Glutamine/proline-rich PQE-1 proteins protect Caenorhabditis elegans neurons from huntingtin polyglutamine neurotoxicity

Unraveling the Puzzle of Therapeutic Peptides: A Promising Frontier in Huntington's Disease Treatment

Huntington's disease (HD) is a neurodegenerative genetic disorder characterized by a mutation in the huntingtin (HTT) gene, resulting in the production of a mutant huntingtin protein (mHTT). The accumulation of mHTT leads to the development of toxic aggregates in neurons, causing cell dysfunction and, eventually, cell death. Peptide therapeutics target various aspects of HD pathology, including mHTT reduction and aggregation inhibition, extended CAG mRNA degradation, and modulation of dysregulated signaling pathways, such as BDNF/TrkB signaling. In addition, these peptide therapeutics also target the detrimental interactions of mHTT with InsP3R1, CaM, or Caspase-6 proteins to mitigate HD. This Perspective provides a detailed perspective on anti-HD therapeutic peptides, highlighting their design, structural characteristics, neuroprotective effects, and specific mechanisms of action. Peptide therapeutics for HD exhibit promise in preclinical models, but further investigation is required to confirm their effectiveness as viable therapeutic strategies, recognizing that no approved peptide therapy for HD currently exists.

Towards Understanding Neurodegenerative Diseases: Insights from Caenorhabditis elegans

The elevated occurrence of debilitating neurodegenerative disorders, such as amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD) and Machado-Joseph disease (MJD), demands urgent disease-modifying therapeutics. Owing to the evolutionarily conserved molecular signalling pathways with mammalian species and facile genetic manipulation, the nematode Caenorhabditis elegans (C. elegans) emerges as a powerful and manipulative model system for mechanistic insights into neurodegenerative diseases. Herein, we review several representative C. elegans models established for five common neurodegenerative diseases, which closely simulate disease phenotypes specifically in the gain-of-function aspect. We exemplify applications of high-throughput genetic and drug screenings to illustrate the potential of C. elegans to probe novel therapeutic targets. This review highlights the utility of C. elegans as a comprehensive and versatile platform for the dissection of neurodegenerative diseases at the molecular level.

Caenorhabditis elegans: A transgenic model for studying age-associated neurodegenerative diseases

Neurodegenerative diseases (NDs) are a heterogeneous group of aging-associated ailments characterized by interrupting cellular proteostasic machinery and the misfolding of distinct proteins to form toxic aggregates in neurons. Neurodegenerative diseases, which include Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and others, are becoming an increasing threat to human health worldwide. The degeneration and death of certain specific groups of neurons are the hallmarks of these diseases. Over the past decades, Caenorhabditis eleganshas beenwidely used as a transgenic model to investigate biological processes related to health and disease. The nematode Caenorhabditis elegans (C. elegans) has developed as a powerful tool for studying disease mechanisms due to its ease of genetic handling and instant cultivation while providing a whole-animal system amendable to several molecular and biochemical techniques. In this review, we elucidate the potential of C. elegans as a versatile platform for systematic dissection of the molecular basis of human disease, focusing on neurodegenerative disorders, and may help better our understanding of the disease mechanisms and search for new therapeutics for these devastating diseases.

Caenorhabditis elegans as a model system to evaluate neuroprotective potential of nano formulations

The impact of neurodegenerative illnesses on society is significant, but the mechanisms leading to neuronal malfunction and death in these conditions remain largely unknown despite identifying essential disease genes. To pinpoint the mechanisms behind the pathophysiology of neurodegenerative diseases, several researchers have turned to nematode C. elegans instead of using mammals. Since C. elegans is transparent, free-living, and amenable to culture, it has several benefits. As a result, all the neurons in C. elegans can be easily identified, and their connections are understood. Human proteins linked to Neurodegeneration can be made to express in them. It is also possible to analyze how C. elegans orthologs of the genes responsible for human neurodegenerative diseases function. In this article, we focused at some of the most important C. elegans neurodegeneration models that accurately represent many elements of human neurodegenerative illness. It has been observed that studies using the adaptable C. elegans have helped us in better understanding of human diseases. These studies have used it to replicate several aspects of human neurodegeneration. A nanotech approach involves engineering materials or equipments interacting with biological systems at the molecular level to trigger physiological responses by increasing stimulation, responding, and interacting with target sites while minimizing side effects, thus revolutionizing the treatment and diagnosis of neurodegenerative diseases. Nanotechnologies are being used to treat neurological disorders and deliver nanoscale drugs. This review explores the current and future uses of these nanotechnologies as innovative therapeutic modalities in treatment of neurodegenerative diseases using C elegans as an experimental model.

Possible Co-Evolution of Polyglutamine and Polyproline in Huntingtin Protein: Proline-Rich Domain as Transient Folding Chaperone

Huntington's disease is an inherited neurodegenerative disorder caused by the overduplication of CAG repeats in the Huntingtin gene. Recent findings revealed that among the orthologs, the expansion of CAG repeats (polyQ) in the Huntingtin gene occurs in tandem with the duplication of CCG repeats (polyP). However, the molecular mechanism of this possible co-evolution remains unknown. We examined the structures of Huntingtin exon 1 (HttEx1) from six species along with five designed mutants. We found that the polyP segments "chaperone" the rest of the HttEx1 by forming ad hoc polyP binding grooves. Such a process elongates the otherwise poorly solvated polyQ domain, while modulating its secondary structure propensity from β-strands to α-helices. This chaperoning effect is achieved mostly through transient hydrogen bond interactions between polyP and the rest of HttEx1, resulting in a striking golden ratio of ∼2:1 between the chain lengths of polyQ and polyP.

