Probing mechanisms that underlie human neurodegenerative diseases in Drosophila

Modeling neurodegenerative and neurodevelopmental disorders in the Drosophila mushroom body

The common fruit fly Drosophila melanogaster provides a powerful platform to investigate the genetic, molecular, cellular, and neural circuit mechanisms of behavior. Research in this model system has shed light on multiple aspects of brain physiology and behavior, from fundamental neuronal function to complex behaviors. A major anatomical region that modulates complex behaviors is the mushroom body (MB). The MB integrates multimodal sensory information and is involved in behaviors ranging from sensory processing/responses to learning and memory. Many genes that underlie brain disorders are conserved, from flies to humans, and studies in Drosophila have contributed significantly to our understanding of the mechanisms of brain disorders. Genetic mutations that mimic human diseases-such as Fragile X syndrome, neurofibromatosis type 1, Parkinson's disease, and Alzheimer's disease-affect MB structure and function, altering behavior. Studies dissecting the effects of disease-causing mutations in the MB have identified key pathological mechanisms, and the development of a complete connectome promises to add a comprehensive anatomical framework for disease modeling. Here, we review Drosophila models of human neurodevelopmental and neurodegenerative disorders via the effects of their underlying mutations on MB structure, function, and the resulting behavioral alterations.

Neuronal LRP4 directs the development, maturation, and cytoskeletal organization of peripheral synapses

Synapse development requires multiple signaling pathways to accomplish the myriad of steps needed to ensure a successful connection. Transmembrane receptors on the cell surface are optimally positioned to facilitate communication between the synapse and the rest of the neuron and often function as synaptic organizers to synchronize downstream signaling events. One such organizer, the LDL receptor-related protein LRP4, is a cell surface receptor most well-studied postsynaptically at mammalian neuromuscular junctions. Recent work, however, has identified emerging roles for LRP4 as a presynaptic molecule, but how LRP4 acts as a presynaptic organizer, what roles LRP4 plays in organizing presynaptic biology, and the downstream mechanisms of LRP4 are not well understood. Here we show that LRP4 functions presynaptically at Drosophila neuromuscular synapses, acting in motor neurons to instruct multiple aspects of pre- and postsynaptic development. Loss of presynaptic LRP4 results in a range of developmental defects, impairing active zone organization, synapse growth, physiological function, microtubule organization, synaptic ultrastructure, and synapse maturation. We further demonstrate that LRP4 promotes most aspects of presynaptic development via a downstream SR-protein kinase, SRPK79D. SRPK79D overexpression suppresses synaptic defects associated with loss of lrp4. These data demonstrate a function for LRP4 as a peripheral synaptic organizer acting presynaptically, highlight a downstream mechanism conserved with its CNS function, and indicate previously unappreciated roles for LRP4 in cytoskeletal organization, synapse maturation, and active zone organization, underscoring its developmental importance.

Spectroscopic features of a perylenediimide probe for sensing amyloid fibrils: in vivo imaging of Aβ-aggregates in a Drosophila model organism

Customised perylenediimide (PDI) chromophores find diverse applications not only as chemosensors, inorganic-organic semiconductors, photovoltaics, photocatalysts, etc., but also in protein surface engineering, bio-sensors and drug delivery systems. This study focuses on the interaction of a custom synthesized phenylalanine derivatized perylenediimide (L-Phe-PDI) dye with a model protein, insulin, and its structurally distinct fibrils to develop fluorescence sensors for fibrillar aggregates and in vivo imaging applications. Detailed photophysical studies revealed that L-Phe-PDI gets aggregated in the presence of insulin and causes emission quenching at pH 7.4, which in the absence of insulin occurs only at pH ∼2. During in vitro incubation of insulin to its fibrils, the fluorescence intensity of the L-Phe-PDI probe is enhanced to ∼150 fold in a two-stage manner, manifesting the pathways of structural transformation to β-sheet rich mature fibrils. The in vivo sensing has further been validated in living models of the Aβ-mutant Drosophila fly, which is known to develop progressive neurodegeneration comparable to that of human brains with Alzheimer's disease (AD). Bioimaging of the L-Phe-PDI treated Aβ-mutant Drosophila documented the blood-brain/blood-retina-barrier cross-over ability of L-Phe-PDI with no toxic effects. Comparison of the fibrillar images from the brain and eye region with the reference thioflavin T (ThT) probe established the uptake of L-Phe-PDI by the aggregate/fibrillar moieties. The samples from L-Phe-PDI-treated flies apparently displayed reduced fibrillar spots, a possible case of L-Phe-PDI-induced disintegration of fibrillar aggregates at large, an observation substantiated by the improved phenotype activities as compared to the untreated flies. The findings reported both in vitro and in vivo with the L-Phe-PDI material for the first time open up avenues to explore the therapeutic potential of custom-designed PDI derivatives for amyloid fibril sensors and bioimaging.

Angiotensin converting enzyme inhibitor captopril prevents neuronal overexpression of amyloid-beta and alpha-synuclein in Drosophila melanogaster genetic models of neurodegenerative diseases

BACKGROUND

Parkinson disease (PD) and Alzheimer's disease (AD) are progressive neurodegenerative disorders characterized by loss of selective neurons in discreet part of the brain. The peptide angiotensin II (Ang II) plays significant role in hippocampal and striatal neurons degeneration through the generation of reactive oxygen species. Blockade of the angiotensin converting enzyme or ATI receptors provides protection in animal models of neurodegenerative diseases. In the present study, the neuroprotective effect of captopril was investigated in Drosophila melanogaster model using the UAS-GAL4 system to express the synuclein and Aβ42 peptide in the flies' neurons.

METHODS

The disease causing human Aβ42 peptide or α-syn was expressed pan-neuronally (elav-GAL4) or dopamine neuron (DDC-GAL4) using the UAS-GAL4 system. Flies were either grown in food media with or without captopril (1, 5, or 10µM). This was followed by fecundity, larva motility, negative geotaxis assay (climbing) and lifespan as a measure of neurodegeneration.

RESULTS

Elav-Gal4<Aβ or DDC-GAL4<α-syn flies displayed significant decrease in larva motility when compared with normal control (w1118) which was reversed by the supplementation of the media with captopril (5 or 10 mM) indicative of neuroprotection. Interestingly, supplementation of flies' media with captopril improved climbing activity in Elav-Gal4<Aβ or DDC-GAL4<α-syn flies when compared with vehicle treated only. Moreover, flies grown on captopril caused no significant change in lifespan.  Conclusion: Findings from this study confirmed the neuroprotective action of captopril in genetic or familial forms of neurodegeneration.

