Early postnatal cardiac changes and premature death in transgenic mice overexpressing a mutant form of serum response factor

Deletion of Interleukin-1β Converting Enzyme Alters Mouse Cardiac Structure and Function

Interleukin-1β converting enzyme (ICE, caspase-1) is a thiol protease that cleaves the pro-inflammatory cytokine precursors of IL-1β and IL-18 into active forms. Given the association between caspase-1 and cardiovascular pathology, we analyzed the hearts of ICE knockout (ICE KO) mice to test the hypothesis that caspase-1 plays a significant role in cardiac morphology and function. We characterized the histological and functional changes in the hearts of ICE KO mice compared to the Wild type. The cardiomyocytes from the neonatal ICE KO mice showed an impaired response to oxidative stress. Subsequently, the hearts from the ICE KO mice were hypertrophied, with a significant increase in the left ventricular and septal wall thickness and a greater LV mass/body weight ratio. The ICE KO mice hearts exhibited irregular myofibril arrangements and disruption of the cristae in the mitochondrial structure. Proapoptotic proteins that were significantly increased in the hearts of ICE KO versus the Wild type included pErk, pJNK, p53, Fas, Bax, and caspase 3. Further, the antiapoptotic proteins Bag-1 and Bcl-2 are activated in ICE KO hearts. Functionally, there was an increase in the left ventricular epicardial diameter and volume in ICE KO. In conclusion, our findings support the important role of caspase-1 in maintaining cardiac health; specifically, a significant decrease in caspase-1 is detrimental to the cardiovascular system.

Interrelation between α-Cardiac Actin Treadmilling and Myocardin-Related Transcription Factor-A Nuclear Shuttling in Cardiomyocytes

Myocardin-related transcription factors (MRTFs) play a central role in the regulation of actin expression and cytoskeletal dynamics that are controlled by Rho GTPases. SRF is a ubiquitous transcription factor strongly expressed in muscular tissues. The depletion of SRF in the adult mouse heart leads to severe dilated cardiomyopathy associated with the down-regulation of target genes encoding sarcomeric proteins including α-cardiac actin. The regulatory triad, composed of SRF, its cofactor MRTFA and actin, plays a major role in the coordination of the nuclear transcriptional response to adapt actin filament dynamics associated with changes in cell shape, and contractile and migratory activities. Most of the knowledge on the regulation of the SRF–MRTF–Actin axis has been obtained in non-muscle cells with α-actin and smooth muscle cells with α-smooth actin. Here, we visualized for the first time by a time-lapse video, the nucleocytoplasmic shuttling of MRTFA induced by serum or pro-hypertrophic agonists such as angiotensin II, phenylephrine and endothelin-1, using an MRTFA-GFP adenovirus in cultures of neonatal rat cardiomyocytes. We showed that an inhibitor of the RhoA/ROCK signaling pathway leads to an α-cardiac actin polymerization disruption and inhibition of MRTFA nucleocytoplasmic shuttling. Moreover, inhibition of the PI3K/Akt signaling pathway also prevents the entry of MRTFA into the nuclei. Our findings point out a central role of the SRF–MRTFA–actin axis in cardiac remodeling.

SRF: a seriously responsible factor in cardiac development and disease

The molecular mechanisms that regulate embryogenesis and cardiac development are calibrated by multiple signal transduction pathways within or between different cell lineages via autocrine or paracrine mechanisms of action. The heart is the first functional organ to form during development, which highlights the importance of this organ in later stages of growth. Knowledge of the regulatory mechanisms underlying cardiac development and adult cardiac homeostasis paves the way for discovering therapeutic possibilities for cardiac disease treatment. Serum response factor (SRF) is a major transcription factor that controls both embryonic and adult cardiac development. SRF expression is needed through the duration of development, from the first mesodermal cell in a developing embryo to the last cell damaged by infarction in the myocardium. Precise regulation of SRF expression is critical for mesoderm formation and cardiac crescent formation in the embryo, and altered SRF levels lead to cardiomyopathies in the adult heart, suggesting the vital role played by SRF in cardiac development and disease. This review provides a detailed overview of SRF and its partners in their various functions and discusses the future scope and possible therapeutic potential of SRF in the cardiovascular system.

Nonsense Mutations in Eukaryotes

Nonsense mutations are a type of mutations which results in a premature termination codon occurrence. In general, these mutations have been considered to be among the most harmful ones which lead to premature protein translation termination and result in shortened nonfunctional polypeptide. However, there is evidence that not all nonsense mutations are harmful as well as some molecular mechanisms exist which allow to avoid pathogenic effects of these mutations. This review addresses relevant information on nonsense mutations in eukaryotic genomes, characteristics of these mutations, and different molecular mechanisms preventing or mitigating harmful effects thereof.

Serum response factor-cofactor interactions and their implications in disease

Serum response factor (SRF), a member of the Mcm1, Agamous, Deficiens, and SRF (MADS) box transcription factor, is widely expressed in all cell types and plays a crucial role in the physiological function and development of diseases. SRF regulates its downstream genes by binding to their CArG DNA box by interacting with various cofactors. However, the underlying mechanisms are not fully understood, therefore attracting increasing research attention due to the importance of this topic. This review's objective is to discuss the new progress in the studies of the molecular mechanisms involved in the activation of SRF and its impacts in physiological and pathological conditions. Notably, we summarized the recent studies on the interaction of SRF with its two main types of cofactors belonging to the myocardin families of transcription factors and the members of the ternary complex factors. The knowledge of these mechanisms will create new opportunities for understanding the dynamics of many traits and disease pathogenesis especially, cardiovascular diseases and cancer that could serve as targets for pharmacological control and treatment of these diseases.

Novel mutations of the SRF gene in Chinese sporadic conotruncal heart defect patients

BACKGROUND

Conotruncal heart defects (CTDs) are a group of congenital heart malformations that cause anomalies of cardiac outflow tracts. In the past few decades, many genes related to CTDs have been reported. Serum response factor (SRF) is a ubiquitous nuclear protein that acts as transcription factor, and SRF was found to be a critical factor in heart development and to be strongly expressed in the myocardium of the developing mouse and chicken hearts. The targeted inactivation of SRF during heart development leads to embryonic lethality and myocardial defects in mice.

METHODS

To illustrate the relationship between SRF and human heart defects, we screened SRF mutations in 527 CTD patients, a cross sectional study. DNA was extracted from peripheral leukocyte cells for target sequencing. The mutations of SRF were detected and validated by Sanger sequencing. The affection of the mutations on wild-type protein was analyzed by in silico softwares. Western blot and real time PCR were used to analyze the changes of the expression of the mutant mRNA and protein. In addition, we carried out dual luciferase reporter assay to explore the transcriptional activity of the mutant SRF.

