Myocardin and ternary complex factors compete for SRF to control smooth muscle gene expression

Molecular regulation of contractile smooth muscle cell phenotype: implications for vascular tissue engineering

The molecular regulation of smooth muscle cell (SMC) behavior is reviewed, with particular emphasis on stimuli that promote the contractile phenotype. SMCs can shift reversibly along a continuum from a quiescent, contractile phenotype to a synthetic phenotype, which is characterized by proliferation and extracellular matrix (ECM) synthesis. This phenotypic plasticity can be harnessed for tissue engineering. Cultured synthetic SMCs have been used to engineer smooth muscle tissues with organized ECM and cell populations. However, returning SMCs to a contractile phenotype remains a key challenge. This review will integrate recent work on how soluble signaling factors, ECM, mechanical stimulation, and other cells contribute to the regulation of contractile SMC phenotype. The signal transduction pathways and mechanisms of gene expression induced by these stimuli are beginning to be elucidated and provide useful information for the quantitative analysis of SMC phenotype in engineered tissues. Progress in the development of tissue-engineered scaffold systems that implement biochemical, mechanical, or novel polymer fabrication approaches to promote contractile phenotype will also be reviewed. The application of an improved molecular understanding of SMC biology will facilitate the design of more potent cell-instructive scaffold systems to regulate SMC behavior.

In vivo role of platelet-derived growth factor-BB in airway smooth muscle proliferation in mouse lung

Airway smooth muscle (ASM) hyperplasia in asthma likely contributes considerably to functional changes. Investigating the mechanisms behind proliferation of these cells may lead to therapeutic benefit. Platelet-derived growth factor (PDGF)-BB is a well known ASM mitogen in vitro but has yet to be directly explored using in vivo mouse models in the context of ASM proliferation and airway responsiveness. To determine the in vivo influence of PDGF-BB on gene transcripts encoding contractile proteins, ASM proliferation, and airway physiology, we used an adenovirus overexpression system and a model of chronic allergen exposure. We used adenovirus technology to selectively overexpress PDGF-BB in the airway epithelium of mice. Outcome measurements, including airway physiology, real time RT-PCR measurements, proliferating cell nuclear antigen staining, and airway smooth muscle quantification, were performed 7 days after exposure. The same outcome measurements were performed 24 hours and 4 weeks after a chronic allergen exposure model. PDGF-BB overexpression resulted in airway hyperresponsiveness, decreased lung compliance, increased airway smooth muscle cell numbers, positive proliferating cell nuclear antigen-stained airway smooth muscle cells, and a reduction in genes encoding contractile proteins. Chronic allergen exposure resulted in elevations in lung lavage PDGF-BB, which were observed in conjunction with changes in gene transcript expression encoding contractile proteins and ASM proliferation. We demonstrate for the first time in vivo that PDGF-BB induces ASM hyperplasia and changes in lung mechanics in mice and that, during periods of allergen exposure changes in lung, PDGF-BB are associated with changes in airway structure and function.

Tanshinone IIA inhibits miR-1 expression through p38 MAPK signal pathway in post-infarction rat cardiomyocytes

Tanshinone IIA is a fat-soluble pharmacologically active ingredient of Danshen, a well-known traditional Chinese medicine used for cardiovascular diseases such as coronary heart disease. Tanshinone IIA has been confirmed to suppress miR-1 and reduce the arrhythmogenesis after myocardial infarction (MI). However, the modulation mechanism is not clear. Tanshinone IIA was administrated daily for 7 days before ligation of the left anterior descending artery (LAD) and lasted for 3 months after LAD. Neonatal cardiomyocytes were exposed to 2% O(2)+95% N(2) condition for 24 h to simulate ischemia in vivo. Protein expression was examined with Western blot and miR-1 level was quantified by Real-time PCR. Our results showed that tanshinone IIA relieved ischemia-induced injury by improving the cardiac function. This beneficial effect may due to the depression of the elevated miR-1 level in ischemic and hypoxic cardiomyocytes, which subsequently restored its target Cx43 protein. Furthermore, tanshinone IIA could inhibit activated p38 MAPK and heart special transcription factors SRF and MEF2, in ischemic and hypoxic cardiomyocytes. Pretreatment with p38 MAPK inhibitor, SB203580 (10 uM), significantly relieved hypoxia-induced miR-1 increment and restored its downstream target Cx43 protein expression. These data suggest that tanshinone IIA play a role in protection cardiomyocytes from ischemic and hypoxic injury. The effect is based on inhibiting miR-1 expression through p38 MAPK signal pathway. This might provide us a new target to explore the novel strategy for ischemic cardioprotection.

MicroRNA and vascular smooth muscle cells

Vascular smooth muscle cells (VSMCs) exhibit extraordinary plasticity during postnatal development. Vascular injury initiates VSMC phenotypic switch from the contractile to proliferative phenotype, which plays a central role in vascular lesion formation and diverse vascular diseases. MicroRNAs (miRNAs) regulate gene expression posttranscriptionally by either degrading target mRNAs or repressing their translation. Emerging evidence has revealed miRNAs are critical regulators in VSMC differentiation from stem cells, phenotypic switch, and various vascular pathogenesis. Here, we review recent advances regarding functions of specific miRNAs in vasculature and discuss possible mechanisms by which miRNAs affect VSMC biology.

Serum response factor utilizes distinct promoter- and enhancer-based mechanisms to regulate cytoskeletal gene expression in macrophages

Cells of the monocyte/macrophage lineage play essential roles in tissue homeostasis and immune responses, but mechanisms underlying the coordinated expression of cytoskeletal genes required for specialized functions of these cells, such as directed migration and phagocytosis, remain unknown. Here, using genetic and genomic approaches, we provide evidence that serum response factor (SRF) regulates both general and cell type-restricted components of the cytoskeletal gene expression program in macrophages. Genome-wide location analysis of SRF in macrophages demonstrates enrichment of SRF binding at ubiquitously expressed target gene promoters, as expected, but also reveals that the majority of SRF binding sites associated with cell type-restricted target genes are at distal inter- and intragenic locations. Most of these distal SRF binding sites are established by the prior binding of the macrophage- and the B cell-specific transcription factor PU.1 and exhibit histone modifications characteristic of enhancers. Consistent with this, representative cytoskeletal target genes associated with these elements require both SRF and PU.1 for full expression. These findings suggest that SRF uses two distinct molecular strategies to regulate programs of cytoskeletal gene expression: a promoter-based strategy for ubiquitously expressed target genes and an enhancer-based strategy at target genes that exhibit cell type-restricted patterns of expression.

The essential function for serum response factor in T-cell development reflects its specific coupling to extracellular signal-regulated kinase signaling

Serum response factor (SRF) recruits members of two families of signal-regulated coactivators, the extracellular signal-regulated kinase (ERK)-regulated ternary complex factors (TCFs) and the actin-regulated myocardin-related transcription factors (MRTFs), to its target genes through its DNA-binding domain. Whether coactivator association is required for SRF function in vivo and whether particular SRF functions reflect specific coupling to one or the other signal pathway have remained largely unexplored. We show that SRF is essential for thymocyte positive selection and thymic T(reg) and NK T-cell development but dispensable for early thymocyte development and negative selection. Expression of wild-type SRF, or mutants lacking the N-terminal phosphorylation sites or C-terminal transcriptional activation domain, restores positive selection in SRF null thymocytes. In contrast, SRF.V194E, which cannot recruit TCF or MRTF family members, is inactive, although it is recruited to target genes. Fusion of a TCF C-terminal activation domain to SRF.V194E effectively restores ERK-dependent single-positive (SP) thymocyte development. The resulting SP thymocytes exhibit normal surface marker expression and proliferation following T-cell receptor cross-linking. Thus, ERK signaling through the TCF pathway to SRF is necessary and sufficient for SRF function in thymocyte positive selection.

