Influence of autophagy genes on ion-channel-dependent neuronal degeneration in Caenorhabditis elegans

Complementary protective effects of autophagy and oxidative response against graphene oxide toxicity in Caenorhabditis elegans

Graphene oxide (GO) exposure may cause damage to C. elegans. However, the role of autophagy and its interactive effect with oxidative response in GO toxicity still remain largely unclear. In the present study, we investigated the protective role of autophagy against GO and its association with oxidative response using C. elegans as an in vivo system. Results indicated that GO exposure induced autophagy in a dose dependent manner in C. elegans. Autophagy inhibitor 3-methyladenine (3-MA) and silencing autophagy genes lgg-1, bec-1 and unc-51 exacerbated the toxicity of GO whereas autophagy activator rapamycin alleviated it. In addition, the antioxidant N-Acetyl-L-cysteine (NAC) effectively suppressed the toxicity of GO with increased resistance to oxidative stress. Worms with RNAi-induced antioxidative genes sod-1, sod-2, sod-3 and sod-4 knockdown were more sensitive to GO. 3-MA increased the expression of superoxide dismutase SOD-3 under GO exposure conditions and exacerbated the toxicity of GO under the anti-oxidation inaction condition by sod-3 RNAi. In contrast, NAC reduced autophagy levels in GO exposed nematodes and increased tolerance to GO in autophagy-defective worms. These results suggested that autophagy and antioxidative response provide complementary protection against GO in C. elegans.

Modelling Parkinson's Disease in C. elegans: Strengths and Limitations

Parkinson's disease (PD) is a common neurodegenerative disease that affects the motor system and progressively worsens with age. Current treatment options for PD mainly target symptoms, due to our limited understanding of the etiology and pathophysiology of PD. A variety of preclinical models have been developed to study different aspects of the disease. The models have been used to elucidate the pathogenesis and for testing new treatments. These models include cell models, non-mammalian models, rodent models, and non-human primate models. Over the past few decades, Caenorhabditis elegans (C. elegans) has been widely adopted as a model system due to its small size, transparent body, short generation time and life cycle, fully sequenced genome, the tractability of genetic manipulation and suitability for large scale screening for disease modifiers. Here, we review studies using C. elegans as a model for PD and highlight the strengths and limitations of the C. elegans model. Various C. elegans PD models, including neurotoxin-induced models and genetic models, are described in detail. Moreover, met.

Noncanonical necrosis in 2 different cell types in a Caenorhabditis elegans NAD+ salvage pathway mutant

Necrosis was once described as a chaotic unregulated response to cellular insult. We now know that necrosis is controlled by multiple pathways in response to many different cellular conditions. In our pnc-1 NAD+ salvage deficient Caenorhabditis elegans model excess nicotinamide induces excitotoxic death in uterine-vulval uv1 cells and OLQ mechanosensory neurons. We sought to characterize necrosis in our pnc-1 model in the context of well-characterized necrosis, apoptosis, and autophagy pathways in C. elegans. We confirmed that calpain and aspartic proteases were required for uv1 necrosis, but changes in intracellular calcium levels and autophagy were not, suggesting that uv1 necrosis occurs by a pathway that diverges from mec-4d-induced touch cell necrosis downstream of effector aspartic proteases. OLQ necrosis does not require changes in intracellular calcium, the function of calpain or aspartic proteases, or autophagy. Instead, OLQ survival requires the function of calreticulin and calnexin, pro-apoptotic ced-4 (Apaf1), and genes involved in both autophagy and axon guidance. In addition, the partially OLQ-dependent gentle nose touch response decreased significantly in pnc-1 animals on poor quality food, further suggesting that uv1 and OLQ necrosis differ downstream of their common trigger. Together these results show that, although phenotypically very similar, uv1, OLQ, and touch cell necrosis are very different at the molecular level.

Increased mitochondrial calcium uptake and concomitant mitochondrial activity by presenilin loss promotes mTORC1 signaling to drive neurodegeneration

Metabolic dysfunction and protein aggregation are common characteristics that occur in age‐related neurodegenerative disease. However, the mechanisms underlying these abnormalities remain poorly understood. We have found that mutations in the gene encoding presenilin in Caenorhabditis elegans, sel‐12, results in elevated mitochondrial activity that drives oxidative stress and neuronal dysfunction. Mutations in the human presenilin genes are the primary cause of familial Alzheimer's disease. Here, we demonstrate that loss of SEL‐12/presenilin results in the hyperactivation of the mTORC1 pathway. This hyperactivation is caused by elevated mitochondrial calcium influx and, likely, the associated increase in mitochondrial activity. Reducing mTORC1 activity improves proteostasis defects and neurodegenerative phenotypes associated with loss of SEL‐12 function. Consistent with high mTORC1 activity, we find that SEL‐12 loss reduces autophagosome formation, and this reduction is prevented by limiting mitochondrial calcium uptake. Moreover, the improvements of proteostasis and neuronal defects in sel‐12 mutants due to mTORC1 inhibition require the induction of autophagy. These results indicate that mTORC1 hyperactivation exacerbates the defects in proteostasis and neuronal function in sel‐12 mutants and demonstrate a critical role of presenilin in promoting neuronal health.

Molecular Basis of Neuronal Autophagy in Ageing: Insights from Caenorhabditis elegans

Autophagy is an evolutionarily conserved degradation process maintaining cell homeostasis. Induction of autophagy is triggered as a response to a broad range of cellular stress conditions, such as nutrient deprivation, protein aggregation, organelle damage and pathogen invasion. Macroautophagy involves the sequestration of cytoplasmic contents in a double-membrane organelle referred to as the autophagosome with subsequent degradation of its contents upon delivery to lysosomes. Autophagy plays critical roles in development, maintenance and survival of distinct cell populations including neurons. Consequently, age-dependent decline in autophagy predisposes animals for age-related diseases including neurodegeneration and compromises healthspan and longevity. In this review, we summarize recent advances in our understanding of the role of neuronal autophagy in ageing, focusing on studies in the nematode Caenorhabditis elegans.

