George and MaryLou Boone Professor
Director, Center for Craniofacial Molecular Biology and Associate Dean, Research
Ostrow School of Dentistry
- Developmental Biology
Research OverviewWe are interested in the molecular and cellular regulatory mechanism of embryonic development and malformations. Congenital birth defects, such as cleft palate, tooth and skull malformations, affect many people around the world. These babies suffer multiple handicaps that significantly compromise the quality of their lives. The causes of congenital birth defects are complex, including multiple genetic and environmental factors (for a review see Chai and Maxson, 2006). Recent advancements in mouse genetics have greatly facilitated the investigation of molecular and cellular mechanism of craniofacial malformations. We have made a strategic decision in which we focus our study on signaling pathways that have been shown to be indispensible in both mouse and humans during craniofacial development. Furthermore, we are developing genetic and pharmacological approaches to prevent and rescue craniofacial malformations in our mutant mouse models. Finally, our translational research will regenerate craniofacial tissues based on the principles we learned from our studies in mouse embryonic development and stem cell biology.
1. Cranial Neural Crest Cells and Craniofacial Malformations
One of the key features of craniofacial development is the formation of cranial neural crest (CNC) cells. The specification, emigration and migration, proliferation, survival and ultimate fate determination of the CNC play an important role in regulating craniofacial development. Unlike the trunk neural crest, CNC cells give rise to an array of cell types during embryonic development. For example, cranial neural crest cells form most of the hard tissues of the head such as bone, cartilage and teeth, whereas hard tissues in the rest of the body are formed from mesoderm cells. Using a two-component genetic system, we developed a comprehensive cell fate tracking system, in which we are able to investigate the functional significance of a signaling molecule in regulating the fate of neural crest cells (Chai et al., 2000). For example, we have found that transforming growth factor-beta (TGF-β) plays an important role in regulating the fate of CNC cells. Conditional inactivation of Tgfbr2 (TGF-β type II receptor) in neural crest cells causes cleft palate (Figure 1) and other craniofacial defects. Significantly, this TGF-β mutant model has provided us the opportunity to discover downstream signaling events and to test how manipulation of TGF-β signaling may prevent and rescue cleft palate in mouse and humans (Ito et al., 2003; Sasaki et al., 2006; Iwata et al., 2010 and unpublished data). Finally, in collaboration with Scott Fraser at the California Institute of Technology, we are taking a dynamic, three-dimensional approach to investigate normal and abnormal craniofacial development (see merged 3D microMRI and microCT image below, Figure 2).
2. Tooth Morphogenesis and Jawbone Regeneration
The tooth development involves continuous interactions between the dental epithelium and the CNC-derived mesenchyme, hence the tooth is an excellent model to investigate tissue-tissue interactions in regulating organogenesis. Recent studies show that BMP/TGF-β, Shh, FGF, and Wnt signaling are involved in regulating tooth development. Specifically, we have discovered that Smad4-mediated BMP/TGF-β signaling plays a crucial role in regulating tooth root development. However, it is unclear whether the inductive signal(s) of root formation resides in the dental epithelium or mesenchyme and how the BMP/TGF-β signaling network controls root development. Currently, we are performing experiments to test the hypotheses that Smad4 mediated BMP/TGF-β signaling and their downstream target genes, such as FGF, Wnt, Nfic, and BMPs are crucial for mediating tissue-tissue interactions and controlling the fate of epithelial and dental mesenchymal cells during root development (Figure 3). In parallel, we have discovered that post-migratory cranial neural crest cells (CNCCs) maintain mesenchymal stem cell (MSC) characteristics and show potential utility for the regeneration of craniofacial structures. We are able to induce the osteogenic differentiation of post-migratory CNCCs, and this differentiation is regulated by BMP and TGF-β pathways. Our study will continue to focus on the investigation of a novel function for post-migratory CNCCs in organ development, and demonstrate the utility of these CNCCs in regenerating craniofacial structures.
3. Tissue-tissue Interaction in Regulating Tongue Morphogenesis
The tongue is an important muscular organ and carries out crucial physiological functions. Our preliminary studies show that CNC cells contribute to the interstitial connective tissue, which ultimately compartmentalizes both intrinsic and extrinsic tongue muscles and serves as their attachments. Occipital somite-derived cells migrate into tongue primordium and give rise to muscle cells in the tongue. The intimate relationship between CNC- and mesoderm-derived cells suggests that tissue-tissue interaction may play an important role in regulating tongue development (Figure 4). TGF-β and its signaling mediator Smad are expressed in both CNC- and mesoderm-derived cells in the tongue. Significantly, disruption of TGF-β signaling in either CNC or mesoderm-derived cells does not adversely affect cell migration into the tongue primordium, indicating that TGF-β signaling is specifically required locally during tongue morphogenesis. We discovered that mutation of Tgfbr2 in CNC cells results in a defect in tongue muscle patterning and microglossia, whereas loss of Tgfbr2 in mesoderm-derived cells results in a myogenic differentiation defect with 100% phenotype penetrance. Taking advantage of our neural crest- or mesoderm-specific Tgfbr2 mutant animal models, we designed studies to test the hypothesis that TGF-β signaling controls the fate of CNC as well as mesoderm-derived cells and regulates tissue-tissue interaction during tongue development. Ultimately, our study will provide a better understanding of how the TGF-β signaling cascade regulates the fate of the CNC- and mesoderm derived cells during normal craniofacial development and how signaling pathway disruption can lead to craniofacial malformations.