Evidence for a Na(+)-Cl(-)-H(+)-HCO3- exchange system in the mammalian lens

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Localization of endogenous and recombinant Na(+)-driven anion exchanger protein NDAE1 from Drosophila melanogaster

Na(+)-dependent Cl(-)/HCO exchange activity helps maintain intracellular pH (pH(i)) homeostasis in many invertebrate and vertebrate cell types. Our laboratory cloned and characterized a Na(+)-dependent Cl(-)/HCO exchanger (NDAE1) from Drosophila melanogaster (Romero MF, Henry D, Nelson S, Harte PJ, and Sciortino CM. J Biol Chem 275: 24552--24559, 2000). In the present study we used immunohistochemical and Western blot techniques to characterize the developmental expression, subcellular localization, and tissue distribution of NDAE1 protein in D. melanogaster. We have shown that a polyclonal antibody raised against the NH(2) terminus of NDAE1 (alpha CWR57) recognizes NDAE1 electrophysiologically characterized in Xenopus oocytes. Moreover, our results begin to delineate the NDAE1 topology, i.e., both the NH(2) and COOH termini are intracellular. NDAE1 is expressed throughout Drosophila development in the central and peripheral nervous systems, sensilla, and the alimentary tract (Malpighian tubules, gut, and salivary glands). Coimmunolabeling of larval tissues with NDAE1 antibody and a monoclonal antibody to the Na(+)-K(+)-ATPase alpha-subunit revealed that the majority of NDAE1 is located at the basolateral membranes of Malpighian tubule cells. These results suggest that NDAE1 may be a key pH(i) regulatory protein and may contribute to basolateral ion transport in epithelia and nervous system of Drosophila.

Cloning and characterization of a Na+-driven anion exchanger (NDAE1). A new bicarbonate transporter

Regulation of intra- and extracellular ion activities (e.g. H(+), Cl(-), Na(+)) is key to normal function of the central nervous system, digestive tract, respiratory tract, and urinary system. With our cloning of an electrogenic Na(+)/HCO(3)(-) cotransporter (NBC), we found that NBC and the anion exchangers form a bicarbonate transporter superfamily. Functionally three other HCO(3)(-) transporters are known: a neutral Na(+)/ HCO(3)(-) cotransporter, a K(+)/ HCO(3)(-) cotransporter, and a Na(+)-dependent Cl(-)-HCO(3)(-) exchanger. We report the cloning and characterization of a Na(+)-coupled Cl(-)-HCO(3)(-) exchanger and a physiologically unique bicarbonate transporter superfamily member. This Drosophila cDNA encodes a 1030-amino acid membrane protein with both sequence homology and predicted topology similar to the anion exchangers and NBCs. The mRNA is expressed throughout Drosophila development and is prominent in the central nervous system. When expressed in Xenopus oocytes, this membrane protein mediates the transport of Cl(-), Na(+), H(+), and HCO(3)(-) but does not require HCO(3)(-). Transport is blocked by the stilbene 4,4'-diisothiocyanodihydrostilbene- 2, 2'-disulfonates and may not be strictly electroneutral. Our functional data suggest this Na(+) driven anion exchanger (NDAE1) is responsible for the Na(+)-dependent Cl(-)-HCO(3)(-) exchange activity characterized in neurons, kidney, and fibroblasts. NDAE1 may be generally important for fly development, because disruption of this gene is apparently lethal to the Drosophila larva.