Functional identification and initial characterization of a fish co-receptor involved in aversive signaling

Chemoreception plays an important role in predator-prey interactions and feeding dynamics. While the chemoreception of attractant or pleasant tasting compounds has been well studied, aversive chemoreceptive signaling has been difficult to investigate behaviorally in an ecological context because these interactions are species- and context-specific and deterrent compounds vary among prey. Therefore, little is known about the molecular mechanism(s) used in detection of aversive compounds. Using the coral reef system, this thesis explores on a molecular level the deterrent mechanism underlying detection by fish predators of an aversive compound, in order to gain a greater understanding of predator-prey interactions in this community. Like other organisms that are sessile or slow-moving, marine sponges have special mechanisms for defense from predation, commonly containing aversive-tasting compounds that defend these organisms from predation. To this end, we sought to identify and characterize a fish chemoreceptor that detects one or more of these compounds.;A behavioral assay demonstrated that many sponge compounds that are known to be deterrent to coral reef predator fish are also deterrent to zebrafish, a freshwater fish whose genome is well-characterized. Two of these groups of deterrent triterpene glycosides, formoside and a mixture of ectyoplasides A and B, caused electrophysiological changes in Xenopus oocytes expressing an entire zebrafish cDNA library, beta2AR, and the ion channel CFTR. Utilizing this electrophysiological bioassay, we fractionated the zebrafish cDNA library and isolated a single cDNA clone encoding RL-TGR, a novel co-receptor involved in the signaling of triterpene glycosides. This co-receptor appears to be structurally and functionally related to receptor activity-modifying proteins (RAMPs), a family of co-receptors that physically associate with and modify the activity of G protein-coupled receptors (GPCRs). Structurally, this protein is predicted to have a single-pass transmembrane domain, a short intracellular domain, and a long extracellular domain. Expression in Xenopus oocytes showed that it responds specifically to triterpene glycosides and no other compound tested in a receptor-mediated manner. Additionally, RL-TGR requires co-expression of a GPCR to enable signaling in oocytes, and both of these receptors may be components of a larger signaling complex, as suggested by immunoblotting evidence. Immunoblotting from expressing Xenopus oocyte membranes demonstrated that this protein is membrane associated. A 40 bp portion of the gene is conserved across multiple fish species, but is not found in any other organism with a published genome, suggesting that the expression of this receptor is limited to fish species. Therefore, this fish gene may have coevolved with organisms that produce triterpene glycoside defensive compounds, which include sponges, echinoderms, and vascular plants.;This work suggests that aversive compounds may be detected by RL-TGR and related proteins in fish. The use of a GPCR and RAMP-like co-receptor complex as a detector of deterrent compounds is a clever mechanism in which to perceive potentially harmful compounds. Instead of necessitating expression of a specific bona fide receptor (with the ability to both bind ligand and transduce signals) for each possible compound an organism might need to detect in its lifetime, an organism would only require expression of a limited number of GPCRs and a suite of co-receptors, which can combine in numerous combinations to specifically and efficiently detect a vast number of deterrent compounds, protecting these organism from potentially harmful compounds.;This interdisciplinary work crosses the boundaries of behavioral neuroscience, chemical ecology, and molecular biology, and unites fields that rarely overlap. The discovery of RL-TGR is significant not only because it defines a new chemoreceptor-ligand pair in a field where few of these interactions are known, but also because the gene encoding RL-TGR is the first identified that encodes a co-receptor which responds to a chemical defense. This finding may lead the way for the identification of many other receptors that mediate chemical defense signaling in both marine and terrestrial environments, as this protein has the potential to represent the first of an entire family of co-receptors that respond to aversive compounds. The further study of RL-TGR and any related co-receptors will deepen our understanding of the molecular mechanisms of chemical defense compounds and their effects on predator-prey interactions.