Reactive oligodendrocyte progenitor cells (re-)myelinate the regenerating zebrafish spinal cord.

Spinal cord injury (SCI) results in loss of neurons, oligodendrocytes and myelin sheaths, all of which are not efficiently restored. The scarcity of oligodendrocytes in the lesion site impairs remyelination of spared fibres, which leaves axons denuded, impedes signal transduction and contributes to permanent functional deficits. In contrast to mammals, zebrafish can functionally regenerate the spinal cord. Yet, little is known about oligodendroglial lineage biology and remyelination capacity after SCI in a regeneration-permissive context. Here, we report that in adult zebrafish, SCI results in axonal, oligodendrocyte and myelin sheath loss. We find that OPCs, the oligodendorocyte progenitor cells, survive the injury, enter a reactive state, proliferate and differentiate into oligodendrocytes. Concomitantly, the oligodendrocyte population is re-established to pre-injury levels within two weeks.Transcriptional profiling revealed that reactive OPCs upregulate the expression of several myelination-related genes. Interestingly, global reduction of axonal tracts and partial re-myelination, relative to pre-injury levels, persist at later stages of regeneration, yet suffices for functional recovery. Taken together, these findings imply that in the zebrafish spinal cord, OPCs replace lost oligodendrocytes and, thus, re-establish myelination during regeneration.