by western blot analysis. The results showed that the longevity was improved in salidroside-fed Drosophila groups as well as the locomotor activity. Less vacuoles in the mushroom body, upregulated level of p-GSK-3β and downregulated p-tau were detected following Sal treatment. These data presented the evidence that Sal was capable of reducing the neurodegeneration in tau transgenic Drosophila and inhibiting neuronal loss. The neuroprotective effects of Sal were associated with its up-regulation of the p-GSK-3β and down-regulation of the p-tau (Zhang, 2016).

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Fernandez-Funez, P., Sanchez-Garcia, J., de Mena, L., Zhang, Y., Levites, Y., Khare, S., Golde, T. E. and Rincon-Limas, D. E. (2016). Holdase activity of secreted Hsp70 masks amyloid-β42 neurotoxicity in Drosophila. Proc Natl Acad Sci U S A 113: E5212-5221. PubMed ID: 27531960

Abstract
Alzheimer's disease (AD) is the most prevalent of a large group of related proteinopathies for which there is currently no cure. This study used Drosophila to explore a strategy to block Aβ42 neurotoxicity through engineering of the Heat shock protein 70 (Hsp70), a chaperone that has demonstrated neuroprotective activity against several intracellular amyloids. To target its protective activity against extracellular Aβ42, a signal peptide was added to Hsp70. This secreted form of Hsp70 (secHsp70) suppresses Aβ42 neurotoxicity in adult eyes, reduces cell death, protects the structural integrity of adult neurons, alleviates locomotor dysfunction, and extends lifespan. SecHsp70 binding to Aβ42 through its holdase domain is neuroprotective, but its ATPase activity is not required in the extracellular space. Thus, the holdase activity of secHsp70 masks Aβ42 neurotoxicity by promoting the accumulation of nontoxic aggregates. Combined with other approaches, this strategy may contribute to reduce the burden of AD and other extracellular proteinopathies (Fernandez-Funez, 2016).

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Niccoli, T., Cabecinha, M., Tillmann, A., Kerr, F., Wong, C.T., Cardenes, D., Vincent, A.J., Bettedi, L., Li, L., Grönke, S. Dols, J. and Partridge, L. (2016). Increased glucose transport into neurons rescues Aβ toxicity in Drosophila. Curr Biol [Epub ahead of print]. PubMed ID: 27524482

Abstract
Glucose hypometabolism is a prominent feature of the brains of patients with Alzheimer's disease (AD). Disease progression is associated with a reduction in glucose transporters in both neurons and endothelial cells of the blood-brain barrier. However, whether increasing glucose transport into either of these cell types offers therapeutic potential remains unknown. Using an adult-onset Drosophila model of Aβ (amyloid beta) toxicity, this study shows that genetic overexpression of a glucose transporter specifically in neurons, rescues lifespan, behavioral phenotypes, and neuronal morphology. This amelioration of Aβ toxicity is associated with a reduction in the protein levels of the unfolded protein response (UPR) negative master regulator Grp78 and an increase in the UPR. Genetic downregulation of Grp78 activity also protects against Aβ toxicity, confirming a causal effect of its alteration on AD-related pathology. Metformin, a drug that stimulates glucose uptake in cells, mimics these effects, with a concomitant reduction in Grp78 levels and rescue of the shortened lifespan and climbing defects of Aβ-expressing flies. These findings demonstrate a protective effect of increased neuronal uptake of glucose against Aβ toxicity and highlight Grp78 as a novel therapeutic target for the treatment of AD (Niccoli, 2016).

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Ray, A., Speese, S. D. and Logan, M. A. (2017) (2017). Glial Draper rescues Abeta toxicity in a Drosophila model of Alzheimer's Disease. J Neurosci 37(49):11881-11893. PubMed ID: 29109235

Abstract

Pathological hallmarks of Alzheimer's disease (AD) include amyloid-beta (Abeta) plaques, neurofibrillary tangles, and reactive gliosis. Glial cells offer protection against AD by engulfing extracellular Abeta peptides, but the repertoire of molecules required for glial recognition and destruction of Abeta are still unclear. This study shows that the highly conserved glial engulfment receptor Draper/MEGF10 provides neuroprotection in an AD model of Drosophila (both sexes). Neuronal expression of human Abeta42arc in adult flies results in robust Abeta accumulation, neurodegeneration, locomotor dysfunction, and reduced lifespan. Notably, all of these phenotypes are more severe in draper mutant animals, while enhanced expression of glial Draper reverses Abeta accumulation, as well as behavioral phenotypes. Stat92E, c-Jun N-terminal Kinase (JNK)/AP-1 signaling, and expression of matrix metalloproteinase-1 (Mmp1) are activated downstream of Draper in glia in response to Abeta42arc exposure. Furthermore, Abeta42-induced upregulation of the phagolysosomal markers Atg8 and p62 was notably reduced in draper mutant flies. Based on these findings, it is proposed that glia clear neurotoxic Abeta peptides in the AD model Drosophila brain through a Draper/STAT92E/JNK cascade that may be coupled to protein degradation pathways such as autophagy or more traditional phagolysosomal destruction methods (Ray, 2017).

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Vivien Chiu, W. Y., Koon, A. C., Ki Ngo, J. C., Edwin Chan, H. Y. and Lau, K. F. (2017) (2017). GULP1/CED-6 ameliorates amyloid-beta toxicity in a Drosophila model of Alzheimer's disease. Oncotarget 8(59): 99274-99283. PubMed ID: 29245900

Abstract

Amyloidogenic processing of APP (see Drosophila Appl) by beta- and gamma-secretases leads to the generation of amyloid-beta peptide (Abeta), and the accumulation of Abeta in senile plaques is a hallmark of Alzheimer's disease (AD). Understanding the mechanisms of APP processing is therefore paramount. Increasing evidence suggests that APP intracellular domain (AICD) interacting proteins influence APP processing. This study characterized the overexpression of AICD interactor GULP1 in a Drosophila AD model expressing human BACE and APP695. Transgenic GULP1 significantly lowered the levels of both Abeta1-40 and Abeta1-42 without decreasing the BACE and APP695 levels. Overexpression of GULP1 also reduced APP/BACE-mediated retinal degeneration, rescued motor dysfunction and extended longevity of the flies. These results indicate that GULP1 regulate APP processing and reduce neurotoxicity in a Drosophila AD model (Vivien Chiu, 2017).

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Oka, M., Fujisaki, N., Maruko-Otake, A., Ohtake, Y., Shimizu, S., Saito, T., Hisanaga, S. I., Iijima, K. M. and Ando, K. (2017). Ca2+/calmodulin-dependent protein kinase II promotes neurodegeneration caused by tau phosphorylated at Ser262/356 in a transgenic Drosophila model of tauopathy. J Biochem 162(5): 335-342. PubMed ID: 28992057

Abstract

Abnormal deposition of the microtubule-associated protein tau is a common pathological feature of multiple neurodegenerative diseases, including Alzheimer's disease (AD), and plays critical roles in their pathogenesis. Disruption of calcium homeostasis and the downstream kinase Ca2+/calmodulin-dependent protein kinase II (CaMKII) coincides with pathological phosphorylation of tau in AD brains. However, it remains unclear whether and how dysregulation of CaMKII affects tau toxicity. Using a Drosophila model, it was found that CaMKII promotes neurodegeneration caused by tau phosphorylated at the AD-associated sites Ser262/356. Overexpression of CaMKII promoted, while RNA-mediated knockdown of CaMKII and inhibition of CaMKII activity by expression of an inhibitory peptide suppressed, tau-mediated neurodegeneration. Blocking tau phosphorylation at Ser262/356 by alanine substitutions suppressed promotion of tau toxicity by CaMKII, suggesting that tau phosphorylation at these sites is required for this phenomenon. However, neither knockdown nor overexpression of CaMKII affected tau phosphorylation levels at Ser262/356, suggesting that CaMKII is not directly involved in tau phosphorylation at Ser262/356 in this model. These results suggest that a pathological cascade of events, including elevated levels of tau phosphorylated at Ser262/356 and aberrant activation of CaMKII, work in concert to promote tau-mediated neurodegeneration (OKA, 2017).

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Moore, B. D., Martin, J., de Mena, L., Sanchez, J., Cruz, P. E., Ceballos-Diaz, C., Ladd, T. B., Ran, Y., Levites, Y., Kukar, T. L., Kurian, J. J., McKenna, R., Koo, E. H., Borchelt, D. R., Janus, C., Rincon-Limas, D., Fernandez-Funez, P. and Golde, T. E. (2018). Short Abeta peptides attenuate Abeta42 toxicity in vivo. J Exp Med 215(1): 283-301. PubMed ID: 29208777

Abstract

Processing of amyloid-beta (Abeta) precursor protein (APP) by gamma-secretase produces multiple species of Abeta: Abeta40, short Abeta peptides (Abeta37-39), and longer Abeta peptides (Abeta42-43). gamma-Secretase modulators, a class of Alzheimer's disease therapeutics, reduce production of the pathogenic Abeta42 but increase the relative abundance of short Abeta peptides. To evaluate the pathological relevance of these peptides, this study expressed Abeta36-40 and Abeta42-43 in Drosophila melanogaster to evaluate inherent toxicity and potential modulatory effects on Abeta42 toxicity. In contrast to Abeta42, the short Abeta peptides were not toxic and, when coexpressed with Abeta42, were protective in a dose-dependent fashion. In parallel, the effects were explored of recombinant adeno-associated virus-mediated expression of Abeta38 and Abeta40 in mice. When expressed in nontransgenic mice at levels sufficient to drive Abeta42 deposition, Abeta38 and Abeta40 did not deposit or cause behavioral alterations. These studies indicate that treatments that lower Abeta42 by raising the levels of short Abeta peptides could attenuate the toxic effects of Abeta42 (Moore, 2018).

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Lee, B. I., Suh, Y. S., Chung, Y. J., Yu, K. and Park, C. B. (2017). Shedding light on Alzheimer's beta-Amyloidosis: Photosensitized methylene blue inhibits self-assembly of beta-amyloid peptides and disintegrates their aggregates. Sci Rep 7(1): 7523. PubMed ID: 28790398

Abstract

Abnormal aggregation of beta-amyloid (Abeta) peptides is a major hallmark of Alzheimer's disease (AD). In spite of numerous attempts to prevent the beta-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. In this study, with the aid of multiple photochemical analyses, it is reported that photoexcited MB molecules can block Abeta42 aggregation in vitro. Furthermore, an in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to Abeta42 oxidation by singlet oxygen (1O2) generated from photoexcited MB. Finally, it was shown that photoexcited MB possess a capability to disaggregate the pre-existing Abeta42 aggregates and reduce Abeta-induced cytotoxicity. This work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future (Lee, 2017). Yang, C. N., Wu, M. F., Liu, C. C., Jung, W. H., Chang, Y. C., Lee, W. P., Shiao, Y. J., Wu, C. L., Liou, H. H., Lin, S. K. and Chan, C. C. (2017). Differential protective effects of connective tissue growth factor against Abeta neurotoxicity on neurons and glia. Hum Mol Genet 26(20): 3909-3921. PubMed ID: 29016849

Abstract

Impaired clearance of amyloid-beta peptide (Abeta; see Drosophila Appl) leads to abnormal extracellular accumulation of this neurotoxic protein that drives neurodegeneration in sporadic Alzheimer's disease (AD). Connective tissue growth factor (CTGF/CCN2) expression is elevated in plaque-surrounding astrocytes in AD patients. However, the role of CTGF in AD pathogenesis remains unclear. This study characterized the neuroprotective activity of CTGF. CTGF facilitates Abeta uptake and subsequent degradation within primary glia and neuroblastoma cells. CTGF enhanced extracellular Abeta degradation via membrane-bound matrix metalloproteinase-14 (MMP14) in glia and extracellular MMP13 in neurons. In the brain of a Drosophila AD model, glial-expression of CTGF reduced Abeta deposits, improved locomotor function, and rescued memory deficits. Neuroprotective potential of CTGF against Abeta42-induced photoreceptor degeneration was disrupted through silencing MMPs. Therefore, CTGF may represent a node for potential AD therapeutics as it intervenes in glia-neuron communication via specific MMPs to alleviate Abeta neurotoxicity in the central nervous system.

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Ando, K., Maruko-Otake, A., Ohtake, Y., Hayashishita, M., Sekiya, M. and Iijima, K. M. (2016). Stabilization of microtubule-unbound Tau via Tau phosphorylation at Ser262/356 by Par-1/MARK contributes to augmentation of AD-related phosphorylation and Aβ42-induced Tau toxicity. PLoS Genet 12: e1005917. PubMed ID: 27023670

Abstract
To prevent the cascade of events leading to neurodegeneration in Alzheimer's disease (AD), it is essential to elucidate the mechanisms underlying the initial events of tau mismetabolism. In this study, using transgenic Drosophila co-expressing human tau and Aβ, tau phosphorylation at AD-related Ser262/356 stabilized microtubule-unbound tau was found in the early phase of tau mismetabolism, leading to neurodegeneration. Aβ increased the level of tau detached from microtubules, independent of the phosphorylation status at GSK3-targeted SP/TP sites. Such mislocalized tau proteins, especially the less phosphorylated species, were stabilized by phosphorylation at Ser262/356 via PAR-1/MARK. Levels of Ser262 phosphorylation were increased by Aβ42, and blocking this stabilization of tau suppressed Aβ42-mediated augmentation of tau toxicity and an increase in the levels of tau phosphorylation at the SP/TP site Thr231, suggesting that this process may be involved in AD pathogenesis. In contrast to PAR-1/MARK, blocking tau phosphorylation at SP/TP sites by knockdown of Sgg/GSK3 did not reduce tau levels, suppress tau mislocalization to the cytosol, or diminish Aβ-mediated augmentation of tau toxicity. These results suggest that stabilization of microtubule-unbound tau by phosphorylation at Ser262/356 via the PAR-1/MARK may act in the initial steps of tau mismetabolism in AD pathogenesis, and that such tau species may represent a potential therapeutic target for AD (Ando, 2016).

