Early mobilization in spinal cord injury promotes changes in microglial dynamics and recovery of motor function

In the acute phase of spinal cord injury, the initial injury triggers secondary damage due to neuroinflammation, leading to the formation of cavities and glial scars that impair nerve regeneration. Following injuries to the central nervous system, early mobilization promotes the recovery of physical function. Therefore, in the present study, we investigated the effects of early mobilization on motor function recovery and neuroinflammation in rats. Early mobilization of rats with complete spinal cord transection resulted in good recovery of hindlimb motor function after 3 weeks. At 1 week after spinal cord injury, the early-mobilized rats expressed fewer inflammatory M1 microglia/macrophages and more anti-inflammatory M2 microglia. In addition, significantly more matrix metalloproteinase 2 (MMP2)-positive cells were observed at the lesion site 1 week after injury in the earlymobilized rats. Multiple labeling studies suggested that many MMP2-positive cells were M2 microglia. MMP9positive cells that highly co-expressed GFAP were also observed more frequently in the early-mobilized rats. The density of growth-associated protein-positive structures in the lesion center was significantly higher in the early-mobilized rats at 3 weeks after spinal cord injury. The present results suggest that early mobilization after spinal cord injury reduced the production of M1 microglia/macrophages while increasing the production of M2 microglia at the lesion site. Early mobilization might also activate the expression of MMP2 in M2 microglia and MMP9 in astrocytes. These cellular dynamics might suppress neuroinflammation at the lesion site, thereby inhibiting the progression of tissue destruction and promoting nerve regeneration to recover motor function.