Muscle-specific Myosin Heavy Chain Research Articles

Inhibition of skeletal muscle differentiation by calciprotein particles in human primary myoblasts.

Sarcopenia is a common complication of chronic kidney disease (CKD) and has a detrimental effect on prognosis. Previous studies have explored the role of secondary calciprotein particles (CPP2) in determining the progression of complications and poor outcomes in patients with CKD. However, no study has demonstrated that CPP2 impairs skeletal myogenesis. Our study revealed that CPP2 exposure inhibits skeletal myogenesis by suppressing myotube formation and expression of skeletal muscle-specific myosin heavy chain and actin in human primary myoblasts. Moreover, CPP2 exposure altered the expression patterns of lineage-determinative transcription factors responsible for regulating myotube differentiation marker genes. This study first demonstrated that CPP2 interferes with myoblast differentiation and myotube formation in vitro.

Intrauterine Nitric Oxide Deficiency Weakens Differentiation of Vascular Smooth Muscle in Newborn Rats.

Nitric oxide (NO) deficiency during pregnancy is a key reason for preeclampsia development. Besides its important vasomotor role, NO is shown to regulate the cell transcriptome. However, the role of NO in transcriptional regulation of developing smooth muscle has never been studied before. We hypothesized that in early ontogeny, NO is important for the regulation of arterial smooth muscle-specific genes expression. Pregnant rats consumed NO-synthase inhibitor L-NAME (500 mg/L in drinking water) from gestational day 10 till delivery, which led to an increase in blood pressure, a key manifestation of preeclampsia. L-NAME reduced blood concentrations of NO metabolites in dams and their newborn pups, as well as relaxations of pup aortic rings to acetylcholine. Using qPCR, we demonstrated reduced abundances of the smooth muscle-specific myosin heavy chain isoform, α-actin, SM22α, and L-type Ca2+-channel mRNAs in the aorta of newborn pups from the L-NAME group compared to control pups. To conclude, the intrauterine NO deficiency weakens gene expression specific for a contractile phenotype of arterial smooth muscle in newborn offspring.

Inducible Deletion of YAP and TAZ in Adult Mouse Smooth Muscle Causes Rapid and Lethal Colonic Pseudo-Obstruction.

Background & AimsYAP (Yap1) and TAZ (Wwtr1) are transcriptional co-activators and downstream effectors of the Hippo pathway, which play crucial roles in organ size control and cancer pathogenesis. Genetic deletion of YAP/TAZ has shown their critical importance for embryonic development of the heart, vasculature, and gastrointestinal mesenchyme. The aim of this study was to determine the functional role of YAP/TAZ in adult smooth muscle cells in vivo.MethodsBecause YAP and TAZ are mutually redundant, we used YAP/TAZ double-floxed mice crossed with mice that express tamoxifen-inducible CreERT2 recombinase driven by the smooth muscle–specific myosin heavy chain promoter.ResultsDouble-knockout of YAP/TAZ in adult smooth muscle causes lethality within 2 weeks, mainly owing to colonic pseudo-obstruction, characterized by severe distension and fecal impaction. RNA sequencing in colon and urinary bladder showed that smooth muscle markers and muscarinic receptors were down-regulated in the YAP/TAZ knockout. The same transcripts also correlated with YAP/TAZ in the human colon. Myograph experiments showed reduced contractility to depolarization by potassium chloride and a nearly abolished muscarinic contraction and spontaneous activity in colon rings of YAP/TAZ knockout.ConclusionsYAP and TAZ in smooth muscle are guardians of colonic contractility and control expression of contractile proteins and muscarinic receptors. The knockout model has features of human chronic intestinal pseudo-obstruction and may be useful for studying this disease.

Slug, a Cancer-Related Transcription Factor, is Involved in Vascular Smooth Muscle Cell Transdifferentiation Induced by Platelet-Derived Growth Factor-BB During Atherosclerosis.

