Immunohistochemical and western analyses of protein arginine N-methyltransferase 3 in the mouse brain

From Environment to Gene Expression: Epigenetic Methylations and One-Carbon Metabolism in Amyotrophic Lateral Sclerosis

The etiology of the neurodegenerative disease amyotrophic lateral sclerosis (ALS) is complex and considered multifactorial. The majority of ALS cases are sporadic, but familial cases also exist. Estimates of heritability range from 8% to 61%, indicating that additional factors beyond genetics likely contribute to ALS. Numerous environmental factors are considered, which may add up and synergize throughout an individual's lifetime building its unique exposome. One level of integration between genetic and environmental factors is epigenetics, which results in alterations in gene expression without modification of the genome sequence. Methylation reactions, targeting DNA or histones, represent a large proportion of epigenetic regulations and strongly depend on the availability of methyl donors provided by the ubiquitous one-carbon (1C) metabolism. Thus, understanding the interplay between exposome, 1C metabolism, and epigenetic modifications will likely contribute to elucidating the mechanisms underlying altered gene expression related to ALS and to developing targeted therapeutic interventions. Here, we review evidence for 1C metabolism alterations and epigenetic methylation dysregulations in ALS, with a focus on the impairments reported in neural tissues, and discuss these environmentally driven mechanisms as the consequences of cumulative exposome or late environmental hits, but also as the possible result of early developmental defects.

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.

A meta-analysis of transcriptomic profiles of Huntington's disease patients

Description of robust transcriptomic alterations in Huntington’s disease is essential to identify targets for biochemical studies and drug development. We analysed publicly available transcriptome data from the brain and blood of 220 HD patients and 241 healthy controls and identified 737 and 661 genes with robustly altered mRNA levels in the brain and blood of HD patients, respectively. In the brain, a subnetwork of 320 genes strongly correlated with HD and was enriched in transport-related genes. Bioinformatical analysis of this subnetwork highlighted CDC42, PAK1, YWHAH, NFY, DLX1, HMGN3, and PRMT3. Moreover, we found that CREB1 can regulate 78.0% of genes whose mRNA levels correlated with HD in the blood of patients. Alterations in protein transport, metabolism, transcriptional regulation, and CDC42-mediated functions are likely central features of HD. Further our data substantiate the role of transcriptional regulators that have not been reported in the context of HD (e.g. DLX1, HMGN3 and PRMT3) and strongly suggest dysregulation of NFY and its target genes across tissues. A large proportion of the identified genes such as CDC42 were also altered in Parkinson’s (PD) and Alzheimer’s disease (AD). The observed dysregulation of CDC42 and YWHAH in samples from HD, AD and PD patients indicates that those genes and their upstream regulators may be interesting therapeutic targets.

Arginine methylation: the promise of a 'silver bullet' for brain tumours?

Despite intense research efforts, our pharmaceutical repertoire against high-grade brain tumours has not been able to increase patient survival for a decade and life expectancy remains at less than 16 months after diagnosis, on average. Inhibitors of protein arginine methyltransferases (PRMTs) have been developed and investigated over the past 15 years and have now entered oncology clinical trials, including for brain tumours. This review collates recent advances in the understanding of the role of PRMTs and arginine methylation in brain tumours. We provide an up-to-date literature review on the mechanisms for PRMT regulation. These include endogenous modulators such as alternative splicing, miRNA, post-translational modifications and PRMT-protein interactions, and synthetic inhibitors. We discuss the relevance of PRMTs in brain tumours with a particular focus on PRMT1, -2, -5 and -8. Finally, we include a future perspective where we discuss possible routes for further research on arginine methylation and on the use of PRMT inhibitors in the context of brain tumours.

