From fruit fly to bedside: translating lessons from Drosophila models of neurodegenerative disease

A perspective on Drosophila genetics and its insight into human neurodegenerative disease

Drosophila has been long appreciated as a classic genetic system for its ability to define gene function in vivo. Within the last several decades, the fly has also emerged as a premiere system for modeling and defining mechanisms of human disease by expressing dominant human disease genes and analyzing the effects. Here I discuss key aspects of this latter approach that first intrigued me to focus my laboratory research on this idea. Differences between the loss-of-function vs. the gain-of-function approach are raised-and the insight of these approaches for appreciating mechanisms that contribute to human neurodegenerative disease. The application of modifier genetics, which is a prominent goal of models of human disease, has implications for how specific genes or pathways intersect with the dominant disease-associated mechanisms. Models of human disease will continue to reveal unanticipated insight into fundamental cellular processes-insight that might be harder to glean from classical genetic methodologies vs modifier genetics of disease.

Animal Models of Neurodegenerative Disease: Recent Advances in Fly Highlight Innovative Approaches to Drug Discovery

Neurodegenerative diseases represent a formidable challenge to global health. As advances in other areas of medicine grant healthy living into later decades of life, aging diseases such as Alzheimer's disease (AD) and other neurodegenerative disorders can diminish the quality of these additional years, owed largely to the lack of efficacious treatments and the absence of durable cures. Alzheimer's disease prevalence is predicted to more than double in the next 30 years, affecting nearly 15 million Americans, with AD-associated costs exceeding $1 billion by 2050. Delaying onset of AD and other neurodegenerative diseases is critical to improving the quality of life for patients and reducing the burden of disease on caregivers and healthcare systems. Significant progress has been made to model disease pathogenesis and identify points of therapeutic intervention. While some researchers have contributed to our understanding of the proteins and pathways that drive biological dysfunction in disease using in vitro and in vivo models, others have provided mathematical, biophysical, and computational technologies to identify potential therapeutic compounds using in silico modeling. The most exciting phase of the drug discovery process is now: by applying a target-directed approach that leverages the strengths of multiple techniques and validates lead hits using Drosophila as an animal model of disease, we are on the fast-track to identifying novel therapeutics to restore health to those impacted by neurodegenerative disease.

Small molecule inhibitors of α-synuclein oligomers identified by targeting early dopamine-mediated motor impairment in C. elegans

BACKGROUND

Parkinson's disease is a disabling neurodegenerative movement disorder characterized by dopaminergic neuron loss induced by α-synuclein oligomers. There is an urgent need for disease-modifying therapies for Parkinson's disease, but drug discovery is challenged by lack of in vivo models that recapitulate early stages of neurodegeneration. Invertebrate organisms, such as the nematode worm Caenorhabditis elegans, provide in vivo models of human disease processes that can be instrumental for initial pharmacological studies.

METHODS

To identify early motor impairment of animals expressing α-synuclein in dopaminergic neurons, we first used a custom-built tracking microscope that captures locomotion of single C. elegans with high spatial and temporal resolution. Next, we devised a method for semi-automated and blinded quantification of motor impairment for a population of simultaneously recorded animals with multi-worm tracking and custom image processing. We then used genetic and pharmacological methods to define the features of early motor dysfunction of α-synuclein-expressing C. elegans. Finally, we applied the C. elegans model to a drug repurposing screen by combining it with an artificial intelligence platform and cell culture system to identify small molecules that inhibit α-synuclein oligomers. Screen hits were validated using in vitro and in vivo mammalian models.

RESULTS

We found a previously undescribed motor phenotype in transgenic α-synuclein C. elegans that correlates with mutant or wild-type α-synuclein protein levels and results from dopaminergic neuron dysfunction, but precedes neuronal loss. Together with artificial intelligence-driven in silico and in vitro screening, this C. elegans model identified five compounds that reduced motor dysfunction induced by α-synuclein. Three of these compounds also decreased α-synuclein oligomers in mammalian neurons, including rifabutin which has not been previously investigated for Parkinson's disease. We found that treatment with rifabutin reduced nigrostriatal dopaminergic neurodegeneration due to α-synuclein in a rat model.

CONCLUSIONS

We identified a C. elegans locomotor abnormality due to dopaminergic neuron dysfunction that models early α-synuclein-mediated neurodegeneration. Our innovative approach applying this in vivo model to a multi-step drug repurposing screen, with artificial intelligence-driven in silico and in vitro methods, resulted in the discovery of at least one drug that may be repurposed as a disease-modifying therapy for Parkinson's disease.

Modeling Parkinson's Disease: Not Only Rodents?

Parkinson’s disease (PD) is a common chronic progressive multifactorial neurodegenerative disease. In most cases, PD develops as a sporadic idiopathic disease. However, in 10%–15% of all patients, Mendelian inheritance of the disease is observed in an autosomal dominant or autosomal recessive manner. To date, mutations in seven genes have been convincingly confirmed as causative in typical familial forms of PD, i.e., SNCA, LRRK2, VPS35, PRKN, PINK1, GBA, and DJ-1. Family and genome-wide association studies have also identified a number of candidate disease genes and a common genetic variability at 90 loci has been linked to risk for PD. The analysis of the biological function of both proven and candidate genes made it possible to conclude that mitochondrial dysfunction, lysosomal dysfunction, impaired exosomal transport, and immunological processes can play important roles in the development of the pathological process of PD. The mechanisms of initiation of the pathological process and its earliest stages remain unclear. The study of the early stages of the disease (before the first motor symptoms appear) is extremely complicated by the long preclinical period. In addition, at present, the possibility of performing complex biochemical and molecular biological studies familial forms of PD is limited. However, in this case, the analysis of the state of the central nervous system can only be assessed by indirect signs, such as the level of metabolites in the cerebrospinal fluid, peripheral blood, and other biological fluids. One of the potential solutions to this problem is the analysis of disease models, in which it is possible to conduct a detailed in-depth study of all aspects of the pathological process, starting from its earliest stages. Many modeling options are available currently. An analysis of studies published in the 2000s suggests that toxic models in rodents are used in the vast majority of cases. However, interesting and important data for understanding the pathogenesis of PD can be obtained from other in vivo models. Within the framework of this review, we will consider various models of PD that were created using various living organisms, from unicellular yeast (Saccharomyces cerevisiae) and invertebrate (Nematode and Drosophila) forms to various mammalian species.

