Faculty Bibliography

Toll-like receptors (TLRs) play critical roles in innate immunity in many animal species. The sole TLR of C. elegans-TOL-1-is required for a pathogen-avoidance behavior, yet how it promotes this behavior is unknown. We show that for pathogen avoidance TOL-1 signaling is required in the chemosensory BAG neurons, where it regulates gene expression and is necessary for their chemosensory function. Genetic studies revealed that TOL-1 acts together with many conserved components of TLR signaling. BAG neurons are activated by carbon dioxide (CO2), and we found that this modality is required for pathogen avoidance. TLR signaling can therefore mediate host responses to microbes through an unexpected mechanism: by promoting the development and function of chemosensory neurons that surveil the metabolic activity of environmental microbes.

A beetle pheromone that lures nematode worms to an insect host can also stop their development or even kill them outright.

Animals from diverse phyla possess neurons that are activated by the product of aerobic respiration, CO2. It has long been thought that such neurons primarily detect the CO2 metabolites protons and bicarbonate. We have determined the chemical tuning of isolated CO2 chemosensory BAG neurons of the nematode Caenorhabditis elegans. We show that BAG neurons are principally tuned to detect molecular CO2, although they can be activated by acid stimuli. One component of the BAG transduction pathway, the receptor-type guanylate cyclase GCY-9, suffices to confer cellular sensitivity to both molecular CO2 and acid, indicating that it is a bifunctional chemoreceptor. We speculate that in other animals, receptors similarly capable of detecting molecular CO2 might mediate effects of CO2 on neural circuits and behavior.

Aerobic metabolism is fundamental for almost all animal life. Cellular consumption of oxygen (O(2)) and production of carbon dioxide (CO(2)) signal metabolic states and physiological stresses. These respiratory gases are also detected as environmental cues that can signal external food quality and the presence of prey, predators and mates. In both contexts, animal nervous systems are endowed with mechanisms for sensing O(2)/CO(2) to trigger appropriate behaviors and maintain homeostasis of internal O(2)/CO(2). Although different animal species show different behavioral responses to O(2)/CO(2), some underlying molecular mechanisms and pathways that function in the detection of respiratory gases are fundamentally similar and evolutionarily conserved. Studies of Caenorhabditis elegans and Drosophila melanogaster have identified roles for cyclic nucleotide signaling and the hypoxia inducible factor (HIF) transcriptional pathway in mediating behavioral responses to respiratory gases. Understanding how simple invertebrate nervous systems detect respiratory gases to control behavior might reveal general principles common to nematodes, insects and vertebrates that function in the molecular sensing of respiratory gases and the neural control of animal behaviors.

To identify molecular mechanisms that function in G-protein signaling, we have performed molecular genetic studies of a simple behavior of the nematode Caenorhabditis elegans, egg laying, which is driven by a pair of serotonergic neurons, the hermaphrodite-specific neurons (HSNs). The activity of the HSNs is regulated by the G(o)-coupled receptor EGL-6, which mediates inhibition of the HSNs by neuropeptides. We report here that this inhibition requires one of three inwardly rectifying K(+) channels encoded by the C. elegans genome: IRK-1. Using ChannelRhodopsin-2-mediated stimulation of HSNs, we observed roles for egl-6 and irk-1 in regulating the excitability of HSNs. Although irk-1 is required for inhibition of HSNs by EGL-6 signaling, we found that other G(o) signaling pathways that inhibit HSNs involve irk-1 little or not at all. These findings suggest that the neuropeptide receptor EGL-6 regulates the potassium channel IRK-1 via a dedicated pool of G(o) not involved in other G(o)-mediated signaling. We conclude that G-protein-coupled receptors that signal through the same G-protein in the same cell might activate distinct effectors and that specific coupling of a G-protein-coupled receptor to its effectors can be determined by factors other than its associated G-proteins.