MicroRNAs in cardiac hypertrophy: angels or devils

MicroRNAs (miRNAs) are short noncoding RNA molecules that can regulate gene expression via affecting mRNA stability or translation efficiency. miRNAs mediate many important cellular processes and emerge as a newly discovered regulator of gene expression. In cardiac hypertrophy, miRNAs expression is aberrantly altered. Some of these miRNAs can promote cardiac hypertrophy, whereas others can inhibit the process. In this review, we summarize the up- and downregulated miRNAs during cardiac hypertrophy and discuss about their roles in cardiac hypertrophy. The studies on miRNAs shed new light on the mechanism of cardiac hypertrophy and suggest that they may be promising therapeutic targets in tackling cardiac hypertrophy.

The actin associated protein palladin is important for the early smooth muscle cell differentiation

Palladin, an actin associated protein, plays a significant role in regulating cell adhesion and cell motility. Palladin is important for development, as knockdown in mice is embryonic lethal, yet its role in the development of the vasculature is unknown. We have shown that palladin is essential for the expression of smooth muscle cells (SMC) marker genes and force development in response to agonist stimulation in palladin deficient SMCs. The goal of the study was to determine the molecular mechanisms underlying palladin's ability to regulate the expression of SMC marker genes. Results showed that palladin expression was rapidly induced in an A404 cell line upon retinoic acid (RA) induced differentiation. Suppression of palladin expression with siRNAs inhibited the expression of RA induced SMC differentiation genes, SM α-actin (SMA) and SM22, whereas over-expression of palladin induced SMC gene expression. Chromatin immunoprecipitation assays provided evidence that palladin bound to SMC genes, whereas co-immunoprecipitation assays also showed binding of palladin to myocardin related transcription factors (MRTFs). Endogenous palladin was imaged in the nucleus, increased with leptomycin treatment and the carboxyl-termini of palladin co-localized with MRTFs in the nucleus. Results support a model wherein palladin contributes to SMC differentiation through regulation of CArG-SRF-MRTF dependent transcription of SMC marker genes and as previously published, also through actin dynamics. Finally, in E11.5 palladin null mouse embryos, the expression of SMA and SM22 mRNA and protein is decreased in the vessel wall. Taken together, our findings suggest that palladin plays a key role in the differentiation of SMCs in the developing vasculature.

Serum response factor is an essential transcription factor in megakaryocytic maturation

Serum response factor (Srf) is a MADS-box transcription factor that is critical for muscle differentiation. Its function in hematopoiesis has not yet been revealed. Mkl1, a cofactor of Srf, is part of the t(1;22) translocation in acute megakaryoblastic leukemia, and plays a critical role in megakaryopoiesis. To test the role of Srf in megakaryocyte development, we crossed Pf4-Cre mice, which express Cre recombinase in cells committed to the megakaryocytic lineage, to Srf(F/F) mice in which functional Srf is no longer expressed after Cre-mediated excision. Pf4-Cre/Srf(F/F) knockout (KO) mice are born with normal Mendelian frequency, but have significant macrothrombocytopenia with approximately 50% reduction in platelet count. In contrast, the BM has increased number and percentage of CD41(+) megakaryocytes (WT: 0.41% ± 0.06%; KO: 1.92% ± 0.12%) with significantly reduced ploidy. KO mice show significantly increased megakaryocyte progenitors in the BM by FACS analysis and CFU-Mk. Megakaryocytes lacking Srf have abnormal stress fiber and demarcation membrane formation, and platelets lacking Srf have abnormal actin distribution. In vitro and in vivo assays reveal platelet function defects in KO mice. Critical actin cytoskeletal genes are down-regulated in KO megakaryocytes. Thus, Srf is required for normal megakaryocyte maturation and platelet production partly because of regulation of cytoskeletal genes.

Nuclear import mechanism for myocardin family members and their correlation with vascular smooth muscle cell phenotype

Myocardin (Mycd), which is essential for the differentiation of the smooth muscle cell lineage, is constitutively located in the nucleus, although its family members, myocardin-related transcription factors A and B (MRTF-A/B), mostly reside in the cytoplasm and translocate to the nucleus in response to Rho signaling. The mechanism for their nuclear import is unclear. Here we investigated the mechanism for the nuclear import of Mycd family members and demonstrated any correlation between such mechanism and the phenotype of vascular smooth muscle cells (VSMCs). In cultured VSMCs, the knockdown of importin β1 inhibited the nuclear import of Mycd and MRTF-A/B. Their NH(2)-terminal basic domain was identified as a binding site for importin α/β1 by in vitro analyses. However, Mycd had a higher affinity for importin α/β1 than did MRTF-A/B, even in the absence of G-actin, and Mycd affinity for importin α1/β1 was stronger than for any other importin α/β1 heterodimers. The binding of Mycd to importin α/β1 was insensitive to G-actin, whereas that of MRTF-A/B was differently inhibited by G-actin. In dedifferentiated VSMCs, the levels of importins α1 and β1 were reduced concomitant with down-regulation of Mycd, serum response factor, and smooth muscle cell markers. By contrast, in differentiated VSMCs, their expressions were up-regulated. Thus, the nuclear import of Mycd family members in VSMCs depends on importin α/β1, and their relative affinities for importin α/β1 heterodimers determine Mycd nuclear import. The expression of Mycd nuclear import machineries is related to the expression levels of VSMC phenotype-dependent smooth muscle cell markers.

Role of serum response factor in the pathogenesis of disease

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds to a DNA cis element known as the CArG box, which is found in the proximal regulatory regions of over 200 experimentally validated target genes. Genetic deletion of SRF is incompatible with life in a variety of animals from different phyla. In mice, loss of SRF throughout the early embryo results in gastrulation defects precluding analyses in individual organ systems. Genetic inactivation studies using conditional or inducible promoters directing the expression of the bacteriophage Cre recombinase have shown a vital role for SRF in such cellular processes as contractility, cell migration, synaptic activity, inflammation, and cell survival. A growing number of experimental and human diseases are associated with changes in SRF expression, suggesting that SRF has a role in the pathogenesis of disease. This review summarizes data from experimental model systems and human pathology where SRF expression is either deliberately or naturally altered.

