The efficiency of remyelination decreases with age, but the molecular mechanisms responsible for this decline remain only partially understood. In this study, we show that remyelination is regulated by age-dependent epigenetic control of gene expression. In demyelinated young brains, new myelin synthesis is preceded by downregulation of oligodendrocyte differentiation inhibitors and neural stem cell markers, and this is associated with recruitment of histone deacetylases (HDACs) to promoter regions. In demyelinated old brains, HDAC recruitment is inefficient, and this allows the accumulation of transcriptional inhibitors and prevents the subsequent surge in myelin gene expression. Defective remyelination can be recapitulated in vivo in mice receiving systemic administration of pharmacological HDAC inhibitors during cuprizone treatment and is consistent with in vitro results showing defective differentiation of oligodendrocyte progenitors after silencing specific HDAC isoforms. Thus, we suggest that inefficient epigenetic modulation of the oligodendrocyte differentiation program contributes to the age-dependent decline in remyelination efficiency.Remyelination is the regenerative process in which new myelin sheaths are restored to demyelinated axons. In the CNS, this process is mediated by the recruitment and differentiation of a widespread population of adult stem and progenitor cells, called oligodendrocyte progenitor cells (OPCs), into myelin sheath-forming oligodendrocytes 1-3 . Remyelination can be a highly efficient process resulting in complete healing in both experimental models and clinical demyelinating diseases, including multiple sclerosis 4-8 . However, for reasons that are not fully understood, remyelination may be incomplete or fail in multiple sclerosis, leaving axons demyelinated and vulnerable to atrophy 9 . For this reason, therapeutic promotion of Published in final edited form as:Nat Neurosci. 2008 September ; 11(9): 1024-1034. doi:10.1038/nn.2172. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript remyelination represents an attractive option for preventing the axonal loss that underlies the progressive deterioration frequently associated with the later stages of the disease 10,11 .One of the most profound factors affecting remyelination is aging: as with other regenerative processes, remyelination becomes less efficient with age 12 , an effect that ismore pronounced inmales than in females 13 . This age-associated effect is due to impairment of OPC recruitment and differentiation 14 , of which inefficient differentiation is the more significant, as increasing the availability of OPCs during remyelination in old animals does not enhance remyelination efficiency 15 . Inefficient OPC differentiation in aging mirrors non-remyelinating plaques in humans with multiple sclerosis, which are replete with oligodendrocyte-lineage cells that fail to differentiate into remyelinating oligodendrocytes 16-18 . Thus, understanding OPC differentiation is central to explaining ...

Timely differentiation of progenitor cells is critical for development. In this study we asked whether global epigenetic mechanisms regulate timing of progenitor cell differentiation into myelin-forming oligodendrocytes in vivo. Histone deacetylation was essential during a specific temporal window of development and was dependent on the enzymatic activity of histone deacetylases, whose expression was detected in the developing corpus callosum. During the first 10 postnatal days, administration of valproic acid (VPA), the specific inhibitor for histone deacetylase activity, resulted in significant hypomyelination with delayed expression of late differentiation markers and retained expression of progenitor markers. Differentiation resumed in VPA-injected rats if a recovery period was allowed. Administration of VPA after myelination onset had no effect on myelin gene expression and was consistent with changes of nucleosomal histones from reversible deacetylation to more stable methylation and chromatin compaction. Together, these data identify global modifications of nucleosomal histones critical for timing of oligodendrocyte differentiation and myelination in the developing corpus callosum.

Mice lacking the expression of proteolipid protein (PLP)/DM20 in oligodendrocytes provide a genuine model for spastic paraplegia (SPG-2). Their axons are well myelinated but exhibit impaired axonal transport and progressive degeneration, which is difficult to attribute to the absence of a single myelin protein. We hypothesized that secondary molecular changes in PLP null myelin contribute to the loss of PLP/DM20-dependent neuroprotection and provide more insight into glia-axonal interactions in this disease model. By gel-based proteome analysis, we identified Ͼ160 proteins in purified myelin membranes, which allowed us to systematically monitor the CNS myelin proteome of adult PLP null mice, before the onset of disease. We identified three proteins of the septin family to be reduced in abundance, but the nicotinamide adenine dinucleotide (NAD ϩ )-dependent deacetylase sirtuin 2 (SIRT2) was virtually absent. SIRT2 is expressed throughout the oligodendrocyte lineage, and immunoelectron microscopy revealed its association with myelin. Loss of SIRT2 in PLP null was posttranscriptional, suggesting that PLP/DM20 is required for its transport into the myelin compartment. Because normal SIRT2 activity is controlled by the NAD ϩ /NADH ratio, its function may be coupled to the axo-glial metabolism and the long-term support of axons by oligodendrocytes.

