Cloning and functional characterization of new splice variants of human Na+‐driven Cl/HCO3 (NDCBE)

In hippocampal neurons, NDCBE is thought to be the major acid‐extrusion mechanism. Little is known about the different splice variants of this transporter, or their function. Our lab cloned the first NDCBE (NDCBE‐B), which encodes a protein of 1044 amino acids (aa). Here, we report the cloning of three new human splice variants of NDCBE. Screening a whole‐brain cDNA library and using 3′ RACE, we cloned NDCBE‐A (human ortholog of a known mouse clone), a 1093aa protein in which 17aa at the extreme C terminus (Ct) of NDCBE‐B are replaced by 66aa. Performing 5′ RACE, we cloned two new variants, NDCBE‐C and ‐D. These are identical to NDCBE‐A and ‐B, but are 53aa shorter at the extreme N terminus. NDCBE‐C/D mRNA are expressed in brain, heart, kidney and skeletal muscle. We expressed NDCBE‐A, ‐B, and ‐D in Xenopus oocytes, and monitored membrane potential (V m ) and pH i using microelectrodes. Oocytes expressing NDCBE‐A or ‐D did not display a V m change when exposed to extracellular CO 2 /HCO 3 − Thus, these transporters are electroneutral. Oocytes expressing NDCBE‐A, ‐B, and ‐D all mediated Na‐dependent pH i recovery from the CO 2 ‐induced acid load. The rates of recovery (dpH i /dt) were nearly identical for the three clones: 13×10 −5 pH unit/s for NDCBE‐A, 9×10 −5 pH unit/s for NDCBE‐B, and 12×10 −5 pH unit/s for NDCBE‐D versus 2×10 −5 pH unit/s for water‐injected oocytes. This work was supported by a NIH grant NS18400.