Myocardin/MKL family of SRF coactivators: key regulators of immediate early and muscle specific gene expression

The actin-associated protein Palladin is required for development of normal contractile properties of smooth muscle cells derived from embryoid bodies

Palladin is a widely expressed actin-associated protein localized at stress fibers, focal adhesions, and other actin-based structures, playing a significant role in cell adhesion and cell motility. Knockout of Palladin in mice is embryonic lethal, demonstrating the importance of Palladin in development yet its role in the vasculature is not known. In the present study, smooth muscle cell (SMC) markers, such as myosin, actin, caldesmon, calponin, and LPP, were down-regulated in embryoid bodies (EBs) derived from embryonic stem cells lacking Palladin. Transgenic embryonic stem cell lines were generated that stably expressed a puromycin-resistance gene under the control of a SM alpha-actin (SMA) promoter. Negative selection was then used to purify SMCs from EBs. Purified SMCs expressing multiple SMC markers were designated APSCs (SMA-puromycin-selected cells). Palladin null APSCs express significantly less myosin, actin, calponin, and h-caldesmon. The filamentous (F) to globular (G) actin ratio, known to regulate myocardin family transcription factors, was also decreased. Palladin null APSCs showed increased cell adhesion and decreased cell motility. Importantly, Palladin null APSCs within collagen gels generated less maximum contractile force when stimulated with endothelin-1, sphingosine 1-phosphate (S1P), and thrombin. Myosin light chains (MLC20) were phosphorylated by lysophosphatidic acid to the same extent in Palladin null and wild type APSCs but myosin content/total protein was reduced by >50%, consistent with the observed decreases in contractility. All together, these results suggest that Palladin is essential for expression of the full complement of contractile proteins necessary for optimal force development of SMCs derived from EBs.

A rare human sequence variant reveals myocardin autoinhibition

Myocardin (MYOCD) is a transcriptional co-activator that promotes cardiac or smooth muscle gene programs through its interaction with myocyte-enhancing factor (MEF2) or serum-response factor (SRF). Isoforms of MYOCD with a truncated amino terminus show increased activity when compared with those with the full-length amino terminus, but how this is achieved remains unknown. We identified a rare human sequence variation in MYOCD in a patient with congenital heart disease that resulted in a missense mutation at codon 259 (K259R). This variation created a hypomorphic cardiac isoform with impaired SRF binding and transactivation capacity but did not impair the smooth muscle isoform of MYOCD, which lacks the amino terminus. Consistent with differential effects of the amino terminus on the K259R mutation, we found that the cardiac-specific amino terminus acted in an autoinhibitory fashion to bind MYOCD via specific negatively charged residues and thereby repressed SRF-dependent MYOCD activity. This effect was exaggerated in the MYOCD-K259R mutant. The amino terminus was sufficient to impair MYOCD-dependent fibroblast conversion into smooth muscle cells as well as cardiomyocyte hypertrophy. These findings identify a novel mechanism that regulates levels of MYOCD-dependent activation of the SRF genetic program differentially in cardiac and smooth muscle.

Serum-induced phosphorylation of the serum response factor coactivator MKL1 by the extracellular signal-regulated kinase 1/2 pathway inhibits its nuclear localization

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related coactivator of the serum response factor (SRF) transcription factor, which has an integral role in differentiation, migration, and proliferation. Serum induces RhoA-dependent translocation of MKL1 from the cytoplasm to the nucleus and also causes a rapid increase in MKL1 phosphorylation. We have mapped a serum-inducible phosphorylation site and found, surprisingly, that its mutation causes constitutive localization to the nucleus, suggesting that phosphorylation of MKL1 inhibits its serum-induced nuclear localization. The key site, serine 454, resembles a mitogen-activated protein kinase phosphorylation site, and its modification was blocked by the MEK1 inhibitor U0126, implying that extracellular signal-regulated kinase 1/2 (ERK1/2) is the serum-inducible kinase that phosphorylates MKL1. Previous results indicated that G-actin binding to MKL1 promotes its nuclear export, and we found that MKL1 phosphorylation is required for its binding to actin, explaining its effect on localization. We propose a model in which serum induction initially stimulates MKL1 nuclear localization due to a decrease in G-actin levels, but MKL1 is then downregulated by nuclear export due to ERK1/2 phosphorylation.