Histone deacetylase 1 (HDAC1) is a nuclear enzyme involved in transcriptional repression. We report here that cytosolic HDAC1 is detected in damaged axons in brains of human patients with Multiple Sclerosis and of mice with cuprizone-induced demyelination, ex vivo models of demyelination and in cultured neurons exposed to glutamate and TNF-α. Nuclear export of HDAC1 is mediated by the interaction with the nuclear receptor CRM-1 and leads to impaired mitochondrial transport. The formation of complexes between exported HDAC1 and members of the kinesin family of motor proteins hinders the interaction with cargo molecules thereby inhibiting mitochondrial movement and inducing localized beadings. This effect is prevented by inhibiting HDAC1 nuclear export with leptomycin B, treating neurons with pharmacological inhibitors of HDAC activity or silencing HDAC1 but not other HDAC isoforms. Together these data identify nuclear export of HDAC1 as a critical event for impaired mitochondrial transport in damaged neurons.

Differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes is regulated by the interplay between extrinsic signals and intrinsic epigenetic determinants. In this study, we analyze the effect that the extracellular ligands sonic hedgehog (Shh) and bone morphogenetic protein 4 (Bmp4), have on histone acetylation and gene expression in cultured OPCs. Shh treatment favored the progression towards oligodendrocytes by decreasing histone acetylation and inducing peripheral chromatin condensation. Bmp4 treatment, in contrast, inhibited the progression towards oligodendrocytes and favored astrogliogenesis by favoring global histone acetylation and retaining euchromatin. Pharmacological treatment or silencing of histone deacetylase 1 (Hdac1) or histone deacetylase 2 (Hdac2) in OPCs did not affect Bmp4-dependent astrogliogenesis, while it prevented Shh-induced oligodendrocyte differentiation and favored the expression of astrocytic genes. Transcriptional profiling of treated OPC, revealed that Bmp4-inhibition of oligodendrocyte differentiation was accompanied by increased levels of Wnt (Tbx3) and Notch-target genes (Jag1, Hes1, Hes5, Hey1 and Hey2), decreased recruitment of Hdac and increased histone acetylation at these loci. Similar up-regulation of Notch-target genes and increased histone acetylation were observed in the corpus callosum of mice infused with Bmp4 during cuprizone-induced demyelination. We conclude that Shh and Bmp4 differentially regulate histone acetylation and chromatin structure in OPCs and that BMP4 acts as a potent inducer of gene expression, including Notch and Wnt target genes, thereby enhancing the cross-talk among signaling pathways that are known to inhibit myelination and repair.

In this study, we address the hypothesis that aging modifies the intrinsic properties of oligodendrocytes, the myelin-forming cells of the brain. According to our model, an "epigenetic memory" is stored in the chromatin of the oligodendrocyte lineage cells and is responsible for the maintenance of a mature phenotype, characterized by low levels of expression of transcriptional inhibitors. We report here an age-related decline of histone deacetylation and methylation, the molecular mechanisms responsible for the establishment and maintenance of this "epigenetic memory" of the differentiated state. We further show that lack of histone methylation and increased acetylation in mature oligodendrocytes are associated with global changes in gene expression, that include the re-expression of bHLH inhibitors (i.e. Hes5 and Id4) and precursor markers (i.e. Sox2). These changes characteristic of the "aging" oligodendrocytes can be recapitulated in vitro, by treating primary oligodendrocyte cultures with histone deacetylase inhibitors. Thus, we conclude that the "epigenetic memory loss" detected in white matter tracts of older mice induces global changes of gene expression that modify the intrinsic properties of aged oligodendrocytes and may functionally modulate the responsiveness of these cells to external stimuli.

The role of epigenetics in modulating gene expression in the development of organs and tissues and in disease states is becoming increasingly evident. Epigenetics refers to the several mechanisms modulating inheritable changes in gene expression that are independent of modifications of the primary DNA sequence and include post-translational modifications of nucleosomal histones, changes in DNA methylation, and the role of microRNA. This review focuses on the epigenetic regulation of gene expression in oligodendroglial lineage cells. The biological effects that posttranslational modifications of critical residues in the N-terminal tails of nucleosomal histones have on oligodendroglial cells are reviewed, and the implications for disease and repair are critically discussed.

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