Molecular pathophysiology of SLC4 bicarbonate transporters

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Abstract
Acid–base (H+ and HCO3 −) transport in the kidney is crucial for maintaining blood pH, cellular pH and excreting metabolic acid. HCO3 − transport in the kidney is mediated by HCO3 − transporter proteins which occur in two gene families in humans, vertebrates and invertebrates (SLC4 and SLC26). Since SLC26 transporters have other, non-HCO3 − transport functions, this review highlights the history and recent advances in the SLC4 transporters in the kidney. The SLC4 gene and protein family (10 genes) contains three types of HCO3 − transporters: Cl−–HCO3 − exchangers, Na+/HCO3 − cotransporters and Na+-driven Cl−–HCO3 − exchangers. Function and human chromosomal location have been determined for most members. Human mutations in AE1 (SLC4A1) and NBCe1 (SLC4A4) are associated with distal and proximal renal tubular acidosis, respectively. Recent advances include the cellular and biophysical mechanisms by which AE1 and NBCe1 mutations lead to renal disease. Mutational and cellular trafficking studies have begun to elucidate the membrane topology and functional domains of AE1 and NBCe1. Knockout mice for AE2 and NBCn1 do not have obvious renal phenotypes. Recently, SLC4A11 (bicarbonate transporter 1) was shown to function as an electrogenic Na+/borate cotransporter unable to transport HCO3 − but involved in cell cycle control. SLC4 HCO3 − transporters play critical roles in systemic and cellular pH homeostasis. Most of the SLC4 members are present at some level in the kidney. Future studies will likely continue to make use of knockout animals, for example mice and zebrafish, human mutations or polymorphisms to elucidate the normal and pathophysiologic roles of these proteins.