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Gerstner, J. R., Lenz, O., Vanderheyden, W. M., Chan, M. T., Pfeiffenberger, C. and Pack, A. I. (2016). Amyloid-β induces sleep fragmentation that is rescued by fatty acid binding proteins in Drosophila. J Neurosci Res [Epub ahead of print]. PubMed ID: 27320125

Abstract
Disruption of sleep/wake activity in Alzheimer's disease (AD) patients significantly affects their quality of life and that of their caretakers and is a major contributing factor for institutionalization. Levels of amyloid-β (Aβ; see Drosophila Appl) have been shown to be regulated by neuronal activity and to correlate with the sleep/wake cycle. Whether consolidated sleep can be disrupted by Aβ alone is not well understood. It was hypothesized that Aβ42 can increase wakefulness and disrupt consolidated sleep. This study shows that flies expressing the human Aβ42 transgene in neurons have significantly reduced consolidated sleep compared with control flies. Fatty acid binding proteins (Fabp) are small hydrophobic ligand carriers that have been clinically implicated in AD. Aβ42 flies that carry a transgene of either the Drosophila Fabp or the mammalian brain-type Fabp show a significant increase in nighttime sleep and long consolidated sleep bouts, rescuing the Aβ42-induced sleep disruption. These studies suggest that alterations in Fabp levels and/or activity may be associated with sleep disturbances in AD. Future work to determine the molecular mechanisms that contribute to Fabp-mediated rescue of Aβ42-induced sleep loss will be important for the development of therapeutics in the treatment of AD (Gerstner, 2016).

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Burnouf, S., Grönke, S., Augustin, H., Dols, J., Gorsky, M.K., Werner, J., Kerr, F., Alic, N., Martinez, P. and Partridge, L. (2016). Deletion of endogenous Tau proteins is not detrimental in Drosophila. Sci Rep 6: 23102. PubMed ID: 26976084

Abstract
Human Tau (hTau) is a highly soluble and natively unfolded protein that binds to microtubules within neurons. Its dysfunction and aggregation into insoluble paired helical filaments is involved in the pathogenesis of Alzheimer's disease (AD), constituting, together with accumulated β-amyloid (Aβ) peptides, a hallmark of the disease. Deciphering both the loss-of-function and toxic gain-of-function of hTau proteins is crucial to further understand the mechanisms leading to neurodegeneration in AD. As the fruit fly Drosophila melanogaster expresses Tau proteins (dTau) that are homologous to hTau, this study aimed to better comprehend dTau functions by generating a specific tau knock-out (KO) fly line using homologous recombination. It was observed that the specific removal of endogenous dTau proteins does not lead to overt, macroscopic phenotypes in flies. Indeed, survival, climbing ability and neuronal function are unchanged in tau KO flies. In addition, any overt positive or negative effect of dTau removal on human Aβ-induced toxicity were not found. Altogether, these results indicate that the absence of dTau proteins has no major functional impact on flies, and suggest that the tau KO strain is a relevant model to further investigate the role of dTau proteins in vivo, thereby giving additional insights into hTau functions (Burnouf, 2016).

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Farca Luna, A. J., Perier, M. and Seugnet, L. (2017). Amyloid precursor protein in Drosophila glia regulates sleep and genes Involved in glutamate recycling. J Neurosci 37(16): 4289-4300. PubMed ID: 28314820

Abstract

Amyloid precursor protein (App) plays a crucial role in Alzheimer's disease via the production and deposition of toxic β-amyloid peptides. App is heavily expressed in neurons, the focus of the vast majority of studies investigating its function. Meanwhile, almost nothing is known about App's function in glia, where it is also expressed, and can potentially participate in the regulation of neuronal physiology. This report investigated whether Appl, the Drosophila homolog of App, could influence sleep-wake regulation when its function is manipulated in glial cells. Appl inhibition in astrocyte-like and cortex glia resulted in higher sleep amounts and longer sleep bout duration during the night, while overexpression had the opposite effect. These sleep phenotypes were not the result of developmental defects, and were correlated with changes in expression in glutamine synthetase (GS) in astrocyte-like glia and in changes in the gap-junction component innexin2 in cortex glia. Downregulating both GS and innexin2, but not either one individually, resulted in higher sleep amounts, similarly to Appl inhibition. Consistent with these results, the expression of GS and innexin2 are increased following sleep deprivation, indicating that GS and innexin2 genes are dynamically linked to vigilance states. Interestingly, the reduction of GS expression and the sleep phenotype observed upon Appl inhibition could be rescued by increasing the expression of the glutamate transporter dEaat1. In contrast, reducing dEaat1 expression severely disrupted sleep. These results associate glutamate recycling, sleep, and a glial function for the App family proteins (Farca Luna, 2017).

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Gorsky, M. K., Burnouf, S., Dols, J., Mandelkow, E. and Partridge, L. (2016). Acetylation mimic of lysine 280 exacerbates human Tau neurotoxicity in vivo. Sci Rep 6: 22685. PubMed ID: 26940749

Abstract
Dysfunction and accumulation of the microtubule-associated human Tau (hTau) protein into intraneuronal aggregates is observed in many neurodegenerative disorders including Alzheimer's disease (AD). Reversible lysine acetylation has recently emerged as a post-translational modification that may play an important role in the modulation of hTau pathology. Acetylated hTau species have been observed within hTau aggregates in human AD brains and multi-acetylation of hTau in vitro regulates its propensity to aggregate. However, whether lysine acetylation at position 280 (K280) modulates hTau-induced toxicity in vivo is unknown. This study generated new Drosophila transgenic models of hTau pathology to evaluate the contribution of K280 acetylation to hTau toxicity, by analysing the respective toxicity of pseudo-acetylated (K280Q) and pseudo-de-acetylated (K280R) mutant forms of hTau. It was observed that mis-expression of pseudo-acetylated K280Q-hTau in the adult fly nervous system potently exacerbated fly locomotion defects and photoreceptor neurodegeneration. In addition, modulation of K280 influenced total hTau levels and phosphorylation without changing hTau solubility. Altogether, these results indicate that pseudo-acetylation of the single K280 residue is sufficient to exacerbate hTau neurotoxicity in vivo, suggesting that acetylated K280-hTau species contribute to the pathological events leading to neurodegeneration in AD (Gorsky, 2016).

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Sofola-Adesakin, O., Khericha, M., Snoeren, I., Tsuda, L. and Partridge, L. (2016). pGluAβ increases accumulation of Aβ in vivo and exacerbates its toxicity. Acta Neuropathol Commun 4: 109. PubMed ID: 27717375

Abstract
Several species of β-amyloid peptides (&Abeta;; see Drosophila Appl) exist as a result of differential cleavage from amyloid precursor protein (APP) to yield various C-terminal Aβ peptides. Several N-terminal modified Aβ peptides have also been identified in Alzheimer's disease (AD) brains, the most common of which is pyroglutamate-modified Aβ (AβpE3-42). AβpE3-42 peptide has an increased propensity to aggregate, appears to accumulate in the brain before the appearance of clinical symptoms of AD, and precedes Aβ1-42 deposition. Moreover, in vitro studies have shown that AβpE3-42 can act as a seed for full length Aβ1-42. This study characterized the Drosophila model of AβpE3-42 toxicity by expressing the peptide in specific sets of neurons using the GAL4-UAS system, and measuring different phenotypic outcomes. AβpE3-42 peptide was found to have an increased propensity to aggregate. Expression of AβpE3-42 in the neurons of adult flies led to behavioural dysfunction and shortened lifespan. Expression of AβpE3-42 constitutively in the eyes led to disorganised ommatidia, and activation of the c-Jun N-terminal kinase (JNK) signaling pathway. The eye disruption was almost completely rescued by co-expressing a candidate Aβ degrading enzyme, neprilysin2 (see Neprilysin 4). Furthermore, neprilysin2 was capable of degrading AβpE3-42. Also, the seeding hypothesis was tested for AβpE3-42 in vivo, and its effect on Aβ1-42 levels were measured. Aβ1-42 levels were significantly increased when Aβ1-42 and AβpE3-42 peptides were co-expressed. Furthermore, AβpE3-42 enhanced Aβ1-42 toxicity in vivo. These findings implicate AβpE3-42 as an important source of toxicity in AD, and suggest that its specific degradation could be therapeutic (Sofola-Adesakin, 2016).

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Xu, W., et al. (2016). Amyloid precursor protein-mediated endocytic pathway disruption induces axonal dysfunction and neurodegeneration. J Clin Invest [Epub ahead of print]. PubMed ID: 27064279

Abstract
The endosome/lysosome pathway is disrupted early in the course of both Alzheimer's disease (AD) and Down syndrome (DS); however, it is not clear how dysfunction in this pathway influences the development of these diseases. This study explored the cellular and molecular mechanisms by which endosomal dysfunction contributes to the pathogenesis of AD and DS. It was determined that full-length amyloid precursor protein (APP; see Drosophila Appl) and its β-C-terminal fragment (β-CTF) act though increased activation of Rab5 to cause enlargement of early endosomes and to disrupt retrograde axonal trafficking of nerve growth factor (NGF) signals. The functional impacts of APP and its various products were investigated in PC12 cells, cultured rat basal forebrain cholinergic neurons (BFCNs), and BFCNs from a mouse model of DS. The full-length wild-type APP (APPWT) and β-CTF both induced endosomal enlargement and disrupted NGF signaling and axonal trafficking. β-CTF alone induced atrophy of BFCNs that was rescued by the dominant-negative Rab5 mutant, Rab5S34N. Moreover, expression of a dominant-negative Rab5 construct markedly reduced APP-induced axonal blockage in Drosophila. Therefore, increased APP and/or β-CTF impact the endocytic pathway to disrupt NGF trafficking and signaling, resulting in trophic deficits in BFCNs. These data strongly support the emerging concept that dysregulation of Rab5 activity contributes importantly to early pathogenesis of AD and DS (Xu, 2016).

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Ping, Y., Hahm, E.T., Waro, G., Song, Q., Vo-Ba, D.A., Licursi, A., Bao, H., Ganoe, L., Finch, K. and Tsunoda, S. (2015). Linking Aβ42-induced hyperexcitability to neurodegeneration, learning and motor deficits, and a shorter lifespan in an Alzheimer's model. PLoS Genet 11: e1005025. PubMed ID: 25774758

Abstract
Alzheimer’s disease (AD) is the most prevalent form of dementia in the elderly. β-amyloid (Aβ) accumulation in the brain is thought to be a primary event leading to eventual cognitive and motor dysfunction in AD. Aβ has been shown to promote neuronal hyperactivity, which is consistent with enhanced seizure activity in mouse models and AD patients. Little, however, is known about whether, and how, increased excitability contributes to downstream pathologies of AD. This study shows that overexpression of human Aβ42 in a Drosophila model indeed induces increased neuronal activity. It was found that the underlying mechanism involves the selective degradation of the A-type K+ channel, Kv4. An age-dependent loss of Kv4 leads to an increased probability of AP firing. Interestingly, it was found that loss of Kv4 alone results in learning and locomotion defects, as well as a shortened lifespan. To test whether the Aβ42-induced increase in neuronal excitability contributes to, or exacerbates, downstream pathologies, Kv4 was transgenically over-expressed to near wild-type levels in Aβ42-expressing animals. It was shown that restoration of Kv4 attenuates age-dependent learning and locomotor deficits, slows the onset of neurodegeneration, and partially rescues premature death seen in Aβ42-expressing animals. The study concludes that Aβ42-induced hyperactivity plays a critical role in the age-dependent cognitive and motor decline of this Aβ42-Drosophila model, and possibly in AD (Ping, 2015).

Highlights

  • Aβ42 induces increased neuronal excitability in 9 days old neuronal cultures.
  • Kv4 current is selectively decreased by Aβ42 expression.
  • Aβ42 induces degradation of Kv4 protein via a pathway dependent on both the proteasome and lysosome.
  • Transgenic restoration of Kv4 rescues hyperexcitability.
  • Loss of Kv4 plays a critical role in Aβ42-induced learning defects.
  • Restoration of Kv4 levels rescues locomotor defects.
  • Restoration of Kv4 slows Aβ42-induced neurodegeneration.
  • Aβ42-induced loss of Kv4 contributes to premature death.

Discussion
Aβ-induced hyperexcitability is indeed intriguing, with interesting implications especially for seizure-like activity and epilepsy, which are potentially associated with AD. Little, however, has been done previously to determine whether Aβ-induced hyperactivity contributes to downstream behavioral pathologies. Recent studies, however, suggest that neuronal hyperactivity may precede neurological dysfunction and may be improved by pharmacologically reducing activity. This study shows that Kv4 channels are specifically down-regulated by Aβ42 expression, while other K+ currents (eg. Kv2 and Kv3) remain unaltered in cultured neurons and in the intact brain. The resulting increase in neuronal excitability is present in the adult brain at an age (8 days AE) before the appearance of locomotor (14–15 days AE) and learning defects (14 days AE), and before the onset of neurodegeneration (25 days AE), supporting the hypothesis that hyperactivity precedes and contributes to these downstream pathologies. The study also shows that increasing Kv4 channel levels in Aβ-expressing animals restores normal excitability to neurons, and as a result, completely rescues learning and locomotor defects, slows neurodegeneration, and slightly increases lifespan. It is significant to note that the expression of a UAS-GFP or UAS-CD8-GFP transgene does not rescue any of these pathologies, suggesting that any rescue effects by UAS-Kv4 are not simply due to the introduction of another UAS target for GAL4 that would dilute the expression of Aβ42; indeed, quantification of Aβ42 is not any lower in Aβ42+Kv4 flies. In future studies, it will be interesting to examine the temporal requirement for reducing excitability with Kv4 expression; for example, is early hyperexcitability more “toxic” to the system than later stage hyperexcitability? (Ping, 2015).