The molecular and functional phenotype of glomerular podocytes reveals key features of contractile smooth muscle cells

The glomerular podocyte is a highly specialized cell, with the ability to ultrafilter blood and support glomerular capillary pressures. However, little is known about either the genetic programs leading to this functionality or the final phenotype. We approached this question utilizing a human conditionally immortalized cell line, which differentiates from a proliferating epithelial phenotype to a differentiated form. We profiled gene expression during several time points during differentiation and grouped the regulated genes into major functional categories. A novel category of genes that was upregulated during differentiation was of smooth muscle-related molecules. We further examined the smooth muscle phenotype and showed that podocytes consistently express the differentiated smooth muscle markers smoothelin and calponin and the specific transcription factor myocardin, both in vitro and in vivo. The contractile contribution of the podocyte to the glomerular capillary is controversial. We demonstrated using two novel techniques that podocytes contract vigorously in vitro when differentiated and in real time were able to demonstrate that angiotensin II treatment decreases monolayer resistance, morphologically correlating with enhanced contractility. We conclude that the mature podocyte in vitro possesses functional apparatus of contractile smooth muscle cells, with potential implications for its in vivo ability to regulate glomerular dynamic and permeability characteristics.

Rho/Rho-associated kinase signal regulates myogenic differentiation via myocardin-related transcription factor-A/Smad-dependent transcription of the Id3 gene

RhoA is known to be involved in myogenic differentiation, but whether it acts as a positive or negative regulator is controversial. To resolve this issue, we investigated the differentiation stage-specific roles of RhoA and its effector, Rho-associated kinase, using C2C12 myoblasts. We found that proliferating myoblasts show high levels of RhoA and serum-response factor activities and strong expression of the downstream target of RhoA, myocardin-related transcription factor-A (MRTF-A or MAL); these activities and expression are markedly lower in differentiating myocytes. We further demonstrated that, in proliferating myoblasts, an increase in MRTF-A, which forms a complex with Smad1/4, strikingly activates the expression level of the Id3 gene; the Id3 gene product is a potent inhibitor of myogenic differentiation. Finally, we found that during differentiation, one of the forkhead transcription factors translocates into the nucleus and suppresses Id3 expression by preventing the association of the MRTF-A-Smad complex with the Id3 promoter, which leads to the enhancement of myogenic differentiation. We conclude that RhoA/Rho-associated kinase signaling plays positive and negative roles in myogenic differentiation, mediated by MRTF-A/Smad-dependent transcription of the Id3 gene in a differentiation stage-specific manner.

Comparative in silico analysis identifies bona fide MyoD binding sites within the Myocyte stress 1 gene promoter

BACKGROUND

Myocyte stress 1 (MS1) is a striated muscle actin binding protein required for the muscle specific activity of the evolutionary ancient myocardin related transcription factor (MRTF)/serum response factor (SRF) transcriptional pathway. To date, little is known about the molecular mechanisms that govern skeletal muscle specific expression of MS1. Such mechanisms are likely to play a major role in modulating SRF activity and therefore muscle determination, differentiation and regeneration. In this study we employed a comparative in silico analysis coupled with an experimental promoter characterisation to delineate these mechanisms.

RESULTS

Analysis of MS1 expression in differentiating C2C12 muscle cells demonstrated a temporal differentiation dependent up-regulation in ms1 mRNA. An in silico comparative sequence analysis identified two conserved putative myogenic regulatory domains within the proximal 1.5 kbp of 5' upstream sequence. Co-transfecting C2C12 myoblasts with ms1 promoter/luciferase reporters and myogenic regulatory factor (MRF) over-expression plasmids revealed specific sensitivity of the ms1 promoter to MyoD. Subsequent mutagenesis and EMSA analysis demonstrated specific targeting of MyoD at two distinct E-Boxes (E1 and E2) within identified evolutionary conserved regions (ECRs, alpha and beta). Chromatin immunoprecipitation (ChIP) analysis indicates that co-ordinated binding of MyoD at E-Boxes located within ECRs alpha and beta correlates with the temporal induction in ms1 mRNA.

