The role of mitochondrial reactive oxygen species in chondrocyte mechanotransduction.

Chondrocytes are mechanosensitive cells able to sense and respond to external mechanical stimuli through the process of mechanotransduction. Previous studies have demonstrated that mechanical stimulation causes mitochondrial deformation leading to mitochondrial reactive oxygen species (ROS) release in a dose-dependent manner. For this reason, we focused on elucidating the role of mitochondrial ROS as anabolic signaling molecules in chondrocyte mechanotransduction. Chondrocyte-seeded agarose gels were subjected to mechanical stimuli and the effect on matrix synthesis, ROS production, and mitogen-activated protein kinases (MAPK) signaling was evaluated. Through the use of ROS-specific staining, superoxide anion was the primary ROS released in response to mechanical stimuli. The anabolic effect of mechanical stimulation was abolished in the presence of electron transport chain inhibitors (complexes I, III, and V) and superoxide anion scavengers. Subsequent studies were centered on the involvement of MAPK pathways (ERK1/2, p38, and JNK) in the mechanotransduction cascade. While disruption of the ERK1/2 pathway had no apparent effect, the anabolic effect of mechanical stimulation was abolished in the presence of p38 and JNK pathway inhibitors. This suggest the involvement of apoptosis stimulating kinase 1 (ASK1), an upstream redox-sensitive MAP3K shared by both the JNK and p38 pathways. Future experiments will focus on the involvement of the thioredoxin-ASK1 complex which disassociates in the presence of oxidative stress, allowing ASK1 to phosphorylate several MAP2Ks. Overall, these findings indicate superoxide anion as the primary ROS released in response to mechanical stimuli and that the resulting anabolic effect on chondrogenic matrix biosynthesis arises from the ROS-dependent activation of the p38 and JNK MAPKs.