An insulin-like signaling pathway affects both longevity and reproduction in Caenorhabditis elegans.

Mutations in daf-2 and age-1 cause a dramatic increase in longevity as well as developmental arrest at the dauer diapause stage in Caenorhabditis elegans. daf-2 and age-1 encode components of an insulin-like signaling pathway. Both daf-2 and age-1 act at a similar point in the genetic epistasis pathway for dauer arrest and longevity and regulate the activity of the daf-16 gene. Mutations in daf-16 cause a dauer-defective phenotype and are epistatic to the diapause arrest and life span extension phenotypes of daf-2 and age-1 mutants. Here we show that mutations in this pathway also affect fertility and embryonic development. Weak daf-2 alleles, and maternally rescued age-1 alleles that cause life span extension but do not arrest at the dauer stage, also reduce fertility and viability. We find that age-1(hx546) has reduced both maternal and zygotic age-1 activity. daf-16 mutations suppress all of the daf-2 and age-1 phenotypes, including dauer arrest, life span extension, reduced fertility, and viability defects. These data show that insulin signaling, mediated by DAF-2 through the AGE-1 phosphatidylinositol-3-OH kinase, regulates reproduction and embryonic development, as well as dauer diapause and life span, and that DAF-16 transduces these signals. The regulation of fertility, life span, and metabolism by an insulin-like signaling pathway is similar to the endocrine regulation of metabolism and fertility by mammalian insulin signaling.     

Caenorhabditis elegans as a model system to study aging of learning and memory

The nematode Caenorhabditis elegans is an excellent model organism to study biological processes relevant to a wide variety of human and rodent disease systems. Previous studies have suggested that mutants of the insulin/insulin-like growth factor-1 pathway show life extension and increased stress resistance in various species, including C. elegans, the fruit fly, and the mouse. It has recently been shown that the life-extending mutants, including the age-1 phosphatidylinositol- 3 OH kinase mutants and the daf-2 insulin-like receptor mutants, display improvement in a type of associative learning behavior called thermotaxis learning behavior. The age-1 mutant shows a dramatic threefold extension of the health-span that ensures thermotaxis learning behavior, suggesting strong neuroprotective actions during aging. The age-1 and daf-2 mutants show resistance to multiple forms of stress and upregulates the genes involved in reactive oxygen species scavenging, heat shock, and P450 drug-detoxification. The life-extending mutants may confer resistance to various stress and diseases in neurons. Therefore, C. elegans provides an emerging system for the prevention of age-related deficits in the nervous system and in learning behaviors. This article discusses the aging of learning and memory and the neuroprotection effects of life-extending mutants on learning behaviors.     

LET-60 RAS modulates effects of insulin/IGF-1 signaling on development and aging in Caenorhabditis elegans

The DAF-2 insulin/insulin-like growth factor 1 (IGF-1) receptor signals via a phosphatidylinositol 3-kinase (PI3K) pathway to control dauer larva formation and adult longevity in Caenorhabditis elegans. Yet epistasis analysis suggests signal bifurcation downstream of DAF-2. We have used epistasis analysis to test whether the Ras pathway (which plays a role in signaling from mammalian insulin receptors) acts downstream of DAF-2. We find that an activated Ras mutation, let-60(n1046gf), weakly suppresses constitutive dauer diapause in daf-2 and age-1 (PI3K) mutants. Moreover, increased Ras pathway signaling partially suppresses the daf-2 mutant feeding defect, while reduced Ras pathway signaling enhances it. By contrast, activated Ras extends the longevity induced by mutation of daf-2, while reduced Ras pathway signaling partially suppresses it. Thus, Ras pathway signaling appears to act with insulin/IGF-1 signaling during larval development, but against it during aging.     

Suppressors of transforming growth factor-beta pathway mutants in the Caenorhabditis elegans dauer formation pathway

The dauer is a developmentally arrested alternative third larval stage of Caenorhabditis elegans. Entry into this state is regulated by environmental cues, including temperature, food, and the concentration of constitutively secreted dauer pheromone. Genetically, three parallel pathways have been found that regulate this process. Of these, the group 2 pathway, which includes the genes daf-1, daf-3, daf-4, daf-5, daf-7, daf-8, and daf-14, mediates the transduction of environmental signals through the ASI chemosensory neuron and encodes a TGF-beta-related signaling pathway. To identify additional genes that function in this pathway, we carried out a screen for suppressors of mutations in daf-1, daf-8, and daf-14. From the total of 36 mutations, seven complementation groups were identified. Three complementation groups correspond to the previously described genes daf-3, daf-5, and daf-12. Three correspond to novel genes scd-1, scd-2, and scd-3. Genetic analysis of these scd genes is presented here. A fourth complementation group was represented by a single mutation sa315, which affects the daf-2/age-1 insulin-related signaling pathway.     