Arginine Methylation in Brain Tumors: Tumor Biology and Therapeutic Strategies

Protein arginine methylation is a common post-translational modification that plays a pivotal role in cellular regulation. Protein arginine methyltransferases (PRMTs) catalyze the modification of target proteins by adding methyl groups to the guanidino nitrogen atoms of arginine residues. Protein arginine methylation takes part in epigenetic and cellular regulation and has been linked to neurodegenerative diseases, metabolic diseases, and tumor progression. Aberrant expression of PRMTs is associated with the development of brain tumors such as glioblastoma and medulloblastoma. Identifying PRMTs as plausible contributors to tumorigenesis has led to preclinical and clinical investigations of PRMT inhibitors for glioblastoma and medulloblastoma therapy. In this review, we discuss the role of arginine methylation in cancer biology and provide an update on the use of small molecule inhibitors of PRMTs to treat glioblastoma, medulloblastoma, and other cancers.

SCYL1 arginine methylation by PRMT1 is essential for neurite outgrowth via Golgi morphogenesis

Arginine methylation is a common posttranslational modification that modulates protein function. SCY1-like pseudokinase 1 (SCYL1) is crucial for neuronal functions and interacts with γ₂-COP to form coat protein complex I (COPI) vesicles that regulate Golgi morphology. However, the molecular mechanism by which SCYL1 is regulated remains unclear. Here, we report that the γ₂-COP-binding site of SCYL1 is arginine-methylated by protein arginine methyltransferase 1 (PRMT1) and that SCYL1 arginine methylation is important for the interaction of SCYL1 with γ₂-COP. PRMT1 was colocalized with SCYL1 in the Golgi fraction. Inhibition of PRMT1 suppressed axon outgrowth and dendrite complexity via abnormal Golgi morphology. Knockdown of SCYL1 by small interfering RNA (siRNA) inhibited axon outgrowth, and the inhibitory effect was rescued by siRNA-resistant SCYL1, but not SCYL1 mutant, in which the arginine methylation site was replaced. Thus, PRMT1 regulates Golgi morphogenesis via SCYL1 arginine methylation. We propose that SCYL1 arginine methylation by PRMT1 contributes to axon and dendrite morphogenesis in neurons.

Protein Arginine Methyltransferases in Cardiovascular and Neuronal Function

The methylation of arginine residues by protein arginine methyltransferases (PRMTs) is a type of post-translational modification which is important for numerous cellular processes, including mRNA splicing, DNA repair, signal transduction, protein interaction, and transport. PRMTs have been extensively associated with various pathologies, including cancer, inflammation, and immunity response. However, the role of PRMTs has not been well described in vascular and neurological function. Aberrant expression of PRMTs can alter its metabolic products, asymmetric dimethylarginine (ADMA), and symmetric dimethylarginine (SDMA). Increased ADMA levels are recognized as an independent risk factor for cardiovascular disease and mortality. Recent studies have provided considerable advances in the development of small-molecule inhibitors of PRMTs to study their function under normal and pathological states. In this review, we aim to elucidate the particular roles of PRMTs in vascular and neuronal function as a potential target for cardiovascular and neurological diseases.

Increase of arginine dimethylation correlates with the progression and prognosis of ALS

OBJECTIVE

To investigate whether arginine methylation is altered in patients with amyotrophic lateral sclerosis (ALS) and how it affects disease severity, progression, and prognosis.

METHODS

We compared the immunoreactivity of protein arginine methyltransferase 1 (PRMT1) and its products, asymmetric dimethylated proteins (ASYM), in postmortem spinal cord. We also measured the concentrations of total l-arginine and methylated arginine residues, including asymmetric dimethyl l-arginine (ADMA), symmetric dimethyl arginine, and monomethyl arginine, in CSF samples from 52 patients with ALS using liquid chromatography-tandem mass spectrometry, and we examined their relationship with the progression and prognosis of ALS.

RESULTS

The immunoreactivity of both PRMT1 (p < 0.0001) and ASYM (p = 0.005) was increased in patients with ALS. The concentration of ADMA in CSF was substantially higher in patients with ALS than in disease controls. The ADMA/l-arginine ratio was correlated with the change of decline in the ALS Functional Rating Scale at 12 months after the time of measurement (r = 0.406, p = 0.010). A Cox proportional hazards model showed that the ADMA/l-arginine ratio was an independent predictor for overall survival. Moreover, a high ADMA/l-arginine ratio predicted poor prognosis, even in a group with normal percentage forced vital capacity.

