Transcriptional programs regulating vascular smooth muscle cell development and differentiation

Role of integrin-linked kinase in the hypoxia-induced phenotypic transition of pulmonary artery smooth muscle cells: Implications for hypoxic pulmonary hypertension

The phenotypic transition of pulmonary artery smooth muscle cells (PASMCs) from a contractile/differentiated to synthetic/de-differentiated phenotype is an important mechanism for the occurrence and development of hypoxic pulmonary hypertension (HPH). Integrin-linked kinase (ILK) is an early hypoxic response factor whose kinase activity is significantly affected during early hypoxia. Myocardin and ETS-like protein 1 (Elk-1) are co-activators of serum response factor (SRF) and can bind to SRF to mediate the phenotypic transition of PASMCs. However, little is known about the role of ILK on the phenotypic transition of these PASMCs. Thus, in our study, we explored the role of ILK in this process. We found that the expression of ILK and myocardin decreased gradually with the increase in hypoxia exposure time in the pulmonary arteries of rats. We observed that hypoxia exposure for 1 h caused an increase in the phosphorylation of Elk-1 but did not affect the expression of ILK, myocardin, or SRF. Exposure to hypoxic treatment for 1 h decreased ILK kinase activity and caused Elk-1 to suppress myocardin binding to SRF and the smooth muscle (SM) α-actin gene promoters. In addition, hypoxia exposure for 24 h decreased the expression of ILK, myocardin, SM α-actin, and calponin but increased the expression of osteopontin. Silencing of the myocardin gene significantly decreased the expression of SM α-actin and calponin but increased the expression of osteopontin. Silencing of the ILK gene significantly decreased the expression of myocardin, SM α-actin, and calponin but increased the expression of osteopontin. ILK overexpression reversed the effects of 24 h of hypoxia on the expression of myocardin, SM α-actin, calponin, and osteopontin and reversed the decrease in binding of myocardin to the SM α-actin promoter caused by 24 h of hypoxia exposure. Thus, our results suggest that ILK initiates the phenotypic transition of PASMCs. The underlying mechanism may involve hypoxia downregulating ILK kinase activity and protein expression, causing Elk-1 to compete with myocardin for binding to the SM α-actin promoter, which downregulates the expression of the downstream target myocardin and results in the phenotypic transition of PASMCs from a contractile to a synthetic phenotype. This may be an important mechanism in the development of HPH.

Serum Response Factor in Muscle Tissues: From Development to Ageing

Skeletal, cardiac and smooth muscle cells share various common characteristic features. During development the embryonic mesodermal layer contribute at different proportions to the formation of these tissues. At the functional level, contractility as well as its decline during ageing, are also common features. Cytoskeletal components of these tissues are characterized by various actin isoforms that govern through their status (polymerised versus monomeric) and their interaction with the myosins the contractile properties of these muscles. Finally, at the molecular level, a set of different transcription factors with the notable exception of Serum Response Factor SRF- which is commonly enriched in the 3 types of muscle- drive and maintain the differentiation of these cells (Myf5, MyoD, Myogenin for skeletal muscle; Nkx2.5, GATA4 for cardiomyocytes). In this review, we will focus on the transcription factor SRF and its role in the homeostasis of cardiac, smooth and skeletal muscle tissues as well as its behaviour during the age related remodelling process of these tissues with a specific emphasis on animal models and human data when available.

Vascular smooth muscle cell differentiation-2010

Vascular smooth muscle cells have attracted considerable interest as a model for a flexible program of gene expression. This cell type arises throughout the embryo body plan via poorly understood signaling cascades that direct the expression of transcription factors and microRNAs which, in turn, orchestrate the activation of contractile genes collectively defining this cell lineage. The discovery of myocardin and its close association with serum response factor has represented a major break-through for the molecular understanding of vascular smooth muscle cell differentiation. Retinoids have been shown to improve the outcome of vessel wall remodeling following injury and have provided further insights into the molecular circuitry that defines the vascular smooth muscle cell phenotype. This review summarizes the progress to date in each of these areas of vascular smooth muscle cell biology.

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.

