Determination of positional identity during limb development and regeneration

在肢体发育和再生过程中，信号分子在确定位置信息中的作用已被研究，结果总结如下。利用逆转录病毒载体，将发育早期在远肢芽中表达的Wnt-5a在鸡胚中过表达，导致胎足长度缩短。除了Wnt-3a外，Wnt家族还参与了顶端外胚层嵴的诱导，而Wnt-10a由于在小鼠和鸡胚胎中的表达模式而成为候选。然而，当Wnt-10a在假定的肢体外胚层中错误表达时，也无法诱导FGF-8的表达。在鸡胚胎中发现了一个新的卷曲家族成员Fz-10，它在肢体间质背后区表达。Xenopus动物帽实验表明，Fz-10受SHH和Wnt-7a的正调控，具有介导Wnt-7a信号通路的活性，而不介导Wnt-3a信号通路的活性。细胞分选实验表明，更多的gpi锚定细胞表面蛋白决定了肢体芽的近端-远端位置值。其中一种蛋白ephrin-A2在近端肢芽中表达，而ephrin-A的受体EphA4在远端肢芽中以互补模式表达。远端肢体芽中ephrin-A2的错误表达导致自足区肢体软骨畸形和部分融合。过表达ephrin-A2的肢体间充质细胞从非表达细胞中分离出来，提示ephrin-Eph相互作用参与了位置依赖性细胞粘附。SHH蛋白诱导的趾重复与HoxD12和HoxD13的持续表达相关，但与BMP-2的瞬时诱导无关。在腿芽中，FGF-4可以诱导前肢芽中的SHH表达，从而上调HoxD13的表达，而不依赖于细胞从身体侧翼区域的迁移。相反，FGF-4在羽芽中不能诱导SHH表达，提示前肢和后肢芽对FGF信号的反应存在差异。少

Roles of signaling molecules in determination of positional information have been studied in the developing and regenerating limb, and the results are summarized below.Wnt-5a expressed in the distal limb bud at early stage of development was over-expressed in the chick embryo using retrovirus vector, resulting in shortening of zeugopod length. Wnt family other than Wnt-3a is implicated in induction of the apical ectodermal ridge, and Wnt-10a is a candidate because of expression pattern both in the mouse and chick embryos. However, Wnt-10a was also unable to induce FGF-8 expression when misexpressed in the presumptive limb ectoderm.A novel frizzled family member, Fz-10, was identified in the chick embryo that expressed in the dorsal- posterior region of the limb mesenchyme. Fz-10 was positively regulated by both sonic hedgehog (SHH) and Wnt-7a, and shown to have an activity to mediate Wnt-7a signaling, but not Wnt-3a signaling, as demonstrated by Xenopus animal cap assay.Signaling throu … More gh GPI-anchored cell surface proteins was shown to determine proximal-distal positional value in the limb bud, as demonstrated by cell sorting assay. One of the proteins, ephrin-A2, is expressed in the proximal limb bud, whereas a receptor for ephrin-A, EphA4, is expressed in the distal limb bud with a complementary pattern. Misexpression of ephrin-A2 in the distal limb bud resulted in malformation and partial fusion of the limb cartilage within the autopod region. Limb mesenchymal cells over-expressing ephrin-A2 are segregated from non-expressing cells, suggesting involvement of ephrin-Eph interaction in position-dependent cell adhesion.SHH protein-induced digit duplication is correlated with persistent expression of HoxD12 and HoxD13, but not with transient induction of BMP-2. In the leg bud, FGF-4 could induce SHH expression in the anterior limb bud, thereby up-regulating HoxD13 expression, independent of cell migration from the body flank region. On the contrary, FGF-4 could not induce SHH expression in the wing bud, suggesting differential responsiveness to FGF signaling between the forelimb and hindlimb buds. Less