An Automated Functional Annotation Pipeline That Rapidly Prioritizes Clinically Relevant Genes for Autism Spectrum Disorder

Human genetic studies have implicated more than a hundred genes in Autism Spectrum Disorder (ASD). Understanding how variation in implicated genes influence expression of co-occurring conditions and drug response can inform more effective, personalized approaches for treatment of individuals with ASD. Rapidly translating this information into the clinic requires efficient algorithms to sort through the myriad of genes implicated by rare gene-damaging single nucleotide and copy number variants, and common variation detected in genome-wide association studies (GWAS). To pinpoint genes that are more likely to have clinically relevant variants, we developed a functional annotation pipeline. We defined clinical relevance in this project as any ASD associated gene with evidence indicating a patient may have a complex, co-occurring condition that requires direct intervention (e.g., sleep and gastrointestinal disturbances, attention deficit hyperactivity, anxiety, seizures, depression), or is relevant to drug development and/or approaches to maximizing efficacy and minimizing adverse events (i.e., pharmacogenomics). Starting with a list of all candidate genes implicated in all manifestations of ASD (i.e., idiopathic and syndromic), this pipeline uses databases that represent multiple lines of evidence to identify genes: (1) expressed in the human brain, (2) involved in ASD-relevant biological processes and resulting in analogous phenotypes in mice, (3) whose products are targeted by approved pharmaceutical compounds or possessing pharmacogenetic variation and (4) whose products directly interact with those of genes with variants recommended to be tested for by the American College of Medical Genetics (ACMG). Compared with 1000 gene sets, each with a random selection of human protein coding genes, more genes in the ASD set were annotated for each category evaluated (p ≤ 1.99 × 10⁻²). Of the 956 ASD-implicated genes in the full set, 18 were flagged based on evidence in all categories. Fewer genes from randomly drawn sets were annotated in all categories (x = 8.02, sd = 2.56, p = 7.75 × 10⁻⁴). Notably, none of the prioritized genes are represented among the 59 genes compiled by the ACMG, and 78% had a pathogenic or likely pathogenic variant in ClinVar. Results from this work should rapidly prioritize potentially actionable results from genetic studies and, in turn, inform future work toward clinical decision support for personalized care based on genetic testing.

Multidimensional Proteomics Identifies Declines in Protein Homeostasis and Mitochondria as Early Signals for Normal Aging and Age-associated Disease in Drosophila

Aging is characterized by a gradual deterioration in proteome. However, how protein dynamics that changes with normal aging and in disease is less well understood. Here, we profiled the snapshots of aging proteome in Drosophila, from head and muscle tissues of post-mitotic somatic cells, and the testis of mitotically-active cells. Our data demonstrated that dysregulation of proteome homeostasis, or proteostasis, might be a common feature associated with age. We further used pulsed metabolic stable isotope labeling analysis to characterize protein synthesis. Interestingly, this study determined an age-modulated decline in protein synthesis with age, particularly in the pathways related to mitochondria, neurotransmission, and proteostasis. Importantly, this decline became dramatically accelerated in Pink1 mutants, a Drosophila model of human age-related Parkinson's disease. Taken together, our multidimensional proteomic study revealed tissue-specific protein dynamics with age, highlighting mitochondrial and proteostasis-related proteins. We suggest that declines in proteostasis and mitochondria early in life are critical signals prior to the onset of aging and aging-associated diseases.

Low-dose ionizing radiation alleviates Aβ42-induced cell death via regulating AKT and p38 pathways in Drosophila Alzheimer's disease models

Ionizing radiation is widely used in medicine and is valuable in both the diagnosis and treatment of many diseases. However, its health effects are ambiguous. Here, we report that low-dose ionizing radiation has beneficial effects in human amyloid-β42 (Aβ42)-expressing Drosophila Alzheimer's disease (AD) models. Ionizing radiation at a dose of 0.05 Gy suppressed AD-like phenotypes, including developmental defects and locomotive dysfunction, but did not alter the decreased survival rates and longevity of Aβ42-expressing flies. The same dose of γ-irradiation reduced Aβ42-induced cell death in Drosophila AD models through downregulation of head involution defective (hid), which encodes a protein that activates caspases. However, 4 Gy of γ-irradiation increased Aβ42-induced cell death without modulating pro-apoptotic genes grim, reaper and hid. The AKT signaling pathway, which was suppressed in Drosophila AD models, was activated by either 0.05 or 4 Gy γ-irradiation. Interestingly, p38 mitogen-activated protein-kinase (MAPK) activity was inhibited by exposure to 0.05 Gy γ-irradiation but enhanced by exposure to 4 Gy in Aβ42-expressing flies. In addition, overexpression of phosphatase and tensin homolog (PTEN), a negative regulator of the AKT signaling pathway, or a null mutant of AKT strongly suppressed the beneficial effects of low-dose ionizing radiation in Aβ42-expressing flies. These results indicate that low-dose ionizing radiation suppresses Aβ42-induced cell death through regulation of the AKT and p38 MAPK signaling pathways, suggesting that low-dose ionizing radiation has hormetic effects on the pathogenesis of Aβ42-associated AD.

Summary: Low-dose ionizing radiation can reduce cell death by regulating AKT/p38 signaling pathway and improve Aβ42-induced symptoms in Drosophila Alzheimer's disease, suggesting that low-dose ionizing radiation may be applicable for treatment.

Modeling Parkinson's disease in adult Drosophila

BACKGROUND

Protein aggregation in neurons is a prominent pathological mark of neurodegeneration. In Parkinson's disease (PD), inclusions of the α-Synuclein (α-Syn) protein form the Lewy bodies in dopaminergic (DA) neurons. Ectopic expression of human α-Syn inDrosophila neurons leads to the protein accumulation, degeneration of DA neurons and locomotor deterioration, and therefore constitutes the present fly PD model. Yet, this model does not enable to study the role of genes, which are essential for normal development, in neurodegeneration.

THE NEW METHOD

Using the Gal80/Gal4/UAS system we optimized the current PD model, such that only the adult stage of the fly is affected by α-Syn expression in the brain.

RESULTS

The symptoms of neurodegeneration typifying the classic model, including reduced locomotor ability, shortened lifespan and the loss of DA neurons, are significantly demonstrated in the novel adult fly PD model.

COMPARISON WITH EXISTING METHOD

The neurodegeneration symptoms exhibited by the innovative model are very similar to those manifested in the recognized one.

CONCLUSIONS

Specific expression of α-Syn in the adult fly brain enables the investigation of developmental genes involved in neurodegeneration, thereby deciphering gene functions and molecular mechanisms. It may further be used for addressing therapeutic targets and treatment platforms specifically during adult stages.

