130-kDa smooth muscle myosin light chain kinase is transcribed from a CArG-dependent, internal promoter within the mouse mylk gene

Itga8-Cre-mediated deletion of YAP and TAZ impairs bladder contractility with minimal inflammation and chondrogenic differentiation

A role of Yes1-associated transcriptional regulator (YAP) and WW domain-containing transcription regulator 1 (TAZ) in vascular and gastrointestinal contractility due to control of myocardin (Myocd) expression, which in turn activates contractile genes, has been demonstrated. Whether this transcriptional hierarchy applies to the urinary bladder is unclear. We found that YAP/TAZ are expressed in human detrusor myocytes and therefore exploited the Itga8-CreERT2 model for the deletion of YAP/TAZ. Recombination occurred in detrusor, and YAP/TAZ transcripts were reduced by >75%. Bladder weights were increased (by ≈22%), but histology demonstrated minimal changes in the detrusor, while arteries in the mucosa were inflamed. Real-time quantitative reverse transcription PCR (RT-qPCR) using the detrusor demonstrated reductions of Myocd (-79 ± 18%) and serum response factor (Srf) along with contractile genes. In addition, the cholinergic receptor muscarinic 2 (Chrm2) and Chrm3 were suppressed (-80 ± 23% and -80 ± 10%), whereas minute increases of Il1b and Il6 were seen. Unlike YAP/TAZ-deficient arteries, SRY (sex-determining region Y)-box 9 (Sox9) did not increase, and no chondrogenic differentiation was apparent. Reductions of smooth muscle myosin heavy chain 11 (Myh11), myosin light-chain kinase gene (Mylk), and Chrm3 were seen at the protein level. Beyond restraining the smooth muscle cell (SMC) program of gene expression, YAP/TAZ depletion silenced SMC-specific splicing, including exon 2a of Myocd. Reduced contractile differentiation was associated with weaker contraction in response to myosin phosphatase inhibition (-36%) and muscarinic activation (reduced by 53% at 0.3 µM carbachol). Finally, short-term overexpression of constitutively active YAP in human embryonic kidney 293 (HEK293) cells increased myocardin (greater than eightfold) along with archetypal target genes, but contractile genes were unaffected or reduced. YAP and TAZ thus regulate myocardin expression in the detrusor, and this is important for SMC differentiation and splicing as well as for contractility.NEW & NOTEWORTHY This study addresses the hypothesis that YAP and TAZ have an overarching role in the transcriptional hierarchy in the smooth muscle of the urinary bladder by controlling myocardin expression. Using smooth muscle-specific and inducible deletion of YAP and TAZ in adult mice, we find that YAP and TAZ control myocardin expression, contractile differentiation, smooth muscle-specific splicing, and bladder contractility. These effects are largely independent of inflammation and chondrogenic differentiation.

The Long Noncoding RNA Cardiac Mesoderm Enhancer-Associated Noncoding RNA (Carmn) Is a Critical Regulator of Gastrointestinal Smooth Muscle Contractile Function and Motility

BACKGROUND & AIMS

Visceral smooth muscle cells (SMCs) are an integral component of the gastrointestinal (GI) tract that regulate GI motility. SMC contraction is regulated by posttranslational signaling and the state of differentiation. Impaired SMC contraction is associated with significant morbidity and mortality, but the mechanisms regulating SMC-specific contractile gene expression, including the role of long noncoding RNAs (lncRNAs), remain largely unexplored. Herein, we reveal a critical role of Carmn (cardiac mesoderm enhancer-associated noncoding RNA), an SMC-specific lncRNA, in regulating visceral SMC phenotype and contractility of the GI tract.

METHODS

Genotype-Tissue Expression and publicly available single-cell RNA sequencing (scRNA-seq) data sets from embryonic, adult human, and mouse GI tissues were interrogated to identify SMC-specific lncRNAs. The functional role of Carmn was investigated using novel green fluorescent protein (GFP) knock-in (KI) reporter/knock-out (KO) mice. Bulk RNA-seq and single nucleus RNA sequencing (snRNA-seq) of colonic muscularis were used to investigate underlying mechanisms.

RESULTS

Unbiased in silico analyses and GFP expression patterns in Carmn GFP KI mice revealed that Carmn is highly expressed in GI SMCs in humans and mice. Premature lethality was observed in global Carmn KO and inducible SMC-specific KO mice due to GI pseudo-obstruction and severe distension of the GI tract, with dysmotility in cecum and colon segments. Histology, GI transit, and muscle myography analysis revealed severe dilation, significantly delayed GI transit, and impaired GI contractility in Carmn KO vs control mice. Bulk RNA-seq of GI muscularis revealed that loss of Carmn promotes SMC phenotypic switching, as evidenced by up-regulation of extracellular matrix genes and down-regulation of SMC contractile genes, including Mylk, a key regulator of SMC contraction. snRNA-seq further revealed SMC Carmn KO not only compromised myogenic motility by reducing contractile gene expression but also impaired neurogenic motility by disrupting cell-cell connectivity in the colonic muscularis. These findings may have translational significance, because silencing CARMN in human colonic SMCs significantly attenuated contractile gene expression, including MYLK, and decreased SMC contractility. Luciferase reporter assays showed that CARMN enhances the transactivation activity of the master regulator of SMC contractile phenotype, myocardin, thereby maintaining the GI SMC myogenic program.

CONCLUSIONS

Our data suggest that Carmn is indispensable for maintaining GI SMC contractile function in mice and that loss of function of CARMN may contribute to human visceral myopathy. To our knowledge this is the first study showing an essential role of lncRNA in the regulation of visceral SMC phenotype.

