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

Abstract

The pharyngeal pouches are a segmental series of epithelial structures that organize the embryonic vertebrate face. In mice and zebrafish that carry mutations in homologs of the DiGeorge syndrome gene TBX1, a lack of pouches correlates with severe craniofacial defects, yet how Tbx1 controls pouch development remains unclear. Using mutant and transgenic rescue experiments in zebrafish, we show that Tbx1 functions in the mesoderm to promote the morphogenesis of pouch-forming endoderm through wnt11r and fgf8a expression. Consistently, compound losses of wnt11r and fgf8a phenocopy tbx1 mutant pouch defects, and mesoderm-specific restoration of Wnt11r and Fgf8a rescues tbx1 mutant pouches. Time-lapse imaging further reveals that Fgf8a acts as a Wnt11r-dependent guidance cue for migrating pouch cells. We therefore propose a two-step model in which Tbx1 coordinates the Wnt-dependent epithelial destabilization of pouch-forming cells with their collective migration towards Fgf8a-expressing mesodermal guideposts.

Reiterative expression of pax1 directs pharyngeal pouch segmentation in medaka (Oryzias latipes)

“…Additionally, we showed that endodermal pax1 plays an indispensable role in segmental pouch formation, except for the first pouch and anterior wall of the second pouch. Importantly, pax1 is required for the endodermal activation of tbx1 and fgf3 transcription, the functions of which in the development and patterning of endodermal pouches were described previously (Piotrowski et al, 2003;Crump et al, 2004;Herzog et al, 2004;Choe and Crump, 2014). The results of our TUNEL assay did not suggest an increase in apoptosis in the pharyngeal endoderm of the pax1 mutants.…”

“…In zebrafish, the expression of fgf3 in the pharyngeal endoderm of tbx1 mutants is not affected, although they lack pouch segmentation (Choe and Crump, 2014). At stage 23 in the wild type, expression of fgf3 was detected in endodermal cells of the first, second and third pouches as well as in the midbrain-hindbrain boundary, and fgf3 expression in the first and third pouches was much weaker than in the second pouch (Fig.…”

“…In zebrafish, wnt11r is reported to be expressed in the pharyngeal mesoderm in a segmental manner, and its signaling initiates the epithelial destabilization of the endoderm to form pouches (Choe et al, 2013;Choe and Crump, 2014). We therefore examined whether wnt11r regulates the reiterative expression of pax1 in medaka endoderm.…”

“…In mice, although mesodermal Tbx1 is required for proper pouch development (Zhang et al, 2006), dynamic expression of Ripply3, which encodes a Tbx1 repressor, regulates the endodermal activity of Tbx1 to form pouches posterior to the second arch (Okubo et al, 2011). In zebrafish, mesodermal tbx1 drives endodermal pouch morphogenesis by upregulating the expression of wnt11r and fgf8 in a cell-autonomous manner (Choe and Crump, 2014). It was also reported that the segmental expression pattern of fgf3 is retained in the pharyngeal endoderm of tbx1 mutants and that both mesodermal and endodermal Tbx1 play roles ensuring proper pouch segmentation (Choe and Crump, 2014).…”

“…In zebrafish, mesodermal tbx1 drives endodermal pouch morphogenesis by upregulating the expression of wnt11r and fgf8 in a cell-autonomous manner (Choe and Crump, 2014). It was also reported that the segmental expression pattern of fgf3 is retained in the pharyngeal endoderm of tbx1 mutants and that both mesodermal and endodermal Tbx1 play roles ensuring proper pouch segmentation (Choe and Crump, 2014). Therefore, the regulatory network for the endodermal expression of tbx1 and fgf3 is crucial for generating the reiterative pattern of pharyngeal segmentation.…”