Although specificity of rescue by Kv4 varies from one pathology to another, the genetically engineered EKO channel that acts as a general activity inhibitor does not ameliorate any of the cognitive, motor, or survival deficits tested. This suggests that general dampening of excitability is not sufficient to replace Kv4 loss. Kv1, the other A-type K+ channel present in Drosophila, however, is able to rescue Aβ42-induced locomotor dysfunction, but, interestingly, not learning or premature death. These results are consistent with the fact that Kv1 and Kv4 share some, but certainly not all, biophysical properties. For example, Kv4 channels have a much more hyperpolarized voltage-operating range than Kv1 channels, making them much more likely to play roles at subthreshold potentials. Also, while both Kv4 and Kv1 channels display fast inactivation, the inactivation rate is voltage-independent for Kv4 channels and voltage-dependent for Kv1 channels. Finally, the subcellular localization of Kv4 and Kv1 channels are thought to be distinct, with Kv4 channels restricted to dendrites and cell bodies, and Kv1 channels localized in axons and nerve terminals (Ping, 2015).

The study also examined whether the loss of Kv4 function alone is sufficient to lead to cognitive and motor pathologies. Previously, it was shown that expression of a dominant-negative Kv4 subunit, DNKv4, results in the elimination of the Kv4 current. Loss of Kv4 function leads to increased excitability and locomotor deficits. In the present study, it was found that expression of DNKv4 also induces learning defects and a shortened lifespan, consistent with a key role for the Aβ42-induced reduction in Kv4 in these downstream pathologies. In mammalian systems, Kv4.2 has been shown to play a role in the induction of long-term potentiation (LTP), and hippocampal dependent learning/memory defects. Loss of Kv4 function alone, however, does not induce any significant neurodegeneration, suggesting that while Aβ42-induced loss of Kv4 exacerbates degeneration, it is not sufficient to trigger neurodegenerative pathway(s) (Ping, 2015).

Previous reports over the years have shown different effects of Aβ on A-type K+ currents in vitro, with some identifying decreases in A-type K+ currents (IA) and others reporting increases in IA. Differences between these studies are likely to be due to a variety of factors including the species of Aβ tested (eg. Aβ1–40, Aβ1–42, Aβ25–35; some studies finding clear differences with different Aβ species, the cell type examined (eg. HEK cells, hippocampal neurons, or cortical neurons), and the time course of the effect (eg. from seconds to days in different studies). For example, the Aβ species applied, the concentration used, and time incubated with cells all affect the assembly state of Aβ, which has also been proposed to have differential downstream effects on K+ currents and excitability/activity. In the future, it will be interesting to see how effects on Kv4 develop, and possibly change, throughout the assembly of Aβ42 from monomers to oligomers, protofibrils, and mature fibrils in vivo (Ping, 2015).

Much remains to be understood about the mechanism by which Kv4 channels are lost in response to Aβ42 expression. In this study, pharmacological and genetic approaches suggest a degradation pathway for Kv4 that depends on both the proteasome and lysosome, similar to the EGF receptor. This scenario is likely to be complicated since previous studies have shown that Aβ directly inhibits the proteasome, and that clearance of Aβ depends on the proteasome. Further study is needed to understand how Kv4 channels are targeted for turnover by Aβ42, and what other component(s) are involved (Ping, 2015).

Further study is also needed to unravel specific mechanisms by which Kv4 channels function in downstream Aβ42 pathologies. For example, how does the loss of Kv4 exacerbate neurodegeneration? One possibility is that cell death is induced by an “excitotoxic” pathway due to excess Ca2+ entry, and ultimately, necrosis. Interestingly, previous studies have shown that Aβ42 induces an increase in various K+ currents that are linked to cell death in vitro, consistent with evidence that efflux of K+ is required as an early step in apoptosis. While it is not clear how to reconcile these findings with ours, it does seem that proper K+ homeostasis is critical for neuronal survival. The role of Kv4 in lifespan, however, is complex, given that neurodegeneration, learning/memory formation, and locomotor activity all contribute to survival. The partial to full rescue of multiple Aβ42-induced pathologies by Kv4, however, underscores the importance of the loss of Kv4 in vivo and suggests that Kv4 is a critical target of Aβ42 in this model, and perhaps in AD (Ping, 2015).

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Blake, M.R., Holbrook, S.D., Kotwica-Rolinska, J., Chow, E.S., Kretzschmar, D. and Giebultowicz, J.M. (2015). Manipulations of amyloid precursor protein cleavage disrupt the circadian clock in aging Drosophila. Neurobiol Dis 77: 117-126. PubMed ID: 25766673

Abstract
Alzheimer's disease (AD) is a neurodegenerative disease characterized by severe cognitive deterioration. While causes of AD pathology are debated, a large body of evidence suggests that increased cleavage of Amyloid Precursor Protein (APP) producing the neurotoxic Amyloid-β (Aβ) peptide plays a fundamental role in AD pathogenesis. One of the detrimental behavioral symptoms commonly associated with AD is the fragmentation of sleep-activity cycles with increased nighttime activity and daytime naps in humans. Sleep-activity cycles, as well as physiological and cellular rhythms, which may be important for neuronal homeostasis, are generated by a molecular system known as the circadian clock. Links between AD and the circadian system are increasingly evident but not well understood. This study examined whether genetic manipulations of APP-like (APPL) protein cleavage in Drosophila melanogaster affect rest-activity rhythms and core circadian clock function in this model organism. It was shown that the increased β-cleavage of endogenous APPL by the β-secretase (dBACE) severely disrupts circadian behavior and leads to reduced expression of clock protein PER in central clock neurons of aging flies. The study's data suggest that behavioral rhythm disruption is not a product of APPL-derived Aβ production but rather may be caused by a mechanism common to both α and β-cleavage pathways. Specifically, it was shown that increased production of the endogenous Drosophila Amyloid Intracellular Domain (dAICD) causes disruption of circadian rest-activity rhythms, while flies overexpressing endogenous APPL maintain stronger circadian rhythms during aging. In summary, this study offers a novel entry point toward understanding the mechanism of circadian rhythm disruption in Alzheimer's disease (Blake, 2015).

Highlights

  • Over-expression of dBACE in clock cells accelerates aging phenotypes and disrupts rest-activity rhythms.
  • Neuronal over-expression of dBACE disrupts behavioral rest-activity rhythms.
  • Over-expression of dBACE dampens the cycling of PER in central pacemaker neurons.
  • Rest-activity rhythms are disrupted by KUZ over-expression.
  • Expression of AICD disrupts rest-activity rhythms.
  • dAICD is capable of entering the nucleus, but is not toxic to central pacemaker neurons.

Discussion
Loss of rest-activity rhythms is a well-established early symptom of AD in humans. Because disruption of circadian rhythms is detrimental to neuronal homeostasis, it is important to understand relationships between AD and circadian rhythms at the cellular and molecular levels. To address this question, this study examined how manipulations of the fly ortholog of APP and its cleaving enzymes affect endogenous rest-activity rhythms and clock mechanism in Drosophila. Over-expression of dBACE was found to disrupt behavioral rest-activity rhythms, and this effect is most severe in aged flies suggesting an age-dependent mechanism. Furthermore, dBACE expression resulted in dampened oscillation of the core clock protein PER in central pacemaker neurons, which are master regulators of rest activity rhythms. Significantly reduced PER levels are observed in the sLNv and lLNv neurons of age 50d flies expressing dBACE in all clock cells (including glia), all neurons, or only in PDF-positive sLNv and lLNv neurons. These data suggest that manipulation of APP-cleavage by dBACE over-expression directly affects the oscillation of PER protein in central pacemaker neurons in a cell-autonomous manner. Since a functional clock mechanism in sLNv is necessary and sufficient to maintain free running activity rhythms, reduced oscillations of PER in these neurons could be responsible for the loss of activity rhythms in age 50d flies. Importantly, the decline in PER levels occurrs only in flies with manipulated dBACE, not in old control flies. This is in agreement with earlier findings that aging does not dampen PER oscillations in pacemaker neurons of wild type flies, while it reduces clock oscillations in peripheral clocks  (Blake, 2015).

While this study reports that the loss of behavioral rhythms after manipulation of dBACE is associated with reduced expression of clock genes in the central pacemaker, other recent work shows that expression of human Aβ peptides leads to disruption of rest activity rhythms without interfering with PER oscillations in the central pacemaker. Even strongly neurotoxic Aβ peptides, such as Aβ42 arctic, do not cause rhythm disruption when expressed in central pacemaker neurons; rather, pan-neuronal expression is required. The fact that even the most neurotoxic Aβ peptides are not capable of dampening PER oscillation in pacemaker neurons suggests that Aβ production does not affect clock oscillations and that it is not Aβ production that causes the phenotype observed in this study upon over-expression of dBACE. This was confirmed by expression of KUZ, whose activity does not increase dAβ production; however, it also leads to disruption of rest-activity rhythms. Similar rhythm disruption by dBACE and KUZ suggests that an excess cleavage product of both pathways might be responsible for the disruption. Like in the mammalian APP cleavage pathway, in Drosophila cleavage of APPL by KUZ or dBACE results in a C-terminal fragment (CTF) that is subsequently cleaved by the ϒ-secretase resulting in the production of dAICD. Indeed, it was shown that expression of dAICD results in an age-dependent decline in rhythmic locomotor activity. As with dBACE and KUZ expression, dAICD expression causes weakening or complete loss of behavioral rhythms while age-matched control flies remain highly rhythmic. In this context, it is worth noting that α-secretase activators are considered for clinical trials to reduce Aβ production in AD patients. However, according to results in this study, this could lead to disruptions of circadian rhythms and sleep patterns thus negatively impacting the lives of patients and their caretakers (Blake, 2015).

This study's data suggest that increased dAICD may be the proximal cause of decay in rest-activity rhythms. The role of AICD in AD is increasingly evident but poorly understood. AICD is able to enter the nucleus and has been implicated in transcriptional regulation that may affect cell death, neurite outgrowth and neuronal excitability. Interestingly, transgenic mice expressing AICD have increased activity of GSK-3, which in flies affects the circadian clock. Over-expression of GSK-3 in Drosophila leads to altered circadian behavior by hyper-phosphorylation of TIMELESS (TIM), a key circadian protein which forms dimers with PER that enter the nucleus and regulate the clock mechanism. Of further interest, increased GSK-3 activity has been implicated in AD, and in Drosophila, increased GSK-3 activity mediates the toxicity of Aβ peptides (Blake, 2015).

Cleavage of APPL likely results in a significant decline in intact APPL, and this could be detrimental as APPL has neuroprotective effects. It was also recently shown that loss of full-length APPL induces cognitive deficits in memory. This study reports that flies over-expressing full-length APPL in central pacemaker neurons maintain stronger behavioral rest-activity rhythms during aging than control flies; however this effect is not observed when APPL is expressed pan-neuronally. This could be caused by negative effects of APPL when expressed in other unspecified neurons, or could be related to driver strength. Overall, the study suggests that the loss of full-length APPL might negatively affect circadian behavior by way of the central pacemaker neurons (Blake, 2015).

Over-expression of dAICD induces a severe phenotype, disrupting rest-activity rhythms as early as age 5d when expressed in central pacemaker neurons and by age 35d with pan-neuronal expression. Taken together these results suggest that while loss of full-length APPL by over-expression of its secretases might negatively impact circadian behavior, the cleavage product dAICD induces the most severe behavioral rest-activity disruption. Interestingly, the observed effect is not likely a product of neurodegeneration as it was previously shown that dAICD has no effect on neurodegeneration, and this study shows that the pacemaker cells appear intact in pdf > dAICD flies. In addition, it was shown that dAICD, like the vertebrate AICD, can be found in the nucleus. Therefore, this study suggests that dAICD may directly or indirectly affect the expression of clock genes. This offers a novel entry point toward understanding the mechanism of circadian rhythm disruption in Alzheimer's disease (Blake, 2015).

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Long, D.M., Blake, M.R., Dutta, S., Holbrook, S.D., Kotwica-Rolinska, J., Kretzschmar, D. and Giebultowicz, J.M. (2014). Relationships between the circadian system and Alzheimer's disease-like symptoms in Drosophila. PLoS One 9: e106068. PubMed ID: 25171136

Abstract
Circadian clocks coordinate physiological, neurological, and behavioral functions into circa 24 hour rhythms, and the molecular mechanisms underlying circadian clock oscillations are conserved from Drosophila to humans. Clock oscillations and clock-controlled rhythms are known to dampen during aging; additionally, genetic or environmental clock disruption leads to accelerated aging and increased susceptibility to age-related pathologies. Neurodegenerative diseases, such as Alzheimer's disease (AD), are associated with a decay of circadian rhythms, but it is not clear whether circadian disruption accelerates neuronal and motor decline associated with these diseases. To address this question, this study utilized transgenic Drosophila expressing various Amyloid-β (Aβ) peptides, which are prone to form aggregates characteristic of AD pathology in humans. The study compared development of AD-like symptoms in adult flies expressing Aβ peptides in the wild type background and in flies with clocks disrupted via a null mutation in the clock gene period (per01). No significant differences were observed in longevity, climbing ability and brain neurodegeneration levels between control and clock-deficient flies, suggesting that loss of clock function does not exacerbate pathogenicity caused by human-derived Aβ peptides in flies. However, AD-like pathologies were found to affect the circadian system in aging flies. It was found that rest/activity rhythms are impaired in an age-dependent manner. Flies expressing the highly pathogenic arctic Aβ peptide show a dramatic degradation of these rhythms in tune with their reduced longevity and impaired climbing ability. At the same time, the central pacemaker remains intact in these flies providing evidence that expression of Aβ peptides causes rhythm degradation downstream from the central clock mechanism (Long, 2014).

Highlights

  • Lifespan reduction caused by Aβ peptides is not exacerbated by the by loss of the clock gene period.
  • Flies expressing Aβ42arc show similar motor decline and neurodegeneration in clock-positive and clock-disrupted backgrounds.
  • Daily locomotor activity rhythms are impaired in aging flies expressing different Aβ peptides.
  • Rhythms in PER cycling continue in lateral neurons of elav>Aβ42arc flies.

