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

Abstract

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 (Vm) and pHi using microelectrodes. Oocytes expressing NDCBE-A or -D did not display a Vm change when exposed to extracellular CO2/HCO3− Thus, these transporters are electroneutral. Oocytes expressing NDCBE-A, -B, and -D all mediated Na-dependent pHi recovery from the CO2-induced acid load. The rates of recovery (dpHi/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.