Myocardin regulates exon usage in smooth muscle cells through induction of splicing regulatory factors

Differentiation of smooth muscle cells (SMCs) depends on serum response factor (SRF) and its co-activator myocardin (MYOCD). The role of MYOCD for the SMC program of gene transcription is well established. In contrast, the role of MYOCD in control of SMC-specific alternative exon usage, including exon splicing, has not been explored. In the current work we identified four splicing factors (MBNL1, RBPMS, RBPMS2, and RBFOX2) that correlate with MYOCD across human SMC tissues. Forced expression of MYOCD family members in human coronary artery SMCs in vitro upregulated expression of these splicing factors. For global profiling of transcript diversity, we performed RNA-sequencing after MYOCD transduction. We analyzed alternative transcripts with three different methods. Exon-based analysis identified 1637 features with differential exon usage. For example, usage of 3´ exons in MYLK that encode telokin increased relative to 5´ exons, as did the 17 kDa telokin to 130 kDa MYLK protein ratio. Dedicated event-based analysis identified 239 MYOCD-driven splicing events. Events involving MBNL1, MCAM, and ACTN1 were among the most prominent, and this was confirmed using variant-specific PCR analyses. In support of a role for RBPMS and RBFOX2 in MYOCD-driven splicing we found enrichment of their binding motifs around differentially spliced exons. Moreover, knockdown of either RBPMS or RBFOX2 antagonized splicing events stimulated by MYOCD, including those involving ACTN1, VCL, and MBNL1. Supporting an in vivo role of MYOCD-SRF-driven splicing, we demonstrate altered Rbpms expression and splicing in inducible and SMC-specific Srf knockout mice. We conclude that MYOCD-SRF, in part via RBPMS and RBFOX2, induce a program of differential exon usage and alternative splicing as part of the broader program of SMC differentiation.

SRF depletion in early life contributes to social interaction deficits in the adulthood

Alterations in social behavior are core symptoms of major developmental neuropsychiatric diseases such as autism spectrum disorders or schizophrenia. Hence, understanding their molecular and cellular underpinnings constitutes the major research task. Dysregulation of the global gene expression program in the developing brain leads to modifications in a number of neuronal connections, synaptic strength and shape, causing unbalanced neuronal plasticity, which may be important substrate in the pathogenesis of neurodevelopmental disorders, contributing to their clinical outcome. Serum response factor (SRF) is a major transcription factor in the brain. The behavioral influence of SRF deletion during neuronal differentiation and maturation has never been studied because previous attempts to knock-out the gene caused premature death. Herein, we generated mice that lacked SRF from early postnatal development to precisely investigate the role of SRF starting in the specific time window before maturation of excitatory synapses that are located on dendritic spine occurs. We show that the time-controlled loss of SRF in neurons alters specific aspects of social behaviors in SRF knock-out mice, and causes deficits in developmental spine maturation at both the structural and functional levels, including downregulated expression of the AMPARs subunits GluA1 and GluA2, and increases the percentage of filopodial/immature dendritic spines. In aggregate, our study uncovers the consequences of postnatal SRF elimination for spine maturation and social interactions revealing novel mechanisms underlying developmental neuropsychiatric diseases.

Transcription factor TEAD1 is essential for vascular development by promoting vascular smooth muscle differentiation

TEAD1 (TEA domain transcription factor 1), a transcription factor known for the functional output of Hippo signaling, is important for tumorigenesis. However, the role of TEAD1 in the development of vascular smooth muscle cell (VSMC) is unknown. To investigate cell-specific role of Tead1, we generated cardiomyocyte (CMC) and VSMC-specific Tead1 knockout mice. We found CMC/VSMC-specific deletion of Tead1 led to embryonic lethality by E14.5 in mice due to hypoplastic cardiac and vascular walls, as a result of impaired CMC and VSMC proliferation. Whole transcriptome analysis revealed that deletion of Tead1 in CMCs/VSMCs downregulated expression of muscle contractile genes and key transcription factors including Pitx2c and myocardin. In vitro studies demonstrated that PITX2c and myocardin rescued TEAD1-dependent defects in VSMC differentiation. We further identified Pitx2c as a novel transcriptional target of TEAD1, and PITX2c exhibited functional synergy with myocardin by directly interacting with myocardin, leading to augment the differentiation of VSMC. In summary, our study reveals a critical role of Tead1 in cardiovascular development in mice, but also identifies a novel regulatory mechanism, whereby Tead1 functions upstream of the genetic regulatory hierarchy for establishing smooth muscle contractile phenotype.

Single nucleotide polymorphisms in the MYLKP1 pseudogene are associated with increased colon cancer risk in African Americans

INTRODUCTION

Pseudogenes are paralogues of functional genes historically viewed as defunct due to either the lack of regulatory elements or the presence of frameshift mutations. Recent evidence, however, suggests that pseudogenes may regulate gene expression, although the functional role of pseudogenes remains largely unknown. We previously reported that MYLKP1, the pseudogene of MYLK that encodes myosin light chain kinase (MLCK), is highly expressed in lung and colon cancer cell lines and tissues but not in normal lung or colon. The MYLKP1 promoter is minimally active in normal bronchial epithelial cells but highly active in lung adenocarcinoma cells. In this study, we further validate MYLKP1 as an oncogene via elucidation of the functional role of MYLKP1 genetic variants in colon cancer risk.

METHODS

Proliferation and migration assays were performed in MYLKP1-transfected colon and lung cancer cell lines (H441, A549) and commercially-available normal lung and colon cells. Fourteen MYLKP1 SNPs (MAFs >0.01) residing within the 4 kb MYLKP1 promoter region, the core 1.4 kb of MYLKP1 gene, and a 4 kb enhancer region were selected and genotyped in a colorectal cancer cohort. MYLKP1 SNP influences on activity of MYLKP1 promoter (2kb) was assessed by dual luciferase reporter assay.

RESULTS

Cancer cell lines, H441 and A549, exhibited increased MYLKP1 expression, increased MYLKP1 luciferase promoter activity, increased proliferation and migration. Genotyping studies identified two MYLKP1 SNPs (rs12490683; rs12497343) that significantly increase risk of colon cancer in African Americans compared to African American controls. Rs12490683 and rs12497343 further increase MYLKP1 promoter activity compared to the wild type MYLKP1 promoter.

CONCLUSION

MYLKP1 is a cancer-promoting pseudogene whose genetic variants differentially enhance cancer risk in African American populations.

