Enhanced Senescent Cell Burden in the Quadriceps, Articular Cartilage and Periarticular Bone Accompanies Atrophy and Osteoarthritis Following ACL Injury in Mice.

Protracted quadriceps weakness and atrophy after anterior cruciate ligament (ACL) injury result in poor knee mechanics and contribute to the development of posttraumatic osteoarthritis. The biological underpinnings of poor functional recovery and osteoarthritis are unknown, limiting the development of evidence-based therapies. The identification of shared cellular pathology between skeletal muscle, bone and articular cartilage would offer powerful therapeutic targets to combat musculoskeletal degeneration; therefore, we sought to define the musculoskeletal cellular senescence burden as a cellular predictor of muscle atrophy and osteoarthritis severity. We hypothesized that cellular senescence would be associated with greater muscle atrophy and osteoarthritis severity. Male and female C57BL/6 mice between 4 and 6 months of age underwent unilateral ACL transection surgery (ACLT), and then quadriceps and whole knees (healthy contralateral and ACLT) were collected at 7, 14 and 28d post-ACLT. Quadriceps fiber size was determined using immunohistochemistry (IHC), and osteoarthritis severity was assessed with Safranin O staining via Osteoarthritis Research Society International (OARSI) scoring. Additionally, cellular senescence was determined by quantification of senescence-associated β-galactosidase (SA β-Gal) and p16 (p16) positive cells using IHC. SA β-Gal expression was greatest at 7 days post-ACLT (ACLT: 18.6±4.3 vs. Healthy: 3.3±2.6 cells/mm , p<0.05) but remained elevated through 28 days post-ACLT. Quadriceps senescent cell density occurred alongside atrophy (7 days: -21%, 14 days: -16%, 28 days: -13%, all p<0.05 vs healthy contralateral). p16+ cell density was elevated within the articular cartilage (ACLT: 32.0±5.8 vs. Healthy: 10.0±2.5 cells/mm , p<0.05) and trabecular bone (ACLT: 435.5±130.6 vs. Healthy: 119.4±8.6 cells/mm , p=0.08) at 28 days post-ACLT. OARSI scoring demonstrated cartilage erosion at 28 days post-ACLT (ACLT: 2.9±1.0 vs Healthy: 0.4±0.4 OARSI score, p<0.05). These findings support an elevated cellular senescence burden within muscle, bone and cartilage following ACL injury associated with musculoskeletal deficits and osteoarthritis. Future research should focus on strategies to target senescent cells to enhance functional recovery and mitigate osteoarthritis following ACL injury.