Braun N, Schikorski T, Zimmermann H
AK Neurochemie, Zoologisches Institut der J. W. Goethe-Universität, Frankfurt am Main, Federal Republic of Germany.
Neuroscience. 1993 Feb;52(3):745-56. doi: 10.1016/0306-4522(93)90423-d.
The cytoplasm of the highly polarized nerve cell is permanently segregated into domains with differing organellar composition. The mechanisms maintaining this segregation are largely unknown. In order to elucidate the potential role of cytoskeletal elements in this process we compared the cytoplasmic segregation within the giant electromotoneuron of the electric catfish (Malapterurus electricus) with the distribution of binding sites for antibodies against elements of the cytoskeleton. Most prominent cytoplasmic segregations include the formation of a subplasmalemmal cortical structure free of Nissl bodies and Golgi cisternae, the separation within the soma of domains containing rough endoplasmic reticulum and filament-rich domains, and the soma-axon transition. The cytoplasmic transition at the axon hillock forms a distinct borderline where Nissl bodies, Golgi cisternae and the bulk of lysosomes abruptly terminate and are excluded from the axoplasm. Synaptic vesicles and mitochondria are free to pass compartmental borders. Tropomyosin, spectrin, and alpha-actinin reveal a rather homogeneous immunofluorescence throughout the neuron. In contrast, neurofilament protein and tubulin display a distinctly increased immunofluorescence in the subplasmalemmal cortical layer, in dendrites as well as in the axon. The increase in immunofluorescence at the axon hillock exactly depicts the small transition zone from the somatic cytoplasm rich in Nissl bodies, Golgi cisternae and lysosomes to the differently structured axoplasm. The picture is similar for beta-tubulin, tyrosinylated and detyrosinylated alpha-tubulin. Detyrosinylated tubulin (glu-tubulin, which is contained in microtubules of increased stability) shows the most prominent enrichment in the axon. The distribution of myosin is comparable to that of neurofilament protein but there is less difference in immunofluorescence between the domains. Our results would be compatible with a role of microtubules together with (the closely associated) neurofilaments in the segregation of neuronal cytoplasmic domains. Active transport as well as stable binding to the somatic cytoskeleton might counteract a homogeneous cytoplasmic distribution of the various classes of organelles by diffusion.
高度极化的神经细胞的细胞质被永久分隔成具有不同细胞器组成的区域。维持这种分隔的机制在很大程度上尚不清楚。为了阐明细胞骨架成分在此过程中的潜在作用，我们将电鲶（电鲇属）的巨型电运动神经元内的细胞质分隔与针对细胞骨架成分的抗体结合位点的分布进行了比较。最显著的细胞质分隔包括形成不含尼氏体和高尔基池的亚质膜皮质结构、在胞体内富含粗面内质网的区域和富含细丝的区域的分离以及胞体 - 轴突过渡。轴丘处的细胞质过渡形成一条明显的边界，尼氏体、高尔基池和大部分溶酶体在此突然终止并被排除在轴质之外。突触小泡和线粒体可自由通过分隔边界。原肌球蛋白、血影蛋白和α - 辅肌动蛋白在整个神经元中显示出相当均匀的免疫荧光。相比之下，神经丝蛋白和微管蛋白在亚质膜皮质层、树突以及轴突中显示出明显增强的免疫荧光。轴丘处免疫荧光的增加准确描绘了从富含尼氏体、高尔基池和溶酶体的胞体细胞质到结构不同的轴质的小过渡区。β - 微管蛋白、酪氨酸化和去酪氨酸化的α - 微管蛋白的情况类似。去酪氨酸化的微管蛋白（谷氨酰胺 - 微管蛋白，包含在稳定性增加的微管中）在轴突中显示出最显著的富集。肌球蛋白的分布与神经丝蛋白的分布相当，但各区域之间的免疫荧光差异较小。我们的结果与微管以及（紧密相关的）神经丝在神经元细胞质区域分隔中的作用相符。主动运输以及与胞体细胞骨架的稳定结合可能通过扩散抵消各类细胞器在细胞质中的均匀分布。
