Serum response factor is crucial for actin cytoskeletal organization and focal adhesion assembly in embryonic stem cells

Smooth Muscle Expression of Lipoma Preferred Partner Is Mediated by an Alternative Intronic Promoter That Is Regulated by Serum Response Factor/Myocardin

Lipoma preferred partner (LPP) was recently recognized as a smooth muscle marker that plays a role in smooth muscle cell migration. In this report, we focus on the transcriptional regulation of the LPP gene. In particular, we investigate whether LPP is directly regulated by serum response factor (SRF). We show that the LPP gene contains 3 evolutionarily conserved CArG boxes and that 1 of these is part of an alternative promoter in intron 2. Quantitative RT-PCR shows that this alternative promoter directs transcription specifically to smooth muscle containing tissues in vivo. By using chromatin immunoprecipitation, we demonstrate that 2 of the CArG boxes, including the promoter-associated CArG box, bind to endogenous SRF in cultured aortic smooth muscle cells. Electrophoretic mobility-shift assays show that the conserved CArG boxes bind SRF in vitro. In reporter experiments, we show that the alternative promoter has transcriptional capacity that is dependent on SRF/myocardin and that the promoter associated CArG box is required for that activity. Finally, we show by quantitative RT-PCR that the alternative promoter is strongly downregulated in SRF-deficient embryonic stem cells and in smooth muscle tissues derived from conditional SRF knockout mice. Collectively, our data demonstrate that expression of LPP in smooth muscle is mediated by an alternative promoter that is regulated by SRF/myocardin.

SrfB, a member of the Serum Response Factor family of transcription factors, regulates starvation response and early development in Dictyostelium

The Serum Response Factor (SRF) is an important regulator of cell proliferation and differentiation. Dictyostelium discoideum srfB gene codes for an SRF homologue and is expressed in vegetative cells and during development under the control of three alternative promoters, which show different cell-type specific patterns of expression. The two more proximal promoters directed gene transcription in prestalk AB, stalk and lower-cup cells. The generation of a strain where the srfB gene has been interrupted (srfB⁻) has shown that this gene is required for regulation of actin–cytoskeleton-related functions, such as cytokinesis and macropinocytosis. The mutant failed to develop well in suspension, but could be rescued by cAMP pulsing, suggesting a defect in cAMP signaling. srfB⁻ cells showed impaired chemotaxis to cAMP and defective lateral pseudopodium inhibition. Nevertheless, srfB⁻ cells aggregated on agar plates and nitrocellulose filters 2 h earlier than wild type cells, and completed development, showing an increased tendency to form slug structures. Analysis of wild type and srfB⁻ strains detected significant differences in the regulation of gene expression upon starvation. Genes coding for lysosomal and ribosomal proteins, developmentally-regulated genes, and some genes coding for proteins involved in cytoskeleton regulation were deregulated during the first stages of development.

Severe intestinal obstruction on induced smooth muscle-specific ablation of the transcription factor SRF in adult mice

BACKGROUND & AIMS

SRF (Serum Response Factor), a widely expressed transcription factor, controls expression of mitogen-responsive and muscle-specific genes, thereby regulating the contractile actin microfilament. Genetic Srf deletion studies showed SRF to be indispensable for in vivo skeletal and cardiac muscle cell development. We now investigated for the first time in vivo SRF functions in smooth muscle cells of adult mice.

METHODS

We conditionally deleted a floxed Srf allele (Srf(flex1)) in adult mice by inducible activation of the CreER(T2) recombinase expressed specifically in smooth muscle cells. Tamoxifen-induced CreER(T2) activity stimulated deletion of exon 1 coding sequences of Srf(flex1), thereby abolishing full-length SRF protein expression in adult smooth muscle cells of the analyzed organs: colon, bladder, and stomach.

RESULTS

Smooth muscle cell-specific ablation of full-length SRF protein in adult mice showed impaired contraction of intestinal smooth muscle, resulting in defective peristalsis. Mutant mice died within 2 weeks of tamoxifen treatment, displaying clear symptoms of ileus paralyticus. Cultured primary SRF-deficient colon smooth muscle cells were viable, but displayed drastic structural alterations and elevated senescence, paralleled by degeneration of the actin microfilament and impaired expression of smooth muscle-specific genes.

CONCLUSIONS

SRF plays a vital role in the contractile activity and cytoskeletal architecture of adult smooth muscle cells and is therefore essential for physiologic functions of the gastrointestinal tract in vivo. Our mouse genetic model may resemble features of human chronic intestinal pseudo-obstruction.

Inducible mouse model of chronic intestinal pseudo-obstruction by smooth muscle-specific inactivation of the SRF gene

BACKGROUND & AIMS

Serum response factor (SRF) regulates the expression of muscle genes and immediate early genes. We investigated the consequences of inactivating this transcription factor SRF in adult gastrointestinal smooth muscle cells.

METHODS

SRF-floxed mice were crossed with SM-CreER(T2)(ki) mice expressing a tamoxifen-inducible recombinase in smooth muscle cells. Tamoxifen was injected into 12-week-old animals to activate the CreER(T2) and excise the SRF gene.

RESULTS

SRF was down-regulated in the smooth muscle cells of the gastrointestinal tract, urinary bladder, and aorta. The mutant mice developed severe dilation of the intestinal tract associated with food stasis and air-fluid levels in the lumen 13 days after tamoxifen treatment. Mutant mice displayed cachexia and died between days 13 and 22. The dilation was associated with a thinning of the muscularis propria and was also observed in the urinary bladder. Ex vivo colonic contraction induced by electric field stimulation and carbachol was impaired in the mutant mice before the occurrence of the dilation phenotype. The expression of several genes, including those encoding smooth muscle actin, the heavy chain of smooth muscle myosin, and smoothelin, was 60% to 70% lower in mutants than in controls, and mutants also had a lower F/G actin ratio.

CONCLUSIONS

SRF plays a central role in maintaining visceral smooth muscle contractile function in adults. Mice with a smooth muscle cell-specific SRF mutation develop a severe motility disorder resembling chronic intestinal pseudo-obstruction in humans and may be used as an inducible model of this disorder.

Actin cytoskeleton dynamics linked to synovial fibroblast activation as a novel pathogenic principle in TNF-driven arthritis

Rheumatoid arthritis is a chronic inflammatory disorder whose origin of defect has been the subject of extensive research during the past few decades. While a number of immune and non-immune cell types participate in the development of chronic destructive inflammation in the arthritic joint, synovial fibroblasts have emerged as key effector cells capable of modulating both joint destruction and propagation of inflammation. Ample evidence of aberrant changes in the morphology and biochemical behaviour of rheumatoid arthritis synovial fibroblasts have established the tissue evading and "transformed" character of this cell type. We have recently demonstrated that actin cytoskeletal rearrangements determine the pathogenic activation of synovial fibroblasts in modelled TNF-mediated arthritis, a finding correlating with similar gene expression changes which we observed in human rheumatoid arthritis synovial fibroblasts. Here, we show that pharmacological inhibition of actin cytoskeleton dynamics alters potential pathogenic properties of the arthritogenic synovial fibroblast, such as proliferation, migration and resistance to apoptosis, indicating novel opportunities for therapeutic intervention in arthritis. Recent advances in this field of research are reviewed and discussed.

