The Chromatin Environment Around Interneuron Genes in Oligodendrocyte Precursor Cells and Their Potential for Interneuron Reprograming

Histone H4 acetylation differentially modulates proliferation in adult oligodendrocyte progenitors

Adult oligodendrocyte progenitors (aOPCs) generate myelinating oligodendrocytes like neonatal progenitors (nOPCs), and they also display unique functional features. Here, using unbiased histone proteomics analysis and ChIP sequencing analysis of PDGFRα+ OPCs sorted from neonatal and adult Pdgfra-H2B-EGFP reporter mice, we identify the activating H4K8ac histone mark as enriched in the aOPCs. We detect increased occupancy of the H4K8ac activating mark at chromatin locations corresponding to genes related to the progenitor state (e.g., Hes5, Gpr17), metabolic processes (e.g., Txnip, Ptdgs), and myelin components (e.g., Cnp, Mog). aOPCs showed higher levels of transcripts related to lipid metabolism and myelin, and lower levels of transcripts related to cell cycle and proliferation compared with nOPCs. In addition, pharmacological inhibition of histone acetylation decreased the expression of the H4K8ac target genes in aOPCs and decreased their proliferation. Overall, this study identifies acetylation of the histone H4K8 as a regulator of the proliferative capacity of aOPCs.

PDGF-BB overexpression in p53 null oligodendrocyte progenitors increases H3K27me3 and induces transcriptional changes which favor proliferation

Proneural gliomas are brain tumors characterized by enrichment of oligodendrocyte progenitor cell (OPC) transcripts and genetic alterations. In this study we sought to identify transcriptional and epigenetic differences between OPCs with Trp53 deletion and PDGF-BB overexpression (BB-p53n) and those carrying only p53 deletion (p53n). In culture, the BB-p53n OPCs display growth characteristics more similar to glioma cells than p53n OPCs. When injected in mouse brains, BB-p53n OPC form tumors, while the p53n OPCs do not. Unbiased histone proteomics and transcriptomic analysis on these OPC populations identified higher levels of the histone H3K27me3 mark and lower levels of the histone H4K20me3. The transcriptome of the BB-p53n OPCs was characterized by higher levels of transcripts related to proliferation and cell adhesion compared to p53n OPCs. Pharmacological inhibition of the enzyme responsible for histone H3K27 trimethylation (EZH2i) in BB-p53n OPCs, reduced cell cycle transcripts and increased the expression of differentiation markers, but was not sufficient to restore their growth characteristics. This suggests that PDGF-BB overexpression in p53n OPCs favors the early stages of transformation, by promoting proliferation and halting differentiation in a H3K27me3-dependent pathway, and favoring growth characteristics in a H3K27me3 independent manner.

The epigenetic landscape of oligodendrocyte progenitors changes with time

SUMMARY Dansu et al. identify distinct histone H4 modifications as potential mechanism underlying the functional differences between adult and neonatal progenitors. While H4K8ac favors the expression of differentiation genes, their expression is halted by H4K20me3. Adult oligodendrocyte progenitors (aOPCs) generate myelinating oligodendrocytes, like neonatal progenitors (nOPCs), but they also display unique functional features. Here, using RNA-sequencing, unbiased histone proteomics analysis and ChIP-sequencing, we define the transcripts and histone marks underlying the unique properties of aOPCs. We describe the lower proliferative capacity and higher levels of expression of oligodendrocyte specific genes in aOPCs compared to nOPCs, as well as the greater levels of H4 histone marks. We also report increased occupancy of the H4K8ac mark at chromatin locations corresponding to oligodendrocyte-specific transcription factors and lipid metabolism genes. Pharmacological inhibition of H4K8ac deposition reduces the levels of these transcripts in aOPCs, rendering their transcriptome more similar to nOPCs. The repressive H4K20me3 mark is also higher in aOPCs compared to nOPCs and pharmacological inhibition of its deposition results in increased levels of genes related to the mature oligodendrocyte state. Overall, this study identifies two histone marks which are important for the unique transcriptional and functional identity of aOPCs.

The stability of the myelinating oligodendrocyte transcriptome is regulated by the nuclear lamina

Oligodendrocytes are specialized cells that insulate and support axons with their myelin membrane, allowing proper brain function. Here, we identify lamin A/C (LMNA/C) as essential for transcriptional and functional stability of myelinating oligodendrocytes. We show that LMNA/C levels increase with differentiation of progenitors and that loss of Lmna in differentiated oligodendrocytes profoundly alters their chromatin accessibility and transcriptional signature. Lmna deletion in myelinating glia is compatible with normal developmental myelination. However, altered chromatin accessibility is detected in fully differentiated oligodendrocytes together with increased expression of progenitor genes and decreased levels of lipid-related transcription factors and inner mitochondrial membrane transcripts. These changes are accompanied by altered brain metabolism, lower levels of myelin-related lipids, and altered mitochondrial structure in oligodendrocytes, thereby resulting in myelin thinning and the development of a progressively worsening motor phenotype. Overall, our data identify LMNA/C as essential for maintaining the transcriptional and functional stability of myelinating oligodendrocytes.

Molecular and functional heterogeneity in dorsal and ventral oligodendrocyte progenitor cells of the mouse forebrain in response to DNA damage

In the developing mouse forebrain, temporally distinct waves of oligodendrocyte progenitor cells (OPCs) arise from different germinal zones and eventually populate either dorsal or ventral regions, where they present as transcriptionally and functionally equivalent cells. Despite that, developmental heterogeneity influences adult OPC responses upon demyelination. Here we show that accumulation of DNA damage due to ablation of citron-kinase or cisplatin treatment cell-autonomously disrupts OPC fate, resulting in cell death and senescence in the dorsal and ventral subsets, respectively. Such alternative fates are associated with distinct developmental origins of OPCs, and with a different activation of NRF2-mediated anti-oxidant responses. These data indicate that, upon injury, dorsal and ventral OPC subsets show functional and molecular diversity that can make them differentially vulnerable to pathological conditions associated with DNA damage.

