Contribution of Schwann Cells to Remyelination in a Naturally Occurring Canine Model of CNS Neuroinflammation

Gliogenesis under pathophysiological conditions is of particular clinical relevance since it may provide evidence for regeneration promoting cells recruitable for therapeutic purposes. There is evidence that neurotrophin receptor p75 (p75(NTR))-expressing cells emerge in the lesioned CNS. However, the phenotype and identity of these cells, and signals triggering their in situ generation under normal conditions and certain pathological situations has remained enigmatic. In the present study, we used a spontaneous, idiopathic and inflammatory CNS condition in dogs with prominent lympho-histiocytic infiltration as a model to study the phenotype of Schwann cells and their relation to Schwann cell remyelination within the CNS. Furthermore, the phenotype of p75(NTR)-expressing cells within the injured CNS was compared to their counter-part in control sciatic nerve and after peripheral nerve injury. In addition, organotypic slice cultures were used to further elucidate the origin of p75(NTR)-positive cells. In cerebral and cerebellar white and grey matter lesions as well as in the brain stem, p75(NTR)-positive cells co-expressed the transcription factor Sox2, but not GAP-43, GFAP, Egr2/Krox20, periaxin and PDGFR-α. Interestingly, and contrary to the findings in control sciatic nerves, p75(NTR)-expressing cells only co-localized with Sox2 in degenerative neuropathy, thus suggesting that such cells might represent dedifferentiated Schwann cells both in the injured CNS and PNS. Moreover, effective Schwann cell remyelination represented by periaxin- and P0-positive mature myelinating Schwann cells, was strikingly associated with the presence of p75(NTR)/Sox2-expressing Schwann cells. Intriguingly, the emergence of dedifferentiated Schwann cells was not affected by astrocytes, and a macrophage-dominated inflammatory response provided an adequate environment for Schwann cells plasticity within the injured CNS. Furthermore, axonal damage was reduced in brain stem areas with p75(NTR)/Sox2-positive cells. This study provides novel insights into the involvement of Schwann cells in CNS remyelination under natural occurring CNS inflammation. Targeting p75(NTR)/Sox2-expressing Schwann cells to enhance their differentiation into competent remyelinating cells appears to be a promising therapeutic approach for inflammatory/demyelinating CNS diseases.