PRMT5-mediated regulation of developmental myelination

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.

A New Acquaintance of Oligodendrocyte Precursor Cells in the Central Nervous System

Oligodendrocyte precursor cells (OPCs) are a heterogeneous multipotent population in the central nervous system (CNS) that appear during embryogenesis and persist as resident cells in the adult brain parenchyma. OPCs could generate oligodendrocytes to participate in myelination. Recent advances have renewed our knowledge of OPC biology by discovering novel markers of oligodendroglial cells, the myelin-independent roles of OPCs, and the regulatory mechanism of OPC development. In this review, we will explore the updated knowledge on OPC identity, their multifaceted roles in the CNS in health and diseases, as well as the regulatory mechanisms that are involved in their developmental stages, which hopefully would contribute to a further understanding of OPCs and attract attention in the field of OPC biology.

SAH is a major metabolic sensor mediating worsening metabolic crosstalk in metabolic syndrome

In this study, we observed worsening metabolic crosstalk in mouse models with concomitant metabolic disorders such as hyperhomocysteinemia (HHcy), hyperlipidemia, and hyperglycemia and in human coronary artery disease by analyzing metabolic profiles. We found that HHcy worsening is most sensitive to other metabolic disorders. To identify metabolic genes and metabolites responsible for the worsening metabolic crosstalk, we examined mRNA levels of 324 metabolic genes in Hcy, glucose-related and lipid metabolic systems. We examined Hcy-metabolites (Hcy, SAH and SAM) by LS-ESI-MS/MS in 6 organs (heart, liver, brain, lung, spleen, and kidney) from C57BL/6J mice. Through linear regression analysis of Hcy-metabolites and metabolic gene mRNA levels, we discovered that SAH-responsive genes were responsible for most metabolic changes and all metabolic crosstalk mediated by Serine, Taurine, and G3P. SAH-responsive genes worsen glucose metabolism and cause upper glycolysis activation and lower glycolysis suppression, indicative of the accumulation of glucose/glycogen and G3P, Serine synthesis inhibition, and ATP depletion. Insufficient Serine due to negative correlation of PHGDH with SAH concentration may inhibit the folate cycle and transsulfurarion pathway and consequential reduced antioxidant power, including glutathione, taurine, NADPH, and NAD+. Additionally, we identified SAH-activated pathological TG loop as the consequence of increased fatty acid (FA) uptake, FA β-oxidation and Ac-CoA production along with lysosomal damage. We concluded that HHcy is most responsive to other metabolic changes in concomitant metabolic disorders and mediates worsening metabolic crosstalk mainly via SAH-responsive genes, that organ-specific Hcy metabolism determines organ-specific worsening metabolic reprogramming, and that SAH, acetyl-CoA, Serine and Taurine are critical metabolites mediating worsening metabolic crosstalk, redox disturbance, hypomethylation and hyperacetylation linking worsening metabolic reprogramming in metabolic syndrome.

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), which form tumors when transplanted in mouse brains, and those carrying only p53 deletion (p53n), which do not. We used unbiased histone proteomics and RNA-seq analysis on these two genetically modified OPC populations and detected higher levels of H3K27me3 in BB-p53n compared to p53n OPCs. The BB-p53n OPC were characterized by higher levels of transcripts related to proliferation and lower levels of those related to differentiation. Pharmacological inhibition of histone H3K27 trimethylation in BB-p53n OPC reduced cell cycle transcripts and increased the expression of differentiation markers. These data suggest that PDGF-BB overexpression in p53 null OPC results in histone post-translational modifications and consequent transcriptional changes favoring proliferation while halting differentiation, thereby promoting the early stages of transformation.

Protein Arginine Methyltransferase 5 Contributes to Paclitaxel-Induced Neuropathic Pain by Activating Transient Receptor Potential Vanilloid 1 Epigenetic Modification in Dorsal Root Ganglion

BACKGROUND

Paclitaxel (PTX), which is a first-line chemotherapy drug used to treat various types of cancers, exhibits peripheral neuropathy as a common side effect that is difficult to treat. Protein arginine methyltransferase 5 (PRMT 5) is a key regulator of the chemotherapy response, as chemotherapy drugs induce PRMT5 expression. However, little is known about the PRMT5-mediated epigenetic mechanisms involved in PTX-induced neuropathic allodynia.

METHODS

Sprague-Dawley rats were intraperitoneally given PTX to induce neuropathic pain. Biochemical analyses were conducted to measure the protein expression levels in the dorsal root ganglion (DRG) of the animals. The von Frey test and hot plate test were used to evaluate nociceptive behaviors.

RESULTS

PTX increased the PRMT5 (mean difference [MD]: 0.68, 95% confidence interval [CI], 0.88-0.48; P < .001 for vehicle)-mediated deposition of histone H3R2 dimethyl symmetric (H3R2me2s) at the transient receptor potential vanilloid 1 ( Trpv1 ) promoter in the DRG. PRMT5-induced H3R2me2s recruited WD repeat domain 5 (WDR5) to increase trimethylation of lysine 4 on histone H3 (H3K4me3) at Trpv1 promoters, thus resulting in TRPV1 transcriptional activation (MD: 0.65, 95% CI, 0.82-0.49; P < .001 for vehicle) in DRG in PTX-induced neuropathic pain. Moreover, PTX increased the activity of NADPH oxidase 4 (NOX4) (MD: 0.66, 95% CI, 0.81-0.51; P < .001 for vehicle), PRMT5-induced H3R2me2s, and WDR5-mediated H3K4me3 in the DRG in PTX-induced neuropathic pain. Pharmacological antagonism and the selective knockdown of PRMT5 in DRG neurons completely blocked PRMT5-mediated H3R2me2s, WDR5-mediated H3K4me3, or TRPV1 expression and neuropathic pain development after PTX injection. Remarkably, NOX4 inhibition not only attenuated allodynia behavior and reversed the above-mentioned signaling but also reversed NOX4 upregulation via PTX.

CONCLUSIONS

Thus, the NOX4/PRMT5-associated epigenetic mechanism in DRG has a dominant function in the transcriptional activation of TRPV1 in PTX-induced neuropathic pain.

PRMT5-mediated homologous recombination repair is essential to maintain genomic integrity of neural progenitor cells

Maintaining genomic stability is a prerequisite for proliferating NPCs to ensure genetic fidelity. Though histone arginine methylation has been shown to play important roles in safeguarding genomic stability, the underlying mechanism during brain development is not fully understood. Protein arginine N-methyltransferase 5 (PRMT5) is a type II protein arginine methyltransferase that plays a role in transcriptional regulation. Here, we identify PRMT5 as a key regulator of DNA repair in response to double-strand breaks (DSBs) during NPC proliferation. Prmt5F/F; Emx1-Cre (cKO-Emx1) mice show a distinctive microcephaly phenotype, with partial loss of the dorsal medial cerebral cortex and complete loss of the corpus callosum and hippocampus. This phenotype is resulted from DSBs accumulation in the medial dorsal cortex followed by cell apoptosis. Both RNA sequencing and in vitro DNA repair analyses reveal that PRMT5 is required for DNA homologous recombination (HR) repair. PRMT5 specifically catalyzes H3R2me2s in proliferating NPCs in the developing mouse brain to enhance HR-related gene expression during DNA repair. Finally, overexpression of BRCA1 significantly rescues DSBs accumulation and cell apoptosis in PRMT5-deficient NSCs. Taken together, our results show that PRMT5 maintains genomic stability by regulating histone arginine methylation in proliferating NPCs.

