Laminin binding to beta1-integrins selectively alters beta1- and beta2-adrenoceptor signalling in cat atrial myocytes

Inhibition of cAMP-Dependent PKA Activates β2-Adrenergic Receptor Stimulation of Cytosolic Phospholipase A2 via Raf-1/MEK/ERK and IP3-Dependent Ca2+ Signaling in Atrial Myocytes

We previously reported in atrial myocytes that inhibition of cAMP-dependent protein kinase (PKA) by laminin (LMN)-integrin signaling activates β2-adrenergic receptor (β2-AR) stimulation of cytosolic phospholipase A2 (cPLA2). The present study sought to determine the signaling mechanisms by which inhibition of PKA activates β2-AR stimulation of cPLA2. We therefore determined the effects of zinterol (0.1 μM; zint-β2-AR) to stimulate ICa,L in atrial myocytes in the absence (+PKA) and presence (-PKA) of the PKA inhibitor (1 μM) KT5720 and compared these results with atrial myocytes attached to laminin (+LMN). Inhibition of Raf-1 (10 μM GW5074), phospholipase C (PLC; 0.5 μM edelfosine), PKC (4 μM chelerythrine) or IP3 receptor (IP3R) signaling (2 μM 2-APB) significantly inhibited zint-β2-AR stimulation of ICa,L in-PKA but not +PKA myocytes. Western blots showed that zint-β2-AR stimulation increased ERK1/2 phosphorylation in-PKA compared to +PKA myocytes. Adenoviral (Adv) expression of dominant negative (dn) -PKCα, dn-Raf-1 or an IP3 affinity trap, each inhibited zint-β2-AR stimulation of ICa,L in + LMN myocytes compared to control +LMN myocytes infected with Adv-βgal. In +LMN myocytes, zint-β2-AR stimulation of ICa,L was enhanced by adenoviral overexpression of wild-type cPLA2 and inhibited by double dn-cPLA2S505A/S515A mutant compared to control +LMN myocytes infected with Adv-βgal. In-PKA myocytes depletion of intracellular Ca2+ stores by 5 μM thapsigargin failed to inhibit zint-β2-AR stimulation of ICa,L via cPLA2. However, disruption of caveolae formation by 10 mM methyl-β-cyclodextrin inhibited zint-β2-AR stimulation of ICa,L in-PKA myocytes significantly more than in +PKA myocytes. We conclude that inhibition of PKA removes inhibition of Raf-1 and thereby allows β2-AR stimulation to act via PKCα/Raf-1/MEK/ERK1/2 and IP3-mediated Ca2+ signaling to stimulate cPLA2 signaling within caveolae. These findings may be relevant to the remodeling of β-AR signaling in failing and/or aging heart, both of which exhibit decreases in adenylate cyclase activity.

Photothermally targeted thermosensitive polymer-masked nanoparticles

The targeted delivery of therapeutic cargos using noninvasive stimuli has the potential to improve efficacy and reduce off-target effects (toxicity). Here, we demonstrate a targeting mechanism that uses a thermoresponsive copolymer to mask a peptide ligand that binds a widely distributed receptor (integrin β1) on the surface of silica core-gold shell nanoparticles. The nanoparticles convert NIR light into heat, which causes the copolymer to collapse, exposing the ligand peptide, allowing cell binding. The use of NIR light could allow targeting of plasmonic nanoparticles deep within tissues. This approach could be extended to a variety of applications including photothermal therapy and drug delivery.

LIMK-Dependent Actin Polymerization in Primary Sensory Neurons Promotes the Development of Inflammatory Heat Hyperalgesia in Rats

Inflammation-induced sensitivity to pain could be reduced by disrupting the actin cytoskeleton in primary sensory neurons.

Integrin and GPCR Crosstalk in the Regulation of ASM Contraction Signaling in Asthma

Airway hyperresponsiveness (AHR) is one of the cardinal features of asthma. Contraction of airway smooth muscle (ASM) cells that line the airway wall is thought to influence aspects of AHR, resulting in excessive narrowing or occlusion of the airway. ASM contraction is primarily controlled by agonists that bind G protein-coupled receptor (GPCR), which are expressed on ASM. Integrins also play a role in regulating ASM contraction signaling. As therapies for asthma are based on symptom relief, better understanding of the crosstalk between GPCRs and integrins holds good promise for the design of more effective therapies that target the underlying cellular and molecular mechanism that governs AHR. In this paper, we will review current knowledge about integrins and GPCRs in their regulation of ASM contraction signaling and discuss the emerging concept of crosstalk between the two and the implication of this crosstalk on the development of agents that target AHR.

