Mechanisms of Node of Ranvier Assembly

髓鞘轴突的Ranvier节点对动作电位的跳跃传导至关重要，是神经元精细域组织特征的一个显著例子。节点由多聚物复合物组成，包括电压门控钠和钾通道、附属β亚基、细胞粘附分子（CAMs）和细胞骨架复合物，包括通道和CAMs结合的锚蛋白G。我们最近的研究表明，节点从不同的来源组装。CAMs，特别是NF186，通过与雪旺细胞相互作用的扩散捕获，从轴突表面现有的池中重新分配，在形成节点上积累。相反，通道和细胞骨架蛋白主要通过轴突运输传递到淋巴结。令人惊讶的是，似乎还有一个钠通道池，在不依赖于转运和锚蛋白g的情况下在节点上运输和积累。最后，我们的研究表明，存在一个活跃的程序，可以从结外位点清除节点蛋白，进一步加强它们在节点上的选择性富集。为了扩展这些发现并进一步阐明淋巴结组装的机制，我们将通过髓鞘共培养的实时成像策略研究淋巴结组件的运输和组装，以及它们如何从结外部位清除。具体而言，我们将：i)确定将成分运输到节点的囊泡是否分为cam特异性囊泡和离子通道特异性囊泡，并表现出任何结构域特异性；ii)表征钠通道成分到节点的运输依赖和独立运输，分别包括锚蛋白G和NF186的作用，并确定钠通道复合物是否在局部组装；i)研究如何从结外部位清除节点成分。以NF186为重点，进一步研究其细胞质段、内吞作用和蛋白水解在清除NF186淋巴结间表面池中的作用。相关性：这些研究将阐明调节Ranvier结组装的机制，这对神经纤维适当传导电脉冲的能力至关重要。因此，本研究的发现可能对我们理解髓鞘纤维疾病的发病机制具有重要意义，包括导致神经传导异常的神经病变，并可能因此导致这些神经系统疾病的新治疗策略。

The nodes of Ranvier of myelinated axons are critical for action potential propagation by saltatory conduction and are a striking example of the exquisite domain organization characteristic of neurons. Nodes are comprised of a multimeric complex that includes voltage gated sodium and potassium channels, accessory beta subunits, cell adhesion molecules (CAMs), and a cytoskeletal complex that includes ankyrin G to which channels and CAMs bind. Our recent studies indicate that the node assembles from distinct sources. CAMs, notably NF186, accumulate at forming nodes by redistribution from existing pools on the axon surface via diffusion trapping from interactions with the Schwann cell. In contrast, channels and cytoskeletal proteins are delivered to the node primarily via axonal transport. Surprisingly, there also appears to be a pool of sodium channels that traffics and accumulates at the node independent of transport and of ankyrin G. Finally, our studies suggest there is an active program to clear nodal proteins from extranodal sites that further reinforces their selective enrichment at the node. To extend these findings and further elucidate the mechanisms of node assembly, we will investigate trafficking and assembly of components of the node, and how they are cleared from extranodal sites, by live imaging strategies in myelinating cocultures. In particular, we will: i) determine whether vesicles that transport components to the nodes segregate into those specific for CAMs and others for ion channels, and exhibit any domain specificity, ii) characterize transport-dependent and -independent trafficking of sodium channel components to the node, including the roles of ankyrin G and NF186, respectively, and determine whether the sodium channel complex assembles locally, and i) investigate how nodal components are cleared from extranodal sites, focusing on NF186 to examine further the role of its cytoplasmic segment and that of endocytosis and proteolysis in clearing the internodal, surface pool of NF186. Relevance: These studies will elucidate the mechanisms that regulate the assembly of the node of Ranvier, which is critical to the ability of nerve fibers to conduct electrical impulses appropriately. Findings in this study may therefore have important implications for our understanding of the pathogenesis of disorders of myelinated fibers, including neuropathies that result in aberrant nerve conduction, and may thereby lead to new therapeutic strategies for these neurological disorders.