ABSTRACT
Schwann cells respond to cyclic adenosine monophosphate (cAMP) halting proliferation and expressing myelin proteins. Here we show that cAMP signaling induces the nuclear shuttling of the class IIa histone deacetylase (HDAC)-4 in these cells, where it binds to the promoter and blocks the expression of c-Jun, a negative regulator of myelination. To do it, HDAC4 does not interfere with the transcriptional activity of MEF2. Instead, by interacting with NCoR1, it recruits HDAC3 and deacetylates histone 3 in the promoter of c-Jun, blocking gene expression. Importantly, this is enough to up-regulate Krox20 and start Schwann cell differentiation program-inducing myelin gene expression. Using conditional knockout mice, we also show that HDAC4 together with HDAC5 redundantly contribute to activate the myelin transcriptional program and the development of myelin sheath in vivo. We propose a model in which cAMP signaling shuttles class IIa HDACs into the nucleus of Schwann cells to regulate the initial steps of myelination in the peripheral nervous system.
Subject(s)
Cyclic AMP/metabolism , Histone Deacetylases/metabolism , Myelin Sheath/metabolism , Nerve Fibers, Myelinated/enzymology , Schwann Cells/enzymology , Sciatic Nerve/enzymology , Transcription, Genetic , Active Transport, Cell Nucleus , Animals , Binding Sites , Cells, Cultured , Early Growth Response Protein 2/genetics , Early Growth Response Protein 2/metabolism , Histone Deacetylases/deficiency , Histone Deacetylases/genetics , MEF2 Transcription Factors/genetics , MEF2 Transcription Factors/metabolism , Mice, Knockout , Myelin Sheath/genetics , Nerve Fibers, Myelinated/ultrastructure , Nuclear Receptor Co-Repressor 1/genetics , Nuclear Receptor Co-Repressor 1/metabolism , Promoter Regions, Genetic , Proto-Oncogene Proteins c-jun/genetics , Proto-Oncogene Proteins c-jun/metabolism , Rats, Wistar , Schwann Cells/ultrastructure , Sciatic Nerve/ultrastructure , Second Messenger Systems , Tissue Culture TechniquesABSTRACT
The number of Schwann cells is fitted to axonal length in peripheral nerves. This relationship is lost when tumorigenic stimuli induce uncontrolled Schwann cell proliferation, generating tumours such us neurofibromas and schwannomas. Schwann cells also re-enter the cell cycle following nerve injury during the process of Wallerian degeneration. In both cases proliferation is finally arrested. We show that in neurofibroma, the induction of Jmjd3 (jumonji domain containing 3, histone lysine demethylase) removes trimethyl groups on lysine-27 of histone-H3 and epigenetically activates the Ink4a/Arf-locus, forcing Schwann cells towards replicative senescence. Remarkably, blocking this mechanism allows unrestricted proliferation, inducing malignant transformation of neurofibromas. Interestingly, our data suggest that in injured nerves, Schwann cells epigenetically activate the same locus to switch off proliferation and enter the senescence programme. Indeed, when this pathway is genetically blocked, Schwann cells fail to drop out of the cell cycle and continue to proliferate. We postulate that the Ink4a/Arf-locus is expressed as part of a physiological response that prevents uncontrolled proliferation of the de-differentiated Schwann cell generated during nerve regeneration, a response that is also activated to avoid overproliferation after tumorigenic stimuli in the peripheral nervous system.
