Clustering of genetically defined allele classes in the Caenorhabditis elegans DAF-2 insulin/IGF-1 receptor

Mutation of an insulin-sensitive Drosophila insulin-like receptor mutant requires methionine metabolism reprogramming to extend lifespan

Insulin/insulin growth factor signaling is a conserved pathway that regulates lifespan across many species. Multiple mechanisms are proposed for how this altered signaling slows aging. To elaborate these causes, we recently developed a series of Drosophila insulin-like receptor (dInr) mutants with single amino acid substitutions that extend lifespan but differentially affect insulin sensitivity, growth and reproduction. Transheterozygotes of canonical dInr mutants (Type I) extend longevity and are insulin-resistant, small and weakly fecund. In contrast, a dominant mutation (dInr 353, Type II) within the Kinase Insert Domain (KID) robustly extends longevity but is insulin-sensitive, full-sized, and highly fecund. We applied transcriptome and metabolome analyses to explore how dInr 353 slows aging without insulin resistance. Type I and II mutants overlap in many pathways but also produce distinct transcriptomic profiles that include differences in innate immune and reproductive functions. In metabolomic analyses, the KID mutant dInr 353 reprograms methionine metabolism in a way that phenocopies dietary methionine restriction, in contrast to canonical mutants which are characterized by upregulation of the transsulfuration pathway. Because abrogation of S-adenosylhomocysteine hydrolase blocks the longevity benefit conferred by dInr 353, we conclude the methionine cycle reprogramming of Type II is sufficient to slow aging. Metabolomic analysis further revealed the Type II mutant is metabolically flexible: unlike aged wildtype, aged dInr 353 adults can reroute methionine toward the transsulfuration pathway, while Type I mutant flies upregulate the trassulfuration pathway continuously from young age. Altered insulin/insulin growth factor signaling has the potential to slow aging without the complications of insulin resistance by modulating methionine cycle dynamics.

Catalytic activities of wild-type C. elegans DAF-2 kinase and dauer-associated mutants

DAF-2, the Caenorhabditis elegans insulin-like receptor homolog, regulates larval development, metabolism, stress response, and lifespan. The availability of numerous daf-2 mutant alleles has made it possible to elucidate the genetic mechanisms underlying these physiological processes. The DAF-2 pathway is significantly conserved with the human insulin/IGF-1 signaling pathway; it includes proteins homologous to human IRS, GRB-2, and PI3K, making it an important model to investigate human pathological conditions. We expressed and purified the kinase domain of wild-type DAF-2 to examine the catalytic activity and substrate specificity of the enzyme. Like the human insulin receptor kinase, DAF-2 kinase phosphorylates tyrosines within specific YxN or YxxM motifs, which are important for recruiting downstream effectors. DAF-2 kinase phosphorylated peptides derived from the YxxM and YxN motifs located in the C-terminal extension of the receptor tyrosine kinase, consistent with the idea that the DAF-2 receptor may possess independent signaling capacity. Unlike the human insulin or IGF-1 receptor kinases, DAF-2 kinase was poorly inhibited by the small-molecule inhibitor linsitinib. We also expressed and purified mutant kinases corresponding to daf-2 alleles that result in partial loss-of-function phenotypes in C. elegans. These mutations caused a complete loss of kinase function in vitro. Our biochemical investigations provide new insights into DAF-2 kinase function, and the approach should be useful for studying other mutations to shed light on DAF-2 signaling in C. elegans physiology.

The ACE inhibitor captopril inhibits ACN-1 to control dauer formation and aging

The renin-angiotensin-aldosterone system (RAAS) plays a well-characterized role regulating blood pressure in mammals. Pharmacological and genetic manipulation of the RAAS has been shown to extend lifespan in Caenorhabditis elegans, Drosophila and rodents, but its mechanism is not well defined. Here, we investigate the angiotensin-converting enzyme (ACE) inhibitor drug captopril, which extends lifespan in worms and mice. To investigate the mechanism, we performed a forward genetic screen for captopril-hypersensitive mutants. We identified a missense mutation that causes a partial loss of function of the daf-2 receptor tyrosine kinase gene, a powerful regulator of aging. The homologous mutation in the human insulin receptor causes Donohue syndrome, establishing these mutant worms as an invertebrate model of this disease. Captopril functions in C. elegans by inhibiting ACN-1, the worm homolog of ACE. Reducing the activity of acn-1 via captopril or RNA interference promoted dauer larvae formation, suggesting that acn-1 is a daf gene. Captopril-mediated lifespan extension was abrogated by daf-16(lf) and daf-12(lf) mutations. Our results indicate that captopril and acn-1 influence lifespan by modulating dauer formation pathways. We speculate that this represents a conserved mechanism of lifespan control.

The ACE-inhibitor drug captopril inhibits ACN-1 to control dauer formation and aging

The renin-angiotensin-aldosterone system (RAAS) plays a well-characterized role regulating blood pressure in mammals. Pharmacological and genetic manipulation of the RAAS has been shown to extend lifespan in C. elegans , Drosophila , and rodents, but its mechanism is not well defined. Here we investigate the angiotensin-converting enzyme (ACE) inhibitor drug captopril, which extends lifespan in worms and mice. To investigate the mechanism, we performed a forward genetic screen for captopril hypersensitive mutants. We identified a missense mutation that causes a partial loss-of-function of the daf-2 receptor tyrosine kinase gene, a powerful regulator of aging. The homologous mutation in the human insulin receptor causes Donohue syndrome, establishing these mutant worms as an invertebrate model of this disease. Captopril functions in C. elegans by inhibiting ACN-1, the worm homolog of ACE. Reducing the activity of acn-1 via captopril or RNAi promoted dauer larvae formation, suggesting acn-1 is a daf gene. Captopril-mediated lifespan extension xwas abrogated by daf-16(lf) and daf-12(lf) mutations. Our results indicate that captopril and acn-1 control aging by modulating dauer formation pathways. We speculate that this represents a conserved mechanism of lifespan control.

Summary Statement

Captopril and acn-1 control aging. By demonstrating they regulate dauer formation and interact with daf genes, including a new DAF-2(A261V) mutant corresponding to a human disease variant, we clarified the mechanism.

Aminopeptidase MNP-1 triggers intestine protease production by activating daf-16 nuclear location to degrade pore-forming toxins in Caenorhabditis elegans

Pore-forming toxins (PFTs) are effective tools for pathogens infection. By disrupting epithelial barriers and killing immune cells, PFTs promotes the colonization and reproduction of pathogenic microorganisms in their host. In turn, the host triggers defense responses, such as endocytosis, exocytosis, or autophagy. Bacillus thuringiensis (Bt) bacteria produce PFT, known as crystal proteins (Cry) which damage the intestinal cells of insects or nematodes, eventually killing them. In insects, aminopeptidase N (APN) has been shown to act as an important receptor for Cry toxins. Here, using the nematode Caenorhabditis elegans as model, an extensive screening of APN gene family was performed to analyze the potential role of these proteins in the mode of action of Cry5Ba against the nematode. We found that one APN, MNP-1, participate in the toxin defense response, since the mnp-1(ok2434) mutant showed a Cry5Ba hypersensitive phenotype. Gene expression analysis in mnp-1(ok2434) mutant revealed the involvement of two protease genes, F19C6.4 and R03G8.6, that participate in Cry5Ba degradation. Finally, analysis of the transduction pathway involved in F19C6.4 and R03G8.6 expression revealed that upon Cry5Ba exposure, the worms up regulated both protease genes through the activation of the FOXO transcription factor DAF-16, which was translocated into the nucleus. The nuclear location of DAF-16 was found to be dependent on mnp-1 under Cry5Ba treatment. Our work provides evidence of new host responses against PFTs produced by an enteric pathogenic bacterium, resulting in activation of host intestinal proteases that degrade the PFT in the intestine.

