Smyd1b is required for skeletal and cardiac muscle function in zebrafish

Social context prevents heat hormetic effects against mutagens during fish development

Since stress can be transmitted to congeners via social metabolites, it is paramount to understand how the social context of abiotic stress influences aquatic organisms' responses to global changes. Here, we integrated the transcriptomic and phenotypic responses of zebrafish embryos to a UV damage/repair assay following scenarios of heat stress, its social context and their combination. Heat stress preceding UV exposure had a hormetic effect through the cellular stress response and DNA repair, rescuing and/or protecting embryos from UV damage. However, experiencing heat stress within a social context negated this molecular hormetic effect and lowered larval fitness. We discuss the molecular basis of interindividual chemical transmission within animal groups as another layer of complexity to organisms' responses to environmental stressors.

Hsf1 is essential for proteotoxic stress response in smyd1b-deficient embryos and fish survival under heat shock

Molecular chaperones play critical roles in post-translational maintenance in protein homeostasis. Previous studies have shown that loss of Smyd1b function results in defective myofibril organization and dramatic upregulation of heat shock protein gene (hsp) expression in muscle cells of zebrafish embryos. To investigate the molecular mechanisms and functional importance of this stress response, we characterized changes of gene expression in smyd1b knockdown and knockout embryos using RNA-seq. The results showed that the top upregulated genes encode mostly cytosolic heat shock proteins. Co-IP assay revealed that the upregulated cytosolic Hsp70s associate with myosin chaperone UNC45b which is critical for myosin protein folding and sarcomere assembly. Strikingly, several hsp70 genes also display muscle-specific upregulation in response to heat shock-induced stress in zebrafish embryos. To investigate the regulation of hsp gene upregulation and its functional significance in muscle cells, we generated heat shock factor 1 (hsf-/-) knockout zebrafish mutants and analyzed hsp gene expression and muscle phenotype in the smyd1b-/-single and hsf1-/-;smyd1b-/- double-mutant embryos. The results showed that knockout of hsf1 blocked the hsp gene upregulation and worsened the muscle defects in smyd1b-/- mutant embryos. Moreover, we demonstrated that Hsf1 is essential for fish survival under heat shock (HS) conditions. Together, these studies uncover a correlation between Smyd1b deficiency and the Hsf1-activated heat shock response (HSR) in regulating muscle protein homeostasis and myofibril assembly and demonstrate that the Hsf1-mediated hsp gene upregulation is vital for the survival of zebrafish larvae under thermal stress conditions.

Heart Morphogenesis Requires Smyd1b for Proper Incorporation of the Second Heart Field in Zebrafish

Background/Objectives: Abnormal development of the second heart field significantly contributes to congenital heart defects, often caused by disruptions in tightly regulated molecular pathways. Smyd1, a gene encoding a protein with SET and MYND domains, is essential for heart and skeletal muscle development. Mutations in SMYD1 result in severe cardiac malformations and misregulation of Hand2 expression in mammals. This study examines the role of Smyd1b in zebrafish cardiac morphogenesis to elucidate its function and the mechanisms underlying congenital heart defects. Methods: Smyd1b (still heart) mutant embryos were analyzed for cardiac defects, and changes in gene expression related to heart development using live imaging, in situ hybridization, quantitative PCR and immunofluorescent comparisons and analysis. Results: Smyd1b mutants displayed severe cardiac defects, including failure to loop, severe edema, and an expansion of cardiac jelly linked to increased has2 expression. Additionally, the expression of key cardiac transcription factors, such as gata4, gata5, and nkx2.5, was notably reduced, indicating disrupted transcriptional regulation. The migration of cardiac progenitors was impaired and the absence of Islet-1-positive cells in the mutant hearts suggests a failed contribution of SHF progenitor cells. Conclusions: These findings underscore the essential role of Smyd1b in regulating cardiac morphogenesis and the development of the second heart field. This study highlights the potential of Smyd1b as a key factor in understanding the genetic and molecular mechanisms underlying congenital heart defects and cardiac development.

Expression of Smyd1b_tv1 by Alternative Splicing in Cardiac Muscle is Critical for Sarcomere Organization in Cardiomyocytes and Heart Function

Smyd1, a member of the Smyd lysine methyltransferase family, plays an important role in myofibrillogenesis of skeletal and cardiac muscles. Loss of Smyd1b (a Smyd1 ortholog) function in zebrafish results in embryonic death from heart malfunction. smyd1b encodes two isoforms, Smyd1b_tv1 and Smyd1b_tv2, differing by 13 amino acids due to alternative splicing. While smyd1 alternative splicing is evolutionarily conserved, the isoform-specific expression and function of Smyd1b_tv1 and Smyd1b_tv2 remained unknown. Here we analyzed their expression and function in skeletal and cardiac muscles. Our analysis revealed expression of smyd1b_tv1 predominately in cardiac and smyd1b_tv2 in skeletal muscles. Using zebrafish models expressing only one isoform, we demonstrated that Smyd1b_tv1 is essential for cardiomyocyte differentiation and fish viability, whereas Smyd1b_tv2 is dispensable for heart development and fish survival. Cellular and biochemical analyses revealed that Smyd1b_tv1 differs from Smyd1b_tv2 in protein localization and binding with myosin chaperones. While Smyd1b_tv2 diffused in the cytosol of muscle cells, Smyd1b_tv1 was localized to M-lines and essential for sarcomere organization in cardiomyocytes. Co-IP analysis revealed a stronger binding of Smyd1b_tv1 with chaperones and cochaperones compared with Smyd1b_tv2. Collectively, these findings highlight the nonequivalence of Smyd1b isoforms in cardiomyocyte differentiation, emphasizing the critical role of Smyd1b_tv1 in cardiac function.

Smyd1: Implications for novel approaches in rhabdomyosarcoma therapy

Rhabdomyosarcoma (RMS), a tumor that consists of poorly differentiated skeletal muscle cells, is the most common soft-tissue sarcoma in children. Despite considerable progress within the last decades, therapeutic options are still limited, warranting the need for novel approaches. Recent data suggest deregulation of the Smyd1 protein, a sumoylation target as well as H3K4me2/3 methyltransferase and transcriptional regulator in myogenesis, and its binding partner skNAC, in RMS cells. Here, we show that despite the fact that most RMS cells express at least low levels of Smyd1 and skNAC, failure to upregulate expression of these genes in reaction to differentiation-promoting signals can always be observed. While overexpression of the Smyd1 gene enhances many aspects of RMS cell differentiation and inhibits proliferation rate and metastatic potential of these cells, functional integrity of the putative Smyd1 sumoylation motif and its SET domain, the latter being crucial for HMT activity, appear to be prerequisites for most of these effects. Based on these findings, we explored the potential for novel RMS therapeutic strategies, employing small-molecule compounds to enhance Smyd1 activity. In particular, we tested manipulation of (a) Smyd1 sumoylation, (b) stability of H3K4me2/3 marks, and (c) calpain activity, with calpains being important targets of Smyd1 in myogenesis. We found that specifically the last strategy might represent a promising approach, given that suitable small-molecule compounds will be available for clinical use in the future.

Epigenetics

Epigenetics is the study of heritable changes to the genome and gene expression patterns that are not caused by direct changes to the DNA sequence. Examples of these changes include posttranslational modifications to DNA-bound histone proteins, DNA methylation, and remodeling of nuclear architecture. Collectively, epigenetic changes provide a layer of regulation that affects transcriptional activity of genes while leaving DNA sequences unaltered. Sequence variants or mutations affecting enzymes responsible for modifying or sensing epigenetic marks have been identified in patients with congenital heart disease (CHD), and small-molecule inhibitors of epigenetic complexes have shown promise as therapies for adult heart diseases. Additionally, transgenic mice harboring mutations or deletions of genes encoding epigenetic enzymes recapitulate aspects of human cardiac disease. Taken together, these findings suggest that the evolving field of epigenetics will inform our understanding of congenital and adult cardiac disease and offer new therapeutic opportunities.

