Cartilage-specific RBPjκ-dependent and -independent Notch signals regulate cartilage and bone development

The Notch signaling pathway has emerged as an important regulator of endochondral bone formation. Although recent studies have examined the role of Notch in mesenchymal and chondro-osteo progenitor cell populations, there has yet to be a true examination of Notch signaling specifically within developing and committed chondrocytes, or a determination of whether cartilage and bone formation are regulated via RBPj kappa-dependent or -independent Notch signaling mechanisms. To develop a complete understanding of Notch signaling during cartilage and bone development we generated and compared general Notch gain-of-function (Rosa-NICDf/+), RBPj kappa-deficient (RBPj kappa(f/f)), and RBPj kappa-deficient Notch gain-of-function (Rosa-NICDf/+; RBPj kappa(f/f)) conditional mutant mice, where activation or deletion of floxed alleles were specifically targeted to mesenchymal progenitors (Prx1Cre) or committed chondrocytes (inducible Col2Cre(ERT2)). These data demonstrate, for the first time, that Notch regulation of chondrocyte maturation is solely mediated via the RBPj kappa-dependent pathway, and that the perichodrium or osteogenic lineage probably influences chondrocyte terminal maturation and turnover of the cartilage matrix. Our study further identifies the cartilage-specific RBPj kappa-independent pathway as crucial for the proper regulation of chondrocyte proliferation, survival and columnar chondrocyte organization. Unexpectedly, the RBPj kappa-independent Notch pathway was also identified as an important long-range cell non-autonomous regulator of perichondral bone formation and an important cartilage-derived signal required for coordinating chondrocyte and osteoblast differentiation during endochondral bone development. Finally, cartilage-specific RBPj kappa-independent Notch signaling likely regulates Ihh responsiveness during cartilage and bone development.