The Discovery of Novel PGK1 Activators as Apoptotic Inhibiting and Neuroprotective Agents

Stroke is the second leading cause of death worldwide and the leading cause of long-term disability that seriously endangers health and quality of human life. Tissue-type fibrinogen activator is currently the only drug approved by FDA for the treatment of ischemic stroke. Neuroprotection is theoretically a common strategy for the treatment of both ischemic and hemorrhagic stroke; therefore, the development of neuroprotective agent has been the focus of research. However, no ideal neuroprotective drug is clinically available. Phosphoglycerate kinase-1 (PGK1) activator has the effect of inhibiting apoptosis and protecting tissue damage, and therefore could be a potential neuroprotective agent. To obtain effective PGK1 activators, we virtually screened a large chemical database and their evaluated the efficacy by the Drosophila oxidative stress model, PGK1 enzymatic activity assay, and oxygen-glucose stripping reperfusion (OGD/R) model. The results showed that compounds 7979989, Z112553128 and AK-693/21087020 are potential PGK1 activators with protective effects against PQ-induced oxidative stress in the Drosophila model and could effectively ameliorate apoptosis induced by OGD/R-induced neuronal cell injury. Additionally, compounds 7979989 and Z112553128 are effective in alleviating LPS-induced cellular inflammation. This study indicated that these compounds are promising lead compounds that provide theoretical and material basis to the neuroprotective drug discovery.

A phospho-switch at Acinus-Serine437 controls autophagic responses to Cadmium exposure and neurodegenerative stress

Neuronal health depends on quality control functions of autophagy, but mechanisms regulating neuronal autophagy are poorly understood. Previously, we showed that in Drosophila starvation-independent quality control autophagy is regulated by acinus (acn) and the Cdk5-dependent phosphorylation of its serine⁴³⁷ (Nandi et al., 2017). Here, we identify the phosphatase that counterbalances this activity and provides for the dynamic nature of acinus-serine⁴³⁷ (acn-S437) phosphorylation. A genetic screen identified six phosphatases that genetically interacted with an acn gain-of-function model. Among these, loss of function of only one, the PPM-type phosphatase Nil (CG6036), enhanced pS437-acn levels. Cdk5-dependent phosphorylation of acn-S437 in nil¹ animals elevates neuronal autophagy and reduces the accumulation of polyQ proteins in a Drosophila Huntington’s disease model. Consistent with previous findings that Cd²⁺ inhibits PPM-type phosphatases, Cd²⁺ exposure elevated acn-S437 phosphorylation which was necessary for increased neuronal autophagy and protection against Cd²⁺-induced cytotoxicity. Together, our data establish the acn-S437 phosphoswitch as critical integrator of multiple stress signals regulating neuronal autophagy.

Neuronal ROS-induced glial lipid droplet formation is altered by loss of Alzheimer's disease-associated genes

A growing list of Alzheimer's disease (AD) genetic risk factors is being identified, but the contribution of each variant to disease mechanism remains largely unknown. We have previously shown that elevated levels of reactive oxygen species (ROS) induces lipid synthesis in neurons leading to the sequestration of peroxidated lipids in glial lipid droplets (LD), delaying neurotoxicity. This neuron-to-glia lipid transport is APOD/E-dependent. To identify proteins that modulate these neuroprotective effects, we tested the role of AD risk genes in ROS-induced LD formation and demonstrate that several genes impact neuroprotective LD formation, including homologs of human ABCA1, ABCA7, VLDLR, VPS26, VPS35, AP2A, PICALM, and CD2AP Our data also show that ROS enhances Aβ42 phenotypes in flies and mice. Finally, a peptide agonist of ABCA1 restores glial LD formation in a humanized APOE4 fly model, highlighting a potentially therapeutic avenue to prevent ROS-induced neurotoxicity. This study places many AD genetic risk factors in a ROS-induced neuron-to-glia lipid transfer pathway with a critical role in protecting against neurotoxicity.

Systematic functional analysis of rab GTPases reveals limits of neuronal robustness to environmental challenges in flies

Rab GTPases are molecular switches that regulate membrane trafficking in all cells. Neurons have particular demands on membrane trafficking and express numerous Rab GTPases of unknown function. Here, we report the generation and characterization of molecularly defined null mutants for all 26 rab genes in Drosophila. In flies, all rab genes are expressed in the nervous system where at least half exhibit particularly high levels compared to other tissues. Surprisingly, loss of any of these 13 nervous system-enriched Rabs yielded viable and fertile flies without obvious morphological defects. However, all 13 mutants differentially affected development when challenged with different temperatures, or neuronal function when challenged with continuous stimulation. We identified a synaptic maintenance defect following continuous stimulation for six mutants, including an autophagy-independent role of rab26. The complete mutant collection generated in this study provides a basis for further comprehensive studies of Rab GTPases during development and function in vivo.

Altered MICOS Morphology and Mitochondrial Ion Homeostasis Contribute to Poly(GR) Toxicity Associated with C9-ALS/FTD

Amyotrophic lateral sclerosis (ALS) manifests pathological changes in motor neurons and various other cell types. Compared to motor neurons, the contribution of the other cell types to the ALS phenotypes is understudied. G4C2 repeat expansion in C9ORF72 is the most common genetic cause of ALS along with frontotemporal dementia (C9-ALS/FTD), with increasing evidence supporting repeat-encoded poly(GR) in disease pathogenesis. Here, we show in Drosophila muscle that poly(GR) enters mitochondria and interacts with components of the Mitochondrial Contact Site and Cristae Organizing System (MICOS), altering MICOS dynamics and intra-subunit interactions. This impairs mitochondrial inner membrane structure, ion homeostasis, mitochondrial metabolism, and muscle integrity. Similar mitochondrial defects are observed in patient fibroblasts. Genetic manipulation of MICOS components or pharmacological restoration of ion homeostasis with nigericin effectively rescue the mitochondrial pathology and disease phenotypes in both systems. These results implicate MICOS-regulated ion homeostasis in C9-ALS pathogenesis and suggest potential new therapeutic strategies.

Spatially-resolved metabolic profiling of living Drosophila in neurodegenerative conditions using 1H magic angle spinning NMR

Drosophila flies are versatile animal models for the study of gene mutations in neuronal pathologies. Their small size allows performing in vivo Magic Angle Spinning (MAS) experiments to obtain high-resolution 1H nuclear magnetic resonance (NMR) spectra. Here, we use spatially-resolved 1H high-resolution MAS NMR to investigate in vivo metabolite contents in different segments of the fly body. A comparative study of metabolic changes was performed for three neurodegenerative disorders: two cell-specific neuronal and glial models of Huntington disease (HD) and a model of glutamate excitotoxicity. It is shown that these pathologies are characterized by specific and sometimes anatomically localized variations in metabolite concentrations. In two cases, the modifications of 1H MAS NMR spectra localized in fly heads were significant enough to allow the creation of a predictive model.