Using Caenorhabditis elegans to Model Therapeutic Interventions of Neurodegenerative Diseases Targeting Microbe-Host Interactions

Emerging evidence from both clinical studies and animal models indicates the importance of the interaction between the gut microbiome and the brain in the pathogenesis of neurodegenerative diseases (NDs). Although how microbes modulate neurodegeneration is still mostly unclear, recent studies have started to probe into the mechanisms for the communication between microbes and hosts in NDs. In this review, we highlight the advantages of using Caenorhabditis elegans (C. elegans) to disentangle the microbe-host interaction that regulates neurodegeneration. We summarize the microbial pro- and anti-neurodegenerative factors identified using the C. elegans ND models and the effects of many are confirmed in mouse models. Specifically, we focused on the role of bacterial amyloid proteins, such as curli, in promoting proteotoxicity and neurodegeneration by cross-seeding the aggregation of endogenous ND-related proteins, such as α-synuclein. Targeting bacterial amyloid production may serve as a novel therapeutic strategy for treating NDs, and several compounds, such as epigallocatechin-3-gallate (EGCG), were shown to suppress neurodegeneration at least partly by inhibiting curli production. Because bacterial amyloid fibrils contribute to biofilm formation, inhibition of amyloid production often leads to the disruption of biofilms. Interestingly, from a list of 59 compounds that showed neuroprotective effects in C. elegans and mouse ND models, we found that about half of them are known to inhibit bacterial growth or biofilm formation, suggesting a strong correlation between the neuroprotective and antibiofilm activities. Whether these potential therapeutics indeed protect neurons from proteotoxicity by inhibiting the cross-seeding between bacterial and human amyloid proteins awaits further investigations. Finally, we propose to screen the long list of antibiofilm agents, both FDA-approved drugs and novel compounds, for their neuroprotective effects and develop new pharmaceuticals that target the gut microbiome for the treatment of NDs. To this end, the C. elegans ND models can serve as a platform for fast, high-throughput, and low-cost drug screens that target the microbe-host interaction in NDs.

In vivo imaging of fluorescent single-walled carbon nanotubes within C. elegans nematodes in the near-infrared window

Caenorhabditis elegans (C. elegans) nematodes serve as a model organism for eukaryotes, especially due to their genetic similarity. Although they have many advantages like their small size and transparency, their autofluorescence in the entire visible wavelength range poses a challenge for imaging and tracking fluorescent proteins or dyes using standard fluorescence microscopy. Herein, near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWCNTs) are utilized for in vivo imaging within the gastrointestinal track of C. elegans. The SWCNTs are biocompatible, and do not affect the worms' viability nor their reproduction ability. The worms do not show any autofluorescence in the NIR range, thus enabling the spectral separation between the SWCNT NIR fluorescence and the strong autofluorescence of the worm gut granules. The worms are fed with ssDNA-SWCNT which are visualized mainly in the intestine lumen. The NIR fluorescence is used in vivo to track the contraction and relaxation in the area of the pharyngeal valve at the anterior of the terminal bulb. These biocompatible, non-photobleaching, NIR fluorescent nanoparticles can advance in vivo imaging and tracking within C. elegans and other small model organisms by overcoming the signal-to-noise challenge stemming from the wide-range visible autofluorescence.

G-Protein-Coupled Receptor SRBC-48 Protects against Dendrite Degeneration and Reduced Longevity Due to Infection

Increasing evidence suggests that deficient immune modulation and microbial infections underline neurodegeneration, but the mechanisms remain obscure. Here, we show that the G-protein-coupled receptor (GPCR) SRBC-48, which belongs to the class BC serpentine receptors, has a protective role in Caenorhabditis elegans dendrite degeneration caused by Pseudomonas aeruginosa infection. Our results indicate that SRBC-48 functions in a cell-autonomous manner in AWC neurons to protect against infection-associated dendrite degeneration. The absence of SRBC-48 results in a reduced lifespan caused by a pathogen infection early in life that induces dendrite degeneration. The decreased longevity in animals deficient in SRBC-48 is due to uncontrolled activation of immune genes, particularly those regulated by the FOXO family transcription factor DAF-16 that is part of the insulin/insulin-like growth factor (IGF)-1 receptor homolog DAF-2. These results reveal how an infection early in life can not only induce dendrite degeneration but also reduce lifespan.

C. elegans electrotaxis behavior is modulated by heat shock response and unfolded protein response signaling pathways

The nematode C. elegans is a leading model to investigate the mechanisms of stress-induced behavioral changes coupled with biochemical mechanisms. Our group has previously characterized C. elegans behavior using a microfluidic-based electrotaxis device, and showed that worms display directional motion in the presence of a mild electric field. In this study, we describe the effects of various forms of genetic and environmental stress on the electrotactic movement of animals. Using exposure to chemicals, such as paraquat and tunicamycin, as well as mitochondrial and endoplasmic reticulum (ER) unfolded protein response (UPR) mutants, we demonstrate that chronic stress causes abnormal movement. Additionally, we report that pqe-1 (human RNA exonuclease 1 homolog) is necessary for the maintenance of multiple stress response signaling and electrotaxis behavior of animals. Further, exposure of C. elegans to several environmental stress-inducing conditions revealed that while chronic heat and dietary restriction caused electrotaxis speed deficits due to prolonged stress, daily exercise had a beneficial effect on the animals, likely due to improved muscle health and transient activation of UPR. Overall, these data demonstrate that the electrotaxis behavior of worms is susceptible to cytosolic, mitochondrial, and ER stress, and that multiple stress response pathways contribute to its preservation in the face of stressful stimuli.

Reactive Species in Huntington Disease: Are They Really the Radicals You Want to Catch?

Huntington disease (HD) is a neurodegenerative condition and one of the so-called rare or minority diseases, due to its low prevalence (affecting 1–10 of every 100,000 people in western countries). The causative gene, HTT, encodes huntingtin, a protein with a yet unknown function. Mutant huntingtin causes a range of phenotypes, including oxidative stress and the activation of microglia and astrocytes, which leads to chronic inflammation of the brain. Although substantial efforts have been made to find a cure for HD, there is currently no medical intervention able to stop or even delay progression of the disease. Among the many targets of therapeutic intervention, oxidative stress and inflammation have been extensively studied and some clinical trials have been promoted to target them. In the present work, we review the basic research on oxidative stress in HD and the strategies used to fight it. Many of the strategies to reduce the phenotypes associated with oxidative stress have produced positive results, yet no substantial functional recovery has been observed in animal models or patients with the disease. We discuss possible explanations for this and suggest potential ways to overcome it.

