Myocardial expression, signaling, and function of GPR22: a protective role for an orphan G protein-coupled receptor

G protein-coupled receptors (GPCRs) play an essential role in the regulation of cardiovascular function. Therapeutic modulation of GPCRs has proven to be beneficial in the treatment of human heart disease. Myocardial "orphan" GPCRs, for which the natural ligand is unknown, represent potential novel therapeutic targets for the treatment of heart disease. Here, we describe the expression pattern, signaling pathways, and possible physiological role of the orphan GPR22. GPR22 mRNA analysis revealed a highly restricted expression pattern, with remarkably abundant and selective expression in the brain and heart of humans and rodents. In the heart, GPR22 mRNA was determined to be expressed in all chambers and was comparable with transcript levels of the beta(1)-adrenergic receptor as assessed by Taqman PCR. GPR22 protein expression in cardiac myocytes and coronary arteries was demonstrated in the rat heart by immunohistochemistry. When transfected into HEK-293 cells, GPR22 coupled constitutively to G(i)/G(o), resulting in the inhibition of adenyl cyclase. No constitutive coupling to G(s) or G(q) was observed. Myocardial mRNA expression of GPR22 was dramatically reduced following aortic banding in mice, suggesting a possible role in response to the stress associated with increased afterload. The absence of detectable GPR22 mRNA expression in the hearts of GPR22(-/-) mice had no apparent effect on normal heart structure or function; however, these mice displayed increased susceptibility to functional decompensation following aortic banding. Thus, we described, for the first time, the expression pattern and signaling for GPR22 and identified a protective role for GPR22 in response to hemodynamic stress resulting from increased afterload.