Repetitive Trans-spinal Magnetic Stimulation Suppresses Microglia to Engulf Synapse and Promotes Nerve Repairment via cGAS-STING Signaling Pathway after Spinal Cord Injury

<b>Background:</b> Spinal cord injury (SCI) is a neurological disorder characterized by progressive neuronal death. Notably, microglia-mediated synapse phagocytosis contributes to the disruption of the surviving neuronal network. Recovery of neurological function after SCI largely relies on the activation and remodeling of neural circuits. Magnetic stimulation has been shown to improve the reconstruction of neural synapses and neural circuits. However, the specific mechanisms by which repetitive trans-spinal magnetic stimulation (rTSMS) modulates microglial phagocytosis of synapses in SCI remain unclear. <b>Methods:</b> A modified version of Allen's method was used to establish an SCI model. Structural recovery was assessed using Hematoxylin-eosin and Nissl staining. Neurological function was evaluated through several assessments: the Basso, Beattie, and Bresnahan scale, the modified Rivlin inclined plate test, the horizontal ladder test, thermal pain assessment, motor evoked potential measurements, and gait analyses. Single-cell RNA sequencing was utilized to elucidate the cellular and molecular mechanisms by which rTSMS promotes recovery after SCI. Additionally, western blotting and immunofluorescence staining were performed to measure microglial phagocytosis of synapses and to investigate the expression of components of the cyclic GMP-AMP synthase (cGAS)-STING pathway. Furthermore, the STING agonist 2,3 cGAMP was used to further explore the role of the cGAS-STING pathway in the effects of rTSMS. <b>Results:</b> rTSMS significantly reduced the lesion area and improved functional recovery in rats subjected to SCI, and these changes correlated with enhanced synapse reservation and axon regeneration. Single-cell RNA sequencing identified microglia as the primary target cells that actively respond to rTSMS. Importantly, rTSMS effectively inhibited the phagocytosis of synapses by overactivated microglia via suppressing the cGAS-STING pathway. Moreover, 2,3 cGAMP counteracted the effects of magnetic stimulation on microglia both <i>in vivo</i> and <i>in vitro</i>. <b>Conclusion:</b> rTSMS mitigates SCI-induced synapse loss and neurological deficits by modulating microglial phagocytosis, a process dependent on the cGAS-STING pathway. These findings provide new insights into the mechanisms by which rTSMS exerts neuroprotective effects in the context of SCI.