Nexilin/NEXN controls actin polymerization in smooth muscle and is regulated by myocardin family coactivators and YAP

Nexilin, encoded by the NEXN gene, is expressed in striated muscle and localizes to Z-discs, influencing mechanical stability. We examined Nexilin/NEXN in smooth muscle cells (SMCs), and addressed if Nexilin localizes to dense bodies and dense bands and whether it is regulated by actin-controlled coactivators from the MRTF (MYOCD, MKL1, MKL2) and YAP/TAZ (YAP1 and WWTR1) families. NEXN expression in SMCs was comparable to that in striated muscles. Immunofluorescence and immunoelectron microscopy suggested that Nexilin localizes to dense bodies and dense bands. Correlations at the mRNA level suggested that NEXN expression might be controlled by actin polymerization. Depolymerization of actin using Latrunculin B repressed the NEXN mRNA and protein in bladder and coronary artery SMCs. Overexpression and knockdown supported involvement of both YAP/TAZ and MRTFs in the transcriptional control of NEXN. YAP/TAZ and MRTFs appeared equally important in bladder SMCs, whereas MRTFs dominated in vascular SMCs. Expression of NEXN was moreover reduced in situations of SMC phenotypic modulation in vivo. The proximal promoter of NEXN conferred control by MRTF-A/MKL1 and MYOCD. NEXN silencing reduced actin polymerization and cell migration, as well as SMC marker expression. NEXN targeting by actin-controlled coactivators thus amplifies SMC differentiation through the actin cytoskeleton, probably via dense bodies and dense bands.

Idiopathic gastroparesis is associated with specific transcriptional changes in the gastric muscularis externa

BACKGROUND

The molecular changes that occur in the stomach that are associated with idiopathic gastroparesis are poorly described. The aim of this study was to use quantitative analysis of mRNA expression to identify changes in mRNAs encoding proteins required for the normal motility functions of the stomach.

METHODS

Full-thickness stomach biopsy samples were collected from non-diabetic control subjects who exhibited no symptoms of gastroparesis and from patients with idiopathic gastroparesis. mRNA was isolated from the muscularis externa and mRNA expression levels were determined by quantitative reverse transcriptase (RT)-PCR.

KEY RESULTS

Smooth muscle tissue from idiopathic gastroparesis patients had decreased expression of mRNAs encoding several contractile proteins, such as MYH11 and MYLK1. Conversely, there was no significant change in mRNAs characteristic of interstitial cells of Cajal (ICCs) such as KIT or ANO1. There was also a significant decrease in mRNA-encoding platelet-derived growth factor receptor α (PDGFRα) and its ligand PDGFB and in Heme oxygenase 1 in idiopathic gastroparesis subjects. In contrast, there was a small increase in mRNA characteristic of neurons. Although there was not an overall change in KIT expression in gastroparesis patients, KIT expression showed a significant correlation with gastric emptying whereas changes in MYLK1, ANO1 and PDGFRα showed weak correlations to the fullness/satiety subscore of patient assessment of upper gastrointestinal disorder-symptom severity index scores.

CONCLUSIONS AND INFERENCES

Our findings suggest that idiopathic gastroparesis is associated with altered smooth muscle cell contractile protein expression and loss of PDGFRα⁺ cells without a significant change in ICCs.

Distinct molecular subtypes of uterine leiomyosarcoma respond differently to chemotherapy treatment

Background

Uterine leiomyosarcoma (ULMS) is an aggressive form of soft tissue tumors. The molecular heterogeneity and pathogenesis of ULMS are not well understood.

Methods

Expression profiling data were used to determine the possibility and optimal number of ULMS molecular subtypes. Next, clinicopathological characters and molecular pathways were analyzed in each subtype to prospect the clinical applications and progression mechanisms of ULMS.

Results

Two distinct molecular subtypes of ULMS were defined based on different gene expression signatures. Subtype I ULMS recapitulated low-grade ULMS, the gene expression pattern of which resembled normal smooth muscle cells, characterized by overexpression of smooth muscle function genes such as LMOD1, SLMAP, MYLK, MYH11. In contrast, subtype II ULMS recapitulated high-grade ULMS with higher tumor weight and invasion rate, and was characterized by overexpression of genes involved in the pathway of epithelial to mesenchymal transition and tumorigenesis, such as CDK6, MAPK13 and HOXA1.

Conclusions

We identified two distinct molecular subtypes of ULMS responding differently to chemotherapy treatment. Our findings provide a better understanding of ULMS intrinsic molecular subtypes, and will potentially facilitate the development of subtype-specific diagnosis biomarkers and therapy strategies for these tumors.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3568-y) contains supplementary material, which is available to authorized users.

Myosin Light Chain Kinase MYLK1: Anatomy, Interactions, Functions, and Regulation

This review discusses and summarizes the results of molecular and cellular investigations of myosin light chain kinase (MLCK, MYLK1), the key regulator of cell motility. The structure and regulation of a complex mylk1 gene and the domain organization of its products is presented. The interactions of the mylk1 gene protein products with other proteins and posttranslational modifications of the mylk1 gene protein products are reviewed, which altogether might determine the role and place of MLCK in physiological and pathological reactions of cells and entire organisms. Translational potential of MLCK as a drug target is evaluated.

Regulator of G protein signaling 4 is a novel target of GATA-6 transcription factor

GATA transcription factors regulate an array of genes important in cell proliferation and differentiation. Here we report the identification of regulator of G protein signaling 4 (RGS4) as a novel target for GATA-6 transcription factor. Although three sites (a, b, c) within the proximal region of rabbit RGS4 promoter for GATA transcription factors were predicted by bioinformatics analysis, only GATA-a site (16 bp from the core TATA box) is essential for RGS4 transcriptional regulation. RT-PCR analysis demonstrated that only GATA-6 was highly expressed in rabbit colonic smooth muscle cells but GATA-4/6 were expressed in cardiac myocytes and GATA-1/2/3 expressed in blood cells. Adenovirus-mediated expression of GATA-6 but not GATA-1 significantly increased the constitutive and IL-1β-induced mRNA expression of the endogenous RGS4 in colonic smooth muscle cells. IL-1β stimulation induced GATA-6 nuclear translocation and increased GATA-6 binding to RGS4 promoter. These data suggest that GATA factor could affect G protein signaling through regulating RGS4 expression, and GATA signaling may develop as a future therapeutic target for RGS4-related diseases.