A comparison of murine smooth muscle cells generated from embryonic versus induced pluripotent stem cells

Smooth muscle cell (SMC) differentiation and dedifferentiation play a critical role in the pathogenesis of cardiovascular diseases. The lack of a good and simple in vitro SMC differentiation system has hampered the progress of SMC field for years. The generation of such an in vitro system would be invaluable for exploring molecular mechanisms of SMC differentiation and dedifferentiation. Recently, the establishment of induced pluripotent stem (iPS) cells has offered a novel therapeutic strategy to generate patient-specific stem cell lines. Here we have investigated whether iPS cells are able to differentiate into SMCs in vitro. Mouse iPS cell (O9 and TT025) monolayers were treated with 10(-5) mol/L all-trans retinoid acid (RA). After 8 days of RA treatment, we found that >40% of the O9 iPS cells expressed the SMC-markers including SMalpha-actin and SM myosin heavy chain. Also, we documented that iPS-derived SMCs acquired SMC functional characteristics including contraction and calcium influx in response to stimuli. Moreover, our results indicated that there were differences in SMC-specific gene expression patterns between SMCs derived from O9 and TT025 iPS as well as normal embryonic stem cells. These differences might be due to disparity in the current iPS technology. Taken together, our data have established a simple iPS-SMC system to generate SMCs in vitro, which has tremendous potential to generate individualized SMCs for vascular tissue engineering and personalized drug screening.

Modulation of pulmonary vascular smooth muscle cell phenotype in hypoxia: role of cGMP-dependent protein kinase and myocardin

We have previously reported that in ovine fetal pulmonary venous smooth muscle cells (FPVSMC), decreased expression of cGMP-dependent protein kinase (PKG) by hypoxia could explain hypoxia-induced SMC phenotype modulation. In this study, we investigated the role of myocardin, a possible downstream effector of PKG, in SMC phenotype modulation induced by 1 and 24 h of hypoxia. Hypoxia for 1 h induced the phosphorylation of E-26-like protein 1 (Elk-1), indicating a quick activation of Elk-1 after hypoxia. Either hypoxia (1 h) or treatment with DT-3, a PKG inhibitor, increased associations of Elk-1 with myosin heavy chain (MHC) gene and serum response factor (SRF), which was paralleled by a decrease in association of myocardin with MHC gene and SRF. Exposure to hypoxia of FPVSMC for 24 h significantly decreased the promoter activity of multiple SMC marker genes, downregulated protein and mRNA expression of myocardin, and upregulated mRNA expression of Elk-1, but had no significant effects on the phosphorylation of Elk-1. Inhibition of myocardin by siRNA transfection downregulated the expression of SMC marker proteins, while overexpression of myocardin prevented the hypoxia-induced decrease in expression of SMC marker proteins. Inhibition of PKG by siRNA transfection downregulated the expression of myocardin, but upregulated that of Elk-1. Overexpression of PKG prevented hypoxia-induced effects on protein expression of myocardin and Elk-1. These data suggest that PKG induces displacement of myocardin from SRF and upregulates myocardin expression, thus activating the SMC genes transcription. The inhibitory effects of hypoxia on PKG may explain hypoxia-induced SMC phenotype modulation by decreasing the effects of PKG on myocardin.

Pod1 induces myofibroblast differentiation in mesenchymal progenitor cells from mouse kidney

The class II basic helix-loop-helix (bHLH) transcription factor Pod1 is expressed in mesenchymal cells including smooth muscle progenitors during development and in interstitial cells in adult organs. To determine the role of Pod1 in mesenchymal cell smooth muscle and myofibroblast differentiation, we examined a kidney progenitor cell line (4E) that endogenously expresses Pod1 and its class I bHLH partner E2A. In vitro-translated Pod1 co-immunoprecipitated E2A and increased E2A binding to a calponin promoter E-box sequence as determined by an electrophoresis mobility shift assay (EMSA). Overexpression of Pod1 and E2A resulted in increased smooth muscle and myofibroblast gene expression including calponin, SM22alpha, alphaSMA, fibronectin, and connective tissue growth factor (CTGF) compared with overexpression of E2A alone. Suppression of Pod1 by siRNA resulted in increased cell proliferation and reduced expression of alphaSMA, fibronectin, and CTGF, and myofibroblast secreted proteins including pro-fibrotic cytokines and inhibitors of matrix metalloproteinases. Examination of the signaling pathways for myofibroblast differentiation including Rho/Rho kinase and p38 MAPK showed that inhibition of actin polymerization by Rho kinase inhibitors decreased nuclear Pod1 levels while inhibition of p38 MAPK decreased Pod1 expression. These results indicate that Pod1 increases myofibroblast differentiation in combination with E2A and promotes a myofibroblast phenotype in mesenchymal progenitor cells.

