Structural and molecular changes in the menisci of experimental models of post-traumatic osteoarthritis

Abstract

Purpose: Meniscal pathology is commonly associated with anterior cruciate ligament (ACL) injuries and substantially increases the risk of osteoarthritis (OA) developing later in life. The mechanisms driving meniscal degeneration after an ACL injury and its subsequent role in osteoarthritis pathogenesis are not clearly defined. The aim of this study was to investigate the structural and molecular progression of meniscal pathology in two mouse models of ACL injury with known differences in OA outcomes: surgical ACL transection (ACLT) and mechanical ACL rupture (ACLR). Methods: ACL injury was induced in the right hindlimb of 10-week-old, male C57BL/6 mice by either surgical transection (ACLT) or mechanical compression (ACLR). Separate un-injured mice and sham-surgery controls were included for comparison. At 1, 2, 4 and 8 weeks post-injury, joints were harvested for histology (n=7) or isolation of the medial and lateral menisci for gene expression (n=5 for ACLT/ACLR; n=2 for un-injured/sham using pooled samples of 3). The medial and lateral menisci were evaluated separately for both outcome measures. Meniscal pathology was assessed histologically using semi-quantitative scores for: proteoglycan loss, structural damage and peripheral bone formation. Quantitative PCR was performed to investigate key molecules implicated in meniscal and wider joint pathology. Results: Meniscal pathology was evident both medially and laterally following ACL injury. Superficial proteoglycan loss was notable from 1 week post-injury, particularly in the medial meniscus. Structural damage was more rapid and severe in the medial meniscus of ACLR joints compared with ACLT but lateral pathology was similar between the two groups. Peripheral bone formation was similarly elevated in the medial meniscus of ACLR joints but no different between the two groups at 8 weeks post-injury. Molecular meniscal changes were generally greater in the medial versus lateral meniscus. Acan, Col2a1, Col10a1, Mmp9, and Mmp13 were all significantly upregulated after ACL injury but were not notably different between the two models. Acan and Col2a1 increases preceded maximal meniscal pathology, while Col10a1, Mmp9, and Mmp13 were found to increase more gradually over time in both ACL injury models. Other genes (Col1a1, Adamts4, Mmp2, Lox, IGF, Il1b) were similarly regulated in all joints exposed to an intervention (ACL-injured and sham joints). Conclusions: The temporo-spatial development of meniscal pathology in the two ACL-injury models mirrored their known differences in OA onset and progression (ACLR-OA develops more rapidly than ACLT-OA). Both ACL injury models induced specific meniscal molecular changes with a greater increase in the medial meniscus compared to the lateral. These changes may be predictive of long term meniscal pathology and have the potential to contribute to the development of medial tibiofemoral OA though reduced mechanical function and increased production of degradative enzymes.