Schwann cell precursors: a favourable cell for myelin repair in the Central Nervous System

Cell transplant therapies are currently under active consideration for a number of degenerative diseases. In the immune-mediated demyelinating-neurodegenerative disease multiple sclerosis (MS), only the myelin sheaths of the CNS are lost, while Schwann cell myelin of the PNS remains normal. This, and the finding that Schwann cells can myelinate CNS axons, has focussed interest on Schwann cell transplants to repair myelin in MS. However, the experimental use of these cells for myelin repair in animal models has revealed a number of problems relating to the incompatibility between peripheral glial cells and the CNS glial environment. Here, we have tested whether these difficulties can be avoided by using an earlier stage of the Schwann cell lineage, the Schwann cell precursor (SCP). For direct comparison of these two cell types, we implanted Schwann cells from post-natal rat nerves and SCPs from embryo day 14 (E14) rat nerves into the CNS under various experimental conditions. Examination 1 and 2 months later showed that in the presence of naked CNS axons, both types of cell form myelin that antigenically and ultrastructurally resembles that formed by Schwann cells in peripheral nerves. In terms of every other parameter we studied, however, the cells in these two implants behaved remarkably differently. As expected from previous work, Schwann cell implants survive poorly unless the cells find axons to myelinate, the cells do not migrate significantly from the implantation site, fail to integrate with host oligodendrocytes and astrocytes, and form little myelin when challenged with astrocyte-rich environment in the retina. Following SCP implantation, on the other hand, the cells survive well, migrate through normal CNS tissue, interface smoothly and intimately with host glial cells and myelinate extensively among the astrocytes of the retina. Furthermore, when implanted at a distance from a demyelinated lesion, SCPs but not Schwann cells migrate through normal CNS tissue to reach the lesion and generate new myelin. These features of SCP implants are all likely to be helpful attributes for a myelin repair cell. Since these cells also form Schwann cell myelin that is arguably likely to be resistant to MS pathology, they share some of the main advantages of Schwann cells without suffering from the disadvantages that render Schwann cells less than ideal candidates for transplantation into MS lesions.