BackgroundHeart attacks and stroke often result from occlusive thrombi following the rupture of vulnerable atherosclerotic plaques. Vascular smooth muscle cells (VSMCs) play a pivotal role in plaque vulnerability because of their switch towards a proinflammatory/macrophage‐like phenotype when in the context of atherosclerosis. The prometastatic transcription factor Slug/Snail2 is a critical regulator of cell phenotypic transition. Here, we aimed to investigate the role of Slug in the transdifferentiation process of VSMCs occurring during atherogenesis.Methods and ResultsIn rat and human primary aortic smooth muscle cells, Slug protein expression is strongly and rapidly increased by platelet‐derived growth factor‐BB (PDGF‐BB). PDGF‐BB increases Slug protein without affecting mRNA levels indicating that this growth factor stabilizes Slug protein. Immunocytochemistry and subcellular fractionation experiments reveal that PDGF‐BB triggers a rapid accumulation of Slug in VSMC nuclei. Using pharmacological tools, we show that the PDGF‐BB–dependent mechanism of Slug stabilization in VSMCs involves the extracellular signal‐regulated kinase 1/2 pathway. Immunohistochemistry experiments on type V and type VI atherosclerotic lesions of human carotids show smooth muscle–specific myosin heavy chain–/Slug‐positive cells surrounding the prothrombotic lipid core. In VSMCs, Slug siRNAs inhibit prostaglandin E2 secretion and prevent the inhibition of cholesterol efflux gene expression mediated by PDGF‐BB, known to be involved in plaque vulnerability and/or thrombogenicity.ConclusionsOur results highlight, for the first time, a role of Slug in aortic smooth muscle cell transdifferentiation and enable us to consider Slug as an actor playing a role in the atherosclerotic plaque progression towards a life‐threatening phenotype. This also argues for common features between acute cardiovascular events and cancer.

Skeletal Muscle Constructs Engineered from Human Embryonic Stem Cell Derived Myogenic Progenitors Exhibit Enhanced Contractile Forces When Differentiated in a Medium Containing EGM-2 Supplements.

Three-dimensional (3D) skeletal muscle constructs engineered from myogenic progenitors derived from human pluripotent stem cells (hPSCs) have a wide range of applications, but to date, such constructs generate lower specific tetanic force than adult human muscles. Methods enhancing functional muscle differentiation and force generation of these constructs are highly desirable. The finding of this study is that addition of the supplements in the endothelial cell growth medium-2 (EGM-2) to the myogenic differentiation medium can substantially enhance contractile force generation. For constructs differentiated for 4 weeks, addition of the EGM-2 supplements in the first 2 weeks leads to tenfold and sevenfold increases in twitch and tetanic forces, respectively. The specific tetanic force generated by these constructs is 33 mN mm-2 , which is significantly higher than previously reported. These constructs show wider myotubes and higher gene expression levels for all skeletal muscle-specific myosin heavy chain isoforms, suggesting that a more mature differentiation stage of the cells underlies the greater contractile force generation. The constructs exposed to these supplements for 4 weeks do not generate as high contractile forces, suggesting that prolonged treatment is not beneficial. These results suggest that temporal conditioning with the EGM-2 supplements assists functional development of hPSC-derived skeletal muscle constructs.

TBP-like protein (TLP) represses myogenesis via inhibition of the myogenin promoter

TBP-like protein (TLP) is one of the metazoan-restricted transcription factors participating in development and differentiation, though the molecular mechanism by which TLP regulates these processes remains unclear. In this study, we investigated the relationship between TLP and myogenesis of mouse C2C12 myoblasts. We found that TLP gene expression decreases during myogenic differentiation. Overexpression and knockdown of TLP revealed that the levels of muscle-specific myosin heavy chain and the myogenic transcription factor myogenin are downregulated by TLP. TLP inhibits the progression of morphological change from myoblasts to myotubes, thereby suppressing myogenesis. We further show that TLP represses the promoter activity of myogenin. The proximal AT-rich sequence of the myogenin promoter is responsible for TLP-mediated transcriptional repression. The results of this study suggest that TLP inhibits myogenesis through downregulation of the myogenin gene.