A PTEN variant uncouples longevity from impaired fitness in Caenorhabditis elegans with reduced insulin/IGF-1 signaling

Insulin/IGF-1 signaling (IIS) regulates various physiological aspects in numerous species. In Caenorhabditis elegans, mutations in the daf-2/insulin/IGF-1 receptor dramatically increase lifespan and immunity, but generally impair motility, growth, and reproduction. Whether these pleiotropic effects can be dissociated at a specific step in insulin/IGF-1 signaling pathway remains unknown. Through performing a mutagenesis screen, we identified a missense mutation daf-18(yh1) that alters a cysteine to tyrosine in DAF-18/PTEN phosphatase, which maintained the long lifespan and enhanced immunity, while improving the reduced motility in adult daf-2 mutants. We showed that the daf-18(yh1) mutation decreased the lipid phosphatase activity of DAF-18/PTEN, while retaining a partial protein tyrosine phosphatase activity. We found that daf-18(yh1) maintained the partial activity of DAF-16/FOXO but restricted the detrimental upregulation of SKN-1/NRF2, contributing to beneficial physiological traits in daf-2 mutants. Our work provides important insights into how one evolutionarily conserved component, PTEN, can coordinate animal health and longevity.

End-of-life targeted degradation of DAF-2 insulin/IGF-1 receptor promotes longevity free from growth-related pathologies

Preferably, lifespan-extending therapies should work when applied late in life without causing undesired pathologies. Reducing insulin/insulin-like growth factor (IGF)-1 signaling (IIS) increases lifespan across species, but the effects of reduced IIS interventions in extreme geriatric ages remains unknown. Using the nematode Caenorhabditis elegans, we engineered the conditional depletion of the DAF-2/insulin/IGF-1 transmembrane receptor using an auxin-inducible degradation (AID) system. This allowed for the temporal and spatial reduction in DAF-2 protein levels at time points after which interventions such as RNAi become ineffective. Using this system, we found that AID-mediated depletion of DAF-2 protein surpasses the longevity of daf-2 mutants. Depletion of DAF-2 during early adulthood resulted in multiple adverse phenotypes, including growth retardation, germline shrinkage, egg retention, and reduced brood size. By contrast, AID-mediated depletion of DAF-2 post-reproduction, or specifically in the intestine in early adulthood, resulted in an extension of lifespan without these deleterious effects. Strikingly, at geriatric ages, when 75% of the population had died, AID-mediated depletion of DAF-2 protein resulted in a doubling in lifespan. Thus, we provide a proof-of-concept that even close to the end of an individual’s lifespan, it is possible to slow aging and promote longevity.

The goal of geroscience, or research into old age, is to promote health during old age, and thus, to increase lifespan. In the body, the groups of biochemical reactions, or ‘pathways’, that allow an organism to sense nutrients, and regulate growth and stress, play major roles in ensuring healthy aging. Indeed, organisms that do not produce a working version of the insulin/IGF-1 receptor, a protein involved in one such pathway, show increased lifespan. In the worm Caenorhabditis elegans, mutations in the insulin/IGF-1 receptor can even double their lifespan. However, it is unclear whether this increase can be achieved once the organism has reached old age.

To answer this question, Venz et al. genetically engineered the nematode worm C. elegans so that they could trigger the rapid degradation of the insulin/IGF-1 receptor either in the entire organism or in a specific tissue. Venz et al. started by aging several C. elegans worms for three weeks, until about 75% had died. At this point, they triggered the degradation of the insulin/IGF-1 receptor in some of the remaining worms, keeping the rest untreated as a control for the experiment.

The results showed that the untreated worms died within a few days, while worms in which the insulin/IGF-1 receptor had been degraded lived for almost one more month. This demonstrates that it is possible to double the lifespan of an organism at the very end of life.

Venz et al.’s findings suggest that it is possible to make interventions to extend an organism’s lifespan near the end of life that are as effective as if they were performed when the organism was younger. This sparks new questions regarding the quality of this lifespan extension: do the worms become younger with the intervention, or is aging simply slowed down?

Aging modulated by the Drosophila insulin receptor through distinct structure-defined mechanisms

Mutations of the Drosophila melanogaster insulin/IGF signaling system slow aging, while also affecting growth and reproduction. To understand this pleiotropy, we produced an allelic series of single codon substitutions in the Drosophila insulin receptor, InR. We generated InR substitutions using homologous recombination and related each to emerging models of receptor tyrosine kinase structure and function. Three mutations when combined as trans-heterozygotes extended lifespan while retarding growth and fecundity. These genotypes reduced insulin-stimulated Akt phosphorylation, suggesting they impede kinase catalytic domain function. Among these genotypes, longevity was negatively correlated with egg production, consistent with life-history trade-off theory. In contrast, one mutation (InR353) was located in the kinase insert domain, a poorly characterized element found in all receptor tyrosine kinases. Remarkably, wild-type heterozygotes with InR353 robustly extended lifespan without affecting growth or reproduction and retained capacity to fully phosphorylate Akt. The Drosophila insulin receptor kinase insert domain contains a previously unrecognized SH2 binding motif. We propose the kinase insert domain interacts with SH2-associated adapter proteins to affect aging through mechanisms that retain insulin sensitivity and are independent of reproduction.

Insulin/IGF-1 signaling and heat stress differentially regulate HSF1 activities in germline development

Germline development is sensitive to nutrient availability and environmental perturbation. Heat shock transcription factor 1 (HSF1), a key transcription factor driving the cellular heat shock response (HSR), is also involved in gametogenesis. The precise function of HSF1 (HSF-1 in C. elegans) and its regulation in germline development are poorly understood. Using the auxin-inducible degron system in C. elegans, we uncovered a role of HSF-1 in progenitor cell proliferation and early meiosis and identified a compact but important transcriptional program of HSF-1 in germline development. Interestingly, heat stress only induces the canonical HSR in a subset of germ cells but impairs HSF-1 binding at its developmental targets. Conversely, insulin/insulin growth factor 1 (IGF-1) signaling dictates the requirement for HSF-1 in germline development and functions through repressing FOXO/DAF-16 in the soma to activate HSF-1 in germ cells. We propose that this non-cell-autonomous mechanism couples nutrient-sensing insulin/IGF-1 signaling to HSF-1 activation to support homeostasis in rapid germline growth.

Mutation of daf-2 extends lifespan via tissue-specific effectors that suppress distinct life-limiting pathologies

In aging Caenorhabditis elegans, as in higher organisms, there is more than one cause of death. C. elegans exhibit early death with a swollen, infected pharynx (P death), and later death with pharyngeal atrophy (p death). Interventions that alter lifespan can differentially affect frequency and timing of each type of death, generating complex survival curve shapes. Here, we use mortality deconvolution analysis to investigate how reduction of insulin/IGF‐1 signaling (IIS), which increases lifespan (the Age phenotype), affects different forms of death. All daf‐2 insulin/IGF‐1 receptor mutants exhibit increased lifespan in the p subpopulation (p Age), while pleiotropic class 2 daf‐2 mutants show an additional marked reduction in P death frequency. The latter is promoted by pharyngeal expression of the IIS‐regulated DAF‐16 FOXO transcription factor, and at higher temperature by reduced pharyngeal pumping rate. Pharyngeal DAF‐16 also promotes p Age in class 2 daf‐2 mutants, revealing a previously unknown role for the pharynx in the regulation of aging. Necropsy analysis of daf‐2 interactions with the daf‐12 steroid receptor implies that previously described opposing effects of daf‐12 on daf‐2 longevity are attributable to internal hatching of larvae, rather than complex interactions between insulin/IGF‐1 and steroid signaling. These findings support the view that wild‐type IIS acts through multiple distinct mechanisms which promote different life‐limiting pathologies, each of which contribute to late‐life mortality. This study further demonstrates the utility of mortality deconvolution analysis to better understand the genetics of lifespan.

