Agonist-induced changes in the phosphorylation of the myosin- binding subunit of myosin light chain phosphatase and CPI17, two regulatory factors of myosin light chain phosphatase, in smooth muscle

Regulation of myosin light-chain phosphorylation and its roles in cardiovascular physiology and pathophysiology

The regulation of muscle contraction is a critical function in the cardiovascular system, and abnormalities may be life-threatening or cause illness. The common basic mechanism in muscle contraction is the interaction between the protein filaments myosin and actin. Although this interaction is primarily regulated by intracellular Ca²⁺, the primary targets and intracellular signaling pathways differ in vascular smooth muscle and cardiac muscle. Phosphorylation of the myosin regulatory light chain (RLC) is a primary molecular switch for smooth muscle contraction. The equilibrium between phosphorylated and unphosphorylated RLC is dynamically achieved through two enzymes, myosin light chain kinase, a Ca²⁺-dependent enzyme, and myosin phosphatase, which modifies the Ca²⁺ sensitivity of contractions. In cardiac muscle, the primary target protein for Ca²⁺ is troponin C on thin filaments; however, RLC phosphorylation also plays a modulatory role in contraction. This review summarizes recent advances in our understanding of the regulation, physiological function, and pathophysiological involvement of RLC phosphorylation in smooth and cardiac muscles.

Perinatal cardiopulmonary adaptation to the thin air of the Alto Andino by a native Altiplano dweller, the llama

Most mammals have a poor tolerance to hypoxia, and prolonged O₂ restriction can lead to organ injury, particularly during fetal and early postnatal life. Nevertheless, the llama (Lama Glama) has evolved efficient mechanisms to adapt to acute and chronic perinatal hypoxia. One striking adaptation is the marked peripheral vasoconstriction measured in the llama fetus in response to acute hypoxia, which allows efficient redistribution of cardiac output toward the fetal heart and adrenal glands. This strong peripheral vasoconstrictor tone is triggered by a carotid body reflex and critically depends on α-adrenergic signaling. A second adaptation is the ability of the llama fetus to protect its brain against hypoxic damage. During hypoxia, in the llama fetus there is no significant increase in brain blood flow. Instead, there is a fall in brain O₂ consumption and temperature, together with a decrease of Na⁺-K⁺-ATPase activity and Na⁺ channels expression, protecting against seizures and neuronal death. Finally, the newborn llama does not develop pulmonary hypertension in response to chronic hypoxia. In addition to maintaining basal pulmonary arterial pressure at normal levels the pulmonary arterial pressor response to acute hypoxia is lower in highland than in lowland llamas. The protection against hypoxic pulmonary arterial hypertension and pulmonary contractile hyperreactivity is partly due to increased hemoxygenase-carbon monoxide signaling and decreased Ca²⁺ sensitization in the newborn llama pulmonary vasculature. These three striking physiological adaptations of the llama allow this species to live and thrive under the chronic influence of the hypobaric hypoxia of life at high altitude.

p116Rip promotes myosin phosphatase activity in airway smooth muscle cells

Myosin phosphatase-Rho interacting protein (p116^Rip ) was originally found as a RhoA-binding protein. Subsequent studies by us and others revealed that p116^Rip facilitates myosin light chain phosphatase (MLCP) activity through direct and indirect manners. However, it is unclear how p116^Rip regulates myosin phosphatase activity in cells. To elucidate the role of p116^Rip in cellular contractile processes, we suppressed the expression of p116^Rip by RNA interference in human airway smooth muscle cells (HASMCs). We found that knockdown of p116^Rip in HASMCs led to increased di-phosphorylated MLC (pMLC), that is phosphorylation at both Ser19 and Thr18. This was because of a change in the interaction between MLCP and myosin, but not an alteration of RhoA/ROCK signaling. Attenuation of Zipper-interacting protein kinase (ZIPK) abolished the increase in di-pMLC, suggesting that ZIPK is involved in this process. Moreover, suppression of p116^Rip expression in HASMCs substantially increased the histamine-induced collagen gel contraction. We also found that expression of the p116^Rip was decreased in the airway smooth muscle tissue from asthmatic patients compared with that from non-asthmatic patients, suggesting a potential role of p116^Rip expression in asthma pathogenesis.

The role of nitric oxide signaling in pulmonary circulation of high- and low-altitude newborn sheep under basal and acute hypoxic conditions

Nitric oxide (NO) is the main vasodilator agent that drives the rapid decrease of pulmonary vascular resistance for the respiratory onset during the fetal to neonatal transition. Nevertheless, the enhanced NO generation by the neonatal pulmonary arterial endothelium does not prevent development of hypoxic pulmonary hypertension in species without an evolutionary story at high altitude. Therefore, this study aims to describe the limits of the NO function at high-altitude during neonatal life in the sheep as an animal model without tolerance to perinatal hypoxia. We studied the effect of blockade of NO synthesis with l-NAME in the cardiopulmonary response of lowland (580 m) and highland (3600 m) newborn lambs basally and under an episode of acute hypoxia. We also determined the pulmonary expression of proteins that mediate the actions of the NO vasodilator pathway in the pulmonary vasoactive tone and remodeling. We observed an enhanced nitrergic function in highland lambs under basal conditions, evidenced as a markedly greater increase in basal mean pulmonary arterial pressure (mPAP) and resistance (PVR) under blockade of NO synthesis. Further, acute hypoxic challenge in lowland lambs infused with l-NAME markedly increased their mPAP and PVR to values greater than baseline, whilst in highland animals under NO synthesis blockade, these variables did not show additional increase in response to low PO₂. Highland animals showed increased pulmonary RhoA expression, decreased PSer188-RhoA fraction, increased PSer311-p65-NFқβ fraction and up-regulated smooth muscle α-actin, relative to lowland controls. Taken together our data suggest that NO-mediated vasodilation is important to keep a low pulmonary vascular resistance under basal conditions and acute hypoxia at low-altitude. At high-altitude, the enhanced nitrergic signaling partially prevents excessive pulmonary hypertension but does not protect against acute hypoxia. The decreased vasodilator efficacy of nitrergic tone in high altitude lambs could be in part due to increased RhoA signaling that opposes to NO action in the hypoxic pulmonary circulation.

Roles of Cyclic AMP Response Element Binding Activation in the ERK1/2 and p38 MAPK Signalling Pathway in Central Nervous System, Cardiovascular System, Osteoclast Differentiation and Mucin and Cytokine Production

There are many downstream targets of mitogen-activated protein kinase (MAPK) signalling that are involved in neuronal development, cellular differentiation, cell migration, cancer, cardiovascular dysfunction and inflammation via their functions in promoting apoptosis and cell motility and regulating various cytokines. It has been reported that cyclic AMP response element-binding protein (CREB) is phosphorylated and activated by cyclic AMP signalling and calcium/calmodulin kinase. Recent evidence also points to CREB phosphorylation by the MAPK signalling pathway. However, the specific roles of CREB phosphorylation in MAPK signalling have not yet been reviewed in detail. Here, we describe the recent advances in the study of this MAPK-CREB signalling axis in human diseases. Overall, the crosstalk between extracellular signal-related kinase (ERK) 1/2 and p38 MAPK signalling has been shown to regulate various physiological functions, including central nervous system, cardiac fibrosis, alcoholic cardiac fibrosis, osteoclast differentiation, mucin production in the airway, vascular smooth muscle cell migration, steroidogenesis and asthmatic inflammation. In this review, we focus on ERK1/2 and/or p38 MAPK-dependent CREB activation associated with various diseases to provide insights for basic and clinical researchers.

