Schwann cell myelination depends on Krox-20/Egr2 and other promyelin transcription factors that are activated by axonal signals and control the generation of myelin-forming cells. Myelin-forming cells remain remarkably plastic and can revert to the immature phenotype, a process which is seen in injured nerves and demyelinating neuropathies. We report that c-Jun is an important regulator of this plasticity. At physiological levels, c-Jun inhibits myelin gene activation by Krox-20 or cyclic adenosine monophosphate. c-Jun also drives myelinating cells back to the immature state in transected nerves in vivo. Enforced c-Jun expression inhibits myelination in cocultures. Furthermore, c-Jun and Krox-20 show a cross-antagonistic functional relationship. c-Jun therefore negatively regulates the myelinating Schwann cell phenotype, representing a signal that functionally stands in opposition to the promyelin transcription factors. Negative regulation of myelination is likely to have significant implications for three areas of Schwann cell biology: the molecular analysis of plasticity, demyelinating pathologies, and the response of peripheral nerves to injury.

Protein phosphorylation regulates virtually all biological processes. Although protein kinases are popular drug targets, targeting protein phosphatases remains a challenge. Here, we describe Sephin1 (selective inhibitor of a holophosphatase), a small molecule that safely and selectively inhibited a regulatory subunit of protein phosphatase 1 in vivo. Sephin1 selectively bound and inhibited the stress-induced PPP1R15A, but not the related and constitutive PPP1R15B, to prolong the benefit of an adaptive phospho-signaling pathway, protecting cells from otherwise lethal protein misfolding stress. In vivo, Sephin1 safely prevented the motor, morphological, and molecular defects of two otherwise unrelated protein-misfolding diseases in mice, Charcot-Marie-Tooth 1B, and amyotrophic lateral sclerosis. Thus, regulatory subunits of phosphatases are drug targets, a property exploited here to safely prevent two protein misfolding diseases.

Notch signaling is central to vertebrate development, and analysis of Notch has provided important insights into pathogenetic mechanisms in the CNS and many other tissues. However, surprisingly little is known about the role of Notch in the development and pathology of Schwann cells and peripheral nerves. Using transgenic mice and cell cultures, we found that Notch has complex and extensive regulatory functions in Schwann cells. Notch promoted the generation of Schwann cells from Schwann cell precursors and regulated the size of the Schwann cell pool by controlling proliferation. Notch inhibited myelination, establishing that myelination is subject to negative transcriptional regulation that opposes forward drives such as Krox20. Notably, in the adult, Notch dysregulation resulted in demyelination; this finding identifies a signaling pathway that induces myelin breakdown in vivo. These findings are relevant for understanding the molecular mechanisms that control Schwann cell plasticity and underlie nerve pathology, including demyelinating neuropathies and tumorigenesis.

Deletion of serine 63 from P0 glycoprotein (P0S63del) causes Charcot-Marie-Tooth 1B neuropathy in humans, and P0S63del produces a similar demyelinating neuropathy in transgenic mice. P0S63del is retained in the endoplasmic reticulum and fails to be incorporated into myelin. Here we report that P0S63del is misfolded and Schwann cells mount a consequential canonical unfolded protein response (UPR), including expression of the transcription factor CHOP, previously associated with apoptosis in ER-stressed cells. UPR activation and CHOP expression respond dynamically to P0S63del levels and are reversible but are associated with only limited apoptosis of Schwann cells. Nonetheless, Chop ablation in S63del mice completely rescues their motor deficit and reduces active demyelination 2-fold. This indicates that signaling through the CHOP arm of the UPR provokes demyelination in inherited neuropathy. S63del mice also provide an opportunity to explore how cells can dysfunction yet survive in prolonged ER stress-important for neurodegeneration related to misfolded proteins.

Missense mutations in 22 genes account for one-quarter of Charcot-Marie-Tooth (CMT) hereditary neuropathies. Myelin Protein Zero (MPZ, P0) mutations produce phenotypes ranging from adult demyelinating (CMT1B) to early onset [Déjérine-Sottas syndrome (DSS) or congenital hypomyelination] to predominantly axonal neuropathy, suggesting gain of function mechanisms. To test this directly, we produced mice in which either the MpzS63C (DSS) or MpzS63del (CMT1B) transgene was inserted randomly, so that the endogenous Mpz alleles could compensate for any loss of mutant P0 function. We show that either mutant allele produces demyelinating neuropathy that mimics the corresponding human disease. However, P0S63C creates a packing defect in the myelin sheath, whereas P0S63del does not arrive to the myelin sheath and is instead retained in the endoplasmic reticulum, where it elicits an unfolded protein response (UPR). This is the first evidence for UPR in association with neuropathy and provides a model to determine whether and how mutant proteins can provoke demyelination from outside of myelin.