Shaping the oligodendrocyte identity by epigenetic control

The Role of Oligodendrocytes in Neurodegenerative Diseases: Unwrapping the Layers

Neurodegenerative diseases (NDs), including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis/motor neuron disease, and multiple sclerosis, are characterized by progressive loss of neuronal structure and function, leading to severe cognitive, motor, and behavioral impairments. They pose a significant and growing challenge due to their rising prevalence and impact on global health systems. The societal and emotional toll on patients, caregivers, and healthcare infrastructures is considerable. While significant progress has been made in elucidating the pathological hallmarks of these disorders, the underlying cellular and molecular mechanisms remain incompletely understood. Increasing evidence implicates oligodendrocytes and their progenitors-oligodendrocyte progenitor cells (OPCs)-in the pathogenesis of several NDs, beyond their traditionally recognized role in demyelinating conditions such as MS. Oligodendrocytes are essential for axonal myelination, metabolic support, and neural circuit modulation in the central nervous system. Disruptions in oligodendrocyte function and myelin integrity-manifesting as demyelination, hypomyelination, or dysmyelination-have been associated with disease progression in various neurodegenerative contexts. This review consolidates recent findings on the role of OPCs in NDs, explores the concept of myelin plasticity, and discusses therapeutic strategies targeting oligodendrocyte dysfunction. By highlighting emerging research in oligodendrocyte biology, this review aims to provide a short overview of its relevance to neurodegenerative disease progression and potential therapeutic advances.

Establishing validated RT-qPCR workflow for the analysis of oligodendrocyte gene expression in the developing murine brain

Myelination is essential for the proper conduction of impulses across neuronal networks. Mature, myelinating glia differentiate from progenitor cells through distinct stages that correspond to oligodendrocyte-specific gene expression markers. Reverse transcription quantiatative PCR (RT-qPCR) is a common technique used to quantify gene expression across cell development; however, a lack of standardization and transparency in methodology may lead to irreproducible data. Here, we have designed and validated RT-qPCR assays for oligodendrocyte genes and reference genes in the developing C57BL6/J mouse brain that align with the MIQE guidelines, including quality controls for primer specificity, temperature dependence, and efficiency. A panel of eight commonly used reference genes was ranked using a series of reference gene stability methods that consistently identified Gapdh, Sdha, Hmbs, Hprt1, and Pgk1 as the top candidates for normalization across brain regions. In the cerebrum, myelin genes peaked in expression at postnatal day 21, which corresponds to the peak of developmental myelination. The gene expression patterns from the brain homogenate were in agreement with previously reported RNA-seq and microarray profiles from oligodendrocyte lineage cells. The validated RT-qPCR assays begin to build a framework for future investigation into the molecular mechanisms that regulate myelination in mouse models of brain development, aging, and disease.

A primary cilium in oligodendrocytes: a fine structure signal of repairs in thalamic Osmotic Demyelination Syndrome (ODS)

A murine osmotic demyelination syndrome (ODS) model of the central nervous system included the relay thalamic ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei. Morphologic comparisons between treatments have revealed oligodendrocyte changes and, already 12 hours following the osmolality restoration, some heavily contrasted oligodendrocytes formed a unique intracellular primary cilium. This unique structure, found in vivo, in mature CNS oligodendrocytes, could account for a local awakening of some of the developmental proteome as it can be expressed in oligodendrocyte precursor cells. This resilience accompanied the emergence of arl13b protein expression along with restoration of nerve cell body axon hillocks shown in a previous issue of this journal. Additionally, the return of several thalamic oligodendrocyte fine features (nucleus, organelles) was shown 36 h later, including some mitosis. Those cell restorations and recognized translational activities comforted that local repairs could again take place, due to oligodendrocyte resilience after ODS instead or added to a postulated immigration of oligodendrocyte precursor cells distant from the sites of myelinolysis.

