Connexins, gap junctions and peripheral neuropathy.

Gap junctions (GJs) have emerged as an important molecular component of peripheral myelinated fibers following the discovery of mutations affecting the GJ protein connexin32 (Cx32) in patients with the X-linked Charcot-Marie-Tooth neuropathy (CMT1X). CMT1X is the second most common CMT form and is caused by over 400 different mutations in the GJB1 gene encoding Cx32. In peripheral nerves, Cx32 is expressed by Schwann cells and forms reflexive GJs through non-compact myelin areas, which allow the diffusion of ions and small molecules including second messengers across apposed cell membranes connecting directly the Schwann cell perinuclear cytoplasm with the adaxonal cell compartment inside the myelin sheath. GJs formed by Cx32 play an important role in the homeostasis of myelinated axons. Patients with CMT1X typically present with a progressive peripheral neuropathy characterized by mixed demyelinating and axonal features electrophysiologically and pathologically, which may be accompanied by transient or chronic CNS myelin dysfunction. Both in vitro and in vivo models of the disease indicate that most Cx32 mutations cause loss of function and inability of the mutant Cx32 to form functional GJs. Increased understanding of CMT1X pathogenesis will lead to the development of effective therapies for this currently incurable disease.