RESULTS

Among the target sequencing results of 527 patients, two novel mutations (Mut1: c.821A > G p.G274D, the adenine(A) was mutated to guanine(G) at position 821 of the SRF gene coding sequences (CDS), lead to the Glycine(G) mutated to Asparticacid(D) at position 274 of the SRF protein amino acid sequences; Mut2: c.880G > T p.G294C, the guanine(G) was mutated to thymine (T) at position 880 of the SRF CDS, lead to the Glycine(G) mutated to Cysteine (C) at position 294 of the SRF protein amino acid sequences.) of SRF (NM_003131.4) were identified. Western blotting and real-time PCR showed that there were no obvious differences between the protein expression and mRNA transcription of mutants and wild-type SRF. A dual luciferase reporter assay showed that both SRF mutants (G274D and G294C) impaired SRF transcriptional activity at the SRF promoter and atrial natriuretic factor (ANF) promoter (p < 0.05), additionally, the mutants displayed reduced synergism with GATA4.

CONCLUSION

These results suggest that SRF-p.G274D and SRF-p.G294C may have potential pathogenic effects.

miR-486 is modulated by stretch and increases ventricular growth

Perturbations in biomechanical stimuli during cardiac development contribute to congenital cardiac defects such as hypoplastic left heart syndrome (HLHS). This study sought to identify stretch-responsive pathways involved in cardiac development. miRNA-Seq identified miR-486 as being increased in cardiomyocytes exposed to cyclic stretch in vitro. The right ventricles (RVs) of patients with HLHS experienced increased stretch and had a trend toward higher miR-486 levels. Sheep RVs dilated from excessive pulmonary blood flow had 60% more miR-486 compared with control RVs. The left ventricles of newborn mice treated with miR-486 mimic were 16.9%-24.6% larger and displayed a 2.48-fold increase in cardiomyocyte proliferation. miR-486 treatment decreased FoxO1 and Smad signaling while increasing the protein levels of Stat1. Stat1 associated with Gata-4 and serum response factor (Srf), 2 key cardiac transcription factors with protein levels that increase in response to miR-486. This is the first report to our knowledge of a stretch-responsive miRNA that increases the growth of the ventricle in vivo.

Regulation of cardiomyocyte maturation during critical perinatal window

A primary limitation in the use of pluripotent stem cell-derived cardiomyocytes (PSC-CMs) for both patient health and scientific investigation is the failure of these cells to achieve full functional maturity. In vivo, cardiomyocytes undergo numerous adaptive structural, functional and metabolic changes during maturation. By contrast, PSC-CMs fail to fully undergo these developmental processes, instead remaining arrested at an embryonic stage of maturation. There is thus a significant need to understand the biological processes underlying proper CM maturation in vivo. Here, we discuss what is known regarding the initiation and coordination of CM maturation. We postulate that there is a critical perinatal window, ranging from embryonic day 18.5 to postnatal day 14 in mice, in which the maturation process is exquisitely sensitive to perturbation. While the initiation mechanisms of this process are unknown, it is increasingly clear that maturation proceeds through interconnected regulatory circuits that feed into one another to coordinate concomitant structural, functional and metabolic CM maturation. We highlight PGC1α, SRF and the MEF2 family as transcription factors that may potentially mediate this cross-talk. We lastly discuss several emerging technologies that will facilitate future studies into the mechanisms of CM maturation. Further study will not only produce a better understanding of its key processes, but provide practical insights into developing a robust strategy to produce mature PSC-CMs.

SRF promotes gastric cancer metastasis through stromal fibroblasts in an SDF1-CXCR4-dependent manner

It has been suggested that the overexpression of serum response factor (SRF) in cancer cells may promote cancer metastasis. However, the exact pathway by which SRF promotes metastasis has not been clarified. Here we showed that SRF promotes gastric cancer (GC) metastasis through stromal fibroblasts in an SDF1-CXCR4-dependent manner. SRF expression was significantly increased in metastatic GCs compared with the non-metastatic GCs (n=50, p=0.013). Immuno-staining indicated that SRF was primarily expressed in a-smooth muscle actin (αSMA)-expressing periglandular fibroblasts in GCs. The conditioned medium (CM) from CCD18Co fibroblasts stably transfected with the SRF vector (CCD18Co-SRF) significantly enhanced migration of MKN45 gastric cancer cells. In contrast, the CM from CCD18Co fibroblasts, in which SRF was downregulated, inhibited mobility of MKN45 cells. Similar results were observed in cultured BGC823 cells even when they were treated with the NIH3T3-SRF CM. When MKN45 cells and SRF-upregulated or downregulated CCD18Co cells were simultaneously co-injected into the tail veins of NOD-SCID mice, a significant increase or decrease was, respectively, observed in the experimental pulmonary metastasis of cancer cells. Furthermore, SRF overexpression significantly upregulated `SMA and stromal cell derived factor1 (SDF1) expression in these fibroblasts, and an anti-SDF1 antibody or the SDF1 receptor CXCR4-specific inhibitor AMD3100 treatment completely reversed the SRF-enhanced migration of cancer cells. Quantitative RT-PCR demonstrated that the expression level of SRF was positively correlated with that of SDF1 in 92 GC samples (r=0.63, p<0.001). In conclusion, SRF promote GC metastasis by facilitating myofibroblast-cancer cell crosstalk in an SDF1-CXCR4 dependent manner, and maybe a therapeutic target.

SIRT2 gene has a classic SRE element, is a downstream target of serum response factor and is likely activated during serum stimulation

The sirtuin proteins are an evolutionarily conserved family of NAD+-dependent deacetylases that regulate various cellular functions. Among the seven sirtuins, SIRT2 is predominantly found in the cytoplasm, and is present in a wide range of tissues. Recent studies indicate that SIRT2 plays an important role in metabolic homeostasis. Several studies indicate that SIRT2 is upregulated under serum deprivation conditions. Since the serum response factor gene usually responds rapidly to serum deprivation and/or serum restoration following deprivation, we hypothesized that a common mechanism may serve to regulate both SIRT2 and SRF during serum stimulation. Using a bioinformatics approach, we searched the SRF binding motif in the SIRT2 gene, and found one classic CArG element (CCATAATAGG) in the SIRT2 gene promoter, which was bound to SRF in the electrophoretic mobility shift assay (EMSA). Serum deprivation induced SIRT2 expression, while SRF and the SRF binding protein, p49/STRAP, repressed SIRT2 gene expression. SIRT2 gene expression was also repressed by the Rho/SRF inhibitor, CCG-1423. These data demonstrate that the classic SRE element in the SIRT2 gene promoter region is functional and therefore, SIRT2 gene is a downstream target of the Rho/SRF signaling pathway. The increased expression of SRF that was observed in the aged heart may affect SIRT2 gene expression and contribute to altered metabolic status in senescence.