DOT1L stimulates MYC/Mondo transcription factor activity by promoting its degradation cycle on chromatin

The proto-oncogene c-MYC is a key representative of the MYC transcription factor network regulating growth and metabolism. MML-1 (Myc- and Mondo-like) is its homolog in C. elegans. The functional and molecular cooperation between c-MYC and H3 lysine 79 methyltransferase DOT1L was demonstrated in several human cancer types, and we have earlier discovered the connection between C. elegans MML-1 and DOT-1.1. Here, we demonstrate the critical role of DOT1L/DOT-1.1 in regulating c-MYC/MML-1 target genes genome-wide by ensuring the removal of "spent" transcription factors from chromatin by the nuclear proteasome. Moreover, we uncover a previously unrecognized proteolytic activity of DOT1L, which may facilitate c-MYC turnover. This new mechanism of c-MYC regulation by DOT1L may lead to the development of new approaches for cancer treatment.

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.

EZH2 Inhibition Sensitizes IDH1R132H-Mutant Gliomas to Histone Deacetylase Inhibitor

Isocitrate Dehydrogenase-1 (IDH1) is commonly mutated in lower-grade diffuse gliomas. The IDH1R132H mutation is an important diagnostic tool for tumor diagnosis and prognosis; however, its role in glioma development, and its impact on response to therapy, is not fully understood. We developed a murine model of proneural IDH1R132H-mutated glioma that shows elevated production of 2-hydroxyglutarate (2-HG) and increased trimethylation of lysine residue K27 on histone H3 (H3K27me3) compared to IDH1 wild-type tumors. We found that using Tazemetostat to inhibit the methyltransferase for H3K27, Enhancer of Zeste 2 (EZH2), reduced H3K27me3 levels and increased acetylation on H3K27. We also found that, although the histone deacetylase inhibitor (HDACi) Panobinostat was less cytotoxic in IDH1R132H-mutated cells (either isolated from murine glioma or oligodendrocyte progenitor cells infected in vitro with a retrovirus expressing IDH1R132H) compared to IDH1-wild-type cells, combination treatment with Tazemetostat is synergistic in both mutant and wild-type models. These findings indicate a novel therapeutic strategy for IDH1-mutated gliomas that targets the specific epigenetic alteration in these tumors.

PRMT5 inhibition shows in vitro efficacy against H3K27M-altered diffuse midline glioma, but does not extend survival in vivo

H3K27-altered Diffuse Midline Glioma (DMG) is a universally fatal paediatric brainstem tumour. The prevalent driver mutation H3K27M creates a unique epigenetic landscape that may also establish therapeutic vulnerabilities to epigenetic inhibitors. However, while HDAC, EZH2 and BET inhibitors have proven somewhat effective in pre-clinical models, none have translated into clinical benefit due to either poor blood-brain barrier penetration, lack of efficacy or toxicity. Thus, there remains an urgent need for new DMG treatments. Here, we performed wider screening of an epigenetic inhibitor library and identified inhibitors of protein arginine methyltransferases (PRMTs) among the top hits reducing DMG cell viability. Two of the most effective inhibitors, LLY-283 and GSK591, were targeted against PRMT5 using distinct binding mechanisms and reduced the viability of a subset of DMG cells expressing wild-type TP53 and mutant ACVR1. RNA-sequencing and phenotypic analyses revealed that LLY-283 could reduce the viability, clonogenicity and invasion of DMG cells in vitro, representing three clinically important phenotypes, but failed to prolong survival in an orthotopic xenograft model. Together, these data show the challenges of DMG treatment and highlight PRMT5 inhibitors for consideration in future studies of combination treatments.

PRMT5 regulates the polysaccharide content by controlling the splicing of thaumatin-like protein in Ganoderma lucidum

IMPORTANCE

PRMT5 contributes to secondary metabolite biosynthesis in Ganoderma lucidum. However, the mechanism through which PRMT5 regulates the biosynthesis of secondary metabolites remains unclear. In the current study, PRMT5 silencing led to a significant decrease in the biosynthesis of polysaccharides from G. lucidum through the action of the alternative splicing of TLP. A shorter TLP2 isoform can directly bind to PGI and regulated polysaccharide biosynthesis. These results suggest that PRMT5 enhances PGI activity by regulating TLP binding to PGI. The results of the current study reveal a novel target gene for PRMT5-mediated alternative splicing and provide a reference for the identification of PRMT5 regulatory target genes.

The arginine methyltransferase Prmt1 coordinates the germline arginine methylome essential for spermatogonial homeostasis and male fertility

Arginine methylation, catalyzed by the protein arginine methyltransferases (PRMTs), is a common post-translational protein modification (PTM) that is engaged in a plethora of biological events. However, little is known about how the methylarginine-directed signaling functions in germline development. In this study, we discover that Prmt1 is predominantly distributed in the nuclei of spermatogonia but weakly in the spermatocytes throughout mouse spermatogenesis. By exploiting a combination of three Cre-mediated Prmt1 knockout mouse lines, we unravel that Prmt1 is essential for spermatogonial establishment and maintenance, and that Prmt1-catalyzed asymmetric methylarginine coordinates inherent transcriptional homeostasis within spermatogonial cells. In conjunction with high-throughput CUT&Tag profiling and modified mini-bulk Smart-seq2 analyses, we unveil that the Prmt1-deposited H4R3me2a mark is permissively enriched at promoter and exon/intron regions, and sculpts a distinctive transcriptomic landscape as well as the alternative splicing pattern, in the mouse spermatogonia. Collectively, our study provides the genetic and mechanistic evidence that connects the Prmt1-deposited methylarginine signaling to the establishment and maintenance of a high-fidelity transcriptomic identity in orchestrating spermatogonial development in the mammalian germline.

Protein arginine methylation in viral infection and antiviral immunity

Protein arginine methyltransferase (PRMT)-mediated arginine methylation is an important post-transcriptional modification that regulates various cellular processes including epigenetic gene regulation, genome stability maintenance, RNA metabolism, and stress-responsive signal transduction. The varying substrates and biological functions of arginine methylation in cancer and neurological diseases have been extensively discussed, providing a rationale for targeting PRMTs in clinical applications. An increasing number of studies have demonstrated an interplay between arginine methylation and viral infections. PRMTs have been found to methylate and regulate several host cell proteins and different functional types of viral proteins, such as viral capsids, mRNA exporters, transcription factors, and latency regulators. This modulation affects their activity, subcellular localization, protein-nucleic acid and protein-protein interactions, ultimately impacting their roles in various virus-associated processes. In this review, we discuss the classification, structure, and regulation of PRMTs and their pleiotropic biological functions through the methylation of histones and non-histones. Additionally, we summarize the broad spectrum of PRMT substrates and explore their intricate effects on various viral infection processes and antiviral innate immunity. Thus, comprehending the regulation of arginine methylation provides a critical foundation for understanding the pathogenesis of viral diseases and uncovering opportunities for antiviral therapy.

