Prmt7 Deficiency Causes Reduced Skeletal Muscle Oxidative Metabolism and Age-Related Obesity

PRMT7 as a unique member of the protein arginine methyltransferase family: A review

Protein arginine methyltransferases (PRMTs) are found in a wide variety of eukaryotic organisms and can regulate gene expression, DNA repair, RNA splicing, and stem cell biology. In mammalian cells, nine genes encode a family of sequence-related enzymes; six of these PRMTs catalyze the formation of ω-asymmetric dimethyl derivatives, two catalyze ω-symmetric dimethyl derivatives, and only one (PRMT7) solely catalyzes ω-monomethylarginine formation. Purified recombinant PRMT7 displays a number of unique enzymatic properties including a substrate preference for arginine residues in R-X-R motifs with additional flanking basic amino acid residues and a temperature optimum well below 37 °C. Evidence has been presented for crosstalk between PRMT7 and PRMT5, where methylation of a histone H4 peptide at R17, a PRMT7 substrate, may activate PRMT5 for methylation of R3. Defects in muscle stem cells (satellite cells) and immune cells are found in mouse Prmt7 homozygous knockouts, while humans lacking PRMT7 are characterized by significant intellectual developmental delays, hypotonia, and facial dysmorphisms. The overexpression of the PRMT7 gene has been correlated with cancer metastasis in humans. Current research challenges include identifying cellular factors that control PRMT7 expression and activity, identifying the physiological substrates of PRMT7, and determining the effect of methylation on these substrates.

Overweight in the Elderly Induces a Switch in Energy Metabolism that Undermines Muscle Integrity

Aging is characterized by a progressive loss of skeletal muscle mass and function (sarcopenia). Obesity exacerbates age-related decline and lead to frailty. Skeletal muscle fat infiltration increases with aging and seems to be crucial for the progression of sarcopenia. Additionally, skeletal muscle plasticity modulates metabolic adaptation to different pathophysiological situations. Thus, cellular bioenergetics and mitochondrial profile were studied in the skeletal muscle of overweight aged people without reaching obesity to prevent this extreme situation. Overweight aged muscle lacked ATP production, as indicated by defects in the phosphagen system, glycolysis and especially mostly by oxidative phosphorylation metabolic pathway. Overweight subjects exhibited an inhibition of mitophagy that was linked to an increase in mitochondrial biogenesis that underlies the accumulation of dysfunctional mitochondria and encourages the onset of sarcopenia. As a strategy to maintain cellular homeostasis, overweight subjects experienced a metabolic switch from oxidative to lactic acid fermentation metabolism, which allows continued ATP production under mitochondrial dysfunction, but without reaching physiological aged basal levels. This ATP depletion induced early signs of impaired contractile function and a decline in skeletal muscle structural integrity, evidenced by lower levels of filamin C. Our findings reveal the main effector pathways at an early stage of obesity and highlight the importance of mitochondrial metabolism in overweight and obese individuals. Exploiting mitochondrial profiles for therapeutic purposes in humans is an ambitious strategy for treating muscle impairment diseases.

Treatment with celastrol protects against obesity through suppression of galanin-induced fat intake and activation of PGC-1α/GLUT4 axis-mediated glucose consumption

Overweight and obesity may cause several metabolic complications, including type 2 diabetes mellitus and hyperlipidemia. Despite years of progress in medicine, there are no highly effective pharmacological treatments for obesity. The natural compound celastrol, a pentacyclic triterpene extracted from the roots of Tripterygium Wilfordi (thunder god vine) plant, exerts various bioactivities including anti-diabetic and anti-obese effects. Although celastrol could decrease food intake and obesity, the detailed mechanism for celastrol is still unclear as yet. Herein, we intended to determine the effect of celastrol on obesity and the underlying mechanisms. In the present study, diet-induced obese mice were treated with 100 μg/kg/d celastrol for the last 21 days, and 3T3-L1 cells were treated with celastrol for 6 h. The present findings showed that celastrol suppresses fat intake, and leads to weight loss by inhibiting galanin and its receptor expression in the hypothalamus of mice fed a high-fat diet. More importantly, in addition to these direct anti-obesity activities, celastrol augmented the PGC-1α and GLUT4 expression in adipocytes and skeletal muscles to increase glucose uptake through AKT and P38 MAPK activation. Celastrol also inhibited gluconeogenic activity through a CREB/PGC-1α pathway. In conclusion, the weight-lowering effects of celastrol are driven by decreased galanin-induced food consumption. Thus, this study contributes to our understanding of the anti-obese role of celastrol, and provides a possibility of using celastrol to treat obesity in clinic.