Myocardin-related transcription factors: critical coactivators regulating cardiovascular development and adaptation

The association of transcriptional coactivators with DNA-binding proteins provides an efficient mechanism to expand and modulate genetic information encoded within the genome. Myocardin-related transcription factors (MRTFs), including myocardin, MRTF-A/MKL1/MAL, and MRTF-B/MKL2, comprise a family of related transcriptional coactivators that physically associate with the MADS box transcription factor, serum response factor, and synergistically activate transcription. MRTFs transduce cytoskeletal signals to the nucleus, activating a subset of serum response factor-dependent genes promoting myogenic differentiation and cytoskeletal organization. MRTFs are multifunctional proteins that share evolutionarily conserved domains required for actin-binding, homo- and heterodimerization, high-order chromatin organization, and transcriptional activation. Mice harboring loss-of-function mutations in myocardin, MRTF-A, and MRTF-B, respectively, display distinct phenotypes, including cell autonomous defects in vascular smooth muscle cell and myoepithelial cell differentiation and function. This article reviews the molecular basis of MRTF function with particular focus on the role MRTFs play in regulating cardiovascular patterning, vascular smooth muscle cell and cardiomyocyte differentiation and in the pathogenesis of congenital heart disease and vascular proliferative syndromes.

Response gene to complement 32, a novel regulator for transforming growth factor-beta-induced smooth muscle differentiation of neural crest cells

We previously developed a robust in vitro model system for vascular smooth muscle cell (VSMC) differentiation from neural crest cell line Monc-1 upon transforming growth factor-beta (TGF-beta) induction. Further studies demonstrated that both Smad and RhoA signaling are critical for TGF-beta-induced VSMC development. To identify downstream targets, we performed Affymetrix cDNA array analysis of Monc-1 cells and identified a gene named response gene to complement 32 (RGC-32) to be important for the VSMC differentiation. RGC-32 expression was increased 5-fold after 2 h and 50-fold after 24 h of TGF-beta induction. Knockdown of RGC-32 expression in Monc-1 cells by small interfering RNA significantly inhibited the expression of multiple smooth muscle marker genes, including SM alpha-actin (alpha-SMA), SM22alpha, and calponin. Of importance, the inhibition of RGC-32 expression correlated with the reduction of alpha-SMA while not inhibiting smooth muscle-unrelated c-fos gene expression, suggesting that RGC-32 is an important protein factor for VSMC differentiation from neural crest cells. Moreover, RGC-32 overexpression significantly enhanced TGF-beta-induced alpha-SMA, SM22alpha, and SM myosin heavy chain promoter activities in both Monc-1 and C3H10T1/2 cells. The induction of VSMC gene promoters by RGC-32 appears to be CArG-dependent. These data suggest that RGC-32 controls VSMC differentiation by regulating marker gene transcription in a CArG-dependent manner. Further studies revealed that both Smad and RhoA signaling are important for RGC-32 activation.

Acute myeloid leukemia-associated Mkl1 (Mrtf-a) is a key regulator of mammary gland function

Transcription of immediate-early genes--as well as multiple genes affecting muscle function, cytoskeletal integrity, apoptosis control, and wound healing/angiogenesis--is regulated by serum response factor (Srf). Extracellular signals regulate Srf in part via a pathway involving megakaryoblastic leukemia 1 (Mkl1, also known as myocardin-related transcription factor A [Mrtf-a]), which coactivates Srf-responsive genes downstream of Rho GTPases. Here we investigate Mkl1 function using gene targeting and show the protein to be essential for the physiologic preparation of the mammary gland during pregnancy and the maintenance of lactation. Lack of Mkl1 causes premature involution and impairs expression of Srf-dependent genes in the mammary myoepithelial cells, which control milk ejection following oxytocin-induced contraction. Despite the importance of Srf in multiple transcriptional pathways and widespread Mkl1 expression, the spectrum of abnormalities associated with Mkl1 absence appears surprisingly restricted.

Inhibition of transforming growth factor beta-enhanced serum response factor-dependent transcription by SMAD7

Transforming growth factor (TGF)-beta is present in large amounts in the airways of patients with asthma and with other diseases of the lung. We show here that TGFbeta treatment increased transcriptional activation of SM22alpha, a smooth muscle-specific promoter, in airway smooth muscle cells, and we demonstrate that this effect stems in part from TGFbeta-induced enhancement of serum response factor (SRF) DNA binding and transcription promoting activity. Overexpression of Smad7 inhibited TGFbeta-induced stimulation of SRF-dependent promoter function, and chromatin immunoprecipitation as well as co-immunoprecipitation assays established that endogenous or recombinant SRF interacts with Smad7 within the nucleus. The SRF binding domain of Smad7 mapped to the C-terminal half of the Smad7 molecule. TGFbeta treatment weakened Smad7 association with SRF, and conversely the Smad7-SRF interaction was increased by inhibition of the TGFbeta pathway through overexpression of a dominant negative mutant of TGFbeta receptor I or of Smad3 phosphorylation-deficient mutant. Our findings thus reveal that SRF-Smad7 interactions in part mediate TGFbeta regulation of gene transcription in airway smooth muscle. This offers potential targets for interventions in treating lung inflammation and asthma.