Phosphorylation of the regulatory light chain of myosin in striated muscle: methodological perspectives

Phosphorylation of the regulatory light chain (RLC) of myosin modulates cellular functions such as muscle contraction, mitosis, and cytokinesis. Phosphorylation defects are implicated in a number of diseases. Here we focus on striated muscle where changes in RLC phosphorylation relate to diseases such as hypertrophic cardiomyopathy and muscular dystrophy, or age-related changes. RLC phosphorylation in smooth muscle and non-muscle cells are covered briefly where relevant. There is much scientific interest in controlling the phosphorylation levels of RLC in vivo and in vitro in order to understand its physiological function in striated muscles. A summary of available and emerging in vivo and in vitro methods is presented. The physiological role of RLC phosphorylation and novel pathways are discussed to highlight the differences between muscle types and to gain insights into disease processes.

Autoregulatory Control of Smooth Muscle Myosin Light Chain Kinase Promoter by Notch Signaling

Smooth muscle myosin light chain kinase (SM-MLCK) is the key enzyme responsible for phosphorylation of regulatory myosin light chain (MLC20), resulting in actin-myosin cross-bridging and force generation in vascular smooth muscle required for physiological vasoreactivity and blood pressure control. In this study, we investigated the combinatorial role of myocardin/serum response factor (SRF) and Notch signaling in the transcriptional regulation of MLCK gene expression. Promoter reporter analyses in rat A10 smooth muscle cells revealed a bimodal pattern of MLCK promoter activity and gene expression upon stimulation with constitutively active Notch1 in presence of myocardin or by Jagged1 ligand stimulation. An initial Notch1-induced increase in MLCK transcription was followed by loss in promoter sensitivity, which could be restored with further Notch1 dose escalation. Real-time PCR analyses revealed that endogenous levels of Hairy Related Transcription (HRT) factor 2 (HRT2) peaked concurrently with inhibitory concentrations of Notch1. Forced expression of HRT2 demonstrated simultaneous repression of both myocardin- and Notch1-induced MLCK promoter activity. HRT2-mediated repression was further confirmed by HRT2 truncations and siHRT2 treatments that rescued MLCK promoter activity and gene expression. Chromatin immunoprecipitation studies revealed both Jagged1 ligand- and Notch1-enhanced myocardin/SRF complex formation at the promoter CArG element. In contrast, heightened levels of HRT2 concomitantly disrupted myocardin/SRF and Notch transcription complex formation at respective CArG and CSL binding elements. Taken together, SM-MLCK promoter activity appears highly sensitive to the relative levels of Notch1 signaling, HRT2, and myocardin. These findings identify a novel Notch-dependent HRT2 autoregulatory circuit coordinating transcriptional regulation of SM-MLCK.

Sp1/Sp3 transcription factors regulate hallmarks of megakaryocyte maturation and platelet formation and function

Sp1 and Sp3 belong to the specificity proteins (Sp)/Krüppel-like transcription factor family. They are closely related, ubiquitously expressed, and recognize G-rich DNA motifs. They are thought to regulate generic processes such as cell-cycle and growth control, metabolic pathways, and apoptosis. Ablation of Sp1 or Sp3 in mice is lethal, and combined haploinsufficiency results in hematopoietic defects during the fetal stages. Here, we show that in adult mice, conditional pan-hematopoietic (Mx1-Cre) ablation of either Sp1 or Sp3 has minimal impact on hematopoiesis, whereas the simultaneous loss of Sp1 and Sp3 results in severe macrothrombocytopenia. This occurs in a cell-autonomous manner as shown by megakaryocyte-specific (Pf4-Cre) double-knockout mice. We employed flow cytometry, cell culture, and electron microscopy and show that although megakaryocyte numbers are normal in bone marrow and spleen, they display a less compact demarcation membrane system and a striking inability to form proplatelets. Through megakaryocyte transcriptomics and platelet proteomics, we identified several cytoskeleton-related proteins and downstream effector kinases, including Mylk, that were downregulated upon Sp1/Sp3 depletion, providing an explanation for the observed defects in megakaryopoiesis. Supporting this notion, selective Mylk inhibition by ML7 affected proplatelet formation and stabilization and resulted in defective ITAM receptor-mediated platelet aggregation.

Cytosolic carboxypeptidase CCP6 is required for megakaryopoiesis by modulating Mad2 polyglutamylation

Ye et al. identify cytosolic carboxypeptidase CCP6 as a protein required for the regulation of bone marrow megakaryopoiesis in mice. The authors find that Mad2 (a core component of spindle checkpoint in mitosis) is a substrate of CCP6 in megakaryocytes and is polyglutamylated by proteins TTLL6 and TTLL4, subsequently affecting the activity of Aurora B kinase. Mad2 is thus additionally implicated in megakaryopoiesis regulation.

Bone marrow progenitor cells develop into mature megakaryocytes (MKs) to produce platelets for hemostasis and other physiological functions. However, the molecular mechanisms underlying megakaryopoiesis are not completely defined. We show that cytosolic carboxypeptidase (CCP) 6 deficiency in mice causes enlarged spleens and increased platelet counts with underdeveloped MKs and dysfunctional platelets. The prominent phenotypes of CCP6 deficiency are different from those of CCP1-deficient mice. We found that CCP6 and tubulin tyrosine ligase-like family (TTLL) members TTLL4 and TTLL6 are highly expressed in MKs. We identify Mad2 (mitotic arrest deficient 2) as a novel substrate for CCP6 and not CCP1. Mad2 can be polyglutamylated by TTLL4 and TTLL6 to modulate the maturation of MKs. CCP6 deficiency causes hyperglutamylation of Mad2 to promote activation of Aurora B, leading to suppression of MK maturation. We reveal that Mad2 polyglutamylation plays a critical role in the regulation of megakaryopoiesis.