Lysine methylation signaling in skeletal muscle biology: from myogenesis to clinical insights

Lysine methylation signaling is well studied for its key roles in the regulation of transcription states through modifications on histone proteins. While histone lysine methylation has been extensively studied, recent discoveries of lysine methylation on thousands of non-histone proteins has broadened our appreciation for this small chemical modification in the regulation of protein function. In this review, we highlight the significance of histone and non-histone lysine methylation signaling in skeletal muscle biology, spanning development, maintenance, regeneration, and disease progression. Furthermore, we discuss potential future implications for its roles in skeletal muscle biology as well as clinical applications for the treatment of skeletal muscle-related diseases.

Down-regulated Smyd1 participated in the inhibition of myoblast differentiation induced by cigarette smoke extract

The histone methyltransferase Smyd1 is essential for muscle development; however, its role in smoking-induced skeletal muscle atrophy and dysfunction has not been investigated thus far. In this study, Smyd1 was overexpressed or knocked down in C2C12 myoblasts by an adenovirus vector and cultured in differentiation medium containing 5% cigarette smoke extract (CSE) for 4 days. CSE exposure resulted in inhibition of C2C12 cell differentiation and downregulation of Smyd1 expression, whereas Smyd1 overexpression reduced the degree of inhibition of myotube differentiation caused by CSE exposure. CSE exposure activated P2RX7-mediated apoptosis and pyroptosis, caused increased intracellular reactive oxygen species (ROS) levels, and impaired mitochondrial biogenesis and increased protein degradation by downregulating PGC1α, whereas Smyd1 overexpression partially restored the altered protein levels caused by CSE exposure. Smyd1 knockdown alone produced a phenotype similar to CSE exposure, and Smyd1 knockdown during CSE exposure aggravated the degree of inhibition of myotube differentiation and the degree of activation of P2RX7. CSE exposure suppressed H3K4me2 expression, and chromatin immunoprecipitation confirmed the transcriptional regulation of P2rx7 by H3K4me2 modification. Our findings suggest that CSE exposure mediates C2C12 cell apoptosis and pyroptosis through the Smyd1-H3K4me2-P2RX7 axis, and inhibits PGC1α expression to impair mitochondrial biosynthesis and increase protein degradation by inhibiting Smyd1 expression, ultimately leading to abnormal C2C12 myoblasts differentiation and impaired myotube formation.

Lysine Methyltransferase SMYD1 Regulates Myogenesis via skNAC Methylation

The SMYD family is a unique class of lysine methyltransferases (KMTases) whose catalytic SET domain is split by a MYND domain. Among these, Smyd1 was identified as a heart- and skeletal muscle-specific KMTase and is essential for cardiogenesis and skeletal muscle development. SMYD1 has been characterized as a histone methyltransferase (HMTase). Here we demonstrated that SMYD1 methylates is the Skeletal muscle-specific splice variant of the Nascent polypeptide-Associated Complex (skNAC) transcription factor. SMYD1-mediated methylation of skNAC targets K1975 within the carboxy-terminus region of skNAC. Catalysis requires physical interaction of SMYD1 and skNAC via the conserved MYND domain of SMYD1 and the PXLXP motif of skNAC. Our data indicated that skNAC methylation is required for the direct transcriptional activation of myoglobin (Mb), a heart- and skeletal muscle-specific hemoprotein that facilitates oxygen transport. Our study revealed that the skNAC, as a methylation target of SMYD1, illuminates the molecular mechanism by which SMYD1 cooperates with skNAC to regulate transcriptional activation of genes crucial for muscle functions and implicates the MYND domain of the SMYD-family KMTases as an adaptor to target substrates for methylation.

Identification of Two Homozygous Variants in MYBPC3 and SMYD1 Genes Associated with Severe Infantile Cardiomyopathy

Mutations in cardiac genes are one of the primary causes of infantile cardiomyopathy. In this study, we report the genetic findings of two siblings carrying variations in the MYBPC3 and SMYD1 genes. The first patient is a female proband exhibiting hypertrophic cardiomyopathy (HCM) and biventricular heart failure carrying a truncating homozygous MYBPC3 variant c.1224-52G>A (IVS13-52G>A) and a novel homozygous variant (c.302A>G; p.Asn101Ser) in the SMYD1 gene. The second patient, the proband's sibling, is a male infant diagnosed with hypertrophic cardiomyopathy and carries the same homozygous MYBPC3 variant. While this specific MYBPC3 variant (c.1224-52G>A, IVS13-52G>A) has been previously reported to be associated with adult-onset hypertrophic cardiomyopathy, this is the first report linking it to infantile cardiomyopathy. In addition, this work describes, for the first time, a novel SMYD1 variant (c.302A>G; p.Asn101Ser) that has never been reported. We performed a histopathological evaluation of tissues collected from both probands and show that these variants lead to myofibrillar disarray, reduced and irregular mitochondrial cristae and cardiac fibrosis. Together, these results provide critical insight into the molecular functionality of these genes in human cardiac physiology.

Epigenetic Modification Factors and microRNAs Network Associated with Differentiation of Embryonic Stem Cells and Induced Pluripotent Stem Cells toward Cardiomyocytes: A Review

More research is being conducted on myocardial cell treatments utilizing stem cell lines that can develop into cardiomyocytes. All of the forms of cardiac illnesses have shown to be quite amenable to treatments using embryonic (ESCs) and induced pluripotent stem cells (iPSCs). In the present study, we reviewed the differentiation of these cell types into cardiomyocytes from an epigenetic standpoint. We also provided a miRNA network that is devoted to the epigenetic commitment of stem cells toward cardiomyocyte cells and related diseases, such as congenital heart defects, comprehensively. Histone acetylation, methylation, DNA alterations, N6-methyladenosine (m⁶a) RNA methylation, and cardiac mitochondrial mutations are explored as potential tools for precise stem cell differentiation.

Dynamics of metabolism and regulation of epigenetics during cardiomyocytes maturation

Maturation is the last step of heart growth that prepares the organ over the lifetime of the mammal for powerful, effective, and sustained pumping. Structural, gene expression, physiological, and functional specialties of cardiomyocytes describe this mechanism as the heart transits from fetus to adult phases. The main cornerstones of maturation of cardiomyocytes are reviewed and primary regulatory mechanisms are summarized to facilitate and organize these cellular activities. During embryonic development, cardiomyocytes proliferate rigorously but leave the cell cycle permanently immediately after the parturition of the child and experience terminal differentiation. The activation of a host of genes specific for the mature heart is correlated with the exit from the cell cycle. Even when exposed to mitogenic stimuli, the bulk of mature cardiomyocytes do not re-join the cell cycle. The reason for this permanent exit from the cell cycle is shown to be linked with stable switching off of the genes of the cell cycle directly involved in the G2/M transition phase and cytokinesis development. Researchers also trying to explain the molecular mechanism involved in stable inhibition of the gene and described structural changes (epigenetic and chromatin) in this mechanism. Substantial developments in the future with advances in the scientific platforms used for cardiomyocyte maturation research will broaden our understanding of this mechanism and result in better maturation of cardiomyocyte-derived pluripotent stem cells and effective treatment approaches for cardiovascular diseases.

Gene expression and functional analysis of Aha1a and Aha1b in stress response in zebrafish

Activator of heat shock protein 90 (hsp90) ATPase (Aha1) is a Hsp90 co-chaperone required for Hsp90 ATPase activation. Aha1 is essential for yeast survival and muscle development in C. elegans under elevated temperature and hsp90-deficeiency induced stress conditions. The roles of Aha1 in vertebrates are poorly understood. Here, we characterized the expression and function of Aha1 in zebrafish. We showed that zebrafish genome contains two aha1 genes, aha1a and aha1b, that show distinct patterns of expression during development. Under the normal physiological conditions, aha1a is primarily expressed in skeletal muscle cells of zebrafish embryos, while aha1b is strongly expressed in the head region. aha1a and aha1b expression increased dramatically in response to heat shock induced stress. In addition, Aha1a-GFP fusion protein exhibited a dynamic translocation in muscle cells in response to heat shock. Moreover, upregulation of aha1 expression was also observed in hsp90a1 knockdown embryos that showed a muscle defect. Genetic studies demonstrated that knockout of aha1a, aha1b or both had no detectable effect on embryonic development, survival, and growth in zebrafish. The aha1a and aha1b mutant embryos showed normal muscle development and stress response in response to heat shock. Single or double aha1a and aha1b mutants could grow into normal reproductive adults with normal skeletal muscle structure and morphology compared with wild type control. Together, data from these studies indicate that Aha1a and Aha1b are involved in stress response. However, they are dispensable in zebrafish embryonic development, growth, and survival.