Diversity and plasticity in signaling pathways that regulate smooth muscle responsiveness: Paradigms and paradoxes for the myosin phosphatase, the master regulator of smooth muscle contraction

A hallmark of smooth muscle cells is their ability to adapt their functions to meet temporal and chronic fluctuations in their demands. These functions include force development and growth. Understanding the mechanisms underlying the functional plasticity of smooth muscles, the major constituent of organ walls, is fundamental to elucidating pathophysiological rationales of failures of organ functions. Also, the knowledge is expected to facilitate devising innovative strategies that more precisely monitor and normalize organ functions by targeting individual smooth muscles. Evidence has established a current paradigm that the myosin light chain phosphatase (MLCP) is a master regulator of smooth muscle responsiveness to stimuli. Cellular MLCP activity is negatively and positively regulated in response to G-protein activation and cAMP/cGMP production, respectively, through the MYPT1 regulatory subunit and an endogenous inhibitor protein named CPI-17. In this article we review the outcomes from two decade of research on the CPI-17 signaling and discuss emerging paradoxes in the view of signaling pathways regulating smooth muscle functions through MLCP.

Physiological vs. pharmacological signalling to myosin phosphorylation in airway smooth muscle

KEY POINTS

Smooth muscle myosin regulatory light chain (RLC) is phosphorylated by Ca²⁺ /calmodulin-dependent myosin light chain kinase and dephosphorylated by myosin light chain phosphatase (MLCP). Tracheal smooth muscle contains significant amounts of myosin binding subunit 85 (MBS85), another myosin phosphatase targeting subunit (MYPT) family member, in addition to MLCP regulatory subunit MYPT1. Concentration/temporal responses to carbachol demonstrated similar sensitivities for bovine tracheal force development and phosphorylation of RLC, MYPT1, MBS85 and paxillin. Electrical field stimulation releases ACh from nerves to increase RLC phosphorylation but not MYPT1 or MBS85 phosphorylation. Thus, nerve-mediated muscarinic responses in signalling modules acting on RLC phosphorylation are different from pharmacological responses with bath added agonist. The conditional knockout of MYPT1 or the knock-in mutation T853A in mice had no effect on muscarinic force responses in isolated tracheal tissues. MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

ABSTRACT

Ca²⁺ /calmodulin activation of myosin light chain kinase (MLCK) initiates myosin regulatory light chain (RLC) phosphorylation for smooth muscle contraction with subsequent dephosphorylation for relaxation by myosin light chain phosphatase (MLCP) containing regulatory (MYPT1) and catalytic (PP1cδ) subunits. RLC phosphorylation-dependent force development is regulated by distinct signalling modules involving protein phosphorylations. We investigated responses to cholinergic agonist treatment vs. neurostimulation by electric field stimulation (EFS) in bovine tracheal smooth muscle. Concentration/temporal responses to carbachol demonstrated tight coupling between force development and RLC phosphorylation but sensitivity differences in MLCK, MYPT1 T853, MYPT1 T696, myosin binding subunit 85 (MBS85), paxillin and CPI-17 (PKC-potentiated protein phosphatase 1 inhibitor protein of 17 kDa) phosphorylations. EFS increased force and phosphorylation of RLC, CPI-17 and MLCK. In the presence of the cholinesterase inhibitor neostigmine, EFS led to an additional increase in phosphorylation of MYPT1 T853, MYPT1 T696, MBS85 and paxillin. Thus, there were distinct pharmacological vs. physiological responses in signalling modules acting on RLC phosphorylation and force responses, probably related to degenerate G protein signalling networks. Studies with genetically modified mice were performed. Expression of another MYPT1 family member, MBS85, was enriched in mouse, as well as bovine tracheal smooth muscle. Carbachol concentration/temporal-force responses were similar in trachea from MYPT1^SM+/+ , MYPT1^SM-/- and the knock-in mutant mice containing nonphosphorylatable MYPT1 T853A with no differences in RLC phosphorylation. Thus, MYPT1 T853 phosphorylation was not necessary for regulation of RLC phosphorylation in tonic airway smooth muscle. Furthermore, MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

Expression of MYPT1, CPI-17 and MLC20 in ileum of neonatal mouse NEC model and its significance

The present study determined the changes in the expression levels of MYPT1, CPI-17 and MLC20 in the ileum of mice with neonatal induced necrotizing enterocolitis (NEC) to provide a basis for a pathogenesis model that includes smooth muscle changes during NEC. A group of 7-day-old BALB/c mice were fed with formula (40 µl/g, 5 times/day) and given hypoxia treatments (5% O₂ and 95% N₂ for 10 min, twice daily) for 4 days to induce NEC and establish a mouse model. A control group of 7-day-old BALB/c mice were left with their mother for the duration of the treatment. After establishing the model, the two groups of mice were sacrificed, and the terminal ileum tissue was collected and subjected to western blot analysis and immunohistochemistry. The results showed the expression levels of MYPT1 and pMYPT1 in the ileum of the mice in the NEC group were lower than those in the control group (P<0.01). The levels of CPI17 and pCPI17 were higher in the NEC group compared with those in the control group. The expression level of MLC20 in NEC group was lower than that in the control group (P<0.01), but the level of pMLC20 in the NEC group was higher (P<0.05). The results of immunohistochemistry showed that the staining intensities of MYPT1, CPI-17 and MLC20 in the NEC group were lighter than those in the control group, and the proportion of positive cells was also lower in the NEC group (P<0.01). Taken together our results suggest that establishment of NEC is accompanied by changes in the protein levels of MYPT1 and pCPI-17, which can regulate smooth muscle contraction in the ileum.

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and pulmonary hypertension.

Altered Redox Balance in the Development of Chronic Hypoxia-induced Pulmonary Hypertension

Normally, the pulmonary circulation is maintained in a low-pressure, low-resistance state with little resting tone. Pulmonary arteries are thin-walled and rely heavily on pulmonary arterial distension and recruitment for reducing pulmonary vascular resistance when cardiac output is elevated. Under pathophysiological conditions, however, active vasoconstriction and vascular remodeling lead to enhanced pulmonary vascular resistance and subsequent pulmonary hypertension (PH). Chronic hypoxia is a critical pathological factor associated with the development of PH resulting from airway obstruction (COPD, sleep apnea), diffusion impairment (interstitial lung disease), developmental lung abnormalities, or high altitude exposure (World Health Organization [WHO]; Group III). The rise in pulmonary vascular resistance increases right heart afterload causing right ventricular hypertrophy that can ultimately lead to right heart failure in patients with chronic lung disease. PH is typically characterized by diminished paracrine release of vasodilators, antimitogenic factors, and antithrombotic factors (e.g., nitric oxide and protacyclin) and enhanced production of vasoconstrictors and mitogenic factors (e.g., reactive oxygen species and endothelin-1) from the endothelium and lung parenchyma. In addition, phenotypic changes to pulmonary arterial smooth muscle cells (PASMC), including alterations in Ca²⁺ homeostasis, Ca²⁺ sensitivity, and activation of transcription factors are thought to play prominent roles in the development of both vasoconstrictor and arterial remodeling components of hypoxia-associated PH. These changes in PASMC function are briefly reviewed in Sect. 1 and the influence of altered reactive oxygen species homeostasis on PASMC function discussed in Sects. 2-4.

The Rho kinases: critical mediators of multiple profibrotic processes and rational targets for new therapies for pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive lung scarring, short median survival, and limited therapeutic options, creating great need for new pharmacologic therapies. IPF is thought to result from repetitive environmental injury to the lung epithelium, in the context of aberrant host wound healing responses. Tissue responses to injury fundamentally involve reorganization of the actin cytoskeleton of participating cells, including epithelial cells, fibroblasts, endothelial cells, and macrophages. Actin filament assembly and actomyosin contraction are directed by the Rho-associated coiled-coil forming protein kinase (ROCK) family of serine/threonine kinases (ROCK1 and ROCK2). As would therefore be expected, lung ROCK activation has been demonstrated in humans with IPF and in animal models of this disease. ROCK inhibitors can prevent fibrosis in these models, and more importantly, induce the regression of already established fibrosis. Here we review ROCK structure and function, upstream activators and downstream targets of ROCKs in pulmonary fibrosis, contributions of ROCKs to profibrotic cellular responses to lung injury, ROCK inhibitors and their efficacy in animal models of pulmonary fibrosis, and potential toxicities of ROCK inhibitors in humans, as well as involvement of ROCKs in fibrosis in other organs. As we discuss, ROCK activation is required for multiple profibrotic responses, in the lung and multiple other organs, suggesting ROCK participation in fundamental pathways that contribute to the pathogenesis of a broad array of fibrotic diseases. Multiple lines of evidence therefore indicate that ROCK inhibition has great potential to be a powerful therapeutic tool in the treatment of fibrosis, both in the lung and beyond.

