HCO ₃ ^--independent conductance with a mutant Na ⁺/HCO ₃ ^- cotransporter (SLC4A4) in a case of proximal renal tubular acidosis with hypokalaemic paralysis

The renal electrogenic Na ⁺/HCO ₃ ^- cotransporter (NBCe1-A) contributes to the basolateral step of transepithelial HCO ₃ ^- reabsorption in proximal tubule epithelia, contributing to the buffering of blood pH. Elsewhere in the body (e.g. muscle cells) NBCe1 variants contribute to, amongst other processes, maintenance of intracellular pH. Others have described a homozygous mutation in NBCe1 (NBCe1-A p.Ala799Val) in an individual with severe proximal renal tubular acidosis (pRTA; usually associated with defective HCO ₃ ^- reabsorption in proximal tubule cells) and hypokalaemic periodic paralysis (hypoPP; usually associated with leaky cation channels in muscle cells). Using biotinylation and two-electrode voltage-clamp on Xenopus oocytes expressing NBCe1, we demonstrate that the mutant NBCe1-A (A _A799V) exhibits a per-molecule transport defect that probably contributes towards the observed pRTA. Furthermore, we find that A _A799V expression is associated with an unusual HCO ₃ ^--independent conductance that, if associated with mutant NBCe1 in muscle cells, could contribute towards the appearance of hypokalaemic paralysis in the affected individual. We also study three novel lab mutants of NBCe1-A: p.Ala799Ile, p.Ala799Gly and p.Ala799Ser. All three exhibit a per-molecule transport defect, but only A _A799I exhibits an A _A799V-like ion conductance. A _A799G and A _A799S exhibit unusual outward rectification in their HCO ₃ ^--dependent conductance and A _A799G exhibits reduced sensitivity to both DIDS and tenidap. A799G is the first mutation shown to affect the apparent tenidap affinity of NBCe1. Finally we show that A _A799V and A _A799I, which accumulate poorly in the plasma membrane of oocytes, exhibit signs of abnormal intracellular accumulation in a non-polarized renal cell-line.