GPR37L1 Expressed in Renal Proximal Tubule Cells Regulates Sodium Transport

Abstract

GPR37L1 is a G protein‐coupled receptor that is expressed mainly in brain glial cells and muscle‐myenteric nerve layers in the GI tract. GPR37L1 knockout (KO) mice have increased systolic blood pressure. Prosaposin (PSAP) is a precursor for saposins A, B, C, and D. PSAP was identified as a ligand for GPR37L1 although recent reports showed conflicting results on the role of PSAP as ligand for GPR37L1. There is no report on the expression or function of GPR37L1 in the kidney. Our studies using immunostaining and immunoblotting showed that GPR37L1 is expressed in the apical membrane of mouse renal proximal tubules but not collecting duct cells; RT‐PCR on proximal tubule and collecting duct cells obtained by laser‐capture microdissection of mouse kidney sections, confirmed these findings. Live‐cell imaging of human renal proximal tubule cells (hRPTC) expressing GPR37L1‐GFP protein showed the internalization of the membrane GPR37L1‐GFP following exposure to prosaptide, a 14 amino acid peptide derived from the neurotropic region in the N‐terminal portion of saposin C. cAMP production was increased in hRPTC expressing GPR37L1 treated with prosaptide (4.2 ± 1.4‐fold, P<0.05). In addition, chronic high salt intake increased the renal expression of PSAP suggesting that PSAP is a ligand for GPR37L1 and has a role in renal function. To determine the role of GPR37L1 in renal Na+ transport, we over‐expressed or silenced the expression of GPR37L1 in hRPTC. Over‐ expression of GPR37L1 in hRPTC increased intracellular Na+ (3.2 ± 0.6‐fold, P<0.001) compared with mock‐transfected cells. Treatment with prosaptide blunted the GPR37L1‐ enhanced increase in intracellular Na+, suggesting that prosaptide binding to GPR37L1 may inhibit Na+ entry into the cell. Treatment with a selective NHE3 inhibitor (S3226) decreased intracellular Na+ in GPR37L1‐over expressing, as well as mock‐transfected hRPTC. Treatment with Na+, K+‐ATPase inhibitor (ouabain) showed no effect on the GPR37L1 enhancement of intracellular Na+ in GPR37L1 over‐expressing hRPTC. Silencing of GPR37L1 expression decreased the basal level of intracellular Na+ in the hRPTC. Immunoblot analyses showed increased phosphorylation of Erk1/2 (1.73 ± 0.11‐fold, P<0.05) and ribosomal S6 protein (1.49 ± 0.08‐fold, P<0.01) in hRPTC over‐expressing GPR37L1, compared with mock‐transfected hRPTC. Taken together, these results show that GPR37L1 is expressed in the renal proximal tubule cells, signals via the Erk1/2 pathway, and plays a role in renal Na+ transport and may be a novel target to design drugs to treat patients with hypertension.