Regulation of cardiac alternans by β-adrenergic signaling pathways

In cat atrial myocytes, β-adrenergic receptor (β-AR) stimulation exerts profound effects on excitation-contraction coupling and cellular Ca(2+) cycling that are mediated by β(1)- and β(2)-AR subtypes coupled to G proteins (G(s) and G(i)). In this study, we determined the effects of β-AR stimulation on pacing-induced Ca(2+) alternans. Ca(2+) alternans was recorded from single cat atrial myocytes with the fluorescent Ca(2+) indicator indo-1. Stable Ca(2+) alternans occurred at an average pacing frequency of 1.7 Hz at room temperature with a mean alternans ratio of 0.43. Nonselective β-AR stimulation as well as selective stimulation of β(1)/G(s), β(2)/G(s) + G(i), and β(2)/G(s) coupled pathways all abolished pacing-induced Ca(2+) alternans. β(1)-AR stimulation abolished alternans through stimulation of PKA and Ca(2+)/calmodulin-dependent protein kinase II, whereas β(2)-AR stimulation exclusively involved PKA and was mediated via G(s), whereas a known second pathway in cat atrial myocytes acting through G(i) and nitric oxide production was not involved in alternans regulation. Inhibition of various mitochondrial functions (dissipation of the mitochondrial membrane potential or inhibition of mitochondrial F(1)/F(0)-ATP synthase, mitochondrial Ca(2+) uptake via the mitochondrial Ca(2+) uniporter, and Ca(2+) extrusion via mitochondrial Na(+)/Ca(2+) exchange) enhanced Ca(2+) alternans; however, β-AR stimulation still abrogated alternans, provided that sufficient cellular ATP was available. Selective inhibition of mitochondrial or glycolytic ATP production did not prevent β-AR stimulation from abolishing Ca(2+) alternans. However, when both ATP sources were depleted, β-AR stimulation failed to decrease Ca(2+) alternans. These results indicate that in atrial myocytes, β-AR stimulation protects against pacing-induced alternans by acting through parallel and complementary signaling pathways.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

Fibronectin increases the force production of mouse papillary muscles via α5β1 integrin

The extracellular matrix (ECM) protein-integrin-cytoskeleton axis plays a central role as a mechanotransducing protein assemblage in many cell types. However, how the process of mechanotransduction and the mechanically generated signals arising from this axis affect myofilament function in cardiac muscle are not completely understood. We hypothesize that ECM proteins can regulate cardiac function through integrin binding, and thereby alter the intracellular calcium concentration ([Ca(2+)](i)) and/or modulate myofilament activation processes. Force measurements made in mouse papillary muscle demonstrated that in the presence of the soluble form of the ECM protein, fibronectin (FN), active force was increased significantly by 40% at 1 Hz, 54% at 2 Hz, 35% at 5 Hz and 16% at 9 Hz stimulation frequencies. Furthermore, increased active force in the presence of FN was associated with 12-33% increase in [Ca(2+)](i) and 20-50% increase in active force per unit Ca(2+). A function blocking antibody for α5 integrin prevented the effects of the FN on the changes in force and [Ca(2+)](i), whereas a function blocking α3 integrin antibody did not reverse the effects of FN. The effects of FN were reversed by an L-type Ca(2+) channel blocker, verapamil or PKA inhibitor. Freshly isolated cardiomyocytes exhibited a 39% increase in contraction force and a 36% increase in L-type Ca(2+) current in the presence of FN. Fibers treated with FN showed a significant increase in the phosphorylation of phospholamban; however, the phosphorylation of troponin I was unchanged. These results demonstrate that FN acts via α5β1 integrin to increase force production in myocardium and that this effect is partly mediated by increases in [Ca(2+)](i) and Ca(2+) sensitivity, PKA activation and phosphorylation of phospholamban.

Laminin enhances beta(2)-adrenergic receptor stimulation of L-type Ca(2+) current via cytosolic phospholipase A(2) signalling in cat atrial myocytes

