CO₂-dependent opening of connexin 43 hemichannels

Sequence and structure comparisons between alpha and beta connexins, Cx43 and Cx26, revealed that Cx43 has a motif, the carbamylation motif, that confers CO₂-sensitivity on a subset of beta connexins. By using a fluorescent dye loading assay, whole cell patch clamp recordings and real-time measurement of ATP release via GRAB_ATP, we have demonstrated that Cx43 hemichannels open in a highly CO₂-sensitive manner over the range 20-70 mmHg. Mutational analysis confirms that the equivalent residues to those in Cx26, known to be involved in mediating the effects of CO₂ on gating of hemichannels and gap junction channels, also mediate Cx43 hemichannel gating. These data predict that Cx43 will be partially open at resting physiological levels of PCO₂. In acute mouse hippocampal slices, we have demonstrated a CO₂-dependent enhancement of synaptic transmission that was blocked by the Cx43-selective mimetic peptide Gap26. Our data resolves an inconsistency in the literature between in vivo studies suggesting that Cx43 hemichannels are at least partially open at rest and in vitro studies performed in the absence of HCO₃^-/CO₂ buffering that show Cx43 hemichannels are shut.