Molecular Mechanisms for K+ channel clustering on myelinated axons

离子通道的特征性定位对于有髓神经中跳跃传导的传播至关重要。电压门控性Na ~+通道主要位于郎维结，而电压门控性K ~+通道主要位于腹主动脉旁结区。从我们目前的研究中，在中枢神经系统中的K+通道集群与髓鞘的轴突上的形成，但不与少突胶质细胞体的协会，这表明，在相反的Na+通道集群，髓鞘膜上的轴突的接触是重要的K+通道集群。为了确定K+通道聚集的分子机制，在正常和脱髓鞘轴突中检查PSD-95的定位。已知PSD-95结合K+通道和NMDA受体，并在突触后密度形成这些蛋白质的簇，PSD-95与K+通道共定位于突触后区。在脱髓鞘的轴突中，PSD-95和K+通道的聚集消失，但通过Western印迹判断的两种蛋白质的总量没有变化，表明它们弥散地重新分布。从使用K+通道亚基和PSD-95的cDNA的转染研究中，我们发现，这些蛋白质在轴突中的适当位置的簇形成可能需要另一个因素。两个新发现的蛋白质的节点周围，Caspr（paranode），和Caspr 2（arctaparanode）的本地化，也进行了检查，并被发现是很好的相关性与K+通道集群和paranodal交界处的形成也是重要的特征本地化。这些结果表明，这两个蛋白质也可能参与K+通道聚类除了PSD-95。

The characteristic localization of ion channels is crucial for the propagation of saltatory conductions in myelinated nerves. The voltage-gated Na+ channels located at Node of Ranvier while the voltage-gated K+ channels are mainly found at juxtaparanodal regions. From our present studies, the K+ channel clustering in the CNS is well correlated with the formation of myelin sheath on the axons, but not with the association of oligodendrocyte cell bodies, suggesting that, in contrast to Na+ channel clustering, contact of the myelin membrane onto the axons is important for K+ channels clustering. To identify the molecular mechanisms for K+ channel clustering, the localization of PSD-95 was examined both in normal and demyelinated axons. PSD-95, which is known to bind K+ channels and NMDA receptors and form clusters of these proteins at post synaptic densities, was colocalized with K+ channel in the juxtaparanodal regions. In demyelinated axons, the clustering of both PSD-95 and K+ channels disappeared, but the total amount of both proteins as judged by the Western blotting did not change, indicating that they redistributed diffusely. From transfection studies using cDNAs of K+ channel subunits and PSD-95, we found that another factors may be required for the cluster formation of these proteins in the appropriate places in the axons. The localizations of two newly identified proteins around the nodes, Caspr (paranode), and Caspr2 (juxtaparanode), were also examined, and were found to be well corelated with K+ channel clustering and the paranodal junctional formation was also important for their characteristic localization. These results suggested that these two proteins may also be involved in K+ channel clustering in addition to PSD-95.