Caenorhabditis elegans as a model system for studying aging-associated neurodegenerative diseases

Neurodegenerative diseases (NDs) are a heterogeneous group of aging-associated disorders characterized by the disruption of cellular proteostasis machinery and the misfolding of distinct protein species to form toxic aggregates in neurons. The increasing prevalence of NDs represents a growing healthcare burden worldwide, a concern compounded by the fact that few, if any, treatments exist to target the underlying cause of these diseases. Consequently, the application of a high-throughput, physiologically relevant model system to studies dissecting the molecular mechanisms governing ND pathology is crucial for identifying novel avenues for the development of targeted therapeutics. The nematode Caenorhabditis elegans (C. elegans) has emerged as a powerful tool for the study of disease mechanisms due to its ease of genetic manipulation and swift cultivation, while providing a whole-animal system amendable to numerous molecular and biochemical techniques. To date, numerous C. elegans models have been generated for a variety of NDs, allowing for the large-scale in vivo study of protein-conformation disorders. Furthermore, the comparatively low barriers to entry in the development of transgenic worm models have facilitated the modeling of rare or "orphan" NDs, thereby providing unparalleled insight into the shared mechanisms underlying these pathologies. In this review, we summarize findings from a comprehensive collection of C. elegans neurodegenerative disease models of varying prevalence to emphasize shared mechanisms of proteotoxicity, and highlight the utility of these models in elucidating the molecular basis of ND pathologies.

PolyQ-independent toxicity associated with novel translational products from CAG repeat expansions

Expanded CAG nucleotide repeats are the underlying genetic cause of at least 14 incurable diseases, including Huntington’s disease (HD). The toxicity associated with many CAG repeat expansions is thought to be due to the translation of the CAG repeat to create a polyQ protein, which forms toxic oligomers and aggregates. However, recent studies show that HD CAG repeats undergo a non-canonical form of translation called Repeat-associated non-AUG dependent (RAN) translation. RAN translation of the CAG sense and CUG anti-sense RNAs produces six distinct repeat peptides: polyalanine (polyAla, from both CAG and CUG repeats), polyserine (polySer), polyleucine (polyLeu), polycysteine (polyCys), and polyglutamine (polyGln). The toxic potential of individual CAG-derived RAN polypeptides is not well understood. We developed pure C. elegans protein models for each CAG RAN polypeptide using codon-varied expression constructs that preserve RAN protein sequence but eliminate repetitive CAG/CUG RNA. While all RAN polypeptides formed aggregates, only polyLeu was consistently toxic across multiple cell types. In GABAergic neurons, which exhibit significant neurodegeneration in HD patients, codon-varied (Leu)₃₈, but not (Gln)₃₈, caused substantial neurodegeneration and motility defects. Our studies provide the first in vivo evaluation of CAG-derived RAN polypeptides in a multicellular model organism and suggest that polyQ-independent mechanisms, such as RAN-translated polyLeu peptides, may have a significant pathological role in CAG repeat expansion disorders.

Glutamatergic nervous system degeneration in a C. elegans TauA152T tauopathy model involves pathways of excitotoxicity and Ca2+ dysregulation

Mutations in the gene encoding Tau (MAPT-microtubule-associated protein tau) cause a group of neurodegenerative diseases called tauopathies. A recently identified Tau variant, p.A152T, has been reported as a risk factor for frontotemporal dementia-related disorders and Alzheimer disease. However, the mechanism for the pathologies still remain poorly understood. Transgenic Caenorhabditis elegans expressing mutant 2N4R-Tau^A152T (Tau^AT) panneuronally show locomotor defects, neurodegeneration and accelerated aging. Here we report that, in Tau^AT animals, the glutamatergic nervous system is at a high risk of progressive neuronal loss. We present genetic data that this loss occurs predominantly through necrosis. The neuronal loss is caused by several determinants, such as altered adenylyl cyclase (type AC9) pathway, prevalence of excitotoxicity-like conditions, aging-related factors and finally dyshomeostasis of intracellular calcium (Ca²⁺). The study provides novel insights into the mechanisms involved in selective loss of glutamatergic neurons in a Tau^AT tauopathy model which could point to new therapeutic targets.

Caenorhabditis elegans as a model system for target identification and drug screening against neurodegenerative diseases

Over the past decades, Caenorhabditis elegans (C. elegans) has been widely used as a model system because of its small size, transparent body, short generation time and lifespan (~3 days and 3 weeks, respectively), completely sequenced genome and tractability to genetic manipulation. Protein misfolding and aggregation are key pathological features in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease and Amyotrophic lateral sclerosis. Animal models, including C. elegans, have been extensively used to discover and validate new drugs against neurodegenerative diseases. The well-defined and genetically tractable nervous system of C. elegans offers an effective model to explore basic mechanistic pathways of neurodegenerative diseases. Recent progress in high-throughput drug screening also provides a powerful approach for identifying chemical modulators of biological processes. Here, we summarize the latest progress of using C. elegans as a model system for target identification and drug screening in neurodegenerative diseases.

Wild type huntingtin toxicity in yeast: Implications for the role of amyloid cross-seeding in polyQ diseases

Proteins with expanded polyglutamine (polyQ) regions are prone to form amyloids, which can cause diseases in humans and toxicity in yeast. Recently, we showed that in yeast non-toxic amyloids of Q-rich proteins can induce aggregation and toxicity of wild type huntingtin (Htt) with a short non-pathogenic polyglutamine tract. Similarly to mutant Htt with an elongated N-terminal polyQ sequence, toxicity of its wild type counterpart was mediated by induced aggregation of the essential Sup35 protein, which contains a Q-rich region. Notably, polymerization of Sup35 was not caused by the initial benign amyloids and, therefore, aggregates of wild type Htt acted as intermediaries in seeding Sup35 polymerization. This exemplifies a protein polymerization cascade which can generate a network of interdependent polymers. Here we discuss cross-seeded protein polymerization as a possible mechanism underlying known interrelations between different polyQ diseases. We hypothesize that similar mechanisms may enable proteins, which possess expanded Q-rich tracts but are not associated with diseases, to promote the development of polyQ diseases.

