Characterization of new genes for BMP signaling pathway.

TAK1是MAP激酶激酶激酶家族的成员，其激活因子TAB1参与爪蟾早期胚胎中胚层诱导和模式形成的骨形态发生蛋白（BMP）信号通路。然而，从受体激活到TAK1激活的事件仍有待确定。使用酵母相互作用筛选来寻找在连接受体和TAB1-TAK1通路中起作用的蛋白质。人类x染色体连接的凋亡抑制蛋白（XIAP）是一个tab1结合蛋白。在哺乳动物细胞中，XIAP不仅与TAB1相关，还与BMP受体相关。向背侧卵裂球注射XIAP mRNA，以tab1 - tak1依赖的方式增强爪蟾胚胎的腹化。此外，缺少tab1结合结构域的截断形式的XIAP部分阻断了由组成活性BMP I型受体诱导的腹侧中表皮标记基因的表达。综上所述，我们发现XIAP作为连接BMP受体和TAB1-TAK1的正调节因子参与BMP信号通路。最近，我们在成骨细胞MC3T3-E1中分离出一种含有小鼠同源盒的基因，远端无5 (mDlx5)，作为bmp诱导基因。过表达mDlx5 mRNA的稳定转染MC3T3-E1显示出各种成骨标志物的增加；碱性磷酸酶活性增加4倍，骨钙素产生增加6倍，细胞外基质矿化。此外，在BMP-4处理的小鼠胚胎和成年小鼠骨折骨中原位诱导mDlx5。与这些观察结果一致，我们还发现在背侧卵裂球中注射mDlx5 mRNA增强了爪蟾胚胎的腹化。我们的研究结果表明，mDlx5是BMP信号通路的靶基因，在成骨和胚胎轴的背-腹侧模式中起着重要的调节作用。

TAK1, a member of the MAP kinase kinase kinase family, and its activator, TAB1, participate in the bone morphogenetic protein (BMP) signaling pathway involved in mesoderm induction and patterning in early Xenopus embryos.However, the events leading from receptor activation to TAK1 activation remain to be identified.A yeast interaction screen was used to search for proteins that function in the pathway linking the receptors and TAB1-TAK1.The human X-chromosome-linked inhibitor of apoptosis protein (XIAP) was isolated as a TAB1-binding protein.XIAP associated not only with TAB1 but also with the BMP receptors in mammalian cells. Injection of XIAP mRNA into dorsal blastomeres enhanced the ventralization of Xenopus embryos in a TAB1-TAK1-dependent manner.Furthermore, a truncated form of XIAP lacking the TAB1-binding domain partially blocked the expression of ventral mesodermal marker genes induced by a constitutively-active BMP type I receptor.Taken together, we identified that XIAP participates in the BMP signaling pathway as a positive regulator linking the BMP receptors and TAB1-TAK1.Recently, we isolated a murine homeobox-containing gene, distal-less 5 (mDlx5), as a BMP-inducible gene in osteoblastic MC3T3-E1 cells.Stable transfectants of MC3T3-E1 that overexpress mDlx5 mRNA showed increase in various osteogenic markers ; a 4-fold increase in alkaline phosphatase activity, a 6-fold increase in osteocalcin production and appearance in mineralization of extracellular matrix.Furthermore, mDlx5 was induced orthotopically in mouse embryos treated with BMP-4 and in fractured bone of adult mice. Consistent with these observations, we also found that injection of mDlx5 mRNA in dorsal blastomeres enhanced the ventralization of Xenopus embryos.Our results indicated that mDlx5 is a target gene of the BMP signaling pathway and acts as an important regulator of both osteogenesis and dorso-ventral patterning of embryonic axis.