Milder degenerative effects of Carfilzomib vs. Bortezomib in the Drosophila model: a link to clinical adverse events

Proteasome inhibitors, e.g. Bortezomib (BTZ) and Carfilzomib (CFZ), have demonstrated clinical efficacy against haematological cancers. Interestingly, several adverse effects are less common, compared to BTZ, in patients treated with CFZ. As the molecular details of these observations remain not well understood we assayed the pathophysiological effects of CFZ vs. BTZ in the Drosophila experimental model. Mass Spectrometry analyses showed that neither CFZ nor BTZ are hydrolysed in flies’ tissues, while at doses inducing similar inhibition of the rate limiting for protein breakdown chymotrypsin-like (CT-L) proteasomal activity, CFZ treatment resulted in less intense increase of oxidative stress or activation of antioxidant and proteostatic modules. Also, despite comparable cardiotoxicity likely due to disrupted mitochondrial function, CFZ did not affect developmental processes, showed minimal neuromuscular defects and reduced to a lesser extent flies’ healthspan. Studies in flies, human cancer cell lines and blood cells isolated from Multiple Myeloma patients treated with CFZ or BTZ revealed, that the increased BTZ toxicity likely relates to partial co-inhibition of the caspase-like (C-L) proteasomal activity Supportively, co-treating flies with CFZ and a C-L selective proteasome inhibitor exacerbated CFZ-mediated toxicity. Our findings provide a reasonable explanation for the differential adverse effects of CFZ and BTZ in the clinic.

Short Aβ peptides attenuate Aβ42 toxicity in vivo

Data demonstrate that short amyloid-β (Aβ) peptides are not toxic in vivo and can partially block toxicity associated with Aβ42 accumulation. Moore et al. further validate the use of γ-secretase modulators that lower Aβ42 and increase short Aβs as potential Alzheimer’s disease therapeutics.

Processing of amyloid-β (Aβ) precursor protein (APP) by γ-secretase produces multiple species of Aβ: Aβ40, short Aβ peptides (Aβ37–39), and longer Aβ peptides (Aβ42–43). γ-Secretase modulators, a class of Alzheimer’s disease therapeutics, reduce production of the pathogenic Aβ42 but increase the relative abundance of short Aβ peptides. To evaluate the pathological relevance of these peptides, we expressed Aβ36–40 and Aβ42–43 in Drosophila melanogaster to evaluate inherent toxicity and potential modulatory effects on Aβ42 toxicity. In contrast to Aβ42, the short Aβ peptides were not toxic and, when coexpressed with Aβ42, were protective in a dose-dependent fashion. In parallel, we explored the effects of recombinant adeno-associated virus–mediated expression of Aβ38 and Aβ40 in mice. When expressed in nontransgenic mice at levels sufficient to drive Aβ42 deposition, Aβ38 and Aβ40 did not deposit or cause behavioral alterations. These studies indicate that treatments that lower Aβ42 by raising the levels of short Aβ peptides could attenuate the toxic effects of Aβ42.

GULP1/CED-6 ameliorates amyloid-β toxicity in a Drosophila model of Alzheimer's disease

Amyloidogenic processing of APP by β- and γ-secretases leads to the generation of amyloid-β peptide (Aβ), and the accumulation of Aβ in senile plaques is a hallmark of Alzheimer’s disease (AD). Understanding the mechanisms of APP processing is therefore paramount. Increasing evidence suggests that APP intracellular domain (AICD) interacting proteins influence APP processing. In this study, we characterized the overexpression of AICD interactor GULP1 in a Drosophila AD model expressing human BACE and APP695. Transgenic GULP1 significantly lowered the levels of both Aβ1-40 and Aβ1-42 without decreasing the BACE and APP695 levels. Overexpression of GULP1 also reduced APP/BACE-mediated retinal degeneration, rescued motor dysfunction and extended longevity of the flies. Our results indicate that GULP1 regulate APP processing and reduce neurotoxicity in a Drosophila AD model.

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.

2D and 3D Mechanobiology in Human and Nonhuman Systems

Mechanobiology involves the investigation of mechanical forces and their effect on the development, physiology, and pathology of biological systems. The human body has garnered much attention from many groups in the field, as mechanical forces have been shown to influence almost all aspects of human life ranging from breathing to cancer metastasis. Beyond being influential in human systems, mechanical forces have also been shown to impact nonhuman systems such as algae and zebrafish. Studies of nonhuman and human systems at the cellular level have primarily been done in two-dimensional (2D) environments, but most of these systems reside in three-dimensional (3D) environments. Furthermore, outcomes obtained from 3D studies are often quite different than those from 2D studies. We present here an overview of a select group of human and nonhuman systems in 2D and 3D environments. We also highlight mechanobiological approaches and their respective implications for human and nonhuman physiology.

Defective Phagocytic Corpse Processing Results in Neurodegeneration and Can Be Rescued by TORC1 Activation

The removal of apoptotic cell corpses is important for maintaining homeostasis. Previously, defects in apoptotic cell clearance have been linked to neurodegeneration. However, the mechanisms underlying this are still poorly understood. In this study, we report that the absence of the phagocytic receptor Draper in glia leads to a pronounced accumulation of apoptotic neurons in the brain of Drosophila melanogaster. These dead cells persist in the brain throughout the lifespan of the organism and are associated with age-dependent neurodegeneration. Our data indicate that corpses persist because of defective phagosome maturation, rather than recognition defects. TORC1 activation, or inhibition of Atg1, in glia is sufficient to rescue corpse accumulation as well as neurodegeneration. These results suggest that phagocytosis of apoptotic neurons by glia during development is essential for brain homeostasis in adult flies. Furthermore, it suggests that TORC1 regulates Draper-mediated phagosome maturation.

SIGNIFICANCE STATEMENT

Previously, defects in dead cell clearance were linked to neurodegeneration, but the exact mechanisms are not well understood. In this study, we report that the absence of an engulfment receptor leads to a pronounced accumulation of dead neurons in the brain of the fruit fly Drosophila melanogaster. These dead cells persist in the brain throughout the lifespan of the organism and are associated with age-dependent neurodegeneration. Our data indicate that corpses persist because of defective degradation of cells rather than recognition of dead cells.

