Acceleration of myosin light chain dephosphorylation and relaxation of smooth muscle by telokin. Synergism with cyclic nucleotide-activated kinase

p90RSK2, a new MLCK mediates contractility in myosin light chain kinase null smooth muscle

Introduction: Phosphorylation of smooth muscle (SM) myosin regulatory light chain (RLC20) is a critical switch leading to SM contraction. The canonical view held that only the short isoform of myosin light chain kinase (MLCK1) catalyzed this reaction. It is now accepted that auxiliary kinases may contribute to vascular SM tone and contractility. We have previously reported that p90 ribosomal S6 kinase (RSK2) functions as such a kinase, in parallel with MLCK1, contributing ∼25% of the maximal myogenic force in resistance arteries. Thus, RSK2 may be instrumental in the regulation of basal vascular tone and blood pressure. Here, we take advantage of a MLCK1 null mouse (mylk1 -/-) to further test our hypothesis that RSK2 can function as an MLCK, playing a significant physiological role in SM contractility. Methods: Using fetal (E14.5-18.5) SM tissues, as embryos die at birth, we investigated the necessity of MLCK for contractility and fetal development and determined the ability of RSK2 kinase to compensate for the lack of MLCK and characterized its signaling pathway in SM. Results and Discussion: Agonists induced contraction and RLC20 phosphorylation in mylk1 -/- SM was attenuated by RSK2 inhibition. The pCa-tension relationships in permeabilized strips of bladder showed no difference in Ca2+ sensitivity in WT vs mylk1 -/- muscles, although the magnitude of force responses was considerably smaller in the absence of MLCK. The magnitude of contractile responses was similar upon addition of GTPγS to activate the RhoA/ROCK pathway or calyculinA to inhibit the myosin phosphatase. The Ca2+-dependent tyrosine kinase, Pyk2, contributed to RSK2-mediated contractility and RLC20 phosphorylation. Proximity-ligation and immunoprecipitation assays demonstrated an association of RSK2, PDK1 and ERK1/2 with MLCK and actin. RSK2, PDK1, ERK1/2 and MLCK formed a signaling complex on the actin filament, positioning them for interaction with adjacent myosin heads. The Ca2+-dependent component reflected the agonist mediated increases in Ca2+, which activated the Pyk2/PDK1/RSK2 signaling cascade. The Ca2+-independent component was through activation of Erk1/2/PDK1/RSK2 leading to direct phosphorylation of RLC20, to increase contraction. Overall, RSK2 signaling constitutes a new third signaling pathway, in addition to the established Ca2+/CaM/MLCK and RhoA/ROCK pathways to regulate SM contractility.

p90RSK2, a new MLCK, rescues contractility in myosin light chain kinase null smooth muscle

Background

Phosphorylation of smooth muscle (SM) myosin regulatory light chain (RLC 20 ) is a critical switch leading to contraction or cell migration. The canonical view held that the only kinase catalyzing this reaction is the short isoform of myosin light chain kinase (MLCK1). Auxiliary kinases may be involved and play a vital role in blood pressure homeostasis. We have previously reported that p90 ribosomal S6 kinase (RSK2) functions as such a kinase, in parallel with the classical MLCK1, contributing ∼25% of the maximal myogenic force in resistance arteries and regulating blood pressure. Here, we take advantage of a MLCK1 null mouse to further test our hypothesis that RSK2 can function as an MLCK, playing a significant physiological role in SM contractility.

Methods

Fetal (E14.5-18.5) SM tissues were used as embryos die at birth. We investigated the necessity of MLCK for contractility, cell migration and fetal development and determined the ability of RSK2 kinase to compensate for the lack of MLCK and characterized it's signaling pathway in SM.

Results

Agonists induced contraction and RLC 20 phosphorylation in mylk1 -/- SM, that was inhibited by RSK2 inhibitors. Embryos developed and cells migrated in the absence of MLCK. The pCa-tension relationships in WT vs mylk1 -/- muscles demonstrated a Ca 2+ -dependency due to the Ca 2+ -dependent tyrosine kinase Pyk2, known to activate PDK1 that phosphorylates and fully activates RSK2. The magnitude of contractile responses was similar upon addition of GTPγS to activate the RhoA/ROCK pathway. The Ca 2+ -independent component was through activation of Erk1/2/PDK1/RSK2 leading to direct phosphorylation of RLC 20 , to increase contraction. RSK2, PDK1, Erk1/2 and MLCK formed a signaling complex on the actin filament, optimally positioning them for interaction with adjacent myosin heads.

Conclusions

RSK2 signaling constitutes a new third signaling pathway, in addition to the established Ca 2+ /CAM/MLCK and RhoA/ROCK pathways to regulate SM contractility and cell migration.

EPAC in Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are major components of blood vessels. They regulate physiological functions, such as vascular tone and blood flow. Under pathological conditions, VSMCs undergo a remodeling process known as phenotypic switching. During this process, VSMCs lose their contractility and acquire a synthetic phenotype, where they over-proliferate and migrate from the tunica media to the tunica interna, contributing to the occlusion of blood vessels. Since their discovery as effector proteins of cyclic adenosine 3′,5′-monophosphate (cAMP), exchange proteins activated by cAMP (EPACs) have been shown to play vital roles in a plethora of pathways in different cell systems. While extensive research to identify the role of EPAC in the vasculature has been conducted, much remains to be explored to resolve the reported discordance in EPAC’s effects. In this paper, we review the role of EPAC in VSMCs, namely its regulation of the vascular tone and phenotypic switching, with the likely involvement of reactive oxygen species (ROS) in the interplay between EPAC and its targets/effectors.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

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.

Synthetic Peptides as cGMP-Independent Activators of cGMP-Dependent Protein Kinase Iα

PKG is a multifaceted signaling molecule and potential pharmaceutical target due to its role in smooth muscle function. A helix identified in the structure of the regulatory domain of PKG Iα suggests a novel architecture of the holoenzyme. In this study, a set of synthetic peptides (S-tides), derived from this helix, was found to bind to and activate PKG Iα in a cyclic guanosine monophosphate (cGMP)-independent manner. The most potent S-tide derivative (S1.5) increased the open probability of the potassium channel KCa1.1 to levels equivalent to saturating cGMP. Introduction of S1.5 to smooth muscle cells in isolated, endothelium-denuded cerebral arteries through a modified reversible permeabilization procedure inhibited myogenic constriction. In contrast, in endothelium-intact vessels S1.5 had no effect on myogenic tone. This suggests that PKG Iα activation by S1.5 in vascular smooth muscle would be sufficient to inhibit augmented arterial contractility that frequently occurs following endothelial damage associated with cardiovascular disease.

Mechanisms of Vascular Smooth Muscle Contraction and the Basis for Pharmacologic Treatment of Smooth Muscle Disorders

The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.

Role of Telokin in Regulating Murine Gastric Fundus Smooth Muscle Tension

Telokin phosphorylation by cyclic GMP-dependent protein kinase facilitates smooth muscle relaxation. In this study we examined the relaxation of gastric fundus smooth muscles from basal tone, or pre-contracted with KCl or carbachol (CCh), and the phosphorylation of telokin S13, myosin light chain (MLC) S19, MYPT1 T853, T696, and CPI-17 T38 in response to 8-Bromo-cGMP, the NO donor sodium nitroprusside (SNP), or nitrergic neurotransmission. We compared MLC phosphorylation and the contraction and relaxation responses of gastric fundus smooth muscles from telokin-/- mice and their wild-type littermates to KCl or CCh, and 8-Bromo-cGMP, SNP, or nitrergic neurotransmission, respectively. We compared the relaxation responses and telokin phosphorylation of gastric fundus smooth muscles from wild-type mice and W/WV mice which lack ICC-IM, to 8-Bromo-cGMP, SNP, or nitrergic neurotransmission. We found that telokin S13 is basally phosphorylated and that 8-Bromo-cGMP and SNP increased basal telokin phosphorylation. In muscles pre-contracted with KCl or CCh, 8-Bromo-cGMP and SNP had no effect on CPI-17 or MYPT1 phosphorylation, but increased telokin phosphorylation and reduced MLC phosphorylation. In telokin-/- gastric fundus smooth muscles, basal tone and constitutive MLC S19 phosphorylation were increased. Pre-contracted telokin-/- gastric fundus smooth muscles have increased contractile responses to KCl, CCh, or cholinergic neurotransmission and reduced relaxation to 8-Bromo-cGMP, SNP, and nitrergic neurotransmission. However, basal telokin phosphorylation was not increased when muscles were stimulated with lower concentrations of SNP or when the muscles were stimulated by nitrergic neurotransmission. SNP, but not nitrergic neurotransmission, increased telokin Ser13 phosphorylation in both wild-type and W/WV gastric fundus smooth muscles. Our findings indicate that telokin may play a role in attenuating constitutive MLC phosphorylation and provide an additional mechanism to augment gastric fundus mechanical responses to inhibitory neurotransmission.

