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内容記述 |
In vertebrate embryogenesis, the most crucial events are the determination of three dimensional body axes and the pattern formation along an individual axis. Consequently, the gastrula signaling centre, organizer, is formed and establishes the body plan. Therefore, the elucidation of the molecular mechanisms of these events is important for understanding early developmental processes.<br /> In vertebrate, the pattern formation along dorsal/ventral (D/V) and anterior/posterior axis is well-known to be regulated by polypeptide growth factors belonging to the transforming growth factor-β (TGF-β) superfamily, bone morphogenetic protein (BMP) subfamily as ventralizing factors and nodal/activin subfamily as dorsalizing factors. BMPs are ubiquitously expressed in animal hemisphere and marginal zone of blastula embryo. Two classes of nodal genes begins to be activated in the dorsal endoderm region in the early blastula stage and the nodal activity activates the genes for other classes of nodal and BMP antagonists, chordin, noggin and follistatin, in the dorsal blastopore Nip region, called Spemann's organizer, from late blastula to early gastrula stage. It is believed that by these stages the gradient of BMP activity, high ventrally and low dorsally, is formed by the BMP antagonists and several agonists and establishes the pattern formation along the D/V axis. How and what factors act as downstream components of the growth factors in this events? He was interested in the downstream mechanisms of BMP signaling regulating pattern formation along D/V axis. Several homeobox genes are known to be induced in response to BMP in early Xenopus development. In particular, Xmsx-1, an amphibian homologue of vertebrate Msx-1 homeobox gene, is well characterized as a target gene of BMP. He focused on the role of Xmsx-1 to understand the downstream events of BMP signaling.<br /> In Part I, using a dominant-negative form of Xmsx-1 (VP-Xmsx-1), which is a fusion protein made with the virus-derived VP16 activation domain, he has examined whether Xmsx-1 activity is required in the endogenous ventralizing pathway. Overexpression of wild type Xmsx-1 in dorsal side resulted in the ventralizing embryo as BMP did. VP-Xmsx-1 induced a secondary body axis, complete with muscle and neural tissues, when overexpressed in ventral blastomeres, suggesting that Xmsx-1 activity is necessary for both mesoderm and ectoderm to be ventralized. Supporting this, VP-Xmsx-1 induced all dorsal markers tested, goosecoid, chordin and Xmyf-5, and repressed the ventrolateral markers, Xwnt-8, Xvent-1 and xGATA-2, in the ventral marginal zone. Conversely, wild type Xmsx-1 repressed the dorsal markers and induced the ventrolateral markers. He has also examined the epistatic relationship between Xmsx-1 and another ventralizing homeobox protein Xvent-1 and show that Xmsx-1 is likely to be acting upstream of Xvent-1 because Xmsx-1 induced Xvent-1 expression but Xvent-1 did not Xmsx-1. Moreover, VP-Xmsx-1-induced dorsalized phenotypes were effectively rescued by coexpression of Xvent-1, while the ventralized Xvent-1 phenotype was not rescued by VP-Xmsx-1. He proposes that Xmsx-1 is required in the BMP-stimulated ventralization pathway that involves the downstream activation of Xvent-1.<br /> In Part II, he used another inhibitory form of Xmsx-1 to examine its role in head formation. Overexpression of any inhibitors of BMP signaling except cerberus, which is a triple inhibitor of BMP, Wnt and Nodal, does not induce an ectoric head structure. Therefore, it is recent belief that inhibition of BMP signaling alone is insufficient and simultaneous inhibition of BMP and Wnt signaling in gastrula stage is required for head formation. Interestingly, ventral overexpression of a dominant Xmsx-1 inhibitor alone induced an ectopic head with eyes and a cement gland in the ventral side of the embryo, suggesting that Xmsx-1 is normally required to suppress head formation in the ventral side. Supporting this observation, he also found that wild-type Xmsx-1 suppresses head formation through the inhibition of nodal signaling, which is known to induce head organizer genes such as cerberus, Xhex, and Xdkk-1. Furthermore, extensive inhibition of BMP signaling by overexpression of multiple subtypes of dominant-negative BMP receptors or multiple extracellular BMP antagonists resulted in an ectopic head formation in the ventral side. He proposes that negative regulation of the BMP/Xmsx-1 signal is involved not only in neural induction and D/V patterning of mesoderm but also in head induction and formation. He further shows that Xmsx-1 physically interacts with Smad4 and the interaction can interrupt the Smad2/4/FAST-1 complex formation, which plays a key role in nodal signaling. He suggests that the inhibition of nodal signaling by Xmsx-1 may occur intracellularly, through the interaction with Smads, at the level of the transcriptional complex activating activin responsive element. |
要旨・審査要旨 / Abstract, Screening Result (311.4 kB)
