Melatonin alleviates rheumatoid arthritis via elimination of damaged mitochondria

Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disease primarily characterized by symmetrical synovial inflammation, leading to joint swelling, pain, and progressive cartilage and bone destruction. Unfortunately, the clinical treatment of RA still faces numerous challenges. Although melatonin (MT), the circadian rhythm hormone, is known to relieve the pathological process of RA, the underlying mechanism remains poorly understood. Herein, we assess the impacts of MT on collagen or K/BxN serum-induced arthritis (two well-established models of RA) and confirm its excellent therapeutic effect. Mechanistically, MT activates MTNR1A (melatonin receptor 1A) to promote mitophagy for the elimination of reactive oxygen (ROS) and leaked mitochondrial DNA triggered by damaged mitochondria, which in turn limits NLRP3 (NLR family pyrin domain containing 3) inflammasome activation and pro-inflammatory cytokine release. Mice with deletion of the autophagy-related gene Atg5 in myeloid cells (atg5^fl/fl Lyz2/LysM-cre) barely display any benefits of MT in K/BxN serum-induced arthritis. Our results indicate that mitophagy promoted by MT is essential to deactivate NLRP3 inflammasome and alleviate the development of arthritis, which provides a candidate for the treatment of RA.Abbreviations: 3-MA: 3-methyladenine; ACP5/TRAP: acid phosphatase 5, tartrate resistant; ANOVA: analysis of variance; ATG5: autophagy releated 5; ATP: adenosine triphosphate; BMD: bone mineral density; BMDMs: bone marrow derived macrophages; BV:TV: bone volume to tissue volume; CASP1: caspase 1; CYCS: cytochrome c; DNM1L/DRP1: dynamin 1 like; EtBr: ethidium bromide; FCCP: carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone; GSDMD: gasdermin D; IL1B/IL-1β: interleukin 1 beta; IL18: interleukin 18; LPS: lipopolysaccharide; MT: melatonin; MTNR1A: melatonin receptor 1A; MTNR1B: melatonin receptor type 1B; MFF: mitochondrial fission factor; mtROS: mitochondrial reactive oxygen species; mt-DNA: mitochondrial DNA; NAC: N-acetylcysteine; NT-GSDMD: N-terminal gasdermin D; NLRP3: NLR family pyrin domain containing 3; Ox-mtDNA: oxidized mitochondrial DNA; PINK1: PTEN induced kinase 1; PRKAA/AMPK: protein kinase AMP-activated catalytic subunit alpha; PRKN: parkin RBR E3 ubiquitin protein ligase; PYCARD/ASC: PYD and CARD domain containing; RA: rheumatoid arthritis; ROS: reactive oxygen; SD: standard deviation; SQSTM1/p62: sequestosome 1; Tb.N: trabecular number; Tb.Th: trabecular thickness; TNF/TNF-α: tumor necrosis factor; ULK1: unc-51 like autophagy activating kinase 1.