Myocardin is differentially required for the development of smooth muscle cells and cardiomyocytes

Plasticity in airway smooth muscle differentiation during mouse lung development

It has been proposed that smooth muscle differentiation may physically sculpt airway epithelial branches in mammalian lungs. Serum response factor (SRF) acts with its co-factor myocardin to activate the expression of contractile smooth muscle markers. In the adult, however, smooth muscle exhibits a variety of phenotypes beyond contractile, and these are independent of SRF/myocardin-induced transcription. To determine whether a similar phenotypic plasticity is exhibited during development, we deleted Srf from the mouse embryonic pulmonary mesenchyme. Srf-mutant lungs branch normally, and the mesenchyme displays mechanical properties indistinguishable from controls. scRNA-seq identified an Srf-null smooth muscle cluster, wrapping the airways of mutant lungs, which lacks contractile smooth muscle markers but retains many features of control smooth muscle. Srf-null embryonic airway smooth muscle exhibits a synthetic phenotype, compared with the contractile phenotype of mature wild-type airway smooth muscle. Our findings identify plasticity in embryonic airway smooth muscle and demonstrate that a synthetic smooth muscle layer promotes airway branching morphogenesis.

Temporal regulation of notch activation improves arteriovenous fistula maturation

BACKGROUND

Arteriovenous fistula (AVF) maturation is a process involving remodeling of venous arm of the AVFs. It is a challenge to balance adaptive AVF remodeling and neointima formation. In this study we temporally controlled Notch activation to promote AVF maturation while avoiding neointima formation.

METHODS

Temporal Notch activation was controlled by regulating the expression of Notch transcription factor, RBP-Jκ, or dnMAML1 (dominant negative MAML2) in vascular smooth muscle cells (VSMCs). AVF mouse model was created and VSMC phenotype dynamic changes during AVF remodeling were determined.

RESULTS

Activated Notch was found in the nuclei of neointimal VSMCs in AVFs from uremic mice. We found that the VSMCs near the anastomosis became dedifferentiated and activated after AVF creation. These dedifferentiated VSMCs regained smooth muscle contractile markers later during AVF remodeling. However, global or VSMC-specific KO of RBP-Jκ at early stage (before or 1 week after AVF surgery) blocked VSMC differentiation and neointima formation in AVFs. These un-matured AVFs showed less intact endothelium and increased infiltration of inflammatory cells. Consequently, the VSMC fate in the neointima was completely shut down, leading to an un-arterialized AVF. In contrast, KO of RBP-Jκ at late stage (3 weeks after AVF surgery), it could not block neointima formation and vascular stenosis. Inhibition of Notch activation at week 1 or 2, could maintain VSMC contractile markers expression and facilitate AVF maturation.

CONCLUSIONS

This work uncovers the molecular and cellular events in each segment of AVF remodeling and found that neither sustained increasing nor blocking of Notch signaling improves AVF maturation. It highlights a novel strategy to improve AVF patency: temporally controlled Notch activation can achieve a balance between adaptive AVF remodeling and neointima formation to improve AVF maturation.

TRANSLATIONAL PERSPECTIVE

Adaptive vascular remodeling is required for AVF maturation. The balance of wall thickening of the vein and neointima formation in AVF determines the fate of AVF function. Sustained activation of Notch signaling in VSMCs promotes neointima formation, while deficiency of Notch signaling at early stage during AVF remodeling prevents VSMC accumulation and differentiation from forming a functional AVFs. These responses also delay EC regeneration and impair EC barrier function with increased inflammation leading to failed vascular remodeling of AVFs. Thus, a strategy to temporal regulate Notch activation will improve AVF maturation.

Appropriate Exogenous Expression Stoichiometry of GATA4 as an Important Factor for Cardiac Reprogramming of Human Dermal Fibroblasts

Reprogramming of human dermal fibroblasts (HDFs) into induced cardiomyocyte-like cells (iCMs) represents a promising strategy for human cardiac regeneration. Different cocktails of cardiac transcription factors can convert HDFs into iCMs, although with low efficiency and immature phenotype. Here, GATA4, MEF2C, TBX5, MESP1, and MYOCD (GMTMeMy for short) were used to reprogram HDFs by retrovirus infection. We found that the exogenous expression stoichiometry of GATA4 (GATA4 stoichiometry) significantly affected reprogramming efficiency. When 1/8 dosage of GATA4 virus (GATA4 dosage) plus MTMeMy was used, the reprogramming efficiency was obviously improved compared with average pooled virus encoding each factor, which measured, by the expression level of cardiac genes, the percentage of cardiac troponin T and alpha-cardiac myosin heavy-chain immunopositive cells and the numbers of iCMs showing calcium oscillation or beating synchronously in co-culture with mouse CMs. In addition, we prepared conditioned maintenance medium (CMM) by CM differentiation of H9 human embryonic stem cell line. We found that compared with traditional maintenance medium (TMM), CMM made iCMs show well-organized sarcomere formation and characteristic calcium oscillation wave earlier. These findings demonstrated that appropriate GATA4 stoichiometry was essential for cardiac reprogramming and some components in CMM were important for maturation of iCMs.