Muscle-Specific Myosin Heavy Chain Shifts in Response to a Long-Term High Fat/High Sugar Diet and Resveratrol Treatment in Nonhuman Primates.

Shifts in myosin heavy chain (MHC) expression within skeletal muscle can be induced by a host of stimuli including, but not limited to, physical activity, alterations in neural activity, aging, and diet or obesity. Here, we hypothesized that both age and a long-term (2 year) high fat/high sugar diet (HFS) would induce a slow to fast MHC shift within the plantaris, soleus, and extensor digitorum longus (EDL) muscles from rhesus monkeys. Furthermore, we tested whether supplementation with resveratrol, a naturally occurring compound that has been attributed with augmenting aerobic potential through mitochondrial proliferation, would counteract any diet-induced MHC changes by promoting a fast to slow isoform switch. In general, we found that MHC isoforms were not altered by aging during mid-life. The HFS diet had the largest impact within the soleus muscle where the greatest slow to fast isoform shifts were observed in both mRNA and protein indicators. As expected, long-term resveratrol treatment counteracted, or blunted, these diet-induced shifts within the soleus muscle. The plantaris muscle also demonstrated a fast-to-slow phenotypic response to resveratrol treatment. In conclusion, diet or resveratrol treatment impacts skeletal muscle phenotype in a muscle-specific manner and resveratrol supplementation may be one approach for promoting the fatigue-resistant MHC (type I) isoform especially if its expression is blunted as a result of a long-term high fat/sugar diet.

5′-flanking sequences of zebrafish fast myosin heavy chain genes regulate unique expression in the anterior, medial subsection and posterior tail somites of the skeletal muscle

In zebrafish, fast muscle-specific myosin heavy chain genes have their unique expression patterns in a well-defined and restricted region of the skeletal muscle. However, the transcriptional regulatory mechanisms involved have remained unclear. Here, we examined the regulation of spatio-temporal expression patterns of myhz1 (myhz1.1, myhz1.2 and myhz1.3) and myhz2 during their development by using transient gene and stable transgenic techniques. Embryos microinjected with different length 5'-flanking sequences of myhz1 conjugated with the enhanced green fluorescent protein (EGFP) gene showed EGFP expression in the anterior and medial subsections of somites, but not in the tail somite region. In contrast, embryos microinjected with different length 5'-flanking sequences of myhz2 showed EGFP expression exclusively at the posterior tail somite domain. Promoter deletion analyses demonstrated that reduced EGFP fluorescence typically is correlated with smaller 5'-flanking sequences. The immunohistochemical observation revealed that zebrafish larvae provided with the transient gene and those from stable transgenic lines consistently expressed EGFP in the fast muscle fibers. r-VISTA plot identified one common conserved region of about 140°bp among myhz1.1, myhz1.2 and myhz1.3. Deletion of this conserved region from the 5'-flanking sequence of each myhz1 markedly reduced EGFP expression in its unique spatial somite region. Deletion mutation analysis demonstrated that myhz2 expression in the tail somite region might be mediated by Tbx (familyoftranscriptionfactorshavingacommonDNA-bindingsequenceknown asT-box) binding elements. In summary, 5'-flanking sequences of myhz1 and myhz2 regulate their unique expression patterns in a well-defined and restricted somite region of the skeletal muscle in zebrafish.

Requirement of miR-9-dependent regulation of Myocd in PASMCs phenotypic modulation and proliferation induced by hepatopulmonary syndrome rat serum.

