ASNA-1 positively regulates insulin secretion in C. elegans and mammalian cells

SPP-5 affects larval arrest via insulin signaling pathway in Caenorhabditis elegans

Diapause is an endocrine-mediated metabolic and growth arrest state in response to unfavorable external environments. The nematode Caenorhabditis elegans can enter diapause/arrest during embryonic, larval, or adult stages when subjected to detrimental external environments. Larval stage 1 (L1) arrest happens when animals hatch without food. Previous work has shown that the insulin pathway plays a prominent role in regulating L1 arrest. However, the downstream signal molecular mechanisms and biomarkers are still missing. In this study, we showed that SaPosin-like Protein family member SPP-5 is significantly upregulated during L1 arrest, suggesting that it could act as an L1 arrest biomarker. Using RNA interference we demonstrated that spp-5  knockdown accelerated larval development, while the overexpression resulted in L1 arrest. Consistently, SPP-5 level was significantly up-regulated in the L1 arrest daf-2(e1370) mutants, and spp-5(RNAi) suppressed the daf-2(e1370) induced L1 arrest. These results suggest that SPP-5 can serve as an L1 arrest biomarker and promote the arrest probably via the insulin signaling pathway.

C. elegans insulin-like peptides

Insulin-like peptides are a group of hormones crucial for regulating metabolism, growth, and development in animals. Invertebrates, such as C. elegans, have been instrumental in understanding the molecular mechanisms of insulin-like peptides. Here, we review the 40 insulin-like peptide genes encoded in the C. elegans genome. Despite the large number, there is only one C. elegans insulin-like peptide receptor, called DAF-2. The insulin and insulin-like growth factor signaling (IIS) pathway is evolutionarily conserved from worms to humans. Thus C. elegans provides an excellent model to understand how these insulin-like peptides function. C. elegans is unique in that it possesses insulin-like peptides that have antagonistic properties, unlike all human insulin-like peptides, which are agonists. This review provides an overview of the current literature on C. elegans insulin-like peptide structures, processing, tissue localization, and regulation. We will also provide examples of insulin-like peptide signaling in C. elegans during growth, development, germline development, learning/memory, and longevity.

The neurohormone tyramine stimulates the secretion of an Insulin-Like Peptide from the intestine to modulate the systemic stress response in C. elegans

The DAF-2/insulin/insulin-like growth factor signaling (IIS) pathway plays an evolutionarily conserved role in regulating reproductive development, lifespan, and stress resistance. In C. elegans , DAF-2/IIS signaling is modulated by an extensive array of insulin-like peptides (ILPs) with diverse spatial and temporal expression patterns. However, the release dynamics and specific functions of these ILPs in adapting to different environmental conditions remain poorly understood. Here, we show that the ILP, INS-3, plays a crucial role in modulating the response to different types of stressors in C. elegans . ins-3 mutants display increased resistance to both heat and oxidative stress; however, under favorable conditions, this advantage is countered by slower reproductive development. ins-3 expression in both neurons and the intestine is downregulated in response to environmental stressors. Conversely, the neurohormone tyramine, which is released during the acute flight response, triggers an upregulation in ins-3 expression. Moreover, we found that tyramine negatively impacts environmental stress resistance by stimulating the release of INS-3 from the intestine. The subsequent release of INS-3 systemically activates the DAF-2 pathway, resulting in the inhibition of cytoprotective mechanisms mediated by DAF-16/FOXO and HSF-1. These studies offer mechanistic insights into the brain-gut communication pathway that weighs adaptive strategies to respond to acute and long-term stress scenarios.

A homeostatic gut-to-brain insulin antagonist restrains neuronally stimulated fat loss

In C. elegans mechanisms by which peripheral organs relay internal state information to the nervous system remain unknown, although strong evidence suggests that such signals do exist. Here we report the discovery of a peptide of the ancestral insulin superfamily called INS-7 that functions as an enteroendocrine peptide and is secreted from specialized cells of the intestine. INS-7 secretion increases during fasting, and acts as a bona fide gut-to-brain homeostatic signal that attenuates neuronally induced fat loss during food shortage. INS-7 functions as an antagonist at the canonical DAF-2 receptor in the nervous system, and phylogenetic analysis suggests that INS-7 bears greater resemblance to members of the broad insulin/relaxin superfamily than to conventional mammalian insulin and IGF peptides. The discovery of an endogenous insulin antagonist secreted by specialized intestinal cell with enteroendocrine functions suggests that much remains to be learned about the intestine and its role in directing neuronal functions.

The C. elegans truncated insulin receptor DAF-2B regulates survival of L1 arrested larvae

We have previously characterized a truncated isoform of the C. elegans insulin-like receptor, DAF-2B, which retains the ligand binding domain but cannot transduce a signal due to the absence of the intracellular signaling domain. DAF-2B modifies insulin / insulin-like growth factor signaling-dependent processes, such as dauer formation and lifespan, by sequestering insulin-like peptides (ILP) and preventing signaling through full length DAF-2 receptors. Here we show that DAF-2B is also important for starvation resistance, as genetic loss of daf-2b reduces survival in arrested first stage larvae (L1). Under fed conditions, we observe daf-2b splicing capacity in both the intestine and the hypodermis, but in starved L1s this becomes predominantly hypodermal. Using a novel splicing reporter system, we observe an increase in the ratio of truncated to full length insulin receptor splicing capacity in starved L1 larvae compared with fed, that may indicate a decrease in whole body insulin responsiveness. Consistent with this, overexpression of DAF-2B from the hypodermis, but not the intestine, confers increased survival to L1 animals under starvation conditions. Our findings demonstrate that the truncated insulin receptor DAF-2B is involved in the response to L1 starvation and promotes survival when expressed from the hypodermis.

The G protein-coupled receptor neuropeptide receptor-15 modulates larval development via the transforming growth factor-β DAF-7 protein in Caenorhabditis elegans

G protein-coupled receptors (GPCRs) are a major class of membrane receptors that modulate a wide range of physiological functions. These receptors transmit extracellular signals, including secreted bioactive peptides, to intracellular signaling pathways. The nematode Caenorhabditis elegans has FMRFamide-like peptides, which are one of the most diverse neuropeptide families, some of which modulate larval development through GPCRs. In this study, we identified the GPCR neuropeptide receptor (NPR)-15, which modulates C. elegans larval development. Our molecular genetic analyses indicated the following: 1) NPR-15 mainly functions in ASI neurons, which predominantly regulate larval development, 2) NPR-15 interacts with GPA-4, a C. elegans Gα subunit, and 3) NPR-15, along with GPA-4, modulates larval development by regulating the production and secretion of the transforming growth factor-β (TGF-β)-like protein DAF-7. The present study is the first report to demonstrate the importance of a GPCR to the direct regulation of a TGF-β-like protein.

A proinsulin-dependent interaction between ENPL-1 and ASNA-1 in neurons is required to maintain insulin secretion in C. elegans

Neuropeptides, including insulin, are important regulators of physiological functions of the organisms. Trafficking through the Golgi is crucial for the regulation of secretion of insulin-like peptides. ASNA-1 (TRC40) and ENPL-1 (GRP94) are conserved insulin secretion regulators in Caenorhabditis elegans (and mammals), and mouse Grp94 mutants display type 2 diabetes. ENPL-1/GRP94 binds proinsulin and regulates proinsulin levels in C. elegans and mammalian cells. Here, we have found that ASNA-1 and ENPL-1 cooperate to regulate insulin secretion in worms via a physical interaction that is independent of the insulin-binding site of ENPL-1. The interaction occurs in DAF-28/insulin-expressing neurons and is sensitive to changes in DAF-28 pro-peptide levels. Consistently, ASNA-1 acted in neurons to promote DAF-28/insulin secretion. The chaperone form of ASNA-1 was likely the interaction partner of ENPL-1. Loss of asna-1 disrupted Golgi trafficking pathways. ASNA-1 localization to the Golgi was affected in enpl-1 mutants and ENPL-1 overexpression partially bypassed the ASNA-1 requirement. Taken together, we find a functional interaction between ENPL-1 and ASNA-1 that is necessary to maintain proper insulin secretion in C. elegans and provides insights into how their loss might cause diabetes in mammals.