In vitro aggregating β-lactamase-polyQ chimeras do not induce toxic effects in an in vivo Caenorhabditis elegans model

BACKGROUND

A series of human diseases are caused by the misfolding and aggregation of specific proteins or peptides into amyloid fibrils; nine of these diseases, referred to as polyglutamine diseases, are associated with proteins carrying an expanded polyglutamine (polyQ) region. While the presence of this latter is thought to be the determinant factor for the development of polyQ diseases, the non-polyQ regions of the host proteins are thought to play a significant modulating role.

METHOD

In order to better understand the role of non-polyQ regions, the toxic effects of model proteins bearing different polyQ regions (containing up to 79 residues) embedded at two distinct locations within the β-lactamase (BlaP) host enzyme were evaluated in Caenorhabditis elegans. This small organism can be advantageous for the validation of in vitro findings, as it provides a multicellular context yet avoids the typical complexity of common studies relying on vertebrate models. Several phenotypic assays were performed in order to screen for potential toxic effects of the different BlaP-polyQ proteins.

RESULTS

Despite the significant in vitro aggregation of BlaP-polyQ proteins with long polyQ regions, none of the BlaP-polyQ chimeras aggregated in the generated transgenic in vivo models.

CONCLUSION

The absence of a toxic effect of the expression of BlaP-polyQ chimeras may find its cause in biochemical mechanisms present in vivo to cope with protein aggregation (e.g. presence of chaperones) or in C. elegans' limitations such as its short lifespan. It is plausible that the aggregation propensities of the different BlaP chimeras containing embedded polyQ sequences are too low in this in vivo environment to permit their aggregation. These experiments emphasize the need for several comparative and in vivo verification studies of biologically relevant in vitro findings, which reveal both the strengths and limitations of widely used model systems.

Food-Derived Antioxidant Polysaccharides and Their Pharmacological Potential in Neurodegenerative Diseases

Oxidative stress is known to impair architecture and function of cells, which may lead to various chronic diseases, and therefore therapeutic and nutritional interventions to reduce oxidative damages represent a viable strategy in the amelioration of oxidative stress-related disorders, including neurodegenerative diseases. Over the past decade, a variety of natural polysaccharides from functional and medicinal foods have attracted great interest due to their antioxidant functions such as scavenging free radicals and reducing oxidative damages. Interestingly, these antioxidant polysaccharides are also found to attenuate neuronal damages and alleviate cognitive and motor decline in a range of neurodegenerative models. It has recently been established that the neuroprotective mechanisms of polysaccharides are related to oxidative stress-related pathways, including mitochondrial function, antioxidant defense system and pathogenic protein aggregation. Here, we first summarize the current status of antioxidant function of food-derived polysaccharides and then attempt to appraise their anti-neurodegeneration activities.

Genetic variation in neurodegenerative diseases and its accessibility in the model organism Caenorhabditis elegans

BACKGROUND

Neurodegenerative diseases (NGDs) such as Alzheimer's and Parkinson's are debilitating and largely untreatable conditions strongly linked to age. The clinical, neuropathological, and genetic components of NGDs indicate that neurodegeneration is a complex trait determined by multiple genes and by the environment.

MAIN BODY

The symptoms of NGDs differ among individuals due to their genetic background, and this variation affects the onset and progression of NGD and NGD-like states. Such genetic variation affects the molecular and cellular processes underlying NGDs, leading to differential clinical phenotypes. So far, we have a limited understanding of the mechanisms of individual background variation. Here, we consider how variation between genetic backgrounds affects the mechanisms of aging and proteostasis in NGD phenotypes. We discuss how the nematode Caenorhabditis elegans can be used to identify the role of variation between genetic backgrounds. Additionally, we review advances in C. elegans methods that can facilitate the identification of NGD regulators and/or networks.

CONCLUSION

Genetic variation both in disease genes and in regulatory factors that modulate onset and progression of NGDs are incompletely understood. The nematode C. elegans represents a valuable system in which to address such questions.

Distinct mechanisms of mutant huntingtin toxicity in different yeast strains

Expansion of polyglutamine stretches in several proteins causes neurodegenerative amyloidoses, including Huntington disease. In yeast, mutant huntingtin (mHtt) with a stretch of 103 glutamine residues (HttQ103) forms toxic aggregates. A range of yeast strains have been used to elucidate the mechanisms of mHtt toxicity, and have revealed perturbations of various unrelated processes. HttQ103 aggregates can induce aggregation of cellular proteins, many of which contain glutamine/asparagine-rich regions, including Sup35 and Def1. In the strain 74-D694 HttQ103, toxicity is related to aggregation-mediated depletion of soluble Sup35 and its interacting partner Sup45. Def1 was also implicated in mHtt toxicity, since its lack detoxified HttQ103 in another yeast strain, BY4741. Here we show that in BY4742, deletion of DEF1 lowers HttQ103 toxicity and decreases the amount of its polymers, but does not affect copolymerization of Sup35. Furthermore, in contrast to 74-D694, increasing the levels of soluble Sup35 and Sup45 does not alleviate toxicity of HttQ103 in BY4742. These data demonstrate a difference in the mechanisms underlying mHtt toxicity in different yeast strains and suggest that in humans with Huntington disease, neurons of different brain compartments and cells in other tissues can also be damaged by different mechanisms.