Rac1 regulates myosin II phosphorylation through regulation of myosin light chain phosphatase

Phosphorylation of regulatory light chain (MLC) activates myosin II, which enables it to promote contractile and motile activities of cells. We report here a novel signaling mechanism that activates MLC phosphorylation and smooth muscle contraction. Contractile agonists activated Rac1, and Rac1 inhibition diminished agonist-induced MLC phosphorylation, thus inhibiting smooth muscle contraction. Rac1 inhibits the activity of MLC phosphatase (MLCP) but not that of MLC kinase, through a phosphatase that targets MYPT1 (a regulatory subunit of MLCP) and CPI-17 (a MLCP specific inhibitor) rather than through the RhoA-Rho dependent kinase (ROCK) pathway. Rac1 inhibition decreased the activity of protein kinase C (PKC), which also contributes to the change in CPI-17 phosphorylation. We propose that activation of Rac1 increases the activity of PKC, which increases the phosphorylation of CPI-17 and MYPT1 by inhibiting the phosphatase that targets these proteins, thereby decreasing the activity of MLCP and increasing phosphorylation of MLC. Our results suggest that Rac1 coordinates with RhoA to increase MLC phosphorylation by inactivation of CPI-17/MYPT1 phosphatase, which decreases MLCP activity thus promoting MLC phosphorylation and cell contraction.

In vivo roles for myosin phosphatase targeting subunit-1 phosphorylation sites T694 and T852 in bladder smooth muscle contraction

KEY POINTS

Force production and maintenance in smooth muscle is largely controlled by myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. MYPT1 is the regulatory subunit of MLCP that biochemically inhibits MLCP activity via T694 or T852 phosphorylation in vitro. Here we separately investigated the contribution of these two phosphorylation sites in bladder smooth muscles by establishing two single point mutation mouse lines, T694A and T852A, and found that phosphorylation of MYPT1 T694, but not T852, mediates force maintenance via inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. Our findings reveal the role of MYPT1 T694/T852 phosphorylation in vivo in regulation of smooth muscle contraction.

ABSTRACT

Force production and maintenance in smooth muscle is largely controlled by different signalling modules that fine tune myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. To investigate the regulation of MLCP activity in vivo, we analysed the role of two phosphorylation sites on MYPT1 (regulatory subunit of MLCP) that biochemically inhibit MLCP activity in vitro. MYPT1 is constitutively phosphorylated at T694 by unidentified kinases in vivo, whereas the T852 site is phosphorylated by RhoA-associated protein kinase (ROCK). We established two mouse lines with alanine substitution of T694 or T852. Isolated bladder smooth muscle from T852A mice displayed no significant changes in RLC phosphorylation or force responses, but force was inhibited with a ROCK inhibitor. In contrast, smooth muscles containing the T694A mutation showed a significant reduction of force along with reduced RLC phosphorylation. The contractile responses of T694A mutant smooth muscle were also independent of ROCK activation. Thus, phosphorylation of MYPT1 T694, but not T852, is a primary mechanism contributing to inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. The constitutive phosphorylation of MYPT1 T694 may provide a mechanism for regulating force maintenance of smooth muscle.

Thromboxane-induced actin polymerization in hypoxic neonatal pulmonary arterial myocytes involves Cdc42 signaling

In hypoxic pulmonary arterial (PA) myocytes, challenge with thromboxane mimetic U46619 induces marked actin polymerization and contraction, phenotypic features of persistent pulmonary hypertension of the newborn (PPHN). Rho GTPases regulate the actin cytoskeleton. We previously reported that U46619-induced actin polymerization in hypoxic PA myocytes occurs independently of the RhoA pathway and hypothesized involvement of the Cdc42 pathway. PA myocytes grown in normoxia or hypoxia for 72 h were stimulated with U46619, then analyzed for Rac/Cdc42 activation by affinity precipitation, phosphatidylinositide-3-kinase (PI3K) activity by phospho-Akt, phospho-p21-activated kinase (PAK) by immunoblot, and association of Cdc42 with neuronal Wiskott Aldrich Syndrome protein (N-WASp) by immunoprecipitation. The effect of Rac or PAK inhibition on filamentous actin was quantified by laser-scanning cytometry and by cytoskeletal fractionation; effects of actin-modifying agents were measured by isometric myography. Basal Cdc42 activity increased in hypoxia, whereas Rac activity decreased. U46619 challenge increased Cdc42 and Rac activity in hypoxic cells, independently of PI3K. Hypoxia increased phospho-PAK, unaltered by U46619. Association of Cdc42 with N-WASp decreased in hypoxia but increased after U46619 exposure. Hypoxia doubled filamentous-to-globular ratios of α- and γ-actin isoforms. Jasplakinolide stabilized γ-filaments, increasing force; cytochalasin D depolymerized all actin isoforms, decreasing force. Rac and PAK inhibition decreased filamentous actin in tissues although without decrease in force. Rho inhibition decreased myosin phosphorylation and force. Hypoxia induces actin polymerization in PA myocytes, particularly increasing filamentous α- and γ-actin, contributing to U46619-induced contraction. Hypoxic PA myocytes challenged with a thromboxane mimetic polymerize actin via the Cdc42 pathway, reflecting increased Cdc42 association with N-WASp. Mechanisms regulating thromboxane-mediated actin polymerization are potential targets for future PPHN pharmacotherapy.

Constitutive phosphorylation of myosin phosphatase targeting subunit-1 in smooth muscle

Smooth muscle contraction initiated by myosin regulatory light chain (RLC) phosphorylation is dependent on the relative activities of Ca(2+)-calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). We have investigated the physiological role of the MLCP regulatory subunit MYPT1 in bladder smooth muscle containing a smooth muscle-specific deletion of MYPT1 in adult mice. Deep-sequencing analyses of mRNA and immunoblotting revealed that MYPT1 depletion reduced the amount of PP1cδ with no compensatory changes in expression of other MYPT1 family members. Phosphatase activity towards phosphorylated smooth muscle heavy meromyosin was proportional to the amount of PP1cδ in total homogenates from wild-type or MYPT1-deficient tissues. Isolated MYPT1-deficient tissues from MYPT1(SM-/-) mice contracted with moderate differences in response to KCl and carbachol treatments, and relaxed rapidly with comparable rates after carbachol removal and only 1.5-fold slower after KCl removal. Measurements of phosphorylated proteins in the RLC signalling and actin polymerization modules during contractions revealed moderate changes. Using a novel procedure to quantify total phosphorylation of MYPT1 at Thr696 and Thr853, we found substantial phosphorylation in wild-type tissues under resting conditions, predicting attenuation of MLCP activity. Reduced PP1cδ activity in MYPT1-deficient tissues may be similar to the attenuated MLCP activity in wild-type tissues resulting from constitutively phosphorylated MYPT1. Constitutive phosphorylation of MYPT1 Thr696 and Thr853 may thus represent a physiological mechanism acting in concert with agonist-induced MYPT1 phosphorylation to inhibit MLCP activity. In summary, MYPT1 deficiency may not cause significant derangement of smooth muscle contractility because the effective MLCP activity is not changed.

