Transcriptional networks regulating the costamere, sarcomere, and other cytoskeletal structures in striated muscle

Mustn1 ablation in skeletal muscle results in functional alterations

Mustn1, a gene expressed exclusively in the musculoskeletal system, was shown in previous in vitro studies to be a key regulator of myogenic differentiation and myofusion. Other studies also showed Mustn1 expression associated with skeletal muscle development and hypertrophy. However, its specific role in skeletal muscle function remains unclear. This study sought to investigate the effects of Mustn1 in a conditional knockout (KO) mouse model in Pax7 positive skeletal muscle satellite cells. Specifically, we investigated the potential effects of Mustn1 on myogenic gene expression, grip strength, alterations in gait, ex vivo investigations of isolated skeletal muscle isometric contractions, and potential changes in the composition of muscle fiber types. Results indicate that Mustn1 KO mice did not present any substantial phenotypic changes or significant variations in genes related to myogenic differentiation and fusion. However, an approximately 10% decrease in overall grip strength was observed in the 2-month-old KO mice in comparison to the control wild type (WT), but this decrease was not significant when normalized by weight. KO mice also generated approximately 8% higher vertical force than WT at 4 months in the hindlimb. Ex vivo experiments revealed decreases in about 20 to 50% in skeletal muscle contractions and about 10%-20% fatigue in soleus of both 2- and 4-month-old KO mice, respectively. Lastly, immunofluorescent analyses showed a persistent increase of Type IIb fibers up to 15-fold in the KO mice while Type I fibers decreased about 20% and 30% at both 2 and 4 months, respectively. These findings suggest a potential adaptive or compensatory mechanism following Mustn1 loss, as well as hinting at an association between Mustn1 and muscle fiber typing. Collectively, Mustn1's complex roles in skeletal muscle physiology requires further research, particularly in terms of understanding the potential role of Mustn1 in muscle repair and regeneration, as well as with influence of exercise. Collectively, these will offer valuable insights into Mustn1's key biological functions and regulatory pathways.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Polycomb protein RYBP activates transcription factor Plagl1 during in vitro cardiac differentiation of mouse embryonic stem cells

RING1 and YY1 binding protein (RYBP) is primarily known to function as a repressor being a core component of the non-canonical polycomb repressive complexes 1 (ncPRC1s). However, several ncPRC1-independent functions of RYBP have also been described. We previously reported that RYBP is essential for mouse embryonic development and that Rybp null mutant embryonic stem cells cannot form contractile cardiomyocytes (CMCs) in vitro. We also showed that PLAGL1, a cardiac transcription factor, which is often mutated in congenital heart diseases (CHDs), is not expressed in Rybp-null mutant CMCs. However, the underlying mechanism of how RYBP regulates Plagl1 expression was not revealed. Here, we demonstrate that RYBP cooperated with NKX2-5 to transcriptionally activate the P1 and P3 promoters of the Plagl1 gene and that this activation is ncPRC1-independent. We also show that two non-coding RNAs residing in the Plagl1 locus can also regulate the Plagl1 promoters. Finally, PLAGL1 was able to activate Tnnt2, a gene important for contractility of CMCs in transfected HEK293 cells. Our study shows that the activation of Plagl1 by RYBP is important for sarcomere development and contractility, and suggests that RYBP, via its regulatory functions, may contribute to the development of CHDs.

Interplay between the Chd4/NuRD Complex and the Transcription Factor Znf219 Control Cardiac Cell Identity

The sarcomere regulates striated muscle contraction. This structure is composed of several myofibril proteins, isoforms of which are encoded by genes specific to either the heart or skeletal muscle. The chromatin remodeler complex Chd4/NuRD regulates the transcriptional expression of these specific sarcomeric programs by repressing genes of the skeletal muscle sarcomere in the heart. Aberrant expression of skeletal muscle genes induced by the loss of Chd4 in the heart leads to sudden death due to defects in cardiomyocyte contraction that progress to arrhythmia and fibrosis. Identifying the transcription factors (TFs) that recruit Chd4/NuRD to repress skeletal muscle genes in the myocardium will provide important information for understanding numerous cardiac pathologies and, ultimately, pinpointing new therapeutic targets for arrhythmias and cardiomyopathies. Here, we sought to find Chd4 interactors and their function in cardiac homeostasis. We therefore describe a physical interaction between Chd4 and the TF Znf219 in cardiac tissue. Znf219 represses the skeletal-muscle sarcomeric program in cardiomyocytes in vitro and in vivo, similarly to Chd4. Aberrant expression of skeletal-muscle sarcomere proteins in mouse hearts with knocked down Znf219 translates into arrhythmias, accompanied by an increase in PR interval. These data strongly suggest that the physical and genetic interaction of Znf219 and Chd4 in the mammalian heart regulates cardiomyocyte identity and myocardial contraction.