Critical Roles of Protein Arginine Methylation in the Central Nervous System

A remarkable post-transitional modification of both histones and non-histone proteins is arginine methylation. Methylation of arginine residues is crucial for a wide range of cellular process, including signal transduction, DNA repair, gene expression, mRNA splicing, and protein interaction. Arginine methylation is modulated by arginine methyltransferases and demethylases, like protein arginine methyltransferase (PRMTs) and Jumonji C (JmjC) domain containing (JMJD) proteins. Symmetric dimethylarginine and asymmetric dimethylarginine, metabolic products of the PRMTs and JMJD proteins, can be changed by abnormal expression of these proteins. Many pathologies including cancer, inflammation and immune responses have been closely linked to aberrant arginine methylation. Currently, the majority of the literature discusses the substrate specificity and function of arginine methylation in the pathogenesis and prognosis of cancers. Numerous investigations on the roles of arginine methylation in the central nervous system (CNS) have so far been conducted. In this review, we display the biochemistry of arginine methylation and provide an overview of the regulatory mechanism of arginine methyltransferases and demethylases. We also highlight physiological functions of arginine methylation in the CNS and the significance of arginine methylation in a variety of neurological diseases such as brain cancers, neurodegenerative diseases and neurodevelopmental disorders. Furthermore, we summarize PRMT inhibitors and molecular functions of arginine methylation. Finally, we pose important questions that require further research to comprehend the roles of arginine methylation in the CNS and discover more effective targets for the treatment of neurological diseases.

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.

Deficiency of MicroRNA-23-27-24 Clusters Exhibits the Impairment of Myelination in the Central Nervous System

Several microRNAs (miRNAs), including miR-23 and miR-27a have been reportedly involved in regulating myelination in the central nervous system. Although miR-23 and miR-27a form clusters in vivo and the clustered miRNAs are known to perform complementary functions, the role of these miRNA clusters in myelination has not been studied. To investigate the role of miR-23-27-24 clusters in myelination, we generated miR-23-27-24 cluster knockout mice and evaluated myelination in the brain and spinal cord. Our results showed that 10-week-old knockout mice had reduced motor function in the hanging wire test compared to the wild-type mice. At 4 weeks, 10 weeks, and 12 months of age, knockout mice showed reduced myelination compared to wild-type mice. The expression levels of myelin basic protein and myelin proteolipid protein were also significantly lower in the knockout mice compared to the wild-type mice. Although differentiation of oligodendrocyte progenitor cells to oligodendrocytes was not inhibited in the knockout mice, the percentage of oligodendrocytes expressing myelin basic protein was significantly lower in 4-week-old knockout mice than that in wild-type mice. Proteome analysis and western blotting showed increased expression of leucine-zipper-like transcription regulator 1 (LZTR1) and decreased expression of R-RAS and phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in the knockout mice. In summary, loss of miR-23-27-24 clusters reduces myelination and compromises motor functions in mice. Further, LZTR1, which regulates R-RAS upstream of the ERK1/2 pathway, a signal that promotes myelination, has been identified as a novel target of the miR-23-27-24 cluster in this study.

The Role of PRMT5 in Immuno-Oncology

Immune checkpoint inhibitor (ICI) therapy has caused a paradigm shift in cancer therapeutic strategy. However, this therapy only benefits a subset of patients. The difference in responses to ICIs is believed to be dependent on cancer type and its tumor microenvironment (TME). The TME is favorable for cancer progression and metastasis and can also help cancer cells to evade immune attacks. To improve the response to ICIs, it is crucial to understand the mechanism of how the TME is maintained. Protein arginine methyltransferase 5 (PRMT5) di-methylates arginine residues in its substrates and has essential roles in the epigenetic regulation of gene expression, signal transduction, and the fidelity of mRNA splicing. Through these functions, PRMT5 can support cancer cell immune evasion. PRMT5 is necessary for regulatory T cell (Treg) functions and promotes cancer stemness and the epithelial–mesenchymal transition. Specific factors in the TME can help recruit Tregs, tumor-associated macrophages, and myeloid-derived suppressor cells into tumors. In addition, PRMT5 suppresses antigen presentation and the production of interferon and chemokines, which are necessary to recruit T cells into tumors. Overall, PRMT5 supports an immunosuppressive TME. Therefore, PRMT5 inhibition would help recover the immune cycle and enable the immune system-mediated elimination of cancer cells.

Coordinated Regulation of Myelination by Growth Factor and Amino-acid Signaling Pathways

Myelin-forming oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) are essential for structural and functional homeostasis of nervous tissue. Albeit with certain similarities, the regulation of CNS and PNS myelination is executed differently. Recent advances highlight the coordinated regulation of oligodendrocyte myelination by amino-acid sensing and growth factor signaling pathways. In this review, we discuss novel insights into the understanding of differential regulation of oligodendrocyte and Schwann cell biology in CNS and PNS myelination, with particular focus on the roles of growth factor-stimulated RHEB-mTORC1 and GATOR2-mediated amino-acid sensing/signaling pathways. We also discuss recent progress on the metabolic regulation of oligodendrocytes and Schwann cells and the impact of their dysfunction on neuronal function and disease.

The epigenetic landscape of oligodendrocyte lineage cells

The epigenetic landscape of oligodendrocyte lineage cells refers to the cell-specific modifications of DNA, chromatin, and RNA that define a unique gene expression pattern of functionally specialized cells. Here, we focus on the epigenetic changes occurring as progenitors differentiate into myelin-forming cells and respond to the local environment. First, modifications of DNA, RNA, nucleosomal histones, key principles of chromatin organization, topologically associating domains, and local remodeling will be reviewed. Then, the relationship between epigenetic modulators and RNA processing will be explored. Finally, the reciprocal relationship between the epigenome as a determinant of the mechanical properties of cell nuclei and the target of mechanotransduction will be discussed. The overall goal is to provide an interpretative key on how epigenetic changes may account for the heterogeneity of the transcriptional profiles identified in this lineage.

A New Insight into MYC Action: Control of RNA Polymerase II Methylation and Transcription Termination

MYC oncoprotein deregulation is a common catastrophic event in human cancer and limiting its activity restrains tumor development and maintenance, as clearly shown via Omomyc, an MYC-interfering 90 amino acid mini-protein. MYC is a multifunctional transcription factor that regulates many aspects of transcription by RNA polymerase II (RNAPII), such as transcription activation, pause release, and elongation. MYC directly associates with Protein Arginine Methyltransferase 5 (PRMT5), a protein that methylates a variety of targets, including RNAPII at the arginine residue R1810 (R1810me2s), crucial for proper transcription termination and splicing of transcripts. Therefore, we asked whether MYC controls termination as well, by affecting R1810me2S. We show that MYC overexpression strongly increases R1810me2s, while Omomyc, an MYC shRNA, or a PRMT5 inhibitor and siRNA counteract this phenomenon. Omomyc also impairs Serine 2 phosphorylation in the RNAPII carboxyterminal domain, a modification that sustains transcription elongation. ChIP-seq experiments show that Omomyc replaces MYC and reshapes RNAPII distribution, increasing occupancy at promoter and termination sites. It is unclear how this may affect gene expression. Transcriptomic analysis shows that transcripts pivotal to key signaling pathways are both up- or down-regulated by Omomyc, whereas genes directly controlled by MYC and belonging to a specific signature are strongly down-regulated. Overall, our data point to an MYC/PRMT5/RNAPII axis that controls termination via RNAPII symmetrical dimethylation and contributes to rewiring the expression of genes altered by MYC overexpression in cancer cells. It remains to be clarified which role this may have in tumor development.