We previously reported that attachment of atrial myocytes to the extracellular matrix protein laminin (LMN), decreases adenylate cyclase (AC)/cAMP and increases beta(2)-adrenergic receptor (AR) stimulation of L-type Ca(2+) current (I(Ca,L)). This study therefore sought to determine whether LMN enhances beta(2)-AR signalling via a cAMP-independent mechanism, i.e. cytosolic phospholipase A(2) (cPLA(2)) signalling. Studies were performed on acutely isolated atrial myocytes plated on uncoated coverslips (LMN) or coverslips coated with LMN (+LMN). As previously reported, 0.1 microm zinterol (zint-beta(2)-AR) stimulation of I(Ca,L) was larger in +LMN than LMN myocytes. In +LMN myocytes, zint-beta(2)-AR stimulation of I(Ca,L) was inhibited by inhibition of cPLA(2) by arachidonyltrifluoromethyl ketone (AACOCF(3); 10 microm), inhibition of G(i) by pertussis toxin and chelation of intracellular Ca(2+) by 10 microm BAPTA-AM. In contrast to zinterol, stimulation of I(Ca,L) by fenoterol (fen-beta(2)-AR), a beta(2)-AR agonist that acts exclusively via G(s) signalling, was smaller in +LMN than LMN myocytes. Arachidonic acid (AA; 5 microm) stimulated I(Ca,L) to a similar extent in LMN and +LMN myocytes. Inhibition of cAMP-dependent protein kinase A (cAMP/PKA) by either 5 mum H89 or 1 microm KT5720 in LMN myocytes mimicked the effects of +LMN myocytes to enhance zint-beta(2)-AR stimulation of I(Ca,L), which was blocked by 10 microm AACOCF(3). In contrast, H89 inhibited fen-beta(2)-AR stimulation of I(Ca,L), which was unchanged by AACOCF(3). Inhibition of ERK1/2 by 1 microm U0126 inhibited zint-beta(2)-AR stimulation of I(Ca,L) in +LMN myocytes and LMN myocytes in which cAMP/PKA was inhibited by KT5720. In LMN myocytes, cytochalasin D prevented inhibition of cAMP/PKA from enhancing zint-beta(2)-AR stimulation of I(Ca,L). We conclude that LMN enhances zint-beta(2)-AR stimulation of I(Ca,L) via G(i)/ERK1/2/cPLA(2)/AA signalling which is activated by concomitant inhibition of cAMP/PKA signalling and dependent on the actin cytoskeleton. These findings provide new insight into the cellular mechanisms by which the extracellular matrix can remodel beta(2)-AR signalling in atrial muscle.

L-type Ca(2+) current in ventricular cardiomyocytes

L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.

Cardiac cushions modulate action potential phenotype during heart development [corrected]

The extracellular matrix plays an important role in cardiac function. Its role in the generation and modulation of electrical activity in the early stages of heart development has not been studied extensively. Our study demonstrates that the extracellular matrix in cardiac cushions can alter the action potential phenotype by direct contact with cardiomyocytes from different regions of the heart. We also demonstrate that fibronectin, an important and abundant component of the cardiac extracellular matrix, partially mimics the effects of the cushion tissue in altering the changes in action potential. Fibronectin increases I(Ca) (2+) and acutely increases cytosolic calcium. These findings suggest that the composition of the cardiac extracellular matrix during development plays an important role in defining patterns of electrical activity in the developing heart.

Laminin acts via focal adhesion kinase/phosphatidylinositol-3' kinase/protein kinase B to down-regulate beta1-adrenergic receptor signalling in cat atrial myocytes

We previously reported that short-term (2 h) plating of cat atrial myocytes on the extracellular matrix protein, laminin (LMN) decreases adenylate cyclase activity and beta(1)-adrenergic receptor (beta(1)-AR) stimulation of L-type Ca(2+) current (I(Ca,L)). The present study sought to determine whether LMN-mediated down-regulation of beta(1) signalling is due to down-regulation of adenylate cyclase and to gain insight into the signalling mechanisms responsible. beta(1)-AR stimulation was achieved by 0.01 microm isoproterenol (isoprenaline) plus 0.1 microm ICI 118551, a selective beta(2)-AR antagonist. Atrial myocytes were plated for at least 2 h on uncoated cover-slips (-LMN) or cover-slips coated with LMN (+LMN). As previously reported, beta(1)-AR stimulation of I(Ca,L) was significantly smaller in +LMN compared to -LMN atrial myocytes. In -LMN myocytes, 10 microm LY294002 (LY), a specific inhibitor of PI-(3)K, had no effect on beta(1)-AR stimulation of I(Ca,L). In +LMN myocytes, however, LY significantly increased beta(1)-AR stimulation of I(Ca,L). Western blots revealed that compared with -LMN myocytes, +LMN myocytes showed a significant increase in Akt phosphorylation at Ser-473, which was prevented by LY. In another approach, +LMN myocytes were infected (multiplicity of infection (MOI), 100; 24 h) with replication-defective adenoviruses (Adv) expressing dominant-negative inhibitors of focal adhesion kinase (FAK) (Adv-FRNK or Adv-Y397F-FAK) or Akt (Adv-dnAkt). Compared with control cells infected with Adv-beta-galactosidase, cells infected with Adv-FRNK, Adv-Y397F-FAK or Adv-dnAkt each exhibited a significantly greater beta(1)-AR stimulation of I(Ca,L). In -LMN myocytes LY had no effect on forskolin (FSK)-stimulated I(Ca,L). However, in +LMN myocytes LY significantly increased FSK-stimulated I(Ca,L). Similar results were obtained in +LMN atrial myocytes infected with Adv-FRNK. We conclude that LMN binding to beta(1)-integrin receptors acts via FAK/PI-(3)K/Akt to inhibit adenylate cyclase activity and thereby down-regulates beta(1)-AR-mediated stimulation of I(Ca,L). These findings provide new insight into the cellular mechanisms by which the extracellular matrix can modulate atrial beta-AR signalling.