Serum response factor deletion 5 regulates phospholamban phosphorylation and calcium uptake

AIMS

Pediatric dilated cardiomyopathy (pDCM) is characterized by unique age-dependent molecular mechanisms that include myocellular responses to therapy. We previously showed that pDCM, but not adult DCM patients respond to phosphodiesterase 3 inhibitors (PDE3i) by increasing levels of the second messenger cAMP and consequent phosphorylation of phospholamban (PLN). However, the molecular mechanisms involved in the differential pediatric and adult response to PDE3i are not clear.

METHODS AND RESULTS

Quantification of serum response factor (SRF) isoforms from the left ventricle of explanted hearts showed that PDE3i treatment affects expression of SRF isoforms in pDCM hearts. An SRF isoform lacking exon 5 (SRFdel5) was highly expressed in the hearts of pediatric, but not adult DCM patients treated with PDE3i. To determine the functional consequence of expression of SRFdel5, we overexpressed full length SRF or SRFdel5 in cultured cardiomyocytes with and without adrenergic stimulation. Compared to a control adenovirus, expression of SRFdel5 increased phosphorylation of PLN, negatively affected expression of the phosphatase that promotes dephosphorylation of PLN (PP2Cε), and promoted faster calcium reuptake, whereas expression of full length SRF attenuated calcium reuptake through blunted phosphorylation of PLN.

CONCLUSIONS

Taken together, these data indicate that expression of SRFdel5 in pDCM hearts in response to PDE3i contributes to improved function through regulating PLN phosphorylation and thereby calcium reuptake.

Bisphenols and phthalates: Plastic chemical exposures can contribute to adverse cardiovascular health outcomes

Phthalates and bisphenols are high production volume chemicals that are used in the manufacturing of consumer and medical products. Given the ubiquity of bisphenol and phthalate chemicals in the environment, biomonitoring studies routinely detect these chemicals in 75-90% of the general population. Accumulating evidence suggests that such chemical exposures may influence human health outcomes, including cardiovascular health. These associations are particularly worrisome for sensitive populations, including fetal, infant and pediatric groups-with underdeveloped metabolic capabilities and developing organ systems. In the presented article, we aimed to review the literature on environmental and clinical exposures to bisphenols and phthalates, highlight experimental work that suggests that these chemicals may exert a negative influence on cardiovascular health, and emphasize areas of concern that relate to vulnerable pediatric groups. Gaps in our current knowledge are also discussed, so that future endeavors may resolve the relationship between chemical exposures and the impact on pediatric cardiovascular physiology.

Evidence for skeletal muscle fiber type-specific expressions of mechanosensors

Mechanosensors govern muscle tissue integrity and constitute a subcellular structure known as costameres. Costameres physically link the muscle extracellular matrix to contractile and signaling 'hubs' inside muscle fibers mainly via integrins and are localized beneath sarcolemmas of muscle fibers. Costameres are the main mechanosensors converting mechanical cues into biological events. However, the fiber type-specific costamere architecture in muscles is unexplored. We hypothesized that fiber types differ in the expression of genes coding for costamere components. By coupling laser microdissection to a multiplex tandem qPCR approach, we demonstrate that type 1 and type 2 fibers indeed show substantial differences in their mechanosensor complexes. We confirmed these data by fiber type population-specific protein analysis and confocal microscopy-based localization studies. We further show that knockdown of the costamere gene integrin-linked kinase (Ilk) in muscle precursor cells results in significantly increased slow-myosin-coding Myh7 gene, while the fast-myosin-coding genes Myh1, Myh2, and Myh4 are downregulated. In parallel, protein synthesis-enhancing signaling molecules (p-mTORSer2448, p < 0.05; p-P70S6KThr389, tendency with p < 0.1) were reduced upon Ilk knockdown. However, overexpression of slow type-inducing NFATc1 in muscle precursor cells did not change Ilk or other costamere gene expressions. In addition, we demonstrate fiber type-specific costamere gene regulation upon mechanical loading and unloading conditions. Our data imply that costamere genes, such as Ilk, are involved in the control of muscle fiber characteristics. Further, they identify costameres as muscle fiber type-specific loading management 'hubs' and may explain adaptation differences of muscle fiber types to mechanical (un)loading.

