Nuclear Hormone Receptor Regulation of MicroRNAs Controls Innate Immune Responses in C. elegans

Nuclear hormone receptors respond to small molecules such as retinoids or steroids and regulate development. Signaling in the conserved p38/PMK-1 MAP kinase pathway regulates innate immunity. In this study, we show that the Caenorhabditis elegans nuclear receptor DAF-12 negatively regulates the defense against pathogens via the downstream let-7 family of microRNAs, which directly target SKN-1, a gene downstream of PMK-1. These findings identify nuclear hormone receptors as components of innate immunity that crosstalk with the p38/PMK-1 MAP kinase pathway.     

WWP-1 Is a Novel Modulator of the DAF-2 Insulin-Like Signaling Network Involved in Pore-Forming Toxin Cellular Defenses in Caenorhabditis elegans

Pore-forming toxins (PFTs) are the single largest class of bacterial virulence factors. The DAF-2 insulin/insulin-like growth factor-1 signaling pathway, which regulates lifespan and stress resistance in Caenorhabditis elegans, is known to mutate to resistance to pathogenic bacteria. However, its role in responses against bacterial toxins and PFTs is as yet unexplored. Here we reveal that reduction of the DAF-2 insulin-like pathway confers the resistance of Caenorhabditis elegans to cytolitic crystal (Cry) PFTs produced by Bacillus thuringiensis. In contrast to the canonical DAF-2 insulin-like signaling pathway previously defined for aging and pathogenesis, the PFT response pathway diverges at 3-phosphoinositide-dependent kinase 1 (PDK-1) and appears to feed into a novel insulin-like pathway signal arm defined by the WW domain Protein 1 (WWP-1). In addition, we also find that WWP-1 not only plays an important role in the intrinsic cellular defense (INCED) against PFTs but also is involved in innate immunity against pathogenic bacteria Pseudomonas aeruginosa and in lifespan regulation. Taken together, our data suggest that WWP-1 and DAF-16 function in parallel within the fundamental DAF-2 insulin/IGF-1 signaling network to regulate fundamental cellular responses in C. elegans.     

HSF-1 regulators DDL-1/2 link insulin-like signaling to heat-shock responses and modulation of longevity

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Highlights? C. elegans DAF-2 insulin-like signaling negatively regulates HSF-1 activity ? DDL-1 and DDL-2 are negative regulators of HSF-1 ? Formation of a complex containing DDL-1/2 and HSF-1 inhibits HSF-1 activity ? DAF-2 signaling promotes DHIC formation by inhibiting DDL-1 phosphorylation     

Antinematode Activity of Violacein and the Role of the Insulin/IGF-1 Pathway in Controlling Violacein Sensitivity in Caenorhabditis elegans

The purple pigment violacein is well known for its numerous biological activities including antibacterial, antiviral, antiprotozoan, and antitumor effects. In the current study we identify violacein as the antinematode agent produced by the marine bacterium Microbulbifer sp. D250, thereby extending the target range of this small molecule. Heterologous expression of the violacein biosynthetic pathway in E. coli and experiments using pure violacein demonstrated that this secondary metabolite facilitates bacterial accumulation in the nematode intestine, which is accompanied by tissue damage and apoptosis. Nematodes such as Caenorhabditis elegans utilise a well-defined innate immune system to defend against pathogens. Using C. elegans as a model we demonstrate the DAF-2/DAF-16 insulin/IGF-1 signalling (IIS) component of the innate immune pathway modulates sensitivity to violacein-mediated killing. Further analysis shows that resistance to violacein can occur due to a loss of DAF-2 function and/or an increased function of DAF-16 controlled genes involved in antimicrobial production (spp-1) and detoxification (sod-3). These data suggest that violacein is a novel candidate antinematode agent and that the IIS pathway is also involved in the defence against metabolites from non-pathogenic bacteria.     

GPA-9 is a novel regulator of innate immunity against Escherichia coli foods in adult Caenorhabditis elegans

Innate immune responses to pathogens are governed by the nervous system. Here, we investigated the molecular mechanism underlying innate immunity in Caenorhabditis elegans against Escherichia coli OP50, a standard laboratory C. elegans food. Longevity was compared in worms fed live or UV‐killed OP50 at low or high density food condition (HDF). Expression of the antimicrobial gene lys‐8 was approximately 5‐fold higher in worms fed live OP50, suggesting activation of innate immunity upon recognition of OP50 metabolites. Lifespan was extended and SOD‐3 mRNA levels were increased in gpa‐9‐overexpressing gpa‐9XS worms under HDF in association with robust induction of insulin/IGF‐1 signaling (IIS). Expression of ins‐7 and daf‐28 that control lys‐8 expression was reduced in gpa‐9XS, indicating that GPA‐9‐mediated immunity is due in part to ins‐7 and daf‐28 downregulation. Our results suggest that OP50 metabolites in amphid neurons elicit innate immunity through the IIS pathway, and identify GPA‐9 as a novel regulator of both the immune system and aging in C. elegans.     