Arrhythmogenic effects of beta2-adrenergic stimulation in the failing heart are attributable to enhanced sarcoplasmic reticulum Ca load

Ventricular tachycardia in heart failure (HF) can initiate by nonreentrant mechanisms such as delayed afterdepolarizations. In an arrhythmogenic rabbit model of HF, we have shown that isoproterenol induces ventricular tachycardia in vivo and aftercontractions and transient inward currents in HF myocytes. To determine whether beta(2)-adrenergic receptor (beta(2)-AR) stimulation contributes, we performed in vivo drug infusion, in vitro myocyte and biochemical studies. Intravenous zinterol (2.5 microg/kg) led to ventricular arrhythmias, including ventricular tachycardia up to 13 beats long in 4 of 6 HF rabbits (versus 0 of 5 controls, P<0.01), an effect blocked by beta(2)-AR antagonist ICI-118,551 (0.2 mg/kg). In field-stimulated myocytes (0.5 to 4 Hz, 37 degrees C), beta(2)-AR stimulation (1 micromol/L zinterol+300 nmol/L beta(1)-AR antagonist CGP-29712A) induced aftercontractions and Ca aftertransients in 88% of HF versus 0% of control myocytes (P<0.01). beta(2)-AR stimulation in HF (but not control) myocytes increased Ca transient amplitude (by 29%), sarcoplasmic reticulum (SR) Ca load (by 28%), the rate of [Ca](i) decline (by 28%; n=12, all P<0.05), and phospholamban phosphorylation at Ser16, but Ca current was unchanged. All of these effects in HF myocytes were blocked by ICI-118,551 (100 nmol/L). Although total beta-AR expression was reduced by 47% in HF rabbit left ventricle, beta(2)-AR number was unchanged, indicating more potent beta(2)-AR-dependent SR Ca uptake and arrhythmogenesis in HF. Human HF myocytes showed similar beta(2)-AR-induced aftercontractions, aftertransients, and enhanced Ca transient amplitude, SR Ca load and twitch [Ca](i) decline rate. Thus, beta(2)-AR stimulation is arrhythmogenic in HF, mediated by SR Ca overload-induced spontaneous SR Ca release and aftercontractions.

Sympathetic activation causes focal adhesion signaling alteration in early compensated volume overload attributable to isolated mitral regurgitation in the dog

We reported that left ventricular (LV) dilatation after 4 weeks of isolated mitral regurgitation (MR) in the dogs is marked by extracellular matrix loss and an increase in adrenergic drive. Given that extracellular matrix proteins and their receptor integrins influence beta-adrenergic receptor (beta-AR) responses in vitro, we tested whether beta1-AR activation modulates focal adhesion (FA) signaling and LV remodeling in these same dogs with isolated MR. Normal dogs were compared with dogs with MR of a 4-week duration and with MR dogs treated with beta(1)-AR blockade (beta(1)-RB) (extended-release metoprolol succinate, 100 mg QD) that was started 24 hours after MR induction. In MR LVs, a decrease in collagen accumulation compared with normal dogs was associated with a decrease in FA kinase tyrosine phosphorylation, along with FA kinase interaction with adapter and cytoskeletal proteins, p130(Cas) and paxillin, respectively, as determined by immunoprecipitation assays. There was increased phosphorylation of stress related molecules p38 mitogen-activated protein kinase (MAPK) and Hsp27 and survival signaling kinases extracellular signal-regulated kinase 1/2 and AKT, with no evidence of cardiomyocyte apoptosis. beta(1)-RB attenuated FA signaling loss and prevented p38 MAPK, Hsp27, and AKT phosphorylation induced by MR and significantly increased LV epicardial collagen content. However, beta(1)-RB did not improve LV endocardial collagen loss or LV dilatation induced by MR. Isolated myocytes from normal and MR dog hearts treated with beta(1)- or beta(2)-AR agonists demonstrated no difference in FA kinase, p38 MAPK, Hsp27, or AKT phosphorylation. These results showed that chronic stimulation of beta(1)-AR during early compensated MR impairs FA signaling that may affect myocyte/fibroblast-extracellular matrix scaffolding necessary for LV remodeling.