Myocardin suppression increases lipid retention and atherosclerosis via downregulation of ABCA1 in vascular smooth muscle cells

Myocardin (MYOCD) plays an important role in cardiovascular disease. However, its underlying impact on atherosclerosis remains to be elucidated. ATP binding cassette transporter A1 (ABCA1), a key membrane-associated lipid transporter which maintains intracellular lipid homeostasis, has a protective function in atherosclerosis progress. The purpose of this study was to investigate whether and how the effect of MYOCD on atherosclerosis is associated with ABCA1 in vascular smooth muscle cells (VSMCs). We found both MYOCD and ABCA1 expression were dramatically decreased in atherosclerotic patient aortas compared to control. MYOCD knockdown inhibited ABCA1 expression in human aortic vascular smooth muscle cells (HAVSMCs), leading to reduced cholesterol efflux and increased intracellular cholesterol contents. MYOCD overexpression exerted the opposite effect. Mechanistically, MYOCD regulates ABCA1 expression in an SRF-dependent manner. Consistently, apolipoprotein E-deficient mice treated with MYOCD shRNA developed more plaques in the aortic sinus, which is associated with reduced ABCA1 expression, increased cholesterol retention in the aorta, and decreased high-density lipoprotein cholesterol levels in the plasma. Our data suggest that MYOCD deficiency exacerbates atherosclerosis by downregulating ABCA1 dependent cholesterol efflux from VSMCs, thereby providing a novel strategy for the therapeutic treatment of atherosclerotic cardiovascular disease.

QKI is a critical pre-mRNA alternative splicing regulator of cardiac myofibrillogenesis and contractile function

The RNA-binding protein QKI belongs to the hnRNP K-homology domain protein family, a well-known regulator of pre-mRNA alternative splicing and is associated with several neurodevelopmental disorders. Qki is found highly expressed in developing and adult hearts. By employing the human embryonic stem cell (hESC) to cardiomyocyte differentiation system and generating QKI-deficient hESCs (hESCs-QKI^del) using CRISPR/Cas9 gene editing technology, we analyze the physiological role of QKI in cardiomyocyte differentiation, maturation, and contractile function. hESCs-QKI^del largely maintain normal pluripotency and normal differentiation potential for the generation of early cardiogenic progenitors, but they fail to transition into functional cardiomyocytes. In this work, by using a series of transcriptomic, cell and biochemical analyses, and the Qki-deficient mouse model, we demonstrate that QKI is indispensable to cardiac sarcomerogenesis and cardiac function through its regulation of alternative splicing in genes involved in Z-disc formation and contractile physiology, suggesting that QKI is associated with the pathogenesis of certain forms of cardiomyopathies.

Discovery and progress of direct cardiac reprogramming

Cardiac disease remains a major cause of death worldwide. Direct cardiac reprogramming has emerged as a promising approach for cardiac regenerative therapy. After the discovery of MyoD, a master regulator for skeletal muscle, other single cardiac reprogramming factors (master regulators) have been sought. Discovery of cardiac reprogramming factors was inspired by the finding that multiple, but not single, transcription factors were needed to generate induced pluripotent stem cells (iPSCs) from fibroblasts. We first reported a combination of cardiac-specific transcription factors, Gata4, Mef2c, and Tbx5 (GMT), that could convert mouse fibroblasts into cardiomyocyte-like cells, which were designated as induced cardiomyocyte-like cells (iCMs). Following our first report of cardiac reprogramming, many researchers, including ourselves, demonstrated an improvement in cardiac reprogramming efficiency, in vivo direct cardiac reprogramming for heart regeneration, and cardiac reprogramming in human cells. However, cardiac reprogramming in human cells and adult fibroblasts remains inefficient, and further efforts are needed. We believe that future research elucidating epigenetic barriers and molecular mechanisms of direct cardiac reprogramming will improve the reprogramming efficiency, and that this new technology has great potential for clinical applications.

The Invalidation of HspB1 Gene in Mouse Alters the Ultrastructural Phenotype of Muscles

Even though abundance of Hsp27 is the highest in skeletal muscle, the relationships between the expression of HspB1 (encoding Hsp27) and muscle characteristics are not fully understood. In this study, we have analysed the effect of Hsp27 inactivation on mouse development and phenotype. We generated a mouse strain devoid of Hsp27 protein by homologous recombination of the HspB1 gene. The HspB1^-/- mouse was viable and fertile, showing neither apparent morphological nor anatomical alterations. We detected a gender dimorphism with marked effects in males, a lower body weight (P < 0.05) with no obvious changes in the growth rate, and a lower plasma lipids profile (cholesterol, HDL and triglycerides, 0.001 < P< 0.05). The muscle structure of the animals was examined by optical microscopy and transmission electron microscopy. Not any differences in the characteristics of muscle fibres (contractile and metabolic type, shape, perimeter, cross-sectional area) were detected except a trend for a higher proportion of small fibres. Different myosin heavy chains electrophoretic profiles were observed in the HspB1^-/- mouse especially the presence of an additional isoform. Electron microscopy revealed ultrastructural abnormalities in the myofibrillar structure of the HspB1^-/- mouse mutant mice (e.g. destructured myofibrils and higher gaps between myofibrils) especially in the m. Soleus. Combined with our previous data, these findings suggest that Hsp27 could directly impact the organization of muscle cytoskeleton at the molecular and ultrastructural levels.