Z-disc protein CHAPb induces cardiomyopathy and contractile dysfunction in the postnatal heart

Aims

The Z-disc is a crucial structure of the sarcomere and is implicated in mechanosensation/transduction. Dysregulation of Z-disc proteins often result in cardiomyopathy. We have previously shown that the Z-disc protein Cytoskeletal Heart-enriched Actin-associated Protein (CHAP) is essential for cardiac and skeletal muscle development. Furthermore, the CHAP gene has been associated with atrial fibrillation in humans. Here, we studied the misregulated expression of CHAP isoforms in heart disease.

Methods and results

Mice that underwent transverse aortic constriction and calcineurin transgenic (Tg) mice, both models of experimental heart failure, displayed a significant increase in cardiac expression of fetal isoform CHAPb. To investigate whether increased expression of CHAPb postnatally is sufficient to induce cardiomyopathy, we generated CHAPb Tg mice under the control of the cardiac-specific αMHC promoter. CHAPb Tg mice displayed cardiac hypertrophy, interstitial fibrosis and enlargement of the left atrium at three months, which was more pronounced at the age of six months. Hypertrophy and fibrosis were confirmed by evidence of activation of the hypertrophic gene program (Nppa, Nppb, Myh7) and increased collagen expression, respectively. Connexin40 and 43 were downregulated in the left atrium, which was associated with delayed atrioventricular conduction. Tg hearts displayed both systolic and diastolic dysfunction partly caused by impaired sarcomere function evident from a reduced force generating capacity of single cardiomyocytes. This co-incided with activation of the actin signalling pathway leading to the formation of stress fibers.

Conclusion

This study demonstrated that the fetal isoform CHAPb initiates progression towards cardiac hypertrophy, which is accompanied by delayed atrioventricular conduction and diastolic dysfunction. Moreover, CHAP may be a novel therapeutic target or candidate gene for screening in cardiomyopathies and atrial fibrillation.

MicroRNA miR-301a is a novel cardiac regulator of Cofilin-2

Calsarcin-1 deficient mice develop dilated cardiomyopathy (DCM) phenotype in pure C57BL/6 genetic background (Cs1-ko) despite severe contractile dysfunction and robust activation of fetal gene program. Here we performed a microRNA microarray to identify the molecular causes of this cardiac phenotype that revealed the dysregulation of several microRNAs including miR-301a, which was highly downregulated in Cs1-ko mice compared to the wild-type littermates. Cofilin-2 (Cfl2) was identified as one of the potential targets of miR-301a using prediction databases, which we validated by luciferase assay and mutation of predicted binding sites. Furthermore, expression of miR-301a contrastingly regulated Cfl2 expression levels in neonatal rat ventricular cardiomyocytes (NRVCM). Along these lines, Cfl2 was significantly upregulated in Cs1-ko mice, indicating the physiological association between miR-301a and Cfl2 in vivo. Mechanistically, we found that Cfl2 activated serum response factor response element (SRF-RE) driven luciferase activity in neonatal rat cardiomyocytes and in C2C12 cells. Similarly, knockdown of miR301a activated, whereas, its overexpression inhibited the SRF-RE driven luciferase activity, further strengthening physiological interaction between miR-301a and Cfl2. Interestingly, the expression of SRF and its target genes was strikingly increased in Cs1-ko suggesting a possible in vivo correlation between expression levels of Cfl2/miR-301a and SRF activation, which needs to be independently validated. In summary, our data demonstrates that miR-301a regulates Cofilin-2 in vitro in NRVCM, and in vivo in Cs1-ko mice. Our findings provide an additional and important layer of Cfl2 regulation, which we believe has an extended role in cardiac signal transduction and dilated cardiomyopathy presumably due to the reported involvement of Cfl2 in these mechanisms.

Overexpression of Striated Muscle Activator of Rho Signaling (STARS) Increases C2C12 Skeletal Muscle Cell Differentiation

Background: Skeletal muscle growth and regeneration depend on the activation of satellite cells, which leads to myocyte proliferation, differentiation and fusion with existing muscle fibers. Skeletal muscle cell proliferation and differentiation are tightly coordinated by a continuum of molecular signaling pathways. The striated muscle activator of Rho signaling (STARS) is an actin binding protein that regulates the transcription of genes involved in muscle cell growth, structure and function via the stimulation of actin polymerization and activation of serum-response factor (SRF) signaling. STARS mediates cell proliferation in smooth and cardiac muscle models; however, whether STARS overexpression enhances cell proliferation and differentiation has not been investigated in skeletal muscle cells.

Results: We demonstrate for the first time that STARS overexpression enhances differentiation but not proliferation in C2C12 mouse skeletal muscle cells. Increased differentiation was associated with an increase in the gene levels of the myogenic differentiation markers Ckm, Ckmt2 and Myh4, the differentiation factor Igf2 and the myogenic regulatory factors (MRFs) Myf5 and Myf6. Exposing C2C12 cells to CCG-1423, a pharmacological inhibitor of SRF preventing the nuclear translocation of its co-factor MRTF-A, had no effect on myotube differentiation rate, suggesting that STARS regulates differentiation via a MRTF-A independent mechanism.

Conclusion: These findings position STARS as an important regulator of skeletal muscle growth and regeneration.

Regulation of Connective Tissue Growth Factor and Cardiac Fibrosis by an SRF/MicroRNA-133a Axis

Myocardial fibrosis contributes to the remodeling of heart and the loss of cardiac function leading to heart failure. SRF is a transcription factor implicated in the regulation of a large variety of genes involved in cardiac structure and function. To investigate the impact of an SRF overexpression in heart, we developed a new cardiac-specific and tamoxifen-inducible SRF overexpression mouse model by the Cre/loxP strategy. Here, we report that a high level overexpression of SRF leads to severe modifications of cardiac cytoarchitecture affecting the balance between cardiomyocytes and cardiac fibroblasts and also a profound alteration of cardiac gene expression program. The drastic development of fibrosis was characterized by intense sirius red staining and associated with an increased expression of genes encoding extracellular matrix proteins such as fibronectin, procollagen type 1α1 and type 3α1 and especially connective tissue growth factor (CTGF). Furthermore miR-133a, one of the most predominant cardiac miRNAs, is strongly downregulated when SRF is overexpressed. By comparison a low level overexpression of SRF has minor impact on these different processes. Investigation with miR-133a, antimiR-133a and AdSRF-VP16 experiments in H9c2 cardiac cells demonstrated that: 1)–miR-133a acts as a repressor of SRF and CTGF expression; 2)–a simultaneous overexpression of SRF by AdSRF-VP16 and inhibition of miR-133a by a specific antimiR increase CTGF expression; 3)–miR-133a overexpression can block the upregulation of CTGF induced by AdSRF-VP16. Taken together, these findings reveal a key role of the SRF/CTGF/miR-133a axis in the regulation of cardiac fibrosis.