Skeletal muscle-specific Prmt1 deletion causes muscle atrophy via deregulation of the PRMT6-FOXO3 axis

Protein arginine methyltransferases (PRMTs) have emerged as important regulators of skeletal muscle metabolism and regeneration. However, the direct roles of the various PRMTs during skeletal muscle remodeling remain unclear. Using skeletal muscle-specific prmt1 knockout mice, we examined the function and downstream targets of PRMT1 in muscle homeostasis. We found that muscle-specific PRMT1 deficiency led to muscle atrophy. PRMT1-deficient muscles exhibited enhanced expression of a macroautophagic/autophagic marker LC3-II, FOXO3 and muscle-specific ubiquitin ligases, TRIM63/MURF-1 and FBXO32, likely contributing to muscle atrophy. The mechanistic study reveals that PRMT1 regulates FOXO3 through PRMT6 modulation. In the absence of PRMT1, increased PRMT6 specifically methylates FOXO3 at arginine 188 and 249, leading to its activation. Finally, we demonstrate that PRMT1 deficiency triggers FOXO3 hyperactivation, which is abrogated by PRMT6 depletion. Taken together, PRMT1 is a key regulator for the PRMT6-FOXO3 axis in the control of autophagy and protein degradation underlying muscle maintenance. Abbreviations: Ad-RNAi: adenovirus-delivered small interfering RNA; AKT: thymoma viral proto-oncogene; AMPK: AMP-activated protein kinase; Baf A1: bafilomycin A₁; CSA: cross-sectional area; EDL: extensor digitorum longus; FBXO32: F-box protein 32; FOXO: forkhead box O; GAS: gatrocnemieus; HDAC: histone deacetylase; IGF: insulin-like growth factor; LAMP: lysosomal-associated membrane protein; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; mKO: Mice with skeletal muscle-specific deletion of Prmt1; MTOR: mechanistic target of rapamycin kinase; MYH: myosin heavy chain; MYL1/MLC1f: myosin, light polypeptide 1; PRMT: protein arginine N-methyltransferase; sgRNA: single guide RNA; SQSTM1: sequestosome 1; SOL: soleus; TA: tibialis anterior; TRIM63/MURF-1: tripartite motif-containing 63; YY1: YY1 transcription factor.

SGTb regulates a surface localization of a guidance receptor BOC to promote neurite outgrowth

Neuritogenesis is a critical event for neuronal differentiation and neuronal circuitry formation during neuronal development and regeneration. Our previous study revealed a critical role of a guidance receptor BOC in a neuronal differentiation and neurite outgrowth. However, regulatory mechanisms for BOC signaling pathway remain largely unexplored. In the current study, we have identified Small glutamine-rich tetratricopeptide repeat (TPR)-containing b (SGTb) as a BOC interacting protein through yeast two-hybrid screening. Like BOC, SGTb is highly expressed in brain and P19 embryonal carcinoma (EC) cells differentiated into neuronal cells. BOC and SGTb proteins co-precipitate in mouse brain and differentiated P19 EC cells. Furthermore, BOC and SGTb co-localize in neurites and especially are concentrated at the tip of neurites in various neuronal cells. SGTb depletion attenuates neuronal differentiation of P19 cells through reduction of the surface level of BOC. Additionally, SGTb depletion causes BOC localization at neurite tip, coinciding with decreased p-JNK levels critical for actin cytoskeleton remodeling. The overexpression of SGTb or BOC restores JNK activation in BOC or SGTb-depleted cells, respectively. Finally, SGTb elevates the level of surface-resident BOC in BOC-depleted cells, restoring JNK activation. Taken together, our data suggest that SGTb interacts with BOC and regulates its surface level and consequent JNK activation, thereby promoting neuronal differentiation and neurite outgrowth.

Exercise-induced Protein Arginine Methyltransferase Expression in Skeletal Muscle

PURPOSE

This study aimed to determine protein arginine methyltransferase 1 (PRMT1), -4 (also known as coactivator-associated arginine methyltransferase 1 [CARM1]), and -5 expression and function during acute, exercise-induced skeletal muscle remodeling in vivo.

