Endocrine Abstracts

Accepted dogma once stated that G protein-coupled receptors (GPCRs) function as monomers, however, over the last decade in vitro experiments have shown GPCRs to function as dimers and higher order oligomers. We have recently reported the first in vivo evidence for the physiological importance of Class A GPCR homodimerisation using the LHCGR as a model receptor. Transgenic co-expression of binding deficient LHCGR (LHCGR^-LH) and signalling deficient LHCGR (LHCGR^-cAMP) reversed the hypogonadism and infertility of male LHCGR null mice. Utilising the LHCGR^-LH and LHCGR^-cAMP as tools for studying cis (same receptor binding hormone and propagating signal) and trans (one receptor partner binding hormone, and one propagating signal) activation through receptor dimerisation, we aimed to interrogate whether these distinct modes of receptor signalling result in ligand bias to a receptor that can couple to multiple G proteins, using the endogenous ligands, LH and hCG. In cells expressing either WT LHCGR or LHCGR^-LH/LHCGR^-cAMP, hCG and LH showed equivalent Gαs- cAMP responses indicating that trans-activation was sufficient to mediate the Gαs response to both ligands. hCG-dependent Gαq signalling was comparable in WT LHCGR and LHCGR^-LH/LHCGR^-cAMP expressing cells. However, LH-dependent Gαq signalling was severely diminished in the trans-activation mode of signalling, revealing a requirement of cis-activation for full LH-dependent Gαq signalling. Assessment of ligand induced receptor-G protein interactions by bioluminescence energy transfer (BRET) also showed equivalent hCG and LH-dependent Gαs coupling, however LH-dependent Gαq receptor-association in the transactivation model was diminished. Interestingly, varying the surface expression of the LHCGR^-LH to the LHCGR^-cAMP indicated that trans-activation to Gαs signalling favours an oligomeric complex containing an excess of LHCGR^-cAMP. This study indicates that cis- and trans- modes of GPCR activation can result in ligand-induced diversification of signalling, and may provide mechanistic insight into how GPCR dimers/oligomers, via trans-activation, can regulate the degree of activity of G-protein signalling.

Declaration of funding

BBSRC grant number BB/1008004/1 and Wellcome Trust grant number 082101/Z/07/Z.