Regulation of intracellular pH by electrogenic Na+/HCO3- co-transporters in embryonic neural stem cell-derived radial glia-like cells.

A stroke causes a hypoxic brain microenvironment that alters neural cell metabolism resulting in cell membrane hyperpolarization and intracellular acidosis. We studied how intracellular pH (pHi) is regulated in differentiated mouse neural progenitor cells during hyperpolarizing conditions, induced by prompt reduction of the extracellular K+ concentration. We found that the radial glia-like population in differentiating embryonic neural progenitor cells, but not neuronal cells, was rapidly acidified under these conditions. However, when extracellular calcium was removed, an instant depolarization and recovery of the pHi, back to normal levels, took place. The rapid recovery phase seen in the absence of calcium, was dependent on extracellular bicarbonate and could be inhibited by S0859, a potent Na/HCO3 cotransporter inhibitor. Immunostaining and PCR data, showed that NBCe1 (SLC4A4) and NBCn1 (SLC4A7) were expressed in the cell population and that the pHi recovery in the radial glial-like cells after calcium removal was mediated mainly by the electrogenic sodium bicarbonate transporter NBCe1 (SLC4A4). Our results indicate that extracellular calcium might hamper pHi regulation and Na/HCO3 cotransporter activity in a brain injury microenvironment. Our findings show that the NBC-type transporters are the main pHi regulating systems prevailing in glia-like progenitor cells and that these calcium sensitive transporters are important for neuronal progenitor cell proliferation, survival and neural stem cell differentiation.