Overexpression of Lifeact-GFP Disrupts F-Actin Organization in Cardiomyocytes and Impairs Cardiac Function

Lifeact-GFP is a frequently used molecular probe to study F-actin structure and dynamic assembly in living cells. In this study, we generated transgenic zebrafish models expressing Lifeact-GFP specifically in cardiac muscles to investigate the effect of Lifeact-GFP on heart development and its application to study cardiomyopathy. The data showed that transgenic zebrafish with low to moderate levels of Lifeact-GFP expression could be used as a good model to study contractile dynamics of actin filaments in cardiac muscles in vivo. Using this model, we demonstrated that loss of Smyd1b, a lysine methyltransferase, disrupted F-actin filament organization in cardiomyocytes of zebrafish embryos. Our studies, however, also demonstrated that strong Lifeact-GFP expression in cardiomyocytes was detrimental to actin filament organization in cardiomyocytes that led to pericardial edema and early embryonic lethality of zebrafish embryos. Collectively, these data suggest that although Lifeact-GFP is a good probe for visualizing F-actin dynamics, transgenic models need to be carefully evaluated to avoid artifacts induced by Lifeact-GFP overexpression.

Epigenetic Regulation of Cardiomyocyte Differentiation from Embryonic and Induced Pluripotent Stem Cells

With the intent to achieve the best modalities for myocardial cell therapy, different cell types are being evaluated as potent sources for differentiation into cardiomyocytes. Embryonic stem cells and induced pluripotent stem cells have great potential for future progress in the treatment of myocardial diseases. We reviewed aspects of epigenetic mechanisms that play a role in the differentiation of these cells into cardiomyocytes. Cardiomyocytes proliferate during fetal life, and after birth, they undergo permanent terminal differentiation. Upregulation of cardiac-specific genes in adults induces hypertrophy due to terminal differentiation. The repression or expression of these genes is controlled by chromatin structural and epigenetic changes. However, few studies have reviewed and analyzed the epigenetic aspects of the differentiation of embryonic stem cells and induced pluripotent stem cells into cardiac lineage cells. In this review, we focus on the current knowledge of epigenetic regulation of cardiomyocyte proliferation and differentiation from embryonic and induced pluripotent stem cells through histone modification and microRNAs, the maintenance of pluripotency, and its alteration during cardiac lineage differentiation.

Copper ions impair zebrafish skeletal myofibrillogenesis via epigenetic regulation

Unbalanced copper (Cu²⁺ ) homeostasis is associated with the developmental defects of vertebrate myogenesis, but the underlying molecular mechanisms remain elusive. In this study, it was found that Cu²⁺ stressed zebrafish embryos and larvae showed reduced locomotor speed as well as loose and decreased myofibrils in skeletal muscle, coupled with the downregulated expression of muscle fiber markers mylpfa and smyhc1l and the irregular arrangement of myofibril and sarcomere. Meanwhile, the Cu²⁺ stressed zebrafish embryos and larvae also showed significant reduction in the expression of H3K4 methyltransferase smyd1b transcripts and H3K4me3 protein as well as in the binding enrichment of H3K4me3 on gene mylpfa promoter in skeletal muscle cells, suggesting that smyd1b-H3K4me3 axis mediates the Cu²⁺ -induced myofibrils specification defects. Additionally, whole genome DNA methylation sequencing unveiled that the gene smyd5 exhibited significant promoter hyper-methylation and increased expression in Cu²⁺ stressed embryos, and the ectopic expression of smyd5 in zebrafish embryos also induced the myofibrils specification defects as those observed in Cu²⁺ stressed embryos. Moreover, Cu²⁺ was shown to suppress myofibrils specification and smyd1b promoter transcriptional activity directly independent of the integral function of copper transporter cox17 and atp7b. All these data may shed light on the linkage of unbalanced copper homeostasis with specific gene promoter methylation and epigenetic histone protein modification as well as the resultant signaling transduction and the myofibrillogenesis defects.

DNA methylation and histone modifications are essential for regulation of stem cell formation and differentiation in zebrafish development

The complex processes necessary for embryogenesis require a gene regulatory network that is complex and systematic. Gene expression regulates development and organogenesis, but this process is altered and fine-tuned by epigenetic regulators that facilitate changes in the chromatin landscape. Epigenetic regulation of embryogenesis adjusts the chromatin structure by modifying both DNA through methylation and nucleosomes through posttranslational modifications of histone tails. The zebrafish is a well-characterized model organism that is a quintessential tool for studying developmental biology. With external fertilization, low cost and high fecundity, the zebrafish are an efficient tool for studying early developmental stages. Genetic manipulation can be performed in vivo resulting in quick identification of gene function. Large-scale genome analyses including RNA sequencing, chromatin immunoprecipitation and chromatin structure all are feasible in the zebrafish. In this review, we highlight the key events in zebrafish development where epigenetic regulation plays a critical role from the early stem cell stages through differentiation and organogenesis.

Smyd1 is essential for myosin expression and sarcomere organization in craniofacial, extraocular, and cardiac muscles

Skeletal and cardiac muscles are striated myofibers that contain highly organized sarcomeres for muscle contraction. Recent studies revealed that Smyd1, a lysine methyltransferase, plays a key role in sarcomere assembly in heart and trunk skeletal muscles. However, Smyd1 expression and function in craniofacial muscles are not known. Here, we analyze the developmental expression and function of two smyd1 paralogous genes, smyd1a and smyd1b, in craniofacial and cardiac muscles of zebrafish embryos. Our data show that loss of smyd1a (smyd1amb5) or smyd1b (smyd1bsa15678) has no visible effects on myogenic commitment and expression of myod and myosin heavy-chain mRNA transcripts in craniofacial muscles. However, myosin heavy-chain protein accumulation and sarcomere organization are dramatically reduced in smyd1bsa15678 single mutant, and almost completely diminish in smyd1amb5; smyd1bsa15678 double mutant, but not in smyd1amb5 mutant. Similar defects are also observed in cardiac muscles of smyd1bsa15678 mutant. Defective craniofacial and cardiac muscle formation is associated with an upregulation of hsp90α1 and unc45b mRNA expression in smyd1bsa15678 and smyd1amb5; smyd1bsa15678 mutants. Together, our studies indicate that Smyd1b, but not Smyd1a, plays a key role in myosin heavy-chain protein expression and sarcomere organization in craniofacial and cardiac muscles. Loss of smyd1b results in muscle-specific stress response.

The Gridlock transcriptional repressor impedes vertebrate heart regeneration by restricting expression of lysine methyltransferase

Teleost zebrafish and neonatal mammalian hearts exhibit the remarkable capacity to regenerate through dedifferentiation and proliferation of pre-existing cardiomyocytes (CMs). Although many mitogenic signals that stimulate zebrafish heart regeneration have been identified, transcriptional programs that restrain injury-induced CM renewal are incompletely understood. Here, we report that mutations in gridlock (grl; also known as hey2), encoding a Hairy-related basic helix-loop-helix transcriptional repressor, enhance CM proliferation and reduce fibrosis following damage. In contrast, myocardial grl induction blunts CM dedifferentiation and regenerative responses to heart injury. RNA sequencing analyses uncover Smyd2 lysine methyltransferase (KMT) as a key transcriptional target repressed by Grl. Reduction in Grl protein levels triggered by injury induces smyd2 expression at the wound myocardium, enhancing CM proliferation. We show that Smyd2 functions as a methyltransferase and modulates the Stat3 methylation and phosphorylation activity. Inhibition of the KMT activity of Smyd2 reduces phosphorylated Stat3 at cardiac wounds, suppressing the elevated CM proliferation in injured grl mutant hearts. Our findings establish an injury-specific transcriptional repression program in governing CM renewal during heart regeneration, providing a potential strategy whereby silencing Grl repression at local regions might empower regeneration capacity to the injured mammalian heart.

