Seleiro E A, Connolly D J, Cooke J
National Institute for Medical Research, Mill Hill, London, UK.
Curr Biol. 1996 Nov 1;6(11):1476-86. doi: 10.1016/s0960-9822(96)00752-x.
Genes of the transforming growth factor beta (TGF beta) superfamily have been implicated in the earliest steps of developmental patterning in vertebrates. In Xenopus, the Vg1 gene is a candidate for the initiator of axis formation: its RNA and protein are broadly but appropriately localized at the start of development, and processed Vg1 protein is a powerful inducer of organized axial tissue in blastular animal caps in vitro and when locally produced in vivo after injection of Vg1 mRNA into blastomeres. Site-specific proteolytic processing occurs ubiquitously for most TGF beta members, producing the active peptide ligand, but is tightly restricted, by unknown mechanisms, for endogenous Vg protein in Xenopus and zebrafish embryos. This restriction may be involved in the spatial localization of activity required for an organizing role.
We have characterized an amniote (chick) orthologue of Vg1, cVg1, and examined its developmental expression. The early expression of cVg1 includes a phase broadly related to the known time and site of axis (primitive streak) initiation; the initial transcription of cVg1 is centred in the posterior marginal zone (PMZ), a region of the blastoderm known to contain the axial organizing activity at this stage. We also observed later neural and paraxial mesodermal expression of cVg1, which has not been described previously for Vg homologues in other vertebrates. We have grafted transfected COS cells, producing processed cVg1 protein, to peripheral positions around the chick early blastoderm. Such grafts initiate formation of morphologically complete primitive streaks, simulating the properties of grafts from the PMZ.
In vertebrate development, Vg genes may be required for an evolutionarily conserved early step in positioning or induction of the axis.
转化生长因子β（TGFβ）超家族的基因与脊椎动物发育模式形成的最早步骤有关。在非洲爪蟾中，Vg1基因是轴形成起始因子的候选基因：其RNA和蛋白质在发育开始时广泛且适当地定位，加工后的Vg1蛋白在体外囊胚动物帽中以及将Vg1 mRNA注射到卵裂球后在体内局部产生时，是有组织的轴向组织的强大诱导剂。大多数TGFβ成员普遍发生位点特异性蛋白水解加工，产生活性肽配体，但对于非洲爪蟾和斑马鱼胚胎中的内源性Vg蛋白，其加工受到未知机制的严格限制。这种限制可能与发挥组织作用所需活性的空间定位有关。
我们鉴定了Vg1的羊膜动物（鸡）直系同源物cVg1，并研究了其发育表达情况。cVg1的早期表达包括一个与轴（原条）起始的已知时间和位点广泛相关的阶段；cVg1的初始转录集中在后缘区（PMZ），即胚盘的一个区域，已知在此阶段该区域含有轴向组织活性。我们还观察到cVg1在后期的神经和近轴中胚层表达，这在其他脊椎动物的Vg同源物中尚未有过描述。我们将产生加工后cVg1蛋白的转染COS细胞移植到鸡早期胚盘周围的外周位置。这种移植引发了形态完整的原条的形成，模拟了来自PMZ的移植的特性。
在脊椎动物发育中，Vg基因可能是轴定位或诱导过程中进化上保守的早期步骤所必需的。