Crucial role of cytoskeleton reorganization in the negative inotropic effect of chromogranin A-derived peptides in eel and frog hearts

Vasostatins (VSs), i.e. the main biologically active peptides generated by the proteolytic processing of chromogranin A (CGA) N-terminus, exert negative inotropism in vertebrate hearts. Here, using isolated working eel (Anguilla anguilla) and frog (Rana esculenta) heart preparations, we have studied the role of the cytoskeleton in the VSs-mediated inotropic response. In both eel and frog hearts, VSs-mediated-negative inotropy was abolished by treatment with inhibitors of cytoskeleton reorganization, such as cytochalasin-D (eel: 10 nM; frog: 1 nM), an inhibitor of actin polymerisation, wortmannin (0.01 nM), an inhibitor of PI3-kinase (PI3-K)/protein kinase B (Akt) signal-transduction cascade, butanedione 2-monoxime (BDM) (eel: 100 nM; frog: 10 nM), an antagonist of myosin ATPase, and N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide (W7) (eel: 100 nM; frog: 1 nM), a calcium-calmodulin antagonist. These results demonstrate that changes in cytoskeletal dynamics play a crucial role in the negative inotropic influence of VSs on eel and frog hearts.

Chronotropic response of cultured neonatal rat ventricular myocytes to short-term fluid shear

Ventricular myocytes are continuously exposed to fluid shear in vivo by relative movement of laminar sheets and adjacent cells. Preliminary observations have shown that neonatal myocytes respond to fluid shear by increasing their beating rate, which could have an arrhythmogenic effect under elevated shear conditions. The objective of this study is to investigate the characteristics of the fluid shear response in cultured myocytes and to study selected potential mechanisms. Cultured neonatal rat ventricular myocytes that were spontaneously beating were subjected to low shear rates (5-50/s) in a fluid flow chamber using standard culture medium. The beating rate was measured from digital microscopic recordings. The myocytes reacted to low shear rates by a graded and reversible increase in their spontaneous beating rate of up to 500%. The response to shear was substantially attenuated in the presence of the beta-adrenergic agonist isoproterenol (by 86+/-8%), as well as after incubation with integrin-blocking RGD peptides (by 92+/-8%). The results suggest that the beta-adrenergic signaling pathway and integrin activation, which are known to interact, may play an important role in the response mechanism.

Bradykinin activates calcium-dependent potassium channels in cultured human airway smooth muscle cells

Bradykinin (BK) is an inflammatory mediator that can cause bronchoconstriction. In this study, we investigated the membrane currents induced by BK in cultured human airway smooth muscle (ASM) cells. Depolarization of the cells induced outward currents, which were inhibited by tetraethylammonium (TEA) in a concentration-dependent manner with an IC50 of 0.33 microM. The currents were increased by elevating intracellular free Ca2+ concentration, suggesting they are calcium-activated potassium channels [I(K(Ca))]. Preexposure to inhibitor of I(K(Ca)) of large conductance (BKCa), iberiotoxin, and small conductance (SKCa), apamin, inhibited the increase of outward current induced by BK. The relative contribution of BKCa was greatest in early passage cells. Both nickel and SKF-96365 (10 microM) inhibited the increase of the I(K(Ca)) induced by BK; however, the l-type Ca2+ channel blocker, nifedipine, had no effect. Activation of the BK-induced current was inhibited by heparin, indicating dependence on intact inositol 1,4,5-triphosphate (IP3)-sensitive intracellular Ca2+ stores. BK also increased inositol phosphate accumulation and induced a transient Ca2+-activated chloride current (CACC) and a sustained nonselective cation current (I(CAT)). In summary, BK activates BKCa, SKCa, CACC, and I(CAT) via IP3-sensitive stores in human ASM.

