Huang Claire Yu-Mei, Zhang Chuansheng, Zollinger Daniel R, Leterrier Christophe, Rasband Matthew N
Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, and.
NeuroCyto, NICN UMR7259, Aix Marseille Université, CNRS, 13344 cedex 15, Marseille, France.
J Neurosci. 2017 Nov 22;37(47):11323-11334. doi: 10.1523/JNEUROSCI.2113-17.2017. Epub 2017 Oct 16.
Axons must withstand mechanical forces, including tension, torsion, and compression. Spectrins and actin form a periodic cytoskeleton proposed to protect axons against these forces. However, because spectrins also participate in assembly of axon initial segments (AISs) and nodes of Ranvier, it is difficult to uncouple their roles in maintaining axon integrity from their functions at AIS and nodes. To overcome this problem and to determine the importance of spectrin cytoskeletons for axon integrity, we generated mice with αII spectrin-deficient peripheral sensory neurons. The axons of these neurons are very long and exposed to the mechanical forces associated with limb movement; most lack an AIS, and some are unmyelinated and have no nodes. We analyzed αII spectrin-deficient mice of both sexes and found that, in myelinated axons, αII spectrin forms a periodic cytoskeleton with βIV and βII spectrin at nodes of Ranvier and paranodes, respectively, but that loss of αII spectrin disrupts this organization. mice have reduced numbers of nodes, disrupted paranodal junctions, and mislocalized Kv1 K channels. We show that the density of nodal βIV spectrin is constant among axons, but the density of nodal αII spectrin increases with axon diameter. Remarkably, mice have intact nociception and small-diameter axons, but severe ataxia due to preferential degeneration of large-diameter myelinated axons. Our results suggest that nodal αII spectrin helps resist the mechanical forces experienced by large-diameter axons, and that αII spectrin-dependent cytoskeletons are also required for assembly of nodes of Ranvier. A periodic axonal cytoskeleton consisting of actin and spectrin has been proposed to help axons resist the mechanical forces to which they are exposed (e.g., compression, torsion, and stretch). However, until now, no vertebrate animal model has tested the requirement of the spectrin cytoskeleton in maintenance of axon integrity. We demonstrate the role of the periodic spectrin-dependent cytoskeleton in axons and show that loss of αII spectrin from PNS axons causes preferential degeneration of large-diameter myelinated axons. We show that nodal αII spectrin is found at greater densities in large-diameter myelinated axons, suggesting that nodes are particularly vulnerable domains requiring a specialized cytoskeleton to protect against axon degeneration.
轴突必须承受机械力，包括拉力、扭力和压力。血影蛋白和肌动蛋白形成一种周期性细胞骨架，其作用是保护轴突免受这些力的影响。然而，由于血影蛋白也参与轴突起始段（AIS）和郎飞结的组装，因此很难将其在维持轴突完整性方面的作用与在AIS和节点处的功能区分开来。为了克服这个问题并确定血影蛋白细胞骨架对轴突完整性的重要性，我们培育了外周感觉神经元αII血影蛋白缺陷的小鼠。这些神经元的轴突非常长，且暴露于与肢体运动相关的机械力之下；大多数缺乏AIS，有些是无髓鞘的且没有节点。我们分析了雌雄两性的αII血影蛋白缺陷小鼠，发现，在有髓鞘的轴突中，αII血影蛋白分别与βIV血影蛋白和βII血影蛋白在郎飞结和旁结处形成周期性细胞骨架，但αII血影蛋白的缺失会破坏这种组织结构。小鼠的节点数量减少，旁结连接被破坏，Kv1钾通道定位错误。我们发现，轴突中节点βIV血影蛋白的密度是恒定的，但节点αII血影蛋白的密度随轴突直径增加。值得注意的是，小鼠的痛觉和小直径轴突完好无损，但由于大直径有髓鞘轴突的优先退化而出现严重共济失调。我们的结果表明，节点αII血影蛋白有助于抵抗大直径轴突所经历的机械力，并且郎飞结的组装也需要αII血影蛋白依赖性细胞骨架。有人提出，由肌动蛋白和血影蛋白组成的周期性轴突细胞骨架有助于轴突抵抗其所承受的机械力（如压缩、扭转和拉伸）。然而，到目前为止，还没有脊椎动物模型测试血影蛋白细胞骨架在维持轴突完整性方面的必要性。我们证明了周期性血影蛋白依赖性细胞骨架在轴突中的作用，并表明外周神经系统（PNS）轴突中αII血影蛋白的缺失会导致大直径有髓鞘轴突的优先退化。我们发现，在大直径有髓鞘轴突中，节点αII血影蛋白的密度更高，这表明节点是特别脆弱的区域，需要专门的细胞骨架来防止轴突退化。
