Hormesis and aging in Caenorhabditis elegans

Hormesis has emerged as an important manipulation for the study of aging. Although hormesis is manifested in manifold combinations of stress and model organism, the mechanisms of hormesis are only partly understood. The increased stress resistance and extended survival caused by hormesis can be manipulated to further our understanding of the roles of intrinsic and induced stress resistance in aging. Genes of the dauer/insulin/insulin-like signaling (IIS) pathway have well-established roles in aging in Caenorhabditis elegans. Here, we discuss the role of some of those genes in the induced stress resistance and induced life extension attributable to hormesis. Mutations in three genes (daf-16, daf-18, and daf-12) block hormetically induced life extension. However, of these three, only daf-18 appears to be required for a full induction of thermotolerance induced by hormesis, illustrating possible separation of the genetic requirements for stress resistance and life extension. Mutations in three other genes of this pathway (daf-3, daf-5, and age-1) do not block induced life extension or induced thermotolerance; daf-5 mutants may be unusually sensitive to hormetic conditions.     

Neurosecretory control of aging in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, an insulin receptor signaling pathway regulates adult life span and developmental arrest at the dauer larval stage. Here we show that the unc-64 and unc-31 genes also function in this pathway. These two genes are involved in mediating Ca2+-regulated secretion. Mutations in unc-64 and unc-31 increase adult life span and cause constitutive dauer formation. Both phenotypes are suppressed by mutations in daf-16, which also suppresses other mutations in this pathway. We present evidence that the site of action of unc-64 is neuronal, suggesting that a neurosecretory signal regulates life span and dauer formation.     

秀丽线虫衰老的遗传调控机制

秀丽线虫生命周期短,衰老表型较为明显,遗传操作灵活便利,是一种理想的衰老研究模式生物。自1988年第一株线虫长寿突变体age-1产生以来,秀丽线虫迅速成为衰老研究的首要模式生物。研究者利用秀丽线虫,通过遗传学研究方法,逐步阐明了多条寿命调节相关通路。     

Advantages and disadvantages of Caenorhabditis elegans for aging research

The nematode Caenorhabditis elegans has been the organism of choice for most aging research, especially genetic approaches to aging. More than 70 longevity genes have been identified, with more to come, and these genes have been the subjects of intense study. I identify the major reasons for this and discuss limitations of this organism for future progress in research on aging.     

Identifying factors that promote functional aging in Caenorhabditis elegans

A major feature of aging is a reduction in muscle strength from sarcopenia, the loss of muscle mass. Sarcopenia impairs physical ability, reduces quality of life and increases the risk of fall and injury. Since aging is a process of stochastic decline, there may be many factors that impinge on the progression of sarcopenia. Possible factors that may promote muscle decline are contraction-related injury and oxidative stress. However, relatively little is understood about the cellular pathways affecting muscle aging, in part because lifespan studies are difficult to conduct in species with large muscles, such as rodents and primates. For this reason, shorter-lived invertebrate models of aging may be more useful for unraveling causes of sarcopenia and functional declines during aging. Recent studies have examined both physiological and genetic factors that affect aging-related declines in Caenorhabditis elegans nematodes. In C. elegans, aging leads to significant functional declines that correlate with muscle deterioration, similar to those documented for longer-lived vertebrates. This article will examine the current research into aging-related functional declines in this species, focusing on recent studies of locomotory and feeding decline during aging in the nematode, C. elegans.     

Pharmacology of delayed aging and extended lifespan of Caenorhabditis elegans

The identification and analysis of compounds that delay aging and extend lifespan is an important aspect of gerontology research; these studies can test theories of aging, lead to the discovery of endogenous systems that influence aging, and establish the foundation for treatments that might delay normal human aging. Here we review studies using the nematode Caenorhabditis elegans to identify and characterize compounds that delay aging and extend lifespan. These studies are considered in four groups: (1) Studies that address the free-radical theory of aging by analyzing candidate compounds with antioxidant activities including vitamin E, tocotrienols, coenzyme Q, and Eukarion-8/134. (2) Studies that analyze plant extracts (blueberry and Ginko biloba) that contain a mixture of compounds. (3) Studies of resveratrol, which was identified in a screen for compounds that affect the activity of the Sir2 protein that influences lifespan. (4) Studies based on screening compound libraries using C. elegans aging as a bioassay, which led to the identification of the anticonvulsant medicines ethosuximide and trimethadione. There has been exciting progress in the analysis of compounds that influence C. elegans aging, and important challenges and opportunities remain in determining the mechanisms of action of these compounds and the relevance of these observations to aging of other animals.     

Genetics, life span, health span, and the aging process in Caenorhabditis elegans

As a tool for measuring the aging process, life span has been invaluable in dissecting the genes that modulate longevity. Studies over the past few decades have identified several hundred genes that can modify life span in model organisms such as yeast, worms, and flies. Yet, despite this vast amount of research, we still do not fully understand how the genes that affect life span influence how an organism ages. How does modulation of the genes that affect life span contribute to the aging process? Does life-span extension result in extension of healthy aging? Here, we will focus primarily on the insulin/IGF-1 signaling pathway in Caenorhabditis elegans because members of this pathway have been shown to be associated with extended life span across phylogeny, from worms to humans. I discuss how this connects to the aging process, age-associated disease, and the potential to increase healthy aging in addition to lengthening life span.     