Phosphate and vascular calcification: Emerging role of the sodium-dependent phosphate co-transporter PiT-1

Elevated serum phosphate is a risk factor for vascular calcification and cardiovascular events in kidney disease as well as in the general population. Elevated phosphate levels drive vascular calcification, in part, by regulating vascular smooth muscle cell (VSMC) gene expression, function, and fate. The type III sodium-dependent phosphate co-transporter, PiT-1, is necessary for phosphate-induced VSMC osteochondrogenic phenotype change and calcification, and has recently been shown to have unexpected functions in cell proliferation and embryonic development.

cGMP-dependent protein kinase and the regulation of vascular smooth muscle cell gene expression: possible involvement of Elk-1 sumoylation

Although the regulation of smooth muscle cell (SMC) gene expression by cGMP-dependent protein kinase (PKG) is now recognized, the mechanisms underlying these effects are not fully understood. In this study, we report that PKG-I stimulates myocardin/serum response factor (SRF)-dependent gene expression in vascular SMCs. The expression of PKG in PKG-deficient cells enhanced myocardin-induced SM22 promoter activity in a concentration-dependent fashion. However, neither SRF nor myocardin expression was affected. To investigate alternative mechanisms, we examined whether PKG affects the phosphorylation of E26-like protein-1 (Elk-1), a SRF/myocardin transcription antagonist. The activation of PKG caused an increase in a higher molecular mass form of phospho-Elk-1 that was determined to be small ubiquitin-related modifier (sumo)ylated Elk-1. PKG increased Elk-1 sumoylation twofold compared with the PKG-deficient cells, and Elk-1 sumoylation was reduced using dominant-negative sumo-conjugating enzyme, DN-Ubc9, confirming PKG-dependent sumoylation of phospho-Elk-1 in vascular SMCs. In addition, PKG stimulated Elk-1 sumoylation in COS-7 cells overexpressing Elk-1, sumo-1, and PKG-I. The increased expression of PKG in vascular SMCs inhibited Elk-1 binding to SMC-specific promoters, SM22 and smooth muscle myosin heavy chain, as measured by EMSA and chromatin immunoprecipitation assay, and PKG suppressed the Elk-1 inhibition of SM22 reporter gene expression. Taken together, these data suggest that PKG-I decreases Elk-1 activity by sumo modification of Elk-1, thereby increasing myocardin-SRF activity on SMC-specific gene expression.

The transcription factor Srf regulates hematopoietic stem cell adhesion

Adhesion properties of hematopoietic stem cells (HSCs) in the bone marrow (BM) niches control their migration and affect their cell-cycle dynamics. The serum response factor (Srf) regulates growth factor-inducible genes and genes controlling cytoskeleton structures involved in cell spreading, adhesion, and migration. We identified a role for Srf in HSC adhesion and steady-state hematopoiesis. Conditional deletion of Srf in BM cells resulted in a 3-fold expansion of the long- and short-term HSCs and multipotent progenitors (MPPs), which occurs without long-term modification of cell-cycle dynamics. Early differentiation steps to myeloid and lymphoid lineages were normal, but Srf loss results in alterations in mature-cell production and severe thrombocytopenia. Srf-null BM cells also displayed compromised engraftment properties in transplantation assays. Gene expression analysis identified Srf target genes expressed in HSCs, including a network of genes associated with cell migration and adhesion. Srf-null stem cells and MPPs displayed impair expression of the integrin network and decreased adherence in vitro. In addition, Srf-null mice showed increase numbers of circulating stem and progenitor cells, which likely reflect their reduced retention in the BM. Altogether, our results demonstrate that Srf is an essential regulator of stem cells and MPP adhesion, and suggest that Srf acts mainly through cell-matrix interactions and integrin signaling.

Distinct effects of voltage- and store-dependent calcium influx on stretch-induced differentiation and growth in vascular smooth muscle

Stretch of the vascular wall stimulates smooth muscle hypertrophy by activating the MAPK and Rho/Rho kinase (ROK) pathways. We investigated the role of calcium in this response. Stretch-stimulated expression of contractile and cytoskeletal proteins in mouse portal vein was inhibited at mRNA and protein levels by blockade of voltage-dependent Ca(2+) entry (VDCE). In contrast, blockade of store-operated Ca(2+) entry (SOCE) did not affect smooth muscle marker expression but decreased global protein synthesis. Activation of VDCE caused membrane translocation of RhoA followed by phosphorylation of its downstream effectors LIMK-2 and cofilin-2. Stretch-activated cofilin-2 phosphorylation depended on VDCE but not on SOCE. VDCE was associated with increased mRNA expression of myocardin, myocyte enhancer factor (MEF) -2A and -2D, and smooth muscle marker genes, all of which depended on ROK activity. SOCE increased ERK1/2 phosphorylation and c-Fos expression but had no effect on phosphorylation of LIMK-2 and cofilin-2 or on myocardin and MEF2 expression. Knockdown of MEF2A or -2D eliminated the VDCE-induced activation of myocardin expression and increased basal c-Jun and c-Fos mRNA levels. These results indicate that MEF2 mediates VDCE-dependent stimulation of myocardin expression via the Rho/ROK pathway. In addition, SOCE activates the expression of immediate-early genes, known to be regulated by MEF2 via Ca(2+)-dependent phosphorylation of histone deacetylases, but this mode of Ca(2+) entry does not affect the Rho/ROK pathway. Compartmentation of Ca(2+) entry pathways appears as one mechanism whereby extracellular and membrane signals influence smooth muscle phenotype regulation, with MEF2 as a focal point.

Contribution of myocardin in the hypoxia-induced phenotypic switching of rat pulmonary arterial smooth muscle cells

BACKGROUND

Hypoxic exposure contributes to the phenotypic switching of smooth muscle cells (SMCs), while the mechanisms involved in this process is not yet fully elucidated. Myocardin as a co-actor of serum reaction factor plays a crucial role in differentiation of SMCs. This study was aimed to investigate the role of myocardin in hypoxia-induced phenotypic switching of rat pulmonary arterial SMCs (PASMCs).

METHODS

Primary PASMCs were cultured under normoxia and hypoxia (3%O(2), 48 h) respectively, and then the cell proliferation was assessed and the expression of SM22α, osteopontin (contractile and synthetic marker of SMCs, respectively), myocardin and platelet-derived growth factor-BB (PDGF-BB) were detected. After pGCSIL-GFP-shMYOCD lentviral vector was transduced to the PASMCs, the expression of myocardin and SM22α were examined. Moreover, myocardin expression in PASMCs treated with medium enriched with PDGF-BB and conditional medium (CM) from normoxia- and hypoxia-exposed PASMCs was assessed.

RESULTS

Exposing PASMCs to hypoxia led to an increased cell numbers and the up-regulation of proliferating cell nuclear antigen (PCNA), osteopontin and PDGF-BB; moreover, a significant down-regulation of SM22α and myocardin was identified. Further analysis revealed that knock-down of myocardin with pGCSIL-GFP-shMYOCD vector followed by a decreased SM22α in the PASMCs, and treatment of PASMCs with either exogenous PDGF-BB or hypoxic CM led to a marked decrease of myocardin.

CONCLUSIONS

Our findings suggest that the decrease of myocardin in PASMCs exposed to hypoxia is partly regulated by the increase of PDGF-BB, which contributes to the phenotypic switching of PASMCs in hypoxic condition.

Myocardin-related transcription factor A is a common mediator of mechanical stress- and neurohumoral stimulation-induced cardiac hypertrophic signaling leading to activation of brain natriuretic peptide gene expression

Subjecting cardiomyocytes to mechanical stress or neurohumoral stimulation causes cardiac hypertrophy characterized in part by reactivation of the fetal cardiac gene program. Here we demonstrate a new common mechanism by which these stimuli are transduced to a signal activating the hypertrophic gene program. Mechanically stretching cardiomyocytes induced nuclear accumulation of myocardin-related transcription factor A (MRTF-A), a coactivator of serum response factor (SRF), in a Rho- and actin dynamics-dependent manner. Expression of brain natriuretic peptide (BNP) and other SRF-dependent fetal cardiac genes in response to acute mechanical stress was blunted in mice lacking MRTF-A. Hypertrophic responses to chronic pressure overload were also significantly attenuated in mice lacking MRTF-A. Mutation of a newly identified, conserved and functional SRF-binding site within the BNP promoter, or knockdown of MRTF-A, reduced the responsiveness of the BNP promoter to mechanical stretch. Nuclear translocation of MRTF-A was also involved in endothelin-1- and angiotensin-II-induced activation of the BNP promoter. Moreover, mice lacking MRTF-A showed significantly weaker hypertrophic responses to chronic angiotensin II infusion than wild-type mice. Collectively, these findings point to nuclear translocation of MRTF-A as a novel signaling mechanism mediating both mechanical stretch- and neurohumoral stimulation-induced BNP gene expression and hypertrophic responses in cardiac myocytes.