Reorganization of the actin cytoskeleton via transcriptional regulation of cytoskeletal/focal adhesion genes by myocardin-related transcription factors (MRTFs/MAL/MKLs)

RhoA is a crucial regulator of stress fiber and focal adhesion formation through the activation of actin nucleation and polymerization. It also regulates the nuclear translocation of myocardin-related transcription factor-A and -B (MRTF-A/B, MAL or MKL 1/2), which are co-activators of serum response factor (SRF). In dominant-negative MRTF-A (DN-MRTF-A)-expressing NIH 3T3 cell lines, the expressions of several cytoskeletal/focal adhesion genes were down-regulated, and the formation of stress fiber and focal adhesion was severely diminished. MRTF-A/B-knockdown cells also exhibited such cytoskeletal defects. In reporter assays, both RhoA and MRTF-A enhanced promoter activities of these genes in a CArG-box-dependent manner, and DN-MRTF-A inhibited the RhoA-mediated activation of these promoters. In dominant-negative RhoA (RhoA-N19)-expressing NIH 3T3 cell lines, the nuclear translocation of MRTF-A/B was predominantly prevented, resulting in the reduced expression of cytoskeletal/focal adhesion proteins. Further, constitutive-active MRTF-A/B increased the expression of endogenous cytoskeletal/focal adhesion proteins, and thereby rescued the defective phenotype of stress fibers and focal adhesions in RhoA-N19 expressing cells. These results indicate that MRTF-A/B act as pivotal mediators of stress fiber and focal adhesion formation via the transcriptional regulation of a subset of cytoskeletal/focal adhesion genes.

Serum response factor contributes selectively to lymphocyte development

Serum response factor (SRF), is a crucial transcription factor for murine embryonic development and for the function of muscle cells and neurons. Gene expression data show that SRF and its transcriptional cofactors are also expressed in lymphocyte precursors and mature lymphocytes. However, the role of SRF in lymphocyte development has not been addressed in vivo so far, attributed in part to early embryonic lethality of conventional Srf-null mice. To determine the in vivo role of SRF in developing lymphocytes, we specifically inactivated the murine Srf gene during T or B cell development using lymphocyte-specific Cre transgenic mouse lines. T cell-specific Srf deletion led to a severe block in thymocyte development at the transition from CD4/CD8 double to single positive stage. The few residual T cells detectable in the periphery retained at least one functional Srf allele, thereby demonstrating the importance of SRF in T cell development. In contrast, deletion of Srf in developing B cells did not interfere with the growth and survival of B cells in general, yet led to a complete loss of marginal zone B cells and a marked reduction of the CD5+ B cell subset. Our study also revealed a contribution of SRF to the expression of the surface molecules IgM, CD19, and the chemokine receptor 4 in B lymphocytes. We conclude that SRF fulfills essential and distinct functions in the differentiation of different types of lymphocytes.

Programming smooth muscle plasticity with chromatin dynamics

Smooth muscle cells (SMCs) possess remarkable phenotypic plasticity that allows rapid adaptation to fluctuating environmental cues. For example, vascular SMCs undergo profound changes in their phenotype during neointimal formation in response to vessel injury or within atherosclerotic plaques. Recent studies have shown that interaction of serum response factor (SRF) and its numerous accessory cofactors with CArG box DNA sequences within promoter chromatin of SMC genes is a nexus for integrating signals that influence SMC differentiation in development and disease. During development, SMC-restricted sets of posttranslational histone modifications are acquired within the CArG box chromatin of SMC genes. These modifications in turn control the chromatin-binding properties of SRF. The histone modifications appear to encode a SMC-specific epigenetic program that is used by extracellular cues to influence SMC differentiation, by regulating binding of SRF and its partners to the chromatin template. Thus, SMC differentiation is dynamically regulated by the interplay between SRF accessory cofactors, the SRF-CArG interaction, and the underlying histone modification program. As such, the inherent plasticity of the SMC lineage offers unique glimpses into how cellular differentiation is dynamically controlled at the level of chromatin within the context of changing microenvironments. Further elucidation of how chromatin regulates SMC differentiation will undoubtedly yield valuable insights into both normal developmental processes and the pathogenesis of several vascular diseases that display detrimental SMC phenotypic behavior.

Hic-5 contributes to epithelial-mesenchymal transformation through a RhoA/ROCK-dependent pathway

Epithelial-mesenchymal transformation (EMT) in response to TGFbeta1 is a coordinated process of tissue morphogenesis that occurs during embryonic development as well as during certain pathologic events including kidney tubulointerstitial fibrosis. It is characterized by the disassembly of cell-cell junctions and dramatic alterations in the actin cytoskeleton that facilitates cell-matrix adhesion and stimulates migration. The focal adhesion adapter protein, Hic-5, has previously been reported to be upregulated during TGFbeta1-induced EMT in mouse mammary epithelial cells and the current study recapitulates this result in both mouse kidney proximal tubule epithelial, MCT, cells and human mammary epithelial, MCF10A, cells. To evaluate a causative role for Hic-5 in EMT, Hic-5 RNA interference (siRNA) was used to prevent Hic-5 expression in response to TGFbeta1 stimulation and was shown to suppress cell migration and actin stress fiber formation. It also resulted in the retention of a robust epithelial cell morphology characterized by elevated E-cadherin protein expression and well-organized adherens junctions. In addition, Hic-5 siRNA treatment led to the suppression of TGFbeta1 induction of RhoA activation. In contrast, forced expression of Hic-5 led to the formation of ROCK-dependent actin stress fibers. Furthermore, the induction of Hic-5 expression in response to TGFbeta1 was shown to be a RhoA/ROCK I-dependent process. Together, these data implicate Hic-5 as a key regulator of EMT and suggest that RhoA stimulated Hic-5 expression in response to TGFbeta1 may be functioning in a feed forward mechanism whereby Hic-5 maintains the mesenchymal phenotype through sustained RhoA activation and signaling.

A critical role of serum response factor in myofibroblast differentiation during experimental oesophageal ulcer healing in rats

BACKGROUND

Myofibroblast differentiation is a key event during wound healing and is triggered primarily by transforming growth factor beta (TGFbeta). Serum response factor (SRF) is a TGFbeta-inducible transcription factor that is important for wound healing. Injection of SRF expression plasmid into rat gastric ulcers significantly accelerated restoration of epithelium and smooth muscle structures.

AIM

To determine the role of SRF in oesophageal ulcer healing, especially in myofibroblast differentiation.

SUBJECTS

Rats (in vivo), oesophageal epithelial cells (Het1A) and fibroblasts (Rat1-R12) (in vitro) were used.

METHODS

Oesophageal ulcers were induced in rats with acetic acid and subsequently treated by local injection of plasmids expressing either SRF or SRF antisense sequence. Rats were killed at 1, 3, 6, 9 and 14 days after treatment and tissues collected. For in vitro studies, both Het1A and Rat1-R12 cells were transfected with the plasmids used in ulcer treatment.

RESULTS

Upregulation of SRF increased the myofibroblast population in ulcer granulation tissue; knockdown of SRF suppressed this event. In addition, ulceration induced SRF and TGFbeta expression coordinately. In vitro studies showed that overexpression of SRF in either oesophageal epithelial cells or fibroblasts was sufficient to induce myofibroblast phenotype. Furthermore, the TGFbeta-induced myofibroblast phenotype required integrin-linked kinase (ILK)-mediated SRF activation, as either knockdown of SRF or inactivation of ILK prevented this action.