CONCLUSION

These findings suggest that the tissue specific and differentiation dependent up-regulation in ms1 mRNA is mediated by temporal binding of MyoD at distinct evolutionary conserved E-Boxes within the ms1 5' upstream sequence. We believe, through its activation of ms1, this is the first study to demonstrate a direct link between MyoD activity and SRF transcriptional signalling, with clear implications for the understanding of muscle determination, differentiation and regeneration.

Myocardin gene regulatory variants as surrogate markers of cardiac hypertrophy - study in a genetically homogeneous population

Myocardin is thought to contribute to heart hypertrophy as assessed in animal models. The aim of this study was to identify polymorphisms on the myocardin gene and investigate possible relationships with left ventricular structure in human hypertrophic cardiomyopathy (HCM). Eighty-four native Cretan individuals (36 patients with HCM and 48 healthy controls) were examined by direct sequencing and subsequent restriction fragment length polymorphism analysis and six polymorphisms were identified in the promoter region at positions -435T>C (rs758187), -629A>T (rs8071072), -1030C>G (rs1233851), -1069A>G, -1166A>G and -1406G>A (rs976906). Allele and haplotype frequencies were not significantly different between patients and controls. However, patients carrying the [-435C;-629T] allelic variant had decreased left ventricular wall thickness (LVWT, p = 0.020) and left ventricular mass (p = 0.006) as compared with the wild-type genotype. Carrier status of this myocardin promoter allelic variant was also associated with significant lower myocardin mRNA levels in peripheral blood (p = 0.039). Thus, a myocardin promoter allelic variant existing in the normal Cretan population was associated with decreased left ventricular mass in HCM patients and decreased myocardin mRNA levels in peripheral blood. Our results may be limited by the limited sample size, but are strengthened by the genetic homogeneity of the Cretan population. Our data suggest that functional natural myocardin promoter variation might be a genetic factor contributing to inter-individual differences in the development of cardiac hypertrophy.

A novel role of Brg1 in the regulation of SRF/MRTFA-dependent smooth muscle-specific gene expression

Serum response factor (SRF) is a key regulator of smooth muscle differentiation, proliferation, and migration. Myocardin-related transcription factor A (MRTFA) is a co-activator of SRF that can induce expression of SRF-dependent, smooth muscle-specific genes and actin/Rho-dependent genes, but not MAPK-regulated growth response genes. How MRTFA and SRF discriminate between these sets of target genes is still unclear. We hypothesized that SWI/SNF ATP-dependent chromatin remodeling complexes, containing Brahma-related gene 1 (Brg1) or Brahma (Brm), may play a role in this process. Results from Western blotting and qRT-PCR analysis demonstrated that dominant negative Brg1 blocked the ability of MRTFA to induce expression of smooth muscle-specific genes, but not actin/Rho-dependent early response genes, in fibroblasts. In addition, dominant negative Brg1 attenuated expression of smooth muscle-specific genes in primary cultures of smooth muscle cells. MRTFA overexpression did not induce expression of smooth muscle-specific genes in SW13 cells, which lack endogenous Brg1 or Brm. Reintroduction of Brg1 or Brm into SW13 cells restored their responsiveness to MRTFA. Immunoprecipitation assays revealed that Brg1, SRF, and MRTFA form a complex in vivo, and Brg1 directly binds MRTFA, but not SRF, in vitro. Results from chromatin immunoprecipitation assays demonstrated that dominant negative Brg1 significantly attenuated the ability of MRTFA to increase SRF binding to the promoters of smooth muscle-specific genes, but not early response genes. Together these data suggest that Brg1/Brm containing SWI/SNF complexes play a critical role in regulating expression of SRF/MRTFA-dependent smooth muscle-specific genes but not SRF/MRTFA-dependent early response genes.

