Aggrecan hypomorphism accelerates the progression of post-traumatic osteoarthritis in mice

ObjectiveThe Agc1CreERT2 mouse line is a powerful tamoxifen-inducible genetic tool used for conditional gene manipulation specifically in cartilage. The aim of this study was to investigate the effects of aggrecan hypomorphism on the progression of post-traumatic osteoarthritis (PT-OA) in Agc1CreERT2 mice. MethodsProteoglycan content in cartilage samples from the knees of E18.5 embryos extracts were quantified by sulfated glycosaminoglycan (sGAG) assay. Destabilization of the medial meniscus (DMM) surgery was performed to induce PT-OA in 12-week-old wild-type, heterozygous Agc1CreERT2/+ and homozygous Agc1CreERT2/CreERT2 mice. Progression of OA was assessed at 4-, 8-, and 12-weeks post-DMM by OARSI, synovitis and osteophyte maturation histopathology scores and micro-computed tomography ({micro}CT). Aggrecan deposition, cartilage matrix-degrading proteases, aggrecan and collagen II degradation neoepitopes were investigated by immunohistochemical staining, and serum C-terminal cross-linked telopeptide of type II collagen (CTX-II) levels by an enzyme-linked immunosorbent assay (ELISA). Chondrocyte apoptosis was analyzed with the terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) assay. The biomechanical properties of articular cartilage (AC) were investigated with indentation-type atomic force microscopy (IT-AFM). ResultsBefore DMM, homozygous Agc1CreERT2/CreERT2 mice had reduced sGAG and aggrecan levels and increased cartilage stiffness. After DMM, they exhibited increased cartilage degradation, synovitis, osteophyte formation and meniscus mineralization, chondrocyte apoptosis and cartilage stiffness compared to wild-type mice. Immunohistochemistry demonstrated increased expression of the aggrecanase ADAMTS-5, the metalloproteinase MMP-13, the aggrecan degradation neoepitope NITEGE and the collagen degradation neoepitope C1,2C in AC. ELISA also revealed elevated serum CTX-II levels. Heterozygous Agc1CreERT2/+ mice also exhibited accelerated PT-OA compared with wild-type mice, characterized by elevated CTX-II levels at 4-weeks, increased synovitis, osteophyte and soft tissue mineralization at 8-weeks, and more severe cartilage degeneration at 12-weeks post-DMM. ConclusionBoth homozygous and heterozygous Agc1CreERT2 mice exhibit increased susceptibility to PT-OA, underscoring the importance of physiological aggrecan expression in maintaining joint homeostasis and regulating joint pathophysiology. These findings indicate that Agc1CreERT2/+ mice are not phenotypically neutral in the DMM model and that this intrinsic susceptibility should be considered when interpreting studies employing inducible, cartilage-specific gene deletion.