Regulator of G-protein signaling-21 (RGS21) is an inhibitor of bitter gustatory signaling found in lingual and airway epithelia.

The gustatory system detects tastants and transmits signals to the brain regarding ingested substances and nutrients. Although tastant receptors and taste signaling pathways have been identified, little is known about their regulation. Because bitter, sweet, and umami taste receptors are G protein-coupled receptors (GPCRs), we hypothesized that regulators of G protein signaling (RGS) proteins may be involved. The recent cloning of RGS21 from taste bud cells has implicated this protein in the regulation of taste signaling; however, the exact role of RGS21 has not been precisely defined. Here, we sought to determine the role of RGS21 in tastant responsiveness. Biochemical analyses confirmed in silico predictions that RGS21 acts as a GTPase-accelerating protein (GAP) for multiple G protein α subunits, including adenylyl cyclase-inhibitory (Gα(i)) subunits and those thought to be involved in tastant signal transduction. Using a combination of in situ hybridization, RT-PCR, immunohistochemistry, and immunofluorescence, we demonstrate that RGS21 is not only endogenously expressed in mouse taste buds but also in lung airway epithelial cells, which have previously been shown to express components of the taste signaling cascade. Furthermore, as shown by reverse transcription-PCR, the immortalized human airway cell line 16HBE was found to express transcripts for tastant receptors, RGS21, and downstream taste signaling components. Over- and underexpression of RGS21 in 16HBE cells confirmed that RGS21 acts to oppose bitter tastant signaling to cAMP and calcium second messenger changes. Our data collectively suggests that RGS21 modulates bitter taste signal transduction.

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

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 (beta(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 beta(2)AR. In HEK293 cells, coexpression of beta(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 beta(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.

RGS21, a regulator of taste and mucociliary clearance?

OBJECTIVES/HYPOTHESIS: Motile cilia of airway epithelial cells help to expel harmful inhaled material. Activation of bitterant-responsive G protein-coupled receptors (GPCRs) is believed to potentiate cilia beat frequency and mucociliary clearance. In this study, we investigated whether regulator of G protein signaling-21 (RGS21) has the potential to modulate signaling pathways connected to airway mucociliary clearance, given that RGS proteins modulate GPCR signaling by acting as GTPase-accelerating proteins (GAPs) for the Gα subunits of heterotrimeric G proteins. STUDY DESIGN: This is a pilot investigation to determine if RGS21, a potential tastant specific RGS gene, is expressed in sinonasal mucosa, and to determine its specific Gα substrate using in vitro biochemical assays with purified proteins. METHODS: Rgs21 expression in sinonasal mucosa was determined using quantitative, real-time PCR and a transgenic mouse expressing RFP from the Rgs21 promoter. Rgs21 was cloned, over-expressed, and purified using multistep protein chromatography. Biochemical and biophysical assays were used to determine if RGS21 could bind and accelerate the hydrolysis of GTP on heterotrimeric Gα subunits. RESULTS: Rgs21 was expressed in sinonasal mucosa and lingual epithelium. Purified recombinant protein directly bound and accelerated GTP hydrolysis on Gα subunits. CONCLUSIONS: Rgs21 is expressed in sinonasal mucosa, is amenable to purification as a recombinant protein, and can bind to Gα(i/o/q) subunits. Furthermore, RGS21 can accelerate the hydrolysis rate of GTP on Gαi subunits. This provides evidence that RGS21 may be a negative regulator of bitterant responses. Future studies will be needed to determine the physiological role of this protein in mucociliary clearance.