CONCLUSIONS

SRF is indispensable for myofibroblast differentiation during oesophageal ulcer healing and is required for TGFbeta-induced myofibroblast transition from either epithelial cells or fibroblasts. ILK is a mediator in TGFbeta-induced SRF activation and subsequent myofibroblast differentiation. ILK is associated with SRF, and TGFbeta enhances this association.

Identification of the glomerular podocyte as a target for growth hormone action

GH excess in both the human and transgenic animal models is characterized by significant changes in blood pressure and renal function. The GH/GH receptor (GHR) axis is also implicated in the development of diabetic nephropathy. However, it is not clear whether GH's actions on renal function are due to indirect actions mediated via changes in blood pressure and vascular tone or due to direct action of GH on the kidney. We hypothesized that functional GHRs are expressed on the glomerular podocyte enabling direct actions of GH on glomerular function. Real-time PCR, immunohistochemistry, and Western blot analysis of murine podocyte cells (MPC-5) and kidney glomeruli demonstrated expression of GHR mRNA and protein. Exposure of both murine and human podocytes to GH (50-500 ng/ml) resulted in an increase in abundance of phosphorylated signal transducer and activator of transcription-5, Janus kinase-2, and ERK1/2 proteins. Exposure of podocytes to GH also caused changes in the intracellular distribution of the Janus kinase-2 adapter protein Src homology 2-Bbeta, stimulation of focal adhesion kinase, increase in reactive oxygen species, and GH-dependent changes in the actin cytoskeleton. We conclude that glomerular podocytes express functional GHRs and that GH increases levels of reactive oxygen species and induces reorganization of the actin cytoskeleton in these cells. These results provide a novel mechanistic link between GH's actions and glomerular dysfunction in disorders such as acromegaly and diabetic glomerulosclerosis.

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.

The bioactive lipid sphingosylphosphorylcholine induces differentiation of mouse embryonic stem cells and human promyelocytic leukaemia cells

Sphingosylphosphorylcholine (SPC) is the major component of high-density lipoproteins (HDL) in blood plasma. The bioactive lipid acts mainly via G protein coupled receptors (GPCRs). Similar to ligands of other GPCRs, SPC has multiple biological roles including the regulation of proliferation, migration, angiogenesis, wound healing and heart rate. Lysophospholipids and their receptors have also been implicated in cell differentiation. A potential role of SPC in stem cell or tumour cell differentiation has been elusive so far. Here we examined the effect of SPC on the differentiation of mouse embryonic stem (ES) cells and of human NB4 promyelocytic leukemia cells, a well established tumour differentiation model. Our data show that mouse embryonic stem cells and NB4 cells express the relevant GPCRs for SPC. We demonstrate both at the level of morphology and of gene expression that SPC induces neuronal and cardiac differentiation of mouse ES cells. Furthermore, SPC induces differentiation of NB4 cells by a mechanism which is critically dependent on the activity of the MEK-ERK cascade. Thus, the bioactive lipid SPC is a novel differentiation inducing agent both for mouse ES cells, but also of certain human tumour cells.

Zyxin is upregulated in the nucleus by thymosin β4 in SiHa cells

Thymosin beta4 is a 43-amino acid actin-binding protein that promotes cell migration and is important in angiogenesis, wound healing, and tumor metastasis. We searched for genes upregulated by thymosin beta4 and identified zyxin as increased in SiHa cells in the presence of exogenously added thymosin beta4 and when thymosin beta4 is overexpressed using adenoviral vectors. Both zyxin and thymosin beta4 show increased localization in the nucleus. We conclude that thymosin beta4 may exert some of its migration promoting activity via increased zyxin expression.

Serum response factor: master regulator of the actin cytoskeleton and contractile apparatus

Serum response factor (SRF) is a highly conserved and widely expressed, single copy transcription factor that theoretically binds up to 1,216 permutations of a 10-base pair cis element known as the CArG box. SRF-binding sites were defined initially in growth-related genes. Gene inactivation or knockdown studies in species ranging from unicellular eukaryotes to mice have consistently shown loss of SRF to be incompatible with life. However, rather than being critical for proliferation and growth, these genetic studies point to a crucial role for SRF in cellular migration and normal actin cytoskeleton and contractile biology. In fact, recent genomic studies reveal nearly half of the >200 SRF target genes encoding proteins with functions related to actin dynamics, lamellipodial/filopodial formation, integrin-cytoskeletal coupling, myofibrillogenesis, and muscle contraction. SRF has therefore emerged as a dispensable transcription factor for cellular growth but an absolutely essential orchestrator of actin cytoskeleton and contractile homeostasis. This review summarizes the recent genomic and genetic analyses of CArG-SRF that support its role as an ancient, master regulator of the actin cytoskeleton and contractile machinery.

The myocardin family of transcriptional coactivators: versatile regulators of cell growth, migration, and myogenesis

The association of transcriptional coactivators with sequence-specific DNA-binding proteins provides versatility and specificity to gene regulation and expands the regulatory potential of individual cis-regulatory DNA sequences. Members of the myocardin family of coactivators activate genes involved in cell proliferation, migration, and myogenesis by associating with serum response factor (SRF). The partnership of myocardin family members and SRF also controls genes encoding components of the actin cytoskeleton and confers responsiveness to extracellular growth signals and intracellular changes in the cytoskeleton, thereby creating a transcriptional-cytoskeletal regulatory circuit. These functions are reflected in defects in smooth muscle differentiation and function in mice with mutations in myocardin family members. This article reviews the functions and mechanisms of action of the myocardin family of coactivators and the physiological significance of transcriptional coactivation in the context of signal-dependent and cell-type-specific gene regulation.

Staying in shape with formins

Formins constitute a diverse protein family present in all eukaryotes examined. They are defined by the presence of a formin homology 2 (FH2) domain, which possesses intrinsic and conserved functions regulating cytoskeletal dynamics. Over the past few years, formins have become recognized as potent nucleators of linear actin filaments that control a large variety of cellular and morphogenetic functions. Here, we review the molecular principles of formin-induced cytoskeletal rearrangements and their consequences for a growing number of biological processes.

BARX2 and estrogen receptor-alpha (ESR1) coordinately regulate the production of alternatively spliced ESR1 isoforms and control breast cancer cell growth and invasion

The estrogen receptor-alpha gene (ESR1) was previously identified as a direct target of the homeobox transcription factor BARX2 in MCF7 cells. Here, we show that BARX2 and ESR1 proteins bind to different ESR1 gene promoters and regulate the expression of alternatively spliced mRNAs that encode 66 and 46 kDa ESR1 protein isoforms. BARX2 increases the expression of both ESR1 isoforms; however, it has a greater effect on the 46 kDa isoform, leading to an increased ratio between the 46 and 66 kDa proteins. BARX2 also influences estrogen-dependent processes such as anchorage-independent growth and modulates the expression of the estrogen-responsive genes SOX5, RBM15, Dynein and Mortalin. In addition, BARX2 expression promotes cellular invasion and increases the expression of active matrix metalloproteinase-9 (MMP9). BARX2 also increases the expression of the tissue inhibitor of metalloproteinase (TIMP) genes, TIMP1 and TIMP3, in cooperation with estrogen signaling. Overall, these data indicate that BARX2 and ESR1 may coordinately regulate cell growth, survival and invasion pathways that are critical to breast cancer progression.

