Role of BMP and FGF signaling during limb development

在之前的研究中，我们通过检测缺乏FGF配体和受体编码基因的小鼠，描述了成纤维细胞生长因子（FGF）信号在肢体发育过程中的重要作用（Lewandoski et al. 2000 Nature Genetics 28:7 7, Sun et al. 2000 Nature Genetics 25:6, Verheyden et al. 2005Development 132:4235）。另一个重要的肢体发育信号通路由骨形态发生蛋白（BMPs）控制，它可以控制肢体生长的所有方面：所有三个轴的早期模式，程序性细胞死亡和骨形成。因此，我们的任务是了解BMP和FGF信号通路在肢体发育过程中如何相互作用。由于我们对肢体发育的普遍兴趣，我们参与了一项合作，该合作证明了islet1介导的β -连环蛋白途径的激活对于后肢发育是必要的。在第二次合作中，我们确定了Dicer基因产物对于后肢芽沿身体前后轴的正确定位是必需的（Zhang， Z. et al . 2011 Dev Biol 351, 254）。最后，在一项合作研究中，我们确定骨盆带及其周围软骨膜的所有元素都是tbx4衍生的。此外，虽然Tbx4被认为主要与后肢相关，但我们在一项研究中表征了两个前肢表达域，在这项研究中，我们重新详细检查了Tbx4的表达，并追踪了具有先前生成但未完全表征的Tbx4等位基因敲入Cre等位基因的表达细胞的命运（Naiche a et al . 2001 Dev Dyn in press）。在目前的工作中，我们正在研究BMP信号作为发生在间充质指间细胞的正常程序性细胞死亡的效应器的作用，从而在出生时没有蹼肢的动物中去除它们并雕刻最终的指骨模式。在之前的研究中，研究人员提出了一种新的遗传模型，其中表面外胚层必须接收BMP信号，导致Fgfs的下调，进而诱导潜在间质细胞凋亡（Pajni-Underwood S. et al . 2007 Development 134: 2359）。因此，我们证明bmp通过调节FGF信号间接控制程序性细胞死亡。然而，需要强调的是，这一发现并不排除BMP信号在控制发育肢体细胞死亡中的直接作用。因此，我们通过在肢体发育的特定区域表达Cre的小鼠品系研究BMP和FGF信号在肢体发育的各个方面的作用来扩展这些研究。例如，检验bmp作为细胞死亡的直接效应器这一假设的唯一方法是仅在经历细胞死亡的谱系中灭活bmp受体，而不影响附近细胞中FGF的表达。我们使用新的Cre系实现了这一点，这些Cre系允许Cre介导的基因失活。这些细胞系都在问：bmp是正常程序性细胞死亡的直接效应器吗？如果不是,程序性细胞死亡是如何控制的?如果是这样，bmp是如何实现这一目标的？在另一项研究中，我们发现了FGFs和BMP之间一个重要的信号传导节点，这对肢体骨骼的正常发育至关重要。我们之前的研究表明，从肢体芽的远端结构分泌的特定FGFs调节肢体的正常生长和模式。在目前的工作中，我们正在产生遗传证据，表明骨形成蛋白的信号传递给骨骼元件的祖先群体，通过控制FGF拮抗剂的表达来调节FGF信号。这两种信号通路的联系不仅是对肢体如何形成模式的独特见解，而且可能为两种通路在其他发育环境或癌症期间如何相互作用提供模型。

In prior efforts, we contributed to a large body of work that described an essential role for Fibroblast Growth Factor (FGF) signaling during limb development by examining mice lacking genes that encode FGF ligands and receptors (Lewandoski et al. 2000 Nature Genetics 28:167, Sun et al 2000 Nature Genetics 25: 6, (Verheyden et al 2005Development 132:4235). Another important signaling pathway during limb development controlled by Bone Morphogenetic Proteins (BMPs), which arguably controls all aspects of limb outgrowth: early patterning in all three axes, programmed cell death and bone formation. Therefore we have set ourselves the task to understand how BMP and FGF signaling pathways interact during limb development. Because of our general interest in limb development we have participated in a collaboration that demonstrated that Islet1-mediated activation of the beta-catenin pathway is necessary for hindlimb initiation (in press). In second collaboration it was determined that the Dicer gne product is required for the proper positioning of the hindlimb bud along the anterior -posterior body axis (Zhang, Z. et al 2011 Dev Biol 351, 254). Finally, also in a collaborative study, we determined that all of the elements of the pelvic girdle and its surrounding perichondrium are Tbx4-derived. Also, although Tbx4 is thought to be associated primarily with the hindlimb, we characterized two forelimb expression domains in a study in which we reexamined the expression of Tbx4 in detail and also traced the fates of Tbx4-expressing cells with a previously generated, but incompletely characterized, allele of Tbx4 knocked in Cre allele (Naiche A et al 2001 Dev Dyn in press). In current work, we are studying the role of BMP signaling as effectors of normal programmed cell death that occurs in mesenchymal interdigit cells, thus removing them and sculpting the final digit pattern in animals that are born without webbed limbs. In previous work produced genetic evidence for a novel model in which the surface ectoderm must receive a BMP signal, resulting in down regulation of Fgfs which in turn induces apoptosis of the underlying mesenchyme (Pajni-Underwood S. et al 2007 Development 134: 2359). Thus we demonstrated that BMPs control programmed cell death indirectly, by regulating FGF signaling. However, it is important to emphasize that this insight does not exclude a direct role for BMP signaling in controlling cell death in the developing limb. Therefore we are extended these studies by studying the role of BMP and FGF signaling in various aspects of limb development using mouse lines that express Cre in specific region of the developing limb. For example the only way to test the hypothesis that BMPs act as direct effectors of cell death is to inactivate BMPs receptors only in the lineage that undergoes cells death, without affecting FGF expression in nearby cells. We have achieved this using new Cre lines that allow Cre-mediated gene inactivation in these lineages. With these lines are asking: are BMPs are direct effectors of normal programmed cell death? If not, how is programmed cell death controlled? If so, how do BMPs achieve this endpont? In another study, we have uncovered an important node of signaling between FGFs and BMP that is essential for normal development of the limb skeleton. Our previous work, cited above, demonstrates that specific FGFs, secreted from a distal structure in the limb bud, regulate the normal outgrowth and patterning of the limb. In current work, we are generating genetic evidence that BMP signaling to the progenitor population of the skeletal elements regulates this FGF signal by controlling the expression of an FGF antagonist. This linking of the two signaling pathways is not only a unique insight into how the limb is patterned but may provide a model for how the two pathways interact in other developmental contexts or during cancer.