Tbx1expression in pharyngeal epithelia is necessary for pharyngeal arch artery development

Abstract

During embryonic life, the initially paired pharyngeal arch arteries (PAAs)follow a precisely orchestrated program of persistence and regression that leads to the formation of the mature aortic arch and great vessels. When this program fails, specific cardiovascular defects arise that may be life threatening or mild, according to the identity of the affected artery. Fourth PAA-derived cardiovascular defects occur commonly in DiGeorge syndrome and velocardiofacial syndrome (22q11DS), and in Tbx1+/–mice that model the 22q11DS cardiovascular phenotype. Tbx1 is expressed in pharyngeal mesoderm, endoderm and ectoderm, and, in addition, we show that it is expressed in precursors of the endothelial cells that line the PAAs, thus expanding the number of tissues in which Tbx1 is potentially required for fourth PAA development. In this study, we have used cell fate mapping and tissue-specific gene deletion, driven by six different Cre lines,to explore Tbx1 gene-dosage requirements in the embryonic pharynx for fourth PAA development. Through this approach, we have resolved the spatial requirements for Tbx1 in this process, and we show pharyngeal epithelia to be a critical tissue. We also thereby demonstrate conclusively that the role of Tbx1 in fourth PAA development is cell non-autonomous.

Tbx1 regulation of myogenic differentiation in the limb and cranial mesoderm

“…Furthermore, we have shown that the Tbx1 requirement is intrinsic to the mesoderm. The loss and hypoplasia of the second and first branchial arch muscles in the Tbx1 mutant mouse may be due to intrinsic defects in the mesoderm or secondary to endodermal defects such as the absence of Fgf8 (Vitelli et al, 2002b;Hu et al, 2004;Kelly et al, 2004;Zhang et al, 2005). The endoderm is known to have a key role in patterning the facial structures signaling to the underlying mesoderm and ectomesenchyme (Couly et al, 2002;Ladher et al, 2005).…”

“…These include endothelial cells, myocytes, smooth muscle cells, and undifferentiated cranial mesenchyme (Brown et al, 2004;Zhang et al, 2005). Therefore, it appears that Tbx1 alone does not specify myogenic fate within the cranial mesoderm.…”

“…To confirm this finding in vivo, we turned to a mouse model in which Tbx1 has been specifically inactivated in the cranial mesoderm driven by Mesp1 cre recombination, Mesp1 Cre/ϩ; Tbx1 flox/⌬E5 (Zhang et al, 2005;Saga et al, 1996). This specifically eliminates Tbx1 expression in branchial arch mesoderm except for a little residual expression in the core mesoderm of the first branchial arch (Fig.…”

“…In its absence, the second arch muscles fail to develop while the mandibular or first arch muscles are hypoplastic and sporadic (Kelly et al, 2004). In the developing head, Tbx1 is expressed in both the endoderm and mesoderm and, therefore, myogenic defects may be a consequence of defective endoderm/ectoderm signaling (Vitelli et al, 2002b;Hu et al, 2004;Ataliotis et al, 2005, Zhang et al, 2005. To analyze whether Tbx1 is intrinsically required in the mesoderm, we performed micromass assays, cell cultures generated by plating single cell suspensions made from the mesoderm and neural crest-derived ectomesenchyme of branchial arch 1, at high density.…”

“…The Mesp cre/ϩ and Mesp1 Cre/ϩ; Tbx1 flox/ϩ mice are described in Saga et al (1996) and Zhang et al (2005), respectively. These embryos are phenotypically normal and were used as controls.…”