Hepatopulmonary syndrome (HPS) is characterized by a triad of severe liver disease, intrapulmonary vascular dilation and hypoxaemia. Pulmonary vascular remodelling (PVR) is a key feature of HPS pathology. Our previous studies have established the role of the pulmonary artery smooth muscle cell (PASMC) phenotypic modulation and proliferation in HPS-associated PVR. Myocardin, a robust transcriptional coactivator of serum response factor, plays a critical role in the vascular smooth muscle cell phenotypic switch. However, the mechanism regulating myocardin upstream signalling remains unclear. In this study, treatment of rat PASMCs with serum drawn from common bile duct ligation rats, which model symptoms of HPS, resulted in a significant increase in miR-9 expression correlated with a decrease in expression of myocardin and the phenotypic markers SM-α-actin and smooth muscle-specific myosin heavy chain (SM-MHC). Furthermore, miRNA functional analysis and luciferase reporter assay demonstrated that miR-9 effectively regulated myocardin expression by directly binding to its 3′-untranslated region. Both the knockdown of miR-9 and overexpression of myocardin effectively attenuated the HPS rat serum-induced phenotype switch and proliferation of PASMCs. Taken together, the findings of our present study demonstrate that miR-9 is required in HPS rat serum-induced phenotypic modulation and proliferation of PASMCs for targeting of myocardin and that miR-9 may serve as a potential therapeutic target in HPS.

Abstract 678: Thoracic Aortic Dissection in Hypertensive Myh11r247c Mice Is Regulated by Gsk3β Signaling Pathway.

Acute ascending aortic dissections cause premature deaths due to pericardial tamponade. Mutations in the gene for smooth muscle specific myosin heavy chain (MYH11) causes aortic dissections with minimal aortic dilation, whereas mutations in fibrillin-1 (FBN1) causing Marfan syndrome is characterized by aneurysm formation before dissections occur. We hypothesize that distinct pathways might be responsible for the aortic dissections. Results: Mice with a homozygous Myh11 missense variant (R247C), Myh11R247C, do not have aortic enlargement and do not die prematurely of aortic dissections. Increased blood pressure induced by high salt diet and L-NAME in the Myh11R247C mice leads to sudden death that occur within 2 weeks due to acute aortic dissection and pericardial tamponade in more than 20% (9/39) of the animals (P=0.02, comparing to the death rate (1/45) in WT mice). Increased SMC proliferation and accumulation of proteoglycans are observed in the aortas of the hypertensive Myh11R247C mice. RNA Seq analysis of the Myh11R247C aortas with and without hypertension indicated targets of the Wnt pathway were increased and pathway analysis with DAVID Bioinformatics Resources identified the focal adhesion pathway was highly involved (P<0.01). Immunoblot analysis of aortic lysates confirmed increased activation of FAK and AKT, along with phosphorylation (inactivation) of GSK3β when hypertension was induced. To further inhibit GSK3β in the hypertensive Myh11R247C, the GSK3β inhibitor SB216763 was given immediately before and with hypertension induction. The death rates in Myh11R247C mutant mice but not in WT mice increased to 50% from 20% (P=0.02, N=22). Conclusion: FAK/AKT/GSK3β signaling pathway probably plays a pivotal role in acute aortic dissections in hypertensive Myh11R247C mutant mice. The SMC proliferation and proteoglycan accumulation could be important cellular pathological changes responsible for aortic dissections.

Neuropilin 1 is essential for gastrointestinal smooth muscle contractility and motility in aged mice.

