Molecular mechanisms which control the cell movements during ascidian neural tube formation

海鞘神经管的结构与脊椎动物的基本相同。海鞘相对简单的结构和丰富的分子资源为分析神经管形成的分子机制提供了良好的实验系统。然而，根据我们以往的研究，脊椎动物和海鞘神经管形成的分子机制是否相似的问题已经被提出。因此，我们重新审视了以下两个主题。对豚鼠钙粘蛋白（CiCadI和CiCadII）基因的重新检测我们已经发现了CiCadII在神经管中的特异性表达。我们试图分析另一种Ciona经典钙粘蛋白cadherin type I （CiCadI）的时空表达模式。利用RT-PCR方法，从原胚期到尾芽期，检测到CiCadI的母系表达和合子表达。这一结果表明，在Ciona发育过程中，两种经典钙粘蛋白被用于神经管的形成。CiCadI和CiCadII在海鞘神经管形成过程中同时使用的结果表明，海鞘与脊椎动物神经管形成的分子机制具有相似性。在脊椎动物神经管的形成过程中，提出了两种类型的机制，即初级神经管和次级神经管的形成。在本研究中，我们用phalloidin染色皮质肌动蛋白网络，并标记神经管细胞，这些细胞携带由神经管特异性启动子驱动的gfp报告基因。神经前体细胞主动后移，细胞重排明显。另一方面，大脑前体细胞并没有沿着前后轴移动太多。脑干前体细胞中含有一定量的肌动蛋白网络。这些区域差异表明神经管中每个区域的机制不同。

Ascidian neural tube has basically the same structure as those of vertebrates. Its relatively simple structure and the abundant molecular resources of the ascidian offer an excellent experiment system for analyzing the molecular mechanisms of neural tube formation. However, according to our previous studies, the question has been raised as to whether or not the molecular mechanisms of vertebrate and ascidian neural tube formation are similar. So we reexamined the following two subjects.1.Reexamination of Ciona intestinalis Cadherin genes (CiCadI and CiCadII)We have already showed the neural-tube specific expression of CiCadII. We tried to analyze the spatial and temporal expression pattern of the other Ciona classic cadherin, Cadherin type I (CiCadI). Using the RT-PCR, CiCadI expressed maternally and its zygotic expression was detected from gastrula stage to the tailbud stage. This result suggested that, during the Ciona development, the two classic cadherin were used in the neural tube formation. The result that both CiCadI and CiCadII were used during the ascidian neural tube formation suggested the similarity in the molecular mechanisms of the ascidian and vertebrates neural tube formation.2.Reexamination of the cell movement during neural tube formationDuring the vertebrate neural tube formation, two types of mechanisms, primary-and secondary-neural tube formation, are proposed. In this study, we stained cortical actin network with phalloidin, and marked the neural tube cells, which carried GFP-reporter gene driven by the neural-tube specific promoter. The nerve-cord precursor cells actively moved posteriorly and made obvious cell rearrangement. On the other hand, brain-precursor cells did not move much along the anterior-posterior axis. The brain-stem precursor cells contained a certain amount of actin network. These regional differences suggested the different mechanism for each region within the neural tube.