The effect and mechanism of 19S proteasome PSMD11/Rpn6 subunit in D-Galactose induced mimetic aging models

Regulating proteasome activity is a potent therapeutic aspect of age-related hearing loss, which has been proven to protect neurons from age-related damaging. PSMD11, subunit of the 19S proteasome regulatory particle, is known to mainly up-regulate proteasome activity and prolong aging. However, the mechanism of PSMD11 in age-related hearing loss has not been deeply explored. In the present study, we explore the function and mechanism of PSMD11 protecting neurons in d-Galactose (D-Gal) mimetic aging models. Age-related pathologies were detected by Taq-PCR, ABR, Transmission electron microscopy, toluidine blue and β-galactosidase staining. The relative expressions of the proteins were explored by Western blotting, oxyblot, immunoprecipitation and immunofluorescence. Flow cytometry was used to manifest the oxidative state. We discovered that proteasome activity was impaired with aging, and that ROS and toxic protein accumulated in D-Gal induced aging models. PSMD11 changed with aging, and was associated with the metabolism of proteasome activity in the D-Gal treated models. Moreover, the knockdown or overexpression of PSMD11 was sufficient to change the oxidative state caused by D-Gal. Our results also demonstrated that PSMD11 could bond to AMPKα1/2 in the auditory cortex and PC12 cells, and AMPKα2 but not AMPKα1 was efficient to regulate the function of PSMD11. Deeper insights into the mechanisms of regulating PSMD11 for the anti-aging process are needed, and may offer novel therapeutic methods for central presbycusis.

Proteomic mapping of Drosophila transgenic elav.L-GAL4/+ brain as a tool to illuminate neuropathology mechanisms

Drosophila brain has emerged as a powerful model system for the investigation of genes being related to neurological pathologies. To map the proteomic landscape of fly brain, in a high-resolution scale, we herein employed a nano liquid chromatography-tandem mass spectrometry technology, and high-content catalogues of 7,663 unique peptides and 2,335 single proteins were generated. Protein-data processing, through UniProt, DAVID, KEGG and PANTHER bioinformatics subroutines, led to fly brain-protein classification, according to sub-cellular topology, molecular function, implication in signaling and contribution to neuronal diseases. Given the importance of Ubiquitin Proteasome System (UPS) in neuropathologies and by using the almost completely reassembled UPS, we genetically targeted genes encoding components of the ubiquitination-dependent protein-degradation machinery. This analysis showed that driving RNAi toward proteasome components and regulators, using the GAL4-elav.L driver, resulted in changes to longevity and climbing-activity patterns during aging. Our proteomic map is expected to advance the existing knowledge regarding brain biology in animal species of major translational-research value and economical interest.

Genetic and genomic studies of pathogenic EXOSC2 mutations in the newly described disease SHRF implicate the autophagy pathway in disease pathogenesis

Missense mutations in the RNA exosome component exosome component 2 (EXOSC2), also known as ribosomal RNA-processing protein 4 (RRP4), were recently identified in two unrelated families with a novel syndrome known as Short stature, Hearing loss, Retinitis pigmentosa and distinctive Facies (SHRF, #OMIM 617763). Little is known about the mechanism of the SHRF pathogenesis. Here we have studied the effect of mutations in EXOSC2/RRP4 in patient-derived lymphoblasts, clustered regularly interspaced short palindromic repeats (CRISPR)-generated mutant fetal keratinocytes and Drosophila. We determined that human EXOSC2 is an essential gene and that the pathogenic G198D mutation prevents binding to other RNA exosome components, resulting in protein and complex instability and altered expression and/or activities of critical genes, including those in the autophagy pathway. In parallel, we generated multiple CRISPR knockouts of the fly rrp4 gene. Using these flies, as well as rrp4 mutants with Piggy Bac (PBac) transposon insertion in the 3'UTR and RNAi flies, we determined that fly rrp4 was also essential, that fly rrp4 phenotypes could be rescued by wild-type human EXOSC2 but not the pathogenic form and that fly rrp4 is critical for eye development and maintenance, muscle ultrastructure and wing vein development. We found that overexpression of the transcription factor MITF was sufficient to rescue the small eye and adult lethal phenotypes caused by rrp4 inhibition. The autophagy genes ATG1 and ATG17, which are regulated by MITF, had similar effect. Pharmacological stimulation of autophagy with rapamycin also rescued the lethality caused by rrp4 inactivation. Our results implicate defective autophagy in SHRF pathogenesis and suggest therapeutic strategies.

Drosophila as a model to understand autophagy deregulation in human disorders

Autophagy has important functions in normal physiology to maintain homeostasis and protect against cellular stresses by the removal of harmful cargos such as dysfunctional organelles, protein aggregates and invading pathogens. The deregulation of autophagy is a hallmark of many diseases and therapeutic targeting of autophagy is highly topical. With the complex role of autophagy in disease it is essential to understand the genetic and molecular basis of the contribution of autophagy to pathogenesis. The model organism, Drosophila, provides a genetically amenable system to dissect out the contribution of autophagy to human disease models. Here we review the roles of autophagy in human disease and how autophagy studies in Drosophila have contributed to the understanding of pathophysiology.

A circuit-dependent ROS feedback loop mediates glutamate excitotoxicity to sculpt the Drosophila motor system

Overproduction of reactive oxygen species (ROS) is known to mediate glutamate excitotoxicity in neurological diseases. However, how ROS burdens can influence neural circuit integrity remains unclear. Here, we investigate the impact of excitotoxicity induced by depletion of Drosophila Eaat1, an astrocytic glutamate transporter, on locomotor central pattern generator (CPG) activity, neuromuscular junction architecture, and motor function. We show that glutamate excitotoxicity triggers a circuit-dependent ROS feedback loop to sculpt the motor system. Excitotoxicity initially elevates ROS, thereby inactivating cholinergic interneurons and consequently changing CPG output activity to overexcite motor neurons and muscles. Remarkably, tonic motor neuron stimulation boosts muscular ROS, gradually dampening muscle contractility to feedback-enhance ROS accumulation in the CPG circuit and subsequently exacerbate circuit dysfunction. Ultimately, excess premotor excitation of motor neurons promotes ROS-activated stress signaling that alters neuromuscular junction architecture. Collectively, our results reveal that excitotoxicity-induced ROS can perturb motor system integrity through a circuit-dependent mechanism.

Model systems inform rare disease diagnosis, therapeutic discovery and pre-clinical efficacy

Model systems have played a large role in understanding human diseases and are instrumental in taking basic research findings to the clinic; however, for rare diseases, model systems play an even larger role. Here, we outline how model organisms are crucial for confirming causal associations, understanding functional mechanisms and developing therapies for disease. As diseases that have been studied extensively through genetics and molecular biology, cystic fibrosis and Rett syndrome are portrayed as primary examples of how genetic diagnosis, model organism development and therapies have led to improved patient health. Considering which model to use, yeast, worms, flies, fish, mice or larger animals requires a careful evaluation of experimental genetic tools and gene pathway conservation. Recent advances in genome editing will aid in confirming diagnoses and developing model systems for rare disease. Genetic or chemical screening for disease suppression may reveal functional pathway members and provide candidate entry points for developing therapies. Model organisms may also be used in drug discovery and as preclinical models as a prelude to testing treatments in patient populations. Now, model organisms will increasingly be used as platforms for understanding variation in rare disease severity and onset, thereby informing therapeutic intervention.

From gene to treatment: supporting rare disease translational research through model systems

Individual rare diseases may affect only a few people, making them difficult to recognize, diagnose or treat by studying humans alone. Instead, model organisms help to validate genetic associations, understand functional pathways and develop therapeutic interventions for rare diseases. In this Editorial, we point to the key parameters in face, construct, predictive and target validity for accurate disease modelling, with special emphasis on rare disease models. Raising the experimental standards for disease models will enhance successful clinical translation and benefit rare disease research.