In vivo epicardial force and strain characterisation in normal and MLP-knockout murine hearts

The study's objective is to quantify in vivo epicardial force and strain in the normal and transgenic myocardium using microsensors.Male mice (n = 39), including C57BL/6 (n = 26), 129/Sv (n = 5), wild-type (WT) C57  ×  129Sv (n = 5), and muscle LIM protein (MLP) knock-out (n = 3), were studied under 1.5% isoflurane anaesthesia. Microsurgery allowed the placement of two piezoelectric crystals at longitudinal epicardial loci at the basal, middle, and apical LV regions, and the independent (and/or concurrent) placement of a cantilever force sensor. The findings demonstrate longitudinal contractile and relaxation strains that ranged between 4.8-9.3% in the basal, middle, and apical regions of C57BL/6 mice, and in the mid-ventricular regions of 129/Sv, WT, and MLP mice. Measured forces ranged between 3.1-8.9 mN. The technique's feasibility is also demonstrated in normal mice following afterload, occlusion-reperfusion challenges.Furthermore, the total mid-ventricular forces developed in MLP mice were significantly reduced compared to the WT controls (5.9  ±  0.4 versus 8.9  ±  0.2 mN, p < 0.0001), possibly owing to the fibrotic and stiffer myocardium. No significant strain differences were noted between WT and MLP mice.The possibility of quantifying in vivo force and strain from the normal murine heart is demonstrated with a potential usefulness in the characterisation of transgenic and diseased mice, where regional myocardial function may be significantly altered.

Inhibition of MLC20 phosphorylation downstream of Ca2+ and RhoA: A novel mechanism involving phosphorylation of myosin phosphatase interacting protein (M-RIP) by PKG and stimulation of MLC phosphatase activity

Previous studies have shown that cGMP-dependent protein kinase (PKG) act on several targets in the contractile pathway to reduce intracellular Ca(2+) and/or augment RhoA-regulated myosin light chain phosphatase (MLCP) activity and cause muscle relaxation. Recent studies have identified a novel protein M-RIP that associates with MYPT1, the regulatory subunit of MLCP. Herein, we examine whether PKG enhance MLCP activity downstream of Ca(2+) and RhoA via phosphorylation of M-RIP in gastric smooth muscle cells. Treatment of permeabilized muscle cells with 10 μM Ca(2+) caused an increase in MLC20 phosphorylation and muscle contraction, but had no effect on Rho kinase activity. Activators of PKG (GSNO or cGMP) decreased MLC20 phosphorylation and contraction in response to 10 μM Ca(2+), implying existence of inhibitory mechanism independent of Ca(2+) and RhoA. The effect of PKG on Ca(2+)-induced MLC20 phosphorylation was attenuated by M-RIP siRNA. Both GSNO and 8-pCPT-cGMP induced phosphorylation of M-RIP; phosphorylation was accompanied by an increase in the association of M-RIP with MYPT1 and MLCP activity. Taken together, these results provide evidence that PKG induces phosphorylation of M-RIP and enhances its association with MYPT1 to augment MLCP activity and MLC20 dephosphorylation and inhibits muscle contraction, downstream of Ca(2+)- or RhoA-dependent pathways.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Cyclic AMP-Rap1A signaling activates RhoA to induce α(2c)-adrenoceptor translocation to the cell surface of microvascular smooth muscle cells

Intracellular signaling by the second messenger cyclic AMP (cAMP) activates the Ras-related small GTPase Rap1 through the guanine exchange factor Epac. This activation leads to effector protein interactions, activation, and biological responses in the vasculature, including vasorelaxation. In vascular smooth muscle cells derived from human dermal arterioles (microVSM), Rap1 selectively regulates expression of G protein-coupled α(2C)-adrenoceptors (α(2C)-ARs) through JNK-c-jun nuclear signaling. The α(2C)-ARs are generally retained in the trans-Golgi compartment and mobilize to the cell surface and elicit vasoconstriction in response to cellular stress. The present study used human microVSM to examine the role of Rap1 in receptor localization. Complementary approaches included murine microVSM derived from tail arteries of C57BL6 mice that express functional α(2C)-ARs and mice deficient in Rap1A (Rap1A-null). In human microVSM, increasing intracellular cAMP by direct activation of adenylyl cyclase by forskolin (10 μM) or selectively activating Epac-Rap signaling by the cAMP analog 8-pCPT-2'-O-Me-cAMP (100 μM) activated RhoA, increased α(2C)-AR expression, and reorganized the actin cytoskeleton, increasing F-actin. The α(2C)-ARs mobilized from the perinuclear region to intracellular filamentous structures and to the plasma membrane. Similar results were obtained in murine wild-type microVSM, coupling Rap1-Rho-actin dynamics to receptor relocalization. This signaling was impaired in Rap1A-null murine microVSM and was rescued by delivery of constitutively active (CA) mutant of Rap1A. When tested in heterologous HEK293 cells, Rap1A-CA or Rho-kinase (ROCK-CA) caused translocation of functional α(2C)-ARs to the cell surface (~4- to 6-fold increase, respectively). Together, these studies support vascular bed-specific physiological role of Rap1 and suggest a role in vasoconstriction in microVSM.

Molecular mechanism of telokin-mediated disinhibition of myosin light chain phosphatase and cAMP/cGMP-induced relaxation of gastrointestinal smooth muscle

Phospho-telokin is a target of elevated cyclic nucleotide concentrations that lead to relaxation of gastrointestinal and some vascular smooth muscles (SM). Here, we demonstrate that in telokin-null SM, both Ca(2+)-activated contraction and Ca(2+) sensitization of force induced by a GST-MYPT1(654-880) fragment inhibiting myosin light chain phosphatase were antagonized by the addition of recombinant S13D telokin, without changing the inhibitory phosphorylation status of endogenous MYPT1 (the regulatory subunit of myosin light chain phosphatase) at Thr-696/Thr-853 or activity of Rho kinase. Cyclic nucleotide-induced relaxation of force in telokin-null ileum muscle was reduced but not correlated with a change in MYPT1 phosphorylation. The 40% inhibited activity of phosphorylated MYPT1 in telokin-null ileum homogenates was restored to nonphosphorylated MYPT1 levels by addition of S13D telokin. Using the GST-MYPT1 fragment as a ligand and SM homogenates from WT and telokin KO mice as a source of endogenous proteins, we found that only in the presence of endogenous telokin, thiophospho-GST-MYPT1 co-precipitated with phospho-20-kDa myosin regulatory light chain 20 and PP1. Surface plasmon resonance studies showed that S13D telokin bound to full-length phospho-MYPT1. Results of a protein ligation assay also supported interaction of endogenous phosphorylated MYPT1 with telokin in SM cells. We conclude that the mechanism of action of phospho-telokin is not through modulation of the MYPT1 phosphorylation status but rather it contributes to cyclic nucleotide-induced relaxation of SM by interacting with and activating the inhibited full-length phospho-MYPT1/PP1 through facilitating its binding to phosphomyosin and thus accelerating 20-kDa myosin regulatory light chain dephosphorylation.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

The promise of inhibition of smooth muscle tone as a treatment for erectile dysfunction: where are we now?

