Role of Rho in Ca(2+)-insensitive contraction and paxillin tyrosine phosphorylation in smooth muscle

Cytoskeletal remodeling slows cross-bridge cycling and ATP hydrolysis rates in airway smooth muscle

During isometric activation of airway smooth muscle (ASM), cross-bridge cycling and ATP hydrolysis rates decline across time even though isometric force is sustained. Thus, tension cost (i.e., ATP hydrolysis rate per unit of force during activation) decreases with time. The "latch-state" hypothesis attributes the dynamic change in cross-bridge cycling and ATP hydrolysis rates to changes in phosphorylation of the regulatory myosin light chain (rMLC20 ). However, we previously showed that in ASM, the extent of rMLC20 phosphorylation remains unchanged during sustained isometric force. As an alternative, we hypothesized that cytoskeletal remodeling within ASM cells results in increased internal loading of contractile proteins that slows cross-bridge cycling and ATP hydrolysis rates. To test this hypothesis, we simultaneously measured isometric force and ATP hydrolysis rate in permeabilized porcine ASM strips activated by Ca2+ (pCa 4.0). The extent of rMLC20 phosphorylation remained unchanged during isometric activation, even though ATP hydrolysis rate (tension cost) declined with time. The effect of cytoskeletal remodeling was assessed by inhibiting actin polymerization using Cytochalasin D (Cyto-D). In Cyto-D treated ASM, isometric force was reduced while ATP hydrolysis rate increased compared to untreated ASM strips. These results indicate that external transmission of force, cross-bridge cycling and ATP hydrolysis rates are affected by internal loading of contractile proteins.

The importance of complete tissue homogenization for accurate stoichiometric measurement of myosin light chain phosphorylation in airway smooth muscle

The standard method for measuring the phosphorylation of the regulatory myosin light chain (MLC20) in smooth muscle is extraction of the light chain using a urea extraction buffer, urea-glycerol gel electrophoresis of the soluble portion of the extract (supernatant) and Western blot analysis. The undissolved portion of the tissue during extraction (the pellet) is usually discarded. Because the pellet contains a finite amount of MLC20, omission of the pellet could result in inaccurate measurement of MLC20 phosphorylation. In this study we compared the level of tracheal smooth muscle MLC20 phosphorylation in the supernatant alone, with that in the complete tissue homogenate (supernatant and pellet) using the standard method. The supernatant fraction showed the well-known double bands representing phosphorylated and un-phosphorylated MLC20. The dissolved pellet fraction showed varying amounts of un-phosphorylated and phosphorylated MLC20. There was a small but statistically significant overestimation of the percent MLC20 phosphorylation if the pellet was not taken into consideration. The overestimation was 7% ± 2% (mean ± SEM) (p < 0.05) in unstimulated muscle and 2% ± 1% (p < 0.05) in acetylcholine (10(-6) mol/L) stimulated muscle. This finding suggests that for accurate estimation of the stoichiometry of MLC20 phosphorylation it is necessary to consider the contribution from the pellet portion of the muscle tissue homogenate.

Rho-kinase mediated cytoskeletal stiffness in skinned smooth muscle

The structurally dynamic cytoskeleton is important in many cell functions. Large gaps still exist in our knowledge regarding what regulates cytoskeletal dynamics and what underlies the structural plasticity. Because Rho-kinase is an upstream regulator of signaling events leading to phosphorylation of many cytoskeletal proteins in many cell types, we have chosen this kinase as the focus of the present study. In detergent skinned tracheal smooth muscle preparations, we quantified the proteins eluted from the muscle cells over time and monitored the muscle's ability to respond to acetylcholine (ACh) stimulation to produce force and stiffness. In a partially skinned preparation not able to generate active force but could still stiffen upon ACh stimulation, we found that the ACh-induced stiffness was independent of calcium and myosin light chain phosphorylation. This indicates that the myosin light chain-dependent actively cycling crossbridges are not likely the source of the stiffness. The results also indicate that Rho-kinase is central to the ACh-induced stiffness, because inhibition of the kinase by H1152 (1 μM) abolished the stiffening. Furthermore, the rate of relaxation of calcium-induced stiffness in the skinned preparation was faster than that of ACh-induced stiffness, with or without calcium, suggesting that different signaling pathways lead to different means of maintenance of stiffness in the skinned preparation.