Cytoplasmic gamma-actin contributes to a compensatory remodeling response in dystrophin-deficient muscle

Dystrophin mechanically links the costameric cytoskeleton and sarcolemma, yet dystrophin-deficient muscle exhibits abnormalities in cell signaling, gene expression, and contractile function that are not clearly understood. We generated new antibodies specific for cytoplasmic gamma-actin and confirmed that gamma-actin most predominantly localized to the sarcolemma and in a faint reticular lattice within normal muscle cells. However, we observed that gamma-actin levels were increased 10-fold at the sarcolemma and within the cytoplasm of striated muscle cells from dystrophin-deficient mdx mice. Transgenic overexpression of the dystrophin homologue utrophin, or functional dystrophin constructs in mdx muscle, restored gamma-actin to normal levels, whereas gamma-actin remained elevated in mdx muscle expressing nonfunctional dystrophin constructs. We conclude that increased cytoplasmic gamma-actin in dystrophin-deficient muscle may be a compensatory response to fortify the weakened costameric lattice through recruitment of parallel mechanical linkages. However, the presence of excessive myoplasmic gamma-actin may also contribute to altered cell signaling or gene expression in dystrophin-deficient muscle.

Defining the mammalian CArGome

Serum response factor (SRF) binds a 1216-fold degenerate cis element known as the CArG box. CArG boxes are found primarily in muscle- and growth-factor-associated genes although the full spectrum of functional CArG elements in the genome (the CArGome) has yet to be defined. Here we describe a genome-wide screen to further define the functional mammalian CArGome. A computational approach involving comparative genomic analyses of human and mouse orthologous genes uncovered >100 hypothetical SRF-dependent genes, including 10 previously identified SRF targets, harboring a conserved CArG element within 4000 bp of the annotated transcription start site (TSS). We PCR-cloned 89 hypothetical SRF targets and subjected each of them to at least two of several validations including luciferase reporter, gel shift, chromatin immunoprecipitation, and mRNA expression following RNAi knockdown of SRF; 60/89 (67%) of the targets were validated. Interestingly, 26 of the validated SRF target genes encode for cytoskeletal/contractile or adhesion proteins. RNAi knockdown of SRF diminishes expression of several SRF-dependent cytoskeletal genes and elicits an attending perturbation in the cytoarchitecture of both human and rodent cells. These data illustrate the power of integrating existing algorithms to interrogate the genome in a relatively unbiased fashion for cis-regulatory element discovery. In this manner, we have further expanded the mammalian CArGome with the discovery of an array of cyto-contractile genes that coordinate normal cytoskeletal homeostasis. We suggest one function of SRF is that of an ancient master regulator of the actin cytoskeleton.

The ternary complex factor Net regulates cell migration through inhibition of PAI-1 expression

Net, Elk-1, and Sap-1 are members of the ternary complex factor (TCF) subfamily of Ets transcription factors. They form ternary complexes with serum response factor (SRF) on serum response elements of immediate early genes such as c-fos and egr-1 and mediate responses to growth factors and mitogen-activated protein kinase signaling. Although the TCFs have been extensively studied as intermediates in signaling cascades, surprisingly little is known about their different target genes and physiological functions. We report that Net homozygous mutant mouse embryonic fibroblasts have a defect in cell migration. This defect results at least in part from increased expression of plasminogen activator inhibitor type 1 (PAI-1), a serine protease inhibitor (serpin) that controls extracellular proteolysis and cell matrix adhesion. The defect in cell migration can be reverted by the addition of a PAI-1 blocking antibody. Net represses PAI-1 promoter activity and binds to a specific region of the promoter containing Ets binding sites in the absence of SRF. We conclude that Net is a negative regulator of PAI-1 expression and is thereby involved in cell migration.

Serum response factor controls neuronal circuit assembly in the hippocampus

Higher organisms rely on multiple modes of memory storage using the hippocampal network, which is built by precisely orchestrated mechanisms of axonal outgrowth, guidance and synaptic targeting. We demonstrate essential roles of the transcription factor serum response factor (SRF), a sensor of cytoskeletal actin dynamics, in all these processes. Conditional deletion of the mouse Srf gene reduced neurite outgrowth and abolished mossy fiber segregation, resulting in ectopic fiber growth inside the pyramidal layer. SRF-deficient mossy fibers aberrantly targeted CA3 somata for synapse formation. Axon guidance assays showed that SRF was a key mediator of ephrin-A and semaphorin guidance cues; in SRF-deficient neurons, these resulted in the formation of F-actin-microtubule rings rather than complete growth cone collapse. Dominant-negative variants of the SRF cofactor megakaryocytic acute leukemia (MAL) severely impeded neurite outgrowth and guidance. These data highlight essential links between SRF-mediated transcription and axon guidance and circuit formation in the hippocampus.

Role of the serum response factor in regulating contractile apparatus gene expression and sarcomeric integrity in cardiomyocytes

The serum response factor (SRF) is a transcriptional regulator required for mesodermal development, including heart formation and function. Previous studies have described the role of SRF in controlling expression of structural genes involved in conferring the myogenic phenotype. Recent studies by us and others have demonstrated embryonic lethal cardiovascular phenotypes in SRF-null animals, but have not directly addressed the mechanistic role of SRF in controlling broad regulatory programs in cardiac cells. In this study, we used a loss-of-function approach to delineate the role of SRF in cardiomyocyte gene expression and function. In SRF-null neonatal cardiomyocytes, we observed severe defects in the contractile apparatus, including Z-disc and stress fiber formation, as well as mislocalization and/or attenuation of sarcomeric proteins. Consistent with this, gene array and reverse transcription-PCR analyses showed down-regulation of genes encoding key cardiac transcriptional regulatory factors and proteins required for the maintenance of sarcomeric structure, function, and regulation. Chromatin immunoprecipitation analysis revealed that at least a subset of these proteins are likely regulated directly by SRF. The results presented here indicate that SRF is an essential coordinator of cardiomyocyte function due to its ability to regulate expression of numerous genes (some previously identified and at least 28 targets newly identified in this study) that are involved in multiple and disparate levels of sarcomeric function and assembly.

Temporally controlled onset of dilated cardiomyopathy through disruption of the SRF gene in adult heart

BACKGROUND

Serum response factor (SRF) is a cardiac transcription factor involved in cell growth and differentiation. We have shown, using the Cre/loxP system, that cardiac-specific disruption of SRF gene in the embryonic heart results in lethal cardiac defects. The role of SRF in adult heart is unknown.

METHODS AND RESULTS

We disrupted SRF in the adult heart using a heart-specific tamoxifen-inducible Cre recombinase. This disruption led to impaired left ventricular function with reduced contractility, subsequently progressing to dilated cardiomyopathy, as demonstrated by serial echocardiography, including tissue Doppler imaging. The cytoarchitecture of cardiomyocytes was altered in the intercalated disks. All mutant mice died from heart failure 10 weeks after treatment. These functional and structural defects were preceded by early alterations in the cardiac gene expression program: major decreases in mRNA levels for cardiac alpha-actin, muscle creatine kinase, and calcium-handling genes.

CONCLUSIONS

SRF is crucial for adult cardiac function and integrity. We suggest that the rapid progression to heart failure in SRF mutant mice results primarily from decreased expression of proteins involved in force generation and transmission, low levels of polymerized actin, and changes in cytoarchitecture, without hypertrophic compensation. These cardiac-specific SRF-deficient mice have the morphological and clinical features of acquired dilated cardiomyopathy in humans and may therefore be used as an inducible model of this disorder.