Increased neointima formation in cysteine-rich protein 2-deficient mice in response to vascular injury

In response to arterial injury, medial vascular smooth muscle cells (VSMCs) proliferate and migrate into the intima, contributing to the development of occlusive vascular disease. The LIM protein cysteine-rich protein (CRP) 2 associates with the actin cytoskeleton and may maintain the cytoarchitecture. CRP2 also interacts with transcription factors in the nucleus to mediate SMC gene expression. To test the hypothesis that CRP2 may be an important regulator of vascular development or function we generated Csrp2 (gene symbol of the mouse CRP2 gene)-deficient (Csrp2(-/-)) mice by targeted mutation. Csrp2(-/-) mice did not have any gross vascular defects or altered expression levels of SM alpha-actin, SM22alpha, or calponin. Following femoral artery injury, CRP2 expression persisted in the vessel wall at 4 days and then decreased by 14 days. Intimal thickening was enhanced 3.4-fold in Csrp2(-/-) compared with wild-type (WT) mice 14 days following injury. Cellular proliferation was similar between WT and Csrp2(-/-) VSMC both in vivo and in vitro. Interestingly, Csrp2(-/-) VSMC migrated more rapidly in response to PDGF-BB and had increased Rac1 activation. Our data demonstrate that CRP2 is not required for vascular development. However, an absence of CRP2 enhanced VSMC migration and increased neointima formation following arterial injury.

Ca2+/calmodulin-dependent protein kinase IV activates cysteine-rich protein 1 through adjacent CRE and CArG elements

Smooth muscle-specific transcription is controlled by a multitude of transcriptional regulators that cooperate to drive expression in a temporospatial manner. Previous analysis of the cysteine-rich protein 1 ( CRP1/Csrp) gene revealed an intronic enhancer that is sufficient for expression in arterial smooth muscle cells and requires a serum response factor-binding CArG element for activity. The presence of a CArG box in smooth muscle regulatory regions is practically invariant; however, it stands to reason that additional elements contribute to the modulation of transcription in concert with the CArG. Because of the potential importance of other regulatory elements for expression of the CRP1 gene, we sought to identify additional motifs within the enhancer that are necessary for expression. In this effort, we identified a conserved cAMP response element (CRE) that, when mutated, diminishes the expression of the enhancer in cultured vascular smooth muscle cells. Using transfection and electrophoretic mobility shift assays, we have shown that the CRE binds the cAMP response element-binding protein (CREB) and is activated by Ca2+/calmodulin-dependent protein kinase IV (CaMKIV), but not by CaMKII. Furthermore, our data demonstrate that CaMKIV stimulates CRP1 expression not only through the CRE but also through the CArG box. These findings represent evidence of a functional CRE within a smooth muscle-specific gene and provide support for a mechanism in which CREB functions as a smooth muscle determinant through CaMKIV activation.

Transforming growth factor-beta1-induced expression of smooth muscle marker genes involves activation of PKN and p38 MAPK

Differentiated vascular smooth muscle cells (SMCs) exhibit a work phenotype characterized by expression of several well documented contractile apparatus-associated proteins. However, SMCs retain the ability to de-differentiate into a proliferative phenotype, which is involved in the progression of vascular diseases such as atherosclerosis and restenosis. Understanding the mechanisms involved in maintaining SMC differentiation is critical for preventing proliferation associated with vascular disease. In this study, the molecular mechanisms through which transforming growth factor-beta1 (TGF-beta1) induces differentiation of SMCs were examined. TGF-beta1 stimulated actin re-organization, inhibited cell proliferation, and up-regulated SMC marker gene expression in PAC-1 SMCs. These effects were blocked by pretreatment of cells with either HA1077 or Y-27632, which inhibit the kinases downstream of RhoA. Moreover, TGF-beta1 activated RhoA and its downstream target PKN. Overexpression of active PKN alone was sufficient to increase the transcriptional activity of the promoters that control expression of smooth muscle (SM) alpha-actin, SM-myosin heavy chain, and SM22alpha. In addition, PKN increased the activities of serum-response factor (SRF), GATA, and MEF2-dependent enhancer-reporters. RNA interference-mediated inhibition of PKN abolished TGF-beta1-induced activation of SMC marker gene promoters. Finally, examination of MAPK signaling demonstrated that TGF-beta1 increased the activity of p38 MAPK, which was required for activation of the SMC marker gene promoters. Co-expression of dominant negative p38 MAPK was sufficient to block PKN-mediated activation of the SMC marker gene promoters as well as the serum-response factor, GATA, and MEF2 enhancers. Taken together, these results identify components of an important intracellular signaling pathway through which TGF-beta1 activates PKN to promote differentiation of SMCs.