Summary: This Editorial discusses the importance of model systems with accurate face, construct, target and predictive validity for rare disease research.

The Role of PI3K/Akt and ERK in Neurodegenerative Disorders

Disruption of Akt and Erk-mediated signal transduction significantly contributes in the pathogenesis of various neurodegenerative diseases (NDs), such as Parkinson's disease, Alzheimer's diseases, Huntington's disease, and many others. These regulatory proteins serve as the regulator of cell survival, motility, transcription, metabolism, and progression of the cell cycle. Therefore, targeting Akt and Erk pathway has been proposed as a reasonable approach to suppress ND progression. This review has emphasized on involvement of Akt/Erk cascade in the neurodegeneration. Akt has been reported to regulate neuronal toxicity through its various substrates like FOXos, GSK3β, and caspase-9 etc. Akt is also involved with PI3K in signaling pathway to mediate neuronal survival. ERK is another kinase which also regulates proliferation, differentiation, and survival of the neural cell. There has also been much progress in developing a therapeutic molecule targeting Akt and Erk signaling. Therefore, improved understanding of the molecular mechanism behind the regulatory aspect of Akt and Erk networks can make strong impact on exploration of the neurodegenerative disease pathogenesis.

Unraveling Novel Mechanisms of Neurodegeneration Through a Large-Scale Forward Genetic Screen in Drosophila

Neurodegeneration is characterized by progressive loss of neurons. Genetic and environmental factors both contribute to demise of neurons, leading to diverse devastating cognitive and motor disorders, including Alzheimer's and Parkinson's diseases in humans. Over the past few decades, the fruit fly, Drosophila melanogaster, has become an integral tool to understand the molecular, cellular and genetic mechanisms underlying neurodegeneration. Extensive tools and sophisticated technologies allow Drosophila geneticists to identify and study evolutionarily conserved genes that are essential for neural maintenance. In this review, we will focus on a large-scale mosaic forward genetic screen on the fly X-chromosome that led to the identification of a number of essential genes that exhibit neurodegenerative phenotypes when mutated. Most genes identified from this screen are evolutionarily conserved and many have been linked to human diseases with neurological presentations. Systematic electrophysiological and ultrastructural characterization of mutant tissue in the context of the Drosophila visual system, followed by a series of experiments to understand the mechanism of neurodegeneration in each mutant led to the discovery of novel molecular pathways that are required for neuronal integrity. Defects in mitochondrial function, lipid and iron metabolism, protein trafficking and autophagy are recurrent themes, suggesting that insults that eventually lead to neurodegeneration may converge on a set of evolutionarily conserved cellular processes. Insights from these studies have contributed to our understanding of known neurodegenerative diseases such as Leigh syndrome and Friedreich's ataxia and have also led to the identification of new human diseases. By discovering new genes required for neural maintenance in flies and working with clinicians to identify patients with deleterious variants in the orthologous human genes, Drosophila biologists can play an active role in personalized medicine.

Aberrant activation of non-coding RNA targets of transcriptional elongation complexes contributes to TDP-43 toxicity

TDP-43 is the major disease protein associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-TDP). Here we identify the transcriptional elongation factor Ell—a shared component of little elongation complex (LEC) and super elongation complex (SEC)—as a strong modifier of TDP-43-mediated neurodegeneration. Our data indicate select targets of LEC and SEC become upregulated in the fly ALS/FTLD-TDP model. Among them, U12 snRNA and a stress-induced long non-coding RNA Hsrω, functionally contribute to TDP-43-mediated degeneration. We extend the findings of Hsrω, which we identify as a chromosomal target of TDP-43, to show that the human orthologue Sat III is elevated in a human cellular disease model and FTLD-TDP patient tissue. We further demonstrate an interaction between TDP-43 and human ELL2 by co-immunoprecipitation from human cells. These findings reveal important roles of Ell-complexes LEC and SEC in TDP-43-associated toxicity, providing potential therapeutic insight for TDP-43-associated neurodegeneration.

TDP-43 is associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitinated inclusions (FTD-TDP). Here, the authors identify the transcriptional elongation factor Ell as a strong modifier of TDP-43-mediated neurodegeneration through the Ell transcriptional elongation complexes LEC and SEC.

Postmitotic cell longevity-associated genes: a transcriptional signature of postmitotic maintenance in neural tissues

Different cell types have different postmitotic maintenance requirements. Nerve cells, however, are unique in this respect as they need to survive and preserve their functional complexity for the entire lifetime of the organism, and failure at any level of their supporting mechanisms leads to a wide range of neurodegenerative conditions. Whether these differences across tissues arise from the activation of distinct cell type-specific maintenance mechanisms or the differential activation of a common molecular repertoire is not known. To identify the transcriptional signature of postmitotic cellular longevity (PMCL), we compared whole-genome transcriptome data from human tissues ranging in longevity from 120 days to over 70 years and found a set of 81 genes whose expression levels are closely associated with increased cell longevity. Using expression data from 10 independent sources, we found that these genes are more highly coexpressed in longer-living tissues and are enriched in specific biological processes and transcription factor targets compared with randomly selected gene samples. Crucially, we found that PMCL-associated genes are downregulated in the cerebral cortex and substantia nigra of patients with Alzheimer's and Parkinson's disease, respectively, as well as Hutchinson-Gilford progeria-derived fibroblasts, and that this downregulation is specifically linked to their underlying association with cellular longevity. Moreover, we found that sexually dimorphic brain expression of PMCL-associated genes reflects sexual differences in lifespan in humans and macaques. Taken together, our results suggest that PMCL-associated genes are part of a generalized machinery of postmitotic maintenance and functional stability in both neural and non-neural cells and support the notion of a common molecular repertoire differentially engaged in different cell types with different survival requirements.

How Surrogate and Chemical Genetics in Model Organisms Can Suggest Therapies for Human Genetic Diseases

Genetic diseases are both inherited and acquired. Many genetic diseases fall under the paradigm of orphan diseases, a disease found in < 1 in 2000 persons. With rapid and cost-effective genome sequencing becoming the norm, many causal mutations for genetic diseases are being rapidly determined. In this regard, model organisms are playing an important role in validating if specific mutations identified in patients drive the observed phenotype. An emerging challenge for model organism researchers is the application of genetic and chemical genetic platforms to discover drug targets and drugs/drug-like molecules for potential treatment options for patients with genetic disease. This review provides an overview of how model organisms have contributed to our understanding of genetic disease, with a focus on the roles of yeast and zebrafish in gene discovery and the identification of compounds that could potentially treat human genetic diseases.

