Ectopic expression of Hoxb3 in the second branchial arch leads to abnormal craniofacial development

Abstract

Craniofacial morphogenesis is orchestrated through molecular interactions between heterogeneous populations of cells in the branchial arches. To understand how Hoxb3 acts in concert with other genes in specifying hindbrain rhombomere and branchial arch identity, we created a gain-offunction mouse mutant using a Hoxb2 rhombomere 4 and second branchial arch specific enhancer element. The mutant mice, named SL2, displayed various craniofacial abnormalities including pointed face, impaired whisker movement, facial paralysis, impaired reaching response and abnormal circling behaviour. The SL2 transgenic mutants have smaller second branchial arches. Skeletal staining showed a partial or complete loss of middle and inner ear structures, most notably the lack of tympanic ring and stapes; the morphology of the mandible and squamous bone were abnormal. In the SL2 mutant, although Hoxb3 is ectopically expressed only in the second arch, both first and second arch-derived structures are affected. We hypothesize that mis-patterned second arch derived cells may affect first arch derived structures during morphogenesis. We found that ectopic expression of Hoxb3 in the second branchial arch resulted in altered expression domains of genes in both first and second branchial arches. In 9.5 and 10.5 dpc embryos, the expression domains of genes involved in the ET1/EdnrA signalling pathway including Gsc, dHAND, Prx2 and Dlx5 were all affected in SL2 mutants. By contrast, the expression domains of genes involved in Bmp and Fgf signaling pathways including Bmp4, Msx1 and Spry1 remain unchanged. Therefore, it is possible that Hoxb3 is specifically involved in the ET1/EdnrA pathway during branchial arch patterning.