Role of cyclic nucleotide-dependent actin cytoskeletal dynamics:Ca(2+)](i) and force suppression in forskolin-pretreated porcine coronary arteries

Initiation of force generation during vascular smooth muscle contraction involves a rise in intracellular calcium ([Ca(2+)]i) and phosphorylation of myosin light chains (MLC). However, reversal of these two processes alone does not account for the force inhibition that occurs during relaxation or inhibition of contraction, implicating that other mechanisms, such as actin cytoskeletal rearrangement, play a role in the suppression of force. In this study, we hypothesize that forskolin-induced force suppression is dependent upon changes in actin cytoskeletal dynamics. To focus on the actin cytoskeletal changes, a physiological model was developed in which forskolin treatment of intact porcine coronary arteries (PCA) prior to treatment with a contractile agonist resulted in complete suppression of force. Pretreatment of PCA with forskolin suppressed histamine-induced force generation but did not abolish [Ca(2+)]i rise or MLC phosphorylation. Additionally, forskolin pretreatment reduced filamentous actin in histamine-treated tissues, and prevented histamine-induced changes in the phosphorylation of the actin-regulatory proteins HSP20, VASP, cofilin, and paxillin. Taken together, these results suggest that forskolin-induced complete force suppression is dependent upon the actin cytoskeletal regulation initiated by the phosphorylation changes of the actin regulatory proteins and not on the MLC dephosphorylation. This model of complete force suppression can be employed to further elucidate the mechanisms responsible for smooth muscle tone, and may offer cues to pathological situations, such as hypertension and vasospasm.

Effect of aging on cellular mechanotransduction

Aging is becoming a critical heath care issue and a burgeoning economic burden on society. Mechanotransduction is the ability of the cell to sense, process, and respond to mechanical stimuli and is an important regulator of physiologic function that has been found to play a role in regulating gene expression, protein synthesis, cell differentiation, tissue growth, and most recently, the pathophysiology of disease. Here we will review some of the recent findings of this field and attempt, where possible, to present changes in mechanotransduction that are associated with the aging process in several selected physiological systems, including musculoskeletal, cardiovascular, neuronal, respiratory systems and skin.

Actin depolymerization factor/cofilin activation regulates actin polymerization and tension development in canine tracheal smooth muscle

The contractile activation of airway smooth muscle tissues stimulates actin polymerization, and the inhibition of actin polymerization inhibits tension development. Actin-depolymerizing factor (ADF) and cofilin are members of a family of actin-binding proteins that mediate the severing of F-actin when activated by dephosphorylation at serine 3. The role of ADF/cofilin activation in the regulation of actin dynamics and tension development during the contractile activation of smooth muscle was evaluated in intact canine tracheal smooth muscle tissues. Two-dimensional gel electrophoresis revealed that ADF and cofilin exist in similar proportions in the muscle tissues, and that approximately 40% of the total ADF/cofilin in unstimulated tissues is phosphorylated. Phospho-ADF/cofilin decreased concurrently with tension development in response to stimulation with acetylcholine (ACh) or potassium depolarization indicating the activation of ADF/cofilin. Expression of an inactive phospho-cofilin mimetic (cofilin S3E) but not wild type cofilin in the smooth muscle tissues inhibited endogenous ADF/cofilin dephosphorylation and ACh-induced actin polymerization. Expression of cofilin S3E in the tissues depressed tension development in response to ACh, but it did not affect myosin light chain phosphorylation. The ACh-induced dephosphorylation of ADF/cofilin required the Ca2+-dependent activation of calcineurin (PP2B). The results indicate that the activation of ADF/cofilin is regulated by contractile stimulation in tracheal smooth muscle and that cofilin activation is required for actin polymerization and tension development in response to contractile stimulation.

Physiologic properties and regulation of the actin cytoskeleton in vascular smooth muscle

Vascular smooth muscle tone plays a fundamental role in regulating blood pressure, blood flow, microcirculation, and other cardiovascular functions. The cellular and molecular mechanisms by which vascular smooth muscle contractility is regulated are not completely elucidated. Recent studies show that the actin cytoskeleton in smooth muscle is dynamic, which regulates force development. In this review, evidence for actin polymerization in smooth muscle upon external stimulation is summarized. Protein kinases such as Abelson tyrosine kinase, focal adhesion kinase, Src, and mitogen-activated protein kinase have been documented to coordinate actin polymerization in smooth muscle. Transmembrane integrins have also been reported to link to signaling pathways modulating actin dynamics. The roles of Rho family of the small proteins that bind to guanosine triphosphate (GTP), also known as GTPases, and the actin-regulatory proteins, including Crk-associated substrate, neuronal Wiskott-Aldrich Syndrome protein, the Arp2/3 complex, and profilin, and heat shock proteins in regulating actin assembly are discussed. These new findings promote our understanding on how smooth muscle contraction is regulated at cellular and molecular levels.

