Deletion of the pH Sensor GPR4 Decreases Renal Acid Excretion

Abstract

epithelial cells, and the effects of its deletion on acid-base homeostasis. We observed GPR4 expression in the kidney cortex, in the outer and inner medulla, in isolated kidney collecting ducts, and in cultured outer and inner medullary collecting duct cells (mOMCD1 and mIMCD3). Cultured mOMCD1 cells exhibited pH-dependent accumulation of intracellular cAMP, characteristic of GPR4 activation; GPR4 knockdown attenuated this accumulation. In vivo, deletion of GPR4 decreased net acid secretion by the kidney and resulted in a nongap metabolic acidosis, indicating that GPR4 is required to maintain acid-base homeostasis. Collectively, these findings suggest that GPR4 is a pH sensor with an important role in regulating acid secretion in the kidney collecting duct. Daily changes in the amount of protein in the diet produce fluctuations in metabolic net acid production. The kidneys adjust to these daily variations, ensuring tight correlation between acid production and excretion. 1,2 In humans, net acid excretion significantly correlates with changes in blood P CO2 , pH, and bicarbonate levels. 2 Physiologic changes of P CO2 , pH, and bicarbonate concentrations can therefore regulate net acid excretion, but this regulation is poorly understood.The recently discovered family of "proton-activated" G protein-coupled receptors (GPCRs) represents candidate pH sensors capable of relaying information about local and/or systemic pH to acidsecreting cells in the kidney. [3][4][5][6][7] Several investigators have demonstrated that GPR4 (G protein-coupled receptor 4), 4,8,9 OGR1 (ovarian cancer G protein-coupled receptor 1, GPR68), 4,10 -12 and TDAG8 (T cell death-associated gene 8, GPR65) can be stimulated by a reduction in extracellular pH to induce second messenger generation. [13][14][15][16] An elegant study by Ludwig et al. 4 demonstrated that mutation of specific putative proton-accepting histidine residues in OGR1 significantly impaired acid-stimulated phosphoinositide generation. Ex vivo studies from OGR1 knockouts indicate that OGR1 is the pH sensor, mediating the release of calcium from the bone during metabolic acidosis. 12 Analysis of aortic rings isolated from GPR4 knockout mice indicated that GPR4 acts as a pH sensor in blood vessels, regulating the outgrowth of new

Collecting Duct Intercalated Cell Function and Regulation

“…The Clone-C cell line is particularly useful because it can switch phenotypes from type B to type A intercalated cells (Figure 2) depending on culture conditions (16,17). Of note, many very relevant studies that have uncovered important regulatory pathways in intercalated cells have been performed in cell lines of inner medullary origin, such as the mIMCD3 cell line (18,19).…”

“…Its function was identified and characterized in cell lines in studies that used the mOMCD1 intercalated cell model (18). GPR4-null mice had decreased kidney net acid secretion and a nongap metabolic acidosis, suggesting that GPR4 is a pH sensor with a likely important role in promoting acid secretion in kidney collecting duct intercalated cells in vivo.…”

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.…”

Attending Rounds

“…When faced with systemic acidosis, the kidneys are able to increase net acid excretion many fold, primarily by increasing ammoniagenesis and urinary ammonium excretion. A recently discovered G-protein-coupled receptor that accepts hydrogen as a ligand may be a sensor that relays information about changes in systemic pH to acid-secreting cells in the kidney (6). Although ammonia entry into collecting duct has been considered to be mostly passive, there is increasing evidence that this involves a specific membrane protein from the family of Rhesus proteins located in the distal nephron (7).…”