Highlighted Article: Novel mechanisms of the Grl-Smyd2 network govern vertebrate CM renewal and heart regeneration, which might be relevant in developing strategies for regeneration interventions in humans.

Role of Muscle-Specific Histone Methyltransferase (Smyd1) in Exercise-Induced Cardioprotection against Pathological Remodeling after Myocardial Infarction

Pathological remodeling is the main detrimental complication after myocardial infarction (MI). Overproduction of reactive oxygen species (ROS) in infarcted myocardium may contribute to this process. Adequate exercise training after MI may reduce oxidative stress-induced cardiac tissue damage and remodeling. SET and MYND domain containing 1 (Smyd1) is a muscle-specific histone methyltransferase which is upregulated by resistance training, may strengthen sarcomere assembly and myofiber folding, and may promote skeletal muscles growth and hypertrophy. However, it remains elusive if Smyd1 has similar functions in post-MI cardiac muscle and participates in exercise-induced cardioprotection. Accordingly, we investigated the effects of interval treadmill exercise on cardiac function, ROS generation, Smyd1 expression, and sarcomere assembly of F-actin in normal and infarcted hearts. Adult male rats were randomly divided into five groups (n = 10/group): control (C), exercise alone (EX), sham-operated (S), MI induced by permanent ligation of left anterior descending coronary artery (MI), and MI with interval exercise training (MI + EX). Exercise training significantly improved post-MI cardiac function and sarcomere assembly of F-actin. The cardioprotective effects were associated with increased Smyd1, Trx1, cTnI, and α-actinin expression as well as upregulated ratio of phosphorylated AMP-activated protein kinase (AMPK)/AMPK, whereas Hsp90, MuRF1, brain natriuretic peptide (BNP) expression, ROS generation, and myocardial fibrosis were attenuated. The improved post-MI cardiac function was associated with increased Smyd1 expression. In cultured H9C2 cardiomyoblasts, in vitro treatment with H₂O₂ (50 µmol/L) or AMP-activated protein kinase (AMPK) agonist (AICAR, 1 mmol/L) or their combination for 4 h simulated the effects of exercise on levels of ROS and Smyd1. In conclusion, we demonstrated a novel role of Smyd1 in association with post-MI exercise-induced cardioprotection. The moderate level of ROS-induced upregulation of Smyd1 may be an important target for modulating post-MI cardiac function and remodeling.

Under construction: The dynamic assembly, maintenance, and degradation of the cardiac sarcomere

The sarcomere is the basic contractile unit of striated muscle and is a highly ordered protein complex with the actin and myosin filaments at its core. Assembling the sarcomere constituents into this organized structure in development, and with muscle growth as new sarcomeres are built, is a complex process coordinated by numerous factors. Once assembled, the sarcomere requires constant maintenance as its continuous contraction is accompanied by elevated mechanical, thermal, and oxidative stress, which predispose proteins to misfolding and toxic aggregation. To prevent protein misfolding and maintain sarcomere integrity, the sarcomere is monitored by an assortment of protein quality control (PQC) mechanisms. The need for effective PQC is heightened in cardiomyocytes which are terminally differentiated and must survive for many years while preserving optimal mechanical output. To prevent toxic protein aggregation, molecular chaperones stabilize denatured sarcomere proteins and promote their refolding. However, when old and misfolded proteins cannot be salvaged by chaperones, they must be recycled via degradation pathways: the calpain and ubiquitin-proteasome systems, which operate under basal conditions, and the stress-responsive autophagy-lysosome pathway. Mutations to and deficiency of the molecular chaperones and associated factors charged with sarcomere maintenance commonly lead to sarcomere structural disarray and the progression of heart disease, highlighting the necessity of effective sarcomere PQC for maintaining cardiac function. This review focuses on the dynamic regulation of assembly and turnover at the sarcomere with an emphasis on the chaperones involved in these processes and describes the alterations to chaperones - through mutations and deficient expression - implicated in disease progression to heart failure.

Function of the MYND Domain and C-Terminal Region in Regulating the Subcellular Localization and Catalytic Activity of the SMYD Family Lysine Methyltransferase Set5

SMYD lysine methyltransferases target histones and nonhistone proteins for methylation and are critical regulators of muscle development and implicated in neoplastic transformation. They are characterized by a split catalytic SET domain and an intervening MYND zinc finger domain, as well as an extended C-terminal domain. Saccharomyces cerevisiae contains two SMYD proteins, Set5 and Set6, which share structural elements with the mammalian SMYD enzymes. Set5 is a histone H4 lysine 5, 8, and 12 methyltransferase, implicated in the regulation of stress responses and genome stability. While the SMYD proteins have diverse roles in cells, there are many gaps in our understanding of how these enzymes are regulated. Here, we performed mutational analysis of Set5, combined with phosphoproteomics, to identify regulatory mechanisms for its enzymatic activity and subcellular localization. Our results indicate that the MYND domain promotes Set5 chromatin association in cells and is required for its role in repressing subtelomeric genes. Phosphoproteomics revealed extensive phosphorylation of Set5, and phosphomimetic mutations enhance Set5 catalytic activity but diminish its ability to interact with chromatin in cells. These studies uncover multiple regions within Set5 that regulate its localization and activity and highlight potential avenues for understanding mechanisms controlling the diverse roles of SMYD enzymes.

Defective sarcomere organization and reduced larval locomotion and fish survival in slow muscle heavy chain 1 (smyhc1) mutants

Zebrafish skeletal muscles are broadly divided into slow-twitch and fast-twitch muscle fibers. The slow fibers, which express a slow fiber-specific myosin heavy chain 1 (Smyhc1), are the first group of muscle fibers formed during myogenesis. To uncover Smyhc1 function in muscle growth, we generated three mutant alleles with reading frame shift mutations in the zebrafish smyhc1 gene using CRISPR. The mutants showed shortened sarcomeres with no thick filaments and M-lines in slow fibers of the mutant embryos. However, the formation of slow muscle precursors and expression of other slow muscle genes were not affected and fast muscles appeared normal. The smyhc1 mutant embryos and larvae showed reduced locomotion and food intake. The mutant larvae exhibited increased lethality of incomplete penetrance. Approximately 2/5 of the homozygous mutants were viable and grew into reproductive adults. These adult mutants displayed a typical pattern of slow and fast muscle fiber distribution, and regained normal slow muscle formation. Together, our studies indicate that Smyhc1 is essential for myogenesis in embryonic slow muscles, and loss of Smyhc1 results in defective sarcomere assembly, reduces larval motility and fish survival, but has no visible impact on muscle growth in juvenile and adult zebrafish that escape the larval lethality.

The SMYD3 methyltransferase promotes myogenesis by activating the myogenin regulatory network

The coordinated expression of myogenic regulatory factors, including MyoD and myogenin, orchestrates the steps of skeletal muscle development, from myoblast proliferation and cell-cycle exit, to myoblast fusion and myotubes maturation. Yet, it remains unclear how key transcription factors and epigenetic enzymes cooperate to guide myogenic differentiation. Proteins of the SMYD (SET and MYND domain-containing) methyltransferase family participate in cardiac and skeletal myogenesis during development in zebrafish, Drosophila and mice. Here, we show that the mammalian SMYD3 methyltransferase coordinates skeletal muscle differentiation in vitro. Overexpression of SMYD3 in myoblasts promoted muscle differentiation and myoblasts fusion. Conversely, silencing of endogenous SMYD3 or its pharmacological inhibition impaired muscle differentiation. Genome-wide transcriptomic analysis of murine myoblasts, with silenced or overexpressed SMYD3, revealed that SMYD3 impacts skeletal muscle differentiation by targeting the key muscle regulatory factor myogenin. The role of SMYD3 in the regulation of skeletal muscle differentiation and myotube formation, partially via the myogenin transcriptional network, highlights the importance of methyltransferases in mammalian myogenesis.