Depletion of serum response factor by RNA interference mimics the mitogenic effects of platelet derived growth factor-BB in vascular smooth muscle cells

Promoters of many smooth muscle-specific genes (SM-genes) contain multiple CArG boxes, which represent a binding site for serum response factor (SRF). Transcriptional control through these regions involves interactions with SRF and specific coactivators such as myocardin. We have previously reported that suppression of SM-gene expression by platelet derived growth factor (PDGF) is associated with redistribution of SRF, leading to lower intra-nuclear levels, and a reduction in SRF transactivation. To further assess the role of SRF depletion on VSMC phenotype, the current study used RNA interference (RNAi). Two SRF-specific sequences constructed as hairpins were stably expressed in rat VSMC. Clones expressing SRF RNAi had no detectable SRF expression by immunoblotting, and showed diminished levels of SM alpha-actin protein and promoter activity. Unexpectedly, depletion of VSMC resulted in increased rates of proliferation and migration. Several genes whose expression is increased by PDGF stimulation, including c-Jun, were similarly induced in cells lacking SRF. Effects of SRF depletion were not attributable to altered PDGF receptor activity or alterations in activation of Akt. These data indicate that loss of SRF transactivation in VSMC, in this case through suppression via RNAi, induces biological responses similar to that seen with PDGF.

Conditional mutagenesis of the murine serum response factor gene blocks cardiogenesis and the transcription of downstream gene targets

Serum response factor (SRF) homozygous-null embryos from our backcross of SRF(LacZ/)(+) "knock-in" mice failed to gastrulate and form mesoderm, similar to the findings of an earlier study (Arsenian, S., Weinhold, B., Oelgeschlager, M., Ruther, U., and Nordheim, A. (1998) EMBO J. 17, 6289-6299). Our use of embryonic stem cells provided a model system that could be used to investigate the specification of multiple embryonic lineages, including cardiac myocytes. We observed the absence of myogenic alpha-actins, SM22alpha, and myocardin expression and the failure to form beating cardiac myocytes in aggregated SRF null embryonic stem cells, whereas the appearance of transcription factors Nkx2-5 and GATA4 were unaffected. To study the role of SRF during heart organogenesis, we then performed cardiac-specific ablation of SRF by crossing the transgenic alpha-myosin heavy chain Cre recombinase line with SRF LoxP-engineered mice. Cardiac-specific ablation of SRF resulted in embryonic lethality due to cardiac insufficiency during chamber maturation. Conditional ablation of SRF also reduced cell survival concomitant with increased apoptosis and reduced cellularity. Significant reductions in SRF (> or =95%), atrial naturetic factor (> or =80%), and cardiac (> or =60%), skeletal (> or =90%), and smooth muscle (> or =75%) alpha-actin transcripts were also observed in the cardiac-conditional knock-out heart. This was consistent with the idea that SRF directs de novo cardiac and smooth muscle gene activities. Finally, quantitation of the knock-in LacZ reporter gene transcripts in the hearts of cardiac-conditional knock-out embryos revealed an approximately 30% reduction in gene activity, indicating SRF gene autoregulation during cardiogenesis.

Neuronal migration in the murine rostral migratory stream requires serum response factor

The central nervous system is fundamentally dependent on guided cell migration, both during development and in adulthood. We report an absolute requirement of the transcription factor serum response factor (SRF) for neuronal migration in the mouse forebrain. Conditional, late-prenatal deletion of Srf causes neurons to accumulate ectopically at the subventricular zone (SVZ), a prime neurogenic region in the brain. SRF-deficient cells of the SVZ exhibit impaired tangential chain migration along the rostral migratory stream into the olfactory bulb. SVZ explants display retarded chain migration in vitro. Regarding target genes, SRF deficiency impairs expression of the beta-actin and gelsolin genes, accompanied by reduced cytoskeletal actin fiber density. At the posttranslational level, cofilin, a key regulator of actin dynamics, displays dramatically elevated inhibitory phosphorylation at Ser-3. Our studies indicate that SRF-controlled gene expression directs both the structure and dynamics of the actin microfilament, thereby determining cell-autonomous neuronal migration.

Regulation of Rho and Rac signaling to the actin cytoskeleton by paxillin during Drosophila development

Paxillin is a prominent focal adhesion docking protein that regulates cell adhesion and migration. Although numerous paxillin-binding proteins have been identified and paxillin is required for normal embryogenesis, the precise mechanism by which paxillin functions in vivo has not yet been determined. We identified an ortholog of mammalian paxillin in Drosophila (Dpax) and have undertaken a genetic analysis of paxillin function during development. Overexpression of Dpax disrupted leg and wing development, suggesting a role for paxillin in imaginal disc morphogenesis. These defects may reflect a function for paxillin in regulation of Rho family GTPase signaling as paxillin interacts genetically with Rac and Rho in the developing eye. Moreover, a gain-of-function suppressor screen identified a genetic interaction between Dpax and cdi in wing development. cdi belongs to the cofilin kinase family, which includes the downstream Rho target, LIM kinase (LIMK). Significantly, strong genetic interactions were detected between Dpax and Dlimk, as well as downstream effectors of Dlimk. Supporting these genetic data, biochemical studies indicate that paxillin regulates Rac and Rho activity, positively regulating Rac and negatively regulating Rho. Taken together, these data indicate the importance of paxillin modulation of Rho family GTPases during development and identify the LIMK pathway as a critical target of paxillin-mediated Rho regulation.

The SARS coronavirus nucleocapsid protein induces actin reorganization and apoptosis in COS-1 cells in the absence of growth factors

In March 2003, a novel coronavirus was isolated from patients exhibiting atypical pneumonia, and was subsequently proven to be the causative agent of the disease now referred to as SARS (severe acute respiratory syndrome). The complete genome of the SARS-CoV (SARS coronavirus) has since been sequenced. The SARS-CoV nucleocapsid (SARS-CoV N) protein shares little homology with other members of the coronavirus family. In the present paper, we show that SARS-CoV N is capable of inducing apoptosis of COS-1 monkey kidney cells in the absence of growth factors by down-regulating ERK (extracellular-signal-regulated kinase), up-regulating JNK (c-Jun N-terminal kinase) and p38 MAPK (mitogen-activated protein kinase) pathways, and affecting their downstream effectors. SARS-CoV N expression also down-regulated phospho-Akt and Bcl-2 levels, and activated caspases 3 and 7. However, apoptosis was independent of the p53 and Fas signalling pathways. Furthermore, activation of the p38 MAPK pathway was found to induce actin reorganization in cells devoid of growth factors. At the cytoskeletal level, SARS-CoV N down-regulated FAK (focal adhesion kinase) activity and also down-regulated fibronectin expression. This is the first report showing the ability of the N protein of SARS-CoV to induce apoptosis and actin reorganization in mammalian cells under stressed conditions.

Control of smooth muscle development by the myocardin family of transcriptional coactivators

Differentiation of smooth muscle cells (SMCs) is accompanied by the transcriptional activation of an array of muscle-specific genes that confer the unique contractile and physiologic properties of this muscle cell type. The majority of smooth muscle genes are controlled by serum response factor (SRF), a widely expressed transcription factor that also regulates genes involved in cell proliferation. Myocardin and myocardin-related transcription factors (MRTFs) interact with SRF and potently stimulate SRF-dependent transcription. Gain- and loss-of-function experiments have shown myocardin to be sufficient and necessary for SMC differentiation. SMCs are highly plastic and can switch between differentiated and proliferative states in response to extracellular cues. Suppression of SMC differentiation by growth factor signaling is mediated, at least in part, by the displacement of myocardin from SRF by growth factor-dependent ternary complex factors. The association of SRF with myocardin and MRTFs provides a molecular basis for the activation of SMC genes by SRF and the responsiveness of the smooth muscle differentiation program to growth factor signaling.