Conserved functions of RNA-binding proteins in muscle

Animals require different types of muscle for survival, for example for circulation, motility, reproduction and digestion. Much emphasis in the muscle field has been placed on understanding how transcriptional regulation generates diverse types of muscle during development. Recent work indicates that alternative splicing and RNA regulation are as critical to muscle development, and altered function of RNA-binding proteins causes muscle disease. Although hundreds of genes predicted to bind RNA are expressed in muscles, many fewer have been functionally characterized. We present a cross-species view summarizing what is known about RNA-binding protein function in muscle, from worms and flies to zebrafish, mice and humans. In particular, we focus on alternative splicing regulated by the CELF, MBNL and RBFOX families of proteins. We discuss the systemic nature of diseases associated with loss of RNA-binding proteins in muscle, focusing on mis-regulation of CELF and MBNL in myotonic dystrophy. These examples illustrate the conservation of RNA-binding protein function and the marked utility of genetic model systems in understanding mechanisms of RNA regulation.

Myocyte enhancer factor 2A promotes proliferation and its inhibition attenuates myogenic differentiation via myozenin 2 in bovine skeletal muscle myoblast

Myocyte enhancer factor 2A (MEF2A) is widely distributed in various tissues or organs and plays crucial roles in multiple biological processes. To examine the potential effects of MEF2A on skeletal muscle myoblast, the functional role of MFE2A in myoblast proliferation and differentiation was investigated. In this study, we found that the mRNA expression level of Mef2a was dramatically increased during the myogenesis of bovine skeletal muscle primary myoblast. Overexpression of MEF2A significantly promoted myoblast proliferation, while knockdown of MEF2A inhibited the proliferation and differentiation of myoblast. RT-PCR and western blot analysis revealed that this positive effect of MEF2A on the proliferation of myoblast was carried out by triggering cell cycle progression by activating CDK2 protein expression. Besides, MEF2A was found to be an important transcription factor that bound to the myozenin 2 (MyoZ2) proximal promoter and performed upstream of MyoZ2 during myoblast differentiation. This study provides the first experimental evidence that MEF2A is a positive regulator in skeletal muscle myoblast proliferation and suggests that MEF2A regulates myoblast differentiation via regulating MyoZ2.

Antagonistic regulation of cell-cycle and differentiation gene programs in neonatal cardiomyocytes by homologous MEF2 transcription factors

Cardiomyocytes acquire their primary specialized function (contraction) before exiting the cell cycle. In this regard, proliferation and differentiation must be precisely coordinated for proper cardiac morphogenesis. Here, we have investigated the complex transcriptional mechanisms employed by cardiomyocytes to coordinate antagonistic cell-cycle and differentiation gene programs through the molecular dissection of the core cardiac transcription factor, MEF2. Knockdown of individual MEF2 proteins, MEF2A, -C, and -D, in primary neonatal cardiomyocytes resulted in radically distinct and opposite effects on cellular homeostasis and gene regulation. MEF2A and MEF2D were absolutely required for cardiomyocyte survival, whereas MEF2C, despite its major role in cardiac morphogenesis and direct reprogramming, was dispensable for this process. Inhibition of MEF2A or -D also resulted in the activation of cell-cycle genes and down-regulation of markers of terminal differentiation. In striking contrast, the regulation of cell-cycle and differentiation gene programs by MEF2C was antagonistic to that of MEF2A and -D. Computational analysis of regulatory regions from MEF2 isoform-dependent gene sets identified the Notch and Hedgehog signaling pathways as key determinants in coordinating MEF2 isoform-specific control of antagonistic gene programs. These results reveal that mammalian MEF2 family members have distinct transcriptional functions in cardiomyocytes and suggest that these differences are critical for proper development and maturation of the heart. Analysis of MEF2 isoform-specific function in neonatal cardiomyocytes has yielded insight into an unexpected transcriptional regulatory mechanism by which these specialized cells utilize homologous members of a core cardiac transcription factor to coordinate cell-cycle and differentiation gene programs.