Reconstituted human myosin light chain phosphatase reveals distinct roles of two inhibitory phosphorylation sites of the regulatory subunit, MYPT1

The myosin light chain phosphatase (MLCP) is a cytoskeleton-associated protein phosphatase-1 (PP1) holoenzyme and a RhoA/ROCK effector, regulating cytoskeletal reorganization. ROCK-induced phosphorylation of the MLCP regulatory subunit (MYPT1) at two sites, Thr696 and Thr853, suppresses the activity, although little is known about the difference in the role. Here, we developed a new method for the preparation of the recombinant human MLCP complex and determined the molecular and cellular basis of inhibitory phosphorylation. The recombinant MLCP partially purified from mammalian cell lysates retained characteristics of the native enzyme, such that it was fully active without Mn²⁺ and sensitive to PP1 inhibitor compounds. Selective thio-phosphorylation of MYPT1 at Thr696 with ROCK inhibited the MLCP activity 30%, whereas the Thr853 thio-phosphorylation did not alter the phosphatase activity. Interference with the docking of phospho-Thr696 at the active site weakened the inhibition, suggesting selective autoinhibition induced by phospho-Thr696. Both Thr696 and Thr853 sites underwent autodephosphorylation. Compared with that of Thr853, phosphorylation of Thr696 was more stable, and it facilitated Thr853 phosphorylation. Endogenous MYPT1 at Thr696 was spontaneously phosphorylated in quiescent human leiomyosarcoma cells. Serum stimulation of the cells resulted in dissociation of MYPT1 from myosin and PP1C in parallel with an increase in the level of Thr853 phosphorylation. The C-terminal domain of human MYPT1(495–1030) was responsible for the binding to the N-terminal portion of myosin light meromyosin. The spontaneous phosphorylation at Thr696 may adjust the basal activity of cellular MLCP and affect the temporal phosphorylation at Thr853 that is synchronized with myosin targeting.

PKC activation increases Ca²⁺ sensitivity of permeabilized lymphatic muscle via myosin light chain 20 phosphorylation-dependent and -independent mechanisms

The contractile activity of muscle cells lining the walls of collecting lymphatics is responsible for generating and regulating flow within the lymphatic system. Activation of PKC signaling contributes to the regulation of smooth muscle contraction by enhancing sensitivity of the contractile apparatus to Ca(2+). It is currently unknown whether PKC signaling contributes to the regulation of lymphatic muscle contraction. We hypothesized that the activation of PKC signaling would increase the sensitivity of the lymphatic myofilament to Ca(2+). To test this hypothesis, we determined the effects of PKC activation with phorbol esters [PMA or phorbol dibutyrate (PDBu)] on the contractile behavior of α-toxin-permeabilized rat mesenteric and cervical lymphatics or the thoracic duct. The addition of PMA or PDBu induced a significant increase in the contractile force of submaximally activated α-toxin-permeabilized lymphatic muscle independent of a change in intracellular Ca(2+) concentration, and the Ca(2+)-force relationship of lymphatic muscle was significantly left shifted, indicating greater myofilament Ca(2+) sensitivity. Phorbol esters increased the maximal rate of force development, whereas the rate of relaxation was reduced. Western blot and immunohistochemistry data indicated that the initial rapid increase in tension development after stimulation by PDBu was associated with myosin light chain (MLC)20 phosphorylation; however, the later, steady-state Ca(2+) sensitization of permeabilized lymphatic muscle was not associated with increased phosphorylation of MLC20 at Ser(19), 17-kDa C-kinase-potentiated protein phosphatase-1 inhibitor at Thr(38), or caldesmon at Ser(789). Thus, these data indicate that PKC-dependent Ca(2+) sensitization of lymphatic muscle may involve MLC20 phosphorylation-dependent and -independent mechanism(s).

Sphingosine-1-phosphate induced contraction of bladder smooth muscle

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that contracts most smooth muscles. Although S1P has been shown to contract bladder smooth muscle, the mechanism(s) by which S1P initiates contraction has not been extensively investigated. The goal of this study was to determine if S1P-induced force generation and myosin light chain (MLC) phosphorylation are dependent on calcium sensitization pathways mediated by protein kinase C (PKC) and Rho kinase (ROCK) and which S1P receptor is important in this response. Bladder smooth muscle strips from rabbit and rat were mounted for isometric force recording and contracted in response to carbachol or S1P in the presence and absence of an inhibitor of PKC (3 µM Bisindolylmaleimide-1) or ROCK (1 µM H-1172). 10 µM S1P produced approximately 40% of the force generated in response to 110 mM KCl in rabbit bladder smooth muscle. S1P, up to 100 µM, did not produce a response in rat bladder smooth muscle, any response evoked was due to solvent (NaOH). S1P-dependent force development was associated with a concomitant increase in Ser(19), but not dual Thr(18)/Ser(19) MLC phosphorylation. Inhibition of PKC decreased force development, whereas inhibition of ROCK abolished S1P-induced force. An inhibitor of the S1P2 receptor, JTE-013, relaxed a S1P-induced contraction; whereas, an agonist with low affinity to the S1P2 receptor, dihydro-S1P, did not elicit a contraction. Our results suggest that S1P contracts rabbit, but not rat, bladder smooth muscle via the S1P2 receptor and is dependent on MLC phosphorylation and myofilament calcium sensitization primarily in response to ROCK activation.

Role of PKC and RhoA/ROCK pathways in the spontaneous phasic activity in the rectal smooth muscle

The role of PKC and RhoA/ROCK pathways in the phasic activities in the rectal smooth muscles (RSM) in the basal state is not known. We examined this issue by determining the effects of PKC inhibitors (calphostin C and Gö-6850) and a ROCK inhibitor (Y-27632) on the slow-rate (~3/min) and fast-rate (~25/min) phasic activities. We also examined the corresponding signal transduction cascades and the PKC and ROCK enzymatic activities in the RSM in the basal state. PKC inhibition with calphostin C and Gö-6850 (10(-5) M) caused a significant decrease (~25%) in slow-rate (but not fast-rate) phasic activity (monitored by frequency and amplitude of contractions) of the RSM. Conversely, ROCK inhibition with Y-27632 (10(-5) M) caused a significant decrease not only in slow-rate, but also fast-rate, phasic activity caused by ROCK inhibition in the RSM. Western blot analysis revealed that the PKC inhibition-induced decrease in RSM phasic activity was associated with decreases in PKCα translocation, phosphorylated (Thr(38)) PKC-potentiated inhibitor (CPI-17), and phosphorylated (Thr(18)/Ser(19)) 20-kDa myosin regulatory light chain. Conversely, decreases in the phasic activity in the RSM by ROCK inhibition were accompanied by the additional decrease in phosphorylated (Thr(696)) myosin phosphatase target subunit 1. Data show that while PKC and RhoA/ROCK pathways play a significant role in slow-rate high-amplitude spontaneous phasic activity, only the RhoA/ROCK pathway primarily mediates fast-rate low-amplitude phasic activity, in the RSM. Such knowledge is important in the understanding of the pathophysiology of large intestinal motility disorders. Relative contributions of the PKC vs. the RhoA/ROCK pathway in the phasic activity remain to be determined.

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.