The role of insulin/IGF-1 signaling in the longevity of model invertebrates, C. elegans and D. melanogaster

Insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway regulates aging in many organisms, ranging from simple invertebrates to mammals, including humans. Many seminal discoveries regarding the roles of IIS in aging and longevity have been made by using the roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. In this review, we describe the mechanisms by which various IIS components regulate aging in C. elegans and D. melanogaster. We also cover systemic and tissue-specific effects of the IIS components on the regulation of lifespan. We further discuss IIS-mediated physiological processes other than aging and their effects on human disease models focusing on C. elegans studies. As both C. elegans and D. melanogaster have been essential for key findings regarding the effects of IIS on organismal aging in general, these invertebrate models will continue to serve as workhorses to help our understanding of mammalian aging. [BMB Reports 2016; 49(2): 81-92]

Epimedium Polysaccharide Alleviates Polyglutamine-Induced Neurotoxicity in Caenorhabditis elegans by Reducing Oxidative Stress

Epimedium has been traditionally used to treat a variety of medical conditions, including neurological disorders. In this study, an acidic polysaccharide EbPS-A1 is isolated from Epimedium brevicornum and found to contain mainly galacturonic acid, galactose, and rhamnose but also arabinose and glucuronic acid. Using Caenorhabditis elegans models, we show that EbPS-A1 is capable of inhibiting behavioral dysfunction mediated by polyglutamine (polyQ), which is implicated in several neurodegenerative disorders such as Huntington's disease. Interestingly, EbPS-A1 does not inhibit polyQ aggregation or extend lifespan in the nematodes; it does, however, improve the survival under increased oxidative stress of both polyQ and wild-type nematodes intoxicated by paraquat. Further studies reveal that EbPS-A1 is capable of not only scavenging free radicals in vitro but also reducing reactive oxygen species levels, enhancing antioxidant enzyme activities, and decreasing lipid peroxidation product in C. elegans models. Together, these results suggest that the protective effect of Epimedium polysaccharide against polyQ-mediated neurotoxicity is likely due to its antioxidant function.

A protein polymerization cascade mediates toxicity of non-pathological human huntingtin in yeast

Several neurodegenerative amyloidoses, including Huntington disease, are caused by expansion of polyglutamine (polyQ) stretches in otherwise unrelated proteins. In a yeast model, an N-terminal fragment of mutant huntingtin with a stretch of 103 glutamine residues aggregates and causes toxicity, while its non-toxic wild type variant with a sequence of 25 glutamines (Htt25Q) does not aggregate. Here, we observed that non-toxic polymers of various proteins with glutamine-rich domains could seed polymerization of Htt25Q, which caused toxicity by seeding polymerization of the glutamine/asparagine-rich Sup35 protein thus depleting the soluble pools of this protein and its interacting partner, Sup45. Importantly, only polymers of Htt25Q, but not of the initial benign polymers, induced Sup35 polymerization, indicating an intermediary role of Htt25Q in cross-seeding Sup35 polymerization. These data provide a novel insight into interactions between amyloidogenic proteins and suggest a possible role for these interactions in the pathogenesis of Huntington and other polyQ diseases.

Using C. elegans to discover therapeutic compounds for ageing-associated neurodegenerative diseases

Age-associated neurodegenerative disorders such as Alzheimer's disease are a major public health challenge, due to the demographic increase in the proportion of older individuals in society. However, the relatively few currently approved drugs for these conditions provide only symptomatic relief. A major goal of neurodegeneration research is therefore to identify potential new therapeutic compounds that can slow or even reverse disease progression, either by impacting directly on the neurodegenerative process or by activating endogenous physiological neuroprotective mechanisms that decline with ageing. This requires model systems that can recapitulate key features of human neurodegenerative diseases that are also amenable to compound screening approaches. Mammalian models are very powerful, but are prohibitively expensive for high-throughput drug screens. Given the highly conserved neurological pathways between mammals and invertebrates, Caenorhabditis elegans has emerged as a powerful tool for neuroprotective compound screening. Here we describe how C. elegans has been used to model various human ageing-associated neurodegenerative diseases and provide an extensive list of compounds that have therapeutic activity in these worm models and so may have translational potential.

Use of Caenorhabditis elegans as a model to study Alzheimer's disease and other neurodegenerative diseases

Advances in research and technology has increased our quality of life, allowed us to combat diseases, and achieve increased longevity. Unfortunately, increased longevity is accompanied by a rise in the incidences of age-related diseases such as Alzheimer’s disease (AD). AD is the sixth leading cause of death, and one of the leading causes of dementia amongst the aged population in the USA. It is a progressive neurodegenerative disorder, characterized by the prevalence of extracellular Aβ plaques and intracellular neurofibrillary tangles, derived from the proteolysis of the amyloid precursor protein (APP) and the hyperphosphorylation of microtubule-associated protein tau, respectively. Despite years of extensive research, the molecular mechanisms that underlie the pathology of AD remain unclear. Model organisms, such as the nematode, Caenorhabditis elegans, present a complementary approach to addressing these questions. C. elegans has many advantages as a model system to study AD and other neurodegenerative diseases. Like their mammalian counterparts, they have complex biochemical pathways, most of which are conserved. Genes in which mutations are correlated with AD have counterparts in C. elegans, including an APP-related gene, apl-1, a tau homolog, ptl-1, and presenilin homologs, such as sel-12 and hop-1. Since the neuronal connectivity in C. elegans has already been established, C. elegans is also advantageous in modeling learning and memory impairments seen during AD. This article addresses the insights C. elegans provide in studying AD and other neurodegenerative diseases. Additionally, we explore the advantages and drawbacks associated with using this model.

Prion-like proteins sequester and suppress the toxicity of huntingtin exon 1

Expansions of preexisting polyglutamine (polyQ) tracts in at least nine different proteins cause devastating neurodegenerative diseases. There are many unique features to these pathologies, but there must also be unifying mechanisms underlying polyQ toxicity. Using a polyQ-expanded fragment of huntingtin exon-1 (Htt103Q), the causal protein in Huntington disease, we and others have created tractable models for investigating polyQ toxicity in yeast cells. These models recapitulate key pathological features of human diseases and provide access to an unrivalled genetic toolbox. To identify toxicity modifiers, we performed an unbiased overexpression screen of virtually every protein encoded by the yeast genome. Surprisingly, there was no overlap between our modifiers and those from a conceptually identical screen reported recently, a discrepancy we attribute to an artifact of their overexpression plasmid. The suppressors of Htt103Q toxicity recovered in our screen were strongly enriched for glutamine- and asparagine-rich prion-like proteins. Separated from the rest of the protein, the prion-like sequences of these proteins were themselves potent suppressors of polyQ-expanded huntingtin exon-1 toxicity, in both yeast and human cells. Replacing the glutamines in these sequences with asparagines abolished suppression and converted them to enhancers of toxicity. Replacing asparagines with glutamines created stronger suppressors. The suppressors (but not the enhancers) coaggregated with Htt103Q, forming large foci at the insoluble protein deposit in which proteins were highly immobile. Cells possessing foci had fewer (if any) small diffusible oligomers of Htt103Q. Until such foci were lost, cells were protected from death. We discuss the therapeutic implications of these findings.