Meta-Analysis of Transcriptome Regulation During Induction to Cardiac Myocyte Fate From Mouse and Human Fibroblasts

Ectopic expression of a defined set of transcription factors (TFs) can directly convert fibroblasts into a cardiac myocyte cell fate. Beside inefficiency in generating induced cardiomyocytes (iCMs), the molecular mechanisms that regulate this process remained to be well defined. The main purpose of this study was to provide better insight on the transcriptome regulation and to introduce a new strategy for candidating TFs for the transdifferentiation process. Eight mouse and three human high quality microarray data sets were analyzed to find differentially expressed genes (DEGs), which we integrated with TF-binding sites and protein-protein interactions to construct gene regulatory and protein-protein interaction networks. Topological and biological analyses of constructed gene networks revealed the main regulators and most affected biological processes. The DEGs could be categorized into two distinct groups, first, up-regulated genes that are mainly involved in cardiac-specific processes and second, down-regulated genes that are mainly involved in fibroblast-specific functions. Gata4, Mef2a, Tbx5, Tead4 TFs were identified as main regulators of cardiac-specific gene expression program; and Trp53, E2f1, Myc, Sfpi1, Lmo2, and Meis1 were identified as TFs which mainly regulate the expression of fibroblast-specific genes. Furthermore, we compared gene expression profiles and identified TFs between mouse and human to find the similarities and differences. In summary, our strategy of meta-analyzing the data of high-throughput techniques by computational approaches, besides revealing the mechanisms involved in the regulation of the gene expression program, also suggests a new approach for increasing the efficiency of the direct reprogramming of fibroblasts into iCMs. J. Cell. Physiol. 232: 2053-2062, 2017. © 2016 Wiley Periodicals, Inc.

G9a inhibits MEF2C activity to control sarcomere assembly

In this study, we demonstrate that the lysine methyltransferase G9a inhibits sarcomere organization through regulation of the MEF2C-HDAC5 regulatory axis. Sarcomeres are essential for muscle contractile function. Presently, skeletal muscle disease and dysfunction at the sarcomere level has been associated with mutations of sarcomere proteins. This study provides evidence that G9a represses expression of several sarcomere genes and its over-expression disrupts sarcomere integrity of skeletal muscle cells. G9a inhibits MEF2C transcriptional activity that is essential for expression of sarcomere genes. Through protein interaction assays, we demonstrate that G9a interacts with MEF2C and its co-repressor HDAC5. In the presence of G9a, calcium signaling-dependent phosphorylation and export of HDAC5 to the cytoplasm is blocked which likely results in enhanced MEF2C-HDAC5 association. Activation of calcium signaling or expression of constitutively active CaMK rescues G9a-mediated repression of HDAC5 shuttling as well as sarcomere gene expression. Our results demonstrate a novel epigenetic control of sarcomere assembly and identifies new therapeutic avenues to treat skeletal and cardiac myopathies arising from compromised muscle function.

The Function of the MEF2 Family of Transcription Factors in Cardiac Development, Cardiogenomics, and Direct Reprogramming

Proper formation of the mammalian heart requires precise spatiotemporal transcriptional regulation of gene programs in cardiomyocytes. Sophisticated regulatory networks have evolved to not only integrate the activities of distinct transcription factors to control tissue-specific gene programs but also, in many instances, to incorporate multiple members within these transcription factor families to ensure accuracy and specificity in the system. Unsurprisingly, perturbations in this elaborate transcriptional circuitry can lead to severe cardiac abnormalities. Myocyte enhancer factor–2 (MEF2) transcription factor belongs to the evolutionarily conserved cardiac gene regulatory network. Given its central role in muscle gene regulation and its evolutionary conservation, MEF2 is considered one of only a few core cardiac transcription factors. In addition to its firmly established role as a differentiation factor, MEF2 regulates wide variety of, sometimes antagonistic, cellular processes such as cell survival and death. Vertebrate genomes encode multiple MEF2 family members thereby expanding the transcriptional potential of this core transcription factor in the heart. This review highlights the requirement of the MEF2 family and their orthologs in cardiac development in diverse animal model systems. Furthermore, we describe the recently characterized role of MEF2 in direct reprogramming and genome-wide cardiomyocyte gene regulation. A thorough understanding of the regulatory functions of the MEF2 family in cardiac development and cardiogenomics is required in order to develop effective therapeutic strategies to repair the diseased heart.