Background and AimsNeuropilin 1 (NRP1) is a non-tyrosine kinase receptor for vascular endothelial growth factor (VEGF) and class 3 semaphorins, playing a role in angiogenesis and neuronal axon guidance, respectively. NRP1 is expressed in smooth muscle cells (SMC) but the functional role of NRP1 in SMC has not been elucidated. We therefore investigated the biological relevance of NRP1 in SMC in vivo by generating mice with SMC-specific Nrp1 deficiency.MethodsConditional gene targeting generated SMC-specific Nrp1 knockout mice (Nrp1SMKO) in which Cre recombinase is driven by the smooth muscle-specific myosin heavy chain (smMHC) promoter.ResultsSMC-specific Nrp1 deficiency resulted in a significant reduction in intestinal length by 6 months, and, by 18 months, in severe constipation, and enlargement of the intestine consistent with chronic intestinal pseudo-obstruction. These effects were associated with significant thinning of the intestinal smooth muscle, and decreased intestinal contractility. Expression of contractile proteins was reduced in Nrp1SMKO mice, including the smMHC isoform, SMB, whereas we observed a significant increase in the expression of the small-conductance calcium-activated potassium channel 3 (SK3/KCa2.3), implicated in negative regulation of smooth muscle contraction.Conclusions Nrp1 deficiency in visceral SMC results in adult-onset defects in gastrointestinal contractility and motility and causes a shift to a less contractile SMC phenotype. These findings indicate a new role for Nrp1 in the maintenance of the visceral SMC contractile phenotype required for normal GI motility in aged mice.

Differential regulation of myosin heavy chains defines new muscle domains in zebrafish

Numerous muscle lineages are formed during myogenesis within both slow- and fast-specific cell groups. In this study, we show that six fast muscle-specific myosin heavy chain genes have unique expression patterns in the zebrafish embryo. The expression of tail-specific myosin heavy chain (fmyhc2.1) requires wnt signaling and is essential for fast muscle organization within the tail. Retinoic acid treatment results in reduced wnt signaling, which leads to loss of the fmyhc2.1 domain. Retinoic acid treatment also results in a shift of muscle identity within two trunk domains defined by expression of fmyhc1.2 and fmyhc1.3 in favor of the anteriormost myosin isoform, fmyhc1.2. In summary, we identify new muscle domains along the anteroposterior axis in the zebrafish that are defined by individual nonoverlapping, differentially regulated expression of myosin heavy chain isoforms.

Stimulatory and inhibitory mechanisms of slow muscle-specific myosin heavy chain gene expression in fish: Transient and transgenic analysis of torafugu MYHM86-2 promoter in zebrafish embryos

The myosin heavy chain gene, MYHM86-2, exhibited restricted expression in slow muscle fibers of torafugu embryos and larvae, suggesting its functional roles for embryonic and larval muscle development. However, the transcriptional mechanisms involved in its expression are still ambiguous. The present study is the first extensive analysis of slow muscle-specific MYHM86-2 promoter in fish for identifying the cis-elements that are crucial for its expression. Combining both transient transfection and transgenic approaches, we demonstrated that the 2614bp 5′-flanking sequences of MYHM86-2 contain a sufficient promoter activity to drive gene expression specific to superficial slow muscle fibers. By cyclopamine treatment, we also demonstrated that the differentiation of such superficial slow muscle fibers depends on hedgehog signaling activity. The deletion analyses defined an upstream fragment necessary for repressing ectopic MYHM86-2 expression in the fast muscle fibers. The transcriptional mechanism that prevents MYHM86-2 expression in the fast muscle fibers is mediated through Sox6 binding elements. We also demonstrated that Sox6 may function as a transcriptional repressor of MYHM86-2 expression. We further discovered that nuclear factor of activated T cells (NFAT) binding elements plays a key role and myocyte enhancer factor-2 (MEF2) binding elements participate in the transcriptional regulation of MYHM86-2 expression.

Developing Cardiac and Skeletal Muscle Share Fast-Skeletal Myosin Heavy Chain and Cardiac Troponin-I Expression