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.

Human T-cell leukemia virus type 1 Tax enhances serum response factor DNA binding and alters site selection

Human T-cell leukemia virus type I (HTLV-1) is the etiological agent of adult T-cell leukemia. The viral transforming protein Tax regulates the transcription of viral and cellular genes by interacting with cellular transcription factors and coactivators. The effects of Tax on cellular gene expression have an important impact on HTLV-1-mediated cellular transformation. Expression of the c-fos cellular oncogene is regulated by serum response factor (SRF), and Tax is known to induce c-fos gene expression by activating SRF-responsive transcription. SRF activates cellular gene expression by binding to a consensus DNA sequence (CArG box) located within a serum response element (SRE). Since SRF activates transcription of many growth regulatory genes, this pathway is likely to have a significant impact on Tax-mediated transformation. Here we demonstrate that Tax interacts with SRF and enhances the binding of SRF to SREs located in the c-fos, Nur77, and viral promoters. Also, we establish that in the presence of Tax, SRF selects more divergent CArG box sequences than in the absence of Tax, revealing a novel mechanism for regulating SRF-responsive gene expression. Finally, increased association of SRF with chromatin and specific promoters was observed in Tax-expressing cells, correlating with increased c-fos and Nur77 mRNA levels in Tax-expressing cells. These results suggest that Tax activates SRF-responsive transcription by enhancing its binding affinity to multiple different SRE sequences.

Cell contact-dependent regulation of epithelial-myofibroblast transition via the rho-rho kinase-phospho-myosin pathway

Epithelial-mesenchymal-myofibroblast transition (EMT), a key feature in organ fibrosis, is regulated by the state of intercellular contacts. Our recent studies have shown that an initial injury of cell-cell junctions is a prerequisite for transforming growth factor-beta1 (TGF-beta1)-induced transdifferentiation of kidney tubular cells into alpha-smooth muscle actin (SMA)-expressing myofibroblasts. Here we analyzed the underlying contact-dependent mechanisms. Ca(2+) removal-induced disruption of intercellular junctions provoked Rho/Rho kinase (ROK)-mediated myosin light chain (MLC) phosphorylation and Rho/ROK-dependent SMA promoter activation. Importantly, myosin-based contractility itself played a causal role, because the myosin ATPase inhibitor blebbistatin or a nonphosphorylatable, dominant negative MLC (DN-MLC) abolished the contact disruption-triggered SMA promoter activation, eliminated the synergy between contact injury and TGF-beta1, and suppressed SMA expression. To explore the responsible mechanisms, we investigated the localization of the main SMA-inducing transcription factors, serum response factor (SRF), and its coactivator myocardin-related transcription factor (MRTF). Contact injury enhanced nuclear accumulation of SRF and MRTF. These processes were inhibited by DN-Rho or DN-MLC. TGF-beta1 strongly facilitated nuclear accumulation of MRTF in cells with reduced contacts but not in intact epithelia. DN-myocardin abrogated the Ca(2+)-removal- +/- TGF-beta1-induced promoter activation. These studies define a new mechanism whereby cell contacts regulate epithelial-myofibroblast transition via Rho-ROK-phospho-MLC-dependent nuclear accumulation of MRTF.