The SRF target gene Fhl2 antagonizes RhoA/MAL-dependent activation of SRF

RhoA signaling regulates the activity of the transcription factor SRF (serum response factor) during muscle differentiation. How RhoA signaling is integrated at SRF target promoters to achieve muscle-lineage-specific expression is largely unknown. Using large-scale expression profiling combined with bioinformatic and biochemical approaches, we identified several SRF target genes, including Fhl2, encoding a transcriptional cofactor that is highly expressed in the heart. SRF binds the Fhl2 promoter in vivo and regulates Fhl2 expression in response to RhoA activation. FHL2 protein and SRF interact physically, and FHL2 binds the promoters of SRF-responsive smooth muscle (SM) genes, but not the promoters of immediate-early genes (IEGs), in response to RhoA. FHL2 antagonizes induction of SM genes, but not IEGs or cardiac genes, by competing with the coactivator MAL/MRTF-A for SRF binding. Our findings identify an autoregulatory mechanism to selectively regulate subsets of RhoA-activated SRF target genes.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.

Characterization of antirrhinum petal development and identification of target genes of the class B MADS box gene DEFICIENS

The class B MADS box transcription factors DEFICIENS (DEF) and GLOBOSA (GLO) of Antirrhinum majus together control the organogenesis of petals and stamens. Toward an understanding of how the downstream molecular mechanisms controlled by DEF contribute to petal organogenesis, we conducted expression profiling experiments using macroarrays comprising >11,600 annotated Antirrhinum unigenes. First, four late petal developmental stages were compared with sepals. More than 500 ESTs were identified that comprise a large number of stage-specifically regulated genes and reveal a highly dynamic transcriptional regulation. For identification of DEF target genes that might be directly controlled by DEF, we took advantage of the temperature-sensitive def-101 mutant. To enhance the sensitivity of the profiling experiments, one petal developmental stage was selected, characterized by increased transcriptome changes that reflect the onset of cell elongation processes replacing cell division processes. Upon reduction of the DEF function, 49 upregulated and 52 downregulated petal target genes were recovered. Eight target genes were further characterized in detail by RT-PCR and in situ studies. Expression of genes responding rapidly toward an altered DEF activity is confined to different petal tissues, demonstrating the complexity of the DEF function regulating diverse basic processes throughout petal morphogenesis.

Restricted inactivation of serum response factor to the cardiovascular system

Serum response factor (SRF) directs programs of gene expression linked to growth and muscle differentiation. To investigate the role of SRF in cardiovascular development, we generated mice in which SRF is knocked out in >80% of cardiomyocytes and >50% of vascular smooth muscle cells (SMC) through SM22alpha-Cre-mediated excision of SRF's promoter and first exon. Mutant mice display vascular patterning, cardiac looping, and SRF-dependent gene expression through embryonic day (e)9.5. At e10.5, attenuation in cardiac trabeculation and compact layer expansion is noted, with an attendant decrease in vascular SMC recruitment to the dorsal aorta. Ultrastructurally, cardiac sarcomeres and Z disks are highly disorganized in mutant embryos. Moreover, SRF mutant mice exhibit vascular SMC lacking organizing actin/intermediate filament bundles. These structural defects in the heart and vasculature coincide with decreases in SRF-dependent gene expression, such that by e11.5, when mutant embryos succumb to death, no SRF-dependent mRNA expression is evident. These results suggest a vital role for SRF in contractile/cytoskeletal architecture necessary for the proper assembly and function of cardiomyocytes and vascular SMC.

Mutant actins that stabilise F-actin use distinct mechanisms to activate the SRF coactivator MAL

Nuclear accumulation of the serum response factor coactivator MAL/MKL1 is controlled by its interaction with G-actin, which results in its retention in the cytoplasm in cells with low Rho activity. We previously identified actin mutants whose expression promotes MAL nuclear accumulation via an unknown mechanism. Here, we show that actin interacts directly with MAL in vitro with high affinity. We identify a further activating mutation, G15S, which stabilises F-actin, as do the activating actins S14C and V159N. The three mutants share several biochemical properties, but can be distinguished by their ability to bind cofilin, ATP and MAL. MAL interaction with actin S14C is essentially undetectable, and that with actin V159N is weakened. In contrast, actin G15S interacts more strongly with MAL than the wild-type protein. Strikingly, the nuclear accumulation of MAL induced by overexpression of actin S14C is substantially dependent on Rho activity and actin treadmilling, while that induced by actin G15S expression is not. We propose a model in which actin G15S acts directly to promote MAL nuclear entry.

The MADS-box transcription factor SrfA is required for actin cytoskeleton organization and spore coat stability during Dictyostelium sporulation

The MADS-box transcription factor SrfA is involved in spore differentiation in Dictyostelium [Development 125 (1998) 3801]. Mutant spores show an altered morphology and loss of viability. A detailed structural analysis of mutant spores has been performed to gain insight into the specific aspects of spore differentiation in which SrfA is involved. Two main structural defects have been observed. One is the formation of high order actin structures, the so-called actin rods. SrfA mutant spores showed the initial stages of rod formation but no mature rods were found in older spores either in the nucleus or the cytoplasm. Moreover, phosphorylation of actin, that is believed to stabilize the actin rods, is strongly reduced in the mutant. The other defect observed was the formation of the spore coat. Young srfA- spores show basically normal trilaminar coat structures suggesting that release of prespore vesicles and basic assembly of the coat takes place in the absence of SrfA. However, the outer layer gets wavier as the spore ages and suffers a progressive degradation suggesting a late defect in the stability of the spore coat. Taken together, these results suggest that SrfA is involved in late events of spore maturation necessary for spore stability.

Evidence for tension-based regulation of Drosophila MAL and SRF during invasive cell migration

Cells migrating through a tissue exert force via their cytoskeleton and are themselves subject to tension, but the effects of physical forces on cell behavior in vivo are poorly understood. Border cell migration during Drosophila oogenesis is a useful model for invasive cell movement. We report that this migration requires the activity of the transcriptional factor serum response factor (SRF) and its cofactor MAL-D and present evidence that nuclear accumulation of MAL-D is induced by cell stretching. Border cells that cannot migrate lack nuclear MAL-D but can accumulate it if they are pulled by other migrating cells. Like mammalian MAL, MAL-D also responds to activated Diaphanous, which affects actin dynamics. MAL-D/SRF activity is required to build a robust actin cytoskeleton in the migrating cells; mutant cells break apart when initiating migration. Thus, tension-induced MAL-D activity may provide a feedback mechanism for enhancing cytoskeletal strength during invasive migration.

Expression profiling of serum inducible genes identifies a subset of SRF target genes that are MKL dependent

Background

Serum Response Factor (SRF) is a transcription factor that is required for the expression of many genes including immediate early genes, cytoskeletal genes, and muscle-specific genes. SRF is activated in response to extra-cellular signals by its association with a diverse set of co-activators in different cell types. In the case of the ubiquitously expressed immediate early genes, the two sets of SRF binding proteins that regulate its activity are the TCF family of proteins that include Elk1, SAP1 and SAP2 and the myocardin-related MKL family of proteins that include MKL1 and MKL2 (also known as MAL, MRTF-A and -B and BSAC). In response to serum or growth factors these two classes of co-activators are activated by different upstream signal transduction pathways. However, it is not clear how they differentially activate SRF target genes.