Generation of MuRF-GFP transgenic zebrafish models for investigating murf gene expression and protein localization in Smyd1b and Hsp90α1 knockdown embryos

Muscle-specific RING-finger proteins (MuRFs) are E3 ubiquitin ligases that play important roles in protein quality control in skeletal and cardiac muscles. Here we characterized murf gene expression and protein localization in zebrafish embryos. We found that the zebrafish genome contains six murf genes, including murf1a, murf1b, murf2a, murf2b, murf3 and a murf2-like gene that are specifically expressed in skeletal and cardiac muscles of zebrafish embryos. To analyze the subcellular localization, we generated transgenic zebrafish models expressing MurF1a-GFP or MuRF2a-GFP fusion proteins. MuRF1a-GFP and MuRF2a-GFP showed distinct patterns of subcellular localization. MuRF1a-GFP displayed a striated pattern of localization in myofibers, whereas MuRF2a-GFP mainly exhibited a random pattern of punctate distribution. The MuRF1a-GFP signal appeared as small dots aligned along the M-lines of the sarcomeres in skeletal myofibers. To determine whether knockdown of smyd1b or hsp90α1 that increased myosin protein degradation could alter murf gene expression or MuRF protein localization, we knocked down smyd1b or hsp90α1 in wild type, Tg(ef1a:MurF1a-GFP) and Tg(ef1a:MuRF2a-GFP) transgenic zebrafish embryos. Knockdown of smyd1b or hsp90α1 had no effect on murf gene expression. However, the sarcomeric distribution of MuRF1a-GFP was abolished in the knockdown embryos. This was accompanied by an increased random punctate distribution of MuRF1a-GFP in muscle cells of zebrafish embryos. Collectively, these studies demonstrate that MuRFs are specifically expressed in developing muscles of zebrafish embryos. The M-line localization MuRF1a is altered by sarcomere disruption in smyd1b or hsp90α1 knockdown embryos.

Myomesin is part of an integrity pathway that responds to sarcomere damage and disease

The structure and function of the sarcomere of striated muscle is well studied but the steps of sarcomere assembly and maintenance remain under-characterized. With the aid of chaperones and factors of the protein quality control system, muscle proteins can be folded and assembled into the contractile apparatus of the sarcomere. When sarcomere assembly is incomplete or the sarcomere becomes damaged, suites of chaperones and maintenance factors respond to repair the sarcomere. Here we show evidence of the importance of the M-line proteins, specifically myomesin, in the monitoring of sarcomere assembly and integrity in previously characterized zebrafish muscle mutants. We show that myomesin is one of the last proteins to be incorporated into the assembling sarcomere, and that in skeletal muscle, its incorporation requires connections with both titin and myosin. In diseased zebrafish sarcomeres, myomesin1a shows an early increase of gene expression, hours before chaperones respond to damaged muscle. We found that myomesin expression is also more specific to sarcomere damage than muscle creatine kinase, and our results and others support the use of myomesin assays as an early, specific, method of detecting muscle damage.

Molecular features of the UNC-45 chaperone critical for binding and folding muscle myosin

Myosin is a motor protein that is essential for a variety of processes ranging from intracellular transport to muscle contraction. Folding and assembly of myosin relies on a specific chaperone, UNC-45. To address its substrate-targeting mechanism, we reconstitute the interplay between Caenorhabditis elegans UNC-45 and muscle myosin MHC-B in insect cells. In addition to providing a cellular chaperone assay, the established system enabled us to produce large amounts of functional muscle myosin, as evidenced by a biochemical and structural characterization, and to directly monitor substrate binding to UNC-45. Data from in vitro and cellular chaperone assays, together with crystal structures of binding-deficient UNC-45 mutants, highlight the importance of utilizing a flexible myosin-binding domain. This so-called UCS domain can adopt discrete conformations to efficiently bind and fold substrate. Moreover, our data uncover the molecular basis of temperature-sensitive UNC-45 mutations underlying one of the most prominent motility defects in C. elegans.

Myosin, a motor protein essential for intracellular transport to muscle contraction, requires a chaperone UNC-45 for folding and assembly. Here authors use in vitro reconstitution and structural biology to characterize the interplay between UNC-45 and muscle myosin MHC-B.

SETD7 Drives Cardiac Lineage Commitment through Stage-Specific Transcriptional Activation

Cardiac development requires coordinated and large-scale rearrangements of the epigenome. The roles and precise mechanisms through which specific epigenetic modifying enzymes control cardiac lineage specification, however, remain unclear. Here we show that the H3K4 methyltransferase SETD7 controls cardiac differentiation by reading H3K36 marks independently of its enzymatic activity. Through chromatin immunoprecipitation sequencing (ChIP-seq), we found that SETD7 targets distinct sets of genes to drive their stage-specific expression during cardiomyocyte differentiation. SETD7 associates with different co-factors at these stages, including SWI/SNF chromatin-remodeling factors during mesodermal formation and the transcription factor NKX2.5 in cardiac progenitors to drive their differentiation. Further analyses revealed that SETD7 binds methylated H3K36 in the bodies of its target genes to facilitate RNA polymerase II (Pol II)-dependent transcription. Moreover, abnormal SETD7 expression impairs functional attributes of terminally differentiated cardiomyocytes. Together, these results reveal how SETD7 acts at sequential steps in cardiac lineage commitment, and they provide insights into crosstalk between dynamic epigenetic marks and chromatin-modifying enzymes.

Modulation of chromatin remodeling proteins SMYD1 and SMARCD1 promotes contractile function of human pluripotent stem cell-derived ventricular cardiomyocyte in 3D-engineered cardiac tissues

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have the ability of differentiating into functional cardiomyocytes (CMs) for cell replacement therapy, tissue engineering, drug discovery and toxicity screening. From a scale-free, co-expression network analysis of transcriptomic data that distinguished gene expression profiles of undifferentiated hESC, hESC-, fetal- and adult-ventricular(V) CM, two candidate chromatin remodeling proteins, SMYD1 and SMARCD1 were found to be differentially expressed. Using lentiviral transduction, SMYD1 and SMARCD1 were over-expressed and suppressed, respectively, in single hESC-VCMs as well as the 3D constructs Cardiac Micro Tissues (CMT) and Tissue Strips (CTS) to mirror the endogenous patterns, followed by dissection of their roles in controlling cardiac gene expression, contractility, Ca²⁺-handling, electrophysiological functions and in vitro maturation. Interestingly, compared to independent single transductions, simultaneous SMYD1 overexpression and SMARCD1 suppression in hESC-VCMs synergistically interacted to increase the contractile forces of CMTs and CTSs with up-regulated transcripts for cardiac contractile, Ca²⁺-handing, and ion channel proteins. Certain effects that were not detected at the single-cell level could be unleashed under 3D environments. The two chromatin remodelers SMYD1 and SMARCD1 play distinct roles in cardiac development and maturation, consistent with the notion that epigenetic priming requires triggering signals such as 3D environmental cues for pro-maturation effects.

