ACTIVIN RECEPTORS AND SMAD2 IN MAMMALIAN GASTRULATION

转化生长因子- β (tgf - β)超家族分子在多种发育过程中发挥关键作用。我们的长期目标是了解tgf - β家族因子调控形态发生的信号机制。中胚层诱导和原肠胚形成是导致体轴形成的关键过程。tgf - β家族的成员，如激活素、Vg-1、nodal和bmp，是脊椎动物早期发育过程中有效的中胚层诱导因子和功能。激活素或BMP激活的信号级联和发育程序在爪蟾和鸡早期胚胎中得到了广泛的研究。然而，尚不清楚这些信号通路在哺乳动物中是否保守。遗传学研究表明，结蛋白和BMP4，而不是合子激活素，对小鼠早期发育至关重要。为了研究激活素I型和II型受体以及下游信号介质Smad2在哺乳动物发育中的功能，我们通过靶向破坏小鼠的相应基因来产生突变小鼠。突变小鼠的遗传和胚胎学研究提供了直接证据，证明激活素受体和Smad2对中胚层形成和原肠胚形成至关重要。为了进一步分析激活素受体和Smad2介导的信号通路在细胞生长和分化、原始条纹形成和原肠胚形成中的作用，我们提出了以下具体目标：(1)了解激活素受体和Smad2在卵筒形成和原肠胚形成过程中如何调节胚胎生长、胚层组织和细胞命运决定。(2)研究激活素受体和Smad2如何调控原始条纹的形成和中胚层图案的形成。3)探讨nodal、BMP4和chordin在原始条纹形成中的作用及其信号通路。本项目将采用胚泡显微注射或四倍体胚胎与胚胎干细胞聚集的嵌合体（或嵌合体）分析、原位杂交、用lacz标记的突变胚胎进行谱系分析、胚胎干细胞介导的基因传递等方法。这些目标一旦实现，将有助于更好地理解控制早期哺乳动物发育的信号机制。获得的关于早期胚胎发育的知识将有助于理解后来的发育过程，如器官发生，并将对理解人类各种形式出生缺陷的遗传基础具有重要意义。

The transforming growth factor-beta (TGF-beta) superfamily molecules play critical roles in a diverse range of developmental processes. Our long term goal is to understand the signaling mechanism by which TGF-beta family factors regulate morphogenesis. Mesoderm induction and gastrulation are key events that lead to the establishment of body axes. Members of the TGF-beta family such as activins, Vg-1, nodal, and BMPs are potent mesoderm inducing factors and function during early development in vertebrate animals. The signaling cascade and the developmental program activated by activin or BMP has been extensively studied in Xenopus and chick early embryos. However, it remains unclear whether these signaling pathways are conserved in mammals. Genetic studies have demonstrated that nodal and BMP4, but not zygotic activins, are essential for early mouse development. To investigate the function of activin type I and type II receptors and a downstream signal mediator Smad2 in mammalian development, mutant mice have been generated by targeted disruption of the corresponding genes in mice. Genetic and embryological studies of the mutant mice have provided direct evidence that activin receptors and Smad2 are essential for mesoderm formation and gastrulation. To further analyze the function of activin receptor- and Smad2- mediated signaling pathways in the regulation of cell growth and differentiation, primitive streak formation and gastrulation, the following specific aims are proposed: (1) To understand how activin receptors and Smad2 regulate embryo growth, germ layer organization, and cell fate determination during egg cylinder formation and gastrulation. (2) To investigate how activin receptors and Smad2 regulate primitive streak formation and mesoderm patterning. 3) To investigate the function of nodal, BMP4, and chordin and their signaling pathways in primitive streak formation. Methods such as chimera (or mosaic) analysis through microinjection of blastocysts or aggregation of tetraploid embryos with ES cells, in situ hybridization, lineage analysis with lacZ-marked mutant ES cells, and ES cell-mediated gene delivery will be applied in this project. These aims, once accomplished, will lead to a better understanding of the signaling mechanism that controls early mammalian development. The knowledge acquired regarding early embryonic development will be useful for understanding later developmental processes such as organogenesis and will have important implications towards understanding the genetic basis of various forms of birth defects in human.