Results

In order to identify the serum-inducible SRF target genes that are specifically dependent on the MKL pathway, we have performed microarray experiments using a cell line that expresses dominant negative MKL1. This approach was used to identify SRF target genes whose activation is MKL-dependent. Twenty-eight of 150 serum-inducible genes were found to be MKL-dependent. The promoters of the serum-inducible genes were analyzed for SRF binding sites and other common regulatory elements. Putative SRF binding sites were found at a higher rate than in a mouse promoter database but were only identified in 12% of the serum-inducible promoters analyzed. Additional partial matches to the consensus SRF binding site were found at a higher than expected rate in the MKL-dependent gene promoters. The analysis for other common regulatory elements is discussed.

Conclusions

These results suggest that a subset of immediate early and SRF target genes are activated by the Rho-MKL pathway. MKL may also contribute to the induction of other SRF target genes however its role is not essential, possibly due to other activation mechanisms such as MAPK phosphorylation of TCFs.

A myocardin-related transcription factor regulates activity of serum response factor in Drosophila

Serum response factor (SRF) regulates genes involved in cell proliferation, migration, cytoskeletal organization, and myogenesis. Myocardin and myocardin-related transcription factors (MRTFs) act as powerful transcriptional coactivators of SRF in mammalian cells. We describe an MRTF from Drosophila, called DMRTF, which shares high homology with the functional domains of mammalian myocardin and MRTFs. DMRTF forms a ternary complex with and stimulates the activity of Drosophila SRF, which has been implicated in branching of the tracheal (respiratory) system and formation of wing interveins. A loss-of-function mutation introduced into the DMRTF locus by homologous recombination results in abnormalities in tracheal branching similar to those in embryos lacking SRF. Misexpression in wing imaginal discs of a dominant negative DMRTF mutant also causes a diminution of wing interveins, whereas overexpression of DMRTF results in excess intervein tissue, abnormalities reminiscent of SRF loss- and gain-of-function phenotypes, respectively. Overexpression of these DMRTF mutants in mesoderm and in the tracheal system also perturbs mesoderm cell migration and tracheal branching, respectively. We conclude that the interaction of MRTFs with SRF represents an ancient protein partnership involved in cytoplasmic outgrowth and cell migration during development.

Serum response factor is a critical requirement for VEGF signaling in endothelial cells and VEGF-induced angiogenesis

Angiogenesis, new capillary blood vessel formation, is essential for embryonic development, wound healing, and cancer growth. Vascular endothelial growth factor (VEGF) induces angiogenesis by activating endothelial cell migration and proliferation. Serum response factor (SRF) is a transcription factor important for embryonic development and activation of immediate early gene expression. The roles of SRF in endothelial cell biology and angiogenesis have not been explored. Here we demonstrate that SRF is a downstream mediator of VEGF signaling in endothelial cells and a critical requirement for VEGF-induced angiogenesis. Knockdown of SRF protein levels in human and rat endothelial cells abolished VEGF-induced in vitro angiogenesis, impaired endothelial cell migration and proliferation, and inhibited VEGF-induced actin polymerization and immediate early gene expression. Injection of SRF antisense expression plasmid into gastric ulcers in rats significantly inhibited in vivo angiogenesis in granulation tissue. Mechanistically, this study also revealed that VEGF promotes SRF expression and nuclear translocation and increases SRF binding activity to DNA in endothelial cells through both Rho-actin and MEK-ERK dependent signaling pathways. These findings have potential therapeutic implications, e.g., local anti-SRF treatment may inhibit angiogenesis crucial for tumor growth.

Elastin induces myofibrillogenesis via a specific domain, VGVAPG

A hallmark of vascular smooth muscle cells (VSMCs) is their dynamic ability to assemble and disassemble contractile proteins into sarcomeric units depending upon their phenotypic state. This phenotypic plasticity plays an important role during vascular development and in obstructive vascular disease. Previously, we showed that the Elastin gene product, tropoelastin, activates myofibrillar organization of VSMCs. Recently, others have suggested that elastin does not have a direct signaling role but rather binds to and alters the interactions of other matrix proteins with their cognate receptors or disrupts the binding of growth factors and cytokines. In contrast, we provide evidence that tropoelastin directly regulates contractile organization of VSMCs. First, we show that a discrete domain within tropoelastin, VGVAPG, induces myofibrillogenesis in a time- and dose-dependent fashion. We confirm specificity using a closely related control peptide that fails to stimulate actin stress fiber formation. Second, the activity of VGVAPG is not affected by the presence or absence of other serum or matrix components. Third, both the elastin hexapeptide and tropoelastin stimulate actin polymerization through a common pertussis toxin-sensitive G protein pathway that activates RhoA-GTPase and results in the conversion of G to F actin. Collectively, these data support a model whereby the elastin gene product, signaling through the VGVAPG domain, directly induces VSMC myofibrillogenesis.

Targeted inactivation of serum response factor in the developing heart results in myocardial defects and embryonic lethality

Serum response factor (SRF) is at the confluence of multiple signaling pathways controlling the transcription of immediate-early response genes and muscle-specific genes. There are active SRF target sequences in more than 50 genes expressed in the three muscle lineages including normal and diseased hearts. However, the role of SRF in heart formation has not been addressed in vivo thus far due to the early requirement of SRF for mesoderm formation. We have generated a conditional mutant of SRF by using Cre-LoxP strategy that will be extremely useful to study the role of SRF in embryonic and postnatal cardiac functions, as well as in other tissues. This report shows that heart-specific deletion of SRF in the embryo by using a new beta MHC-Cre transgenic mouse line results in lethal cardiac defects between embryonic day 10.5 (E10.5) and E13.5, as evidenced by abnormally thin myocardium, dilated cardiac chambers, poor trabeculation, and a disorganized interventricular septum. At E9.5, we found a marked reduction in the expression of essential regulators of heart development, including Nkx2.5, GATA4, myocardin, and the SRF target gene c-fos prior to overt maldevelopment. We conclude that SRF is crucial for cardiac differentiation and maturation, acting as a global regulator of multiple developmental genes.

SRF regulates Bcl-2 expression and promotes cell survival during murine embryonic development

The transcription factor serum response factor (SRF) controls the expression of genes involved in cellular proliferation and differentiation. Interestingly, SRF also promotes cell survival by regulating the expression of antiapoptotic genes. In in vitro differentiating murine embryonic stem (ES) cells, SRF deficiency leads to increased apoptosis. Loss of SRF correlates with impaired expression of the antiapoptotic Bcl-2 and Bcl-xl genes. SRF binds the Bcl-2 promoter in vivo and activates Bcl-2 transcription. Reconstituting Bcl-2 in Srf-/- ES cells rescues these cells from apoptosis, demonstrating that SRF-dependent Bcl-2 expression is critical for ES cell survival. At the multicellular level, SRF deficiency leads to impaired cavitation and reduced Bcl-2 expression in embryoid bodies (EBs) and inappropriate apoptosis in both EBs and pregastrulation mouse embryos. Thus, our data from genetic and cellular studies uncover SRF-regulated Bcl-2 expression as a novel mechanism that is important for cell survival during early murine embryogenesis.

