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Myocardial Bmp2 overexpression leads to ectopic cardiac EMT and promotes chamber cardiomyocyte proliferation and immaturity

ABSTRACT: Defective valve and septa formation constitute a substantial part of congenital heart defects affecting the neonate or the adult. During cardiac development, restricted myocardial Bmp2 expression is a key signal for the specification and patterning of the valve-forming field in the atrio-ventricular canal (AVC) region and the initiation of the epithelial-mesenchyme transition (EMT) that gives rise to the valve primordia. We have generated a mouse transgenic line conditionally expressing Bmp2. Nkx2.5Cre-driven Bmp2 overexpression in the myocardium leads to foetal lethality, due to valve and chamber dysmorphogenesis, and rescue of the AVC specification defect of Bmp2-null embryos. Nkx2.5Cre/+;Bmp2tg/+ transgenic embryos show ventricular septal defect, thickened valves, enlarged trabeculae and dilated ventricles, whose endocardium is able to undergo EMT when explanted onto collagen gels. Gene profile and marker analysis indicates that cellular proliferation is increased and chamber differentiation and patterning is impaired in Nkx2.5Cre/+;Bmp2tg/+ embryos, but the ventricular-specific gene expression program is not abolished. We obtained similar results using a second myocardial driver (cTnTCre) to activate Bmp2 expression, but not with an endothelial-specific one (Tie2Cre), as Tie2Cre/+;Bmp2tg/+ mice are normal and survive to adulthood, indicating that Bmp2 must emanate from the myocardium to exert its EMT-driving and cardiomyocyte maturation blocking effect. Forced Bmp2 expression in vitro stimulates EBs proliferation and blocks their progression into cardiomyogenesis, an effect partially rescued by Noggin. These data show that widespread myocardial Bmp2 expression directs ectopic valve primordium formation and maintains chamber myocardium and early cardiac progenitors in a primitive, proliferative state, identifying Bmp2 as a potential factor for the expansion of immature cardiomyocytes.

ORGANISM(S): Mus

PROVIDER: GSE100810 | GEO | 2018/04/11

REPOSITORIES: GEO

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Project description:Rationale: A critical step in heart development is the coordinated formation of nodal myocardium and cushion tissue from the atrioventricular canal (AVC). After its specification, the myocardium of the AVC aligns the chambers, forms the AV node, and induces the formation of the mesenchymal AV cushions, primordia of valves and septa, while it resists working myocardial differentiation. Objective: To assess what roles Tbx2 and Tbx3, two closely related T-box transcription factors expressed in the AVC, play in these processes. Methods and Results: We analyzed mice ectopically expressing Tbx3 in the atrial myocardium by genome-wide microarray and expression analysis. We found a prominent role for Tbx3 in defining the nodal phenotype by repressing working myocardial genes (sarcomeric, mitochondrial, fast conduction) and cell proliferation regulators, and in inducing node-associated genes. Moreover, there was a striking induction of genes associated with endocardial cushions and mesenchyme. Using gain-of-function models, we found that in the developing heart both Tbx2 and Tbx3 induce ectopic Bmp2 and Tgfb2 expression and endocardial cushion formation. Analysis of compound Tbx2/Tbx3 mutant embryos revealed that upon loss of more than two functional alleles, expansion of the AV myocardium does not occur and AV cushions fail to form. Conclusions: Tbx2 and Tbx3 locally stimulate development of the AVC myocardium, induce the AV nodal phenotype therein and trigger AV cushion formation from the overlying AV endocardium, providing a mechanism for the colocalization and coordination of these two important processes in heart development. Nppa-Cre4 (Cre4) mice were crossed with CT mice to obtain efficient activation of Tbx3 in atria of double transgenic Cre4-CT mice, as previously described (Hoogaars et al., 2007). To investigate the gene expression profile of atria of Cre4-CT mice, we performed whole genome microarray analysis using Sentrix Mouse-6 oligonucleotide beadchips. We compared the atrial gene expression profiles of six male double transgenic Cre4-CT mice and six male Cre4 control mice.

Project description:Malformations of the cardiovascular system are the most common type of birth defect in humans, affecting predominantly the formation of valves and septa. During heart valve and septa formation, cells from the atrio-ventricular canal (AVC) and outflow tract (OFT) regions of the heart undergo an epithelial-to-mesenchymal transformation (EMT) and invade the underlying extracellular matrix to give rise to endocardial cushions. Subsequent maturation of newly formed mesenchyme cells leads to thin stress-resistant leaflets. TWIST1 is a basic helix-loop-helix transcription factor expressed in newly formed mesenchyme cells of the AVC and OFT that has been shown to play roles in cell survival, cell proliferation and differentiation. However, the role and downstream targets of TWIST1 during heart valve formation remain unclear. To identify genes important for heart valve development downstream of Twist1 we performed global gene expression profiling of AVC, OFT, atria and ventricles of the embryonic day 10.5 mouse heart by tag-sequencing (Tag-seq). Using this resource we identified a novel set of 1246 genes, including 201 regulators of transcription, enriched in the valve forming regions of the heart. We compared these genes to a Tag-seq library from the Twist1 null developing valves revealing significant gene expression changes. These changes were consistent with a role of TWIST1 in controlling differentiation of mesenchymal cells following their transformation from endothelium in the mouse. To study the role of TWIST1 at the DNA level we performed chromatin immunoprecipitation and identified novel direct targets of TWIST1 in the developing heart valves. Our findings are consistent with a role for TWIST1 in the differentiation of AVC mesenchyme post-EMT in the mouse, and suggest that TWIST1 exerts its function by direct DNA binding to activate valve specific gene expression. Profiled the AVC, OFT, atria and ventricles of the embryonic day 10.5 mouse heart by tag-sequencing (Tag-seq) (no replicates). We also produced a Tag-seq library from Twist1 null developing valves to reveal the gene expression changes associated with loss of this gene.

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Myocardial Bmp2 overexpression leads to ectopic cardiac EMT and promotes chamber cardiomyocyte proliferation and immaturity

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