RNA-binding proteins with basic-acidic dipeptide (BAD) domains self-assemble and aggregate in Alzheimer's disease

The U1 small nuclear ribonucleoprotein 70 kDa (U1-70K) and other RNA-binding proteins (RBPs) are mislocalized to cytoplasmic neurofibrillary Tau aggregates in Alzheimer's disease (AD), yet the co-aggregation mechanisms are incompletely understood. U1-70K harbors two disordered low-complexity domains (LC1 and LC2) that are necessary for aggregation in AD brain extracts. The LC1 domain contains highly repetitive basic (Arg/Lys) and acidic (Asp/Glu) residues, referred to as a basic-acidic dipeptide (BAD) domain. We report here that this domain shares many of the properties of the Gln/Asn-rich LC domains in RBPs that also aggregate in neurodegenerative disease. These properties included self-assembly into oligomers and localization to nuclear granules. Co-immunoprecipitations of recombinant U1-70K and deletions lacking the LC domain(s) followed by quantitative proteomic analyses were used to resolve functional classes of U1-70K-interacting proteins that depend on the BAD domain for their interaction. Within this interaction network, we identified a class of RBPs with BAD domains nearly identical to that found in U1-70K. Two members of this class, LUC7L3 and RBM25, required their respective BAD domains for reciprocal interactions with U1-70K and nuclear granule localization. Strikingly, a significant proportion of RBPs with BAD domains had elevated insolubility in the AD brain proteome. Furthermore, we show that the BAD domain of U1-70K can interact with Tau from AD brains but not from other tauopathies. These findings highlight a mechanistic role for BAD domains in stabilizing RBP interactions and in potentially mediating co-aggregation with the pathological AD-specific Tau isoforms.

Pathways to neurodegeneration: lessons learnt from unbiased genetic screens in Drosophila

Neurodegenerative diseases are a complex set of disorders that are known to be caused by environmental as well as genetic factors. In the recent past, mutations in a large number of genes have been identified that are linked to several neurodegenerative diseases. The pathogenic mechanisms in most of these disorders are unknown. Recently, studies of genes that are linked to neurodegeneration in Drosophila, the fruit flies, have contributed significantly to our understanding of mechanisms of neuroprotection and degeneration. In this review, we focus on forward genetic screens in Drosophila that helped in identification of novel genes and pathogenic mechanisms linked to neurodegeneration. We also discuss identification of four novel pathways that contribute to neurodegeneration upon mitochondrial dysfunction.

Drosophila models of amyotrophic lateral sclerosis with defects in RNA metabolism

The fruit fly Drosophila Melanogaster has been widely used to study neurodegenerative diseases. The conservation of nervous system biology coupled with the rapid life cycle and powerful genetic tools in the fly have enabled the identification of novel therapeutic targets that have been validated in vertebrate model systems and human patients. A recent example is in the study of the devastating motor neuron degenerative disease amyotrophic lateral sclerosis (ALS). Mutations in genes that regulate RNA metabolism are a major cause of inherited ALS, and functional analysis of these genes in the fly nervous system has shed light on how mutations cause disease. Importantly, unbiased genetic screens have identified key pathways that contribute to ALS pathogenesis such as nucleocytoplasmic transport and stress granule assembly. In this review, we will discuss the utilization of Drosophila models of ALS with defects in RNA metabolism.

Loss of the Drosophila m-AAA mitochondrial protease paraplegin results in mitochondrial dysfunction, shortened lifespan, and neuronal and muscular degeneration

The progressive accumulation of dysfunctional mitochondria is implicated in aging and in common diseases of the elderly. To oppose this occurrence, organisms employ a variety of strategies, including the selective degradation of oxidatively damaged and misfolded mitochondrial proteins. Genetic studies in yeast indicate that the ATPase Associated with diverse cellular Activities (AAA⁺) family of mitochondrial proteases account for a substantial fraction of this protein degradation, but their metazoan counterparts have been little studied, despite the fact that mutations in the genes encoding these proteases cause a variety of human diseases. To begin to explore the biological roles of the metazoan mitochondrial AAA⁺ protease family, we have created a CRISPR/Cas9 allele of the Drosophila homolog of SPG7, which encodes an inner membrane-localized AAA⁺ protease known as paraplegin. Drosophila SPG7 mutants exhibited shortened lifespan, progressive locomotor defects, sensitivity to chemical and environmental stress, and muscular and neuronal degeneration. Ultrastructural examination of photoreceptor neurons indicated that the neurodegenerative phenotype of SPG7 mutants initiates at the synaptic terminal. A variety of mitochondrial defects accompanied the degenerative phenotypes of SPG7 mutants, including altered axonal transport of mitochondria, accumulation of electron-dense material in the matrix of flight muscle mitochondria, reduced activities of respiratory chain complexes I and II, and severely swollen and dysmorphic mitochondria in the synaptic terminals of photoreceptors. Drosophila SPG7 mutants recapitulate key features of human diseases caused by mutations in SPG7, and thus provide a foundation for the identification of Drosophila paraplegin substrates and strategies that could be used to ameliorate the symptoms of these diseases.

Neuronal-specific impairment of heparan sulfate degradation in Drosophila reveals pathogenic mechanisms for Mucopolysaccharidosis type IIIA

Mucopolysaccharidosis type IIIA (MPS IIIA) is a lysosomal storage disorder resulting from the deficit of the N-sulfoglucosamine sulfohydrolase (SGSH) enzyme that leads to accumulation of partially-degraded heparan sulfate. MPS IIIA is characterized by severe neurological symptoms, clinically presenting as Sanfilippo syndrome, for which no effective therapy is available. The lysosomal SGSH enzyme is conserved in Drosophila and we have identified increased levels of heparan sulfate in flies with ubiquitous knockdown of SGSH/CG14291. Using neuronal specific knockdown of SGSH/CG14291 we have also observed a higher abundance of Lysotracker-positive puncta as well as increased expression of GFP tagged Ref(2)P supporting disruption to lysosomal function. We have also observed a progressive defect in climbing ability, a hallmark of neurological dysfunction. Genetic screens indicate proteins and pathways that can functionally modify the climbing phenotype, including autophagy-related proteins (Atg1 and Atg18), superoxide dismutase enzymes (Sod1 and Sod2) and heat shock protein (HSPA1). In addition, reducing heparan sulfate biosynthesis by knocking down sulfateless or slalom expression significantly worsens the phenotype; an important observation given that substrate inhibition is being evaluated clinically as a treatment for MPS IIIA. Identifying the cellular pathways that can modify MPS IIIA neuropathology is an essential step in the development of novel therapeutic approaches to prevent and/or ameliorate symptoms in children with Sanfilippo syndrome.

Drosophila melanogaster White Mutant w1118 Undergo Retinal Degeneration

Key scientific discoveries have resulted from genetic studies of Drosophila melanogaster, using a multitude of transgenic fly strains, the majority of which are constructed in a genetic background containing mutations in the white gene. Here we report that white mutant flies from w1118 strain undergo retinal degeneration. We observed also that w1118 mutants have progressive loss of climbing ability, shortened life span, as well as impaired resistance to various forms of stress. Retinal degeneration was abolished by transgenic expression of mini-white+ in the white null background w1118 . We conclude that beyond the classical eye-color phenotype, mutations in Drosophila white gene could impair several biological functions affecting parameters like mobility, life span and stress tolerance. Consequently, we suggest caution and attentiveness during the interpretation of old experiments employing white mutant flies and when planning new ones, especially within the research field of neurodegeneration and neuroprotection. We also encourage that the use of w1118 strain as a wild-type control should be avoided.