The FMRFamide-like peptide FLP-1 modulates larval development by regulating the production and secretion of the insulin-like peptide DAF-28 in Caenorhabditis elegans

The FMRFamide-like peptides (FLPs) are conserved in both free-living and parasitic nematodes. This molecular genetic study verified the relevance of the flp-1 gene, which is conserved in many nematode species, to the larval development of the free-living soil nematode Caenorhabditis elegans. Using C. elegans as a model, we found that: (1) FLP-1 suppressed larval development, resulting in diapause; (2) the secretion of FLP-1, which is produced in AVK head neurons, was suppressed by the presence of food (Escherichia coli) as an environmental factor to continue larval development; (3) the FLP-1 reduced the production and secretion of DAF-28, which is produced in ASI head neurons and is the predominant insulin-like peptide (INS) present. FLP-1 is conserved in many species of plant-parasitic root-knot nematodes that cause severe damage to crops. Therefore, our findings may provide insight into the development of new nematicides that can disturb their infection and development.

Identification of C. elegans ASNA-1 domains and tissue requirements that differentially influence platinum sensitivity and growth control

ASNA1 plays an essential role in cisplatin chemotherapy response, type 2 diabetes, and heart disease. It is also an important biomarker in the treatment response of many diseases. Biochemically, ASNA1 has two mutually exclusive redox-modulated roles: a tail-anchored protein (TAP) targeting function in the reduced state and a holdase/chaperone function in the oxidized state. Assigning biochemical roles of mammalian ASNA1 to biomedical functions is crucial for successful therapy development. Our previous work showed the relevance of the C. elegans ASNA-1 homolog in modeling cisplatin response and insulin secretion. Here we analyzed two-point mutants in highly conserved residues in C. elegans ASNA-1 and determined their importance in separating the cisplatin response function from its roles in insulin secretion. asna-1(ΔHis164) and asna-1(A63V) point mutants, which both preferentially exist in the oxidized state, displayed cisplatin sensitivity phenotype as well as TAP insertion defect but not an insulin secretion defect. Further, using targeted depletion we analyzed the tissue requirements of asna-1 for C. elegans growth and development. Somatic depletion of ASNA-1 as well as simultaneous depletion of ASNA-1 in neurons and intestines resulted in an L1 arrest. We concluded that, targeting single residues in ASNA-1 affecting Switch I/Switch II domain function, in comparison to complete knockdown counteracted cisplatin resistance without jeopardizing other important biological functions. Taken together, our study shows that effects on health caused by ASNA1 mutations can have different biochemical bases.

The FMRFamide-like peptide FLP-2 is involved in the modulation of larval development and adult lifespan by regulating the secretion of the insulin-like peptide INS-35 in Caenorhabditis elegans

In the animal kingdom, neuropeptides regulate diverse physiological functions. In invertebrates, FMRFamide and its related peptides, a family of neuropeptides, play an important role as neurotransmitters. The FMRFamide-like peptides (FLPs) are one of the most diverse neuropeptide families and are conserved in nematodes. Our screen for flp genes of the free-living soil nematode Caenorhabditis elegans revealed that the flp-2 gene is involved in larval development. The gene is also conserved in plant-parasitic root-knot nematodes. Our molecular genetic analyses of the C. elegans flp-2 gene demonstrated as follows: 1) the production and secretion of FLP-2, produced in the head neurons, are controlled by environmental factors (growth density and food); 2) the FLP-2 is involved in not only larval development but also adult lifespan by regulating the secretion of one of the insulin-like peptides INS-35, produced in the intestine. These findings provide new insight into the development of new nematicides.

A gliclazide complex based on palladium towards Alzheimer's disease: promising protective activity against Aβ-induced toxicity in C. elegans

A new palladium coordination compound based on gliclazide with the chemical formula [Pd(glz)₂] (where glz = gliclazide) has been synthesized and characterised. The structural characterization reveals that this material consists of mononuclear units formed by a Pd²⁺ ion coordinated to two molecules of the glz ligand, in which palladium ions exhibit a distorted plane-square coordination sphere. This novel material behaves like a good and selective inhibitor of butyrylcholinesterase, one of the most relevant therapeutic targets against Alzheimer's disease. Analysis of the enzyme kinetics showed a mixed mode of inhibition, the title compound being capable of interacting with both the free enzyme and the enzyme-substrate complex. Finally, the palladium compound shows promising protective activity against Aβ-induced toxicity in the Caenorhabditis elegans model, which has never been reported.

Alternative redox forms of ASNA-1 separate insulin signaling from tail-anchored protein targeting and cisplatin resistance in C. elegans

Cisplatin is a frontline cancer therapeutic, but intrinsic or acquired resistance is common. We previously showed that cisplatin sensitivity can be achieved by inactivation of ASNA-1/TRC40 in mammalian cancer cells and in Caenorhabditis elegans. ASNA-1 has two more conserved functions: in promoting tail-anchored protein (TAP) targeting to the endoplasmic reticulum membrane and in promoting insulin secretion. However, the relation between its different functions has remained unknown. Here, we show that ASNA-1 exists in two redox states that promote TAP-targeting and insulin secretion separately. The reduced state is the one required for cisplatin resistance: an ASNA-1 point mutant, in which the protein preferentially was found in the oxidized state, was sensitive to cisplatin and defective for TAP targeting but had no insulin secretion defect. The same was true for mutants in wrb-1, which we identify as the C. elegans homolog of WRB, the ASNA1/TRC40 receptor. Finally, we uncover a previously unknown action of cisplatin induced reactive oxygen species: cisplatin induced ROS drives ASNA-1 into the oxidized form, and selectively prevents an ASNA-1-dependent TAP substrate from reaching the endoplasmic reticulum. Our work suggests that ASNA-1 acts as a redox-sensitive target for cisplatin cytotoxicity and that cisplatin resistance is likely mediated by ASNA-1-dependent TAP substrates. Treatments that promote an oxidizing tumor environment should be explored as possible means to combat cisplatin resistance.

Molecular characterization and analysis of the ATPase ASNA1 homolog gene of Eimeria tenella in a drug sensitive strain and drug resistant strains

The widespread use of drugs has exacerbated the resistance of Eimeria tenalla to anti-coccidial drugs. Using RNA-seq, we previously found the ATPase ASNA1 homolog of E. tenella (EtASNA1) was differentially expressed in resistant strains and drug sensitive (DS) strain. In our study, we used western blotting and quantitative real-time PCR (qRT-PCR) to analyze the translational and transcriptional levels of EtASNA1 in a diclazuril-resistant (DZR) strain, maduramicin-resistant (MRR) strain, salinomycin-resistant (SMR) strain, and DS strain and found EtASNA1 was highly expressed in three drug-resistant strains. The qRT-PCR and western blotting results also showed that the expression levels of EtASNA1 increased with increasing drug concentration, and the transcription levels of the DZR strains isolated from the field were higher than those of the DS strain. In addition, we used in vivo and in vitro tests to analyze the changes of EtASNA1 expression after DZR, MRR, and DS strain infections in chickens, and in vitro inoculation of DF-1 cells in the presence of drugs. The addition of drugs caused expression to be upregulated. The results of qRT-PCR and western blotting also showed that the expression levels of EtASNA1 in second-generation merozoites (SM) and unsporulated oocysts (UO) were significantly higher than those in the other two developmental stages. The immunofluorescence localization of EtASNA1 indicated that the protein was distributed throughout the sporozoites (SZ) and SM, except for the refractile bodies of SZ. In vitro inhibition experiments showed that anti-EtASNA1 antibody incubation significantly inhibited SZ invasion of DF-1 cells. The above results showed that EtASNA1 may be related to host cell invasion of E. tenella and may be involved in the development of E. tenella resistance to some drugs.