Animal models of neurodegenerative diseases

The prevalence of neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), increases with age, and the number of affected patients is expected to increase worldwide in the next decades. Accurately understanding the etiopathogenic mechanisms of these diseases is a crucial step for developing disease-modifying drugs able to preclude their emergence or at least slow their progression. Animal models contribute to increase the knowledge on the pathophysiology of neurodegenerative diseases. These models reproduce different aspects of a given disease, as well as the histopathological lesions and its main symptoms. The purpose of this review is to present the main animal models for AD, PD, and Huntington's disease.

Histone methylation restrains the expression of subtype-specific genes during terminal neuronal differentiation in Caenorhabditis elegans

Although epigenetic control of stem cell fate choice is well established, little is known about epigenetic regulation of terminal neuronal differentiation. We found that some differences among the subtypes of Caenorhabditis elegans VC neurons, particularly the expression of the transcription factor gene unc-4, require histone modification, most likely H3K9 methylation. An EGF signal from the vulva alleviated the epigenetic repression of unc-4 in vulval VC neurons but not the more distant nonvulval VC cells, which kept unc-4 silenced. Loss of the H3K9 methyltransferase MET-2 or H3K9me2/3 binding proteins HPL-2 and LIN-61 or a novel chromodomain protein CEC-3 caused ectopic unc-4 expression in all VC neurons. Downstream of the EGF signaling in vulval VC neurons, the transcription factor LIN-11 and histone demethylases removed the suppressive histone marks and derepressed unc-4. Behaviorally, expression of UNC-4 in all the VC neurons caused an imbalance in the egg-laying circuit. Thus, epigenetic mechanisms help establish subtype-specific gene expression, which are needed for optimal activity of a neural circuit.

Deciphering the roles of trehalose and Hsp104 in the inhibition of aggregation of mutant huntingtin in a yeast model of Huntington's disease

Despite the significant amount of experimental data available on trehalose, the molecular mechanism responsible for its intracellular stabilising properties has not emerged yet. The repair of cellular homeostasis in many protein-misfolding diseases by trehalose is credited to the disaccharide being an inducer of autophagy, a mechanism by which aggregates of misfolded proteins are cleared by the cell. In this work, we expressed the pathogenic N-terminal fragment of huntingtin in Δnth1 mutant (unable to degrade trehalose) of Saccharomyces cerevisiae BY4742 strain. We show that the presence of trehalose resulted in the partitioning of the mutant huntingtin in the soluble fraction of the cell. This led to reduced oxidative stress and improved cell survival. The beneficial effect was independent of the expression of the major cellular antioxidant enzyme, superoxide dismutase. Additionally, trehalose led to the overexpression of the heat shock protein, Hsp104p, in mutant huntingtin-expressing cells, and resulted in rescue of the endocytotic defect in the yeast cell. We propose that at least in the initial stages of aggregation, trehalose functions as a stabiliser, increasing the level of monomeric mutant huntingtin protein, with its concomitant beneficial effects, in addition to its role as an inducer of autophagy.

Modeling neurodegenerative diseases in Caenorhabditis elegans

Neurodegenerative diseases which include Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington disease (HD), and others are becoming an increasing threat to human health worldwide. The degeneration and death of certain specific groups of neurons are the hallmarks of these diseases. Despite the research progress in identification of several disease-related genes, the mechanisms underlying the neurodegeneration in these diseases remain unclear. Given the molecular conservation in neuronal signaling between Caenorhabditis elegans and vertebrates, an increasing number of research scientists have used the nematode to study this group of diseases. This review paper will focus on the model system that has been established in C. elegans to investigate the pathogenetic roles of those reported disease-related genes in AD, PD, ALS, HD and others. The progress in C. elegans provides useful information of the genetic interactions and molecular pathways that are critical in the disease process, and may help better our understanding of the disease mechanisms and search for new therapeutics for these devastating diseases.

Experimental models for identifying modifiers of polyglutamine-induced aggregation and neurodegeneration

Huntington's disease (HD) typifies a class of inherited neurodegenerative disorders in which a CAG expansion in a single gene leads to an extended polyglutamine tract and misfolding of the expressed protein, driving cumulative neural dysfunction and degeneration. HD is invariably fatal with symptoms that include progressive neuropsychiatric and cognitive impairments, and eventual motor disability. No curative therapies yet exist for HD and related polyglutamine diseases; therefore, substantial efforts have been made in the drug discovery field to identify potential drug and drug target candidates for disease-modifying treatment. In this context, we review here a range of early-stage screening approaches based in in vitro, cellular, and invertebrate models to identify pharmacological and genetic modifiers of polyglutamine aggregation and induced neurodegeneration. In addition, emerging technologies, including high-content analysis, three-dimensional culture models, and induced pluripotent stem cells are increasingly being incorporated into drug discovery screening pipelines for protein misfolding disorders. Together, these diverse screening strategies are generating novel and exciting new probes for understanding the disease process and for furthering development of therapeutic candidates for eventual testing in the clinical setting.