MicroRNA Clusters in the Adult Mouse Heart: Age-Associated Changes

The microRNAs and microRNA clusters have been implicated in normal cardiac development and also disease, including cardiac hypertrophy, cardiomyopathy, heart failure, and arrhythmias. Since a microRNA cluster has from two to dozens of microRNAs, the expression of a microRNA cluster could have a substantial impact on its target genes. In the present study, the configuration and distribution of microRNA clusters in the mouse genome were examined at various inter-microRNA distances. Three important microRNA clusters that are significantly impacted during adult cardiac aging, the miR-17-92, miR-106a-363, and miR-106b-25, were also examined in terms of their genomic location, RNA transcript character, sequence homology, and their relationship with the corresponding microRNA families. Multiple microRNAs derived from the three clusters potentially target various protein components of the cdc42-SRF signaling pathway, which regulates cytoskeleton dynamics associated with cardiac structure and function. The data indicate that aging impacted the expression of both guide and passenger strands of the microRNA clusters; nutrient stress also affected the expression of the three microRNA clusters. The miR-17-92, miR-106a-363, and miR-106b-25 clusters are likely to impact the Cdc42-SRF signaling pathway and thereby affect cardiac morphology and function during pathological conditions and the aging process.

Novel genetic variants in differentiated thyroid cancer and assessment of the cumulative risk

A genome-wide association study (GWAS) performed on a high-incidence Italian population followed by replications on low-incidence cohorts suggested a strong association of differentiated thyroid cancer (DTC) with single nucleotide polymorphisms (SNPs) at 9q22.33, 2q35, 20q11.22-q12 and 14q24.3. Moreover, six additional susceptibility loci were associated with the disease only among Italians. The present study had two aims, first to identify loci involved in DTC risk and then to assess the cumulative effect of the SNPs identified so far in the Italian population. The combined analysis of the previous GWAS and the present Italian study provided evidence of association with rs7935113 (GALNTL4, OR = 1.36, 95%CI 1.20–1.53, p-value = 7.41 × 10⁻⁷) and rs1203952 (FOXA2, OR = 1.29, 95%CI 1.16–1.44, p-value = 4.42 × 10⁻⁶). Experimental ENCODE and eQTL data suggested that both SNPs may influence the closest genes expression through a differential recruitment of transcription factors. The assessment of the cumulative risk of eleven SNPs showed that DTC risk increases with an increasing number of risk alleles (p-trend = 3.13 × 10⁻⁴⁷). Nonetheless, only a small fraction (about 4% on the disease liability scale) of DTC is explained by these SNPs. These data are consistent with a polygenic model of DTC predisposition and highlight the importance of association studies in the discovery of the disease hereditability.

Ca²⁺ signal-induced cardiomyocyte hypertrophy through activation of myocardin

Hypertrophic growth of cardiomyocytes in response to pressure overload is an important stage during the development of many cardiac diseases. Ca(2+) overload as well as subsequent activation of Ca(2+) signaling pathways has been reported to induce cardiac hypertrophy. Myocardin, a transcription cofactor of serum response factor (SRF), is a key transducer of hypertrophic signals. However, the direct role of myocardin in Ca(2+) signal-induced cardiomyocyte hypertrophy has not been explained clearly. In the present study, we discovered that embryonic rat heart-derived H9c2 cells responded to the stimulation of calcium ionophore A23187 with a cell surface area enlargement and an increased expression of cardiac hypertrophy marker genes. Increased Ca(2+) also induces an organization of sarcomeres in neonatal rat cardiomyocytes, as revealed by α-actinin staining. Increased Ca(2+) could upregulate the expression of myocardin. Knockdown of myocardin by shRNA attenuates hypertrophic responses triggered by increased intracellular Ca(2+), suggesting that Ca(2+) signals induce cardiomyocyte hypertrophy partly through activation of myocardin. Furthermore, A23187 treatment directly activates myocardin promoter, chelation of Ca(2+) by EGTA inhibits this activation and knockdown of myocardin expression using shRNA also abrogates A23187-induced ANF and SK-α-actin promoter activity. CSA (calcineurin inhibitor) and KN93 (CaMKII inhibitor) inhibit A23187-induced the increase in myocardin expression. These results suggest that myocardin plays a critical role in Ca(2+) signal-induced cardiomyocyte hypertrophy, which may serve as a novel mechanism that is important for cardiac hypertrophy.

Pak1 is required to maintain ventricular Ca²⁺ homeostasis and electrophysiological stability through SERCA2a regulation in mice

BACKGROUND

Impaired sarcoplasmic reticular Ca(2+) uptake resulting from decreased sarcoplasmic reticulum Ca(2+)-ATPase type 2a (SERCA2a) expression or activity is a characteristic of heart failure with its associated ventricular arrhythmias. Recent attempts at gene therapy of these conditions explored strategies enhancing SERCA2a expression and the activity as novel approaches to heart failure management. We here explore the role of Pak1 in maintaining ventricular Ca(2+) homeostasis and electrophysiological stability under both normal physiological and acute and chronic β-adrenergic stress conditions.

METHODS AND RESULTS

Mice with a cardiomyocyte-specific Pak1 deletion (Pak1(cko)), but not controls (Pak1(f/f)), showed high incidences of ventricular arrhythmias and electrophysiological instability during either acute β-adrenergic or chronic β-adrenergic stress leading to hypertrophy, induced by isoproterenol. Isolated Pak1(cko) ventricular myocytes correspondingly showed aberrant cellular Ca(2+) homeostasis. Pak1(cko) hearts showed an associated impairment of SERCA2a function and downregulation of SERCA2a mRNA and protein expression. Further explorations of the mechanisms underlying the altered transcriptional regulation demonstrated that exposure to control Ad-shC2 virus infection increased SERCA2a protein and mRNA levels after phenylephrine stress in cultured neonatal rat cardiomyocytes. This was abolished by the Pak1-knockdown in Ad-shPak1-infected neonatal rat cardiomyocytes and increased by constitutive overexpression of active Pak1 (Ad-CAPak1). We then implicated activation of serum response factor, a transcriptional factor well known for its vital role in the regulation of cardiogenesis genes in the Pak1-dependent regulation of SERCA2a.

CONCLUSIONS

These findings indicate that Pak1 is required to maintain ventricular Ca(2+) homeostasis and electrophysiological stability and implicate Pak1 as a novel regulator of cardiac SERCA2a through a transcriptional mechanism.

Overexpression of p49/STRAP alters cellular cytoskeletal structure and gross anatomy in mice

Background

The protein p49/STRAP (SRFBP1) is a transcription cofactor of serum response factor (SRF) which regulates cytoskeletal and muscle-specific genes.