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 neuronal ceroid lipofuscinoses: Opportunities from model systems

The neuronal ceroid lipofuscinoses are a group of severe and progressive neurodegenerative disorders, generally with childhood onset. Despite the fact that these diseases remain fatal, significant breakthroughs have been made in our understanding of the genetics that underpin these conditions. This understanding has allowed the development of a broad range of models to study disease processes, and to develop new therapeutic approaches. Such models have contributed significantly to our knowledge of these conditions. In this review we will focus on the advantages of each individual model, describe some of the contributions the models have made to our understanding of the broader disease biology and highlight new techniques and approaches relevant to the study and potential treatment of the neuronal ceroid lipofuscinoses. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

Drosophila and experimental neurology in the post-genomic era

For decades, the fruit fly, Drosophila melanogaster, has been among the premiere genetic model systems for probing fundamental neurobiology, including elucidation of mechanisms responsible for human neurologic disorders. Flies continue to offer virtually unparalleled versatility and speed for genetic manipulation, strong genomic conservation, and a nervous system that recapitulates a range of cellular and network properties relevant to human disease. I focus here on four critical challenges emerging from recent advances in our understanding of the genomic basis of human neurologic disorders where innovative experimental strategies are urgently needed: (1) pinpointing causal genes from associated genomic loci; (2) confirming the functional impact of allelic variants; (3) elucidating nervous system roles for novel or poorly studied genes; and (4) probing network interactions within implicated regulatory pathways. Drosophila genetic approaches are ideally suited to address each of these potential translational roadblocks, and will therefore contribute to mechanistic insights and potential breakthrough therapies for complex genetic disorders in the coming years. Strategic collaboration between neurologists, human geneticists, and the Drosophila research community holds great promise to accelerate progress in the post-genomic era.

Total cysteine and glutathione determination in hemolymph of individual adult D. melanogaster

Determination of thiols, glutathione (GSH) and cysteine (Cys) are important due to their roles in oxidative stress and aging. Oxidants such as soluble O2 and H2O2 promote oxidation of thiols to disulfide (SS) bonded dimers affecting quantitation accuracy. The method presented here reduces disulfide-bonded species followed by fluorescence labelling of the 29.5 (±18.2) nL hemolymph volumes of individual adult Drosophila Melanogaster. The availability of only tens of nanoliter (nL) samples that are also highly volume variant requires efficient sample handling to improve thiol measurements while minimizing sample dilution. The optimized method presented here utilizes defined lengths of capillaries to meter tris(2-carboxyethyl)phosphine reducing reagent and monobromobimane derivatizing reagent volumes enabling Cys and GSH quantitation with only 20-fold dilution. The nL assay developed here was optimized with respect to reagent concentrations, sample dilution, reaction times and temperatures. Separation and identification of the nL thiol mixtures were obtained with capillary electrophoresis-laser induced fluorescence. To demonstrate the capability of this method total Cys and total GSH were measured in the hemolymph collected from individual adult D. Melanogaster. The thiol measurements were used to compare a mutant fly strain with a non-functional cystine-glutamate transporter (xCT) to its background control. The mutant fly, genderblind (gb), carries a non-functional gene for a protein similar to mammalian xCT whose function is not fully understood. Average concentrations obtained for mutant and control flies are 2.19 (±0.22) and 1.94 (±0.34) mM Cys and 2.14 (±0.60) and 2.08 (±0.71) mM GSH, respectively, and are not significantly different (p>0.05). Statistical analysis showed significant differences in total GSH of males and females independent of the xCT mutation. Overall, the method demonstrates an approach for effective chemical characterization of thiols in nL sample volumes.

Emerging roles for hnRNPs in post-transcriptional regulation: what can we learn from flies?

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a highly conserved family of RNA-binding proteins able to associate with nascent RNAs in order to support their localization, maturation and translation. Research over this last decade has remarked the importance of gene regulatory processes at post-transcriptional level, highlighting the emerging roles of hnRNPs in several essential biological events. Indeed, hnRNPs are key factors in regulating gene expression, thus, having a number of roles in many biological pathways. Moreover, failure of the activities catalysed by hnRNPs affects various biological processes and may underlie several human diseases including cancer, diabetes and neurodegenerative syndromes. In this review, we summarize some of hnRNPs' roles in the model organism Drosophila melanogaster, particularly focusing on their participation in all aspects of post-transcriptional regulation as well as their conserved role and involvement in the aetiology of human pathologies.

Engineering animal models of dystonia

Dystonia is a neurological disorder characterized by abnormal involuntary movements that are prolonged and often cause twisting and turning. Several genetically modified worms, fruit flies, and rodents have been generated as models of genetic dystonias, in particular DYT1, DYT11, and DYT12 dystonias. Although these models do not show overt dystonic symptoms, the rodent models exhibit motor deficits in specialized behavioral tasks, such as the rotarod and beam-walking tests. For example, in a rodent model of DYT12 dystonia, which is generally stress triggered, motor deficits are observed only after the animal is stressed. Moreover, in a rodent model of DYT1 dystonia, the motor and electrophysiological deficits can be rescued by trihexyphenidyl, a common anticholinergic medication used to treat dystonic symptoms in human patients. Biochemically, the DYT1 and DYT11 animal models also share some similarities to patients, such as a reduction in striatal D2 dopamine receptor and binding activities. In addition, conditional knockout mouse models for DYT1 and DYT11 dystonia demonstrate that loss of the causal dystonia-related proteins in the striatum leads to motor deficits. Interestingly, loss of the DYT1 dystonia causal protein in Purkinje cells shows an improvement in motor performance, suggesting that gene therapy targeting of the cerebellum or intervention in its downstream pathways may be useful. Finally, recent studies using DYT1 dystonia worm and mouse models led to a potential novel therapeutic agent, which is currently undergoing clinical trials. These results indicate that genetic animal models are powerful tools to elucidate the pathophysiology and to further develop new therapeutics for dystonia.

Drosophila as a screening tool to study human neurodegenerative diseases

In an aging society, research involving neurodegenerative disorders is of paramount importance. Over the past few years, research on Alzheimer's and Parkinson's diseases has made tremendous progress. Experimental studies, however, rely mostly on transgenic animal models, preferentially using mice. Although experiments on mice have enormous advantages, they also have some inherent limitations, some of which can be overcome by the use of Drosophila melanogaster as an experimental animal. Among the major advantages of using the fly is its small genome, which can also be modified very easily. The fact that its genome lends itself to diverse alterations (e. g. mutagenesis, transposons) has made the fly a useful organism to perform large-scale and genome-wide screening approaches. This has opened up an entirely new field of experimental research aiming to elucidate genetic interactions and screen for modifiers of disease processes in vivo. Here, we provide a brief overview of how flies can be used to analyze molecular mechanisms underlying human neurodegenerative diseases.