Calcium ions trigger the exposure of phosphatidylserine on the surface of necrotic cells

Intracellular Ca²⁺ level is under strict regulation through calcium channels and storage pools including the endoplasmic reticulum (ER). Mutations in certain ion channel subunits, which cause mis-regulated Ca²⁺ influx, induce the excitotoxic necrosis of neurons. In the nematode Caenorhabditis elegans, dominant mutations in the DEG/ENaC sodium channel subunit MEC-4 induce six mechanosensory (touch) neurons to undergo excitotoxic necrosis. These necrotic neurons are subsequently engulfed and digested by neighboring hypodermal cells. We previously reported that necrotic touch neurons actively expose phosphatidylserine (PS), an “eat-me” signal, to attract engulfing cells. However, the upstream signal that triggers PS externalization remained elusive. Here we report that a robust and transient increase of cytoplasmic Ca²⁺ level occurs prior to the exposure of PS on necrotic touch neurons. Inhibiting the release of Ca²⁺ from the ER, either pharmacologically or genetically, specifically impairs PS exposure on necrotic but not apoptotic cells. On the contrary, inhibiting the reuptake of cytoplasmic Ca²⁺ into the ER induces ectopic necrosis and PS exposure. Remarkably, PS exposure occurs independently of other necrosis events. Furthermore, unlike in mutants of DEG/ENaC channels, in dominant mutants of deg-3 and trp-4, which encode Ca²⁺ channels, PS exposure on necrotic neurons does not rely on the ER Ca²⁺ pool. Our findings indicate that high levels of cytoplasmic Ca²⁺ are necessary and sufficient for PS exposure. They further reveal two Ca²⁺-dependent, necrosis-specific pathways that promote PS exposure, a “two-step” pathway initiated by a modest influx of Ca²⁺ and further boosted by the release of Ca²⁺ from the ER, and another, ER-independent, pathway. Moreover, we found that ANOH-1, the worm homolog of mammalian phospholipid scramblase TMEM16F, is necessary for efficient PS exposure in thapsgargin-treated worms and trp-4 mutants, like in mec-4 mutants. We propose that both the ER-mediated and ER-independent Ca²⁺ pathways promote PS externalization through activating ANOH-1.

Necrosis is a type of cell death that exhibits distinct morphological features such as cell and organelle swelling. Necrotic cells expose phosphatidylserine (PS)–a type of phospholipid—on their outer surfaces. Receptor molecules on phagocytes detect PS on necrotic cells and subsequently initiate the engulfment process. As necrosis is associated with stroke, cancer, neurodegenerative diseases, and heart diseases, studying necrotic cell clearance has important medical relevance. In the model organism the nematode C. elegans, we previously identified membrane proteins that promote the exposure of PS on necrotic cell surfaces by studying neurons that are induced to undergo necrosis by dominant mutations in ion channels. Here, in C. elegans, we have discovered that the necrotic insults trigger an increase of the cytoplasmic calcium ion (Ca²⁺), which in turn promotes PS externalization on necrotic cell surfaces. Furthermore, we have identified two different mechanisms that increase cytoplasmic Ca²⁺ levels, one dependent on the Ca²⁺ contribution from the endoplasmic reticulum (ER), the other independent of the ER. The Ca²⁺ signal targets ANOH-1, a worm homolog of mammalian proteins capable of externalizing PS, for promoting PS exposure on necrotic cells. Our findings reveal novel upstream regulatory mechanisms that promote necrotic cell clearance in animals.

Comprehensive Perspectives on Experimental Models for Parkinson's Disease

Parkinson’s disease (PD) ranks second among the most common neurodegenerative diseases, characterized by progressive and selective loss of dopaminergic neurons. Various cross-species preclinical models, including cellular models and animal models, have been established through the decades to study the etiology and mechanism of the disease from cell lines to nonhuman primates. These models are aimed at developing effective therapeutic strategies for the disease. None of the current models can replicate all major pathological and clinical phenotypes of PD. Selection of the model for PD largely relies on our interest of study. In this review, we systemically summarized experimental PD models, including cellular and animal models used in preclinical studies, to understand the pathogenesis of PD. This review is intended to provide current knowledge about the application of these different PD models, with focus on their strengths and limitations with respect to their contributions to the assessment of the molecular pathobiology of PD and identification of the therapeutic strategies for the disease.

C. elegans to model autophagy-related human disorders

Autophagy is a highly conserved degradation process that clears damaged intracellular macromolecules and organelles in order to maintain cellular health. Dysfunctional autophagy is fundamentally linked to the development of various human disorders and pathologies. The use of the nematode Caenorhabditis elegans as a model system to study autophagy has improved our understanding of its regulation and function in organismal physiology. Here, we review the genetic, functional, and regulatory conservation of the autophagy pathway in C. elegans and we describe tools to quantify and study the autophagy process in this incredibly useful model organism. We further discuss how these nematodes have been modified to model autophagy-related human diseases and underscore the important insights obtained from such models. Altogether, we highlight the strengths of C. elegans as an exceptional tool to understand the genetic and molecular foundations underlying autophagy-related human diseases.

Tau Abnormalities and Autophagic Defects in Neurodegenerative Disorders; A Feed-forward Cycle

Abnormal deposition of misfolded proteins is a neuropathological characteristic shared by many neurodegenerative disorders including Alzheimer’s disease (AD). Generation of excessive amounts of aggregated proteins and impairment of degradation systems for misfolded proteins such as autophagy can lead to accumulation of proteins in diseased neurons. Molecules that contribute to both these effects are emerging as critical players in disease pathogenesis. Furthermore, impairment of autophagy under disease conditions can be both a cause and a consequence of abnormal protein accumulation. Specifically, disease-causing proteins can impair autophagy, which further enhances the accumulation of abnormal proteins. In this short review, we focus on the relationship between the microtubule-associated protein tau and autophagy to highlight a feed-forward mechanism in disease pathogenesis.

TOR Signaling in Caenorhabditis elegans Development, Metabolism, and Aging

The Target of Rapamycin (TOR or mTOR) is a serine/threonine kinase that regulates growth, development, and behaviors by modulating protein synthesis, autophagy, and multiple other cellular processes in response to changes in nutrients and other cues. Over recent years, TOR has been studied intensively in mammalian cell culture and genetic systems because of its importance in growth, metabolism, cancer, and aging. Through its advantages for unbiased, and high-throughput, genetic and in vivo studies, Caenorhabditis elegans has made major contributions to our understanding of TOR biology. Genetic analyses in the worm have revealed unexpected aspects of TOR functions and regulation, and have the potential to further expand our understanding of how growth and metabolic regulation influence development. In the aging field, C. elegans has played a leading role in revealing the promise of TOR inhibition as a strategy for extending life span, and identifying mechanisms that function upstream and downstream of TOR to influence aging. Here, we review the state of the TOR field in C. elegans, and focus on what we have learned about its functions in development, metabolism, and aging. We discuss knowledge gaps, including the potential pitfalls in translating findings back and forth across organisms, but also describe how TOR is important for C. elegans biology, and how C. elegans work has developed paradigms of great importance for the broader TOR field.