Lessons learned from the transgenic Huntington's disease rats

Huntington's disease (HD) is a fatal inherited disorder leading to selective neurodegeneration and neuropsychiatric symptoms. Currently, there is no treatment to slow down or to stop the disease. There is also no therapy to effectively reduce the symptoms. In the investigation of novel therapies, different animal models of Huntington's disease, varying from insects to nonhuman primates, have been created and used. Few years ago, the first transgenic rat model of HD, carrying a truncated huntingtin cDNA fragment with 51 CAG repeats under control of the native rat huntingtin promoter, was introduced. We have been using this animal model in our research and review here our experience with the behavioural, neurophysiological, and histopathological phenotype of the transgenic Huntington's disease rats with relevant literature.

Mutations in the pqe-1 gene enhance transgene expression in Caenorhabditis elegans

Although various genetic tools have been developed and used as transgenes, the expression of the transgenes often is hampered by negative regulators. Disrupting such negative regulators of gene expression is potentially a way to overcome the common problem of low expression of transgenes. To find such regulators whose mutations enhance transgene expression in Caenorhabditis elegans, we took advantage of a newly developed reporter transgene, lin-11pAΔ::venus. This transgene induces expression of a fluorescent protein, Venus, in specific neurons including AIZ, where the expression was stochastic. The frequency of reporter expression in AIZ seemed to be correlated with the strength of transgene expression. By using this system, in which a moderate increase of expression was converted to all-or-none expression states, we describe here a forward genetic screen for mutations that enhance the expression of transgenes. Through the screen, we found that mutations in the pqe-1 gene, which encodes a Q/P-rich nuclear protein with an exonuclease domain, increase the chance of reporter expression in AIZ. The fluorescence intensity in RIC, in which all lin-11pAΔ::venus animals show reporter expression, was increased in pqe-1 mutants, suggesting that pqe-1 reduces the expression level of the transgene. Expression of transgenes with other promoters, 3'UTR, or reporter genes was also enhanced by the pqe-1 mutation, suggesting that the effect was not specific to a particular type of transgenes, whereas the effect did not seem to extend to endogenous genes. We propose that pqe-1 mutants can be used to increase the expression of various useful transgenes.

Operant-based instrumental learning for analysis of genetically modified models of Huntington's disease

Huntington's disease is the result of an expanded CAG repeat in the gene that codes for the protein huntingtin and results in a progressive sequelae of motor, cognitive and psychiatric symptoms. The development of genetically modified rodent models of Huntington's disease has led to the need for sensitive behavioural phenotyping. Operant tests for rodents have been developed that can determine the functional deficits in these genetically modified models, from motor, cognitive and emotional domains. The current review discusses tests that employ operant equipment, an automated and highly flexible method for testing rodents. Different operant paradigms are examined in relation to their relevance to Huntington's disease symptomology, as well as summarising research to date on genetic models with these tests.

Proteomic analysis of wild-type and mutant huntingtin-associated proteins in mouse brains identifies unique interactions and involvement in protein synthesis

Huntington disease is a neurodegenerative disorder caused by a CAG repeat amplification in the gene huntingtin (HTT) that is reflected by a polyglutamine expansion in the Htt protein. Nearly 20 years of research have uncovered roles for Htt in a wide range of cellular processes, and many of these discoveries stemmed from the identification of Htt-interacting proteins. However, no study has employed an impartial and comprehensive strategy to identify proteins that differentially associate with full-length wild-type and mutant Htt in brain tissue, the most relevant sample source to the disease condition. We analyzed Htt affinity-purified complexes from wild-type and HTT mutant juvenile mouse brain from two different biochemical fractions by tandem mass spectrometry. We compared variations in protein spectral counts relative to Htt to identify those proteins that are the most significantly contrasted between wild-type and mutant Htt purifications. Previously unreported Htt interactions with Myo5a, Prkra (PACT), Gnb2l1 (RACK1), Rps6, and Syt2 were confirmed by Western blot analysis. Gene Ontology analysis of these and other Htt-associated proteins revealed a statistically significant enrichment for proteins involved in translation among other categories. Furthermore, Htt co-sedimentation with polysomes in cytoplasmic mouse brain extracts is dependent upon the presence of intact ribosomes. Finally, wild-type or mutant Htt overexpression inhibits cap-dependent translation of a reporter mRNA in an in vitro system. Cumulatively, these data support a new role for Htt in translation and provide impetus for further study into the link between protein synthesis and Huntington disease pathogenesis.

Control of nonapoptotic developmental cell death in Caenorhabditis elegans by a polyglutamine-repeat protein

Death is a vital developmental cell fate. In Caenorhabditis elegans, programmed death of the linker cell, which leads gonadal elongation, proceeds independently of caspases and apoptotic effectors. To identify genes promoting linker-cell death, we performed a genome-wide RNA interference screen. We show that linker-cell death requires the gene pqn-41, encoding an endogenous polyglutamine-repeat protein. pqn-41 functions cell-autonomously and is expressed at the onset of linker-cell death. pqn-41 expression is controlled by the mitogen-activated protein kinase kinase SEK-1, which functions in parallel to the zinc-finger protein LIN-29 to promote cellular demise. Linker-cell death is morphologically similar to cell death associated with normal vertebrate development and polyglutamine-induced neurodegeneration. Our results may therefore provide molecular inroads to understanding nonapoptotic cell death in metazoan development and disease.

Identification of common genetic modifiers of neurodegenerative diseases from an integrative analysis of diverse genetic screens in model organisms

BACKGROUND

An array of experimental models have been developed in the small model organisms C. elegans, S. cerevisiae and D. melanogaster for the study of various neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and expanded polyglutamine diseases as exemplified by Huntington's disease (HD) and related ataxias. Genetic approaches to determine the nature of regulators of the disease phenotypes have ranged from small scale to essentially whole genome screens. The published data covers distinct models in all three organisms and one important question is the extent to which shared genetic factors can be uncovered that affect several or all disease models. Surprisingly it has appeared that there may be relatively little overlap and that many of the regulators may be organism or disease-specific. There is, however, a need for a fully integrated analysis of the available genetic data based on careful comparison of orthologues across the species to determine the real extent of overlap.