Glutamine's protection against cellular injury is dependent on heat shock factor-1

Glutamine (GLN) has been shown to protect cells, tissues, and whole organisms from stress and injury. Enhanced expression of heat shock protein (HSP) has been hypothesized to be responsible for this protection. To date, there are no clear mechanistic data confirming this relationship. This study tested the hypothesis that GLN-mediated activation of the HSP pathway via heat shock factor-1 (HSF-1) is responsible for cellular protection. Wild-type HSF-1 (HSF-1^+/+) and knockout (HSF-1^–/–) mouse fibroblasts were used in all experiments. Cells were treated with GLN concentrations ranging from 0 to 16 mM and exposed to heat stress injury in a concurrent treatment model. Cell viability was assayed with phenazine methosulfate plus tetrazolium salt, HSP-70, HSP-25, and nuclear HSF-1 expression via Western blot analysis, and HSF-1/heat shock element (HSE) binding via EMSA. GLN significantly attenuated heat-stress induced cell death in HSF-1^+/+ cells in a dose-dependent manner; however, the survival benefit of GLN was lost in HSF-1^–/– cells. GLN led to a dose-dependent increase in HSP-70 and HSP-25 expression after heat stress. No inducible HSP expression was observed in HSF-1^–/– cells. GLN increased unphosphorylated HSF-1 in the nucleus before heat stress. This was accompanied by a GLN-mediated increase in HSF-1/HSE binding and nuclear content of phosphorylated HSF-1 after heat stress. This is the first demonstration that GLN-mediated cellular protection after heat-stress injury is related to HSF-1 expression and cellular capacity to activate an HSP response. Furthermore, the mechanism of GLN-mediated protection against injury appears to involve an increase in nuclear HSF-1 content before stress and increased HSF-1 promoter binding and phosphorylation. knockout cells; amino acid; heat stress mechanism     

Origin of Toll-Like Receptor-Mediated Innate Immunity

Toll-related receptors (TLR) have been found in four animal phyla: Nematoda, Arthropoda, Echinodermata, and Chordata. No TLR has been identified thus far in acoelomates. TLR genes play a pivotal role in the innate immunity in both fruit fly and mammals. The prevailing view is that TLR-mediated immunity is ancient. The two pseudocoelomate TLRs, one each from Caenorhabditis elegans and Strongyloides stercoralis, were distinct from the coelomate ones. Further, the only TLR gene (Tol-1) in Ca. elegans did not appear to play a role in innate immunity. We argue that TLR-mediated innate immunity developed only in the coelomates, after they split from pseudocoelomates and acoelomates. We hypothesize that the function of TLR-mediated immunity is to prevent microbial infection in the body cavity present only in the coelomates. Phylogenetic analysis showed that almost all arthropod TLRs form a separate cluster from the mammalian counterparts. We further hypothesize that TLR-mediated immunity developed independently in the protostomia and deuterostomia coelomates.     

A trade-off switch of two immunological memories in Caenorhabditis elegans reinfected by bacterial pathogens

Recent studies have suggested that innate immune responses exhibit characteristics associated with memory linked to modulations in both vertebrates and invertebrates. However, the diverse evolutionary paths taken, particularly within the invertebrate taxa, should lead to similarly diverse innate immunity memory processes. Our understanding of innate immune memory in invertebrates primarily comes from studies of the fruit fly Drosophila melanogaster, the generality of which is unclear. Caenorhabditis elegans typically inhabits soil harboring a variety of fatal microbial pathogens; for this invertebrate, the innate immune system and aversive behavior are the major defensive strategies against microbial infection. However, their characteristics of immunological memory remains infantile. Here we discovered an immunological memory that promoted avoidance and suppressed innate immunity during reinfection with bacteria, which we revealed to be specific to the previously exposed pathogens. During this trade-off switch of avoidance and innate immunity, the chemosensory neurons AWB and ADF modulated production of serotonin and dopamine, which in turn decreased expression of the innate immunity-associated genes and led to enhanced avoidance via the downstream insulin-like pathway. Therefore, our current study profiles the immune memories during C. elegans reinfected by pathogenic bacteria and further reveals that the chemosensory neurons, the neurotransmitter(s), and their associated molecular signaling pathways are responsible for a trade-off switch between the two immunological memories.