Defective sarcomere assembly in smyd1a and smyd1b zebrafish mutants

Two smyd1 paralogues, smyd1a and smyd1b, have been identified in zebrafish. Although Smyd1b function has been reported in fast muscle, its function in slow muscle and the function of Smyd1a, in general, are uncertain. In this study, we generated 2 smyd1a mutant alleles and analyzed the muscle defects in smyd1a and smyd1b single and double mutants in zebrafish. We demonstrated that knockout of smyd1a alone had no visible effect on muscle development and fish survival. This was in contrast to the smyd1b mutant, which exhibited skeletal and cardiac muscle defects, leading to early embryonic lethality. The smyd1a and smyd1b double mutants, however, showed a stronger muscle defect compared with smyd1a or smyd1b mutation alone, namely, the complete disruption of sarcomere organization in slow and fast muscles. Immunostaining revealed that smyd1a; smyd1b double mutations had no effect on myosin gene expression but resulted in a dramatic reduction of myosin protein levels in muscle cells of zebrafish embryos. This was accompanied by the up-regulation of hsp40 and hsp90-α1 gene expression. Together, our studies indicate that both Smyd1a and Smyd1b partake in slow and fast muscle development although Smyd1b plays a dominant role compared with Smyd1a.-Cai, M., Han, L., Liu, L., He, F., Chu, W., Zhang, J., Tian, Z., Du, S. Defective sarcomere assembly in smyd1a and smyd1b zebrafish mutants.

Genetic Mutations in jamb, jamc, and myomaker Revealed Different Roles on Myoblast Fusion and Muscle Growth

Myoblast fusion is a vital step for skeletal muscle development, growth, and regeneration. Loss of Jamb, Jamc, or Myomaker (Mymk) function impaired myoblast fusion in zebrafish embryos. In addition, mymk mutation hampered fish muscle growth. However, the effect of Jamb and Jamc deficiency on fish muscle growth is not clear. Moreover, whether jamb;jamc and jamb;mymk double mutations have stronger effects on myoblast fusion and muscle growth remains to be investigated. Here, we characterized the muscle development and growth in jamb, jamc, and mymk single and double mutants in zebrafish. We found that although myoblast fusion was compromised in jamb and jamc single or jamb;jamc double mutants, these mutant fish showed no defect in muscle cell fusion during muscle growth. The mutant fish were able to grow into adults that were indistinguishable from the wild-type sibling. In contrast, the jamb;mymk double mutants exhibited a stronger muscle phenotype compared to the jamb and jamc single and double mutants. The jamb;mymk double mutant showed reduced growth and partial lethality, similar to a mymk single mutant. Single fiber analysis of adult skeletal myofibers revealed that jamb, jamc, or jamb;jamc mutants contained mainly multinucleated myofibers, whereas jamb;mymk double mutants contained mostly mononucleated fibers. Significant intramuscular adipocyte infiltration was found in skeletal muscles of the jamb;mymk mutant. Collectively, these studies demonstrate that although Jamb, Jamc, and Mymk are all involved in myoblast fusion during early myogenesis, they have distinct roles in myoblast fusion during muscle growth. While Mymk is essential for myoblast fusion during both muscle development and growth, Jamb and Jamc are dispensable for myoblast fusion during muscle growth.

Loss of SMYD1 Results in Perinatal Lethality via Selective Defects within Myotonic Muscle Descendants

SET and MYND Domain 1 (SMYD1) is a cardiac and skeletal muscle-specific, histone methyl transferase that is critical for both embryonic and adult heart development and function in both mice and men. We report here that skeletal muscle-specific, myogenin (myoG)-Cre-mediated conditional knockout (CKO) of Smyd1 results in perinatal death. As early as embryonic day 12.5, Smyd1 CKOs exhibit multiple skeletal muscle defects in proliferation, morphology, and gene expression. However, all myotonic descendants are not afflicted equally. Trunk muscles are virtually ablated with excessive accumulation of brown adipose tissue (BAT), forelimb muscles are disorganized and improperly differentiated, but other muscles, such as the masseter, are normal. While expression of major myogenic regulators went unscathed, adaptive and innate immune transcription factors critical for BAT development/physiology were downregulated. Whereas classical mitochondrial BAT accumulation went unscathed following loss of SMYD1, key transcription factors, including PRDM16, UCP-1, and CIDE-a that control skeletal muscle vs. adipose fate, were downregulated. Finally, in rare adults that survive perinatal lethality, SMYD1 controls specification of some, but not all, skeletal muscle fiber-types.

Maternal obesity impairs fetal cardiomyocyte contractile function in sheep

Obesity is a major public health problem worldwide. In the United States, one-third of women of reproductive age are obese. Human studies show that maternal obesity (MO) predisposes offspring to cardiovascular disease. However, the underlying mechanisms remain unclear. Given the similarities between pregnancy in sheep and humans, we studied sheep to examine the impact of MO on fetal cardiomyocyte contractility at term. We observed that MO impaired cardiomyocyte contractility by reducing peak shortening and shortening/relengthening velocity, prolonging time to relengthening. MO disrupted Ca²⁺ homeostasis in fetal cardiomyocytes, increasing intracellular Ca²⁺ and inducing cellular Ca²⁺ insensitivity. The Ca²⁺-release channel was impaired, but Ca²⁺ uptake was unaffected by MO. The upstream kinases that phosphorylate the Ca²⁺-release channel-ryanodine receptor-2, PKA, and calmodulin-dependent protein kinase II-were activated in MO fetuses. Contractile dysfunction was associated with an increased ratio of myosin heavy chain (MHC)-β to MHC-α and upregulated cardiac troponin (cTn)-T and tropomyosin, as well as cTn-I phosphorylation. In summary, this is the first characterization of the effects of MO on fetal cardiomyocyte contractility. Our findings indicate that MO impairs fetal cardiomyocyte contractility through altered intracellular Ca²⁺ handling, overloading fetal cardiomyocyte intracellular Ca²⁺ and aberrant myofilament protein composition. These mechanisms may contribute to developmental programming by MO of offspring cardiac function and predisposition to later life cardiovascular disease in the offspring.-Wang, Q., Zhu, C., Sun, M., Maimaiti, R., Ford, S. P., Nathanielsz, P. W., Ren, J., Guo, W. Maternal obesity impairs fetal cardiomyocyte contractile function in sheep.

Zebrafish VCAP1X2 regulates cardiac contractility and proliferation of cardiomyocytes and epicardial cells

Sarcomeric signaling complexes are important to sustain proper sarcomere structure and function, however, the mechanisms underlying these processes are not fully elucidated. In a gene trap experiment, we found that vascular cell adhesion protein 1 isoform X2 (VCAP1X2) mutant embryos displayed a dilated cardiomyopathy phenotype, including reduced cardiac contractility, enlarged ventricular chamber and thinned ventricular compact layer. Cardiomyocyte and epicardial cell proliferation was decreased in the mutant heart ventricle, as was the expression of pAKT and pERK. Contractile dysfunction in the mutant was caused by sarcomeric disorganization, including sparse myofilament, blurred Z-disc, and decreased gene expression for sarcomere modulators (smyd1b, mypn and fhl2a), sarcomeric proteins (myh6, myh7, vmhcl and tnnt2a) and calcium regulators (ryr2b and slc8a1a). Treatment of PI3K activator restored Z-disc alignment while injection of smyd1b mRNA restored Z-disc alignment, contractile function and cardiomyocyte proliferation in ventricles of VCAP1X2 mutant embryos. Furthermore, injection of VCAP1X2 variant mRNA rescued all phenotypes, so long as two cytosolic tyrosines were left intact. Our results reveal two tyrosine residues located in the VCAP1X2 cytoplasmic domain are essential to regulate cardiac contractility and the proliferation of ventricular cardiomyocytes and epicardial cells through modulating pAKT and pERK expression levels.

