Glucocerebrosidase reduces the spread of protein aggregation in a Drosophila melanogaster model of neurodegeneration by regulating proteins trafficked by extracellular vesicles
Abnormal protein aggregation within neurons is a key pathologic feature of Parkinson's disease (PD). The spread of brain protein aggregates is associated with clinical disease progression, but how this occurs remains unclear. Mutations in glucosidase, beta acid 1 (GBA), which encodes glucocerebrosidase (GCase), are the most penetrant common genetic risk factor for PD and dementia with Lewy bodies and associate with faster disease progression. To explore how GBA mutations influence pathogenesis, Drosophila model of GBA deficiency (Gba1b) was created that manifests neurodegeneration and accelerated protein aggregation. Proteomic analysis of Gba1b mutants revealed dysregulation of proteins involved in extracellular vesicle (EV) biology, and altered protein composition of EVs was found from Gba1b mutants. Accordingly, it was hypothesized that GBA may influence pathogenic protein aggregate spread via EVs. It was found that accumulation of ubiquitinated proteins and Ref(2)P, Drosophila homologue of mammalian p62, were reduced in muscle and brain tissue of Gba1b flies by ectopic expression of wildtype GCase in muscle. Neuronal GCase expression also rescued protein aggregation both cell-autonomously in brain and non-cell-autonomously in muscle. Muscle-specific GBA expression reduced the elevated levels of EV-intrinsic proteins and Ref(2)P found in EVs from Gba1b flies. Perturbing EV biogenesis through neutral sphingomyelinase (nSMase), an enzyme important for EV release and ceramide metabolism, enhanced protein aggregation when knocked down in muscle, but did not modify Gba1b mutant protein aggregation when knocked down in neurons. Lipidomic analysis of nSMase knockdown on ceramide and glucosylceramide levels suggested that Gba1b mutant protein aggregation may depend on relative depletion of specific ceramide species often enriched in EVs. Finally, ectopically expressed GCase was identified within isolated EVs. Together, these findings suggest that GCase deficiency promotes accelerated protein aggregate spread between cells and tissues via dysregulated EVs, and EV-mediated trafficking of GCase may partially account for the reduction in aggregate spread (Jewett, 2021).
Many genetic influences of PD have now been identified, and much work has been focused on how these genes lead to protein aggregation through mechanisms such as protein misfolding and autophagy defects. However, none of these genes have been implicated in cell-to-cell spread of pathogenic protein aggregates, which closely correlates with clinical disease progression. Proteomic analysis and non-cell-autonomous rescue of protein aggregation in Gba1b mutants has led to the hypothesize that GBA mutations may influence the rate of propagation of protein aggregates between neurons. This work suggests a link between GBA mutations and faster spread of intracellular protein aggregates via a novel EV-mediated mechanism, possibly explaining the recent clinical finding that GBA mutations accelerate the progression of clinical disease. Using a Drosophila model of GBA deficiency that manifests accelerated protein aggregation, this study found that expressing WT GCase in specific tissues of a GBA-deficient fly can not only rescue protein aggregation cell-autonomously and in distant tissues, but also rescue alterations in protein cargo observed in EVs isolated from Gba1b mutant hemolymph. Interestingly, ectopically expressed WT GCase itself was found within EVs of GBA-deficient flies, suggesting that the non-cell-autonomous rescue due to GCase expression is mediated by both reduction in aggregated proteins in EVs and trafficking of GCase via EVs to distant cells and tissues. Perturbing EV biogenesis through decreased expression of ESCRT-independent nSMase affected protein aggregation in local tissues in a tissue-dependent manner, and further decreased a subset of Cer species already reduced in Gba1b mutants. Interestingly, this subset of Cer species is known to be enriched in EV membranes. Together, these findings suggest that mutations in GBA result in the accelerated spread of protein aggregates through changes in cellular lipid composition and dysregulation of proteins trafficked by EVs (Jewett, 2021).
Although the model of GBA mutations promoting spread of protein aggregates via EVs is novel, the idea that proteostasis can be maintained in a non-cell-autonomous fashion is well supported in the literature. For example, in C. elegans, misfolded α-synuclein accumulating in endo-lysosomal vesicles was found to be transmitted from muscle to the hypodermis, a nearby tissue, for degradation. It is possible that a non-cell-autonomous mechanism is necessary because certain tissues may be more efficient in reducing protein aggregation. This has been previously described, where overexpression of FOXO in Drosophila muscle decreased aging-related protein aggregates in muscle as well as brain and other distant tissues, but FOXO overexpression in adipose tissue was unable to prevent protein aggregation in muscle. In the curren model, overexpressing dGba1b in Drosophila muscle or neuronal tissue prevented accumulation of protein aggregates throughout the organism, however overexpression of WT GCase in midgut and fat body did not significantly reduce protein aggregation in the brain. These discrepancies could be due to tissue-specific biogenesis of EVs, which could depend on factors such as metabolic rate or endovesicular trafficking. Although dGba1b is expressed in all tissues, a second homologue of human GBA1, dGba1a, is expressed only in the midgut. Gba1b mutants retain ~25% expression of dGba1a. Deficiency of dGba1a was found to extend lifespan and does not result in significant accumulation of GlcCer, suggesting that there can be significant tissue-specific differences in function for GCase that could influence EV biogenesis (Jewett, 2021).
The unexpected results from perturbation of EV biogenesis suggest that the EV-mediated regulation of protein aggregation is tissue-specific and complex. Because an increase in EV-intrinsic proteins and alteration of protein cargo were observed in Gba1b mutants, it is anticipated that genetic perturbations decreasing the biogenesis of EVs might rescue protein aggregation non-cell-autonomously by reducing the production of dysregulated EVs. However, decreased expression of ESCRT-independent nSMase in muscle did not rescue protein aggregation in heads, suggesting that a tissue-specific decrease in biogenesis of dysregulated EVs is not sufficient to reduce protein aggregation in the rest of the organism, and the cargo of EVs may need to be corrected to reduce spread of protein aggregation. In contrast, decreased expression of nSMase in the nervous system had no effect on protein aggregation in the head. This difference in outcome in perturbation of EV biogenesis in muscle and neurons could be due to cell-specific compensatory mechanisms or intrinsic metabolic demands and solicits further investigation (Jewett, 2021).
A possible explanation for why decreased muscle expression of nSMase enhanced cell-autonomous protein aggregation and EV protein cargo alterations is that both GCase and nSMase enzymatically produce Cer. If GCase-deficient phenotypes are dependent on a relative reduction in Cer, decreased nSMase expression could exacerbate Gba1b mutant phenotypes. Indeed, lipidomic analysis of alterations in Cer metabolism due to nSMase knockdown revealed a further decrease in a subset of Cer species that were already significantly decreased in Gba1b mutants compared to controls. The further reduction in Cer species due to nSMase knockdown correlates with enhancement of cell-autonomous protein aggregation and EV cargo alterations, suggesting that accelerated protein aggregation in Gba1b mutants is mediated by Cer deficiency rather than GlcCer accumulation, as nSMase knockdown had a much more modest effect on the significantly increased levels of GlcCer species in Gba1b mutants compared to controls (Jewett, 2021).
Cer has been implicated in the composition and biogenesis of EVs, and nSMase knockdown further altered EV cargo in Gba1b mutants, suggesting that decreased Cer levels may directly influence EV biogenesis in Gba1b mutants. However, Cer species were not globally decreased, suggesting that the regulation of Cer metabolism is complex and may be more dependent on specific Cer species. Interestingly, only 1 of the 9 Cer species significantly increased in Gba1b mutants versus controls had a monounsaturated fatty acyl group, while all 5 of the Cer species significantly decreased in Gba1b mutants versus controls had a monounsaturated fatty acyl group, suggesting GBA influences the metabolism of specific subset of Cer species that may be implicated in Gba1b mutant phenotypes. This subset of Cer species is enriched in species with long chain monounsaturated fatty acyl chains. Interestingly, lipids with monounsaturated fatty acyl groups are an abundant component in mammalian exosome membranes. Investigating the alterations in lipid composition of EVs resulting from GCase deficiency and nSMase knockdown will be important in elucidating the role of Cer metabolism in Gba1b mutant phenotypes (Jewett, 2021).
This work suggests that GCase deficiency influences EV biogenesis to promote faster propagation of pathogenic protein aggregates throughout the tissues of an organism, which may be a compensatory response to cell-autonomous lysosomal stress. In the initial characterization of the Drosophila GBA-deficient model, accelerated insoluble ubiquitinated protein aggregates, accumulation of Ref(2)P, and oligomerization of ectopically expressed human α-synuclein in was found Gba1b mutants, suggesting an impairment in lysosomal degradation. A similar GBA-deficient Drosophila model also found evidence of lysosomal dysfunction, including enlarged lysosomes in GBA-deficient brains. However, proteomic analysis of Gba1b mutants did not support a profound impairment in autophagy, but instead suggested dysregulation of EVs with altered protein cargo which could be suppressed locally with knockdown of genes encoding ESCRT machinery important for EV biogenesis. Based on these results, it is believed that the initial observations of increased insoluble ubiquitinated proteins and Ref(2)P in Gba1b mutants are due to lysosomal stress. One possible explanation for the proteomic findings is that there may be a compensatory increase in EV biogenesis and packaging of autophagy substrates within EVs for discard outside of the cell in Gba1b mutants. Such an increase may have prevented detection od defects in autophagy. Upregulation of EV biogenesis may be cell-autonomously neuroprotective in the setting of lysosomal stress, particularly in aggregation-prone neurodegenerative diseases such as PD. It was recently demonstrated in a human neuronal cell culture model of PD that inhibiting macroautophagy protects against α-synuclein-induced cell death by promoting the release of α-synuclein-containing EVs. However, it remains possible that upregulating EV biogenesis may relieve lysosomal stress within cells containing aggregate-prone proteins, while simultaneously promoting the spread of protein aggregates between cells and throughout the organism (Jewett, 2021).
This work suggests a novel mechanism for GBA in reducing the spread of pathogenic protein aggregation from cell-to-cell via regulation of EV protein cargo, but many key questions remain. To better understand the progression of neurodegenerative diseases, it is important to uncover the mechanisms by which GCase deficiency alters EV protein content and biogenesis, identify the specific changes in EVs facilitating propagation of pathogenic protein aggregates, and determine how these changes influence recipient cells internalizing dysregulated EVs. GCase is a critical enzyme in ceramide metabolism, hydrolyzing glucosylceramide into glucose and ceramide. Ceramides are a key component of EV membranes, and alterations in ceramide metabolism due to GCase deficiency may directly influence EV biogenesis and protein cargo trafficked via EVs. Further studies using this Drosophila model and mammalian cell culture models should better elucidate how GCase deficiency alters the protein cargo of EVs to induce propagation of pathogenic protein aggregates, as well as whether endogenous GCase is enzymatically functional when trafficked to distant tissues via EVs. Understanding this mechanism could have broad implications in understanding the pathogenesis of aggregate-prone neurodegenerative diseases and reveal new therapeutic targets to slow or halt disease progression (Jewett, 2021).
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Fellgett, A., Middleton, C. A., Munns, J., Ugbode, C., Jaciuch, D., Wilson, L., Chawla, S. and Elliott, C. J. H. (2021). Multiple Pathways of LRRK2-G2019S/Rab10 Interaction in Dopaminergic Neurons. J Parkinsons Dis. PubMed ID: 34250948
Abstract
Inherited mutations in the LRRK2 protein are the common causes of Parkinson's disease, but the mechanisms by which increased kinase activity of mutant LRRK2 leads to pathological events remain to be determined. In vitro assays (heterologous cell culture, phospho-protein mass spectrometry) suggest that several Rab proteins might be directly phosphorylated by LRRK2-G2019S. An in vivo screen of Rab expression in dopaminergic neurons in young adult Drosophila demonstrated a strong genetic interaction between LRRK2-G2019S and Rab10. To determine if Rab10 is necessary for LRRK2-induced pathophysiological responses in the neurons that control movement, vision, circadian activity, and memory. These four systems were chosen because they are modulated by dopaminergic neurons in both humans and flies. LRRK2-G2019S was expressed in Drosophila dopaminergic neurons and the effects of Rab10 depletion on Proboscis Extension, retinal neurophysiology, circadian activity pattern ('sleep'), and courtship memory determined in aged flies. Rab10 loss-of-function rescued LRRK2-G2019S induced bradykinesia and retinal signaling deficits. Rab10 knock-down, however, did not rescue the marked sleep phenotype which results from dopaminergic LRRK2-G2019S. Courtship memory is not affected by LRRK2, but is markedly improved by Rab10 depletion. Anatomically, both LRRK2-G2019S and Rab10 are seen in the cytoplasm and at the synaptic endings of dopaminergic neurons. It is concluded that, in Drosophila dopaminergic neurons, Rab10 is involved in some, but not all, LRRK2-induced behavioral deficits. Therefore, variations in Rab expression may contribute to susceptibility of different dopaminergic nuclei to neurodegeneration seen in people with Parkinson's disease (Fellgett, 2021).
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Pallos, J., Jeng, S., McWeeney, S. and Martin, I. (2015). Neurobiol Dis: 105390. PubMed ID: 33984508
Abstract
Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common genetic cause of late-onset autosomal dominant Parkinson's disease. The pathogenic G2019S mutation enhances LRRK2 kinase activity and induces neurodegeneration in C. elegans, Drosophila and rodent models through unclear mechanisms. Gene expression profiling has the potential to provide detailed insight into the biological pathways modulated by LRRK2 kinase activity in vivo. Prior studies have surveyed the effects of LRRK2 G2019S on genome-wide mRNA expression in complex brain tissues with high cellular heterogeneity, limiting their power to detect more restricted gene expression changes occurring in a cell type-specific manner. This study used translating ribosome affinity purification (TRAP) coupled to RNA-seq to profile dopamine neuron-specific gene expression changes caused by LRRK2 G2019S in the Drosophila CNS. A modest number of genes were differentially expressed in the presence of mutant LRRK2 that represent a broad range of molecular functions including DNA repair (RfC3), mRNA metabolism and translation (Ddx1 and lin-28), calcium homeostasis (MCU), and other categories (Ugt37c1, disp, l(1)G0196, CG6602, CG1126 and CG11068). Further analysis on a subset of these genes revealed that LRRK2 G2019S did not alter their expression across the whole brain, consistent with dopamine neuron-specific effects uncovered by the TRAP approach that may offer insight into the neurodegenerative process. This is the first study to profile the effects of LRRK2 G2019S on DA neuron gene expression in vivo. Beyond providing a set of differentially expressed gene candidates relevant to LRRK2, this study demonstrates the effective use of TRAP to perform high-resolution assessment of dopamine neuron gene expression for the study of PD (Pallos, 2021).
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Sen, A., Kalvakuri, S., Bodmer, R. and Cox, R. T. (2015). Clueless, a protein required for mitochondrial function, interacts with the PINK1-Parkin complex in Drosophila. Dis Model Mech 8: 577-589. PubMed ID: 26035866
Abstract
Loss of mitochondrial function often leads to neurodegeneration and is thought to be one of the underlying causes of neurodegenerative diseases such as Parkinson's disease. However, the precise events linking mitochondrial dysfunction to neuronal death remain elusive. PTEN-induced putative kinase 1 (PINK1) and Parkin (Park), either of which, when mutated, are responsible for early-onset PD, mark individual mitochondria for destruction at the mitochondrial outer membrane. The specific molecular pathways that regulate signaling between the nucleus and mitochondria to sense mitochondrial dysfunction under normal physiological conditions are not well understood. This study shows that Drosophila Clueless (Clu), a highly conserved protein required for normal mitochondrial function, can associate with Translocase of the outer membrane (TOM) 20, Porin and PINK1, and is thus located at the mitochondrial outer membrane. Previous studies have found that clu genetically interacts with park in Drosophila female germ cells. This study shows that clu also genetically interacts with PINK1, and epistasis analysis places clu downstream of PINK1 and upstream of park. In addition, Clu forms a complex with PINK1 and Park, further supporting that Clu links mitochondrial function with the PINK1-Park pathway. Lack of Clu causes PINK1 and Park to interact with each other, and clu mutants have decreased mitochondrial protein levels, suggesting that Clu can act as a negative regulator of the PINK1-Park pathway. Taken together, these results suggest that Clu directly modulates mitochondrial function, and that Clu's function contributes to the PINK1-Park pathway of mitochondrial quality control (Sen, 2015).