RESULTS

We carried out an integrated analysis using C. elegans as the baseline model organism since this is the most widely studied in this context. Combination of data from 28 published studies using small to large scale screens in all three small model organisms gave a total of 950 identifications of genetic regulators. Of these 624 were separate genes with orthologues in C. elegans. In addition, 34 of these genes, which all had human orthologues, were found to overlap across studies. Of the common genetic regulators some such as chaperones, ubiquitin-related enzymes (including the E3 ligase CHIP which directly links the two pathways) and histone deacetylases were involved in expected pathways whereas others such as the peroxisomal acyl CoA-oxidase suggest novel targets for neurodegenerative disease therapy

CONCLUSIONS

We identified a significant number of overlapping regulators of neurodegenerative disease models. Since the diseases have, as an underlying feature, protein aggregation phenotypes it was not surprising that some of the overlapping genes encode proteins involved in protein folding and protein degradation. Interestingly, however, some of the overlapping genes encode proteins that have not previously featured in targeted studies of neurodegeneration and this information will form a useful resource to be exploited in further studies of potential drug-targets.

Inhibition of polyglutamine-mediated proteotoxicity by Astragalus membranaceus polysaccharide through the DAF-16/FOXO transcription factor in Caenorhabditis elegans

Late-onset neurodegenerative diseases are characterized by progressive accumulation of aggregation-prone proteins and global disruption of the proteostasis network, e.g. abnormal polyQ (polyglutamine) aggregation in Huntington's disease. Astragalus membranaceus polysaccharide (astragalan) has recently been shown to modulate aging and proteotoxic stress pathways. Using Caenorhabditis elegans models, we now show that astragalan not only reduces polyQ aggregation, but also alleviates the associated neurotoxicity. We also reveal that astragalan can extend the adult lifespan of wild-type and polyQ nematodes, indicating a connection of its anti-aging benefit with the toxicity-suppressing effect. Further examination demonstrates that astragalan can extend the lifespan of daf-2 and age-1, but not daf-16, mutant nematodes of the insulin-like aging and stress pathway, suggesting a lifespan-regulation signalling independent of DAF (abnormal dauer formation)-2/IGF-1R (insulin-like growth factor 1 receptor), but dependent on the DAF-16/FOXO (forkhead box O) transcription factor, a pivotal integrator of divergent signalling pathways related to both lifespan regulation and stress resistance. We also show that a subset of DAF-16 downstream genes are regulated by astragalan, including the DAF-16 transcriptional target gene scl-20, which is itself constitutively up-regulated in transgenic polyQ nematodes. These findings, together with our previous work on LEA (late embryogenesis abundant) proteins and trehalose, provide a revealing insight into the potential of stress and lifespan regulators in the prevention of proteotoxic disorders.

DEG/ENaC but not TRP channels are the major mechanoelectrical transduction channels in a C. elegans nociceptor

Many nociceptors detect mechanical cues, but the ion channels responsible for mechanotransduction in these sensory neurons remain obscure. Using in vivo recordings and genetic dissection, we identified the DEG/ENaC protein, DEG-1, as the major mechanotransduction channel in ASH, a polymodal nociceptor in Caenorhabditis elegans. But DEG-1 is not the only mechanotransduction channel in ASH: loss of deg-1 revealed a minor current whose properties differ from those expected of DEG/ENaC channels. This current was independent of two TRPV channels expressed in ASH. Although loss of these TRPV channels inhibits behavioral responses to noxious stimuli, we found that both mechanoreceptor currents and potentials were essentially wild-type in TRPV mutants. We propose that ASH nociceptors rely on two genetically distinct mechanotransduction channels and that TRPV channels contribute to encoding and transmitting information. Because mammalian and insect nociceptors also coexpress DEG/ENaCs and TRPVs, the cellular functions elaborated here for these ion channels may be conserved.

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.

Neuronal GPCR controls innate immunity by regulating noncanonical unfolded protein response genes

The unfolded protein response (UPR), which is activated when unfolded or misfolded proteins accumulate in the endoplasmic reticulum, has been implicated in the normal physiology of immune defense and in several human diseases, including diabetes, cancer, neurodegenerative disease, and inflammatory disease. In this study, we found that the nervous system controlled the activity of a noncanonical UPR pathway required for innate immunity in Caenorhabditis elegans. OCTR-1, a putative octopamine G protein-coupled catecholamine receptor (GPCR, G protein-coupled receptor), functioned in sensory neurons designated ASH and ASI to actively suppress innate immune responses by down-regulating the expression of noncanonical UPR genes pqn/abu in nonneuronal tissues. Our findings suggest a molecular mechanism by which the nervous system may sense inflammatory responses and respond by controlling stress-response pathways at the organismal level.

Disrupted-in-Schizophrenia 1-mediated axon guidance involves TRIO-RAC-PAK small GTPase pathway signaling

Defects in neuronal connectivity of the brain are well documented among schizophrenia patients. Although the schizophrenia susceptibility gene Disrupted-in-Schizophrenia 1 (DISC1) has been implicated in various neurodevelopmental processes, its role in regulating axonal connections remains elusive. Here, a heterologous DISC1 transgenic system in the relatively simple and well-characterized Caenorhabditis elegans motor neurons has been established to investigate whether DISC1 regulates axon guidance during development. Transgenic DISC1 in C. elegans motor neurons is enriched in the migrating growth cones and causes guidance defects of their growing axons. The abnormal axonal phenotypes induced by DISC1 are similar to those by gain-of-function rac genes. In vivo genetic interaction studies revealed that the UNC-73/TRIO-RAC-PAK signaling pathway is activated by ectopic DISC1 in C. elegans motor axons. Using in vitro GST pull-down and coimmunoprecipitation assays, we found that DISC1 binds specifically to the amino half of spectrin repeats of TRIO, thereby preventing TRIO's amino half of spectrin repeats from interacting with its first guanine nucleotide exchange factor (GEF) domain, GEF1, and facilitating the recruitment of RAC1 to TRIO. In cultured mammalian cells, RAC1 is activated by increased TRIO's GEF activity when DISC1 is present. These results together indicate that the TRIO-RAC-PAK signaling pathway can be exploited and modulated by DISC1 to regulate axonal connectivity in the developing brain.