Myocardin regulates vascular smooth muscle cell inflammatory activation and disease

OBJECTIVE

Atherosclerosis, the cause of 50% of deaths in westernized societies, is widely regarded as a chronic vascular inflammatory disease. Vascular smooth muscle cell (VSMC) inflammatory activation in response to local proinflammatory stimuli contributes to disease progression and is a pervasive feature in developing atherosclerotic plaques. Therefore, it is of considerable therapeutic importance to identify mechanisms that regulate the VSMC inflammatory response.

APPROACH AND RESULTS

We report that myocardin, a powerful myogenic transcriptional coactivator, negatively regulates VSMC inflammatory activation and vascular disease. Myocardin levels are reduced during atherosclerosis, in association with phenotypic switching of smooth muscle cells. Myocardin deficiency accelerates atherogenesis in hypercholesterolemic apolipoprotein E(-/-) mice. Conversely, increased myocardin expression potently abrogates the induction of an array of inflammatory cytokines, chemokines, and adhesion molecules in VSMCs. Expression of myocardin in VSMCs reduces lipid uptake, macrophage interaction, chemotaxis, and macrophage-endothelial tethering in vitro, and attenuates monocyte accumulation within developing lesions in vivo. These results demonstrate that endogenous levels of myocardin are a critical regulator of vessel inflammation.

CONCLUSIONS

We propose myocardin as a guardian of the contractile, noninflammatory VSMC phenotype, with loss of myocardin representing a critical permissive step in the process of phenotypic transition and inflammatory activation, at the onset of vascular disease.

Myocardin in biology and disease

Myocardin (MYOCD) is a potent transcriptional coactivator that functions primarily in cardiac muscle and smooth muscle through direct contacts with serum response factor (SRF) over cis elements known as CArG boxes found near a number of genes encoding for contractile, ion channel, cytoskeletal, and calcium handling proteins. Since its discovery more than 10 years ago, new insights have been obtained regarding the diverse isoforms of MYOCD expressed in cells as well as the regulation of MYOCD expression and activity through transcriptional, post-transcriptional, and post-translational processes. Curiously, there are a number of functions associated with MYOCD that appear to be independent of contractile gene expression and the CArG-SRF nucleoprotein complex. Further, perturbations in MYOCD gene expression are associated with an increasing number of diseases including heart failure, cancer, acute vessel disease, and diabetes. This review summarizes the various biological and pathological processes associated with MYOCD and offers perspectives to several challenges and future directions for further study of this formidable transcriptional coactivator.

Role of Krüppel-like factor 4 and its binding proteins in vascular disease

Krüppel-like factor 4(KLF4) is a zinc-finger transcription factor that plays a key role in cellular differentiation and proliferation during normal development and in various diseases, such as cancer. The results of recent studies have revealed that KLF4 is expressed in multiple vascular cell types, including phenotypically modulated smooth muscle cells(SMCs), endothelial cells and monocytes/macrophages and contributes to the progression of vascular diseases by activating or repressing the transcription of multiple genes via its associations with a variety of partner proteins. For example, KLF4 decreases the expression of markers of SMC differentiation by interacting with serum response factor, ELK1 and histone deacetylases. KLF4 also suppresses SMC proliferation by associating with p53. In addition, KLF4 enhances arterial medial calcification in concert with RUNX2. Furthermore, endothelial KLF4 represses arterial inflammation by binding to nuclear factor-κB. This article summarizes the role of KLF4 in vascular disease with a particular focus on in vivo studies and reviews recent progress in our understanding of the regulatory mechanisms involved in KLF4- mediated gene transcription.

Re-expression and epigenetic modification of maspin induced apoptosis in MCF-7 cells mediated by myocardin

Breast cancer is the leading cause of cancer death in women worldwide. It is well known that oncogene activation and anti-oncogene inactivation affect the development and progression of breast cancer, but the role of oncogene activation and anti-oncogene inactivation in breast cancer is still not fully understood. We now report that maspin acts as a tumor suppressor gene to induce MCF-7 cell apoptosis. In addition, maspin promoter hypermethylation and histone hypoacetylation lead to silencing of maspin gene expression in MCF-7 cells. Moreover, DNA methyltransferase (DNMT) inhibitor 5-aza-2'-deoxycytidine (5-aza-dc) and/or the histone deacetylase (HDAC) inhibitor Trichostatin A (TSA) strongly up-regulated the expression of maspin in MCF-7 cells. Notably, myocardin can promote the re-expression of maspin in MCF-7 cells. Luciferase assay shows that myocardin activates the transcription of maspin promoter by CArG box. More importantly, 5-aza-dc/TSA and myocardin synergetically enhance re-expression of maspin and augment maspin-mediated apoptosis in MCF-7 cells. Thus, these data reveal the new insight that myocardin meditates apoptosis in breast cancer through affecting maspin re-expression and epigenetic modification to regulate the development of breast cancer, thereby raising the possibility of its use in breast cancer therapy.

