Microglial CX3CR1 signaling mediates synaptic pruning in degenerative cervical myelopathy

Degenerative cervical myelopathy (DCM) encompasses several conditions that cause compression of the cervical spinal cord. While the pathobiology underlying compression-induced neural degeneration remains incompletely understood, microglial synaptic pruning, driven by CX3CR1, is thought to be a main contributor. The current study builds on previous work demonstrating that Cx3cr1^-/- mice exhibit improved gait following DCM and seeks to determine if CX3CR1-mediated synaptic pruning underlies this improvement. We compared C57BL/6 (WT, wild-type) and Cx3cr1^-/- mice across 12-weeks of DCM. Locomotor and pain function were assessed using CatWalk and Von Frey outcome measures, respectively. Synaptic and electrophysiological changes were characterized at baseline and after 4-weeks of DCM using pre- and post-synaptic markers, motor evoked potentials (MEPs), and whole-cell patch-clamp recordings. Cx3cr1^-/- and WT mice showed differences in locomotion at baseline. Normalization of all groups revealed that Cx3cr1^-/- mice exhibited improved locomotion across 12-weeks and greater mechanical sensitivity on Von Frey. 4-weeks post-DCM induction, the microglia of Cx3cr1^-/- mice displayed reduced engulfment of excitatory and inhibitory synapses in the dorsal and ventral horns. This was accompanied by a disruption to the dorsal horn excitation-to-inhibition (E/I) balance, driven primarily by a loss of inhibitory transmission as confirmed by whole-cell patch clamp recording, and altered corticospinal-neuromuscular excitability, as confirmed by motor evoked potentials. Together, our findings reveal CX3CR1-mediated microglial synaptic pruning as a potential novel contributor to early DCM pathobiology. Specifically, our results suggest that an absence of pruning shifts the synaptic E/I balance in the dorsal horn, which may underlie the gait benefits observed in Cx3cr1^-/- mice.