Morphogenesis of vertebrate tissues and organs occurs through well-orchestrated tissue interactions. During development of the upper jaw, signaling interactions among the forebrain, the facial ectoderm, and the intervening neural crest cells regulate development of the upper jaw. Our research has revealed that signals from the brain and the neural crest cells act together to induce expression of Sonic hedgehog (SHH) in the Frontonasal Ectodermal Zone (FEZ), a signaling center located in the surface ectoderm of the Frontonasal Process (FNP) that directs patterned growth of the upper jaw anlagen. Our preliminary data also suggest that Pre-B-cell leukemia homeobox (PBX) transcription factors are involved in inducing and maintaining the pattern of SHH expression in the FEZ by an interacting genetic network. The research proposed in this application is designed to uncover the gene regulatory mechanisms that are activated by signals from the forebrain and the neural crest, as well as, to investigate the role of PBX transcription factors in regulating SHH expression in the FEZ. Specifically, we hypothesize that 1) SHH signaling from the forebrain induces ?competence? in surface ectoderm cells to express SHH; 2) subsequently, upon arrival, neural crest cells induce transcription of SHH in competent cells of the FEZ, and 3) PBX transcription factors participate in regulating expression of SHH in the FEZ. We will test this hypothesis in three specific aims using avian and murine embryos. In each Aim, we will use chick embryos because they allow us to manipulate the signaling interactions between the brain and the FEZ (Aim 1), the neural crest cells and the ectoderm (Aim 2), or expression of PBX1 and PBX3 (Aim 3). We will use a variety of genomic approaches (ATAC-seq, ChIP-seq, GRO-seq) to evaluate changes in chromatin and activated transcriptional regulatory elements (TREs) at the SHH locus.
In Aim 3, we will also assess morphological, cellular, and molecular outcomes of altering PBX gene expression, and we will use mouse embryos to take advantage of data already generated in the Selleri laboratory. The results of the work proposed in this application will shed light on the molecular aspects of facial development, and will aid understanding of disease processes that occur in this region. Allelic variants in SHH, or SHH pathway members, are associated with human dysmorphologies of the craniofacial complex in diseases such as Holoprosencephaly and cleft lip with our without cleft palate. Recently, PBX genes have been identified as risk variants in patients with craniofacial dysmorphology. Hence, our innovative approach combining studies with a solid foundation in developmental biology with chromatin analyses will yield information that is directly applicable to understanding human dysmorphology.
While the tissue interactions that regulate development of the upper jaw are known, the regulation of gene expression remains elusive. Yet, this information is essential for understanding the structural dysmorphologies, such as cleft lip and palate, that occur in this region. In this work, we will utilize our knowledge of the tissue interactions that control morphogenesis to uncover the genetic regulatory landscape that is responsible for facial morphogenesis. !