Results

Two conserved domains were found in the p49/STRAP protein. The SRF-binding domain was at its N-terminus and was highly conserved among mammalian species, xenopus and zebrafish. A BUD22 domain was found at its C-terminus in three sequence databases. The BUD22 domain was conserved among mammalian p49/STRAP proteins, and yeast cellular morphogenesis proteins, which is involved in ribosome biogenesis that affects growth rate and cell size. The endogenous p49/SRAP protein was localized mainly in the nucleus but also widely distributed in the cytoplasm, and was in close proximity to the actin. Transfected GFP-p49/STRAP protein co-localized with nucleolin within the nucleolus. Overexpression of p49/STRAP reduced actin content in cultured cells and resulted in smaller cell size versus control cells. Increased expression of p49/STRAP in transgenic mice resulted in newborns with malformations, which included asymmetric abdominal and thoracic cavities, and substantial changes in cardiac morphology. p49/STRAP altered the expression of certain muscle-specific genes, including that of the SRF gene, which is a key regulator of cardiac genes at the developmental, structural and maintenance level and has two SRE binding sites.

Conclusions

Since p49/STRAP is a co-factor of SRF, our data suggest that p49/STRAP likely regulates cell size and morphology through SRF target genes. The function of its BUD22 domain warrants further investigation. The observed increase in p49/STRAP expression during cellular aging may contribute to observed morphological changes in senescence.

Dysbindin is a potent inducer of RhoA-SRF-mediated cardiomyocyte hypertrophy

Dysbindin activates RhoA–SRF and MEK1–ERK1 signaling pathways in cardiomyocytes to promote cardiac hypertrophy.

Dysbindin is an established schizophrenia susceptibility gene thoroughly studied in the context of the brain. We have previously shown through a yeast two-hybrid screen that it is also a cardiac binding partner of the intercalated disc protein Myozap. Because Dysbindin is highly expressed in the heart, we aimed here at deciphering its cardiac function. Using a serum response factor (SRF) response element reporter-driven luciferase assay, we identified a robust activation of SRF signaling by Dysbindin overexpression that was associated with significant up-regulation of SRF gene targets, such as Acta1 and Actc1. Concurrently, we identified RhoA as a novel binding partner of Dysbindin. Further phenotypic and mechanistic characterization revealed that Dysbindin induced cardiac hypertrophy via RhoA–SRF and MEK1–ERK1 signaling pathways. In conclusion, we show a novel cardiac role of Dysbindin in the activation of RhoA–SRF and MEK1–ERK1 signaling pathways and in the induction of cardiac hypertrophy. Future in vivo studies should examine the significance of Dysbindin in cardiomyopathy.

Histone deacetylase signaling in cardioprotection

Cardiovascular disease (CVD) represents a major challenge for health care systems, both in terms of the high mortality associated with it and the huge economic burden of its treatment. Although CVD represents a diverse range of disorders, they share common compensatory changes in the heart at the structural, cellular, and molecular level that, in the long term, can become maladaptive and lead to heart failure. Treatment of adverse cardiac remodeling is therefore an important step in preventing this fatal progression. Although previous efforts have been primarily focused on inhibition of deleterious signaling cascades, the stimulation of endogenous cardioprotective mechanisms offers a potent therapeutic tool. In this review, we discuss class I and class II histone deacetylases, a subset of chromatin-modifying enzymes known to have critical roles in the regulation of cardiac remodeling. In particular, we discuss their molecular modes of action and go on to consider how their inhibition or the stimulation of their intrinsic cardioprotective properties may provide a potential therapeutic route for the clinical treatment of CVD.

Regulation of fetal gene expression in heart failure

During the processes leading to adverse cardiac remodeling and heart failure, cardiomyocytes react to neurohumoral stimuli and biomechanical stress by activating pathways that induce pathological hypertrophy. The gene expression patterns and molecular changes observed during cardiac hypertrophic remodeling bare resemblance to those observed during fetal cardiac development. The re-activation of fetal genes in the adult failing heart is a complex biological process that involves transcriptional, posttranscriptional and epigenetic regulation of the cardiac genome. In this review, the mechanistic actions of transcription factors, microRNAs and chromatin remodeling processes in regulating fetal gene expression in heart failure are discussed.

Identification of temporal and region-specific myocardial gene expression patterns in response to infarction in swine

Molecular mechanisms associated with pathophysiological changes in ventricular remodelling due to myocardial infarction (MI) remain poorly understood. We analyzed changes in gene expression by microarray technology in porcine myocardial tissue at 1, 4, and 6 weeks post-MI.MI was induced by coronary artery ligation in 9 female pigs (30-40 kg). Animals were randomly sacrificed at 1, 4, or 6 weeks post-MI (n = 3 per group) and 3 healthy animals were also included as control group. Total RNA from myocardial samples was hybridized to GeneChip® Porcine Genome Arrays. Functional analysis was obtained with the Ingenuity Pathway Analysis (IPA) online tool. Validation of microarray data was performed by quantitative real-time PCR (qRT-PCR).More than 8,000 different probe sets showed altered expression in the remodelling myocardium at 1, 4, or 6 weeks post-MI. Ninety-seven percent of altered transcripts were detected in the infarct core and 255 probe sets were differentially expressed in the remote myocardium. Functional analysis revealed 28 genes de-regulated in the remote myocardial region in at least one of the three temporal analyzed stages, including genes associated with heart failure (HF), systemic sclerosis and coronary artery disease. In the infarct core tissue, eight major time-dependent gene expression patterns were recognized among 4,221 probe sets commonly altered over time. Altered gene expression of ACVR2B, BID, BMP2, BMPR1A, LMNA, NFKBIA, SMAD1, TGFB3, TNFRSF1A, and TP53 were further validated.The clustering of similar expression patterns for gene products with related function revealed molecular footprints, some of them described for the first time, which elucidate changes in biological processes at different stages after MI.

Epigenetic regulation of cardiac development and function by polycomb group and trithorax group proteins

Heart disease is a leading cause of death and disability in developed countries. Heart disease includes a broad range of diseases that affect the development and/or function of the cardiovascular system. Some of these diseases, such as congenital heart defects, are present at birth. Others develop over time and may be influenced by both genetic and environmental factors. Many of the known heart diseases are associated with abnormal expression of genes. Understanding the factors and mechanisms that regulate gene expression in the heart is essential for the detection, treatment, and prevention of heart diseases. Polycomb Group (PcG) and Trithorax Group (TrxG) proteins are special families of chromatin factors that regulate developmental gene expression in many tissues and organs. Accumulating evidence suggests that these proteins are important regulators of development and function of the heart as well. A better understanding of their roles and functional mechanisms will translate into new opportunities for combating heart disease.