Many animals possess neurons specialized for the detection of carbon dioxide (CO(2)), which acts as a cue to elicit behavioral responses and is also an internally generated product of respiration that regulates animal physiology. In many organisms how such neurons detect CO(2) is poorly understood. We report here a mechanism that endows C. elegans neurons with the ability to detect CO(2). The ETS-5 transcription factor is necessary for the specification of CO(2)-sensing BAG neurons. Expression of a single ETS-5 target gene, gcy-9, which encodes a receptor-type guanylate cyclase, is sufficient to bypass a requirement for ets-5 in CO(2)-detection and transforms neurons into CO(2)-sensing neurons. Because ETS-5 and GCY-9 are members of gene families that are conserved between nematodes and vertebrates, a similar mechanism might act in the specification of CO(2)-sensing neurons in other phyla.

CO(2) is both a critical regulator of animal physiology and an important sensory cue for many animals for host detection, food location, and mate finding. The free-living soil nematode Caenorhabditis elegans shows CO(2) avoidance behavior, which requires a pair of ciliated sensory neurons, the BAG neurons. Using in vivo calcium imaging, we show that CO(2) specifically activates the BAG neurons and that the CO(2)-sensing function of BAG neurons requires TAX-2/TAX-4 cyclic nucleotide-gated ion channels and the receptor-type guanylate cyclase GCY-9. Our results delineate a molecular pathway for CO(2) sensing and suggest that activation of a receptor-type guanylate cyclase is an evolutionarily conserved mechanism by which animals detect environmental CO(2)

Biogenic amines such as serotonin and dopamine are intercellular signaling molecules that function widely as neurotransmitters and neuromodulators. We have identified in the nematode Caenorhabditis elegans three ligand-gated chloride channels that are receptors for biogenic amines: LGC-53 is a high-affinity dopamine receptor, LGC-55 is a high-affinity tyramine receptor, and LGC-40 is a low-affinity serotonin receptor that is also gated by choline and acetylcholine. lgc-55 mutants are defective in a behavior that requires endogenous tyramine, which indicates that this ionotropic tyramine receptor functions in tyramine signaling in vivo. Our studies suggest that direct activation of membrane chloride conductances is a general mechanism of action for biogenic amines in the modulation of C. elegans behavior

Egg-laying behavior of the Caenorhabditis elegans hermaphrodite is regulated by G protein signaling pathways. Here we show that the egg laying-defective mutant egl-6(n592) carries an activating mutation in a G protein-coupled receptor that inhibits C. elegans egg-laying motor neurons in a G(o)-dependent manner. Ligands for EGL-6 are Phe-Met-Arg-Phe-NH(2) (FMRFamide)-related peptides encoded by the genes flp-10 and flp-17. flp-10 is expressed in both neurons and non-neuronal cells. The major source of flp-17 peptides is a pair of presumptive sensory neurons, the BAG neurons. Genetic analysis of the egl-6 pathway revealed that the EGL-6 neuropeptide signaling pathway functions redundantly with acetylcholine to inhibit egg-laying. The retention of embryos in the uterus of the C. elegans hermaphrodite is therefore under the control of a presumptive sensory system and is inhibited by the convergence of signals from neuropeptides and the small-molecule neurotransmitter acetylcholine

Octopamine biosynthesis requires tyrosine decarboxylase to convert tyrosine into tyramine and tyramine beta-hydroxylase to convert tyramine into octopamine. We identified and characterized a Caenorhabditis elegans tyrosine decarboxylase gene, tdc-1, and a tyramine beta-hydroxylase gene, tbh-1. The TBH-1 protein is expressed in a subset of TDC-1-expressing cells, indicating that C. elegans has tyraminergic cells that are distinct from its octopaminergic cells. tdc-1 mutants have behavioral defects not shared by tbh-1 mutants. We show that tyramine plays a specific role in the inhibition of egg laying, the modulation of reversal behavior, and the suppression of head oscillations in response to anterior touch. We propose a model for the neural circuit that coordinates locomotion and head oscillations in response to anterior touch. Our findings establish tyramine as a neurotransmitter in C. elegans, and we suggest that tyramine is a genuine neurotransmitter in other invertebrates and possibly in vertebrates as well