Stress-induced Cdk5 activity enhances cytoprotective basal autophagy in Drosophila melanogaster by phosphorylating acinus at serine437

Cdk5 is a post-mitotic kinase with complex roles in maintaining neuronal health. The various mechanisms by which Cdk5 inhibits and promotes neurodegeneration are still poorly understood. Here, we show that in Drosophila melanogaster Cdk5 regulates basal autophagy, a key mechanism suppressing neurodegeneration. In a targeted screen, Cdk5 genetically interacted with Acinus (Acn), a primarily nuclear protein, which promotes starvation-independent, basal autophagy. Loss of Cdk5, or its required cofactor p35, reduces S437-Acn phosphorylation, whereas Cdk5 gain-of-function increases pS437-Acn levels. The phospho-mimetic S437D mutation stabilizes Acn and promotes basal autophagy. In p35 mutants, basal autophagy and lifespan are reduced, but restored to near wild-type levels in the presence of stabilized AcnS437D. Expression of aggregation-prone polyQ-containing proteins or the Amyloid-β42 peptide, but not alpha-Synuclein, enhances Cdk5-dependent phosphorylation of S437-Acn. Our data indicate that Cdk5 is required to maintain the protective role of basal autophagy in the initial responses to a subset of neurodegenerative challenges.

Overexpression of Drosophila frataxin triggers cell death in an iron-dependent manner

Friedreich ataxia (FRDA) is the most important autosomal recessive ataxia in the Caucasian population. FRDA patients display severe neurological and cardiac symptoms that reflect a strong cellular and axonal degeneration. FRDA is caused by a loss of function of the mitochondrial protein frataxin which impairs the biosynthesis of iron-sulfur clusters and in turn the catalytic activity of several enzymes in the Krebs cycle and the respiratory chain leading to a diminished energy production. Although FRDA is due to frataxin depletion, overexpression might also be very helpful to better understand cellular functions of frataxin. In this work, we have increased frataxin expression in neurons to elucidate specific roles that frataxin might play in these tissues. Using molecular, biochemical, histological and behavioral methods, we report that frataxin overexpression is sufficient to increase oxidative phosphorylation, modify mitochondrial morphology, alter iron homeostasis and trigger oxidative stress-dependent cell death. Interestingly, genetic manipulation of mitochondrial iron metabolism by silencing mitoferrin successfully improves cell survival under oxidative-attack conditions, although enhancing antioxidant defenses or mitochondrial fusion failed to ameliorate frataxin overexpression phenotypes. This result suggests that cell degeneration is directly related to enhanced incorporation of iron into the mitochondria. Drosophila frataxin overexpression might also provide an alternative approach to identify processes that are important in FRDA such as changes in mitochondrial morphology and oxidative stress induced cell death.

The Glia-Neuron Lactate Shuttle and Elevated ROS Promote Lipid Synthesis in Neurons and Lipid Droplet Accumulation in Glia via APOE/D

Elevated reactive oxygen species (ROS) induce the formation of lipids in neurons that are transferred to glia, where they form lipid droplets (LDs). We show that glial and neuronal monocarboxylate transporters (MCTs), fatty acid transport proteins (FATPs), and apolipoproteins are critical for glial LD formation. MCTs enable glia to secrete and neurons to absorb lactate, which is converted to pyruvate and acetyl-CoA in neurons. Lactate metabolites provide a substrate for synthesis of fatty acids, which are processed and transferred to glia by FATP and apolipoproteins. In the presence of high ROS, inhibiting lactate transfer or lowering FATP or apolipoprotein levels decreases glial LD accumulation in flies and in primary mouse glial-neuronal cultures. We show that human APOE can substitute for a fly glial apolipoprotein and that APOE4, an Alzheimer's disease susceptibility allele, is impaired in lipid transport and promotes neurodegeneration, providing insights into disease mechanisms.

Taking Stock of the Drosophila Research Ecosystem

With a century-old history of fundamental discoveries, the fruit fly has long been a favored experimental organism for a wide range of scientific inquiries. But Drosophila is not a “legacy” model organism; technical and intellectual innovations continue to revitalize fly research and drive advances in our understanding of conserved mechanisms of animal biology. Here, we provide an overview of this “ecosystem” and discuss how to address emerging challenges to ensure its continued productivity. Drosophila researchers are fortunate to have a sophisticated and ever-growing toolkit for the analysis of gene function. Access to these tools depends upon continued support for both physical and informational resources. Uncertainty regarding stable support for bioinformatic databases is a particular concern, at a time when there is the need to make the vast knowledge of functional biology provided by this model animal accessible to scientists studying other organisms. Communication and advocacy efforts will promote appreciation of the value of the fly in delivering biomedically important insights. Well-tended traditions of large-scale tool development, open sharing of reagents, and community engagement provide a strong basis for coordinated and proactive initiatives to improve the fly research ecosystem. Overall, there has never been a better time to be a fly pusher.

A novel iron (II) preferring dopamine agonist chelator D-607 significantly suppresses α-syn- and MPTP-induced toxicities in vivo

Here, we report the characterization of a novel hybrid D₂/D₃ agonist and iron (II) specific chelator, D-607, as a multi-target-directed ligand against Parkinson's disease (PD). In our previously published report, we showed that D-607 is a potent agonist of dopamine (DA) D₂/D₃ receptors, exhibits efficacy in a reserpinized PD animal model and preferentially chelates to iron (II). As further evidence of its potential as a neuroprotective agent in PD, the present study reveals D-607 to be protective in neuronal PC12 cells against 6-OHDA toxicity. In an in vivo Drosophila melanogaster model expressing a disease-causing variant of α-synuclein (α-Syn) protein in fly eyes, the compound was found to significantly suppress toxicity compared to controls, concomitant with reduced levels of aggregated α-Syn. Furthermore, D-607 was able to rescue DAergic neurons from MPTP toxicity in mice, a well-known PD neurotoxicity model, following both sub-chronic and chronic MPTP administration. Mechanistic studies indicated that possible protection of mitochondria, up-regulation of hypoxia-inducible factor, reduction in formation of α-Syn aggregates and antioxidant activity may underlie the observed neuroprotection effects. These observations strongly suggest that D-607 has potential as a promising multifunctional lead molecule for viable symptomatic and disease-modifying therapy for PD.

Drosophila melanogaster As a Model Organism to Study RNA Toxicity of Repeat Expansion-Associated Neurodegenerative and Neuromuscular Diseases

For nearly a century, the fruit fly, Drosophila melanogaster, has proven to be a valuable tool in our understanding of fundamental biological processes, and has empowered our discoveries, particularly in the field of neuroscience. In recent years, Drosophila has emerged as a model organism for human neurodegenerative and neuromuscular disorders. In this review, we highlight a number of recent studies that utilized the Drosophila model to study repeat-expansion associated diseases (READs), such as polyglutamine diseases, fragile X-associated tremor/ataxia syndrome (FXTAS), myotonic dystrophy type 1 (DM1) and type 2 (DM2), and C9ORF72-associated amyotrophic lateral sclerosis/frontotemporal dementia (C9-ALS/FTD). Discoveries regarding the possible mechanisms of RNA toxicity will be focused here. These studies demonstrate Drosophila as an excellent in vivo model system that can reveal novel mechanistic insights into human disorders, providing the foundation for translational research and therapeutic development.