Regulation of 130-kDa smooth muscle myosin light chain kinase expression by an intronic CArG element

The mylk1 gene encodes a 220-kDa nonmuscle myosin light chain kinase (MLCK), a 130-kDa smooth muscle MLCK (smMLCK), as well as the non-catalytic product telokin. Together, these proteins play critical roles in regulating smooth muscle contractility. Changes in their expression are associated with many pathological conditions; thus, it is important to understand the mechanisms regulating expression of mylk1 gene transcripts. Previously, we reported a highly conserved CArG box, which binds serum response factor, in intron 15 of mylk1. Because this CArG element is near the promoter that drives transcription of the 130-kDa smMLCK, we examined its role in regulating expression of this transcript. Results show that deletion of the intronic CArG region from a β-galactosidase reporter gene abolished transgene expression in mice in vivo. Deletion of the CArG region from the endogenous mylk1 gene, specifically in smooth muscle cells, decreased expression of the 130-kDa smMLCK by 40% without affecting expression of the 220-kDa MLCK or telokin. This reduction in 130-kDa smMLCK expression resulted in decreased phosphorylation of myosin light chains, attenuated smooth muscle contractility, and a 24% decrease in small intestine length that was associated with a significant reduction of Ki67-positive smooth muscle cells. Overall, these data show that the CArG element in intron 15 of the mylk1 gene is necessary for maximal expression of the 130-kDa smMLCK and that the 130-kDa smMLCK isoform is specifically required to regulate smooth muscle contractility and small intestine smooth muscle cell proliferation.

Inactivation of Serum Response Factor Contributes To Decrease Vascular Muscular Tone and Arterial Stiffness in Mice

Rationale:

Vascular smooth muscle (SM) cell phenotypic modulation plays an important role in arterial stiffening associated with aging. Serum response factor (SRF) is a major transcription factor regulating SM genes involved in maintenance of the contractile state of vascular SM cells.

Objective:

We investigated whether SRF and its target genes regulate intrinsic SM tone and thereby arterial stiffness.

Methods and Results:

The SRF gene was inactivated SM-specific knockout of SRF (SRF SMKO ) specifically in vascular SM cells by injection of tamoxifen into adult transgenic mice. Fifteen days later, arterial pressure and carotid thickness were lower in SRF SMKO than in control mice. The carotid distensibility/pressure and elastic modulus/wall stress curves showed a greater arterial elasticity in SRF SMKO without modification in collagen/elastin ratio. In SRF SMKO , vasodilation was decreased in aorta and carotid arteries, whereas a decrease in contractile response was found in mesenteric arteries. By contrast, in mice with inducible SRF overexpression, the in vitro contractile response was significantly increased in all arteries. Without endothelium, the contraction was reduced in SRF SMKO compared with control aortic rings owing to impairment of the NO pathway. Contractile components (SM-actin and myosin light chain), regulators of the contractile response (myosin light chain kinase, myosin phosphatase target subunit 1, and protein kinase C–potentiated myosin phosphatase inhibitor) and integrins were reduced in SRF SMKO .

Conclusions:

SRF controls vasoconstriction in mesenteric arteries via vascular SM cell phenotypic modulation linked to changes in contractile protein gene expression. SRF-related decreases in vasomotor tone and cell-matrix attachment increase arterial elasticity in large arteries.

Brain-derived neurotrophic factor induces matrix metalloproteinase 9 expression in neurons via the serum response factor/c-Fos pathway

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in the regulation of the transcription of genes that encode proplasticity proteins. In the present study, we provide evidence that stimulation of rat primary cortical neurons with BDNF upregulates matrix metalloproteinase 9 (MMP-9) mRNA and protein levels and increases enzymatic activity. The BDNF-induced MMP-9 transcription was dependent on extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and c-Fos expression. Overexpression of AP-1 dimers in neurons led to MMP-9 promoter activation, with the most potent being those that contained c-Fos, whereas knockdown of endogenous c-Fos by small hairpin RNA (shRNA) reduced BDNF-mediated MMP-9 transcription. Additionally, mutation of the proximal AP-1 binding site in the MMP-9 promoter inhibited the activation of MMP-9 transcription. BDNF stimulation of neurons induced binding of endogenous c-Fos to the proximal MMP-9 promoter region. Furthermore, as the c-Fos gene is a known target of serum response factor (SRF), we investigated whether SRF contributes to MMP-9 transcription. Inhibition of SRF and its cofactors by either overexpression of dominant negative mutants or shRNA decreased MMP-9 promoter activation. In contrast, MMP-9 transcription was not dependent on CREB activity. Finally, we showed that neuronal activity stimulates MMP-9 transcription in a tyrosine kinase receptor B (TrkB)-dependent manner.

Notch transcriptional control of vascular smooth muscle regulatory gene expression and function

Notch receptors and ligands mediate heterotypic cell signaling that is required for normal vascular development. Dysregulation of select Notch receptors in mouse vascular smooth muscle (VSM) and in genetic human syndromes causes functional impairment in some regional circulations, the mechanistic basis of which is undefined. In this study, we used a dominant-negative Mastermind-like (DNMAML1) to block signaling through all Notch receptors specifically in VSM to more broadly test a functional role for this pathway in vivo. Mutant DNMAML1-expressing mice exhibited blunted blood pressure responses to vasoconstrictors, and their aortic, femoral, and mesenteric arteries had reduced contractile responses to agonists and depolarization in vitro. The mutant arteries had significant and specific reduction in the expression and activity of myosin light chain kinase (MLCK), a primary regulator of VSM force production. Conversely, activated Notch signaling in VSM cells induced endogenous MLCK transcript levels. We identified MLCK as a direct target of activated Notch receptor as demonstrated by an evolutionarily conserved Notch-responsive element within the MLCK promoter that binds the Notch receptor complex and is required for transcriptional activity. We conclude that Notch signaling through the transcriptional control of key regulatory proteins is required for contractile responses of mature VSM. Genetic or pharmacological manipulation of Notch signaling is a potential strategy for modulating arterial function in human disease.

