About 75% of birth defects involve the head, face, and oral tissues. Although orofacial clefts and other craniofacial malformations have clear environmental and genetic causes, insufficient information exists concerning the mechanisms of craniofacial development to enable the majority of these defects to be detected or prevented pre-natally. Our goal is to develop animal models of craniofacial malformations that will lead to mechanistic insight into the diagnosis and treatment of related human birth defects. The evolution and formation of the craniofacial skeleton relies on a specialized population of cells, the neural crest, arising at the margins of the neural tube during embryogenesis. While these cells contain significant intrinsic patterning information, they also rely on signals supplied by the surface ectoderm of the facial prominences to fulfill their growth and patterning potential. A number of studies, largely initiated using avian model systems, have demonstrated that several signal transduction pathways operating between the ectoderm and mesenchyme are critical for morphogenesis of the face. Indeed, manipulation of signaling cascades initiated by secreted factors belonging to the Fgf, Hedgehog (Hh), Wnt, and BMP families can alter the size and shape of the chick face. These studies have been much more challenging in the mouse due to its in utero development. Moreover, mouse gene knockouts affecting critical components of these signal transduction pathways die early in embryogenesis. Therefore, to study the function of these signaling events in later developmental processes - such as facial morphogenesis - it has been necessary to employ conditional gene knockout technology. A number of Cre recombinase transgenes can target gene expression in the developing facial ectoderm, but most are limited by the extent and/or timing of their expression. Recently, we generated a new Cre recombinase transgene, Crect, which circumvents many of these problems. Crect can mediate recombination in the entire embryonic ectoderm prior to E9.5 and is highly specific for this tissue layer and its derivatives. Preliminary data obtained using conditional alleles of Fgf8, Ctnnb1 (?-catenin), and Shh with Crect have shown the importance of the expression of these signaling molecules in the ectoderm for facial patterning. These studies have also revealed extensive cross-talk between these pathways in shaping the face. Thus, the goal of this application is to determine how these proteins function individually and as a network in the ectoderm to regulate craniofacial formation.
In Aim I we will use Crect to investigate the role of Wnt/?-catenin signaling in the ectoderm for development of the face.
In Aim II we will perform a similar analysis on the Hh pathway.
In Aim III we will test the interplay of these two pathways with Fgf8. The results of these analyses will reveal how these signaling pathways interact to control mouse craniofacial morphogenesis and will provide significant insight into the regulation of facial growth and patterning pertinent to the study of human birth defects and facial reconstruction.
Birth defects affect ~ 3% of all infants born in the US - with about 75% of these involving the head, face, and oral tissues - and the presence of a major birth defect will frequently reduce the quality of life for both the child and the parents. Insufficient information exists concerning the mechanisms of craniofacial development to enable the majority of these defects to be detected or prevented pre-natally. We are using animal model systems to determine how normal and abnormal craniofacial development proceeds and to identify new mechanisms that mediate face formation so that we may apply this knowledge to understand and ultimately treat the origins of human facial birth defects.
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