Transcriptome dysregulation by anthrax lethal toxin plays a key role in induction of human endothelial cell cytotoxicity

We have investigated how Bacillus anthracis lethal toxin (LT) triggers caspase-3 activation and the formation of thick actin cables in human endothelial cells. By DNA array analysis we show that LT has a major impact on the cell transcriptome and we identify key host genes involved in LT cytotoxic effects. Indeed, upregulation of TRAIL and downregulation of XIAP both participate in LT-induced caspase-3 activation. LT induces a downregulation of the immediate early gene and master regulator of transcription egr1. Importantly, its re-expression in LT-intoxicated cells blocks caspase-3 activation. In parallel, we found that the formation of actin cables induced by LT occurs in the absence of direct activation of RhoA/ROCK signalling. We show that knock-down of cortactin and rhophilin-2 under conditions of calponin-1 expression defines the minimal set of genes regulated by LT for actin cable formation. Together our data establish that the modulation of the cell transcriptome by LT plays a key role in triggering human endothelial cell toxicity.

Activation and repression of cellular immediate early genes by serum response factor cofactors

The induction of expression of many cellular immediate early genes (IEG) involves the transcription factor serum response factor (SRF). Two families of SRF coactivators have also been implicated in IEG induction, the ternary complex factors (TCFs), ELK1, Sap1, and Net, and the myocardin-related factors, MKL1 and MKL2. We found that serum induction of some SRF target genes is preferentially regulated by MKL1/2, whereas others are redundantly activated by both TCFs and MKL1/2. Yet ELK1 can also repress transcription. Binding of ELK1 and MKL1 to SRF has been found to be mutually exclusive in vitro, suggesting that ELK1 could repress expression of IEGs by blocking MKL1 binding. We characterized the in vivo binding of MKL1 and ELK1 to target genes and found an inverse relationship of serum-induced MKL1 binding and serum-decreased ELK1 binding. However, experiments with short hairpin RNA-mediated MKL1/2 depletion and expression of a nuclear MKL1 (N100) variant in stably transfected cells failed to alter ELK1 binding, suggesting that ELK1 binding to target genes is regulated independently of MKL1/2. Nevertheless, we found that short interfering RNA-mediated depletion of TCFs increased target gene expression in cells containing the N100 MKL1 activator, most notably in cells under continuous growth conditions. These results indicate that the TCFs can function both as activators and repressors of target gene expression depending upon the cellular growth conditions.

The proliferation and differentiation of placental-derived multipotent cells into smooth muscle cells on fibrillar collagen

Type I collagen constitutes a major portion of the extracellular matrix (ECM) in arterial wall and it is the major substrate for cell growth and differentiation. The goal of this study was to evaluate the differentiation and proliferation of placenta-derived multipotent cells (PDMCs) on polymerized type I collagen fibrils and monomer collagen. PDMCs grown on both polymerized collagen and monomer collagen with transforming growth factor (TGF)-beta treatment increases the expression of smooth muscle cell (SMC)-specific markers, including calponin, alpha-smooth muscle actin (alpha-SMA) and smooth muscle-myosin heavy chain (SM-MHC). Polymerized collagen increased the expressions of p21(CIP1) and p27(KIP1); decreased cyclin A, cyclin D1, cyclin-dependent protein kinase 2 (Cdk2); and led to G(0)/G(1) arrest in PDMCs. Furthermore, PDMC-differentiated SMCs exhibited significant collagen contractility in the presence or absence of endothelin-1 (ET-1) stimulation. By using specific inhibitors and small interfering RNA (siRNA), we demonstrated that p38 MAPK pathway and serum response factor (SRF)-DNA binding activity is critical for the polymerized collagen-induced PDMC differentiation into SMCs. Thus, polymerized collagen exhibits the great potential in inducing PDMCs differentiation into SMCs, and exerts anti-proliferative effect on PDMC-differentiated SMCs.

Linking actin dynamics and gene transcription to drive cellular motile functions

Numerous physiological and pathological stimuli promote the rearrangement of the actin cytoskeleton, thereby modulating cellular motile functions. Although it seems intuitively obvious that cell motility requires coordinated protein biosynthesis, until recently the linkage between cytoskeletal actin dynamics and correlated gene activities remained unknown. This knowledge gap was filled in part by the discovery that globular actin polymerization liberates myocardin-related transcription factor (MRTF) cofactors, thereby inducing the nuclear transcription factor serum response factor (SRF) to modulate the expression of genes encoding structural and regulatory effectors of actin dynamics. This insight stimulated research to better understand the actin-MRTF-SRF circuit and to identify alternative mechanisms that link cytoskeletal dynamics and genome activity.

MicroRNA regulatory networks in cardiovascular development

The heart, more than any other organ, requires precise functionality on a second-to-second basis throughout the lifespan of the organism. Even subtle perturbations in cardiac structure or function have catastrophic consequences, resulting in lethal forms of congenital and adult heart disease. Such intolerance of the heart to variability necessitates especially robust regulatory mechanisms to govern cardiac gene expression. Recent studies have revealed central roles for microRNAs (miRNAs) as governors of gene expression during cardiovascular development and disease. The integration of miRNAs into the genetic circuitry of the heart provides a rich and robust array of regulatory interactions to control cardiac gene expression. miRNA regulatory networks also offer opportunities for therapeutically modulating cardiac function through the manipulation of pathogenic and protective miRNAs. We discuss the roles of miRNAs as regulators of cardiac form and function, unresolved questions in the field, and issues for the future.

Neural crest cell-specific deletion of Rac1 results in defective cell-matrix interactions and severe craniofacial and cardiovascular malformations

The small GTP-binding protein Rac1, a member of the Rho family of small GTPases, has been implicated in regulation of many cellular processes including adhesion, migration and cytokinesis. These functions have largely been attributed to its ability to reorganize cytoskeleton. While the function of Rac1 is relatively well known in vitro, its role in vivo has been poorly understood. It has previously been shown that in neural crest cells (NCCs) Rac1 is required in a stage-specific manner to acquire responsiveness to mitogenic EGF signals. Here we demonstrate that mouse embryos lacking Rac1 in neural crest cells (Rac1/Wnt1-Cre) showed abnormal craniofacial development including regional ectodermal detachment associated with mesenchymal acellularity culminating in cleft face at E12. Rac1/Wnt1-Cre mutants also displayed inappropriate remodelling of pharyngeal arch arteries and defective outflow tract septation resulting in the formation of a common arterial trunk ('persistent truncus arteriosus' or PTA). The mesenchyme around the aortic sac also developed acellular regions, and the distal aortic sac became grossly dysmorphic, forming a pair of bilateral, highly dilated arterial structures connecting to the dorsal aortas. Smooth muscle cells lacking Rac1 failed to differentiate appropriately, and subpopulations of post-migratory NCCs demonstrated aberrant cell death and attenuated proliferation. These novel data demonstrate that while Rac1 is not required for normal NCC migration in vivo, it plays a critical cell-autonomous role in post-migratory NCCs during craniofacial and cardiac development by regulating the integrity of the craniofacial and pharyngeal mesenchyme.