Angiotensin II, from vasoconstrictor to growth factor: a paradigm shift

Angiotensin II (Ang II) is today considered as one of the essential factors in the pathophysiology of cardiovascular disease, producing acute, hemodynamic and chronic, pleiotropic effects. Although now it is widely accepted that these chronic effects are important, Ang II was initially considered only a short-acting, vasoactive hormone. This view was modified a quarter of a century ago when Dr. Owens and his group published a paper in Circulation Research with initial evidence that Ang II can act as a growth factor that regulates cell hypertrophy. They showed in series of elegant experiments that Ang II promotes hypertrophy and hyperploidy of cultured rat aortic smooth muscle cells. However, Ang II had no effect on hyperplasia. These findings led to a paradigm shift in our understanding of the roles of growth factors and vasoactive substances in cardiovascular pathology and helped to redirect basic and clinical renin-angiotensin system research over the next twenty-five years. Ang II is now known to be a pleiotropic hormone that utilizes multiple signaling pathways to influence most processes that contribute to the development and progression of cardiovascular diseases, ranging from hypertrophy, endothelial dysfunction, cardiac remodeling, fibrosis, and inflammation to oxidative stress.

Overexpression of myocardin induces partial transdifferentiation of human-induced pluripotent stem cell-derived mesenchymal stem cells into cardiomyocytes

Mesenchymal stem cells (MSCs) derived from human‐induced pluripotent stem cells (iPSCs) show superior proliferative capacity and therapeutic potential than those derived from bone marrow (BM). Ectopic expression of myocardin further improved the therapeutic potential of BM‐MSCs in a mouse model of myocardial infarction. The aim was of this study was to assess whether forced myocardin expression in iPSC‐MSCs could further enhance their transdifferentiation to cardiomyocytes and improve their electrophysiological properties for cardiac regeneration. Myocardin was overexpressed in iPSC‐MSCs using viral vectors (adenovirus or lentivirus). The expression of smooth muscle cell and cardiomyocyte markers, and ion channel genes was examined by reverse transcription‐polymerase chain reaction (RT‐PCR), immunofluorescence staining and patch clamp. The conduction velocity of the neonatal rat ventricular cardiomyocytes cocultured with iPSC‐MSC monolayer was measured by multielectrode arrays recording plate. Myocardin induced the expression of α‐MHC, GATA4, α‐actinin, cardiac MHC, MYH11, calponin, and SM α‐actin, but not cTnT, β‐MHC, and MLC2v in iPSC‐MSCs. Overexpression of myocardin in iPSC‐MSC enhanced the expression of SCN9A and CACNA1C, but reduced that of KCa3.1 and Kir2.2 in iPSC‐MSCs. Moreover, BK_Ca, I_Kir, I_Cl, I_to and I_Na.TTX were detected in iPSC‐MSC with myocardin overexpression; while only BK_Ca, I_Kir, I_Cl, IK_DR, and IK_Ca were noted in iPSC‐MSC transfected with green florescence protein. Furthermore, the conduction velocity of iPSC‐MSC was significantly increased after myocardin overexpression. Overexpression of myocardin in iPSC‐MSCs resulted in partial transdifferentiation into cardiomyocytes phenotype and improved the electrical conduction during integration with mature cardiomyocytes.

Forced myocardin expression in human‐induced pluripotent stem cell (hiPSC)‐derived mesenchymal stem cells lead to partial transdifferentiation into cardiomyocytes and smooth muscle cells phenotypes through modification in ion channel expression profile and electrical conduction velocity.

microRNA-1 regulates sarcomere formation and suppresses smooth muscle gene expression in the mammalian heart

microRNA-1 (miR-1) is an evolutionarily conserved, striated muscle-enriched miRNA. Most mammalian genomes contain two copies of miR-1, and in mice, deletion of a single locus, miR-1-2, causes incompletely penetrant lethality and subtle cardiac defects. Here, we report that deletion of miR-1-1 resulted in a phenotype similar to that of the miR-1-2 mutant. Compound miR-1 knockout mice died uniformly before weaning due to severe cardiac dysfunction. miR-1-null cardiomyocytes had abnormal sarcomere organization and decreased phosphorylation of the regulatory myosin light chain-2 (MLC2), a critical cytoskeletal regulator. The smooth muscle-restricted inhibitor of MLC2 phosphorylation, Telokin, was ectopically expressed in the myocardium, along with other smooth muscle genes. miR-1 repressed Telokin expression through direct targeting and by repressing its transcriptional regulator, Myocardin. Our results reveal that miR-1 is required for postnatal cardiac function and reinforces the striated muscle phenotype by regulating both transcriptional and effector nodes of the smooth muscle gene expression network.

DOI:http://dx.doi.org/10.7554/eLife.01323.001

MicroRNAs are tiny RNAs that do not encode proteins. Instead, they regulate the expression of genes by preventing protein-encoding messenger RNAs from being translated into protein. MicroRNAs are expressed throughout the body, including the heart, where the most abundant microRNA is called miR-1. This is encoded by two nearly identical genes: miR-1-1 and miR-1-2.

