T-box Gene Tbx5 Research Articles

Investigation of T-box gene function in patterning second heart field progenitor cells

The heart is derived from two sources of progenitor cells: the first heart field giving rise to the left ventricle and atria, and the Second Heart Field (SHF) that progressively contributes to the right ventricle, outflow tract and parts of the atria. Perturbation of SHF cell addition leads to a spectrum of congenital heart defects, exemplified by conotruncal and atrioventricular septal defects observed in mouse embryos lacking the T-box transcription factor Tbx1, encoded by the major 22q11.2 deletion (DiGeorge) syndrome candidate gene. Tbx1 is expressed in the SHF where it regulates proliferation, differentiation and epithelial properties. Tbx1 is also required for the segregation of distinct anterior and posterior subpopulations of the SHF giving rise to arterial and venous pole myocardium, respectively. Here we show using genetic lineage tracing and immunofluorescence labeling that a second T-box gene Tbx5 , implicated in Holt-Oram syndrome, is activated in Tbx1-positive pharyngeal mesoderm in response to retinoic acid signaling at the onset of progenitor cell segregation. Loss of retinoic acid signaling results in failure to establish the posterior SHF domain and atrioventricular septal defects. SHF cells coexpressing Tbx1 and Tbx5 subsequently downregulate Tbx1 expression, in a Tbx1-dependent manner, leading to the emergence of a transcriptional boundary between anterior and posterior SHF domains. Analysis of Tbx5 lineage and conditional loss of function implicate de novo activation of Tbx5 in boundary formation. Our results provide new insights into progenitor cell patterning during SHF deployment and the origins of common forms of congenital heart defects.

T-box gene tbx5 is essential for formation of the pectoral limb bud

The T-box genes Tbx4 and Tbx5 have been shown to have key functions in the specification of the identity of the vertebrate forelimb (Tbx5) and hindlimb (Tbx4). Here we show that in zebrafish, Tbx5 has an additional early function that precedes the formation of the limb bud itself. Functional knockdown of zebrafish tbx5 through the use of an antisense oligonucleotide resulted in a failure to initiate fin bud formation, leading to the complete loss of pectoral fins. The function of the tbx5 gene in the development of zebrafish forelimbs seems to involve the directed migration of individual lateral-plate mesodermal cells into the future limb-bud-producing region. The primary defect seen in the tbx5-knockdown phenotype is similar to the primary defects described in known T-box-gene mutants such as the spadetail mutant of zebrafish and the Brachyury mutant of the mouse, which both similarly exhibit an altered migration of mesodermal cells. A common function for many of the T-box genes might therefore be in mediating the proper migration and/or changes in adhesive properties of early embryonic cells.

Ventricular Expression of tbx5 Inhibits Normal Heart Chamber Development

The T-box gene tbx5 is expressed in the developing heart, forelimb, eye, and liver in vertebrate embryos during critical stages of morphogenesis and patterning. In humans, mutations in the TBX5 gene have been associated with Holt–Oram syndrome, which is characterized by developmental anomalies in the heart and forelimbs. In chicken and mouse embryos, tbx5 expression is initiated at the earliest stages of heart formation throughout the heart primordia and is colocalized with other cardiac transcription factors such as nkx-2.5 and GATA4. As the heart differentiates, tbx5 expression is restricted to the posterior sinoatrial segments of the heart, consistent with the timing of atrial chamber determination. The correlation between tbx5 expression and atrial lineage determination was examined in retinoic acid (RA)-treated chicken embryos. tbx5 expression is maintained throughout the hearts of RA-treated embryos under conditions that also expand atrial-specific gene expression. The downstream effects of persistent tbx5 expression in the ventricles were examined directly in transgenic mice. Embryos that express tbx5 driven by a β-myosin heavy chain promoter throughout the primitive heart tube were generated. Loss of ventricular-specific gene expression and retardation of ventricular chamber morphogenesis were observed in these embryos. These studies provide direct evidence for an essential role for tbx5 in early heart morphogenesis and chamber-specific gene expression.

Tbx5 and Tbx4 genes determine the wing/leg identity of limb buds.

Much progress has been made in understanding limb development. Most genes are expressed equally and in the same pattern in the fore- and hindlimbs, which nevertheless develop into distinct structures. The T-box genes Tbx5 and Tbx4, on the other hand, are expressed differently in chick wing (Tbx5) and leg (Tbx4) buds. Molecular analysis of the optomotor blind gene, which belongs to the same family of transcription factors, has revealed that this gene is involved in the transdetermination of Drosophila wing and leg imaginal discs. In addition, expression of Tbx5 and Tbx4 correlates well with the identity of ectopic limb buds induced by fibroblast growth factor. Thus, it is thought that Tbx5 and Tbx4 might be involved in determining limb identity. Another candidate is the Pitx1 gene, which encodes a bicoid-type homeodomain transcription factor that is expressed in leg buds. Here we determine the importance of these factors in establishing limb identity.

Isolation of a Drosophila T-box gene closely related to human TBX1.

T-box genes, in all metazoans studied from nematode to man, exist in small gene families. They encode transcription factors with a novel, large, and highly conserved DNA binding domain termed the T-domain. In all cases studied, T-box genes have important developmental roles. Two familial diseases, Holt-Oram syndrome and ulnar-mammary syndrome, were recently shown to be caused by mutations in the human T-box genes TBX5 and TBX3, respectively. T-box genes were first identified in Drosophila and mouse. Two of the three known Drosophila T-box genes show a close sequence homology to mammalian genes. Similarities in the phenotypes of fly and mammalian mutants can be taken as evidence of functional conservation. We report here the isolation of a fourth Drosophila T-box gene, optomotor-blind-related gene-1 (org-1), closely related to mouse and human TBX1. We localized TBX1 to chromosomal band 22q11, confirming a recent report, and discuss TBX1 as a candidate gene for DiGeorge and related syndromes.