Biophysical basis for airway hyperresponsiveness

Airway hyperresponsiveness is the excessive narrowing of the airway lumen caused by stimuli that would cause little or no narrowing in the normal individual. It is one of the cardinal features of asthma, but its mechanisms remain unexplained. In asthma, the key end-effector of acute airway narrowing is contraction of the airway smooth muscle cell that is driven by myosin motors exerting their mechanical effects within an integrated cytoskeletal scaffolding. In just the past few years, however, our understanding of the rules that govern muscle biophysics has dramatically changed, as has their classical relationship to airway mechanics. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that in a dynamic setting nonclassical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt (remodel) its internal microstructure rapidly in response to its ever-changing mechanical environment. Here, we consider some of these emerging concepts and, in particular, focus on structural remodeling of the airway smooth muscle cell as it relates to excessive airway narrowing in asthma.

Mechanotransduction, asthma, and airway smooth muscle

Excessive force generation by airway smooth muscle is the main culprit in excessive airway narrowing during an asthma attack. The maximum force the airway smooth muscle can generate is exquisitely sensitive to muscle length fluctuations during breathing, and is governed by complex mechanotransduction events that can best be studied by a hybrid approach in which the airway wall is modeled in silico so as to set a dynamic muscle load comparable to that experienced in vivo.

Airway smooth muscle as a target of asthma therapy: history and new directions

Ultimately, asthma is a disease characterized by constriction of airway smooth muscle (ASM). The earliest approach to the treatment of asthma comprised the use of xanthines and anti-cholinergics with the later introduction of anti-histamines and anti-leukotrienes. Agents directed at ion channels on the smooth muscle membrane (Ca2+ channel blockers, K+ channel openers) have been tried and found to be ineffective. Functional antagonists, which modulate intracellular signalling pathways within the smooth muscle (beta-agonists and phosphodiesterase inhibitors), have been used for decades with success, but are not universally effective and patients continue to suffer with exacerbations of asthma using these drugs. During the past several decades, research energies have been directed into developing therapies to treat airway inflammation, but there have been no substantial advances in asthma therapies targeting the ASM. In this manuscript, excitation-contraction coupling in ASM is addressed, highlighting the current treatment of asthma while proposing several new directions that may prove helpful in the management of this disease.

Airway smooth muscle as a regulator of immune responses and bronchomotor tone

The traditional view of airway smooth muscle (ASM) in asthma, as a purely contractile tissue, seems to be inadequate. Compelling evidence now suggests that ASM plays an important role in regulating bronchomotor tone, in perpetuating airway inflammation, and in remodeling of the airways. This article reviews three distinct functions of ASM cells: the process of excitation-contraction coupling, with a particular focus on the role of cytokines in modulating calcium responses; the processes of smooth muscle cell proliferation and migration; and the synthetic and immunomodulatory function of ASM cells. This article also discusses how altered synthetic function contributes to airway remodeling.

Silencing of p21-activated kinase attenuates vimentin phosphorylation on Ser-56 and reorientation of the vimentin network during stimulation of smooth muscle cells by 5-hydroxytryptamine

Vimentin intermediate filaments undergo spatial reorganization in endothelial cells and fibroblasts in response to stimulation with platelet-derived growth factor and epidermal growth factor. In the present study, the vimentin network exhibited a curved filamentous structure in unstimulated smooth muscle cells. Vimentin filaments became straight and were arranged along the long axis of cells upon stimulation with 5-hydroxytryptamine (5-HT; serotonin). Stimulation of smooth muscle cells with 5-HT also induced phosphorylation of vimentin on Ser-56. Treatment of cells with small interfering RNA selectively down-regulated the expression of PAK1 (p21-activated kinase 1) without affecting the content of smooth muscle alpha-actin. The silencing of PAK1 inhibited the site-specific phosphorylation and spatial rearrangement of the vimentin network in response to stimulation with 5-HT. Neither the disruption of stress fibres by cytochalasin D nor the inhibition of protein tyrosine phosphorylation affects the spatial reorganization of vimentin intermediate filaments in response to stimulation with 5-HT. In addition, stimulation of smooth muscle cells with 5-HT increased the ratio of soluble to insoluble vimentin. PAK1 silencing attenuated increases in the ratio of soluble to insoluble vimentin upon stimulation with 5-HT. These results suggest that the PAK-mediated site-specific phosphorylation of vimentin may play a role in regulating the reorganization of vimentin intermediate filaments during stimulation of smooth muscle cells with 5-HT.