Mice that lack the miR-1-2 gene have various heart abnormalities, but generally survive because they still produce some miR-1 from their remaining miR-1-1 gene. Now, Heidersbach et al. have generated the first mice that specifically lack both miR-1 genes, and shown that these animals die before weaning.

When viewed under the electron microscope, heart muscle from miR-1 double knockout mice lacks the characteristic ‘striped’, or striated, appearance of normal heart muscle. Additionally, miR-1 double knockout hearts have some gene expression characteristics more similar to the smooth muscle found in the gut and in the walls of blood vessels. Smooth muscle differs from striated muscle in that it lacks sarcomeres: these are bands of fibrous proteins, such as myosin, that are essential for muscle contraction.

In normal mice, an enzyme called MLCK contributes to the formation and function of sarcomeres by adding phosphate groups to myosin molecules. By contrast, in smooth muscle an enzyme called Telokin promotes phosphate group removal, and thus affects the function of sarcomeres. Heidersbach et al. showed that miR-1 interacts directly with Telokin mRNA to prevent its expression in the heart, and simultaneously represses a protein called Myocardin, which directly activates transcription of Telokin. However, when miR-1 is absent, as in the miR-1 double knockout mice, Telokin is expressed in heart muscle, along with many other genes characteristic of smooth muscle.

As well as improving our understanding of the development and functioning of the heart, these findings should shed new light on the role of microRNAs in maintaining the patterns of gene expression that characterize unique cell fates.

DOI:http://dx.doi.org/10.7554/eLife.01323.002

Cell division cycle 7 mediates transforming growth factor-β-induced smooth muscle maturation through activation of myocardin gene transcription

Smooth muscle (SM) development consists of several processes, including cell fate determination, differentiation, and maturation. The molecular mechanisms controlling SM early differentiation have been studied extensively. However, little is known about the mechanism underlying SM maturation. Cell division cycle 7 (Cdc7) has been shown to regulate cell fate determination in the initial phase of transforming growth factor-β (TGF-β)-induced SM differentiation. Our present study indicates that Cdc7 also regulates SM maturation. Knockdown of Cdc7 suppresses TGF-β-induced expression of SM myosin heavy chain, a late marker of SM differentiation. Cdc7 overexpression, on the other hand, enhances SM myosin heavy chain expression. Interestingly, Cdc7 activates the mRNA expression and promoter activity of myocardin (Myocd), a master regulator of SM differentiation, whose transcription is blocked in the initial phase of the differentiation because TGF-β does not induce Myocd mRNA until after the early SM markers are induced. These data suggest that Cdc7 mediates TGF-β-induced SM maturation via activation of Myocd transcription. Mechanistically, Cdc7 physically and functionally interacts with Nkx2.5 to regulate Myocd promoter activity. Cdc7 appears to enhance Nkx2.5 binding to Myocd promoter, leading to Myocd activation. Taken together, our studies demonstrate that Cdc7 regulates the initial and late phase of SM differentiation through distinct mechanisms.

Activation of MRTF-A-dependent gene expression with a small molecule promotes myofibroblast differentiation and wound healing

Myocardin-related transcription factors (MRTFs) regulate cellular contractility and motility by associating with serum response factor (SRF) and activating genes involved in cytoskeletal dynamics. We reported previously that MRTF-A contributes to pathological cardiac remodeling by promoting differentiation of fibroblasts to myofibroblasts following myocardial infarction. Here, we show that forced expression of MRTF-A in dermal fibroblasts stimulates contraction of a collagen matrix, whereas contractility of MRTF-A null fibroblasts is impaired under basal conditions and in response to TGF-β1 stimulation. We also identify an isoxazole ring-containing small molecule, previously shown to induce smooth muscle α-actin gene expression in cardiac progenitor cells, as an agonist of myofibroblast differentiation. Isoxazole stimulates myofibroblast differentiation via induction of MRTF-A-dependent gene expression. The MRTF-SRF signaling axis is activated in response to skin injury, and treatment of dermal wounds with isoxazole accelerates wound closure and suppresses the inflammatory response. These results reveal an important role for MRTF-SRF signaling in dermal myofibroblast differentiation and wound healing and suggest that targeting MRTFs pharmacologically may prove useful in treating diseases associated with inappropriate myofibroblast activity.

miR-1/133a clusters cooperatively specify the cardiomyogenic lineage by adjustment of myocardin levels during embryonic heart development

miRNAs are small RNAs directing many developmental processes by posttranscriptional regulation of protein-coding genes. We uncovered a new role for miR-1-1/133a-2 and miR-1-2/133a-1 clusters in the specification of embryonic cardiomyocytes allowing transition from an immature state characterized by expression of smooth muscle (SM) genes to a more mature fetal phenotype. Concomitant knockout of miR-1-1/133a-2 and miR-1-2/133a-1 released suppression of the transcriptional co-activator myocardin, a major regulator of SM gene expression, but not of its binding partner SRF. Overexpression of myocardin in the embryonic heart essentially recapitulated the miR-1/133a mutant phenotype at the molecular level, arresting embryonic cardiomyocytes in an immature state. Interestingly, the majority of postulated miR-1/133a targets was not altered in double mutant mice, indicating that the ability of miR-1/133a to suppress target molecules strongly depends on the cellular context. Finally, we show that myocardin positively regulates expression of miR-1/133a, thus constituting a negative feedback loop that is essential for early cardiac development.

