Bacterial Siderophores Promote Animal Host Iron Acquisition and Growth.

Siderophores are small molecules produced by bacteria that bind ferric iron in the surrounding environment with extraordinary affinity. A new study provides evidence that a simple animal host, Caenorhabditis elegans, co-opts siderophores to promote its own iron acquisition and growth.

Chemosensation of bacterial secondary metabolites modulates neuroendocrine signaling and behavior of C. elegans.

Discrimination between pathogenic and beneficial microbes is essential for host organism immunity and homeostasis. Here, we show that chemosensory detection of two secondary metabolites produced by Pseudomonas aeruginosa modulates a neuroendocrine signaling pathway that promotes avoidance behavior in the simple animal host Caenorhabditis elegans. Secondary metabolites phenazine-1-carboxamide and pyochelin activate a G-protein-signaling pathway in the ASJ chemosensory neuron pair that induces expression of the neuromodulator DAF-7/TGF-ß. DAF-7, in turn, activates a canonical TGF-ß signaling pathway in adjacent interneurons to modulate aerotaxis behavior and promote avoidance of pathogenic P. aeruginosa. Our data provide a chemical, genetic, and neuronal basis for how the behavior and physiology of a simple animal host can be modified by the microbial environment and suggest that secondary metabolites produced by microbes may provide environmental cues that contribute to pathogen recognition and host survival.

IRE1 Sulfenylation by Reactive Oxygen Species Coordinates Cellular Stress Signaling.

In this issue of Molecular Cell, Hourihan et al. (2016) show that sulfenylation of IRE1 by reactive oxygen species inhibits the role of IRE1 in the Unfolded Protein Response and activates a SKN-1/Nrf2-dependent response to oxidative stress.

Immediate activation of chemosensory neuron gene expression by bacterial metabolites is selectively induced by distinct cyclic GMP-dependent pathways in Caenorhabditis elegans.

Dynamic gene expression in neurons shapes fundamental processes in the nervous systems of animals. However, how neuronal activation by different stimuli can lead to distinct transcriptional responses is not well understood. We have been studying how microbial metabolites modulate gene expression in chemosensory neurons of Caenorhabditis elegans. Considering the diverse environmental stimuli that can activate chemosensory neurons of C. elegans, we sought to understand how specific transcriptional responses can be generated in these neurons in response to distinct cues. We have focused on the mechanism of rapid (<6 min) and selective transcriptional induction of daf-7, a gene encoding a TGF-ß ligand, in the ASJ chemosensory neurons in response to the pathogenic bacterium Pseudomonas aeruginosa. DAF-7 is required for the protective behavioral avoidance of P. aeruginosa by C. elegans. Here, we define the involvement of two distinct cyclic GMP (cGMP)-dependent pathways that are required for daf-7 expression in the ASJ neuron pair in response to P. aeruginosa. We show that a calcium-independent pathway dependent on the cGMP-dependent protein kinase G (PKG) EGL-4, and a canonical calcium-dependent signaling pathway dependent on the activity of a cyclic nucleotide-gated channel subunit CNG-2, function in parallel to activate rapid, selective transcription of daf-7 in response to P. aeruginosa metabolites. Our data suggest that fast, selective early transcription of neuronal genes require PKG in shaping responses to distinct microbial stimuli in a pair of C. elegans chemosensory neurons.

Global transcriptional regulation of innate immunity by ATF-7 in C. elegans.

The nematode Caenorhabditis elegans has emerged as a genetically tractable animal host in which to study evolutionarily conserved mechanisms of innate immune signaling. We previously showed that the PMK-1 p38 mitogen-activated protein kinase (MAPK) pathway regulates innate immunity of C. elegans through phosphorylation of the CREB/ATF bZIP transcription factor, ATF-7. Here, we have undertaken a genomic analysis of the transcriptional response of C. elegans to infection by Pseudomonas aeruginosa, combining genome-wide expression analysis by RNA-seq with ATF-7 chromatin immunoprecipitation followed by sequencing (ChIP-Seq). We observe that PMK-1-ATF-7 activity regulates a majority of all genes induced by pathogen infection, and observe ATF-7 occupancy in regulatory regions of pathogen-induced genes in a PMK-1-dependent manner. Moreover, functional analysis of a subset of these ATF-7-regulated pathogen-induced target genes supports a direct role for this transcriptional response in host defense. The genome-wide regulation through PMK-1- ATF-7 signaling reveals a striking level of control over the innate immune response to infection through a single transcriptional regulator.