Aging Regulated Through a Stability Model of Insulin/Insulin Growth Factor Receptor Function

Mutations of the insulin-like receptor in Drosophila extend lifespan. New research suggests this receptor operates in two modes. The first extends lifespan while slowing reproduction and reducing growth. The second strongly extends lifespan without impairing growth or reproduction; it confers longevity assurance. The mutation that confers longevity assurance resides in the kinase insert domain, which contains a potential SH2 binding site for substrate proteins. We apply a recent model for the function of receptor tyrosine kinases to propose how insulin receptor structure can modulate aging. This concept hypothesizes that strong insulin-like ligands promote phosphorylation of high threshold substrate binding sites to robustly induce reproduction, which impairs survival as a consequence of trade-offs. Lower levels of receptor stimulation provide less kinase dimer stability, which reduces reproduction and extends lifespan by avoiding reproductive costs. Environmental conditions that favor diapause alter the expression of insulin ligands to further repress the stability of the interacting kinase domains, block phosphorylation of low threshold substrates and thus induce a unique molecular program that confers longevity assurance. Mutations of the insulin receptor that block low-phosphorylation site interactions, such as within the kinase insert domain, can extend lifespan while maintaining overall dimer stability. These flies are long-lived while maintaining reproduction and growth. The kinase insert domain of Drosophila provides a novel avenue from which to seek signaling of the insulin/insulin-like growth factor system of humans that modulate aging without impacting reproduction and growth, or incurring insulin resistance pathology.

Identification of Uric Acid Gluconucleoside-Ascaroside Conjugates in Caenorhabditis elegans by Combining Synthesis and MicroED

Few nucleoside-derived natural products have been identified from animals, despite the ubiquity of nucleosides in living organisms. Here, we use a combination of synthesis and the emerging electron microscopy technique microcrystal electron diffraction to determine the structures of several N3-(β-glucopyranosyl)uric acid derivatives in Caenorhabditis elegans. These noncanonical gluconucleosides further integrate an ascaroside moiety, for which we present a shortened synthetic route. The production of a phosphorylated gluconucleoside is influenced by evolutionarily conserved insulin signaling.

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.

Human insulin modulates α-synuclein aggregation via DAF-2/DAF-16 signalling pathway by antagonising DAF-2 receptor in C. elegans model of Parkinson's disease

Insulin-signalling is an important pathway in multiple cellular functions and organismal ageing across the taxa. A strong association of insulin-signalling with Parkinson’s disease (PD) has been proposed but the exact nature of molecular events and genetic associations are yet to be understood. We employed transgenic C. elegans strain harboring human α-synuclein::YFP transgene, towards studying the aggregation pattern of α-synuclein, a PD-associated endpoint, under human insulin (Huminsulin®) treatment and DAF-16/DAF-2 knockdown conditions, independently and in combination. The aggregation was increased when DAF-16 was knocked-down independently or alongwith a co-treatment of Human insulin (HumINS) and decreased when DAF-2 was knocked-down independently or alongwith a co-treatment of HumINS; whereas HumINS treatment per se, reduced the aggregation. Our results depicted that HumINS decreases α-synuclein aggregation via DAF-2/DAF-16 pathway by acting as an antagonist for DAF-2 receptor. Knockdown of reported DAF-2 agonist (INS-6) and antagonists (INS-17 and INS-18) also resulted in a similar effect on α-synuclein aggregation. Further by utilizing bioinformatics tools, we compared the differences between the binding sites of probable agonists and antagonists on DAF-2 including HumINS. Our results suggest that HumINS treatment and DAF-16 expression play a protective role against α-synuclein aggregation and its associated effects.

Requirement for translocon-associated protein (TRAP) α in insulin biogenesis

The mechanistic basis for the biogenesis of peptide hormones and growth factors is poorly understood. Here, we show that the conserved endoplasmic reticulum membrane translocon-associated protein α (TRAPα), also known as signal sequence receptor 1, plays a critical role in the biosynthesis of insulin. Genetic analysis in the nematode Caenorhabditis elegans and biochemical studies in pancreatic β cells reveal that TRAPα deletion impairs preproinsulin translocation while unexpectedly disrupting distal steps in insulin biogenesis including proinsulin processing and secretion. The association of common intronic single-nucleotide variants in the human TRAPα gene with susceptibility to type 2 diabetes and pancreatic β cell dysfunction suggests that impairment of preproinsulin translocation and proinsulin trafficking may contribute to the pathogenesis of type 2 diabetes.

C. elegans PTEN and AMPK block neuroblast divisions by inhibiting a BMP-insulin-PP2A-MAPK pathway

Caenorhabditis elegans that hatch in the absence of food stop their postembryonic development in a process called L1 arrest. Intriguingly, we find that the postembryonic Q neuroblasts divide and migrate during L1 arrest in mutants that have lost the energy sensor AMP-activated protein kinase (AMPK) or the insulin/IGF-1 signaling (IIS) negative regulator DAF-18/PTEN. We report that DBL-1/BMP works upstream of IIS to promote agonistic insulin-like peptides during L1 arrest. However, the abnormal Q cell divisions that occur during L1 arrest use a novel branch of the IIS pathway that is independent of the terminal transcription factor DAF-16/FOXO. Using genetic epistasis and drug interactions we show that AMPK functions downstream of, or in parallel with DAF-18/PTEN and IIS to inhibit PP2A function. Further, we show that PP2A regulates the abnormal Q cell divisions by activating the MPK-1/ERK signaling pathway via LIN-45/RAF, independently of LET-60/RAS. PP2A acts as a tumor suppressor in many oncogenic signaling cascades. Our work demonstrates a new role for PP2A that is needed to induce neuroblast divisions during starvation and is regulated by both insulin and AMPK.

Function and regulation of the Caenorhabditis elegans Rab32 family member GLO-1 in lysosome-related organelle biogenesis

Cell type-specific modifications of conventional endosomal trafficking pathways lead to the formation of lysosome-related organelles (LROs). C. elegans gut granules are intestinally restricted LROs that coexist with conventional degradative lysosomes. The formation of gut granules requires the Rab32 family member GLO-1. We show that the loss of glo-1 leads to the mistrafficking of gut granule proteins but does not significantly alter conventional endolysosome biogenesis. GLO-3 directly binds to CCZ-1 and they both function to promote the gut granule association of GLO-1, strongly suggesting that together, GLO-3 and CCZ-1 activate GLO-1. We found that a point mutation in GLO-1 predicted to spontaneously activate, and function independently of it guanine nucleotide exchange factor (GEF), localizes to gut granules and partially restores gut granule protein localization in ccz-1(-) and glo-3(-) mutants. CCZ-1 forms a heterodimeric complex with SAND-1(MON1), which does not function in gut granule formation, to activate RAB-7 in trafficking pathways to conventional lysosomes. Therefore, our data suggest a model whereby the function of a Rab GEF can be altered by subunit exchange. glo-3(-) mutants, which retain low levels of GLO-3 activity, generate gut granules that lack GLO-1 and improperly accumulate RAB-7 in a SAND-1 dependent process. We show that GLO-1 and GLO-3 restrict the distribution of RAB-7 to conventional endolysosomes, providing insights into the segregation of pathways leading to conventional lysosomes and LROs.

Food perception without ingestion leads to metabolic changes and irreversible developmental arrest in C. elegans

BACKGROUND

Developmental physiology is very sensitive to nutrient availability. For instance, in the nematode Caenorhabditis elegans, newly hatched L1-stage larvae require food to initiate postembryonic development. In addition, larvae arrested in the dauer diapause, a non-feeding state of developmental arrest that occurs during the L3 stage, initiate recovery when exposed to food. Despite the essential role of food in C. elegans development, the contribution of food perception versus ingestion on physiology has not been delineated.

RESULTS

We used a pharmacological approach to uncouple the effects of food (bacteria) perception and ingestion in C. elegans. Perception was not sufficient to promote postembryonic development in L1-stage larvae. However, L1 larvae exposed to food without ingestion failed to develop upon return to normal culture conditions, instead displaying an irreversible arrest phenotype. Inhibition of gene expression during perception rescued subsequent development, demonstrating that the response to perception without feeding is deleterious. Perception altered DAF-16/FOXO subcellular localization, reflecting activation of insulin/IGF signaling (IIS). The insulin-like peptide daf-28 was specifically required, suggesting perception in chemosensory neurons, where it is expressed, regulates peptide synthesis and possibly secretion. However, genetic manipulation of IIS did not modify the irreversible arrest phenotype caused by food perception, revealing that wild-type function of the IIS pathway is not required to produce this phenotype and that other pathways affected by perception of food in the absence of its ingestion are likely to be involved. Gene expression and Nile red staining showed that food perception could alter lipid metabolism and storage. We found that starved larvae sense environmental polypeptides, with similar molecular and developmental effects as perception of bacteria. Environmental polypeptides also promoted recovery from dauer diapause, suggesting that perception of polypeptides plays an important role in the life history of free-living nematodes.

