| Cartilage development within the limb skeleton is initiated by the condensation of mesenchymal progenitor cells within the developing limb bud. These progenitors undergo chondrogenesis to generate immature chondrocytes and the cartilage template from which individual skeletal elements form. Chondrocytes must undergo a highly organized process of proliferation, differentiation, and cell death in order to achieve normal skeletal growth and function. These processes of chondrocyte proliferation, hypertrophic and terminal hypertrophic differentiation, and cartilage matrix turnover are intricately regulated by developmental signaling pathways that include: Ihh/PTHrP, Wnt/beta-catenin, BMPs and FGFs. Importantly, many of these genetic pathways are defective in congenital and age related cartilage diseases (ie. chondrodysplasias and osteoarthritis), and therefore elucidating the molecular mechanisms by which cartilage develops is crucial to our understanding of cartilage disorders, the repair response to cartilage injury, and age associated changes observed in growth plate and joint cartilages. Recently, our group and others identified the Notch signaling pathway as yet another important regulator of cartilage development.;The work presented here set out to determine the molecular mechanisms by which Notch signaling regulates chondrocyte proliferation, hypertrophic differentiation, and terminal chondrocyte maturation using both in vivo and in vitro approaches. Specifically, we relied heavily upon mouse genetic approaches using the Cre/loxP system to generate various Notch-related gain- and loss-of-function mouse models for our analyses. Our studies established, for the first time, that RBPjkappa-dependent Notch signaling regulates both the onset of chondrocyte hypertrophy and terminal chondrocyte maturation. Surprisingly, the RBPjkappa-independent pathway was identified as a regulator of chondrocyte proliferation, and a long-range, cell non-autonomous regulator of perichondral bone formation. Further studies established Sox9, the master transcriptional regulator of chondrogeneis, as a target of RBPjkappa-dependent Notch signaling important for Notch-mediated regulation of chondrocyte hypertrophy. Finally, HES1 was identified as at least one of the mediators of RBPjkappa-dependent Notch signaling in the regulation of Sox9 and the initiation of hypertrophy. Taken together, this work provides a strong foundation for our molecular understanding of how Notch signaling regulates cartilage development, allowing future studies to bring even further clarification to the role of Notch signaling in both normal cartilage development and disease. |