Predictive screening for regulators of conserved functional gene modules (gene batteries) in mammals

Background

The expression of gene batteries, genomic units of functionally linked genes which are activated by similar sets of cis- and trans-acting regulators, has been proposed as a major determinant of cell specialization in metazoans. We developed a predictive procedure to screen the mouse and human genomes and transcriptomes for cases of gene-battery-like regulation.

Results

In a screen that covered ~40 per cent of all annotated protein-coding genes, we identified 21 co-expressed gene clusters with statistically supported sharing of cis-regulatory sequence elements. 66 predicted cases of over-represented transcription factor binding motifs were validated against the literature and fell into three categories: (i) previously described cases of gene battery-like regulation, (ii) previously unreported cases of gene battery-like regulation with some support in a limited number of genes, and (iii) predicted cases that currently lack experimental support. The novel predictions include for example Sox 17 and RFX transcription factor binding sites that were detected in ~10% of all testis specific genes, and HNF-1 and 4 binding sites that were detected in ~30% of all kidney specific genes respectively. The results are publicly available at .

Conclusion

21 co-expressed gene clusters were enriched for a total of 66 shared cis-regulatory sequence elements. A majority of these predictions represent novel cases of potential co-regulation of functionally coupled proteins. Critical technical parameters were evaluated, and the results and the methods provide a valuable resource for future experimental design.

Cell Cycle-mediated Regulation of Smooth Muscle α-Actin Gene Transcription in Fibroblasts and Vascular Smooth Muscle Cells Involves Multiple Adenovirus E1A-interacting Cofactors

Expression of smooth muscle alpha-actin in growth factor-induced myofibroblasts and in differentiated vascular smooth muscle cells is transcriptionally controlled by multiple positive or negative trans-acting factors interacting with distinct cis-elements in the 5'-flanking region of the gene. Because none of the transcriptional regulators reported to date is smooth muscle cell- or myofibroblast-specific per se, the dynamic interplay among many factors interacting at specific sites along the promoter appears to be a signature feature of smooth muscle alpha-actin gene regulation in these cell types. Herein, the ability of the adenovirus E1A 12 S protein to bind and functionally inactivate specific cell regulatory factors has been exploited to identify several previously unknown coactivators of the mouse smooth muscle alpha-actin promoter in rodent fibroblasts and vascular smooth muscle cells. In transient cotransfection assays, ectopic expression of wild type E1A suppressed promoter activity in a dose- and cis-element-dependent manner. In asynchronous cells, N-terminal E1A mutants defective in CREB-binding protein (CBP) and p300 binding capacity exhibited markedly reduced inhibitory activity toward a smooth muscle alpha-actin promoter driven by a composite TEF-1-, SRF-, and Sp1/3-regulated enhancer. In synchronized cells, however, a more complex mutant E1A inhibitory pattern indicated that collaboration between CBP/p300 and the retinoblastoma family of pocket proteins was required to produce a fully functional enhancer. Cotransfection experiments conducted with Rb(-/-) fibroblasts demonstrated the necessity of pRB in augmenting smooth muscle alpha-actin enhancer/promoter activity. Physical interaction studies with the use of purified wild type and mutant E1A proteins confirmed that CBP, p300, and pRB were targets of E1A binding in nuclear extracts of vascular smooth muscle cells and/or fibroblasts. Collectively, these results suggest that a repertoire of E1A-interacting proteins, namely CBP/p300 and pRB, serve to integrate the activities of multiple trans-acting factors to control smooth muscle alpha-actin gene transcription in a cell type- and cell cycle-dependent manner.

Mef2c is activated directly by Ets transcription factors through an evolutionarily conserved endothelial cell-specific enhancer

Members of the Myocyte Enhancer Factor 2 (MEF2) family of transcription factors play key roles in the development and differentiation of numerous cell types during mammalian development, including the vascular endothelium. Mef2c is expressed very early in the development of the endothelium, and genetic studies in mice have demonstrated that mef2c is required for vascular development. However, the transcriptional pathways involving MEF2C during endothelial cell development have not been defined. As a first step towards identifying the transcriptional factors upstream of MEF2C in the vascular endothelium, we screened for transcriptional enhancers from the mouse mef2c gene that regulate vascular expression in vivo. In this study, we identified a transcriptional enhancer from the mouse mef2c gene sufficient to direct expression to the vascular endothelium in transgenic embryos. This enhancer is active in endothelial cells within the developing vascular system from very early stages in vasculogenesis, and the enhancer remains robustly active in the vascular endothelium during embryogenesis and in adulthood. This mef2c endothelial cell enhancer contains four perfect consensus Ets transcription factor binding sites that are efficiently bound by Ets-1 protein in vitro and are required for enhancer function in transgenic embryos. Thus, these studies identify mef2c as a direct transcriptional target of Ets factors via an evolutionarily conserved transcriptional enhancer and establish a direct link between these two early regulators of vascular gene expression during endothelial cell development in vivo.

