Cartilage-Specific Has2 Deletion Uncovers an Important Role for Hyaluronan in Cartilage and Joint Integrity

Background: Hyaluronan (HA) is a critical extracellular matrix component that we have demonstrated to be important for embryonic endochondral bone formation and postnatal synovial joint formation, supporting normal articular cartilage (AC) architecture and chondrocyte function. Although the embryonic requirement for Hyaluronan Synthase 2 (Has2), the main HA-producing enzyme in skeletal tissues, has been extensively investigated, the cartilage-cell-specific roles of Has2 and HA in maintaining postnatal cartilage and joint integrity are not well-defined. Methods: In this study, we used a tamoxifen-inducible, cartilage-specific Has2 conditional knockout mouse model (AggrecanCreERT2Cre/+; Has2^fl/fl). A total of 20 male mice were collected, followed with tamoxifen administered at 3 weeks of age and tissues analyzed at early and late post-induction time points using histological and matrix-based assessments. Results: Administration of tamoxifen at 3 weeks of age resulted in near-complete absence of HA in AC and growth late (GP) at 4 weeks, one week after the induction, as confirmed by highly specific HA staining Hyaluronan binding protein (HABP) immunohistochemistry. These early changes establish that Has2-dependent HA synthesis is indispensable for maintaining matrix integrity, columnar organization, and postnatal GP maturation. We further extended these findings into later developmental stages, showing that by 11 weeks of age (8 weeks after induction), tibial joints exhibit AC surface irregularity, proteoglycan depletion, disrupted zonal architecture, and changes in the osteochondral unit consistent with early degenerative features. Conclusions: Taken together, these data suggest that HA deficiency triggered in early postnatal life is associated with increased cartilage vulnerability, supporting an important role for Has2 in cartilage maturation and long-term joint integrity.