An essential role of myosin light-chain kinase in the regulation of agonist- and fluid flow-stimulated Ca2+ influx in endothelial cells

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Trpml controls actomyosin contractility and couples migration to phagocytosis in fly macrophages

Phagocytes use their actomyosin cytoskeleton to migrate as well as to probe their environment by phagocytosis or macropinocytosis. Although migration and extracellular material uptake have been shown to be coupled in some immune cells, the mechanisms involved in such coupling are largely unknown. By combining time-lapse imaging with genetics, we here identify the lysosomal Ca²⁺ channel Trpml as an essential player in the coupling of cell locomotion and phagocytosis in hemocytes, the Drosophila macrophage-like immune cells. Trpml is needed for both hemocyte migration and phagocytic processing at distinct subcellular localizations: Trpml regulates hemocyte migration by controlling actomyosin contractility at the cell rear, whereas its role in phagocytic processing lies near the phagocytic cup in a myosin-independent fashion. We further highlight that Vamp7 also regulates phagocytic processing and locomotion but uses pathways distinct from those of Trpml. Our results suggest that multiple mechanisms may have emerged during evolution to couple phagocytic processing to cell migration and facilitate space exploration by immune cells.

Synthetic mRNAs Drive Highly Efficient iPS Cell Differentiation to Dopaminergic Neurons

Proneural transcription factors (TFs) drive highly efficient differentiation of pluripotent stem cells to lineage-specific neurons. However, current strategies mainly rely on genome-integrating viruses. Here, we used synthetic mRNAs coding two proneural TFs (Atoh1 and Ngn2) to differentiate induced pluripotent stem cells (iPSCs) into midbrain dopaminergic (mDA) neurons. mRNAs coding Atoh1 and Ngn2 with defined phosphosite modifications led to higher and more stable protein expression, and induced more efficient neuron conversion, as compared to mRNAs coding wild-type proteins. Using these two modified mRNAs with morphogens, we established a 5-day protocol that can rapidly generate mDA neurons with >90% purity from normal and Parkinson's disease iPSCs. After in vitro maturation, these mRNA-induced mDA (miDA) neurons recapitulate key biochemical and electrophysiological features of primary mDA neurons and can provide high-content neuron cultures for drug discovery. Proteomic analysis of Atoh1-binding proteins identified the nonmuscle myosin II (NM-II) complex as a new binding partner of nuclear Atoh1. The NM-II complex, commonly known as an ATP-dependent molecular motor, binds more strongly to phosphosite-modified Atoh1 than the wild type. Blebbistatin, an NM-II complex antagonist, and bradykinin, an NM-II complex agonist, inhibited and promoted, respectively, the transcriptional activity of Atoh1 and the efficiency of miDA neuron generation. These findings established the first mRNA-driven strategy for efficient iPSC differentiation to mDA neurons. We further identified the NM-II complex as a positive modulator of Atoh1-driven neuron differentiation. The methodology described here will facilitate the development of mRNA-driven differentiation strategies for generating iPSC-derived progenies widely applicable to disease modeling and cell replacement therapy. Stem Cells Translational Medicine 2019;8:112&12.

Integration of TRPC6 and NADPH oxidase activation in lysophosphatidylcholine-induced TRPC5 externalization

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), activate canonical transient receptor potential 6 (TRPC6) channels, and the subsequent increase in intracellular Ca²⁺ leads to TRPC5 activation. The goal of this study is to elucidate the steps in the pathway between TRPC6 activation and TRPC5 externalization. Following TRPC6 activation by lysoPC, extracellular regulated kinase (ERK) is phosphorylated. This leads to phosphorylation of p47^phox and subsequent NADPH oxidase activation with increased production of reactive oxygen species. ERK activation requires TRPC6 opening and influx of Ca²⁺ as evidenced by the failure of lysoPC to induce ERK phosphorylation in TRPC6^-/- endothelial cells. ERK siRNA blocks the lysoPC-induced activation of NADPH oxidase, demonstrating that ERK activation is upstream of NADPH oxidase. The reactive oxygen species produced by NADPH oxidase promote myosin light chain kinase (MLCK) activation with phosphorylation of MLC and TRPC5 externalization. Downregulation of ERK, NADPH oxidase, or MLCK with the relevant siRNA prevents TRPC5 externalization. Blocking MLCK activation prevents the prolonged rise in intracellular calcium levels and preserves endothelial migration in the presence of lysoPC.

Ca2+Influx Does Not Trigger Glucose-Induced Traffic of the Insulin Granules and Alteration of Their Distribution

This study investigated mechanisms by which glucose increases readily releasable secretory granules via acting on preexocytotic steps, i.e., intracellular granule movement and granule access to the plasma membrane using a pancreatic beta-cell line, MIN6. Glucose-induced activation of the movement occurred at a substimulatory concentration with regard to insulin output. Glucose activation of the movement was inhibited by pretreatment with thapsigargin plus acetylcholine to suppress intracellular Ca2+ mobilization. Inhibitors of calmodulin and myosin light chain kinase also suppressed glucose activation of the movement. Simultaneous addition of glucose with Ca2+ channel blockers or the ATP-sensitive K+ channel opener diazoxide failed to suppress the traffic activation, and addition of these substances on top of glucose stimulation resulted in a further increase. Although stimulatory glucose had minimal changes in the intracellular granule distribution, inhibition of Ca2+ influx revealed increases by glucose of the granules in the cell periphery. In contrast, high K+ depolarization decreased the peripheral granules. Glucose-induced granule margination was abolished when the protein kinase C activity was downregulated. These findings indicate that preexocytotic control of insulin release is regulated by distinct mechanisms from Ca2+ influx, which triggers insulin exocytosis. The nature of the regulation by glucose may explain a part of potentiating effects of the hexose independent of the closure of the ATP-sensitive K+ channel.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Structure-Function Analysis of the Non-Muscle Myosin Light Chain Kinase (nmMLCK) Isoform by NMR Spectroscopy and Molecular Modeling: Influence of MYLK Variants