A model of chemotaxis and associative learning in C. elegans

The nematode C. elegans has attracted a great deal of interest from the neuroscience community due to the simplicity of its nervous system, which in the hermaphrodite is composed of just 302 neurons. C. elegans is known to engage in a number of sophisticated behaviours such as chemo- and thermotaxis. Experimental work has shown that these behaviours can be modified by experience and that C. elegans is capable of associative learning. In this paper, we focus on the chemotactic response of C. elegans to sodium chloride mediated by the ASE sensory neurons. We construct a biophysical model of the ASEL and ASER neurons that captures the time course of the ASE responses in response to up- and down-steps in NaCl concentration. We use this model to show that the time course of the ASE responses provide sufficient temporal resolution to successfully drive chemotaxis in C. elegans via steering, pirouettes and control of final turn angle. We show that these different locomotion strategies are individually capable of driving chemotaxis and that by working together they produce the best chemotactic response. We find that there is a separation into upward and downward drives mediated by the left and right ASE neurons. We show that the connectivity from ASEL and ASER must be of opposite polarity and that ASER, and the concomitant ability to sense when the worm is moving down the gradient, is more important for chemotaxis than ASEL, findings that are consistent with existing modelling studies in the literature. Finally, we examine associative learning in the network and show that experimental data can be explained by changes that occur at either the synaptic or sensory neuron level, the choice of which has distinct consequences for network function.     

The Age-1 and Daf-2 Genes Function in a Common Pathway to Control the Lifespan of Caenorhabditis Elegans

Recessive mutations in two genes, daf-2 and age-1, extend the lifespan of Caenorhabditis elegans significantly. The daf-2 gene also regulates formation of an alternative developmental state called the dauer. Here we asked whether these two genes function in the same or different lifespan pathways. We found that the longevity of both age-1 and daf-2 mutants requires the activities of the same two genes, daf-16 and daf-18. In addition, the daf-2(e1370); age-1(hx546) double mutant did not live significantly longer than the daf-2 single mutant. We also found that, like daf-2 mutations, the age-1(hx546) mutation affects certain aspects of dauer formation. These findings suggest that age-1 and daf-2 mutations do act in the same lifespan pathway and extend lifespan by triggering similar if not identical processes.     

Metabolism, physiology and stress defense in three aging Ins/IGF-1 mutants of the nematode Caenorhabditis elegans

The insulin/insulin-like growth factor-1 (Ins/IGF-1) pathway regulates the aging rate of the nematode Caenorhabditis elegans. We describe other features of the three Ins/IGF-1 mutants daf-2, age-1 and aap-1. We show that the investigated Ins/IGF-1 mutants all have a reduced body volume, reduced reproductive capacity, increased ATP concentrations and an elevated stress resistance. We also observed that heat production is lower in these mutants, although the respiration rate was similar or higher compared with wild-type individuals, suggesting a metabolic shift in these mutants.     

Protein oxidation during aging of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has proven a robust genetic model for studies of aging, including the roles of oxidative stress and protein damage. In this review, we focus on the genetics of select long-lived (e.g., age-1, daf-2, daf-16) and short-lived (e.g., mev-1) mutants that have proven useful in revealing the relationships that exist among oxidative stress, life span, and protein oxidation. The former are known to control an insulin/IGF-1-like pathway in C. elegans, while the latter affect mitochondrial function.     

A heady message for lifespan regulation

Mutant Caenorhabditis elegans in which the age-1 and daf-2 genes (involved in insulin-receptor-like signalling) are expressed at low levels exhibit extended lifespan. Wolkow and colleagues recently showed that restricted re-expression of age-1 and daf-2 genes in neurons of these mutants rescues wild-type lifespan as effectively as ubiquitous re-expression. Low levels of insulin-like signalling in neurons might control longevity by enhancing protection against free radical damage. However, in mammalian cells (including neurons) reduced insulin-like signalling is generally thought to be deleterious to antioxidant defence and to neuron survival. Here we discuss the new work and several hypotheses to explain this apparent contradiction.     