CONCLUSION

There was an enhancement of arginine dimethylation in patients with ALS, and the ADMA/l-arginine ratio predicted disease progression and prognosis in such patients.

Protein Arginine Methyltransferase 3 Enhances Chemoresistance in Pancreatic Cancer by Methylating hnRNPA1 to Increase ABCG2 Expression

Pancreatic cancer is poorly responsive to chemotherapy due to intrinsic or acquired resistance. Our previous study showed that epigenetic modifying enzymes including protein arginine methyltransferase 3 (PRMT3) are dysregulated in gemcitabine (GEM)-resistant pancreatic cancer cells. Here, we attempt to elucidate the role of PRMT3 in chemoresistance. Overexpression of PRMT3 led to increased resistance to GEM in pancreatic cancer cells, whereas reduction of PRMT3 restored GEM sensitivity in resistant cells. We identified a novel PRMT3 target, ATP-binding cassette subfamily G member 2 (ABCG2), which is known to play a critical role in drug resistance. PRMT3 overexpression upregulated ABCG2 expression by increasing its mRNA stability. Mass spectrometric analysis identified hnRNPA1 as a PRMT3 interacting protein, and methylation of hnRNPA1 at R31 by PRMT3 in vivo and in vitro. The expression of methylation-deficient hnRNPA1-R31K mutant reduced the RNA binding activity of hnRNPA1 and the expression of ABCG2 mRNA. Taken together, this provides the first evidence that PRMT3 methylates the RNA recognition motif (RRM) of hnRNPA1 and promotes the binding between hnRNPA1 and ABCG2 to enhance drug resistance. Inhibition of PRMT3 could be a novel strategy for the treatment of GEM-resistant pancreatic cancer.

Arginine Methylation: The Coming of Age

Arginine methylation is a common post-translational modification functioning as an epigenetic regulator of transcription and playing key roles in pre-mRNA splicing, DNA damage signaling, mRNA translation, cell signaling, and cell fate decision. Recently, a wealth of studies using transgenic mouse models and selective PRMT inhibitors helped define physiological roles for protein arginine methyltransferases (PRMTs) linking them to diseases such as cancer and metabolic, neurodegenerative, and muscular disorders. This review describes the recent molecular advances that have been uncovered in normal and diseased mammalian cells.

Transient expression of Xpn, an XLMR protein related to neurite extension, during brain development and participation in neurite outgrowth

KIAA2022 has been implicated as a gene responsible for expressing X-linked mental retardation (XLMR) proteins in humans. However, the functional role of KIAA2022 in the human brain remains unclear. Here, we revealed that depletion of Kiaa2022 inhibits neurite outgrowth of PC12 cells, indicating that the gene participates in neurite extension. Thus, we termed Kiaa2022 as an XLMR protein related to neurite extension (Xpn). Using the mouse brain as a model and ontogenetic analysis of Xpn by real-time PCR, we clearly demonstrated that Xpn is expressed transiently during the late embryonic and perinatal stages. In situ hybridization histochemistry further revealed that Xpn-expressing neurons could be categorized ontogenetically into three types. The first type showed transient expression of Xpn during development. The second type maximally expressed Xpn during the late embryonic or perinatal stage. Thereafter, Xpn expression in this type of neuron decreased gradually throughout development. Nevertheless, a significant level of Xpn expression was detected even into adulthood. The third type of neurons initiated expression of Xpn during the embryonic stage, and continued to express the gene throughout the remaining developmental stages. Subsequent immunohistochemical analysis revealed that Xpn was localized to the nucleus and cytoplasm throughout brain development. Our findings indicate that Xpn may participate in neural circuit formation during developmental stages via nuclear and cytoplasmic Xpn. Moreover, disturbances of this neuronal circuit formation may play a role in the pathogenesis of mental retardation.