Ten years ago, the inhibition of Rho kinase by intracavernosal injection of Y-27632 was found to induce an erectile response. This effect did not require activation of nitric oxide-mediated signaling, introducing a novel target pathway for the treatment of erectile dysfunction (ED), with potential added benefit in cases where nitric oxide bioavailability is attenuated (and thus phosphodiesterase type 5 (PDE5) inhibitors are less efficacious). Rho-kinase antagonists are currently being developed and tested for a wide range of potential uses. The inhibition of this calcium-sensitizing pathway results in blood vessel relaxation. It is also possible that blockade of additional smooth muscle contractile signaling mechanisms may have the same effect. In this review, we conducted an extensive search of pertinent literature using PUBMED. We have outlined the various pathways involved in the maintenance of penile smooth muscle tone and discussed the current potential benefit for the pharmacological inhibition of these targets for the treatment of ED.

Relaxin regulates myofibroblast contractility and protects against lung fibrosis

Myofibroblasts are specialized contractile cells that participate in tissue fibrosis and remodeling, including idiopathic pulmonary fibrosis (IPF). Mechanotransduction, a process by which mechanical stimuli are converted into biochemical signals, regulates myofibroblast differentiation. Relaxin is a peptide hormone that mediates antifibrotic effects through regulation of collagen synthesis and turnover. In this study, we demonstrate enhanced myofibroblast contraction in bleomycin-induced lung fibrosis in mice and in fibroblastic foci of human subjects with IPF, using phosphorylation of the regulatory myosin light chain (MLC(20)) as a biomarker of in vivo cellular contractility. Compared with wild-type mice, relaxin knockout mice express higher lung levels of phospho-MLC(20) and develop more severe bleomycin-induced lung fibrosis. Exogenous relaxin inhibits MLC(20) phosphorylation and bleomycin-induced lung fibrosis in both relaxin knockout and wild-type mice. Ex vivo studies of IPF lung myofibroblasts demonstrate decreases in MLC(20) phosphorylation and reduced contractility in response to relaxin. Characterization of the signaling pathway reveals that relaxin regulates MLC(20) dephosphorylation and lung myofibroblast contraction by inactivating RhoA/Rho-associated protein kinase through a nitric oxide/cGMP/protein kinase G-dependent mechanism. These studies identify a novel antifibrotic role of relaxin involving the inhibition of the contractile phenotype of lung myofibroblasts and suggest that targeting myofibroblast contractility with relaxin-like peptides may be of therapeutic benefit in the treatment of fibrotic lung disease.

The cAMP-responsive Rap1 guanine nucleotide exchange factor, Epac, induces smooth muscle relaxation by down-regulation of RhoA activity

Agonist activation of the small GTPase, RhoA, and its effector Rho kinase leads to down-regulation of smooth muscle (SM) myosin light chain phosphatase activity, an increase in myosin light chain (RLC(20)) phosphorylation and force. Cyclic nucleotides can reverse this process. We report a new mechanism of cAMP-mediated relaxation through Epac, a GTP exchange factor for the small GTPase Rap1 resulting in an increase in Rap1 activity and suppression of RhoA activity. An Epac-selective cAMP analog, 8-pCPT-2'-O-Me-cAMP ("007"), significantly reduced agonist-induced contractile force, RLC(20), and myosin light chain phosphatase phosphorylation in both intact and permeabilized vascular, gut, and airway SMs independently of PKA and PKG. The vasodilator PGI(2) analog, cicaprost, increased Rap1 activity and decreased RhoA activity in intact SMs. Forskolin, phosphodiesterase inhibitor isobutylmethylxanthine, and isoproterenol also significantly increased Rap1-GTP in rat aortic SM cells. The PKA inhibitor H89 was without effect on the 007-induced increase in Rap1-GTP. Lysophosphatidic acid-induced RhoA activity was reduced by treatment with 007 in WT but not Rap1B null fibroblasts, consistent with Epac signaling through Rap1B to down-regulate RhoA activity. Isoproterenol-induced increase in Rap1 activity was inhibited by silencing Epac1 in rat aortic SM cells. Evidence is presented that cooperative cAMP activation of PKA and Epac contribute to relaxation of SM. Our findings demonstrate a cAMP-mediated signaling mechanism whereby activation of Epac results in a PKA-independent, Rap1-dependent Ca(2+) desensitization of force in SM through down-regulation of RhoA activity. Cyclic AMP inhibition of RhoA is mediated through activation of both Epac and PKA.

Hypersensitivity to thromboxane receptor mediated cerebral vasomotion and CBF oscillations during acute NO-deficiency in rats

BACKGROUND

Low frequency (4-12 cpm) spontaneous fluctuations of the cerebrovascular tone (vasomotion) and oscillations of the cerebral blood flow (CBF) have been reported in diseases associated with endothelial dysfunction. Since endothelium-derived nitric oxide (NO) suppresses constitutively the release and vascular effects of thromboxane A(2) (TXA(2)), NO-deficiency is often associated with activation of thromboxane receptors (TP). In the present study we hypothesized that in the absence of NO, overactivation of the TP-receptor mediated cerebrovascular signaling pathway contributes to the development of vasomotion and CBF oscillations.

METHODOLOGY/PRINCIPAL FINDINGS

Effects of pharmacological modulation of TP-receptor activation and its downstream signaling pathway have been investigated on CBF oscillations (measured by laser-Doppler flowmetry in anesthetized rats) and vasomotion (measured by isometric tension recording in isolated rat middle cerebral arteries, MCAs) both under physiological conditions and after acute inhibition of NO synthesis. Administration of the TP-receptor agonist U-46619 (1 µg/kg i.v.) to control animals failed to induce any changes of the systemic or cerebral circulatory parameters. Inhibition of the NO synthesis by nitro-L-arginine methyl ester (L-NAME, 100 mg/kg i.v.) resulted in increased mean arterial blood pressure and a decreased CBF accompanied by appearance of CBF-oscillations with a dominant frequency of 148±2 mHz. U-46619 significantly augmented the CBF-oscillations induced by L-NAME while inhibition of endogenous TXA(2) synthesis by ozagrel (10 mg/kg i.v.) attenuated it. In isolated MCAs U-46619 in a concentration of 100 nM, which induced weak and stable contraction under physiological conditions, evoked sustained vasomotion in the absence of NO, which effect could be completely reversed by inhibition of Rho-kinase by 10 µM Y-27632.

CONCLUSION/SIGNIFICANCE

These results suggest that hypersensitivity of the TP-receptor-Rho-kinase signaling pathway contributes to the development of low frequency cerebral vasomotion which may propagate to vasospasm in pathophysiological states associated with NO-deficiency.

Vascular smooth muscle phenotypic diversity and function

The control of force production in vascular smooth muscle is critical to the normal regulation of blood flow and pressure, and altered regulation is common to diseases such as hypertension, heart failure, and ischemia. A great deal has been learned about imbalances in vasoconstrictor and vasodilator signals, e.g., angiotensin, endothelin, norepinephrine, and nitric oxide, that regulate vascular tone in normal and disease contexts. In contrast there has been limited study of how the phenotypic state of the vascular smooth muscle cell may influence the contractile response to these signaling pathways dependent upon the developmental, tissue-specific (vascular bed) or disease context. Smooth, skeletal, and cardiac muscle lineages are traditionally classified into fast or slow sublineages based on rates of contraction and relaxation, recognizing that this simple dichotomy vastly underrepresents muscle phenotypic diversity. A great deal has been learned about developmental specification of the striated muscle sublineages and their phenotypic interconversions in the mature animal under the control of mechanical load, neural input, and hormones. In contrast there has been relatively limited study of smooth muscle contractile phenotypic diversity. This is surprising given the number of diseases in which smooth muscle contractile dysfunction plays a key role. This review focuses on smooth muscle contractile phenotypic diversity in the vascular system, how it is generated, and how it may determine vascular function in developmental and disease contexts.