Discussion
Associations between AD and impaired daily rhythms are well documented in humans, yet the causes and consequences of AD-related loss of circadian sleep/activity rhythms have not been teased apart. One of the unanswered questions is whether age-related decline of the circadian system contributes to AD progression. This study tested directly whether total arrhythmia caused by mutation in the core clock gene per would exacerbate AD-like phenotypes observed in an AD fly model. It was shown that premature death, progressive locomotor deficits, and vacuolization in the brain occurs with similar timing and intensity in flies with genetically disrupted clock mechanism as in control flies. Consistent with previous reports, the severity of symptoms is proportional to the pathogenicity of the expressed human Aβ fragments. However, within each genotype, symptoms in clock-deficient flies are similar to those in clock-competent flies. While this study's data show that disruption of the clock via removal of the core clock repressor PER does not exacerbate AD symptoms, it cannot be ruled out that disabling the positive clock arm could be more detrimental. A recent report showed that loss of the positive element BMAL1 causes brain neurodegeneration in mice. It was previously demonstrated that the loss of per accelerates death, locomotor impairments, and brain vacuolization in neurodegeneration-prone sniffer and swiss cheese fly mutants. However, the underlying molecular mechanism that mediates this effect is not known. The AD model used in this study is based on the expression of human Aβ peptides, which have been reported to accumulate into insoluble forms in aging flies. Because the disruption of the circadian clock does not affect the pathogenicity of these peptides, the study assumes that it has no effect on Aβ aggregation or clearance. In sum, this study's data show that the molecular and behavioral arrhythmia characteristic for per-null flies is not detrimental in this AD fly model (Long, 2014).

However, the study shows that associations between AD and altered behavioral rhythms, observed in humans and AD model mice, also extend to fly AD models. Pan-neuronal expression of Aβ42 causes age-dependent impairment of circadian rest/activity rhythms, such that a reduced fraction of 50-days old elav>Aβ42 flies remain rhythmic in constant darkness compared to controls. A more dramatic disruption of circadian rhythms is observed in elav>Aβ42arc. In LD, 5-day old flies of this genotype show bimodal activity with an attenuated morning activity peak, while no activity peaks are detected in 15-day old flies, rather they are active around the clock, including nighttime when control flies have prolonged rest. In another related study, a loss of locomotor activity rhythms in elav>Aβ42arc flies even at young age was shown, similar to findings in this study. Together, these results demonstrate that AD model flies have rest/activity rhythm degradation reminiscent of the behavioral degradation observed in humans with AD (Long, 2014).

Loss of rest/activity rhythms in elav>Aβ42arc flies formed the basis of investigation of the functional status of central pacemaker neurons, which are necessary and sufficient for the activity rhythms, at least in young flies. Immunocytochemistry of PDF-positive pacemaker neurons sLNv and lLNv shows the correct number and arborization pattern in elav>Aβ42arc flies. Moreover these neurons show nuclear peak and trough of the core clock protein PER indistinguishable from wild type flies. Similar observations have been published earlier, and it was additionally shown that even expression of the more pathogenic tandem Aβ42 construct leaves molecular oscillations in pacemaker neurons intact. Together, these data show dissociation between functioning molecular pacemaker and disrupted circadian coordination of rest/activity rhythms. This suggests that behavioral rhythm degradation observed in humans and mouse AD models may occur despite the presence of a functional central clock. Importantly, strong body temperature rhythms have been reported in AD patients again suggesting that the central clock may be intact in AD. This is reminiscent of the situation in very old flies and mammals, which show degradation of rest/activity rhythms while their central pacemaker neurons continue to show molecular oscillations (Long, 2014).

While AD-related degradation of behavioral rhythms is not caused by malfunction of the central clock, other contributing factors remain to be investigated. Aβ related arrhythmicity might be due to non-cell-autonomous toxicity as focused expression of toxic peptides in clock containing cells does not affect behavioral rhythmicity, but expression outside of the pacemaker neurons may affect their synaptic connections. Additionally, downstream neuronal or humoral output pathways leading from the central pacemaker network to the motor centers could be adversely affected by Aβ aggregates. For example, recent studies reporting a direct measurement of neuronal activity in elav>Aβ42arc flies reveal increased latency and decreased response stability of the pathways leading from the giant fiber system in the brain into motor neurons of the thoracic ganglia. It is possible that neuronal deficits of this kind can disable output pathways from the central clock leading to fragmented rather than consolidated sleep. This may lead to a vicious cycle as sleep deprivation increases amyloid peptides in mice and Aβ aggregation disrupts the sleep/wake cycle. As flies provide a powerful toolkit to study both AD and circadian rhythms, studies at the intersection of chronobiology and AD should help to provide insights into the mechanisms underlying AD-related pathologies (Long, 2014).

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Wang, X., Ma, Y., Zhao, Y., Chen, Y., Hu, Y., Chen, C., Shao, Y. and Xue, L. (2015). APLP1 promotes dFoxO-dependent cell death in Drosophila. Apoptosis 20: 778-786. PubMed ID: 25740230

Abstract
The amyloid precursor like protein-1 (APLP1), an engineered human gene into the Drosophila genome, belongs to the amyloid precursor protein family that also includes the Drosophila amyloid precursor protein (APP) and the amyloid precursor like protein-2 (APLP2). Though the three proteins share similar structures and undergo the same cleavage processing by α-, β- and γ-secretases, APLP1 shows divergent subcellular localization from that of APP and APLP2, and thus, may perform distinct roles in vivo. While extensive studies have been focused on APP, which is implicated in the pathogenesis of Alzheimer’s disease, the functions of APLP1 remain largely elusive. This study reports that the expression of APLP1 in Drosophila induces cell death and produces developmental defects in wing and thorax. This function of APLP1 depends on the transcription factor dFoxO, as the depletion of dFoxO abrogates APLP1-induced cell death and adult defects. Consistently, APLP1 up-regulates the transcription of dFoxO target hid and reaper-two well known pro-apoptotic genes. Thus, the present study provides the first in vivo evidence that APLP1 is able to induce cell death, and that FoxO is a crucial downstream mediator of APLP1’s activity (Wang, 2015).

Highlights

  • APLP1 induces caspase-dependent cell death in Drosophila.
  • APLP1 up-regulates dFoxO target gene expression.

Discussion
Amyloid pecursor like protein-1(APLP1) is a mammalian paralog of amyloid precursor protein (APP). While APP has been extensively studied for its involvement in the Alzheimer’s disease, few studies have been directed to APLP1 and its in vivo functions remain largely unknown. This study investigated the in vivo functions of APLP1 using Drosophila as a model organism. It was found that ectopic expression of APLP1 induces cell death and developmental defects in the nervous and non-nervous system. Genetic studies characterize the transcription factor dFoxO as a critical downstream factor that mediates APLP1’s activity, for the depletion of dFoxO significantly suppresses APLP1-induced cell death in larval discs and associated phenotypes in adults. Further, it was shown that APLP1 is able to up-regulate the transcription of dFoxO target genes hid and reaper (Wang, 2015).

APLP1 has been reported to function mainly in the nervous system, as high expression level of APLP1 is detected in the developing central and peripheral nervous systems, yet a weak expression signal of APLP1 is also observed in organs like heart, lung, liver and kidney in mouse embryos, implying a role of APLP1 in the development of non-neuronal tissues. Consistent with this explanation, RNAi mediated knockdown of APLP1 in WI-38 and MCF7 cells dramatically reduced the proliferation of these cells. This study showed that expression of APLP1 could induce cell death and developmental defects in both neuronal and non-neuronal systems in Drosophila, and thus, provides further evidence for the function of APLP1 in non-neuronal cells (Wang, 2015).

Earlier studies have also shown that loss of APLP1 diminishes stress induced apoptosis in neuroblastoma cells, whereas ectopic expression of APLP1 moderately enhances cell death upon stress stimulation. However, expression of APLP1 alone is not sufficient to induce neuroblastoma cell death , suggesting APLP1 induces cell death in a context dependent manner. Data from this study demonstrates that APLP1 by itself is sufficient to induce cell death. APLP1 has been reported to be a direct transcriptional target of the p53 tumor suppressor, which suggests a possible involvement of APLP1 in p53-induced cell death. p53 is known to interact with the transcriptional factor FoxO, and MDM2 is known to act downstream of p53 to promote FoxO ubiquitination and degradation. In the present study, it was shown that FoxO mediates APLP1-induced cell death. The exact relationship between APLP1, FoxO and p53 in cell death will require further investigation. Overall, this study highlights a novel function of APLP1 in promoting FoxO-mediated cell death in vivo, which will shed light on the role of APLP1 in mammalian cells (Wang, 2015).

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Omata, Y., Lim, Y.M., Akao, Y. and Tsuda, L. (2014). Age-induced reduction of autophagy-related gene expression is associated with onset of Alzheimer's disease. Am J Neurodegener Dis 3: 134-142. PubMed ID: 25628964

Abstract
Aging is a major risk factor for Alzheimer's disease (AD). Aggregation of amyloid beta (Aβ) in cerebral cortex and hippocampus is a hallmark of AD. Many factors have been identified as causative elements for onset and progression of AD; for instance, tau seems to mediate the neuronal toxicity of Aβ, and downregulation of macroautophagy (autophagy) is thought to be a causative element of AD pathology. Expression of autophagy-related genes is reduced with age, which leads to increases in oxidative stress and aberrant protein accumulation. This study found that expression of the autophagy-related genes atg1, atg8a, and atg18 in Drosophila melanogaster is regulated with aging as well as their own activities. In addition, the level of atg18 is maintained by dfoxo (foxo) and dsir2 (sir2) activities in concert with aging. These results indicate that some autophagy-related gene expression is regulated by foxo/sir2-mediated aging processes. It was further found that reduced autophagy activity correlates with late-onset neuronal dysfunction caused by neuronal induction of Aβ. These data support the idea that age-related dysfunction of autophagy is a causative element in onset and progression of AD (Omata, 2014).

Highlights

  • Expression of autophagy-related genes is reduced with aging.
  • Expression of atg18 is reduced in dfoxo and dsir2 mutants.
  • Initiation factor 4E-binding protein (4E-BP) negatively regulates autophagy-related gene expression.
  • Aβ transgenic flies produce Aβ42 and exhibit reduced locomotor activity.
  • Expression of autophagy-related genes affects development of AD.
  • Expression of autophagy-related genes correlates with neuronal toxicity caused by Aβ42.

Discussion
This study shows that expression of autophagy-related genes is regulated by age-related signaling. dsir2 (a Drosophila SIRT1 homolog) and dfoxo are required to maintain atg18 expression during aging, suggesting that, among autophagy-related genes, this gene specifically is regulated by foxo/sir2 activity. Interestingly, aging seems to affect expression of all autophagy-related genes tested, suggesting that aging and foxo/sir2 may act at different levels to regulate autophagy-related gene expression (Omata, 2014).

Previous studies show that sir2, foxo and 4E-BP are involved in regulating the Drosophila lifespan. Data from this study, however, indicate that 4E-BP antagonizes expression of autophagy-related genes. 4E-BP is believed to be controlled by TOR signaling. Therefore, the negative effect of 4E-BP on autophagy-related gene expression may be mediated through the effect of TOR signaling pathway, which also seems to antagonize autophagy-related gene expression (Omata, 2014). 

Autophagy is highly correlated with lysosomal activity, and the autophagy-lysosome pathway is thought to be involved in many cellular processes. Earlier studies indicate that lysosomal activity affects expression of autophagy-related genes. The lysosome nutrient sensing (LYNUS) machinery is responsible for sensing whether there are sufficient nutrients. Under a sufficient nutrient status, the mammalian target of rapamycin complex 1 (mTORC1, a member of the LYNUS machinery) phosphorylates transcription factor EB (TFEB) on the lysosomal surface and inhibits its nuclear localization. In this way, TFEB is unable to induce expression of lysosomal and autophagy-related genes under nutrient sufficient conditions. These results suggest that the level of autophagy-related genes might be regulated by the state of lysosome formation and autophagy itself. Here, expression of autophagy-related genes is affected by the activity of other autophagy-related genes as well as their own activity, suggesting that auto-feedback regulation is part of the mechanism used to maintain expression of autophagy-related genes in Drosophila (Omata, 2014). 

It was observed that reducing the expression of autophagy-related genes strongly enhances the neuronal toxicity caused by Aβ expression. Furthermore, reducing atg1 expression using the Df(atg1)/+ heterozygote shows a more severe enhancement of Aβ-dependent neuronal toxicity than reducing atg18 expression using the Df(atg18)/+ heterozygote. Interestingly, atg1 also demonstrates strong auto-feedback regulation, as reducing expression of atg1 results in further defects in expression of atg genes. Therefore, it is possible that a drastic reduction in expression of many atg genes may contribute to the neuronal toxicity of Aβ42, and that aging and autophagy may be determinants of AD onset (Omata, 2014).

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Saitoh, Y., Fujikake, N., Okamoto, Y., Popiel, H.A., Hatanaka, Y., Ueyama, M., Suzuki, M., Gaumer, S., Murata, M., Wada, K. and Nagai, Y. (2015). p62 plays a protective role in the autophagic degradation of polyglutamine protein oligomers in polyglutamine disease model flies. J Biol Chem 290: 1442-1453. PubMed ID: 25480790

Abstract
Oligomer formation and accumulation of pathogenic proteins are key events in the pathomechanisms of many neurodegenerative diseases, such as Alzheimer disease, ALS, and the polyglutamine (polyQ) diseases. The autophagy-lysosome degradation system may have therapeutic potential against these diseases because it can degrade even large oligomers. Although p62/sequestosome 1 plays a physiological role in selective autophagy of ubiquitinated proteins, whether p62 recognizes and degrades pathogenic proteins in neurodegenerative diseases has remained unclear. This study used Drosophila models of neurodegenerative diseases to elucidate the role of p62 in such pathogenic conditions in vivo. It was found that p62 predominantly co-localizes with cytoplasmic polyQ protein aggregates in the MJDtr-Q78 polyQ disease model flies. Loss of p62 function results in significant exacerbation of eye degeneration in these flies. Immunohistochemical analyses revealed enhanced accumulation of cytoplasmic aggregates by p62 knockdown in the MJDtr-Q78 flies, similarly to knockdown of autophagy-related genes (Atgs). Knockdown of both p62 and Atgs did not show any additive effects in the MJDtr-Q78 flies, implying that p62 function is mediated by autophagy. Biochemical analyses showed that loss of p62 function delays the degradation of the MJDtr-Q78 protein, especially its oligomeric species. It was also found that loss of p62 function exacerbates eye degeneration in another polyQ disease fly model as well as in ALS model flies. The study therefore concludes that p62 plays a protective role against polyQ-induced neurodegeneration, by the autophagic degradation of polyQ protein oligomers in vivo, indicating its therapeutic potential for the polyQ diseases and possibly for other neurodegenerative diseases (Saitoh, 2015).

