Gene Variants in Sodium Bicarbonate Cotransporter NBCe2 Increase Sodium and Bicarbonate Transport in Human Renal Proximal Tubule

Abstract

RationaleThe sodium bicarbonate cotransporter NBCe2 (encoded by SLC4A5) is involved in the regulation of renal tubular sodium bicarbonate transport. We hypothesized that human SLC4A5 single nucleotide polymorphisms (SNPs, rs10177833 and rs7571842) increase NBCe2 expression and renal proximal tubule (RPT) sodium transport and may be a cause of salt sensitivity of blood pressure.ObjectiveTo characterize the function of wild‐type (WT) and homozygous variants (HV) of SLC4A5.Methods and ResultsThe expressions of NBCe2 mRNA and protein were not different between RPT cells (RPTCs) carrying WT (n=4) or HV SLC4A5 (n=6) before or after dopaminergic or angiotensin (II and III) stimulation. However, total transcellular (luminal to basolateral) RPTC sodium transport, NHE3 protein expression, and Cl−/HCO3− exchanger activity were higher in HV than WT SLC4A5. Bicarbonate‐dependent pH recovery after an intracellular acid load, as well as NBCe2 mRNA and protein, was increased when intracellular sodium was elevated (5.2±1.1 mM) in HV SLC4A5 compared with WT SLC4A5 (pH recovery Vmax 0.22±0.02 and Km 6.39±0.81 in HV vs Vmax 0.28±0.02 and Km 4.30±0.53 in WT, P<0.05, P<0.05, n=3). The increased NBCe2‐specific bicarbonate‐dependent pH recovery in HV was verified in stable SLC4A5 shRNA cell lines (n=8/12/group). RPTCs carrying HV SLC4A5 had increased binding of HNF4A to SLC4A5 DNA (1.52±0.10‐fold, N=3, P<0.05, t‐test), using purified HNF4A and ChIP and oligo binding assays. Additionally, the difference between HV and WT in bicarbonate‐dependent pH recovery was abolished by two HNF4A antagonists. RPTCs isolated from human urine of salt‐sensitive subjects who are HV for SLC4A5 had increased bicarbonate dependent pH recovery, relative to salt‐resistant subjects who are WT for SLC4A5 (n=4/group).ConclusionsNBCe2 activity is stimulated by an increase in intracellular sodium and is hyper‐responsive in RPTCs carrying HV SLC4A5 through an aberrant HNF4A‐mediated mechanism, which may be a key mechanism in the etiology of human salt sensitivity.