Employment of gene expression profiling to identify transcriptional regulators of hepatic stellate cells

Activated hepatic stellate cells (HSC) play a central role in scar formation that leads to liver fibrosis. The molecular mechanisms underlying this process are not fully understood. Microarray and bioinformatics analyses have proven to be useful in identifying transcription factors that regulate cellular processes such as cell differentiation. Using oligonucleotide microarrays, we performed transcriptional analyses of activated human HSC cultured on Matrigel-coated tissue culture dishes. Examination of microarray data following Matrigel-induced deactivation of HSC revealed a significant down-regulation of myocardin, an important transcriptional regulator in smooth and cardiac muscle development. Thus, gene expression profiling as well as functional assays of activated HSC have provided the first evidence of the involvement of myocardin in HSC activation.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

Nonmuscle myosin light-chain kinase deficiency attenuates atherosclerosis in apolipoprotein E-deficient mice via reduced endothelial barrier dysfunction and monocyte migration

BACKGROUND

Endothelial dysfunction and monocyte migration are key events in the pathogenesis of atherosclerosis. Nonmuscle myosin light-chain kinase (nmMLCK), the predominant MLCK isoform in endothelial cells, has been shown to contribute to vascular inflammation by altering endothelial barrier function. However, its impact on atherogenesis remains unknown.

METHODS AND RESULTS

We investigated the role of nmMLCK in the development of atherosclerotic lesions in apolipoprotein E-deficient (apoE(-/-)) mice fed an atherogenic diet for 12 weeks. Histopathological examination demonstrated that nmMLCK deficiency (apoE(-/-)nmmlck(-/-)) reduced the size of aortic lesions by 53%, lipid contents by 44%, and macrophage deposition by 40%. Western blotting and reverse-transcription polymerase chain reaction revealed the expression of nmMLCK in aortic endothelial cells and peripheral blood monocytes. Measurements of transendothelial electric resistance indicated that nmMLCK deficiency attenuated endothelial barrier dysfunction caused by thrombin, oxidized low-density lipoprotein, and tumor necrosis factor α. In monocytes, nmMLCK deficiency reduced their migration in response to the chemokine monocyte chemoattractant protein-1. Further mechanistic studies showed that nmMLCK acted through both myosin light chain phosphorylation-coupled and -uncoupled pathways; the latter involved Rous sacracoma virus homolog genes-encoded tyrosine kinases (Src) signaling. Moreover, depletion of Src via gene silencing, site-specific mutagenesis, or pharmacological inhibition of Src greatly attenuated nmMLCK-dependent endothelial barrier dysfunction and monocyte migration.

CONCLUSIONS

Nonmuscle myosin light-chain kinase contributes to atherosclerosis by regulating endothelial barrier function and monocyte migration via mechanisms involving not only kinase-mediated MLC phosphorylation but also Src activation.

A transcribed pseudogene of MYLK promotes cell proliferation

Pseudogenes are considered nonfunctional genomic artifacts of catastrophic pathways. Recent evidence, however, indicates novel roles for pseudogenes as regulators of gene expression. We tested the functionality of myosin light chain kinase pseudogene (MYLKP1) in human cells and tissues by RT-PCR, promoter activity, and cell proliferation assays. MYLKP1 is partially duplicated from the original MYLK gene that encodes nonmuscle and smooth muscle myosin light chain kinase (smMLCK) isoforms and regulates cell contractility and cytokinesis. Despite strong homology with the smMLCK promoter (∼ 89.9%), the MYLKP1 promoter is minimally active in normal bronchial epithelial cells but highly active in lung adenocarcinoma cells. Moreover, MYLKP1 and smMLCK exhibit negatively correlated transcriptional patterns in normal and cancer cells with MYLKP1 strongly expressed in cancer cells and smMLCK highly expressed in non-neoplastic cells. For instance, expression of smMLCK decreased (19.5 ± 4.7 fold) in colon carcinoma tissues compared to normal colon tissues. Mechanistically, MYLKP1 overexpression inhibits smMLCK expression in cancer cells by decreasing RNA stability, leading to increased cell proliferation. These studies provide strong evidence for the functional involvement of pseudogenes in carcinogenesis and suggest MYLKP1 as a potential novel diagnostic or therapeutic target in human cancers.

Regulation of serum response factor activity and smooth muscle cell apoptosis by chromodomain helicase DNA-binding protein 8

Serum response factor (SRF) is a widely expressed protein that plays a key role in the regulation of smooth muscle differentiation, proliferation, migration, and apoptosis. It is generally accepted that one mechanism by which SRF regulates these diverse functions is through pathway-specific cofactor interactions. A novel SRF cofactor, chromodomain helicase DNA binding protein 8 (CHD8), was isolated from a yeast two-hybrid screen using SRF as bait. CHD8 is highly expressed in adult smooth muscle tissues. Coimmunoprecipitation assays from A10 smooth muscle cells demonstrated binding of endogenous SRF and CHD8. Data from GST-pulldown assays indicate that the NH(2)-terminus of CHD8 can interact directly with the MADS domain of SRF. Adenoviral-mediated knockdown of CHD8 in smooth muscle cells resulted in attenuated expression of SRF-dependent, smooth muscle-specific genes. Knockdown of CHD8, SRF, or CTCF, a previously described binding partner of CHD8, in A10 VSMCs also resulted in a marked induction of apoptosis. Mechanistically, apoptosis induced by CHD8 knockdown was accompanied by attenuated expression of the anti-apoptotic proteins, Birc5, and CARD10, whereas SRF knockdown attenuated expression of CARD10 and Mcl-1, but not Birc5, and CTCF knockdown attenuated expression of Birc5. These data suggest that CHD8 plays a dual role in smooth muscle cells modulating SRF activity toward differentiation genes and promoting cell survival through interactions with both SRF and CTCF to regulate expression of Birc5 and CARD10.