Integrin Receptor Activation Triggers Converging Regulation of Cav1.2 Calcium Channels by c-Src and Protein Kinase A Pathways

L-type, voltage-gated Ca2+ channels (CaL) play critical roles in brain and muscle cell excitability. Here we show that currents through heterologously expressed neuronal and smooth muscle CaL channel isoforms are acutely potentiated following alpha5beta1 integrin activation. Only the alpha1C pore-forming channel subunit is critical for this process. Truncation and site-directed mutagenesis strategies reveal that regulation of Cav1.2 by alpha5beta1 integrin requires phosphorylation of alpha1C C-terminal residues Ser1901 and Tyr2122. These sites are known to be phosphorylated by protein kinase A (PKA) and c-Src, respectively, and are conserved between rat neuronal (Cav1.2c) and smooth muscle (Cav1.2b) isoforms. Kinase assays are consistent with phosphorylation of these two residues by PKA and c-Src. Following alpha5beta1 integrin activation, native CaL channels in rat arteriolar smooth muscle exhibit potentiation that is completely blocked by combined PKA and Src inhibition. Our results demonstrate that integrin-ECM interactions are a common mechanism for the acute regulation of CaL channels in brain and muscle. These findings are consistent with the growing recognition of the importance of integrin-channel interactions in cellular responses to injury and the acute control of synaptic and blood vessel function.

Costameres, focal adhesions, and cardiomyocyte mechanotransduction

Mechanotransduction refers to the cellular mechanisms by which load-bearing cells sense physical forces, transduce the forces into biochemical signals, and generate appropriate responses leading to alterations in cellular structure and function. This process affects the beat-to-beat regulation of cardiac performance but also affects the proliferation, differentiation, growth, and survival of the cellular components that comprise the human myocardium. This review focuses on the experimental evidence indicating that the costamere and its structurally related structure the focal adhesion complex are critical cytoskeletal elements involved in cardiomyocyte mechanotransduction. Biochemical signals originating from the extracellular matrix-integrin-costameric protein complex share many common features with those signals generated by growth factor receptors. The roles of key regulatory kinases and other muscle-specific proteins involved in mechanotransduction and growth factor signaling are discussed, and issues requiring further study in this field are outlined.

Integrin signaling in inflammatory and neuropathic pain in the rat

Many painful conditions are associated with alterations in the extracellular matrix (ECM) of affected tissues. While several integrins, the receptors for ECM proteins, are present on sensory neurons that mediate pain, the possible role of these cell adhesion molecules in inflammatory or neuropathic pain has not been explored. We found that the intradermal injection of peptide fragments of domains of laminin and fibronectin important for adhesive signaling selectively inhibited the hyperalgesia caused by prostaglandin E2 (PGE2) and epinephrine (EPI), respectively. The block of EPI hyperalgesia was mimicked by other peptides containing the RGD integrin-binding sequence. Monoclonal antibodies (mAbs) against the alpha1 or alpha3 integrin subunits, which participate in laminin binding, selectively blocked PGE2 hyperalgesia, while a mAb against the alpha5 subunit, which participates in fibronectin binding, blocked only EPI-induced hyperalgesia. A mAb against the beta1 integrin subunit, common to receptors for both laminin and fibronectin, inhibited hyperalgesia caused by both agents, as did the knockdown of beta1 integrin expression by intrathecal injection of antisense oligodeoxynucleotides. The laminin peptide, but not the fibronectin peptides, also reversibly abolished the longer lasting inflammatory hyperalgesia induced by carrageenan. Finally, the neuropathic hyperalgesia caused by systemic administration of the cancer chemotherapy agent taxol was reversibly inhibited by antisense knockdown of beta1 integrin. These results strongly implicate specific integrins in the maintenance of inflammatory and neuropathic hyperalgesia.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Role of the sensory neuron cytoskeleton in second messenger signaling for inflammatory pain

Prostaglandin E(2) (PGE(2)) and epinephrine act directly on nociceptors to produce mechanical hyperalgesia through protein kinase A (PKA) alone or through a combination of PKA, protein kinase C epsilon (PKCepsilon), and extracellular signal-regulated kinase (ERK), respectively. Disruptors of the cytoskeleton (microfilaments, microtubules, and intermediate filaments) markedly attenuated the hyperalgesia in rat paws caused by injection of epinephrine or its downstream mediators. In contrast, the hyperalgesia induced by PGE(2) or its mediators was not affected by any of the cytoskeletal disruptors. These effects were mimicked in vitro, as measured by enhancement of the tetrodotoxin-resistant sodium current. When PGE(2) hyperalgesia was shifted to dependence on PKCepsilon and ERK as well as PKA, as when the tissue is "primed" by prior treatment with carrageenan, it too became dependent on an intact cytoskeleton. Thus, inflammatory mediator-induced mechanical hyperalgesia was differentially dependent on the cytoskeleton such that cytoskeletal dependence correlated with mediation by PKCepsilon and ERK.

