Carbonic Anhydrases CA4 and CA14 Both Enhance AE3-Mediated Cl--HCO3- Exchange in Hippocampal Neurons

Carbonic anhydrase (CA) activity in the brain extracellular space is attributable mainly to isoforms CA4 and CA14. In brain, these enzymes have been studied mostly in the context of buffering activity-dependent extracellular pH transients. Yet evidence from others has suggested that CA4 acts in a complex with anion exchangers (AEs) to facilitate Cl--HCO3- exchange in cotransfected cells. To investigate whether CA4 or CA14 plays such a role in hippocampal neurons, we studied NH4+-induced alkalinization of the cytosol, which is mitigated by Cl- entry and HCO3- exit. The NH4+-induced alkalinization was enhanced when the extracellular CAs were inhibited by the poorly permeant CA blocker, benzolamide, or by inhibitory antibodies specific for either CA4 or CA14. The NH4+-induced alkalinization was also increased with inhibition of anion exchange by 4,4*-diisothiocyanostilbene-2,2*-disulfonic acid, or by eliminating Cl- from the medium. No effect of benzolamide was seen under these conditions, in which no Cl--HCO3- exchange was possible. Quantitative PCR on RNA from the neuronal cultures indicated that AE3 was the predominant AE isoform. Single-cell PCR also showed that Slc4a3 (AE3) transcripts were abundant in isolated neurons. In hippocampal neurons dissociated from AE3-null mice, the NH4+-induced alkalinization was much larger than that seen in neurons from wild-type mice, suggesting little or no Cl--HCO3- exchange in the absence of AE3. Benzolamide had no effect on the NH4+-induced alkalinization in the AE3 knock-out neurons. Our results indicate that CA4 and CA14 both play important roles in the regulation of intracellular pH in hippocampal neurons, by facilitating AE3-mediated Cl--HCO3- exchange.