Kosaras B, Kirschner D A
Neurology Research, Children's Hospital, Boston, MA 02115.
J Neurocytol. 1990 Apr;19(2):187-99. doi: 10.1007/BF01217297.
The radial component is a structural specialization within CNS myelin that is believed to stabilize the apposition of membranes in the internode. Previous observations on thin sections and freeze-fracture replicas show that this junctional complex consists of linear, particulate strands that run parallel to the nerve fibre axis and radially through the myelin sheath, but details on its molecular organization are lacking. The objective of our current study was to gain further insight into its arrangement and composition by examining its fine-structure and incidence in: myelin with known deficits in protein composition (e.g., shiverer, transgenic shiverer, myelin deficient and jimpy mutant mice); isolated CNS myelin, which has been shown by X-ray diffraction to be more stable than intact CNS myelin; and human white matter, in which this junctional complex has not yet been described. Our results confirm the localization and general appearance of the radial component as previously reported. In addition, we found that: (1) the radial component occurs abundantly in human CNS myelin where it has a complex subunit structure; (2) the constituent junctional unit of this structure is organized as a pair of globular domains (each approximately 40 A diameter) at the extracellular apposition which is linked by approximately 15 A diameter filaments extending through the bilayer to approximately 25 A globular domains in the adjacent cytoplasmic apposition; (3) the radial component is present with apparently normal structure in the sparse, compact myelin of murine mutants containing either different amounts of MBP or no PLP which indicates that neither of these proteins is necessary for junctional integrity; (4) the radial component is present in purified CNS myelin membranes which may account for the stability of these membranes; and (5) the radial component is structurally resistant to Triton, which suggests a method for its further biochemical characterization. Finally, from an analysis of images from tilted transverse and longitudinal sections, we have reconstructed a model of its three-dimensional, supramolecular organization.
放射状成分是中枢神经系统髓鞘内的一种结构特化，据信其可稳定节间膜的并置。先前对薄切片和冷冻断裂复制品的观察表明，这种连接复合体由平行于神经纤维轴并径向穿过髓鞘的线性颗粒链组成，但缺乏其分子组织的详细信息。我们当前研究的目的是通过检查其精细结构和发生率，进一步深入了解其排列和组成，具体涉及：蛋白质组成存在已知缺陷的髓鞘（例如，颤抖鼠、转基因颤抖鼠、髓鞘缺陷和跳跃突变小鼠）；经X射线衍射显示比完整中枢神经系统髓鞘更稳定的分离中枢神经系统髓鞘；以及尚未描述这种连接复合体的人类白质。我们的结果证实了先前报道的放射状成分的定位和总体外观。此外，我们发现：（1）放射状成分大量存在于人类中枢神经系统髓鞘中，具有复杂的亚基结构；（2）该结构的组成连接单元在细胞外并置处组织为一对球状结构域（每个直径约40埃），由直径约15埃的细丝连接，细丝穿过双层延伸至相邻细胞质并置处约25埃的球状结构域；（3）在含有不同量髓鞘碱性蛋白或不含髓鞘蛋白脂蛋白的小鼠突变体的稀疏、致密髓鞘中，放射状成分以明显正常的结构存在，这表明这两种蛋白质对于连接完整性都不是必需的；（4）放射状成分存在于纯化的中枢神经系统髓鞘膜中，这可能解释了这些膜的稳定性；（5）放射状成分在Triton处理下结构稳定，这提示了一种对其进行进一步生化表征的方法。最后，通过对倾斜的横向和纵向切片图像的分析，我们重建了其三维超分子组织模型。