Behavioral testing regimens in genetic-based animal models of Parkinson's disease: cogencies and caveats

Although the onset and progression of Parkinson's disease (PD) is fundamentally sporadic, identification of several of the genes implicated in the disease has provided significant insight concerning patho-physiological mechanisms potentially underlying sporadic PD. Moreover, such studies have caused a revolution in the way researchers view the disease. Since single genes responsible for rare familial forms of the disease have only been identified within the past few years, animal models based on these defects have only recently been generated, thereby not leaving a lot of time for their evaluation and subsequent improvement. The current article provides an extensive review of the major motor and non-motor behavioral tests used in genetically-induced Parkinsonian animals. Moreover, we assess the insights concerning the etiopathogenesis of PD generated from use of such tests and how these have improved available treatment strategies for alleviating aspects of sporadic and non-sporadic parkinsonism.

Axon degeneration and regeneration: insights from Drosophila models of nerve injury

Axon degeneration is the pivotal pathological event of acute traumatic neural injury as well as many chronic neurodegenerative diseases. It is an active cellular program and yet molecularly distinct from cell death. Much effort is devoted toward understanding the nature of axon degeneration and promoting axon regeneration. However, the fundamental mechanisms of self-destruction of damaged axons remain unclear, and there are still few treatments for traumatic brain injury (TBI) or spinal cord injury (SCI). Genetically approachable model organisms such as Drosophila melanogaster, the fruit fly, have proven exceptionally successful in modeling human neurodegenerative diseases. More recently, this success has been extended into the field of acute axon injury and regeneration. In this review, we discuss recent findings, focusing on how these models hold promise for accelerating mechanistic insight into axon injury and identifying potential therapeutic targets for TBI and SCI.

Drosophila Answers to TDP-43 Proteinopathies

Initially implicated in the pathogenesis of CFTR and HIV-1 transcription, nuclear factor TDP-43 was subsequently found to be involved in the origin and development of several neurodegenerative diseases. In 2006, in fact, it was reported for the first time the cytoplasmic accumulation of TDP-43 in ubiquitin-positive inclusions of ALS and FTLD patients, suggesting the presence of a shared underlying mechanism for these diseases. Today, different animal models of TDP-43 proteinopathies are available in rodents, nematodes, fishes, and flies. Although these models recapitulate several of the pathological features found in patients, the mechanisms underpinning the progressive neuronal loss observed in TDP-43 proteinopathies remain to be characterized. Compared to other models, Drosophila are appealing because they combine the presence of a sophisticated brain with the possibility to investigate quickly and massively phenotypic genetic modifiers as well as possible therapeutic strategies. At present, the development of TDP-43-related Drosophila models has further strengthened the hypothesis that both TDP-43 “loss-of-function” and “gain-of-function” mechanisms can contribute to disease. The aim of this paper is to describe and compare the results obtained in a series of transgenic and knockout flies, along with the information they have generated, towards a better understanding of the mechanisms underlying TDP-43 proteinopathies.

Polyglutamine misfolding in yeast: toxic and protective aggregation

Protein misfolding is associated with many human diseases, including neurodegenerative diseases, such as Alzheimer disease, Parkinson disease and Huntington disease. Protein misfolding often results in the formation of intracellular or extracellular inclusions or aggregates. Even though deciphering the role of these aggregates has been the object of intense research activity, their role in protein misfolding diseases is unclear. Here, I discuss the implications of studies on polyglutamine aggregation and toxicity in yeast and other model organisms. These studies provide an excellent experimental and conceptual paradigm that contributes to understanding the differences between toxic and protective trajectories of protein misfolding. Future studies like the ones discussed here have the potential to transform basic concepts of protein misfolding in human diseases and may thus help to identify new therapeutic strategies for their treatment.

Stem cells: a model for screening, discovery and development of drugs

The identification of normal and cancerous stem cells and the recent advances made in isolation and culture of stem cells have rapidly gained attention in the field of drug discovery and regenerative medicine. The prospect of performing screens aimed at proliferation, directed differentiation, and toxicity and efficacy studies using stem cells offers a reliable platform for the drug discovery process. Advances made in the generation of induced pluripotent stem cells from normal or diseased tissue serves as a platform to perform drug screens aimed at developing cell-based therapies against conditions like Parkinson’s disease and diabetes. This review discusses the application of stem cells and cancer stem cells in drug screening and their role in complementing, reducing, and replacing animal testing. In addition to this, target identification and major advances in the field of personalized medicine using induced pluripotent cells are also discussed.

Model organisms reveal insight into human neurodegenerative disease: ataxin-2 intermediate-length polyglutamine expansions are a risk factor for ALS

Model organisms include yeast Saccromyces cerevisae and fly Drosophila melanogaster. These systems have powerful genetic approaches, as well as highly conserved pathways, both for normal function and disease. Here, we review and highlight how we applied these systems to provide mechanistic insight into the toxicity of TDP-43. TDP-43 accumulates in pathological aggregates in ALS and about half of FTD. Yeast and fly studies revealed an interaction with the counterparts of human Ataxin-2, a gene whose polyglutamine repeat expansion is associated with spinocerebellar ataxia type 2. This finding raised the hypothesis that repeat expansions in ataxin-2 may associate with diseases characterized by TDP-43 pathology such as ALS. DNA analysis of patients revealed that intermediate-length polyglutamine expansions in ataxin-2 are a risk factor for ALS, such that repeat lengths are greater than normal, but lower than that associated with spinocerebellar ataxia type 2 (SCA2), and are more frequent in ALS patients than in matched controls. Moreover, repeat expansions associated with ALS are interrupted CAA-CAG sequences as opposed to the pure CAG repeat expansions typically associated with SCA2. These studies provide an example of how model systems, when extended to human cells and human patient tissue, can reveal new mechanistic insight into disease.

Transforming Growth Factor-Beta Signaling in the Neural Stem Cell Niche: A Therapeutic Target for Huntington's Disease

The neural stem cell niches possess the regenerative capacity to generate new functional neurons in the adult brain, suggesting the possibility of endogenous neuronal replacement after injury or disease. Huntington disease (HD) is a neurodegenerative disease and characterized by neuronal loss in the basal ganglia, leading to motor, cognitive, and psychological disabilities. Apparently, in order to make use of the neural stem cell niche as a therapeutic concept for repair strategies in HD, it is important to understand the cellular and molecular composition of the neural stem cell niche under such neurodegenerative conditions. This paper mainly discusses the current knowledge on the regulation of the hippocampal neural stem cell niche in the adult brain and by which mechanism it might be compromised in the case of HD.

