Caenorhabditis elegans has recently been developed as a model for microbial pathogenesis, yet little is known about its immunological defenses. Previous work implicated insulin signaling in mediating pathogen resistance in a manner dependent on the transcriptional regulator DAF-16, but the mechanism has not been elucidated. We present evidence that C. elegans, like mammalian phagocytes, produces reactive oxygen species (ROS) in response to pathogens. Signs of oxidative stress occur in the intestine-the site of the host-pathogen interface-suggesting that ROS release is localized to this tissue. Evidence includes the accumulation of lipofuscin, a pigment resulting from oxidative damage, at this site. In addition, SOD-3, a superoxide dismutase regulated by DAF-16, is induced in intestinal tissue after exposure to pathogenic bacteria. Moreover, we show that the oxidative stress response genes sod-3 and ctl-2 are required for DAF-16-mediated resistance to Enterococcus faecalis using a C. elegans killing assay. We propose a model whereby C. elegans responds to pathogens by producing ROS in the intestine while simultaneously inducing a DAF-16-dependent oxidative stress response to protect adjacent tissues. Because insulin-signaling mutants overproduce oxidative stress response enzymes, the model provides an explanation for their increased resistance to pathogens.
Caenorhabditis elegans exhibits a food-associated behavior that is modulated by the past cultivation temperature. Mutations in INS-1, the homolog of human insulin, caused the defect in this integrative behavior. Mutations in DAF-2/insulin receptor and AGE-1/phosphatidylinositol 3 (PI-3)-kinase partially suppressed the defect of ins-1 mutants, and a mutation in DAF-16, a forkhead-type transcriptional factor, caused a weak defect. In addition, mutations in the secretory protein HEN-1 showed synergistic effects with INS-1. Expression of AGE-1 in any of the three interneurons, AIY, AIZ, or RIA, rescued the defect characteristic of age-1 mutants. Calcium imaging revealed that starvation induced INS-1-mediated down-regulation of AIZ activity. Our results suggest that INS-1, in cooperation with HEN-1, antagonizes the DAF-2 insulin-like signaling pathway to modulate interneuron activity required for food-associated integrative behavior. The secreted peptide hormone insulin modulates neural plasticity. Insulin and insulin receptors are expressed in several regions of the rat brain (Havrankova et al. 1978a,b), insulin receptors localize to post-synapses (Abbott et al. 1999), and insulin can produce long-term depression (LTD) of synaptic transmission through endocytosis of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in rat hippocampal CA1 neurons (Man et al. 2000). In addition, Phosphatidylinositol 3 (PI-3)-kinase that functions in the insulin signaling pathway is thought to induce long-term potentiation (LTP) of synaptic transmission in the dentate gyrus of rat (Kelly and Lynch 2000). These alterations by proteins of the insulin signaling pathway may be involved in learning and memory, but what kind of behavior the insulin signaling pathway modulates has been largely unknown.The nematode Caenorhabditis elegans is well suited for the analysis of the molecular and cellular mechanisms underlying neural plasticity because of its accessible genetics, stereotyped behavioral responses, and its simple nervous system consisting of 302 neurons whose connections are entirely known (White et al. 1986). Recently, physiological analysis of the neural circuit in live worms has become possible by the use of cameleon, a genetically encodable calcium indicator, to measure Ca 2+ concentration changes (Miyawaki et al. 1997;Kimura et al. 2004).C. elegans exhibits thermotaxis, an integrative behavior in which well-fed animals in a thermal gradient are attracted to their cultivation temperature, whereas starved animals avoid it (Hedgecock and Russell 1975;Mohri et al. 2005;Rankin 2005). This food-associated behavioral plasticity, regarded the most complex behavior in C. elegans, is an ideal behavioral paradigm for comprehensive study of neural plasticity at the molecular, physiological, and behavioral levels. In this study, we show that in cooperation with a secreted protein HEN-1, an insulin homolog INS-1, and insulin-like signaling pathway modulate neuronal activity of interneurons to execute thermotaxis behavior in...