Bacteria and the aging and longevity of Caenorhabditis elegans.

The molecular genetic analysis of longevity of Caenorhabditis elegans has yielded fundamental insights into evolutionarily conserved pathways and processes governing the physiology of aging. Recent studies suggest that interactions between C. elegans and its microbial environment may influence the aging and longevity of this simple host organism. Experimental evidence supports a role for bacteria in affecting longevity through distinct mechanisms--as a nutrient source, as a potential pathogen that induces double-edged innate immune and stress responses, and as a coevolved sensory stimulus that modulates neuronal signaling pathways regulating longevity. Motivating this review is the anticipation that the molecular genetic dissection of the integrated host immune, stress, and neuroendocrine responses to microbes in C. elegans will uncover basic insights into the cellular and organismal physiology that governs aging and longevity.

Host-microbe interactions and the behavior of Caenorhabditis elegans.

Microbes are ubiquitous in the natural environment of Caenorhabditis elegans. Bacteria serve as a food source for C. elegans but may also cause infection in the nematode host. The sensory nervous system of C. elegans detects diverse microbial molecules, ranging from metabolites produced by broad classes of bacteria to molecules synthesized by specific strains of bacteria. Innate recognition through chemosensation of bacterial metabolites or mechanosensation of bacteria can induce immediate behavioral responses. The ingestion of nutritive or pathogenic bacteria can modulate internal states that underlie long-lasting behavioral changes. Ingestion of nutritive bacteria leads to learned attraction and exploitation of the bacterial food source. Infection, which is accompanied by activation of innate immunity, stress responses, and host damage, leads to the development of aversive behavior. The integration of a multitude of microbial sensory cues in the environment is shaped by experience and context. Genetic, chemical, and neuronal studies of C. elegans behavior in the presence of bacteria have defined neural circuits and neuromodulatory systems that shape innate and learned behavioral responses to microbial cues. These studies have revealed the profound influence that host-microbe interactions have in governing the behavior of this simple animal host.

Age-Dependent Neuroendocrine Signaling from Sensory Neurons Modulates the Effect of Dietary Restriction on Longevity of Caenorhabditis elegans.

Dietary restriction extends lifespan in evolutionarily diverse animals. A role for the sensory nervous system in dietary restriction has been established in Drosophila and Caenorhabditis elegans, but little is known about how neuroendocrine signals influence the effects of dietary restriction on longevity. Here, we show that DAF-7/TGFß, which is secreted from the C. elegans amphid, promotes lifespan extension in response to dietary restriction in C. elegans. DAF-7 produced by the ASI pair of sensory neurons acts on DAF-1/TGFß receptors expressed on interneurons to inhibit the co-SMAD DAF-3. We find that increased activity of DAF-3 in the presence of diminished or deleted DAF-7 activity abrogates lifespan extension conferred by dietary restriction. We also observe that DAF-7 expression is dynamic during the lifespan of C. elegans, with a marked decrease in DAF-7 levels as animals age during adulthood. We show that this age-dependent diminished expression contributes to the reduced sensitivity of aging animals to the effects of dietary restriction. DAF-7 signaling is a pivotal regulator of metabolism and food-dependent behavior, and our studies establish a molecular link between the neuroendocrine physiology of C. elegans and the process by which dietary restriction can extend lifespan.

Mutations in Nonessential eIF3k and eIF3l Genes Confer Lifespan Extension and Enhanced Resistance to ER Stress in Caenorhabditis elegans.

The translation initiation factor eIF3 is a multi-subunit protein complex that coordinates the assembly of the 43S pre-initiation complex in eukaryotes. Prior studies have demonstrated that not all subunits of eIF3 are essential for the initiation of translation, suggesting that some subunits may serve regulatory roles. Here, we show that loss-of-function mutations in the genes encoding the conserved eIF3k and eIF3l subunits of the translation initiation complex eIF3 result in a 40% extension in lifespan and enhanced resistance to endoplasmic reticulum (ER) stress in Caenorhabditis elegans. In contrast to previously described mutations in genes encoding translation initiation components that confer lifespan extension in C. elegans, loss-of-function mutations in eif-3.K or eif-3.L are viable, and mutants show normal rates of growth and development, and have wild-type levels of bulk protein synthesis. Lifespan extension resulting from EIF-3.K or EIF-3.L deficiency is suppressed by a mutation in the Forkhead family transcription factor DAF-16. Mutations in eif-3.K or eif-3.L also confer enhanced resistance to ER stress, independent of IRE-1-XBP-1, ATF-6, and PEK-1, and independent of DAF-16. Our data suggest a pivotal functional role for conserved eIF3k and eIF3l accessory subunits of eIF3 in the regulation of cellular and organismal responses to ER stress and aging.