PRMT1 is required for the generation of MHC-associated microglia and remyelination in the central nervous system

PRMT1 regulates MHC-associated microglia cluster during de/remyelination.

Remyelination failure in multiple sclerosis leads to progressive demyelination and inflammation, resulting in neurodegeneration and clinical decline. Microglia are innate immune cells that can acquire a regenerative phenotype to promote remyelination, yet little is known about the regulators controlling the regenerative microglia activation. Herein, using a cuprizone (CPZ)-diet induced de- and remyelination mice model, we identify PRMT1 as a driver for MHC-associated microglia population required for remyelination in the central nervous system. The loss of PRMT1, but not PRMT5, in microglia resulted in impairment of the remyelination with a reduction of oligoprogenitor cell number and prolonged microgliosis and astrogliosis. Using single-cell RNA sequencing, we found eight distinct microglial clusters during the CPZ diet, and PRMT1 depleted microglia hindered the formation of the MHC-associated cluster, expressing MHCII and CD11c. Mechanistically, PRMT1-KO microglia displayed reduced the H3K27ac peaks at the promoter regions of the MHC- and IFN-associated genes and further suppressed gene expression during CPZ diet. Overall, our findings demonstrate that PRMT1 is a critical regulator of the MHC- and IFN-associated microglia, necessary for central nervous system remyelination.

Developmental Cues and Molecular Drivers in Myelinogenesis: Revisiting Early Life to Re-Evaluate the Integrity of CNS Myelin

The mammalian central nervous system (CNS) coordinates its communication through saltatory conduction, facilitated by myelin-forming oligodendrocytes (OLs). Despite the fact that neurogenesis from stem cell niches has caught the majority of attention in recent years, oligodendrogenesis and, more specifically, the molecular underpinnings behind OL-dependent myelinogenesis, remain largely unknown. In this comprehensive review, we determine the developmental cues and molecular drivers which regulate normal myelination both at the prenatal and postnatal periods. We have indexed the individual stages of myelinogenesis sequentially; from the initiation of oligodendrocyte precursor cells, including migration and proliferation, to first contact with the axon that enlists positive and negative regulators for myelination, until the ultimate maintenance of the axon ensheathment and myelin growth. Here, we highlight multiple developmental pathways that are key to successful myelin formation and define the molecular pathways that can potentially be targets for pharmacological interventions in a variety of neurological disorders that exhibit demyelination.

A Glance at the Molecules That Regulate Oligodendrocyte Myelination

Oligodendrocyte (OL) myelination is a critical process for the neuronal axon function in the central nervous system. After demyelination occurs because of pathophysiology, remyelination makes repairs similar to myelination. Proliferation and differentiation are the two main stages in OL myelination, and most factors commonly play converse roles in these two stages, except for a few factors and signaling pathways, such as OLIG2 (Oligodendrocyte transcription factor 2). Moreover, some OL maturation gene mutations induce hypomyelination or hypermyelination without an obvious function in proliferation and differentiation. Herein, three types of factors regulating myelination are reviewed in sequence.

ACTL6a coordinates axonal caliber recognition and myelination in the peripheral nerve

Cells elaborate transcriptional programs in response to external signals. In the peripheral nerves, Schwann cells (SC) sort axons of given caliber and start the process of wrapping their membrane around them. We identify Actin-like protein 6a (ACTL6a), part of SWI/SNF chromatin remodeling complex, as critical for the integration of axonal caliber recognition with the transcriptional program of myelination. Nuclear levels of ACTL6A in SC are increased by contact with large caliber axons or nanofibers, and result in the eviction of repressive histone marks to facilitate myelination. Without Actl6a the SC are unable to coordinate caliber recognition and myelin production. Peripheral nerves in knockout mice display defective radial sorting, hypo-myelination of large caliber axons, and redundant myelin around small caliber axons, resulting in a clinical motor phenotype. Overall, this suggests that ACTL6A is a key component of the machinery integrating external signals for proper myelination of the peripheral nerve.

PRMT5 Interacting Partners and Substrates in Oligodendrocyte Lineage Cells

The protein arginine methyl transferase PRMT5 is an enzyme expressed in oligodendrocyte lineage cells and responsible for the symmetric methylation of arginine residues on histone tails. Previous work from our laboratory identified PRMT5 as critical for myelination, due to its transcriptional regulation of genes involved in survival and early stages of differentiation. However, besides its nuclear localization, PRMT5 is found at high levels in the cytoplasm of several cell types, including oligodendrocyte progenitor cells (OPCs) and yet, its interacting partners in this lineage, remain elusive. By using mass spectrometry on protein eluates from extracts generated from primary oligodendrocyte lineage cells and immunoprecipitated with PRMT5 antibodies, we identified 1196 proteins as PRMT5 interacting partners. These proteins were related to molecular functions such as RNA binding, ribosomal structure, cadherin and actin binding, nucleotide and protein binding, and GTP and GTPase activity. We then investigated PRMT5 substrates using iTRAQ-based proteomics on cytosolic and nuclear protein extracts from CRISPR-PRMT5 knockdown immortalized oligodendrocyte progenitors compared to CRISPR-EGFP controls. This analysis identified a similar number of peptides in the two subcellular fractions and a total number of 57 proteins with statistically decreased symmetric methylation of arginine residues in the CRISPR-PRMT5 knockdown compared to control. Several PRMT5 substrates were in common with cancer cell lines and related to RNA processing, splicing and transcription. In addition, we detected ten oligodendrocyte lineage specific substrates, corresponding to proteins with high expression levels in neural tissue. They included: PRC2C, a proline-rich protein involved in methyl-RNA binding, HNRPD an RNA binding protein involved in regulation of RNA stability, nuclear proteins involved in transcription and other proteins related to migration and actin cytoskeleton. Together, these results highlight a cell-specific role of PRMT5 in OPC in regulating several other cellular processes, besides RNA splicing and metabolism.

Histone Acetylation and Methylation Underlie Oligodendroglial and Myelin Susceptibility in Schizophrenia

Schizophrenia is a complex neuropsychiatric disorder affected by both genetic and epigenetic factors. Except for neuronal dysfunction, oligodendroglial abnormalities also contribute to the disease pathogenesis, characterized by a robust dysregulation of oligodendrocyte and myelin related genes. Accumulating evidence shows that histone modifications play important roles in transcriptional regulation of the genes crucial for oligodendrocyte differentiation and myelination. Specifically, the histone acetylation and methylation were two well-recognized histone modification abnormalities in the schizophrenic brains. In this mini-review, we will describe the dynamic changes of histone acetylation and methylation in schizophrenia, which may coordinate and induce deleterious epigenetic memory in oligodendroglial cells, and further lead to oligodendrocyte and myelin deficits. Precise modulation of histone modification status in oligodendroglial cells needs to secure the balance of epigenetic marks, which may revise the therapeutic strategy for the white matter etiology of neuropsychiatric disorders.

