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

Abstract

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, Cx32^DN, that coassembles with Cx32^WT, 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.