The effects of altered BMP4 signaling in first branchial-arch-derived murine embryonic orofacial tissues

The first branchial arch (BA1), which is derived from cranial neural crest (CNC) cells, gives rise to various orofacial tissues. Cre mice are widely used for the determination of CNC and exploration of gene functions in orofacial development. However, there is a lack of Cre mice specifically marked BA1's cells. Pax2-Cre allele was previously generated and has been widely used in the field of inner ear development. Here, by compounding Pax2-Cre and R26R-mTmG mice, we found a specific expression pattern of Pax2(+) cells that marked BA1's mesenchymal cells and the BA1-derivatives. Compared to Pax2-Cre; R26R-mTmG allele, GFP(+) cells were abundantly found both in BA1 and second branchial arch in Wnt1-Cre;R26R-mTmG mice. As BMP4 signaling is required for orofacial development, we over-activated Bmp4 by using Pax2-Cre; pMes-BMP4 strain. Interestingly, our results showed bilateral hyperplasia between the upper and lower teeth. We also compare the phenotypes of Wnt1-Cre; pMes-BMP4 and Pax2-Cre; pMes-BMP4 strains and found severe deformation of molar buds, palate, and maxilla-mandibular bony structures in Wnt1-Cre; pMes-BMP4 mice; however, the morphology of these orofacial organs were comparable between controls and Pax2-Cre; pMes-BMP4 mice except for bilateral hyperplastic tissues. We further explore the properties of the hyperplastic tissue and found it is not derived from Runx2(+) cells but expresses Msx1, and probably caused by abnormal cell proliferation and altered expression pattern of p-Smad1/5/8. In sum, our findings suggest altering BMP4 signaling in BA1-specific cell lineage may lead to unique phenotypes in orofacial regions, further hinting that Pax2-Cre mice could be a new model for genetic manipulation of BA1-derived organogenesis in the orofacial region.