Autophagy in C. elegans development

Autophagy involves the sequestration of cytoplasmic contents in a double-membrane structure referred to as the autophagosome and the degradation of its contents upon delivery to lysosomes. Autophagy activity has a role in multiple biological processes during the development of the nematode Caenorhabditis elegans. Basal levels of autophagy are required to remove aggregate prone proteins, paternal mitochondria, and spermatid-specific membranous organelles. During larval development, autophagy is required for the remodeling that occurs during dauer development, and autophagy can selectively degrade components of the miRNA-induced silencing complex, and modulate miRNA-mediated silencing. Basal levels of autophagy are important in synapse formation and in the germ line, to promote the proliferation of proliferating stem cells. Autophagy activity is also required for the efficient removal of apoptotic cell corpses by promoting phagosome maturation. Finally, autophagy is also involved in lipid homeostasis and in the aging process. In this review, we first describe the molecular complexes involved in the process of autophagy, its regulation, and mechanisms for cargo recognition. In the second section, we discuss the developmental contexts where autophagy has been shown to be important. Studies in C. elegans provide valuable insights into the physiological relevance of this process during metazoan development.

Methods to Determine the Role of Autophagy Proteins in C. elegans Aging

This chapter describes methods for the analysis of autophagy proteins in C. elegans aging. We discuss the strains to be considered, the methods for the delivery of double-stranded RNA, and the methods to measure autophagy levels, autophagic flux, and degradation by autophagy.

6-hydroxydopamine (6-OHDA) Oxidative Stress Assay for Observing Dopaminergic Neuron Loss in Caenorhabditis elegans

The nematode Caenorhabditis elegans is a powerful genetic model that can be used to investigate neuronal death. Research using C. elegans has been crucial to characterize cell death programmes that are conserved in mammals. Many neuronal signaling components, such as those mediating dopaminergic neurotransmission, are preserved as well. Dopaminergic neurons are progressively lost in Parkinson's disease and an important risk factor to develop this disease appears to be oxidative stress, the increased occurrence of highly reactive oxygen species. Oxidative stress-induced dopaminergic neurodegeneration is mimicked in animal models by treatment with 6-hydroxydopamine (6-OHDA), a dopamine analog, which is specifically taken up into dopaminergic neurons. After exposing C. elegans to 6-OHDA, the loss of fluorescently labeled dopaminergic neurons can be easily monitored. An organisms' sensitivity to oxidative stress is thought to be influenced by basal levels of intrinsic oxidative stress and the ability to counteract oxidative stress and oxidative stress-induced damage. The C. elegans '6-OHDA model' led to the discovery of novel genes that are required to protect dopaminergic neurons and it has helped to determine the effects of conserved cell death and cell engulfment pathways in dopaminergic neurodegeneration. Here, we describe a simple protocol that allows for the easy detection of dopaminergic neuron loss after 6-OHDA treatment in C. elegans.

Approaches for Studying Autophagy in Caenorhabditis elegans

Macroautophagy (hereafter referred to as autophagy) is an intracellular degradative process, well conserved among eukaryotes. By engulfing cytoplasmic constituents into the autophagosome for degradation, this process is involved in the maintenance of cellular homeostasis. Autophagy induction triggers the formation of a cup-shaped double membrane structure, the phagophore, which progressively elongates and encloses materials to be removed. This double membrane vesicle, which is called an autophagosome, fuses with lysosome and forms the autolysosome. The inner membrane of the autophagosome, along with engulfed compounds, are degraded by lysosomal enzymes, which enables the recycling of carbohydrates, amino acids, nucleotides, and lipids. In response to various factors, autophagy can be induced for non-selective degradation of bulk cytoplasm. Autophagy is also able to selectively target cargoes and organelles such as mitochondria or peroxisome, functioning as a quality control system. The modification of autophagy flux is involved in developmental processes such as resistance to stress conditions, aging, cell death, and multiple pathologies. So, the use of animal models is essential for understanding these processes in the context of different cell types throughout the entire lifespan. For almost 15 years, the nematode Caenorhabditis elegans has emerged as a powerful model to analyze autophagy in physiological or pathological contexts. This review presents a rapid overview of physiological processes involving autophagy in Caenorhabditis elegans, the different assays used to monitor autophagy, their drawbacks, and specific tools for the analyses of selective autophagy.

Antioxidant response is a protective mechanism against nutrient deprivation in C. elegans

Animals often experience periods of nutrient deprivation; however, the molecular mechanisms by which animals survive starvation remain largely unknown. In the nematode Caenorhabditis elegans, the nuclear receptor DAF-12 acts as a dietary and environmental sensor to orchestrate diverse aspects of development, metabolism, and reproduction. Recently, we have reported that DAF-12 together with co-repressor DIN-1S is required for starvation tolerance by promoting fat mobilization. In this report, we found that genetic inactivation of the DAF-12 signaling promoted the production of reactive oxygen species (ROS) during starvation. ROS mediated systemic necrosis, thereby inducing organismal death. The DAF-12/DIN-1S complex up-regulated the expression of antioxidant genes during starvation. The antioxidant enzyme GST-4 in turn suppressed ROS formation, thereby conferring worm survival. Our findings highlight the importance of antioxidant response in starvation tolerance and provide a novel insight into multiple organisms survive and adapt to periods of nutrient deprivation.

Detection of Autophagy in Caenorhabditis elegans

Autophagy is a dynamic and catabolic process that results in the breakdown and recycling of cellular components through the autophagosomal-lysosomal pathway. Many autophagy genes identified in yeasts and mammals have orthologs in the nematode Caenorhabditis elegans. In recent years, gene inactivation by RNA interference (RNAi) and chromosomal mutations has been useful to probe the functions of autophagy in C. elegans, and a role for autophagy has been shown to contribute to multiple processes, such as the adaptation to stress, longevity, cell death, cell growth control, clearance of aggregation-prone proteins, degradation of P granules during embryogenesis, and apoptotic cell clearance. Here, we discuss some of these roles and describe methods that can be used to study autophagy in C. elegans. Specifically, we summarize how to visualize autophagy in embryos, larva, or adults, how to detect the lipidation of the ubiquitin-like modifier LGG-1 by western blot, and how to inactivate autophagy genes by RNAi.