METHODS

C57BL/6 mice were assigned to one of three experimental groups: sedentary, acute bout of exercise, or acute exercise followed by 3 h of recovery. Mice in the exercise groups performed a single bout of treadmill running at 15 m·min for 90 min. Hindlimb muscles were collected, and quantitative real-time polymerase chain reaction and Western blotting were used to examine exercise-induced gene expression.

RESULTS

The PRMT gene expression and global enzyme activity were muscle-specific, generally being higher (P < 0.05) in slow, oxidative muscle, as compared with faster, more glycolytic tissue. Despite the significant activation of canonical exercise-induced signaling involving AMP-activated protein kinase and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), PRMT expression and activity at the whole muscle level were unchanged. However, subcellular analyses revealed a significant exercise-evoked myonuclear translocation of PRMT1 before the nuclear accumulation of PGC-1α. Acute physical activity also augmented (P < 0.05) the targeted methyltransferase activities of the PRMT in the myonuclear compartment, suggesting that PRMT-mediated histone arginine methylation is part of the early signals that drive muscle plasticity. Finally, basal PGC-1α asymmetric dimethylarginine status, as well as constitutive interactions between PGC-1α and PRMT1 or CARM1 may contribute to the exercise-induced muscle remodeling process.

CONCLUSIONS

The present study provides the first evidence that PRMT activity is selectively augmented during the initial activation of exercise-induced skeletal muscle remodeling in vivo. These data support the emergence of PRMTs as important players in the regulation of skeletal muscle plasticity.

Cardiac specific PRMT1 ablation causes heart failure through CaMKII dysregulation

Dysregulation of Ca²⁺/calmodulin-dependent protein kinase (CaMK)II is closely linked with myocardial hypertrophy and heart failure. However, the mechanisms that regulate CaMKII activity are incompletely understood. Here we show that protein arginine methyltransferase 1 (PRMT1) is essential for preventing cardiac CaMKII hyperactivation. Mice null for cardiac PRMT1 exhibit a rapid progression to dilated cardiomyopathy and heart failure within 2 months, accompanied by cardiomyocyte hypertrophy and fibrosis. Consistently, PRMT1 is downregulated in heart failure patients. PRMT1 depletion in isolated cardiomyocytes evokes hypertrophic responses with elevated remodeling gene expression, while PRMT1 overexpression protects against pathological responses to neurohormones. The level of active CaMKII is significantly elevated in PRMT1-deficient hearts or cardiomyocytes. PRMT1 interacts with and methylates CaMKII at arginine residues 9 and 275, leading to its inhibition. Accordingly, pharmacological inhibition of CaMKII restores contractile function in PRMT1-deficient mice. Thus, our data suggest that PRMT1 is a critical regulator of CaMKII to maintain cardiac function.

The mechanisms that regulate the activity of Ca^2 +/calmodulin-dependent protein kinase II (CaMKII) in the context of heart failure are incompletely understood. Here the authors show that protein arginine methyltransferase 1 (PRMT1) prevents cardiac hyperactivation of CaMKII and heart failure development by methylating CaMKII at arginine residues 9 and 275.

Central galanin receptor 2 mediates galanin action to promote systemic glucose metabolism of type 2 diabetic rats

Although recent results of our and other studies have showed that galanin (GAL) is an antidiabetic and anti-inflammatory neuropeptide, the molecular mechanism how central GAL regulates energy homeostasis and insulin sensitivity is still not fully understood. The aim of this study was to investigate whether central type 2 of GAL receptors (GALR2) are involved in the regulation of systemic glucose metabolism and its underlying mechanisms. In the present study, type 2 diabetic rats were intracerebroventricularly (i.c.v.) given 100 nM/kg/d GALR2 agonist M1145 or GALR2 antagonist M871 in 5 μl artificial cerebrospinal fluid once a day for consecutive 21 days. Then insulin resistance indexes, inflammatory factor and many genes associated with the function of glucose metabolism were examined in peripheral tissues. The present findings showed that the intracerebroventricular injection of M1145 or M871 respectively increased or decreased glucose infusion rates in hyperinsulinemic euglycemic clamp tests, but attenuated or enhanced the plasma inflammatory factors and glucose concentration in type 2 diabetic rats. Moreover, administration of M1145 markedly increased PGC-1α and GLUT4 expression in skeletal muscles and adipocytes of type 2 diabetic rats. In conclusion, activation of central GALR2 promotes glucose metabolism and ameliorates insulin resistance mainly through the PGC-1α/GLUT4 pathways. The central GALR2 is crucial to whole-body insulin sensitivity and energy homeostasis.

