DiGeorge Syndrome Gene tbx1 Functions through wnt11r to Regulate Heart Looping and Differentiation

Abstract

DiGeorge syndrome (DGS) is the most common microdeletion syndrome, and is characterized by congenital cardiac, craniofacial and immune system abnormalities. The cardiac defects in DGS patients include conotruncal and ventricular septal defects. Although the etiology of DGS is critically regulated by TBX1 gene, the molecular pathways underpinning TBX1's role in heart development are not fully understood. In this study, we characterized heart defects and downstream signaling in the zebrafish tbx1−/− mutant, which has craniofacial and immune defects similar to DGS patients. We show that tbx1−/− mutants have defective heart looping, morphology and function. Defective heart looping is accompanied by failure of cardiomyocytes to differentiate normally and failure to change shape from isotropic to anisotropic morphology in the outer curvatures of the heart. This is the first demonstration of tbx1's role in regulating heart looping, cardiomyocyte shape and differentiation, and may explain how Tbx1 regulates conotruncal development in humans. Next we elucidated tbx1's molecular signaling pathway guided by the cardiac phenotype of tbx1−/− mutants. We show for the first time that wnt11r (wnt11 related), a member of the non-canonical Wnt pathway, and its downstream effector gene alcama (activated leukocyte cell adhesion molecule a) regulate heart looping and differentiation similarly to tbx1. Expression of both wnt11r and alcama are downregulated in tbx1−/− mutants. In addition, both wnt11r −/− mutants and alcama morphants have heart looping and differentiation defects similar to tbx1−/− mutants. Strikingly, heart looping and differentiation in tbx1−/− mutants can be partially rescued by ectopic expression of wnt11r or alcama, supporting a model whereby heart looping and differentiation are regulated by tbx1 in a linear pathway through wnt11r and alcama. This is the first study linking tbx1 and non-canonical Wnt signaling and extends our understanding of DGS and heart development.

TBX1 regulates epithelial polarity and dynamic basal filopodia in the second heart field

“…Altered basal extracellular matrix properties might also impact on the formation and duration of filopodia. Increased circularity of cardiomyocytes in tbx1 mutant zebrafish hearts has recently been reported (Choudhry and Trede, 2013). This appears to reflect a block in the transition from a rounded morphology in the early heart tube to an anisotropic elongated shape in the outer curvature of the fish heart.…”

Tbx1 controls the morphogenesis of pharyngeal pouch epithelia through mesodermal Wnt11r and Fgf8a

“…Third, restoration of Wnt11r in this mesoderm partially rescued tbx1 −/− pouch defects, and did so more effectively when combined with Fgf8a. Recently, a role for Wnt11r downstream of Tbx1 has been reported for the looping of the heart in zebrafish ( Choudhry and Trede, 2013 ). This dual requirement of Tbx1 in the outpocketing of pouch endoderm and looping of heart mesoderm might reflect a common regulation of wnt11r in the nkx2.5 -positive mesoderm, with this mesodermal subpopulation not only contributing to the myocardium but also organizing the adjacent pouch endoderm.…”

Continuous addition of progenitors forms the cardiac ventricle in zebrafish

“…Impaired TBX1 in humans results in DiGeorge Syndrome (Scambler, 2010) with variable cardiac defects including Tetralogy of Fallot, OFT defects, and an interrupted aortic arch, defects recapitulated by Tbx1-mutant mice (Jerome and Papaioannou, 2001;Lindsay et al, 2001;Merscher et al, 2001). The zebrafish tbx1 mutant van gogh (vgo) displays among other phenotypes defects in the pharyngeal arches and a smaller BA, underlining the conserved function of Tbx1 in CPF control (Choudhry et al, 2013;Piotrowski et al, 2003). The 12.8 kilobases (kb) upstream of the murine Tbx1 gene as transgenic reporter principally recapitulate endogenous expression through separable Forkhead factor-binding enhancers that drive pharyngeal/anterior endoderm versus mesoderm expression, including activity in the OFT (Hu et al, 2004;Maeda et al, 2006;Yamagishi et al, 2003).…”