Locally expressed IGF1 propeptide improves mouse heart function in induced dilated cardiomyopathy by blocking myocardial fibrosis and SRF-dependent CTGF induction

Cardiac fibrosis is critically involved in the adverse remodeling accompanying dilated cardiomyopathies (DCMs), which leads to cardiac dysfunction and heart failure (HF). Connective tissue growth factor (CTGF), a profibrotic cytokine, plays a key role in this deleterious process. Some beneficial effects of IGF1 on cardiomyopathy have been described, but its potential role in improving DCM is less well characterized. We investigated the consequences of expressing a cardiac-specific transgene encoding locally acting IGF1 propeptide (muscle-produced IGF1; mIGF1) on disease progression in a mouse model of DCM [cardiac-specific and inducible serum response factor (SRF) gene disruption] that mimics some forms of human DCM. Cardiac-specific mIGF1 expression substantially extended the lifespan of SRF mutant mice, markedly improved cardiac functions, and delayed both DCM and HF. These protective effects were accompanied by an overall improvement in cardiomyocyte architecture and a massive reduction of myocardial fibrosis with a concomitant amelioration of inflammation. At least some of the beneficial effects of mIGF1 transgene expression were due to mIGF1 counteracting the strong increase in CTGF expression within cardiomyocytes caused by SRF deficiency, resulting in the blockade of fibroblast proliferation and related myocardial fibrosis. These findings demonstrate that SRF plays a key role in the modulation of cardiac fibrosis through repression of cardiomyocyte CTGF expression in a paracrine fashion. They also explain how impaired SRF function observed in human HF promotes fibrosis and adverse cardiac remodeling. Locally acting mIGF1 efficiently protects the myocardium from these adverse processes, and might thus represent a therapeutic avenue to counter DCM.

Cardiac expression of ms1/STARS, a novel gene involved in cardiac development and disease, is regulated by GATA4

Ms1/STARS is a novel muscle-specific actin-binding protein that specifically modulates the myocardin-related transcription factor (MRTF)-serum response factor (SRF) regulatory axis within striated muscle. This ms1/STARS-dependent regulatory axis is of central importance within the cardiac gene regulatory network and has been implicated in cardiac development and postnatal cardiac function/homeostasis. The dysregulation of ms1/STARS is associated with and causative of pathological cardiac phenotypes, including cardiac hypertrophy and cardiomyopathy. In order to gain an understanding of the mechanisms governing ms1/STARS expression in the heart, we have coupled a comparative genomic in silico analysis with reporter, gain-of-function, and loss-of-function approaches. Through this integrated analysis, we have identified three evolutionarily conserved regions (ECRs), α, SINA, and DINA, that act as cis-regulatory modules and confer differential cardiac cell-specific activity. Two of these ECRs, α and DINA, displayed distinct regulatory sensitivity to the core cardiac transcription factor GATA4. Overall, our results demonstrate that within embryonic, neonatal, and adult hearts, GATA4 represses ms1/STARS expression with the pathologically associated depletion of GATA4 (type 1/type 2 diabetic models), resulting in ms1/STARS upregulation. This GATA4-dependent repression of ms1/STARS expression has major implications for MRTF-SRF signaling in the context of cardiac development and disease.

Identification of New SRF Binding Sites in Genes Modulated by SRF Over-Expression in Mouse Hearts

Background:

To identify in vivo new cardiac binding sites of serum response factor (SRF) in genes and to study the response of these genes to mild over-expression of SRF, we employed a cardiac-specific, transgenic mouse model, with mild over-expression of SRF (Mild-O SRF Tg).

Methodology:

Microarray experiments were performed on hearts of Mild-O-SRF Tg at 6 months of age. We identified 207 genes that are important for cardiac function that were differentially expressed in vivo. Among them the promoter region of 192 genes had SRF binding motifs, the classic CArG or CArG-like (CArG-L) elements. Fifty-one of the 56 genes with classic SRF binding sites had not been previously reported. These SRF-modulated genes were grouped into 12 categories based on their function. It was observed that genes associated with cardiac energy metabolism shifted toward that of carbohydrate metabolism and away from that of fatty acid metabolism. The expression of genes that are involved in transcription and ion regulation were decreased, but expression of cytoskeletal genes was significantly increased. Using public databases of mouse models of hemodynamic stress (GEO database), we also found that similar altered expression of the SRF-modulated genes occurred in these hearts with cardiac ischemia or aortic constriction as well.

Conclusion and significance:

SRF-modulated genes are actively regulated under various physiological and pathological conditions. We have discovered that a large number of cardiac genes have classic SRF binding sites and were significantly modulated in the Mild-O-SRF Tg mouse hearts. Hence, the mild elevation of SRF protein in the heart that is observed during typical adult aging may have a major impact on many SRF-modulated genes, thereby affecting cardiac structure and performance. The results from our study could help to enhance our understanding of SRF regulation of cellular processes in the aged heart.

Regulation of cardiac microRNAs by serum response factor

Serum response factor (SRF) regulates certain microRNAs that play a role in cardiac and skeletal muscle development. However, the role of SRF in the regulation of microRNA expression and microRNA biogenesis in cardiac hypertrophy has not been well established. In this report, we employed two distinct transgenic mouse models to study the impact of SRF on cardiac microRNA expression and microRNA biogenesis. Cardiac-specific overexpression of SRF (SRF-Tg) led to altered expression of a number of microRNAs. Interestingly, downregulation of miR-1, miR-133a and upregulation of miR-21 occurred by 7 days of age in these mice, long before the onset of cardiac hypertrophy, suggesting that SRF overexpression impacted the expression of microRNAs which contribute to cardiac hypertrophy. Reducing cardiac SRF level using the antisense-SRF transgenic approach (Anti-SRF-Tg) resulted in the expression of miR-1, miR-133a and miR-21 in the opposite direction. Furthermore, we observed that SRF regulates microRNA biogenesis, specifically the transcription of pri-microRNA, thereby affecting the mature microRNA level. The mir-21 promoter sequence is conserved among mouse, rat and human; one SRF binding site was found to be in the mir-21 proximal promoter region of all three species. The mir-21 gene is regulated by SRF and its cofactors, including myocardin and p49/Strap. Our study demonstrates that the downregulation of miR-1, miR-133a, and upregulation of miR-21 can be reversed by one single upstream regulator, SRF. These results may help to develop novel therapeutic interventions targeting microRNA biogenesis.

MicroRNAs in cardiac hypertrophy: angels or devils

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

Serum response factor: Look into the gut

Serum response factor (SRF) is a transcription factor that regulates many genes involved in cellular activities such as proliferation, migration, differentiation, angiogenesis, and apoptosis. Although it has only been known for about two decades, SRF has been studied extensively. To date, over a thousand SRF studies have been published, but it still remains a hot topic. Due to its critical role in mesoderm-derived tissues, most of the SRF studies focused on muscle structure/function, cardiovascular development/maintenance, and smooth muscle generation/repair. Recently, SRF has received more attention in the digestive field and several important discoveries have been made. This review will summarize what we have learned about SRF in the gastrointestinal tract and provide insights into possible future directions in this area.