Discussion
Mitochondrial function is intimately linked to cellular health. These organelles provide the majority of ATP for the cell in addition to being the sites for major metabolic pathways such as fatty acid β-oxidation and heme biosynthesis. In addition, mitochondria are crucial for apoptosis, and they can irreparably damage the cell via oxidation when their biochemistry is abnormally altered. Given these many roles, tissues and cell types with high energy demands, such as neurons, are particularly sensitive to changes in mitochondrial function. This is also true for germ cell mitochondria because mitochondria are inherited maternally from the egg's cytoplasm and are thus the sole source of energy for the newly developing embryo (Sen, 2015).
Mitochondrial biology is complex owing to the dynamic nature of the organelle and the fact that most of the proteins required for function are encoded in the nucleus. In addition to the metabolites they provide, mitochondria undergo regulated fission, fusion and transport along microtubules. Because mitochondria cannot be made de novo, and tend to accumulate oxidative damage due to their biochemistry, they are subject to organelle and protein quality-control measures that involve mitochondrial and cytoplasmic proteases, as well as a specialized organelle-specific autophagy called mitophagy. However, the specific molecular signaling pathways between the nucleus and mitochondria that are used to sense which individual mitochondria are damaged during normal cellular homeostasis in vivo are not well understood.
This study used the Drosophila ovary to identify genes regulating mitochondrial function and have characterized mitochondrial dynamics during Drosophila oogenesis. Germ cells contain large numbers of mitochondria that can be visualized at the single organelle level, making this system useful for studying genes that control mitochondrial function (Sen, 2015).
The gene clueless (clu) is crucial for mitochondrial localization in germ cells. Clu has homologs in many different species, and shows 53% amino acid identity to the human homolog, CLUH. The molecular role of Clu is not known. The yeast homolog, Clu1p, was found to interact with the eukaryotic initiation factor 3 (eIF3) complex in yeast and bind mRNA; however, the significance of this is not clear. CLUH has also been shown to bind mRNA. Flies mutant for clu are weak, uncoordinated, short-lived, and male and female sterile. Lack of Clu causes a sharp decrease in ATP, increased mitochondrial oxidative damage and changes in mitochondrial ultrastructure. Levels of Clu protein are homogeneously high in the cytoplasm and it is also found in large mitochondrially-associated particles. Although Clu clearly has an effect on mitochondria function, whether this is direct or indirect has not yet been established (Sen, 2015).
Parkin (Park), an E3 ubiquitin ligase, acts with PTEN-induced putative kinase 1 (PINK1) to target mitochondria for mitophagy. clu genetically interacts with park, and Clu particles are absent in park mutants, indicating that Clu might play a role in Park's mechanism. park and PINK1 have been identified as genes that, when mutated, cause early-onset forms of Parkinson's disease. Upon mitochondrial depolarization, PINK1 is stabilized on the mitochondrial outer membrane, recruiting Park, which then goes on to ubiquitinate many surface proteins, thus marking and targeting that mitochondrion for mitophagy. Before their biochemical interaction was recognized, PINK1 was placed upstream of park in a genetic pathway in Drosophila. Understanding Park and PINK1's role in mitochondrial quality control has shed light on the neurodegeneration underlying Parkinson's disease (Sen, 2015).
This study shows that Clu's mitochondrial role is well conserved, because the human homolog, CLUH, can rescue the fly mutant. Clu peripherally associates with mitochondria because it forms a complex with the mitochondrial outer-membrane proteins Porin and Translocase of the outer membrane (TOM) 20, supporting that the loss of mitochondrial function caused by lack of Clu is a direct effect. In addition, this study found that clu genetically interacts with PINK1 and, using epistasis, clu was placed upstream of park, but downstream of PINK1. Clu forms a complex with PINK1, and is able to interact with Park after the mitochondrial membrane potential is disrupted. Finally, lack of Clu causes PINK1 and Park to interact with each other, as well as causing a decrease in mitochondrial proteins, which suggests that Clu negatively regulates PINK1-Park function. Taken together, these data identify Clu as a mitochondrially-associated protein that plays a direct role in maintaining mitochondrial function and that binds TOM20, and support a role for Clu linking mitochondrial function to the PINK1-Park pathway (Sen, 2015).
Drosophila Clu is a large, highly conserved protein that shares its Clu and tetratricopeptide repeat (TPR) domains with its human homolog, CLUH. Expressing CLUH in flies that are mutant for clu rescues the mutant phenotypes; thus, the human protein can use the fly machinery to fulfill the role of Clu. To date, all the evidence supports the idea that Clu has a role in mitochondrial function; however, it has been unclear how direct it is. In this study, using IPs showed that Clu can associate with three proteins located on the mitochondrial outer membrane, TOM20, Porin and PINK1. Thus, Clu is not only a cytoplasmic protein, but can also be a peripherally associated mitochondrial protein, supporting the idea that this highly conserved protein directly affects mitochondrial function (Sen, 2015).
clu mutants share many phenotypes with park and PINK1 mutant flies, including flight muscle defects and sterility. Mitochondria are also mislocalized in PINK1 mutant germ cells, similarly to park mutants, and form large knotted clumps that include circularized mitochondria, which is consistent with increased fusion events. Mitochondria in clu mutant germ cells, on the other hand, do not show any signs of changes in fission or fusion. clu also genetically interacts with PINK1 and park, with double heterozygotes having clumped mitochondria in germ cells and a loss of Clu particles, and double knockdown of clu with PINK1 or park in flight muscle causing an increase in abnormal wing posture. Park functions in a pathway with PINK1 to elicit a mitophagic response, and overexpressing park can rescue PINK1 phenotypes in Drosophila. Using S2R+ cells and clu RNAi knockdown, this study found that overexpressing Park, but not PINK1, causes mitochondria to disperse. In adult flies, overexpressing full-length clu rescues the abnormal wing phenotype as well as mitochondrial phenotypes of PINK1 mutants, and overexpressing full-length clu or CLUH in PINK1, but not park, mutants rescues their thoracic indentation. These results place clu upstream of park, but downstream of PINK1. PINK1 stabilization on the mitochondrial outer membrane signals for Park to translocate to the organelle and subsequently ubiquitinate different proteins on the mitochondrial surface. Thus, it is somewhat surprising in Drosophila that loss of PINK1 can be rescued by increased amounts of Park, and suggests that there might be additional roles that Park plays in the cell. The data support the idea that an excess of Park overcomes deficits in mitochondrial function because it can rescue a loss of Clu as well. Mitochondrial clumping seems to be one of the responses to mitochondrial damage, in this system and in human tissue culture cells; thus, the dispersal upon Park overexpression in clu-RNAi-treated S2R+ cells is likely a sign of better mitochondrial health (Sen, 2015).
This study shows that Clu reciprocally immunoprecipitates with overexpressed PINK1 under normal cell culture conditions. PINK1 has been shown to directly bind TOM20, and Clu can also form a complex with TOM20, suggesting that all three proteins are found in close proximity at the mitochondrial membrane. Clu still immunoprecipitates with PINK1 when PINK1 is no longer targeted to the mitochondrial outer membrane (PINK1ΔMTS). This result indicates that Clu forms a complex with PINK1 independent of TOM20 or any other mitochondrial outer membrane proteins. Under normal conditions, PINK1 degradation happens so quickly that there are undetectable levels found at the outer mitochondrial membrane. Therefore, how is it possible that Clu is found in a complex with PINK1 in the absence of mitochondrial damage? It is likely that overexpressed PINK1 overwhelms the normal degradation process, thus becoming aberrantly stabilized at the outer mitochondrial membrane. Alternatively, it is possible that low levels of mitochondrial damage could account for the PINK1 being stabilized at the outer membrane, and then being able to interact with Clu (Sen, 2015).
Mitophagy ultimately leads to mitochondrial degradation in the lysosome. Currently, the literature involving Park and PINK1 uses mitochondrial protein levels as a read-out of mitophagy. However, recent data shows that different mitochondrial proteins have different half-lives, likely depending on what type of protein quality-control mechanism they use. Recent papers have examined protein half-life and found that Drosophila and yeast mitochondrial proteins, particularly those of Complex I in the case of flies, have increased half-lives when mitophagy proteins are missing. In addition, mitochondrial protein quality control does not always require destruction of the entire mitochondrion, but can selectively destroy certain proteins. For the mitochondrial proteins examined, all were greatly reduced in clu and PINK1 mutants, but not substantially altered in park mutants. This suggests that the turnover of the mitochondrial proteinsexamined is more sensitive to the absence of clu and PINK1 than park. This study found that Park and PINK1 form a complex in the absence of Clu. Thus, Clu is not necessary for this interaction, and loss of Clu causes a PINK1-Park interaction. This, plus the fact that Clu can be found at the outer mitochondrial membrane in a complex with both PINK1 and Park, suggests that Clu can influence mitochondrial quality or function, perhaps by regulating mitochondrial protein levels (Sen, 2015).
Yeast Clu1p was identified as a component of the eukaryotic initiation factor 3 (eIF3) complex and as an mRNA-binding protein. From IP and mass spectrometry data of the current study, there evidence that Clu can associate with the ribosome as well. Although CCCP is commonly used to force mitophagy and mitochondrial protein turnover, this treatment might not mimic the more subtle damage and changes mitochondria likely face in vivo. Mitochondrial protein import, for example, requires an intact mitochondrial membrane potential. Given the curent data, it is possible that Clu could function in co-translational import of proteins, as well as act as a sensor to couple PINK1-Park complex activation to how well protein import occurs. This would help explain why this study found that loss of Clu triggers a PINK1-Park interaction. In addition, Park and PINK1 directly interact with Porin and TOM20, respectively, placing them and Clu at the same place at the outer mitochondrial membrane. Recently, CLUH has been found to bind mRNAs for nuclear-encoded mitochondrial proteins, supporting a potential role in co-translational import. Further experiments are required to understand the precise relationship between Clu, TOM20, PINK1 and Park (Sen, 2015).
Mitochondria clearly undergo targeted destruction and require robust quality-control mechanisms, which are very active areas of investigation. PINK1 and Park's molecular mechanisms are particularly relevant to Parkinson's disease, given that inherited mutations in PARK2 and PINK1 can cause early-onset Parkinsonism. The molecular mechanisms that control mitophagy are becoming increasingly complex, involving membrane and cell biology; however, to date, the field has yet to visualize and understand the role of basal mitophagy levels in vivo. In the future, studying mitochondria and Clu function in Drosophila germ cells could lead to a better understand the role of mitochondrial protein turnover and quality control in the normal life cycle of tissues (Sen, 2015).
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Ham, S. J., Lee, D., Xu, W. J., Cho, E., Choi, S., Min, S., Park, S. and Chung, J. (2021). (2021). Loss of UCHL1 rescues the defects related to Parkinson's disease by suppressing glycolysis. Sci Adv 7(28). PubMed ID: 34244144
The role of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1; also called PARK5) in the pathogenesis of Parkinson's disease (PD) has been controversial. This study finds that the loss of UCHL1 destabilizes pyruvate kinase (PKM) and mitigates the PD-related phenotypes induced by PTEN-induced kinase 1 (PINK1) or Parkin loss-of-function mutations in Drosophila and mammalian cells. In UCHL1 knockout cells, cellular pyruvate production and ATP levels are diminished, and the activity of AMP-activated protein kinase (AMPK) is highly induced. Consequently, the activated AMPK promotes the mitophagy mediated by Unc-51-like kinase 1 (ULK1) and FUN14 domain-containing 1 (FUNDC1), which underlies the effects of UCHL1 deficiency in rescuing PD-related defects. Furthermore, this study identified tripartite motif-containing 63 (TRIM63) as a previously unknown E3 ligase of PKM and demonstrate its antagonistic interaction with UCHL1 to regulate PD-related pathologies. These results suggest that UCHL1 is an integrative factor for connecting glycolysis and PD pathology (Ham, 2021).
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Inoue, E., Suzuki, T., Shimizu, Y., Sudo, K., Kawasaki, H. and Ishida, N. (2021). Saffron ameliorated motor symptoms, short life span and retinal degeneration in Parkinson's disease fly models. Gene 799: 145811. PubMed ID: 34224829
Parkinson's disease (PD) is a common neurodegenerative disorder with motor symptoms linked to the loss of dopaminergic neurons in the brain. α-Synuclein is an aggregation-prone neural protein that plays a role in the pathogenesis of PD. In a previous paper, it was found that saffron; the stigma of Crocus sativus Linne (Iridaceae), and its constituents (crocin and crocetin) suppressed aggregation of α-synuclein and promoted the dissociation of α-synuclein fibrils in vitro. This study investigated the effect of dietary saffron and its constituent, crocetin, in vivo on a fly PD model overexpressing several mutant α-synuclein in a tissue-specific manner. Saffron and crocetin significantly suppressed the decrease of climbing ability in the Drosophila overexpressing A30P (A30P fly PD model) or G51D (G51D fly PD model) mutated α-synuclein in neurons. Saffron and crocetin extended the life span in the G51D fly PD model. Saffron suppressed the rough-eyed phenotype and the dispersion of the size histogram of the ocular long axis in the eye of A30P fly PD model. Saffron had a cytoprotective effect on a human neuronal cell line with α-synuclein fibrils. These data showed that saffron and its constituent crocetin have protective effects on the progression of PD disease in animals in vivo and suggest that saffron and crocetin can be used to treat PD (Inoue, 2021).
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Sarkar, S., Olsen, A. L., Sygnecka, K., Lohr, K. M. and Feany, M. B. (2021). alpha-synuclein impairs autophagosome maturation through abnormal actin stabilization. PLoS Genet 17(2): e1009359. PubMed ID: 33556113
Vesicular trafficking defects, particularly those in the autophagolysosomal system, have been strongly implicated in the pathogenesis of Parkinson's disease and related α-synucleinopathies. However, mechanisms mediating dysfunction of membrane trafficking remain incompletely understood. Using a Drosophila model of α-synuclein neurotoxicity with widespread and robust pathology, this study found that human α-synuclein expression impairs autophagic flux in aging adult neurons. Genetic destabilization of the actin cytoskeleton rescues F-actin accumulation, promotes autophagosome clearance, normalizes the autophagolysosomal system, and rescues neurotoxicity in α-synuclein transgenic animals through an Arp2/3 dependent mechanism. Similarly, mitophagosomes accumulate in human α-synuclein-expressing neurons, and reversal of excessive actin stabilization promotes both clearance of these abnormal mitochondria-containing organelles and rescue of mitochondrial dysfunction. These results suggest that Arp2/3 dependent actin cytoskeleton stabilization mediates autophagic and mitophagic dysfunction and implicate failure of autophagosome maturation as a pathological mechanism in Parkinson's disease and related α-synucleinopathies (Sarkar, 2021).
