Fang H, Elinson R P
Department of Zoology, University of Toronto, 25 Harbord Street, Toronto, Ontario, M5S 3G5, Canada.
Dev Biol. 1999 Jan 15;205(2):233-9. doi: 10.1006/dbio.1998.9078.
The homeobox gene otx2 is a key regulator for specifying the rostral part of the vertebrate head. In Xenopus, otx2 directly controls the differentiation of the cement gland, the anterior-most organ formed in the tadpole. Since embryos of a direct developing frog, Eleutherodactylus coqui, lack a cement gland, we are interested in whether altered expression of the otx2 gene is involved in this evolutionary change. We have cloned the E. coqui homologue of otx2, Ecotx2. The homeodomain of the Ecotx2 protein is identical to the mouse and zebrafish Otx2 proteins and differs by a single amino acid from the Xenopus Otx2 protein. Study of the spatiotemporal expression pattern shows that Ecotx2 RNA is progressively restricted to the anterior region of the embryo during gastrulation and becomes further restricted to the future forebrain and midbrain during neural development. In Xenopus, in addition to the conserved expression in the anterior neuroectoderm, the expression in ectoderm expands more anteriorly to the cement gland primordium. This anterior expansion of otx2 expression is not found in E. coqui, correlating with the loss of a cement gland. When misexpressed in Xenopus laevis ectoderm, Ecotx2 can activate expression of the cement-gland-specific genes XCG and XAG1, indicating that the function of activating the pathway of cement gland formation is retained by the Ecotx2 protein. Our results indicate that there are modifications in the pathway of cement gland formation, upstream of otx2 expression, in the development of E. coqui.
同源框基因otx2是确定脊椎动物头部前端部分的关键调节因子。在非洲爪蟾中，otx2直接控制着蝌蚪中形成的最前端器官——黏腺的分化。由于直接发育的青蛙——寇氏雨蛙的胚胎缺乏黏腺，我们感兴趣的是otx2基因表达的改变是否参与了这一进化变化。我们已经克隆了otx2的寇氏雨蛙同源物Ecotx2。Ecotx2蛋白的同源结构域与小鼠和斑马鱼的Otx2蛋白相同，与非洲爪蟾的Otx2蛋白仅相差一个氨基酸。对时空表达模式的研究表明，在原肠胚形成过程中，Ecotx2 RNA逐渐局限于胚胎的前部区域，在神经发育过程中进一步局限于未来的前脑和中脑。在非洲爪蟾中，除了在前神经外胚层中的保守表达外，在外胚层中的表达向前扩展到黏腺原基。在寇氏雨蛙中未发现otx2表达的这种向前扩展，这与黏腺的缺失相关。当在非洲爪蟾外胚层中错误表达时，Ecotx2可以激活黏腺特异性基因XCG和XAG1的表达，表明Ecotx2蛋白保留了激活黏腺形成途径的功能。我们的结果表明，在寇氏雨蛙的发育过程中，在otx2表达的上游，黏腺形成途径存在修饰。
