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

Abstract

Elongation of the vertebrate heart occurs by progressive addition of second heart field (SHF) cardiac progenitor cells from pharyngeal mesoderm to the poles of the heart tube. The importance of these cells in the etiology of congenital heart defects has led to extensive research into the regulation of SHF deployment by signaling pathways and transcription factors. However, the basic cellular features of these progenitor cells, including epithelial polarity, cell shape and cell dynamics, remain poorly characterized. Here, using immunofluorescence, live imaging and embryo culture, we demonstrate that SHF cells constitute an atypical, apicobasally polarized epithelium in the dorsal pericardial wall, characterized by apical monocilia and dynamic actin-rich basal filopodia. We identify the 22q11.2 deletion syndrome gene Tbx1, required in the SHF for outflow tract development, as a regulator of the epithelial properties of SHF cells. Cell shape changes in mutant embryos include increased circularity, a reduced basolateral membrane domain and impaired filopodial activity, and are associated with elevated aPKCζ levels. Activation of aPKCζ in embryo culture similarly impairs filopodia activity and phenocopies proliferative defects and ectopic differentiation observed in the SHF of Tbx1 null embryos. Our results reveal that epithelial and progenitor cell status are coupled in the SHF, identifying control of cell shape as a regulatory step in heart tube elongation and outflow tract morphogenesis.

Spatial regulation of cell cohesion by Wnt5a during second heart field progenitor deployment

“…4 & 5). In a recent report, E-cadherin is also found to be expressed in SHF cells in the rostral, but not the caudal, SpM (Francou et al, 2014). Therefore, we reason that there may be an increasing gradient of cell cohesion from the caudal SpM to rostral SpM/ distal OFT, which may act as a pulling force to deploy SHF cells rostrally into the OFT (Fig.…”

Cyp26 Enzymes Facilitate Second Heart Field Progenitor Addition and Maintenance of Ventricular Integrity

“…Disruption of this regulatory network is thought to underlie malformations in DiGeorge Syndrome, at least in the posterior pharyngeal mesoderm [62]. Additionally, cell polarity and tight junction defects have recently been described in SHF progenitors of Tbx1 null mice [63]. Although previous studies have found RA signaling negatively regulates tbx1 expression during early somitogenesis [64,65], via RT-qPCR and ISH, we found that tbx1 expression is not reduced or significantly altered in Cyp26-deficient embryos at these later stages (S5N–S5R Fig).…”

On the role of mechanics in driving mesenchymal-to-epithelial transitions

“…These cells aggregate to line the innermost lumen of the heart, and polarize their actin cytoskeleton, N-cadherin and integrin expression and NaK-ATPase [58, 68]. Additional trunk mesoderm cells contribute to the outflow tract and the right ventricle of the growing heart; disruption of this MET may have a role in congenital heart defects associated with DiGeorge Syndrome and basal filopodia activity appears crucial to outflow tract development [69]. Both of these processes involve restructuring ECM, directed migration, and polarized actomyosin, suggesting that microenvironment mechanics or intercellular forces play a role in these METs.…”