Dysregulation of axonal sodium channel isoforms after adult-onset chronic demyelination

Altered expression of ion channel isoforms at the node of Ranvier in P0-deficient myelin mutants

To elucidate the impact of myelinating Schwann cells on the molecular architecture of the node of Ranvier, we investigated the nodal expression of voltage-gated sodium channel (VGSC) isoforms and the localization of paranodal and juxtaparanodal membrane proteins in a severely affected Schwann cell mutant, the mouse deficient in myelin protein zero (P0). The abnormal myelin formation and compaction was associated with immature nodal cluster types of VGSC. Most strikingly, P0-deficient motor nerves displayed an ectopic nodal expression of the Na(v)1.8 isoform, where it is coexpressed with the ubiquitous Na(v)1.6 channel. Furthermore, Caspr was distributed asymmetrically or was even absent in the mutant nerve fibers. The potassium channel K(v)1.2 and Caspr2 were not confined to juxtaparanodes, but often protruding into the paranodes. Thus, deficiency of P0 leads to dysregulation of nodal VGSC isoforms and to altered localization of paranodal and juxtaparanodal components of the nodal complex.

The local differentiation of myelinated axons at nodes of Ranvier

Efficient and rapid propagation of action potentials in myelinated axons depends on the molecular specialization of the nodes of Ranvier. The nodal region is organized into several distinct domains, each of which contains a unique set of ion channels, cell-adhesion molecules and cytoplasmic adaptor proteins. Voltage-gated Na+ channels - which are concentrated at the nodes - are separated from K+ channels - which are clustered at the juxtaparanodal region - by a specialized axoglial contact that is formed between the axon and the myelinating cell at the paranodes. This local differentiation of myelinated axons is tightly regulated by oligodendrocytes and myelinating Schwann cells, and is achieved through complex mechanisms that are used by another specialized cell-cell contact - the synapse.

Polarized domains of myelinated axons

The entire length of myelinated axons is organized into a series of polarized domains that center around nodes of Ranvier. These domains, which are crucial for normal saltatory conduction, consist of distinct multiprotein complexes of cell adhesion molecules, ion channels, and scaffolding molecules; they also differ in their diameter, organelle content, and rates of axonal transport. Juxtacrine signals from myelinating glia direct their sequential assembly. The composition, mechanisms of assembly, and function of these molecular domains will be reviewed. I also discuss similarities of this domain organization to that of polarized epithelia and present emerging evidence that disorders of domain organization and function contribute to the axonopathies of myelin and other neurologic disorders.