Dynamic Lamin B1-Gene Association During Oligodendrocyte Progenitor Differentiation

Differentiation of oligodendrocytes (OL) from progenitor cells (OPC) is the result of a unique program of gene expression, which is further regulated by the formation of topological domains of association with the nuclear lamina. In this study, we show that cultured OPC were characterized by progressively declining levels of endogenous Lamin B1 (LMNB1) during differentiation into OL. We then identify the genes dynamically associated to the nuclear lamina component LMNB1 during this transition, using a well established technique called DamID, which is based on the ability of a bacterially-derived deoxyadenosine methylase (Dam), to modify genomic regions in close proximity. We expressed a fusion protein containing Dam and LMNB1 in OPC (OPCLMNB1-Dam) and either kept them proliferating or differentiated them into OL (OLLMNB1-Dam) and identified genes that were dynamically associated to LMNB1 with differentiation. Importantly, we identified Lss, the gene encoding for lanosterol synthase, a key enzyme in cholesterol synthesis, as associated to the nuclear lamina in OLLMNB1-Dam. This finding could at least in part explain the lipid dysregulation previously reported for mouse models of ADLD characterized by persistent LMNB1 expression in oligodendrocytes.

Could the inhibitor of DNA binding 2 and 4 play a role in white matter injury?

White matter injury (WMI) prevents the normal development of myelination, leading to central nervous system myelination disorders and the production of chronic sequelae associated with WMI, such as chronic dyskinesia, cognitive impairment and cerebral palsy. This results in a large emotional and socioeconomic burden. Decreased myelination in preterm infant WMI is associated with the delayed development or destruction of oligodendrocyte (OL) lineage cells, particularly oligodendrocyte precursor cells (OPCs). The development of cells from the OL lineage involves the migration, proliferation and different stages of OL differentiation, finally leading to myelination. A series of complex intrinsic, extrinsic and epigenetic factors regulate the OPC cell cycle withdrawal, OL lineage progression and myelination. We focus on the inhibitor of DNA binding 2 (ID2), because it is widely involved in the different stages of OL differentiation and genesis. ID2 is a key transcription factor for the normal development of OL lineage cells, and the pathogenesis of WMI is closely linked with OL developmental disorders. ID4, another family member of the IDs protein, also plays a similar role in OL differentiation and genesis. ID2 and ID4 belong to the helix-loop-helix family; they lack the DNA-binding sequences and inhibit oligodendrogenesis and OPC differentiation. In this review, we mainly discuss the roles of ID2 in OL development, especially during OPC differentiation, and summarize the ID2-mediated intracellular and extracellular signaling pathways that regulate these processes. We also discuss ID4 in relation to bone morphogenetic protein signaling and oligodendrogenesis. It is likely that these developmental mechanisms are also involved in the myelin repair or remyelination in human neurological diseases.

5-Aza Exposure Improves Reprogramming Process Through Embryoid Body Formation in Human Gingival Stem Cells

Embryoid bodies (EBs) are three-dimensional aggregates formed by pluripotent stem cells, including embryonic stem cells and induced pluripotent stem cells. They are used as an in vitro model to evaluate early extraembryonic tissue formation and differentiation process. In the adult organisms, cell differentiation is controlled and realized through the epigenetic regulation of gene expression, which consists of various mechanisms including DNA methylation. One demethylating agent is represented by 5-Azacytidine (5-Aza), considered able to induce epigenetic changes through gene derepression. Human gingival mesenchymal stem cells (hGMSCs), an easily accessible stem cells population, migrated from neural crest. They are particularly apt as an in vitro study model in regenerative medicine and in systemic diseases. The ability of 5-Aza treatment to induce hGMSCs toward a dedifferentiation stage and in particular versus EBs formation was investigated. For this purpose hGMSCs were treated for 48 h with 5-Aza (5 μM). After treatment, hGMSCs are organized as round 3D structures (EBs-hGMSCs). At light and transmission electron microscopy, the cells at the periphery of EBs-hGMSCs appear elongated, while ribbon-shaped cells and smaller cells with irregular shape surrounded by extracellular matrix were present in the center. By RT-PCR, EBs-hGMSCs expressed specific transcription markers related to the three germ layers as MAP-2, PAX-6 (ectoderm), MSX-1, Flk-1 (mesoderm), GATA-4, and GATA-6 (endoderm). Moreover, in EB-hGMSCs the overexpression of DNMT1 and ACH3 other than the down regulation of p21 was detectable. Immunofluorescence staining also showed a positivity for specific etodermal and mesodermal markers. In conclusion, 5-Aza was able to induce the direct conversion of adult hGMSCs into cells of three embryonic lineages: endoderm, ectoderm, and mesoderm, suggesting their possible application in autologous cell therapy for clinical organ repair.

