SEQUENCE AND EXPRESSION OF AMPHIOXUS ALKALI MYOSIN LIGHT-CHAIN (AMPHIMLC-ALK) THROUGHOUT DEVELOPMENT - IMPLICATIONS FOR VERTEBRATE MYOGENESIS
SEQUENCE AND EXPRESSION OF AMPHIOXUS ALKALI MYOSIN LIGHT-CHAIN (AMPHIMLC-ALK) THROUGHOUT DEVELOPMENT - IMPLICATIONS FOR VERTEBRATE MYOGENESIS
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DOI：
10.1006/dbio.1995.1313
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发表时间：
1995-10-01
影响因子：
2.7
通讯作者：
HOLLAND, ND
中科院分区：
文献类型：
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作者：
HOLLAND, LZ;PACE, DA;HOLLAND, ND
下脊索文文鱼被广泛认为是与脊椎动物关系最近的无脊椎动物，是理解复杂脊椎动物身体结构的基因复制与进化之间关系的关键生物。在四足脊椎动物中，碱肌球蛋白轻链基因（MLC-alk）编码与肌球蛋白重链球形头相关的蛋白质，构成了一个大家族，具有不同阶段、组织和纤维类型的特异性表达，被认为是由单个祖先基因的重复产生的。在节肢动物、软体动物和线虫等原口无脊椎动物中，只发现了一个MLC-alk基因，但在后口无脊椎动物和低等脊椎动物中，这类基因的数量尚不清楚。本报告描述了文昌鱼碱肌球蛋白轻链基因（AmphiMLC-alk）在发育过程中的序列和表达，从而填补了理解基因复制与肌肉细胞类型多样性增加之间关系的主要空白。从文昌鱼幼鱼cDNA文昌鱼文库中分离到一个全长1 kb的AmphiMLC-alk克隆。它编码了一个149个氨基酸的蛋白质，与脊椎动物胚胎形式的MLC-alk最密切相关。Southern blot分析发现AmphiMLC-alk基因只有1个拷贝，提示这是文昌鱼中唯一的MLC-alk基因。Northern blot分析表明，该基因只产生一个转录本，在发育的所有阶段和成人中表达。原位杂交显示，该基因最初表达于神经胚体2-5号体的肌组中，随后在1号体的肌组中表达，并在新形成的体的肌组中逐渐表达。肌瘤细胞的表达在幼虫发育过程中一直持续到成虫阶段，因为肌瘤细胞分化为纹状的、单核的肌肉细胞——与脊椎动物的纹状肌细胞不同，文文鱼的肌瘤细胞不会变成多核的。在晚期幼虫和成虫的肌瘤中，AmphiMLC-alk的表达定位于肌瘤的内侧边缘和细胞的末端。这是首次在任何动物的肌肉细胞中证明MLC转录本的细胞内定位。AmphiMLC-alk在平滑肌和非肌瘤来源的横纹肌中也有表达。这些表达数据与Southern blot分析一致，表明文昌鱼中只有一个MLC-alk基因。因此，祖先脊椎动物的MLC-alk基因的重复可能发生在脊椎动物和文昌鱼谱系分裂之后。我们得出的结论是，在多核时期，多轴肌组织的发展先于多个MLC-alk亚型的进化。由于文昌鱼的肌肉发生在结构和基因水平上与脊椎动物的肌肉发生相似，但远比脊椎动物的肌肉发生简单，因此了解文昌鱼的肌肉发生可以深入了解脊椎动物肌肉发生的进化史和详细机制。(C) 1995学术出版社，Inc。
The lower chordate amphioxus, widely considered the closest living invertebrate relative of the vertebrates, is a key organism for understanding the relationship between gene duplications and evolution of the complex vertebrate body plan. In tetrapod vertebrates, the alkali myosin light chain genes (MLC-alk), which code for proteins associated with the globular head of the myosin heavy chain, constitute a large family with stage-, tissue-, and fiber-type-specific expression of different isoforms thought to have arisen by duplication of a single ancestral gene. In protostome invertebrates, e.g., arthropods, molluscs, and nematodes, only one MLC-alk gene has been found, but the number of such genes in deuterostome invertebrates and lower vertebrates is unknown. The present report, describing the sequence and expression throughout development of the amphioxus gene for alkali myosin light chain (AmphiMLC-alk), thus fills a major gap in understanding the relation between gene duplication and increasing diversity of muscle-cell types. A full-length clone (1 kb) of AmphiMLC-alk was isolated from a larval amphioxus cDNA Library. It coded for a 149-amino-acid protein most closely related to the vertebrate embryonic form of MLC-alk. Southern blot analysis revealed only one copy of AmphiMLC-alk and suggested that it is the only MLC-alk gene in amphioxus. Northern blot analysis indicated that this gene produces only one transcript, which is expressed at all stages of development and in adults. In situ hybridizations showed expression initially in the myotomes of somites 2-5 of neurula embryos and soon thereafter in the myotomes of somite 1 and of newly forming somites progressively added posteriorly. Myotomal expression continues throughout larval development and into the adult stage as the myotomal cells differentiate into striated, mononucleate muscle cells-unlike vertebrate striated muscle cells, those of amphioxus never become multinucleate. In late larvae and adults myotomal expression of AmphiMLC-alk is localized along the medial edge of the myotome and at the ends of the cells. This is the first demonstration of intracellular localization of MLC transcripts in muscle cells of any animal. Expression of AmphiMLC-alk was also detected in smooth muscles as well as in striated muscles not derived from the myotome. These expression data are consistent with the Southern blot analysis in suggesting that there is only one MLC-alk gene in amphioxus. Thus, duplication of an ancestral vertebrate MLC-alk gene probably occurred after the vertebrate and amphioxus lineages split. We conclude that development of a segmented axial musculature preceded the evolution of multiple MLC-alk isoforms, which evidently arose about the time of multinucleation. Since myogenesis in amphioxus is similar to but far simpler than myogenesis in vertebrates at both the structural and gene levels, an understanding of myogenesis in amphioxus can give insights into both the evolutionary history and the detailed mechanisms of vertebrate myogenesis. (C) 1995 Academic Press, Inc.