GATA-6 regulates genes promoting synthetic functions in vascular smooth muscle cells

OBJECTIVE

Previous studies suggested the zinc-finger transcription factor GATA-6 inhibits vascular smooth muscle cell (VSMC) proliferation and promotes the contractile VSMC phenotype. The objective of this study was to identify bona fide target genes regulated by GATA-6 in VSMCs.

METHODS AND RESULTS

Microarray analyses were performed comparing mRNA from rat aortic smooth muscle cells (SMCs) infected with either adenovirus encoding a dominant-negative GATA-6/engrailed fusion protein or with control adenovirus. These studies identified 122 genes differentially expressed by at least 2-fold, including multiple genes involved in cell-cell signaling and cell-matrix interactions. Among these, endothelin-1 and the angiotensin type(1a) (AT(1a)) receptor are known to be induced in VSMCs in response to inflammatory stimuli and to be expressed in a GATA-dependent manner in cardiac myocytes in response to hemodynamic stress. Consistent with these findings, the endothelin-1 and AT(1a) receptor promoters were activated by forced expression of GATA-6 and repressed by forced expression of GATA-6/engrailed. Surprisingly, genes encoding SMC contractile proteins were not altered, and myocardin-induced SMC differentiation was not impaired in GATA-6(-/-) embryonic stem cells.

CONCLUSIONS

These data demonstrate that in VSMCs, GATA-6 regulates a set of genes associated with synthetic SMC functions and suggest that this transcriptional pathway may be independent from myocardin-induced SMC differentiation. An unbiased microarray screen of genes regulated by GATA-6 in VSMCs identified multiple genes involved in cell-cell signaling and cell-matrix interactions. The endothelin-1 and the AT1a receptor genes were shown to be direct GATA-6 target genes. These data suggest that GATA-6 plays a role in promoting synthetic functions in VSMCs.

HRC is a direct transcriptional target of MEF2 during cardiac, skeletal, and arterial smooth muscle development in vivo

The HRC gene encodes the histidine-rich calcium-binding protein, which is found in the lumen of the junctional sarcoplasmic reticulum (SR) of cardiac and skeletal muscle and within calciosomes of arterial smooth muscle. The expression of HRC in cardiac, skeletal, and smooth muscle raises the possibility of a common transcriptional mechanism governing its expression in all three muscle cell types. In this study, we identified a transcriptional enhancer from the HRC gene that is sufficient to direct the expression of lacZ in the expression pattern of endogenous HRC in transgenic mice. The HRC enhancer contains a small, highly conserved sequence that is required for expression in all three muscle lineages. Within this conserved region is a consensus site for myocyte enhancer factor 2 (MEF2) proteins that we show is bound efficiently by MEF2 and is required for transgene expression in all three muscle lineages in vivo. Furthermore, the entire HRC enhancer sequence lacks any discernible CArG motifs, the binding site for serum response factor (SRF), and we show that the enhancer is not activated by SRF. Thus, these studies identify the HRC enhancer as the first MEF2-dependent, CArG-independent transcriptional target in smooth muscle and represent the first analysis of the transcriptional regulation of an SR gene in vivo.

Calcineurin initiates smooth muscle differentiation in neural crest stem cells

The process of vascular smooth muscle cell (vSMC) differentiation is critical to embryonic angiogenesis. However, despite its importance, the vSMC differentiation program remains largely undefined. Murine gene disruption studies have identified several gene products that are necessary for vSMC differentiation, but these methodologies cannot establish whether or not a factor is sufficient to initiate the differentiation program. A gain-of-function system consisting of normal vSMC progenitor cells would serve as a useful complement to whole animal loss-of-function studies. We use such a system here, namely freshly isolated rat neural crest stem cells (NCSCs), to show that activation of the calcineurin signaling pathway is sufficient to drive these cells toward a smooth muscle fate. In addition, we present data suggesting that transforming growth factor (TGF)-beta1, which also causes NCSCs to differentiate into smooth muscle, activates calcineurin signaling in NCSCs, leading to a model in which activation of calcineurin signaling is the mechanism by which TGF-beta1 causes SMC differentiation in these cells.

