Characterization of human SLC4A10 as an electroneutral Na/HCO3 cotransporter (NBCn2) with Cl- self-exchange activity

The SLC4A10 gene product, commonly known as NCBE, is highly expressed in rodent brain and has been characterized by others as a Na+-driven Cl-HCO3 exchanger. However, some of the earlier data are not consistent with Na+-driven Cl-HCO3 exchange activity. In the present study, northern blot analysis showed that, also in humans, NCBE transcripts are predominantly expressed in brain. In some human NCBE transcripts, splice cassettes A and/or B, originally reported in rats and mice, are spliced out. In brain cDNA, we found evidence of a unique partial splice of cassette B that is predicted to produce an NCBE protein with a novel C terminus containing a protein kinase C phosphorylation site. We used pH-sensitive microelectrodes to study the molecular physiology of human NCBE expressed in Xenopus oocytes. In agreement with others we found that NCBE mediates the 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid-sensitive, Na+-dependent transport of HCO3-. For the first time, we demonstrated that this transport process is electroneutral. Using Cl--sensitive microelectrodes positioned at the oocyte surface, we found that, unlike both human and squid Na+-driven Cl-HCO3 exchangers, human NCBE does not normally couple the net influx of HCO3-to a net efflux of Cl-. Moreover we found that that the Cl-36 efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na+ and HCO3-, actually represents a CO2/HCO3-stimulated Cl-self-exchange not coupled to either Na+ or net HCO3-transport. We propose to rename NCBE as the second electroneutral Na/HCO3 cotransporter, NBCn2.