Body size,insulin/IGF signaling and aging in the nematode Caenorhabditis elegans

A number of recent studies of aging in Drosophila, mice and dogs have shown an association between reduced body size and increased lifespan. It is unclear (a) whether such an association is a general feature of animal species; and (b) whether the association reflects an effect of body size on aging, or pleiotropic effects of common determinants of growth and aging. To address these issues, we have studied the relationship between size and lifespan in the nematode Caenorhabditis elegans, and surveyed related findings in Drosophila. In C. elegans, we compared 12 wild isolates with varying body size and lifespan, but saw no correspondence between these traits. We also examined aging in giant and dwarf mutants, but observed only reduced lifespan in all cases. In a comparison of 15 long-lived daf-2 insulin/IGF receptor mutants, we saw a positive correlation between body length and lifespan, and up to a 28% increase in daf-2 mutant body volume. Thus, in C. elegans, insulin/IGF signaling may limit growth rather than promote it. Studies of Drosophila show no consistent correlation between body size and lifespan. These results indicate that the negative correlation between body size and lifespan seen in some mammals is not typical of invertebrates, but support the view that co-variation of size and longevity may occur via effects on insulin/IGF signaling.     

Synchronous growth and aging of Caenorhabditis elegans in the presence of fluorodeoxyuridine.

Fluorodeoxyuridine (FUdR), an inhibitor of DNA synthesis, was examined for its ability to prevent a synchronous population of C. elegans from reproducing without otherwise interfering with the organism's post-maturational development and aging. When a synchronized population was exposed to 400 micrometer FUdR just as the population reached sexual maturity, the FUdR induced complete sterility within five hours by preventing eggs from hatching. Any larvae that hatched from eggs made before the FUdR was added remained small in the presence of FUdR and were easily removed by filtration or sedimentation. FUdR-sterilized adults showed no morphological abnormalities. Age-associated changes seen in controls also occurred in FUdR-treated worms, including atrophy of the gonads, increased pigmentation, sluggishness and increased transparency. Life span was not shortened by FUdR treatment. Our observations suggest that treatment with FUdR under carefully controlled conditions is a reasonable way to maintain synchronously aging populations of C. elegans.     

An hypothesis concerning control networks and aging in Drosophila melanogaster and Caenorhabditis elegans

To explain trends emerging from the study of longevity mutants, a modification of the reactive oxygen species (ROS) model of aging is suggested. ROS do not appear to be produced in greater quantities during cellular activity unless specific factors are also present. These include raised cytosolic calcium and sodium ions, nitric oxide (NO) or dopamine. Metabolically active cells that are repeatedly exposed to these factors, especially in combination, show the most ROS damage and may contribute most to aging. This explains the importance of neurons, which is highlighted by genetic studies, and points to which cells are the most aging-sensitive. Control pathways disrupted in long-lived mutants are those which control one or more of these ROS promoting factors. The daf-2/daf-16 pathway may interact with these factors in several ways. Changes in control networks may be propagated from a relatively small number of cells/junctions by neural connectivity and hormonal means.     

Essential roles for four cytoplasmic intermediate filament proteins in Caenorhabditis elegans development

The structural proteins of the cytoplasmic intermediate filaments (IFs) arise in the nematode Caenorhabditis elegans from eight reported genes and an additional three genes now identified in the complete genome. With the use of double-stranded RNA interference (RNAi) for all 11 C. elegans genes encoding cytoplasmic IF proteins, we observe phenotypes for the five genes A1, A2, A3, B1, and C2. These range from embryonic lethality (B1) and embryonic/larval lethality (A3) to larval lethality (A1 and A2) and a mild dumpy phenotype of adults (C2). Phenotypes A2 and A3 involve displaced body muscles and paralysis. They probably arise by reduction of hypodermal IFs that participate in the transmission of force from the muscle cells to the cuticle. The B1 phenotype has multiple morphogenetic defects, and the A1 phenotype is arrested at the L1 stage. Thus, at least four IF genes are essential for C. elegans development. Their RNAi phenotypes are lethal defects due to silencing of single IF genes. In contrast to C. elegans, no IF genes have been identified in the complete Drosophila genome, posing the question of how Drosophila can compensate for the lack of these proteins, which are essential in mammals and C. elegans. We speculate that the lack of IF proteins in Drosophila can be viewed as cytoskeletal alteration in which, for instance, stable microtubules, often arranged as bundles, substitute for cytoplasmic IFs.     