Rhabditophanes diutinus a parthenogenetic clade IV nematode with dauer larvae

Comparative studies using non-parasitic model species such as Caenorhabditis elegans, have been very helpful in investigating the basic biology and evolution of parasitic nematodes. However, as phylogenetic distance increases, these comparisons become more difficult, particularly when outside of the nematode clade to which C. elegans belongs (V). One of the reasons C. elegans has nevertheless been used for these comparisons, is that closely related well characterized free-living species that can serve as models for parasites of interest are frequently not available. The Clade IV parasitic nematodes Strongyloides are of great research interest due to their life cycle and other unique biological features, as well as their medical and veterinary importance. Rhabditophanes, a closely related free-living genus, forms part of the Strongyloidoidea nematode superfamily. Rhabditophanes diutinus (= R. sp. KR3021) was included in the recent comparative genomic analysis of the Strongyloididae, providing some insight into the genomic nature of parasitism. However, very little is known about this species, limiting its usefulness as a research model. Here we provide a species description, name the species as R. diutinus and investigate its life cycle and subsequently gene expression in multiple life stages. We identified two previously unreported starvation induced life stages: dauer larvae and arrested J2 (J2A) larvae. The dauer larvae are morphologically similar to and are the same developmental stage as dauers in C. elegans and infective larvae in Strongyloides. As in C. elegans and Strongyloides, dauer formation is inhibited by treatment with dafachronic acid, indicating some genetic control mechanisms are conserved. Similarly, the expression patterns of putative dauer/infective larva control genes resemble each other, in particular between R. diutinus and Strongyloides spp. These findings illustrate and increase the usefulness of R. diutinus as a non-parasitic, easy to work with model species for the Strongyloididae for studying the evolution of parasitism as well as many aspects of the biology of Strongyloides spp, in particular the formation of infective larvae.

Parasitic worms are an issue of great medical, veterinary, agricultural and economic importance, yet little is known about how worms become parasites. Comparative studies with non-parasitic model species like C. elegans have been useful, however, this usefulness decreases as the evolutionary distance between the species increases. One way to combat this is by having more well-studied closely related species to parasites of interest. To address this, we provide information about Rhabditophanes diutinus, a free-living nematode that is part of the same superfamily as the medically and veterinary important Strongyloides parasites. We provide analysis on its life cycle, in particular on two starvation induced life stages, along with gene expression data. Overall, this important information illustrates and improves the use of R. diutinus, as a non-parasitic model species for studying parasite evolution and basic biology within Strongyloides.

Starvation Responses Throughout the Caenorhabditiselegans Life Cycle

Caenorhabditis elegans survives on ephemeral food sources in the wild, and the species has a variety of adaptive responses to starvation. These features of its life history make the worm a powerful model for studying developmental, behavioral, and metabolic starvation responses. Starvation resistance is fundamental to life in the wild, and it is relevant to aging and common diseases such as cancer and diabetes. Worms respond to acute starvation at different times in the life cycle by arresting development and altering gene expression and metabolism. They also anticipate starvation during early larval development, engaging an alternative developmental program resulting in dauer diapause. By arresting development, these responses postpone growth and reproduction until feeding resumes. A common set of signaling pathways mediates systemic regulation of development in each context but with important distinctions. Several aspects of behavior, including feeding, foraging, taxis, egg laying, sleep, and associative learning, are also affected by starvation. A variety of conserved signaling, gene regulatory, and metabolic mechanisms support adaptation to starvation. Early life starvation can have persistent effects on adults and their descendants. With its short generation time, C. elegans is an ideal model for studying maternal provisioning, transgenerational epigenetic inheritance, and developmental origins of adult health and disease in humans. This review provides a comprehensive overview of starvation responses throughout the C. elegans life cycle.

ENPL-1, the Caenorhabditis elegans homolog of GRP94, promotes insulin secretion via regulation of proinsulin processing and maturation

Insulin/IGF signaling in Caenorhabditis elegans is crucial for proper development of the dauer larva and growth control. Mutants disturbing insulin processing, secretion and downstream signaling perturb this process and have helped identify genes that affect progression of type 2 diabetes. Insulin maturation is required for its proper secretion by pancreatic β cells. The role of the endoplasmic reticulum (ER) chaperones in insulin processing and secretion needs further study. We show that the C. elegans ER chaperone ENPL-1/GRP94 (HSP90B1), acts in dauer development by promoting insulin secretion and signaling. Processing of a proinsulin likely involves binding between the two proteins via a specific domain. We show that, in enpl-1 mutants, an unprocessed insulin exits the ER lumen and is found in dense core vesicles, but is not secreted. The high ER stress in enpl-1 mutants does not cause the secretion defect. Importantly, increased ENPL-1 levels result in increased secretion. Taken together, our work indicates that ENPL-1 operates at the level of insulin availability and is an essential modulator of insulin processing and secretion.

Glucose-Regulated Protein 94 (GRP94): A Novel Regulator of Insulin-Like Growth Factor Production

Mammals have two insulin-like growth factors (IGF) that are key mediators of somatic growth, tissue differentiation, and cellular responses to stress. Thus, the mechanisms that regulate the bioavailability of IGFs are important in both normal and aberrant development. IGF-I levels are primarily controlled via the growth hormone-IGF axis, in response to nutritional status, and also reflect metabolic diseases and cancer. One mechanism that controls IGF bioavailablity is the binding of circulating IGF to a number of binding proteins that keep IGF in a stable, but receptor non-binding state. However, even before IGF is released from the cells that produce it, it undergoes an obligatory association with a ubiquitous chaperone protein, GRP94. This binding is required for secretion of a properly folded, mature IGF. This chapter reviews the known aspects of the interaction and highlights the specificity issues yet to be determined. The IGF–GRP94 interaction provides a potential novel mechanism of idiopathic short stature, involving the obligatory chaperone and not just IGF gene expression. It also provides a novel target for cancer treatment, as GRP94 activity can be either inhibited or enhanced.

An alternatively spliced, non-signaling insulin receptor modulates insulin sensitivity via insulin peptide sequestration in C. elegans

In the nematode C. elegans, insulin signaling regulates development and aging in response to the secretion of numerous insulin peptides. Here, we describe a novel, non-signaling isoform of the nematode insulin receptor (IR), DAF-2B, that modulates insulin signaling by sequestration of insulin peptides. DAF-2B arises via alternative splicing and retains the extracellular ligand binding domain but lacks the intracellular signaling domain. A daf-2b splicing reporter revealed active regulation of this transcript through development, particularly in the dauer larva, a diapause stage associated with longevity. CRISPR knock-in of mScarlet into the daf-2b genomic locus confirmed that DAF-2B is expressed in vivo and is likely secreted. Genetic studies indicate that DAF-2B influences dauer entry, dauer recovery and adult lifespan by altering insulin sensitivity according to the prevailing insulin milieu. Thus, in C. elegans alternative splicing at the daf-2 locus generates a truncated IR that fine-tunes insulin signaling in response to the environment.

Biallelic Variants in ASNA1, Encoding a Cytosolic Targeting Factor of Tail-Anchored Proteins, Cause Rapidly Progressive Pediatric Cardiomyopathy

Supplemental Digital Content is available in the text.

Pediatric cardiomyopathies are a clinically and genetically heterogeneous group of heart muscle disorders associated with high morbidity and mortality. Although knowledge of the genetic basis of pediatric cardiomyopathy has improved considerably, the underlying cause remains elusive in a substantial proportion of cases.