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Reiszadeh S. J., Ramesh, S. R., Finkelstein, D. I. and Haddadi, M. (2020). alpha-Synuclein E46K Mutation and Involvement of Oxidative Stress in a Drosophila Model of Parkinson's Disease. Parkinsons Dis 2021: 6621507. PubMed ID: 34285796
Parkinson's disease (PD) is an age-associated neurodegenerative condition in which some genetic variants are known to increase disease susceptibility on interaction with environmental factors inducing oxidative stress. Different mutations in the SNCA gene (synuclein α) are reported as the major genetic contributors to PD. E46K mutation pathogenicity has not been investigated as intensive as other SNCA gene mutations including A30P and A53T. In this study, based on the GAL4-UAS binary genetic tool, transgenic Drosophila melanogaster flies expressing wild-type and E46K-mutated copies of the human SNCA gene were constructed. Overexpression of human α-synuclein in the central nervous system of these transgenic flies led to disorganized ommatidia structures and loss of dopaminergic neurons. E46K α-synuclein caused remarkable climbing defects, reduced survivorship, higher ethanol sensitivity, and increased PQ-mediated mortality. A noticeable decline in activity of catalase and superoxide dismutase enzymes besides considerable increase in the levels of lipid peroxidation and reactive oxygen species was observed in head capsule homogenates of α-synuclein-expressing flies, which indicates obvious involvement of oxidative stress as a causal factor in SNCA (E46K) neurotoxicity. In all the investigations, E46K copy of the SNCA gene was found to impose more severe defects when compared to wild-type SNCA. It can be concluded that the constructed Drosophila models developed PD-like symptoms that facilitate comparative studies of molecular and cellular pathways implicated in the pathogenicity of different α-synuclein mutations (Reiszadeh, 2021).
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Sarkar, S., Murphy, M. A., Dammer, E. B., Olsen, A. L., Rangaraju, S., Fraenkel, E. and Feany, M. B. (2020). Comparative proteomic analysis highlights metabolic dysfunction in α-synucleinopathy. NPJ Parkinsons Dis 6(1): 40. PubMed ID: 33311497
The synaptic protein α-synuclein is linked through genetics and neuropathology to the pathogenesis of Parkinson's disease and related disorders. However, the mechanisms by which α-synuclein influences disease onset and progression are incompletely understood. To identify pathogenic pathways and therapeutic targets proteomic analysis was performed in a highly penetrant new Drosophila model of α-synucleinopathy. 476 significantly upregulated and 563 significantly downregulated proteins were identified in heads from α-synucleinopathy model flies compared to controls. Multiple complementary analyses was used to identify and prioritize genes and pathways within the large set of differentially expressed proteins for functional studies. Gene Ontology enrichment analysis was performed, the proteomic changes were integrated with human Parkinson's disease genetic studies, and the α-synucleinopathy proteome was compared with that of tauopathy model flies, which are relevant to Alzheimer's disease and related disorders. These approaches identified GTP cyclohydrolase (GCH1) and folate metabolism as candidate mediators of α-synuclein neurotoxicity. In functional validation studies, it was found that the knockdown of Drosophila Gch1 enhanced locomotor deficits in α-synuclein transgenic flies, while folate supplementation protected from α-synuclein toxicity. This integrative analysis suggested that mitochondrial dysfunction was a common downstream mediator of neurodegeneration. Accordingly, Gch1 knockdown enhanced metabolic dysfunction in α-synuclein transgenic fly brains while folate supplementation partially normalized brain bioenergetics. An integrative approach was outlined and implemented to identify and validate potential therapeutic pathways using comparative proteomics and genetics and capitalizing on the facile genetic and pharmacological tools available in Drosophila (Sarkar, 2020).
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Liu, Q., Bautista-Gomez, J., Higgins, D. A., Yu, J. and Xiong, Y. (2021). Dysregulation of the AP2M1 phosphorylation cycle by LRRK2 impairs endocytosis and leads to dopaminergic neurodegeneration. Sci Signal 14(693). PubMed ID: 34315807
Mutations in the kinase LRRK2 and impaired endocytic trafficking are both implicated in the pathogenesis of Parkinson's disease (PD). Expression of the PD-associated LRRK2 mutant in mouse dopaminergic neurons was shown to disrupt clathrin-mediated endocytic trafficking. This study explored the molecular mechanism linking LRRK2 to endocytosis and found that LRRK2 bound to and phosphorylated the μ2 subunit of the adaptor protein AP2 (AP2M1), a core component of the clathrin-mediated endocytic machinery. Analysis of human SH-SY5Y cells and mouse neurons and tissues revealed that loss of LRRK2 abundance or kinase function resulted in decreased phosphorylation of AP2M1, which is required for the initial formation of clathrin-coated vesicles (CCVs). In contrast, overexpression of LRRK2 or expression of a Parkinson's disease-associated gain-of-function mutant LRRK2 (G2019S) inhibited the uncoating of AP2M1 from CCVs at later stages and prevented new cycles of CCV formation. Thus, the abundance and activity of LRRK2 must be calibrated to ensure proper endocytosis. Dysregulated phosphorylation of AP2M1 from the brain but not thyroid tissues of LRRK2 knockout and G2019S-knockin mice suggests a tissue-specific regulatory mechanism of endocytosis. Furthermore, this study found that LRRK2-dependent phosphorylation of AP2M1 mediated dopaminergic neurodegeneration in a Drosophila model of PD. Together, these findings provide a mechanistic link between LRRK2, AP2, and endocytosis in the pathogenesis of PD.
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Xie, J., Chen, S., Bopassa, J. C. and Banerjee, S. (2021). Drosophila tubulin polymerization promoting protein mutants reveal pathological correlates relevant to human Parkinson's disease. Sci Rep. 11(1):13614. PubMed ID: 34193896
Parkinson's disease (PD) is a progressive neurodegenerative disorder with no known cure. PD is characterized by locomotion deficits, nigrostriatal dopaminergic neuronal loss, mitochondrial dysfunctions and formation of α-Synuclein aggregates. A well-conserved and less understood family of Tubulin Polymerization Promoting Proteins (TPPP) is also implicated in PD and related disorders, where TPPP exists in pathological aggregates in neurons in patient brains. However, there are no in vivo studies on mammalian TPPP to understand the genetics and neuropathology linking TPPP aggregation or neurotoxicity to PD. The only Drosophila homolog of human TPPP is named Ringmaker (Ringer). This study reports that adult ringer mutants display progressive locomotor disabilities, reduced lifespan and neurodegeneration. Importantly, the findings reveal that Ringer is associated with mitochondria and ringer mutants have mitochondrial structural damage and dysfunctions. Adult ringer mutants also display progressive loss of dopaminergic neurons. Together, these phenotypes of ringer mutants recapitulate some of the salient features of human PD patients, thus allowing utilization of ringer mutants as a fly model relevant to PD, and further exploration of its genetic and molecular underpinnings to gain insights into the role of human TPPP in PD (Xie, 2021)
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Terriente-Felix, A., Wilson, E. L. and Whitworth, A. J. (2020). Drosophila phosphatidylinositol-4 kinase fwd promotes mitochondrial fission and can suppress Pink1/parkin phenotypes. PLoS Genet 16(10): e1008844. PubMed ID: 33085661
Balanced mitochondrial fission and fusion play an important role in shaping and distributing mitochondria, as well as contributing to mitochondrial homeostasis and adaptation to stress. In particular, mitochondrial fission is required to facilitate degradation of damaged or dysfunctional units via mitophagy. Two Parkinson's disease factors, PINK1 and Parkin, are considered key mediators of damage-induced mitophagy, and promoting mitochondrial fission is sufficient to suppress the pathological phenotypes in Drosophila Pink1/parkin mutants. Additional factors were sought that impinge on mitochondrial dynamics and which may also suppress Pink1/parkin phenotypes. The Drosophila phosphatidylinositol 4-kinase IIIβ homologue, Four wheel drive (Fwd), promotes mitochondrial fission downstream of the pro-fission factor Drp1. Previously described only as male sterile, this study identified several new phenotypes in fwd mutants, including locomotor deficits and shortened lifespan, which are accompanied by mitochondrial dysfunction. Finally, fwd overexpression can suppress locomotor deficits and mitochondrial disruption in Pink1/parkin mutants, consistent with its function in promoting mitochondrial fission. Together these results shed light on the complex mechanisms of mitochondrial fission and further underscore the potential of modulating mitochondrial fission/fusion dynamics in the context of neurodegeneration (Terriente-Felix, 2020).
Mitochondria are dynamic organelles that are transported to the extremities of the cell and frequently undergo fusion and fission events that influence their size, branching and degradation. Many of the core components of the mitochondrial fission and fusion machineries have been well characterised. There include the pro-fusion factors Mfn1/2 and Opa1, and pro-fission factors Drp1 and Mff. Maintaining an appropriate balance of fission and fusion, as well as transport dynamics, is crucial for cellular health and survival as mutations in many of the core components cause severe neurological conditions in humans and model organisms. Recently, a role for phosphatidylinositol 4-phosphate [PI(4)P] in mitochondrial fission has been elucidated in cultured cells (Nagashima, 2020; Science 367(6484): 1366-1371), but the in vivo consequences have not yet been described (Terriente-Felix, 2020).
The mitochondrial fission/fusion cycle has been linked to the selective removal of damaged mitochondria through the process of autophagy (termed mitophagy), in which defective mitochondria are engulfed into autophagosomes and degraded by lysosomes. Two genes that have been firmly linked to the mitophagy process are PINK1 and PRKN. Mutations in these genes cause autosomal-recessive juvenile parkinsonism, associated with degeneration of midbrain dopaminergic neurons and motor impairments, among other symptoms and pathologies. Studies from a wide variety of model systems have shown various degrees of mitochondrial dysfunction associated with mutation of PINK1/PRKN homologues including disrupted fission/fusion. Drosophila have proven to be a fruitful model for investigating the function of the conserved homologues Pink1 and parkin, with these mutants exhibiting robust mitochondrial disruption and neuromuscular phenotypes. Importantly, several studies have shown that the pathological consequences of loss of Pink1 or parkin can be largely suppressed by genetic manipulations that increase mitochondrial fission or reduce fusion (Terriente-Felix, 2020).
To identify genes involved in mitochondrial quality control and homeostasis, an RNAi screen was performed in Drosophila S2 cells to identify kinases and phosphatases that phenocopy or suppress hyperfused mitochondria caused by loss of Pink1 (Pogson, 2014). This study identified the phosphatidylinositol 4-kinase IIIβ homologue, four wheel drive (fwd), whose knockdown phenocopied Pink1 RNAi, resulting in excess mitochondrial fusion. Drosophila mutant for fwd have been reported to be viable but male sterile due to incomplete cytokinesis during spermatogenesis. While muscle-specific knockdown has shown to impact neuromuscular junction formation (Forrest, 2013), no other organismal phenotypes or mitochondrial involvement have been described to date. Thus, this study sought to better understand the role of Fwd in mitochondrial homeostasis (Terriente-Felix, 2020).
This study has characterised fwd mutants for organismal phenotypes associated with Pink1/parkindysfunction and analysed the impact on mitochondrial form and function. Genetic interactions were investigated between fwd and Pink1/parkin, as well as with mitochondrial fission/fusion factors. It was found that loss of fwd inhibited mitochondrial function, causing increased mitochondrial length and branching, and decreased respiratory capacity. These effects were associated with shortened lifespan and dramatically reduced locomotor ability, similar to Pink1 and parkin mutants. Furthermore, fwd overexpression was sufficient to significantly suppress Pink1/parkin mutant locomotor deficits and mitochondrial phenotypes. Interestingly, it was found that the mitochondrial and locomotion phenotypes in fwd mutants can be rescued by loss of pro-fusion factors Marf and Opa1, but the pro-fission activity of Drp1 appears to require fwd. These results support a role for fwd in regulating mitochondrial morphology, specifically in facilitating mitochondrial fission, and further substantiate the important contribution of aberrant mitochondrial fission/fusion dynamics in Pink1/parkinphenotypes (Terriente-Felix, 2020).
Previous work identified fwd as a gene whose knockdown induces mitochondrial hyperfusion in cultured cells, similar to loss of Pink1 (Pogson, 2014). This study has validated that the genetic loss or knockdown of fwd also causes excess mitochondrial fusion in neuronal cells in vivo, leading to increased mitochondrial length and branching. As mitochondrial fission/fusion dynamics have been shown to be important for mitochondrial homeostasis, it is not surprising that this also has an impact on respiration at the organismal level and on organismal fitness and vitality. While fwd mutants have mainly been characterised for their male sterility phenotype, this study describes new organismal phenotypes associated with loss of fwd: profound locomotor deficits and shortened lifespan. Interestingly, while the data reveal a stronger requirement for fwd in the nervous system compared to the musculature to maintain normal motor behaviour, fwd is required in muscle for neuromuscular junction formation (Brill, 200). Furthermore, consistent with the observations on lifespan, Fwd overexpression has previously been shown to confer increased lifespan. Thus, Fwd clearly has a more widespread role in organismal vitality than previously appreciated (Terriente-Felix, 2020).
The robust locomotor phenotype allowed a test of the genetic relationship between fwd and core components of the mitochondrial fission/fusion machinery. Given the excess mitochondrial fusion upon loss of fwd, suppression of the organismal phenotypes by reduction of fusion factors Marf and Opa1 was expected. However, it was surprising that overexpression of the fission factor Drp1 was unable to ameliorate organismal phenotypes or even the increased mitochondrial length, though it was able to revert the increased branching caused by loss of fwd. These results suggested that Drp1 requires Fwd to drive mitochondrial fission. Consistent with this, Drp1 overexpression was no longer able to rescue Pink1/parkin mutant phenotypes in the absence of fwd. These genetic experiments strongly hint at a functional link between Drp1 and Fwd but do not illuminate the molecular mechanism underpinning it. Fwd is the Drosophila homologue of phosphatidylinositol 4-kinase IIIβ [PI(4)KB], which mediates the phosphorylation of phosphatidylinositol to generate phosphatidylinositol 4-phosphate [PI(4)P]. PI(4)P is one of the most abundant phosphoinositides, which is usually concentrated in the trans-Golgi network; thus, the mechanism by which PI(4)P may influence mitochondrial dynamics is not immediately obvious. However, while this manuscript was in preparation, Godi, 1999 (Nat Cell Biol 1(5): 280-287) reported that Golgi-derived PI(4)P-containing vesicles were required for the final stages of mitochondrial fission (Nagashima, 2020; Science 367(6484): 1366-1371). In that study, the authors found that loss of PI(4)KIIIβ led to hyperfusion and increased branching of the mitochondrial network, consistent with what was observed in this study. Moreover, Nagashima described that while Drp1 was still recruited, it was unable to fully execute the scission event, although the reason is unclear, leading to extended mitochondrial constriction sites. Genetic evidence that the action of Drp1 requires Fwd is consistent with these findings, and provides an in vivo validation of Nagashima's results. Currently, it is unclear why Drp1 overexpression was able to revert the increased branching caused by loss of fwd but the mechanisms of branch formations are not well understood. It is interesting to note that while Nagashima suggest a universal role for PI(4)P in mitochondrial fission, the current in vivo analysis reveals that while fwd affected mitochondrial morphology in the nervous system, it appeared to have a much more limited role in the musculature. These tissue-specific requirements were borne out in the strong locomotor deficits caused by neuronal loss of fwd but much less so by knockdown in muscles. Clearly, further work is required to better understand the complexities of regulated fission/fusion events in different cell contexts in vivo (Terriente-Felix, 2020).
