Murine T-box transcription factor Tbx20 acts as a repressor during heart development, and is essential for adult heart integrity, function and adaptation

Abstract

The genetic hierarchies guiding lineage specification and morphogenesis of the mammalian embryonic heart are poorly understood. We now show by gene targeting that murine T-box transcription factor Tbx20 plays a central role in these pathways, and has important activities in both cardiac development and adult function. Loss of Tbx20 results in death of embryos at mid-gestation with grossly abnormal heart morphogenesis. Underlying these disturbances was a severely compromised cardiac transcriptional program,defects in the molecular pre-pattern, reduced expansion of cardiac progenitors and a block to chamber differentiation. Notably, Tbx20-null embryos showed ectopic activation of Tbx2 across the whole heart myogenic field. Tbx2 encodes a transcriptional repressor normally expressed in non-chamber myocardium, and in the atrioventricular canal it has been proposed to inhibit chamber-specific gene expression through competition with positive factor Tbx5. Our data demonstrate a repressive activity for Tbx20 and place it upstream of Tbx2 in the cardiac genetic program. Thus, hierarchical,repressive interactions between Tbx20 and other T-box genes and factors underlie the primary lineage split into chamber and non-chamber myocardium in the forming heart, an early event upon which all subsequent morphogenesis depends. Additional roles for Tbx20 in adult heart integrity and contractile function were revealed by in-vivo cardiac functional analysis of Tbx20 heterozygous mutant mice. These data suggest that mutations in human cardiac transcription factor genes, possibly including TBX20,underlie both congenital heart disease and adult cardiomyopathies.

Genetic Networks Governing Heart Development

“…Together, however, these factors provide the combinatorial coding for cardiogenesis (discussed further below). It is noteworthy that although knockout of kernel factors generally leads to arrested heart development owing to defective tissue growth and patterning, phenotypes can show distinct features (Watt et al 2004;Stennard et al 2005;Prall et al 2007), indicating overlapping and unique roles within the kernel and in the flow of information from upstream regulators to downstream functions. Limited combinations of cardiac TFs centered on GATA4, TBX5, and MEF2C, with and without the inclusion of specific microRNAs (miRNAs) or small molecules modulating signaling and epigenetic states, can reprogram noncardiac embryonic or adult cells to a cardiomyocyte fate (Ieda et al 2010;Chen et al 2014).…”

BMP/SMAD1 signaling sets a threshold for the left/right pathway in lateral plate mesoderm and limits availability of SMAD4

“…Whole-mount in situ hybridization was performed as described (Stennard et al 2005) with minor modifications. Nodal, Lefty1/ 2, Dante, and Plunc antisense riboprobes were cloned by RT-PCR into pGEM-T Easy vector (Promega) from 8.5 dpc mouse embryo cDNAs.…”

Novel tool to suppress cell proliferation in vivo demonstrates that myocardial and coronary vascular growth represent distinct developmental programs

“…For example, deletion of Tbx20 leads to severe defects in cardiac chamber morphogenesis. Failure to form a multichambered heart was reported to be due to defective linear heart tube elongation, inappropriate gene expression, and reduced cell proliferation (Singh et al, 2005;Stennard et al, 2005). Whether the defects in heart tube elongation and/or gene expression are due to reduced cell proliferation (or vice versa) is unclear from the reported analyses.…”