Role of MAO-B in innate immunity during inflammatory diseases in mouse models

Since the NLRP3 inflammasome plays a crucial role in inflammatory diseases, its targeting has become relevant for the development of novel treatments. Several signals activate the NLRP3 inflammasome, and a few studies have reported that mitochondrial reactive oxygen species (ROS) are involved in this process. However, the source of mitochondrial ROS and their specific role in NLRP3 triggering are unclear. Monoamine oxidase B (MAO-B) is an enzyme that produces H2O2 in its metabolic activity. Therefore, we hypothesized that MAO-B has a non-redundant role in the inflammatory response by macrophages. Here, we show that H2O2 produced by the mitochondrial enzyme MAO-B plays a non-superfluous role in sustaining NLRP3 inflammasome activation. Mechanistically, MAO-B-dependent ROS formation caused mitochondrial dysfunction and NF-κB induction, resulting in NLRP3 and pro-IL-1β overexpression. Both, in vitro and in vivo, MAO-B inhibition by rasagiline prevented IL-1β and IL-6 secretion, which is another important pro-inflammatory cytokine, and MAO-B deficient mice showed an impaired response to LPS-mediated endotoxemia. In addition, we targeted MAO-B to inhibit different inflammatory pathways. We demonstrated that inhibition of MAO-B dampens the IL-6 release via the NOD2-NF-κB activation pathway in macrophages. The calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, also known as “pseudogout”, is characterized by CPPD crystal deposition in connective tissues, often driving several severe clinical symptoms. The pathogenic mechanism of calcium crystal deposition is only partially elucidated, however the NLRP3 inflammasome was identified as a key player. No drug is currently available to prevent crystal deposition or to target the pathogenic effects that result in the clinical manifestations. Based on our previous findings, we evaluated the efficacy of two MAO-B inhibitors (iMAO-Bs), rasagiline and safinamide, as anti-inflammatory drugs in an in vitro and in vivo model of CPPD crystal deposition. We showed that in vitro MAO-B inhibition prevented IL-1β secretion and NLRP3 activation under LPS and CPPD crystal stimulus. To validate these data, was tested iMAO-Bs in a preventive and therapeutic target to dampen inflammation in mouse models of human diseases . We observed that iMAO-Bs treatment reduced the release of several cytokines (IL-6, IL-1β, IL1-α, IL-17, IL-12p70, TNF-α and IFN-γ), chemokines (MCP-1, CXCL1, CXCL10, CCL3, CCL4, CXCL2 and CCL5) and growth factors (G-CSF and GM-CSF) under CPPD crystal-induction in the murine air pouch model mice in a preventive protocol. Besides, iMAOB-treated mice display a significant decrease in the number of monocytes and neutrophils recruited at the site of injury. Next, we used a CPPD-induced arthritis mouse model, and we demonstrated the therapeutic efficacy of iMAO-B in preventing the acute inflammatory response, as detected by the reduction of ankle swelling and gene expression of the main pro-inflammatory cytokines and chemokines. Our findings highlight MAO-B as a promising target for the treatment of pseudogout. Notably, they represent a significant opportunity of using iMAO-Bs to develop new therapies at low risk and a fraction of the cost. Indeed, iMAO-B are emerging as safe and cheap anti-inflammatory drugs that will greatly benefit many of these patients with limited treatment options.