A key role of mitochondrial fission/fusion dynamics is in contributing to a quality control mechanism of mitochondrial sorting to eliminate dysfunctional units via mitophagy. A substantial body of evidence from cellular models indicates that mammalian PINK1 and Parkin act to promote damage-induced mitophagy, and some in vivo evidence from Drosophila also supports this. However, the precise nature of PINK1/Parkin-mediated mitochondrial turnover in vivo is debated with contradictory results emerging. Nevertheless, interventions to combat the decline in mitochondrial homeostasis remain a key challenge to combatting PINK1/PRKN related pathologies. One mechanism that seems to provide substantial benefit in physiological contexts is through augmenting mitochondrial fission, which presumably facilitates the flux of damaged mitochondrial components towards turnover. This study, provide further evidence that augmenting a pro-fission pathway is beneficial against Pink1 and parkin dysfunction. As phosphoinositides can be interconverted by the action of multiple enzymes that may be druggable, these findings suggest another potential route towards a therapeutic intervention (Terriente-Felix, 2020).
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Solana-Manrique, C., Sanz, F. J., Ripolles, E., Bano, M. C., Torres, J., Munoz-Soriano, V. and Paricio, N. (2020). Enhanced activity of glycolytic enzymes in Drosophila and human cell models of Parkinson's disease based on DJ-1 deficiency. Free Radic Biol Med. PubMed ID: 32726690
Parkinson's disease (PD) is a neurodegenerative debilitating disorder characterized by progressive disturbances in motor, autonomic and psychiatric functions. One of the genes involved in familial forms of the disease is DJ-1, whose mutations cause early-onset PD. Besides, it has been shown that an over-oxidized and inactive form of the DJ-1 protein is found in brains of sporadic PD patients. Interestingly, the DJ-1 protein plays an important role in cellular defense against oxidative stress and also participates in mitochondrial homeostasis. Flies mutant for the DJ-1β gene, the Drosophila ortholog of human DJ-1, exhibited disease-related phenotypes such as motor defects, increased reactive oxygen species production and high levels of protein carbonylation. The present study demonstrated that DJ-1β mutants also show a significant increase in the activity of several regulatory glycolytic enzymes. Similar results were obtained in DJ-1-deficient SH-SY5Y neuroblastoma cells, thus suggesting that loss of DJ-1 function leads to an increase in the glycolytic rate. In such a scenario, an enhancement of the glycolytic pathway could be a protective mechanism to decrease ROS production by restoring ATP levels, which are decreased due to mitochondrial dysfunction. The results also show that meclizine and dimethyl fumarate, two FDA-approved compounds with different clinical applications, are able to attenuate PD-related phenotypes in both models. Moreover, it was found that they may exert their beneficial effect by increasing glycolysis through the activation of key glycolytic enzymes. Taken together, these results are consistent with the idea that increasing glycolysis could be a potential disease-modifying strategy for PD, as recently suggested. Besides, they also support further evaluation and potential repurposing of meclizine and dimethyl fumarate as modulators of energy metabolism for neuroprotection in PD.
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Fernandez-Cruz, I., Sanchez-Diaz, I., Narvaez-Padilla, V. and Reynaud, E. (2020). Rpt2 proteasome subunit reduction causes Parkinson's disease like symptoms in Drosophila, IBRO Rep 9: 65-77. PubMed ID: 32715147
The dysfunction of the proteasome-ubiquitin system is commonly reported in several neurodegenerative diseases. Post mortem samples of brains of patients with Parkinson´s disease present cytoplasmic inclusions that are rich in proteins such as ubiquitin, Tau, and α-synuclein. In Parkinson´s disease, a specific reduction of some of the proteasome subunits has also been reported. However, the specific role of the different proteasome subunits in dopaminergic neuron degeneration has not been thoroughly explored. In this work, the Gal4/UAS system was used to test fourteen Drosophila melanogaster RNAi lines from the Bloomington Drosophila Stock Center. Each of these lines targets a different proteasome subunit. To identify the strains that were able to induce neurodegeneration, the expression of these lines was driven to the eye, and they were catagorized as a function of the extent of neurodegeneration that they induced. The targeted proteasomal subunits are conserved in mammals and therefore may be relevant to study proteasome related diseases. The RNAi line among the regulatory subunits with the most penetrant phenotype targeted the proteasomal subunit Rpt2 and its phenotypes were further characterized. Rpt2 knockdown in the Drosophila central nervous system reduced the activity of the proteasome, augmented the amount of insoluble ubiquitinated protein, and elicited motor and non-motor phenotypes that were similar to the ones found in Drosophila and other models for Parkinson's disease. When Rpt2 is silenced pan-neurally, third instar larvae have locomotion dysfunctions and die during pupation. Larval lethality was avoided using the Gal80-Gal4 system to induce the expression of the Rpt2 RNAi to dopaminergic neurons only after pupation. The reduction of Rpt2 in adult dopaminergic neurons causes reduced survival, hyperactivity, neurodegeneration, and sleep loss; probably recapitulating some of the sleep disorders that Parkinson's disease patients have before the appearance of locomotion disorders (Fernandez-Cruz, 2020).
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Ingles-Prieto, A., Furthmann, N., Crossman, S. H., Tichy, A. M., Hoyer, N., Petersen, M., Zheden, V., Biebl, J., Reichhart, E., Gyoergy, A., Siekhaus, D. E., Soba, P., Winklhofer, K. F. and Janovjak, H. (2021). Optogenetic delivery of trophic signals in a genetic model of Parkinson's disease. PLoS Genet 17(4): e1009479. PubMed ID: 33857132
Optogenetics has been harnessed to shed new mechanistic light on current and future therapeutic strategies. This has been to date achieved by the regulation of ion flow and electrical signals in neuronal cells and neural circuits that are known to be affected by disease. In contrast, the optogenetic delivery of trophic biochemical signals, which support cell survival and are implicated in degenerative disorders, has never been demonstrated in an animal model of disease. This study reengineered the human and Drosophila melanogaster REarranged during Transfection (hRET and dRET) receptors to be activated by light, creating one-component optogenetic tools termed Opto-hRET and Opto-dRET. Upon blue light stimulation, these receptors robustly induced the MAPK/ERK proliferative signaling pathway in cultured cells. In PINK1B9 flies that exhibit loss of PTEN-induced putative kinase 1 (PINK1), a kinase associated with familial Parkinson's disease (PD), light activation of Opto-dRET suppressed mitochondrial defects, tissue degeneration and behavioral deficits. In human cells with PINK1 loss-of-function, mitochondrial fragmentation was rescued using Opto-dRET via the PI3K/NF-kappaB pathway. These results demonstrate that a light-activated receptor can ameliorate disease hallmarks in a genetic model of PD. The optogenetic delivery of trophic signals is cell type-specific and reversible and thus has the potential to inspire novel strategies towards a spatio-temporal regulation of tissue repair.
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Olsen, A. L. and Feany, M. B. (2021). Parkinson's disease risk genes act in glia to control neuronal alpha-synuclein toxicity. Neurobiol Dis 159: 105482. PubMed ID: 34390834
Idiopathic Parkinson's disease is the second most common neurodegenerative disease and is estimated to be approximately 30% heritable. Genome wide association studies have revealed numerous loci associated with risk of development of Parkinson's disease. The majority of genes identified in these studies are expressed in glia at either similar or greater levels than their expression in neurons, suggesting that glia may play a role in Parkinson's disease pathogenesis. The role of individual glial risk genes in Parkinson's disease development or progression is unknown, however. It was hypothesized that some Parkinson's disease risk genes exert their effects through glia. A Drosophila model of α-synucleinopathy was developed in which gene expression can be individually expressed in neurons and glia. Human wild type α-synuclein is expressed in all neurons, and these flies develop the hallmarks of Parkinson's disease, including motor impairment, death of dopaminergic and other neurons, and α-synuclein aggregation. In these flies, a candidate genetic screen was performed, using RNAi to knockdown 14 well-validated Parkinson's disease risk genes in glia, and the effect on locomotion was measured in order to identify glial modifiers of the &alpha-synuclein phenotype. Four modifiers were identified: aux, Lrrk, Ric, and Vps13, orthologs of the human genes GAK, LRRK2, RIT2, and VPS13C, respectively. Knockdown of each gene exacerbated neurodegeneration as measured by total and dopaminergic neuron loss. Knockdown of each modifier also increased α-synuclein oligomerization. These results suggest that some Parkinson's disease risk genes exert their effects in glia and that glia can influence neuronal α-synuclein proteostasis in a non-cell-autonomous fashion. Further, this study provides proof of concept that this novel Drosophila α-synucleinopathy model can be used to study glial modifier genes, paving the way for future large unbiased screens to identify novel glial risk factors that contribute to PD risk and progression.
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Han, Y., Zhuang, N. and Wang, T. (2021). Roles of PINK1 in regulation of systemic growth inhibition induced by mutations of PTEN in Drosophila. Cell Rep 34(12): 108875. PubMed ID: 33761355
The maintenance of mitochondrial homeostasis requires PTEN-induced kinase 1 (PINK1)-dependent mitophagy, and mutations in PINK1 are associated with Parkinson's disease (PD). PINK1 is also downregulated in tumor cells with PTEN mutations. However, there is limited information concerning the role of PINK1 in tissue growth and tumorigenesis. This study shows that the loss of pink1 caused multiple growth defects independent of its pathological target, Parkin. Moreover, knocking down pink1 in muscle cells induced hyperglycemia and limited systemic organismal growth by the induction of Imaginal morphogenesis protein-Late 2 (ImpL2). Similarly, disrupting PTEN activity in multiple tissues impaired systemic growth by reducing pink1 expression, resembling wasting-like syndrome in cancer patients. Furthermore, the re-expression of PINK1 fully rescued defects in carbohydrate metabolism and systemic growth induced by the tissue-specific pten mutations. These data suggest a function for PINK1 in regulating systemic growth in Drosophila and shed light on its role in wasting in the context of PTEN mutations.
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Krzystek, T. J., Banerjee, R., Thurston, L., Huang, J., Swinter, K., Rahman, S. N., Falzone, T. L. and Gunawardena, S. (2021). Differential mitochondrial roles for alpha-synuclein in DRP1-dependent fission and PINK1/Parkin-mediated oxidation. Cell Death Dis 12(9): 796. PubMed ID: 34404758
Summary:
Mitochondria are highly dynamic organelles with strict quality control processes that maintain cellular homeostasis. Within axons, coordinated cycles of fission-fusion mediated by dynamin related GTPase protein (DRP1) and mitofusins (MFN), together with regulated motility of healthy mitochondria anterogradely and damaged/oxidized mitochondria retrogradely, control mitochondrial shape, distribution and size.
This study has isolated the mechanistic role of α-syn in mitochondrial homeostasis in vivo in a humanized Drosophila model of Parkinson's disease (PD). It was shown that excess α-syn causes fragmented mitochondria, which persists with either truncation of the C-terminus (α(1-120)) or deletion of the NAC region (α(ΔNAC)). Using in vivo oxidation reporters Mito-roGFP2-ORP1/GRX1 and MitoTimer, it was found that α-mediated fragments were oxidized/damaged, but α(1-120)-induced fragments were healthy, suggesting that the C-terminus is required for oxidation. α-mediated oxidized fragments showed biased retrograde motility, but α(1-120)-mediated healthy fragments did not, demonstrating that the C-terminus likely mediates the retrograde motility of oxidized mitochondria. Depletion/inhibition or excess DRP1-rescued α-syn-mediated fragmentation, oxidation, and the biased retrograde motility, indicating that DRP1-mediated fragmentation is likely upstream of oxidation and motility changes. Further, excess PINK/Parkin, two PD-associated proteins that function to coordinate mitochondrial turnover via induction of selective mitophagy, rescued α-syn-mediated membrane depolarization, oxidation and cell death in a C-terminus-dependent manner, suggesting a functional interaction between &alpha-syn and PINK/Parkin. Taken together, these findings identify distinct roles for α-syn in mitochondrial homeostasis, highlighting a previously unknown pathogenic pathway for the initiation of PD.
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Guo, Q., Wang, B., Wang, X., Smith, W. W., Zhu, Y. and Liu, Z. (2021). Activation of Nrf2 in Astrocytes Suppressed PD-Like Phenotypes via Antioxidant and Autophagy Pathways in Rat and Drosophila Models. Cells 10(8). PubMed ID: 34440619
Summary:
The oxidative-stress-induced impairment of autophagy plays a critical role in the pathogenesis of Parkinson's disease (PD). This study investigated whether the alteration of Nrf2 in astrocytes protected against 6-OHDA (6-hydroxydopamine)- and rotenone-induced PD-like phenotypes, using 6-OHDA-induced rat PD and rotenone-induced Drosophila PD models. In the PD rat model, Nrf2 expression was significantly higher in astrocytes than in neurons. CDDO-Me (CDDO methyl ester, an Nrf2 inducer) administration attenuated PD-like neurodegeneration mainly through Nrf2 activation in astrocytes by activating the antioxidant signaling pathway and enhancing autophagy in the substantia nigra and striatum. In the PD Drosophila model, the overexpression of Nrf2 in glial cells displayed more protective effects than such overexpression in neurons. Increased Nrf2 expression in glial cells significantly reduced oxidative stress and enhanced autophagy in the brain tissue. The administration of the Nrf2 inhibitor ML385 reduced the neuroprotective effect of Nrf2 through the inhibition of the antioxidant signaling pathway and autophagy pathway. The autophagy inhibitor 3-MA partially reduced the neuroprotective effect of Nrf2 through the inhibition of the autophagy pathway, but not the antioxidant signaling pathway. Moreover, Nrf2 knockdown caused neurodegeneration in flies. Treatment with CDDO-Me attenuated the Nrf2-knockdown-induced degeneration in the flies through the activation of the antioxidant signaling pathway and increased autophagy. An autophagy inducer, rapamycin, partially rescued the neurodegeneration in Nrf2-knockdown Drosophila by enhancing autophagy. These results indicate that the activation of the Nrf2-linked signaling pathways in glial cells plays an important neuroprotective role in PD models.
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Putz, S. M., Kram, J., Rauh, E., Kaiser, S., Toews, R., Lueningschroer-Wang, Y., Rieger, D. and Raabe, T. (2021). Loss of p21-activated kinase Mbt/PAK4 causes Parkinson-like phenotypes in Drosophila. Dis Model Mech 14(6). PubMed ID: 34125184
Summary:
Parkinson's disease (PD) provokes bradykinesia, resting tremor, rigidity and postural instability, and also non-motor symptoms such as depression, anxiety, sleep and cognitive impairments. Similar phenotypes can be induced in Drosophila melanogaster through modification of PD-relevant genes or the administration of PD-inducing toxins. Recent studies correlated deregulation of human p21-activated kinase 4 (PAK4) with PD, leaving open the question of a causative relationship of mutations in this gene for manifestation of PD symptoms. To determine whether flies lacking the PAK4 homolog Mushroom bodies tiny (Mbt) show PD-like phenotypes, a variety of PD criteria was tested. This study demonstrated that mbt mutant flies show PD-like phenotypes including age-dependent movement deficits, reduced life expectancy and fragmented sleep. They also react to a stressful situation with higher immobility, indicating an influence of Mbt on emotional behavior. Loss of Mbt function has a negative effect on the number of dopaminergic protocerebral anterior medial (PAM) neurons, most likely caused by a proliferation defect of neural progenitors. The age-dependent movement deficits are not accompanied by a corresponding further loss of PAM neurons. Previous studies highlighted the importance of a small PAM subgroup for age-dependent PD motor impairments. This study shows that impaired motor skills are caused by a lack of Mbt in this PAM subgroup. In addition, a broader re-expression of Mbt in PAM neurons improves life expectancy. Conversely, selective Mbt knockout in the same cells shortens lifespan. It is concluded that mutations in Mbt/PAK4 can play a causative role in the development of PD phenotypes.
