The Proton-activated G Protein Coupled Receptor OGR1 Acutely Regulates the Activity of Epithelial Proton Transport Proteins

Abstract

The Ovarian cancer G protein-coupled Receptor 1 (OGR1; GPR68) is proton-sensitive in the pH range of 6.8 - 7.8. However, its physiological function is not defined to date. OGR1 signals via inositol trisphosphate and intracellular calcium, albeit downstream events are unclear. To elucidate OGR1 function further, we transfected HEK293 cells with active OGR1 receptor or a mutant lacking 5 histidine residues (H5Phe-OGR1). An acute switch of extracellular pH from 8 to 7.1 (10 nmol/l vs 90 nmol/l protons) stimulated NHE and H⁺-ATPase activity in OGR1-transfected cells, but not in H5Phe-OGR1-transfected cells. ZnCl₂ and CuCl₂ that both inhibit OGR1 reduced the stimulatory effect. The activity was blocked by chelerythrine, whereas the ERK1/2 inhibitor PD 098059 had no inhibitory effect. OGR1 activation increased intracellular calcium in transfected HEK293 cells. We next isolated proximal tubules from kidneys of wild-type and OGR1-deficient mice and measured the effect of extracellular pH on NHE activity in vitro. Deletion of OGR1 affected the pH-dependent proton extrusion, however, in the opposite direction as expected from cell culture experiments. Upregulated expression of the pH-sensitive kinase Pyk2 in OGR1 KO mouse proximal tubule cells may compensate for the loss of OGR1. Thus, we present the first evidence that OGR1 modulates the activity of two major plasma membrane proton transport systems. OGR1 may be involved in the regulation of plasma membrane transport proteins and intra- and/or extracellular pH.

Emerging roles for the pH-sensing G protein-coupled receptors in response to acidotic stress

“…Two of the proton-sensing GPCRs, GPR4 and GPR68, have been studied as pH sensors in the renal system. 58,59 GPR4 and GPR68 are expressed in the lung and kidney, which may both be necessary to successfully buffer the circulatory system and maintain pH homeostasis.…”

“…60 GPR68 has also been reported to alter proton export of HEK293 cells by stimulating the Na + /H + exchanger and H + -ATPase. 58 The activation of GPR68 by acidosis was found to stimulate this effect through a cluster of extracellular histidine residues and the G q /PKC signaling pathway. 58 In GPR68-null mice the expression of the pH-sensitive kinase Pyk2 in the kidney proximal tubules was upregulated which might compensate for GPR68 deficiency.…”

“…58 The activation of GPR68 by acidosis was found to stimulate this effect through a cluster of extracellular histidine residues and the G q /PKC signaling pathway. 58 In GPR68-null mice the expression of the pH-sensitive kinase Pyk2 in the kidney proximal tubules was upregulated which might compensate for GPR68 deficiency. 58 Taken together, GPR4 and GPR68 may both be necessary for successful systemic pH buffering by controlling renal acid excretion.…”

“…58 In GPR68-null mice the expression of the pH-sensitive kinase Pyk2 in the kidney proximal tubules was upregulated which might compensate for GPR68 deficiency. 58 Taken together, GPR4 and GPR68 may both be necessary for successful systemic pH buffering by controlling renal acid excretion.…”

Role of extracellular proton-sensing OGR1 in regulation of insulin secretion and pancreatic β-cell functions

“…However, recent studies suggested that the OGR1 family of GPCRs is also involved in the physiological events as well. For example, GPR4 deficiency causes vascular abnormality during development [58] and decreased net acid secretion by the kidney [59], and OGR1 deficiency enhances pH-dependent proton extrusion in proximal tubules form the kidney [60] and changes insulin secretion and insulin sensitivity [61]. In the following sections, we will focus on the role of OGR1 in insulin secretion and β-cell differentiation.…”