Endoplasmic Reticulum Chaperone Glucose-Regulated Protein 94 Is Essential for Proinsulin Handling

Although endoplasmic reticulum (ER) chaperone binding to mutant proinsulin has been reported, the role of protein chaperones in the handling of wild-type proinsulin is underinvestigated. Here, we have explored the importance of glucose-regulated protein 94 (GRP94), a prominent ER chaperone known to fold insulin-like growth factors, in proinsulin handling within β-cells. We found that GRP94 coimmunoprecipitated with proinsulin and that inhibition of GRP94 function and/or expression reduced glucose-dependent insulin secretion, shortened proinsulin half-life, and lowered intracellular proinsulin and insulin levels. This phenotype was accompanied by post-ER proinsulin misprocessing and higher numbers of enlarged insulin granules that contained amorphic material with reduced immunogold staining for mature insulin. Insulin granule exocytosis was accelerated twofold, but the secreted insulin had diminished bioactivity. Moreover, GRP94 knockdown or knockout in β-cells selectively activated protein kinase R-like endoplasmic reticulum kinase (PERK), without increasing apoptosis levels. Finally, GRP94 mRNA was overexpressed in islets from patients with type 2 diabetes. We conclude that GRP94 is a chaperone crucial for proinsulin handling and insulin secretion.

Pervasive Positive and Negative Feedback Regulation of Insulin-Like Signaling in Caenorhabditis elegans

The Caenorhabditis elegans insulin-like signaling network supports homeostasis and developmental plasticity. The genome encodes 40 insulin-like peptides and one known receptor. Feedback regulation has been reported, but the extent of feedback and its effect on signaling dynamics in response to changes in nutrient availability has not been determined. We measured messenger RNA expression for each insulin-like peptide, the receptor daf-2, components of the PI3K pathway, and its transcriptional effectors daf-16/FoxO and skn-1/Nrf at high temporal resolution during transition from a starved, quiescent state to a fed, growing state in wild type and mutants affecting daf-2/InsR and daf-16/FoxO. We also analyzed the effect of temperature on insulin-like gene expression. We found that most PI3K pathway components and insulin-like peptides are affected by signaling activity, revealing pervasive positive and negative feedback regulation at intra- and intercellular levels. Reporter gene analysis demonstrated that the daf-2/InsR agonist daf-28 positively regulates its own transcription and that the putative agonist ins-6 cross-regulates DAF-28 protein expression through feedback. Our results show that positive and negative feedback regulation of insulin-like signaling is widespread, giving rise to an organismal FoxO-to-FoxO signaling network that supports homeostasis during fluctuations in nutrient availability.

Pattern Formation in the Longevity-Related Expression of Heat Shock Protein-16.2 in Caenorhabditis elegans

Aging in Caenorhabditis elegans is controlled, in part, by the insulin-like signaling and heat shock response pathways. Following thermal stress, expression levels of small heat shock protein-16.2 show a spatial patterning across the 20 intestinal cells that reside along the length of the worm. Here, we present a hypothesized mechanism that could lead to this patterned response and develop a mathematical model of this system to test our hypothesis. We propose that the patterned expression of heat shock protein is caused by a diffusion-driven instability within the pseudocoelom, or fluid-filled cavity, that borders the intestinal cells in C. elegans. This instability is due to the interactions between two classes of insulin-like peptides that serve antagonistic roles. We examine output from the developed model and compare it to experimental data on heat shock protein expression. Given biologically bounded parameters, the model presented is capable of producing patterns similar to what is observed experimentally and provides a first step in mathematically modeling aging-related mechanisms in C. elegans.

Sphingosine Kinase Regulates Neuropeptide Secretion During the Oxidative Stress-Response Through Intertissue Signaling

The Nrf2 antioxidant transcription factor promotes redox homeostasis in part through reciprocal signaling between neurons and neighboring cells, but the signals involved in intertissue signaling in response to Nrf2 activation are not well defined. In Caenorhabditis elegans, activation of SKN-1/Nrf2 in the intestine negatively regulates neuropeptide secretion from motor neurons. Here, we show that sphingosine kinase (SPHK-1) functions downstream of SKN-1/Nrf2 in the intestine to regulate neuropeptide secretion from motor neurons during the oxidative stress response in C. elegans hermaphrodites. SPHK-1 localizes to mitochondria in the intestine and SPHK-1 mitochondrial localization and kinase activity are essential for its function in regulating motor neuron function. SPHK-1 is recruited to mitochondria from cytosolic pools and its mitochondrial abundance is negatively regulated by acute or chronic SKN-1 activation. Finally, the regulation of motor function by SKN-1 requires the activation of the p38 MAPK cascade in the intestine and occurs through controlling the biogenesis or maturation of dense core vesicles in motor neurons. These findings show that the inhibition of SPHK-1 in the intestine by SKN-1 negatively regulates neuropeptide secretion from motor neurons, revealing a new mechanism by which SPHK-1 signaling mediates its effects on neuronal function in response to oxidative stress.SIGNIFICANCE STATEMENT Neurons are highly susceptible to damage by oxidative stress, yet have limited capacity to activate the SKN-1/Nrf2 oxidative stress response, relying instead on astrocytes to provide redox homeostasis. In Caenorhabditis elegans, intertissue signaling from the intestine plays a key role in regulating neuronal function during the oxidative stress response. Here, through a combination of genetic, behavioral, and fluorescent imaging approaches, we found that sphingosine kinase functions in the SKN-1/Nrf2 pathway in the intestine to regulate neuropeptide biogenesis and secretion in motor neurons. These results implicate sphingolipid signaling as a new component of the oxidative stress response and suggest that C. elegans may be a genetically tractable model to study non-cell-autonomous oxidative stress signaling to neurons.

C. elegans Locomotion: Finding Balance in Imbalance

The excitation-inhibition (E-I) imbalance in neural circuits represents a hallmark of several neuropsychiatric disorders. The tiny nematode Caenorhabditis elegans has emerged as an excellent system to study the molecular mechanisms underlying this imbalance in neuronal circuits. The C. elegans body wall muscles receive inputs from both excitatory cholinergic and inhibitory GABAergic motor neurons at neuromuscular junctions (NMJ), making it an excellent model for studying the genetic and molecular mechanisms required for maintaining E-I balance at the NMJ. The cholinergic neurons form dyadic synapses wherein they synapse onto ipsilateral body wall muscles allowing for muscle contraction as well as onto GABAergic motor neurons that in turn synapse on the contralateral body wall muscles causing muscle relaxation. An alternating wave of contraction and relaxation mediated by excitatory and inhibitory signals maintains locomotion in C. elegans. This locomotory behavior requires an intricate balance between the excitatory cholinergic signaling and the inhibitory GABAergic signaling mechanisms.Studies on the C. elegans NMJ have provided insights into several molecular mechanisms that could regulate this balance in neural circuits. This review provides a discussion on multiple genetic factors including neuropeptides and their receptors, cell adhesion molecules, and other molecular pathways that have been associated with maintaining E-I balance in C. elegans motor circuits. Further, it also discusses the implications of these studies that could help us in understanding the role of E-I balance in mammalian neural circuits and how changes in this balance could give rise to brain disorders.

The endoplasmic reticulum: A hub of protein quality control in health and disease

One third of the eukaryotic proteome is synthesized at the endoplasmic reticulum (ER), whose unique properties provide a folding environment substantially different from the cytosol. A healthy, balanced proteome in the ER is maintained by a network of factors referred to as the ER quality control (ERQC) machinery. This network consists of various protein folding chaperones and modifying enzymes, and is regulated by stress response pathways that prevent the build-up as well as the secretion of potentially toxic and aggregation-prone misfolded protein species. Here, we describe the components of the ERQC machinery, investigate their response to different forms of stress, and discuss the consequences of ERQC break-down.

A Bystander Mechanism Explains the Specific Phenotype of a Broadly Expressed Misfolded Protein

Misfolded proteins in transgenic models of conformational diseases interfere with proteostasis machinery and compromise the function of many structurally and functionally unrelated metastable proteins. This collateral damage to cellular proteins has been termed 'bystander' mechanism. How a single misfolded protein overwhelms the proteostasis, and how broadly-expressed mutant proteins cause cell type-selective phenotypes in disease are open questions. We tested the gain-of-function mechanism of a R37C folding mutation in an endogenous IGF-like C.elegans protein DAF-28. DAF-28(R37C) is broadly expressed, but only causes dysfunction in one specific neuron, ASI, leading to a distinct developmental phenotype. We find that this phenotype is caused by selective disruption of normal biogenesis of an unrelated endogenous protein, DAF-7/TGF-β. The combined deficiency of DAF-28 and DAF-7 biogenesis, but not of DAF-28 alone, explains the gain-of-function phenotype—deficient pro-growth signaling by the ASI neuron. Using functional, fluorescently-tagged protein, we find that, in animals with mutant DAF-28/IGF, the wild-type DAF-7/TGF-β is mislocalized to and accumulates in the proximal axon of the ASI neuron. Activation of two different branches of the unfolded protein response can modulate both the developmental phenotype and DAF-7 mislocalization in DAF-28(R37C) animals, but appear to act through divergent mechanisms. Our finding that bystander targeting of TGF-β explains the phenotype caused by a folding mutation in an IGF-like protein suggests that, in conformational diseases, bystander misfolding may specify the distinct phenotypes caused by different folding mutations.