Highlights

  • p62 co-localizes with cytoplasmic polyQ protein aggregates.
  • Loss of p62 function causes exacerbation of eye degeneration in polyQ disease model flies.
  • Loss of p62 function results in an increase in cytoplasmic polyQ protein aggregates.
  • Loss of autophagic function causes the exacerbation of eye degeneration accompanied by the enhanced accumulation of cytoplasmic polyQ protein aggregates.
  • Protective role of p62 against polyQ protein toxicity is dependent on autophagy.
  • Loss of p62 function delays the degradation of the polyQ protein in vivo.
  • p62 plays a protective role in various neurodegenerative disease model flies.

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Lim, J.Y., Reighard, C.P. and Crowther, D.C. (2015). The pro-domains of neurotrophins, including BDNF, are linked to Alzheimer's disease through a toxic synergy with Aβ. Hum Mol Genet 24: 3929-3938. PubMed ID: 25954034

Abstract
Brain-derived neurotrophic factor (BDNF) has a crucial role in learning and memory by promoting neuronal survival and modulating synaptic connectivity. BDNF levels are lower in the brains of individuals with Alzheimer's disease (AD), suggesting a pathogenic involvement. The Drosophila orthologue of BDNF is the highly conserved Neurotrophin 1 (DNT1). BDNF and DNT1 have the same overall protein structure and can be cleaved, resulting in the conversion of a full-length polypeptide into separate pro- and mature-domains. While the BDNF mature-domain is neuroprotective, the role of the pro-domain is less clear. This study identified a synergistic toxic interaction between the amyloid-β peptide (Aβ1–42) and the pro-domains of both DNT1 and BDNF in flies and mammalian cells. Specifically, it was shown that DNT1 pro-domain acquires a neurotoxic activity in the presence of Aβ1–42. In contrast, DNT1 mature-domain is protective against Aβ1–42 toxicity. Likewise, in SH-SY5Y cell culture, BDNF pro-domain is toxic only in the presence of Aβ1–42. Western blots indicate that this synergistic interaction likely results from the Aβ1–42-induced upregulation of the BDNF pro-domain receptor p75NTR. The clinical relevance of these findings is underlined by a greater than thirty fold increase in the ratio of BDNF pro- to mature-domains in the brains of individuals with AD. This unbalanced BDNF pro:mature-domain ratio in patients represents a possible biomarker of AD and may offer a target for therapeutic intervention (Lim, 2015).

Highlights

  • Drosophila DNT1 mRNA levels are high during development but suppressed in adulthood.
  • The subdomains of DNT1 have differential effects on neurotoxic phenotypes in the fly.
  • Co-expression of various domains of DNT1 does not change the abundance or conformation of Aβ.
  • The pro-domain of BDNF and Aβ1–42 exhibit synergistic toxicity in SH-SY5Y human neuroblastoma cell cultures.
  • An elevated ratio of pro- to mature-domain of BDNF correlates with disease status in elderly individuals.

Discussion
NTs are a major class of molecules promoting neuronal survival in vertebrates. They are synthesized as larger precursor forms that are proteolytically processed to yield a mature, biologically active ligand. Among the NTs, BDNF has emerged as a major regulator of synaptic plasticity, neuronal survival and differentiation, and also as a potential molecular target for the treatment of neurological disease. Several studies indicate that the cortex and hippocampus, areas of the brain associated with learning and memory, not only exhibit extensive amyloid pathology but also show decreased levels of BDNF in AD. Interestingly, precursor and mature forms of BDNF are significantly decreased in preclinical and early stages of AD, and this reduction correlates with clinical neuropsychological scores. Low levels of BDNF may favor AD pathogenesis by failing to adequately support neurones and allowing them to succumb to other toxic insults. Furthermore, several studies have linked polymorphisms, specifically Val66Met and Cys270Thr, in the pro-domain of BDNF to an increased risk for AD (Lim, 2015).

BDNF exerts many of its neuroprotective effects by binding to the TrkB receptor, a member of the tumor necrosis factor receptor family. Indeed, some reports indicate that Aβ may have a negative effect on neuronal survival by down-regulating the TrkB receptor. For example, levels of TrkB are reduced in the temporal and frontal cortex of AD brain. In addition, BDNF-induced TrkB autophosphorylation and the activation of the downstream enzymes AKT and ERK are all suppressed in the hippocampus of APP/PS1 mice (Lim, 2015).

The consequences of BDNF binding to its alternative receptor p75NTR and sortilin are less well understood, although they are thought to include promotion of myelination and neuronal migration but also neuronal process retraction and neuronal apoptosis. It has been proposed that the balance between cell death and survival is determined by the relative activity of the precursor versus mature forms of BDNF; indeed, in dorsal root ganglion lesion models in neonatal rats, the signaling appears to be affected by the relative levels of the relevant receptors, namely TrkB, p75NTR and sortilin. Notably, BDNF promotes the death of cultured neurons in vitro when p75NTR is upregulated and TrkB downregulated. Precursor BDNF can also cause cell death in both in vitro and in vivo model systems and the prevailing view is that the apoptotic signal is generated by the pro-domain. Similar domain-specific activities have also been observed for NGF; specifically, the precursor form of this related NT preferentially activates p75NTR resulting in apoptosis, while mature NGF preferentially activates TrkA receptor with neurotrophic effects (Lim, 2015).

Mammalian NTs are similar in many ways to their insect orthologues. Sequence analysis has identified Drosophila neurotrophin (DNT1), also called Spatzle 2 (Spz 2), as the closest fly orthologue of human BDNF. Like the NTs, the Spz polypeptides are synthesized with a signal peptide, followed by a pro-domain and then the cysteine knot-containing mature domain. The characteristic NT cysteine knot, formed by antiparallel β-sheets held together by three disulfide bonds, is conserved in the crystal structures of both Spz and NGF. There is also functional conservation between DNT1, Spz and the mammalian NTs in the nervous system. Indeed, during Drosophila embryogenesis DNT1 is expressed in neurons and muscle cells where it promotes neuronal survival and suppresses apoptosis. This study shows that the high levels of endogenous DNT1 mRNA that are present during larval development are rapidly suppressed at the beginning of adult life in both control flies and similarly in those expressing various isoforms of the Aβ peptide (Lim, 2015).

Because of the functional similarities between mammalian and Drosophila NTs, this study analyzed the interaction between Aβ and DNT1 in the fruit fly. It was found that transgenic expression of the DNT1 mature-domain protects flies against Aβ toxicity and that this benefit is not due to spurious reductions in Aβ levels. Similarly, in human SH-SY5Y culture, the mature-domain of BDNF protects cells from Aβ toxicity. In both the fly and the mammalian models, the pro-domains of both DNT1 and BDNF, while being harmless alone, are nevertheless toxic when added in combination with Aβ. In SH-SY5Y cells, this synergistic toxic interaction between Aβ and the BDNF pro-domain is only apparent for the Met66 variant and is absent for the wild-type Val66 isoform. This finding provides mechanistic underpinning for the clinical observation that the Met66 polymorphism is linked to poor prognosis, particularly in individuals with high Aβ burden on PiB photon emission tomography brain scans (Lim, 2015).

The expression of the receptor p75NTR is upregulated in both SH-SY5Y cells following Aβ treatment and also in transgenic mice expressing the human APPswe transgene. Aβ treatment also enhances sortilin expression via p75NTR, which is thought to activate the downstream effectors JNK and Rho, resulting in apoptotic cell death. Furthermore, the accumulation of Aβ in humans is also accompanied by an increased hippocampal membrane-associated p75NTR. However, it seems unlikely that Aβ binds directly to p75NTR, rather this study suggests that Aβ amplifies the pro-apoptotic signaling of pro-domain BDNF by upregulating p75NTR in the human SH-SY5Y cells (Lim, 2015).

Next, analysis of post-mortem hippocampal tissue from age-matched healthy elderly and patients with AD underlines the clinical significance of experimental findings in this study. As expected, the mature-domain of BDNF is reduced in AD when compared with control brain tissue. In analysis of 10 cases and controls, it was found that control subjects fall into two groups—those with levels of BDNF mature-domain that are up to 20 times higher than AD cases; however, an equal number of controls have mature-domain levels that are equivalent to AD patients. The separation between cases and controls is better when BDNF pro-domain levels are measured: cases have elevated levels of pro-domain, spread over almost 2 orders of magnitude while all but one of the controls are closely grouped. The average pro-domain level is ∼16-fold higher in AD cases when compared with controls. These reciprocal changes in the levels of pro- and mature-BDNF are surprising considering that the peptides are generated stoichiometrically. Receptor-mediated clearance is an unlikely explanation because the levels of TrkB are low and p75 high in AD cases when compared with controls. Conceivably, in AD the BDNF pro-domain is being stabilized by binding to another protein; this intriguing possibility requires further investigation (Lim, 2015).

When these relative changes in pro- and mature-domains are combined as a ratio, it was found that cases and controls have an average 30-fold difference and only one of the controls overlaps with the AD range. This particular control individual is interesting because, despite being symptom-free and relatively young (aged 63), she was the only control to have accumulated significant Aβ. In fact, she had the highest level of Aβ amongst all cases and controls. That this control individual's pro:mature domain ratio was in the AD range raises the tantalizing possibility that disease changes can be predicted before the onset of symptoms (Lim, 2015).

In conclusion, this study shows for the first time that the pro- and mature-domains of NTs have opposing effects on Aβ neurotoxicity in vitro and in vivo. The mature-domains of NTs protect against Aβ1–42 toxicity, whereas the pro-domains acquir a toxic role in the presence of Aβ1–42. In case of post-mortem clinical brain samples, the ratio of pro- to mature-domain of BDNF is significantly higher in patients with AD when compared with controls. Taken together, the finding that patients with AD have elevated levels of BDNF pro-domain underlines the importance of its synergistic toxic interaction with Aβ. Conceivably, individuals with an unfavourable pro:mature domain ratio could be targeted with therapy aimed at restoring a more neurotrophic environment in the brain (Lim, 2015).

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Ott, S., Dziadulewicz, N. and Crowther, D.C. (2015). Iron is a specific cofactor for distinct oxidation- and aggregation-dependent Aβ toxicity mechanisms in a Drosophila model. Dis Model Mech 8: 657-667. PubMed ID: 26035384

Abstract
Metals, including iron, are present at high concentrations in amyloid plaques in individuals with Alzheimer's disease, where they are also thought to be cofactors in generating oxidative stress and modulating amyloid formation. This study presents data from several Drosophila models of neurodegenerative proteinopathies indicating that the interaction between iron and amyloid beta peptide (Aβ) is specific and is not seen for other aggregation-prone polypeptides. The interaction with iron is likely to be important in the dimerisation of Aβ and is mediated by three N-terminal histidines. Transgenic fly lines systematically expressing all combinations of His>Ala substitutions in Aβ were generated and used to study the pathological role of these residues. Developmental eye phenotypes, longevity and histological examinations indicate that the N-terminal histidines have distinct position-dependent and -independent mechanisms. The former mediate the toxic effects of metals and Aβ aggregation under non-oxidising conditions and the latter are relevant under oxidising conditions. Understanding how Aβ mediates neurotoxic effects in vivo will help to better target pathological pathways using aggregation blockers and metal-modifying agents (Ott, 2015).

Highlights

  • The iron-binding protein ferritin is a specific suppressor of Aβ toxicity in vivo.
  • Iron modifies Aβ aggregation in vitro and in vivo.
  • Position-dependent effects of His>Ala substitutions on Aβ toxicity in a non-oxidising environment.
  • Position-dependent effects of His>Ala substitutions on the metal-dependent component of Aβ toxicity in a non-oxidising environment.
  • Position-independent effects of His>Ala substitutions on Aβ toxicity in an oxidising environment.

Discussion
A number of neurodegenerative disorders are characterized by the abnormal metabolism of metals, such as copper and iron; however, the relevance of this observation to pathological mechanisms remains unclear. Consequently, this study used several models of protein aggregation disorders to investigate the role of metals. In an earlier study using a Drosophila model of Aβ toxicity, it was found that the iron-binding protein ferritin and other iron-chelators are powerful suppressors of both Aβ toxicity and the associated markers of oxidative damage. Consequently, this study asked whether ferritin expression also rescues disease-related phenotypes in other Drosophila models of common neurodegenerative disorders. Specifically, flies expressing tau, the Q48 peptide and TDP43, in addition to various isoforms of Aβ, were analyzed (Ott, 2015).

The inclusion of these particular polypeptides in the study is justified by varying degrees of evidence linking their pathogenicity to interactions with metal cofactors. Considering tau, its hyperphosphorylation and deposition as neurofibrillary tangles are characteristic of AD and other tauopathies. Some investigators have highlighted the role of metal binding in generating tau deposits and the consequent neuronal microtubule dysfunction. It is thought that this process is induced by the ability of Fe3+ to bind to phosphorylated tau, causing it to aggregate. Furthermore, tau, like Aβ, may form complexes with iron and copper, resulting in the generation of reactive oxygen species, in turn favouring tau phosphorylation (Ott, 2015).

Similarly, in HD there is evidence for abnormal metal metabolism, with clinical MRI scans showing iron accumulation in susceptible brain regions. Iron levels tend to be higher in advanced disease and in patients with longer polyQ repeat lengths. In mouse and fly models of HD there is good evidence that both copper and iron metabolism are disturbed and that chelator therapy may be beneficial. In vitro, metals promote oxidative damage, with a truncated huntingtin polypeptide interacting with both Cu2+ and Fe3+ (Ott, 2015).

Finally, familial amyotrophic lateral sclerosis (ALS) may result from mutations in the genes for TDP43, FUS, C9ORF72 and, classically, SOD1. Although there is considerable evidence that copper plays a role in the pathogenicity of SOD1 variants, the role of metals in ALS as a whole is less clear. For example, mice expressing disease-linked variants of TDP43 show abnormal metabolism of a number of metals, although iron appears to be normal (Ott, 2015).