Detection of Autophagy in Caenorhabditis elegans Using GFP::LGG-1 as an Autophagy Marker

In yeast and mammalian cells, the autophagy protein Atg8/LC3 (microtubule-associated proteins 1A/1B light chain 3B encoded by MAP1LC3B) has been the marker of choice to detect double-membraned autophagosomes that are produced during the process of autophagy. A lipid-conjugated form of Atg8/LC3B is localized to the inner and outer membrane of the early-forming structure known as the phagophore. During maturation of autophagosomes, Atg8/LC3 bound to the inner autophagosome membrane remains in situ as the autophagosomes fuse with lysosomes. The nematode Caenorhabditis elegans is thought to conduct a similar process, meaning that tagging the nematode ortholog of Atg8/LC3-known as LGG-1-with a fluorophore has become a widely accepted method to visualize autophagosomes. Under normal growth conditions, GFP-modified LGG-1 displays a diffuse expression pattern throughout a variety of tissues, whereas, when under conditions that induce autophagy, the GFP::LGG-1 tag labels positive punctate structures, and its overall level of expression increases. Here, we present a protocol for using fluorescent reporters of LGG-1 coupled to GFP to monitor autophagosomes in vivo. We also discuss the use of alternative fluorescent markers and the possible utility of the LGG-1 paralog LGG-2.

Guidelines for monitoring autophagy in Caenorhabditis elegans

The cellular recycling process of autophagy has been extensively characterized with standard assays in yeast and mammalian cell lines. In multicellular organisms, numerous external and internal factors differentially affect autophagy activity in specific cell types throughout the stages of organismal ontogeny, adding complexity to the analysis of autophagy in these metazoans. Here we summarize currently available assays for monitoring the autophagic process in the nematode C. elegans. A combination of measuring levels of the lipidated Atg8 ortholog LGG-1, degradation of well-characterized autophagic substrates such as germline P granule components and the SQSTM1/p62 ortholog SQST-1, expression of autophagic genes and electron microscopy analysis of autophagic structures are presently the most informative, yet steady-state, approaches available to assess autophagy levels in C. elegans. We also review how altered autophagy activity affects a variety of biological processes in C. elegans such as L1 survival under starvation conditions, dauer formation, aging, and cell death, as well as neuronal cell specification. Taken together, C. elegans is emerging as a powerful model organism to monitor autophagy while evaluating important physiological roles for autophagy in key developmental events as well as during adulthood.

A LAPF/phafin1-like protein regulates TORC1 and lysosomal membrane permeabilization in response to endoplasmic reticulum membrane stress

The controlled permeabilization of lysosomes and vacuoles may represent an ancient manner of programmed cell death. It is shown that TORC1 is required for lysosomal membrane permeabilization and death of yeast cells that have been exposed to antifungals, and that a novel FYVE-domain protein regulates TORC1 signaling in these conditions.

Lysosomal membrane permeabilization (LMP) is a poorly understood regulator of programmed cell death that involves leakage of luminal lysosomal or vacuolar hydrolases into the cytoplasm. In Saccharomyces cerevisiae, LMP can be induced by antifungals and endoplasmic reticulum stressors when calcineurin also has been inactivated. A genome-wide screen revealed Pib2, a relative of LAPF/phafin1 that regulates LMP in mammals, as a pro-LMP protein in yeast. Pib2 associated with vacuolar and endosomal limiting membranes in unstressed cells in a manner that depended on its FYVE domain and on phosphatidylinositol 3-phosphate (PI(3)P) biosynthesis. Genetic experiments suggest that Pib2 stimulates the activity of TORC1, a vacuole-associated protein kinase that is sensitive to rapamycin, in a pathway parallel to the Ragulator/EGO complex containing the GTPases Gtr1 and Gtr2. A hyperactivating mutation in the catalytic subunit of TORC1 restored LMP to the gtr1∆ and pib2∆ mutants and also prevented the synthetic lethality of the double mutants. These findings show novel roles of PI(3)P and Pib2 in the regulation of TORC1, which in turn promoted LMP and nonapoptotic death of stressed cells. Rapamycin prevented the death of the pathogenic yeast Candida albicans during exposure to fluconazole plus a calcineurin inhibitor, suggesting that TORC1 broadly promotes sensitivity to fungistats in yeasts.

Death Associated Protein Kinase (DAPK) -mediated neurodegenerative mechanisms in nematode excitotoxicity

Background

Excitotoxicity (the toxic overstimulation of neurons by the excitatory transmitter Glutamate) is a central process in widespread neurodegenerative conditions such as brain ischemia and chronic neurological diseases. Many mechanisms have been suggested to mediate excitotoxicity, but their significance across diverse excitotoxic scenarios remains unclear. Death Associated Protein Kinase (DAPK), a critical molecular switch that controls a range of key signaling and cell death pathways, has been suggested to have an important role in excitotoxicity. However, the molecular mechanism by which DAPK exerts its effect is controversial. A few distinct mechanisms have been suggested by single (sometimes contradicting) studies, and a larger array of potential mechanisms is implicated by the extensive interactome of DAPK.

Results

Here we analyze a well-characterized model of excitotoxicity in the nematode C. elegans to show that DAPK is an important mediator of excitotoxic neurodegeneration across a large evolutionary distance. We further show that some proposed mechanisms of DAPK’s action (modulation of synaptic strength, involvement of the DANGER-related protein MAB-21, and autophagy) do not have a major role in nematode excitotoxicity. In contrast, Pin1/PINN-1 (a DAPK interaction-partner and a peptidyl-prolyl isomerase involved in chronic neurodegenerative conditions) suppresses neurodegeneration in our excitotoxicity model.

Conclusions

Our studies highlight the prominence of DAPK and Pin1/PINN-1 as conserved mediators of cell death processes in diverse scenarios of neurodegeneration.

