Cloning, localization, and functional expression of the electrogenic Na⁺ bicarbonate cotransporter (NBCe1) from zebrafish

Mutations in the electrogenic Na⁺/nHCO₃^- cotransporter (NBCe1, SLC4A4) cause severe proximal renal tubular acidosis, glaucoma, and cataracts in humans, indicating NBCe1 has a critical role in acid-base homeostasis and ocular fluid transport. To better understand the homeostatic roles and protein ontogeny of NBCe1, we have cloned, localized, and downregulated NBCe1 expression in zebrafish, and examined its transport characteristics when expressed in Xenopus oocytes. Zebrafish NBCe1 (zNBCe1) is 80% identical to published mammalian NBCe1 cDNAs. Like other fish NBCe1 clones, zebrafish NBCe1 is most similar to the pancreatic form of mammalian NBC (Slc4a4-B) but appears to be the dominant isoform found in zebrafish. In situ hybridization of embryos demonstrated mRNA expression in kidney pronephros and eye by 24 h postfertilization (hpf) and gill and brain by 120 hpf. Immunohistochemical labeling demonstrated expression in adult zebrafish eye and gill. Morpholino knockdown studies demonstrated roles in eye and brain development and caused edema, indicating altered fluid and electrolyte balance. With the use of microelectrodes to measure membrane potential (V_m), voltage clamp (VC), intracellular pH (pHi), or intracellular Na⁺ activity (aNa_i), we examined the function of zNBCe1 expressed in Xenopus oocytes. Zebrafish NBCe1 shared transport properties with mammalian NBCe1s, demonstrating electrogenic Na⁺ and HCO₃ ^- transport as well as similar drug sensitivity, including inhibition by 4,4′-diiso-thiocyano-2,2′-disulfonic acid stilbene and tenidap. These data indicate that NBCe1 in zebrafish shares many characteristics with mammalian NBCe1, including tissue distribution, importance in systemic water and electrolyte balance, and electrogenic transport of Na⁺ and HCO ₃^-. Thus zebrafish promise to be useful model system for studies of NBCe1 physiology.