The InteractiveFly: Drosophila as a Model for Human Diseases


Alzheimer's disease
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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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Krupp, S., Tam, O., Gale Hammell, M. and Dubnau, J.. (22). 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 Flower^LoseB 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 Flower^LoseB 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 kuz^e29-4/+ 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 FasII^e76 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>FasII^e76/+ larvae. This study therefore investigated whether removing one copy of Appl could rescue the memory deficits of homozygous i>FasII^e76 mutants. This resulted in a significant improvement in performance, as did one copy of the i>FasII^e76 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, Aβ 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 (tau^wt) 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 tau^wt 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 Tau^WT, 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 Tau^WT 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 Tau^WT 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 Aβ 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

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 (PrP^C) can act as a cell-surface receptor for amyloid-beta (Abeta) peptide; however, a role for PrP^C in the pathogenesis of Alzheimer's disease (AD) is contested. This study has expressed a range of Abeta isoforms and PrP^C in the Drosophila brain. Co-expression of Abeta and PrP^C 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 PrP^C individually did not lead to these phenotypic changes. In vitro studies revealed that substoichiometric amounts of PrP^C trap Abeta as oligomeric assemblies and fragment-preformed Abeta fibers. The ability of membrane-anchored PrP^C 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 PrP^C in AD. This study provides a second animal model linking PrP^C expression with Abeta toxicity and supports a role for PrP^C in AD pathogenesis. Blocking the interaction of Abeta and PrP^C 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).