Activity-dependent CO2 production in the axon triggers opening of Connexin32 in the Schwann cell paranode.

Loss of function mutations of Cx32, which is expressed in Schwann cells, cause X-linked Charcot-Marie-Tooth disease, a slowly progressive peripheral neuropathy. Action potential propagation causes Cx32 hemichannels in the Schwann cell paranode to open. As Cx32 hemichannels are directly sensitive to CO2, we have tested whether CO2 produced in the axon, as a consequence of the energetic demands of action potential propagation, might gate Cx32 hemichannels. Using isolated sciatic nerve from the mouse, we found that the critical components required for intercellular CO2 signaling are present (nodal mitochondria, the source of CO2; a CO2-permeable aquaporin, AQP1; paranodal Cx32; and carbonic anhydrase). We have used a membrane impermeant fluorescent dye, FITC, to demonstrate the opening of Cx32 in Schwann cells in response to an external CO2 stimulus or during action potential propagation in the isolated nerve. Pharmacological manipulations of AQP1 or carbonic anhydrase activity altered Cx32 gating during action potential firing. Expression of a modified Cx32 subunit, Cx32DN, that coassembles with Cx32WT, revealed that the activity-dependent dye loading of Schwann cells depended upon CO2 binding to Cx32. CO2 can, therefore, mediate neuron-to-glia signaling via connexins. CO2 permeable aquaporins and carbonic anhydrase are key components of this signaling mechanism.