Induction of human cardiomyocyte-like cells from fibroblasts by defined factors

Heart disease remains a leading cause of death worldwide. Owing to the limited regenerative capacity of heart tissue, cardiac regenerative therapy has emerged as an attractive approach. Direct reprogramming of human cardiac fibroblasts (HCFs) into cardiomyocytes may hold great potential for this purpose. We reported previously that induced cardiomyocyte-like cells (iCMs) can be directly generated from mouse cardiac fibroblasts in vitro and vivo by transduction of three transcription factors: Gata4, Mef2c, and Tbx5, collectively termed GMT. In the present study, we sought to determine whether human fibroblasts also could be converted to iCMs by defined factors. Our initial finding that GMT was not sufficient for cardiac induction in HCFs prompted us to screen for additional factors to promote cardiac reprogramming by analyzing multiple cardiac-specific gene induction with quantitative RT-PCR. The addition of Mesp1 and Myocd to GMT up-regulated a broader spectrum of cardiac genes in HCFs more efficiently compared with GMT alone. The HCFs and human dermal fibroblasts transduced with GMT, Mesp1, and Myocd (GMTMM) changed the cell morphology from a spindle shape to a rod-like or polygonal shape, expressed multiple cardiac-specific proteins, increased a broad range of cardiac genes and concomitantly suppressed fibroblast genes, and exhibited spontaneous Ca(2+) oscillations. Moreover, the cells matured to exhibit action potentials and contract synchronously in coculture with murine cardiomyocytes. A 5-ethynyl-2'-deoxyuridine assay revealed that the iCMs thus generated do not pass through a mitotic cell state. These findings demonstrate that human fibroblasts can be directly converted to iCMs by defined factors, which may facilitate future applications in regenerative medicine.

Myocardin regulates vascular response to injury through miR-24/-29a and platelet-derived growth factor receptor-β

OBJECTIVE

Myocardin, a potent transcriptional coactivator of serum response factor, is involved in vascular development and promotes a contractile smooth muscle phenotype. Myocardin levels are reduced during vascular injury, in association with phenotypic switching of smooth muscle cells (SMCs). However, the direct role of myocardin in vascular disease is unclear.

APPROACH AND RESULTS

We show that re-expression of myocardin prevents the vascular injury response in murine carotid arteries, with reduced neointima formation due to decreased SMC migration and proliferation. Myocardin reduced SMC migration by downregulating platelet-derived growth factor receptor-β (PDGFRB) expression. Pdgfrb was regulated by myocardin-induced miR-24 and miR-29a expression, and antagonizing these microRNAs restored SMC migration. Furthermore, using miR-24 and miR-29a mimics, we demonstrated that miR-29a directly regulates Pdgfrb expression at the 3' untranslated region while miR-24 has an indirect effect on Pdgfrb levels. Myocardin heterozygous-null mice showed an augmented neointima formation with increased SMC migration and proliferation, demonstrating that endogenous levels of myocardin are a critical regulator of vessel injury responses.

CONCLUSIONS

Our results extend the function of myocardin from a developmental role to a pivotal regulator of SMC phenotype in response to injury, and this transcriptional coactivator may be an attractive target for novel therapeutic strategies in vascular disease.

A novel in vitro model system for smooth muscle differentiation from human embryonic stem cell-derived mesenchymal cells

The objective of this study was to develop a novel in vitro model for smooth muscle cell (SMC) differentiation from human embryonic stem cell-derived mesenchymal cells (hES-MCs). We found that hES-MCs were differentiated to SMCs by transforming growth factor-β (TGF-β) in a dose- and time-dependent manner as demonstrated by the expression of SMC-specific genes smooth muscle α-actin, calponin, and smooth muscle myosin heavy chain. Under normal growth conditions, however, the differentiation capacity of hES-MCs was very limited. hES-MC-derived SMCs had an elongated and spindle-shaped morphology and contracted in response to the induction of carbachol and KCl. KCl-induced calcium transient was also evident in these cells. Compared with the parental cells, TGF-β-treated hES-MCs sustained the endothelial tube formation for a longer time due to the sustained SMC phenotype. Mechanistically, TGF-β-induced differentiation was both Smad- and serum response factor/myocardin dependent. TGF-β regulated myocardin expression via multiple signaling pathways including Smad2/3, p38 MAPK, and PI3K. Importantly, we found that a low level of myocardin was present in mesoderm prior to SMC lineage determination, and a high level of myocardin was not induced until the differentiation process was initiated. Taken together, our study characterized a novel SMC differentiation model that can be used for studying human SMC differentiation from mesoderm during vascular development.

Derivation and maturation of synthetic and contractile vascular smooth muscle cells from human pluripotent stem cells

AIMS

Embryonic vascular smooth muscle cells (vSMCs) have a synthetic phenotype; in adults, they commit to the mature contractile phenotype. Research shows that human pluripotent stem cells (hPSCs) differentiate into vSMCs, but nobody has yet documented their maturation into the synthetic or contractile phenotypes. This study sought to control the fate decisions of hPSC derivatives to guide their maturation towards a desired phenotype.