Skeletal muscle derived stem cells (MDSCs) transplanted into injured myocardium can differentiate into fast skeletal muscle specific myosin heavy chain (sk-fMHC) and cardiac specific troponin-I (cTn-I) positive cells sustaining recipient myocardial function. We have recently found that MDSCs differentiate into a cardiomyocyte phenotype within a three-dimensional gel bioreactor. It is generally accepted that terminally differentiated myocardium or skeletal muscle only express cTn-I or sk-fMHC, respectively. Studies have shown the presence of non-cardiac muscle proteins in the developing myocardium or cardiac proteins in pathological skeletal muscle. In the current study, we tested the hypothesis that normal developing myocardium and skeletal muscle transiently share both sk-fMHC and cTn-I proteins. Immunohistochemistry, western blot, and RT-PCR analyses were carried out in embryonic day 13 (ED13) and 20 (ED20), neonatal day 0 (ND0) and 4 (ND4), postnatal day 10 (PND10), and 8 week-old adult female Lewis rat ventricular myocardium and gastrocnemius muscle. Confocal laser microscopy revealed that sk-fMHC was expressed as a typical striated muscle pattern within ED13 ventricular myocardium, and the striated sk-fMHC expression was lost by ND4 and became negative in adult myocardium. cTn-I was not expressed as a typical striated muscle pattern throughout the myocardium until PND10. Western blot and RT-PCR analyses revealed that gene and protein expression patterns of cardiac and skeletal muscle transcription factors and sk-fMHC within ventricular myocardium and skeletal muscle were similar at ED20, and the expression patterns became cardiac or skeletal muscle specific during postnatal development. These findings provide new insight into cardiac muscle development and highlight previously unknown common developmental features of cardiac and skeletal muscle.

Glucocorticoid Receptor Signaling Is Essential for Mesoderm Formation and Muscle Development in Zebrafish

Glucocorticoid receptor (GR) signaling is thought to play a key role in embryogenesis, but its specific developmental effects remain unclear. Cortisol is the primary ligand for GR activation in teleosts, and in zebrafish (Danio rerio), the prehatch embryo content of this steroid is of maternal origin. Using early zebrafish developmental stages, we tested the hypothesis that GR signaling is critical for embryo growth and hatching. In zebrafish, maternal GR mRNA is degraded quickly, followed by zygotic synthesis of the receptor. GR protein is widely expressed throughout early development, and we were able to knockdown this protein using morpholino oligonucleotides. This led to a more than 70% reduction in mRNA abundance of matrix metalloproteinase-13 (mmp13), a glucocorticoid-responsive gene. The GR morphants displayed delayed somitogenesis, defects in somite and tail morphogenesis, reduced embryo size, and rarely survived after hatch. This correlated with altered expression of myogenic markers, including myogenin, myostatin, and muscle-specific myosin heavy chain and troponin genes. A key finding was a 70-90% reduction in the mRNA abundance of bone morphogenetic proteins (BMP), including bmp2a, bmp2b, and bmp4 in GR morphants. Bioinformatics analysis confirmed multiple putative glucocorticoid response elements upstream of these BMP genes. GR morphants displayed reduced expression of BMP-modulated genes, including eve1 and pax3. Zebrafish GR mRNA injection rescued the GR morphant phenotype and reversed the disrupted expression of BMP and myogenic genes. Our results for the first time indicate that GR signaling is essential for zebrafish muscle development, and we hypothesize a role for BMP morphogens in this process.

Cell Aspect Ratio Alters Stem Cell Traction Stresses and Lineage

Adult mesenchymal stem cells (MSCs) “feel” the stiffness of their environment and differentiate in response to it; thus aberrant stiffness resulting from fibrosis in muscle dystrophies could misdirect MSCs into the wrong lineage. Conversely, MSCs have been shown to respond in a myosin-dependent manner to adipogenic and osteogenic media when cell spread area changes from 103 to 104 μm2 or when cultured in specific shapes, e.g. circles versus rectangles, and polygons, respectively. When both cues are present in a disparate fashion, e.g. highly elongated cells similar to muscle despite the presence of an abnormally stiff microenvironment, we hypothesized that a myosin contraction-dependent balance could induce a subset of MSCs to differentiate in to a muscle-like phenotype despite residing in a dystrophic-like stiffness. To regulate MSC morphology, we patterned fibronectin in shapes of varying aspect ratios but common area on polyacrylamide substrates of known stiffness. MSCs spread to the patterns and localized their focal adhesion in a stiffness and shape-dependent manner. Using traction force microscopy, we found that strain energy from cell-generated forces scaled with stiffness, but decreased as a function of cell elongation with isotropic cell patterns producing the highest contractile energy in contrast to our hypothesis. Muscle-specific myosin heavy chain (mMHC), an indicator of early muscle differentiation, also was expressed in a stiffness and elongation dependent manner. On muscle-like stiffness of 11 kiloPascals (kPa), cells with only minimal elongation, i.e. 1:1 and 3:1 patterns, expressed mMHC most strongly. In contrast on osteogenic-like matrices of 34 kPa, highest MHC expression corresponded to the most elongated patterns. These shape- and stiffness-dependent lineage changes with muscle markers correlated to contractility-based observations suggest that muscle induction may be possible in non-permissive stiffer environments and could prove beneficial to treat fibrotic muscle diseases.

