Pharmacology and physiological function of the orphan GPRC6A receptor.

The G protein-coupled receptor GPRC6A (GPCR, Class C, group 6, subtype A) is a Gq/11 -coupled receptor widely expressed in human and rodent tissues. The proposed endogenous ligands are L-amino acids, divalent cations, osteocalcin and testosterone. This MiniReview provides an updated overview of the literature including the latest in vitro and in vivo studies. GPRC6A forms homodimers, it undergoes constitutive internalization, and very interestingly, the reason for the intracellular retention of the human receptor has been revealed. Multiple physiological functions of GPRC6A have been suggested based on studies using three different global GPRC6A knockout (KO) mouse models where exon II, exon VI or the full locus has been deleted. The newest studies on the full locus GPRC6A KO model show intact glucose and bone homoeostasis with a minor reduction in serum osteocalcin levels. Unfortunately, the physiological function of the receptor remains elusive due to a general lack of consensus/validation of reported phenotypes of the different KO models, and more research is thus warranted to uncover the physiological function. Recent discoveries of human genetic variants that cause either a premature stop codon or an intracellular retention of the receptor point towards human population studies as the preferred approach to continue studies on the function of GPRC6A.