Serum response factor binding sites differ in three human cell types

The serum response factor (SRF) is essential for embryonic development and maintenance of muscle cells and neurons. The mechanism by which this factor controls these divergent pathways is unclear. Here we present a genome-wide view of occupancy of SRF at its binding sites with a focus on those that vary with cell type. We used chromatin immunoprecipitation (ChIP) in combination with human promoter microarrays to identify 216 putative SRF binding sites in the human genome. We performed independent quantitative PCR validation at over half of these sites that resulted in 146 sites we assert to be true binding sites at over 90% confidence. Nearly half of the sites are bound by SRF in only one of the three cell types we tested, providing strong evidence for the diverse roles for SRF in different cell types. We also explore possible mechanisms controlling differential binding of SRF in these cell types by assaying cofactor binding, DNA methylation, histone methylation, and histone acetylation at a subset of sites bound preferentially in smooth muscle cells. Although we did not see a strong correlation between SRF binding and epigenetics modifications, at these sites, we propose that SRF cofactors may play an important role in determining cell-dependent SRF binding sites. ELK4 (previously known as SAP-1 [SRF-associated protein-1]) is ubiquitously expressed. Therefore, we expected it to occupy sites where SRF binding is common in all cell types. Indeed, 90% of SRF sites also bound by ELK4 were common to all three cell types. Together, our data provide a more complete understanding of the regulatory network controlled by SRF.

Role of megakaryoblastic acute leukemia-1 in ERK1/2-dependent stimulation of serum response factor-driven transcription by BDNF or increased synaptic activity

Serum response factor (SRF)-mediated transcription contributes to developmental and adult brain plasticity. Therefore, we investigated the role of a newly identified SRF coactivator, MKL1, in the regulation of SRF-driven transcription in rat forebrain neurons. MKL1 expression was found in newborn rat cortical or hippocampal neurons in culture as well as in adult rat forebrain. Immunostaining demonstrated constitutive nuclear localization of MKL1 in the CA1 region of the hippocampus, in the deep layers of the neocortex, and in cultured neurons. Overexpression of MKL1 in primary cortical neurons elevated SRF-driven transcription and enhanced its stimulation by BDNF. In addition, inhibition of endogenous MKL1 by overexpression of a dominant-negative MKL1 mutant or by small interfering RNA reduced BDNF activation of SRF-driven transcription. In neurons, endogenous MKL1 was associated with SRF-regulated chromatin regions of several endogenous genes including c-fos, JunB, Srf, and Cyr61. BDNF activation of MKL1/SRF-driven transcription was dependent on the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway, which also led to MKL1 phosphorylation. Finally, synaptic activity stimulation of SRF-driven transcription was reduced by inhibition of endogenous MKL1. Conversely, synaptic activity enhanced transcription by overexpressed MKL1. MKL1 regulation by synaptic activity was mediated through the NMDA receptor-activated ERK1/2. These results suggest that neuronal MKL1 contributes to SRF-regulated gene expression induced by BDNF or synaptic activity. In addition, MKL1 appears as a novel mediator of the signaling between ERK1/2 and SRF. Moreover, MKL1 is a likely regulator of SRF-driven transcription programs that underlie neuronal plasticity.

Requirement of a myocardin-related transcription factor for development of mammary myoepithelial cells

The mammary gland consists of a branched ductal system comprised of milk-producing epithelial cells that form ductile tubules surrounded by a myoepithelial cell layer that provides contractility required for milk ejection. Myoepithelial cells bear a striking resemblance to smooth muscle cells, but they are derived from a different embryonic cell lineage, and little is known of the mechanisms that control their differentiation. Members of the myocardin family of transcriptional coactivators cooperate with serum response factor to activate smooth muscle gene expression. We show that female mice homozygous for a loss-of-function mutation of the myocardin-related transcription factor A (MRTF-A) gene are unable to effectively nurse their offspring due to a failure in maintenance of the differentiated state of mammary myoepithelial cells during lactation, resulting in apoptosis of this cell population, a consequent inability to release milk, and premature involution. The phenotype of MRTF-A mutant mice reveals a specific and essential role for MRTF-A in mammary myoepithelial cell differentiation and points to commonalities in the transcriptional mechanisms that control differentiation of smooth muscle and myoepithelial cells.

