@misc{oai:ir.soken.ac.jp:00001370,
 author = {倉田, 智子 and クラタ, トモコ and KURATA, Tomoko},
 month = {2016-02-17, 2016-02-17},
 note = {During the process of neurulation in vertebrates, the ectoderm is differentiated into several distinct tissue types including neural plate, epidermis, and neural crest.  Recent works, mainly in Xenopzts, have focused upon molecular mechanisms that can direct ectoderm to neural fates.  In the models, neural induction is initially caused by inhibition of BMP activities in ectoderm by secreted BMP antagonists Noggin, Chordin and Follisitalin that are induced in Spemann's organizer.  Dorsal ectoderm that has low BMP signaling differentiated into neural plate and ventral ectoderm that has high BMP signaling differentiated into epidermis.  However, neural crest induction is more complicated.  Neural crest is induced at the border between prospective neural plate and prospective epidermis.  One model suggests that an intermediate level of BMP signaling that is generated by the balance between the BMPs and BMP antagonists plays a role in establishing the neural crest fate in Xenopus.  It was also reported that canonical Wnt signaling and fibroblast growth factor (FGF) signaling enhances neural crest induction, cooperating with BMP antagonists.  Several transcriptional factors that able to induced neural crest makers such as Zic-related genes were reported.  But many of them can also induce neural plate makers and they are expressed in not only neural crest region but also neural plate, so it is complicated to understand how ectoderm diverge to neural plate and neural crest fates.  In addition, positioning mechanism of neural-epidermal border and mechanism of neural crest induction are poorly understood.<br />  In order to understand of molecular mechanisms of ectodermal patterning, she first focused on the in vivo BMP activity that is a basis of the neural and epidermal induction.  She performed visualization of endogenous BMP signaling using an antibody that preferentially recognizes BMP-stimulated form of Smads.  BMP signaling system composed of several BMP ligands and receptors, and there are several negative regulators.  Regulation of downstream target genes and their roles are complex.  Therefore it is quite difficult to evaluate BMP signals by analyzing BMP target genes. A preferred method to evaluate BMP signaling in situ is to detect activated forms of intracellular signaling molecules specific for BMP.  Smads 1, 5, and 8 are best characterized signaling components of BMP signals and believed to be mediating a major part of BMP activity.<br />  BMP signaling was observed uniformly in early blastula, but was restricted to the ventral side of the embryo from the late blastula stage.  At gastrula in ventral ectoderm (prospective epidermis) and ventral mesoderm were stained intensively, and dorsal ectoderm (prospective neural plate) and dorsal mesoderm were less stained.  These results support the proposed roles of BMPs as ventralizing factor and anti-neurulizing factors in Xenopus embryos.  From late gastrula, a gradient of staining becomes evident in the dorsal ectoderm, along the anterior-posterior axis.  In early neurula staining was gradually reduced along the DV axis in all three germ layers again indicating the presence of a BMP signaling gradient.  During the neural tube forming stages, staining was observed at the dorsal part of neural tube.  The location of staining in dorsal neural tube is also consistent with previous findings showing that BMP family members act as dorsalizing factors of the neural tube.  The distinct gradient of staining was not observed in neural tube.  So it seems that BMP activity is a short-range signal at least in the dorsal neural tube.<br />  Next she performed the functional analysis of a newly isolated homeobox gene that expressed neural-epidermal border.  She identified a novel NK-1 class homeobox gene named Nbx.  Expression of Nbx was detected at neural-epidermal border at neural crest forming stages and partially overlapped with neural crest makers.  Nbx has an Eh1 repressor motif and act as a transcriptional suppressor.  The gain-of-function analysis showed that Nbx suppressed neural plate makers.  The inhibition of neural induction by Nbx overexpression caused expansion of epidermal maker into the neural plate, and suppressed neural crest induction at early neurula.  In later stages, however enhanced expression of neural crest maker was observed at the injected region.  Nbx is not likely to be a direct neural crest inducer because Nbx could not induce neural crest maker alone in the animal caps.  Interestingly, co-injection of dominant negative form of BMP receptor and Nbx caused melanophore induction efficiently in animal caps.   Overexpression of a dominant-negative form of Nbx (VP-Nbx-GR) expanded the neural plate markers such as Sox2 and Otx2, and suppressed neural crest marker Slug.  Therefore, she speculated that Nbx may be an essential transcription factor to regulate neural-epidermal border by inhibiting the neural plate fate and direct to neural crest induction.<br />  The pattern of BMP signaling visualized in this work supports the model of neural and epidermal induction by BMP activity.  Furthermore she had demonstrated that a novel homeobox gene Nbx may be essential for rigorous regional specification on neural-epidermal border and neural crest induction in the downstream process of pattern formation by BMP activity., application/pdf, 総研大甲第688号},
 title = {Molecular mechanism of ectodermal patterning in Xenopus laveis},
 year = {}
}