Tissue expression pattern of PMK-2 p38 MAPK is established by the miR-58 family in C. elegans.

Analyses of gene expression profiles in evolutionarily diverse organisms have revealed a role for microRNAs in tuning tissue-specific gene expression. Here, we show that the relatively abundant and constitutively expressed miR-58 family of microRNAs sharply defines the tissue-specific expression of the broadly transcribed gene encoding PMK-2 p38 MAPK in Caenorhabditis elegans. Whereas PMK-2 functions redundantly with PMK-1 in the nervous system to regulate neuronal development and behavioral responses to pathogenic bacteria, the miR-58, miR-80, miR-81, and miR-82 microRNAs function redundantly to destabilize pmk-2 mRNA in non-neuronal cells with switch-like potency. Our data suggest a role for the miR-58 family in the establishment of neuronal-specific gene expression in C. elegans, and support a more general role for microRNAs in the establishment of tissue-specific gene expression.

Behavioral avoidance of pathogenic bacteria by Caenorhabditis elegans.

The simple animal host Caenorhabditis elegans utilizes its nervous system to respond to diverse microbial cues, and can engage in a protective behavioral avoidance response to environmental pathogens. This behavior bears hallmarks of an immune response, with sensors and recognition systems that trigger a protective response following a learning experience. Neuronal circuits required for aversive learning have been defined, revealing conserved signaling modules with dual roles in immunity and neuronal responses to pathogenic bacteria. Identification of natural polymorphisms that modulate avoidance behavior has enabled an improved understanding of host-microbe interactions at the molecular level. We review here these findings and discuss how the microbial cues and host responses defined in C. elegans may provide insight into evolutionarily diverse host-microbe interactions.

Natural polymorphisms in C. elegans HECW-1 E3 ligase affect pathogen avoidance behaviour.

Heritable variation in behavioural traits generally has a complex genetic basis, and thus naturally occurring polymorphisms that influence behaviour have been defined only in rare instances. The isolation of wild strains of Caenorhabditis elegans has facilitated the study of natural genetic variation in this species and provided insights into its diverse microbial ecology. C. elegans responds to bacterial infection with conserved innate immune responses and, although lacking the immunological memory of vertebrate adaptive immunity, shows an aversive learning response to pathogenic bacteria. Here, we report the molecular characterization of naturally occurring coding polymorphisms in a C. elegans gene encoding a conserved HECT domain-containing E3 ubiquitin ligase, HECW-1. We show that two distinct polymorphisms in neighbouring residues of HECW-1 each affect C. elegans behavioural avoidance of a lawn of Pseudomonas aeruginosa. Neuron-specific rescue and ablation experiments and genetic interaction analysis indicate that HECW-1 functions in a pair of sensory neurons to inhibit P. aeruginosa lawn avoidance behaviour through inhibition of the neuropeptide receptor NPR-1 (ref. 10), which we have previously shown promotes P. aeruginosa lawn avoidance behaviour. Our data establish a molecular basis for natural variation in a C. elegans behaviour that may undergo adaptive changes in response to microbial pathogens.

An essential role for XBP-1 in host protection against immune activation in C. elegans.