FHL3 is an actin-binding protein that regulates alpha-actinin-mediated actin bundling: FHL3 localizes to actin stress fibers and enhances cell spreading and stress fiber disassembly

Four and a half LIM domain (FHL) proteins are members of the LIM protein superfamily. Several FHL proteins function as co-activators of CREM/CREB transcription factors and the androgen receptor. FHL3 is highly expressed in skeletal muscle, but its function is unknown. FHL3 localized to the nucleus in C2C12 myoblasts and, following integrin engagement, exited the nucleus and localized to actin stress fibers and focal adhesions. In mature skeletal muscle FHL3 was found at the Z-line. Actin was identified as a potential FHL3 binding partner in yeast two-hybrid screening of a skeletal muscle library. FHL3 complexed with actin both in vitro and in vivo as shown by glutathione S-transferase pull-down assays and co-immunoprecipitation of recombinant and endogenous proteins. FHL3 promoted cell spreading and when overexpressed in spread C2C12 cells disrupted actin stress fibers. Increased FHL3 expression was detected in highly motile cells migrating into an artificial wound, compared with non-motile cells. The molecular mechanism by which FHL3 induced actin stress fiber disassembly was demonstrated by low speed actin co-sedimentation assays and electron microscopy. FHL3 inhibited alpha-actinin-mediated actin bundling. These studies reveal FHL3 as a significant regulator of actin cytoskeletal dynamics in skeletal myoblasts.

Nitric oxide signalling by selective beta(2)-adrenoceptor stimulation prevents ACh-induced inhibition of beta(2)-stimulated Ca(2+) current in cat atrial myocytes

The present study determined the effects of acetylcholine (ACh) on the L-type Ca(2+) current (I(Ca,L)) stimulated by beta(1)- or beta(2)-adrenergic receptor (AR) agonists in cat atrial myocytes. When isoproterenol (ISO; 0.1 microM) plus the beta(2)-AR antagonist ICI 118,551 (ISO-beta(1)-AR stimulation) or 0.1 microM fenoterol, a beta(2)-AR agonist (FEN-beta(2)-AR stimulation) increased I(Ca,L), ACh (1 microM) inhibited I(Ca,L) by -60 +/- 4 and -63 +/- 6 %, respectively. When ISO plus the beta(1)-AR antagonist atenolol (ISO-beta(2)-AR stimulation) or 1 microM zinterol (ZIN-beta(2)-AR stimulation) increased I(Ca,L), ACh-induced inhibition of I(Ca,L) was significantly smaller, at -21 +/- 3 and -24 +/- 3 %, respectively. L-N(5)-(1-iminoethyl)ornithine (L-NIO, 10 microM), an inhibitor of nitric oxide (NO) synthase, enhanced ACh-induced inhibition of I(Ca,L) when stimulated by ZIN-beta(2)-ARs, but not when stimulated by ISO-beta(1)-ARs or FEN-beta(2)-ARs. Haemoglobin (50 microM), a NO scavenger, also enhanced ACh-induced inhibition when I(Ca,L) was stimulated by ZIN-beta(2)-ARs, but not when stimulated by FEN-beta(2)-ARs. ACh-induced inhibition of I(Ca,L) stimulated by ZIN-beta(2)-ARs was not affected by 10 microM 1H-[1,2,4] oxadiazolo[4,3-a] quinoxaline-1-one (ODQ) a guanylate cyclase inhibitor, but was significantly enhanced by 500 microM reduced glutathione or 100 microM dithiothreitol, agents that act as sinks for S-nitrosylation. ACh-induced inhibition was smaller when I(Ca,L) was stimulated by spermine/NO, a NO donor, than by milrinone, a phosphodiesterase type III inhibitor. ISO (ISO-beta(1)/beta(2)-AR stimulation) increased I(Ca,L) and even though ISO releases NO, ACh prominently inhibited I(Ca,L). This inhibitory effect of ACh was enhanced by L-NIO. Stimulation of ZIN-beta(2)-ARs increased intracellular NO, whereas ISO-beta(1)-ARs or FEN-beta(2)-ARs failed to increase intracellular NO. These results indicate that in atrial myocytes, NO released by selective beta(2)-AR stimulation prevents ACh-induced inhibition of I(Ca,L) stimulated by beta(2)-ARs. NO acts via a cGMP-independent, S-nitrosylation mechanism. Although FEN acts via beta(2)-ARs, it fails to stimulate G(i)-/NO signalling and preferentially stimulates G(s)-/adenylate cyclase signalling, similar to beta(1)-ARs. These findings indicate that NO signalling modulates muscarinic receptor inhibition of atrial function stimulated by beta(2)-ARs.