Modeling human trinucleotide repeat diseases in Drosophila

Drosophila is a powerful model system to study human trinucleotide repeat diseases. Findings in Drosophila models highlighted importance of host proteins, chaperons, and protein clearance pathways in polyglutamine diseases as well as that of RNA-binding proteins in noncoding repeat RNA toxicity diseases. Recent novel aspects revealed in Drosophila models include pleiotropic Ataxin 2 interactions, antisense transcription in trinucleotide repeat diseases, contribution of CAG RNA in polyglutamine diseases, and the role of RNA foci in CUG expansion diseases. Drosophila models have been also used for repeat stability studies.

Drosophila melanogaster in the study of human neurodegeneration

Human neurodegenerative diseases are devastating illnesses that predominantly affect elderly people. The majority of the diseases are associated with pathogenic oligomers from misfolded proteins, eventually causing the formation of aggregates and the progressive loss of neurons in the brain and nervous system. Several of these proteinopathies are sporadic and the cause of pathogenesis remains elusive. Heritable forms are associated with genetic defects, suggesting that the affected protein is causally related to disease formation and/or progression. The limitations of human genetics, however, make it necessary to use model systems to analyse affected genes and pathways in more detail. During the last two decades, research using the genetically amenable fruitfly has established Drosophila melanogaster as a valuable model system in the study of human neurodegeneration. These studies offer reliable models for Alzheimer's, Parkinson's, and motor neuron diseases, as well as models for trinucleotide repeat expansion diseases, including ataxias and Huntington's disease. As a result of these studies, several signalling pathways including phosphatidylinositol 3-kinase (PI3K)/Akt and target of rapamycin (TOR), c-Jun N-terminal kinase (JNK) and bone morphogenetic protein (BMP) signalling, have been shown to be deregulated in models of proteinopathies, suggesting that two or more initiating events may trigger disease formation in an age-related manner. Moreover, these studies also demonstrate that the fruitfly can be used to screen chemical compounds for their potential to prevent or ameliorate the disease, which in turn can directly guide clinical research and the development of novel therapeutic strategies for the treatment of human neurodegenerative diseases.

Transgenic Drosophila models of Alzheimer's disease and tauopathies

Alzheimer's disease (AD) is the most common form of senile dementia. Aggregation of the amyloid-beta42 peptide (Abeta42) and tau proteins are pathological hallmarks in AD brains. Accumulating evidence suggests that Abeta42 plays a central role in the pathogenesis of AD, and tau acts downstream of Abeta42 as a modulator of the disease progression. Tau pathology is also observed in frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) and other related diseases, so called tauopathies. Although most cases are sporadic, genes associated with familial AD and FTDP-17 have been identified, which led to the development of transgenic animal models. Drosophila has been a powerful genetic model system used in many fields of biology, and recently emerges as a model for human neurodegenerative diseases. In this review, we will summarize key features of transgenic Drosophila models of AD and tauopathies and a number of insights into disease mechanisms as well as therapeutic implications gained from these models.

A neurodegenerative disease affecting synaptic connections in Drosophila mutant for the tumor suppressor morphogen Patched

The tumor suppressor morphogen, Patched (Ptc), has an extensive homology to the Niemann-Pick-C 1 (NPC1) protein. The NPC disease is a paediatric, progressive and fatal neurodegenerative disorder thought to be due to an abnormal accumulation of cholesterol in neurons. Here, we report that patched mutant adults develop a progressive neurodegenerative disease and their brain contains membranous and lamellar inclusions. There is also a significant reduction in the number of synaptic terminals in the brain of the mutant adults. Interestingly, feeding cholesterol to wild type flies generates inclusions in the brain, but does not cause the disease. However, feeding cholesterol to mutant flies increases synaptic connections and suppresses the disease. Our results suggest that sequestration of cholesterol in the mutant brain in the form of membranous material and inclusions affects available pool of cholesterol for cellular functions. This, in turn, negatively affects the synaptic number and contributes to the disease-state. Consistent with this, in ptc mutants there is a reduction in the pool of cholesterol esters, and cholesterol-mediated suppression of the disease accompanies an increase in cholesterol esters. We further show that Ptc does not function directly in this process since gain of function for Hedgehog also induces the same disease with a reduction in the level of cholesterol esters. We believe that loss of function for ptc causes neurodegeneration via two distinct ways: de-repression of genes that interfere with lipid trafficking, and de-repression of genes outside of the lipid trafficking; the functions of both classes of genes ultimately converge on synaptic connections.

Genetic screen identifies serpin5 as a regulator of the toll pathway and CHMP2B toxicity associated with frontotemporal dementia

Frontotemporal dementia (FTD) is the most common form of dementia before 60 years of age. Rare pathogenic mutations in CHMP2B, which encodes a component of the endosomal sorting complex required for transport (ESCRT-III), are associated with FTD linked to chromosome 3 (FTD3). Animal models of FTD3 have not yet been reported, and what signaling pathways are misregulated by mutant CHMP2B in vivo is unknown. Here we report the establishment of a Drosophila model of FTD3 and show the genetic interactions between mutant CHMP2B and other components of ESCRT. Through an unbiased genome-wide screen, we identified 29 modifier loci and found that serpin5 (Spn5), a largely uncharacterized serine protease inhibitor, suppresses the melanization phenotype induced by mutant CHMP2B in the fly eye. We also found that Spn5 is a negative regulator of the Toll pathway and functions extracellularly, likely by blocking the proteolytic activation of Spaetzle, the Toll receptor ligand. Moreover, Spn5 inhibited activation of the Toll pathway by mutant CHMP2B. Our findings identify Spn5 as a regulator of the Toll pathway and CHMP2B toxicity and show that the Toll pathway is a major signaling pathway misregulated by mutant CHMP2B in vivo. This fly model will be useful to further dissect genetic pathways that are potentially relevant to the pathogenesis and treatment of FTD.

Drosophila melanogaster as a model organism of brain diseases

Drosophila melanogaster has been utilized to model human brain diseases. In most of these invertebrate transgenic models, some aspects of human disease are reproduced. Although investigation of rodent models has been of significant impact, invertebrate models offer a wide variety of experimental tools that can potentially address some of the outstanding questions underlying neurological disease. This review considers what has been gleaned from invertebrate models of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, metabolic diseases such as Leigh disease, Niemann-Pick disease and ceroid lipofuscinoses, tumor syndromes such as neurofibromatosis and tuberous sclerosis, epilepsy as well as CNS injury. It is to be expected that genetic tools in Drosophila will reveal new pathways and interactions, which hopefully will result in molecular based therapy approaches.