Cyclosporine up-regulates Krüppel-like factor-4 (KLF4) in vascular smooth muscle cells and drives phenotypic modulation in vivo

Cyclosporine A (CSA, calcineurin inhibitor) has been shown to block both vascular smooth muscle cell (VSMC) proliferation in cell culture and vessel neointimal formation following injury in vivo. The purpose of this study was to determine molecular and pathological effects of CSA on VSMCs. Using real-time reverse transcription-polymerase chain reaction, Western blot analysis, and immunofluorescence microscopy, we show that CSA up-regulated the expression of Krüppel-like factor-4 (KLF4) in VSMCs. KLF4 plays a key role in regulating VSMC phenotypic modulation. KLF4 antagonizes proliferation, facilitates migration, and down-regulates VSMC differentiation marker gene expression. We show that the VSMC differentiation marker genes smooth muscle alpha-actin (ACTA2), transgelin (TAGLN), smoothelin (SMTN), and myocardin (MYOCD) are all down-regulated by CSA in VSMC monoculture, whereas cyclin-dependent kinase inhibitor-1A (CDKN1A) and matrix metalloproteinase-3 (MMP3) are up-regulated. CSA did not affect the abundance of the VSMC microRNA (MIR) markers MIR143 and MIR145. Administration of CSA to rat carotid artery in vivo resulted in acute and transient suppression of ACTA2, TAGLN, SMTN, MYOCD, and smooth muscle myosin heavy chain (MYH11) mRNA levels. The tumor suppressor genes KLF4, p53, and CDKN1A, however, were up-regulated, as well as MMP3, MMP9, and collagen-VIII. CSA-treated arteries showed remarkable remodeling, including breakdown of the internal elastic lamina and reorientation of VSMCs, as well as increased KLF4 immunostaining in VSMCs and endothelial cells. Altogether, these data show that cyclosporin up-regulates KLF4 expression and promotes phenotypic modulation of VSMCs.

Role of Krüppel-like factor 4 in phenotypic switching and proliferation of vascular smooth muscle cells

Phenotypic switching and proliferation of vascular smooth muscle cells (VSMCs) are critical components in the development of many vascular proliferation diseases such as atherosclerosis and restenosis after percutaneous coronary interventions. Krüppel-like factor 4 (KLF4) has been shown to play a key role in VSMC proliferation and differentiation. The focus of this review is to provide an overview for understanding the physiological and pathobiological roles of KLF4 in phenotypic switching and proliferation of VSMCs.

Epithelial Protein Lost in Neoplasm alpha (Eplin-alpha) is transcriptionally regulated by G-actin and MAL/MRTF coactivators

Epithelial Protein Lost in Neoplasm alpha is a novel cytoskeleton-associated tumor suppressor whose expression inversely correlates with cell growth, motility, invasion and cancer mortality. Here we show that Eplin-alpha transcription is regulated by actin-MAL-SRF signalling. Upon signal induction, the coactivator MAL/MRTF is released from a repressive complex with monomeric actin, binds the transcription factor SRF and activates target gene expression. In a transcriptome analysis with a combination of actin binding drugs which specifically and differentially interfere with the actin-MAL complex (Descot et al., 2009), we identified Eplin to be primarily controlled by monomeric actin. Further analysis revealed that induction of the Eplin-alpha mRNA and its promoter was sensitive to drugs and mutant actins which stabilise the repressive actin-MAL complex. In contrast, the Eplin-beta isoform remained unaffected. Knockdown of MRTFs or dominant negative MAL which inhibits SRF-mediated transcription impaired Eplin-alpha expression. Conversely, constitutively active mutant actins and MAL induced Eplin-alpha. MAL and SRF were bound to a consensus SRF binding site of the Eplin-alpha promoter; the recruitment of MAL to this region was enhanced severalfold upon induction. The tumor suppressor Eplin-alpha is thus a novel cytoskeletal target gene transcriptionally regulated by the actin-MAL-SRF pathway, which supports a role in cancer biology.

Cartilage Oligomeric Matrix Protein Maintains the Contractile Phenotype of Vascular Smooth Muscle Cells by Interacting With α 7 β 1 Integrin

Rational : Vascular smooth muscle cells (VSMCs) switching from a contractile/differentiated to a synthetic/dedifferentiated phenotype has an essential role in atherosclerosis, postangioplastic restenosis and hypertension. However, how normal VSMCs maintain the differentiated state is less understood.

Objective : We aimed to indentify the effect of cartilage oligomeric matrix protein (COMP), a normal vascular extracellular matrix, on modulation of VSMCs phenotype.

Methods and Results : We demonstrated that COMP was associated positively with the expression of VSMC differentiation marker genes during phenotype transition. Knockdown of COMP by small interfering (si)RNA favored dedifferentiation. Conversely, adenoviral overexpression of COMP markedly suppressed platelet-derived growth factor-BB-elicited VSMC dedifferentiation, characterized by altered VSMC morphology, actin fiber organization, focal adhesion assembly, and the expression of phenotype-dependent markers. Whereas α 7 integrin coimmunoprecipitated with COMP in normal rat VSMCs and vessels, neutralizing antibody or siRNA against α 7 integrin inhibited VSMC adhesion to COMP, which indicated that α 7 β 1 integrin is a potential receptor for COMP. As well, blocking or interference by siRNA of α 7 integrin completely abolished the effect of COMP on conserving the contractile phenotype. In accordance, ectopic adenoviral overexpression of COMP greatly retarded VSMC phenotype switching, rescued contractility of carotid artery ring, and inhibited neointima formation in balloon-injured rats.

Conclusions : Our data suggest that COMP is essential for maintaining a VSMC contractile phenotype and the protective effects of COMP are mainly mediated through interaction with α 7 β 1 integrin. Investigations to identify the factors affecting the expression and integrity of COMP may provide a novel therapeutic target for vascular disorders.

Oxidized low density lipoprotein-induced transdifferentiation of bone marrow-derived smooth muscle-like cells into foam-like cells in vitro

Oxidized-low density lipoprotein (ox-LDL) is believed to contribute to atherogenesis in part by being taken up into smooth muscle cells (SMC) via specific scavenger receptors; however, it is not clear whether ox-LDL receptor(s) are expressed in bone marrow-derived smooth muscle-like cells (SMLCs) and whether they play a role in the process of SMLC development. Therefore, we examined the ox-LDL-induced transdifferentiation of SMLCs that is mediated by lectin-like ox-LDL receptor-1 (LOX-1). Smooth muscle progenitor cells (SMPCs) from bone marrow mesenchymal stem cells (BMSCs) were isolated using a tissue-specific promoter sorting method with a mouse SM22_ promoter (_480 bp)/green fluorescent protein recombinant plasmid. The SMPCs were myocardin+CD105+KDR+CD45(-)CD34(-), but did not express SMC-specific markers alpha-smooth muscle actin (alpha-SMA), SM22, smooth muscle myosin heavy chain (SM-MHC) and smoothelin. After long-term culture with platelet-derived growth factor-BB (PDGF-BB), SMPCs expressed alpha-SMA, SM22 and SM-MHC and differentiated into SMLCs. When SMLCs were incubated with different concentrations of ox-LDL, LOX-1 expression on the surface of SMLCs gradually increased with the increase of the ox-LDL concentration, but myocardin and SMC-specific marker genes decreased, accordingly. Furthermore, receptor-mediated endocytosis was enhanced and lipid droplets accumulated in the cytoplasm of SMLCs. A subpopulation of myocardin+CD105+KDR+CD45(-)CD34(-) SMPCs exist in BMSCs that can differentiate into SMLCs under induction with PDGF-BB. Moreover, LOX-1 contributes to the ox-LDL-induced transdifferentiation of bone marrow-derived SMLCs into foam-like cells.