Tools and methods to analyze autophagy in C. elegans

For a long time, autophagy has been mainly studied in yeast or mammalian cell lines, and assays for analyzing autophagy in these models have been well described. More recently, the involvement of autophagy in various physiological functions has been investigated in multicellular organisms. Modification of autophagy flux is involved in developmental processes, resistance to stress conditions, aging, cell death and multiple pathologies. So, the use of animal models is essential to understand these processes in the context of different cell types and during the whole life. For ten years, the nematode Caenorhabditis elegans has emerged as a powerful model to analyze autophagy in physiological or pathological contexts. In this article, we present some of the established approaches and the emerging tools available to monitor and manipulate autophagy in C. elegans, and discuss their advantages and limitations.

Tetraspanin (TSP-17) protects dopaminergic neurons against 6-OHDA-induced neurodegeneration in C. elegans

Parkinson's disease (PD), the second most prevalent neurodegenerative disease after Alzheimer's disease, is linked to the gradual loss of dopaminergic neurons in the substantia nigra. Disease loci causing hereditary forms of PD are known, but most cases are attributable to a combination of genetic and environmental risk factors. Increased incidence of PD is associated with rural living and pesticide exposure, and dopaminergic neurodegeneration can be triggered by neurotoxins such as 6-hydroxydopamine (6-OHDA). In C. elegans, this drug is taken up by the presynaptic dopamine reuptake transporter (DAT-1) and causes selective death of the eight dopaminergic neurons of the adult hermaphrodite. Using a forward genetic approach to find genes that protect against 6-OHDA-mediated neurodegeneration, we identified tsp-17, which encodes a member of the tetraspanin family of membrane proteins. We show that TSP-17 is expressed in dopaminergic neurons and provide genetic, pharmacological and biochemical evidence that it inhibits DAT-1, thus leading to increased 6-OHDA uptake in tsp-17 loss-of-function mutants. TSP-17 also protects against toxicity conferred by excessive intracellular dopamine. We provide genetic and biochemical evidence that TSP-17 acts partly via the DOP-2 dopamine receptor to negatively regulate DAT-1. tsp-17 mutants also have subtle behavioral phenotypes, some of which are conferred by aberrant dopamine signaling. Incubating mutant worms in liquid medium leads to swimming-induced paralysis. In the L1 larval stage, this phenotype is linked to lethality and cannot be rescued by a dop-3 null mutant. In contrast, mild paralysis occurring in the L4 larval stage is suppressed by dop-3, suggesting defects in dopaminergic signaling. In summary, we show that TSP-17 protects against neurodegeneration and has a role in modulating behaviors linked to dopamine signaling.

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons. While hereditary forms are known, most cases are attributable to a combination of genetic and environmental risk factors. In PD models, dopaminergic neurodegeneration can be triggered by neurotoxins such as 6-hydroxydopamine (6-OHDA). This drug, which is taken up by the presynaptic dopamine reuptake transporter (DAT-1), also causes the selective death of C. elegans dopaminergic neurons. We found that TSP-17, a member of the tetraspanin family of membrane proteins, protects dopaminergic neurons from 6-OHDA-induced degeneration. We provide evidence that TSP-17 inhibits the C. elegans dopamine transporter DAT-1, leading to increased neuronal 6-OHDA uptake in tsp-17 mutants. TSP-17 also protects against toxicity conferred by excessive intracellular dopamine. TSP-17 interacts with the DOP-2 dopamine receptor, possibly as part of a pathway that negatively regulates DAT-1. tsp-17 mutants have subtle behavioral phenotypes that are partly conferred by aberrant dopamine signaling. In summary, we have used C. elegans genetics to model key aspects of PD.

Necrosis in C. elegans

To use Caenorhabditis elegans to study the mechanisms for initiation and execution of necrosis, the experimentalist should be familiar with the established models of necrosis in C. elegans and the genetic and molecular tools available. We present a summary of two contrasting models for studying necrosis in C. elegans and outline the methods for scoring necrosis in each. These methods are useful for the study of necrosis under other conditions in C. elegans and for comparative studies both between established and new necrosis models. We also present a list of the genetic and drug tools available for perturbing pathways known to be important for initiation or execution of necrosis and describe their use in C. elegans. Specifically, we outline methods to inhibit autophagy, to perturb calcium homeostasis, and to disrupt lysosomal function in the C. elegans system.

Necrotic cell death and neurodegeneration: The involvement of endocytosis and intracellular trafficking

Necrosis, one of the two main types of cell death, contributes critically in many devastating pathological conditions in human, including stroke, ischemia, trauma and neurodegenerative diseases. However, unlike apoptosis, the molecular mechanisms underlying necrotic cell death and neurodegeneration are poorly understood. Caenorhabditis elegans offers a powerful platform for a thorough and systematic dissection of the molecular basis of necrotic cell death. Similarly to humans, neuronal necrosis can be induced by several well-characterized genetic lesions and by adverse environmental conditions in the nematode. The availability of precisely-defined C. elegans neurodegeneration models provides a unique opportunity for comprehensive delineation of the cellular and molecular mechanisms mediating necrotic cell death. Through genetic dissection of such models, we recently uncovered an unexpected requirement for specific proteins involved in endocytosis and intracellular trafficking, in the execution of necrosis. Moreover, initiation of necrotic cell death is accompanied by a sharp increase in the formation of early and recycling endosomes, which subsequently disintegrate during the final stage of cell death. These findings implicate endocytic and intracellular trafficking processes in the cellular destruction during necrosis. Indeed, endocytosis synergizes with two other essential cellular processes, autophagy and lysosomal proteolysis to facilitate necrotic neurodegeneration. In this commentary, we consider the contribution of endocytosis and intracellular trafficking to cell injury and discuss the crosstalk between these processes and other molecular mechanisms that mediate necrosis.

Dynamic regulation of autophagy and endocytosis for cell remodeling during early development

Fertilization triggers cell remodeling from each gamete to a totipotent zygote. Using Caenorhabditis elegans as a model system, it has been revealed that lysosomal degradation pathways play important roles in cellular remodeling during this developmental transition. Endocytosis and autophagy, two pathways leading to the lysosomes, are highly upregulated during this period. A subset of maternal membrane proteins is selectively endocytosed and degraded in the lysosomes before the first mitotic cell division. Autophagy is also induced shortly after fertilization and executes the degradation of paternally inherited embryonic organelles, e.g. mitochondria and membranous organelles. This mechanism underlies the maternal inheritance of the mitochondrial genome. Autophagy is also required for the removal of extra P-granule (germ granules in C. elegans) components in somatic cells of early embryos and thereby for the specific distribution of P-granules to germ cells. This review focuses on recent advances in the study of the physiological roles and mechanisms of lysosomal pathways during early development in C. elegans.

