Microglia Promote Inhibitory Synapse Phagocytosis in the Spinal Cord Dorsal Horn and Modulate Pain-Like Behaviors in a Murine Cancer-Induced Bone Pain Model

BACKGROUND: The microglial activation has been implicated in cancer-induced bone pain. Recent studies have revealed that microglia mediate synaptic pruning in the central nervous system, where the cluster of differentiation 47-signal regulatory protein α (CD47-SIRPα) axis creates a "don't eat me" signal and elicits an antiphagocytic effect to protect synapses against elimination. To date, the synaptic phagocytosis in microglia has never been investigated in the murine cancer-induced bone pain model. The present experiments sought to explore whether microglia phagocytize synapses in mice with bone cancer pain as well as the possible mechanisms. METHODS: Male C3H/HeN mice were used to induce bone cancer pain. Minocycline and S-ketamine were injected into D14. The number of spontaneous flinches (NSF) and paw withdrawal mechanical thresholds (PWMT) were measured on D0, D4, D7, D10, D14, D21, and D28. Hematoxylin and eosin staining presented bone lesions. Western blotting examined the Gephyrin, CD47, and SIRPα expression. Flow cytometry evaluated the proportion of SIRPα + cells in the spine. Immunofluorescence and 3-dimensional reconstruction showed the Gephyrin puncta inside microglial lysosomes. RESULTS: Mice embedded with tumor cells induced persistent spontaneous pain and mechanical hyperalgesia. Hematoxylin and eosin staining revealed bone destruction and tumor infiltration in marrow cavities. Microglia underwent a responsive and proliferative burst (t = -16.831, P < .001). Western blotting manifested lowered Gephyrin expression in the tumor group (D4, D7, D10, D14, D21, and D28: P < .001). Immunofluorescence and 3-dimensional reconstruction showed larger volumes of Gephyrin puncta inside microglial lysosomes (t = -23.273, P < .001; t = -27.997, P < .001). Treatment with minocycline or S-ketamine exhibited pain relief and antiphagocytic effects (t = -6.191, P < .001, t = -7.083, P < .001; t = -20.767, P < .001, t = -17.080, P < .001; t = 11.789, P < .001, t = 16.777, P < .001; t = 8.868, P < .001, t = 21.319, P < .001). Last but not least, the levels of CD47 and SIRPα proteins were downregulated (D10: P = .004, D14, D21, and D28: P < .001; D10, D14, D21, and D28: P < .001). Flow cytometry and immunofluorescence substantiated reduced microglial SIRPα (t = 11.311, P < .001; t = 12.189, P < .001). CONCLUSIONS: Microglia-mediated GABAergic synapse pruning in the spinal cord dorsal horn in bone cancer pain mice, which might be associated with the declined CD47-SIRPα signal. Our research uncovered an innovative mechanism that highlighted microglia-mediated synaptic phagocytosis in a murine cancer-induced bone pain model.