Crucial role of nrf3 in smooth muscle cell differentiation from stem cells

RATIONALE

Nuclear factor erythroid 2-related factor (Nrf)3, a member of the cap 'N' collar family of transcription factors that bind to the DNA-antioxidant responsive elements, is involved in reactive oxygen species balancing and in muscle precursor migration during early embryo development.

OBJECTIVE

To investigate the functional role of Nrf3 in smooth muscle cell (SMC) differentiation in vitro and in vivo.

METHODS AND RESULTS

Nrf3 was upregulated significantly following 1 to 8 days of SMC differentiation. Knockdown of Nrf3 resulted in downregulation of smooth muscle specific markers expression, whereas enforced expression of Nrf3 enhanced SMC differentiation in a dose-dependent manner. SMC-specific transcription factor myocardin, but not serum response factor, was significantly upregulated by Nrf3 overexpression. Strikingly, the binding of SRF and myocardin to the promoter of smooth muscle differentiation genes was dramatically increased by Nrf3 overexpression, and Nrf3 can directly bind to the promoters of SMC differentiation genes as demonstrated by chromatin immunoprecipitation assay. Moreover, NADPH-derived reactive oxygen species production during SMC differentiation was further enhanced by Nrf3 overexpression through upregulation of NADPH oxidase and inhibition of antioxidant signaling pathway. In addition, Nrf3 was involved in the endoplasmic reticulum stressor induced SMC differentiation.

CONCLUSION

Our findings demonstrate for the first time that Nrf3 has a crucial role in SMC differentiation from stem cells indicating that Nrf3 could be a potential target for manipulation of stem cell differentiation toward vascular lineage.

Intronic CArG box regulates cysteine-rich protein 2 expression in the adult but not in developing vasculature

OBJECTIVE

An absence of cysteine-rich protein 2 (CRP2) enhances vascular smooth muscle cell (VSMC) migration and increases neointima formation after arterial injury; therefore, CRP2 plays an important role in the response to vascular injury. The goal of the present study was to elucidate the molecular mechanisms that preserve CRP2 expression in the adult vasculature and thus might serve to inhibit the response to injury.

METHODS AND RESULTS

We generated a series of transgenic mice harboring potential Csrp2 regulatory regions with a lacZ reporter. We determined that the 12-kb first intron was necessary for transgene activity in adult but not in developing vasculature. Within the intron we identified a 6.3-kb region that contains 2 CArG boxes. Serum response factor preferentially bound to CArG2 box in gel mobility shift and chromatin immunoprecipitation assays; additionally, serum response factor coactivator myocardin factors activated CRP2 expression via the CArG2 box. Mutational analysis revealed that CArG2 box was important in directing lacZ expression in VSMC of adult vessels.

CONCLUSIONS

Although CRP2 expression during development is independent of CArG box regulatory sites, CRP2 expression in adult VSMC requires CArG2 element within the first intron. Our results suggest that distinct mechanisms regulate CRP2 expression in VSMC that are controlled by separate embryonic and adult regulatory modules.

Myocardin functions as an effective inducer of growth arrest and differentiation in human uterine leiomyosarcoma cells

Myocardin is an important transcriptional regulator in smooth and cardiac muscle development. We noticed that the expression of myocardin was markedly downregulated in human uterine leiomyosarcoma cells. Restoration of myocardin expression induced the reexpression of smooth muscle marker proteins and the formation of well-developed actin fibers. A concomitant increase in the expression of a cyclin-dependent kinase inhibitor, p21, led to significantly reduced cell proliferation, via p21's inhibition of the G(1)-S transition. A p21 promoter-reporter assay showed that myocardin markedly increased p21's promoter activity. Furthermore, a serum response factor (SRF)-binding cis-element CArG box in the p21 promoter region was required for this myocardin effect. Chromatin immunoprecipitation and DNA-protein binding assays showed that myocardin indirectly bound to the CArG box in the p21 promoter through the interaction with SRF. Furthermore, immunohistochemistry revealed that the levels of myocardin and p21 were both lower in leiomyosarcoma samples than in normal smooth muscle tissue. Taken together, our results indicate that the downregulation of myocardin expression facilitates cell cycle progression via the reduction of p21 expression in human leimyosarcomas and suggest that myocardin could be a useful therapeutic target for this disease.

Ureter myogenesis: putting Teashirt into context

After the basic shape of the mammalian ureter is established, its epithelia mature and a coat of smooth muscle cells differentiate around nascent urothelia. The ureter actively propels tubular fluid from the renal pelvis to the bladder, and this peristalsis, which starts in the fetal period, requires coordinated smooth muscle contraction. Teashirt-3 (Tshz3) is expressed in smooth muscle cell precursors that form the wall of the forming mammalian ureter. The Teashirt gene family was first identified in Drosophila where Teashirt (Tsh) protein acts as a transcription factor directing embryonic anterior-posterior patterning and leg and eye development. In fly embryonic renal tubules, Tsh is expressed in mesodermally derived stellate cells intercalating between principal cells, and a paralogue, tiptop, is expressed in forming tubules. Teashirt is a component of several gene networks in flies and it is notable that similar networks control mammalian renal tract development. Null mutation of Tshz3 in mice leads to failure of functional muscularization in the top of the ureter and this is followed by congenital hydronephrosis. A signaling pathway can be envisaged, starting with sonic hedgehog secreted by the nascent ureteric urothelium and ending with ureteric smooth muscle cell differentiation, with Tshz3 downstream of bone morphogenetic protein 4 and upstream of myocardin and smooth muscle cell contractile protein synthesis. The phenotype of Tshz3 mutant mice resembles that of human congenital pelviureteric junction obstruction, and we suggest these individuals may have mutations of genes encoding molecules in the differentiation pathway mediated by Tshz3.

Phenylephrine induces elevated RhoA activation and smooth muscle alpha-actin expression in Pkd2+/- vascular smooth muscle cells

The mechanisms underlying vascular complications in autosomal-dominant polycystic kidney disease (ADPKD) have not been fully elucidated. However, molecular components altered in Pkd mutant vascular smooth muscle cells (VSMCs) are gradually being identified. Pkd2(+/-) arterial smooth muscles show elevated levels of (1) phenylephrine (PE)-induced, Ca(2+)-independent vasocontraction and (2) smooth muscle alpha-actin (SMA) expression. As these two processes are heavily influenced by RhoA signaling and by cellular filamentous-to-globular (F/G)-actin dynamics, we examined PE-induced changes in RhoA activation and the F/G-SMA ratio in wild-type (wt) and Pkd2(+/-) VSMCs; we further tested the hypothesis that the abnormal response to PE and the resultant elevation in the F/G-SMA ratio contribute to the exuberant SMA expression in Pkd2(+/-) VSMCs. GTP-RhoA and F/G-SMA in mouse aortic media and primary cultured VSMCs were determined using RhoA activation and in vivo F-to-G-actin assays. Myocardin-related transcription factor-A (MRTF-A) (SMA transcription coactivator) was localized by immunofluorescence, nuclear MRTF-A quantified by western analysis using nuclear extracts and SMA expression by luciferase reporter assay. PE induced a >3-fold higher RhoA activation in Pkd2(+/-) than in wt VSMCs and higher levels of downstream p-LIMK and p-cofilin. Moreover, Pkd2(+/-) VSMCs showed a higher baseline and PE-induced F/G-SMA ratio. The F/G-SMA elevation enhanced nuclear translocation of MRTF-A, which upregulated SMA transcription. In summary, PE-induced RhoA hyperactivation and defects in F-to-G SMA balance likely have a role in the abnormal vasocontraction and SMA expression in Pkd2(+/-) arteries. These defects could potentially contribute to the genesis of vascular complications in ADPKD, thus providing new areas for further research and therapeutic targeting.