CONCLUSIONS

We conclude that actual ingestion of food is required to initiate postembryonic development in C. elegans. We also conclude that polypeptides are perceived as a food-associated cue in this and likely other animals, initiating a signaling and gene regulatory cascade that alters metabolism in anticipation of feeding and development, but that this response is detrimental if feeding does not occur.

Modulatory upregulation of an insulin peptide gene by different pathogens in C. elegans

When an animal is infected, its innate immune response needs to be tightly regulated across tissues and coordinated with other aspects of organismal physiology. Previous studies with Caenorhabditis elegans have demonstrated that insulin-like peptide genes are differentially expressed in response to different pathogens. They represent prime candidates for conveying signals between tissues upon infection. Here, we focused on one such gene, ins-11 and its potential role in mediating cross-tissue regulation of innate immune genes. While diverse bacterial intestinal infections can trigger the up-regulation of ins-11 in the intestine, we show that epidermal infection with the fungus Drechmeria coniospora triggers an upregulation of ins-11 in the epidermis. Using the Shigella virulence factor OpsF, a MAP kinase inhibitor, we found that in both cases, ins-11 expression is controlled cell autonomously by p38 MAPK, but via distinct transcription factors, STA-2/STAT in the epidermis and HLH-30/TFEB in the intestine. We established that ins-11, and the insulin signaling pathway more generally, are not involved in the regulation of antimicrobial peptide gene expression in the epidermis. The up-regulation of ins-11 in the epidermis does, however, affect intestinal gene expression in a complex manner, and has a deleterious effect on longevity. These results support a model in which insulin signaling, via ins-11, contributes to the coordination of the organismal response to infection, influencing the allocation of resources in an infected animal.

Low Concentrations of Caffeine and Its Analogs Extend the Lifespan of Caenorhabditis elegans by Modulating IGF-1-Like Pathway

Caffeine has been reported to delay aging and protect aging-associated disorders in Caenorhabditis elegans. However, the effects of low concentration of caffeine and its analogs on lifespan are currently missing. Herein, we report that at much lower concentrations (as low as 10 μg/ml), caffeine extended the lifespan of C. elegans without affecting food intake and reproduction. The effect of caffeine was dependent on IGF-1-like pathway, although the insulin receptor homolog, daf-2 allele, e1371, was dispensable. Four caffeine analogs, 1-methylxanthine, 7-methylxanthine, 1,3-dimethylxanthine, and 1,7-dimethylxanthine, also extended lifespan, whereas 3-methylxanthine and 3,7-dimethylxanthine did not exhibit lifespan-extending activity.

Age-Dependence and Aging-Dependence: Neuronal Loss and Lifespan in a C. elegans Model of Parkinson's Disease

It is often assumed, but not established, that the major neurodegenerative diseases, such as Parkinson's disease, are not just age-dependent (their incidence changes with time) but actually aging-dependent (their incidence is coupled to the process that determines lifespan). To determine a dependence on the aging process requires the joint probability distribution of disease onset and lifespan. For human Parkinson's disease, such a joint distribution is not available, because the disease cuts lifespan short. To acquire a joint distribution, we resorted to an established C. elegans model of Parkinson's disease in which the loss of dopaminergic neurons is not fatal. We find that lifespan is not correlated with the loss of individual neurons. Therefore, neuronal loss is age-dependent and aging-independent. We also find that a lifespan-extending intervention into insulin/IGF1 signaling accelerates the loss of specific dopaminergic neurons, while leaving death and neuronal loss times uncorrelated. This suggests that distinct and compartmentalized instances of the same genetically encoded insulin/IGF1 signaling machinery act independently to control neurodegeneration and lifespan in C. elegans. Although the human context might well be different, our study calls attention to the need to maintain a rigorous distinction between age-dependence and aging-dependence.

Proteome-wide Changes in Protein Turnover Rates in C. elegans Models of Longevity and Age-Related Disease

The balance between protein synthesis and protein breakdown is a major determinant of protein homeostasis, and loss of protein homeostasis is one of the hallmarks of aging. Here we describe pulsed SILAC-based experiments to estimate proteome-wide turnover rates of individual proteins. We applied this method to determine protein turnover rates in Caenorhabditis elegans models of longevity and Parkinson's disease, using both developing and adult animals. Whereas protein turnover in developing, long-lived daf-2(e1370) worms is about 30% slower than in controls, the opposite was observed in day 5 adult worms, in which protein turnover in the daf-2(e1370) mutant is twice as fast as in controls. In the Parkinson's model, protein turnover is reduced proportionally over the entire proteome, suggesting that the protein homeostasis network has a strong ability to adapt. The findings shed light on the relationship between protein turnover and healthy aging.

Untangling Longevity, Dauer, and Healthspan in Caenorhabditis elegans Insulin/IGF-1-Signalling

The groundbreaking discovery that lower levels of insulin/IGF-1 signaling (IIS) can induce lifespan extension was reported 24 years ago in the nematode Caenorhabditis elegans. In this organism, mutations in the insulin/IGF-1 receptor gene daf-2 or other genes in this pathway can double lifespan. Subsequent work has revealed that reduced IIS (rIIS) extends lifespan across diverse species, possibly including humans. In C. elegans, IIS also regulates development into the diapause state known as dauer, a quiescent larval form that enables C. elegans to endure harsh environments through morphological adaptation, improved cellular repair, and slowed metabolism. Considerable progress has been made uncovering mechanisms that are affected by C. elegans rIIS. However, from the beginning it has remained unclear to what extent rIIS extends C. elegans lifespan by mobilizing dauer-associated mechanisms in adults. As we discuss, recent work has shed light on this question by determining that rIIS can extend C. elegans lifespan comparably through downstream processes that are either dauer-related or -independent. Importantly, these two lifespan extension programs can be distinguished genetically. It will now be critical to tease apart these programs, because each may involve different longevity-promoting mechanisms that may be relevant to higher organisms. A recent analysis of organismal "healthspan" has questioned the value of C. elegans rIIS as a paradigm for understanding healthy aging, as opposed to simply extending life. We discuss other work that argues strongly that C. elegans rIIS is indeed an invaluable model and consider the likely possibility that dauer-related processes affect parameters associated with health under rIIS conditions. Together, these studies indicate that C. elegans and analyses of rIIS in this organism will continue to provide unexpected and exciting results, and new paradigms that will be valuable for understanding healthy aging in humans.

Lifespan extension by peroxidase and dual oxidase-mediated ROS signaling through pyrroloquinoline quinone in C. elegans

Reactive oxygen species (ROS), originally characterized based on their harmful effects on cells or organisms, are now recognized as important signal molecules regulating various biological processes. In particular, low levels of ROS released from mitochondria extend lifespan. Here, we identified a novel mechanism of generating appropriate levels of ROS at the plasma membrane through a peroxidase and dual oxidase (DUOX) system, which could extend lifespan in Caenorhabditis elegans A redox co-factor, pyrroloquinoline quinone (PQQ), activates the C. elegans DUOX protein BLI-3 to produce the ROS H₂O₂ at the plasma membrane, which is subsequently degraded by peroxidase (MLT-7), eventually ensuring adequate levels of ROS. These ROS signals are transduced mainly by the oxidative stress transcriptional factors SKN-1 (Nrf2 or NFE2L2 in mammals) and JUN-1, and partially by DAF-16 (a FOXO protein homolog). Cell biology experiments demonstrated a similarity between the mechanisms of PQQ-induced activation of human DUOX1 and DUOX2 and that of C. elegans BLI-3, suggesting that DUOXs are potential targets of intervention for lifespan extension. We propose that low levels of ROS, fine-tuned by the peroxidase and dual oxidase system at the plasma membrane, act as second messengers to extend lifespan by the effect of hormesis.