Size-dependent heterogeneity of contractile Ca2+ sensitization in rat arterial smooth muscle

Each segment along arterial vessels adapts to different circumstances, including blood pressure and sympathetic innervation. PKC and Rho-associated kinase (ROCK) Ca(2+)-sensitizing pathways leading to myosin phosphatase inhibition are critically involved in α(1)-adrenoceptor-mediated vascular smooth muscle contraction in distinctive time-dependent manners. We tested whether the amplitude and time course of each pathway varies dynamically between arterial segments. Using pharmacological approaches, we determined the time-dependent roles of Ca(2+) release, Ca(2+) influx, PKC and ROCK in α(1)-agonist-induced contraction and phosphorylation of key proteins in denuded rat small mesenteric artery, midsized caudal artery and thoracic aorta. SR Ca(2+) release and voltage-dependent Ca(2+) influx were essential for the initial rising and late sustained phases, respectively, of phenylephrine-induced contraction, regardless of arterial size. In small mesenteric arteries, α(1A)-subtype-specific antagonists and inhibitors of PKC, but not ROCK, markedly reduced the initial and late phases of contraction in a non-additive manner and suppressed phosphorylation of myosin light chain (MLC) and CPI-17, but not myosin targeting subunit of myosin light chain phosphatase (MYPT1). In aorta, an α(1D)-specific antagonist reduced both the initial and late phases of contraction with a significant decrease in MLC but not CPI-17 or MYPT1 phosphorylation. ROCK inhibitors, but not PKC inhibitors, suppressed the sustained phase of contraction with a decrease in MLC and MYPT1 phosphorylation in the aorta. The effect of ROCK inhibitors was additive with the α(1D)-antagonist. The results for midsized arteries were intermediate. Thus, the PKC-CPI-17 Ca(2+)-sensitizing pathway, which is dependent on PKC subtype and a Ca(2+)-handling mechanism, and is downstream of α(1A) receptors, plays a major role in α(1)-agonist-induced contraction of small resistance arteries in the splanchnic vascular beds. The effect of PKC and ROCK increases and decreases, respectively, with decreasing arterial size.

Mechanisms underlying potentiation of endothelin-1-induced myofilament Ca(2+) sensitization after subarachnoid hemorrhage

Increased vascular smooth muscle contractility has an important role in the development of cerebral vasospasm after subarachnoid hemorrhage (SAH). Myofilament Ca(2+) sensitivity is a major determinant of smooth muscle contractility. We investigated changes in the Ca(2+)-sensitizing effect of endothelin-1 (ET-1) and the mechanisms underlying ET-1-induced Ca(2+) sensitization after SAH using a rabbit SAH model. After SAH, the contractile response to ET-1 was enhanced, and the ET(A) receptor expression was upregulated in the basilar artery. In α-toxin-permeabilized preparations, ET-1 induced enhanced and prolonged contraction after SAH, suggesting that ET-1-induced Ca(2+) sensitization is potentiated after SAH. Endothelin-1-induced Ca(2+) sensitization became less sensitive to inhibitors of Rho-associated coiled-coil protein kinase (ROCK) and protein kinase C (PKC) after SAH. The expression of PKCα, ROCK2, PKC-potentiated phosphatase inhibitor of 17 kDa (CPI-17) and myosin phosphatase target subunit 1 (MYPT1) was upregulated, and the level of phosphorylation of CPI-17 and MYPT1 was elevated after SAH. This study demonstrated for the first time that the Ca(2+)-sensitizing effect of ET-1 on myofilaments is potentiated after SAH. The increased expression and activity of PKCα, ROCK2, CPI-17, and MYPT1, as well as the upregulation of ET(A) receptor expression are suggested to underlie the enhanced and prolonged Ca(2+) sensitization induced by ET-1.

Phorbol 12,13-dibutyrate-induced, protein kinase C-mediated contraction of rabbit bladder smooth muscle

Contraction of bladder smooth muscle is predominantly initiated by M(3) muscarinic receptor-mediated activation of the G(q/11)-phospholipase C β-protein kinase C (PKC) and the G(12/13)-RhoGEF-Rho kinase (ROCK) pathways. However, these pathways and their downstream effectors are not well understood in bladder smooth muscle. We used phorbol 12,13-dibutyrate (PDBu), and 1,2-dioctanoyl-sn-glycerol (DOG), activators of PKC, in this investigation. We were interested in dissecting the role(s) of PKC and to clarify the signaling pathways in bladder smooth muscle contraction, especially the potential cross-talk with ROCK and their downstream effectors in regulating myosin light chain phosphatase activity and force. To achieve this goal, the study was performed in the presence or absence of the PKC inhibitor bisindolylmaleimide-1 (Bis) or the ROCK inhibitor H-1152. Phosphorylation levels of Thr(38)-CPI-17 and Thr(696)/Thr(850) myosin phosphatase target subunit (MYPT1) were measured during PDBu or DOG stimulation using site specific antibodies. PDBu-induced contraction in bladder smooth muscle involved both activation of PKC and PKC-dependent activation of ROCK. CPI-17 as a major downstream effector, is phosphorylated by PKC and ROCK during PDBu and DOG stimulation. Our results suggest that Thr(696) and Thr(850)-MYPT1 phosphorylation are not involved in the regulation of a PDBu-induced contraction. The results also demonstrate that bladder smooth muscle contains a constitutively active isoform of ROCK that may play an important role in the regulation of bladder smooth muscle basal tone. Together with the results from our previous study, we developed a working model to describe the complex signaling pathways that regulate contraction of bladder smooth muscle.

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.

Vasorelaxant effect of Cinnamomi ramulus ethanol extract via rho-kinase signaling pathway

The Rho-kinase (ROCK) signaling pathway is substantially involved in vascular contraction. This study investigated the vasodilatory effects and possible mechanisms of Cinnamomi ramulus ethanol extract (CRE), with the hypothesis that the CRE vasodilatory effect involves RhoA and the ROCK signaling pathway in rat aortic preparations. CRE (0.05-1 mg/ml) dose-dependently relaxed the vascular contraction induced by phenylephrine and calpeptin in an endothelium-independent manner. Measurement of the expression levels of ROCK-related signaling molecules in response to calpeptin revealed that CRE completely inhibited RhoA and ROCK2 protein expressions. Furthermore, CRE dephosphorylated the subsequent downstream targets myosin phosphatase targeting subunit 1 (MYPT-1), protein kinase C potentiated phosphatase inhibitor protein-17 kDa (CPI-17) and myosin light chain 20 kDa (MLC20). We conclude that the vasorelaxation effect of CRE occurs via downregulation of ROCK signal molecules.

3',4'-Dihydroxyflavonol reduces vascular contraction through Ca²⁺ desensitization in permeabilized rat mesenteric artery

3',4'-Dihydroxyflavonol (DiOHF) exerts endothelium-independent relaxation in rat aortic rings. In this study, we hypothesized that DiOHF reduces vascular contraction through Ca²⁺ desensitization in permeabilized third-order branches of rat mesenteric arteries. The third-order branches of rat mesenteric arteries were permeabilized with β-escin and subjected to tension measurement. Cumulative addition of phenylephrine (0.3-30 μM) produced concentration-dependent vascular contraction of endothelium-intact and endothelium-denuded arterial rings, which were inhibited by pretreatment with DiOHF (10, 30, or 100 μM). In addition, DiOHF dose-dependently decreased vascular contractions induced by 3.0 μM phenylephrine. β-Escin-permeabilized third-order branches of mesenteric arteries were contracted with Ca²⁺, NaF, or guanosine-5'-(γ-thio)triphosphate (GTPγS) 30 min after pretreatment with DiOHF or vehicle. Pretreatment with DiOHF for 30 min inhibited vascular contraction induced by cumulative additions of Ca²⁺ (pCa 9.0-6.0) or NaF (4.0-16.0 mM) in permeabilized arterial rings. Cumulative addition of DiOHF also reduced vascular contraction induced by Ca²⁺-controlled solution of pCa 6.0, 16.0 mM NaF, or 100 μM GTPγS in permeabilized arterial rings. DiOHF inhibited the increase in vascular tension provoked by calyculin A, even though it did not affect vascular tension already produced by calyculin A. DiOHF accelerated the relaxation induced by rapidly lowering Ca²⁺. DiOHF reduced vascular contraction through Ca²⁺ desensitization in permeabilized third-order branches of rat mesenteric arteries. These results suggest that DiOHF may have a therapeutic potential in the treatment of cardiovascular diseases.