Therapeutic Strategies for Leukodystrophic Disorders Resulting from Perinatal Asphyxia: Focus on Myelinating Oligodendrocytes

Perinatal asphyxia results from the action of different risk factors like complications during pregnancy, preterm delivery, or long and difficult labor. Nowadays, it is still the leading cause of neonatal brain injury known as hypoxic-ischemic encephalopathy (HIE) and resulting neurological disorders. A temporal limitation of oxygen, glucose, and trophic factors supply results in alteration of neural cell differentiation and functioning and/or leads to their death. Among the affected cells are oligodendrocytes, responsible for myelinating the central nervous system (CNS) and formation of white matter. Therefore, one of the major consequences of the experienced HIE is leukodystrophic diseases resulting from oligodendrocyte deficiency or malfunctioning. The therapeutic strategies applied after perinatal asphyxia are aimed at reducing brain damage and promoting the endogenous neuroreparative mechanisms. In this review, we focus on the biology of oligodendrocytes and discuss present clinical treatments in the context of their efficiency in preserving white matter structure and preventing cognitive and behavioral deficits after perinatal asphyxia.

Molecular Neuropathology of Astrocytes and Oligodendrocytes in Alcohol Use Disorders

Postmortem studies reveal structural and molecular alterations of astrocytes and oligodendrocytes in both the gray and white matter (GM and WM) of the prefrontal cortex (PFC) in human subjects with chronic alcohol abuse or dependence. These glial cellular changes appear to parallel and may largely explain structural and functional alterations detected using neuroimaging techniques in subjects with alcohol use disorders (AUDs). Moreover, due to the crucial roles of astrocytes and oligodendrocytes in neurotransmission and signal conduction, these cells are very likely major players in the molecular mechanisms underpinning alcoholism-related connectivity disturbances between the PFC and relevant interconnecting brain regions. The glia-mediated etiology of alcohol-related brain damage is likely multifactorial since metabolic, hormonal, hepatic and hemodynamic factors as well as direct actions of ethanol or its metabolites have the potential to disrupt distinct aspects of glial neurobiology. Studies in animal models of alcoholism and postmortem human brains have identified astrocyte markers altered in response to significant exposures to ethanol or during alcohol withdrawal, such as gap-junction proteins, glutamate transporters or enzymes related to glutamate and gamma-aminobutyric acid (GABA) metabolism. Changes in these proteins and their regulatory pathways would not only cause GM neuronal dysfunction, but also disturbances in the ability of WM axons to convey impulses. In addition, alcoholism alters the expression of astrocyte and myelin proteins and of oligodendrocyte transcription factors important for the maintenance and plasticity of myelin sheaths in WM and GM. These changes are concomitant with epigenetic DNA and histone modifications as well as alterations in regulatory microRNAs (miRNAs) that likely cause profound disturbances of gene expression and protein translation. Knowledge is also available about interactions between astrocytes and oligodendrocytes not only at the Nodes of Ranvier (NR), but also in gap junction-based astrocyte-oligodendrocyte contacts and other forms of cell-to-cell communication now understood to be critical for the maintenance and formation of myelin. Close interactions between astrocytes and oligodendrocytes also suggest that therapies for alcoholism based on a specific glial cell type pathology will require a better understanding of molecular interactions between different cell types, as well as considering the possibility of using combined molecular approaches for more effective therapies.

