Hirono Moritoshi, Ogawa Yasuhiro, Misono Kaori, Zollinger Daniel R, Trimmer James S, Rasband Matthew N, Misonou Hiroaki
Graduate School of Brain Science, Doshisha University, Kyoto 619-0225, Japan.
Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, and.
J Neurosci. 2015 May 6;35(18):7082-94. doi: 10.1523/JNEUROSCI.3778-14.2015.
In myelinated axons, K(+) channels are clustered in distinct membrane domains to regulate action potentials (APs). At nodes of Ranvier, Kv7 channels are expressed with Na(+) channels, whereas Kv1 channels flank nodes at juxtaparanodes. Regulation of axonal APs by K(+) channels would be particularly important in fast-spiking projection neurons such as cerebellar Purkinje cells. Here, we show that BK/Slo1 channels are clustered at the paranodal junctions of myelinated Purkinje cell axons of rat and mouse. The paranodal junction is formed by a set of cell-adhesion molecules, including Caspr, between the node and juxtaparanodes in which it separates nodal from internodal membrane domains. Remarkably, only Purkinje cell axons have detectable paranodal BK channels, whose clustering requires the formation of the paranodal junction via Caspr. Thus, BK channels occupy this unique domain in Purkinje cell axons along with the other K(+) channel complexes at nodes and juxtaparanodes. To investigate the physiological role of novel paranodal BK channels, we examined the effect of BK channel blockers on antidromic AP conduction. We found that local application of blockers to the axon resulted in a significant increase in antidromic AP failure at frequencies above 100 Hz. We also found that Ni(2+) elicited a similar effect on APs, indicating the involvement of Ni(2+)-sensitive Ca(2+) channels. Furthermore, axonal application of BK channel blockers decreased the inhibitory synaptic response in the deep cerebellar nuclei. Thus, paranodal BK channels uniquely support high-fidelity firing of APs in myelinated Purkinje cell axons, thereby underpinning the output of the cerebellar cortex.
在有髓轴突中，钾离子通道聚集在不同的膜结构域以调节动作电位（APs）。在郎飞结处，Kv7通道与钠离子通道共同表达，而Kv1通道则位于旁结处郎飞结的两侧。钾离子通道对轴突动作电位的调节在快速放电投射神经元（如小脑浦肯野细胞）中尤为重要。在此，我们表明大电导钙激活钾通道（BK/Slo1通道）聚集在大鼠和小鼠有髓浦肯野细胞轴突的旁结连接处。旁结连接是由一组细胞粘附分子（包括接触蛋白相关蛋白（Caspr））在结和旁结之间形成的，它将结间膜结构域与结内的膜结构域分隔开来。值得注意的是，只有浦肯野细胞轴突有可检测到的旁结BK通道，其聚集需要通过Caspr形成旁结连接。因此，BK通道与其他位于结和旁结处的钾离子通道复合体一起占据了浦肯野细胞轴突中的这一独特结构域。为了研究新型旁结BK通道的生理作用，我们检测了BK通道阻滞剂对逆行动作电位传导的影响。我们发现，将阻滞剂局部应用于轴突会导致在频率高于100Hz时逆行动作电位失败率显著增加。我们还发现镍离子（Ni(2+)）对动作电位有类似影响，表明镍离子敏感的钙离子通道参与其中。此外，向轴突应用BK通道阻滞剂会降低小脑深部核团中的抑制性突触反应。因此，旁结BK通道独特地支持有髓浦肯野细胞轴突中动作电位的高保真发放，从而支撑小脑皮质的输出。