METHODS AND RESULTS

The long-term differentiation of hPSCs, including the integration-free-induced PSC line, in high serum with platelet-derived growth factor-BB (PDGF-BB) and transforming growth factor-β1, allowed us to induce the synthetic vSMC (Syn-vSMC) phenotype with increased extracellular matrix (ECM) protein expression and reduced expression of contractile proteins. By monitoring the expression of two contractile proteins, smooth muscle myosin heavy chain (SMMHC) and elastin, we show that serum starvation and PDGF-BB deprivation caused maturation towards the contractile vSMC (Con-vSMC) phenotype. Con-vSMCs differ distinctively from Syn-vSMC derivatives in their condensed morphology, prominent filamentous arrangement of cytoskeleton proteins, production and assembly of elastin, low proliferation, numerous and active caveolae, enlarged endoplasmic reticulum, and ample stress fibres and bundles, as well as their high contractility. When transplanted subcutaneously into nude mice, the human Con-vSMCs aligned next to the host's growing functional vasculature, with occasional circumferential wrapping and vascular tube narrowing.

CONCLUSION

We control hPSC differentiation into synthetic or contractile phenotypes by using appropriate concentrations of relevant factors. Deriving Con-vSMCs from an integration-free hiPSC line may prove useful for regenerative therapy involving blood vessel differentiation and stabilization.

Myocardin overexpression is sufficient for promoting the development of a mature smooth muscle cell-like phenotype from human embryonic stem cells

Background

Myocardin is thought to have a key role in smooth muscle cell (SMC) development by acting on CArG-dependent genes. However, it is unclear whether myocardin-induced SMC maturation and increases in agonist-induced calcium signalling are also associated with increases in the expression of non-CArG-dependent SMC-specific genes. Moreover, it is unknown whether myocardin promotes SMC development from human embryonic stem cells.

Methodology/Principal

Findings The effects of adenoviral-mediated myocardin overexpression on SMC development in human ESC-derived embryoid bodies were investigated using immunofluorescence, flow cytometry and real time RT-PCR. Myocardin overexpression from day 10 to day 28 of embryoid body differentiation increased the number of smooth muscle α-actin⁺ and smooth muscle myosin heavy chain⁺ SMC-like cells and increased carbachol-induced contractile function. However, myocardin was found to selectively regulate only CArG-dependent SMC-specific genes. Nevertheless, myocardin expression appeared to be sufficient to specify the SMC lineage.

Conclusions/Significance

Myocardin increases the development and maturation of SMC-like cells from human embryonic stem cells despite not activating the full repertoire of SMC genes. These findings have implications for vascular tissue engineering and other applications requiring large numbers of functional SMCs.

The actin-MRTF-SRF gene regulatory axis and myofibroblast differentiation

Cardiac fibroblasts are responsible for necrotic tissue replacement and scar formation after myocardial infarction (MI) and contribute to remodeling in response to pathological stimuli. This response to insult or injury is largely due to the phenotypic plasticity of fibroblasts. When fibroblasts encounter environmental disturbances, whether biomechanical or humoral, they often transform into smooth muscle-like, contractile cells called "myofibroblasts." The signals that control myofibroblast differentiation include the transforming growth factor (TGF)-β1-Smad pathway and Rho GTPase-dependent actin polymerization. Recent evidence implicates serum response factor (SRF) and the myocardin-related transcription factors (MRTFs) as key mediators of the contractile gene program in response to TGF-β1 or RhoA signaling. This review highlights the function of myofibroblasts in cardiac remodeling and the role of the actin-MRTF-SRF signaling axis in regulating this process.

In search of novel targets for heart disease: myocardin and myocardin-related transcriptional cofactors

Growing evidence suggests that gene-regulatory networks, which are responsible for directing cardiovascular development, are altered under stress conditions in the adult heart. The cardiac gene regulatory network is controlled by cardioenriched transcription factors and multiple-cell-signaling inputs. Transcriptional coactivators also participate in gene-regulatory circuits as the primary targets of both physiological and pathological signals. Here, we focus on the recently discovered myocardin-(MYOCD) related family of transcriptional cofactors (MRTF-A and MRTF-B) which associate with the serum response transcription factor and activate the expression of a variety of target genes involved in cardiac growth and adaptation to stress via overlapping but distinct mechanisms. We discuss the involvement of MYOCD, MRTF-A, and MRTF-B in the development of cardiac dysfunction and to what extent modulation of the expression of these factors in vivo can correlate with cardiac disease outcomes. A close examination of the findings identifies the MYOCD-related transcriptional cofactors as putative therapeutic targets to improve cardiac function in heart failure conditions through distinct context-dependent mechanisms. Nevertheless, we are in support of further research to better understand the precise role of individual MYOCD-related factors in cardiac function and disease, before any therapeutic intervention is to be entertained in preclinical trials.