Kinase-related protein/telokin inhibits Ca2+-independent contraction in Triton-skinned guinea pig taenia coli

KRP (kinase-related protein), also known as telokin, has been proposed to inhibit smooth muscle contractility by inhibiting the phosphorylation of the rMLC (regulatory myosin light chain) by the Ca2+-activated MLCK (myosin light chain kinase). Using the phosphatase inhibitor microcystin, we show in the present study that KRP also inhibits Ca2+-independent rMLC phosphorylation and smooth muscle contraction mediated by novel Ca2+-independent rMLC kinases. Incubating KRP-depleted Triton-skinned taenia coli with microcystin at pCa>8 induced a slow contraction reaching 90% of maximal force (Fmax) at pCa 4.5 after approximately 25 min. Loading the fibres with KRP significantly slowed down the force development, i.e. the time to reach 50% of Fmax was increased from 8 min to 35 min. KRP similarly inhibited rMLC phosphorylation of HMM (heavy meromyosin) in vitro by MLCK or by the constitutively active MLCK fragment (61K-MLCK) lacking the myosin-docking KRP domain. A C-terminally truncated KRP defective in myosin binding inhibited neither force nor HMM phosphorylation. Phosphorylated KRP inhibited the rMLC phosphorylation of HMM in vitro and Ca2+-insensitive contractions in fibres similar to unphosphorylated KRP, whereby the phosphorylation state of KRP was not altered in the fibres. We conclude that (i) KRP inhibits not only MLCK-induced contractions, but also those elicited by Ca2+-independent rMLC kinases; (ii) phosphorylation of KRP does not modulate this effect; (iii) binding of KRP to myosin is essential for this inhibition; and (iv) KRP inhibition of rMLC phosphorylation is most probably due to the shielding of the phosphorylation site on the rMLC.

Increased degradation of MYPT1 contributes to the development of tolerance to nitric oxide in porcine pulmonary artery

Myosin phosphatase target subunit 1 (MYPT1) is the regulatory subunit of myosin light chain phosphatase (MLCP). It plays a critical role in vasodilatation induced by cGMP-elevating agents such as nitric oxide (NO). The present study was performed to determine the role of MYPT1 in the development of tolerance of the pulmonary artery to NO. Incubation of isolated porcine pulmonary arteries for 24 or 48 h with DETA NONOate (DETA NO) significantly reduced protein levels of MYPT1 and the leucine zipper-positive (LZ+) isoform of MYPT1 but not that of PP1cdelta. The extent of reduction in total MYPT1 protein level was comparable to that of MYPT1 (LZ+). The decrease in MYPT1 protein caused by 48-h DETA NO incubation was prevented by ODQ, an inhibitor of guanylyl cyclase, and by inhibitors of proteasomes (MG-132 and lactacystin) but was not affected by the inhibitor of protein synthesis, cycloheximide. A reduction in MYPT1 protein was also obtained with 8-bromo-cGMP, but this was prevented by Rp-8-bromo-PET-cGMP [inhibitor of cGMP-dependent protein kinase (PKG)]. Incubation for 48 h with DETA NO also reduced dephosphorylation of myosin light chain and relaxation of the artery in response to DETA NO, which was prevented by MG-132. These results suggest that the reduction in MYPT1 protein contributes to the development of tolerance of pulmonary arteries to NO. This may result from increased degradation of MYPT1 after prolonged PKG activation.

Mlck1a is expressed in zebrafish thrombocytes and is an essential component of thrombus formation

BACKGROUND

We have used the advantages of the zebrafish model system to demonstrate which of the vertebrate myosin light chain kinase (MLCK) genes is expressed in thrombocytes and important for thrombus formation.

METHODS AND RESULTS

Here we report that Mlck1a is an essential component of thrombus formation. Phylogenetic data revealed four zebrafish orthologous for three human MLCK genes. To investigate expression of the zebrafish mlck genes in thrombocytes we compared GFP-tagged platelets with other cells by microarray analysis, and showed that mlck1a expression was 4.5-fold enriched in platelets. Furthermore, mlck1a mRNA and mRNA for the platelet-specific cd41 co-localized in thrombi. Expression of other mlck subtypes was lower in GFP-tagged platelets (mlck1b; 0.77-fold enriched) and absent in thrombi (mlck1b, -2, -3). To investigate the role of Mlck1a in thrombus formation, we knocked down mlck1a using two morpholinos. This resulted in impaired morphology changes of platelets adhering on fibrinogen. In a thrombosis model, in which thrombocytes adhere to the vessel wall damaged by laser irradiation, thrombus formation was slowed down in mlck1a-deficient embryos.

CONCLUSION

We conclude that Mlck1a is the subtype of MLCK that contributes to platelet shape change and thrombus formation.

Regulation of smooth muscle contraction by small GTPases

Next to changes in cytosolic [Ca(2+)], members of the Rho subfamily of small GTPases, in particular Rho and its effector Rho kinase, also known as ROK or ROCK, emerged as key regulators of smooth muscle function in health and disease. In this review, we will focus on the regulation of the contractile machinery by Rho/ROK signaling and its interaction with PKC and cyclic nucleotide signaling. We will briefly discuss the emerging evidence that remodeling of cortical actin is necessary for contraction.

Degradation of leucine zipper-positive isoform of MYPT1 may contribute to development of nitrate tolerance

AIMS

A depressed cGMP-dependent protein kinase (PKG) activity is implicated in nitrate tolerance. The present study determines whether the leucine zipper-positive (LZ+) isoform of myosin phosphatase target subunit 1 (MYPT1), a key target protein for PKG actions, is involved in the development of nitrate tolerance.

METHODS AND RESULTS

Nitrate tolerance in in vitro preparations was obtained by a 24 h incubation with nitroglycerin (NTG). Nitrate tolerance in in vivo preparations was obtained by subcutaneous injection of mice with NTG, and the aortas were used. Protein levels of total MYPT1, MYPT1 (LZ+), PP1Cdelta, myosin light chain (MLC), and phosphorylated MLC were determined by Western blot analysis. Isometric vessel tension was determined by an organ chamber technique. Protein levels of MYPT1 (LZ+), but not of PP1Cdelta, were significantly reduced in in vitro and in vivo nitrate-tolerant arteries. The decrease in the MYPT1 (LZ+) protein level of coronary artery was also induced by a nitric oxide donor and a cGMP analogue, which was prevented by the inhibitors of soluble guanylyl cyclase and PKG. The decrease in MYPT1 (LZ+) protein levels was not affected by the inhibitor of protein synthesis, but was prevented by the inhibitors of proteasomes. The diminished inhibition of dephosphorylation of MLC as well as the attenuated relaxation of porcine coronary artery and mouse aorta to NTG was improved by proteasome inhibitors.

CONCLUSION

This study demonstrates that a reduction in the protein level of MYPT1 (LZ+) is involved in nitrate tolerance. This may result in part from a proteasome-dependent degradation of MYPT1 (LZ+).

The role of the calponin homology domain of smoothelin-like 1 (SMTNL1) in myosin phosphatase inhibition and smooth muscle contraction

In this study, we provide further insight into the contribution of the smoothelin-like 1 (SMTNL1) calponin homology (CH)-domain on myosin light chain phosphatase (SMPP-1M) activity and smooth muscle contraction. SMTNL1 protein was shown to have inhibitory effects on SMPP-1M activity but not on myosin light chain kinase (MLCK) activity. Treatment of beta-escin permeabilized rabbit, ileal smooth muscle with SMTNL1 had no effect on the time required to reach half-maximal force (t(1/2)) during stimulation with pCa6.3 solution. The addition of recombinant SMTNL1 protein to permeabilized, smooth muscle strips caused a significant decrease in contractile force. While the calponin homology (CH)-domain was essential for maximal SMTNL1-associated relaxation, it alone did not cause significant changes in force. SMTNL1 was poorly dephosphorylated by PP-1C in the presence of the myosin targeting subunit (MYPT1), suggesting that phosphorylated SMTNL1 does not possess "substrate trapping" properties. Moreover, while full-length SMTNL1 could suppress SMPP-1M activity toward LC(20) in vitro, truncated SMTNL1 lacking the CH-domain was ineffective. In summary, our findings suggest an important role for the CH-domain in mediating the effects of SMTNL1 on smooth muscle contraction.

