Hoxb3 Regulates Jag1 Expression in Pharyngeal Epithelium and Affects Interaction With Neural Crest Cells

Craniofacial morphogenesis depends on proper migration of neural crest cells and their interactions with placodes and other cell types.Hoxgenes provide positional information and are important in patterning the neural crest and pharyngeal arches (PAs) for coordinated formation of craniofacial structures.Hoxgenes are expressed in the surface ectoderm and epibranchial placodes, their roles in the pharyngeal epithelium and their downstream targets in regulating PA morphogenesis have not been established. We altered theHoxcode in the pharyngeal region of theHoxb3Tg/+mutant, in whichHoxb3is driven to ectopically expressed inHoxb2domain in the second pharyngeal arch (PA2). In the transgenic mutant, ectopicHoxb3expression was restricted to the surface ectoderm, including the proximal epibranchial placodal region and the distal pharyngeal epithelium. TheHoxb3Tg/+mutants displayed hypoplasia of PA2, multiple neural crest-derived facial skeletal and nerve defects. Interestingly, we found that in theHoxb3Tg/+mutant, expression of the Notch ligandJag1was specifically up-regulated in the ectodermal pharyngeal epithelial cells of PA2. By molecular experiments, we demonstrated that Hoxb3 could bind to an upstream genomic site S2 and directly regulateJag1expression. In theHoxb3Tg/+mutant, elevated expression ofJag1in the pharyngeal epithelium led to abnormal cellular interaction and deficiency of neural crest cells migrating into PA2. In summary, we showed thatHoxb3regulates Jag1 expression and proposed a model of pharyngeal epithelium and neural crest interaction during pharyngeal arch development.