Histone deacetylase 7 (HDAC7) regulates myocyte migration and differentiation

Class IIa HDACs including HDAC7 play a role in gene expression, cell differentiation, and animal development through their association with transcription factors such as myogenic enhancer factors 2 (MEF2s). In this study, we show that endogenous HDAC7 localizes to both the nucleus and the cytoplasm of C2C12 myoblasts but is exclusively retained in the cytoplasm of myotubes after completion of differentiation process. To elucidate the role of differential distribution of HDAC7 during myogenesis, we examined the effects of stably expressed HDAC7 mutants on myogenesis. Expression of nuclear-retained HDAC7 mutants significantly inhibits myogenesis in C2C12 cells and reduces the expression of muscle-specific myosin heavy chain (MHC) and myogenin. The inhibition in myocyte differentiation can be partially relieved by introduction of a mutation disrupting HDAC7:MEF2 interaction. Since phosphorylation of HDAC7 plays an important role in its nucleocytoplasmic shuttling, we further investigated the expression and distribution of phosphorylated HDAC7. To our surprise, the phosphorylation levels of HDAC7 at S344 and S479 were slightly decreased upon differentiation, whereas the phosphorylation of S178 was unchanged. Interestingly, a significant fraction of pS344- and/or pS479-HDAC7 localized to plasma membrane of myotubes. In addition, Ser178-phosphorylated (pS178) HDAC7 displays a predominately actin filament-like structure before muscle differentiation. Consistent with this notion, HDAC7 partially colocalized with actin filaments; in particular, pS178-HDAC7 largely colocalized with actin filaments as indicated by phalloidin counter staining in myocytes. Furthermore, C2C12 cells expressing nuclear-retained HDAC7 display defects in migration. Our results provide novel insight into the mechanisms that regulate myocyte differentiation and migration by controlling the subcellular distribution of HDAC7 in differentiating myoblasts.

Thymine DNA Glycosylase Represses Myocardin-induced Smooth Muscle Cell Differentiation by Competing with Serum Response Factor for Myocardin Binding

Myocardin is a serum response factor (SRF) co-activator that regulates transcription of many smooth muscle-specific genes and is essential for development of vascular smooth muscle. We used a yeast two-hybrid screen, with myocardin as bait in a search for factors that regulate myocardin transcriptional activity. From this screen, thymine DNA glycosylase (TDG) was identified as a myocardin-associated protein. TDG was originally identified as an enzyme involved in base excision repair of T:G mismatches caused by spontaneous deamination of methylated cytosines. However, TDG has also been shown to act as a transcriptional co-activator or co-repressor. The interaction between TDG and myocardin was confirmed in vitro by glutathione S-transferase pull down and in vivo by co-immunoprecipitation assays. We found that TDG abrogates myocardin induced expression of smooth muscle-specific genes and represses the trans-activation of the promoters of myocardin of these genes. Overexpression of TDG in SMCs down-regulated smooth muscle marker expression. Conversely, depletion of endogenous TDG in SMCs increased smooth muscle-specific myosin heavy chain (SM MHC) and Telokin gene expression. Glutathione S-transferase pull-down assays demonstrated that TDG binds to a region of myocardin that includes the SRF binding domain. Furthermore, TDG was found to compete with SRF for binding to myocardin in vitro and in vivo, suggesting that TDG can inhibit expression of smooth muscle-specific genes, at least in part, through disrupting SRF/myocardin interactions. Finally, we demonstrated that the glycosylase activity of TDG is not required for its inhibitory effects on myocardin function. This study reveals a previously unsuspected role for the repair enzyme TDG as a repressor of smooth muscle differentiation via competing with SRF for binding to myocardin.