Phosphorylation-dependent autoinhibition of myosin light chain phosphatase accounts for Ca2+ sensitization force of smooth muscle contraction

The reversible regulation of myosin light chain phosphatase (MLCP) in response to agonist stimulation and cAMP/cGMP signals plays an important role in the regulation of smooth muscle (SM) tone. Here, we investigated the mechanism underlying the inhibition of MLCP induced by the phosphorylation of myosin phosphatase targeting subunit (MYPT1), a regulatory subunit of MLCP, at Thr-696 and Thr-853 using glutathione S-transferase (GST)-MYPT1 fragments having the inhibitory phosphorylation sites. GST-MYPT1 fragments, including only Thr-696 and only Thr-853, inhibited purified MLCP (IC(50) = 1.6 and 60 nm, respectively) when they were phosphorylated with RhoA-dependent kinase (ROCK). The activities of isolated catalytic subunits of type 1 and type 2A phosphatases (PP1 and PP2A) were insensitive to either fragment. Phospho-GST-MYPT1 fragments docked directly at the active site of MLCP, and this was blocked by a PP1/PP2A inhibitor microcystin (MC)-LR or by mutation of the active sites in PP1. GST-MYPT1 fragments induced a contraction of beta-escin-permeabilized ileum SM at constant pCa 6.3 (EC(50) = 2 microm), which was eliminated by Ala substitution of the fragment at Thr-696 or by ROCK inhibitors or 8Br-cGMP. GST-MYPT1-(697-880) was 5-times less potent than fragments including Thr-696. Relaxation induced by 8Br-cGMP was not affected by Ala substitution at Ser-695, a known phosphorylation site for protein kinase A/G. Thus, GST-MYPT1 fragments are phosphorylated by ROCK in permeabilized SM and mimic agonist-induced inhibition and cGMP-induced activation of MLCP. We propose a model in which MYPT1 phosphorylation at Thr-696 and Thr-853 causes an autoinhibition of MLCP that accounts for Ca(2+) sensitization of smooth muscle force.

Nitric oxide-induced biphasic mechanism of vascular relaxation via dephosphorylation of CPI-17 and MYPT1

Nitric oxide (NO) from endothelium is a major mediator of vasodilatation through cGMP/PKG signals that lead to a decrease in Ca(2+) concentration. In addition, NO-mediated signals trigger an increase in myosin light chain phosphatase (MLCP) activity. To evaluate the mechanism of NO-induced relaxation through MLCP deinhibition, we compared time-dependent changes in Ca(2+), myosin light chain (MLC) phosphorylation and contraction to changes in phosphorylation levels of CPI-17 at Thr38, RhoA at Ser188, and MYPT1 at Ser695, Thr696 and Thr853 in response to sodium nitroprusside (SNP)-induced relaxation in denuded rabbit femoral artery. During phenylephrine (PE)-induced contraction, SNP reduced CPI-17 phosphorylation to a minimal value within 15 s, in parallel with decreases in Ca(2+) and MLC phosphorylation, followed by a reduction of contractile force having a latency period of about 15 s. MYPT1 phosphorylation at Ser695, the PKG-target site, increased concurrently with relaxation. Phosphorylation of RhoA, MYPT1 Thr696 and Thr853 differed significantly at 5 min but not within 1 min of SNP exposure. Inhibition of Ca(2+) release delayed SNP-induced relaxation while inhibition of Ca(2+) channel, BK(Ca) channel or phosphodiesterase-5 did not. Pretreatment of resting artery with SNP suppressed an increase in Ca(2+), contractile force and phosphorylation of MLC, CPI-17, MYPT1 Thr696 and Thr853 at 10 s after PE stimulation, but had no effect on phorbol ester-induced CPI-17 phosphorylation. Together, these results suggest that NO production suppresses Ca(2+) release, which causes an inactivation of PKC and rapid CPI-17 dephosphorylation as well as MLCK inactivation, resulting in rapid MLC dephosphorylation and relaxation.

Losartan decreases p42/44 MAPK signaling and preserves LZ+ MYPT1 expression

Heart failure is associated with impairment in nitric oxide (NO) mediated vasodilatation, which has been demonstrated to result from a reduction in the relative expression of the leucine zipper positive (LZ+) isoform of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase. Further, captopril preserves normal LZ+ MYPT1 expression, the sensitivity to cGMP-mediated vasodilatation and modulates the expression of genes in the p42/44 MAPK and p38 MAPK signaling cascades. This study tests whether angiotensin receptor blockade (ARB) with losartan decreases p42/44 MAPK or p38 MAPK signaling and preserves LZ+ MYPT1 expression in a rat infarct model of heart failure. In aortic smooth muscle, p42/44 MAPK activation increases and LZ+ MYPT1 expression falls after LAD ligation. Losartan treatment decreases the activation of p42/44 MAPK to the uninfarcted control level and preserves normal LZ+ MYPT1 expression. The expression and activation of p38 MAPK, however, is low and does not change following LAD ligation or with losartan therapy. These data suggest that either reducing or blocking the effects of circulating angiotensin II, both decreases the activation of the p42/44 MAPK signaling cascade and preserves LZ+ MYPT1 expression. Thus, the ability of ACE-inhibitors and ARBs to modulate the vascular phenotype, to preserve normal flow mediated vasodilatation may explain the beneficial effects of these drugs compared to other forms of afterload reduction in the treatment of heart failure.

The potential role of MLC phosphatase and MAPK signalling in the pathogenesis of vascular dysfunction in heart failure

The clinical syndrome of heart failure is associated with both a resting vasoconstriction and reduced sensitivity to nitric oxide mediated vasodilatation, and this review will focus on the role of myosin light chain (MLC) phosphatase in the pathogenesis of the vascular abnormalities of heart failure. Nitric oxide mediates vasodilatation by an activation of guanylate cyclase and an increase in the production of cGMP, which leads to the activation of the type I cGMP-dependent protein kinase (PKGI). PKGI then activates a number of targets that produce smooth muscle relaxation including MLC phosphatase. MLC phosphatase is a holoenzyme consisting of three subunits; a 20 kD subunit of unknown function, an approximately 38-kD catalytic subunit and a myosin targeting subunit (MYPT1). Alternative splicing of a 31 bp 3 exon generates MYPT1 isoforms, which differ by a COOH-terminus leucine zipper (LZ). Further, PKGI-mediated activation of MLC phosphatase requires the expression of a LZ+ MYPT1. Congestive heart failure is associated with a decrease in LZ+ MYPT1 expression, which results in a decrease in the sensitivity to cGMP-mediated smooth muscle relaxation. Beyond their ability to reduce afterload, angiotensin converting enzyme (ACE) inhibitors have a number of beneficial effects that include maintaining the expression of the LZ+ MYPT1 isoform, thereby conserving normal sensitivity to cGMP-mediated vasodilatation, as well as differentially regulating genes associated with mitogen activated protein kinase (MAPK) signalling. ACE inhibition reduces circulating angiotensin II and thus limits the downstream activation of MAPK signalling pathways, possibly preventing the alteration of the vascular phenotype to preserve normal vascular function.

Thromboxane A2-induced bi-directional regulation of cerebral arterial tone

Myosin light chain phosphatase plays a critical role in modulating smooth muscle contraction in response to a variety of physiologic stimuli. A downstream target of the RhoA/Rho-kinase and nitric oxide (NO)/cGMP/cyclic GMP-dependent kinase (cGKI) pathways, myosin light chain phosphatase activity reflects the sum of both calcium sensitization and desensitization pathways through phosphorylation and dephosphorylation of the myosin phosphatase targeting subunit (MYPT1). As cerebral blood flow is highly spatio-temporally modulated under normal physiologic conditions, severe perturbations in normal cerebral blood flow, such as in cerebral vasospasm, can induce neurological deficits. In nonpermeabilized cerebral vessels stimulated with U-46619, a stable mimetic of endogenous thromboxane A2 implicated in the etiology of cerebral vasospasm, we observed significant increases in contractile force, RhoA activation, regulatory light chain phosphorylation, as well as phosphorylation of MYPT1 at Thr-696, Thr-853, and surprisingly Ser-695. Inhibition of nitric oxide signaling completely abrogated basal MYPT1 Ser-695 phosphorylation and significantly increased and potentiated U-46619-induced MYPT1 Thr-853 phosphorylation and contractile force, indicating that NO/cGMP/cGKI signaling maintains basal vascular tone through active inhibition of calcium sensitization. Surprisingly, a fall in Ser-695 phosphorylation did not result in an increase in phosphorylation of the Thr-696 site. Although activation of cGKI with exogenous cyclic nucleotides inhibited thromboxane A2-induced MYPT1 membrane association, RhoA activation, contractile force, and regulatory light chain phosphorylation, the anticipated decreases in MYPT1 phosphorylation at Thr-696/Thr-853 were not observed, indicating that the vasorelaxant effects of cGKI are not through dephosphorylation of MYPT1. Thus, thromboxane A2 signaling within the intact cerebral vasculature induces "buffered" vasoconstrictions, in which both the RhoA/Rho-kinase calcium-sensitizing and the NO/cGMP/cGKI calcium-desensitizing pathways are activated.