The Ubiquitin Ligase CHIP Integrates Proteostasis and Aging by Regulation of Insulin Receptor Turnover

Aging is attended by a progressive decline in protein homeostasis (proteostasis), aggravating the risk for protein aggregation diseases. To understand the coordination between proteome imbalance and longevity, we addressed the mechanistic role of the quality-control ubiquitin ligase CHIP, which is a key regulator of proteostasis. We observed that CHIP deficiency leads to increased levels of the insulin receptor (INSR) and reduced lifespan of worms and flies. The membrane-bound INSR regulates the insulin and IGF1 signaling (IIS) pathway and thereby defines metabolism and aging. INSR is a direct target of CHIP, which triggers receptor monoubiquitylation and endocytic-lysosomal turnover to promote longevity. However, upon proteotoxic stress conditions and during aging, CHIP is recruited toward disposal of misfolded proteins, reducing its capacity to degrade the INSR. Our study indicates a competitive relationship between proteostasis and longevity regulation through CHIP-assisted proteolysis, providing a mechanistic concept for understanding the impact of proteome imbalance on aging.

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The ubiquitin ligase CHIP triggers insulin receptor turnover

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Insulin receptor level is linked to insulin and IGF1 signaling and longevity

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Engagement of CHIP in protein quality control limits insulin receptor degradation

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Proteotoxic stress aggravates insulin receptor stability, drives aging, and shortens lifespan

Caenorhabditis elegans DAF-2 as a Model for Human Insulin Receptoropathies

Human exome sequencing has dramatically increased the rate of identification of disease-associated polymorphisms. However, examining the functional consequences of those variants has created an analytic bottleneck. Insulin-like signaling in Caenorhabditis elegans has long provided a model to assess consequences of human insulin signaling mutations, but this has not been evaluated in the context of current genetic tools. We have exploited strains derived from the Million Mutation Project (MMP) and gene editing to explore further the evolutionary relationships and conservation between the human and C. elegans insulin receptors. Of 40 MMP alleles analyzed in the C. elegans insulin-like receptor gene DAF-2, 35 exhibited insulin-like signaling indistinguishable from wild-type animals, indicating tolerated mutations. Five MMP alleles proved to be novel dauer-enhancing mutations, including one new allele in the previously uncharacterized C-terminus of DAF-2. CRISPR-Cas9 genome editing was used to confirm the phenotypic consequence of six of these DAF-2 mutations and to replicate an allelic series of known human disease mutations in a highly conserved tyrosine kinase active site residue, demonstrating the utility of C. elegans for directly modeling human disease. Our results illustrate the challenges associated with prediction of the phenotypic consequences of amino acid substitutions, the value of assaying mutant isoform function in vivo, and how recently developed tools and resources afford the opportunity to expand our understanding even of highly conserved regulatory modules such as insulin signaling. This approach may prove generally useful for modeling phenotypic consequences of candidate human pathogenic mutations in conserved signaling and developmental pathways.

Context-dependent regulation of feeding behaviour by the insulin receptor, DAF-2, in Caenorhabditis elegans

Insulin signalling plays a significant role in both developmental programmes and pathways modulating the neuronal signalling that controls adult behaviour. Here, we have investigated insulin signalling in food-associated behaviour in adult C. elegans by scoring locomotion and feeding on and off bacteria, the worm's food. This analysis used mutants (daf-2, daf-18) of the insulin signalling pathway, and we provide evidence for an acute role for insulin signalling in the adult nervous system distinct from its impact on developmental programmes. Insulin receptor daf-2 mutants move slower than wild type both on and off food and showed impaired locomotory responses to food deprivation. This latter behaviour is manifest as a failure to instigate dispersal following prolonged food deprivation and suggests a role for insulin signalling in this adaptive response. Insulin receptor daf-2 mutants are also deficient in pharyngeal pumping on food and off food. Pharmacological analysis showed the pharynx of daf-2 is selectively compromised in its response to 5-HT compared to the excitatory neuropeptide FLP-17. By comparing the adaptive pharyngeal behaviour in intact worms and isolated pharyngeal preparations, we determined that an insulin-dependent signal extrinsic to the pharyngeal system is involved in feeding adaptation. Hence, we suggest that reactive insulin signalling modulates both locomotory foraging and pharyngeal pumping as the animal adapts to the absence of food. We discuss this in the context of insulin signalling directing a shift in the sensitivity of neurotransmitter systems to regulate the worm's response to changes in food availability in the environment.

Synthetic Ligands of Cannabinoid Receptors Affect Dauer Formation in the Nematode Caenorhabditis elegans

Under adverse environmental conditions the nematode Caenorhabditis elegans can enter an alternate developmental stage called the dauer larva. To identify lipophilic signaling molecules that influence this process, we screened a library of bioactive lipids and found that AM251, an antagonist of the human cannabinoid (CB) receptor, suppresses dauer entry in daf-2 insulin receptor mutants. AM251 acted synergistically with glucose supplementation indicating that the metabolic status of the animal influenced the activity of this compound. Similarly, loss of function mutations in the energy-sensing AMP-activated kinase subunit, aak-2, enhanced the dauer-suppressing effects of AM251, while constitutive activation of aak-2 in neurons was sufficient to inhibit AM251 activity. Chemical epistasis experiments indicated that AM251 acts via G-protein signaling and requires the TGF-β ligand DAF-7, the insulin peptides DAF-28 and INS-6, and a functional ASI neuron to promote reproductive growth. AM251 also required the presence of the SER-5 serotonin receptor, but in vitro experiments suggest that this may not be via a direct interaction. Interestingly, we found that other antagonists of mammalian CB receptors also suppress dauer entry, while the nonselective CB receptor agonist, O-2545, not only inhibited the activity of AM251, but also was able to promote dauer entry when administered alone. Since worms do not have obvious orthologs of CB receptors, the effects of synthetic CBs on neuroendocrine signaling in C. elegans are likely to be mediated via another, as yet unknown, receptor mechanism. However, we cannot exclude the existence of a noncanonical CB receptor in C. elegans.

Increased Protein Stability and Decreased Protein Turnover in the Caenorhabditis elegans Ins/IGF-1 daf-2 Mutant

In Caenorhabditis elegans, cellular proteostasis is likely essential for longevity. Autophagy has been shown to be essential for lifespan extension of daf-2 insulin/IGF mutants. Therefore, it can be hypothesized that daf-2 mutants achieve this phenotype by increasing protein turnover. However, such a mechanism would exert a substantial energy cost. By using classical ³⁵S pulse-chase labeling, we observed that protein synthesis and degradation rates are decreased in young adults of the daf-2 insulin/IGF mutants. Although reduction of protein turnover may be energetically favorable, it may lead to accumulation and aggregation of damaged proteins. As this has been shown not to be the case in daf-2 mutants, another mechanism must exist to maintain proteostasis in this strain. We observed that proteins isolated from daf-2 mutants are more soluble in acidic conditions due to increased levels of trehalose. This suggests that trehalose may decrease the potential for protein aggregation and increases proteostasis in the daf-2 mutants. We postulate that daf-2 mutants save energy by decreasing protein turnover rates and instead stabilize their proteome by trehalose.

Metabolic Youth in Middle Age: Predicting Aging in Caenorhabditis elegans Using Metabolomics

Many mutations and allelic variants are known that influence the rate at which animals age, but when in life do such variants diverge from normal patterns of aging? Is this divergence visible in their physiologies? To investigate these questions, we have used (1)H NMR spectroscopy to study how the metabolome of the nematode Caenorhabditis elegans changes as it grows older. We identify a series of metabolic changes that, collectively, predict the age of wild-type worms. We then show that long-lived mutant daf-2(m41) worms are metabolically youthful compared to wild-type worms, but that this relative youth only appears in middle age. Finally, we show that metabolic age predicts the timing and magnitude of differences in age-specific mortality between these strains. Thus, the future mortality of these two genotypes can be predicted long before most of the worms die.