Correct protein folding and localization ensures cellular health. Dedicated proteostasis machinery assists in protein folding and protects against misfolding. Yet, folding mutations cause many conformational diseases, including neurodegenerative diseases and certain types of diabetes and cancer. Misfolded disease-related proteins interfere with proteostasis machinery, causing global misfolding in the cell. How this global mechanism leads to the specific phenotypes in different conformational diseases is unknown. Moreover, mutant misfolded proteins that only damage specific cell-types in disease often lose this cell-selectivity when overexpressed in genetic models. Here we use an endogenous folding mutation in a C. elegans secreted IGF-like protein, DAF-28, that causes dysfunction in one neuron and a specific developmental phenotype, despite expression in many cells. We find that misfolding of mutant DAF-28 causes mislocalization and defective function of another, wild-type growth factor that is expressed in the affected neuron, the TGF-β protein DAF-7. Decrease in DAF-7 function explains the observed developmental phenotype. This targeting of the bystander protein DAF-7 by the misfolded mutant DAF-28 is specific and is not caused by the global stress. Our data suggest that rather than global effects, it is the selective targeting of specific susceptible bystander proteins that defines the specific phenotypes in conformational diseases.

Asna1/TRC40 that mediates membrane insertion of tail-anchored proteins is required for efficient release of Herpes simplex virus 1 virions

BACKGROUND

Herpes simplex virus type 1 (HSV1), a member of the alphaherpesvirinae, can cause recurrent facial lesions and encephalitis. Two membrane envelopment processes, one at the inner nuclear membrane and a second at cytoplasmic membranes are crucial for a productive viral infection. Depending on the subfamily, herpesviruses encode more than 11 different transmembrane proteins including members of the tail-anchored protein family. HSV1 encodes three tail-anchored proteins pUL34, pUL56 and pUS9 characterized by a single hydrophobic region positioned at their C-terminal end that needs to be released from the ribosome prior to posttranslational membrane insertion. Asna1/TRC40 is an ATPase that targets tail-anchored proteins to the endoplasmic reticulum in a receptor-dependent manner. Cell biological data point to a critical and general role of Asna1/TRC40 in tail-anchored protein biogenesis. With this study, we aimed to determine the importance of the tail-anchored insertion machinery for HSV1 infection.

METHODS

To determine protein-protein interactions, the yeast-two hybrid system was applied. Asna1/TRC40 was depleted using RNA interference. Transient transfection and virus infection experiments followed by indirect immunofluorescence analysis were applied to analyse the localization of viral proteins as well as the impact of Asna1/TRC40 depletion on virus infection.

RESULTS

All HSV1 tail-anchored proteins specifically bound to Asna1/TRC40 but independently localized to their target membranes. While non-essential for cell viability, Asna1/TRC40 is required for efficient HSV1 replication. We show that early events of the replication cycle like virion entry and overall viral gene expression were unaffected by depletion of Asna1/TRC40. Furthermore, equal amounts of infectious virions were formed and remained cell-associated. This indicated that both nuclear egress of capsids that requires the essential tail-anchored protein pUL34, and secondary envelopment to form infectious virions were successfully completed. Despite large part of the virus life cycle proceeding normally, viral propagation was more than 10 fold reduced. We show that depletion of Asna1/TRC40 specifically affected a step late in infection during release of infectious virions to the extracellular milieu.

CONCLUSIONS

Asna1/TRC40 is required at a late step of herpesviral infection for efficient release of mature virions to the extracellular milieu. This study reveals novel tools to decipher exocytosis of newly formed virions as well as hitherto unknown cellular targets for antiviral therapy.

Secretagogin affects insulin secretion in pancreatic β-cells by regulating actin dynamics and focal adhesion

Secretagogin (SCGN), a Ca²⁺-binding protein having six EF-hands, is selectively expressed in pancreatic β-cells and neuroendocrine cells. Previous studies suggested that SCGN enhances insulin secretion by functioning as a Ca²⁺-sensor protein, but the underlying mechanism has not been elucidated. The present study explored the mechanism by which SCGN enhances glucose-induced insulin secretion in NIT-1 insulinoma cells. To determine whether SCGN influences the first or second phase of insulin secretion, we examined how SCGN affects the kinetics of insulin secretion in NIT-1 cells. We found that silencing SCGN suppressed the second phase of insulin secretion induced by glucose and H₂O₂, but not the first phase induced by KCl stimulation. Recruitment of insulin granules in the second phase of insulin secretion was significantly impaired by knocking down SCGN in NIT-1 cells. In addition, we found that SCGN interacts with the actin cytoskeleton in the plasma membrane and regulates actin remodelling in a glucose-dependent manner. Since actin dynamics are known to regulate focal adhesion, a critical step in the second phase of insulin secretion, we examined the effect of silencing SCGN on focal adhesion molecules, including FAK (focal adhesion kinase) and paxillin, and the cell survival molecules ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt. We found that glucose- and H₂O₂-induced activation of FAK, paxillin, ERK1/2 and Akt was significantly blocked by silencing SCGN. We conclude that SCGN controls glucose-stimulated insulin secretion and thus may be useful in the therapy of Type 2 diabetes.

Diapause is associated with a change in the polarity of secretion of insulin-like peptides

The insulin/IGF-1 signalling (IIS) pathway plays an important role in the regulation of larval diapause, the long-lived growth arrest state called dauer arrest, in Caenorhabditis elegans. In this nematode, 40 insulin-like peptides (ILPs) have been identified as putative ligands of the IIS pathway; however, it remains unknown how ILPs modulate larval diapause. Here we show that the secretory polarity of INS-35 and INS-7, which suppress larval diapause, is changed in the intestinal epithelial cells at larval diapause. These ILPs are secreted from the intestine into the body cavity during larval stages. In contrast, they are secreted into the intestinal lumen and degraded during dauer arrest, only to be secreted into the body cavity again when the worms return to developmental growth. The process that determines the secretory polarity of INS-35 and INS-7, thus, has an important role in the modulation of larval diapause.

Insulin-like peptides INS-7 and INS-35 suppress larval diapause in Caenorhabditis elegans via unknown mechanism. Here, Matsunaga et al. show that the secretory polarity of both peptides changes in diapause, when these peptides are secreted into the intestinal lumen instead of the body cavity like in other larval stages.

Asna1/TRC40 Controls β-Cell Function and Endoplasmic Reticulum Homeostasis by Ensuring Retrograde Transport

Type 2 diabetes (T2D) is characterized by insulin resistance and β-cell failure. Insulin resistance per se, however, does not provoke overt diabetes as long as compensatory β-cell function is maintained. The increased demand for insulin stresses the β-cell endoplasmic reticulum (ER) and secretory pathway, and ER stress is associated with β-cell failure in T2D. The tail recognition complex (TRC) pathway, including Asna1/TRC40, is implicated in the maintenance of endomembrane trafficking and ER homeostasis. To gain insight into the role of Asna1/TRC40 in maintaining endomembrane homeostasis and β-cell function, we inactivated Asna1 in β-cells of mice. We show that Asna1(β-/-) mice develop hypoinsulinemia, impaired insulin secretion, and glucose intolerance that rapidly progresses to overt diabetes. Loss of Asna1 function leads to perturbed plasma membrane-to-trans Golgi network and Golgi-to-ER retrograde transport as well as to ER stress in β-cells. Of note, pharmacological inhibition of retrograde transport in isolated islets and insulinoma cells mimicked the phenotype of Asna1(β-/-) β-cells and resulted in reduced insulin content and ER stress. These data support a model where Asna1 ensures retrograde transport and, hence, ER and insulin homeostasis in β-cells.