Considering this evidence that iron is more or less important in the pathological actions of a series of aggregation-prone polypeptides, it is remarkable that, in this study, iron chelation by ferritin was beneficial only for flies expressing Aβ. The finding in this study that ferritin offers Aβ-specific rescue from both longevity and eye phenotypes indicates that an important component of Aβ toxicity in vivo is not generic. Thus, the overall toxicity of an aggregating polypeptide seems to be composed of a core generic component, which is likely to be related to the presence of oligomeric aggregates. To this core are added peptide-specific effects that are modulated by environmental cofactors – in this case the presence of iron or other metals. One way in which metals could enhance Aβ toxicity is by stabilising particular conformers in the aggregation pathway. The formation of an initial dimer is likely to be a fundamental step and this was investigated in vivo by expressing either the normal monomeric peptide or a ‘pre-dimerised’ tandem Aβ peptide. It has been previously shown that both tandem Aβ42 and tandem Aβ40 aggregate rapidly in vivo; however, of these, only the 42 amino acid isoform is able to generate stable oligomeric aggregates and exhibit pronounced toxicity. This study presents tentative evidence in vitro that amyloid generation by partially purified, recombinant Aβ and by tandem Aβ responds differently to the addition of iron. Specifically, the addition of iron to a preparation of partially purified tandem Aβ does not cause the slowing of ThT amyloid signal that is observed with the monomeric peptide preparation. Concordant with these in vitro results, it was found that the toxicity associated with tandem Aβ in the fly is not suppressed by co-expression of ferritin. Furthermore, ferritin has no effect on the number of tandem Aβ deposits in the brain, whereas flies expressing monomeric Aβ consistently have up to 25% fewer deposits in the presence of ferritin. Importantly, this reduced plaque load is not due to decreased Aβ transgene expression or peptide production. In summary, these results suggest that, both in vitro and in vivo, iron is interacting with Aβ to accelerate a step that is redundant in the tandem peptide; this step is likely to be the formation of dimeric aggregates. These results in a Drosophila model are reminiscent of the reduction in plaque load in tg2576 mice treated with the broad-spectrum metal chelator clioquinol and support the relevance of this study for mammalian systems (Ott, 2015).

After evaluating the specificity of iron for Aβ-induced toxicity and its effect on aggregation, potential interactions between Aβ and iron were studied. Such interactions are thought be mediated by the three N-terminal histidine residues at positions 6, 13 and 14. The study systematically assessed how each of the seven possible combinations of His>Ala substitutions modifies the elaboration of three key disease-linked phenotypes, namely: (1) survival in an oxidising environment; (2) survival in a non-oxidising environment; and (3) the proportion of Aβ toxicity that is metal dependent (Ott, 2015).

To assess the response of the flies to oxidative stress, survival after oral challenge with hydrogen peroxide was measured. Whereas control flies are robust when treated in this way, there is a marked reduction in survival after 78 and 96?h when Aβ is expressed. Remarkably, only the number of histidines in a particular Aβ variant determine the susceptibility to oxidative stress. This histidine dose dependency and position independence makes it unlikely that the mechanism involves specific changes in peptide conformation or overall aggregation propensity; indeed, the hydrogen peroxide feeding experiments are performed in 6-day-old flies in which there is no detectable peptide aggregation. Moreover, the predicted aggregation propensities of the various peptide isoforms are essentially identical. A possible molecular scale interpretation of these results is that the formation of redox-active metal-peptide complexes is rather flexible and peptide promiscuous. In this model, histidines from distinct peptides could co-ordinate to a shared metal ion and generate a redox-active complex. An alternative explanation might be that there is an oxidative reaction in the presence of Aβ that uses histidines as a substrate, thereby generating toxic oxygen species. In either case, these results predict that reducing total brain Aβ levels might be most effective in reducing the oxidative damage in AD (Ott, 2015).

By contrast, the remaining two disease-linked phenotypes are sensitive to the positions of each of the His>Ala substitutions. In a non-oxidising environment the survival of flies expressing the various peptides is determined by position-dependent factors. Some in vitro studies have shown that the histidines at positions 13 and 14 interact similarly with copper in models of amyloid fibrils, whereas others suggest that the H14A substitution in synthetic Aβ42 reduces toxicity, possibly by preventing the interaction of the peptide with cell membranes. The longevity effects observed in this study, although relatively modest, are nevertheless both robust and significant, and present with a contrasting observation. Specifically, it was found that H14A enhances toxicity, as evidenced by a reduction in longevity, whereas the H13A substitution has the opposite effect. Interestingly, the H6A substitution has a modulating role in the fly model; in particular, H6A acts to amplify the longevity effects of substitutions at positions 13 and 14. Accordingly, H14A reduces the median survival and H6/14A has the shortest lifespan of all the fly lines. By contrast, Drosophila expressing the H13A variant live longer than wild-type flies and H6/13A live the longest of all the peptide-expressing lines. H6 also modulates the accumulation of peptide deposits in the brain, with H6/14A showing remarkably high levels of Aβ deposition, higher than H14A alone. The use of the φC31 system to target all of the Aβ transgenes to the same 51D genomic insertion site in the same acceptor fly line makes it highly unlikely that the phenotypic differences observed in this study are due to artefactual fluctuations in transgene expression levels. Furthermore, the genetic background of the experimental flies should be essentially identical. It should also be noted that the density of Aβ deposits is comparable between lines despite highly significant differences in median survival (Ott, 2015).

The modulating role of H6 is seen most clearly in the third phenotype, where the degree to which the metal chelator clioquinol prolongs the longevity of the various fly lines was measured. When the percentage increase in median survival upon treatment with clioquinol is calculated, it is apparent that the histidines at positions 13 and 14 each have their position-dependent effects on the clioquinol response. However, there is an overwhelming additive effect of histidine at position 6, such that each corresponding H6A variant shows a 25-30% reduction in clioquinol responsiveness. The implication of these results is that much of the metal-mediated toxicity, in a non-oxidising environment, is dependent on H6A and that, thereafter, there are smaller position-dependent effects of residues 13 and 14. A possible mechanistic interpretation is that metal interactions with H6 initiate metal-mediated Aβ dimerisation and, thereafter, histidines 13 and 14 determine the particular conformations of downstream aggregates and their toxicity. These position-dependent properties of individual histidine residues make it likely that particular peptide conformations are mediating the combined Aβ-iron effects; such structures might be amenable as therapeutic targets (Ott, 2015). 

In summary, this study's analysis of the effects of metals and their specific interactions with Aβ in vivo indicates that dimerisation of the peptide, perhaps mediated by H6, is an early step in generating toxicity. The subsequent toxic consequences of Aβ aggregation are then largely determined by the histidines at positions 13 and 14. In a strongly oxidising environment the importance of peptide aggregation is complemented, if not overwhelmed, by the histidine-dependent oxidative damage mediated by Aβ (Ott, 2015).

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Sofola-Adesakin, O., Castillo-Quan, J.I., Rallis, C., Tain, L.S., Bjedov, I., Rogers, I., Li, L., Martinez, P., Khericha, M., Cabecinha, M., Bähler, J. and Partridge, L. (2014). Lithium suppresses Aβ pathology by inhibiting translation in an adult Drosophila model of Alzheimer's disease. Front Aging Neurosci 6: 190. PubMed ID: 25126078

Abstract
The greatest risk factor for Alzheimer's disease (AD) is age, and changes in the ageing nervous system are likely contributors to AD pathology. Amyloid beta (Aβ) accumulation, which occurs as a result of the amyloidogenic processing of amyloid precursor protein (APP), is thought to initiate the pathogenesis of AD, eventually leading to neuronal cell death. An adult-onset Drosophila model of AD has been developed earlier in which mutant Aβ42 accumulation leads to increased mortality and neuronal dysfunction in the adult flies. Furthermore, lithium reduces Aβ42 protein, but not mRNA, and is able to rescue Aβ42-induced toxicity. This study investigated the mechanism/s by which lithium modulates Aβ42 protein levels and Aβ42 induced toxicity in the fly model. It was found that lithium causes a reduction in protein synthesis in Drosophila and hence the level of Aβ42. At both the low and high doses tested, lithium rescues the locomotory defects induced by Aβ42, but it rescues lifespan only at lower doses, suggesting that long-term, high-dose lithium treatment may have induced toxicity. Lithium also down-regulates translation in the fission yeast Schizosaccharomyces pombe associated with increased chronological lifespan. These data highlight a role for lithium and reduced protein synthesis as potential therapeutic targets for AD pathogenesis (Sofola-Adesakin, 2014).

Highlights

  • Lithium reduces Aβ load in arctic Aβ42 expressing flies through protein clearance/degradation-independent mechanisms.
  • Lithium down-regulates overall protein synthesis/translation.
  • Lithium also inhibits protein synthesis in fission yeast.
  • Lithium extended lifespan of flies expressing Aβ.

Discussion
Human life expectancy continues to increase at a steady rate in most countries worldwide, and has done so by almost 3 months per year in the last 160 years. Therefore, it is of great importance to tackle ageing-related diseases such as AD, because they are becoming increasingly prevalent. Because age is the biggest risk factor for AD, interventions that promote general increases in health during ageing could also be important and beneficial in AD (Sofola-Adesakin, 2014).

Lithium is becoming increasingly implicated as a drug that can ameliorate ageing and neurodegenerative diseases. Several groups have shown that it extends lifespan in model organisms such as the nematode worm C. elegans and Drosophila. This study shows that lithium also extends lifespan in fission yeast Schizosaccharomyces pombe, highlighting that this effect is conserved over large evolutionary distances. Fission yeast is an ideal organism for genetic screens, and future work should identify the molecular targets of lithium both for control of protein synthesis and of lifespan. Furthermore, slightly higher levels of lithium present in the drinking water have been reported as associated with reduced mortality in a Japanese human population (Sofola-Adesakin, 2014).

A substantial body of work has demonstrated that several neurodegenerative diseases and neurological disorders, including but not confined to stroke, schizophrenia, Fragile X syndrome, Huntington's disease and Parkinson's disease, benefit from the therapeutic properties of lithium. In addition, several studies have investigated whether lithium has a beneficial effect in AD pathogenesis. Clinical trials conducted with lithium have yielded conflicting results; some have found benefits, whilst others have not. Interestingly, a correlative study conducted in patients with bipolar disorder, suggests that patients that have been on chronic lithium treatment show a reduced incidence of AD in comparison to patients that have not been on treatment. And a small-scale clinical trial on mild cognitive impaired (MCI) patients finds that low doses of lithium slow cognitive decline. The investigators suggest that a reason for the previous conflicting data on the efficacy of lithium is probably attributable to the pathological states/stages at which the patients were given lithium. It is becoming increasingly evident that drug trials are most likely to yield positive effects when initiated early, at the MCI stage (Sofola-Adesakin, 2014).

Results from this study add to existing data suggesting that lithium could be beneficial in ameliorating Aβ toxicity, and should be considered for a potential large-scale trial on MCI patients. It has the added advantage of being an already approved drug, used to treat bipolar patients. It does have side-effects, but these are minimal at the low doses used in a recent small-scale clinical study. It was also found that there are limits to the beneficial/therapeutic benefits of lithium in fission yeast in chronological lifespan—lithium is unable to increase chronological lifespan at higher doses as well as in the Drosophila AD model. Previously, it was shown that administering both 30 and 100 mM lithium into the fly food is effective in modulating Aβ neuronal toxicity as evidenced by the improved locomotor function in young flies. These lithium concentrations were initially chosen based on another study in which it was shown that lithium concentrations ranging from 10 to 100 mM lithium in the fly food translates to roughly 0.05–0.4 mM in the fly tissue, so well below the toxic levels in patients and mice. This study shows that both 25 and 100 mM lithium reduce Aβ levels in a dose dependent manner at an early time point. It was also found that lower doses of lithium (10 and 25 mM) rescue the shortened longevity of the Aβ flies, but 100 mM lithium is unable to extend lifespan when given to the flies throughout adulthood. It will be important to determine the therapeutic thresholds for lithium in patients that could offer therapeutic benefits without overt side effects (Sofola-Adesakin, 2014).

Similar to the published data on the role of GSK-3 inhibition in down-regulating translation in HCC1806 cells, it was found that lithium is able to reduce translation in fission yeast and flies, suggesting that perhaps some of the effect of lithium on translation down-regulation is via GSK-3 inhibition. However, this is correlative and future work will involve carrying out epistasis interactions between lithium and GSK-3, and identifying molecular targets of GSK-3 and lithium for control of protein synthesis. Nonetheless, this study highlights the potential benefits of lithium through down-regulation of translation, associated with extension of lifespan in very distantly related organisms. By reducing protein synthesis, lithium may reduce the increased proteostatic burden in ageing, a recognized hallmark of ageing. Lithium is also of specific benefit in AD, because of its ability to down-regulate translation, and hence levels of proteins involved in promoting the presence of toxic Aβ (Sofola-Adesakin, 2014).

The mutant Arctic Aβ42 protein present in the transgenic flies used in this study has a propensity to aggregate faster than wild type Aβ. However, both soluble and insoluble Aβ in the Arctic Aβ42 flies were observed, and the ability of lithium to reduce translation of the Aβ peptide without affecting its clearance may lower the level of soluble Aβ. In a wider context, lithium might be beneficial in ameliorating toxicity of AD by lowering expression of APP and of proteins that are involved in the generation of Aβ from APP. The AD model used in this study does not express full length APP, and may therefore not include other potential/additional benefits of lithium on Aβ toxicity. As well as the increased ratio of Aβ42 to Aβ 40 peptide observed in familial AD cases with APP mutations, increased levels of APP could also contribute to AD pathogenesis. Indeed, patients with Down syndrome have a high risk of developing AD possibly due to trisomy of the APP gene which leads to increased APP expression. Also, several mutations in the APP promoter region have been found to significantly increase APP expression in SH-SY5Y cells, and are associated with risk for AD (Sofola-Adesakin, 2014).