Beta 2-adrenergic receptor signaling acts via NO release to mediate ACh-induced activation of ATP-sensitive K+ current in cat atrial myocytes

In atrial myocytes, an initial exposure to isoproterenol (ISO) acts via cAMP to mediate a subsequent acetylcholine (ACh)-induced activation of ATP-sensitive K(+) current (I(K,ATP)). In addition, beta-adrenergic receptor (beta-AR) stimulation activates nitric oxide (NO) release. The present study determined whether the conditioning effect of beta-AR stimulation acts via beta(1)- and/or beta(2)-ARs and whether it is mediated via NO signaling. 0.1 microM ISO plus ICI 118,551 (ISO-beta(1)-AR stimulation) or ISO plus atenolol (ISO-beta(2)-AR stimulation) both increased L-type Ca(2+) current (I(Ca,L)) markedly, but only ISO-beta(2)-AR stimulation mediated ACh-induced activation of I(K,ATP). 1 microM zinterol (beta(2)-AR agonist) also increased I(Ca,L) and mediated ACh-activated I(K,ATP). Inhibition of NO synthase (10 microM L-NIO), guanylate cyclase (10 microM ODQ), or cAMP-PKA (50 microM Rp-cAMPs) attenuated zinterol-induced stimulation of I(Ca,L) and abolished ACh-activated I(K,ATP). Spermine-NO (100 microM; an NO donor) mimicked beta(2)-AR stimulation, and its effects were abolished by Rp-cAMPs. Intracellular dialysis of 20 microM protein kinase inhibitory peptide (PKI) abolished zinterol-induced stimulation of I(Ca,L). Measurements of intracellular NO ([NO](i)) using the fluorescent indicator DAF-2 showed that ISO-beta(2)-AR stimulation or zinterol increased [NO](i). L-NIO (10 microM) blocked ISO- and zinterol-induced increases in [NO](i). ISO-beta(1)-AR stimulation failed to increase [NO](i). Inhibition of G(i)-protein by pertussis toxin significantly inhibited zinterol-mediated increases in [NO](i). Wortmannin (0.2 microM) or LY294002 (10 microM), inhibitors of phosphatidylinositol 3'-kinase (PI-3K), abolished the effects of zinterol to both mediate ACh-activated I(K,ATP) and stimulate [NO](i). We conclude that both beta(1)- and beta(2)-ARs stimulate cAMP. beta(2)-ARs act via two signaling pathways to stimulate cAMP, one of which is mediated via G(i)-protein and PI-3K coupled to NO-cGMP signaling. Only beta(2)-ARs acting exclusively via NO signaling mediate ACh-induced activation of I(K,ATP). NO signaling also contributes to beta(2)-AR stimulation of I(Ca,L). The differential effects of beta(1)- and beta(2)-ARs can be explained by the coupling of these two beta-ARs to different effector signaling pathways.

Disruption of cytoskeletal integrity impairs Gi-mediated signaling due to displacement of Gi proteins

beta1 integrins play a crucial role as cytoskeletal anchorage proteins. In this study, the coupling of the cytoskeleton and intracellular signaling pathways was investigated in beta1 integrin deficient (-/-) embryonic stem cells. Muscarinic inhibition of the L-type Ca2+ current (ICa) and activation of the acetylcholine-activated K+ current (IK,ACh) was found to be absent in beta1 integrin-/- cardiomyocytes. Conversely, beta adrenoceptor-mediated modulation of ICa was unaffected by the absence of beta1 integrins. This defect in muscarinic signaling was due to defective G protein coupling. This was supported by deconvolution microscopy, which demonstrated that Gi exhibited an atypical subcellular distribution in the beta1 integrin-/- cardiomyocytes. A critical role of the cytoskeleton was further demonstrated using cytochalasin D, which displaced Gi and impaired muscarinic signaling. We conclude that cytoskeletal integrity is required for correct localization and function of Gi-associated signaling microdomains.

Integrins and the myocardium

Extracellular matrix provides a structural, chemical, and mechanical substrate that is essential in cardiac development, growth, and responses to pathophysiological signals. Transmembrane receptors termed integrins provide a dynamic interaction of environmental cues and intracellular events. Integrins orchestrate multiple functions in the intact organism including organogenesis, regulation of gene expression, cell proliferation, differentiation, migration, and death. They are expressed in all cellular components of the cardiovascular system, including the vasculature, blood, cardiac myocytes and nonmuscle cardiac cells. The focus of this review will be on the role of integrins in the myocardium. We will provide background on integrin structure and function, discuss how the expression of integrins is critical to the form and function of the developing and postnatal myocardium, and review the known data on integrins as signaling molecules in the heart. Finally, we will offer insights to the future research directions into this important family of extracellular matrix receptors in the myocardium.