Identification of FHOD1-binding proteins and mechanisms of FHOD1-regulated actin dynamics

Formin homology-2-domain containing protein 1 (FHOD1) regulates gene transcription, actin-cytoskeleton structure, and cell migration. To gain insight into the mechanisms by which FHOD1 mediates these diverse activities, a yeast-two-hybrid screen was performed to identify FHOD1-binding proteins. Three proteins specifically interacted with the carboxy-terminal two-thirds of FHOD1, which includes the FH1, FH2, and diaphanous activating domains (DAD). The newly identified FHOD1-binding proteins are protein kinase C binding protein 1 (PRKCBP1), cyclophilin B, and an isoform of WASP-interacting SH3-domain protein/diaphanous-interacting protein 1 (WISH/DIP1), named WISH-B. The proline-rich FH1 domain of FHOD1 was sufficient to interact with the central portion of PRKCP1 and full-length cyclophilin B. The FH1 domain also interacted with full-length WISH-B, but the extreme amino-terminus was sufficient to associate with WISH-B as well. WISH-B altered the solubility of FHOD1 in vitro and a truncation mutant containing the amino-terminal 227 residues of WISH-B disrupted FHOD1-induced stress fibers. WISH-B did not affect FHOD1-induced gene transcription through the serum response factor (SRF) recognition site on the skeletal alpha actin promoter (SkA). However, stabilization of F-actin prevented FHOD1 dependent activation of this promoter in presence of high, but not low serum concentrations. Thus, the identification of a new FHOD1-binding protein provides insight into the mechanisms by which FHOD1 regulates actin polymerization and transcription.

Serum response factor function and dysfunction in smooth muscle

Tight control of smooth muscle cell (SM) proliferation, differentiation, and apoptosis requires a balance between signaling and transcriptional events. Recent developments in vascular research revealed that serum response factor (SRF) function is important for the regulation of each of these processes. The cloning and characterization of several SM specific genes and the discovery that SRF is central for their expression fueled studies aimed at understanding the role of molecular partners including co-activators and co-repressors. Perturbations of pathways involving SRF are associated with abnormalities in the myogenic program and aberrant phenotypic consequences. Surprisingly, studies on airway SM have remained an underrepresented area of investigation. Our laboratory described a novel regulatory mechanism of SRF function in airway myocytes by modulation of its subcellular localization. This review summarizes current knowledge on the structure and function of this essential transcription factor as well different modes of regulating SRF expression and activity that are becoming key players in directing SM function in health and disease.

A role for VASP in RhoA-Diaphanous signalling to actin dynamics and SRF activity

Vasodilator-stimulated phosphoprotein (VASP) is involved in multiple actin-mediated processes, including regulation of serum response factor (SRF) activity. We used the SRF transcriptional assay to define functional domains in VASP and to show that they coincide with those required for F-actin accumulation, as determined by a quantitative FACS assay. We identified inactive VASP mutants that can interfere both with F-actin assembly and with SRF activation by wild-type VASP. These VASP mutants also inhibit actin-based motility of Vaccinia virus and Shigella flexneri. VASP-induced F-actin accumulation and SRF activation require both functional Rho and its effector mDia, and conversely, mDia-mediated SRF activation is critically dependent on functional VASP. VASP and mDia also associate physically in vivo. These findings show that VASP and mDia function cooperatively downstream of Rho to control F-actin assembly and SRF activity.

Actin dynamics control SRF activity by regulation of its coactivator MAL

Rho GTPases regulate the transcription factor SRF via their ability to induce actin polymerization. SRF activity responds to G actin, but the mechanism of this has remained unclear. We show that Rho-actin signaling regulates the subcellular localization of the myocardin-related SRF coactivator MAL, rearranged in t(1;22)(p13;q13) AML. The MAL-SRF interaction displays the predicted properties of a Rho-regulated SRF cofactor. MAL is predominantly cytoplasmic in serum-starved cells, but accumulates in the nucleus following serum stimulation. Activation of the Rho-actin signaling pathway is necessary and sufficient to promote MAL nuclear accumulation. MAL N-terminal sequences, including two RPEL motifs, are required for the response to signaling, while other regions mediate its nuclear export (or cytoplasmic retention) and nuclear import. MAL associates with unpolymerized actin through its RPEL motifs. Constitutively cytoplasmic MAL derivatives interfere with MAL redistribution and Rho-actin signaling to SRF. MAL associates with several SRF target promoters regulated via the Rho-actin pathway.

Serum response factor plays an important role in the mechanically overloaded plantaris muscle of rats

Molecular signaling pathways linking the hypertrophy after mechanical overloading in vivo have not been identified. Using western blot analysis, immunoprecipitation, and immunohistochemistry, we investigated the effect of the mechanical overloading state on RhoA, serum response factor (SRF), and MyoD in the rat plantaris muscle. Adult male rats (10 weeks of age) were used in this experiment. Compensatory enlargement of the plantaris muscle was induced in one leg of each rat by surgical removal of the ipsilateral soleus and gastrocnemius muscles. In the normal plantaris muscle of rats, slight expression of RhoA and SRF was observed in the quiescent satellite cells possessing CD34 and c-Met. Western blotting using the homogenate of whole muscle clearly showed that mechanical overloading of the plantaris muscle significantly increased the amount of RhoA during 3-6 days postsurgery. Threonine phosphorylation of SRF occurred at 2-4 h after mechanical overloading. The most marked increase in SRF protein was observed in the hypertrophied muscle at 6 days postsurgery. At 2 days postoperation, SRF immunoreactivity was not detected in the proliferating satellite cells possessing bromodeoxyuridine and in the infiltrating macrophages expressing ED1 in the overloaded muscle by surgical removal. The SRF protein was colocalized with RhoA, FAK, and myogenin but not Myf-5 in many mononuclear cells at 6 days of functional overload. At this time, MyoD immunoreactivity was detected in the cytoplasm of mononuclear cells (possibly satellite cell-derived myoblasts) possessing SRF protein at the nucleus. These results suggest that the signaling pathway through RhoA-FAK-SRF is important to the differentiation of satellite cells by interacting MyoD and myogenin in the hypertrophied muscle of rats.

Mutant actins demonstrate a role for unpolymerized actin in control of transcription by serum response factor

Signal-induced activation of the transcription factor serum response factor (SRF) requires alterations in actin dynamics. SRF activity can be inhibited by ectopic expression of beta-actin, either because actin itself participates in SRF regulation or as a consequence of cytoskeletal perturbations. To distinguish between these possibilities, we studied actin mutants. Three mutant actins, G13R, R62D, and a C-terminal VP16 fusion protein, were shown not to polymerize in vivo, as judged by two-hybrid, immunofluorescence, and cell fractionation studies. These actins effectively inhibited SRF activation, as did wild-type actin, which increased the G-actin level without altering the F:G-actin ratio. Physical interaction between SRF and actin was not detectable by mammalian or yeast two-hybrid assays, suggesting that SRF regulation involves an unidentified cofactor. SRF activity was not blocked upon inhibition of CRM1-mediated nuclear export by leptomycin B. Two actin mutants were identified, V159N and S14C, whose expression favored F-actin formation and which strongly activated SRF in the absence of external signals. These mutants seemed unable to inhibit SRF activity, because their expression did not reduce the absolute level of G-actin as assessed by DNase I binding. Taken together, these results provide strong evidence that G-actin, or a subpopulation of it, plays a direct role in signal transduction to SRF.