Identification of RL-TGR, a coreceptor involved in aversive chemical signaling

Abstract

Chemical signaling plays an important role in predator–prey interactions and feeding dynamics. Like other organisms that are sessile or slow moving, some marine sponges contain aversive compounds that defend these organisms from predation. We sought to identify and characterize a fish chemoreceptor that detects one of these compounds. Using expression cloning in Xenopus oocytes coexpressing the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, the beta-2 adrenergic receptor (β 2 AR), and fractions of a zebrafish cDNA library, we isolated a cDNA clone encoding receptor activity–modifying protein (RAMP)-like triterpene glycoside receptor (RL-TGR), a novel coreceptor involved in signaling in response to triterpene glycosides. This coreceptor appears to be structurally and functionally related to RAMPs, a family of coreceptors that physically associate with and modify the activity of G protein–coupled receptors (GPCRs). In membranes from formoside-responsive oocytes, RL-TGR was immunoprecipitated in an apparent complex with β 2 AR. In HEK293 cells, coexpression of β 2 AR induced the trafficking of RL-TGR from the cytoplasm to the plasma membrane. These results suggest that RL-TGR in the predatory fish physically associates with the β 2 AR or another, more physiologically relevant GPCR and modifies its pharmacology to respond to triterpene glycosides found in sponges that serve as a potential food source for the fish. RL-TGR forms a coreceptor that responds to a chemical defense compound in the marine environment, and its discovery might lead the way to the identification of other receptors that mediate chemical defense signaling.

Receptor Activity-Modifying Proteins (RAMPs): New Insights and Roles

“…These studies include efforts to understand pharmacology and receptor trafficking, and can provide valuable insight into evolutionary mechanisms (99–101). Furthermore, a receptor activity–modifying protein (RAMP)-like triterpene glycoside receptor has been identified from a zebrafish cDNA library (102). Its functional relationship to RAMPs is not known.…”

Zebrafish aversive taste co-receptor is expressed in both chemo- and mechanosensory cells and plays a role in lateral line development

“…One such system of chemoreception involving receptor and co-receptor was recently discovered in zebrafish. Through heterologous expression in Xenopus oocytes, a small, transmembrane co-receptor from zebrafish was found to be involved in the recognition of a family of compounds that act as feeding deterrents 5 . RAMP-like triterpene glycoside receptor (RL-TGR) is a 96 amino acid protein that, when complexed with an unidentified zebrafish G-protein coupled receptor (GPCR), transduces a signal in the presence of triterpene glycoside ligands.…”

“…The ligands could be cues from the external environment as is the case for chemical compounds in food or they could be secreted by cells as in the case of interleukins, growth factors, etc. 5 , 6 .…”

Taste-mediated behavioral and electrophysiological responses by the predatory fish Ariopsis felis to deterrent pigments from Aplysia californica ink

“…Other studies of detection of toxic or deterrent compounds by fish show evidence of neural pathways for these stimuli that are independent of pathways for chemoattractants and feeding stimulants. For example, channel catfish have an independent gustatory pathway for quinine (Ogawa et al 1997), rainbow trout and Arctic char have gustatory pathways for tetrodotoxin and saxitoxin that are independent from those for amino acids and bile salts (Yamamori et al 1988), and zebrafish have specialized receptor sites for denatonium and sponge deterrents (Ishimaru et al 2005;Oike et al 2007;Cohen et al 2008Cohen et al , 2010. Thus, some deterrents may be processed by gustatory systems with specific fiber types, at least some that are insensitive to major classes of chemical stimuli such as bile salts and amino acids.…”

“…Rainbow trout and Arctic char have receptor types and gustatory pathways for tetrodotoxin and saxitoxin that are both independent from each other and from amino acids and bile salts (Yamamori et al 1988). Zebrafish have receptors for sponge deterrents (Cohen et al 2008(Cohen et al , 2010 and denatonium (Ishimaru et al 2005;Oike et al 2007). An independent pathway for quinine occurs in the gustatory pathway of channel catfish (Ogawa et al 1997).…”