Conservation and divergence in developmental networks: a view from Drosophila myogenesis

Understanding developmental networks has recently been enhanced through the identification of a large number of conserved essential regulators. Interspecies comparisons of the transcriptional networks regulated by these factors are still at a rather early stage, with limited global data available. Here we use the accumulating phenotypic information from multiple species to provide initial insights into the wiring and rewiring of developmental networks, with particular emphasis on myogenesis, a highly conserved developmental process. This review highlights the most recent findings on the transcriptional program driving Drosophila myogenesis and compares this with vertebrates, revealing emerging themes that may be applicable to other developmental contexts.

Disruption of actin cytoskeleton mediates loss of tensile stress induced early phenotypic modulation of vascular smooth muscle cells in organ culture

Aorta organ culture has been widely used as an ex vivo model for studying vessel pathophysiology. Recent studies show that the vascular smooth muscle cells (VSMCs) in organ culture undergo drastic dedifferentiation within the first few hours (termed early phenotypic modulation). Loss of tensile stress to which aorta is subject in vivo is the cause of this early phenotypic modulation. However, no underlying molecular mechanism has been discovered thus far. The purpose of the present study is to identify intracellular signals involved in the early phenotypic modulation of VSMC in organ culture. We find that the drastic VSMC dedifferentiation is accompanied by accelerated actin cytoskeleton dynamics and downregulation of SRF and myocardin. Among the variety of signal pathways examined, increasing actin polymerization by jasplakinolide is the only one hindering VSMC dedifferentiation in organ culture. Moreover, jasplakinolide reverses actin dynamics during organ culture. Latrunculin B (disrupting actin cytoskeleton) and jasplakinolide respectively suppressed and enhanced the expression of VSMC markers, SRF, myocardin, and CArG-box-mediated SMC promoters in PAC1, a VSMC line. These results identify actin cytoskeleton degradation as a major intracellular signal for loss of tensile stress-induced early phenotypic modulation of VSMC in organ culture. This study suggests that disrupting actin cytoskeleton integrity may contribute to the pathogenesis of vascular diseases.

Smooth(ing) muscle differentiation by microRNAs

In a recent report in Nature, Cordes et al. (2009) demonstrate that miR-143 and miR-145 modulate smooth muscle cell (SMC) plasticity in part by regulating key transcription factors involved in SMC fate determination.

Tanshinone IIA protects against sudden cardiac death induced by lethal arrhythmias via repression of microRNA-1

BACKGROUND AND PURPOSE

Tanshinone IIA is an active component of a traditional Chinese medicine based on Salvia miltiorrhiza, which reduces sudden cardiac death by suppressing ischaemic arrhythmias. However, the mechanisms underlying the anti-arrhythmic effects remain unclear.

EXPERIMENTAL APPROACH

A model of myocardial infarction (MI) in rats by ligating the left anterior descending coronary artery was used. Tanshinone IIA or quinidine was given daily, before (7 days) and after (3 months) MI; cardiac electrical activity was monitored by ECG recording. Whole-cell patch-clamp techniques were used to measure the inward rectifying K(+) current (I(K1)) in rat isolated ventricular myocytes. Kir2.1 and serum response factor (SRF) levels were analysed by Western blot and microRNA-1 (miR-1) level was determined by real-time RT-PCR.

KEY RESULTS

Tanshinone IIA decreased the incidence of arrhythmias induced by acute cardiac ischaemia and mortality in rats 3 months after MI. Tanshinone IIA restored the diminished I(K1) current density and Kir2.1 protein after MI in rat ventricular myocytes, while quinidine further inhibited I(K1)/Kir2.1. MiR-1 was up-regulated in MI, possibly due to the concomitant increase in SRF, a transcriptional activator of the miR-1 gene, accounting for decreased Kir2.1. Treatment with tanshinone IIA prevented increased SRF and hence increased miR-1 post-MI, whereas quinidine did not.

CONCLUSIONS AND IMPLICATIONS

Down-regulation of miR-1 and consequent recovery of Kir2.1 may account partially for the efficacy of tanshinone IIA in suppressing ischaemic arrhythmias and cardiac mortality. These finding support the proposal that miR-1 could be a potential therapeutic target for the prevention of ischaemic arrhythmias.

Phosphorylation of myocardin by extracellular signal-regulated kinase

The contractile phenotype of smooth muscle (SM) cells is controlled by serum response factor (SRF), which drives the expression of SM-specific genes including SM alpha-actin, SM22, and others. Myocardin is a cardiac and SM-restricted coactivator of SRF that is necessary for SM gene transcription. Growth factors inducing proliferation of SM cells inhibit SM gene transcription, in a manner dependent on the activation of extracellular signal-regulated kinases ERK1/2. In this study, we found that ERK1/2 phosphorylates mouse myocardin (isoform B) at four sites (Ser(812), Ser(859), Ser(866), and Thr(893)), all of which are located within the transactivation domain of myocardin. The single mutation of each site either to alanine or to aspartate has no effect on the ability of myocardin to activate SRF. However, the phosphomimetic mutation of all four sites to aspartate (4xD) significantly impairs activation of SRF by myocardin, whereas the phosphodeficient mutation of all four sites to alanine (4xA) has no effect. This translates to a reduced ability of the 4xD (but not of 4xA) mutant of myocardin to stimulate expression of SM alpha-actin and SM22, as assessed by corresponding promoter, mRNA, or protein assays. Furthermore, we found that phosphorylation of myocardin at these sites impairs its interaction with acetyltransferase, cAMP response element-binding protein-binding protein, which is known to promote the transcriptional activity of myocardin. In conclusion, we describe a novel mode of modulation of SM gene transcription by ERK1/2 through a direct phosphorylation of myocardin.

Myocardin-dependent activation of the CArG box-rich smooth muscle gamma-actin gene: preferential utilization of a single CArG element through functional association with the NKX3.1 homeodomain protein

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds a 10-bp element known as the CArG box, located in the proximal regulatory region of hundreds of target genes. SRF activates target genes in a cell- and context-dependent manner by assembling unique combinations of cofactors over CArG elements. One particularly strong SRF cofactor, myocardin (MYOCD), acts as a component of a molecular switch for smooth muscle cell (SMC) differentiation by activating cytoskeletal and contractile genes harboring SRF-binding CArG elements. Here we report that the human ACTG2 promoter, containing four conserved CArG elements, displays SMC-specific basal activity and is highly induced in the presence of MYOCD. Stable transfection of a non-SMC cell type with Myocd elicits elevations in endogenous Actg2 mRNA. Gel shift and luciferase assays reveal a strong bias for MYOCD-dependent transactivation through CArG2 of the human ACTG2 promoter. Substitution of CArG2 with other CArGs, including a consensus CArG element, fails to reconstitute full MYOCD-dependent ACTG2 promoter stimulation. Mutation of an adjacent binding site for NKX3.1 reduces MYOCD-dependent transactivation of the ACTG2 promoter. Co-immunoprecipitation, glutathione S-transferase pulldown, and luciferase assays show a physical and functional association between MYOCD and NKX3.1; no such functional relationship is evident with the related NKX2.5 transcription factor despite its interaction with MYOCD. These results demonstrate the ability of MYOCD to discriminate among several juxtaposed CArG elements, presumably through its novel partnership with NKX3.1, to optimally transactivate the human ACTG2 promoter.