The Novel Protease Activities of JMJD5–JMJD6–JMJD7 and Arginine Methylation Activities of Arginine Methyltransferases Are Likely Coupled

The surreptitious discoveries of the protease activities on arginine-methylated targets of a subfamily of Jumonji domain-containing family including JMJD5, JMJD6, and JMJD7 pose several questions regarding their authenticity, function, purpose, and relations with others. At the same time, despite several decades of efforts and massive accumulating data regarding the roles of the arginine methyltransferase family (PRMTs), the exact function of this protein family still remains a mystery, though it seems to play critical roles in transcription regulation, including activation and inactivation of a large group of genes, as well as other biological activities. In this review, we aim to elucidate that the function of JMJD5/6/7 and PRMTs are likely coupled. Besides roles in the regulation of the biogenesis of membrane-less organelles in cells, they are major players in regulating stimulating transcription factors to control the activities of RNA Polymerase II in higher eukaryotes, especially in the animal kingdom. Furthermore, we propose that arginine methylation by PRMTs could be a ubiquitous action marked for destruction after missions by a subfamily of the Jumonji protein family.

Dysregulation of the Nitric Oxide/Dimethylarginine Pathway in Hypoxic Pulmonary Vasoconstriction—Molecular Mechanisms and Clinical Significance

The pulmonary circulation responds to hypoxia with vasoconstriction, a mechanism that helps to adapt to short-lived hypoxic episodes. When sustained, hypoxic pulmonary vasoconstriction (HPV) may become deleterious, causing right ventricular hypertrophy and failure, and contributing to morbidity and mortality in the late stages of several chronic pulmonary diseases. Nitric oxide (NO) is an important endothelial vasodilator. Its release is regulated, amongst other mechanisms, by the presence of endogenous inhibitors like asymmetric dimethylarginine (ADMA). Evidence has accumulated in recent years that elevated ADMA may be implicated in the pathogenesis of HPV and in its clinical sequelae, like pulmonary arterial hypertension (PAH). PAH is one phenotypic trait in experimental models with disrupted ADMA metabolism. In high altitude, elevation of ADMA occurs during long-term exposure to chronic or chronic intermittent hypobaric hypoxia; ADMA is significantly associated with high altitude pulmonary hypertension. High ADMA concentration was also reported in patients with chronic obstructive lung disease, obstructive sleep apnoea syndrome, and overlap syndrome, suggesting a pathophysiological role for ADMA-mediated impairment of endothelium-dependent, NO-mediated pulmonary vasodilation in these clinically relevant conditions. Improved understanding of the molecular (dys-)regulation of pathways controlling ADMA concentration may help to dissect the pathophysiology and find novel therapeutic options for these diseases.

Protein Arginine Methyltransferases in Neuromuscular Function and Diseases

Neuromuscular diseases (NMDs) are characterized by progressive loss of muscle mass and strength that leads to impaired body movement. It not only severely diminishes the quality of life of the patients, but also subjects them to increased risk of secondary medical conditions such as fall-induced injuries and various chronic diseases. However, no effective treatment is currently available to prevent or reverse the disease progression. Protein arginine methyltransferases (PRMTs) are emerging as a potential therapeutic target for diverse diseases, such as cancer and cardiovascular diseases. Their expression levels are altered in the patients and molecular mechanisms underlying the association between PRMTs and the diseases are being investigated. PRMTs have been shown to regulate development, homeostasis, and regeneration of both muscle and neurons, and their association to NMDs are emerging as well. Through inhibition of PRMT activities, a few studies have reported suppression of cytotoxic phenotypes observed in NMDs. Here, we review our current understanding of PRMTs’ involvement in the pathophysiology of NMDs and potential therapeutic strategies targeting PRMTs to address the unmet medical need.

Identifying oligodendrocyte enhancers governing Plp1 expression

Oligodendrocytes (OLs) produce myelin in the central nervous system (CNS), which accelerates the propagation of action potentials and supports axonal integrity. As a major component of CNS myelin, proteolipid protein 1 (Plp1) is indispensable for the axon-supportive function of myelin. Notably, this function requires the continuous high-level expression of Plp1 in OLs. Equally important is the controlled expression of Plp1, as illustrated by Pelizaeus-Merzbacher disease for which the most common cause is PLP1 overexpression. Despite a decade-long search, promoter-distal OL enhancers that govern Plp1 remain elusive. We have recently developed an innovative method that maps promoter-distal enhancers to genes in a principled manner. Here, we applied it to Plp1, uncovering two OL enhancers for it (termed Plp1-E1 and Plp1-E2). Remarkably, clustered regularly interspaced short palindromic repeats (CRISPR) interference epigenome editing showed that Plp1-E1 and Plp1-E2 do not regulate two genes in their vicinity, highlighting their exquisite specificity to Plp1. Assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and chromatin immunoprecipitation with high-throughput sequencing (ChIP-seq) data show that Plp1-E1 and Plp1-E2 are OL-specific enhancers that are conserved among human, mouse and rat. Hi-C data reveal that the physical interactions between Plp1-E1/2 and PLP1 are among the strongest in OLs and specific to OLs. We also show that Myrf, a master regulator of OL development, acts on Plp1-E1 and Plp1-E2 to promote Plp1 expression.

Remyelination in PNS and CNS: current and upcoming cellular and molecular strategies to treat disabling neuropathies

Myelin is a lipid-rich nerve cover that consists of glial cell's plasmalemma layers and accelerates signal conduction. Axon-myelin contact is a source for many developmental and regenerative signals of myelination. Intra- or extracellular factors including both enhancers and inhibitors are other factors affecting the myelination process. Myelin damages are observed in several congenital and hereditary diseases, physicochemical conditions, infections, or traumatic insults, and remyelination is known as an intrinsic response to injuries. Here we discuss some molecular events and conditions involved in de- and remyelination and compare the phenomena of remyelination in CNS and PNS. We have explained applying some of these molecular events in myelin restoration. Finally, the current and upcoming treatment strategies for myelin restoration are explained in three groups of immunotherapy, endogenous regeneration enhancement, and cell therapy to give a better insight for finding the more effective rehabilitation strategies considering the underlying molecular events of a lesion formation and its current condition.

The Structure and Functions of PRMT5 in Human Diseases

Since the discovery of protein arginine methyltransferase 5 (PRMT5) and the resolution of its structure, an increasing number of papers have investigated and delineated the structural and functional role of PRMT5 in diseased conditions. PRMT5 is a type II arginine methyltransferase that catalyzes symmetric dimethylation marks on histones and non-histone proteins. From gene regulation to human development, PRMT5 is involved in many vital biological functions in humans. The role of PRMT5 in various cancers is particularly well-documented, and investigations into the development of better PRMT5 inhibitors to promote tumor regression are ongoing. Notably, emerging studies have demonstrated the pathological contribution of PRMT5 in the progression of inflammatory diseases, such as diabetes, cardiovascular diseases, and neurodegenerative disorders. However, more research in this direction is needed. Herein, we critically review the position of PRMT5 in current literature, including its structure, mechanism of action, regulation, physiological and pathological relevance, and therapeutic strategies.