The LIM domain protein nTRIP6 acts as a co-repressor for the transcription factor MEF2C in myoblasts

The transcription factor Myocyte enhancer factor 2C (MEF2C) plays a key role in the late differentiation of skeletal muscle progenitor cells, the so-called myoblasts. During myoblast differentiation, both MEF2C expression and transcriptional activity are regulated. We have reported that nTRIP6, the nuclear isoform of the focal adhesion LIM domain protein TRIP6, acts as an adaptor transcriptional co-activator for several transcription factors. It interacts with the promoter-bound transcription factors and consequently mediates the recruitment of other co-activators. Based on a described interaction between MEF2C and TRIP6 in a yeast-two-hybrid screen, we hypothesised a co-regulatory function of nTRIP6 for MEF2C. In proliferating myoblasts, nTRIP6 interacted with MEF2C and was recruited together with MEF2C to the MEF2-binding regions of the MEF2C target genes Myom2, Mb, Tnni2 and Des. Silencing nTRIP6 or preventing its interaction with MEF2C increased MEF2C transcriptional activity and increased the expression of these MEF2C target genes. Thus, nTRIP6 acts as a co-repressor for MEF2C. Mechanistically, nTRIP6 mediated the recruitment of the class IIa histone deacetylase HDAC5 to the MEF2C-bound promoters. In conclusion, our results unravel a transcriptional co-repressor function for nTRIP6. This adaptor co-regulator can thus exert either co-activator or co-repressor functions, depending on the transcription factor it interacts with.

Cardioprotection by Thymosin Beta 4

Treatment with thymosin beta 4 (Tβ4) reduces infarct volume and preserves cardiac function in preclinical models of cardiac ischemic injury. These effects stem in part from decreased infarct size, but additional benefits are likely due to specific antifibrotic and proangiogenic activities. Injected or transgenic Tβ4 increase blood vessel growth in large and small animal models, consistent with Tβ4 converting hibernating myocardium to an actively contractile state following ischemia. Tβ4 and its degradation products have antifibrotic effects in in vitro assays and in animal models of fibrosis not related to cardiac injury. This large number of pleiotropic effects results from Tβ4's many interactions with cellular signaling pathways, particularly indirect regulation of cellular motility and movement via the SRF-MRTF-G-actin transcriptional pathway. Variation in effects and effect sizes in animal models may potentially be due to variable distribution of Tβ4. Preclinical studies of PK/PD relationships and a reliable pharmacodynamic biomarker would facilitate clinical development of Tβ4.

The Chromatin Remodeling Complex Chd4/NuRD Controls Striated Muscle Identity and Metabolic Homeostasis

Heart muscle maintains blood circulation, while skeletal muscle powers skeletal movement. Despite having similar myofibrilar sarcomeric structures, these striated muscles differentially express specific sarcomere components to meet their distinct contractile requirements. The mechanism responsible is still unclear. We show here that preservation of the identity of the two striated muscle types depends on epigenetic repression of the alternate lineage gene program by the chromatin remodeling complex Chd4/NuRD. Loss of Chd4 in the heart triggers aberrant expression of the skeletal muscle program, causing severe cardiomyopathy and sudden death. Conversely, genetic depletion of Chd4 in skeletal muscle causes inappropriate expression of cardiac genes and myopathy. In both striated tissues, mitochondrial function was also dependent on the Chd4/NuRD complex. We conclude that an epigenetic mechanism controls cardiac and skeletal muscle structural and metabolic identities and that loss of this regulation leads to hybrid striated muscle tissues incompatible with life.

