Abstract
<jats:p> <jats:italic>Objective:</jats:italic> The <jats:italic> Agc1 <jats:sup>CreERT2</jats:sup> </jats:italic> 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 <jats:italic> Agc1 <jats:sup>CreERT2</jats:sup> </jats:italic> mice. <jats:italic>Methods:</jats:italic> Proteoglycan levels in rib cartilage extracts were examined by sulfated glycosaminoglycan (sGAG) assay. Destabilization of the medial meniscus (DMM) surgery was employed to induce PT-OA in 12-week-old wild-type, heterozygous <jats:italic> Agc1 <jats:sup>CreERT2/+</jats:sup> </jats:italic> and homozygous <jats:italic> Agc1 <jats:sup>CreERT2/CeERT2</jats:sup> </jats:italic> 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 (µCT). Aggrecan deposition, cartilage proteases, aggrecan and collagen II degradation neoepitopes were investigated by immunohistochemical (IHC) staining, and serum CTX-II levels by enzyme-linked immunosorbent assay (ELISA). Cell death was analyzed with terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) assay. Cartilage biomechanical properties were investigated with indentation-type atomic force microscopy (IT-AFM). <jats:italic>Results:</jats:italic> Before DMM, homozygous <jats:italic> Agc1 <jats:sup>CreERT2/CreERT2</jats:sup> </jats:italic> mice had reduced sGAG and aggrecan levels and increased cartilage stiffness. After DMM, they exhibited increased cartilage degradation, synovitis, osteophyte and meniscus mineralization, chondrocyte apoptosis and cartilage stiffness compared to wild-type mice. IHC demonstrated increased expression of the aggrecanase ADAMTS-5, the metalloproteinase MMP-13, the aggrecan degradation neoepitope NITEGE and the collagen degradation marker C1,2C in the articular cartilage (AC). ELISA revealed elevated serum CTX-II levels. Heterozygous <jats:italic> Agc1 <jats:sup>CreERT2/+</jats:sup> </jats:italic> mice also exhibited accelerated PT-OA compared to 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. <jats:italic>Conclusion:</jats:italic> Both homo- and heterozygous <jats:italic> Agc1 <jats:sup>CreERT2</jats:sup> </jats:italic> mice display enhanced susceptibility to PT-OA demonstrating the crucial role of normal aggrecan levels in regulating joint pathophysiology. Our results suggest that the use of <jats:italic> Agc1 <jats:sup>CreERT2/+</jats:sup> </jats:italic> mice for inducible, cartilage-specific gene deletion may inherently affect the interpretation of results in the DMM model. </jats:p>