Endoderm‐specific deletion of Tbx1 reveals an FGF‐independent role for Tbx1 in pharyngeal apparatus morphogenesis

Abstract

Background The T-box transcription factor Tbx1, is essential for the normal development of multiple organ systems in the embryo. One of the most striking phenotypes in Tbx1−/− embryos is the failure of the caudal pharyngeal pouches to evaginate from the foregut endoderm. Despite considerable interest in the role of Tbx1 in development, the mechanisms whereby Tbx1 controls caudal pouch formation have remained elusive. In particular, the question as to how Tbx1 expression in the pharyngeal endoderm regulates pharyngeal pouch morphogenesis in the mouse embryo is not known. Results To address this question, we produced mouse embryos in which Tbx1 was specifically deleted from the pharyngeal endoderm and as expected, embryos failed to form caudal pharyngeal pouches. To determine the molecular mechanism, we examined expression of Fgf3 and Fgf8 ligands and downstream effectors. Although Fgf8 expression is greatly reduced in Tbx1-deficient endoderm, FGF signaling levels are unaffected. Furthermore, pouch morphogenesis is only partially perturbed by the loss of both Fgf3 and Fgf8 from the endoderm, indicating that neither are required for pouch formation. Conclusions Tbx1 deletion from the pharyngeal endoderm is sufficient to cause caudal pharyngeal arch segmentation defects by FGF-independent effectors that remain to be identified.

Dynamic epithelia of the developing vertebrate face

“…In mice, Tbx1 expression appears to mark nascent pouch regions while the related Tbx2 and Tbx3 factors are expressed in inter-pouch zones of endoderm [21,22]. However, Tbx1 was found to be dispensable for early segmental Fgf activity in zebrafish and endodermal Fgf activity in mice [15,23], instead having important requirements in the mesoderm for the subsequent morphogenesis of endodermal segments as discussed below.…”

Tbx1 and Foxi3 genetically interact in the third pharyngeal pouch endoderm required for thymus and parathyroid development

“…Global inactivation of tbx1 and restoration in the PE failed to rescue pouch defects in zebrafish mutant larvae (Choe and Crump, 2014). In contrast, inactivation of Tbx1 in the PE results in failed segmentation of the distal PA in mice (Arnold et al, 2006a;Jackson et al, 2014). Cell proliferation is not affected when Tbx1 is inactivated in the endoderm using Sox17 2A-iCre/+ , but pouches do not form in these mutant embryos regardless (Jackson et al, 2014).…”

“…Previous studies have shown that global or endoderm inactivation of Tbx1 resulted in failed segmentation of PA2-PA6 (Arnold et al, 2006b;Jackson et al, 2014;Jerome and Papaioannou, 2001;Lindsay et al, 2001;Merscher et al, 2001). To test the mechanism by which segmentation was disrupted, Tbx1 -/mutant embryos were generated and immunofluorescence with antibodies to Ecadherin and ZO-1 was performed on coronal tissue sections.…”

“…Tbx1 is expressed in the mesoderm of the head region in early mouse embryos and then throughout the endoderm, mesoderm, and distal ectoderm of the PA, while each arch forms, until mouse embryonic day (E)10.5 (Chapman et al, 1996;Garg et al, 2001;Lindsay et al, 2001). Tissue specific inactivation of Tbx1 has been performed using the Cre-loxP system (Arnold et al, 2006a;Arnold et al, 2006b;Jackson et al, 2014;Racedo et al, 2017;Xu et al, 2005). It was found that Tbx1 is required in all three tissues for the development of the derivative organs affected in the null mutant embryos (Arnold et al, 2006b;Choe and Crump, 2014;Racedo et al, 2017;van Bueren et al, 2010).…”

Cellular organization and boundary formation in craniofacial development

“…In Tbx1 −/− mice and mice lacking Tbx1 specifically from the endoderm, pharyngeal pouches fail to evaginate from the foregut endoderm. Loss of Fgf3 and Fgf8 from the pharyngeal endoderm only partially disrupted pouch morphogenesis indicating that Fgf signaling is not required for pouch formation (Jackson, Kasah, Mansour, Morrow, & Basson, ). However , Fgf8 is required, together with Fgf3 , for segmentation of the pharyngeal endoderm into pouches in zebrafish (Crump, Maves, Lawson, Weinstein, & Kimmel, ).…”