H3K9ac and HDAC2 Activity Are Involved in the Expression of Monocarboxylate Transporter 1 in Oligodendrocyte

Recently, it is reported that monocarboxylate transporter 1 (MCT1) plays crucial role in oligodendrocyte differentiation and myelination. We found that MCT1 is strongly expressed in oligodendrocyte but weakly expressed in oligodendrocyte precursors (OPCs), and the underlying mechanisms remain elusive. Histone deacetylases (HDACs) activity is required for induction of oligodendrocyte differentiation and maturation. We asked whether HDACs are involved in the regulation of MCT1 expression. This work revealed that the acetylation level of histone H3K9 (H3K9ac) was much higher in mct1 gene (Slc16a1) promoter in OPCs than that in oligodendrocyte. H3K9ac regulates MCT1 expression was confirmed by HDAC acetyltransferase inhibitors trichostatin A and curcumin. Of note, there was a negative correlation between H3K9ac and MCT1 expression in oligodendrocyte. Further, we found that the levels of HDAC1, 2, and 3 protein in oligodendrocyte were obviously higher than those in OPCs. However, specific knockdown of HDAC2 but not HDAC1 and HDAC3 significantly decreased the expression of MCT1 in oligodendrocyte. Conversely, overexpression of HDAC2 remarkably enhanced the expression of MCT1. The results imply that HDAC2 is involved in H3K9ac modification which regulates the expression of MCT1 during the development of oligodendrocyte.

Emerging Role for Methylation in Multiple Sclerosis: Beyond DNA

Multiple Sclerosis (MS) is a chronic inflammatory disease of the central nervous system. The inflammatory and neurodegenerative pathways driving MS are modulated by DNA, lysine, and arginine methylation, as evidenced by studies made possible by novel tools for methylation detection or loss of function. We present evidence that MS is associated with genetic variants and metabolic changes that impact on methylation. Further, we comprehensively review current understanding of how methylation can impact on central nervous system (CNS) resilience and neuroregenerative potential, as well as inflammatory versus regulatory T helper (Th) cell balance. These findings are discussed in the context of therapeutic relevance for MS, with broad implications in other neurologic and immune-mediated diseases.

Histone deacetylase inhibition is cytotoxic to oligodendrocyte precursor cells in vitro and in vivo

Histone deacetylase (HDAC) inhibition mediated by small molecule HDAC inhibitors (HDACi) has demonstrated divergent effects including toxicity towards transformed cell lines, neuroprotection in neurological disease models, and inhibition of oligodendrocyte precursor cell (OPC) differentiation to mature oligodendrocytes (OL). However, it remains unknown if transient HDAC inhibition may promote OPC survival. Using mouse cortical OPC primary cultures, we investigated the effects of the FDA approved pan-HDACi suberoylanilide hydroxamic acid (SAHA) on OPC survival. Initial studies showed differences in the HDAC expression pattern of multiple HDAC isoforms in OPCs relative to their terminally differentiated progeny cells, OLs and astrocytes. Treatment of OPCs with SAHA for up to 72h using a maximum concentration either at or lower than those necessary for cytotoxicity in most transformed cell lines resulted in over 67% reduction in viability relative to vehicle-treated OPCs. This was at least partly due to increased apoptosis as SAHA-treated cells displayed activated caspase 3 and were protected by the general caspase inhibitor Q-VD-OPH. Additionally, SAHA treatment of whole mice at postnatal day 5 induced apoptosis of cortical OPCs. These results suggest that SAHA negatively impacts OPC survival and may be detrimental to the myelinating brain and spinal cord. Such toxicity may be relevant in a clinical context as SAHA is currently involved in numerous clinical trials and is in consideration for use in the treatment of psychiatric and neurodegenerative conditions.

Hdac3 Interaction with p300 Histone Acetyltransferase Regulates the Oligodendrocyte and Astrocyte Lineage Fate Switch

Establishment and maintenance of CNS glial cell identity ensures proper brain development and function, yet the epigenetic mechanisms underlying glial fate control remain poorly understood. Here, we show that the histone deacetylase Hdac3 controls oligodendrocyte-specification gene Olig2 expression and functions as a molecular switch for oligodendrocyte and astrocyte lineage determination. Hdac3 ablation leads to a significant increase of astrocytes with a concomitant loss of oligodendrocytes. Lineage tracing indicates that the ectopic astrocytes originate from oligodendrocyte progenitors. Genome-wide occupancy analysis reveals that Hdac3 interacts with p300 to activate oligodendroglial lineage-specific genes, while suppressing astroglial differentiation genes including NFIA. Furthermore, we find that Hdac3 modulates the acetylation state of Stat3 and competes with Stat3 for p300 binding to antagonize astrogliogenesis. Thus, our data suggest that Hdac3 cooperates with p300 to prime and maintain oligodendrocyte identity while inhibiting NFIA and Stat3-mediated astrogliogenesis, and thereby regulates phenotypic commitment at the point of oligodendrocyte-astrocytic fate decision.