Enteroviral proteases: structure, host interactions and pathogenicity

Enteroviruses are common human pathogens, and infections are particularly frequent in children. Severe infections can lead to a variety of diseases, including poliomyelitis, aseptic meningitis, myocarditis and neonatal sepsis. Enterovirus infections have also been implicated in asthmatic exacerbations and type 1 diabetes. The large disease spectrum of the closely related enteroviruses may be partially, but not fully, explained by differences in tissue tropism. The molecular mechanisms by which enteroviruses cause disease are poorly understood, but there is increasing evidence that the two enteroviral proteases, 2A^pro and 3C^pro, are important mediators of pathology. These proteases perform the post‐translational proteolytic processing of the viral polyprotein, but they also cleave several host‐cell proteins in order to promote the production of new virus particles, as well as to evade the cellular antiviral immune responses. Enterovirus‐associated processing of cellular proteins may also contribute to pathology, as elegantly demonstrated by the 2A^pro‐mediated cleavage of dystrophin in cardiomyocytes contributing to Coxsackievirus‐induced cardiomyopathy. It is likely that improved tools to identify targets for these proteases will reveal additional host protein substrates that can be linked to specific enterovirus‐associated diseases. Here, we discuss the function of the enteroviral proteases in the virus replication cycle and review the current knowledge regarding how these proteases modulate the infected cell in order to favour virus replication, including ways to avoid detection by the immune system. We also highlight new possibilities for the identification of protease‐specific cellular targets and thereby a way to discover novel mechanisms contributing to disease. Copyright © 2016 John Wiley & Sons, Ltd.

MEF2D deficiency in neonatal cardiomyocytes triggers cell cycle re-entry and programmed cell death in vitro

The cardiomyocyte cell cycle is a poorly understood process. Mammalian cardiomyocytes permanently withdraw from the cell cycle shortly after birth but can re-enter the cell cycle and proliferate when subjected to injury within a brief temporal window in the neonatal period. Thus, investigating the mechanisms of cell cycle regulation in neonatal cardiomyocytes may provide critical insight into the molecular events that prevent adult myocytes from proliferating in response to injury or stress. MEF2D is a key transcriptional mediator of pathological remodeling in the adult heart downstream of various stress-promoting insults. However, the specific gene programs regulated by MEF2D in cardiomyocytes are unknown. By performing genome-wide transcriptome analysis using MEF2D-depleted neonatal cardiomyocytes, we found a significant impairment in the cell cycle, characterized by the up-regulation of numerous positive cell cycle regulators. Expression of Pten, the primary negative regulator of PI3K/Akt, was significantly reduced in MEF2D-deficient cardiomyocytes and found to be a direct target gene of MEF2D. Consistent with these findings mutant cardiomyocytes showed activation of the PI3K/Akt survival pathway. Paradoxically, prolonged deficiency of MEF2D in neonatal cardiomyocytes did not trigger proliferation but instead resulted in programmed cell death, which is likely mediated by the E2F transcription factor. These results demonstrate a critical role for MEF2D in cell cycle regulation of post-mitotic, neonatal cardiomyocytes in vitro.

MicroRNAs in the Myocyte Enhancer Factor 2 (MEF2)-regulated Gtl2-Dio3 Noncoding RNA Locus Promote Cardiomyocyte Proliferation by Targeting the Transcriptional Coactivator Cited2

Understanding cell cycle regulation in postmitotic cardiomyocytes may lead to new therapeutic approaches to regenerate damaged cardiac tissue. We have demonstrated previously that microRNAs encoded by the Gtl2-Dio3 noncoding RNA locus function downstream of the MEF2A transcription factor in skeletal muscle regeneration. We have also reported expression of these miRNAs in the heart. Here we investigated the role of two Gtl2-Dio3 miRNAs, miR-410 and miR-495, in cardiac muscle. Overexpression of miR-410 and miR-495 robustly stimulated cardiomyocyte DNA synthesis and proliferation. Interestingly, unlike our findings in skeletal muscle, these miRNAs did not modulate the activity of the WNT signaling pathway. Instead, these miRNAs targeted Cited2, a coactivator required for proper cardiac development. Consistent with miR-410 and miR-495 overexpression, siRNA knockdown of Cited2 in neonatal cardiomyocytes resulted in robust proliferation. This phenotype was associated with reduced expression of Cdkn1c/p57/Kip2, a cell cycle inhibitor, and increased expression of VEGFA, a growth factor with proliferation-promoting effects. Therefore, miR-410 and miR-495 are among a growing number of miRNAs that have the ability to potently stimulate neonatal cardiomyocyte proliferation.

