Determination of positional identity during limb development and regeneration

在发育和再生肢体中，信号分子在决定位置信息中的作用进行了研究，结果总结如下：利用逆转录病毒载体，在发育早期在远端肢芽中表达的WNT-5a在鸡胚胎中过表达，导致斑足类动物长度缩短。除了WNT-3a外，WNT-3a家族还参与了顶端外胚层脊的诱导，WNT-10a是候选家族，因为WNT-10a在小鼠和鸡胚胎中的表达模式。然而，WNT-10a在假定的肢体外胚层中错误表达时也不能诱导成纤维细胞生长因子-8的表达。在鸡胚中发现了一个新的卷曲家族成员FZ-10，该成员在肢体间充质的背部-后部区域表达。FZ-10受到Sonic Hedgehog(SHH)和WNT-7a的正向调节，并且具有介导Wnt-7a信号的活性，而不是Wnt-3a信号，如非洲爪哇动物帽子分析所证明的那样。Signating Thrus…细胞分选试验表明，更多的GPI锚定的细胞表面蛋白决定了肢芽中近端-远端的位置值。其中一种蛋白质，ePhin-A2，在近端的肢芽中表达，而ePhin-A的受体EphA4，在远端的肢芽中以互补的模式表达。在四肢远端芽中的错误表达导致肢体软骨的畸形和自体骨区的部分融合。肢体间充质细胞过度表达ePhin-A2的细胞与不表达的细胞分离，提示ePhin-Eph相互作用参与了位置依赖的细胞黏附。SHH蛋白诱导的数字复制与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，成纤维细胞生长因子-4可以诱导前肢芽中SHH的表达，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，成纤维细胞生长因子-4不能诱导翼芽中SHH的表达，提示前肢芽和后肢芽对成纤维细胞生长因子信号的反应不同。较少

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