Length adaptation of airway smooth muscle: a stochastic model of cytoskeletal dynamics

To account for cytoskeleton remodeling as well as smooth muscle length adaptation, here we represent the cytoskeleton as a two-dimensional network of links (contractile filaments or stress fibers) that connect nodes (dense plaques or focal adhesions). The network evolves in continuous turnover with probabilities of link formation and dissolution. The probability of link formation increases with the available fraction of contractile units, increases with the degree of network activation, and decreases with increasing distance between nodes, d, as 1/d(s), where s controls the distribution of link lengths. The probability of link dissolution decays with time to mimic progressive cytoskeleton stabilization. We computed network force (F) as the vector summation of link forces exerted at all nodes, unloaded shortening velocity (V) as being proportional to the average link length, and network compliance (C) as the change in network length per change in elastic force. Imposed deformation caused F to decrease transiently and then recover dynamically; recovery ability decreased with increasing time after activation, mimicking observed biological behavior. Isometric contractions showed small sensitivity of F to network length, thus maintaining high force over a wide range of lengths; V and C increased with increasing length. In these behaviors, link length regulation, as described by the parameter s, was found to be crucial. Concerning length adaptation, all phenomena reported thus far in the literature were captured by this extremely simple network model.

ROK-induced cross-link formation stiffens passive muscle: reversible strain-induced stress softening in rabbit detrusor

Passive mechanical properties of strips of rabbit detrusor smooth muscle were examined and found by cyclic loading in a calcium-free solution to display viscoelastic softening and strain-induced stress softening (strain softening). Strain softening, or the Mullins effect, is a loss of stiffness attributed to the breakage of cross-links, and appeared irreversible in detrusor even after the return of spontaneous rhythmic tone during 120 min of incubation in a calcium-containing solution. However, 3 min of KCl or carbachol (CCh)-induced contraction permitted rapid regeneration of the passive stiffness lost to strain softening, and 3 μM of the RhoA kinase (ROK) inhibitor Y-27632 prevented this regeneration. The degree of ROK-induced passive stiffness was inversely dependent on muscle length over a length range where peak CCh-induced force was length independent. Thus rabbit detrusor displayed variable passive stiffness both strain- and activation-history dependent. In conclusion, activation of ROK by KCl or CCh increased passive stiffness softened by muscle strain and thereby attributed to cross-links that remained stable during tissue incubation in a calcium-free solution. Degradation of this signaling system could potentially contribute to urinary incontinence.

Effects of S1P on myoblastic cell contraction: possible involvement of Ca-independent mechanisms

Sphingosine-1-phosphate (S1P) is a lipid mediator, which affects many essential processes such as cell proliferation, differentiation and contraction in many cell types. We have previously demonstrated that the lipid mediator elicits Ca(2+) transients in a myoblastic cell line (C2C12) by interacting with its specific receptors (S1PR(s)). In the present study, we wanted to correlate the Ca(2+) response with activation of myoblastic cell contractility. C2C12 cells were first investigated for the expression and cellular organization of cytoskeletal proteins by immunoconfocal microscopy. We found that myoblasts exhibited a quite immature cytoskeleton, with filamentous actin dispersed as a web-like structure within the cytoplasm. To evaluate intracellular Ca(2+) mobilization, the cells were loaded with a fluorescent Ca(2+) indicator (Fluo-3), stimulated with S1P and simultaneously observed with differential interference contrast and fluorescence optics. Exogenous S1P-induced myoblastic cell contraction was temporally unrelated to S1P-induced intracellular Ca(2+) increase; cell contraction occurred within 5-8 s from stimulation, whereas intracellular Ca(2+) increase was evident only after 15-25 s. To support the Ca(2+) independence of myoblastic cell contraction, the cells were pretreated with a Ca(2+) chelator, BAPTA/AM, prior to stimulation with S1P. In these experimental conditions, the myoblasts were still able to contract, whereas the S1P-induced Ca(2+) transients were completely abolished. On the contrary, when C2C12 cells were induced to differentiate into skeletal myotubes, they responded to S1P with a rapid cell contraction concurrent with an increase in the intracellular Ca(2+). These data suggest that Ca(2+)-independent mechanism of cell contraction may be replaced by Ca(2+)-dependent ones during skeletal muscle differentiation.