Caenorhabditis elegans was recently developed as a model system to study both pathogen virulence mechanisms and host defense responses. We previously demonstrated that C. elegans produces reactive oxygen species (ROS) in response to exposure to the important gram-positive nosocomial pathogen Enterococcus faecalis. We also presented evidence of oxidative stress and upregulation of stress responses after exposure to the pathogen. As in mammalian systems, this new work shows that production of ROS for innate immune functions occurs via an NADPH oxidase. Specifically, reducing expression of a dual oxidase, Ce-Duox1/BLI-3, causes a decrease in ROS production in response to E. faecalis. We also present evidence that reduction of expression of Ce-Duox1/BLI-3 increases susceptibility to this pathogen, specifically when expression is reduced in the intestine and the hypodermis. Ce-Duox1/ BLI-3 was previously characterized as having a role in cuticle cross-linking. Two C. elegans mutants with point mutations in the peroxidase domain that exhibit severe cuticle defects were discovered to be unaffected in ROS production or pathogen susceptibility. These results demonstrate an important biological role for the peroxidase domain in cuticle cross-linking that is unrelated to ROS production. To further demonstrate the protective effects of the pathogen-induced ROS production, we show that antioxidants that scavenge ROS increase the sensitivity of the nematode to the infection, in stark contrast to their longevity-promoting effects under nonpathogenic conditions. In conclusion, we postulate that the generation of ROS by NADPH oxidases in the barrier epithelium is an ancient, highly conserved innate immune defense mechanism.
During infection, damage can occur to the host as an outcome of both pathogen virulence mechanisms and host defense strategies. Using aggregation of a model polyglutamine-containing protein as an indicator in Caenorhabditis elegans, we show that protein damage occurs specifically at the site of the host-pathogen interaction, the intestine, in response to various bacterial pathogens. We demonstrate that the insulin signaling pathway and the heat shock transcription factor (HSF-1) influence the amount of aggregation that occurs, in addition to heat shock proteins and oxidative stress enzymes. We also show that addition of the antioxidants epigallocatechin gallate and ␣-lipoic acid reduces polyglutamine aggregation. The influence of oxidative stress enzymes and exogenous antioxidants on protein aggregation suggests that reactive oxygen species produced by the host are a source of protein damage during infection. We propose a model in which heat shock proteins and oxidative stress enzymes regulated by insulin signaling and HSF-1 are required for tissue protection during infection, to minimize the effects of protein damage occurring as a result of host-pathogen interactions.Innate immunity is comprised of strategies that allow an organism to immediately defend itself against an invading pathogen. In addition to mechanisms that actively destroy the infecting agent, the organism must protect itself from damage. Damage to the host can occur from virulence mechanisms of the pathogen, but also as a side effect of the immune response of the host. One pathway that we have shown contributes greatly to pathogen resistance in the model host Caenorhabditis elegans is insulin signaling (1). In addition to increased pathogen resistance, loss of insulin signaling in C. elegans results in several cytoprotective phenotypes such as stress resistance (oxidative stress, heat stress) and long lifespan. The insulin signaling pathway consists of a receptor, DAF-2, that when stimulated activates a phosphatidylinositol 3-kinase signaling cascade that culminates in the phosphorylation and down-regulation of the transcription factor DAF-16. A mutation in DAF-2, or any of the other upstream signaling components prevents inhibition of DAF-16, causing greater transcriptional activity (reviewed in Refs. 2-5).It was previously shown that pathogen resistance mediated by increased DAF-16 activity is dependent, at least in part, on the heat shock transcription factor HSF-1. 2 The loss of hsf-1 in a daf-2 mutant, or a mutant that overexpresses daf-16, causes a reduction in pathogen resistance and the overexpression of hsf-1 increases pathogen resistance (6). HSF-1 likely mediates these effects by controlling the expression of genes encoding heat shock proteins (HSPs). HSPs are protein chaperones that bind to unfolded or damaged proteins and prevent aggregation until they can be refolded or recycled (7-11). HSPs regulated by HSF-1 were demonstrated to be protective against bacterial pathogens (6) and we showed that ROS, a possible source of cellular da...
The intestine plays an essential role in organism-wide regulatory networks in both vertebrates and invertebrates. In Caenorhabditis elegans, class 1 flr genes (flr-1, flr-3 and flr-4) act in the intestine and control growth rates and defecation cycle periods, while class 2 flr genes (flr-2, flr-5, flr-6 and flr-7) are characterized by mutations that suppress the slow growth of class 1 flr mutants. This study revealed that flr-2 gene controls antibacterial defense and intestinal color, confirming that flr-2 regulates intestinal functions. flr-2 encoded the only glycoprotein hormone alpha subunit in C. elegans and was expressed in certain neurons. Furthermore, FLR-2 bound to another secretory protein GHI-1, which belongs to a family of lipid-and lipopolysaccharide-binding proteins. A ghi-1 deletion mutation partially suppressed the short defecation cycle periods of class 1 flr mutants, and this effect was enhanced by flr-2 mutations. Thus, FLR-2 acts as a signaling molecule for the neural control of intestinal functions, which is achieved in a functional network involving class 1 and class 2 flr genes as well as ghi-1. These results are informative to studies of glycoprotein hormone signaling in higher animals.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.