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Bhat, S., Guthrie, D. A., Kasture, A., El-Kasaby, A., Cao, J., Bonifazi, A., Ku, T., Giancola, J. B., Hummel, T., Freissmuth, M. and Newman, A. H. (2021). Tropane-Based Ibogaine Analog Rescues Folding-Deficient Serotonin and Dopamine Transporters. ACS Pharmacol Transl Sci 4(2): 503-516. PubMed ID: 33860180
Summary:
Missense mutations that give rise to protein misfolding are rare, but collectively, defective protein folding diseases are consequential. Folding deficiencies are amenable to pharmacological correction (pharmacochaperoning), but the underlying mechanisms remain enigmatic. Ibogaine and its active metabolite noribogaine correct folding defects in the dopamine transporter (DAT), but they rescue only a very limited number of folding-deficient DAT mutant proteins, which give rise to infantile Parkinsonism and dystonia. In this study, a series of analogs was generated by reconfiguring the complex ibogaine ring system and exploring the structural requirements for binding to wild-type transporters, as well as for rescuing two equivalent synthetic folding-deficient mutants, SERT-PG(601,602)AA and DAT-PG(584,585)AA. The most active tropane-based analog (9b) was also an effective pharmacochaperone in vivo in Drosophila harboring the DAT-PG(584,585)AA mutation and rescued 6 out of 13 disease-associated human DAT mutant proteins in vitro. Hence, a novel lead pharmacochaperone has been identified that demonstrates medication development potential for patients harboring DAT mutations.
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Buck, S. A., Steinkellner, T., Aslanoglou, D., Villeneuve, M., Bhatte, S. H., Childers, V. C., Rubin, S. A., De Miranda, B. R., O'Leary, E. I., Neureiter, E. G., Fogle, K. J., Palladino, M. J., Logan, R. W., Glausier, J. R., Fish, K. N., Lewis, D. A., Greenamyre, J. T., McCabe, B. D., Cheetham, C. E. J., Hnasko, T. S. and Freyberg, Z. (2021).Vesicular glutamate transporter modulates sex differences in dopamine neuron vulnerability to age-related neurodegeneration.. Aging Cell: e13365. PubMed ID: 33909313
Summary:
Age is the greatest risk factor for Parkinson's disease (PD) which causes progressive loss of dopamine (DA) neurons, with males at greater risk than females. Intriguingly, some DA neurons are more resilient to degeneration than others. Increasing evidence suggests that vesicular glutamate transporter (VGLUT) expression in DA neurons plays a role in this selective vulnerability. We investigated the role of DA neuron VGLUT in sex- and age-related differences in DA neuron vulnerability using the genetically tractable Drosophila model. This study found sex differences in age-related DA neurodegeneration and its associated locomotor behavior, where males exhibit significantly greater decreases in both DA neuron number and locomotion during aging compared with females. Dynamic changes in DA neuron VGLUT expression mediate these age- and sex-related differences, as a potential compensatory mechanism for diminished DA neurotransmission during aging. Importantly, female Drosophila possess higher levels of VGLUT expression in DA neurons compared with males, and this finding is conserved across flies, rodents, and humans. Moreover, this study showed that diminishing VGLUT expression in DA neurons eliminates females' greater resilience to DA neuron loss across aging. This offers a new mechanism for sex differences in selective DA neuron vulnerability to age-related DA neurodegeneration. Finally, in mice, this study showed that the ability of DA neurons to achieve optimal control over VGLUT expression is essential for DA neuron survival. These findings lay the groundwork for the manipulation of DA neuron VGLUT expression as a novel therapeutic strategy to boost DA neuron resilience to age- and PD-related neurodegeneration.
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Yamaguchi, A., Ishikawa, K. I., Inoshita, T., Shiba-Fukushima, K., Saiki, S., Hatano, T., Mori, A., Oji, Y., Okuzumi, A., Li, Y., Funayama, M., Imai, Y., Hattori, N. and Akamatsu, W. (2020). Identifying Therapeutic Agents for Amelioration of Mitochondrial Clearance Disorder in Neurons of Familial Parkinson Disease. Stem Cell Reports 14(6): 1060-1075. PubMed ID: 32470327
Summary:
Parkinson disease (PD) is a neurodegenerative disorder caused by the progressive loss of midbrain dopaminergic neurons, and mitochondrial dysfunction is involved in its pathogenesis. This study aimed to establish an imaging-based, semi-automatic, high-throughput system for the quantitative detection of disease-specific phenotypes in dopaminergic neurons from induced pluripotent stem cells (iPSCs) derived from patients with familial PD having Parkin or PINK1 mutations, which exhibit abnormal mitochondrial homeostasis. The proposed system recapitulates the deficiency of mitochondrial clearance, ROS accumulation, and increasing apoptosis in these familial PD-derived neurons. 320 compounds were screened for their ability to ameliorate multiple phenotypes, and four candidate drugs were identified. Some of these drugs improved the locomotion defects and reduced ATP production caused by PINK1 inactivation in Drosophila and were effective for idiopathic PD-derived neurons with impaired mitochondrial clearance. These findings suggest that the proposed high-throughput system has potential for identifying effective drugs for familial and idiopathic PD (Yamaguchi, 2020).
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Xie, J., Chen, S., Bopassa, J. C. and Banerjee, S. (2021).Drosophila tubulin polymerization promoting protein mutants reveal pathological correlates relevant to human Parkinson's disease. Sci Rep. 11(1):13614. PubMed ID: 34193896
Summary:
Parkinson's disease (PD) is a progressive neurodegenerative disorder with no known cure. PD is characterized by locomotion deficits, nigrostriatal dopaminergic neuronal loss, mitochondrial dysfunctions and formation of α-Synuclein aggregates. A well-conserved and less understood family of Tubulin Polymerization Promoting Proteins (TPPP) is also implicated in PD and related disorders, where TPPP exists in pathological aggregates in neurons in patient brains. However, there are no in vivo studies on mammalian TPPP to understand the genetics and neuropathology linking TPPP aggregation or neurotoxicity to PD. The only Drosophila homolog of human TPPP is named Ringmaker (Ringer). This study reports that adult ringer mutants display progressive locomotor disabilities, reduced lifespan and neurodegeneration. Importantly, the findings reveal that Ringer is associated with mitochondria and ringer mutants have mitochondrial structural damage and dysfunctions. Adult ringer mutants also display progressive loss of dopaminergic neurons. Together, these phenotypes of ringer mutants recapitulate some of the salient features of human PD patients, thus allowing utilization of ringer mutants as a fly model relevant to PD, and further exploration of its genetic and molecular underpinnings to gain insights into the role of human TPPP in PD.
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Carvajal-Oliveros, A., Domínguez-Baleon, C., Zarate, R. V., Campusano, J. M., Narvaez-Padilla, V. and Reynaud, E. (2021). Nicotine suppresses Parkinson's disease like phenotypes induced by Synphilin-1 overexpression in Drosophila melanogaster by increasing tyrosine hydroxylase and dopamine levels. Sci Rep 11(1): 9579. PubMed ID: 33953275
It has been observed that there is a lower Parkinson's disease (PD) incidence in tobacco users. Nicotine is a cholinergic agonist and is the principal psychoactive compound in tobacco linked to cigarette addiction. Different studies have shown that nicotine has beneficial effects on sporadic and genetic models of PD. This work evaluated nicotine's protective effect in a Drosophila melanogaster model for PD where Synphilin-1 (Sph-1) is expressed in dopaminergic neurons. Nicotine has a moderate effect on dopaminergic neuron survival that becomes more evident as flies age. Nicotine is beneficial on fly survival and motility increasing tyrosine hydroxylase and dopamine levels, suggesting that cholinergic agonists may promote survival and metabolic function of the dopaminergic neurons that express Sph-1. The Sph-1 expressing fly is a good model for the study of early-onset phenotypes such as olfaction loss one of the main non-motor symptom related to PD. Thd data suggest that nicotine is an interesting therapeutic molecule whose properties should be explored in future research on the phenotypic modulators of the disease and for the development of new treatments.
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Qiao, J. D. and Mao, Y. L. (2020). Knockout of PINK1 altered the neural connectivity of Drosophila dopamine PPM3 neurons at input and output sites. Invert Neurosci 20(3): 11. PubMed ID: 32766952
Abstract
Impairment of the dopamine system is the main cause of Parkinson disease (PD). PTEN-induced kinase 1 (PINK1) is possibly involved in pathogenesis of PD. However, its role in dopaminergic neurons has not been fully established yet. In the present investigation, the PINK1 knockout Drosophila model to explore the role of PINK1 in dopaminergic neurons. Electrophysiological and behavioral tests indicated that PINK1 elimination enhances the neural transmission from the presynaptic part of dopaminergic neurons in the protocerebral posterior medial region 3 (PPM3) to PPM3 neurons (which are homologous to those in the substantia nigra in humans). Firing properties of the action potential in PPM3 neurons were also altered in the PINK1 knockout genotypes. Abnormal motor ability was also observed in these PINK1 knockout animals. These results indicate that knockout of PINK1 could alter both the input and output properties of PPM3 neurons (Qiao, 2020).
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Wan, Z., Xu, J., Huang, Y., Zhai, Y., Ma, Z., Zhou, B. and Cao, Z. (2020). Elevating bioavailable iron levels in mitochondria suppresses the defective phenotypes caused by PINK1 loss-of-function in Drosophila melanogaster. Biochem Biophys Res Commun 532(2): 285-291. PubMed ID: 32873392
Abstract
Parkinson's disease (PD) is the second most common progressive neurodegenerative disease, which is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Iron deposit was found in the SNpc of PD patients and animal models, however, the mechanisms involved in disturbed iron metabolism remain unknown. Identifying the relationship between iron metabolism and PD is important for finding new therapeutic strategies. This study found that transgenic overexpression (OE) of Drosophila mitoferrin (dmfrn) or knockdown of Fer3HCH significantly mitigated the reduced mitochondrial aconitase activity, abnormal wing posture, flight deficits and mitochondrial morphology defects associated with PINK1 loss-of-function (LOF). Further work demonstrated that dmfrn OE or Fer3HCH knockdown significantly rescued the impaired mitochondrial respiration in PINK1 LOF flies, indicating that dmfrn or Fer3HCH may rescue PINK1 LOF phenotypes through elevating mitochondrial bioavailable iron levels to promote mitochondrial respiration (Wan, 2020).
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Rahul, Naz, F., Jyoti, S. and Siddique, Y. H. (2020). Effect of kaempferol on the transgenic Drosophila model of Parkinson's disease. Sci Rep 10(1): 13793. PubMed ID: 32796885
Abstract
The present study was aimed to study the effect of kaempferol, on the transgenic Drosophila model of Parkinson's disease. Kaempferol was added in the diet at final concentration of 10, 20, 30 and 40 µM and the effect was studied on various cognitive and oxidative stress markers. The results of the study showed that kaempferol, delayed the loss of climbing ability as well as the activity of PD flies in a dose dependent manner compared to unexposed PD flies. A dose-dependent reduction in oxidative stress markers was also observed. Histopathological examination of fly brains using anti-tyrosine hydroxylase immunostaining has revealed a significant dose-dependent increase in the expression of tyrosine hydroxylase in PD flies exposed to kaempferol. Molecular docking results revealed that kaempferol binds to human alpha synuclein at specific sites that might results in the inhibition of alpha synuclein aggregation and prevents the formation of Lewy bodies (Rahul, 2020).
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Blosser, J. A., Podolsky, E. and Lee, D. (2020). L-DOPA-Induced Dyskinesia in a Genetic Drosophila Model of Parkinson's Disease. Exp Neurobiol 29(4): 273-284. PubMed ID: 32921640
Abstract
Motor symptoms in Parkinson's disease (PD) are directly related to the reduction of a neurotransmitter dopamine. Therefore, its precursor L-DOPA became the gold standard for PD treatment. However, chronic use of L-DOPA causes uncontrollable, involuntary movements, called L-DOPA-induced dyskinesia (LID) in the majority of PD patients. LID is complicated and very difficult to manage. Current rodent and non-human primate models have been developed to study LID mainly using neurotoxins. Therefore, it is necessary to develop a LID animal model with defects in genetic factors causing PD in order to study the relation between LID and PD genes such as α-synuclein. This study first showed that a low concentration of L-DOPA (100 μM) rescues locomotion defects (i.e., speed, angular velocity, pause time) in Drosophila larvae expressing human mutant α-synuclein (A53T). This A53T larval model of PD was used to further examine dyskinetic behaviors. High concentrations of L-DOPA (5 or 10 mM) causes hyperactivity such as body bending behavior (BBB) in A53T larva, which resembles axial dyskinesia in rodents. Using ImageJ plugins and other third party software, dyskinetic BBB has been accurately and efficiently quantified. Further, a dopamine agonist pramipexole (PRX) partially rescues BBB caused by high L-DOPA. This Drosophila genetic LID model will provide an important experimental platform to examine molecular and cellular mechanisms underlying LID, to study the role of PD causing genes in the development of LID, and to identify potential targets to slow/reverse LID pathology (Blosser, 2020).
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Sun, L., Zhang, J., Chen, W., Chen, Y., Zhang, X., Yang, M., Xiao, M., Ma, F., Yao, Y., Ye, M., Zhang, Z., Chen, K., Chen, F., Ren, Y., Ni, S., Zhang, X., Yan, Z., Sun, Z. R., Zhou, H. M., Yang, H., Xie, S., Haque, M. E., Huang, K. and Yang, Y. (2020). Attenuation of epigenetic regulator SMARCA4 and ERK-ETS signaling suppresses aging-related dopaminergic degeneration. Aging Cell 19(9): e13210. PubMed ID: 32749068
Abstract
How complex interactions of genetic, environmental factors and aging jointly contribute to dopaminergic degeneration in Parkinson's disease (PD) is largely unclear. This study applied frequent gene co-expression analysis on human patient substantia nigra-specific microarray datasets to identify potential novel disease-related genes. In vivo Drosophila studies validated two of 32 candidate genes, a chromatin-remodeling factor SMARCA4 and a biliverdin reductase BLVRA. Inhibition of SMARCA4 was able to prevent aging-dependent dopaminergic degeneration not only caused by overexpression of BLVRA but also in four most common Drosophila PD models. Furthermore, down-regulation of SMARCA4 specifically in the dopaminergic neurons prevented shortening of life span caused by α-synuclein and LRRK2. Mechanistically, aberrant SMARCA4 and BLVRA converged on elevated ERK-ETS activity, attenuation of which by either genetic or pharmacological manipulation effectively suppressed dopaminergic degeneration in Drosophila in vivo. Down-regulation of SMARCA4 or drug inhibition of MEK/ERK also mitigated mitochondrial defects in PINK1 (a PD-associated gene)-deficient human cells. These findings underscore the important role of epigenetic regulators and implicate a common signaling axis for therapeutic intervention in normal aging and a broad range of age-related disorders including PD (Sun, 2020).
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Chauhan, N., Shrivastava, N. K., Agrawal, N. and Shakarad, M. N. (2020). Wnt2 overexpression protects against PINK1 mutant induced mitochondrial dysfunction and oxidative stress. Wing patterning in faster developing Drosophila is associated with high ecdysone titer and wingless expression. Mech Dev: 103626. PubMed ID: 32526278
Abstract
'Developmental robustness' is the ability of biological systems to maintain a stable phenotype despite genetic, environmental or physiological perturbations. In holometabolous insects, accurate patterning and development is guaranteed by alignment of final gene expression patterns in tissues at specific developmental stage such as molting and pupariation, irrespective of individual rate of development. In the present study, faster developing Drosophila melanogaster populations were used that show reduction of ~22% in egg to adult development time. Flies from the faster developing population exhibit phenotype constancy, although significantly small in size. The reduction in development time in faster developing flies is possibly due to coordination between higher ecdysteroid release and higher expression of developmental genes. The two together might be ensuring appropriate pattern formation and early exit at each development stage in the populations selected for faster pre-adult development compared to their ancestral controls. This study reports that apart from plasticity in the rate of pattern progression, alteration in the level of gene expression may be responsible for pattern integrity even under reduced development time.