Autophagy and ageing: insights from invertebrate model organisms

Ageing in diverse species ranging from yeast to humans is associated with the gradual, lifelong accumulation of molecular and cellular damage. Autophagy, a conserved lysosomal, self-destructive process involved in protein and organelle degradation, plays an essential role in both cellular and whole-animal homeostasis. Accumulating evidence now indicates that autophagic degradation declines with age and this gradual reduction of autophagy might have a causative role in the functional deterioration of biological systems during ageing. Indeed, loss of autophagy gene function significantly influences longevity. Moreover, genetic or pharmacological manipulations that extend lifespan in model organisms often activate autophagy. Interestingly, conserved signalling pathways and environmental factors that regulate ageing, such as the insulin/IGF-1 signalling pathway and oxidative stress response pathways converge on autophagy. In this article, we survey recent findings in invertebrates that contribute to advance our understanding of the molecular links between autophagy and the regulation of ageing. In addition, we consider related mechanisms in other organisms and discuss their similarities and idiosyncratic features in a comparative manner.

Diapause formation and downregulation of insulin-like signaling via DAF-16/FOXO delays axonal degeneration and neuronal loss

Axonal degeneration is a key event in the pathogenesis of neurodegenerative conditions. We show here that mec-4d triggered axonal degeneration of Caenorhabditis elegans neurons and mammalian axons share mechanistical similarities, as both are rescued by inhibition of calcium increase, mitochondrial dysfunction, and NMNAT overexpression. We then explore whether reactive oxygen species (ROS) participate in axonal degeneration and neuronal demise. C. elegans dauers have enhanced anti-ROS systems, and dauer mec-4d worms are completely protected from axonal degeneration and neuronal loss. Mechanistically, downregulation of the Insulin/IGF-1-like signaling (IIS) pathway protects neurons from degenerating in a DAF-16/FOXO–dependent manner and is related to superoxide dismutase and catalase-increased expression. Caloric restriction and systemic antioxidant treatment, which decrease oxidative damage, protect C. elegans axons from mec-4d-mediated degeneration and delay Wallerian degeneration in mice. In summary, we show that the IIS pathway is essential in maintaining neuronal homeostasis under pro-degenerative stimuli and identify ROS as a key intermediate of neuronal degeneration in vivo. Since axonal degeneration represents an early pathological event in neurodegeneration, our work identifies potential targets for therapeutic intervention in several conditions characterized by axonal loss and functional impairment.

Axonal degeneration and neuronal loss are currently considered crucial pathological factors in neurodegenerative diseases. Therefore, delaying or blocking these procesess is key for neuroprotection. In this work, we used an in vivo approach combining invertebrate (C. elegans) and vertebrate (mice) model systems to identify a novel and unexpected player in the mechanisms of axonal degeneration. Here, we demonstrate that both neuronal somas and axons degenerate through a step dependent on oxidative stress that can be efficiently delayed by genetic downregulation of a pathway controlling oxidative stress resistance. Impressively, we discovered that diapause formation, which is a state related to hibernating conditions, fully prevents neuronal degeneration. We uncovered new players in the degenerative mechanisms of neurons with relevance for several conditions associated to axonal degeneration, such as multiple sclerosis, motoneuron, and Parkinson diseases, offering novel potential targets for neuroprotection.

Hyperactivation of the mammalian degenerin MDEG promotes caspase-8 activation and apoptosis

Intracellular calcium overload plays a critical role in numerous pathological syndromes such as heart failure, brain ischemia, and stroke. Hyperactivation of the acid-sensing ion channels including degenerin/epithelial amiloride-sensitive sodium (DEG/ENaC) channels has been shown to elevate intracellular calcium and cause subsequent neuronal cell death that is independent of the canonical Egl-1/Ced-9/Ced-4/Ced-3 apoptotic pathway in Caenorhabditis elegans. In mammalian cells, hyperactivation of the DEG/ENaC channels can also lead to cell death, although the underlying mechanism remains largely unknown. Here, we use a tetracycline-inducible system to express the hyperactivation mutant of a mammalian DEG/ENaC channel protein, MDEG G430F, in murine kidney epithelial cells deficient in the key mitochondrial apoptotic proteins Bax and Bak. Remarkably, expression of MDEG G430F induces increased intracellular calcium, reactive oxygen species (ROS) production, and cell death. The MDEG G430F-induced cell death is blocked by the intracellular calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl ester), ROS scavengers, and the caspase inhibitor z-VAD-fmk (where z and fmk are benzyloxycarbonyl and fluoromethyl ketone). Mechanistically, the intracellular calcium overload and ROS increase lead to the inhibition of proteasomal and autophagic protein degradation, which promotes the accumulation of protein aggregates containing caspase-8 and subsequent caspase-8 activation. As protein aggregation upon the inhibition of proteasomal and autophagic degradation pathways is mediated by the ubiquitin-binding protein SQSTM1/p62 and the autophagy-related protein LC3, silencing of p62 and LC3 protects cells from MDEG G430F-induced cell death. Our results uncover a new mechanism of caspase-8-mediated apoptosis induced by intracellular calcium overload that is dependent on the autophagy-related proteins LC3 and p62 upon hyperactivation of DEG/ENaC channels.

Allorecognition triggers autophagy and subsequent necrosis in the cnidarian Hydractinia symbiolongicarpus

Transitory fusion is an allorecognition phenotype displayed by the colonial hydroid Hydractinia symbiolongicarpus when interacting colonies share some, but not all, loci within the allorecognition gene complex (ARC). The phenotype is characterized by an initial fusion followed by subsequent cell death resulting in separation of the two incompatible colonies. We here characterize this cell death process using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and continuous in vivo digital microscopy. These techniques reveal widespread autophagy and subsequent necrosis in both colony and grafted polyp assays. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays and ultrastructural observations revealed no evidence of apoptosis. Pharmacological inhibition of autophagy using 3-methyladenine (3-MA) completely suppressed transitory fusion in vivo in colony assays. Rapamycin did not have a significant effect in the same assays. These results establish the hydroid allorecognition system as a novel model for the study of cell death.

Autophagy in toxicology: cause or consequence?

