Microglial Nr4a1 deficiency and neuronal C3 deposition mediate TMJ inflammation-induced hippocampal excessive synaptic pruning and depression-like behaviors in mice.

Temporomandibular joint (TMJ) inflammation-induced depression-like behaviors represent a common clinical complication in oral medicine. Microglia-mediated aberrant synaptic pruning in the hippocampus (HPC) plays a pivotal role in the pathogenesis of emotional disorders. However, the underlying mechanisms of TMJ inflammation-induced depression-like behaviors remain elusive, causing a severe lack of therapeutic targets for this condition. Here, we demonstrated that complete Freund's adjuvant (CFA)-induced TMJ inflammation model induced depression-like behaviors in mice, concomitant with hippocampal microglial activation and abnormal synaptic pruning. Minocycline-mediated suppression of microglial activity ameliorated TMJ inflammation-induced pathological synaptic remodeling. In addition, nuclear receptor subfamily 4 group A member 1 (Nr4a1) expression was significantly downregulated in microglia of HPC following CFA treatment. Nr4a1 silencing promoted the expression of lysosomal protein Cluster of Differentiation 68 (CD68) on microglial surfaces by activating nuclear factor kappa-light-chain-enhancer of activated B Cells (NF-κB) signaling pathway, whereas Nr4a1 overexpression suppressed microglial phagocytic activity. Meanwhile, following TMJ inflammation, complement 3 (C3) expression was upregulated in the HPC, with immunofluorescence (IF) revealing C3 localization in neurons and colocalization between synaptic terminals and microglia accompanied by synaptic loss. Notably, the overexpression of C3 also promoted excessive synaptic pruning in the HPC and induced depression-like behaviors. Collectively, Nr4a1 deficiency in the HPC activated microglia to mediate neuronal synaptic loss through a C3-dependent mechanism, ultimately contributing to emotional disorders in TMJ inflammation. This study elucidated the pivotal role of microglial Nr4a1 and neuronal C3 in synaptic pruning processes, suggesting that targeting these molecules may offer a potential therapeutic strategy for post-TMJ inflammation emotional complications.