Differential transcriptional analysis between red and white skeletal muscle of Chinese Meishan pigs

In order to better understand and elucidate the major determinants of red and white muscle phenotypic properties, the global gene expression profiling was performed in white (longissimus doris) and red (soleus) skeletal muscle of Chinese Meishan pigs using the Affymetrix Porcine Genechip. 550 transcripts at least 1.5-fold difference were identified at p < 0.05 level, with 323 showing increased expression and 227 decreased expression in red muscle. Quantitative real-time PCR validated the differential expression of eleven genes (alpha-Actin, ART3, GATA-6, HMOX1, HSP, MYBPH, OCA2, SLC12A4, TGFB1, TGFB3 and TNX). Twenty eight signaling pathways including ECM-receptor interaction, focal adhesion, TGF-beta signaling pathway, MAPK signaling pathway, Wnt signaling pathway, mTOR signaling pathway, insulin signaling pathway and cell cycle, were identified using KEGG pathway database. Our findings demonstrate previously unrecognized changes in gene transcription between red and white muscle, and some potential cascades identified in the study merit further investigation.

Gene expression profiling and functional assays of activated hepatic stellate cells suggest that myocardin has a role in activation

BACKGROUND

Myofibroblast-like cells derived from transdifferentiated hepatic stellate cells (HSC) play a central role in scar formation that leads to liver fibrosis. The molecular mechanisms underlying this process are not fully understood.

AIM

Our aim was to identify genes that are differentially regulated by HSC activation and to explore their function.

METHODS

Using oligonucleotide microarrays, we performed transcriptional analysis of the human HSC cell line, LI90, cultured on Matrigel. Microarray data were validated by quantitative real-time polymerase chain reaction and Western blotting. The function of myocardin was assessed by myocardin RNAi and overexpression.

RESULTS

Examination of Matrigel-induced deactivation of LI90 cells revealed marked downregulation of myocardin, an important transcriptional regulator in smooth and cardiac muscle development. Small interfering RNA-mediated suppression of myocardin expression in both activated LI90 and rat activated HSC resulted in loss of the phenotypic characteristics of myofibroblasts and significantly impaired the production of activated HSC markers, such as alpha-smooth muscle actin and extracellular matrix proteins like type I collagen. Overexpression of myocardin led to the upregulation of these marker genes. Myocardin was upregulated in rat primary HSC during in vitro activation and in the fibrotic liver of a dimethylnitrosamine-induced fibrosis rat model.

CONCLUSIONS

This study demonstrates that myocardin is involved in the activation of HSC; myocardin may serve as a novel therapeutic target in the treatment of liver fibrosis.

The smooth muscle cell-restricted KCNMB1 ion channel subunit is a direct transcriptional target of serum response factor and myocardin

Large conductance calcium-activated potassium (MaxiK) channels play a pivotal role in maintaining normal arterial tone by regulating the excitation-contraction coupling process. MaxiK channels comprise alpha and beta subunits encoded by Kcnma and the cell-restricted Kcnmb genes, respectively. Although the functionality of MaxiK channel subunits has been well studied, the molecular regulation of their transcription and modulation in smooth muscle cells (SMCs) is incomplete. Using several model systems, we demonstrate down-regulation of Kcnmb1 mRNA upon SMC phenotypic modulation in vitro and in vivo. As part of a broad effort to define all functional CArG elements in the genome (i.e. the CArGome), we discovered two conserved CArG boxes located in the proximal promoter and first intron of the human KCNMB1 gene. Gel shift and chromatin immunoprecipitation assays confirmed serum response factor (SRF) binding to both CArG elements. A luciferase assay showed myocardin (MYOCD)-mediated transactivation of the KCNMB1 promoter in a CArG element-dependent manner. In vivo analysis of the human KCNMB1 promoter disclosed activity in embryonic heart and aortic SMCs; mutation of both conserved CArG elements completely abolished in vivo promoter activity. Forced expression of MYOCD increased Kcnmb1 expression in a variety of rodent and human non-SMC lines with no effect on expression of the Kcnma1 subunit. Conversely, knockdown of Srf resulted in decreases of endogenous Kcnmb1. Functional studies demonstrated MYOCD-induced, iberiotoxin-sensitive potassium currents in porcine coronary SMCs. These results reveal the first ion channel subunit as a direct target of SRF-MYOCD transactivation, providing further insight into the role of MYOCD as a master regulator of the SMC contractile phenotype.

Expression of CPI-17 in smooth muscle during embryonic development and in neointimal lesion formation

Ca(2+) sensitivity of smooth muscle (SM) contraction is determined by CPI-17, an inhibitor protein for myosin light chain phosphatase (MLCP). CPI-17 is highly expressed in mature SM cells, but the expression level varies under pathological conditions. Here, we determined the expression of CPI-17 in embryonic SM tissues and arterial neointimal lesions using immunohistochemistry. As seen in adult animals, the predominant expression of CPI-17 was detected at SM tissues on mouse embryonic sections, whereas MLCP was ubiquitously expressed. Compared with SM alpha-actin, CPI-17 expression doubled in arterial SM from embryonic day E10 to E14. Like SM alpha-actin and other SM marker proteins, CPI-17 was expressed in embryonic heart, and the expression was down-regulated at E17. In adult rat, CPI-17 expression level was reduced to 30% in the neointima of injured rat aorta, compared with the SM layers, whereas the expression of MLCP was unchanged in both regions. Unlike other SM proteins, CPI-17 was detected at non-SM organs in the mouse embryo, such as embryonic neurons and epithelium. Thus, CPI-17 expression is reversibly controlled in response to the phenotype transition of SM cells that restricts the signal to differentiated SM cells and particular cell types.

The serum response factor and a putative novel transcription factor regulate expression of the immediate-early gene Arc/Arg3.1 in neurons

The immediate-early effector gene Arc/Arg3.1 is robustly upregulated by synaptic activity associated with learning and memory. Here we show in primary cortical neuron culture that diverse stimuli induce Arc expression through new transcription. Searching for regulatory regions important for Arc transcription, we found nine DNaseI-sensitive nucleosome-depleted sites at this genomic locus. A reporter gene encompassing these sites responded to synaptic activity in an NMDA receptor-dependent manner, consistent with endogenous Arc mRNA. Responsiveness mapped to two enhancer regions approximately 6.5 kb and approximately 1.4 kb upstream of Arc. We dissected these regions further and found that the proximal enhancer contains a functional and conserved "Zeste-like" response element that binds a putative novel nuclear protein in neurons. Therefore, activity regulates Arc transcription partly by a novel signaling pathway. We also found that the distal enhancer has a functional and highly conserved serum response element. This element binds serum response factor, which is recruited by synaptic activity to regulate Arc. Thus, Arc is the first target of serum response factor that functions at synapses to mediate plasticity.