The detection and compensatory response to the accumulation of unfolded proteins in the endoplasmic reticulum (ER), termed the unfolded protein response (UPR), represents a conserved cellular homeostatic mechanism with important roles in normal development and in the pathogenesis of disease. The IRE1-XBP1/Hac1 pathway is a major branch of the UPR that has been conserved from yeast to human. X-box binding protein 1 (XBP1) is required for the differentiation of the highly secretory plasma cells of the mammalian adaptive immune system, but recent work also points to reciprocal interactions between the UPR and other aspects of immunity and inflammation. We have been studying innate immunity in the nematode Caenorhabditis elegans, having established a principal role for a conserved PMK-1 p38 mitogen-activated protein kinase (MAPK) pathway in mediating resistance to microbial pathogens. Here we show that during C. elegans development, XBP-1 has an essential role in protecting the host during activation of innate immunity. Activation of the PMK-1-mediated response to infection with Pseudomonas aeruginosa induces the XBP-1-dependent UPR. Whereas a loss-of-function xbp-1 mutant develops normally in the presence of relatively non-pathogenic bacteria, infection of the xbp-1 mutant with P. aeruginosa leads to disruption of ER morphology and larval lethality. Unexpectedly, the larval lethality phenotype on pathogenic P. aeruginosa is suppressed by loss of PMK-1-mediated immunity. Furthermore, hyperactivation of PMK-1 causes larval lethality in the xbp-1 mutant even in the absence of pathogenic bacteria. Our data establish innate immunity as a physiologically relevant inducer of ER stress during C. elegans development and indicate that an ancient, conserved role for XBP-1 may be to protect the host organism from the detrimental effects of mounting an innate immune response to microbes.

Physiological IRE-1-XBP-1 and PEK-1 signaling in Caenorhabditis elegans larval development and immunity.

Endoplasmic reticulum (ER) stress activates the Unfolded Protein Response, a compensatory signaling response that is mediated by the IRE-1, PERK/PEK-1, and ATF-6 pathways in metazoans. Genetic studies have implicated roles for UPR signaling in animal development and disease, but the function of the UPR under physiological conditions, in the absence of chemical agents administered to induce ER stress, is not well understood. Here, we show that in Caenorhabditis elegans XBP-1 deficiency results in constitutive ER stress, reflected by increased basal levels of IRE-1 and PEK-1 activity under physiological conditions. We define a dynamic, temperature-dependent requirement for XBP-1 and PEK-1 activities that increases with immune activation and at elevated physiological temperatures in C. elegans. Our data suggest that the negative feedback loops involving the activation of IRE-1-XBP-1 and PEK-1 pathways serve essential roles, not only at the extremes of ER stress, but also in the maintenance of ER homeostasis under physiological conditions.

A decline in p38 MAPK signaling underlies immunosenescence in Caenorhabditis elegans.

The decline in immune function with aging, known as immunosenescence, has been implicated in evolutionarily diverse species, but the underlying molecular mechanisms are not understood. During aging in Caenorhabditis elegans, intestinal tissue deterioration and the increased intestinal proliferation of bacteria are observed, but how innate immunity changes during C. elegans aging has not been defined. Here we show that C. elegans exhibits increased susceptibility to bacterial infection with age, and we establish that aging is associated with a decline in the activity of the conserved PMK-1 p38 mitogen-activated protein kinase pathway, which regulates innate immunity in C. elegans. Our data define the phenomenon of innate immunosenescence in C. elegans in terms of the age-dependent dynamics of the PMK-1 innate immune signaling pathway, and they suggest that a cycle of intestinal tissue aging, immunosenescence, and bacterial proliferation leads to death in aging C. elegans.

Caenorhabditis elegans NPR-1-mediated behaviors are suppressed in the presence of mucoid bacteria.

Caenorhabditis elegans exhibits a diverse range of behaviors in response to bacteria. The presence of bacterial food influences C. elegans aerotaxis, aggregation, locomotion, and pathogen avoidance behaviors through the activity of the NPR-1 neuropeptide receptor. Here, we show that mucoid strains of bacteria that produce an exopolysaccharide matrix do not induce NPR-1-dependent behaviors. In the presence of mucoid strains of bacteria, the C. elegans laboratory wild-type (WT) strain N2 exhibits behaviors characteristic of wild isolates and mutants with reduced NPR-1 activity. Specifically, N2 exhibits lawn bordering and roaming behavior on mucoid nonpathogenic bacteria and loss of pathogen avoidance on mucoid Pseudomonas aeruginosa. Alginate biosynthesis by laboratory and clinical isolates of mucoid P. aeruginosa is necessary and sufficient to attenuate NPR-1-mediated behavior and it suppresses C. elegans pathogen avoidance behavior. Our data suggest that the specific interaction with nonmucoid bacteria induces NPR-1-dependent behaviors of C. elegans. These observations provide an example of how exopolysaccharide matrix biosynthesis by a community of bacteria may inhibit specific host responses to microbes.

Neuroendocrine gene expression coupling of interoceptive bacterial food cues to foraging behavior of C. elegans.