Glomulin is predominantly expressed in vascular smooth muscle cells in the embryonic and adult mouse

Mutations in the glomulin gene result in dominantly inherited vascular lesions of the skin known as glomuvenous malformations (GVMs). These lesions are histologically distinguished by their distended vein-like channels containing characteristic 'glomus cells', which appear to be incompletely or improperly differentiated vascular smooth muscle cells (VSMCs). The function of glomulin is currently unknown. We studied glomulin expression during murine development (E9.5 days post-coitum until adulthood) by non-radioactive in situ hybridization. Glomulin was first detected at E10.5 dpc in cardiac outflow tracts. Later, it showed strong expression in VSMCs as well as a limited expression in the perichondrium. At E11.5-14.5 dpc glomulin RNA was most abundant in the walls of the large vessels. At E16.5 dpc expression was also detectable in smaller arteries and veins. The high expression of glomulin in murine vasculature suggests an important role for glomulin in blood vessel development and/or maintenance, which is supported by the vascular phenotype seen in GVM patients with mutations in this gene.

Megakaryoblastic Leukemia Factor-1 Transduces Cytoskeletal Signals and Induces Smooth Muscle Cell Differentiation from Undifferentiated Embryonic Stem Cells

The SAP domain transcription factor myocardin plays a critical role in the transcriptional program regulating smooth muscle cell differentiation. In this report, we describe the capacity of myocardin to physically associate with megakaryoblastic leukemia factor-1 (MKL1) and characterize the function of MKL1 in smooth muscle cells (SMCs). The MKL1 gene is expressed in most human tissues and myocardin and MKL are co-expressed in SMCs. MKL1 and myocardin physically associate via conserved leucine zipper domains. Overexpression of MKL1 transactivates serum response factor (SRF)-dependent SMC-restricted transcriptional regulatory elements including the SM22alpha promoter, smooth muscle myosin heavy chain promoter/enhancer, and SM-alpha-actin promoter/enhancer in non-SMCs. Moreover, forced expression of MKL1 and SRF in undifferentiated SRF(-/-) embryonic stem cells activates multiple endogenous SMC-restricted genes at levels equivalent to, or exceeding, myocardin. Forced expression of a dominant-negative MKL1 mutant reduces myocardin-induced activation of the SMC-specific SM22alpha promoter. In NIH3T3 fibroblasts MKL1 localizes to the cytoplasm and translocates to the nucleus in response to serum stimulation, actin treadmilling, and RhoA signaling. In contrast, in SMCs MKL1 is observed exclusively in the nucleus regardless of serum conditions or RhoA signaling. However, when actin polymerization is disrupted MKL1 translocates from the nucleus to the cytoplasm in SMCs. Together, these data were consistent with a model wherein MKL1 transduces signals from the cytoskeleton to the nucleus in SMCs and regulates SRF-dependent SMC differentiation autonomously or in concert with myocardin.

Identification of a CArG-independent region of the cysteine-rich protein 2 promoter that directs expression in the developing vasculature

Cysteine-rich protein (CRP)2 is a member of the LIM-only CRP family that is expressed in vascular smooth muscle cells (VSMC). To gain insight into the transcription of CSRP2 (gene name for CRP2) in VSMC, we analyzed the 5'-flanking sequence of the CSRP2 gene. We showed previously that 4,855 bp of the 5'-flanking sequence of the CSRP2 gene directed lacZ reporter gene expression, primarily in the VSMC of transgenic mice. To further define the regulatory sequences important for CSRP2 expression in VSMC, a series of promoter constructs containing deletions of the 5'-flanking sequence upstream of a nuclear-localized lacZ reporter gene were generated and analyzed. Similar to that observed in the -4855CSRP2-lacZ mice, beta-galactosidase reporter activity was detected in the developing great vessels, aorta, intersegmental arteries, umbilical vessels, endocardial cushions, and neural tube in the -3513-, -2663-, -795-, and -664CSRP2-lacZ lines. However, an internal deletion of bp -573 to -550 abolished the vascular, but not the neural tube, staining. Interestingly, no CArG box [CC(A/T)6GG] was present in the -795-bp fragment. Cotransfection experiments showed that dominant-negative serum response factor (SRF) did not repress CSRP2 promoter activity, which was different from the repressive effect of dominant-negative SRF on the SM22 alpha promoter. Our data suggest the presence of a VSMC-specific element(s) within bp -573 to -550 of the CSRP2 5'-flanking sequence; however, in contrast to many other smooth muscle genes, transcriptional regulation of the CSRP2 gene is not dependent on SRF.