PGC-1-related coactivator: immediate early expression and characterization of a CREB/NRF-1 binding domain associated with cytochrome c promoter occupancy and respiratory growth

PGC-1-related coactivator (PRC) was initially characterized as a transcriptional coactivator that shares structural and functional features with PGC-1alpha. Both coactivators interact with nuclear respiratory factor 1 (NRF-1) and activate NRF-1 target genes required for respiratory chain expression. Here, we establish that PRC belongs to the class of immediate early genes that are rapidly induced in the transition from quiescence to proliferative growth. As observed for other members of this class, the rapid serum induction of PRC mRNA does not require de novo protein synthesis and inhibition of protein synthesis stabilizes PRC mRNA, leading to its superinduction. Previous work indicated that PRC activation of cytochrome c expression occurs through cis-acting elements that bind both NRF-1 and CREB. Here, we demonstrate that, like NRF-1, CREB binds PRC in vitro and exists in a complex with PRC in cell extracts. Both CREB and NRF-1 bind the same sites on PRC, and the interaction with CREB requires the CREB b-Zip DNA binding domain. Moreover, a CREB/NRF-1 interaction domain on PRC is required for its trans activation of the cytochrome c promoter and a PRC subfragment containing this domain inhibits respiratory growth on galactose when expressed in trans from a lentivirus vector. Finally, PRC associates with the cytochrome c promoter in vivo and its occupancy of the promoter is markedly elevated in response to serum induction of quiescent fibroblasts. The results establish that PRC is an immediate early gene product that can target key transcription factors as an early event in the program of cellular proliferation.

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.

Developmental expression of the SRF co-activator MAL in brain: role in regulating dendritic morphology

The dynamic changes in dendritic morphology displayed by developing and mature neurons have stimulated interest in deciphering the signaling pathways involved. Recent studies have identified megakaryocytic acute leukemia (MAL), a serum response factor (SRF) co-activator, as a key component of a signaling pathway linking changes in the actin cytoskeleton to SRF-mediated transcription. To help define the role of this pathway in regulating dendritic morphology, we have characterized the pattern of MAL expression in the developing and adult brain, and have examined its role in regulating dendritic morphology in cultured cortical neurons. In histological studies of mouse brain, we found prominent expression of MAL in neurons in adult hippocampus and cerebral cortex. MAL immunostaining revealed localization of this protein in neuronal cell bodies and apical dendrites. During development, an increase in MAL expression occurs during the second post-natal week. Expression of dominant negative MAL constructs or MAL siRNA in cortical neurons grown in primary culture reduces the number of dendritic processes and decreases the basal level of SRF-mediated transcription. Taken together, these findings indicate that the MAL-SRF signaling pathway plays a key role in regulating dendritic morphology.

Mechanical stretch modulates the promoter activity of the profibrotic factor CCN2 through increased actin polymerization and NF-kappaB activation

The connective tissue growth factor known as CCN2 is an inducible, profibrotic molecule that becomes aberrantly expressed in mechanical overload-bearing tissues. In this study, we found that CCN2 gene expression is rapidly induced in cyclically stretched bladder smooth muscle cells (SMCs) in vitro and in the detrusor muscle of a mechanically overloaded bladder in a rat model of experimental urethral obstruction. The activity of CCN2 promoter constructs, transiently transfected into cultured SMCs, was increased (up to 6-fold) by continuous cyclic stretching. Molecular analyses of the CCN2 promoter by serial construct deletions, cis-element mutagenesis, and electrophoretic mobility shift assays revealed that a highly conserved NF-kappaB binding site located within the CCN2 proximal promoter region is responsible for the activation of the promoter by stretch. Chromatin immunoprecipitation assays showed that NF-kappaB binds to the endogenous CCN2 promoter in both stretched cells and mechanically overloaded bladder tissues. Furthermore, stretch-dependent CCN2 promoter activity was significantly reduced upon inhibition of either phosphatidylinositol 3-kinase, p38 stress-activated kinase, or RhoA GTPase and was completely abolished upon inhibition of actin polymerization. Concordantly, actin polymerization was increased in either mechanically stretched cells or overloaded bladder tissues. Incubation of cultured SMCs with a cell-penetrating peptide containing the N-terminal sequence, Ac-EEED, of smooth muscle alpha-actin, altered both actin cytoskeleton organization and stretch-mediated nuclear relocation of NF-kappaB, and subsequently, it reduced CCN2 promoter activity. Thus, mechanical stretch-induced changes in actin dynamics mediate NF-kappaB activation and induce CCN2 gene expression, which probably initiates the fibrotic reactions observed in mechanical overload-associated pathologies.