Cellular identity through the lens of direct lineage reprogramming

Direct lineage reprogramming challenges our traditional view on basic aspects of cellular identity, and in particular on processes crucial for identity acquisition. This is partly because in direct lineage reprogramming but not during natural differentiation processes changing cellular identity can occur in the absence of mitosis. Only recently, technologies emerged to deconstruct the cellular and molecular processes governing the transitory states a cell passes through on the journey from its original identity to the new target cell fate. Here we discuss arising concepts on the nature of these transitory states and the challenges and decisions cells must conquer to reach their new cellular identity.

Life-long oligodendrocyte development and plasticity

Oligodendrocyte precursor cells (OPCs) originate in localized germinal zones in the embryonic neural tube, then migrate and proliferate to populate the entire central nervous system, both white and gray matter. They divide and generate myelinating oligodendrocytes (OLs) throughout postnatal and adult life. OPCs express NG2 and platelet-derived growth factor receptor alpha subunit (PDGFRα), two functionally important cell surface proteins, which are also widely used as markers for OPCs. The proliferation of OPCs, their terminal differentiation into OLs, survival of new OLs, and myelin synthesis are orchestrated by signals in the local microenvironment. We discuss advances in our mechanistic understanding of paracrine effects, including those mediated through PDGFRα and neuronal activity-dependent signals such as those mediated through AMPA receptors in OL survival and myelination. Finally, we review recent studies supporting the role of new OL production and “adaptive myelination” in specific behaviours and cognitive processes contributing to learning and long-term memory formation. Our article is not intended to be comprehensive but reflects the authors’ past and present interests.

Direct reprogramming of oligodendrocyte precursor cells into GABAergic inhibitory neurons by a single homeodomain transcription factor Dlx2

Oligodendrocyte precursor cells (NG2 glia) are uniformly distributed proliferative cells in the mammalian central nervous system and generate myelinating oligodendrocytes throughout life. A subpopulation of OPCs in the neocortex arises from progenitor cells in the embryonic ganglionic eminences that also produce inhibitory neurons. The neuronal fate of some progenitor cells is sealed before birth as they become committed to the oligodendrocyte lineage, marked by sustained expression of the oligodendrocyte transcription factor Olig2, which represses the interneuron transcription factor Dlx2. Here we show that misexpression of Dlx2 alone in postnatal mouse OPCs caused them to switch their fate to GABAergic neurons within 2 days by downregulating Olig2 and upregulating a network of inhibitory neuron transcripts. After two weeks, some OPC-derived neurons generated trains of action potentials and formed clusters of GABAergic synaptic proteins. Our study revealed that the developmental molecular logic can be applied to promote neuronal reprogramming from OPCs.

Epigenetic regulation of oligodendrocyte differentiation: From development to demyelinating disorders

The maintenance of progenitor states or the differentiation of progenitors into specific lineages requires epigenetic remodeling of the gene expression program. In the central nervous system, oligodendrocyte progenitors (OPCs) give rise to oligodendrocytes (OLs), whose main function has been thought to be to produce myelin, a lipid-rich structure insulating the axons. However, recent findings suggest diverse OL transcriptional states, which might imply additional functions. The differentiation of OPCs into postmitotic OLs is a highly regulated and sensitive process and requires temporal waves of gene expression through epigenetic remodeling of the genome. In this review, we will discuss recent advances in understanding the events shaping the chromatin landscape through histone modifications and long noncoding RNAs during OPC differentiation, in physiological and pathological conditions. We suggest that epigenetic regulation plays a fundamental role in governing the accessibility of transcriptional machinery to DNA sequences, which ultimately determines functional outcomes in OLs.

The cerebral cortex is a substrate of multiple interactions between GABAergic interneurons and oligodendrocyte lineage cells

In the cerebral cortex, GABAergic interneurons and oligodendrocyte lineage cells share different characteristics and interact despite being neurons and glial cells, respectively. These two distinct cell types share common embryonic origins and are born from precursors expressing similar transcription factors. Moreover, they highly interact with each other through different communication mechanisms during development. Notably, cortical oligodendrocyte precursor cells (OPCs) receive a major and transient GABAergic synaptic input, preferentially from parvalbumin-expressing interneurons, a specific interneuron subtype recently recognized as highly myelinated. In this review, we highlight the similarities and interactions between GABAergic interneurons and oligodendrocyte lineage cells in the cerebral cortex and suggest potential roles of this intimate interneuron-oligodendroglia relationship in cortical construction. We also propose new lines of research to understand the role of the close link between interneurons and oligodendroglia during cortical development and in pathological conditions such as schizophrenia.

The effects of developmental and current niches on oligodendrocyte precursor dynamics and fate

Oligodendrocyte precursor cells (OPCs), whose primary function is to generate myelinating oligodendrocytes, are distributed widely throughout the developing and mature central nervous system. They originate from several defined subdomains in the embryonic germinal zones at different developmental stages and in the adult. While many phenotypic differences have been observed among OPCs in different anatomical regions and among those arising from different germinal zones, we know relatively little about the molecular and cellular mechanisms by which the historical and current niches shape the behavior of oligodendrocyte lineage cells. This minireview will discuss how the behavior of oligodendrocyte lineage cells is influenced by the developmental niches from which subpopulations of OPCs emerge, by the current niches surrounding OPCs in different regions, and in pathological states that cause deviations from the normal density of oligodendrocyte lineage cells and myelin.