Prowashonupana barley dietary fibre reduces body fat and increases insulin sensitivity in Caenorhabditis elegans model

Prowashonupana barley (PWB) is high in β-glucan with moderate content of resistant starch. PWB reduced intestinal fat deposition (IFD) in wild type Caenorhabditis elegans (C. elegans, N2), and in sir-2.1 or daf-16 null mutants, and sustained a surrogate marker of lifespan, pharyngeal pumping rate (PPR), in N2, sir-2.1, daf-16, or daf-16/daf-2 mutants. Hyperglycaemia (2% glucose) reversed or reduced the PWB effect on IFD in N2 or daf-16/daf-2 mutants with a sustained PPR. mRNA expression of cpt-1, cpt-2, ckr-1, and gcy-8 were dose-dependently reduced in N2 or daf-16 mutants, elevated in daf-16/daf-2 mutants with reduction in cpt-1, and unchanged in sir-2.1 mutants. mRNA expressions were increased by hyperglycaemia in N2 or daf-16/daf-2 mutants, while reduced in sir-2.1 or daf-16 mutants. The effects of PWB in the C. elegans model appeared to be primarily mediated via sir-2.1, daf-16, and daf-16/daf-2. These data suggest that PWB and β-glucans may benefit hyperglycaemia-impaired lipid metabolism.

Oat consumption reduced intestinal fat deposition and improved health span in Caenorhabditis elegans model

In addition to their fermentable dietary fiber and the soluble β-glucan fiber, oats have unique avenanthramides that have anti-inflammatory and antioxidant properties that reduce coronary heart disease in human clinical trials. We hypothesized that oat consumption will increase insulin sensitivity, reduce body fat, and improve health span in Caenorhabditis elegans through a mechanism involving the daf-2 gene, which codes for the insulin/insulin-like growth factor-1–like receptor, and that hyperglycemia will attenuate these changes. Caenorhabditis elegans wild type (N2) and the null strains sir-2.1, daf-16, and daf-16/daf-2 were fed Escherichia coli (OP50) and oat flakes (0.5%, 1.0%, or 3%) with and without 2% glucose. Oat feeding decreased intestinal fat deposition in N2, daf-16, or daf-16/daf-2 strains (P < .05); and glucose did not affect intestinal fat deposition response. The N2, daf-16, or sir-2.1 mutant increased the pharyngeal pumping rate (P < .05), a surrogate marker of life span, following oat consumption. Oat consumption increased ckr-1, gcy-8, cpt-1, and cpt-2 mRNA expression in both the N2 and the sir-2.1 mutant, with significantly higher expression in sir-2.1 than in N2 (P < .01). Additional glucose further increased expression 1.5-fold of the 4 genes in N2 (P < .01), decreased the expression of all except cpt-1 in the daf-16 mutant, and reduced mRNA expression of the 4 genes in the daf-16/daf-2 mutant (P < .01). These data suggest that oat consumption reduced fat storage and increased ckr-1, gcy-8, cpt-1, or cpt-2 through the sir-2.1 genetic pathway. Oat consumption may be a beneficial dietary intervention for reducing fat accumulation, augmenting health span, and improving hyperglycemia-impaired lipid metabolism.

Dauer-independent insulin/IGF-1-signalling implicates collagen remodelling in longevity

Interventions that delay ageing mobilize mechanisms that protect and repair cellular components, but it is unknown how these interventions might slow the functional decline of extracellular matrices, which are also damaged during ageing. Reduced insulin/IGF-1 signalling (rIIS) extends lifespan across the evolutionary spectrum, and in juvenile Caenorhabditis elegans also allows the transcription factor DAF-16/FOXO to induce development into dauer, a diapause that withstands harsh conditions. It has been suggested that rIIS delays C. elegans ageing through activation of dauer-related processes during adulthood, but some rIIS conditions confer robust lifespan extension unaccompanied by any dauer-like traits. Here we show that rIIS can promote C. elegans longevity through a program that is genetically distinct from the dauer pathway, and requires the Nrf (NF-E2-related factor) orthologue SKN-1 acting in parallel to DAF-16. SKN-1 is inhibited by IIS and has been broadly implicated in longevity, but is rendered dispensable for rIIS lifespan extension by even mild activity of dauer-related processes. When IIS is decreased under conditions that do not induce dauer traits, SKN-1 most prominently increases expression of collagens and other extracellular matrix genes. Diverse genetic, nutritional, and pharmacological pro-longevity interventions delay an age-related decline in collagen expression. These collagens mediate adulthood extracellular matrix remodelling, and are needed for ageing to be delayed by interventions that do not involve dauer traits. By genetically delineating a dauer-independent rIIS ageing pathway, our results show that IIS controls a broad set of protective mechanisms during C. elegans adulthood, and may facilitate elucidation of processes of general importance for longevity. The importance of collagen production in diverse anti-ageing interventions implies that extracellular matrix remodelling is a generally essential signature of longevity assurance, and that agents promoting extracellular matrix youthfulness may have systemic benefit.

Regulated spatial organization and sensitivity of cytosolic protein oxidation in Caenorhabditis elegans

Cells adjust their behavior in response to redox events by regulating protein activity through the reversible formation of disulfide bridges between cysteine thiols. However, the spatial and temporal control of these modifications remains poorly understood in multicellular organisms. Here, we measured the protein thiol-disulfide balance in live C. elegans using a genetically-encoded redox sensor and found that it is specific to tissues and patterned spatially within a tissue. Insulin signaling regulates the sensor's oxidation at both of these levels. Unexpectedly, we found that isogenic individuals exhibit large differences in the sensor's thiol-disulfide balance. This variation contrasts with the general view that glutathione acts as the main cellular redox buffer. Indeed, our work suggests that glutathione converts small changes in its oxidation level into large changes in its redox potential. We therefore propose that glutathione facilitates the sensitive control of the thioldisulfide balance of target proteins in response to cellular redox events.

DAF-16/FoxO directly regulates an atypical AMP-activated protein kinase gamma isoform to mediate the effects of insulin/IGF-1 signaling on aging in Caenorhabditis elegans

The DAF-16/FoxO transcription factor controls growth, metabolism and aging in Caenorhabditis elegans. The large number of genes that it regulates has been an obstacle to understanding its function. However, recent analysis of transcript and chromatin profiling implies that DAF-16 regulates relatively few genes directly, and that many of these encode other regulatory proteins. We have investigated the regulation by DAF-16 of genes encoding the AMP-activated protein kinase (AMPK), which has α, β and γ subunits. C. elegans has 5 genes encoding putative AMP-binding regulatory γ subunits, aakg-1-5. aakg-4 and aakg-5 are closely related, atypical isoforms, with orthologs throughout the Chromadorea class of nematodes. We report that ∼75% of total γ subunit mRNA encodes these 2 divergent isoforms, which lack consensus AMP-binding residues, suggesting AMP-independent kinase activity. DAF-16 directly activates expression of aakg-4, reduction of which suppresses longevity in daf-2 insulin/IGF-1 receptor mutants. This implies that an increase in the activity of AMPK containing the AAKG-4 γ subunit caused by direct activation by DAF-16 slows aging in daf-2 mutants. Knock down of aakg-4 expression caused a transient decrease in activation of expression in multiple DAF-16 target genes. This, taken together with previous evidence that AMPK promotes DAF-16 activity, implies the action of these two metabolic regulators in a positive feedback loop that accelerates the induction of DAF-16 target gene expression. The AMPK β subunit, aakb-1, also proved to be up-regulated by DAF-16, but had no effect on lifespan. These findings reveal key features of the architecture of the gene-regulatory network centered on DAF-16, and raise the possibility that activation of AMP-independent AMPK in nutritionally replete daf-2 mutant adults slows aging in C. elegans. Evidence of activation of AMPK subunits in mammals suggests that such FoxO-AMPK interactions may be evolutionarily conserved.

