Chronic demyelination and myelin repair after spinal cord injury in mice: A potential link for glutamatergic axon activity

Our prior work examining endogenous repair after spinal cord injury (SCI) in mice revealed that large numbers of new oligodendrocytes (OLs) are generated in the injured spinal cord, with peak oligodendrogenesis between 4 and 7 weeks post‐injury (wpi). We also detected new myelin formation over 2 months post‐injury (mpi). Our current work significantly extends these results, including quantification of new myelin through 6 mpi and concomitant examination of indices of demyelination. We also examined electrophysiological changes during peak oligogenesis and a potential mechanism driving OL progenitor cell (OPC) contact with axons. Results reveal peak in remyelination occurs during the 3rd mpi, and that myelin generation continues for at least 6 mpi. Further, motor evoked potentials significantly increased during peak remyelination, suggesting enhanced axon potential conduction. Interestingly, two indices of demyelination, nodal protein spreading and Nav1.2 upregulation, were also present chronically after SCI. Nav1.2 was expressed through 10 wpi and nodal protein disorganization was detectable throughout 6 mpi suggesting chronic demyelination, which was confirmed with EM. Thus, demyelination may continue chronically, which could trigger the long‐term remyelination response. To examine a potential mechanism that may initiate post‐injury myelination, we show that OPC processes contact glutamatergic axons in the injured spinal cord in an activity‐dependent manner. Notably, these OPC/axon contacts were increased 2‐fold when axons were activated chemogenetically, revealing a potential therapeutic target to enhance post‐SCI myelin repair. Collectively, results show the surprisingly dynamic nature of the injured spinal cord over time and that the tissue may be amenable to treatments targeting chronic demyelination. Main Points Spontaneous robust myelin repair occurs long‐term after spinal cord injury and is regulated, at least in part, by active glutamatergic axons. Demyelination also occurs but declines after peak remyelination, suggesting effective myelin repair.