Drosophila and genome-wide association studies: a review and resource for the functional dissection of human complex traits

Human genome-wide association studies (GWAS) have successfully identified thousands of susceptibility loci for common diseases with complex genetic etiologies. Although the susceptibility variants identified by GWAS usually have only modest effects on individual disease risk, they contribute to a substantial burden of trait variation in the overall population. GWAS also offer valuable clues to disease mechanisms that have long proven to be elusive. These insights could lead the way to breakthrough treatments; however, several challenges hinder progress, making innovative approaches to accelerate the follow-up of results from GWAS an urgent priority. Here, we discuss the largely untapped potential of the fruit fly, Drosophila melanogaster, for functional investigation of findings from human GWAS. We highlight selected examples where strong genomic conservation with humans along with the rapid and powerful genetic tools available for flies have already facilitated fine mapping of association signals, elucidated gene mechanisms, and revealed novel disease-relevant biology. We emphasize current research opportunities in this rapidly advancing field, and present bioinformatic analyses that systematically explore the applicability of Drosophila for interrogation of susceptibility signals implicated in more than 1000 human traits, based on all GWAS completed to date. Thus, our discussion is targeted at both human geneticists seeking innovative strategies for experimental validation of findings from GWAS, as well as the Drosophila research community, by whom ongoing investigations of the implicated genes will powerfully inform our understanding of human disease.

Mass Histology to Quantify Neurodegeneration in Drosophila

Progressive neurodegenerative diseases like Alzheimer's disease (AD) or Parkinson's disease (PD) are an increasing threat to human health worldwide. Although mammalian models have provided important insights into the underlying mechanisms of pathogenicity, the complexity of mammalian systems together with their high costs are limiting their use. Therefore, the simple but well-established Drosophila model-system provides an alternative for investigating the molecular pathways that are affected in these diseases. Besides behavioral deficits, neurodegenerative diseases are characterized by histological phenotypes such as neuronal death and axonopathy. To quantify neuronal degeneration and to determine how it is affected by genetic and environmental factors, we use a histological approach that is based on measuring the vacuoles in adult fly brains. To minimize the effects of systematic error and to directly compare sections from control and experimental flies in one preparation, we use the 'collar' method for paraffin sections. Neurodegeneration is then assessed by measuring the size and/or number of vacuoles that have developed in the fly brain. This can either be done by focusing on a specific region of interest or by analyzing the entire brain by obtaining serial sections that span the complete head. Therefore, this method allows one to measure not only severe degeneration but also relatively mild phenotypes that are only detectable in a few sections, as occurs during normal aging.

Modeling congenital disease and inborn errors of development in Drosophila melanogaster

Fly models that faithfully recapitulate various aspects of human disease and human health-related biology are being used for research into disease diagnosis and prevention. Established and new genetic strategies in Drosophila have yielded numerous substantial successes in modeling congenital disorders or inborn errors of human development, as well as neurodegenerative disease and cancer. Moreover, although our ability to generate sequence datasets continues to outpace our ability to analyze these datasets, the development of high-throughput analysis platforms in Drosophila has provided access through the bottleneck in the identification of disease gene candidates. In this Review, we describe both the traditional and newer methods that are facilitating the incorporation of Drosophila into the human disease discovery process, with a focus on the models that have enhanced our understanding of human developmental disorders and congenital disease. Enviable features of the Drosophila experimental system, which make it particularly useful in facilitating the much anticipated move from genotype to phenotype (understanding and predicting phenotypes directly from the primary DNA sequence), include its genetic tractability, the low cost for high-throughput discovery, and a genome and underlying biology that are highly evolutionarily conserved. In embracing the fly in the human disease-gene discovery process, we can expect to speed up and reduce the cost of this process, allowing experimental scales that are not feasible and/or would be too costly in higher eukaryotes.

FlyBase portals to human disease research using Drosophila models

The use of Drosophila melanogaster as a model for studying human disease is well established, reflected by the steady increase in both the number and proportion of fly papers describing human disease models in recent years. In this article, we highlight recent efforts to improve the availability and accessibility of the disease model information in FlyBase (http://flybase.org), the model organism database for Drosophila. FlyBase has recently introduced Human Disease Model Reports, each of which presents background information on a specific disease, a tabulation of related disease subtypes, and summaries of experimental data and results using fruit flies. Integrated presentations of relevant data and reagents described in other sections of FlyBase are incorporated into these reports, which are specifically designed to be accessible to non-fly researchers in order to promote collaboration across model organism communities working in translational science. Another key component of disease model information in FlyBase is that data are collected in a consistent format ­­– using the evolving Disease Ontology (an open-source standardized ontology for human-disease-associated biomedical data) – to allow robust and intuitive searches. To facilitate this, FlyBase has developed a dedicated tool for querying and navigating relevant data, which include mutations that model a disease and any associated interacting modifiers. In this article, we describe how data related to fly models of human disease are presented in individual Gene Reports and in the Human Disease Model Reports. Finally, we discuss search strategies and new query tools that are available to access the disease model data in FlyBase.

Drosophila Collection:Drosophila melanogaster is well established as a model for studying human disease. Here, we highlight recent efforts to enhance the availability and accessibility of disease model data in FlyBase, the model organism database for Drosophila.

Membrane protein trafficking in Drosophila photoreceptor cells

Membrane protein trafficking occurs throughout the lifetime of neurons and includes the initial protein synthesis and anterograde transport to the plasma membrane as well as internalization, degradation, and recycling of plasma membrane proteins. Defects in protein trafficking can result in neuronal degeneration and underlie blinding diseases such as retinitis pigmentosa as well as other neuronal disorders. Drosophila photoreceptor cells have emerged as a model system for identifying the components and mechanisms involved in membrane protein trafficking in neurons. Here we summarize the current knowledge about trafficking of three Drosophila phototransduction proteins, the visual pigment rhodopsin and the two light-activated ion channels TRP (transient receptor potential) and TRPL (TRP-like). Despite some common requirements shared by rhodopsin and TRP, details in the trafficking of these proteins differ considerably, suggesting the existence of several trafficking pathways for these photoreceptor proteins.

Development and diabetes on the fly

We review the use of a model organism to study the effects of a slow course, degenerative disease: namely, diabetes mellitus. Development and aging are biological phenomena entailing reproduction, growth, and differentiation, and then decline and progressive loss of functionality leading ultimately to failure and death. It occurs at all biological levels of organization, from molecular interactions to organismal well being and homeostasis. Yet very few models capable of addressing the different levels of complexity in these chronic, developmental phenomena are available to study, and model organisms are an exception and a welcome opportunity for these approaches. Genetic model organisms, like the common fruit fly, Drosophila melanogaster, offer the possibility of studying the panoply of life processes in normal and diseased states like diabetes mellitus, from a plethora of different perspectives. These long-term aspects are now beginning to be characterized.