Arginine methylation of RNA-binding proteins regulates cell function and differentiation

Arginine methylation is a post-translational modification that regulates protein function. RNA-binding proteins are an important class of cell-function mediators, some of which are methylated on arginine. Early studies of RNA-binding proteins and arginine methylation are briefly introduced, and the enzymes that mediate this post-translational modification are described. We review the most common RNA-binding domains and briefly discuss how they associate with RNAs. We address the following groups of RNA-binding proteins: hnRNP, Sm, Piwi, Vasa, FMRP, and HuD. hnRNPs were the first RNA-binding proteins found to be methylated on arginine. The Sm proteins function in RNA processing and germ cell specification. The Piwi proteins are largely germ cell specific and are also required for germ cell production, as is Vasa. FMRP participates in germ cell formation in Drosophila, but is more widely known for its neuronal function. Similarly, HuD plays a role in nervous system development and function. We review the effects of arginine methylation on the function of each protein, then conclude by addressing remaining questions and future directions of arginine methylation as an important and emerging area of regulation.

Plasma corticosterone activates SGK1 and induces morphological changes in oligodendrocytes in corpus callosum

Repeated stressful events are known to be associated with onset of depression. Further, stress activates the hypothalamic–pituitary–adrenocortical (HPA) system by elevating plasma cortisol levels. However, little is known about the related downstream molecular pathway. In this study, by using repeated water-immersion and restraint stress (WIRS) as a stressor for mice, we attempted to elucidate the molecular pathway induced by elevated plasma corticosterone levels. We observed the following effects both, in vivo and in vitro: (1) repeated exposure to WIRS activates the 3-phosphoinositide-dependent protein kinase (PDK1)–serum glucocorticoid regulated kinase (SGK1)–N-myc downstream-regulated gene 1 (NDRG1)–adhesion molecule (i.e., N-cadherin, α-catenin, and β-catenin) stabilization pathway via an increase in plasma corticosterone levels; (2) the activation of this signaling pathway induces morphological changes in oligodendrocytes; and (3) after recovery from chronic stress, the abnormal arborization of oligodendrocytes and depression-like symptoms return to the control levels. Our data strongly suggest that these abnornalities of oligodendrocytes are possibly related to depression-like symptoms.

HuD interacts with survival motor neuron protein and can rescue spinal muscular atrophy-like neuronal defects

Spinal muscular atrophy is an autosomal-recessive neuromuscular disease caused by disruption of the survival of motor neuron (SMN) gene, which promotes cytoplasmic assembly of the splicing core machinery. It remains unclear how a deficiency in SMN results in a disorder leading to selective degeneration of lower motor neurons. We report here that SMN interacts with RNA-binding protein HuD in neurites of motorneuron-derived MN-1 cells. This interaction is mediated through the Tudor domain of SMN and, importantly, naturally occurring Tudor mutations found in patients with severe spinal muscular atrophy (SMA) completely abrogate the interaction, underscoring its relevance to the disease process. We also characterized a regulatory pathway involving coactivator-associated arginine methyltransferase 1 (CARM1) and HuD. Specifically, we show that CARM1 expression is rapidly downregulated, at the protein level, following induction of differentiation through retinoid and neurotrophic signaling. Using purified proteins, we demonstrate that methylation of HuD by CARM1 reduces its interaction with the p21(cip1/waf1) mRNA, showing that CARM1 can directly influence RNA-binding activity. We further demonstrate that this CARM1-dependent regulatory switch mainly controls the activity of HuD in promoting cell-cycle exit, whereas the interaction between HuD and SMN is required for proper recruitment of HuD and its mRNA targets in neuronal RNA granules. Finally, we were able to rescue SMA-like defects in a hypomorphic Smn knockdown MN-1 cell line through overexpression of HuD. Together, these findings extend our understanding of specific role(s) of SMN in motor neurons and provide crucial insights into potential new avenues for SMA therapeutic strategies.