Swiss cheese et allii, some of the first neurodegenerative mutants isolated in Drosophila

Drosophila has only recently become a model organism to study progressive neurodegeneration, mainly using transgenic flies expressing human disease genes. However, classical forward genetics isolating and characterizing fly mutants that show characteristic features of progressive neurodegeneration can also provide a useful tool to get insights into the mechanisms of neurodegeneration. Interestingly, the first such mutants have been already isolated in the 1970s, and this review focuses on the description of four such mutants originally isolated by Martin Heisenberg.

A drosophila model for amyotrophic lateral sclerosis reveals motor neuron damage by human SOD1

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease that leads to loss of motor function and early death. About 5% of cases are inherited, with the majority of identified linkages in the gene encoding copper, zinc-superoxide dismutase (SOD1). Strong evidence indicates that the SOD1 mutations confer dominant toxicity on the protein. To provide new insight into mechanisms of ALS, we have generated and characterized a model for familial ALS in Drosophila with transgenic expression of human SOD1. Expression of wild type or disease-linked (A4V, G85R) mutants of human SOD1 selectively in motor neurons induced progressive climbing deficits. These effects were accompanied by defective neural circuit electrophysiology, focal accumulation of human SOD1 protein in motor neurons, and a stress response in surrounding glia. However, toxicity was not associated with oligomerization of SOD1 and did not lead to neuronal loss. These studies uncover cell-autonomous injury by SOD1 to motor neurons in vivo, as well as non-autonomous effects on glia, and provide the foundation for new insight into injury and protection of motor neurons in ALS.

The 3R principle and the use of non-human primates in the study of neurodegenerative diseases: the case of Parkinson's disease

The aim of this paper is to offer an ethical perspective on the use of non-human primates in neurobiological studies, using the Parkinson's disease (PD) as an important case study. We refer, as theoretical framework, to the 3R principle, originally proposed by Russell and Burch [Russell, W.M.S., Burch, R.L., 1959. The Principles of Humane Experimental Technique. Universities Federation for Animal Welfare Wheathampstead, England (reprinted in 1992)]. Then, the use of non-human primates in the study of PD will be discussed in relation to the concepts of Replacement, Reduction, and Refinement. Replacement and Reduction result to be the more problematic concept to be applied, whereas Refinement offers relatively more opportunities of improvement. However, although in some cases the 3R principle shows its applicative limits, its value, as conceptual and inspirational tool remains extremely valuable. It suggests to the researchers a series of questions, both theoretical and methodological, which can have the results of improving the quality of life on the experimental models, the quality of the scientific data, and the public perception from the non-scientist community.

Axonal injury and regeneration in the adult brain of Drosophila

Drosophila melanogaster is a leading genetic model system in nervous system development and disease research. Using the power of fly genetics in traumatic axonal injury research will significantly speed up the characterization of molecular processes that control axonal regeneration in the CNS. We developed a versatile and physiologically robust preparation for the long-term culture of the whole Drosophila brain. We use this method to develop a novel Drosophila model for CNS axonal injury and regeneration. We first show that, similar to mammalian CNS axons, injured adult wild-type fly CNS axons fail to regenerate, whereas adult-specific enhancement of protein kinase A activity increases the regenerative capacity of lesioned neurons. Combined, these observations suggest conservation of neuronal regeneration mechanisms after injury. We next exploit this model to explore pathways that induce robust regeneration and find that adult-specific activation of c-Jun N-terminal protein kinase signaling is sufficient for de novo CNS axonal regeneration injury, including the growth of new axons past the lesion site and into the normal target area.

Hemolymph amino acid analysis of individual Drosophila larvae

One of the most widely used transgenic animal models in biology is Drosophila melanogaster, the fruit fly. Chemical information from this exceedingly small organism is usually accomplished by studying populations to attain sample volumes suitable for standard analysis methods. This paper describes a direct sampling technique capable of obtaining 50-300 nL of hemolymph from individual Drosophila larvae. Hemolymph sampling performed under mineral oil and in air at 30 s intervals up to 120 s after piercing larvae revealed that the effect of evaporation on amino acid concentrations is insignificant when the sample was collected within 60 s. Qualitative and quantitative amino acid analyses of obtained hemolymph were carried out in two optimized buffer conditions by capillary electrophoresis with laser-induced fluorescence detection after derivatizing with fluorescamine. Thirteen amino acids were identified from individual hemolymph samples of both wild-type (WT) control and the genderblind (gb) mutant larvae. The levels of glutamine, glutamate, and taurine in the gb hemolymph were significantly lower at 35%, 38%, and 57% of WT levels, respectively. The developed technique that samples only the hemolymph fluid is efficient and enables accurate organism-level chemical information while minimizing errors associated with possible sample contaminations, estimations, and effects of evaporation compared to the traditional hemolymph-sampling techniques.

Human cytomegalovirus immediate-early-gene expression disrupts embryogenesis in transgenic Drosophila

Intrauterine infection with human cytomegalovirus (HCMV) is the leading viral cause of birth defects involving the central nervous system. Due to the highly species specific nature of the virus, its course of natural infection cannot be studied in animal models. Here we introduce a novel transgenic Drosophila model system for studying the effects of the major viral regulatory genes, the immediate-early genes, on normal embryonic development. We show that ectopic expression of the immediate-early genes in Drosophila led to increased embryonic lethality manifested in disintegration of the embryos. Further analysis suggested that immediate-early gene expression interfered with adherens junction maintenance, leading to the disruption of embryonic epithelial integrity. Owing to the evolutionary conservation of developmental mechanisms from invertebrates to mammals, we anticipate that the studies in Drosophila will be relevant also to humans and will ultimately provide a versatile system for studying different aspects of viral-host interactions.

ESCRT-III dysfunction causes autophagosome accumulation and neurodegeneration

Defects in the endosomal-lysosomal pathway have been implicated in a number of neurodegenerative disorders. A key step in the endocytic regulation of transmembrane proteins occurs in a subset of late-endosomal compartments known as multivesicular bodies (MVBs), whose formation is controlled by endosomal sorting complex required for transport (ESCRT). The roles of ESCRT in dendritic maintenance and neurodegeneration remain unknown. Here, we show that mSnf7-2, a key component of ESCRT-III, is highly expressed in most mammalian neurons. Loss of mSnf7-2 in mature cortical neurons caused retraction of dendrites and neuronal cell loss. mSnf7-2 binds to CHMP2B, another ESCRT-III subunit, in which a rare dominant mutation is associated with frontotemporal dementia linked to chromosome 3 (FTD3). Ectopic expression of the mutant protein CHMP2B(Intron5) also caused dendritic retraction prior to neurodegeneration. CHMP2B(Intron5) was associated more avidly than CHMP2B(WT) with mSnf7-2, resulting in sequestration of mSnf7-2 in ubiquitin-positive late-endosomal vesicles in cortical neurons. Moreover, loss of mSnf7-2 or CHMP2B(Intron5) expression caused the accumulation of autophagosomes in cortical neurons and flies. These findings indicate that ESCRT-III dysfunction is associated with the autophagy pathway, suggesting a novel neurodegeneration mechanism that may have important implications for understanding FTD and other age-dependent neurodegenerative diseases.