A peroxiredoxin, PRDX-2, is required for insulin secretion and insulin/IIS-dependent regulation of stress resistance and longevity

Peroxiredoxins (Prx) are abundant thiol peroxidases with a conserved anti-ageing role. In contrast to most animals, the nematode worm, Caenorhabditis elegans, encodes a single cytosolic 2-Cys Prx, PRDX-2, rendering it an excellent model for examining how peroxiredoxins affect animal physiology and ageing. Our previous work revealed that, although PRDX-2 protects against the toxicity of peroxides, enigmatically, prdx-2-mutant animals are hyper-resistant to other forms of oxidative stress. Here, we have investigated the basis for this increased resistance. Mammalian FOXO and Nrf2 transcription factors directly promote the expression of a range of detoxification enzymes. We show that the FOXO orthologue, DAF-16, and the Nrf2 orthologue, SKN-1, are required for the increased stress resistance of prdx-2-mutant worms. Our data suggest that PRDX-2 is required for normal levels of insulin secretion and hence the inhibition of DAF-16 and SKN-1 by insulin/IGF-1-like signalling (IIS) under nutrient-rich conditions. Intriguingly, loss of PRDX-2 increases DAF-16 and SKN-1 activities sufficiently to increase arsenite resistance without initiating other IIS-inhibited processes. Together, these data suggest that loss of peroxiredoxin function may increase stress resistance by reducing insulin secretion, but that further changes in insulin signalling are required for the reprogramming of development and fat metabolism. In addition, we reveal that the temperature-dependent prolongevity function of PRDX-2 is required for the extended lifespan associated with several pathways, including further reductions in IIS.

Loss of C. elegans GON-1, an ADAMTS9 Homolog, Decreases Secretion Resulting in Altered Lifespan and Dauer Formation

ADAMTS9 is a metalloprotease that cleaves components of the extracellular matrix and is also implicated in transport from the endoplasmic reticulum (ER) to the Golgi. It has been reported that an ADAMTS9 gene variant is associated with type 2 diabetes. The underlying pathology of type 2 diabetes is insulin resistance and beta-cell dysfunction. However, the molecular mechanisms underlying ADAMTS9 function in beta cells and peripheral tissues are unknown. We show that loss of C. elegans GON-1, an ADAMTS9 homolog, alters lifespan and dauer formation. GON-1 loss impairs secretion of proteins such as insulin orthologs and TGF-beta, and additionally impacts insulin/IGF-1 signaling in peripheral tissues. The function of the GON domain, but not the protease domain, is essential for normal lifespan and dauer formation in these scenarios. We conclude that the GON domain is critical for ADAMTS9/GON-1 function across species, which should help the understanding of type 2 diabetes in humans.

The C. elegans Hypodermis Couples Progenitor Cell Quiescence to the Dietary State

The nutritional status of an organism can greatly impact the function and behavior of stem and progenitor cells [1]. However, the regulatory circuits that inform these cells about the dietary environment remain to be elucidated. Newly hatched C. elegans larvae (L1s) halt development in "L1 arrest" or "L1 diapause" until ample food is encountered and triggers stem and progenitor cells to exit from quiescence [2]. The insulin/insulin-like growth factor signaling (IIS) pathway plays a key role in this reactivation [3, 4], but its site(s) of action have not been elucidated nor have the nutrient molecule(s) that stimulate the pathway been identified. By tissue-specifically modulating the activity of its components, we demonstrate that the IIS pathway acts in the hypodermis to regulate nutrition-responsive reactivation of neural and mesodermal progenitor cells. We identify ethanol, a likely component of the natural Caenorhabditis habitat, and amino acids as nutrients that synergistically reactivate somatic progenitor cells and upregulate expression of insulin-like genes in starved L1 larvae. The hypodermis likely senses the availability of amino acids because forced activation of the amino-acid-responsive Rag-TORC1 (target of rapamycin complex 1) pathway in this tissue can also release somatic progenitor cell quiescence in the presence of ethanol. Finally, there appears to be crosstalk between the IIS and Rag-TORC1 pathways because constitutive activation of the IIS pathway requires Rag to promote reactivation. This work demonstrates that ethanol and amino acids act as dietary cues via the IIS and Rag-TORC1 pathways in the hypodermis to coordinately control progenitor cell behavior.

Kynurenic acid is a nutritional cue that enables behavioral plasticity

The kynurenine pathway of tryptophan metabolism is involved in the pathogenesis of several brain diseases, but its physiological functions remain unclear. We report that kynurenic acid, a metabolite in this pathway, functions as a regulator of food-dependent behavioral plasticity in C. elegans. The experience of fasting in C. elegans alters a variety of behaviors, including feeding rate, when food is encountered post-fast. Levels of neurally produced kynurenic acid are depleted by fasting, leading to activation of NMDA-receptor-expressing interneurons and initiation of a neuropeptide-y-like signaling axis that promotes elevated feeding through enhanced serotonin release when animals re-encounter food. Upon refeeding, kynurenic acid levels are eventually replenished, ending the elevated feeding period. Because tryptophan is an essential amino acid, these findings suggest that a physiological role of kynurenic acid is in directly linking metabolism to activity of NMDA and serotonergic circuits, which regulate a broad range of behaviors and physiologies.

Regulation of body fat in Caenorhabditis elegans

Over the past decade, studies conducted in Caenorhabditis elegans have helped to uncover the ancient and complex origins of body fat regulation. This review highlights the powerful combination of genetics, pharmacology, and biochemistry used to study energy balance and the regulation of cellular fat metabolism in C. elegans. The complete wiring diagram of the C. elegans nervous system has been exploited to understand how the sensory nervous system regulates body fat and how food perception is coupled with the production of energy via fat metabolism. As a model organism, C. elegans also offers a unique opportunity to discover neuroendocrine factors that mediate direct communication between the nervous system and the metabolic tissues. The coming years are expected to reveal a wealth of information on the neuroendocrine control of body fat in C. elegans.

Metabolism of minor isoforms of prion proteins: Cytosolic prion protein and transmembrane prion protein

Transmissible spongiform encephalopathy or prion disease is triggered by the conversion from cellular prion protein to pathogenic prion protein. Growing evidence has concentrated on prion protein configuration changes and their correlation with prion disease transmissibility and pathogenicity. In vivo and in vitro studies have shown that several cytosolic forms of prion protein with specific topological structure can destroy intracellular stability and contribute to prion protein pathogenicity. In this study, the latest molecular chaperone system associated with endoplasmic reticulum-associated protein degradation, the endoplasmic reticulum resident protein quality-control system and the ubiquitination proteasome system, is outlined. The molecular chaperone system directly correlates with the prion protein degradation pathway. Understanding the molecular mechanisms will help provide a fascinating avenue for further investigations on prion disease treatment and prion protein-induced neurodegenerative diseases.

