AP-2α and AP-2β cooperatively function in the craniofacial surface ectoderm to regulate chromatin and gene expression dynamics during facial development

The facial surface ectoderm is essential for normal development of the underlying cranial neural crest cell populations, providing signals that direct appropriate growth, patterning, and morphogenesis. Despite the importance of the ectoderm as a signaling center, the molecular cues and genetic programs implemented within this tissue are understudied. Here, we show that removal of two members of the AP-2 transcription factor family, AP-2α and AP-2ß, within the early embryonic ectoderm of the mouse leads to major alterations in the craniofacial complex. Significantly, there are clefts in both the upper face and mandible, accompanied by fusion of the upper and lower jaws in the hinge region. Comparison of ATAC-seq and RNA-seq analyses between controls and mutants revealed significant changes in chromatin accessibility and gene expression centered on multiple AP-2 binding motifs associated with enhancer elements within these ectodermal lineages. In particular, loss of these AP-2 proteins affects both skin differentiation as well as multiple signaling pathways, most notably the WNT pathway. We also determined that the mutant clefting phenotypes that correlated with reduced WNT signaling could be rescued by Wnt1 ligand overexpression in the ectoderm. Collectively, these findings highlight a conserved ancestral function for AP-2 transcription factors in ectodermal development and signaling, and provide a framework from which to understand the gene regulatory network operating within this tissue that directs vertebrate craniofacial development.

Anomalous incisor morphology indicates tissue-specific roles for Tfap2a and Tfap2b in tooth development

Mice possess two types of teeth that differ in their cusp patterns; incisors have one cusp and molars have multiple cusps. The patterning of these two types of teeth relies on fine-tuning of the reciprocal molecular signaling between dental epithelial and mesenchymal tissues during embryonic development. The AP-2 transcription factors, particularly Tfap2a and Tfap2b, are essential components of such epithelial-mesenchymal signaling interactions that coordinate craniofacial development in mice and other vertebrates, but little is known about their roles in the regulation of tooth development and shape. Here we demonstrate that incisors and molars differ in their temporal and spatial expression of Tfap2a and Tfap2b. At the bud stage, Tfap2a is expressed in both the epithelium and mesenchyme of the incisors and molars, but Tfap2b expression is restricted to the molar mesenchyme, only later appearing in the incisor epithelium. Tissue-specific deletions show that loss of the epithelial domain of Tfap2a and Tfap2b affects the number and spatial arrangement of the incisors, notably resulting in duplicated lower incisors. In contrast, deletion of these two genes in the mesenchymal domain has little effect on tooth development. Collectively these results implicate epithelial expression of Tfap2a and Tfap2b in regulating the extent of the dental lamina associated with patterning the incisors and suggest that these genes contribute to morphological differences between anterior (incisor) and posterior (molar) teeth within the mammalian dentition.

Transcription factor AP2 epsilon (Tfap2e) regulates neural crest specification in Xenopus

Transcription factors Pax3 and Zic1 are two important regulators of cell fate decision at the neural plate border, where they act synergistically to promote neural crest (NC) formation. To understand the role of these factors in NC development, we performed a microarray analysis to identify downstream targets of Pax3 and Zic1 in Xenopus embryos. Among the genes identified was a member of transcription factor activator protein 2 (Tfap2) family, Tfap2 epsilon (Tfap2e). Tfap2e is first expressed at early neurula stage in NC progenitors and Rohon-Beard sensory neurons, and persists in a subset of migrating cranial NC cells as they populate the pharyngeal arches. This is in contrast to other species in which Tfap2e is not detected in the early NC lineage. Tfap2e morpholino-mediated knockdown results in a loss of NC progenitors and an expansion of the neural plate. Tfap2e is also sufficient to activate NC-specific genes in animal cap explants, and gain-of-function experiments in the whole embryo indicate that Tfap2e can promote NC formation. We propose that Tfap2e is a novel player in the gene regulatory network controlling NC specification in Xenopus downstream of Pax3 and Zic1.