Telokin expression and the effect of hypoxia on its phosphorylation status in smooth muscle cells from small and large pulmonary arteries

Small pulmonary arteries (SPA), <500 microm diameter of the cat, constrict when exposed to hypoxia, whereas larger arteries (large pulmonary arteries; LPA), >800 microm diameter, show little or no response. It is unknown why different contractile responses occur within the same vascular bed, but activator or repressor proteins within the smooth muscle cell (SMC) can modify myosin phosphatase and myosin light chain kinase (MLCK), thereby influencing the phosphorylation state of myosin light chain (MLC) and ultimately, contraction. Telokin, a protein with a sequence identical to the COOH-terminal domain of MLCK, is expressed in smooth muscle where in its phosphorylated state it inhibits myosin phosphatase, binds to unphosphorylated myosin, and helps maintain smooth muscle relaxation. We measured telokin mRNA and telokin protein in smooth muscle from different diameter feline pulmonary arteries and sought to determine whether changes in the phosphorylation status of telokin and MLC occurred during hypoxia. In pulmonary arteries, telokin expression varied inversely with artery diameter, but cerebral arteries showed neither telokin protein nor telokin mRNA. Although telokin and MLC were distributed uniformly throughout the SPA muscle cell cytoplasm, they were not colocalized. During hypoxia, telokin dephosphorylated, and MLC became increasingly phosphorylated in SPA SMC, whereas in LPA SMC there was no change in either telokin or MLC phosphorylation. When LPA SMC were exposed to phenylephrine, MLC phosphorylation increased with no change in telokin phosphorylation. These results suggest that in SPA, phosphorylated telokin may help maintain relaxation under unstimulated conditions, whereas in LPA, telokin's function remains undetermined.

A mathematical model of airway and pulmonary arteriole smooth muscle

Airway hyperresponsiveness is a major characteristic of asthma and is believed to result from the excessive contraction of airway smooth muscle cells (SMCs). However, the identification of the mechanisms responsible for airway hyperresponsiveness is hindered by our limited understanding of how calcium (Ca2+), myosin light chain kinase (MLCK), and myosin light chain phosphatase (MLCP) interact to regulate airway SMC contraction. In this work, we present a modified Hai-Murphy cross-bridge model of SMC contraction that incorporates Ca2+ regulation of MLCK and MLCP. A comparative fit of the model simulations to experimental data predicts 1), that airway and arteriole SMC contraction is initiated by fast activation by Ca2+ of MLCK; 2), that airway SMC, but not arteriole SMC, is inhibited by a slower activation by Ca2+ of MLCP; and 3), that the presence of a contractile agonist inhibits MLCP to enhance the Ca2+ sensitivity of airway and arteriole SMCs. The implication of these findings is that murine airway SMCs exploit a Ca2+-dependent mechanism to favor a default state of relaxation. The rate of SMC relaxation is determined principally by the rate of release of the latch-bridge state, which is predicted to be faster in airway than in arteriole. In addition, the model also predicts that oscillations in calcium concentration, commonly observed during agonist-induced smooth muscle contraction, cause a significantly greater contraction than an elevated steady calcium concentration.

The regulation of smooth muscle contractility by zipper-interacting protein kinase

Smooth muscle contractility is mainly regulated by phosphorylation of the 20 kDa myosin light chains (LC20), a process that is controlled by the opposing activities of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). Recently, intensive research has revealed that various protein kinase networks including Rho-kinase, integrin-linked kinase, zipper-interacting protein kinase (ZIPK), and protein kinase C (PKC) are involved in the regulation of LC20 phosphorylation and have important roles in modulating smooth muscle contractile responses to Ca2+ (i.e., Ca2+ sensitization and Ca2+ desensitization). Here, we review the general background and structure of ZIPK and summarize our current understanding of its involvement in a number of cell processes including cell death (apoptosis), cell motility, and smooth muscle contraction. ZIPK has been found to induce the diphosphorylation of LC20 at Ser-19 and Thr-18 in a Ca2+-independent manner and to regulate MLCP activity directly through its phosphorylation of the myosin-targeting subunit of MLCP or indirectly through its phosphorylation of the PKC-potentiated inhibitory protein of MLCP. Future investigations of ZIPK function in smooth muscle will undoubtably focus on determining the mechanisms that regulate its cellular activity, including the identification of upstream signaling pathways, the characterization of autoinhibitory domains and regulatory phosphorylation sites, and the development of specific inhibitor compounds.

A minor role for Ca2+ sensitization in sustained contraction through activation of muscarinic receptor in circular muscle of rat distal colon

We previously demonstrated that Ca(2+) sensitization has an essential role for carbachol-induced contraction in the longitudinal muscle of the rat distal colon. In the present study, we extended these studies to clarify the role of Ca(2+) sensitization in contraction induced by the activation of muscarinic receptors in the circular muscle of the rat distal colon. Carbachol induced a rapid phasic contraction followed by a sustained contraction that was significantly lower than the phasic and was superimposed with the rhythmic contractions. The extent of increase in intracellular Ca(2+) concentration that was measured simultaneously with tension recording was dissociated from the phasic contraction, whereas it exhibited to a similar extent as sustained contraction. In alpha-toxin-permeabilized preparations, Ca(2+) induced contraction comprising a rapid phasic and a subsequent low sustained component. After Ca(2+)-induced sustained contraction reached a constant level, guanosine triphosphate (GTP) addition resulted in the enhancement of contractile force in a concentration-dependent manner. Carbachol in the presence of GTP caused a further minimal increase in tension (Ca(2+) sensitization). Chelerythrine, a protein kinase C (PKC) inhibitor, inhibited carbachol-induced Ca(2+) sensitization but not GTP-induced Ca(2+) sensitization. In contrast, Y-27632, a Rho kinase inhibitor, inhibited GTP-induced Ca(2+) sensitization but not that induced by carbachol. Phorbol 12,13-dibutyrate, a PKC activator, increased the sustained contraction. These results suggest that the activation of muscarinic receptor with carbachol induces Ca(2+) sensitization via activation of PKC, but this action is minor in the circular muscle of the rat distal colon as a result of limited coupling between muscarinic receptors and Ca(2+) sensitization via the PKC pathway.

Calcium-independent tacrine-induced relaxation of rat gastric corpus smooth muscles

Tacrine, a non-competitive reversible acetylcholinesterase and butyrylcholineserase inhibitor, caused a concentration-dependent tonic contraction of gastric smooth muscle preparations in the concentration range 1 × 10−7 mol/L – 1 × 10−5 mol/L, whereas concentrations higher than 2 × 10−5 mol/L induced a biphasic effect; a short-time contraction was followed by a prolonged relaxation. To shed some light on the mechanism underlying this untypical relaxation, the amplitude of mechanical reactions caused by tacrine were compared with those of tacrine in the presence of atropine, ipratropium, metrifonate, TTX, nifedipine, D-600, caffeine, apamin, and charybdotoxin. The results obtained revealed that the relaxation was neither cholinergic in nature, nor mediated by the influence of the drug on intramural neuronal structures. It was not influenced by processes inducing changes in cytosolic Ca2+ levels. This assumption was confirmed by experiments with permeabilized muscle preparations that were pre-contracted in a solution with pCa 5.5. Tacrine relaxed the smooth muscles in spite of the constant intracellular Ca2+ concentration resulting from the permeabilization. These findings argue that tacrine at concentrations higher than 2 × 10−5 mol/L has a desensitizing effect on the contractile apparatus of gastric corpus smooth muscle preparations towards Ca2+.