Arginine methyltransferase PRMT5 methylates and stabilizes KLF5 via decreasing its phosphorylation and ubiquitination to promote basal-like breast cancer

Krüppel-like factor 5 (KLF5) is an oncogenic factor that is highly expressed in basal-like breast cancer (BLBC) and promotes cell proliferation, survival, migration, stemness, and tumor growth; however, its posttranslational modifications are poorly defined. Protein arginine methyltransferase 5 (PRMT5) is also an oncogene implicated in various carcinomas, including breast cancer. In this study, we found that PRMT5 interacts with KLF5 and catalyzes the di-methylation of KLF5 at Arginine 57 (R57) in a methyltransferase activity-dependent manner in BLBC cells. Depletion or pharmaceutical inhibition (using PJ-68) of PRMT5 decreased the expression of KLF5 and its downstream target genes in vitro and in vivo. PRMT5-induced KLF5R57me2 antagonizes GSK3β-mediated KLF5 phosphorylation and subsequently Fbw7-mediated KLF5 ubiquitination and coupled degradation. Functionally, PRMT5 promotes breast cancer stem cell maintenance and proliferation, at least partially, by stabilizing KLF5. PRMT5 and KLF5 protein levels were positively correlated in clinical BLBCs. Taken together, PRMT5 methylates KLF5 to prevent its phosphorylation, ubiquitination, and degradation, and thus promotes breast cancer stem cell maintenance and proliferation. These findings suggest that PRMT5 is a potential therapeutic target for BLBC.

Oligodendrocyte progenitors as environmental biosensors

The past decade has seen an important revision of the traditional concept of the role and function of glial cells. From "passive support" for neurons, oligodendrocyte lineage cells are now recognized as metabolic exchangers with neurons, a cellular interface with blood vessels and responders to gut-derived metabolites or changes in the social environment. In the developing brain, the differentiation of neonatal oligodendrocyte progenitors (nOPCs) is required for normal brain function. In adulthood, the differentiation of adult OPCs (aOPCs) serves an important role in learning, behavioral adaptation and response to myelin injury. Here, we propose the concept of OPCs as environmental biosensors, which "sense" chemical and physical stimuli over time and adjust to the new challenges by modifying their epigenome and consequent transcriptome. Because epigenetics defines the ability of the cell to "adapt" gene expression to changes in the environment, we propose a model of OPC differentiation resulting from time-dependent changes of the epigenomic landscape in response to declining mitogens, raising hormone levels, neuronal activity, changes in space constraints or stiffness of the extracellular matrix. We propose that the intrinsically different functional properties of aOPCs compared to nOPCs result from the accrual of "epigenetic memories" of distinct events, which are "recorded" in the nuclei of OPCs as histone and DNA marks, defining a "unique epigenomic landscape" over time.

Combination of a Sindbis-SARS-CoV-2 Spike Vaccine and αOX40 Antibody Elicits Protective Immunity Against SARS-CoV-2 Induced Disease and Potentiates Long-Term SARS-CoV-2-Specific Humoral and T-Cell Immunity

The COVID-19 pandemic caused by the coronavirus SARS-CoV-2 is a major global public threat. Currently, a worldwide effort has been mounted to generate billions of effective SARS-CoV-2 vaccine doses to immunize the world's population at record speeds. However, there is still a demand for alternative effective vaccines that rapidly confer long-term protection and rely upon cost-effective, easily scaled-up manufacturing. Here, we present a Sindbis alphavirus vector (SV), transiently expressing the SARS-CoV-2 spike protein (SV.Spike), combined with the OX40 immunostimulatory antibody (αOX40) as a novel, highly effective vaccine approach. We show that SV.Spike plus αOX40 elicits long-lasting neutralizing antibodies and a vigorous T-cell response in mice. Protein binding, immunohistochemical, and cellular infection assays all show that vaccinated mice sera inhibits spike functions. Immunophenotyping, RNA Seq transcriptome profiles, and metabolic analysis indicate a reprogramming of T cells in vaccinated mice. Activated T cells were found to mobilize to lung tissue. Most importantly, SV.Spike plus αOX40 provided robust immune protection against infection with authentic coronavirus in transgenic mice expressing the human ACE2 receptor (hACE2-Tg). Finally, our immunization strategy induced strong effector memory response, potentiating protective immunity against re-exposure to SARS-CoV-2 spike protein. Our results show the potential of a new Sindbis virus-based vaccine platform to counteract waning immune response, which can be used as a new candidate to combat SARS-CoV-2. Given the T-cell responses elicited, our vaccine is likely to be effective against variants that are proving challenging, as well as serve as a platform to develop a broader spectrum pancoronavirus vaccine. Similarly, the vaccine approach is likely to be applicable to other pathogens.

TET1-mediated DNA hydroxymethylation regulates adult remyelination in mice

The mechanisms regulating myelin repair in the adult central nervous system (CNS) are unclear. Here, we identify DNA hydroxymethylation, catalyzed by the Ten-Eleven-Translocation (TET) enzyme TET1, as necessary for myelin repair in young adults and defective in old mice. Constitutive and inducible oligodendrocyte lineage-specific ablation of Tet1 (but not of Tet2), recapitulate this age-related decline in repair of demyelinated lesions. DNA hydroxymethylation and transcriptomic analyses identify TET1-target in adult oligodendrocytes, as genes regulating neuro-glial communication, including the solute carrier (Slc) gene family. Among them, we show that the expression levels of the Na+/K+/Cl- transporter, SLC12A2, are higher in Tet1 overexpressing cells and lower in old or Tet1 knockout. Both aged mice and Tet1 mutants also present inefficient myelin repair and axo-myelinic swellings. Zebrafish mutants for slc12a2b also display swellings of CNS myelinated axons. Our findings suggest that TET1 is required for adult myelin repair and regulation of the axon-myelin interface.

Combination of a Sindbis-SARS-CoV-2 spike vaccine and αOX40 antibody elicits protective immunity against SARS-CoV-2 induced disease and potentiates long-term SARS-CoV-2-specific humoral and T-cell immunity

The COVID-19 pandemic caused by the coronavirus SARS-CoV-2 is a major global public threat. Currently, a worldwide effort has been mounted to generate billions of effective SARS-CoV-2 vaccine doses to immunize the world's population at record speeds. However, there is still demand for alternative effective vaccines that rapidly confer long-term protection and rely upon cost-effective, easily scaled-up manufacturing. Here, we present a Sindbis alphavirus vector (SV), transiently expressing the SARS-CoV-2 spike protein (SV.Spike), combined with the OX40 immunostimulatory antibody (αOX40) as a novel, highly effective vaccine approach. We show that SV.Spike plus αOX40 elicits long-lasting neutralizing antibodies and a vigorous T-cell response in mice. Protein binding, immunohistochemical and cellular infection assays all show that vaccinated mice sera inhibits spike functions. Immunophenotyping, RNA Seq transcriptome profiles and metabolic analysis indicate a reprogramming of T-cells in vaccinated mice. Activated T-cells were found to mobilize to lung tissue. Most importantly, SV.Spike plus αOX40 provided robust immune protection against infection with authentic coronavirus in transgenic mice expressing the human ACE2 receptor (hACE2-Tg). Finally, our immunization strategy induced strong effector memory response, potentiating protective immunity against re-exposure to SARS-CoV-2 spike protein. Our results show the potential of a new Sindbis virus-based vaccine platform to counteract waning immune response that can be used as a new candidate to combat SARS-CoV-2. Given the strong T-cell responses elicited, our vaccine is likely to be effective against variants that are proving challenging, as well as, serve as a platform to develop a broader spectrum pancoronavirus vaccine. Similarly, the vaccine approach is likely to be applicable to other pathogens.

