Oxidative stress by the mitochondrial monoamine oxidase B mediates calcium pyrophosphate crystal-induced arthritis

Objective: Calcium pyrophosphate (CPP) crystal deposition in the joints is associated with a heterogeneous set of debilitating syndromes characterized by inflammation and pain, for which no effective therapies are currently available. As we found that the mitochondrial enzyme monoamine oxidase B (MAO-B) plays a fundamental role in promoting inflammatory pathways, this study aims at assessing the efficacy of two clinical-grade inhibitors (iMAO-Bs) in preclinical models of this disease, to pave the way for a novel treatment. Methods: We tested our hypothesis in two murine models of CPP-induced arthritis, by measuring cytokine and chemokine levels, along with immune cell recruitment. iMAO-Bs (rasagiline and safinamide) were administered either before or after crystal injection. To elucidate the molecular mechanism, we challenged in vitro primed macrophages with CPP crystals and assessed the impact of iMAO-Bs in dampening proinflammatory cytokines and in preserving mitochondrial function. Results: Both in preventive and therapeutic in vivo protocols, iMAO-Bs blunted the release of proinflammatory cytokines (interleukin (IL)-6 and IL1-β) and chemokines (CXCL10, CXCL1, CCL2 and CCL5) (n>6 mice/group). Importantly, they also significantly reduced ankle swelling (50.3% vs 17.1% [P<0.001] and 23.1% [P=0.005] for rasagiline and safinamide, respectively). Mechanistically, iMAO-Bs dampened the burst of reactive oxygen species (ROS) and the mitochondrial dysfunction triggered by CPP crystals in isolated macrophages. Moreover, iMAO-Bs blunted cytokine secretion and NLRP3 inflammasome activation through inhibition of the NF-κB and STAT3 pathways. Conclusion: iMAO-Bs dampen inflammation in murine models of crystal-induced arthropathy, thereby uncovering MAO-B as a promising target to treat these diseases.