Molecular Mechanisms for K+ channel clustering on myelinated axons

离子通道的特征定位对于有髓神经中跳跃性传导的传播至关重要。电压门控性Na+通道位于兰维尔结节，而电压门控性K+通道主要位于旁旁结区。根据我们目前的研究，中枢神经系统K+通道聚集与轴突上髓鞘的形成有很好的相关性，但与少突胶质细胞胞体的联系不相关，这表明与Na+通道聚集相反，髓鞘膜与轴突的接触对K+通道聚集是重要的。为了确定K+通道聚集的分子机制，研究了PSD-95在正常轴突和脱髓鞘轴突中的定位。PSD-95能结合K+通道和NMDA受体，并在突触后密度形成这些蛋白质簇，它与K+通道共定位于旁结节区域。在脱髓鞘轴突中，PSD-95和K+通道的聚集消失，但Western blotting判断这两种蛋白的总量没有变化，表明它们重新分布。通过使用K+通道亚基的cDNA和PSD-95的转染研究，我们发现这些蛋白可能需要其他因素才能在轴突中的适当位置形成簇。我们还研究了两种新发现的结节周围蛋白CASPR(旁阳极)和Caspr2(旁旁阳极)的定位，发现它们与K+通道聚集有很好的相关性，结旁连接的形成也是它们的特征定位的重要因素。这些结果表明，除了PSD-95外，这两种蛋白还可能参与K+通道聚集。

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.