Connexins in the Nervous System

This chapter reviews the localizations and physiological roles of connexins in neurons and glia of the central and peripheral nervous systems. Cx32 forms gap junctions in noncompact myelin in Schwann cells, which are thought to form a reflexive communication pathway connecting the outer and inner myelin layers. Cx29 is also expressed in myelinating Schwann cells, but does not appear to form gap junctions; its role remains to be elucidated. Mutations in CX32 cause an X-linked form of the inherited neuropathy Charcot-Marie-Tooth disease; most such mutations are likely to act through loss of function. Connexins may also play an important role in proliferating Schwann cells. Oligodendrocytes express Cx32, Cx47, and Cx29, while astrocytes express Cx43, Cx30, and possibly Cx26. Although astrocytes are extensively coupled to each other in vivo, oligodendrocyte coupling in vivo is demonstrable only to astrocytes, most via heterotypic Cx43–Cx47 or Cx30–Cx32 junctions. These junctions, along with those between astrocytes, may play a role in spatial buffering of Kþ ions and neurotransmitters, and may influence severity of tissue damage during ischemia. Mutations in CX47 cause Pelizaeus Merzbacher–like disease while mutations in CX43 cause oculodentodigital dysplasia. Only Cx36 and Cx45 have been definitively identified in nonretinal brain neurons, where they form electrical synapses; neuron–neuron coupling may play a role in the pathogenesis of epilepsy.