A Hypothesis for Examining Skeletal Muscle Biopsy-Derived Sarcolemmal nNOSμ as Surrogate for Enteric nNOSα Function

The pathophysiology of gastrointestinal motility disorders is controversial and largely unresolved. This provokes empiric approaches to patient management of these so-called functional gastrointestinal disorders. Preliminary evidence demonstrates that defects in neuronal nitric oxide synthase (nNOS) expression and function, the enzyme that synthesizes nitric oxide (NO), the key inhibitory neurotransmitter mediating mechano-electrical smooth muscle relaxation, is the major pathophysiological basis for sluggishness of oro-aboral transit of luminal contents. This opinion is an ansatz of the potential of skeletal muscle biopsy and examining sarcolemmal nNOSμ to provide complementary insights regarding nNOSα expression, localization, and function within enteric nerve terminals, the site of stimulated de novo NO synthesis. The main basis of this thesis is twofold: (a) the molecular similarity of the structures of nNOS α and μ, similar mechanisms of localizations to “active zones” of nitrergic synthesis, and same mechanisms of electron transfers during NO synthesis and (b) pragmatic difficulty to routinely obtain full-thickness biopsies of gastrointestinal tract, even in patients presenting with the most recalcitrant manifestations of stasis and delayed transit of luminal contents. This opinion attempts to provoke dialog whether this approach is feasible as a surrogate to predict catalytic potential of nNOSα and defects in nitrergic neurotransmission. This discussion makes an assumption that similar molecular mechanisms of nNOS defects shall be operant in both the enteric nerve terminals and the skeletal muscles. These overlaps of skeletal and gastrointestinal dysfunction are largely unknown, thus meriting that the thesis be validated in future by proof-of-principle experiments.

Costamere proteins and their involvement in myopathic processes

Muscle fibres are very specialised cells with a complex structure that requires a high level of organisation of the constituent proteins. For muscle contraction to function properly, there is a need for not only sarcomeres, the contractile structures of the muscle fibre, but also costameres. These are supramolecular structures associated with the sarcolemma that allow muscle adhesion to the extracellular matrix. They are composed of protein complexes that interact and whose functions include maintaining cell structure and signal transduction mediated by their constituent proteins. It is important to improve our understanding of these structures, as mutations in various genes that code for costamere proteins cause many types of muscular dystrophy. In this review, we provide a description of costameres detailing each of their constituent proteins, such as dystrophin, dystrobrevin, syntrophin, sarcoglycans, dystroglycans, vinculin, talin, integrins, desmin, plectin, etc. We describe as well the diseases associated with deficiency thereof, providing a general overview of their importance.

MEF2 transcription factors regulate distinct gene programs in mammalian skeletal muscle differentiation

Skeletal muscle differentiation requires precisely coordinated transcriptional regulation of diverse gene programs that ultimately give rise to the specialized properties of this cell type. In Drosophila, this process is controlled, in part, by MEF2, the sole member of an evolutionarily conserved transcription factor family. By contrast, vertebrate MEF2 is encoded by four distinct genes, Mef2a, -b, -c, and -d, making it far more challenging to link this transcription factor to the regulation of specific muscle gene programs. Here, we have taken the first step in molecularly dissecting vertebrate MEF2 transcriptional function in skeletal muscle differentiation by depleting individual MEF2 proteins in myoblasts. Whereas MEF2A is absolutely required for proper myoblast differentiation, MEF2B, -C, and -D were found to be dispensable for this process. Furthermore, despite the extensive redundancy, we show that mammalian MEF2 proteins regulate a significant subset of nonoverlapping gene programs. These results suggest that individual MEF2 family members are able to recognize specific targets among the entire cohort of MEF2-regulated genes in the muscle genome. These findings provide opportunities to modulate the activity of MEF2 isoforms and their respective gene programs in skeletal muscle homeostasis and disease.