Localization of telokin at the intercalated discs of cardiac myocytes

Telokin is identical in sequence to the C-terminal portion of myosin light chain kinase but is expressed independently. We have used monoclonal antibodies specific to the non-telokin portion of myosin light chain kinase and to telokin, immunofluorescence microscopy and image reconstruction to demonstrate the presence of telokin in cardiac myocytes and to study its subcellular distribution. Antibodies to telokin labeled the intercalated discs of adult cardiac myocytes and similar structures in isolated intercalated disc preparations. Antibodies specific to the non-telokin portion of myosin light chain kinase did not label intercalated discs in either of these preparations. Western blots of isolated intercalated discs with anti-telokin revealed a 23kDa protein that co-migrates with purified telokin on SDS-PAGE. Deconvolution, reconstruction and analysis of fluorescence images of isolated intercalated discs labeled with anti-telokin and anti-beta-catenin, anti-gamma-catenin or anti-connexin43 indicated that telokin is only partially co-localized with these proteins at the discs.

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.

Signaling for contraction and relaxation in smooth muscle of the gut

Phosphorylation of Ser19 on the 20-kDa regulatory light chain of myosin II (MLC20) by Ca2+/calmodulin-dependent myosin light-chain kinase (MLCK) is essential for initiation of smooth muscle contraction. The initial [Ca2+]i transient is rapidly dissipated and MLCK inactivated, whereas MLC20 and muscle contraction are well maintained. Sustained contraction does not reflect Ca2+ sensitization because complete inhibition of MLC phosphatase activity in the absence of Ca2+ induces smooth muscle contraction. This contraction is suppressed by staurosporine, implying participation of a Ca2+-independent MLCK. Thus, sustained contraction, as with agonist-induced contraction at experimentally fixed Ca2+ concentrations, involves (a) G protein activation, (b) regulated inhibition of MLC phosphatase, and (c) MLC20 phosphorylation via a Ca2+-independent MLCK. The pathways that lead to inhibition of MLC phosphatase by G(q/13)-coupled receptors are initiated by sequential activation of Galpha(q)/alpha13, RhoGEF, and RhoA, and involve Rho kinase-mediated phosphorylation of the regulatory subunit of MLC phosphatase (MYPT1) and/or PKC-mediated phosphorylation of CPI-17, an endogenous inhibitor of MLC phosphatase. Sustained MLC20 phosphorylation is probably induced by the Ca2+-independent MLCK, ZIP kinase. The pathways initiated by G(i)-coupled receptors involve sequential activation of Gbetagamma(i), PI 3-kinase, and the Ca2+-independent MLCK, integrin-linked kinase. The last phosphorylates MLC20 directly and inhibits MLC phosphatase by phosphorylating CPI-17. PKA and PKG, which mediate relaxation, act upstream to desensitize the receptors (VPAC2 and NPR-C), inhibit adenylyl and guanylyl cyclase activities, and stimulate cAMP-specific PDE3 and PDE4 and cGMP-specific PDE5 activities. These kinases also act downstream to inhibit (a) initial contraction by inhibiting Ca2+ mobilization and (b) sustained contraction by inhibiting RhoA and targets downstream of RhoA. This increases MLC phosphatase activity and induces MLC20 dephosphorylation and muscle relaxation.

Modulation of myosin filament activation by telokin in smooth muscle liberation of myosin kinase and phosphatase from supramolecular complexes

The mechanism of telokin action on reversible phosphorylation of turkey gizzard myosin was investigated using a native-like filamentous myosin. This myosin contained endogenous calmodulin (CaM) and myosin light chain kinase (MLCK) at a molar ratio to myosin of about 1 to 40 or less depending on the initial extractions conditions. These levels were sufficient to fully phosphorylate myosin within 20-40 s or less after addition of [gamma-32P]ATP, but when the ATP was depleted, they became dephosphorylated indicating the presence of myosin light chain phosphatase (MLCP). Addition of telokin at the 1 to 1 or higher molar ratio to myosin caused a three- to five-fold inhibition of the initial phosphorylation rates (without reduction of the overall extent of phosphorylation) and produced a similar increase in the rate of dephosphorylation. The inhibition was also observed for myosin filaments free of MLCK and CaM together with constitutively active MLCKs produced by digestion, or by expression of a truncated mammalian kinase as well as for the wild-type enzyme. Thus, neither N- nor C-terminal of MLCK was necessary for interaction of myosin with telokin and the inhibition resulted from telokin-induced change of myosin head configuration within the filament that prevented their ordered, paracrystaline-like, aggregation. Sedimentation of the filamentous myosin in glycerol gradients showed that this change made the filaments less compact and facilitated release of the endogenous MLCK/CaM complex. For a mixture of the filaments with or without the complex, the configuration change resulted in an increase of the phosphorylation rate but not in its inhibition. The increase of the rate resulting from the liberation of the complex was also observed in mixtures of the filamentous myosin with added isolated regulatory light chain (ReLC) or soluble myosin head subfragment. This observation reinforces the above conclusions. The acceleration of the MLCP activity by telokin was shown to result from dissociation of its catalytic subunit from a MLCK/MLCP complex bound to the filamentous myosin. Analogous desensitizing effects of telokin were also demonstrated for the contraction and relaxation cycle of Triton-skinned fibers from guinea pig Teania coli. Taken together, our results indicate that telokin acted as an effective modulator or chaperone of the myosin filament and a scheme for its action in smooth muscle was proposed.

Agonist- and depolarization-induced signals for myosin light chain phosphorylation and force generation of cultured vascular smooth muscle cells

Phosphorylation of myosin light chain (MLC) and contraction of differentiated smooth muscle cells in vascular walls are regulated by Ca2+ -dependent activation of MLC kinase, and by Rho-kinase- or protein-kinases-C-dependent inhibition of MLC phosphatase (MLCP). We examined regulatory pathways for MLC kinase and MLCP in cultured vascular smooth muscle cells (VSMCs), and for isometric force generation of VSMCs reconstituted in collagen fibers. Protein levels of RhoA, Rho-kinase and MYPT1 (a regulatory subunit of MLCP) were upregulated in cultured VSMCs, whereas a MLCP inhibitor protein, CPI-17, was downregulated. Endothelin-1 evoked a steady rise in levels of Ca2+, MLC phosphorylation and the contractile force of VSMCs, whereas angiotensin-II induced transient signals. Also, Thr853 phosphorylation of MYPT1 occurred in response to stimuli, but neither agonist induced phosphorylation of MYPT1 at Thr696. Unlike fresh aortic tissues, removal of Ca2+ or addition of voltage-dependent Ca2+ -channel blocker did not inhibit contractions of reconstituted VSMC fibers induced by agonists or even high concentrations of extracellular K+ ions. Inhibitors of Ins(1,4,5)P3-receptor and Rho-kinase antagonized agonist-induced or high-K+ -induced contraction in both reconstituted fibers and fresh tissues. These results indicate that both Ins(1,4,5)P3-induced Ca2+ release and Rho-kinase-induced MYPT1 phosphorylation at Thr853 play pivotal roles in MLC phosphorylation of cultured VSMCs where either Ca2+ -influx or CPI-17-MLCP signaling is downregulated.

Vectorial activation of smooth muscle myosin filaments and its modulation by telokin

Smooth muscle myosin copurifies with myosin light chain kinase (MLCK) and calmodulin (CaM) as well as with variable amounts of myosin phosphatase. Therefore, myosin filaments formed in vitro also contain relatively high levels of these enzymes. Thus these filaments may be considered to be native-like because they are similar to those expected to exist in vivo. These endogenous enzymes are present at high concentrations relative to myosin, sufficient for rapid phosphorylation and dephosphorylation of the filaments at rates comparable to those observed for contraction and relaxation in intact muscle strips. The phosphorylation by MLCK/CaM complex appears to exhibit some directionality and is not governed by a random diffusional process. For the mixtures of myosin filaments with and without the endogenous MLCK/CaM complex, the complex preferentially phosphorylates its own parent filament at a higher rate than the neighboring filaments. This selective or vectorial-like activation is lost or absent when myosin filaments are dissolved at high ionic strength. Similar vectorial-like activation is exhibited by the reconstituted filament suspensions, but the soluble systems composed of isolated regulatory light chain or soluble myosin head subfragments exhibit normal diffusional kinetic behavior. At physiological concentrations, kinase related protein (telokin) effectively modulates the activation process by reducing the phosphorylation rate of the filaments without affecting the overall phosphorylation level. This results from telokin-induced liberation of the active MLCK/CaM complex from the filaments, so that the latter can also activate the neighboring filaments via a slower diffusional process. When this complex is bound at insufficient levels, this actually results in acceleration of the initial phosphorylation rates. In short, I suggest that in smooth muscle, telokin plays a chaperone role for myosin and its filaments.

