Wnt signaling regulates atrioventricular canal formation upstream of BMP and Tbx2

In the developing heart, the atrioventricular canal (AVC) is essential for separation and alignment of the cardiac chambers, for valve formation, and serves to delay the electrical impulse from the atria to the ventricles. Defects in various aspects of its formation are the most common form of congenital heart defects. Using mutant and transgenic approaches in zebrafish, this study demonstrates that Wnt/β-catenin signaling is both sufficient and required for the induction of BMP4 and Tbx2b expression in the AVC and consequently the proper patterning of the myocardium. Furthermore, genetic analysis shows that Wnt/β-catenin signaling is upstream and in a linear pathway with BMP and Tbx2 during AVC specification.     

Tbx1基因功能下调影响斑马鱼Tbx20和Tbx2表达

目的采用吗啡啉修饰反义寡核苷酸显微注射方法下调斑马鱼Tbx1基因表达,研究斑马鱼Tbx1基因功能下调对其他两个T盒基因Tbx20和Tbx2表达的影响。方法采用吗啡啉修饰的反义寡核苷酸显微注射方法抑制斑马鱼Tbx1基因表达,分别将2.5、5、8、10 ng吗啡啉反义寡核苷酸在斑马鱼0-4细胞期注入胚胎,并构建Tbx20,骨形成蛋白2b（Bmp2b）和Tbx2反义RNA探针,进行整体原位杂交,观察Tbx1基因下调对Tbx20、Bmp2b及Tbx2表达的影响。结果Tbx1吗啡啉寡核苷酸显微注射组胚胎表现出鳃弓、耳囊、心血管系统和胸腺的发育异常。Tbx1基因下调导致Tbx20的表达出现改变,Tbx20在心脏的表达与对照组相比明显下调,神经元的表达范围明显缩小;Tbx1基因功能下调会导致Bmp2b在心脏和咽囊的表达减低,Bmp2b在后部咽囊的表达较前部咽囊减低得更为明显;Tbx1基因功能下调胚胎,Tbx2在鳃弓的表达模式发生改变,48 hpf,Tbx2在鳃弓的表达出现从后向前逐渐减低,鳃弓的表达范围较对照组明显缩小。结论Tbx1在发育过程中,会对其他T盒基因,如Tbx20和Tbx2具有激活或抑制的调控作用。Tbx1对Tbx20的作用可能是通过影响Bmp2b的途径,继发地影响Tbx20的表达。Tbx1基因功能下调,会改变Tbx2在鳃弓的表达模式。     

Tbx5 and Tbx20 act synergistically to control vertebrate heart morphogenesis

Members of the T-box family of proteins play a fundamental role in patterning the developing vertebrate heart; however, the precise cellular requirements for any one family member and the mechanism by which individual T-box genes function remains largely unknown. In this study, we have investigated the cellular and molecular relationship between two T-box genes, Tbx5 and Tbx20. We demonstrate that blocking Tbx5 or Tbx20 produces phenotypes that display a high degree of similarity, as judged by overall gross morphology, molecular marker analysis and cardiac physiology, implying that the two genes are required for and have non-redundant functions in early heart development. In addition, we demonstrate that although co-expressed, Tbx5 and Tbx20 are not dependent on the expression of one another, but rather have a synergistic role during early heart development. Consistent with this proposal, we show that TBX5 and TBX20 can physically interact and map the interaction domains, and we show a cellular interaction for the two proteins in cardiac development, thus providing the first evidence for direct interaction between members of the T-box gene family.     

Tbx20 regulation of cardiac cell proliferation and lineage specialization during embryonic and fetal development in vivo

TBX20 gain-of-function mutations in humans are associated with congenital heart malformations and myocardial defects. However the effects of increased Tbx20 function during cardiac chamber development and maturation have not been reported previously. CAG-CAT-Tbx20 transgenic mice were generated for Cre-dependent induction of Tbx20 in myocardial lineages in the developing heart. βMHCCre-mediated overexpression of Tbx20 in fetal ventricular cardiomyocytes results in increased thickness of compact myocardium, induction of cardiomyocyte proliferation, and increased expression of Bmp10 and pSmad1/5/8 at embryonic day (E) 14.5. βMHCCre-mediated Tbx20 overexpression also leads to increased expression of cardiac conduction system (CCS) genes Tbx5, Cx40, and Cx43 throughout the ventricular myocardium. In contrast, Nkx2.5Cre mediated overexpression of Tbx20 in the embryonic heart results in reduced cardiomyocyte proliferation, increased expression of a cell cycle inhibitor, p21(CIP1), and decreased expression of Tbx2, Tbx5, and N-myc1 at E9.5, concomitant with decreased phospho-ERK1/2 expression. Together, these analyses demonstrate that Tbx20 differentially regulates cell proliferation and cardiac lineage specification in embryonic versus fetal cardiomyocytes. Induction of pSmad1/5/8 at E14.5 and inhibition of dpERK expression at E9.5 are consistent with selective Tbx20 regulation of these pathways in association with stage-specific effects on cardiomyocyte proliferation. Together, these in vivo data support distinct functions for Tbx20 in regulation of cardiomyocyte lineage maturation and cell proliferation at embryonic and fetal stages of heart development.     

T-box genes coordinate regional rates of proliferation and regional specification during cardiogenesis

Mutations in T-box genes are the cause of several congenital diseases and are implicated in cancer. Tbx20-null mice exhibit severely hypoplastic hearts and express Tbx2, which is normally restricted to outflow tract and atrioventricular canal, throughout the heart. Tbx20 mutant hearts closely resemble those seen in mice overexpressing Tbx2 in myocardium, suggesting that upregulation of Tbx2 can largely account for the cardiac phenotype in Tbx20-null mice. We provide evidence that Tbx2 is a direct target for repression by Tbx20 in developing heart. We have also found that Tbx2 directly binds to the Nmyc1 promoter in developing heart, and can repress expression of the Nmyc1 promoter in transient transfection studies. Repression of Nmyc1 (N-myc) by aberrantly regulated Tbx2 can account in part for the observed cardiac hypoplasia in Tbx20 mutants. Nmyc1 is required for growth and development of multiple organs, including the heart, and overexpression of Nmyc1 is associated with childhood tumors. Despite its clinical relevance, the factors that regulate Nmyc1 expression during development are unknown. Our data present a paradigm by which T-box proteins regulate regional differences in Nmyc1 expression and proliferation to effect organ morphogenesis. We present a model whereby Tbx2 directly represses Nmyc1 in outflow tract and atrioventricular canal of the developing heart, resulting in relatively low proliferation. In chamber myocardium, Tbx20 represses Tbx2, preventing repression of Nmyc1 and resulting in relatively high proliferation. In addition to its role in regulating regional proliferation, we have found that Tbx20 regulates expression of a number of genes that specify regional identity within the heart, thereby coordinating these two important aspects of organ development.