How to make an oligodendrocyte

Oligodendrocytes produce myelin, an insulating sheath required for the saltatory conduction of electrical impulses along axons. Oligodendrocyte loss results in demyelination, which leads to impaired neurological function in a broad array of diseases ranging from pediatric leukodystrophies and cerebral palsy, to multiple sclerosis and white matter stroke. Accordingly, replacing lost oligodendrocytes, whether by transplanting oligodendrocyte progenitor cells (OPCs) or by mobilizing endogenous progenitors, holds great promise as a therapeutic strategy for the diseases of central white matter. In this Primer, we describe the molecular events regulating oligodendrocyte development and how our understanding of this process has led to the establishment of methods for producing OPCs and oligodendrocytes from embryonic stem cells and induced pluripotent stem cells, as well as directly from somatic cells. In addition, we will discuss the safety of engrafted stem cell-derived OPCs, as well as approaches by which to modulate their differentiation and myelinogenesis in vivo following transplantation.

p38 Mitogen-Activated Protein Kinase Pathway Regulates Genes during Proliferation and Differentiation in Oligodendrocytes

We have previously shown that p38 mitogen-activated protein kinase (p38 MAPK) is important for oligodendrocyte (OLG) differentiation and myelination. However, the precise cellular mechanisms by which p38 regulates OLG differentiation remain largely unknown. To determine whether p38 functions in part through transcriptional events in regulating OLG identity, we performed microarray analysis on differentiating oligodendrocyte progenitors (OLPs) treated with a p38 inhibitor. Consistent with a role in OLG differentiation, pharmacological inhibition of p38 down-regulated the transcription of genes that are involved in myelin biogenesis, transcriptional control and cell cycle. Proliferation assays showed that OLPs treated with the p38 inhibitor retained a proliferative capacity which could be induced upon application of mitogens demonstrating that after two days of p38-inhibition OLGs remained poised to continue mitosis. Together, our results suggest that the p38 pathway regulates gene transcription which can coordinate OLG differentiation. Our microarray dataset will provide a useful resource for future studies investigating the molecular mechanisms by which p38 regulates oligodendrocyte differentiation and myelination.

Regulatory RNAs and control of epigenetic mechanisms: expectations for cognition and cognitive dysfunction

The diverse functions of noncoding RNAs (ncRNAs) can influence virtually every aspect of the transcriptional process including epigenetic regulation of genes. In the CNS, regulatory RNA networks and epigenetic mechanisms have broad relevance to gene transcription changes involved in long-term memory formation and cognition. Thus, it is becoming increasingly clear that multiple classes of ncRNAs impact neuronal development, neuroplasticity, and cognition. Currently, a large gap exists in our knowledge of how ncRNAs facilitate epigenetic processes, and how this phenomenon affects cognitive function. In this review, we discuss recent findings highlighting a provocative role for ncRNAs including lncRNAs and piRNAs in the control of epigenetic mechanisms involved in cognitive function. Furthermore, we discuss the putative roles for these ncRNAs in cognitive disorders such as schizophrenia and Alzheimer's disease.

Dynamics of ten-eleven translocation hydroxylase family proteins and 5-hydroxymethylcytosine in oligodendrocyte differentiation

The ten-eleven translocation (TET) family of methylcytosine dioxygenases catalyze oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and promote DNA demethylation. Despite the abundance of 5hmC and TET proteins in the brain, little is known about their role in oligodendrocytes (OLs). Here, we analyzed TET expression during OL development in vivo and in vitro, and found that three TET family members possess unique subcellular and temporal expression patterns. Furthermore, the level of 5hmC exhibits dynamic changes during OL maturation, which implies that 5hmC modification may play a role in the expression of critical genes necessary for OL maturation. siRNA-mediated silencing of the TET family proteins in OLs demonstrated that each of the TET proteins is required for OL differentiation. However, based on their unique domain structures, we speculate that the three TET members may function by different mechanisms. In summary, we have established the temporal expression of TET proteins and the dynamic level of 5hmC during OL development and demonstrate that all three TET members are necessary for OL differentiation.