Animal internal state is modulated by nutrient intake, resulting in behavioral responses to changing food conditions. The neural mechanisms by which internal states are generated and maintained are not well understood. Here, we show that in the nematode Caenorhabditis elegans, distinct cues from bacterial food - interoceptive signals from the ingestion of bacteria and gustatory molecules sensed from nearby bacteria - act antagonistically on the expression of the neuroendocrine TGF-beta ligand DAF-7 from the ASJ pair of sensory neurons to modulate foraging behavior. A positive-feedback loop dependent on the expression of daf-7 from the ASJ neurons acts to promote transitions between roaming and dwelling foraging states and influence the persistence of roaming states. SCD-2, the C. elegans ortholog of mammalian anaplastic lymphoma kinase (ALK), which has been implicated in the central control of metabolism of mammals, functions in the AIA interneurons to regulate foraging behavior and cell-non-autonomously control the expression of DAF-7 from the ASJ neurons. Our data establish how a dynamic neuroendocrine daf-7 expression feedback loop regulated by SCD-2 functions to couple sensing and ingestion of bacterial food to foraging behavior. We further suggest that this neuroendocrine feedback loop underlies previously characterized exploratory behaviors in C. elegans. Our data suggest that the expression of daf-7 from the ASJ neurons contributes to and is correlated with an internal state of 'unmet need' that regulates exploratory foraging behavior in response to bacterial cues in diverse physiological contexts.

Bacteria Are a Major Determinant of Orsay Virus Transmission and Infection in Caenorhabditis elegans.

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for Caenorhabditis elegans and represent an integral part of the natural environment of C. elegans. We determined the effects of bacteria isolated with C. elegans from its natural environment on the transmission of Orsay virus in C. elegans using quantitative virus transmission and host susceptibility assays. We observed that Ochrobactrum species promoted Orsay virus transmission, whereas Pseudomonas lurida MYb11 attenuated virus transmission relative to the standard laboratory bacterial food Escherichia coli OP50. We found that pathogenic Pseudomonas aeruginosa strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a C. elegans population on P. lurida MYb11 compared with Ochrobactrum vermis MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in presence of P. aeruginosa PA14. Genetic analysis of the determinants of P. aeruginosa required for attenuation of C. elegans susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the C. elegans microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.

Bacteria are a major determinant of Orsay virus transmission and infection in Caenorhabditis elegans.

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for Caenorhabditis elegans and represent an integral part of the natural environment of C. elegans. We determined the effects of bacteria isolated with C. elegans from its natural environment on the transmission of Orsay virus in C. elegans using quantitative virus transmission and host susceptibility assays. We observed that Ochrobactrum species promoted Orsay virus transmission, whereas Pseudomonas lurida MYb11 attenuated virus transmission relative to the standard laboratory bacterial food Escherichia coli OP50. We found that pathogenic Pseudomonas aeruginosa strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a C. elegans population on P. lurida MYb11 compared with Ochrobactrum vermis MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in the presence of P. aeruginosa PA14. Genetic analysis of the determinants of P. aeruginosa required for attenuation of C. elegans susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the C. elegans microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.

Germline mitotic quiescence and cell death are induced in Caenorhabditis elegans by exposure to pathogenic Pseudomonas aeruginosa.

The impact of exposure to microbial pathogens on animal reproductive capacity and germline physiology is not well understood. The nematode Caenorhabditis elegans is a bacterivore that encounters pathogenic microbes in its natural environment. How pathogenic bacteria affect host reproductive capacity of C. elegans is not well understood. Here, we show that exposure of C. elegans hermaphrodites to the Gram-negative pathogen Pseudomonas aeruginosa causes a marked reduction in brood size with concomitant reduction in the number of nuclei in the germline and gonad size. We define 2 processes that are induced that contribute to the decrease in the number of germ cell nuclei. First, we observe that infection with P. aeruginosa leads to the induction of germ cell apoptosis. Second, we observe that this exposure induces mitotic quiescence in the proliferative zone of the C. elegans gonad. Importantly, these processes appear to be reversible; when animals are removed from the presence of P. aeruginosa, germ cell apoptosis is abated, germ cell nuclei numbers increase, and brood sizes recover. The reversible germline dynamics during exposure to P. aeruginosa may represent an adaptive response to improve survival of progeny and may serve to facilitate resource allocation that promotes survival during pathogen infection.