Kinetics of relaxation by cGMP/cGKI signaling in fundus smooth muscle

cGMP-dependent kinase I (cGKI) is a major mediator of smooth muscle relaxation and exists in two isoforms, α and β. Both isoforms are supposed to mediate their effects via different intracellular signaling pathways. To verify this concept, the kinetics of relaxation mediated by either isoform was analyzed in gastric fundus smooth muscle from mice. Muscles from mice that express selectively the Iα or Iβ isoform of cGKI in smooth muscle (sm-cGKIα or sm-cGKIβ mice) were compared to muscles from conventional cGKI(-/-) mice. Fundus muscles were contracted by carbachol and then relaxed by 8-Br-cGMP or by electrical field stimulation (EFS). The time course of relaxation by 8-Br-cGMP was not different between muscles from sm-cGKIα and sm-cGKIβ mice. EFS induced a fast transient relaxation in muscles from sm-cGKIα and sm-cGKIβ mice that was blocked by the NO synthase inhibitor L-NAME. Recovery from this relaxation was about 4-times slower in muscles from sm-cGKIα mice than in muscles from sm-cGKIβ mice. The different kinetic of recovery from relaxation after EFS in sm-cGKIα and sm-cGKIβ mice suggests that different signaling pathways exist for each cGKI isoform in vivo in fundus muscles.

Honokiol attenuates vascular contraction through the inhibition of the RhoA/Rho-kinase signalling pathway in rat aortic rings

Objectives

Honokiol is a small-molecule polyphenol isolated from the species Magnolia obovata. We hypothesized that honokiol attenuated vascular contractions through the inhibition of the RhoA/Rho-kinase signalling pathway.

Methods

Rat aortic rings were denuded of endothelium, mounted in organ baths, and subjected to contraction or relaxation. Phosphorylation of 20 kDa myosin light chains (MLC20), myosin phosphatase targeting subunit 1 (MYPT1) and protein kinase C (PKC)-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase (MLCP) of 17 kDa (CPI17) were examined by immunoblot. We also measured the amount of guanosine triphosphate RhoA as a marker for RhoA activation.

Key findings

Pretreatment with honokiol dose-dependently inhibited the concentration–response curves in response to sodium fluoride (NaF) or thromboxane A2 agonist U46619. Honokiol decreased the phosphorylation levels of MLC20, MYPT1Thr855 and CPI17Thr38 as well as the activation of RhoA induced by 8.0 mm NaF or 30 nm U46619.

Conclusions

These results demonstrated that honokiol attenuated vascular contraction through the inhibition of the RhoA/Rho-kinase signalling pathway.

HMC05 attenuates vascular contraction through inhibition of RhoA/Rho-kinase signaling pathway

AIM OF THE STUDY

HMC05, an extract from eight different herbal mixtures, has been developed to treat cardiovascular disease. This extract has a vasorelaxant and anti-atherosclerotic action. We hypothesized that HMC05 attenuates vascular contraction through inhibition of the RhoA/Rho-kinase signaling pathway.

MATERIALS AND METHODS

Rat aortic ring preparations were mounted in organ baths and subjected to contraction and relaxation. Phosphorylation of 20 kDa myosin light chains (MLC(20)) and myosin phosphatase targeting subunit 1 (MYPT1) were examined by immunoblot. We also measured the amount of GTP RhoA as a marker for RhoA activation.

RESULTS

In endothelium-denuded aortic ring preparations, HMC05 relaxed vascular contraction induced by 6.0 mM NaF, 100 nM phenylephrine, 30 nM thromboxane A(2) agonist U46619 or 1.0 μM protein kinase C (PKC) activator phorbol-12,13-dibutyrate (PDBu) in a decreasing order. HMC05 relaxed aortic ring preparations precontracted with sodium fluoride (NaF) whether endothelium was intact or denuded. Pre-incubation with HMC05 for 30 min dose-dependently inhibited the NaF-induced contractile response. In vascular strips, HMC05 decreased the phosphorylation level of both MLC(20) and MYPT1(Thr855) induced by 6.0 mM NaF. Furthermore, HMC05 decreased the amount of GTP RhoA activated by NaF.

CONCLUSIONS

HMC05 attenuates vascular contraction through inhibition of the RhoA/Rho-kinase signaling pathway. HMC05 may be useful for the treatment and/or prevention of cardiovascular diseases associated with activation of RhoA/Rho-kinase signaling pathway.

Differences in the mechanism of collagen lattice contraction by myofibroblasts and smooth muscle cells

Both rat derived vascular smooth muscle cells (SMC) and human myofibroblasts contain α smooth muscle actin (SMA), but they utilize different mechanisms to contract populated collagen lattices (PCLs). The difference is in how the cells generate the force that contracts the lattices. Human dermal fibroblasts transform into myofibroblasts, expressing α-SMA within stress fibers, when cultured in lattices that remain attached to the surface of a tissue culture dish. When attached lattices are populated with rat derived vascular SMC, the cells retain their vascular SMC phenotype. Comparing the contraction of attached PCLs when they are released from the culture dish on day 4 shows that lattices populated with rat vascular SMC contract less than those populated with human myofibroblast. PCL contraction was evaluated in the presence of vanadate and genistein, which modify protein tyrosine phosphorylation, and ML-7 and Y-27632, which modify myosin ATPase activity. Genistein and ML-7 had no affect upon either myofibroblast or vascular SMC-PCL contraction, demonstrating that neither protein tyrosine kinase nor myosin light chain kinase was involved. Vanadate inhibited myofibroblast-PCL contraction, consistent with a role for protein tyrosine phosphatase activity with myofibroblast-generated forces. Y-27632 inhibited both SMC and myofibroblast PCL contraction, consistent with a central role of myosin light chain phosphatase.

Rho kinase modulates postnatal adaptation of the pulmonary circulation through separate effects on pulmonary artery endothelial and smooth muscle cells

At birth, pulmonary vasodilation occurs concomitant with the onset of air-breathing life. Whether and how Rho kinase (ROCK) modulates the perinatal pulmonary vascular tone remains incompletely understood. To more fully characterize the separate and interactive effects of ROCK signaling, we hypothesized that ROCK has discrete effects on both pulmonary artery (PA): 1) endothelial cell (PAEC) nitric oxide (NO) production and contractile state; and 2) smooth muscle cell tone independent of endothelial NO synthase (eNOS) activity. To test these hypotheses, NO production and endothelial barrier function were determined in fetal PAEC under baseline hypoxia and following exposure to normoxia with and without treatment with Y-27632, a specific pharmacological inhibitor of ROCK. In acutely instrumented, late-gestation ovine fetuses, eNOS was inhibited by nitro-l-arginine infusion into the left PA (LPA). Subsequently, fetal lambs were mechanically ventilated (MV) with 100% oxygen in the absence (control period) and presence of Y-27632. In PAEC, treatment with Y-27632 had no effect on cytosolic calcium but did increase normoxia-induced NO production. Moreover, acute normoxia increased PAEC barrier function, an effect that was potentiated by Y-27632. In fetal lambs, MV during the control period had no effect on LPA flow. In contrast, MV after Y-27632 increased LPA flow and fetal arterial P(O)₂ (Pa(O₂)) and decreased PA pressure. In conclusion, ROCK activity modulates vascular tone in the perinatal pulmonary circulation via combined effects on PAEC NO production, barrier function, and smooth muscle tone. ROCK inhibition may represent a novel treatment strategy for neonatal pulmonary vascular disease.

