Analysis of the function of the nodal gene during embryo

Nodal是一种转化生长因子（TGF - β）样蛋白，我们是在一种逆转录病毒插入突变的基础上发现的，它对早期发育至关重要，在原肠胚形成过程中起着重要作用。我们最近对小鼠胚胎发育中的节点信号的研究主要集中在分析一个功能降低的靶向突变等位基因（一种低形态）。由该次形等位基因和完全无等位基因组成的复合杂合子胚胎能够发育过无等位基因纯合子的原肠胚发育阻滞期，但随后表现出各种发育异常。这使我们能够确定在原肠胚形成开始后发生的发育过程中淋巴结的关键作用，包括前/后和左/右体轴的模式，以及中线中胚层和肠胚层的发育。然而，由于在许多节点表达域的活动普遍减少，我们的分析并没有深入了解这些发育事件需要哪些节点表达。为了进一步剖析这些不同的节点功能的时间和空间域，我们产生了一个条件突变（floxed）等位基因。携带该等位基因的小鼠正在与表达Cre重组酶的转基因菌株结合使用，这些菌株是我们开发或从其他研究者那里获得的，用于发育中的胚胎的各个区域。对在前胚期胚胎外胚层（即产生所有胚层的细胞）中表达Cre的菌株所获得的胚胎进行的初步分析，揭示了先前观察到的次胚等位基因的所有相同表型。在胚胎外谱系中表达Cre可能不需要早期淋巴结功能的可能性，现在正在通过在胚胎外内脏内胚层中特异性表达Cre的菌株来解决。目前正在对具有其他组织特异性的Cre菌株进行进一步的研究。

Nodal is a transforming growth factor (TGF)-beta like protein that we discovered on the basis of a retroviral insertional mutation and showed to be essential for early development, playing a major role in gastrulation. Our recent work on nodal signaling in mouse embryonic development has focused on the analysis of a targeted mutant allele with reduced function (a hypomorph). Embryos that are compound heterozygotes for this hypomorphic allele and a completely null allele are capable of developing past the block at gastrulation seen in homozygotes for the null allele, but then show a variety of developmental abnormalities. This has allowed us to establish a critical role for nodal in developmental processes occurring after gastrulation begins, including patterning of the anterior/posterior and left/right body axes, and the development of the midline mesendoderm and gut endoderm. However, because there is a generalized reduction of activity in each of the many domains of nodal expression our analysis did not provide insight into where nodal expression is required for these developmental events. To dissect these various temporal and spatial domains of nodal function further, we generated a conditional mutant (floxed) allele. Mice carrying this allele are being used in conjunction with transgenic strains expressing Cre recombinase in various regions of the developing embryo, which we have developed or obtained from other investigators. Preliminary analysis of embryos obtained from breeding floxed mice with a strain expressing Cre in the pregastrulation embryonic epiblast, the cells that will give rise to all the germ layers, has revealed all of the same phenotypes observed earlier for the hypomorphic allele. The possibility that expression in extraembryonic lineages may not be required for early nodal function is now being addressed by the use of a strain expressing Cre specifically in the extraembryonic visceral endoderm. Additional ongoing work is being performed with Cre strains with other tissue specificities.