Flamme I, Breier G, Risau W
Max-Planck-Institut für physiologische und klinische Forschung, W. G. Kerckhoff Institut, Abteilung molekulare Zellbiologie, Bad Nauheim, Federal Republic of Germany.
Dev Biol. 1995 Jun;169(2):699-712. doi: 10.1006/dbio.1995.1180.
Vasculogenesis, the de novo formation of embryonic blood vessels from their angioblastic precursors in situ, is supposed to be under the control of polypeptide growth factors and their receptors. The receptor tyrosine kinase flk-1 and its high-affinity ligand vascular endothelial growth factor (VEGF) represent an endothelial specific signal transduction system expressed during embryonic vascular growth in the mouse. We have cloned the quail homologs of VEGF and flk-1 using PCR and have investigated their expression pattern in vivo. As shown by Northern analysis and reverse transcription PCR, VEGF and flk-1 mRNA (3.9 and 5.8 kb, respectively) were already present in the unincubated blastodisc at low levels and were largely upregulated during gastrulation at Embryonic Day 1. As detected by in situ hybridization, flk-1 mRNA was initially present in the entire mesoderm of Day 1 embryos but from Day 2 on was restricted to endothelial cells. At Day 2 VEGF was ubiquitously expressed in the embryo proper and was mainly restricted to the vascularized part (area vasculosa) in the yolk sac. Later on VEGF expression was detected in all organs. In the kidney VEGF mRNA was mainly localized to the glomeruli. This pattern of expression is consistent with the pattern found during mouse embryogenesis. We have recently established an in vitro model of vasculogenesis in which hemangioblastic precursors are induced in cell cultures from the unincubated quail blastodisc by basic fibroblast growth factor (bFGF) and give rise to blood vessels in vitro. Taking advantage of this in vitro model we examined whether FGF and VEGF act in concert during vasculogenesis. We found that the flk-1 receptor mRNA is dramatically upregulated within 24 hr upon the addition of FGF to quail blastodisc cell cultures. This inducibility in response to FGF is confined to the first 24 hr of culture. The early expression of the flk-1 mRNA may characterize the differentiation of hemangioblastic precursors from pluripotent epiblast cells which in vivo is initiated during gastrulation. Thus, the time course and the pattern of expression during embryogenesis in different species suggest a major role for the VEGF/flk-1 signal transduction system in vasculogenesis and angiogenesis.
血管生成是指胚胎血管从其原位的成血管细胞前体重新形成，据推测受多肽生长因子及其受体的控制。受体酪氨酸激酶flk-1及其高亲和力配体血管内皮生长因子（VEGF）代表了在小鼠胚胎血管生长过程中表达的一种内皮特异性信号转导系统。我们利用聚合酶链反应（PCR）克隆了鹌鹑的VEGF和flk-1同源物，并研究了它们在体内的表达模式。如Northern印迹分析和逆转录PCR所示，VEGF和flk-1 mRNA（分别为3.9 kb和5.8 kb）在未孵化的胚盘期就已低水平存在，并在胚胎第1天的原肠胚形成期大量上调。原位杂交检测显示，flk-1 mRNA最初存在于第1天胚胎的整个中胚层，但从第2天开始局限于内皮细胞。在第2天，VEGF在胚胎本体中广泛表达，主要局限于卵黄囊的血管化部分（血管区）。后来在所有器官中都检测到了VEGF的表达。在肾脏中，VEGF mRNA主要定位于肾小球。这种表达模式与在小鼠胚胎发育过程中发现的模式一致。我们最近建立了一个血管生成的体外模型，其中通过碱性成纤维细胞生长因子（bFGF）在未孵化的鹌鹑胚盘细胞培养物中诱导成血管细胞前体，并在体外形成血管。利用这个体外模型，我们研究了FGF和VEGF在血管生成过程中是否协同作用。我们发现，在鹌鹑胚盘细胞培养物中加入FGF后24小时内，flk-1受体mRNA显著上调。这种对FGF的诱导性仅限于培养的前24小时。flk-1 mRNA的早期表达可能表征了多能外胚层细胞中成血管细胞前体的分化，在体内这种分化在原肠胚形成期开始。因此，不同物种胚胎发育过程中的时间进程和表达模式表明VEGF/flk-1信号转导系统在血管生成和血管形成中起主要作用。