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.

Requirement of myocardin-related transcription factor-B for remodeling of branchial arch arteries and smooth muscle differentiation

Myocardin and the myocardin-related transcription factors (MRTFs) A and B act as coactivators for serum response factor, which plays a key role in cardiovascular development. To determine the functions of MRTF-B in vivo, we generated MRTF-B mutant mice by targeted inactivation of the MRTF-B gene. We show that mice homozygous for an MRTF-B loss-of-function mutation die during mid-gestation from a spectrum of cardiovascular defects that includes abnormal patterning of the branchial arch arteries, double-outlet right ventricle, ventricular septal defects, and thin-walled myocardium. These abnormalities are accompanied by a failure in differentiation of smooth muscle cells within the branchial arch arteries, which are derived from the neural crest. The phenotype of MRTF-B mutant mice is distinct from that of mice lacking myocardin, revealing unique roles for these serum response factor coactivators in the development of different subsets of smooth muscle cells in vivo.

Nuclear translocation of the SRF co-activator MAL in cortical neurons: role of RhoA signalling

Although it is well established that RhoA signaling pathways play key roles in regulating neuronal morphology, their involvement in other aspects of neuronal function has received little attention. Recent studies have elucidated a novel intracellular signaling pathway used by RhoA to elicit activation of serum response factor (SRF)-mediated transcription. In this pathway, activation of RhoA triggers nuclear translocation of the SRF co-activator, megakaryocytic acute leukemia (MAL). In assessing whether RhoA regulates transcription in neurons via this pathway, we have found that a constitutively active form of Tech (transcript-enriched in cortex and hippocampus), a RhoA guanine nucleotide exchange factor (GEF) that is expressed in forebrain neurons, stimulates SRF reporter activity in extracts of primary cortical cultures and induces nuclear translocation of MAL in cortical neurons. Both of these responses appear to be mediated by Tech's activation of RhoA as they are not mimicked by a mutant Tech construct lacking RhoA GEF activity and are blocked by C3 transferase, a selective inhibitor of RhoA. Furthermore, Tech-induced increases in SRF activity are suppressed by a dominant negative MAL construct. These findings demonstrate that RhoA signaling pathways are able to regulate transcription in neurons by triggering translocation of the SRF co-activator MAL.

Muscle-specific signaling mechanism that links actin dynamics to serum response factor

Myocardin and the myocardin-related transcription factors (MRTFs) MRTF-A and MRTF-B are coactivators for serum response factor (SRF), which regulates genes involved in cell proliferation, migration, cytoskeletal dynamics, and myogenesis. MRTF-A has been shown to translocate to the nucleus and activate SRF in response to Rho signaling and actin polymerization. Previously, we described a muscle-specific actin-binding protein named striated muscle activator of Rho signaling (STARS) that also activates SRF through a Rho-dependent mechanism. Here we show that STARS activates SRF by inducing the nuclear translocation of MRTFs. The STARS-dependent nuclear import of MRTFs requires RhoA and actin polymerization, and the actin-binding domain of STARS is necessary and sufficient for this activity. A knockdown of endogenous STARS expression by using small interfering RNA significantly reduced SRF activity in differentiated C2C12 skeletal muscle cells and cardiac myocytes. The ability of STARS to promote the nuclear localization of MRTFs and SRF-mediated transcription provides a potential muscle-specific mechanism for linking changes in actin dynamics and sarcomere structure with striated muscle gene expression.

