Novel signals controlling embryonic Schwann cell development, myelination and dedifferentiation.

Immature Schwann cells found in perinatal rodent nerves are generated from Schwann cell precursors (SCPs) that originate from the neural crest. Immature Schwann cells generate the myelinating and non-myelinating Schwann cells of adult nerves. When axons degenerate following injury, Schwann cells demyelinate, proliferate and dedifferentiate to assume a molecular phenotype similar to that of immature cells, a process essential for successful nerve regeneration. Increasing evidence indicates that Schwann cell dedifferentiation involves activation of specific receptors, intracellular signalling pathways and transcription factors in a manner analogous to myelination. We have investigated the roles of Notch and the transcription factor c-Jun in development and after nerve transection. In vivo, Notch signalling regulates the transition from SCP to Schwann cell, times Schwann cell generation, controls Schwann cell proliferation and acts as a brake on myelination. Notch is elevated in injured nerves where it accelerates the rate of dedifferentiation. Likewise, the transcription factor c-Jun is required for Schwann cell proliferation and death and is down-regulated by Krox-20 on myelination. Forced expression of c-Jun in Schwann cells prevents myelination, and in injured nerves, c-Jun is required for appropriate dedifferentiation, the re-emergence of the immature Schwann cell state and nerve regeneration. Thus, both Notch and c-Jun are negative regulators of myelination. The growing realisation that myelination is subject to negative as well as positive controls and progress in molecular identification of negative regulators is likely to impact on our understanding of demyelinating disease and mechanisms that control nerve repair.