The miR-58 microRNA family is regulated by insulin signaling and contributes to lifespan regulation in Caenorhabditis elegans

microRNAs regulate diverse biological processes such as development and aging by promoting degradation or inhibiting translation of their target mRNAs. In this study, we have found that the miR-58 family microRNAs regulate lifespan in C.elegans. Intriguingly, members of the miR-58 family affect lifespan differently, sometimes in opposite directions, and have complex genetic interactions. The abundances of the miR-58 family miRNAs are up-regulated in the long-lived daf-2 mutant in a daf-16-dependent manner, indicating that these miRNAs are effectors of insulin signaling in C. elegans. We also found that miR-58 is regulated by insulin signaling and partially required for the lifespan extension mediated by reduced insulin signaling,germline ablation, dietary restriction, and mild mitochondrial dysfunction. We further identified the daf-21, ins-1, and isw-1 mRNAs as endogenous targets of miR-58. Our study shows that miRNAs function in multiple lifespan extension mechanisms,and that the seed sequence is not the dominant factor defining the role of a miRNA in lifespan regulation.     

Genetic Analysis of the Roles of Daf-28 and Age-1 in Regulating Caenorhabditis Elegans Dauer Formation

Based on environmental cues, the nervous system of Caenorhabditis elegans regulates formation of the dauer larva, an alternative larval form specialized for long-term survival under harsh conditions. Mutations that cause constitutive or defective dauer formation (Daf-c or Daf-d) have been identified and the genes ordered in a branched pathway. Most Daf-c mutations also affect recovery from the dauer stage. The semi-dominant mutation daf-28(sa191) is Daf-c but has no apparent effect on dauer recovery. We use this unique aspect of daf-28(sa191) to characterize the effects of several Daf-d and synthetic Daf-c mutations on dauer recovery. We present double mutant analysis that indicates that daf-28(sa191) acts at a novel point downstream in the genetic pathway for dauer formation. We also show that daf-28(sa191) causes a modest increase (12-13%) in life span. The phenotypes and genetic interactions of daf-28(sa191) are most similar to those of daf-2 and daf-23 mutations, which also cause a dramatic increase in life span. We present mapping and complementation data that suggest that daf-23 is the same gene as age-1, identified previously by mutations that extend life span. We find that age-1 alleles are also Daf-c at 27°.     

Molecular Basis for Antioxidant Enzymes in Mediating Copper Detoxification in the Nematode Caenorhabditis elegans

Antioxidant enzymes play a major role in defending against oxidative damage by copper. However, few studies have been performed to determine which antioxidant enzymes respond to and are necessary for copper detoxification. In this study, we examined both the activities and mRNA levels of SOD, CAT, and GPX under excessive copper stress in Caenorhabditis elegans, which is a powerful model for toxicity studies. Then, taking advantage of the genetics of this model, we assessed the lethal concentration (LC₅₀) values of copper for related mutant strains. The results showed that the SOD, CAT, and GPX activities were significantly greater in treated groups than in controls. The mRNA levels of sod-3, sod-5, ctl-1, ctl-2, and almost all gpx genes were also significantly greater in treated groups than in controls. Among tested mutants, the sod-5, ctl-1, gpx-3, gpx-4, and gpx-6 variants exhibited hypersensitivity to copper. The strains with SOD or CAT over expression were reduced sensitive to copper. Mutations in daf-2 and age-1, which are involved in the insulin/insulin-like growth factor-1 signaling pathway, result in reduced sensitivity to stress. Here, we showed that LC₅₀ values for copper in daf-2 and age-1 mutants were significantly greater than in N2 worms. However, the LC₅₀ values in daf-16;daf-2 and daf-16;age-1 mutants were significantly reduced than in daf-2 and age-1 mutants, implying that reduced copper sensitivity is influenced by DAF-16-related functioning. SOD, CAT, and GPX activities and the mRNA levels of the associated copper responsive genes were significantly increased in daf-2 and age-1 mutants compared to N2. Additionally, the activities of SOD, CAT, and GPX were greater in these mutants than in N2 when treated with copper. Our results not only support the theory that antioxidant enzymes play an important role in copper detoxification but also identify the response and the genes involved in these processes.