Myocardin is sufficient for a smooth muscle-like contractile phenotype

BACKGROUND

Myocardin (Myocd) is a strong coactivator that binds the serum response factor (SRF) transcription factor over CArG elements embedded within smooth muscle cell (SMC) and cardiac muscle cyto-contractile genes. Here, we sought to ascertain whether Myocd-mediated gene expression confers a structural and physiological cardiac or SMC phenotype.

METHODS AND RESULTS

Adenoviral-mediated expression of Myocd in the BC(3)H1 cell line induces cardiac and SMC genes while suppressing both skeletal muscle markers and cell growth. Immunofluorescence microscopy shows that SRF and a SMC-like cyto-contractile apparatus are elevated with Myocd overexpression. A short hairpin RNA to Srf impairs BC(3)H1 cyto-architecture; however, cotransduction with Myocd results in complete restoration of the cyto-architecture. Electron microscopic studies demonstrate a SMC ultrastructural phenotype with no evidence for cardiac sarcomerogenesis. Biochemical and time-lapsed videomicroscopy assays reveal clear evidence for Myocd-induced SMC-like contraction.

CONCLUSIONS

Myocd is sufficient for the establishment of a SMC-like contractile phenotype.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Hprt-targeted transgenes provide new insights into smooth muscle-restricted promoter activity

Mouse telokin and SM22alpha promoters have previously been shown to direct smooth muscle cell-specific expression of transgenes in vivo in adult mice. However, the activity of these promoters is highly dependent on the integration site of the transgene. In the current study, we found that the ectopic expression of telokin promoter transgenes could be abolished by flanking the transgene with insulator elements from the H19 gene. However, the insulator elements did not increase the proportion of mouse lines that exhibited consistent, detectable levels of transgene expression. In contrast, when transgenes were targeted to the hprt locus, both telokin and SM22alpha promoters resulted in reproducible patterns and levels of transgene expression in all lines of mice examined. Telokin promoter transgene expression was restricted to smooth muscle tissues in adult and embryonic mice. As reported previously, SM22alpha transgenes were expressed at high levels specifically in arterial smooth muscle cells; however, in contrast to randomly integrated transgenes, the hprt-targeted SM22alpha transgenes were also expressed at high levels in smooth muscle cells in veins, bladder, and gallbladder. Using hprt-targeted transgenes, we further analyzed elements within the telokin promoter required for tissue specific activity in vivo. Analysis of these transgenes revealed that the CArG element in the telokin promoter is required for promoter activity in all tissues and that the CArG element and adjacent AT-rich region are sufficient to drive transgene expression in bladder but not intestinal smooth muscle cells.

Tumor Necrosis Factor-induced Long Myosin Light Chain Kinase Transcription Is Regulated by Differentiation-dependent Signaling Events

Myosin light chain kinase (MLCK) is expressed as long and short isoforms from unique transcriptional start sites within a single gene. Tumor necrosis factor (TNF) augments intestinal epithelial long MLCK expression, which is critical to cytoskeletal regulation. We found that TNF increases long MLCK mRNA transcription, both in human enterocytes in vitro and murine enterocytes in vivo.5'-RACE identified two novel exons, 1A and 1B, which encode alternative long MLCK transcriptional start sites. Chromatin immunoprecipitation (ChIP) and site-directed mutagenesis identified two essential Sp1 sites upstream of the exon 1A long MLCK transcriptional start site. Analysis of deletion and truncation mutants showed that a 102-bp region including these Sp1 sites was necessary for basal transcription. A promoter construct including 4-kb upstream of exon 1A was responsive to TNF, AP-1, or NFkappaB, but all except NFkappaB responses were absent in a shorter 2-kb construct, and all responses were absent in a 1-kb construct. Electrophoretic mobility shift assays, ChIP, and site-directed mutagenesis explained these data by identifying three functional AP-1 sites between 2- and 4-kb upstream of exon 1A and two NFkappaB sites between 1- and 2-kb upstream of exon 1A. Analysis of differentiating epithelia showed that only well differentiated enterocytes activated the 4-kb long MLCK promoter in response to TNF, and consensus promoter reporters demonstrated that TNF-induced NFkappaB activation decreased during differentiation while TNF-induced AP-1 activation increased. Thus either AP-1 or NFkappaB can up-regulate long MLCK transcription, but the mechanisms by which TNF up-regulates intestinal epithelial long MLCK transcription from exon 1A are differentiation-dependent.

Regulation of myosin light chain kinase and telokin expression in smooth muscle tissues

The mylk1 gene is a large gene spanning approximately 250 kb and comprising at least 31 exons. The mylk1 gene encodes at least four protein products: two isoforms of the 220-kDa myosin light chain kinase (MLCK), a 130-kDa MLCK, and telokin. Transcripts encoding these products are derived from four independent promoters within the mylk1 gene. The kinases expressed from the mylk1 gene have been extensively characterized and function to regulate the activity of nonmuscle and smooth muscle myosin II. Activation of these myosin motors by MLCK modulates a variety of contractile processes, including smooth muscle contraction, cell adhesion, migration, and proliferation. Dysregulation of these processes contributes to a number of diseases. The noncatalytic gene product telokin also has been shown to modulate contraction in smooth muscle cells through its ability to inhibit myosin light chain phosphatase. Given the crucial role of the products of the mylk1 gene in regulating numerous contractile processes, it seems intuitive that alterations in the transcriptional activity of the mylk1 gene also will have a significant impact on many physiological and pathological processes. In this review we highlight some of the recent studies that have described the transcriptional regulation of mylk1 gene products in smooth muscle tissues and discuss the implications of these findings for regulation of expression of other smooth muscle-specific genes.