The ability of lithium to down-regulate translation could therefore be beneficial at several stages in AD pathogenesis. Lithium might also have therapeutic benefits for other neurodegenerative disorders that are caused by over-expression of wild type or mutant forms of proteins such as α-synuclein in Parkinson's disease. Lithium could also reduce the production of mis-translated polypeptides, and free proteases or/and chaperones that can then participate in cellular proteostasis. Furthermore, diseases where protein turnover is compromised by loss of function of the degradation machinery could also benefit from lowering the burden of protein production hence reducing cellular stress. This could be particularly important in lysosomal storage diseases, where the intrinsic function of the degradative machinery is compromised. Moreover, induction of autophagy in some cases increases the load of an already dysfunctional lysosome, worsening the cellular proteostatic stress. Hence lowering the production of proteins could again be a viable mechanism to restore proteostasis. Other neurodegenerative models where the role of lithium in lowering protein translation could be beneficial are, for example, the Drosophila models of Pink1 and Parkin, which do not include the over-expression of toxic proteins. Flies lacking either of these proteins accumulate unfolded proteins in mitochondria, resulting in mitochondria impairment. It would be interesting to study whether lithium could ameliorate mitochondrial stress by reducing the production of the proteins accumulating in the mitochondria of the Pink1 or parkin null flies. Lithium could hence be a useful drug with an overall benefit for health during ageing and protection against AD and other neurodegenerative diseases (Sofola-Adesakin, 2014).

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Allan, K., Perez, K.A., Barnham, K.J., Camakaris, J. and Burke, R. (2014). A commonly used Drosophila model of Alzheimer's disease generates an aberrant species of amyloid-β with an additional N-terminal glutamine residue. FEBS Lett 588: 3739-3743. PubMed ID: 25171862

Abstract
Expression of human amyloid-β (Aβ) in Drosophila is frequently used to investigate its toxicity in vivo. This study expressed Aβ1–42 in the fly using a secretion signal derived from the Drosophila necrotic gene, as described in several previous publications. Surface-enhanced laser desorption/ionization TOF MS analysis revealed that the Aβ produced contains an additional glutamine residue at the N-terminus. AβQ+1–42 was found to have increased protein abundance and to cause more severe neurodegenerative effects than wild type Aβ1–42 as assessed by locomotor activity and lifespan assays. These data reveal that a commonly used model of Alzheimer’s disease generates incorrect Aβ peptide (Allan, 2014).

Highlights

  • Amyloid-β (Aβ) peptide produced with the Necrotic signal peptide contains an additional N-terminal glutamine.
  • Aβ produced with the Necrotic signal peptide gives dose-dependent neurodegenerative phenotypes.

Discussion
Alzheimer’s disease (AD) is characterized by the presence of brain extracellular plaques rich in amyloid-β (Aβ), a peptide derived from the proteolytic cleavage of Amyloid-β Precursor Protein (AβPP). Differential processing of AβPP can produce several different Aβ species of varying lengths. Aβ42 is significantly more cytotoxic than the abundant Aβ40 normally found in healthy brains and the Aβ42: Aβ40 ratio is considered to be an important factor in neuronal health and AD progression (Allan, 2014).

When attempting to replicate AD-like neurodegeneration in the vinegar fly Drosophila, the need for AβPP processing has been circumvented by directly expressing Aβ in fly neuronal tissues. A secretory signal sequence at the N-terminus of the Aβ peptide, allowing its secretion from producing cells, is known to be essential for inducing age-dependent phenotypes including reduced adult lifespan, reduced locomotor activity, learning defects, deposition of amyloid plaques and visible brain and retinal degeneration. This model system has since been used to investigate the effect of single amino acid substitutions in the Aβ42 peptide sequence and numerous other aspects of AD including pharmacological and genetic modification of Aβ-mediated toxicity (Allan, 2014).

This study demonstrates that one of the signal peptides frequently used to generate secreted Aβ, derived from the fly Necrotic protein, actually produces an aberrant Aβ1–42 peptide with an additional glutamine residue at the N-terminus, referred to as AβQ+1–42. A similar situation has been seen with a C. elegans Aβ transgenic line that produces a truncated form of Aβ, Aβ3–42, which begins with glutamic acid. Using transgenic lines with equivalent transcription levels, it was found that a wild type Aβ1–42 construct gives dramatically lower peptide levels and no detectable neurodegenerative phenotypes compared to the aberrant AβQ+1–42 form (Allan, 2014).

Since TSpAβ1–42 could not be expressed at sufficient levels for repeating the mass spectrometry analysis performed on NSpAβ1–42, this study cannot confirm that TSpAβ1–42 is being correctly processed. However, Aβ1–42 constructs using the pre-proenkephalin signal sequence (also predicted to cleave correctly) are toxic when expressed at high levels. Therefore the study proposes that the addition of an N-terminal glutamine enhances Aβ abundance and therefore toxicity compared to wild type Aβ, a property revealed when the two species are transcribed at similar levels (Allan, 2014).

The slightly higher NSpAβQ+1–42 transcript levels do not account for the strong difference in peptide levels seen between the NSpAβ1–42 and TSpAβ1–42 constructs. Possibly, NSpAβQ+1–42 is more resistant to degradation and/or clearance. In the human AD brain, N-terminally truncated Aβ3–42 and Aβ11–42 peptides represent ∼50% of total Aβ found in senile plaques. Both peptides begin with a glutamic acid residue which can undergo post-translational cyclization to produce pyroglutamate (pE). pE-modified Aβ displays increased stability due to reduced protease susceptibility. Glutamine can also undergo cyclization. This study therefore hypothesizes that aberrant signal peptide cleavage from the NSpAβ1–42 construct results in a cyclized pQ form of Aβ that displays greater stability and hence increased toxicity. These findings emphasize the importance of determining the species of Aβ being generated experimentally and the transcription levels of each different construct (Allan, 2014).

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Hu, Y., Han, Y., Shao, Y., Wang, X., Ma, Y., Ling, E. and Xue, L. (2015). Gr33a modulates Drosophila male courtship preference. Sci Rep 5: 7777. PubMed ID: 25586066

Abstract
In any gamogenetic species, attraction between individuals of the opposite sex promotes reproductive success that guarantees their thriving. Consequently, mate determination between two sexes is effortless for an animal. However, choosing a spouse from numerous attractive partners of the opposite sex needs deliberation. In Drosophila melanogaster, both younger virgin females and older ones are equally liked options to males; nevertheless, when given options, males prefer younger females to older ones. Non-volatile cuticular hydrocarbons (CHCs), considered as major pheromones in Drosophila, constitute females' sexual attraction that act through males' gustatory receptors (Grs) to elicit male courtship. To date, only a few putative Grs are known to play roles in male courtship. This study reports that loss of Gr33a function or abrogating the activity of Gr33a neurons does not disrupt male-female courtship, but eliminates males' preference for younger mates. Furthermore, ectopic expression of human amyloid precursor protein (APP) in Gr33a neurons abolishes males' preference behavior. Such function of APP is mediated by the transcription factor forkhead box O (dFoxO). These results not only provide mechanistic insights into Drosophila male courtship preference, but also establish a novel Drosophila model for Alzheimer's disease (AD) (Hu, 2015).

Highlights

  • The roles of Grs in males' courtship preference behavior.
  • Gr33a is required for males' preference for younger mates.
  • Activity of Gr33a neurons is essential for males' preference for younger mates.
  • Expression of APP in Gr33a neurons results in dFoxO-dependent choice defect.
  • Female CHC profiles change with age.

Discussion
Drosophila male courtship choice has been frequently applied for studying decision making in animals, yet most of the past studies have focused on male courtship choices between likes and dislikes, such as court towards females vs. males, or virgin vs. non-virgin females. The choice behavior between two equally-liked options: mature virgin females, whether younger or older, has been previously shown to be similarly attractive to naive males; nevertheless, when given the option, males turn out to be picky and prefer younger virgin females to older ones. This study shows that a gustatory receptor, Gr33a, is necessary for males' preference for younger mates. Gr33a is thought to be necessary to inhibit homosexual behavior; its role in heterosexual behavior, however, is rarely pondered. This study reveals the critical role of Gr33a in males' preference for younger mates. Furthermore, ectopic expression of APP in Gr33a neurons eliminates males' preference behavior, and such function is mediated by dFoxO, a recently reported downstream factor of APP. Therefore, this study demonstrates the genetic interaction of APP and dFoxO in Gr33a neurons, which modulates males' preference for younger mates (Hu, 2015).

APP is identified as a potential causative protein of AD, a common progressive neurodegenerative disorder, in which cognitive decline is the prime symptom. Although Drosophila has long been utilized for building AD models to investigate the pathogenesis and possible cure for AD, accepted Drosophila AD models are limited to locomotion model and life span model, which have little correlation with cognitive ability. Findings in this study, however, offer the possibility for establishing a novel Drosophila AD model that is related to cognitive ability (Hu, 2015).

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Lau, H.C., Lee, I.K., Ko, P.W., Lee, H.W., Huh, J.S., Cho, W.J. and Lim, J.O. (2015). Non-invasive screening for Alzheimer's disease by sensing salivary sugar using Drosophila cells expressing gustatory receptor (Gr5a) immobilized on an extended gate ion-sensitive field-effect transistor (EG-ISFET) biosensor. PLoS One 10: e0117810. PubMed ID: 25714733

Abstract
Body fluids are often used as specimens for medical diagnosis. With the advent of advanced analytical techniques in biotechnology, the diagnostic potential of saliva has been the focus of many studies. A previous study reports the presence of excess salivary sugars, in patients with Alzheimer’s disease (AD). This study developed a highly sensitive, cell-based biosensor to detect trehalose levels in patient saliva. The developed biosensor relies on the overexpression of sugar sensitive gustatory receptors (Gr5a) in Drosophila cells to detect the salivary trehalose. The cell-based biosensor is built on the foundation of an improved extended gate ion-sensitive field-effect transistor (EG-ISFET). Using an EG-ISFET, instead of a traditional ion-sensitive field-effect transistor (ISFET), results in an increase in the sensitivity and reliability of detection. The biosensor is designed with the gate terminals segregated from the conventional ISFET device. This design allows the construction of an independent reference and sensing region for simultaneous and accurate measurements of samples from controls and patients respectively. To investigate the efficacy of the cell-based biosensor for AD screening, 20 saliva samples were collected from each of the following groups: participants diagnosed with AD, participants diagnosed with Parkinson’s disease (PD), and a control group composed of healthy individuals. Then, the response generated from the interaction of the salivary trehalose of the saliva samples and the Gr5a in the immobilized cells on an EG-ISFET sensor was studies. The cell-based biosensor significantly distinguishes salivary sugar, trehalose of the AD group from the PD and control groups. Based on these findings, the study propose that salivary trehalose might be a potential biomarker for AD and could be detected using the cell-based EG-ISFET biosensor developed in this study. The cell-based EG-ISFET biosensor provides a sensitive and direct approach for salivary sugar detection and may be used in the future as a screening method for AD (Lau, 2015).

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Frenkel-Pinter, M., Tal, S., Scherzer-Attali, R., Abu-Hussien, M., Alyagor, I., Eisenbaum, T., Gazit, E. and Segal, D. (2016). Naphthoquinone-tryptophan hybrid inhibits aggregation of the Tau-derived peptide PHF6 and reduces neurotoxicity. J Alzheimers Dis [Epub ahead of print]. PubMed ID: 26836184

Abstract

Tauopathies, such as Alzheimer's disease (AD), are a group of disorders characterized neuropathologically by intracellular toxic accumulations of abnormal protein aggregates formed by misfolding of the microtubule-associated protein tau. Since protein self-assembly appears to be an initial key step in the pathology of this group of diseases, intervening in this process can be both a prophylactic measure and a means for modifying the course of the disease for therapeutic purposes. Aromatic small molecules can be effective inhibitors of aggregation of various protein assemblies, by binding to the aromatic core in aggregation-prone motifs and preventing their self-assembly. Specifically, series of small aromatic naphthoquinone-tryptophan hybrid molecules were designed as candidate aggregation inhibitors of β-sheet based assembly, and their efficacy was demonstrated toward inhibiting aggregation of the amyloid-β peptide, another culprit of AD, as well as of various other aggregative proteins involved in other protein misfolding diseases. This study tested whether a leading naphthoquinone-tryptophan hybrid molecule, namely NQTrp, can be repurposed as an inhibitor of the aggregation of the tau protein in vitro and in vivo. The molecule was shown to inhibit the in vitro assembly of PHF6, the aggregation-prone fragment of tau protein, reduces hyperphosphorylated tau deposits and ameliorates tauopathy-related behavioral defect in an established transgenic Drosophila model expressing human tau. It is suggested that NQTrp, or optimized versions of it, could act as novel disease modifying drugs for AD and other tauopathies (Frenkel-Pinter, 2016).

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Bouge, A. L. and Parmentier, M. L. (2016). Tau excess impairs mitosis and kinesin-5 function, leading to aneuploidy and cell death. Dis Model Mech [Epub ahead of print]. PubMed ID: 26822478

Abstract
In neurodegenerative diseases like Alzheimer's disease (AD), cell cycle defects and associated aneuploidy have been described. However, the importance of these defects in the physiopathology of AD and the underlying mechanistic processes are largely unknown in particular with respect to the microtubule-binding protein Tau, which is found in excess in the brain and cerebrospinal fluid of patients. Although it has long been known that Tau is phosphorylated during mitosis to generate a lower affinity for microtubules, there has been no indication that an excess of this protein could affect mitosis. The effect of an excess of human Tau (hTau) protein on cell mitosis was studied in vivo. Using the Drosophila developing wing disc epithelium as a model, this study shows that an excess of hTau induces a mitotic arrest, with the presence of monopolar spindles. This mitotic defect leads to aneuploidy and apoptotic cell death. The mechanism of action of hTau was studied and it was found that the MT-binding domain of hTau is responsible for these defects. hTau effects occur via the inhibition of the function of the kinesin Klp61F, the Drosophila homologue of kinesin-5 (also called Eg5 or KIF11). This deleterious effect of hTau is also found in other Drosophila cell types (neuroblasts) and tissues (the developing eye disc) as well as in human Hela cells.By demonstrating that microtubule-bound Tau inhibits the Eg5/KIF11 kinesin and cell mitosis, this work provides a new framework to consider the role of Tau in neurodegenerative diseases (Bouge, 2016).