Polydendrocytes in development and myelin repair

Polydendrocytes (NG2 cells) are a distinct type of glia that populate the developing and adult central nervous systems (CNS). In the adult CNS, they retain mitotic activity and represent the largest proliferating cell population. Genetic and epigenetic mechanisms regulate the fate of polydendrocytes, which give rise to both oligodendrocytes and astrocytes. In addition, polydendrocytes actively differentiate into myelin-forming oligodendrocytes in response to demyelination. This review summarizes the current knowledge regarding polydendrocyte development, which provides an important basis for understanding the mechanisms that lead to the remyelination of demyelinated lesions.

Identification of conserved and novel microRNAs in cashmere goat skin by deep sequencing

MicroRNAs (miRNAs) are a class of small RNAs that play significant roles in regulating the expression of the post-transcriptional skin and hair follicle gene. In recent years, extensive studies on these microRNAs have been carried out in mammals such as mice, rats, pigs and cattle. By comparison, the number of microRNAs that have been identified in goats is relatively low; and in particular, the miRNAs associated with the processes of skin and hair follicle development remain largely unknown. In this study, areas of skin where the cashmere grows in anagen were sampled. A total of 10,943,292 reads were obtained using Solexa sequencing, a high-throughput sequencing technology. From 10,644,467 reads, we identified 3,381 distinct reads and after applying the classification statistics we obtained 316 miRNAs. Among them, using conservative identification, we found that 68 miRNAs (55 of these are confirmed to match known sheep and goat miRNAs in miRBase ) are conserved in goat and have been reported in NCBI; the remaining 248 miRNA were conserved in other species but have not been reported in goat. Furthermore, we identified 22 novel miRNAs. Both the known and novel miRNAs were confirmed by a second sequencing using the same method as was used in the first. This study confirmed the authenticity of 316 known miRNAs and the discovery of 22 novel miRNAs in goat. We found that the miRNAs that were co-expressed in goat and sheep were located in the same region of the respective chromosomes and may play an essential role in skin and follicle development. Identificaton of novel miRNAs resulted in significant enrichment of the repertoire of goat miRNAs.

Specification and maintenance of oligodendrocyte precursor cells from neural progenitor cells: involvement of microRNA-7a

A gain-of-function study showed that miR-7a promoted the generation of oligodendrocytes (OL) and retained the cells in their precursor stage. Inhibiting miR-7a reduced oligodendrogenesis but expanded neuronal population. miR-7a might exert these effects by repressing the expression of proneural genes and regulators for OL differentiation.

The generation of myelinating cells from multipotential neural stem cells in the CNS requires the initiation of specific gene expression programs in oligodendrocytes (OLs). We reasoned that microRNAs (miRNAs) could play an important role in this process by regulating genes crucial for OL development. Here we identified miR-7a as one of the highly enriched miRNAs in oligodendrocyte precursor cells (OPCs), overexpression of which in either neural progenitor cells (NPCs) or embryonic mouse cortex promoted the generation of OL lineage cells. Blocking the function of miR-7a in differentiating NPCs led to a reduction in OL number and an expansion of neuronal populations simultaneously. We also found that overexpression of this miRNA in purified OPC cultures promoted cell proliferation and inhibited further maturation. In addition, miR-7a might exert the effects just mentioned partially by directly repressing proneuronal differentiation factors including Pax6 and NeuroD4, or proOL genes involved in oligodendrocyte maturation. These results suggest that miRNA pathway is essential in determining cell fate commitment for OLs and thus providing a new strategy for modulating this process in OL loss diseases.

Neuroconnectivity and valproic acid: the myelin hypothesis

Neuropsychiatric medications that directly alter the epigenome, such as valproic acid, can under certain conditions reactivate critical developmental periods and thus impact adult neuroconnectivity. In animal models valproic acid was shown to inhibit the process of postnatal myelination and to replicate age-dependent decline in remyelination efficiency. The human central nervous system's myelination process, unlike that of non-human primates commonly used in the experimental models, is an intricate heterochronous process that continues well into adult life and which probably underlies later life neurocognitive changes and plasticity. Chronic exposure to valproic acid, especially in patients with epilepsy and neuropsychiatric disorders, may profoundly affect this process and its developmental trajectory. Further studies using novel MRI methods that allow in vivo mapping of myelination trajectories across the lifespan are urgently required to address the potential effects of valproic acid on brain development.