Further delineation of the phenotype caused by loss of function mutations in PRMT7

PRMT7 encodes for an arginine methyltransferase that methylates arginine residues on various protein substrates and has been shown to play a role in various developmental processes. Mutations in PRMT7 have been recently shown to be implicated in a phenotype with intellectual disability, short stature and brachydactyly, and considered to be a phenocopy of pseudohypoparathyroidism. We report a patient with short stature, psychomotor delay, hearing loss and brachydactyly, for whom whole exome sequencing detected two mutations in PRMT7 and parental segregation studies detected biallelic mutation inheritance. Few patients with biallelic PRMT7 mutations have been reported so far in the literature. We report a new patient and review all reported cases to date to delineate the clinical manifestations that may help in diagnosis this disorder, known as Short Stature, Brachydactyly, Intellectual Developmental Disability, and Seizures syndrome, allowing appropriate management and genetic counselling.

Ginsenoside Rg1 augments oxidative metabolism and anabolic response of skeletal muscle in mice

Background

The ginsenoside Rg1 has been shown to exert various pharmacological activities with health benefits. Previously, we have reported that Rg1 promoted myogenic differentiation and myotube growth in C2C12 myoblasts. In this study, the in vivo effect of Rg1 on fiber-type composition and oxidative metabolism in skeletal muscle was examined.

Methods

To examine the effect of Rg1 on skeletal muscle, 3-month-old mice were treated with Rg1 for 5 weeks. To assess muscle strength, grip strength tests were performed, and the lower hind limb muscles were harvested, followed by various detailed analysis, such as histological staining, immunoblotting, immunostaining, and real-time quantitative reverse transcription polymerase chain reaction. In addition, to verify the in vivo data, primary myoblasts isolated from mice were treated with Rg1, and the Rg1 effect on myotube growth was examined by immunoblotting and immunostaining analysis.

Results

Rg1 treatment increased the expression of myosin heavy chain isoforms characteristic for both oxidative and glycolytic muscle fibers; increased myofiber sizes were accompanied by enhanced muscle strength. Rg1 treatment also enhanced oxidative muscle metabolism with elevated oxidative phosphorylation proteins. Furthermore, Rg1-treated muscles exhibited increased levels of anabolic S6 kinase signaling.

Conclusion

Rg1 improves muscle functionality via enhancing muscle gene expression and oxidative muscle metabolism in mice.

Phe71 in Type III Trypanosomal Protein Arginine Methyltransferase 7 (TbPRMT7) Restricts the Enzyme to Monomethylation

Protein arginine methyltransferase 7 (PRMT7) is unique within the PRMT family as it is the only isoform known to exclusively make monomethylarginine (MMA). Given its role in epigenetics, the mechanistic basis for the strict monomethylation activity is under investigation. It is thought that PRMT7 enzymes are unable to add a second methyl group because of steric hindrance in the active site that restricts them to monomethylation. To test this, we probed the active site of trypanosomal PRMT7 (TbPRMT7) using accelerated molecular dynamics, site-directed mutagenesis, kinetic, binding, and product analyses. Both the dynamics simulations and experimental results show that the mutation of Phe71 to Ile converts the enzyme from a type III methyltransferase into a mixed type I/II, that is, an enzyme that can now perform dimethylation. In contrast, the serine and alanine mutants of Phe71 preserve the type III behavior of the native enzyme. These results are inconsistent with a sterics-only model to explain product specificity. Instead, molecular dynamics simulations of these variants bound to peptides show hydrogen bonding between would-be substrates and Glu172 of TbPRMT7. Only in the case of the Phe71 to Ile mutation is this interaction between MMA and the enzyme maintained, and the geometry for optimal S_N2 methyl transfer is obtained. The results of these studies highlight the benefit of combined computational and experimental methods in providing a better understanding for how product specificity is dictated by PRMTs.