Zebrafish Embryonic Slow Muscle Is a Rapid System for Genetic Analysis of Sarcomere Organization by CRISPR/Cas9, but Not NgAgo

Zebrafish embryonic slow muscle cells, with their superficial localization and clear sarcomere organization, provide a useful model system for genetic analysis of muscle cell differentiation and sarcomere assembly. To develop a quick assay for testing CRISPR-mediated gene editing in slow muscles of zebrafish embryos, we targeted a red fluorescence protein (RFP) reporter gene specifically expressed in slow muscles of myomesin-3-RFP (Myom3-RFP) zebrafish embryos. We demonstrated that microinjection of RFP-sgRNA with Cas9 protein or Cas9 mRNA resulted in a mosaic pattern in loss of RFP expression in slow muscle fibers of the injected zebrafish embryos. To uncover gene functions in sarcomere organization, we targeted two endogenous genes, slow myosin heavy chain-1 (smyhc1) and heat shock protein 90 α1 (hsp90α1), which are specifically expressed in zebrafish muscle cells. We demonstrated that injection of Cas9 protein or mRNA with respective sgRNAs targeted to smyhc1 or hsp90a1 resulted in a mosaic pattern of myosin thick filament disruption in slow myofibers of the injected zebrafish embryos. Moreover, Myom3-RFP expression and M-line localization were also abolished in these defective myofibers. Given that zebrafish embryonic slow muscles are a rapid in vivo system for testing genome editing and uncovering gene functions in muscle cell differentiation, we investigated whether microinjection of Natronobacterium gregoryi Argonaute (NgAgo) system could induce genetic mutations and muscle defects in zebrafish embryos. Single-strand guide DNAs targeted to RFP, Smyhc1, or Hsp90α1 were injected with NgAgo mRNA into Myom3-RFP zebrafish embryos. Myom3-RFP expression was analyzed in the injected embryos. The results showed that, in contrast to the CRISPR/Cas9 system, injection of the NgAgo-gDNA system did not affect Myom3-RFP expression and sarcomere organization in myofibers of the injected embryos. Sequence analysis failed to detect genetic mutations at the target genes. Together, our studies demonstrate that zebrafish embryonic slow muscle is a rapid model for testing gene editing technologies in vivo and uncovering gene functions in muscle cell differentiation.

Chaperones and the Proteasome System: Regulating the Construction and Demolition of Striated Muscle

Protein folding factors (chaperones) are required for many diverse cellular functions. In striated muscle, chaperones are required for contractile protein function, as well as the larger scale assembly of the basic unit of muscle, the sarcomere. The sarcomere is complex and composed of hundreds of proteins and the number of proteins and processes recognized to be regulated by chaperones has increased dramatically over the past decade. Research in the past ten years has begun to discover and characterize the chaperones involved in the assembly of the sarcomere at a rapid rate. Because of the dynamic nature of muscle, wear and tear damage is inevitable. Several systems, including chaperones and the ubiquitin proteasome system (UPS), have evolved to regulate protein turnover. Much of our knowledge of muscle development focuses on the formation of the sarcomere but recent work has begun to elucidate the requirement and role of chaperones and the UPS in sarcomere maintenance and disease. This review will cover the roles of chaperones in sarcomere assembly, the importance of chaperone homeostasis and the cooperation of chaperones and the UPS in sarcomere integrity and disease.

The Smyd Family of Methyltransferases: Role in Cardiac and Skeletal Muscle Physiology and Pathology

Protein methylation plays a pivotal role in the regulation of various cellular processes including chromatin remodeling and gene expression. SET and MYND domain-containing proteins (Smyd) are a special class of lysine methyltransferases whose catalytic SET domain is split by an MYND domain. The hallmark feature of this family was thought to be the methylation of histone H3 (on lysine 4). However, several studies suggest that the role of the Smyd family is dynamic, targeting unique histone residues associated with both transcriptional activation and repression. Smyd proteins also methylate several non-histone proteins to regulate various cellular processes. Although we are only beginning to understand their specific molecular functions and role in chromatin remodeling, recent studies have advanced our understanding of this relatively uncharacterized family, highlighting their involvement in development, cell growth and differentiation and during disease in various animal models. This review summarizes our current knowledge of the structure, function and methylation targets of the Smyd family and provides a compilation of data emphasizing their prominent role in cardiac and skeletal muscle physiology and pathology.

Proteomic and microRNA Transcriptome Analysis revealed the microRNA-SmyD1 network regulation in Skeletal Muscle Fibers performance of Chinese perch

Fish myotomes are comprised of anatomically segregated fast and slow muscle fibers that possess different metabolic and contractile properties. Although the expression profile properties in fast and slow muscle fibers had been investigated at the mRNA levels, a comprehensive analysis at proteomic and microRNA transcriptomic levels is limited. In the present study, we first systematically compared the proteomic and microRNA transcriptome of the slow and fast muscles of Chinese perch (Siniperca chuatsi). Total of 2102 proteins were identified in muscle tissues. Among them, 99 proteins were differentially up-regulated and 400 were down-regulated in the fast muscle compared with slow muscle. MiRNA microarrays revealed that 199 miRNAs identified in the two types of muscle fibers. Compared with the fast muscle, the 32 miRNAs was up-regulated and 27 down-regulated in the slow muscle. Specifically, expression of miR-103 and miR-144 was negatively correlated with SmyD1a and SmyD1b expression in fast and slow muscles, respectively. The luciferase reporter assay further verified that the miR-103 and miR-144 directly regulated the SmyD1a and SmyD1b expression by targeting their 3′-UTR. The constructed miRNA-SmyD1 interaction network might play an important role in controlling the development and performance of different muscle fiber types in Chinese perch.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

Mouse myofibers lacking the SMYD1 methyltransferase are susceptible to atrophy, internalization of nuclei and myofibrillar disarray

The Smyd1 gene encodes a lysine methyltransferase specifically expressed in striated muscle. Because Smyd1-null mouse embryos die from heart malformation prior to formation of skeletal muscle, we developed a Smyd1 conditional-knockout allele to determine the consequence of SMYD1 loss in mammalian skeletal muscle. Ablation of SMYD1 specifically in skeletal myocytes after myofiber differentiation using Myf6(cre) produced a non-degenerative myopathy. Mutant mice exhibited weakness, myofiber hypotrophy, prevalence of oxidative myofibers, reduction in triad numbers, regional myofibrillar disorganization/breakdown and a high percentage of myofibers with centralized nuclei. Notably, we found broad upregulation of muscle development genes in the absence of regenerating or degenerating myofibers. These data suggest that the afflicted fibers are in a continual state of repair in an attempt to restore damaged myofibrils. Disease severity was greater for males than females. Despite equivalent expression in all fiber types, loss of SMYD1 primarily affected fast-twitch muscle, illustrating fiber-type-specific functions for SMYD1. This work illustrates a crucial role for SMYD1 in skeletal muscle physiology and myofibril integrity.

The chromatin-binding protein Smyd1 restricts adult mammalian heart growth

All terminally differentiated organs face two challenges, maintaining their cellular identity and restricting organ size. The molecular mechanisms responsible for these decisions are of critical importance to organismal development, and perturbations in their normal balance can lead to disease. A hallmark of heart failure, a condition affecting millions of people worldwide, is hypertrophic growth of cardiomyocytes. The various forms of heart failure in human and animal models share conserved transcriptome remodeling events that lead to expression of genes normally silenced in the healthy adult heart. However, the chromatin remodeling events that maintain cell and organ size are incompletely understood; insights into these mechanisms could provide new targets for heart failure therapy. Using a quantitative proteomics approach to identify muscle-specific chromatin regulators in a mouse model of hypertrophy and heart failure, we identified upregulation of the histone methyltransferase Smyd1 during disease. Inducible loss-of-function studies in vivo demonstrate that Smyd1 is responsible for restricting growth in the adult heart, with its absence leading to cellular hypertrophy, organ remodeling, and fulminate heart failure. Molecular studies reveal Smyd1 to be a muscle-specific regulator of gene expression and indicate that Smyd1 modulates expression of gene isoforms whose expression is associated with cardiac pathology. Importantly, activation of Smyd1 can prevent pathological cell growth. These findings have basic implications for our understanding of cardiac pathologies and open new avenues to the treatment of cardiac hypertrophy and failure by modulating Smyd1.