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Xia, S. R., Wen, X. Y., Fan, X. L., Chen, X. R., Wei, Z. W., Li, Q. H. and Sun, L. (2020). Wnt2 overexpression protects against PINK1 mutant induced mitochondrial dysfunction and oxidative stress. Mol Med Rep 21(6): 2633-2641. PubMed ID: 32323790
Abstract
The PTEN induced putative kinase 1 (PINK1) mutation is the second most common cause of autosomal recessive adolescent Parkinson's disease (PD). Furthermore, mitochondrial disorders and oxidative stress are important mechanisms in the pathogenesis of PD. Numerous members of the Wnt family have been found to be associated with neurodegenerative diseases. Therefore, the present study investigated the role of the Wnt2 gene in PINK1B9 transgenic flies, which is a PD model, and its underlying mechanism. It was identified that overexpression of Wnt2 reduced the abnormality rate of PD transgenic Drosophila and improved their flight ability, while other intervention groups had no significant effect. Furthermore, an increase in ATP concentration normalized mitochondrial morphology, and increased the mRNA expression levels of NADHubiquinone oxidoreductase chain 1 (ND1), ND42, ND75, succinate dehydrogenase complex subunits B, Cytochrome b and Cyclooxygenase 1, which are associated with Wnt2 overexpression. Moreover, overexpression of Wnt2 in PD transgenic Drosophila resulted in the downregulation of reactive oxygen species and malondialdehyde production, and increased manganese superoxide dismutase (MnSOD), while glutathione was not significantly affected. It was found that overexpression of Wnt2 did not alter the protein expression of betacatenin in PINK1B9 transgenic Drosophila, but did increase the expression levels of PPARG coactivator 1alpha (PGC1alpha) and forkhead box subgroup O (FOXO). Collectively, the present results indicated that the Wnt2 gene may have a protective effect on PD PINK1B9 transgenic Drosophila. Thus, it was speculated that the reduction of oxidative stress and the restoration of mitochondrial function via Wnt2 overexpression may be related to the PGC1alpha/FOXO/MnSOD signaling pathway in PINK1 mutant transgenic Drosophila (Xia, 2020).
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Shiba-Fukushima, K., Inoshita, T., Sano, O., Iwata, H., Ishikawa, K. I., Okano, H., Akamatsu, W., Imai, Y. and Hattori, N. (2020). A Cell-Based High-Throughput Screening Identified Two Compounds that Enhance PINK1-Parkin Signaling. iScience 23(5): 101048. PubMed ID: 32335362
Abstract
Early-onset Parkinson's disease-associated PINK1-Parkin signaling maintains mitochondrial health. Therapeutic approaches for enhancing PINK1-Parkin signaling present a potential strategy for treating various diseases caused by mitochondrial dysfunction. This study reports two chemical enhancers of PINK1-Parkin signaling, identified using a robust cell-based high-throughput screening system. These small molecules, T0466 and T0467, activate Parkin mitochondrial translocation in dopaminergic neurons and myoblasts at low doses that do not induce mitochondrial accumulation of PINK1. Moreover, both compounds reduce unfolded mitochondrial protein levels, presumably through enhanced PINK1-Parkin signaling. These molecules also mitigate the locomotion defect, reduced ATP production, and disturbed mitochondrial Ca(2+) response in the muscles along with the mitochondrial aggregation in dopaminergic neurons through reduced PINK1 activity in Drosophila. These results suggested that T0466 and T0467 may hold promise as therapeutic reagents in Parkinson's disease and related disorders (Shiba-Fukushima, 2020).
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Petridi, S., Middleton, C. A., Ugbode, C., Fellgett, A., Covill, L. and Elliott, C. J. H. (2020). In Vivo Visual Screen for Dopaminergic Rab <--> LRRK2-G2019S Interactions in Drosophila Discriminates Rab10 from Rab3. G3 (Bethesda). PubMed ID: 32321836
Abstract
LRRK2 mutations cause Parkinson's, but the molecular link from increased kinase activity to pathological neurodegeneration remains undetermined. Previous in vitro assays indicate that LRRK2 substrates include at least 8 Rab GTPases. This hypothesis was examined in vivo in a functional, electroretinogram screen, expressing each Rab with/without LRRK2-G2019S in selected Drosophila dopaminergic neurons. The screen discriminated Rab10 from Rab3. The strongest Rab/LRRK2-G2019S interaction is with Rab10; the weakest with Rab3. Rab10 is expressed in a different set of dopaminergic neurons from Rab3. Thus, anatomical and physiological patterns of Rab10 are related. It is concluded that Rab10 is a valid substrate of LRRK2 in dopaminergic neurons in vivo. It is proposed that variations in Rab expression contribute to differences in the rate of neurodegeneration recorded in different dopaminergic nuclei in Parkinson's (Petridi, 2020).
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Xu, Y., Xie, M., Xue, J., Xiang, L., Li, Y., Xiao, J., Xiao, G. and Wang, H. L. (2020). EGCG ameliorates neuronal and behavioral defects by remodeling gut microbiota and TotM expression in Drosophila models of Parkinson's disease. Faseb j. PubMed ID: 32157731
Abstract
Parkinson's disease (PD) is the second most common neurodegenerative disease. Eigallocatechin-3-gallate (EGCG), the major polyphenol in green tea, is known to exert a beneficial effect on PD patients. Although some mechanisms were suggested to underlie this intervention, it remains unknown if the EGCG-mediated protection was achieved by remodeling gut microbiota. In the present study, 0.1 mM or 0.5 mM EGCG was administered to the Drosophila melanogaster with PINK1 (PTEN induced putative kinase 1) mutations, a prototype PD model, and their behavioral performances, as well as neuronal/mitochondrial morphology (only for 0.5 mM EGCG treatment) were determined. According to the results, the mutant PINK1(B9) flies exhibited dopaminergic, survival, and behavioral deficits, which were rescued by EGCG supplementation. Meanwhile, EGCG resulted in profound changes in gut microbial compositions in PINK1(B9) flies, restoring the abundance of a set of bacteria. Notably, EGCG protection was blunted when gut microbiota was disrupted by antibiotics. Four bacterial strains were isolated from fly guts and the supplementation of individual Lactobacillus plantarum or Acetobacter pomorum strain exacerbated the neuronal and behavioral dysfunction of PD flies, which could not be rescued by EGCG. Transcriptomic analysis identified TotM as the central gene responding to EGCG or microbial manipulations. Genetic ablation of TotM blocked the recovery activity of EGCG, suggesting that EGCG-mediated protection warrants TotM. Apart from familial form, EGCG was also potent in improving sporadic PD symptoms induced by rotenone treatment, wherein gut microbiota shared regulatory roles. Together, these results suggest the relevance of the gut microbiota-TotM pathway in EGCG-mediated neuroprotection, providing insight into indirect mechanisms underlying nutritional intervention of Parkinson's disease (Xu, 2020).
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Pirooznia, S. K., Yuan, C., Khan, M. R., Karuppagounder, S. S., Wang, L., Xiong, Y., Kang, S. U., Lee, Y., Dawson, V. L. and Dawson, T. M. (2020). PARIS induced defects in mitochondrial biogenesis drive dopamine neuron loss under conditions of parkin or PINK1 deficiency. Mol Neurodegener 15(1): 17. PubMed ID: 32138754
Abstract
Mutations in PINK1 and parkin cause autosomal recessive Parkinson's disease (PD). Evidence placing PINK1 and parkin in common pathways regulating multiple aspects of mitochondrial quality control is burgeoning. However, compelling evidence to causatively link specific PINK1/parkin dependent mitochondrial pathways to dopamine neuron degeneration in PD is lacking. This study examined how PINK1/parkin mediated regulation of the pathogenic substrate PARIS impacts dopaminergic mitochondrial network homeostasis and neuronal survival in Drosophila. The UAS-Gal4 system was employed for cell-type specific expression of the various transgenes. Effects on dopamine neuronal survival and function were assessed by anti-TH immunostaining and negative geotaxis assays. Defects in mitochondrial biogenesis were shown to drive adult onset progressive loss of dopamine neurons and motor deficits in Drosophila models of PINK1 or parkin insufficiency. Such defects result from PARIS dependent repression of dopaminergic PGC-1alpha and its downstream transcription factors NRF1 and TFAM that cooperatively promote mitochondrial biogenesis. Dopaminergic accumulation of human or Drosophila PARIS recapitulates these neurodegenerative phenotypes that are effectively reversed by PINK1, parkin or PGC-1alpha overexpression in vivo. PARIS is the only co-substrate of PINK1 and parkin to specifically accumulate in the DA neurons and cause neurodegeneration and locomotor defects stemming from disrupted dopamine signaling. These findings identify a highly conserved role for PINK1 and parkin in regulating mitochondrial biogenesis and promoting mitochondrial health via the PARIS/ PGC-1alpha axis. The Drosophila models described in this study effectively recapitulate the cardinal PD phenotypes and thus will facilitate identification of novel regulators of mitochondrial biogenesis for physiologically relevant therapeutic interventions (Pirooznia, 2020).
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Casu, M. A., Mocci, I., Isola, R., Pisanu, A., Boi, L., Mulas, G., Greig, N. H., Setzu, M. D. and Carta, A. R. (2020). Neuroprotection by the Immunomodulatory Drug Pomalidomide in the Drosophila LRRK2(WD40) Genetic Model of Parkinson's Disease. Front Aging Neurosci 12: 31. PubMed ID: 32116655
Abstract
The search for new disease-modifying drugs for Parkinson's disease (PD) is a slow and highly expensive process, and the repurposing of drugs already approved for different medical indications is becoming a compelling alternative option for researchers. Genetic variables represent a predisposing factor to the disease and mutations in leucine-rich repeat kinase 2 (LRRK2) locus have been correlated to late-onset autosomal-dominant PD. The common fruit fly Drosophila melanogaster carrying the mutation LRRK2 loss-of-function in the WD40 domain (LRRK2(WD40)), is a simple in vivo model of PD and is a valid tool to first evaluate novel therapeutic approaches to the disease. Recent studies have suggested a neuroprotective activity of immunomodulatory agents in PD models. In this study the immunomodulatory drug Pomalidomide (POM), a Thalidomide derivative, was examined in the Drosophila LRRK2(WD40) genetic model of PD. Mutant and wild type flies received increasing POM doses (1, 0.5, 0.25 mM) through their diet from day 1 post eclosion, until postnatal day (PN) 7 or 14, when POM's actions were evaluated by quantifying changes in climbing behavior as a measure of motor performance, the number of brain dopaminergic neurons and T-bars, mitochondria integrity. LRRK2(WD40) flies displayed a spontaneous age-related impairment of climbing activity, and POM significantly and dose-dependently improved climbing performance both at PN 7 and PN 14. LRRK2(WD40) fly motor disability was underpinned by a progressive loss of dopaminergic neurons in posterior clusters of the protocerebrum, which are involved in the control of locomotion, by a low number of T-bars density in the presynaptic bouton active zones. POM treatment fully rescued the cell loss in all posterior clusters at PN 7 and PN 14 and significantly increased the T-bars density. Moreover, several damaged mitochondria with dilated cristae were observed in LRRK2(WD40) flies treated with vehicle but not following POM. This study demonstrates the neuroprotective activity of the immunomodulatory agent POM in a genetic model of PD. POM is an FDA-approved clinically available and well-tolerated drug used for the treatment of multiple myeloma. If further validated in mammalian models of PD, POM could rapidly be clinically tested in humans (Casu, 2020).
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Ham, S. J., Lee, D., Yoo, H., Jun, K., Shin, H. and Chung, J. (2020). Decision between mitophagy and apoptosis by Parkin via VDAC1 ubiquitination. Proc Natl Acad Sci U S A. PubMed ID: 32047033
Abstract
VDAC1 is a critical substrate of Parkin responsible for the regulation of mitophagy and apoptosis. This study demonstrates that VDAC1 can be either mono- or polyubiquitinated by Parkin in a PINK1-dependent manner. VDAC1 deficient with polyubiquitination (VDAC1 Poly-KR) hampers mitophagy, but VDAC1 deficient with monoubiquitination (VDAC1 K274R) promotes apoptosis by augmenting the mitochondrial calcium uptake through the mitochondrial calcium uniporter (MCU) channel. The transgenic flies expressing Drosophila Porin K273R, corresponding to human VDAC1 K274R, show Parkinson disease (PD)-related phenotypes including locomotive dysfunction and degenerated dopaminergic neurons, which are relieved by suppressing MCU and mitochondrial calcium uptake. To further confirm the relevance of these findings in PD, a missense mutation of Parkin was discovered in PD patients, T415N, which lacks the ability to induce VDAC1 monoubiquitination but still maintains polyubiquitination. Interestingly, Drosophila Parkin T433N, corresponding to human Parkin T415N, fails to rescue the PD-related phenotypes of Parkin-null flies. Taken together, these results suggest that VDAC1 monoubiquitination plays important roles in the pathologies of PD by controlling apoptosis (Ham, 2020).
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Lee, J. J., Andreazza, S. and Whitworth, A. J. (2020). The STING pathway does not contribute to behavioural or mitochondrial phenotypes in Drosophila Pink1/parkin or mtDNA mutator models. Sci Rep 10(1): 2693. PubMed ID: 32060339
Abstract
Mutations in PINK1 and Parkin/PRKN cause the degeneration of dopaminergic neurons in familial forms of Parkinson's disease but the precise pathogenic mechanisms are unknown. The PINK1/Parkin pathway has been described to play a central role in mitochondrial homeostasis by signalling the targeted destruction of damaged mitochondria, however, how disrupting this process leads to neuronal death was unclear until recently. An elegant study in mice revealed that the loss of Pink1 or Prkn coupled with an additional mitochondrial stress resulted in the aberrant activation of the innate immune signalling, mediated via the cGAS/STING pathway, causing degeneration of dopaminergic neurons and motor impairment. Genetic knockout of Sting was sufficient to completely prevent neurodegeneration and accompanying motor deficits. To determine whether Sting plays a conserved role in Pink1/parkin related pathology, tests were performed for genetic interactions between Sting and Pink1/parkin in Drosophila. Surprisingly, loss of Sting, or its downstream effector Relish, was insufficient to suppress the behavioural deficits or mitochondria disruption in the Pink1/parkin mutants. Thus, it is concluded that phenotypes associated with loss of Pink1/parkin are not universally due to aberrant activation of the STING pathway (Lee, 2020).
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Diana, A., Collu, M., Casu, M. A., Mocci, I., Aguilar-Santelises, M. and Setzu, M. D. (2020). Improvements of motor performances in the Drosophila LRRK2 loss-of-function model of Parkinson's disease: Effects of dialyzed leucocyte Extracts from Human Serum. Brain Sci 10(1). PubMed ID: 31947539
Abstract
Within neurodegenerative syndromes, Parkinson's disease (PD) is typically associated with its locomotor defects, sleep disturbances and related dopaminergic (DA) neuron loss. The fruit fly, Drosophila melanogaster, with leucine-rich repeat kinase 2 mutants (LRRK2) loss-of-function in the WD40 domain, provides mechanistic insights into corresponding human behaviour, possibly disclosing some physiopathologic features of PD in both genetic and sporadic forms. Moreover, several data support the boosting impact of innate and adaptive immunity pathways for driving the progression of PD. In this context, human dialyzable leukocyte extracts (DLE) have been extensively used to transfer antigen-specific information that influences the activity of various immune components, including inflammatory cytokines. Hence, the main goal of this study was to ascertain the therapeutic potential of DLE from male and female donors on D. melanogaster LRRK2 loss-of-function, as compared to D. melanogaster wild-type (WT), in terms of rescuing physiological parameters, such as motor and climbing activities, which are severely compromised in the mutant flies. Finally, in search of the anatomical structures responsible for restored functions in parkinsonian-like mutant flies, this study found a topographical correlation between improvement of locomotor performances and an increased number of dopaminergic neurons in selective areas of LRRK2 mutant brains (Diana, 2020).