Aging is an important problem for human health and is regulated by complex gene regulatory networks. In a simple nematode worm (Caenorhabditis elegans) mutation of the insulin/IGF-1 receptor daf-2 dramatically extends lifespan. This is due to the increased activity of DAF-16, a FoxO transcription factor, leading to altered expression of genes, many encoding other regulatory proteins. We have focused on one such protein, AMP-activated protein kinase (AMPK), that is important for regulating cellular homeostasis under conditions of low energy availability (e.g. starvation). We find that DAF-16 binds to the promoter of aakg-4 (a gene encoding an atypical γ subunit of AMPK) and increases its expression. Inhibition of aakg-4 leads to down-regulation of multiple DAF-16 target genes and shortens the life of daf-2 mutants. Taken together with a previous report showing that AMPK activates DAF-16, this suggests that AAKG-4 and DAF-16 are involved in a positive feedback loop which accelerates effects of DAF-16 on gene expression, and could contribute to longevity. This study defines a new part of the complex gene regulatory network in which DAF-16 acts to control aging. FoxO-AMPK interactions are present in higher animals, where they could potentially also influence aging.

Nonautonomous regulation of neuronal migration by insulin signaling, DAF-16/FOXO, and PAK-1

Neuronal migration is essential for nervous system development in all organisms and is regulated in the nematode, C. elegans, by signaling pathways that are conserved in humans. Here, we demonstrate that the insulin/IGF-1-PI3K signaling pathway modulates the activity of the DAF-16/FOXO transcription factor to regulate the anterior migrations of the hermaphrodite-specific neurons (HSNs) during embryogenesis of C. elegans. When signaling is reduced, DAF-16 is activated and promotes migration; conversely, when signaling is enhanced, DAF-16 is inactivated, and migration is inhibited. We show that DAF-16 acts nonautonomously in the hypodermis to promote HSN migration. Furthermore, we identify PAK-1, a p21-activated kinase, as a downstream mediator of insulin/IGF-1-DAF-16 signaling in the nonautonomous control of HSN migration. Because a FOXO-Pak1 pathway was recently shown to regulate mammalian neuronal polarity, our findings indicate that the roles of FOXO and Pak1 in neuronal migration are most likely conserved from C. elegans to higher organisms.

Targeted heritable mutation and gene conversion by Cas9-CRISPR in Caenorhabditis elegans

We have achieved targeted heritable genome modification in Caenorhabditis elegans by injecting mRNA of the nuclease Cas9 and Cas9 guide RNAs. This system rapidly creates precise genomic changes, including knockouts and transgene-instructed gene conversion.

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.

Influence of steroid hormone signaling on life span control by Caenorhabditis elegans insulin-like signaling

Sterol-sensing nuclear receptors and insulin-like growth factor signaling play evolutionarily conserved roles in the control of aging. In the nematode Caenorhabditis elegans, bile acid-like steroid hormones known as dafachronic acids (DAs) influence longevity by binding to and regulating the activity of the conserved nuclear receptor DAF-12, and the insulin receptor (InsR) ortholog DAF-2 controls life span by inhibiting the FoxO transcription factor DAF-16. How the DA/DAF-12 pathway interacts with DAF-2/InsR signaling to control life span is poorly understood. Here we specifically investigated the roles of liganded and unliganded DAF-12 in life span control in the context of reduced DAF-2/InsR signaling. In animals with reduced daf-2/InsR activity, mutations that either reduce DA biosynthesis or fully abrogate DAF-12 activity shorten life span, suggesting that liganded DAF-12 promotes longevity. In animals with reduced DAF-2/InsR activity induced by daf-2/InsR RNAi, both liganded and unliganded DAF-12 promote longevity. However, in daf-2/InsR mutants, liganded and unliganded DAF-12 act in opposition to control life span. Thus, multiple DAF-12 activities influence life span in distinct ways in contexts of reduced DAF-2/InsR signaling. Our findings establish new roles for a conserved steroid signaling pathway in life span control and elucidate interactions among DA biosynthetic pathways, DAF-12, and DAF-2/InsR signaling in aging.

Insulin/IGF-1 signaling, including class II/III PI3Ks, β-arrestin and SGK-1, is required in C. elegans to maintain pharyngeal muscle performance during starvation

In C. elegans, pharyngeal pumping is regulated by the presence of bacteria. In response to food deprivation, the pumping rate rapidly declines by about 50–60%, but then recovers gradually to baseline levels on food after 24 hr. We used this system to study the role of insulin/IGF-1 signaling (IIS) in the recovery of pharyngeal pumping during starvation. Mutant strains with reduced function in the insulin/IGF-1 receptor, DAF-2, various insulins (INS-1 and INS-18), and molecules that regulate insulin release (UNC-64 and NCA-1; NCA-2) failed to recover normal pumping rates after food deprivation. Similarly, reduction or loss of function in downstream signaling molecules (e.g., ARR-1, AKT-1, and SGK-1) and effectors (e.g., CCA-1 and UNC-68) impaired pumping recovery. Pharmacological studies with kinase and metabolic inhibitors implicated class II/III phosphatidylinositol 3-kinases (PI3Ks) and glucose metabolism in the recovery response. Interestingly, both over- and under-activity in IIS was associated with poorer recovery kinetics. Taken together, the data suggest that optimum levels of IIS are required to maintain high levels of pharyngeal pumping during starvation. This work may ultimately provide insights into the connections between IIS, nutritional status and sarcopenia, a hallmark feature of aging in muscle.

Stress response pathways protect germ cells from omega-6 polyunsaturated fatty acid-mediated toxicity in Caenorhabditis elegans

Polyunsaturated fatty acids serve both structural and functional roles as membrane components and precursors for a number of different factors involved in inflammation and signaling. These fatty acids are required in the human diet, although excess dietary intake of omega-6 fatty polyunsaturated fatty acids may have a negative influence on human health. In the model nematode, Caenorhabditis elegans, dietary exposure to dihomo-gamma-linolenic acid (DGLA), an omega-6 fatty acid, causes the destruction of germ cells and leads to sterility. In this study we used genetic and microscopic approaches to further characterize this phenomenon. We found that strains carrying mutations in genes involved in lipid homeostasis enhanced sterility phenotypes, while mutations reducing the activity of the conserved insulin/IGF signaling pathway suppressed sterility phenotypes. Exposure to a mild heat stress prior to omega-6 fatty acid treatment led to an adaptive or hormetic response, resulting in less sterility. Mutations in skn-1 and knockdown of genes encoding phase II detoxification enzymes led to increased sterility in the presence of dietary DGLA. Thus, detoxification systems and genetic changes that increase overall stress responses protect the germ cells from destruction. Microscopic analyses revealed that dietary DGLA leads to deterioration of germ cell membranes in the proliferative and transition zones of the developing germ line. Together, these data demonstrate that specific omega-6 polyunsaturated fatty acids, or molecules derived from them, are transported to the germ line where they disrupt the rapidly expanding germ cell membranes, leading to germ cell death.

Noncanonical control of C. elegans germline apoptosis by the insulin/IGF-1 and Ras/MAPK signaling pathways

The insulin/IGF-1 pathway controls a number of physiological processes in the nematode worm Caenorhabditis elegans, including development, aging and stress response. We previously found that the Akt/PKB ortholog AKT-1 dampens the apoptotic response to genotoxic stress in the germline by negatively regulating the p53-like transcription factor CEP-1. Here, we report unexpected rearrangements to the insulin/IGF-1 pathway, whereby the insulin-like receptor DAF-2 and 3-phosphoinositide-dependent protein kinase PDK-1 oppose AKT-1 to promote DNA damage-induced apoptosis. While DNA damage does not affect phosphorylation at the PDK-1 site Thr350/Thr308 of AKT-1, it increased phosphorylation at Ser517/Ser473. Although ablation of daf-2 or pdk-1 completely suppressed akt-1-dependent apoptosis, the transcriptional activation of CEP-1 was unaffected, suggesting that daf-2 and pdk-1 act independently or downstream of cep-1 and akt-1. Ablation of the akt-1 paralog akt-2 or the downstream target of the insulin/IGF-1 pathway daf-16 (a FOXO transcription factor) restored sensitivity to damage-induced apoptosis in daf-2 and pdk-1 mutants. In addition, daf-2 and pdk-1 mutants have reduced levels of phospho-MPK-1/ERK in their germ cells, indicating that the insulin/IGF-1 pathway promotes Ras signaling in the germline. Ablation of the Ras effector gla-3, a negative regulator of mpk-1, restored sensitivity to apoptosis in daf-2 mutants, suggesting that gla-3 acts downstream of daf-2. In addition, the hypersensitivity of let-60/Ras gain-of-function mutants to damage-induced apoptosis was suppressed to wild-type levels by ablation of daf-2. Thus, insulin/IGF-1 signaling selectively engages AKT-2/DAF-16 to promote DNA damage-induced germ cell apoptosis downstream of CEP-1 through the Ras pathway.