A compact unc45b-promoter drives muscle-specific expression in zebrafish and mouse

Summary: Gene therapeutic approaches to cure genetic diseases require tools to express the rescuing gene exclusively within the affected tissues. Viruses are often chosen as gene transfer vehicles but they have limited capacity for genetic information to be carried and transduced. In addition, to avoid off‐target effects the therapeutic gene should be driven by a tissue‐specific promoter in order to ensure expression in the target organs, tissues, or cell populations. The larger the promoter, the less space will be left for the respective gene. Thus, there is a need for small but tissue‐specific promoters. Here, we describe a compact unc45b promoter fragment of 195 bp that retains the ability to drive gene expression exclusively in skeletal and cardiac muscle in zebrafish and mouse. Remarkably, the described unc45b promoter fragment not only drives muscle‐specific expression but presents heat‐shock inducibility, allowing a temporal and spatial quantity control of (trans)gene expression. Here, we demonstrate that the transgenic expression of the smyd1b gene driven by the unc45b promoter fragment is able to rescue the embryonically lethal heart and skeletal muscle defects in smyd1b‐deficient flatline mutant zebrafish. Our findings demonstrate that the described muscle‐specific unc45b promoter fragment might be a valuable tool for the development of genetic therapies in patients suffering from myopathies. genesis 54:431–438, 2016. © 2016 The Authors. Genesis Published by Wiley Periodicals, Inc.

SMYD1 and G6PD modulation are critical events for miR-206-mediated differentiation of rhabdomyosarcoma

Rhadomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood. RMS cells resemble fetal myoblasts but are unable to complete myogenic differentiation. In previous work we showed that miR-206, which is low in RMS, when induced in RMS cells promotes the resumption of differentiation by modulating more than 700 genes. To better define the pathways involved in the conversion of RMS cells into their differentiated counterpart, we focused on 2 miR-206 effectors emerged from the microarray analysis, SMYD1 and G6PD. SMYD1, one of the most highly upregulated genes, is a H3K4 histone methyltransferase. Here we show that SMYD1 silencing does not interfere with the proliferative block or with the loss anchorage independence imposed by miR-206, but severely impairs differentiation of ERMS, ARMS, and myogenic cells. Thus SMYD1 is essential for the activation of muscle genes. Conversely, among the downregulated genes, we found G6PD, the enzyme catalyzing the rate-limiting step of the pentose phosphate shunt. In this work, we confirmed that G6PD is a direct target of miR-206. Moreover, we showed that G6PD silencing in ERMS cells impairs proliferation and soft agar growth. However, G6PD overexpression does not interfere with the pro-differentiating effect of miR-206, suggesting that G6PD downmodulation contributes to - but is not an absolute requirement for - the tumor suppressive potential of miR-206. Targeting cancer metabolism may enhance differentiation. However, therapeutic inhibition of G6PD is encumbered by side effects. As an alternative, we used DCA in combination with miR-206 to increase the flux of pyruvate into the mitochondrion by reactivating PDH. DCA enhanced the inhibition of RMS cell growth induced by miR-206, and sustained it upon miR-206 de-induction. Altogether these results link miR-206 to epigenetic and metabolic reprogramming, and suggest that it may be worth combining differentiation-inducing with metabolism-directed approaches.

Defective myogenesis in the absence of the muscle-specific lysine methyltransferase SMYD1

The SMYD (SET and MYND domain) family of lysine methyltransferases harbor a unique structure in which the methyltransferase (SET) domain is intervened by a zinc finger protein-protein interaction MYND domain. SMYD proteins methylate both histone and non-histone substrates and participate in diverse biological processes including transcriptional regulation, DNA repair, proliferation and apoptosis. Smyd1 is unique among the five family members in that it is specifically expressed in striated muscles. Smyd1 is critical for development of the right ventricle in mice. In zebrafish, Smyd1 is necessary for sarcomerogenesis in fast-twitch muscles. Smyd1 is expressed in the skeletal muscle lineage throughout myogenesis and in mature myofibers, shuttling from nucleus to cytosol during myoblast differentiation. Because of this expression pattern, we hypothesized that Smyd1 plays multiple roles at different stages of myogenesis. To determine the role of Smyd1 in mammalian myogenesis, we conditionally eliminated Smyd1 from the skeletal muscle lineage at the myoblast stage using Myf5(cre). Deletion of Smyd1 impaired myoblast differentiation, resulted in fewer myofibers and decreased expression of muscle-specific genes. Muscular defects were temporally restricted to the second wave of myogenesis. Thus, in addition to the previously described functions for Smyd1 in heart development and skeletal muscle sarcomerogenesis, these results point to a novel role for Smyd1 in myoblast differentiation.

Loss of function of myosin chaperones triggers Hsf1-mediated transcriptional response in skeletal muscle cells

Background

Mutations in myosin chaperones Unc45b and Hsp90aa1.1 as well as in the Unc45b-binding protein Smyd1b impair formation of myofibrils in skeletal muscle and lead to the accumulation of misfolded myosin. The concomitant transcriptional response involves up-regulation of the three genes encoding these proteins, as well as genes involved in muscle development. The transcriptional up-regulation of unc45b, hsp90aa1.1 and smyd1b is specific to zebrafish mutants with myosin folding defects, and is not triggered in other zebrafish myopathy models.

Results

By dissecting the promoter of unc45b, we identify a Heat shock factor 1 (Hsf1) binding element as a mediator of unc45b up-regulation in myofibers lacking myosin folding proteins. Loss-of-function of Hsf1 abolishes unc45b up-regulation in mutants with defects in myosin folding.

Conclusions

Taken together, our data show that skeletal muscle cells respond to defective myosin chaperones with a complex gene program and suggest that this response is mediated by Hsf1 activation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0825-8) contains supplementary material, which is available to authorized users.

The Effects of Hsp90α1 Mutations on Myosin Thick Filament Organization

Heat shock protein 90α plays a key role in myosin folding and thick filament assembly in muscle cells. To assess the structure and function of Hsp90α and its potential regulation by post-translational modification, we developed a combined knockdown and rescue assay in zebrafish embryos to systematically analyze the effects of various mutations on Hsp90α function in myosin thick filament organization. DNA constructs expressing the Hsp90α1 mutants with altered putative ATP binding, phosphorylation, acetylation or methylation sites were co-injected with Hsp90α1 specific morpholino into zebrafish embryos. Myosin thick filament organization was analyzed in skeletal muscles of the injected embryos by immunostaining. The results showed that mutating the conserved D90 residue in the Hsp90α1 ATP binding domain abolished its function in thick filament organization. In addition, phosphorylation mimicking mutations of T33D, T33E and T87E compromised Hsp90α1 function in myosin thick filament organization. Similarly, K287Q acetylation mimicking mutation repressed Hsp90α1 function in myosin thick filament organization. In contrast, K206R and K608R hypomethylation mimicking mutations had not effect on Hsp90α1 function in thick filament organization. Given that T33 and T87 are highly conserved residues involved post-translational modification (PTM) in yeast, mouse and human Hsp90 proteins, data from this study could indicate that Hsp90α1 function in myosin thick filament organization is potentially regulated by PTMs involving phosphorylation and acetylation.

Still Heart Encodes a Structural HMT, SMYD1b, with Chaperone-Like Function during Fast Muscle Sarcomere Assembly

The vertebrate sarcomere is a complex and highly organized contractile structure whose assembly and function requires the coordination of hundreds of proteins. Proteins require proper folding and incorporation into the sarcomere by assembly factors, and they must also be maintained and replaced due to the constant physical stress of muscle contraction. Zebrafish mutants affecting muscle assembly and maintenance have proven to be an ideal tool for identification and analysis of factors necessary for these processes. The still heart mutant was identified due to motility defects and a nonfunctional heart. The cognate gene for the mutant was shown to be smyd1b and the still heart mutation results in an early nonsense codon. SMYD1 mutants show a lack of heart looping and chamber definition due to a lack of expression of heart morphogenesis factors gata4, gata5 and hand2. On a cellular level, fast muscle fibers in homozygous mutants do not form mature sarcomeres due to the lack of fast muscle myosin incorporation by SMYD1b when sarcomeres are first being assembled (19hpf), supporting SMYD1b as an assembly protein during sarcomere formation.