PRMT1-p53 Pathway Controls Epicardial EMT and Invasion

Epicardial cells are cardiac progenitors that give rise to the majority of cardiac fibroblasts, coronary smooth muscle cells, and pericytes during development. An integral phase of epicardial fate transition is epithelial-to-mesenchymal transition (EMT) that confers motility. We uncover an essential role for the protein arginine methyltransferase 1 (PRMT1) in epicardial invasion and differentiation. Using scRNA-seq, we show that epicardial-specific deletion of Prmt1 reduced matrix and ribosomal gene expression in epicardial-derived cell lineages. PRMT1 regulates splicing of Mdm4, which is a key controller of p53 stability. Loss of PRMT1 leads to accumulation of p53 that enhances Slug degradation and blocks EMT. During heart development, the PRMT1-p53 pathway is required for epicardial invasion and formation of epicardial-derived lineages: cardiac fibroblasts, coronary smooth muscle cells, and pericytes. Consequently, this pathway modulates ventricular morphogenesis and coronary vessel formation. Altogether, our study reveals molecular mechanisms involving the PRMT1-p53 pathway and establish its roles in heart development.

PRMT5: a putative oncogene and therapeutic target in prostate cancer

Protein arginine methyltransferase 5 (PRMT5) was discovered two decades ago. The first decade focused on the biochemical characterization of PRMT5 as a regulator of many cellular processes in a healthy organism. However, over the past decade, evidence has accumulated to suggest that PRMT5 may function as an oncogene in multiple cancers via both epigenetic and non-epigenetic mechanisms. In this review, we focus on recent progress made in prostate cancer, including the role of PRMT5 in the androgen receptor (AR) expression and signaling and DNA damage response, particularly DNA double-strand break repair. We also discuss how PRMT5-interacting proteins that are considered PRMT5 cofactors may cooperate with PRMT5 to regulate PRMT5 activity and target gene expression, and how PRMT5 can interact with other epigenetic regulators implicated in prostate cancer development and progression. Finally, we suggest that targeting PRMT5 may be employed to develop multiple therapeutic approaches to enhance the treatment of prostate cancer.

Protein arginine methyltransferase 5: a potential cancer therapeutic target

BACKGROUND

PRMT5 is a type II protein arginine methyltransferase that methylates histone or non-histone proteins. Arginine methylation by PRMT5 has been implicated in gene transcription, ribosome biogenesis, RNA transport, pre-mRNA splicing and signal transduction. High expression of PRMT5 has been observed in various cancers and PRMT5 overexpression has been reported to improve cancer cell survival, proliferation, migration and metabolism and to inhibit cancer cell apoptosis. In addition, PRMT5 has been found to be required for cancer stem cell survival, self-renewal and differentiation. Several microRNAs have been shown to regulate PRMT5 expression. As PRMT5 has oncogene-like properties, several PRMT5 inhibitors have been used to explore their efficacy as potential drugs for different types of cancer, and three of them are now being tested in clinical trials.

CONCLUSIONS

In this review, we summarize current knowledge on the role of PRMT5 in cancer development and progression, including its functions and underlying mechanisms. In addition, we highlight the rapid development of PRMT5 inhibitors and summarize ongoing clinical trials for cancer therapy. By affecting both tumor cells and the tumor microenvironment, PRMT5 inhibitors may serve as effective anti-cancer agents, especially when combined with immune therapies.

Oligodendroglial Epigenetics, from Lineage Specification to Activity-Dependent Myelination

Oligodendroglial cells are the myelinating cells of the central nervous system. While myelination is crucial to axonal activity and conduction, oligodendrocyte progenitor cells and oligodendrocytes have also been shown to be essential for neuronal support and metabolism. Thus, a tight regulation of oligodendroglial cell specification, proliferation, and myelination is required for correct neuronal connectivity and function. Here, we review the role of epigenetic modifications in oligodendroglial lineage cells. First, we briefly describe the epigenetic modalities of gene regulation, which are known to have a role in oligodendroglial cells. We then address how epigenetic enzymes and/or marks have been associated with oligodendrocyte progenitor specification, survival and proliferation, differentiation, and finally, myelination. We finally mention how environmental cues, in particular, neuronal signals, are translated into epigenetic modifications, which can directly influence oligodendroglial biology.

The Role of the PRMT5-SND1 Axis in Hepatocellular Carcinoma

Arginine methylation is an essential post-translational modification (PTM) deposited by protein arginine methyltransferases (PRMTs) and recognized by Tudor domain-containing proteins. Of the nine mammalian PRMTs, PRMT5 is the primary enzyme responsible for the deposition of symmetric arginine methylation marks in cells. The staphylococcal nuclease and Tudor domain-containing 1 (SND1) effector protein is a key reader of the marks deposited by PRMT5. Both PRMT5 and SND1 are broadly expressed and their deregulation is reported to be associated with a range of disease phenotypes, including cancer. Hepatocellular carcinoma (HCC) is an example of a cancer type that often displays elevated PRMT5 and SND1 levels, and there is evidence that hyperactivation of this axis is oncogenic. Importantly, this pathway can be tempered with small-molecule inhibitors that target PRMT5, offering a therapeutic node for cancer, such as HCC, that display high PRMT5–SND1 axis activity. Here we summarize the known activities of this writer–reader pair, with a focus on their biological roles in HCC. This will help establish a foundation for treating HCC with PRMT5 inhibitors and also identify potential biomarkers that could predict sensitivity to this type of therapy.

HDAC6 in Diseases of Cognition and of Neurons

Central nervous system (CNS) neurodegenerative diseases are characterized by faulty intracellular transport, cognition, and aggregate regulation. Traditionally, neuroprotection exerted by histone deacetylase (HDAC) inhibitors (HDACi) has been attributed to the ability of this drug class to promote histone acetylation. However, HDAC6 in the healthy CNS functions via distinct mechanisms, due largely to its cytoplasmic localization. Indeed, in healthy neurons, cytoplasmic HDAC6 regulates the acetylation of a variety of non-histone proteins that are linked to separate functions, i.e., intracellular transport, neurotransmitter release, and aggregate formation. These three HDAC6 activities could work independently or in synergy. Of particular interest, HDAC6 targets the synaptic protein Bruchpilot and neurotransmitter release. In pathological conditions, HDAC6 becomes abundant in the nucleus, with deleterious consequences for transcription regulation and synapses. Thus, HDAC6 plays a leading role in neuronal health or dysfunction. Here, we review recent findings and novel conclusions on the role of HDAC6 in neurodegeneration. Selective studies with pan-HDACi are also included. We propose that an early alteration of HDAC6 undermines synaptic transmission, while altering transport and aggregation, eventually leading to neurodegeneration.