Regulation of Skeletal Muscle Plasticity by Protein Arginine Methyltransferases and Their Potential Roles in Neuromuscular Disorders

Protein arginine methyltransferases (PRMTs) are a family of enzymes that catalyze the methylation of arginine residues on target proteins, thereby mediating a diverse set of intracellular functions that are indispensable for survival. Indeed, full-body knockouts of specific PRMTs are lethal and PRMT dysregulation has been implicated in the most prevalent chronic disorders, such as cancers and cardiovascular disease (CVD). PRMTs are now emerging as important mediators of skeletal muscle phenotype and plasticity. Since their first description in muscle in 2002, a number of studies employing wide varieties of experimental models support the hypothesis that PRMTs regulate multiple aspects of skeletal muscle biology, including development and regeneration, glucose metabolism, as well as oxidative metabolism. Furthermore, investigations in non-muscle cell types strongly suggest that proteins, such as peroxisome proliferator-activated receptor-γ coactivator-1α, E2F transcription factor 1, receptor interacting protein 140, and the tumor suppressor protein p53, are putative downstream targets of PRMTs that regulate muscle phenotype determination and remodeling. Recent studies demonstrating that PRMT function is dysregulated in Duchenne muscular dystrophy (DMD), spinal muscular atrophy (SMA), and amyotrophic lateral sclerosis (ALS) suggests that altering PRMT expression and/or activity may have therapeutic value for neuromuscular disorders (NMDs). This review summarizes our understanding of PRMT biology in skeletal muscle, and identifies uncharted areas that warrant further investigation in this rapidly expanding field of research.

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.

Apigenin enhances skeletal muscle hypertrophy and myoblast differentiation by regulating Prmt7

Apigenin, a natural flavone abundant in various plant-derived foods including parsley and celery, has been shown to prevent inflammation and inflammatory diseases. However, the effect of apigenin on skeletal muscle hypertrophy and myogenic differentiation has not previously been elucidated. Here, we investigated the effects of apigenin on quadricep muscle weight and running distance using C57BL/6 mice on an accelerating treadmill. Apigenin stimulated mRNA expression of MHC (myosin heavy chain) 1, MHC2A, and MHC2B in the quadricep muscles of these animals. GPR56 (G protein-coupled receptor 56) and its ligand collagen III were upregulated by apigenin supplementation, together with enhanced PGC-1α, PGC-1α1, PGC-1α4, IGF1, and IGF2 expression. Prmt7 protein expression increased in conjunction with Akt and mTORC1 activation. Apigenin treatment also upregulated FNDC5 (fibronectin type III domain containing 5) mRNA expression and serum irisin levels. Furthermore, apigenin stimulated C2C12 myogenic differentiation and upregulated total MHC, MHC2A, and MHC2B expression. These events were attributable to an increase in Prmt7-p38-myoD expression and Akt and S6K1 phosphorylation. We also observed that Prmt7 regulates both PGC-1α1 and PGC-1α4 expression, resulting in a subsequent increase in GPR56 expression and mTORC1 activation. Taken together, these findings suggest that apigenin supplementation can promote skeletal muscle hypertrophy and myogenic differentiation by regulating the Prmt7-PGC-1α-GPR56 pathway, as well as the Prmt7-p38-myoD pathway, which may contribute toward the prevention of skeletal muscle weakness.

Mapping arginine methylation in the human body and cardiac disease

PURPOSE

Arginine methylation (ArgMe) is one of the most ubiquitous PTMs, and hundreds of proteins undergo ArgMe in, for example, brain. However, the scope of ArgMe in many tissues, including the heart, is currently underexplored. Here, we aimed to (i) identify proteins undergoing ArgMe in human organs, and (ii) expose the relevance of ArgMe in cardiac disease.

EXPERIMENTAL DESIGN

The publicly available proteomic data is used to search for ArgMe in 13 human tissues. To induce H9c2 cardiac-like cell hypertrophy glucose is used.

RESULTS

The results show that ArgMe is mainly tissue-specific; nevertheless, the authors suggest an embryonic origin of core ArgMe events. In the heart, 103 mostly novel ArgMe sites in 58 nonhistone proteins are found. The authors provide compelling evidence that cardiac protein ArgMe is relevant to cardiomyocyte ontology, and important for proper cardiac function. This is highlighted by the fact that genetic mutations affecting methylated arginine positions are often associated with cardiac disease, including hypertrophic cardiomyopathy. The pilot experimental data suggesting significant changes in ArgMe profiles of H9c2 cells upon induction of cell hypertrophy using glucose is provided.

CONCLUSIONS AND CLINICAL RELEVANCE

The work calls for in-depth investigation of ArgMe in normal and diseased tissues using methods including clinical proteomics.