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Imai, Y., Inoshita, T., Meng, H., Shiba-Fukushima, K., Hara, K. Y., Sawamura, N. and Hattori, N. (2019). Light-driven activation of mitochondrial proton-motive force improves motor behaviors in a Drosophila model of Parkinson's disease. Commun Biol 2: 424. PubMed ID: 31799427
Abstract
Mitochondrial degeneration is considered one of the major causes of Parkinson's disease (PD). Improved mitochondrial functions are expected to be a promising therapeutic strategy for PD. This study introduced a light-driven proton transporter, Delta-rhodopsin (dR), to Drosophila mitochondria, where the mitochondrial proton-motive force (Deltap) and mitochondrial membrane potential are maintained in a light-dependent manner. The loss of the PD-associated mitochondrial gene CHCHD2 resulted in reduced ATP production, enhanced mitochondrial peroxide production and lower Ca(2+)-buffering activity in dopaminergic (DA) terminals in flies. These cellular defects were improved by the light-dependent activation of mitochondrion-targeted dR (mito-dR). Moreover, mito-dR reversed the pathology caused by the CHCHD2 deficiency to suppress alpha-synuclein aggregation, DA neuronal loss, and elevated lipid peroxidation in brain tissue, improving motor behaviors. This study suggests the enhancement of Deltap by mito-dR as a therapeutic mechanism that ameliorates neurodegeneration by protecting mitochondrial functions (Imai, 2019).
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Chung, H. J., Islam, M. S., Rahman, M. M. and Hong, S. T. (2019).Neuroprotective function of Omi to alpha-synuclein-induced neurotoxicity. Neurobiol Dis: 104706. PubMed ID: 31837423
Abstract
The main pathological hallmark of Parkinson's disease (PD) is the presence of Lewy bodies, which mainly consist of aggregated alpha-synuclein. Based on the neurotoxicity of oligomeric alpha-synuclein and its significance in the aetiology of PD, there has been decades of effort to elucidate an enzyme specifically degrading oligomeric alpha-synuclein. This study reports an enzyme, Omi, which specifically recognizes and precisely degrades oligomeric alpha-synuclein but not monomeric alpha-synuclein. After enzymatic and functional analyses of Omi in in vitro, an in vivo assay system of dual gene interaction was developed in Drosophila to investigate further the etiological role of Omi in PD. Pan-neuronal expression of Omi rescued Parkinsonism in a Drosophila model of PD, while Knockout of Omi exacerbated Parkinsonism. Expression of Omi counteracted the alpha-synuclein-induced retinal degeneration, providing additional evidence for Omi's protective role oligomeric alpha-synuclein. This work reports identification of the catabolic pathway of oligomeric alpha-synuclein as well as showing how Omi functions as the key molecule in the recognition and degradation of toxic oligomeric alpha-synuclein, a possible cause of neurodegeneration in PD, without affecting monomeric alpha-synuclein which is a native essential molecule for the normal function of neurons.
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Yedlapudi, D., Xu, L., Luo, D., Marsh, G. B., Todi, S. V. and Dutta, A. K. (2019). Targeting alpha synuclein and amyloid beta by a multifunctional, brain-penetrant dopamine D2/D3 agonist D-520: Potential therapeutic application in Parkinson's disease with dementia. Sci Rep 9(1): 19648. PubMed ID: 31873106
Abstract
A significant number of people with Parkinson's disease (PD) develop dementia in addition to cognitive dysfunction and are diagnosed as PD with dementia (PDD). This is characterized by cortical and limbic alpha synuclein (alpha-syn) accumulation, and high levels of diffuse amyloid beta (Abeta) plaques in the striatum and neocortical areas. In this regard, this study evaluated the effect of a brain-penetrant, novel multifunctional dopamine D2/D3 agonist, D-520 on the inhibition of Abeta aggregation and disintegration of alpha-syn and Abeta aggregates in vitro using purified proteins and in a cell culture model that produces intracellular Abeta-induced toxicity. The effect of D-520 was evaluated in a Drosophila model of Abeta1-42 toxicity. It is reported that D-520 inhibits the formation of Abeta aggregates in vitro and promotes the disaggregation of both alpha-syn and Abeta aggregates. Finally, in an in vivo Drosophila model of Abeta1-42 dependent toxicity, D-520 exhibited efficacy by rescuing fly eyes from retinal degeneration caused by Abeta toxicity. These data indicate the potential therapeutic applicability of D-520 in addressing motor dysfunction and neuroprotection in PD and PDD, as well as attenuating dementia in people with PDD.
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Sim, J. P. L., Ziyin, W., Basil, A. H., Lin, S., Chen, Z., Zhang, C., Zeng, L., Cai, Y. and Lim, K. L. (2019). Identification of PP2A and S6 kinase as modifiers of Leucine-rich repeat kinase-induced neurotoxicity. Neuromolecular Med. PubMed ID: 31664682
Abstract
Mutations in LRRK2 are currently recognized as the most common monogenetic cause of Parkinsonism. The elevation of kinase activity of LRRK2 that frequently accompanies its mutations is widely thought to contribute to its toxicity. Accordingly, many groups have developed LRRK2-specific kinase inhibitors as a potential therapeutic strategy. Given that protein phosphorylation is a reversible event, this study sought to elucidate the phosphatase(s) that can reverse LRRK2-mediated phosphorylation, with the view that targeting this phosphatase(s) may similarly be beneficial. Using an unbiased RNAi phosphatase screen conducted in a Drosophila LRRK2 model, PP2A was identified as a genetic modulator of LRRK2-induced neurotoxicity. Further, ribosomal S6 kinase (S6K), a target of PP2A, was also identified as a novel regulator of LRRK2 function. Finally, modulation of PP2A or S6K activities were shown to ameliorate LRRK2-associated disease phenotype in Drosophila.
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Man Anh, H., Linh, D. M., My Dung, V. and Thi Phuong Thao, D. (2019). Evaluating dose- and time-dependent effects of vitamin C treatment on a Parkinson's disease fly model. Parkinsons Dis 2019: 9720546. PubMed ID: 30719278
Abstract
Parkinson's disease (PD) is a common neurodegenerative disorder and characterized by progressive locomotive defects and loss of dopaminergic neurons (DA neuron). Currently, there is no potent therapy to cure PD, and the medications merely support to control the symptoms. It is difficult to develop an effective treatment, since the PD onset mechanism of PD is still unclear. Oxidative stress is considered as a major cause of neurodegenerative diseases, and there is increasing evidence for the association between PD and oxidative stress. Therefore, antioxidant treatment may be a promising therapy for PD. Drosophila with knockdown of dUCH, a homolog of UCH-L1 which is a PD-related gene, exhibited PD-like phenotypes including progressive locomotive impairments and DA neuron degeneration. Moreover, knockdown of dUCH led to elevated level of ROS. Thus, dUCH knockdown flies can be used as a model for screening of potential antioxidants for treating PD. Previous studies demonstrated that curcumin at 1 mM and vitamin C at 0.5 mM could improve PD-like phenotypes induced by this knockdown. With the purpose of further investigating the efficiency of vitamin C in PD treatment, dUCH knockdown Drosophila model was used to examine the dose- and time-dependent effects of vitamin C on PD-like phenotypes. The results showed that although vitamin C exerted neuroprotective effects, high doses of vitamin C and long-term treatment with this antioxidant also resulted in side effects on physiology. It is suggested that dose-dependent effects of vitamin C should be considered when used for treating PD (Man Anh, 2019).
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Aggarwal, A., Reichert, H. and VijayRaghavan, K. (2019). A locomotor assay reveals deficits in heterozygous Parkinson's disease model and proprioceptive mutants in adult Drosophila. Proc Natl Acad Sci U S A. PubMed ID: 31748267
Abstract
Severe locomotor impairment is a common phenotype of neurodegenerative disorders such as Parkinson's disease (PD). Drosophila models of PD, studied for more than a decade, have helped in understanding the interaction between various genetic factors, such as parkin and PINK1, in this disease. To characterize locomotor behavioral phenotypes for these genes, fly climbing assays have been widely used. While these simple current assays for locomotor defects in Drosophila mutants measure some locomotor phenotypes well, it is possible that detection of subtle changes in behavior is important to understand the manifestation of locomotor disorders. This study introduces a climbing behavior assay which provides such fine-scale behavioral data and tests this proposition for the Drosophila model. This inexpensive, fully automated assay was used to quantitatively characterize the climbing behavior at high parametric resolution in 3 contexts. First, wild-type flies were characterized, and a hitherto unknown sexual dimorphism in climbing behavior was uncovered. Second, climbing behavior was studied of heterozygous mutants of genes implicated in the fly PD model, and previously unreported prominent locomotor defects were revealed in some of these heterozygous fly lines. Finally, locomotor defects were studied in a homozygous proprioceptory mutation (Trp-gamma1) known to affect fine motor control in Drosophila. Moreover, aberrant geotactic behavior was identified in Trp-gamma1 mutants, thereby opening up a finer assay for geotaxis and its genetic basis. This assay is therefore a cost-effective, general tool for measuring locomotor behaviors of wild-type and mutant flies in fine detail and can reveal subtle motor defects (Aggarwal, 2019).
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Mori, A., Hatano, T., Inoshita, T., Shiba-Fukushima, K., Koinuma, T., Meng, H., Kubo, S. I., Spratt, S., Cui, C., Yamashita, C., Miki, Y., Yamamoto, K., Hirabayashi, T., Murakami, M., Takahashi, Y., Shindou, H., Nonaka, T., Hasegawa, M., Okuzumi, A., Imai, Y. and Hattori, N. (2019). Parkinson's disease-associated iPLA2-VIA/PLA2G6 regulates neuronal functions and alpha-synuclein stability through membrane remodeling. Proc Natl Acad Sci U S A 116(41): 20689-20699. PubMed ID: 31548400
Abstract
Mutations in the iPLA2-VIA/PLA2G6 gene are responsible for PARK14-linked Parkinson's disease (PD) with alpha-synucleinopathy. However, it is unclear how iPLA2-VIA mutations lead to alpha-synuclein (alpha-Syn) aggregation and dopaminergic (DA) neurodegeneration. This study reports that iPLA2-VIA-deficient Drosophila exhibits defects in neurotransmission during early developmental stages and progressive cell loss throughout the brain, including degeneration of the DA neurons. Lipid analysis of brain tissues reveals that the acyl-chain length of phospholipids is shortened by iPLA2-VIA loss, which causes endoplasmic reticulum (ER) stress through membrane lipid disequilibrium. The introduction of wild-type human iPLA2-VIA or the mitochondria-ER contact site-resident protein C19orf12 in iPLA2-VIA-deficient flies rescues the phenotypes associated with altered lipid composition, ER stress, and DA neurodegeneration, whereas the introduction of a disease-associated missense mutant, iPLA2-VIA A80T, fails to suppress these phenotypes. The acceleration of alpha-Syn aggregation by iPLA2-VIA loss is suppressed by the administration of linoleic acid, correcting the brain lipid composition. These findings suggest that membrane remodeling by iPLA2-VIA is required for the survival of DA neurons and alpha-Syn stability (Mori, 2019).
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Ding, Y., Kong, D., Zhou, T., Yang, N. D., Xin, C., Xu, J., Wang, Q., Zhang, H., Wu, Q., Lu, X., Lim, K., Ma, B., Zhang, C., Li, L. and Huang, W. (2019). alpha-Arbutin protects against Parkinson's disease-associated mitochondrial dysfunction in vitro and in vivo. Neuromolecular Med. PubMed ID: 31401719
Abstract
Parkinson's disease (PD), the most common neurodegenerative movement disorder, is characterized by the progressive loss of dopaminergic neurons in substantia nigra. The underlying mechanisms of PD pathogenesis have not been fully illustrated and currently PD remains incurable. Accumulating evidences suggest that mitochondrial dysfunction plays pivotal role in the dopaminergic neuronal death. Therefore, discovery of novel and safe agent for rescuing mitochondrial dysfunction would benefit PD treatment. This study demonstrated that alpha-Arbutin (Arb), a natural polyphenol extracted from Ericaceae species, displayed significant protective effect on the rotenone (Rot)-induced mitochondrial dysfunction and apoptosis of human neuroblastoma cell (SH-SY5Y). It was further found that the neuroprotective effect of Arb was associated with ameliorating oxidative stress, stabilizing of mitochondrial membrane potential, and enhancing adenosine triphosphate production. To investigate the underlying mechanism, the AMP-activated protein kinase and autophagy pathway was checked, were found to be involved in the neuroprotection of Arb. Moreover, the protective effect of Arb was explored in Drosophila PD model, and Arb was found to rescue parkin deficiency-induced motor function disability and mitochondrial abnormality of Drosophila. Taken together, this study demonstrated that Arb got excellent neuroprotective effect on PD models both in vitro and in vivo and Arb might serve as a potent therapeutic agent for the treatment of PD (DingY, 2019).
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Wu, S., Tan, K. J., Govindarajan, L. N., Stewart, J. C., Gu, L., Ho, J. W. H., Katarya, M., Wong, B. H., Tan, E. K., Li, D., Claridge-Chang, A., Libedinsky, C., Cheng, L. and Aw, S. S. (2019). Fully automated leg tracking of Drosophila neurodegeneration models reveals distinct conserved movement signatures. PLoS Biol 17(6): e3000346. PubMed ID: 31246996
Abstract
Some neurodegenerative diseases, like Parkinsons Disease (PD) and Spinocerebellar ataxia 3 (SCA3), are associated with distinct, altered gait and tremor movements that are reflective of the underlying disease etiology. Drosophila melanogaster models of neurodegeneration have led to understanding of the molecular mechanisms of disease. However, it is unknown whether specific gait and tremor dysfunctions also occur in fly disease mutants. To answer this question, a machine-learning image-analysis program, Feature Learning-based LImb segmentation and Tracking (FLLIT), was developed that automatically tracks leg claw positions of freely moving flies recorded on high-speed video, producing a series of gait measurements. Notably, unlike other machine-learning methods, FLLIT generates its own training sets and does not require user-annotated images for learning. Using FLLIT, high-throughput and high-resolution analysis of gait and tremor features were carried out in Drosophila neurodegeneration mutants for the first time. Fly models of PD and SCA3 exhibited markedly different walking gait and tremor signatures, which recapitulated characteristics of the respective human diseases. Selective expression of mutant SCA3 in dopaminergic neurons led to a gait signature that more closely resembled those of PD flies. This suggests that the behavioral phenotype depends on the neurons affected rather than the specific nature of the mutation. Different mutations produced tremors in distinct leg pairs, indicating that different motor circuits were affected. Using this approach, fly models can be used to dissect the neurogenetic mechanisms that underlie movement disorders (Wu, 2019).