Research on autophagy and its effects on cell metabolism and physiology has increased dramatically during recent years. Multiple forms of autophagy have been characterized, and many of the genes involved in the regulation of this process have been identified. The importance of autophagy for embryonic development and maintenance of tissue homeostasis in the adult organism has been demonstrated convincingly, and several human diseases have been linked to deficiencies in autophagy. Most often, autophagy serves as a protective mechanism, but persistent activation of autophagy can result in cell death. This is true for many toxic agents. In fact, there are ample examples of cross talk between autophagy and other modes of cell death after exposure to toxicants. However, the relative contribution of autophagy to the overall toxicity of these compounds is not always clear, and further research is needed to clarify the toxicological significance of this process.

Endocytosis and intracellular trafficking contribute to necrotic neurodegeneration in C. elegans

Unlike apoptosis, necrotic cell death is characterized by marked loss of plasma membrane integrity. Leakage of cytoplasmic material to the extracellular space contributes to cell demise, and is the cause of acute inflammatory responses, which typically accompany necrosis. The mechanisms underlying plasma membrane damage during necrotic cell death are not well understood. We report that endocytosis is critically required for the execution of necrosis. Depletion of the key endocytic machinery components dynamin, synaptotagmin and endophilin suppresses necrotic neurodegeneration induced by diverse genetic and environmental insults in C. elegans. We used genetically encoded fluorescent markers to monitor the formation and fate of specific types of endosomes during cell death in vivo. Strikingly, we find that the number of early and recycling endosomes increases sharply and transiently upon initiation of necrosis. Endosomes subsequently coalesce around the nucleus and disintegrate during the final stage of necrosis. Interfering with kinesin-mediated endosome trafficking impedes cell death. Endocytosis synergizes with autophagy and lysosomal proteolytic mechanisms to facilitate necrotic neurodegeneration. These findings demonstrate a prominent role for endocytosis in cellular destruction during neurodegeneration, which is likely conserved in metazoans.

Shared developmental roles and transcriptional control of autophagy and apoptosis in Caenorhabditis elegans

Autophagy is a lysosome-mediated self-degradation process of eukaryotic cells that, depending on the cellular milieu, can either promote survival or act as an alternative mechanism of programmed cell death (PCD) in terminally differentiated cells. Despite the important developmental and medical implications of autophagy and the main form of PCD, apoptosis, orchestration of their regulation remains poorly understood. Here, we show in the nematode Caenorhabditis elegans, that various genetic and pharmacological interventions causing embryonic lethality trigger a massive cell death response that has both autophagic and apoptotic features. The two degradation processes are also redundantly required for normal development and viability in this organism. Furthermore, the CES-2-like basic region leucine-zipper (bZip) transcription factor ATF-2, an upstream modulator of the core apoptotic cell death pathway, is able to directly regulate the expression of at least two key autophagy-related genes, bec-1/ATG6 and lgg-1/ATG8. Thus, the two cell death mechanisms share a common method of transcriptional regulation. Together, these results imply that under certain physiological and pathological conditions, autophagy and apoptosis are co-regulated to ensure the proper morphogenesis and survival of the developing organism. The identification of apoptosis and autophagy as compensatory cellular pathways in C. elegans might help us to understand how dysregulated PCD in humans can lead to diverse pathologies, including cancer, neurodegeneration and diabetes.

Genetic control of necrosis - another type of programmed cell death

Necrosis has been thought to be an accidental or uncontrolled type of cell death rather than programmed. Recent studies from diverse organisms show that necrosis follows a stereotypical series of cellular and molecular events: swelling of organelles, increases in reactive oxygen species and cytoplasmic calcium, a decrease in ATP, activation of calpain and cathepsin proteases, and finally rupture of organelles and plasma membrane. Genetic and chemical manipulations demonstrate that necrosis can be inhibited, indicating that necrosis can indeed be controlled and follows a specific 'program.' This review highlights recent findings from C. elegans, yeast, Dictyostelium, Drosophila, and mammals that collectively provide evidence for conserved mechanisms of necrosis.

What can plant autophagy do for an innate immune response?

Autophagy plays an established role in the execution of senescence, starvation, and stress responses in plants. More recently, an emerging role for autophagy has been discovered during the plant innate immune response. Recent papers have shown autophagy to restrict, and conversely, to also promote programmed cell death (PCD) at the site of pathogen infection. These initial studies have piqued our excitement, but they have also revealed gaps in our understanding of plant autophagy regulation, in our ability to monitor autophagy in plant cells, and in our ability to manipulate autophagic activity. In this review, we present the most pressing questions now facing the field of plant autophagy in general, with specific focus on autophagy as it occurs during a plant-pathogen interaction. To begin to answer these questions, we place recent findings in the context of studies of autophagy and immunity in other systems, and in the context of the mammalian immune response in particular.

Non-apoptotic cell death in Caenorhabditis elegans

The simple nematode worm Caenorhabditis elegans has been instrumental in deciphering the molecular mechanisms underlying apoptosis. Beyond apoptosis, several paradigms of non-apoptotic cell death, either genetically or extrinsically triggered, have also been described in C. elegans. Remarkably, non-apoptotic cell death in worms and pathological cell death in humans share numerous key features and mechanistic aspects. Such commonalities suggest that similarly to apoptosis, non-apoptotic cell death mechanisms are also conserved, and render the worm a useful organism, in which to model and dissect human pathologies. Indeed, the genetic malleability and the sophisticated molecular tools available for C. elegans have contributed decisively to advance our understanding of non-apoptotic cell death. Here, we review the literature on the various types of non-apoptotic cell death in C. elegans and discuss the implications, relevant to pathological conditions in humans.

Protein metabolism and lifespan in Caenorhabditis elegans

Lifespan of the versatile model system Caenorhabditis elegans can be extended by a decrease of insulin/IGF-1 signaling, TOR signaling, mitochondrial function, protein synthesis and dietary intake. The exact molecular mechanisms by which these modulations confer increased life expectancy are yet to be determined but increased stress resistance and improved protein homeostasis seem to be of major importance. In this chapter, we explore the interactions among several genetic pathways and cellular functions involved in lifespan extension and their relation to protein homeostasis in C. elegans. Several of these processes have been associated, however some relevant data are conflicting and further studies are needed to clarify these interactions. In mammals, protein homeostasis is also implicated in several neurodegenerative diseases, many of which can be modeled in C. elegans.

