Completion of myelin compaction, but not the attachment of oligodendroglial processes triggers K(+) channel clustering.

The characteristic localization of ion channels is crucial for the propagation of saltatory conduction in myelinated nerves. Voltage-gated Na(+) channels are located at nodes of Ranvier while voltage-gated K(+) channels are mainly found at juxtaparanodal regions. Recently, a humoral factor secreted by oligodendrocytes has been reported to induce clustering of Na(+) channels in CNS axons. However, the molecular mechanisms for K(+) channel clustering as well as the role of oligodendrocytes are still uncertain. To clarify whether myelin sheath itself can induce the distinct distribution of K(+) channels, we have investigated the localization of K(+) channels in adult and developing mouse optic nerves. The CNS axons from chronic demyelinating and hypomyelinating mice were also examined to determine if myelin sheaths were required for the maintenance of clusters. In all cases, the K(+) channel clustering correlated well with compact myelin, but not with the presence of oligodendrocytes, suggesting that, in contrast to Na(+) channel clustering, the formation of compact myelin is required for initiation as well as maintenance of K(+) channel clustering. In addition, postsynaptic density protein-95 (PSD-95) or its highly related protein was found colocalized with K(+) channels, suggesting that it may interact with K(+) channels to form clusters at juxtaparanodal regions.