Roles of protein arginine methyltransferase 1 (PRMT1) in brain development and disease

BACKGROUND

Protein arginine methyltransferase 1 (PRMT1), a major type I arginine methyltransferase in mammals, methylates histone and non-histone proteins to regulate various cellular functions such as transcription, DNA damage response, and signal transduction.

SCOPE OF REVIEW

This review summarizes previous and recent studies on PRMT1 functions in major cell types of the central nervous system. We also discuss the potential involvement of PRMT1 in neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal dementia. Also, we raise key questions that must be addressed in the future to more precisely understand the roles of PRMT1.

MAJOR CONCLUSIONS

Recent studies revealed that PRMT1 is essential for the development of neurons, astrocytes, and oligodendrocytes, although further investigation using cell type-specific PRMT1-deficient animals is required. In addition, the relevance of PRMT1 in neurodegenerative diseases will continue to be a hot topic.

GENERAL SIGNIFICANCE

PRMT1 is important for neural development and neurodegenerative diseases.

Oligodendrocyte Intrinsic miR-27a Controls Myelination and Remyelination

Remyelination requires the generation of new oligodendrocytes (OLs), which are derived from oligodendrocyte progenitor cells (OPCs). Maturation of OPCs into OLs is a multi-step process. Here, we describe a microRNA expressed by OLs, miR-27a, as a regulator of OL development and survival. Increased levels of miR-27a were found in OPCs associated with multiple sclerosis (MS) lesions and in animal models of demyelination. Increased levels of miR-27a led to inhibition of OPC proliferation by cell-cycle arrest, as well as impaired differentiation of human OPCs (hOPCs) and myelination by dysregulating the Wnt-β-catenin signaling pathway. In vivo administration of miR-27a led to suppression of myelinogenic signals, leading to loss of endogenous myelination and remyelination. Our findings provide evidence supporting a critical role for a steady-state level of OL-specific miR-27a in supporting multiple steps in the complex process of OPC maturation and remyelination.

Loss of PRMT1 in the central nervous system (CNS) induces reactive astrocytes and microglia during postnatal brain development

PRMT1, a major arginine methyltransferase, plays critical roles in transcription, DNA damage response, and cell proliferation. Although we have previously discovered the crucial roles of PRMT1 for oligodendrocyte lineage progression in the central nervous system of neural stem cell-specific PRMT1 conditional knockout (PRMT1-CKO) mice, the context of other glial cell states that may cause the hypomyelination phenotype in PRMT1-CKO mice has not been explored so far. Here, we performed RNA-seq of the neonatal cortices of PRMT1-CKO mice to reveal overall gene expression changes and show the up-regulation of inflammatory signaling which is generally mediated by astrocytes and microglia in advance of the myelination defects. In particular, qRT-PCR analyses revealed Interleukin-6 (Il-6), a major central nervous system cytokine, was dramatically increased in the PRMT1-CKO brains. The gene expression changes led to augmentation of glial fibrillary acidic protein and Vimentin protein levels in PRMT1-CKO mice, showing severe reactive astrogliosis after birth. We further show that IBA1-positive and CD68-positive activated microglia were increased in PRMT1-CKO mice, in spite of intact Prmt1 gene expression in purified microglia from the mutant mice. Our results indicate that PRMT1 loss in the neural stem cell lineage causes disruptive changes in all glial types perturbing postnatal brain development and myelination.

Convergent epigenetic regulation of glial plasticity in myelin repair and brain tumorigenesis: A focus on histone modifying enzymes

Brain regeneration and tumorigenesis are complex processes involving in changes in chromatin structure to regulate cellular states at the molecular and genomic level. The modulation of chromatin structure dynamics is critical for maintaining progenitor cell plasticity, growth and differentiation. Oligodendrocyte precursor cells (OPC) can be differentiated into mature oligodendrocytes, which produce myelin sheathes to permit saltatory nerve conduction. OPCs and their primitive progenitors such as pri-OPC or pre-OPC are highly adaptive and plastic during injury repair or brain tumor formation. Recent studies indicate that chromatin modifications and epigenetic homeostasis through histone modifying enzymes shape genomic regulatory landscape conducive to OPC fate specification, lineage differentiation, maintenance of myelin sheaths, as well as brain tumorigenesis. Thus, histone modifications can be convergent mechanisms in regulating OPC plasticity and malignant transformation. In this review, we will focus on the impact of histone modifying enzymes in modulating OPC plasticity during normal development, myelin regeneration and tumorigenesis.

Transcriptional regulators of the Golli/myelin basic protein locus integrate additive and stealth activities

Myelin is composed of plasma membrane spirally wrapped around axons and compacted into dense sheaths by myelin-associated proteins. Myelin is elaborated by neuroepithelial derived oligodendrocytes in the central nervous system (CNS) and by neural crest derived Schwann cells in the peripheral nervous system (PNS). While some myelin proteins accumulate in only one lineage, myelin basic protein (Mbp) is expressed in both. Overlapping the Mbp gene is Golli, a transcriptional unit that is expressed widely both within and beyond the nervous system. A super-enhancer domain within the Golli/Mbp locus contains multiple enhancers shown previously to drive reporter construct expression specifically in oligodendrocytes or Schwann cells. In order to determine the contribution of each enhancer to the Golli/Mbp expression program, and to reveal if functional interactions occur among them, we derived mouse lines in which they were deleted, either singly or in different combinations, and relative mRNA accumulation was measured at key stages of early development and at maturity. Although super-enhancers have been shown previously to facilitate interaction among their component enhancers, the enhancers investigated here demonstrated largely additive relationships. However, enhancers demonstrating autonomous activity strictly in one lineage, when missing, were found to significantly reduce output in the other, thus revealing cryptic "stealth" activity. Further, in the absence of a key oligodendrocyte enhancer, Golli accumulation was markedly and uniformly attenuated in all cell types investigated. Our observations suggest a model in which enhancer-mediated DNA-looping and potential super-enhancer properties underlie Golli/Mbp regulatory organization.

Post-translational modifications of the Fragile X Mental Retardation Protein in neuronal function and dysfunction

The Fragile X Mental Retardation Protein (FMRP) is an RNA-binding protein essential to the regulation of local translation at synapses. In the mammalian brain, synapses are constantly formed and eliminated throughout development to achieve functional neuronal networks. At the molecular level, thousands of proteins cooperate to accomplish efficient neuronal communication. Therefore, synaptic protein levels and their functional interactions need to be tightly regulated. FMRP generally acts as a translational repressor of its mRNA targets. FMRP is the target of several post-translational modifications (PTMs) that dynamically regulate its function. Here we provide an overview of the PTMs controlling the FMRP function and discuss how their spatiotemporal interplay contributes to the physiological regulation of FMRP. Importantly, FMRP loss-of-function leads to Fragile X syndrome (FXS), a rare genetic developmental condition causing a range of neurological alterations including intellectual disability (ID), learning and memory impairments, autistic-like features and seizures. Here, we also explore the possibility that recently reported missense mutations in the FMR1 gene disrupt the PTM homoeostasis of FMRP, thus participating in the aetiology of FXS. This suggests that the pharmacological targeting of PTMs may be a promising strategy to develop innovative therapies for patients carrying such missense mutations.