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Kilian, J.G., Hsu, H.W., Mata, K., Wolf, F.W. and Kitazawa, M. (2017). Astrocyte transport of glutamate and neuronal activity reciprocally modulate tau pathology in Drosophila. Neuroscience [Epub ahead of print]. PubMed ID: 28215745

Abstract
Abnormal buildup of the microtubule associated protein tau is a major pathological hallmark of Alzheimer's disease (AD) and various tauopathies. The mechanisms by which pathological tau accumulates and spreads throughout the brain remain largely unknown. It is known that a restoration of the major astrocytic glutamate transporter, GLT1, ameliorates a buildup of tau pathology and rescues cognition in a mouse model of AD. In this study, it was hypothesized that aberrant extracellular glutamate and abnormal neuronal excitatory activities promote tau pathology. Consequently, the genetic interactions between tau and the GLT1 homolog dEaat1 were investigated in Drosophila melanogaster. Neuronal-specific overexpression of human wildtype tau markedly shortens lifespan and impairs motor behavior. RNAi depletion of dEaat1 in astrocytes worsens these phenotypes, whereas overexpression of dEaat1 improves them. However, the synaptic neuropil appears unaffected, and there is no major neuronal loss with tau overexpression in combination with dEaat1 depletion. To mimic glutamate-induced aberrant excitatory input in neurons, repeated depolarization of neurons via transgenic TrpA1 was applied to the adult Drosophila optic nerves, and the change of tau deposits was examined. Repeated depolarization significantly increases the accumulation of tau in these neurons. The study propose that increased neuronal excitatory activity exacerbates tau-mediated neuronal toxicity and behavioral deficits (Killian, 2017).

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Li, A., Hooli, B., Mullin, K., Tate, R.E., Bubnys, A., Kirchner, R., Chapman, B., Hofmann, O., Hide, W. and Tanzi, R.E. (2017). Silencing of the Drosophila ortholog of Sox5 leads to abnormal neuronal development and behavioral impairment. Hum Mol Genet [Epub ahead of print]. PubMed ID: 28186563

Abstract
SOX5 encodes a transcription factor that is expressed in multiple tissues including heart, lung and brain. Mutations in SOX5 have been previously found in patients with amyotrophic lateral sclerosis (ALS) and developmental delay, intellectual disability and dysmorphic features. To characterize the neuronal role of SOX5, this study silenced the Drosophila ortholog of SOX5, Sox102F, by RNAi in various neuronal subtypes in Drosophila. Silencing of Sox102F leads to misorientated and disorganized michrochaetes, neurons with shorter dendritic arborization (DA) and reduced complexity, diminished larval peristaltic contractions, loss of neuromuscular junction bouton structures, impaired olfactory perception, and severe neurodegeneration in brain. Silencing of SOX5 in human SH-SY5Y neuroblastoma cells results in a significant repression of WNT signaling activity and altered expression of WNT-related genes. Samples of SOX5 variants reveals several variants that show significant association with Alzheimer’s disease disease status. These findings indicate that SOX5 is a novel candidate gene for AD with important role in neuronal function. The genetic findings warrant further studies to identify and characterize SOX5 variants that confer risk for AD, ALS and intellectual disability.

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Cutler, T., Sarkar, A., Moran, M., Steffensmeier, A., Puli, O.R., Mancini, G., Tare, M., Gogia, N. and Singh, A. (2015). Drosophila eye model to study neuroprotective role of CREB Binding Protein (CBP) in Alzheimer's disease. PLoS One 10: e0137691. PubMed ID: 25714733

Abstract
The progressive neurodegenerative disorder Alzheimer's disease (AD) manifests as loss of cognitive functions, and finally leads to death of the affected individual. AD may result from accumulation of amyloid plaques. These amyloid plaques comprising of amyloid-beta 42 (Aβ42) polypeptides results from the improper cleavage of amyloid precursor protein (APP) in the brain. The Aβ42 plaques have been shown to disrupt the normal cellular processes and thereby trigger abnormal signaling which results in the death of neurons. However, the molecular-genetic mechanism(s) responsible for Aβ42 mediated neurodegeneration is yet to be fully understood. This study utilized Gal4/UAS system to develop a transgenic fruit fly model for Aβ42 mediated neurodegeneration. Targeted misexpression of human Aβ42 in the differentiating photoreceptor neurons of the developing eye of transgenic fly triggers neurodegeneration. This progressive neurodegenerative phenotype resembles Alzheimer's like neuropathology. The histone acetylase, CREB Binding Protein (CBP), was identified as a genetic modifier of Aβ42 mediated neurodegeneration. Targeted misexpression of CBP along with Aβ42 in the differentiating retina can significantly rescue neurodegeneration. It was found that gain-of-function of CBP rescues Aβ42 mediated neurodegeneration by blocking cell death. Misexpression of Aβ42 affects the targeting of axons from retina to the brain but misexpression of full length CBP along with Aβ42 can restore this defect. The CBP protein has multiple domains and is known to interact with many different proteins. Structure-function analysis using truncated constructs lacking one or more domains of CBP protein in transgenic flies reveals that Bromo, HAT and polyglutamine (BHQ) domains together are required for the neuroprotective function of CBP. This BHQ domain of CBP has not been attributed to promote survival in any other neurodegenerative disorders. In conclusion, the study identifies CBP as a genetic modifier of Aβ42 mediated neurodegeneration. Furthermore, BHQ domain of CBP is responsible for its neuroprotective function (Cutler, 2015).

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Jonson, M., Pokrzywa, M., Starkenberg, A., Hammarstrom, P. and Thor, S. (2015). Systematic Aβ analysis in Drosophila reveals high toxicity for the 1-42, 3-42 and 11-42 peptides, and emphasizes N- and C-terminal residues. PLoS One 10: e0133272. PubMed ID: 25714733

Abstract
Brain amyloid plaques are a hallmark of Alzheimer's disease (AD), and primarily consist of aggregated Aβ peptides. While Aβ 1-40 and Aβ 1-42 are the most abundant, a number of other Aβ peptides have also been identified. Studies have indicated differential toxicity for these various Aβ peptides, but in vivo toxicity has not been systematically tested. To address this issue, this study generated improved transgenic Drosophila UAS strains expressing 11 pertinent Aβ peptides. UAS transgenic flies were generated by identical chromosomal insertion, hence removing any transgenic position effects, and crossed to a novel and robust Gal4 driver line. Using this improved Gal4/UAS set-up, survival and activity assays revealed that Aβ 1-42 severely shortens lifespan and reduces activity. N-terminal truncated peptides are quite toxic, with 3-42 similar to 1-42, while 11-42 shows a pronounced but less severe phenotype. N-terminal mutations in 3-42 (E3A) or 11-42 (E11A) result in reduced toxicity for 11-42, and reduced aggregation for both variants. Strikingly, C-terminal truncation of Aβ (1-41, -40, -39, -38, -37) are non-toxic. In contrast, C-terminal extension to 1-43 results in reduced lifespan and activity, but not to the same extent as 1-42. Mutating residue 42 in 1-42 (A42D, A42R and A42W) greatly reduces Aβ accumulation and toxicity. Histological and biochemical analysis reveals strong correlation between in vivo toxicity and brain Aβ aggregate load, as well as amount of insoluble Aβ. This systematic Drosophila in vivo and in vitro analysis reveals crucial N- and C-terminal specificity for Aβ neurotoxicity and aggregation, and underscores the importance of residues 1-10 and E11, as well as a pivotal role of A42 (Jonson, 2015).

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Haddadi, M., Nongthomba, U., Jahromi, S. R. and Ramesh, S. R. (2015). Transgenic Drosophila model to study apolipoprotein E4-induced neurodegeneration. Behav Brain Res [Epub ahead of print]. PubMed ID: 26706888

Abstract
The ε4 isoform of Apolipoprotein E (ApoE4) that is involved in neuron-glial lipid metabolism has been demonstrated as the main genetic risk factor in late-onset of Alzheimer's disease. However, the mechanism underlying ApoE4-mediated neurodegeneration remains unclear. This study created a transgenic model of neurodegenerative disorder by expressing ε3 and ε4 isoforms of human ApoE in the Drosophila. The genetic models exhibited progressive neurodegeneration, shortened lifespan and memory impairment. Genetic interaction studies between amyloid precursor protein and ApoE in axon pathology of the disease revealed that over expression of hApoE in Appl-expressing neurons of Drosophila brain causes neurodegeneration. This Drosophila model may facilitate analysis of the molecular and cellular events implicated in hApoE4 neurotoxicity.

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Ali, Y. O., Ruan, K. and Zhai, R. G. (2012). NMNAT suppresses tau-induced neurodegeneration by promoting clearance of hyperphosphorylated tau oligomers in a Drosophila model of tauopathy. Hum Mol Genet 21(2): 237-250. PubMed ID: 21965302

Abstract

Tauopathies, including Alzheimer's disease, are a group of neurodegenerative diseases characterized by abnormal tau hyperphosphorylation that leads to formation of neurofibrillary tangles. Drosophila models of tauopathy display prominent features of the human disease including compromised lifespan, impairments of learning, memory and locomotor functions and age-dependent neurodegeneration visible as vacuolization. This study used a Drosophila model of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), in order to study the neuroprotective capacity of a recently identified neuronal maintenance factor, nicotinamide mononucleotide (NAD) adenylyl transferase (NMNAT), a protein that has both NAD synthase and chaperone function. NMNAT is essential for maintaining neuronal integrity under normal conditions and has been shown to protect against several neurodegenerative conditions. However, its protective role in tauopathy has not been examined. This study shows that overexpression of NMNAT significantly suppresses both behavioral and morphological deficits associated with tauopathy by means of reducing the levels of hyperphosphorylated tau oligomers. Importantly, the protective activity of NMNAT protein is independent of its NAD synthesis activity, indicating a role for direct protein-protein interaction. Next, it was shown that NMNAT interacts with phosphorylated tau in vivo and promotes the ubiquitination and clearance of toxic tau species. Consequently, apoptosis activation was significantly reduced in brains overexpressing NMNAT, and neurodegeneration was suppressed. This report on the molecular basis of NMNAT-mediated neuroprotection in tauopathies opens future investigation of this factor in other protein foldopathies (Ali, 2012).

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Kong, Y., Wu, J. and Yuan, L. (2014). MicroRNA expression analysis of adult-onset Drosophila Alzheimer's disease model. Curr Alzheimer Res 11: 882-891. PubMed ID: 25274109

Abstract
Alzheimer's disease (AD) is the most common reason for dementia in elderly population. Its neuropathological features include senile plaques, neurofibril tangles and neuronal death. Many AD animal models have been established including yeast, Caenorhabditis elegans, Drosophila melanogaster, mice, rats and non-human primates. Drosophila AD models are much more efficient for genetic manipulation and screening assay than mammals. microRNAs (miRNAs) are ~22nt small RNA molecules that fine-tune gene expression at posttranscriptional level. The dysregulation of miRNAs could participate in AD progression by influencing targets' expression and functions. However, miRNA expression profile of AD flies has not yet been investigated. Using the latest µParaflo™ miRNA microarray assay, this study found that 17 miRNAs are consistently dysregulated in adult-onset AD Drosophila brains: eight of which are upregulated (miR- 8, miR-13b, miR-277, miR-279, miR-981, miR-995, miR-998, miR-1017) and nine are downregulated (let-7, miR-1, miR-9a, miR-184, miR-193, miR-263b, miR-276a, miR-285, miR-289). KEGG pathway annotations using DIANA miRPath or targets predicted by Targetscan identifies 7 pathways (Valine, leucine and isoleucine degradation; MAPK signaling pathway; Dorso-ventral axis formation; Propanoate metabolism; Sphingolipid metabolism; Lysine degradation; Jak- STAT signaling pathway) which might be influenced by these miRNAs. Integrative miRNA/mRNA regulatory network analysis reveals functional cluster with transaminase activity to be potentially regulated by miRNAs in AD. Taken together, dysregulation of miRNA profile may participate in AD pathogenesis by interrupting the metabolism of amino acids in the brain (Kong, 2014).

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Reviews

Fernandez-Funez, P., de Mena, L. and Rincon-Limas, D.E. (2015). Modeling the complex pathology of Alzheimer's disease in Drosophila. Exp Neurol [Epub ahead of print]. PubMed ID: 26024860

Chen, K.F. and Crowther, D.C. (2015). Insights into amyloid disease from fly models. Essays Biochem 56: 69-83. PubMed ID: 25131587

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More in IF

Role of Nck-associated protein 1 (Nap1) in Alzheimer's disease

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Recent Updates

Bolus, H., Crocker, K., Boekhoff-Falk, G. and Chtarbanova, S. (2020). Modeling Neurodegenerative Disorders in Drosophila melanogaster. Int J Mol Sci 21(9). PubMed ID: 32357532

Jeon, Y., Lee, J. H., Choi, B., Won, S. Y. and Cho, K. S. (2020). Genetic Dissection of Alzheimer's Disease Using Drosophila Models. Int J Mol Sci 21(3). PubMed ID: 32019113

Khanna, M.R. and Fortini, M.E. (2015). Transcriptomic analysis of Drosophila mushroom body neurons lacking amyloid-β precursor-like protein activity. J Alzheimers Dis 46: 913-928. PubMed ID: 26402626

Shaw, J.L., Zhang, S. and Chang, K.T. (2015). Bidirectional regulation of amyloid precursor protein-induced memory defects by Nebula/DSCR1: A protein upregulated in Alzheimer's disease and Down syndrome. J Neurosci 35: 11374-11383. PubMed ID: 26269644

Bourdet, I., Lampin-Saint-Amaux, A., Preat, T. and Goguel, V. (2015). Amyloid-β peptide exacerbates the memory deficit caused by amyloid precursor protein loss-of-function in Drosophila. PLoS One 10: e0135741. PubMed ID: 26274614

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Date revised: 25 September 2023

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