Age-related changes in climbing behavior and neural circuit physiology in Drosophila

Identifying the cellular and molecular basis for functional decline remains key to understanding aging. To this end, we have characterized age-dependent changes in climbing and the electrophysiology of the giant fiber circuitry in wild type (Wt) and mutant flies with altered lifespan (methuselah and fragile-X). Our data demonstrate a gradual decline in climbing in Wt and methuselah flies aged 5-45 days. In contrast, fragile-X flies climbed poorly even at 5 days and failed completely at 45 days. We then examined whether synaptic transmission to indirect flight muscles along the giant fiber circuit was altered with aging. At 5 days, the dorsal longitudinal muscle (DLM) in Wt flies followed high frequency stimulation well (at 130 Hz or above). At 35 and 45 days, these flies only followed 60-80 Hz. Methuselah flies did not follow stimuli as well as the Wt flies did at 5 and 25 days, but they were similar to Wt flies at older ages. Fragile-X flies responded poorly even at 5 days (40 Hz) and worsened at 35 days (30 Hz). Unlike DLMs, the tergotrochanteral muscle followed high frequency stimuli relatively well in all genotypes, suggesting that the peripheral interneuron along the DLM pathway or the DLM muscular synapse is prone to age-dependent functional decline. These studies reveal subcellular structures as potential targets of aging, indicating that the giant fiber pathway can be used as a model circuit for quantitative studies of aging in flies as well as fly models of age-related human neurological disorders.

Characterizing pathogenic processes in Batten disease: Use of small eukaryotic model systems

The neuronal ceroid lipofuscinoses (NCLs) are neurodegenerative disorders. Nevertheless, small model organisms, including those lacking a nervous system, have proven invaluable in the study of mechanisms that underlie the disease and in studying the functions of the conserved proteins associated to each disease. From the single-celled yeast, Saccharomyces cerevisiae and Schizosaccharomyces pombe, to the worm, Caenorhabditis elegans and the fruitfly, Drosophila melanogaster, biochemical and, in particular, genetic studies on these organisms have provided insight into the NCLs.

Polyglutamine proteins at the pathogenic threshold display neuron-specific aggregation in a pan-neuronal Caenorhabditis elegans model

The basis of neuron-specific pathogenesis, resulting from the expression of misfolded proteins, is poorly understood and of central importance to an understanding of the cell-type specificity of neurodegenerative disease. In this study, we developed a new model for neuron-specific polyQ pathogenesis in Caenorhabditis elegans by pan-neuronal expression that exhibits polyQ length-dependent aggregation, neurotoxicity, and a pathogenic threshold at a length of 35-40 glutamines. Analysis of specific neurons in C. elegans revealed that only at the threshold length, but not at shorter or longer lengths, polyQ proteins can exist in a soluble state in certain lateral neurons or in an aggregated state in motor neurons of the same animal. These results provide direct experimental evidence that the expression of a single species of a toxic misfolded protein can exhibit a range of neuronal consequences.

MBNL1 and CUGBP1 modify expanded CUG-induced toxicity in a Drosophila model of myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by a CTG expansion in the 3' UTR of the dystrophia myotonica protein kinase (DMPK) gene. It has been hypothesized that the pathogenesis in DM1 is triggered by a toxic gain of function of the expanded DMPK RNA. This expanded RNA is retained in nuclear foci where it sequesters and induces alterations in the levels of RNA-binding proteins (RNA-BP). To model DM1 and study the implication of RNA-BP in CUG-induced toxicity, we have generated a Drosophila DM1 model expressing a non-coding mRNA containing 480 interrupted CUG repeats; i.e. [(CUG)20CUCGA]24. This (iCUG)480 transcript accumulates in nuclear foci and its expression leads to muscle wasting and degeneration in Drosophila. We also report that altering the levels of two RNA-BP known to be involved in DM1 pathogenesis, MBNL1 and CUGBP1, modify the (iCUG)480 degenerative phenotypes. Expanded CUG-induced toxicity in Drosophila is suppressed when MBNL1 expression levels are increased, and enhanced when MBNL1 levels are reduced. In addition, (iCUG)480 also causes a decrease in the levels of soluble MBNL1 that is sequestered in the CUG-containing nuclear foci. In contrast, increasing the levels of CUGBP1 worsens (iCUG)480-induced degeneration even though CUGBP1 distribution is not altered by the expression of the expanded triplet repeat. Our data supports a mechanism for DM1 pathogenesis in which decreased levels of MBNL and increased levels of CUGBP mediate the RNA-induced toxicity observed in DM1. Perhaps more importantly, they also provide proof of the principle that CUG-induced muscle toxicity can be suppressed.

Modeling Polyglutamine Pathogenesis in C. elegans

A growing number of human neurodegenerative diseases are associated with disruption of cellular protein folding homeostasis, leading to the appearance of misfolded proteins and deposition of protein aggregates and inclusions. Recent years have been witness to widespread development of invertebrate systems (specifically Drosophila and Caenorhabditis elegans) to model these disorders, bringing the many advantages of such systems, particularly the power of genetic analysis in a metazoan, to bear on these problems. In this chapter, we describe our studies using the nematode, C. elegans, as a model to study polyglutamine expansions as occur in Huntington's disease and related ataxias. Using fluorescently tagged polyglutamine repeats of different lengths, we have examined the dynamics of aggregate formation both within individual cells and over time throughout the lifetime of individual organisms, identifying aging as an important physiological determinant of aggregation and toxicity. Expanding on these observations, we demonstrate that a genetic pathway regulating longevity can alter the time course of aging-related polyglutamine-mediated phenotypes. To identify novel targets and better understand how cells sense and respond to the appearance of misfolded and aggregation-prone proteins, we use a genome-wide RNA interference-based genetic screen to identify modifiers of age-dependent polyglutamine aggregation. Throughout these studies, we used fluorescence-based, live-cell biological and biophysical methods to study the behavior of these proteins in a complex multicellular environment.