Fluoride induces vascular contraction through activation of RhoA/Rho kinase pathway in isolated rat aortas

We hypothesized that fluoride induces vascular contraction through activation of the RhoA/Rho kinase pathway in isolated rat aortas. Rat aortic rings were mounted in organ baths and contracted with sodium fluoride (NaF). We measured the amount of GTP-RhoA as well as vascular tension. We also determined the level of phosphorylation of the myosin light chain (MLC(20)), myosin phosphatase targeting subunit 1 (MYPT1) and PKC-potentiated inhibitory protein for heterotrimeric MLCP of 17kDa (CPI17). In both physiological salt solution and Ca(2+)-free solution, NaF increased vascular tension and MLC(20) phosphorylation in dose-dependent manners. NaF increased not only phosphorylation level of MYPT1(Thr855) and CPI17(Thr38), but also the amount of GTP-RhoA. Both H1152 and Y27632, inhibitors of Rho kinase, but not Ro31-8220, an inhibitor of PKC, attenuated NaF-induced contraction and phosphorylation level of MLC(20), MYPT1(Thr855) and CPI17(Thr38). In conclusion, fluoride induces vascular contraction through activation of the RhoA/Rho kinase pathway.

Androgens induce nongenomic stimulation of colonic contractile activity through induction of calcium sensitization and phosphorylation of LC20 and CPI-17

We show that androgens, testosterone and 5alpha-dihydrotestosterone (DHT), acutely (approximately 40 min) provoke the mechanical potentiation of spontaneous and agonist-induced contractile activity in mouse colonic longitudinal smooth muscle. The results using flutamide, finasteride, cycloheximide, and actinomycin D indicate that androgen-induced potentiation is dependent on androgen receptors, requires reduction of testosterone to DHT, and occurs independently of transcriptional and translational events. Using permeabilized colonic smooth muscle preparations, we could demonstrate that mechanical potentiation is entirely due to calcium sensitization of contractile machinery. In addition, DHT (10 nm) increased phosphorylation of both 20-kDa myosin light chain (LC(20)) [regulatory myosin light chain, (MLC)] and CPI-17 (an endogenous inhibitor of MLC phosphatase). Paralleling these findings, inhibition of Rho-associated Rho kinase (ROK) and/or protein kinase C (PKC) with, respectively, Y27632 and chelerythrine, prevented LC(20) phosphorylation and abolished calcium sensitization. In addition, inhibition of ROK prevents CPI-17 phosphorylation, indicating that ROK is located upstream PKC-mediated CPI-17 modulation in the signalling cascade. Additionally, androgens induce a rapid activation of RhoA and its translocation to the plasma membrane to activate ROK. The results demonstrate that androgens induce sensitization of colonic smooth muscle to calcium through activation of ROK, which in turn, activates PKC to induce CPI-17 phosphorylation. Activation of this pathway induces a potent steady stimulation of LC(20) by inhibiting MLC phosphatase and displacing the equilibrium of the regulatory subunit towards its phosphorylated state. This is the first demonstration that colonic smooth muscle is a physiological target for androgen hormones, and that androgens modulate force generation of smooth muscle contractile machinery through nongenomic calcium sensitization pathways.

Calcium sensitization induced by sodium fluoride in permeabilized rat mesenteric arteries

It was hypothesized that NaF induces calcium sensitization in Ca(2+)-controlled solution in permeabilized rat mesenteric arteries. Rat mesenteric arteries were permeabilized with beta-escin and subjected to tension measurement. NaF potentiated the concentration-response curves to Ca(2+) (decreased EC(50) and increased E(max)). Cumulative addition of NaF (4.0, 8.0 and 16 mM) also increased vascular tension in Ca(2+)-controlled solution at pCa 7.0 or pCa 6.5, but not at pCa 8.0. NaF-induced vasocontraction and GTPgammaS-induced vasocontraction were not additive. NaF-induced vasocontraction at pCa 7.0 was inhibited by pretreatment with Rho kinase inhibitors H1152 or Y27632 but not with a MLCK inhibitor ML-7 or a PKC inhibitor Ro31-8220. NaF induces calcium sensitization in a Ca(2+)-dependent manner in beta-escin-permeabilized rat mesenteric arteries. These results suggest that NaF is an activator of the Rho kinase signaling pathway during vascular contraction.

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.

Carbachol-induced rabbit bladder smooth muscle contraction: roles of protein kinase C and Rho kinase

Smooth muscle contraction is regulated by phosphorylation of the myosin light chain (MLC) catalyzed by MLC kinase and dephosphorylation catalyzed by MLC phosphatase. Agonist stimulation of smooth muscle results in the inhibition of MLC phosphatase activity and a net increase in MLC phosphorylation and therefore force. The two pathways believed to be primarily important for inhibition of MLC phosphatase activity are protein kinase C (PKC)-catalyzed CPI-17 phosphorylation and Rho kinase (ROCK)-catalyzed myosin phosphatase-targeting subunit (MYPT1) phosphorylation. The goal of this study was to determine the roles of PKC and ROCK and their downstream effectors in regulating MLC phosphorylation levels and force during the phasic and sustained phases of carbachol-stimulated contraction in intact bladder smooth muscle. These studies were performed in the presence and absence of the PKC inhibitor bisindolylmaleimide-1 (Bis) or the ROCK inhibitor H-1152. Phosphorylation levels of Thr(38)-CPI-17 and Thr(696)/Thr(850)-MYPT1 were measured at different times during carbachol stimulation using site-specific antibodies. Thr(38)-CPI-17 phosphorylation increased concurrently with carbachol-stimulated force generation. This increase was reduced by inhibition of PKC during the entire contraction but was only reduced by ROCK inhibition during the sustained phase of contraction. MYPT1 showed high basal phosphorylation levels at both sites; however, only Thr(850) phosphorylation increased with carbachol stimulation; the increase was abolished by the inhibition of either ROCK or PKC. Our results suggest that during agonist stimulation, PKC regulates MLC phosphatase activity through phosphorylation of CPI-17. In contrast, ROCK phosphorylates both Thr(850)-MYPT1 and CPI-17, possibly through cross talk with a PKC pathway, but is only significant during the sustained phase of contraction. Last, our results demonstrate that there is a constitutively activate pool of ROCK that phosphorylates MYPT1 in the basal state, which may account for the high resting levels of MLC phosphorylation measured in rabbit bladder smooth muscle.

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.

Ca2+ sensitization via phosphorylation of myosin phosphatase targeting subunit at threonine-855 by Rho kinase contributes to the arterial myogenic response

Ca(2+) sensitization has been postulated to contribute to the myogenic contraction of resistance arteries evoked by elevation of transmural pressure. However, the biochemical evidence of pressure-induced increases in phosphorylated myosin light chain phosphatase (MLCP) targeting subunit 1 (MYPT1) and/or 17 kDa protein kinase C (PKC)-potentiated protein phosphatase 1 inhibitor protein (CPI-17) required to sustain this view is not currently available. Here, we determined whether Ca(2+) sensitization pathways involving Rho kinase (ROK)- and PKC-dependent phosphorylation of MYPT1 and CPI-17, respectively, contribute to the myogenic response of rat middle cerebral arteries. ROK inhibitors (Y27632, 0.03-10 micromol l(-1); H1152, 0.001-0.3 micromol l(-1)) and PKC inhibitors (GF109203X, 3 micromol l(-1); Gö6976; 10 micromol l(-1)) suppressed myogenic vasoconstriction between 40 and 120 mmHg. An improved, highly sensitive 3-step Western blot method was developed for detection and quantification of MYPT1 and CPI-17 phosphorylation. Increasing pressure from 10 to 60 or 100 mmHg significantly increased phosphorylation of MYPT1 at threonine-855 (T855) and myosin light chain (LC(20)). Phosphorylation of MYPT1 at threonine-697 (T697) and CPI-17 were not affected by pressure. Pressure-evoked elevations in MYPT1-T855 and LC(20) phosphorylation were reduced by H1152, but MYPT1-T697 phosphorylation was unaffected. Inhibition of PKC with GF109203X did not affect MYPT1 or LC(20) phosphorylation at 100 mmHg. Our findings provide the first direct, biochemical evidence that a Ca(2+) sensitization pathway involving ROK-dependent phosphorylation of MYPT1 at T855 (but not T697) and subsequent augmentation of LC(20) phosphorylation contributes to myogenic control of arterial diameter in the cerebral vasculature. In contrast, suppression of the myogenic response by PKC inhibitors cannot be attributed to block of Ca(2+) sensitization mediated by CPI-17 or MYPT1 phosphorylation.