Gene expression changes within Müller glial cells in retinitis pigmentosa

PURPOSE

Retinitis pigmentosa (RP) is a progressive retinal degeneration in which the retina loses nearly all of its photoreceptor cells and undergoes major structural changes. Little is known regarding the role the resident glia, the Müller glia, play in the progression of the disease. In this article, we define gene expression changes in Müller glial cells (MGCs) from two different mouse models of RP, the retinal degeneration 1 (rd1) and rhodopsin knockout (Rhod-ko) models. The RNA repertoire of single MGCs was comprehensively profiled, and a comparison was made between MGCs from wild-type (WT) and mutant retinas. Two time points were chosen for analysis, one at the peak of rod photoreceptor death and one during the period of cone photoreceptor death.

METHODS

Retinas were dissociated, and single MGCs were chosen under a dissecting microscope using a micropipette. Single cell cDNAs were generated and genome-wide profiles were obtained by hybridization to Affymetrix arrays. A comparison was made among all samples to discover the changes in gene expression during the periods of rod and cone photoreceptor death.

RESULTS

MGCs respond to retinal degeneration by undergoing gliosis, a process marked by the upregulation of glial fibrillary acidic protein (Gfap). Many additional transcripts were found to change. These can be placed into functional clusters, such as retinal remodeling, stress response, and immune-related response.

CONCLUSIONS

A high degree of heterogeneity among the individual cells was observed, possibly due to their different spatial proximities to dying cells and/or inherent heterogeneity among MGCs.

MicroRNAs: novel regulators of oligodendrocyte differentiation and potential therapeutic targets in demyelination-related diseases

MicroRNAs (miRNAs or miRs) are a class of endogenous small non-coding RNAs that consist of about 22 nucleotides and play critical roles in various biological processes, including cell proliferation, differentiation, apoptosis, and tumorigenesis. In recent years, some specific miRNA, such as miR-219, miR-138, miR-9, miR-23, and miR-19b were found to participate in the regulation of oligodendrocyte (OL) differentiation and myelin maintenance, as well as in the pathogenesis of demyelination-related diseases (e.g., multiple sclerosis, ischemic stroke, and leukodystrophy). These miRNAs control their target mRNA or regulate the protein levels of some signaling pathways, and participate in OL differentiation and the pathogenesis of demyelination-related diseases. During pathologic processes, the expression levels of specific miRNAs are dynamically altered. Therefore, miRNAs act as diagnostic and prognostic indicators of defects in OL differentiation and demyelination-related diseases, and they can provide potential targets for therapeutic drug development.

Epigenetic epidemiology in psychiatry: A translational neuroscience perspective

Accumulating evidence from the field of neuroscience indicates a crucial role for epigenetic regulation of gene expression in development and aging of nervous system and suggests that aberrations in the epigenetic machinery are involved in the etiology of psychiatric disorders. Epidemiologic evidence on epigenetics in psychiatry, however, is currently very sparsely available, but is consistent with a mediating role for epigenetic mechanisms in bringing together inherited and acquired risk factors into a neurodevelopmental etiological model of psychiatric disorders. Here, we review evidence from the epidemiological and neuroscience literature, and aim to converge the evidence into an etiological model of psychiatric disorders that encompasses environmental, genetic and epigenetic contributions. Given the dynamic nature of the epigenetic machinery and the potential reversibility of epigenetic modifications, future well-designed interdisciplinary and translational studies will be of key importance in order to identify new targets for prevention and therapeutic strategies.

