Injurious Loading of Articular Cartilage Compromises Chondrocyte Respiratory Function

Abstract

Objective Determine whether repeatedly overloading healthy cartilage disrupts mitochondrial function in a manner similar to that associated with osteoarthritis pathogenesis. Methods We exposed normal articular cartilage on bovine osteochondral explants to 1 day or 7 consecutive days of cyclic axial compression (0.25 or 1.0 MPa, 0.5 Hz, 3 hours) and evaluated effects on chondrocyte viability, ATP concentration, reactive oxygen species (ROS) production, indicators of oxidative stress, respiration, and mitochondrial membrane potential. Results Neither 0.25 nor 1.0 MPa cyclic compression caused extensive chondrocyte death, macroscopic tissue damage, or overt changes in stress-strain behavior. After one day of loading, differences in respiratory activities between the 0.25 and 1.0 MPa groups were minimal; after 7 loading days, however, respiratory activity and ATP levels were suppressed in the 1.0 MPa group relative to the 0.25 MPa group, an effect prevented with pretreatment with 10 mM N-acetylcysteine. These changes were accompanied by increased proton leakage and decreases in mitochondrial membrane potential as well as by increased ROS formation indicated by dihydroethidium staining and glutathione oxidation. Conclusion Repeated overloading leads to chondrocyte oxidant-dependent mitochondrial dysfunction. This mitochondrial dysfunction may contribute to destabilization of cartilage during various stages of OA in distinct ways by disrupting chondrocyte anabolic responses to mechanical stimuli.

Targeting mitochondrial responses to intra-articular fracture to prevent posttraumatic osteoarthritis

“…This effect was prevented to varying degrees with either NAC or amobarbital treatments. We also noted that proton leakage, similar to previous observations from human arthritic tissue (11, 19), was present 6 months after IAF, was not present in cells isolated from animals receiving either NAC or amobarbital (Fig. 7E).…”

“…7C) and uncoupled maximal (Fig. 7D) oxygen consumption rates (OCRs) were increased in the cells isolated from IAF animals, similar to previously published results with chondrocytes harvested from osteoarthritic patients (11, 19). This effect was prevented to varying degrees with either NAC or amobarbital treatments.…”

“…Because NAC is readily taken up by cells and rapidly supplements the intracellular thiols oxidized by mechanical injury (11, 20), it represents a strong candidate for translation– as well as for testing whether the treatment effects observed could be derived from an antioxidant rather than an inhibitor of mitochondrial activity. In articular tissue harvested 1 week after IAF, we verified that NAC treatment increased total glutathione (GSH) (n = 10 for ORIF and n = 7 for ORIF + NAC; Fig.…”

“…After 6 months of normal activity after IAF, articular tissue was analyzed for proteoglycan (PG) content and mitochondrial energetic changes associated with PTOA (11, 19). The benefits of amobarbital and NAC, 6 months after any treatments were applied, included stronger safranin O staining for PG throughout the entire depth of the cartilage and within each zone, as quantified by our automated routine (n as described in Figs.…”

“…Although the value of this early window has been recognized and descriptions of early intracellular pathology have noted mitochondrial involvement (9–11), the specific mechanisms linking acute injury, mitochondrial response, and chronic disease are only partially understood. What is known is that immediately after injurious cartilage impact, articular chondrocytes within and around impact sites respond with a burst of mitochondrial activity and produce damaging amounts of reactive oxygen species (ROS) (12–16).…”