GABAergic synapses suppress intestinal innate immunity via insulin signaling in Caenorhabditis elegans

GABAergic neurotransmission constitutes a major inhibitory signaling mechanism that plays crucial roles in central nervous system physiology and immune cell immunomodulation. However, its roles in innate immunity remain unclear. Here, we report that deficiency in the GABAergic neuromuscular junctions (NMJs) of Caenorhabditis elegans results in enhanced resistance to pathogens, whereas pathogen infection enhances the strength of GABAergic transmission. GABAergic synapses control innate immunity in a manner dependent on the FOXO/DAF-16 but not the p38/PMK-1 pathway. Our data reveal that the insulin-like peptide INS-31 level was dramatically decreased in the GABAergic NMJ GABA_AR-deficient unc-49 mutant compared with wild-type animals. C. elegans with ins-31 knockdown or loss of function exhibited enhanced resistance to Pseudomonas aeruginosa PA14 exposure. INS-31 may act downstream of GABAergic NMJs and in body wall muscle to control intestinal innate immunity in a cell-nonautonomous manner. Our results reveal a signaling axis of synapse–muscular insulin–intestinal innate immunity in vivo.

Innate immunity, an evolutionally conserved behavior, constitutes the first defense line of multiple organisms to prevent microbial infections (1). The nematode Caenorhabditis elegans has been used as a model host for human opportunistic pathogen Pseudomonas aeruginosa infection (2) to identify evolutionarily conserved mechanisms of innate immunity. Typically, p38/PMK-1 mitogen-activated protein kinases (MAPKs) (3) and insulin/insulin-like signaling (IIS)/DAF-2 signaling cascades are recognized as two key components of the C. elegans intestinal innate immune response upon P. aeruginosa strain PA14 infection (4), as they are in mammals (3, 4). Moreover, increasing evidence has revealed several neural mechanisms as also being involved in the regulation of innate immunity. For example, G protein–coupled receptor (GPCR) NPR-1– and soluble guanylate cyclase GCY-35–expressing sensory neurons actively suppress the immune response of nonneuronal tissues (5). Additionally, a putative octopamine GPCR, OCTR-1, which is expressed and functions in the C. elegans sensory neurons ASH and ASI (6), down-regulates the unfolded protein response genes pqn/abu to further suppress the immune response of nonneuronal tissues (5, 6).Recent studies demonstrate that dopaminergic signaling inhibits innate immunity (7) whereas neuronal acetylcholine stimulates muscarinic signaling in the epithelium and activates the epithelial canonical Wnt pathway to promote the ability to defend against bacterial infection (8). Moreover, insulin-like peptide INS-7 secreted by the nervous system functions in a cell-nonautonomous manner to activate the IIS/DAF-2 pathway and modulate the intestinal innate immunity of C. elegans (9).GABAergic signaling constitutes a major inhibitory neurotransmission system that plays crucial roles in the central nervous system, especially for maintaining the balance between excitation and inhibition of neuronal networks (10). Disruption of this balance is not only linked to several neuropsychiatric disorders including schizophrenia, autism, and epilepsy (11) but also implicated in autoimmune disease (12). Up to date, multiple lines of evidence have shown that GABAergic signaling cell-autonomously modulates the immune response in immune cells (13–15). However, the roles of GABAergic synapses in innate immunity remain unknown.Here, we found that the nematode C. elegans harboring a deficiency in GABAergic neuromuscular junctions (NMJs) exhibits enhanced resistance to pathogens. P. aeruginosa PA14 infection increases synaptic expression of GABAergic synaptic components at the nerve cord of worms and enhances the strength of GABAergic transmission. Moreover, we identified an insulin-like peptide, INS-31, acting downstream of GABAergic NMJs and in body wall muscle (BWM) to control intestinal innate immunity in a cell-nonautonomous manner. This work reveals a signaling axis of synapse–muscular insulin–intestinal innate immunity in vivo.     

Characterization of a life-extending mutation in age-2, a new aging gene in Caenorhabditis elegans

We have generated a life-extending mutation, yw23, in Caenorhabditis elegans. The mutation is in what appears to be a new aging gene, which we have designated age-2. When homozygous, yw23 produces an increase of mean and maximum life span of about 20% over that of the wild-type strain, N2. Strain HG23 [age-2(yw23)] was obtained by screening for longer life spans among 430 lines of nematodes two generations after exposure to the mutagen ethylmethanesulfonate. Strain HG231 [age-2(yw23)] was obtained after a single out-crossing of HG23 to N2. When compared with N2, HG231 exhibits normal motility, slightly higher swimming rates, reduced fertility (especially at higher temperatures), somewhat longer development times, and a slightly larger size at the time of first egg laying. A Gompertz analysis suggests that HG231 extends life span by reducing the initial mortality rate. In genetic crosses, yw23 complements other known aging mutants in C. elegans genes-age-1, daf-2, spe-26, clk-1, clk-2, clk-3, and gro-1. A double-mutant strain, HG284, combining mutations in age-1 and age-2, lives longer than animals with individual mutations in either age-1 or age-2, and exhibits a longer life span at 25 degrees C than at 20 degrees C.