MURC, a muscle-restricted coiled-coil protein that modulates the Rho/ROCK pathway, induces cardiac dysfunction and conduction disturbance

We identified a novel muscle-restricted putative coiled-coil protein, MURC, which is evolutionarily conserved from frog to human. MURC was localized to the cytoplasm with accumulation in the Z-line of the sarcomere in the murine adult heart. MURC mRNA expression in the heart increased during the developmental process from the embryonic stage to adulthood. In response to pressure overload, MURC mRNA expression increased in the hypertrophied heart. Using the yeast two-hybrid system, we identified the serum deprivation response (SDPR) protein, a phosphatidylserine-binding protein, as a MURC-binding protein. MURC induced activation of the RhoA/ROCK pathway, which modulated serum response factor-mediated atrial natriuretic peptide (ANP) expression and myofibrillar organization. SDPR augmented MURC-induced transactivation of the ANP promoter in cardiomyocytes, and RNA interference of SDPR attenuated the action of MURC on the ANP promoter. Transgenic mice expressing cardiac-specific MURC (Tg-MURC) exhibited cardiac contractile dysfunction and atrioventricular (AV) conduction disturbances with atrial chamber enlargement, reduced thickness of the ventricular wall, and interstitial fibrosis. Spontaneous episodes of atrial fibrillation and AV block were observed in Tg-MURC mice. These findings indicate that MURC modulates RhoA signaling and that MURC plays an important role in the development of cardiac dysfunction and conduction disturbance with increased vulnerability to atrial arrhythmias.

Alternative splicing and nonsense-mediated mRNA decay regulate gene expression of serum response factor

Serum response factor (SRF) is an important transcription factor that regulates a variety of genes in many tissues during development, maturation and aging. The SRF protein also controls the expression of SRF target genes, including the SRF gene itself. However, it is incompletely established how SRF isoforms contribute to the regulation of SRF gene expression. In the present study, we report the identification of three novel SRF isoforms in human tissue. We found that one novel isoform, SRF-triangle up3, contained a premature termination codon (PTC), which was a target of nonsense-mediated mRNA decay (NMD). By contrast, the SRF-triangle up345 isoform protein was able to specifically bind to the serum response element, and to repress the SRF gene promoter activity. Therefore, we propose that SRF isoforms regulate expression of the SRF gene via two different mechanisms. One mechanism is to reduce the abundance of SRF transcripts via coupled alternative splicing and NMD, the other one is to regulate the SRF gene expression via a feedback mechanism in which the SRF isoform proteins bind to the SRF gene promoter region. Analysis of hundreds of SRF cDNA clones derived from human hearts of fetuses, young adults, old and very old individuals revealed that SRF isoform transcripts were increased in the human heart with advancing age. Our data indicate that the SRF isoforms were differentially expressed in the human versus mouse cardiac muscle. Alternative splicing and NMD likely maintain a delicate balance of SRF transcripts and/or proteins among the full-length SRF form and various SRF isoforms that are critical to the regulation of many SRF target genes, including the SRF gene itself.

Maintaining serum response factor activity in the older heart equal to that of the young adult is associated with better cardiac response to isoproterenol stress

To understand the effect of transcription regulation in modulating cardiac aging, we sought to study the role of serum response factor (SRF), a key transcription factor in the heart that is normally increased with senescence and also in congestive heart failure. A Tet-Off gene expression system was used for cardiac-specific over-expression of a mutant SRF protein. In these binary transgenic mice, there is no age-related increase in SRF protein expression; in fact, there appeared to be a mild reduction of SRF protein (Mild-R SRF Tg). The older, middle-aged (15 mo) Mild-R SRF Tg mice appeared healthier and were better able to maintain their left ventricular systolic pressure (LVSP) in response to moderate â-adrenergic stimulation compared with age-matched Non-Tg mice, which demonstrated a negative ionotropic response. The Mild-R SRF Tg hearts had lower mRNA expression of BNP (p < 0.05), and the sodium calcium exchanger (p < 0.05), compared to Non-Tg. Mild-R SRF Tg had higher mRNA levels of SERCA2 (p < 0.05) and ryanodine receptor 2 (p < 0.05) compared to Non-Tg hearts. These findings suggest that preventing the age-associated increase in SRF is associated with better preserved intracellular calcium handling and functional response to stress; it might be advantageous for the older adult heart. This mouse model could be helpful in elucidating the molecular mechanisms underlying certain age-related changes in cardiac reserve capacity and response to stress.

Re-employment of developmental transcription factors in adult heart disease

A finite number of transcription factors constitute a combinatorial code that orchestrates cardiac development and the specification and differentiation of myocytes. Many, if not all of these same transcription factors are re-employed in the adult heart in response to disease stimuli that promote hypertrophic enlargement and/or dilated cardiomyopathy, as part of the so-called "fetal gene program". This review will discuss the transcription factors that regulate the hypertrophic growth response of the adult heart, with a special emphasis on those regulators that participate in cardiac development.

Conditional inactivation of the murine serum response factor in the pancreas leads to severe pancreatitis

The Serum Response Factor (SRF) is widely expressed transcription factor acting at the confluence of multiple signaling pathways and has been implicated in the control of differentiation, growth, and cell death. In the present study, we found that SRF is expressed in the developing and adult pancreas. To explore the possible role of SRF in this organ, we have generated mutant mice with conditional disruption of the Srf gene. Such mutants presented normal development of both the exocrine and endocrine pancreas indicating that SRF is dispensable for pancreas ontogenesis. However, after weaning, these mice developed profound morphological alterations of the exocrine pancreas, which were reminiscent of severe pancreatitis. In these mice, massive acinar injury, Nuclear Factor Kappa B activation and proinflammatory cytokines release led to complete destruction of the exocrine pancreas and its replacement by adipose tissue. Despite these changes, the organization and function of the endocrine islets of Langerhans remained well-preserved. This new animal model of spontaneous pancreatitis could prove a valuable tool to gain further insight into the physiopathology of this disease.

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.

Microarray and phosphokinase screenings leading to studies on ERK and JNK regulation of connective tissue growth factor expression by angiotensin II 1a and bradykinin B2 receptors in Rat1 fibroblasts

Rat1 fibroblasts stably transfected with the rat angiotensin II (AngII) AT1a and bradykinin (BK) B2 receptor cDNAs gained the ability to bind Ang II and BK. Wild-type Rat1 cells bound neither ligand. Exposure to either effector led to characteristic Galphai and Galphaq signal cascades, the release of arachidonic acid (ARA), and the intracellular accumulation of inositol phosphates (IP). Microarray analyses in response to BK or AngII showed that both receptors markedly induce the CCN family genes, CTGF (CCN2) and Cyr61 (CCN1), as well as the vasculature-related genes, Cnn1 and Egr1. Real time PCR confirmed the increased expression of connective tissue growth factor (CTGF) mRNA. Combined sequence-based analysis of gene promoter regions with statistical prevalence analyses identified CREB, SRF, and ATF-1, downstream targets of ERK, and JNK, as prominent products of genes that are regulated by ligand binding to the BK or AngII receptors. The binding of AngII or BK markedly stimulated the phosphorylation and thus the activation of ERK2, JNK, and p38MAPK. A BKB2R and an AT1aR chimera which displayed only negligible G-protein-related signaling were constructed. Both mutant receptors continued to activate these kinases and stimulate CTGF expression. Inhibitors of ERK1/2 and JNK but not p38MAPK inhibited the BK- and AngII-stimulated expression of CTGF in cells expressing either the WT or mutant receptors, illustrating that ERK and JNK participate in the control of CTGF expression in a manner that appears to be independent of G-protein. Conversely, addition of BK or AngII to the cell line expressing WT AT1aR and BKB2R downregulated the expression of collagen alpha1(I) (COL1A1) mRNA. However, these effectors did not have this effect in cells expressing the mutant receptors. Thus, a robust G-protein related response is necessary for BK or AngII to affect COL1A1 expression.