In vitro cardiomyogenic differentiation of adult human bone marrow mesenchymal stem cells. The role of 5-azacytidine

The aim of this study is to investigate the ability of adult human bone marrow mesenchymal stem cells to differentiate towards a cardiomyogenic phenotype in vitro. Bone marrow samples have been aspirated from 30 patients undergoing open heart surgery. Mesenchymal stem cells were isolated and cultured in enriched medium. Second passaged cells were treated with 10 microM 5-azacytidine for 24 h. Selected surface antigens were analyzed by flow cytometry. Morphologic characteristics were analyzed by confocal and electron microscopy. Expression of cytoskeletal protein vimentin and muscle specific myosin heavy chain were analyzed by immunohistochemistry. Expression of alpha-cardiac actin, beta-myosin heavy chain and cardiac troponin-T was detected by reverse transcriptase polymerase chain reaction. Mesenchymal stem cells were spindle-shaped with irregular processes. Cells treated with 5-azacytidine have assumed a stick-like morphology. They were connecting with adjoining cells forming myotube-like structures. Numerous myofilaments were detected in induced cells running in a parallel fashion without forming sarcomeres that were immunohistochemically positive for myosin heavy chain and vimentin. The mRNAs of alpha-cardiac actin, beta-myosin heavy chain and troponin-T were expressed in both induced and uninduced cells. These results indicate that adult human bone marrow mesenchymal stem cells can differentiate towards a cardiomyogenic lineage in vitro.

Spontaneous Contraction of Pseudoglandular-Stage Human Airspaces Is Associated with the Presence of Smooth Muscle-α-Actin and Smooth Muscle-Specific Myosin Heavy Chain in Recently Differentiated Fetal Human Airway Smooth Muscle

Background: Recent investigations demonstrating that pseudoglandular-stage airspaces contract spontaneously suggest that the production of contractile proteins by airway wall smooth muscle (ASM) is an important factor in the functional and structural differentiation of ASM. Aims: Ouraim was to determine if smooth muscle (SM)-myosin heavy chain (MHC) myofilaments, the ‘motor’ underlying SM contraction, and SM-α-actin myofilaments were distributed simultaneously in pseudoglandular-stage human lungs and to further define the nature of fetal airway contractions. Methods: Immunohistochemically stained sections of fetal lung (14 fetuses, 10.1–17 weeks gestation) were analysed by computer-assisted morphometry to determine airspace dimensions and detect SM-MHC- and SM-α-actin-ASM. Lung tissue from the same fetuses was also placed in explant culture to observe airway contractions using videomicroscopy. We found that the smallest airspaces were just as likely to be invested by a layer of SM-MHC-positive ASM as by a layer of SM-α-actin-positive ASM. In addition, larger airways or airways from more mature fetal lungs were more likely to be invested by either SM-MHC- or SM-α-actin-positive ASM. Spontaneous airspace contractions were peristalsis-like and variable in amplitude. The time interval between contractions was temperature dependent (mean ± SEM, 44 ± 7.5 s at 37°C), shortened by carbachol and increased by nitric oxide (NO)-donating drugs. Conclusions: These observations suggest that ASM differentiation is characterised by the simultaneous production of SM-α-actin and SM-MHC myofilaments and that the presence of these proteins is likely to be responsible for cholinergic- and NO-sensitive spontaneous contractions of fetal human airspaces.