Muscle Of Xenopus Xtbx6 Gene Function And The Corresponding Upstream And Downstream Molecular Mechanism

T-box gene belongs to a transcriptional gene family, and plays veryimportant and diverse roles in different organgenesis process atdifferent stage of the developmental embryos. We cloned xTbx6 and examinedthe expression pattern of xTbx6. We find that xTbx6 is initiated from earlygastrula stage (St10+) at ventral and lateral mesoderm. Gain-of andloss-of function study demonstrated that xTbx6 can ectopically induce themuscle formation and is essential for the specification of dorsal mesoderm.Our data modify related xTbx6 data of Japanese group. From the analysisof the sequence, expression pattern, and function, we confirm that xTbx6is the homologe of mouse Tbx6. We further provide evidence that relativelow BMP activity is sufficient and necessary for the function of Tbx6.Thus xTbx6 could read out the BMP gradient and pattern the whole mesodermfrom dorsal to ventral.Keyword: Somitogenesis; Gastrulation; BMP; T-box; Tbx6Chapter II Xenopus Tbx6 Mediates Presomatic Mesoderm Cell Adhesion during SomitogenesisAs a member of T-box gene family,Tbx6 is involved in somite cell fatedecision and endows adhesion properties to mesoderm cells. The phenotypic 3similarity between Tbx6 hypomorphic mutant mouse and the clinical defectseen in the human disease Klippel-Feil Syndrome (KFS) and spondylocostaldysostosis reinforces the notion that the Tbx6 expression level iscritical for somitogenesis. In Xenopus, Tbx6 is expressed in thepresomatic mesoderm (PSM) and is down regulated when the presomaticmesoderm cells differentiate into muscle. Whether the down regulation ofTbx6 is necessary for muscle differentiation, and whether it plays rolesin regulating cell adhesion during somitogenesis, are questions addressedhere. We now show that increasing or reducing Tbx6 expression levelchanges adhesion and morphology of PSM cells, and paraxial protocadherin(PAPC) acts downstream of Tbx6 to mediate cell adhesion. However,prolonging Tbx6 expression in somitomere (anterior part of PSM) does notretard muscle differentiation. These data provide cues to elucidate themolecular mechanism of vertebrate somitogenesis.Key words: T-box gene, PAPC, presomatic mesoderm, Tbx6, adhesion Chapter III Multiple Signaling Pathways Control xTbx6 Expression during Xenopus MyogenesisTbx6 is critical for somite specification and initiates myogenesis. Ithas been shown that Activin/Nodal, VegT/ Nodal, FGF, and BMP signalingpathways are involved early in specifying mesoderm or later in patterningmesoderm, and Xnot plays roles in setting up the boundary betweennotochord and paraxial mesoderm. In this work, we introduced the dominantnegative forms of above genes into embryos to evaluate if they areresponsible for regulating Tbx6 expression. The results showed that, (1)Activin/Nodal and VegT/Nodal signals are necessary for both initiationand maintenance of Tbx6 expression, and Nodal is sufficient to induce 4ectopic Tbx6 expression; (2) while FGF signal is necessary for theinitiation and maintenance of Tbx6, it is not sufficient to induce Tbx6expression; (3) BMP is also necessary for the expression of Tbx6, and theinduction of Tbx6 expression by BMP is dose dependent; and (4) Xnot haveno effect on the expression of Tbx6. Our results suggest that severalsignaling pathways are involved in regulating Tbx6 expression, and pavethe route to reveal the molecular mechanism of how myogenesis isinitiated.