Vanadate activates Rho A translocation in association with contracting effects in ileal longitudinal smooth muscle of guinea pig

We characterized the effects of vanadate, an inhibitor of tyrosine phosphatase, on the tension, the level of myosin light chain (MLC) phosphorylation, and Rho A activation in intact ileal longitudinal smooth muscle of the guinea pig to study the role of tyrosine phosphorylation in contraction signaling. Vanadate exerted a sustained contraction with a slow onset of tension development, in a concentration-dependent manner. The contractile effects of vanadate were accompanied by increases in the level of MLC phosphorylation. The tyrosine kinase inhibitor genistein; the MLC kinase inhibitor 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-9); and the Rho kinase inhibitor (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride, monohydrate (Y-27632) inhibited the vanadate-induced contraction and MLC phosphorylation. Vanadate caused Rho A translocation from the cytosol to the membrane fraction, which was inhibited by genistein, but not by ML-9 and Y-27632. These data indicate that vanadate induces Rho A activation probably via protein tyrosine phosphorylation and the subsequent contraction through increases in the level of MLC phosphorylation.

Bronchospasm and its biophysical basis in airway smooth muscle

Airways hyperresponsiveness is a cardinal feature of asthma but remains unexplained. In asthma, the airway smooth muscle cell is the key end-effector of bronchospasm and acute airway narrowing, but in just the past five years our understanding of the relationship of responsiveness to muscle biophysics has dramatically changed. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that non-classical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt rapidly to changes in its dynamic microenvironment. These newly discovered phenomena have been described empirically, but a mechanistic basis to explain them is only beginning to emerge.

Electron microscopic study of actin polymerization in airway smooth muscle

Actin polymerization as part of the normal smooth muscle response to various stimuli has been reported. The actin dynamics are believed to be necessary for cytoskeletal remodeling in smooth muscle in its adaptation to external stress and strain and for maintenance of optimal contractility. We have shown in our previous studies in airway smooth muscle that myosins polymerized in response to contractile activation as well as to adaptation at longer cell lengths. We postulated that the same response could be elicited from actins under the same conditions. In the present study, actin filament formation was quantified electron microscopically in cell cross sections. Nanometer resolution allowed us to examine regional distribution of filaments in a cell cross section. Airway smooth muscle bundles were fixed in relaxed and activated states at two lengths; muscle preparations were also fixed after a period of oscillatory strain, a condition known to cause depolymerization of myosin filaments. The results indicate that contractile activation and increased cell length nonsynergistically enhanced actin polymerization; the extent of actin polymerization was substantially less than that of myosin polymerization. Oscillatory strain increased thin filament formation. Although thin filament density was found higher in cytoplasmic areas near dense bodies, contractile activation did not preferentially enhance actin polymerization in these areas. It is concluded that actin thin filaments are dynamic structures whose length and number are regulated by the cell in response to changes in extracellular environment and that polymerization and depolymerization of thin filaments occur uniformly across the whole cell cross section.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

Cytoskeletal remodeling of the airway smooth muscle cell: a mechanism for adaptation to mechanical forces in the lung

Airway smooth muscle is continuously subjected to mechanical forces caused by changes in lung volume during breathing. These mechanical oscillations have profound effects on airway smooth muscle contractility both in vivo and in vitro. Alterations in airway smooth muscle properties in response to mechanical forces may result from adaptive changes in the organization of the actin cytoskeleton. Recent advances suggest that in airway smooth muscle, two cytosolic signaling proteins that associate with focal adhesion complexes, focal adhesion kinase (FAK) and paxillin, are involved in transducing external mechanical signals. FAK and paxillin regulate changes in the organization of the actin cytoskeleton and the activation of contractile proteins. Actin is in a dynamic state in airway smooth muscle and undergoes polymerization and depolymerization during the contraction-relaxation cycle. The organization of the cytoskeletal proteins, vinculin, talin, and alpha-actinin, which mediate linkages between actin filaments and transmembrane integrins, is also regulated by contractile stimulation in airway smooth muscle. The fluidity of the cytoskeletal structure of the airway smooth muscle cell may be fundamental to its ability to adapt and respond to the mechanical forces imposed on it in the lung during breathing.

Smooth muscle cell migration stimulated by interleukin 6 is associated with cytoskeletal reorganization

BACKGROUND

Interleukin 6 (IL-6) is elevated in the arterial wall in atherosclerosis and restenosis after angioplasty. An important contributor to these pathologies is migration of vascular smooth muscle cells (VSMC), which is often associated with cytoskeletal reorganization initiated by growth factors and chemokines. We recently reported that IL-6 stimulated migration of VSMC. Here, we examined the cytoskeleton of VSMC and cytoskeletal associated proteins to determine potential mechanisms associated with IL-6 induced migration.

MATERIALS AND METHODS

Studies were performed in VSMC cultured from rat aortas.