Ins-4 and daf-28 function redundantly to regulate C. elegans L1 arrest

Caenorhabditis elegans larvae reversibly arrest development in the first larval stage in response to starvation (L1 arrest or L1 diapause). Insulin-like signaling is a critical regulator of L1 arrest. However, the C. elegans genome encodes 40 insulin-like peptides, and it is unknown which peptides participate in nutritional control of L1 development. Work in other contexts has revealed that insulin-like genes can promote development ("agonists") or developmental arrest ("antagonists"), suggesting that such agonists promote L1 development in response to feeding. We measured mRNA expression dynamics with high temporal resolution for all 40 insulin-like genes during entry into and recovery from L1 arrest. Nutrient availability influences expression of the majority of insulin-like genes, with variable dynamics suggesting complex regulation. We identified thirteen candidate agonists and eight candidate antagonists based on expression in response to nutrient availability. We selected ten candidate agonists (daf-28, ins-3, ins-4, ins-5, ins-6, ins-7, ins-9, ins-26, ins-33 and ins-35) for further characterization in L1 stage larvae. We used destabilized reporter genes to determine spatial expression patterns. Expression of candidate agonists is largely overlapping in L1 stage larvae, suggesting a role of the intestine, chemosensory neurons ASI and ASJ, and the interneuron PVT in control of L1 development. Transcriptional regulation of candidate agonists is most significant in the intestine, as if internal nutrient status is a more important influence on transcription than sensory perception. Phenotypic analysis of single and compound deletion mutants did not reveal effects on L1 developmental dynamics, though simultaneous disruption of ins-4 and daf-28 increases survival of L1 arrest. Furthermore, overexpression of ins-4, ins-6 or daf-28 alone decreases survival and promotes cell division during starvation. These results suggest extensive functional overlap among insulin-like genes in nutritional control of L1 development while highlighting the role of ins-4, daf-28 and to a lesser extent ins-6.

Loss of a neural AMP-activated kinase mimics the effects of elevated serotonin on fat, movement, and hormonal secretions

AMP-activated protein kinase (AMPK) is an evolutionarily conserved master regulator of metabolism and a therapeutic target in type 2 diabetes. As an energy sensor, AMPK activity is responsive to both metabolic inputs, for instance the ratio of AMP to ATP, and numerous hormonal cues. As in mammals, each of two genes, aak-1 and aak-2, encode for the catalytic subunit of AMPK in C. elegans. Here we show that in C. elegans loss of aak-2 mimics the effects of elevated serotonin signaling on fat reduction, slowed movement, and promoting exit from dauer arrest. Reconstitution of aak-2 in only the nervous system restored wild type fat levels and movement rate to aak-2 mutants and reconstitution in only the ASI neurons was sufficient to significantly restore dauer maintenance to the mutant animals. As in elevated serotonin signaling, inactivation of AAK-2 in the ASI neurons caused enhanced secretion of dense core vesicles from these neurons. The ASI neurons are the site of production of the DAF-7 TGF-β ligand and the DAF-28 insulin, both of which are secreted by dense core vesicles and play critical roles in whether animals stay in dauer or undergo reproductive development. These findings show that elevated levels of serotonin promote enhanced secretions of systemic regulators of pro-growth and differentiation pathways through inactivation of AAK-2. As such, AMPK is not only a recipient of hormonal signals but can also be an upstream regulator. Our data suggest that some of the physiological phenotypes previously attributed to peripheral AAK-2 activity on metabolic targets may instead be due to the role of this kinase in neural serotonin signaling.

An insulin-to-insulin regulatory network orchestrates phenotypic specificity in development and physiology

Insulin-like peptides (ILPs) play highly conserved roles in development and physiology. Most animal genomes encode multiple ILPs. Here we identify mechanisms for how the forty Caenorhabditis elegans ILPs coordinate diverse processes, including development, reproduction, longevity and several specific stress responses. Our systematic studies identify an ILP-based combinatorial code for these phenotypes characterized by substantial functional specificity and diversity rather than global redundancy. Notably, we show that ILPs regulate each other transcriptionally, uncovering an ILP-to-ILP regulatory network that underlies the combinatorial phenotypic coding by the ILP family. Extensive analyses of genetic interactions among ILPs reveal how their signals are integrated. A combined analysis of these functional and regulatory ILP interactions identifies local genetic circuits that act in parallel and interact by crosstalk, feedback and compensation. This organization provides emergent mechanisms for phenotypic specificity and graded regulation for the combinatorial phenotypic coding we observe. Our findings also provide insights into how large hormonal networks regulate diverse traits.

A new tool in C. elegans reveals changes in secretory protein metabolism in ire-1-deficient animals

We recently showed that the ire-1/xbp-1 arm of the UPR plays a crucial role in maintaining basic endoplasmic reticulum (ER) functions required for the metabolism of secreted proteins even during unstressed growth conditions. During these studies we realized that although C. elegans is a powerful system to study the genetics of many cellular processes; it lacks effective tools for tracking the metabolism of secreted proteins at the cell and organism levels. Here, we outline how genetic manipulations and expression analysis of a DAF-28::GFP translational fusion transgene can be combined to infer different steps in the life cycle of secretory proteins. We demonstrate how we have used this tool to reveal folding defects, clearance defects, and secretion defects in ire-1 and xbp-1 mutants. We believe that further studies using this tool will deepen the understanding of secretory protein metabolism.

A functional, genome-wide evaluation of liposensitive yeast identifies the "ARE2 required for viability" (ARV1) gene product as a major component of eukaryotic fatty acid resistance

The toxic subcellular accumulation of lipids predisposes several human metabolic syndromes, including obesity, type 2 diabetes, and some forms of neurodegeneration. To identify pathways that prevent lipid-induced cell death, we performed a genome-wide fatty acid sensitivity screen in Saccharomyces cerevisiae. We identified 167 yeast mutants as sensitive to 0.5 mm palmitoleate, 45% of which define pathways that were conserved in humans. 63 lesions also impacted the status of the lipid droplet; however, this was not correlated to the degree of fatty acid sensitivity. The most liposensitive yeast strain arose due to deletion of the "ARE2 required for viability" (ARV1) gene, encoding an evolutionarily conserved, potential lipid transporter that localizes to the endoplasmic reticulum membrane. Down-regulation of mammalian ARV1 in MIN6 pancreatic β-cells or HEK293 cells resulted in decreased neutral lipid synthesis, increased fatty acid sensitivity, and lipoapoptosis. Conversely, elevated expression of human ARV1 in HEK293 cells or mouse liver significantly increased triglyceride mass and lipid droplet number. The ARV1-induced hepatic triglyceride accumulation was accompanied by up-regulation of DGAT1, a triglyceride synthesis gene, and the fatty acid transporter, CD36. Furthermore, ARV1 was identified as a transcriptional of the protein peroxisome proliferator-activated receptor α (PPARα), a key regulator of lipid homeostasis whose transcriptional targets include DGAT1 and CD36. These results implicate ARV1 as a protective factor in lipotoxic diseases due to modulation of fatty acid metabolism. In conclusion, a lipotoxicity-based genetic screen in a model microorganism has identified 75 human genes that may play key roles in neutral lipid metabolism and disease.

The control of cell growth and body size in Caenorhabditis elegans

One of the most important ways in which animal species vary is in their size. Individuals of the largest animal ever thought to have lived, the blue whale (Balaenoptera musculus), can reach a weight of 190 t and a length of over 30 m. At the other extreme, among the smallest multicellular animals are males of the parasitic wasp, Dicopomorpha echmepterygis, which even as adults are just 140 μm in length. In terms of volume, these species differ by more than 14 orders of magnitude. Since size has such profound effects on an organism's ecology, anatomy and physiology, an important task for evolutionary biology and ecology is to account for why organisms grow to their characteristic sizes. Equally, a full description of an organism's development must include an explanation of how its growth and body size are regulated. Here I review research on how these processes are controlled in the nematode, Caenorhabditis elegans. Analyses of small and long mutants have revealed that in the worm, DBL-1, a ligand in the TGFβ superfamily family, promotes growth in a dose-dependent manner. DBL-1 signaling affects body size by stimulating the growth of syncytial hypodermal cells rather than controlling cell division. Signals from chemosensory neurons and from the gonad also modulate body size, in part, independently of DBL-1-mediated signaling. Organismal size and morphology is heavily influenced by the cuticle, which acts as the exoskeleton. Finally, I summarize research on several genes that appear to regulate body size by cell autonomously regulating cell growth throughout the worm.

