Swalla B J, Badgett M R, Jeffery W R
Department of Zoology, University of Texas, Austin 78712.
Development. 1991 Feb;111(2):425-36. doi: 10.1242/dev.111.2.425.
The myoplasm of ascidian eggs is a localized cytoskeletal domain that is segregated to presumptive larval tail muscle cells during embryonic development. We have identified a cytoskeletal protein recognized by a vertebrate neurofilament monoclonal antibody (NN18) which is concentrated in the myoplasm in eggs and embryos of a variety of ascidian species. The NN18 antigen is localized in the periphery of unfertilized eggs, segregates with the myoplasm after fertilization, and enters the larval tail muscle cells during embryonic development. Western blots of one-dimensional and two-dimensional gels showed that the major component recognized by NN18 antibody is a 58 x 10(3) Mr protein (p58), which exists in at least three different isoforms. The enrichment of p58 in the Triton X-100-insoluble fraction of eggs and its reticular staining pattern in eggs and embryos suggests that it is a cytoskeletal protein. In subsequent experiments, p58 was used as a marker to determine whether changes in the myoplasm occur in eggs of anural ascidian species, i.e. those exhibiting a life cycle lacking tadpole larvae with differentiated muscle cells. Although p58 was localized in the myoplasm in eggs of four urodele ascidian species that develop into swimming tadpole larvae, this protein was distributed uniformly in eggs of three anural ascidian species. The eggs of two of these anural species contained the actin lamina, another component of the myoplasm, whereas the third anural species lacked the actin lamina. There was no detectible localization of p58 after fertilization or segregation into muscle lineage cells during cleavage of anural eggs. NN18 antigen was uniformly distributed in pre-vitellogenic oocytes and then localized in the perinuclear zone during vitellogenesis of urodele and anural ascidians. Subsequently, NN18 antigen was concentrated in the peripheral cytoplasm of post-vitellogenic oocytes and mature eggs of urodele, but not anural, ascidians. It is concluded that the myoplasm of ascidian eggs contains an intermediate filament-like cytoskeletal network which is missing in anural species that have modified or eliminated the tadpole larva.
海鞘卵的肌质是一个局部化的细胞骨架区域，在胚胎发育过程中会被分隔到推定的幼虫尾部肌肉细胞中。我们鉴定出一种被脊椎动物神经丝单克隆抗体（NN18）识别的细胞骨架蛋白，它在多种海鞘物种的卵和胚胎的肌质中富集。NN18抗原位于未受精卵的周边，受精后与肌质一起分隔，并在胚胎发育过程中进入幼虫尾部肌肉细胞。一维和二维凝胶的蛋白质印迹分析表明，NN18抗体识别的主要成分是一种58×10³ Mr的蛋白质（p58），它至少以三种不同的异构体形式存在。p58在卵的Triton X-100不溶性部分中的富集及其在卵和胚胎中的网状染色模式表明它是一种细胞骨架蛋白。在随后的实验中，p58被用作标记物，以确定无尾海鞘物种的卵中肌质是否发生变化，即那些表现出缺乏具有分化肌肉细胞的蝌蚪幼虫的生命周期的物种。尽管p58在四种发育成游泳蝌蚪幼虫的有尾海鞘物种的卵的肌质中定位，但这种蛋白质在三种无尾海鞘物种的卵中均匀分布。其中两种无尾物种的卵含有肌质的另一种成分肌动蛋白层，而第三种无尾物种则缺乏肌动蛋白层。在无尾卵的分裂过程中，受精后或分隔到肌肉谱系细胞中均未检测到p58的定位。NN18抗原在卵黄发生前的卵母细胞中均匀分布，然后在有尾和无尾海鞘的卵黄发生过程中定位在核周区域。随后，NN18抗原集中在有尾海鞘而非无尾海鞘的卵黄发生后的卵母细胞和成熟卵的周边细胞质中。结论是，海鞘卵的肌质包含一个中间丝样的细胞骨架网络，而在已经改变或消除蝌蚪幼虫的无尾物种中则不存在。
