Foxi transcription factors promote pharyngeal arch development by regulating formation of FGF signaling centers

Abstract

The bones of the vertebrate face develop from transient embryonic branchial arches that are populated by cranial neural crest cells. We have characterized a mouse mutant for the Forkhead family transcription factor Foxi3, which is expressed in branchial ectoderm and endoderm. Foxi3 mutant mice are not viable and display severe branchial arch-derived facial skeleton defects, including absence of all but the most distal tip of the mandible and complete absence of the inner, middle and external ear structures. Although cranial neural crest cells of Foxi3 mutants are able to migrate, populate the branchial arches and display some elements of correct proximo distal patterning, they succumb to apoptosis from embryonic day 9.75 onwards. We show this cell death correlates with a delay in expression of Fgf8 in branchial arch ectoderm and a failure of neural crest cells in the arches to express FGF responsive genes. Zebrafish foxi1 is also expressed in branchial arch ectoderm and endoderm, and morpholino knockdown of foxi1 also causes apoptosis of neural crest in the branchial arches. We show that heat shock induction of fgf3 in zebrafish arch tissue can rescue cell death in foxi1 morphants. Our results suggest that Foxi3 may play a role in the establishment of signaling centers in the branchial arches that are required for neural crest survival, patterning and the subsequent development of branchial arch derivatives.

Suppression of epithelial differentiation by Foxi3 is essential for molar crown patterning

“…The Foxi3 −/− phenotype is embryonic lethal, with severely affected development of branchial arch derivatives (Edlund et al, 2014), precluding analysis of all mandibular teeth and maxillary molars. We detected structures resembling upper incisors in Foxi3 −/− embryos in an outbred ICR background, but when the mice were maintained in an inbred C57Bl/6 background, no signs of tooth development were observed (data not shown).…”

“…In support of this hypothesis, several adhesion molecules, such as cadherins and claudins, were differentially expressed in the microarray. The branchial arch phenotype of the Foxi3 KO has been speculated to result from a defect in cell adhesion (Edlund et al, 2014). In addition to Foxi3 cKO, the dental cord does not form in the molars of the conditional epithelial Shh KO (Dassule et al, 2000).…”

“…K14-cre43;Foxi3 +/− males (Andl et al, 2004;Edlund et al, 2014) were crossed with Foxi3 floxed/floxed females (Edlund et al, 2014;Foxi3 tm1.1Akg/J ; The Jackson Laboratory stock 024843) to obtain K14-cre43;Foxi3 −/floxed mice (Foxi3 cKO). For analyzing the adult tooth phenotype, cleaned skulls were used in micro-CT scan.…”

“…Foxi3 is expressed in the dental epithelium at all stages of development, but is downregulated in the differentiated ameloblasts (Shirokova et al, 2013). Foxi3 knockout (KO) mice die at birth as a result of craniofacial malformations, which arise from the lack of branchial arch-derived structures (Edlund et al, 2014).…”

Inner ear development: building a spiral ganglion and an organ of Corti out of unspecified ectoderm

“…One of the most important preplacodal transcription factors is Eya1/Six1, which is now known to regulate the expression of downstream pro-neurogenic factors such as Neurog1 (Ahmed et al, 2012). Several factors are selectively expressed in the otic placode such as Foxi factors that are essential for Fgf signaling (Edlund et al, 2014). Fgf's and their recetors are, in turn, essential for placode differentiation (Pirvola et al, 2000).…”