17beta-estradiol attenuates vascular contraction through inhibition of RhoA/Rho kinase pathway

We hypothesized that 17beta-estradiol attenuates vascular contraction through inhibition of RhoA/Rho kinase pathway. Rat aortic rings were contracted with cumulative addition of U46619, NaF, KCl or PDBu 30 min after pretreatment with 17beta-estradiol (10, 30, and 100 microM) or vehicle. We measured the amount of GTP RhoA and the level of phosphorylation of the myosin light chain (MLC(20)), myosin phosphatase targeting subunit 1 (MYPT1) and PKC-potentiated inhibitory protein for heterotrimeric MLCP of 17 kDa (CPI17). Pretreatment with 17beta-estradiol dose-dependently inhibited the concentration-response curves in response to U46619, NaF or KCl, but not to PDBu. 17beta-Estradiol decreased not only the level of phosphorylation of MYPT1(Thr855) and CPI17(Thr38) as well as MLC(20), but also the activity of RhoA induced by U46619 or NaF. However, 17beta-estradiol did not affect the level of phosphorylation of CPI17 induced by PDBu. 17beta-Estradiol attenuates vascular contraction through inhibition of RhoA/Rho kinase pathway.

Isoflavone attenuates vascular contraction through inhibition of the RhoA/Rho-kinase signaling pathway

Isoflavones decrease blood pressure, improve lipid profiles, and restore vascular function. We hypothesized that isoflavone attenuates vascular contraction by inhibiting RhoA/Rho-kinase signaling pathway. Rat aortic rings were denuded of endothelium, mounted in organ baths, and contracted with 11,9 epoxymethano-prostaglandin F(2alpha) (U46619), a thromboxane A2 analog, or KCl 30 min after the pretreatment with genistein (4',5,7-trihydroxyisoflavone), daidzein (4',7-dihydroxyisoflavone), or vehicle. We determined the phosphorylation level of the myosin light chain (MLC(20)), myosin phosphatase-targeting subunit 1 (MYPT1), and protein kinase C-potentiated inhibitory protein for heterotrimeric myosin light-chain phosphatase of 17 kDa (CPI17) by means of the Western blot. We also measured the amount of GTP RhoA as a marker regarding RhoA activation. The cumulative additions of U46619 or KCl increased vascular tension in a concentration-dependent manner, which were inhibited by pretreatment with genistein or daidzein. Both U46619 (30 nM) and KCl (50 mM) increased MLC(20) phosphorylation levels, which were inhibited by genistein and daidzein. Furthermore, both genistein and daidzein decreased the amount of GTP RhoA activated by either U46619 or KCl. U46619 (30 nM) increased phosphorylation of the MYPT1(Thr855) and CPI17(Thr38), which were also inhibited by genistein or daidzein. However, neither genistein nor daidzein inhibited phorbol 12,13-dibutyrate-induced vascular contraction and CPI17 phosphorylation. In conclusion, isoflavone attenuates vascular contraction, at least in part, through inhibition of the RhoA/Rho-kinase signaling pathway.

Myosin light chain kinase activation and calcium sensitization in smooth muscle in vivo

Ca(2+)/calmodulin (CaM)-dependent phosphorylation of myosin regulatory light chain (RLC) in smooth muscle by myosin light chain kinase (MLCK) and dephosphorylation by myosin light chain phosphatase (MLCP) are subject to modulatory cascades that influence the sensitivity of RLC phosphorylation and hence contraction to intracellular Ca(2+) concentration ([Ca(2+)](i)). We designed a CaM-sensor MLCK containing smooth muscle MLCK fused to two fluorescent proteins linked by the MLCK CaM-binding sequence to measure kinase activation in vivo and expressed it specifically in mouse smooth muscle. In phasic bladder muscle, there was greater RLC phosphorylation and force relative to MLCK activation and [Ca(2+)](i) with carbachol (CCh) compared with KCl treatment, consistent with agonist-dependent inhibition of MLCP. The dependence of force on MLCK activity was nonlinear such that at higher concentrations of CCh, force increased with no change in the net 20% activation of MLCK. A significant but smaller amount of MLCK activation was found during the sustained contractile phase. MLCP inhibition may occur through RhoA/Rho-kinase and/or PKC with phosphorylation of myosin phosphatase targeting subunit-1 (MYPT1) and PKC-potentiated phosphatase inhibitor (CPI-17), respectively. CCh treatment, but not KCl, resulted in MYPT1 and CPI-17 phosphorylation. Both Y27632 (Rho-kinase inhibitor) and calphostin C (PKC inhibitor) reduced CCh-dependent force, RLC phosphorylation, and phosphorylation of MYPT1 (Thr694) without changing MLCK activation. Calphostin C, but not Y27632, also reduced CCh-induced phosphorylation of CPI-17. CCh concentration responses showed that phosphorylation of CPI-17 was more sensitive than MYPT1. Thus the onset of agonist-induced contraction in phasic smooth muscle results from the rapid and coordinated activation of MLCK with hierarchical inhibition of MLCP by CPI-17 and MYPT1 phosphorylation.

G(q)-dependent signalling by the lysophosphatidic acid receptor LPA(3) in gastric smooth muscle: reciprocal regulation of MYPT1 phosphorylation by Rho kinase and cAMP-independent PKA

The present study characterized the signalling pathways initiated by the bioactive lipid, LPA (lysophosphatidic acid) in smooth muscle. Expression of LPA(3) receptors, but not LPA(1) and LPA(2), receptors was demonstrated by Western blot analysis. LPA stimulated phosphoinositide hydrolysis, PKC (protein kinase C) and Rho kinase (Rho-associated kinase) activities: stimulation of all three enzymes was inhibited by expression of the G(alphaq), but not the G(alphai), minigene. Initial contraction and MLC(20) (20 kDa regulatory light chain of myosin II) phosphorylation induced by LPA were abolished by inhibitors of PLC (phospholipase C)-beta (U73122) or MLCK (myosin light-chain kinase; ML-9), but were not affected by inhibitors of PKC (bisindolylmaleimide) or Rho kinase (Y27632). In contrast, sustained contraction, and phosphorylation of MLC(20) and CPI-17 (PKC-potentiated inhibitor 17 kDa protein) induced by LPA were abolished selectively by bisindolylmaleimide. LPA-induced activation of IKK2 {IkappaB [inhibitor of NF-kappaB (nuclear factor kappaB)] kinase 2} and PKA (protein kinase A; cAMP-dependent protein kinase), and degradation of IkappaBalpha were blocked by the RhoA inhibitor (C3 exoenzyme) and in cells expressing dominant-negative mutants of IKK2(K44A) or RhoA(N19RhoA). Phosphorylation by Rho kinase of MYPT1 (myosin phosphatase targeting subunit 1) at Thr(696) was masked by phosphorylation of MYPT1 at Ser(695) by PKA derived from IkappaB degradation via RhoA, but unmasked in the presence of PKI (PKA inhibitor) or C3 exoenzyme and in cells expressing IKK2(K44A). We conclude that LPA induces initial contraction which involves activation of PLC-beta and MLCK and phosphorylation of MLC(20), and sustained contraction which involves activation of PKC and phosphorylation of CPI-17 and MLC(20). Although Rho kinase was activated, phosphorylation of MYPT1 at Thr(696) by Rho kinase was masked by phosphorylation of MYPT1 at Ser(695) via cAMP-independent PKA derived from the NF-kappaB pathway.