PRMT3 is essential for dendritic spine maturation in rat hippocampal neurons

Protein arginine N-methyltransferase 3 (PRMT3) is a cytoplasmic enzyme that utilizes S-adenosyl-L-methionine (AdoMet) to methylate specific proteins, most of which contain GAR (glycine-arginine rich) motifs. PRMT3 has been shown to play a role in the proper maturation of the 80S ribosome by binding to and catalyzing the methylation of rpS2, a component of the 40S ribosomal subunit. However, the other roles of PRMT3 are fairly unclear, particularly in the brain, which is abundant in methylated proteins. In this study, we perturbed PRMT3 expression in cultured rat hippocampal neurons by transiently introducing siRNA oligonucleotides that were designed to hybridize with PRMT3 mRNA and then we examined the morphological and functional effects of neuronal PRMT3 depletion. PRMT3-defective neurons showed deformed spines without any change in spine number; less BDNF-induced protein translation of alphaCaMKII; and diminished rpS2 protein stability. Furthermore, overexpression of a methylation-resistant rpS2, whose methylated arginine residues were deleted, produced phenotypes that were similar to those associated with PRMT3 downregulation. These findings demonstrated that PRMT3 possibly plays a pivotal role in neuronal translation by interaction with rpS2 and that it contributes to activity-dependent changes in the dendritic spines.

Alternative Splicing Yields Protein Arginine Methyltransferase 1 Isoforms with Distinct Activity, Substrate Specificity, and Subcellular Localization

PRMT1 is the predominant member of a family of protein arginine methyltransferases (PRMTs) that have been implicated in various cellular processes, including transcription, RNA processing, and signal transduction. It was previously reported that the human PRMT1 pre-mRNA was alternatively spliced to yield three isoforms with distinct N-terminal sequences. Close inspection of the genomic organization in the 5'-end of the PRMT1 gene revealed that it can produce up to seven protein isoforms, all varying in their N-terminal domain. A detailed biochemical characterization of these variants revealed that unique N-terminal sequences can influence catalytic activity as well as substrate specificity. In addition, our results uncovered the presence of a functional nuclear export sequence in PRMT1v2. Finally, we find that the relative balance of PRMT1 isoforms is altered in breast cancer.

Ribosomal protein rpS2 is hypomethylated in PRMT3-deficient mice

PRMT3 is a type I arginine methyltransferase that resides in the cytoplasm. A large proportion of this cystosolic PRMT3 is found associated with ribosomes. It is tethered to the ribosomes through its interaction with rpS2, which is also its substrate. Here we show that mouse embryos with a targeted disruption of PRMT3 are small in size but survive after birth and attain a normal size in adulthood, thus displaying Minute-like characteristics. The ribosome protein rpS2 is hypomethylated in the absence of PRMT3, demonstrating that it is a bona fide, in vivo PRMT3 substrate that cannot be modified by other PRMTs. Finally, the levels 40 S, 60 S, and 80 S monosomes and polyribosomes are unaffected by the loss of PRMT3, but there are additional as yet unidentified proteins that co-fractionate with ribosomes that are also dedicated PRMT3 substrates.

Specific regional distribution of protein arginine methyltransferase 8 (PRMT8) in the mouse brain

The regional distribution of PRMT8 transcript was examined in mouse brain using in situ hybridization (ISH) histochemistry. The PRMT8 cRNA probe was specifically hybridized with CNS and the signals were observed only in the neurons. The distribution of the neurons expressing PRMT8 mRNA was not even throughout the brain. All of the regions related to general somatosensory system expressed PRMT8 mRNA strongly. Most of the relay nuclei intervening the special somatosensory system, such as the auditory, visual, and vestibular systems, were packed with PRMT8 mRNA expressing neurons. Forebrain limbic areas and thalamic nuclei relevant to limbic areas were also strongly labeled with the probe. Some areas related to the motor system, such as the caudate putamen, Purkinje cells, inferior olivary nucleus and cerebellar nuclei expressed PRMT8 mRNA strongly. These findings suggest that PRMT8 is chiefly involved in the somatosensory and limbic systems, and a part of motor system.