The overall goal of this thesis project was to investigate the role of mitochondria (MT) in the very early pathogenesis of posttraumatic osteoarthritis (PTOA) and to test mitoprotection as a strategy to prevent chondrocyte death and cartilage degeneration after cartilage injury. Ankle sprain is the most common athletic injury, and the most common cause of end-stage ankle osteoarthritis (OA). Evidence suggests that the magnitude of the initial cartilage/subchondral bone injury is the most important factor in the development of sprain-associated PTOA. However, most PTOA models utilize the knee and rely on joint destabilization or intraarticular fracture to initiate disease. These models cause rapid progression of cartilage pathology with severe synovitis and do not reflect the likely etiology of ankle PTOA resulting from a high-speed impact injury. Therefore, the first aim of this dissertation research was to develop a clinically relevant large animal model of impact-induced talocrural (ankle) PTOA. A minimally invasive surgical approach was used to apply rapid impact injuries to the equine talus. Twelve weeks after injury, the severity of cartilage lesions was positively correlated to peak impact stress. The significance of this work is that it allows us to directly link in vitro mechanistic studies to in vivo longitudinal studies of disease development, using the same impact system. This model will also allow preclinical testing of disease modifying OA drugs.
The second aim of this thesis was to study MT function of chondrocytes within their native extracellular matrix immediately following a single, rapid impact, which simulates an injury expected to initiate PTOA in vivo. Fresh cartilage explants were subjected to injury at varying stress rates. MT respiratory rate and control were assessed by microrespirometry. Functional integrity of the inner MT membrane was investigated using polarity-sensitive fluorescent probes on confocal microscopy. We found that injury resulted in decreased basal and maximal chondrocyte respiration as well as MT depolarization within hours of cartilage impact, indicating that MT dysfunction is an acute response of articular cartilage to injury. The response of chondrocytes differed between two areas of the same joint; chondrocytes from a non-weight bearing articular surface (the distal patellofemoral groove) were more sensitive to MT dysfunction and chondrocyte death than the main weight-bearing surface of the knee (the medial femoral condyle), indicating regional differences in mechanotransduction. These findings suggest that MT may represent an early therapeutic target in the prevention of PTOA. The third aim of this thesis work was therefore to investigate mitoprotection as a strategy to prevent chondrocyte death and cartilage degeneration in vitro. SS-31 is a highly targeted peptide antioxidant that prevents MT respiratory dysfunction, MT-mediated apoptosis, and ROS production by stabilizing the MT-specific phospholipid cardiolipin. Cartilage was injured, then treated with SS-31 at 0, 1, or 6 hours after injury, and cultured for 1 week. We found that SS-31 prevented impact induced chondrocyte death, apoptosis, and cartilage matrix degradation at 1 day and 1 week after injury. Our findings indicate that mitoprotective therapy within 6 hours after joint injury may be a useful strategy to prevent PTOA.