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Doktor, B., Damulewicz, M. and Pyza, E. (2019). Effects of MUL1 and PARKIN on the circadian clock, brain and behaviour in Drosophila Parkinson's disease models. BMC Neurosci 20(1): 24. PubMed ID: 31138137
Abstract
Mutants which carry mutations in genes encoding mitochondrial ligases MUL1 and PARKIN are convenient Drosophila models of Parkinson's disease (PD). In several studies it has been shown that in Parkinson's disease sleep disturbance occurs, which may be the result of a disturbed circadian clock. This study found that the ROS level was higher, while the anti-oxidant enzyme SOD1 level was lower in mul1(A6) and park(1) mutants than in the white mutant used as a control. Moreover, mutations of both ligases affected circadian rhythms and the clock. The expression of clock genes per, tim and clock and the level of PER protein were changed in the mutants. Moreover, expression of ATG5, an autophagy protein also involved in circadian rhythm regulation, was decreased in the brain and in PDF-immunoreactive large ventral lateral clock neurons. The observed changes in the molecular clock resulted in a longer period of locomotor activity rhythm, increased total activity and shorter sleep at night. Finally, the lack of both ligases led to decreased longevity and climbing ability of the flies. It is concluded that all of the changes observed in the brains of these Drosophila models of PD, in which mitochondrial ligases MUL1 and PARKIN do not function, may explain the mechanisms of some neurological and behavioural symptoms of PD (Doktor, 2019).
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Sakai, R., Suzuki, M., Ueyama, M., Takeuchi, T., Minakawa, E. N., Hayakawa, H., Baba, K., Mochizuki, H. and Nagai, Y. (2019). E46K mutant alpha-synuclein is more degradation resistant and exhibits greater toxic effects than wild-type alpha-synuclein in Drosophila models of Parkinson's disease. PLoS One 14(6): e0218261. PubMed ID: 31242217
Abstract
Alpha-synuclein (alphaSyn) plays key roles in the pathogenesis of Parkinson's disease (PD). The mechanisms underlying the variance in the clinical phenotypes of familial PD caused by missense mutations in the alphaSyn gene remain elusive. This study established novel Drosophila models expressing either wild-type (WT) alphaSyn or one of five alphaSyn mutants (A30P, E46K, H50Q, G51D, and A53T) using site-specific transgenesis, which express transgenes at equivalent levels. Expression of either WT or mutant alphaSyn in the compound eyes by the GMR-GAL4 driver caused mild rough eye phenotypes with no obvious difference among the mutants. Upon pan-neuronal expression by the nSyb-GAL4 driver, these alphaSyn-expressing flies showed a progressive decline in locomotor function. Notably, it was found that E46K, H50Q, G51D, and A53T alphaSyn-expressing flies showed earlier onset of locomotor dysfunction than WT alphaSyn-expressing flies, suggesting their enhanced toxic effects. Whereas mRNA levels of WT and mutant alphaSyn were almost equivalent, it was found that protein expression levels of E46K alphaSyn were higher than those of WT alphaSyn. In vivo chase experiments using the drug-inducible GMR-GeneSwitch driver demonstrated that degradation of E46K alphaSyn protein was significantly slower than WT alphaSyn protein, indicating that the E46K alphaSyn mutant gains resistance to degradation in vivo. It is therefore conclude that the novel site-specific transgenic fly models expressing either WT or mutant alphaSyn are useful to explore the mechanisms by which different alphaSyn mutants gain toxic functions in vivo (Sakai, 2019).
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Cackovic, J., Gutierrez-Luke, S., Call, G. B., Juba, A., O'Brien, S., Jun, C. H. and Buhlman, L. M. (2018). Vulnerable parkin loss-of-function Drosophila dopaminergic neurons have advanced mitochondrial aging, mitochondrial network loss and transiently reduced autophagosome recruitment. Front Cell Neurosci 12: 39. PubMed ID: 29497364
Abstract
Selective degeneration of substantia nigra dopaminergic (DA) neurons is a hallmark pathology of familial Parkinson's disease (PD). While the mechanism of degeneration is elusive, abnormalities in mitochondrial function and turnover are strongly implicated. An Autosomal Recessive-Juvenile Parkinsonism (AR-JP) Drosophila melanogaster model exhibits DA neurodegeneration as well as aberrant mitochondrial dynamics and function. Disruptions in mitophagy have been observed in parkin loss-of-function models, and changes in mitochondrial respiration have been reported in patient fibroblasts. Whether loss of parkin causes selective DA neurodegeneration in vivo as a result of lost or decreased mitophagy is unknown. This study employs the use of fluorescent constructs expressed in Drosophila DA neurons that are functionally homologous to those of the mammalian substantia nigra. Evidence is provided that degenerating DA neurons in parkin loss-of-function mutant flies have advanced mitochondrial aging and that mitochondrial networks are fragmented and contain swollen organelles. This study also found that mitophagy initiation is decreased in park (Drosophila parkin/PARK2 ortholog) homozygous mutants, but autophagosome formation is unaffected, and mitochondrial network volumes are decreased. As the fly ages, autophagosome recruitment becomes similar to control, while mitochondria continue to show signs of damage, and climbing deficits persist. Interestingly, aberrant mitochondrial morphology, aging and mitophagy initiation were not observed in DA neurons that do not degenerate. These results suggest that parkin is important for mitochondrial homeostasis in vulnerable Drosophila DA neurons, and that loss of parkin-mediated mitophagy may play a role in degeneration of relevant DA neurons or motor deficits in this model (Cackovic, 2018).
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Ran, D., Xie, B., Gan, Z., Sun, X., Gu, H. and Yang, J. (2018). Melatonin attenuates hLRRK2-induced long-term memory deficit in a Drosophila model of Parkinson's disease. Biomed Rep 9(3): 221-226. PubMed ID: 30271597
Abstract
As the most common genetic cause of Parkinson's disease (PD), the role of human leucine-rich repeat kinase 2 (hLRRK2) in the efficacy of PD treatment is a focus of study. A previous study demonstrated that mushroom body (MB) expression of hLRRK2 in Drosophila could recapitulate the clinical feature of sleep disturbances observed in PD patients, and melatonin (MT) treatment could attenuate the hLRRK2-induced sleep disorders and synaptic dysfunction, suggesting the therapeutic potential of MT in PD patients carrying hLRRK2 mutations; however, no further study into the impacts on memory deficit was conducted. Therefore, in the current paper, the study of the effects of MT on hLRRK2 flies was continued, to determine its potential role in the improvement of memory deficit in PD. To achieve this, the Drosophila learning and memory phases, including short- and long-term memory, were recorded; furthermore, the effect of MT on calcium channel activity during neurotransmission was detected using electrophysiology patch clamp recordings. It was demonstrated that MT treatment reversed hLRRK2-induced long-term memory deficits in Drosophila; furthermore, MT reduced MB calcium channel activities. These findings suggest that MT may exerts therapeutic effects on the long-term memory of PD patients via calcium channel modulation, thus providing indication of its potential to maintain cognitive function in PD patients (Ran, 2018).
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Himmelberg, M. M., West, R. J. H., Elliott, C. J. H. and Wade, A. R. (2018). Abnormal visual gain control and excitotoxicity in early-onset Parkinson's disease Drosophila models. J Neurophysiol. 119(3):957-970 PubMed ID: 29142100
Abstract
The excitotoxic theory of Parkinson's disease (PD) hypothesises that a pathophysiological degeneration of dopaminergic neurons stems from neural hyperactivity at early stages of disease, leading to mitochondrial stress and cell death. Recent research has harnessed the visual system of Drosophila PD models to probe this hypothesis. This study investigated whether abnormal visual sensitivity and excitotoxicity occur in early-onset PD Drosophila models DJ-1Delta72, DJ1-Delta93, and PINK15. An electroretinogram was used to record steady state visually evoked potentials driven by temporal contrast stimuli. At 1 day of age, all early-onset PD mutants had a twofold increase in response amplitudes when compared to w- controls. Further, excitotoxicity was found to occur in older early-onset PD models after increased neural demand is applied via visual stimulation. In an additional analysis, a linear discriminant analysis was used to test whether there were subtle variations in neural gain control that could be used to classify Drosophila into their correct age and genotype. The discriminant analysis was highly accurate, classifying Drosophila into their correct genotypic class at all age groups at 50-70% accuracy (20% chance baseline). Differences in cellular processes link to subtle alterations in neural network operation in young flies, all of which lead to the same pathogenic outcome. These data are the first to demonstrate abnormal gain control and excitotoxicity in early-onset PD Drosophila mutants. It is concluded that early-onset PD mutations may be linked to more sensitive neuronal signalling in prodromal animals that may cause the expression of PD symptomologies later in life (Himmelberg, 2017).
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Valadas, J. S., Esposito, G., Vandekerkhove, D., Miskiewicz, K., Deaulmerie, L., Raitano, S., Seibler, P., Klein, C. and Verstreken, P. (2018). ER lipid defects in neuropeptidergic neurons impair sleep patterns in Parkinson's disease. Neuron 98(6): 1155-1169.e1156. PubMed ID: 29887339
Abstract
Parkinson's disease patients report disturbed sleep patterns long before motor dysfunction. In parkin and pink1 models, this study has identified circadian rhythm and sleep pattern defects and has mapped these to specific neuropeptidergic neurons in fly models and in hypothalamic neurons differentiated from patient induced pluripotent stem cells (iPSCs). Parkin and Pink1 control the clearance of mitochondria by protein ubiquitination. Although major defects were not observed in mitochondria of mutant neuropeptidergic neurons, excess of endoplasmic reticulum-mitochondrial contacts was found. These excessive contact sites cause abnormal lipid trafficking that depletes phosphatidylserine from the endoplasmic reticulum (ER) and disrupts the production of neuropeptide-containing vesicles. Feeding mutant animals phosphatidylserine rescues neuropeptidergic vesicle production and acutely restores normal sleep patterns in mutant animals. Hence, sleep patterns and circadian disturbances in Parkinson's disease models are explained by excessive ER-mitochondrial contacts, and blocking their formation or increasing phosphatidylserine levels rescues the defects in vivo (Valadas, 2018).
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Prasad, V., Wasser, Y., Hans, F., Goswami, A., Katona, I., Outeiro, T. F., Kahle, P. J., Schulz, J. B. and Voigt, A. (2018). Monitoring alpha-synuclein multimerization in vivo. Faseb j: fj201800148RRR. PubMed ID: 30252534
Abstract
The pathophysiology of Parkinson's disease is characterized by the abnormal accumulation of alpha-synuclein (alpha-Syn), eventually resulting in the formation of Lewy bodies and neurites in surviving neurons in the brain. Although alpha-Syn aggregation has been extensively studied in vitro, there is limited in vivo knowledge on alpha-Syn aggregation. This study used the powerful genetics of Drosophila melanogaster and developed an in vivo assay to monitor alpha-Syn accumulation by using a bimolecular fluorescence complementation assay. Both genetic and pharmacologic manipulations affected alpha-Syn accumulation. Interestingly, it was also found that alterations in the cellular protein degradation mechanisms strongly influenced alpha-Syn accumulation. Administration of compounds identified as risk factors for Parkinson's disease, such as rotenone or heavy metal ions, had only mild or even no impact on alpha-Syn accumulation in vivo. Finally, this study showed that increasing phosphorylation of alpha-Syn at serine 129 enhances the accumulation and toxicity of alpha-Syn. Altogether, this study establishes a novel model to study alpha-Syn accumulation and illustrates the complexity of manipulating proteostasis in vivo (Prasad, 2018).
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Lee, K. S., Huh, S., Lee, S., Wu, Z., Kim, A. K., Kang, H. Y. and Lu, B. (2018). Altered ER-mitochondria contact impacts mitochondria calcium homeostasis and contributes to neurodegeneration in vivo in disease models. Proc Natl Acad Sci U S A. PubMed ID: 30185553
Abstract
Calcium (Ca(2+)) homeostasis is essential for neuronal function and survival. Altered Ca(2+) homeostasis has been consistently observed in neurological diseases. How Ca(2+) homeostasis is achieved in various cellular compartments of disease-relevant cell types is not well understood. This study shows in Drosophila Parkinson's disease (PD) models that Ca(2+) transport from the endoplasmic reticulum (ER) to mitochondria through the ER-mitochondria contact site (ERMCS) critically regulates mitochondrial Ca(2+) (mito-Ca(2+)) homeostasis in dopaminergic (DA) neurons, and that the PD-associated PINK1 protein modulates this process. In PINK1 mutant DA neurons, the ERMCS is strengthened and mito-Ca(2+) level is elevated, resulting in mitochondrial enlargement and neuronal death. Miro, a well-characterized component of the mitochondrial trafficking machinery, mediates the effects of PINK1 on mito-Ca(2+) and mitochondrial morphology, apparently in a transport-independent manner. Miro overexpression mimics PINK1 loss-of-function effect, whereas inhibition of Miro or components of the ERMCS, or pharmacological modulation of ERMCS function, rescued PINK1 mutant phenotypes. Mito-Ca(2+) homeostasis is also altered in the LRRK2-G2019S model of PD and the PAR-1/MARK model of neurodegeneration, and genetic or pharmacological restoration of mito-Ca(2+) level is beneficial in these models. Our results highlight the importance of mito-Ca(2+) homeostasis maintained by Miro and the ERMCS to mitochondrial physiology and neuronal integrity. Targeting this mito-Ca(2+) homeostasis pathway holds promise for a therapeutic strategy for neurodegenerative diseases (Lee, 2018).
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Issa, A. R., Sun, J., Petitgas, C., Mesquita, A., Dulac, A., Robin, M., Mollereau, B., Jenny, A., Cherif-Zahar, B. and Birman, S. (2018). The lysosomal membrane protein LAMP2A promotes autophagic flux and prevents SNCA-induced Parkinson disease-like symptoms in the Drosophila brain. Autophagy. PubMed ID: 29989488
Abstract
The autophagy-lysosome pathway plays a fundamental role in the clearance of aggregated proteins and protection against cellular stress and neurodegenerative conditions. Alterations in autophagy processes, including macroautophagy and chaperone-mediated autophagy (CMA), have been described in Parkinson disease (PD). CMA is a selective autophagic process that depends on LAMP2A (Lysosomal associated membrane protein 2A), a mammal and bird-specific membrane glycoprotein that translocates cytosolic proteins containing a KFERQ-like peptide motif across the lysosomal membrane. Drosophila reportedly lack CMA and use endosomal microautophagy (eMI) as an alternative selective autophagic process. This study reports that neuronal expression of human LAMP2A protected Drosophila against starvation and oxidative stress, and delayed locomotor decline in aging flies without extending their lifespan. LAMP2A also prevented the progressive locomotor and oxidative defects induced by neuronal expression of PD-associated human SNCA (synuclein alpha) with alanine-to-proline mutation at position 30 (SNCA(A30P)). LAMP2A expression stimulated selective autophagy in the adult brain and not in the larval fat body. Noteworthy, neurally expressed LAMP2A markedly upregulated levels of Drosophila Atg5, a key macroautophagy initiation protein, and of the Atg5-containing complex, and that it increased the density of Atg8a/LC3-positive puncta, which reflects the formation of autophagosomes. Furthermore, LAMP2A efficiently prevented accumulation of the autophagy defect marker Ref(2)P/p62 in the adult brain under acute oxidative stress. These results indicate that LAMP2A can promote autophagosome formation and potentiate autophagic flux in the Drosophila brain, leading to enhanced stress resistance and neuroprotection (Issa, 2018).
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Basso, V., Marchesan, E., Peggion, C., Chakraborty, J., von Stockum, S., Giacomello, M., Ottolini, D., Debattisti, V., Caicci, F., Tasca, E., Pegoraro, V., Angelini, C., Antonini, A., Bertoli, A., Brini, M. and Ziviani, E. (2018). Regulation of ER-mitochondria contacts by Parkin via Mfn2. Pharmacol Res. PubMed ID: 30219582
Abstract
Parkin
, an E3 ubiquitin ligase and a 