A Bhaskaran

c-Jun is a negative regulator of myelination

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.

Krox-20 inhibits Jun-NH2-terminal kinase/c-Jun to control Schwann cell proliferation and death

The transcription factor Krox-20 controls Schwann cell myelination. Schwann cells in Krox-20 null mice fail to myelinate, and unlike myelinating Schwann cells, continue to proliferate and are susceptible to death. We find that enforced Krox-20 expression in Schwann cells cell-autonomously inactivates the proliferative response of Schwann cells to the major axonal mitogen β–neuregulin-1 and the death response to TGFβ or serum deprivation. Even in 3T3 fibroblasts, Krox-20 not only blocks proliferation and death but also activates the myelin genes periaxin and protein zero, showing properties in common with master regulatory genes in other cell types. Significantly, a major function of Krox-20 is to suppress the c-Jun NH2-terminal protein kinase (JNK)–c-Jun pathway, activation of which is required for both proliferation and death. Thus, Krox-20 can coordinately control suppression of mitogenic and death responses. Krox-20 also up-regulates the scaffold protein JNK-interacting protein 1 (JIP-1). We propose this as a possible component of the mechanism by which Krox-20 regulates JNK activity during Schwann cell development.

Novel signals controlling embryonic Schwann cell development, myelination and dedifferentiation

Abstract  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.

p38 MAPK Activation Promotes Denervated Schwann Cell Phenotype and Functions as a Negative Regulator of Schwann Cell Differentiation and Myelination

Physical damage to the peripheral nerves triggers Schwann cell injury response in the distal nerves in an event termed Wallerian degeneration: the Schwann cells degrade their myelin sheaths and dedifferentiate, reverting to a phenotype that supports axon regeneration and nerve repair. The molecular mechanisms regulating Schwann cell plasticity in the PNS remain to be elucidated. Using both in vivo and in vitro models for peripheral nerve injury, here we show that inhibition of p38 mitogen-activated protein kinase (MAPK) activity in mice blocks Schwann cell demyelination and dedifferentiation following nerve injury, suggesting that the kinase mediates the injury signal that triggers distal Schwann cell injury response. In myelinating cocultures, p38 MAPK also mediates myelin breakdown induced by Schwann cell growth factors, such as neuregulin and FGF-2. Furthermore, ectopic activation of p38 MAPK is sufficient to induce myelin breakdown and drives differentiated Schwann cells to acquire phenotypic features of immature Schwann cells. We also show that p38 MAPK concomitantly functions as a negative regulator of Schwann cell differentiation: enforced p38 MAPK activation blocks cAMP-induced expression of Krox 20 and myelin proteins, but induces expression of c-Jun. As expected of its role as a negative signal for myelination, inhibition of p38 MAPK in cocultures promotes myelin formation by increasing the number as well as the length of individual myelin segments. Altogether, our data identify p38 MAPK as an important regulator of Schwann cell plasticity and differentiation.

Regulation of the myelin gene periaxin provides evidence for Krox-20-independent myelin-related signalling in Schwann cells

We investigated the role of Krox-20 (Egr2), a transcription factor that regulates myelination, in controlling the myelin-associated protein periaxin. In developing Schwann cells, periaxin immunoreactivity appeared at least 2 days before Krox-20-immunopositive nuclei. Consistent with this, in Krox-20 null mice periaxin was upregulated on schedule, albeit to a lower level. In culture Krox-20 and periaxin were upregulated by cAMP as expected for myelin genes. Only those cells with the highest periaxin levels also expressed Krox-20, while other periaxin-positive cells remained Krox-20-negative. Furthermore, cAMP elevated periaxin even in Krox-20 null cells. We also found that in culture enforced Krox-20 expression induced expression of periaxin mRNA and protein in the absence of cAMP elevating agents, and that this induction was inhibited by the co-repressor NAB2. These findings reveal a dual mechanism for periaxin regulation and suggest that the role of Krox-20 is to amplify an earlier Krox-20-independent activation of the periaxin gene. Thus the axonal signals responsible for myelination are only partially transduced in Schwann cells by mechanisms that depend on Krox-20.

Interactions between Notch, c-Jun and Krox-20 in the control of early Schwann cell development and myelination

receptors plays an important role in myelination. In keeping with this, expression of the laminin alpha2 chain has been reported on axons at the time ofmyelination, and abnormalities of CNS myelination have been described in mice with mutant laminin alpha2 (dy/dy mice). Laminins are recognised by the alpha6beta1 integrin, and I will describe experiments using expression of dominant negative integrins in transgenicmice to examine the role of this receptor in vivo. The results suggest that beta1 integrin is dispensible for myelination in the CNS. However, parallel experiments using myelinating co-cultures show the opposite, and reveal that the timing of integrin loss is critical; prior to myelination no effect is seen but inhibition using antibodies blocks myelination if started after the initiation of the axo-glial interaction. They also show that loss of signalling by the mitogen PDGF is also required for myelination. Together, these results suggest that integrin/ growth factor interactions are important during myelination, with the timing of signalling being an important part of the regulatory process.