A new approach to transcription factor screening for reprogramming of fibroblasts to cardiomyocyte-like cells

The simultaneous overexpression of several transcription factors has emerged as a successful strategy to convert fibroblasts into other cell types including pluripotent cells, neurons, and cardiomyocytes. The selection and screening of factors are critical, and have often involved testing a large pool of transcription factors, followed by successive removal of single factors. Here, to identify a cardiac transcription factor combination facilitating mouse fibroblast reprogramming into cardiomyocytes, we directly screened all triplet combinations of 10 candidate factors combined with a Q-PCR assay reporting induction of multiple cardiac-specific genes. Through this screening method the combination of Tbx5, Mef2c, and Myocd was identified to upregulate a broader spectrum of cardiac genes compared to the combination of Tbx5, Mef2c, and Gata4 that was recently shown to induce reprogramming of fibroblasts into cardiomyocytes. Cells cotransduced with Tbx5, Mef2c, Myocd expressed cardiac contractile proteins, had cardiac-like potassium and sodium currents and action potentials could be elicited. In summary the alternative screening approach that is presented here avoided the elimination of transcription factors whose potency is masked in complex transcription factor mixes. Furthermore, our results point to the importance of verifying multiple lineage specific genes when assessing reprogramming.

Islet1-expressing cardiac progenitor cells: a comparison across species

Adult mammalian cardiac stem cells express the LIM-homeodomain transcription factor Islet1 (Isl1). They are considered remnants of Isl1-positive embryonic cardiac progenitor cells. During amniote heart development, Isl1-positive progenitor cells give rise mainly to the outflow tract, the right ventricle, and parts of the atria. This led to the hypothesis that the development of the right ventricle of the amniote heart depends on the recruitment of additional cells to the primary heart tube. The region from which these additional, Isl1-positive cells originate is called second heart field, as opposed to the first heart field whose cells form the primary heart tube. Here, we review the available data about Isl1 in different species, demonstrating that Isl1 is an important component of the core transcription factor network driving early cardiogenesis in animals of the two clades, deuterostomes, and protostomes. The data support the view of a single cardiac progenitor cell population that includes Isl1-expressing cells and which differentiates into the various cardiac lineages during embryonic development in vertebrates but not in other phyla of the animal kingdom.

Transforming growth factor-β and smooth muscle differentiation

Transforming growth factor (TGF)-β family members are multifunctional cytokines regulating diverse cellular functions such as growth, adhesion, migration, apoptosis, and differentiation. TGF-βs elicit their effects via specific type I and type II serine/threonine kinase receptors and intracellular Smad transcription factors. Knockout mouse models for the different components of the TGF-β signaling pathway have revealed their critical roles in smooth muscle cell (SMC) differentiation. Genetic studies in humans have linked mutations in these signaling components to specific cardiovascular disorders such as aorta aneurysm and congenital heart diseases due to SMC defects. In this review, the current understanding of TGF-β function in SMC differentiation is highlighted, and the role of TGF-β signaling in SMC-related diseases is discussed.

Targeted gene-silencing reveals the functional significance of myocardin signaling in the failing heart

Background

Myocardin (MYOCD), a potent transcriptional coactivator of smooth muscle (SM) and cardiac genes, is upregulated in failing myocardium in animal models and human end-stage heart failure (HF). However, the molecular and functional consequences of myocd upregulation in HF are still unclear.

Methodology/Principal Findings

The goal of the present study was to investigate if targeted inhibition of upregulated expression of myocd could influence failing heart gene expression and function. To this end, we used the doxorubicin (Dox)-induced diastolic HF (DHF) model in neonatal piglets, in which, as we show, not only myocd but also myocd-dependent SM-marker genes are highly activated in failing left ventricular (LV) myocardium. In this model, intra-myocardial delivery of short-hairpin RNAs, designed to target myocd variants expressed in porcine heart, leads on day 2 post-delivery to: (1) a decrease in the activated expression of myocd and myocd-dependent SM-marker genes in failing myocardium to levels seen in healthy control animals, (2) amelioration of impaired diastolic dysfunction, and (3) higher survival rates of DHF piglets. The posterior restoration of elevated myocd expression (on day 7 post-delivery) led to overexpression of myocd-dependent SM-marker genes in failing LV-myocardium that was associated with a return to altered diastolic function.

Conclusions/Significance

These data provide the first evidence that a moderate inhibition (e.g., normalization) of the activated MYOCD signaling in the diseased heart may be promising from a therapeutic point of view.

Epigenetic control of smooth muscle cell differentiation and phenotypic switching in vascular development and disease

The vascular smooth muscle cell (SMC) in adult animals is a highly specialized cell whose principal function is contraction. However, this cell displays remarkable plasticity and can undergo profound changes in phenotype during repair of vascular injury, during remodeling in response to altered blood flow, or in various disease states. There has been extensive progress in recent years in our understanding of the complex mechanisms that control SMC differentiation and phenotypic plasticity, including the demonstration that epigenetic mechanisms play a critical role. In addition, recent evidence indicates that SMC phenotypic switching in adult animals involves the reactivation of embryonic stem cell pluripotency genes and that mesenchymal stem cells may be derived from SMC and/or pericytes. This review summarizes the current state of our knowledge in this field and identifies some of the key unresolved challenges and questions that we feel require further study.