Gel-based protease proteomics for identifying the novel calpain substrates in dopaminergic neuronal cell

Calpains are a family of calcium-dependent cysteine proteases that are ubiquitously expressed in mammals and play critical roles in neuronal death by catalyzing substrate proteolysis. Here, we developed two-dimensional gel electrophoresis-based protease proteomics to identify putative calpain substrates. To accomplish this, cellular lysates from neuronal cells were first separated by pI, and the immobilized sample on a gel strip was incubated with a recombinant calpain and separated by molecular weight. Among 25 altered protein spots that were differentially expressed by at least 2-fold, we confirmed that arsenical pump-driving ATPase, optineurin, and peripherin were cleaved by calpain using in vitro and in vivo cleavage assays. Furthermore, we found that all of these substrates were cleaved in MN9D cells treated with either ionomycin or 1-methyl-4-phenylpyridinium, both of which cause a calcium-mediated calpain activation. Their cleavage was blocked by calcium chelator or calpain inhibitors. In addition, calpain-mediated cleavage of these substrates and its inhibition by calpeptin were confirmed in a middle cerebral artery occlusion model of cerebral ischemia, as well as a stereotaxic brain injection model of Parkinson disease. Transient overexpression of each protein was shown to attenuate 1-methyl-4-phenylpyridinium-induced cell death, indicating that these substrates may confer protection of varying magnitudes against dopaminergic injury. Taken together, the data indicate that our protease proteomic method has the potential to be applicable for identifying proteolytic substrates affected by diverse proteases. Moreover, the results described here will help us decipher the molecular mechanisms underlying the progression of neurodegenerative disorders where protease activation is critically involved.

Extrasynaptic muscarinic acetylcholine receptors on neuronal cell bodies regulate presynaptic function in Caenorhabditis elegans

Acetylcholine (ACh) is a potent neuromodulator in the brain, and its effects on cognition and memory formation are largely performed through muscarinic acetylcholine receptors (mAChRs). mAChRs are often preferentially distributed on specialized membrane regions in neurons, but the significance of mAChR localization in modulating neuronal function is not known. Here we show that the Caenorhabditis elegans homolog of the M1/M3/M5 family of mAChRs, gar-3, is expressed in cholinergic motor neurons, and GAR-3-GFP fusion proteins localize to cell bodies where they are enriched at extrasynaptic regions that are in contact with the basal lamina. The GAR-3 N-terminal extracellular domain is necessary and sufficient for this asymmetric distribution, and mutation of a predicted N-linked glycosylation site within the N-terminus disrupts GAR-3-GFP localization. In transgenic animals expressing GAR-3 variants that are no longer asymmetrically localized, synaptic transmission at neuromuscular junctions is impaired and there is a reduction in the abundance of the presynaptic protein sphingosine kinase at release sites. Finally, GAR-3 can be activated by endogenously produced ACh released from neurons that do not directly contact cholinergic motor neurons. Together, our results suggest that humoral activation of asymmetrically localized mAChRs by ACh is an evolutionarily conserved mechanism by which ACh modulates neuronal function.

Depletion of the ER chaperone ENPL-1 sensitizes C. elegans to the anticancer drug cisplatin

Cisplatin is an essential chemotherapeutic drug in the treatment of many cancers. Its use, however, is limited by the development of resistance in many tumors. The ability to re-sensitize resistant tumors could significantly strengthen cisplatin therapy in patients. Caenorhabditis elegans is a suitable model for studying the cytoplasmic role of cisplatin in tumor cells. We have previously shown that the ATPase ASNA-1 has similar roles as a factor governing cisplatin sensitivity in mammalian tumor cells and C. elegans. Here we study the endoplasmic reticulum (ER) resident chaperone ENPL-1/GRP94 and find that its depletion makes worms sensitive to cisplatin. Elevated ER stress levels in enpl-1 mutants is the likely cause of this sensitivity because a correlation can be made between cisplatin sensitivity and the high ER stress levels. We also find that asna-1 mutants have elevated unfolded protein response (UPR) activity and that the intrinsically cisplatin resistant wild-type worms become sensitive when ER stress is high. We conclude that enpl-1 is a cisplatin sensitizing factor and suggest that manipulation of its levels or of UPR activity will enhance the effects of cisplatin based cancer therapy.

The ire-1 ER stress-response pathway is required for normal secretory-protein metabolism in C. elegans

The unfolded protein response (UPR) allows cells to cope with endoplasmic reticulum (ER) stress by adjusting the capacity of the ER to the load of ER-associated tasks. The UPR is important for maintaining ER homeostasis under extreme ER stress. UPR genes are important under normal growth conditions as well, but what they are required for under these conditions is less clear. Using C. elegans, we show that the ire-1/xbp-1 arm of the UPR plays a crucial role in maintaining ER plasticity and function also in the absence of external ER stress. We find that during unstressed growth conditions, loss of ire-1 or xbp-1 compromises basic ER functions required for the metabolism of secreted proteins, including translation, folding and secretion. Notably, by compromising ER-associated degradation (ERAD) and phagocytosis, loss of ire-1 hinders the clearance of misfolded proteins from the ER as well as the clearance of proteins that were secreted into the pseudocoleom. Whereas the basal activity of the UPR is beneficial under normal conditions, it accelerates the pathology caused by toxic Aβ protein in a C. elegans model of Alzheimer's disease. Taken together, our findings indicate that UPR genes are critical for maintaining secretory protein metabolism under normal growth conditions.

To grow or not to grow: nutritional control of development during Caenorhabditis elegans L1 arrest

It is widely appreciated that larvae of the nematode Caenorhabditis elegans arrest development by forming dauer larvae in response to multiple unfavorable environmental conditions. C. elegans larvae can also reversibly arrest development earlier, during the first larval stage (L1), in response to starvation. "L1 arrest" (also known as "L1 diapause") occurs without morphological modification but is accompanied by increased stress resistance. Caloric restriction and periodic fasting can extend adult lifespan, and developmental models are critical to understanding how the animal is buffered from fluctuations in nutrient availability, impacting lifespan. L1 arrest provides an opportunity to study nutritional control of development. Given its relevance to aging, diabetes, obesity and cancer, interest in L1 arrest is increasing, and signaling pathways and gene regulatory mechanisms controlling arrest and recovery have been characterized. Insulin-like signaling is a critical regulator, and it is modified by and acts through microRNAs. DAF-18/PTEN, AMP-activated kinase and fatty acid biosynthesis are also involved. The nervous system, epidermis, and intestine contribute systemically to regulation of arrest, but cell-autonomous signaling likely contributes to regulation in the germline. A relatively small number of genes affecting starvation survival during L1 arrest are known, and many of them also affect adult lifespan, reflecting a common genetic basis ripe for exploration. mRNA expression is well characterized during arrest, recovery, and normal L1 development, providing a metazoan model for nutritional control of gene expression. In particular, post-recruitment regulation of RNA polymerase II is under nutritional control, potentially contributing to a rapid and coordinated response to feeding. The phenomenology of L1 arrest will be reviewed, as well as regulation of developmental arrest and starvation survival by various signaling pathways and gene regulatory mechanisms.

A novel sphingolipid-TORC1 pathway critically promotes postembryonic development in Caenorhabditis elegans

Regulation of animal development in response to nutritional cues is an intensely studied problem related to disease and aging. While extensive studies indicated roles of the Target of Rapamycin (TOR) in sensing certain nutrients for controlling growth and metabolism, the roles of fatty acids and lipids in TOR-involved nutrient/food responses are obscure. Caenorhabditis elegans halts postembryonic growth and development shortly after hatching in response to monomethyl branched-chain fatty acid (mmBCFA) deficiency. Here, we report that an mmBCFA-derived sphingolipid, d17iso-glucosylceramide, is a critical metabolite in regulating growth and development. Further analysis indicated that this lipid function is mediated by TORC1 and antagonized by the NPRL-2/3 complex in the intestine. Strikingly, the essential lipid function is bypassed by activating TORC1 or inhibiting NPRL-2/3. Our findings uncover a novel lipid-TORC1 signaling pathway that coordinates nutrient and metabolic status with growth and development, advancing our understanding of the physiological roles of mmBCFAs, ceramides, and TOR. DOI:http://dx.doi.org/10.7554/eLife.00429.001.