RESULTS

IL-6 significantly stimulated VSMC migration. IL-6 induced actin polymerization, and tyrosine phosphorylation of focal adhesion-associated cytoskeletal proteins including focal adhesion kinase (FAK) and paxillin. Cytochalasin D, an inhibitor of actin polymerization, blocked phosphorylation of FAK and paxillin as well as cell motility induced by the cytokine.

CONCLUSIONS

Collectively, these data demonstrate for the first time that IL-6 stimulates VSMC motility which correlated with induction of actin cytoskeletal reorganization and tyrosine phosphorylation of FAK and paxillin.

Mechanical stress increases RhoA activation in airway smooth muscle cells

Cultured airway smooth muscle cells subjected to cyclic strain respond with increased cytoskeletal organization and contractility resembling effects described with RhoA activation. To test the hypothesis that strain increases cell cytoskeletal organization through RhoA, cells were subjected to strain in the presence of known activators or inhibitors of RhoA. Ten percent cyclic deformational strain (serum-free conditions) increased F-actin staining (152% over control), and this effect was enhanced by serum or lysophosphatidic acid (180%), but decreased (68%) with Clostridium botulinum toxin inhibition of RhoA or with the Rho kinase inhibitor Y27632 (67%). When cells expressing the dominant negative N17-RhoA isoform were subjected to strain, F-actin staining was disorganized and cells failed to elongate or migrate relative to strain direction. When cells expressing a green fluorescent protein (GFP)-RhoA fusion protein were subjected to strain, GFP showed up to 25% greater cell membrane staining than control cells. Finally, strain caused a 4-fold increase in RhoA activation (Rhotekin binding assay), and a 3-fold increase myosin phosphatase phosphorylation that was inhibited by Y27632. We conclude that mechanical stress activates RhoA, an event that may increase airway smooth muscle contractility.

Rho kinase mediates serum-induced contraction in fibroblast fibers independent of myosin LC20 phosphorylation

Fibroblasts form fibers when grown in culture medium containing native type 1 collagen. The contractile forces generated can be precisely quantified and used to analyze the signal transduction pathways regulating fibroblast contraction. Calf serum (30%) induces a sustained contraction that is accompanied by a transient increase in intracellular calcium ([Ca(2+)](i)). W-7, a calmodulin inhibitor, KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, and ML-7, a myosin light-chain kinase inhibitor, had no effects on either the contraction or the [Ca(2+)](i) responses. Neither genistein, a tyrosine kinase inhibitor, nor calphostin C, a protein kinase C inhibitor, had major effects on force or [Ca(2+)](i). In contrast, the Rho kinase inhibitors (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632) and HA1077 depressed the contraction in a dose-dependent manner without affecting the [Ca(2+)](i) response. Stress fiber formation was also suppressed by Y-27632. Surprisingly, calf serum, Y-27632, and calf serum plus Y-27632 did not alter mono- or diphosphorylation of the myosin regulatory light chain (MRLC) compared with control untreated fibers. These results suggest that the sustained contraction of NIH 3T3 fibroblast fibers induced by calf serum is mediated by Rho kinase but is independent of a sustained increase in [Ca(2+)](i), calcium/calmodulin- or protein kinase C-dependent pathways, or increases in MRLC phosphorylation.

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Gö6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.

The focal adhesion protein paxillin regulates contraction in canine tracheal smooth muscle

The adapter protein paxillin localizes to the focal adhesions of adherent cells and has been implicated in the regulation of cytoskeletal organization and cell motility. Paxillin undergoes tyrosine phosphorylation in response to the contractile stimulation of tracheal smooth muscle. We therefore hypothesized that paxillin may be involved in regulating smooth muscle contraction. Tracheal smooth muscle strips were treated with paxillin antisense oligonucleotides to inhibit the expression of paxillin protein selectively. Paxillin antisense or sense was introduced into muscle strips by reversible permeabilization and strips were incubated with antisense or sense for 3 days. Paxillin antisense selectively depressed paxillin expression, but it did not affect the expression of vinculin, focal adhesion kinase, myosin light chain kinase, myosin heavy chain or myosin light chain. Tension development in response to stimulation with ACh or KCl was markedly depressed in paxillin-depleted muscle strips. Active force and paxillin protein expression were restored by incubation of antisense-treated strips in the absence of oligonucleotides. The depletion of paxillin did not inhibit the increase in intracellular free Ca2+, myosin light chain phosphorylation or myosin ATPase activity in response to contractile stimulation. The concentration of G-actin was significantly lower in unstimulated paxillin-depleted smooth muscle tissues than in normal tissues. While stimulation with acetylcholine caused a decrease in G-actin in normal muscle strips, it caused little change in the G-actin concentration in paxillin-depleted muscle strips, suggesting that paxillin is necessary for normal actin dynamics in smooth muscle. We conclude that paxillin is required for active tension development in smooth muscle, but that it does not regulate increases in intracellular Ca2+, myosin light chain phosphorylation or myosin ATPase activity during contractile stimulation. Paxillin may be important in regulating actin filament dynamics and organization during smooth muscle contraction.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Selected contribution: roles of focal adhesion kinase and paxillin in the mechanosensitive regulation of myosin phosphorylation in smooth muscle