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.

Zipzap/p200 is a novel zinc finger protein contributing to cardiac gene regulation

Serum response factor (SRF) plays an important role in the regulation of immediate-early genes and muscle-specific genes, while SRF cofactors may contribute significantly to assist in tissue-specific, development-stage related regulation of SRF-target genes. We recently cloned a novel SRF cofactor, termed zipzap/p200, which is a zinc finger protein yet to be characterized. We determined that zipzap/p200 is a 200-kDa protein with two classic C2H2 zinc fingers at the carboxyl terminus where the nucleotide sequence was highly conserved among human, mouse, and rat. The zipzap gene was expressed in multiple tissues and at multiple ages, including the fetal and adult heart. The zipzap protein interacted with SRF in vivo and was found in protein complexes containing SRF and other SRF cofactors, including p49/strap and Nkx2.5. Zipzap/p200 activated the promoter of cardiac genes and potentiated the effect of myocardin on ANF promoter activity. Therefore, zipzap may serve as a transcription co-activator for the regulation of cardiac gene expression. Our data support the notion that a number of SRF cofactors may participate in gene regulation and thereby contribute to the delicate control of gene expression in complex biological processes.

hnulp1, a basic helix-loop-helix protein with a novel transcriptional repressive domain, inhibits transcriptional activity of serum response factor

Many bHLH proteins are involved in cardiac development and cardiovascular diseases. Herein, we identified and characterized the human homologue (hnulp1) of mouse gene nulp1. The predicted protein contains a bHLH domain and a DUF654 domain in N-terminal and C-terminal, respectively. Northern blot analysis shows that a 2.3-kb transcript expressed broadly in early human embryonic and adult tissues, especially with a higher level in adult heart. hnulp1 is a transcription repressor when fused to GAL4 DNA-binding domain and co-transfected with VP-16, in which DUF654 motif represents the basal transcriptional repressive activity. Treatment of cells with trichostatin A can relieve this repression, suggesting that the DUF654 motif acts through increasing deacetylase activity at the GAL4-driven promoter. Overexpression of hnulp1 protein in COS-7 cells inhibits the transcriptional activity of serum response factor (SRF), suggesting that hnulp1 may act as a novel bHLH transcriptional repressor in SRF signaling pathway to mediate cellular functions.

Myocardin induces cardiomyocyte hypertrophy

In response to stress signals, postnatal cardiomyocytes undergo hypertrophic growth accompanied by activation of a fetal gene program, assembly of sarcomeres, and cellular enlargement. We show that hypertrophic signals stimulate the expression and transcriptional activity of myocardin, a cardiac and smooth muscle-specific coactivator of serum response factor (SRF). Consistent with a role for myocardin as a transducer of hypertrophic signals, forced expression of myocardin in cardiomyocytes is sufficient to substitute for hypertrophic signals and induce cardiomyocyte hypertrophy and the fetal cardiac gene program. Conversely, a dominant-negative mutant form of myocardin, which retains the ability to associate with SRF but is defective in transcriptional activation, blocks cardiomyocyte hypertrophy induced by hypertrophic agonists such as phenylephrine and leukemia inhibitory factor. Myocardin-dependent hypertrophy can also be partially repressed by histone deacetylase 5, a transcriptional repressor of myocardin. These findings identify myocardin as a nuclear effector of hypertrophic signaling pathways that couples stress signals to a transcriptional program for postnatal cardiac growth and remodeling.

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.

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.

Kruppel-like factor 4 abrogates myocardin-induced activation of smooth muscle gene expression

Platelet-derived growth factor BB (PDGF-BB) has been shown to be an extremely potent negative regulator of smooth muscle cell (SMC) differentiation. Moreover, previous studies have demonstrated that the Kruppel-like transcription factor (KLF) 4 potently represses the expression of multiple SMC genes. However, the mechanisms whereby KLF4 suppresses SMC gene expression are not known, nor is it clear whether KLF4 contributes to PDGF-BB-induced down-regulation of SMC genes. The goals of the present studies were to determine the molecular mechanisms by which KLF4 represses expression of SMC genes and whether it contributes to PDGF-BB-induced suppression of these genes. Results demonstrated that KLF4 markedly repressed both myocardin-induced activation of SMC genes and expression of myocardin. KLF4 was rapidly up-regulated in PDGF-BB-treated, cultured SMC, and a small interfering RNA to KLF4 partially blocked PDGF-BB-induced SMC gene repression. Both PDGF-BB and KLF4 markedly reduced serum response factor binding to CArG containing regions within intact chromatin. Finally, KLF4, which is normally not expressed in differentiated SMC in vivo, was rapidly up-regulated in vivo in response to vascular injury. Taken together, results indicate that KLF4 represses SMC genes by both down-regulating myocardin expression and preventing serum response factor/myocardin from associating with SMC gene promoters, and suggest that KLF4 may be a key effector of PDGF-BB and injury-induced phenotypic switching of SMC.

Identification of a novel serum response factor cofactor in cardiac gene regulation

The transcription factor serum response factor (SRF) plays an important role in the regulation of a variety of cardiac genes during development and during adult aging. A novel SRF cofactor, herein called p49/STRAP, for SRF-dependent transcription regulation-associated protein, was recently identified in our laboratory. This protein interacted mainly with the transcriptional activation domain of the SRF protein and was found to bind to SRF or to the complex of SRF and another cofactor, such as myocardin or Nkx2.5. The expression of p49/STRAP affected the promoter activity of SRF target genes in a non-uniform manner. For example, p49 activated MLC2v and cardiac actin promoters when it was co-transfected with SRF, but it repressed atrial natriuretic factor promoter activity, which was strongly induced by myocardin. The p49/STRAP mRNA was observed to be highly expressed in fetal, adult, and senescent human hearts, and also in hearts of young adult and old mice, suggesting that p49/STRAP may be an important SRF cofactor in the transcriptional regulation of mammalian cardiac muscle genes throughout the life span.