Type II arginine methyltransferase PRMT5 regulates gene expression of inhibitors of differentiation/DNA binding Id2 and Id4 during glial cell differentiation

PRMT5 is a type II protein arginine methyltranferase that catalyzes monomethylation and symmetric dimethylation of arginine residues. PRMT5 is functionally involved in a variety of biological processes including embryo development and circadian clock regulation. However, the role of PRMT5 in oligodendrocyte differentiation and central nervous system myelination is unknown. Here we show that PRMT5 expression gradually increases throughout postnatal brain development, coinciding with the period of active myelination. PRMT5 expression was observed in neurons, astrocytes, and oligodendrocytes. siRNA-mediated depletion of PRMT5 in mouse primary oligodendrocyte progenitor cells abrogated oligodendrocyte differentiation. In addition, the PRMT5-depleted oligodendrocyte progenitor and C6 glioma cells expressed high levels of the inhibitors of differentiation/DNA binding, Id2 and Id4, known repressors of glial cell differentiation. We observed that CpG-rich islands within the Id2 and Id4 genes were bound by PRMT5 and were hypomethylated in PRMT5-deficient cells, suggesting that PRMT5 plays a role in gene silencing during glial cell differentiation. Our findings define a role of PRMT5 in glial cell differentiation and link PRMT5 to epigenetic changes during oligodendrocyte differentiation.

Unwrapping myelination by microRNAs

Myelination of axons by oligodendrocytes and Schwann cells in the central and peripheral nervous system, respectively, is essential for normal neuronal functions, and its failure results in devastating demyelinating diseases. During development, both oligodendrocyte and Schwann cell precursors undergo a temporally well-defined series of molecular and structural changes, ultimately culminating in the cessation of proliferation and the elaboration of a highly complex myelin sheath. Recent studies have demonstrated a critical role of microRNAs (miRNAs) in the progression of oligodendrocyte and Schwann cell precursors to the myelinating state-depletion of miRNAs from either cell type results in an arrest in differentiation and lack of myelination. Furthermore, these studies have begun to elucidate the dynamic regulation of miRNA expression and the complexity of miRNA-mediated gene regulation during differentiation of myelinating cells. In this review, the authors highlight the recent understanding of functional links of miRNAs to regulatory networks for central and peripheral myelination, as well as perspectives on the role of miRNAs in demyelinating diseases.

Epigenetic modifiers are necessary but not sufficient for reprogramming non-myelinating cells into myelin gene-expressing cells

Background

Modifications on specific histone residues and DNA methylation play an essential role in lineage choice and cellular reprogramming. We have previously shown that histone modifications or combinatorial codes of transcription factors (TFs) are critical for the differentiation of multipotential progenitors into myelinating oligodendrocytes. In this study we asked whether combining global manipulation of DNA methylation and histone acetylation together with the expression of oligodendrocyte- specific TFs, was sufficient to switch the identity of fibroblasts into myelin gene-expressing cells.

Methodology/Principal Findings

Transfection of six oligodendrocyte-specific TFs (Olig1, Olig2, Sox10, Mash1, E47 and Nkx2.2) into NIH3T3 fibroblasts was capable of inducing expression of myelin gene promoter-driven reporters, but did not activate endogenous myelin gene expression. These results suggested the existence of a transcriptionally incompetent chromatin conformation in NIH3T3 fibroblasts. Using chromatin immunoprecipitation (ChIP) analysis, we compared the histone code on the conserved regions of myelin genes (i.e. Mbp and Mag) in differentiating oligodendrocyte progenitors and NIH3T3 fibroblasts. Chromatin at myelin gene loci was characterized by the presence of repressive histone modifications (me3K9H3 and me3K27H3) in NIH3T3 fibroblasts and active histone marks (me3K4H3 and AcH3) in oligodendrocyte lineage cells. To induce a transcriptionally competent chromatin signature, NIH3T3 fibroblasts were treated with 5-azadeoxy-citidine (5-AzaC) to decrease DNA methylation, and trichostatin A (TSA) or sirtinol, to favor histone acetylation. Treatment with 5-AzaC/TSA but not sirtinol, resulted in the detection of endogenous myelin gene transcripts in fibroblasts, although not to the levels detected in myelinating cells. Transfection of oligodendrocyte-specific TFs after 5-AzaC/TSA treatment did not further increase myelin gene expression, nor did it reprogram the transcriptional network of NIH3T3 fibroblasts into that of oligodendrocytes.

Conclusions/Significance

These results suggest that reprogramming of fibroblasts into myelin gene-expressing cells not only requires transcriptional activation, but also chromatin manipulations that go beyond histone acetylation and DNA methylation.