The increase in intracellular Ca(2+) and myosin light chain (MLC) phosphorylation in response to the contractile activation of tracheal smooth muscle is greater at longer muscle lengths (21). However, MLC phosphorylation can also be stimulated by Ca(2+)-insensitive signaling pathways (19). The cytoskeletal proteins paxillin and focal adhesion kinase (FAK) mediate a Ca(2+)-independent length-sensitive signaling pathway in tracheal smooth muscle (30). We used alpha-toxin-permeabilized tracheal smooth muscle strips to determine whether the length sensitivity of MLC phosphorylation can be regulated by a Ca(2+)-insensitive signaling pathway and whether the length sensitivity of active tension depends on the length sensitivity of myosin activation. Although active tension remained length sensitive, ACh-induced MLC phosphorylation was the same at optimal muscle length (L(o)) and 0.5 L(o) when intracellular Ca(2+) was maintained at pCa 7. MLC phosphorylation was also the same at L(o) and 0.5 L(o) in strips stimulated with 10 microM Ca(2+). In contrast, the Ca(2+)-insensitive tyrosine phosphorylation of FAK and paxillin stimulated by ACh was higher at L(o) than at 0.5 L(o). We conclude that the length-sensitivity of MLC phosphorylation depends on length-dependent changes in intracellular Ca(2+) but that length-dependent changes in MLC phosphorylation are not the primary mechanism for the length sensitivity of active tension.

Pharmacological evidence for capacitative Ca(2+) entry in cannulated and pressurized skeletal muscle arterioles

Arteriolar myogenic tone shows a marked dependency on extracellular Ca(2+). The contribution played by mechanisms such as intracellular Ca(2+) release and capacitative entry, however, are less certain. The present studies aimed to demonstrate functional evidence for involvement of such mechanisms in myogenic tone and reactivity. Single cremaster arterioles were denuded of endothelium, pressurized under no-flow conditions and loaded with fura 2-AM for measurement of changes in intracellular Ca(2+) [Ca(2+)](i). The cell permeable, putative, IP(3) receptor antagonist 2APB (2 aminoethoxydiphenyl borate) was used to determine the possible role of IP(3) receptor-mediated mechanisms in arteriolar myogenic tone and reactivity. Arterioles dilated in response to increasing concentrations of 2APB (1 - 300 microM) without a concomitant change in global [Ca(2+)](i). Also 2APB (50 microM) completely inhibited the myogenic constriction in response to a step change in luminal pressure (50 - 120 mmHg) with no apparent effect on pressure-mediated increases in [Ca(2+)](i). 2APB markedly attenuated the constrictor response and [Ca(2+)](i) increase stimulated by phenylephrine but not KCl. Capacitative Ca(2+) influx in arterioles was demonstrated either by re-addition of extracellular [Ca(2+)] following pre-treatment with 1 or 10 microM nifedipine in Ca(2+) free buffer or exposure of vessels to thapsigargin (1 microM) to induce store depletion. In both cases 2APB inhibited the increase in [Ca(2+)](i). Capacitative Ca(2+) entry showed an inverse relationship with intraluminal pressure over the range 10 - 120 mmHg. Consistent with an effect on a Ca(2+) entry pathway, 2APB had no effect on intracellular (caffeine releasable) Ca(2+) stores while decreasing the rate of Mn(2+) quench of fura 2 fluorescence. The results provide functional evidence for capacitative Ca(2+) entry in intact arteriolar smooth muscle. The effectiveness of 2APB in inhibiting both non-voltage gated Ca(2+) entry and responsiveness to an acute pressure step is consistent with the involvement of an axis involving IP(3)-mediated and or capacitative Ca(2+) entry mechanisms in myogenic reactivity. Given the lack of effect of 2APB on pressure-induced changes in global [Ca(2+)](i) it is suggested that such mechanisms participate on a localized level to couple the myogenic stimulus to contraction.