Patterns of gene expression differentially regulated by platelet-derived growth factor and hypertrophic stimuli in vascular smooth muscle cells: markers for phenotypic modulation and response to injury

In vascular smooth muscle cells (VSMC), platelet-derived growth factor (PDGF) suppresses expression of multiple smooth muscle contractile proteins, useful markers of differentiation. Conversely, hypertrophic agents induce expression of these genes. The goal of this study was to employ genomic approaches to identify classes of genes differentially regulated by PDGF and hypertrophic stimuli. Changes in gene expression were determined using Affymetrix RAE-230 GeneChips in rat aortic VSMC stimulated with PDGF. For comparison with a model hypertrophic stimulus, a microarray was performed with VSMC stably expressing constitutively active Galpha(16), which strongly induces smooth muscle marker expression. We identified 75 genes whose expression was increased by exposure to PDGF and decreased by expression of Galpha(16) and 97 genes whose expression was decreased by PDGF and increased by Galpha(16). These genes included many smooth muscle-specific proteins; several extracellular matrix, cytoskeletal, and chemotaxis-related proteins; cell signaling molecules; and transcription factors. Changes in gene expression for many of these were confirmed by PCR or immunoblotting. The contribution of signaling pathways activated by PDGF to the gene expression profile was examined in VSMC stably expressing gain-of-function H-Ras or myristoylated Akt. Among the genes that were confirmed to be differentially regulated were CCAAT/enhancer-binding protein delta, versican, and nexilin. All of these genes also had altered expression in injured aortas, consistent with a role for PDGF in the response of injured VSMC. These data indicate that genes that are differentially regulated by PDGF and hypertrophic stimuli may represent families of genes and potentially be biomarkers for vascular injury.

Requirement for serum response factor for skeletal muscle growth and maturation revealed by tissue-specific gene deletion in mice

Serum response factor (SRF) controls the transcription of muscle genes by recruiting a variety of partner proteins, including members of the myocardin family of transcriptional coactivators. Mice lacking SRF fail to form mesoderm and die before gastrulation, precluding an analysis of the roles of SRF in muscle tissues. To investigate the functions of SRF in skeletal muscle development, we conditionally deleted the Srf gene in mice by skeletal muscle-specific expression of Cre recombinase. In mice lacking skeletal muscle SRF expression, muscle fibers formed, but failed to undergo hypertrophic growth after birth. Consequently, mutant mice died during the perinatal period from severe skeletal muscle hypoplasia. The myopathic phenotype of these mutant mice resembled that of mice expressing a dominant negative mutant of a myocardin family member in skeletal muscle. These findings reveal an essential role for the partnership of SRF and myocardin-related transcription factors in the control of skeletal muscle growth and maturation in vivo.

AKAP12alpha, an atypical serum response factor-dependent target gene

We recently identified three AKAP12 isoforms that are differentially regulated by distinct promoters. During a screen to identify molecular determinants distinguishing the activities of these promoters, we found a potential binding site for the serum response factor (SRF) in the promoter of the ubiquitously expressed AKAP12alpha isoform. SRF is an evolutionarily conserved transcription factor that governs disparate programs of gene expression linked to cellular growth and differentiation. Using a combination of reporter assays and RNA interference, we demonstrate that SRF is required for AKAP12alpha expression. SRF regulates the activity of the AKAP12alpha promoter through two conserved CArG boxes that bind SRF with different affinities. Unlike other SRF-dependent genes, AKAP12alpha is not regulated by growth or differentiation stimuli. Molecular analysis of the AKAP12alpha SRF-binding sites, or CArG boxes, indicates that sequences flanking these sites are the determinants of sensitivity to SRF-activating signals. Specifically, the AKAP12alpha CArG boxes are shielded from growth stimulation by the absence of a binding site for Ets transcription factors. Similarly, sensitivity to the differentiation-associated co-factor, myocardin, was also determined by responsive flanking sequence; however, unlike growth stimuli, sensitivity to myocardin was found to also be dependent on a consensus CArG box. Collectively, our data demonstrate that AKAP12alpha belongs to a novel class of atypical SRF-dependent target genes. Furthermore, we provide new insight into the role of flanking sequences in determining sensitivity to SRF-myocardin activity.