Molecular aspects of arterial smooth muscle contraction: focus on Rho

The vascular smooth muscle cell is a highly specialized cell whose primary function is contraction and relaxation. It expresses a variety of contractile proteins, ion channels, and signalling molecules that regulate contraction. Upon contraction, vascular smooth muscle cells shorten, thereby decreasing the diameter of a blood vessel to regulate the blood flow and pressure. Contractile activity in vascular smooth muscle cells is initiated by a Ca(2+)-calmodulin interaction to stimulate phosphorylation of the light chain of myosin. Ca(2+)-sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of the light chain by myosin phosphatase, thereby maintaining force. Removal of Ca(2+) from the cytosol and stimulation of myoson phosphatase initiate the relaxation of vascular smooth muscle.

Smooth muscle of telokin-deficient mice exhibits increased sensitivity to Ca2+ and decreased cGMP-induced relaxation

Cyclic nucleotides can relax smooth muscle without a change in [Ca2+]i, a phenomenon termed Ca2+ desensitization, contributing to vasodilation, gastrointestinal motility, and airway resistance. The physiological importance of telokin, a 17-kDa smooth muscle-specific protein and target for cyclic nucleotide-induced Ca2+ desensitization, was determined in telokin null mice bred to a congenic background. Telokin null ileal smooth muscle homogenates compared to wild type exhibited an approximately 30% decrease in myosin light-chain phosphatase (MLCP) activity, which was reflected in a significant leftward shift (up to 2-fold at pCa 6.3) of the Ca2+ force relationship accompanied by an increase in myosin light-chain phosphorylation. No difference in the Ca2+ force relationship occurred in telokin WT and knockout (KO) aortas, presumably reflecting the normally approximately 5-fold lower telokin content in aorta vs. ileum smooth muscle. Ca2+ desensitization of contractile force by 8-Br-cGMP was attenuated by 50% in telokin KO intestinal smooth muscle. The rate of force relaxation reflecting MLCP activity, in the presence of 50 microM 8-Br-cGMP, was also significantly slowed in telokin KO vs. WT ileum and was rescued by recombinant telokin. Normal thick filaments in telokin KO smooth muscles indicate that telokin is not required for filament formation or stability. Results indicate that a primary role of telokin is to modulate force through increasing MLCP activity and that this effect is further potentiated through phosphorylation by cGMP in telokin-rich smooth tissues.

Convergence of Ca2+-desensitizing mechanisms activated by forskolin and phenylephrine pretreatment, but not 8-bromo-cGMP

Contractile stimuli can sensitize myosin to Ca2+ by activating RhoA kinase (ROK) and PKC that inhibit myosin light chain phosphatase (MLCP) activity. Relaxant stimuli, acting through PKA and PKG (cyclic nucleotide-dependent protein kinases), and pretreatment with contractile agents such as phenylephrine (PE), can desensitize myosin to Ca2+. It is unknown precisely how these stimuli cause Ca2+ desensitization. To test the hypothesis that PKA, PKG, and PE pretreatment signaling systems converge to cause relaxation by inhibition of ROK in intact, isolated tissues, we examined the effects of forskolin (FSK; PKA activation), 8-bromo-cGMP (8br-cGMP; PKG activation), and PE pretreatment on KCl-induced force maintenance in rabbit arteries, a response nearly completely dependent on ROK activation. PE pretreatment and agents activating PKA and PKG caused Ca2+ desensitization by inhibiting KCl-induced tonic force and MLC phosphorylation without inhibiting intracellular [Ca2+]. At pCa 5 in beta-escin-permeabilized muscle, FSK and 8b-cGMP accelerated the relaxation rate when tissues were returned to pCa 9, suggesting that both agents can elevate MLCP activity. However, a component of the Ca2+ desensitization attributed to PKG activation in intact tissues appeared to involve a MLC phosphorylation-independent component. Inhibition of KCl-induced tonic force by the ROK inhibitor, Y-27632, and by PE pretreatment, were synergistically potentiated by 8b-cGMP, but not FSK. FSK and PE pretreatment, but not 8b-cGMP, inhibited the KCl-induced increase in site-specific myosin phosphatase target protein-1 phosphorylation at Thr853. These data support the hypothesis that PKA and PE pretreatment converge on a common Ca2+-desensitization pathway, but that PKG can act by a mechanism different from that activated by PKA and PE pretreatment.

Nitric oxide relaxes circular smooth muscle of rat distal colon through RhoA/Rho-kinase independent Ca2+ desensitisation

1. The aim of this study in circular smooth muscle of rat distal colon was to determine whether Ca(2+) desensitisation, in addition to mechanisms lowering cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)), was involved in the relaxation elicited by nitric oxide (NO). Changes in isometric tension and [Ca(2+)](cyt) were recorded simultaneously in fura-2-loaded strips. 2. In methacholine (10(-5) M)-precontracted preparations, exogenous NO (10(-4) M), adenosine 5'-triphosphate (ATP; 10(-3) M) and electrical field stimulation (EFS; 1 ms, 40 V, 4 Hz, 1 min) induced a decrease in smooth muscle tension, which was accompanied by a fall in [Ca(2+)](cyt). 3. The sarcoplasmic/endoplasmic reticulum Ca(2+) ATP-ase (SERCA) inhibitor thapsigargin (10(-6) M) did not exert an influence on the decrease in tension produced by exogenous NO, but significantly attenuated the fall in [Ca(2+)](cyt). Both the relaxation and the fall in [Ca(2+)](cyt) to ATP and EFS were unaffected by thapsigargin. 4. Calyculin-A (10(-6) M), a myosin light chain phosphatase (MLCP) inhibitor, significantly reduced the decrease in tension elicited by exogenous NO, but did not alter the fall in [Ca(2+)](cyt) to exogenous NO. Inactivating RhoA by exoenzyme C3 (2 mug ml(-1)) or inhibiting Rho-kinase with (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride monohydrate (Y-27632; 10(-5) M) had no effect on the decrease of both tension and [Ca(2+)](cyt) generated by exogenous NO. 5. This paper demonstrates that a RhoA/Rho-kinase independent Ca(2+) desensitisation pathway contributes to the relaxation by NO in circular smooth muscle strips of rat distal colon.

Regulation of smooth muscle-specific gene expression by homeodomain proteins, Hoxa10 and Hoxb8

Smooth muscle cells arise from different populations of precursor cells during embryonic development. The mechanisms that specify the smooth muscle cell phenotype in each of these populations of cells are largely unknown. In many tissues and organs, homeodomain transcription factors play a key role in directing cell specification. However, little is known about how these proteins regulate smooth muscle differentiation. Using degenerate reverse transcription-PCR coupled to cDNA library screening we identified two homeodomain proteins, Hoxa10 and Hoxb8, which are expressed in adult mouse smooth muscle tissues. All three of the previously described transcripts of the Hoxa10 gene, Hoxa10-1, Hoxa10-2, and Hoxa10-3, were identified. Hoxa10-1 directly activated the smooth muscle-specific telokin promoter but did not activate the SM22alpha, smooth muscle alpha-actin, or smooth muscle myosin heavy chain promoters. Small interfering RNA-mediated knock-down of Hoxa10-1 demonstrated that Hoxa10-1 is required for high levels of telokin expression in smooth muscle cells from uterus and colon. On the other hand, Hoxb8 inhibited the activity of the telokin, SM22alpha, and smooth muscle alpha-actin promoters. Cotransfection of Hoxa10-1 together with Hoxa10-2 or Hoxb8 suggested that Hoxa10-2 and Hoxb8 act as competitive inhibitors of Hoxa10-1. Results from gel mobility shift assays demonstrated that Hoxa10-1, Hoxa10-2, and Hoxb8 bind directly to multiple sites in the telokin promoter. Mutational analysis of telokin promoter reporter genes demonstrated that the three homeodomain protein binding sites located between -80 and -75, +2 and +6, and +14 and +17 were required for maximal promoter activation by Hoxa10-1 and maximal inhibition by Hoxb8. Together these data demonstrate that the genes encoding smooth muscle-restricted proteins are direct transcriptional targets of clustered homeodomain proteins and that different homeodomain proteins have distinct effects on the promoters of these genes.