Historical perspective on airway smooth muscle: the saga of a frustrated cell

Despite the lack of a clearly defined physiological function, airway smooth muscle receives substantial attention because of its involvement in the pathogenesis of asthma. Recent investigations have turned to the ways in which the muscle is influenced by its dynamic microenvironment. Ordinarily, airway smooth muscle presents little problem, even when maximally activated, because unending mechanical perturbations provided by spontaneous tidal breathing put airway smooth muscle in a perpetual state of "limbo," keeping its contractile machinery off balance and unable to achieve its force-generating potential. The dynamic microenvironment affects airway smooth muscle in at least two ways: by acute changes associated with disruption of myosin binding and by chronic changes associated with plastic restructuring of contractile and cytoskeletal filament organization. Plastic restructuring can occur when dynamic length changes occur between sequential contractile events or within a single contractile event. Impairment of these normal responses of airway smooth muscle to its dynamic environment may be implicated in airway hyperresponsiveness in asthma.

Invited review: focal adhesion and small heat shock proteins in the regulation of actin remodeling and contractility in smooth muscle

Smooth muscle cells are able to adapt rapidly to chemical and mechanical signals impinging on the cell surface. It has been suggested that dynamic changes in the actin cytoskeleton contribute to the processes of contractile activation and mechanical adaptation in smooth muscle. In this review, evidence for functionally important changes in actin polymerization during smooth muscle contraction is summarized. The functions and regulation of proteins associated with "focal adhesion complexes" (membrane-associated dense plaques) in differentiated smooth muscle, including integrins, focal adhesion kinase (FAK), c-Src, paxillin, and the 27-kDa small heat shock protein (HSP27) are described. Integrins in smooth muscles are key elements of mechanotransduction pathways that communicate with and are regulated by focal adhesion proteins that include FAK, c-Src, and paxillin as well as proteins known to mediate cytoskeletal remodeling. Evidence that functions of FAK and c-Src protein kinases are closely intertwined is discussed as well as evidence that focal adhesion proteins mediate key signal transduction events that regulate actin remodeling and contraction. HSP27 is reviewed as a potentially significant effector protein that may regulate actin dynamics and cross-bridge function in response to activation of p21-activated kinase and the p38 mitogen-activated protein kinase signaling pathway by signaling pathways linked to integrin proteins. These signaling pathways are only part of a large number of yet to be defined pathways that mediate acute adaptive responses of the cytoskeleton in smooth muscle to environmental stimuli.

Depletion of focal adhesion kinase by antisense depresses contractile activation of smooth muscle

Focal adhesion kinase (FAK) undergoes tyrosine phosphorylation in response to the contractile stimulation of tracheal smooth muscle. We hypothesized that FAK may play an important role in signaling pathways that regulate smooth muscle contraction. FAK antisense or FAK sense was introduced into muscle strips by reversible permeabilization, and strips were incubated with antisense or sense for 7 days. Antisense decreased FAK expression compared with that in untreated and sense-treated tissues, but it did not affect the expression of vinculin or myosin light chain kinase. Increases in force, intracellular free Ca2+ and myosin light chain phosphorylation in response to stimulation with ACh or KCl were depressed in FAK-depleted tissues, but FAK depletion did not affect the activation of permeabilized tracheal muscle strips with Ca2+. The tyrosine phosphorylation of paxillin, a substrate for FAK, was also significantly reduced in FAK-depleted strips. We conclude that FAK is a necessary component of the signaling pathways that regulate smooth muscle contraction and that FAK plays a role in regulating intracellular free Ca2+ and myosin light chain phosphorylation.

Molecular mechanisms of phenotypic plasticity in smooth muscle cells

Morphological, functional, molecular and cell biology studies have revealed a striking multifunctional nature of individual smooth muscle cells (SMC). SMCs manifest phenotypic plasticity in response to changes in environment and functional requirements, acquiring a range of structural and functional properties bounded by two extremes, called "synthetic" and "contractile." Each phenotypic state is characterized by expression of a unique set of structural, contractile, and receptor proteins and isoforms that correlate with differing patterns of gene expression. Recent studies have identified signaling pathways and transcription factors (e.g., RhoA GTPase/ROCK, also known as Rho kinase, and serum response factor) that regulate the transcriptional activities of genes encoding proteins associated with the contractile apparatus. Mechanical plasticity of contractile-state smooth muscle further extends SMC functional diversity. This may also be regulated, in part, by the RhoA GTPase/ROCK pathway, via reorganization of cytoskeletal and contractile proteins. Future studies that define transcriptional and posttranscriptional mechanisms of SMC plasticity are necessary to fully understand the role of SMC in the pathogenesis and morbidity of human diseases of the airways, vasculature, and gastrointestinal tract.