TDP-1/TDP-43 regulates stress signaling and age-dependent proteotoxicity in Caenorhabditis elegans

TDP-43 is a multifunctional nucleic acid binding protein linked to several neurodegenerative diseases including Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia. To learn more about the normal biological and abnormal pathological role of this protein, we turned to Caenorhabditis elegans and its orthologue TDP-1. We report that TDP-1 functions in the Insulin/IGF pathway to regulate longevity and the oxidative stress response downstream from the forkhead transcription factor DAF-16/FOXO3a. However, although tdp-1 mutants are stress-sensitive, chronic upregulation of tdp-1 expression is toxic and decreases lifespan. ALS–associated mutations in TDP-43 or the related RNA binding protein FUS activate the unfolded protein response and generate oxidative stress leading to the daf-16–dependent upregulation of tdp-1 expression with negative effects on neuronal function and lifespan. Consistently, deletion of endogenous tdp-1 rescues mutant TDP-43 and FUS proteotoxicity in C. elegans. These results suggest that chronic induction of wild-type TDP-1/TDP-43 by cellular stress may propagate neurodegeneration and decrease lifespan.

TAR DNA Binding Protein 43 (TDP-43) is implicated in several human age-dependent neurodegenerative disorders, but until now little was known about TDP-43's role in the aging process. Here we used the nematode Caenorhabditis elegans to study the role of the TDP-43 orthologue tdp-1 in aging and neurodegeneration. In this study we discovered that tdp-1 is a stress-responsive gene acting within the Insulin/IGF signaling pathway to regulate lifespan and the response to oxidative stress. We found that, although worms missing tdp-1 were stress-sensitive, elevated expression of tdp-1 was toxic. We asked if tdp-1 also responded to the stress caused by toxic proteins found in Amyotrophic Lateral Sclerosis (ALS). Using worm models for ALS, we discovered that mutant TDP-43 generated oxidative stress and induced tdp-1 expression with negative consequences on neuronal function and lifespan. Consistently, removing tdp-1 rescued toxicity in our worm ALS models. tdp-1's role in the cellular stress response likely reflects an ancient adaptation to deal with unfavorable environmental conditions that is inappropriately activated and maintained by genetic mutations leading to proteotoxic and oxidative stress. We predict that similar mechanisms may exist in humans, helping explain the involvement of TDP-43 in a growing number of neurodegenerative disorders.

TAR DNA Binding Protein 43 (TDP-43) is implicated in several human age-dependent neurodegenerative disorders, but until now little was known about TDP-43's role in the aging process. Here we used the nematode Caenorhabditis elegans to study the role of the TDP-43 orthologue tdp-1 in aging and neurodegeneration. In this study we discovered that tdp-1 is a stress-responsive gene acting within the Insulin/IGF signaling pathway to regulate lifespan and the response to oxidative stress. We found that, although worms missing tdp-1 were stress-sensitive, elevated expression of tdp-1 was toxic. We asked if tdp-1 also responded to the stress caused by toxic proteins found in Amyotrophic Lateral Sclerosis (ALS). Using worm models for ALS, we discovered that mutant TDP-43 generated oxidative stress and induced tdp-1 expression with negative consequences on neuronal function and lifespan. Consistently, removing tdp-1 rescued toxicity in our worm ALS models. tdp-1's role in the cellular stress response likely reflects an ancient adaptation to deal with unfavorable environmental conditions that is inappropriately activated and maintained by genetic mutations leading to proteotoxic and oxidative stress. We predict that similar mechanisms may exist in humans, helping explain the involvement of TDP-43 in a growing number of neurodegenerative disorders.

Absence of effects of Sir2 overexpression on lifespan in C. elegans and Drosophila

Overexpression of sirtuins (NAD(+)-dependent protein deacetylases) has been reported to increase lifespan in budding yeast (Saccharomyces cerevisiae), Caenorhabditis elegans and Drosophila melanogaster. Studies of the effects of genes on ageing are vulnerable to confounding effects of genetic background. Here we re-examined the reported effects of sirtuin overexpression on ageing and found that standardization of genetic background and the use of appropriate controls abolished the apparent effects in both C. elegans and Drosophila. In C. elegans, outcrossing of a line with high-level sir-2.1 overexpression abrogated the longevity increase, but did not abrogate sir-2.1 overexpression. Instead, longevity co-segregated with a second-site mutation affecting sensory neurons. Outcrossing of a line with low-copy-number sir-2.1 overexpression also abrogated longevity. A Drosophila strain with ubiquitous overexpression of dSir2 using the UAS-GAL4 system was long-lived relative to wild-type controls, as previously reported, but was not long-lived relative to the appropriate transgenic controls, and nor was a new line with stronger overexpression of dSir2. These findings underscore the importance of controlling for genetic background and for the mutagenic effects of transgene insertions in studies of genetic effects on lifespan. The life-extending effect of dietary restriction on ageing in Drosophila has also been reported to be dSir2 dependent. We found that dietary restriction increased fly lifespan independently of dSir2. Our findings do not rule out a role for sirtuins in determination of metazoan lifespan, but they do cast doubt on the robustness of the previously reported effects of sirtuins on lifespan in C. elegans and Drosophila.

O-linked-N-acetylglucosamine cycling and insulin signaling are required for the glucose stress response in Caenorhabditis elegans

In a variety of organisms, including worms, flies, and mammals, glucose homeostasis is maintained by insulin-like signaling in a robust network of opposing and complementary signaling pathways. The hexosamine signaling pathway, terminating in O-linked-N-acetylglucosamine (O-GlcNAc) cycling, is a key sensor of nutrient status and has been genetically linked to the regulation of insulin signaling in Caenorhabditis elegans. Here we demonstrate that O-GlcNAc cycling and insulin signaling are both essential components of the C. elegans response to glucose stress. A number of insulin-dependent processes were found to be sensitive to glucose stress, including fertility, reproductive timing, and dauer formation, yet each of these differed in their threshold of sensitivity to glucose excess. Our findings suggest that O-GlcNAc cycling and insulin signaling are both required for a robust and adaptable response to glucose stress, but these two pathways show complex and interdependent roles in the maintenance of glucose-insulin homeostasis.

Sensitive and precise quantification of insulin-like mRNA expression in Caenorhabditis elegans

Insulin-like signaling regulates developmental arrest, stress resistance and lifespan in the nematode Caenorhabditis elegans. However, the genome encodes 40 insulin-like peptides, and the regulation and function of individual peptides is largely uncharacterized. We used the nCounter platform to measure mRNA expression of all 40 insulin-like peptides as well as the insulin-like receptor daf-2, its transcriptional effector daf-16, and the daf-16 target gene sod-3. We validated the platform using 53 RNA samples previously characterized by high density oligonucleotide microarray analysis. For this set of genes and the standard nCounter protocol, sensitivity and precision were comparable between the two platforms. We optimized conditions of the nCounter assay by varying the mass of total RNA used for hybridization, thereby increasing sensitivity up to 50-fold and reducing the median coefficient of variation as much as 4-fold. We used deletion mutants to demonstrate specificity of the assay, and we used optimized conditions to assay insulin-like gene expression throughout the C. elegans life cycle. We detected expression for nearly all insulin-like genes and find that they are expressed in a variety of distinct patterns suggesting complexity of regulation and specificity of function. We identified insulin-like genes that are specifically expressed during developmental arrest, larval development, adulthood and embryogenesis. These results demonstrate that the nCounter platform provides a powerful approach to analyzing insulin-like gene expression dynamics, and they suggest hypotheses about the function of individual insulin-like genes.