Redundancy in regulation of chondrogenesis in MIA/CD-RAP-deficient mice

• Chondrogenesis in MIA-deficient (MIA−/−) mouse embryos was analyzed in vivo.
• Initial delayed hypertrophic differentiation was compensated in MIA−/− embryos.
• Chondrogenesis in MIA−/− mice is modified by enhanced Sox9 and AP-2α expression.
• Reduced AP1 and CRE activity enables hypertrophic differentiation in MIA−/− mice.
• Viability and functional integrity is reached by molecular redundancy in MIA−/− mice.

Recent in vitro analysis of MIA/CD-RAP-deficient (MIA−/−) mesenchymal stem cells revealed altered chondrogenic differentiation, characterised by enhanced proliferation and delayed differentiation. However, adult MIA−/− mice develop normally and show only ultrastructural defects of the cartilage but no major abnormalities. We therefore focused, in this study, on chondrogenesis in vivo in MIA−/− mouse embryos to reveal potential molecular changes during embryogenesis and possible redundant mechanisms, which explain the almost normal phenotype despite MIA/CD-RAP loss. In situ hybridisation analysis revealed larger expression areas of Col2a1 and Sox9 positive, proliferating chondrocytes at day 15.5 and 16.5 of embryogenesis in MIA−/− mice. The initially diminished zone of Col10a1-expressing hypertrophic chondrocytes at day 15.5 was compensated at day 16.5 in MIA−/− embryos. Supported by in vitro studies using mesenchymal stem cells, we discovered that chondrogenesis in MIA−/− mice is modified by enhanced Sox9, Sox6 and AP-2α expression. Finally, we identified reduced AP1 and CRE activity, analysed by reporter gene- and electrophoretic mobility shift assays, important for redundancy mechanism which rescued delayed hypertrophic differentiation and allows normal development of MIA−/− mice. In summary, as observed in other knockout models of molecules important for cartilage development and differentiation, viability and functional integrity is reached by remarkable molecular redundancy in MIA/CD-RAP knockout mice.