Death-associated protein kinase (DAPK) and signal transduction: fine-tuning of autophagy in Caenorhabditis elegans homeostasis

Autophagy is an evolutionarily conserved lysosomal pathway used to degrade and recycle long-lived proteins and cytoplasmic organelles. This homeostatic ability makes autophagy an important pro-survival mechanism in response to several stresses, such as nutrient starvation, hypoxia, damaged mitochondria, protein aggregation and pathogens. However, several recent studies have highlighted that autophagy also acts as a pro-death mechanism. What on the surface seem like conflicting roles of autophagy may be explained by the fact that the decision between pro-survival and pro-death is determined by the level of activation. A better understanding of autophagy signaling pathways will be helpful to elucidate how the level of autophagy is precisely regulated under different conditions and eventually how the final outcome is decided. In this review, we briefly discuss the pro-survival and pro-death roles of autophagy, and then discuss the mechanism by which autophagy is regulated, mainly focusing on death-associated protein kinase in the nematode Caenorhabditis elegans.

All-you-can-eat: autophagy in neurodegeneration and neuroprotection

Autophagy is the major pathway involved in the degradation of proteins and organelles, cellular remodeling, and survival during nutrient starvation. Autophagosomal dysfunction has been implicated in an increasing number of diseases from cancer to bacterial and viral infections and more recently in neurodegeneration. While a decrease in autophagic activity appears to interfere with protein degradation and possibly organelle turnover, increased autophagy has been shown to facilitate the clearance of aggregation-prone proteins and promote neuronal survival in a number of disease models. On the other hand, too much autophagic activity can be detrimental as well and lead to cell death, suggesting the regulation of autophagy has an important role in cell fate decisions. An increasing number of model systems are now available to study the role of autophagy in the central nervous system and how it might be exploited to treat disease. We will review here the current knowledge of autophagy in the central nervous system and provide an overview of the various models that have been used to study acute and chronic neurodegeneration.

Molecular time-course and the metabolic basis of entry into dauer in Caenorhabditis elegans

When Caenorhabditis elegans senses dauer pheromone (daumone), signaling inadequate growth conditions, it enters the dauer state, which is capable of long-term survival. However, the molecular pathway of dauer entry in C. elegans has remained elusive. To systematically monitor changes in gene expression in dauer paths, we used a DNA microarray containing 22,625 gene probes corresponding to 22,150 unique genes from C. elegans. We employed two different paths: direct exposure to daumone (Path 1) and normal growth media plus liquid culture (Path 2). Our data reveal that entry into dauer is accomplished through the multi-step process, which appears to be compartmentalized in time and according to metabolic flux. That is, a time-course of dauer entry in Path 1 shows that dauer larvae formation begins at post-embryonic stage S4 (48 h) and is complete at S6 (72 h). Our results also suggest the presence of a unique adaptive metabolic control mechanism that requires both stage-specific expression of specific genes and tight regulation of different modes of fuel metabolite utilization to sustain the energy balance in the context of prolonged survival under adverse growth conditions. It is apparent that worms entering dauer stage may rely heavily on carbohydrate-based energy reserves, whereas dauer larvae utilize fat or glyoxylate cycle-based energy sources. We created a comprehensive web-based dauer metabolic database for C. elegans (www.DauerDB.org) that makes it possible to search any gene and compare its relative expression at a specific stage, or evaluate overall patterns of gene expression in both paths. This database can be accessed by the research community and could be widely applicable to other related nematodes as a molecular atlas.

Autophagy protects against hypoxic injury in C. elegans

Macroautophagy has been implicated in a variety of pathological processes. Hypoxic/ischemic cellular injury is one such process in which autophagy has emerged as an important regulator. In general, autophagy is induced after a hypoxic/ischemic insult; however, whether the induction of autophagy promotes cell death or recovery is controversial and appears to be context dependent. We have developed C. elegans as a genetically tractable model for the study of hypoxic cell injury. Both necrosis and apoptosis are mechanisms of cell death following hypoxia in C. elegans. However, the role of autophagy in hypoxic injury in C. elegans has not been examined. Here, we found that RNAi knockdown of the C. elegans homologs of beclin 1/Atg6 (bec-1) and LC3/Atg8 (lgg-1, lgg-2), and mutation of Atg1 (unc-51) decreased animal survival after a severe hypoxic insult. Acute inhibition of autophagy by the type III phosphatidylinositol 3-kinase inhibitors, 3-methyladenine and Wortmannin, also sensitized animals to hypoxic death. Hypoxia-induced neuronal and myocyte injury as well as necrotic cellular morphology were increased by RNAi knockdown of BEC-1. Hypoxia increased the expression of a marker of autophagosomes in a bec-1-dependent manner. Finally, we found that the hypoxia hypersensitive phenotype of bec-1(RNAi) animals could be blocked by loss-of-function mutations in either the apoptosis or necrosis pathway. These results argue that inhibition of autophagy sensitizes C. elegans and its cells to hypoxic injury and that this sensitization is blocked or circumvented when either of the two major cell-death mechanisms is inhibited.

The cell biology of autophagy in metazoans: a developing story

The cell biological phenomenon of autophagy (or ;self-eating') has attracted increasing attention in recent years. In this review, we first address the cell biological functions of autophagy, and then discuss recent insights into the role of autophagy in animal development, particularly in C. elegans, Drosophila and mouse. Work in these and other model systems has also provided evidence for the involvement of autophagy in disease processes, such as neurodegeneration, tumorigenesis, pathogenic infection and aging. Insights gained from investigating the functions of autophagy in normal development should increase our understanding of its roles in human disease and its potential as a target for therapeutic intervention.

Molecular characterization of Pegarn: a Drosophila homolog of UNC-51 kinase

We have isolated and characterized the gene encoding a Drosophila melanogaster homolog of Caenorhabditis elegans UNC-51 (uncoordinated movement-51): Pegarn. Developmental Northern blot shows the Pegarn gene is expressed at all stages of development. The protein is detected throughout the Drosophila third instar larval central nervous system (CNS) in axons projecting out from the ventral ganglion and in the optic anlagen of the optic lobe. Heterozygous Pegarn mutant embryos show defects in larval axonal neuronal patterning, but survive to adulthood. Homozygous mutants have an even more deformed pattern of neuronal development and do not survive through the larval stages. The data from this research suggest the critical roles of Pegarn in CNS and PNS axonal formation in Drosophila melanogaster and indicates its similar role in other multicellular species.