Myocardin is a critical serum response factor cofactor in the transcriptional program regulating smooth muscle cell differentiation

The SAP family transcription factor myocardin functionally synergizes with serum response factor (SRF) and plays an important role in cardiac development. To determine the function of myocardin in the smooth muscle cell (SMC) lineage, we mapped the pattern of myocardin gene expression and examined the molecular mechanisms underlying transcriptional activity of myocardin in SMCs and embryonic stem (ES) cells. The human and murine myocardin genes were expressed in vascular and visceral SMCs at levels equivalent to or exceeding those observed in the heart. During embryonic development, the myocardin gene was expressed abundantly in a precise, developmentally regulated pattern in SMCs. Forced expression of myocardin transactivated multiple SMC-specific transcriptional regulatory elements in non-SMCs. By contrast, myocardin-induced transactivation was not observed in SRF(-/-) ES cells but could be rescued by forced expression of SRF or the SRF DNA-binding domain. Furthermore, expression of a dominant-negative myocardin mutant protein or small-interfering-RNA-induced myocardin knockdown significantly reduced SM22 alpha promoter activity in SMCs. Most importantly, forced expression of myocardin activated expression of the SM22 alpha, smooth muscle alpha-actin, and calponin-h1 genes in undifferentiated mouse ES cells. Taken together, these data demonstrate that myocardin plays an important role in the SRF-dependent transcriptional program that regulates SMC development and differentiation.

Binding of serum response factor to CArG box sequences is necessary but not sufficient to restrict gene expression to arterial smooth muscle cells

Serum response factor (SRF) plays an important role in regulating smooth muscle cell (SMC) development and differentiation. To understand the molecular mechanisms underlying the activity of SRF in SMCs, the two CArG box-containing elements in the arterial SMC-specific SM22alpha promoter, SME-1 and SME-4, were functionally and biochemically characterized. Mutations that abolish binding of SRF to the SM22alpha promoter totally abolish promoter activity in transgenic mice. Moreover, a multimerized copy of either SME-1 or SME-4 subcloned 5' of the minimal SM22alpha promoter (base pairs -90 to +41) is necessary and sufficient to restrict transgene expression to arterial SMCs in transgenic mice. In contrast, a multimerized copy of the c-fos SRE is totally inactive in arterial SMCs and substitution of the c-fos SRE for the CArG motifs within the SM22alpha promoter inactivates the 441-base pair SM22alpha promoter in transgenic mice. Deletion analysis revealed that the SME-4 CArG box alone is insufficient to activate transcription in SMCs and additional 5'-flanking nucleotides are required. Nuclear protein binding assays revealed that SME-4 binds SRF, YY1, and four additional SMC nuclear proteins. Taken together, these data demonstrate that binding of SRF to specific CArG boxes is necessary, but not sufficient, to restrict transgene expression to SMCs in vivo.

Analysis of SM22alpha-deficient mice reveals unanticipated insights into smooth muscle cell differentiation and function

SM22alpha is a 22-kDa smooth muscle cell (SMC) lineage-restricted protein that physically associates with cytoskeletal actin filament bundles in contractile SMCs. To examine the function of SM22alpha, gene targeting was used to generate SM22alpha-deficient (SM22(-/-LacZ)) mice. The gene targeting strategy employed resulted in insertion of the bacterial lacZ reporter gene at the SM22alpha initiation codon, permitting precise analysis of the temporal and spatial pattern of SM22alpha transcriptional activation in the developing mouse. Northern and Western blot analyses confirmed that the gene targeting strategy resulted in a null mutation. Histological analysis of SM22(+/-LacZ) embryos revealed detectable beta-galactosidase activity in the unturned embryonic day 8.0 embryo in the layer of cells surrounding the paired dorsal aortae concomitant with its expression in the primitive heart tube, cephalic mesenchyme, and yolk sac vasculature. Subsequently, during postnatal development, beta-galactosidase activity was observed exclusively in arterial, venous, and visceral SMCs. SM22alpha-deficient mice are viable and fertile. Their blood pressure and heart rate do not differ significantly from their control SM22alpha(+/-) and SM22alpha(+/+) littermates. The vasculature and SMC-containing tissues of SM22alpha-deficient mice develop normally and appear to be histologically and ultrastructurally similar to those of their control littermates. Taken together, these data demonstrate that SM22alpha is not required for basal homeostatic functions mediated by vascular and visceral SMCs in the developing mouse. These data also suggest that signaling pathways that regulate SMC specification and differentiation from local mesenchyme are activated earlier in the angiogenic program than previously recognized.