MicroRNAs miR-143 and miR-145 modulate cytoskeletal dynamics and responsiveness of smooth muscle cells to injury

Vascular injury triggers dedifferentiation and cytoskeletal remodeling of smooth muscle cells (SMCs), culminating in vessel occlusion. Serum response factor (SRF) and its coactivator, myocardin, play a central role in the control of smooth muscle phenotypes by regulating the expression of cytoskeletal genes. We show that SRF and myocardin regulate a cardiovascular-specific microRNA (miRNA) cluster encoding miR-143 and miR-145. To assess the functions of these miRNAs in vivo, we systematically deleted them singly and in combination in mice. Mice lacking both miR-143 and miR-145 are viable and do not display overt abnormalities in smooth muscle differentiation, although they show a significant reduction in blood pressure due to reduced vascular tone. Remarkably, however, neointima formation in response to vascular injury is profoundly impeded in mice lacking these miRNAs, due to disarray of actin stress fibers and diminished migratory activity of SMCs. These abnormalities reflect the regulation of a cadre of modulators of SRF activity and actin dynamics by miR-143 and miR-145. Thus, miR-143 and miR-145 act as integral components of the regulatory network whereby SRF controls cytoskeletal remodeling and phenotypic switching of SMCs during vascular disease.

Uncoupled investigation of scaffold modulus and mesh size on smooth muscle cell behavior

Although scaffold material properties are known to critically impact cell behavior, it has proven difficult to correlate specific cell responses to isolated scaffold parameters, inhibiting rational design of scaffold material properties. The aim of this study was to validate a systematic approach for evaluating the influence of initial scaffold modulus and mesh size on cell extracellular matrix (ECM) deposition and phenotype. Poly(ethylene glycol) diacrylate (PEGDA) hydrogels were selected for this study because of their tunable material properties. Following screening of six distinct PEGDA hydrogels, three formulations were identified which permitted uncoupled investigation of scaffold mesh size and modulus within the target incremental modulus range of approximately 100-300 kPa. Smooth muscle cells (SMCs) were encapsulated within these three formulations, and cell ECM deposition and phenotype were evaluated following 21 days of culture. Although elastin content appeared to be correlated with scaffold mesh size and modulus to a similar degree, levels of collagen and serum response factor (SRF), a key regulator of SMC phenotype, were more strongly correlated with mesh size. To gain insight into the cell signaling underlying these observed correlations, variations in cell metabolic state and in RhoA signaling were semi-quantitatively evaluated. Both RhoA activity, which is largely modulated by scaffold mechanics in 2D, and cell metabolic activity were highly correlated with hydrogel mesh size. These results indicate that the effects of scaffold mechanics on RhoA activity in 3D may be distinct from those in 2D and underscore the need for uncoupled investigation of scaffold parameters on cell behavior. Furthermore, the present data suggest that RhoA signaling and cell metabolic regulation may be closely linked.

miR-145 and miR-143 regulate smooth muscle cell fate and plasticity

MicroRNAs (miRNAs) are regulators of myriad cellular events, but evidence for a single miRNA that can efficiently differentiate multipotent stem cells into a specific lineage or regulate direct reprogramming of cells into an alternative cell fate has been elusive. Here we show that miR-145 and miR-143 are co-transcribed in multipotent murine cardiac progenitors before becoming localized to smooth muscle cells, including neural crest stem-cell-derived vascular smooth muscle cells. miR-145 and miR-143 were direct transcriptional targets of serum response factor, myocardin and Nkx2-5 (NK2 transcription factor related, locus 5) and were downregulated in injured or atherosclerotic vessels containing proliferating, less differentiated smooth muscle cells. miR-145 was necessary for myocardin-induced reprogramming of adult fibroblasts into smooth muscle cells and sufficient to induce differentiation of multipotent neural crest stem cells into vascular smooth muscle. Furthermore, miR-145 and miR-143 cooperatively targeted a network of transcription factors, including Klf4 (Kruppel-like factor 4), myocardin and Elk-1 (ELK1, member of ETS oncogene family), to promote differentiation and repress proliferation of smooth muscle cells. These findings demonstrate that miR-145 can direct the smooth muscle fate and that miR-145 and miR-143 function to regulate the quiescent versus proliferative phenotype of smooth muscle cells.

SRF in angiogenesis: branching the vascular system

Cell cytoskeleton proteins are fundamental to cell shape, cell adhesion and cell motility, and therefore play an important role during angiogenesis. One of the major regulators of cytoskeletal protein expression is serum response factor (SRF), a MADS-box transcription factor that regulates multiple genes implicated in cell growth, migration, cytoskeletal organization, energy metabolism and myogenesis. Recent data have demonstrated a crucial role of SRF downstream of VEGF and FGF signalling during sprouting angiogenesis, regulating endothelial cell (EC) migration, actin polymerisation, tip cell morphology, EC junction assembly and vascular integrity. Here, we review the role of SRF in the regulation of angiogenesis and EC function, integrate SRF function into a broader mechanism regulating branching morphogenesis, and discuss future directions and perspectives of SRF in EC biology.

Impact of endothelial cells and mechanical conditioning on smooth muscle cell extracellular matrix production and differentiation

The aim of the current study was to explore the separate and coupled effects of endothelial cell (EC) presence and mechanical conditioning on smooth muscle cell (SMC) responses by combining bilayered poly(ethylene glycol) diacrylate (PEGDA) hydrogels with a pulsatile flow bioreactor. Each construct was composed of an outer PEGDA layer containing SMC and an inner PEGDA layer, either with or without EC. After an initial 3 days of static culture, EC(+) and EC(-) constructs were each further divided into two subgroups, half of which received mechanical conditioning mimetic of late gestation (mean pressures of approximately 50 mmHg and peak-to-trough pressure differentials of approximately 20 mmHg at approximately 140-180 beats/min) and half of which were cultured statically. After 18 additional days of culture, the SMC-containing layer of each construct was harvested, and western blots and quantitative histology were conducted to compare collagen type I, collagen type III, and elastin levels among treatment groups. SMC differentiation was evaluated by focusing on SMC marker calponin h1 and direct regulators of its gene expression-the transcription factor serum response factor (SRF) and two of its binding partners, myocardin and Elk-1. Combined EC and pulsatile flow conditioning increased elastin production, but decreased collagen type I deposition. Further, combined EC presence and mechanical stimulation increased SRF levels and the ratio of myocardin to active, phosphorylated Elk-1. This modulation of SRF and its binding partners appeared to result in a net increase in SMC differentiation, as evidenced by an associated increase in calponin h1 levels.

Complexity in transcription control at the activation domain-mediator interface

Transcript elongation by polymerase II paused at the Egr1 promoter is activated by mitogen-activated protein kinase phosphorylation of the ternary complex factor (TCF) ELK1 bound at multiple upstream sites and subsequent phospho-ELK1 interaction with mediator through the MED23 subunit. Consequently, Med23 knockout (KO) nearly eliminates Egr1 (early growth response factor 1) transcription in embryonic stem (ES) cells, leaving a paused polymerase at the promoter. Med23 KO did not, however, eliminate Egr1 transcription in fibroblasts. Chromatin immunoprecipitation analysis and direct visualization of fluorescently labeled TCF derivatives and mediator subunits revealed that three closely related TCFs bound to the same control regions. The relative amounts of these TCFs, which responded differently to the loss of MED23, differed in ES cells and fibroblasts. Transcriptome analysis suggests that most genes expressed in both cell types, such as Egr1, are regulated by alternative transcription factors in the two cell types that respond differently to the same signal transduction pathways.