FOX genes encode a large family of winged helix/forkhead transcription factors that have been shown to play multiple roles during development. While mutations in a number of FOX genes are known to cause craniofacial defects, how members of this large family coordinate development of the craniofacial skeleton remains unclear. In situ analyses in mice have led to the proposal that FOX genes form a complex expression code, much like the Dlx or Hox genes, that pattern the craniofacial primordia, yet this model remains to be tested at the functional level.
In this study, we find that the homologous FOX genes of zebrafish (foxc1a, foxc1b, foxd1, foxd2, foxf1, foxf2a, foxf2b, foxl1 and foxl2) are also expressed in distinct patterns within the neural-crest-derived pharyngeal arches that are the precursors to the facial skeleton. By manipulating major signaling pathways, we show that these distinct expression patterns result from differential sensitivity of FOX enhancers to Hh, Fgf, Bmp and Edn1 signaling. This suggests that FOX genes act as integrators of multiple signaling cascades in the cranial preskeletal mesenchyme. Next, we use knockout mutants and conditional transgenic misexpression approaches to show that FOX genes are required in patterning and growth of cartilage in distinct regions of the developing face, suggested that FOX genes act in region-specific manners to regulate the development of craniofacial primordia.
In summary, our evidence in zebrafish supports a model in which different members of the FOX family act very locally to specify cartilage elements of the facial skeleton. Our work indicates that expression and function of FOX genes specify major patterning domains of zebrafish pharyngeal arches.