Tau and spectraplakins promote synapse formation and maintenance through Jun kinase and neuronal trafficking

The mechanisms regulating synapse numbers during development and ageing are essential for normal brain function and closely linked to brain disorders including dementias. Using Drosophila, we demonstrate roles of the microtubule-associated protein Tau in regulating synapse numbers, thus unravelling an important cellular requirement of normal Tau. In this context, we find that Tau displays a strong functional overlap with microtubule-binding spectraplakins, establishing new links between two different neurodegenerative factors. Tau and the spectraplakin Short Stop act upstream of a three-step regulatory cascade ensuring adequate delivery of synaptic proteins. This cascade involves microtubule stability as the initial trigger, JNK signalling as the central mediator, and kinesin-3 mediated axonal transport as the key effector. This cascade acts during development (synapse formation) and ageing (synapse maintenance) alike. Therefore, our findings suggest novel explanations for intellectual disability in Tau deficient individuals, as well as early synapse loss in dementias including Alzheimer's disease.

Uncoupling neuronal death and dysfunction in Drosophila models of neurodegenerative disease

Common neurodegenerative proteinopathies, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are characterized by the misfolding and aggregation of toxic protein species, including the amyloid beta (Aß) peptide, microtubule-associated protein Tau (Tau), and alpha-synuclein (αSyn) protein. These factors also show toxicity in Drosophila; however, potential limitations of prior studies include poor discrimination between effects on the adult versus developing nervous system and neuronal versus glial cell types. In addition, variable expression paradigms and outcomes hinder systematic comparison of toxicity profiles. Using standardized conditions and medium-throughput assays, we express human Tau, Aß or αSyn selectively in neurons of the adult Drosophila retina and monitor age-dependent changes in both structure and function, based on tissue histology and recordings of the electroretinogram (ERG), respectively. We find that each protein causes a unique profile of neurodegenerative pathology, demonstrating distinct and separable impacts on neuronal death and dysfunction. Strikingly, expression of Tau leads to progressive loss of ERG responses whereas retinal architecture and neuronal numbers are largely preserved. By contrast, Aß induces modest, age-dependent neuronal loss without degrading the retinal ERG. αSyn expression, using a codon-optimized transgene, is characterized by marked retinal vacuolar change, progressive photoreceptor cell death, and delayed-onset but modest ERG changes. Lastly, to address potential mechanisms, we perform transmission electron microscopy (TEM) to reveal potential degenerative changes at the ultrastructural level. Surprisingly, Tau and αSyn each cause prominent but distinct synaptotoxic profiles, including disorganization or enlargement of photoreceptor terminals, respectively. Our findings highlight variable and dynamic properties of neurodegeneration triggered by these disease-relevant proteins in vivo, and suggest that Drosophila may be useful for revealing determinants of neuronal dysfunction that precede cell loss, including synaptic changes, in the adult nervous system.

PINK1 and Parkin cooperatively protect neurons against constitutively active TRP channel-induced retinal degeneration in Drosophila

Calcium has an important role in regulating numerous cellular activities. However, extremely high levels of intracellular calcium can lead to neurotoxicity, a process commonly associated with degenerative diseases. Despite the clear role of calcium cytotoxicity in mediating neuronal cell death in this context, the pathological mechanisms remain controversial. We used a well-established Drosophila model of retinal degeneration, which involves the constitutively active TRP^P365 channels, to study calcium-induced neurotoxicity. We found that the disruption of mitochondrial function was associated with the degenerative process. Further, increasing autophagy flux prevented cell death in Trp^P365 mutant flies, and this depended on the PINK1/Parkin pathway. In addition, the retinal degeneration process was also suppressed by the coexpression of PINK1 and Parkin. Our results provide genetic evidence that mitochondrial dysfunction has a key role in the pathology of cellular calcium neurotoxicity. In addition, the results demonstrated that maintaining mitochondrial homeostasis via PINK1/Parkin-dependent mitochondrial quality control can potentially alleviate cell death in a wide range of neurodegenerative diseases.

Morgan's legacy: fruit flies and the functional annotation of conserved genes

In 1915, "The Mechanism of Mendelian Heredity" was published by four prominent Drosophila geneticists. They discovered that genes form linkage groups on chromosomes inherited in a Mendelian fashion and laid the genetic foundation that promoted Drosophila as a model organism. Flies continue to offer great opportunities, including studies in the field of functional genomics.

Drosophila as an In Vivo Model for Human Neurodegenerative Disease

With the increase in the ageing population, neurodegenerative disease is devastating to families and poses a huge burden on society. The brain and spinal cord are extraordinarily complex: they consist of a highly organized network of neuronal and support cells that communicate in a highly specialized manner. One approach to tackling problems of such complexity is to address the scientific questions in simpler, yet analogous, systems. The fruit fly, Drosophila melanogaster, has been proven tremendously valuable as a model organism, enabling many major discoveries in neuroscientific disease research. The plethora of genetic tools available in Drosophila allows for exquisite targeted manipulation of the genome. Due to its relatively short lifespan, complex questions of brain function can be addressed more rapidly than in other model organisms, such as the mouse. Here we discuss features of the fly as a model for human neurodegenerative disease. There are many distinct fly models for a range of neurodegenerative diseases; we focus on select studies from models of polyglutamine disease and amyotrophic lateral sclerosis that illustrate the type and range of insights that can be gleaned. In discussion of these models, we underscore strengths of the fly in providing understanding into mechanisms and pathways, as a foundation for translational and therapeutic research.

The promises of neurodegenerative disease modeling

The rise in the prevalence of neurodegenerative diseases parallels the rapid increase in human lifespan. Despite intensive research, the molecular and cellular mechanisms underlying the onset and progression of these devastating diseases with age are still poorly understood. Many aspects of these diseases have been modelled successfully in experimental animals such as the mouse, the zebrafish Brachydanio rero, the nematode worm Caenorhaditis elegans and the fruit fly Drosophila melanogaster. This review will focus on the advantages offered by the genetic tools available in Drosophila for combining powerful strategies in order to tackle the causative factors of these complex pathologies and help to elaborate efficient drugs to treat them.

Gut-neuron interaction via Hh signaling regulates intestinal progenitor cell differentiation in Drosophila

Intestinal homeostasis is maintained by intestinal stem cells (ISCs) and their progenies. A complex autonomic nervous system spreads over posterior intestine. However, whether and how neurons regulate posterior intestinal homeostasis is largely unknown. Here we report that neurons regulate Drosophila posterior intestinal homeostasis. Specifically, downregulation of neuronal Hedgehog (Hh) signaling inhibits the differentiation of ISCs toward enterocytes (ECs), whereas upregulated neuronal Hh signaling promotes such process. We demonstrate that, among multiple sources of Hh ligand, those secreted by ECs induces similar phenotypes as does neuronal Hh. In addition, intestinal JAK/STAT signaling responds to activated neuronal Hh signaling, suggesting that JAK/STAT signaling acts downstream of neuronal Hh signaling in intestine. Collectively, our results indicate that neuronal Hh signaling is essential for the determination of ISC fate.