The MYLK gene encodes the multifunctional enzyme, myosin light chain kinase (MLCK), involved in isoform-specific non-muscle and smooth muscle contraction and regulation of vascular permeability during inflammation. Three MYLK SNPs (P21H, S147P, V261A) alter the N-terminal amino acid sequence of the non-muscle isoform of MLCK (nmMLCK) and are highly associated with susceptibility to acute lung injury (ALI) and asthma, especially in individuals of African descent. To understand the functional effects of SNP associations, we examined the N-terminal segments of nmMLCK by ¹H-¹⁵N heteronuclear single quantum correlation (HSQC) spectroscopy, a 2-D NMR technique, and by in silico molecular modeling. Both NMR analysis and molecular modeling indicated SNP localization to loops that connect the immunoglobulin-like domains of nmMLCK, consistent with minimal structural changes evoked by these SNPs. Molecular modeling analysis identified protein-protein interaction motifs adversely affected by these MYLK SNPs including binding by the scaffold protein 14-3-3, results confirmed by immunoprecipitation and western blot studies. These structure-function studies suggest novel mechanisms for nmMLCK regulation, which may confirm MYLK as a candidate gene in inflammatory lung disease and advance knowledge of the genetic underpinning of lung-related health disparities.

TRPV4 calcium entry and surface expression attenuated by inhibition of myosin light chain kinase in rat pulmonary microvascular endothelial cells

In previous studies, blockade or gene deletion of either myosin light chain kinase (MLCK) or the mechanogated transient receptor potential vanilloid 4 (TRPV4) channel attenuated mechanical lung injury. To determine their effects on calcium entry, rat pulmonary microvascular endothelial cells (RPMVEC) were labeled with fluo-4 and calcium entry initiated with the TRPV4 agonist, 4α-phorbol 12, 13-didecanoate (4αPDD). Mean calcium transients peaked at ∼25 sec and persisted ∼500 sec. The 4αPDD response was essentially abolished in calcium-free media, or after pretreatment with the MLCK inhibitor, ML-7. ML-7 also attenuated the 4αPDD-induced inward calcium current measured directly using whole-cell patch clamp. Pretreatment with dynasore, an inhibitor of dynamin produced an initial calcium transient followed by a 4αPDD transient of unchanged peak intensity. Automated averaging of areas under the curve (AUC) of calcium transients in individual cells indicated total calcium activity with a relationship between treatment groups of ML-7 + 4αPDD < 4αPDD only < dynasore + 4αPDD. Measurement of biotinylated surface TRPV4 protein indicated a significant reduction after ML-7 pretreatment, but no significant change with dynasore treatment. RPMVEC monolayer electrical resistances were decreased by only 3% with 10 μmol/L 4αPDD and the response was dose-related. Dynasore alone produced a 29% decrease in resistance, but neither ML-7 nor dynasore affected the subsequent 4αPDD resistance response. These studies suggest that MLCK may inhibit mechanogated calcium responses through reduced surface expression of stretch activated TRPV4 channels in the plasma membrane.

Myosin light chain kinase controls voltage-dependent calcium channels in vascular smooth muscle

The Ca(2+)-dependent kinase myosin light chain kinase (MLCK) is the activator of smooth muscle contraction. In addition, it has been reported to be involved in Ca(2+) channel regulation in cultured cells, and we previously showed that the MLCK inhibitor ML-7 decreases arginine vasopressin (AVP)-induced Ca(2+) influx in rat aorta. This study was designed to investigate whether MLCK is involved in Ca(2+) regulation in resistance artery smooth muscle cell, which plays a major role in the control of blood pressure. As ML compounds were shown to have off-target effects, MLCK was downregulated by transfection with a small interfering RNA targeting MLCK (MLCK-siRNA) in rat small resistance mesenteric artery (RMA) and in the rat embryonic aortic cell line A7r5. Noradrenaline-induced contraction and Ca(2+) signal were significantly depressed in MLCK-siRNA compared to scramble-siRNA-transfected RMA. Contraction and Ca(2+) signal induced by high KCl and voltage-activated Ca(2+) current were also significantly decreased in MLCK-siRNA-transfected RMA, suggesting that MLCK depletion modifies voltage-operated Ca(2+) channels. KCl- and AVP-induced Ca(2+) signals and voltage-activated Ca(2+) current were decreased in MLCK-depleted A7r5 cells. Eventually, real-time quantitative PCR analysis indicated that in A7r5, MLCK controlled mRNA expression of CaV1.2 (L-type) and CaV3.1 (T-type) voltage-dependent Ca(2+) channels. Our results suggest that MLCK controls the transcription of voltage-dependent Ca(2+) channels in vascular smooth muscle cells.

Inhibitor κB kinase 2 is a myosin light chain kinase in vascular smooth muscle

RATIONALE

Myosin light chain (MLC) phosphorylation determines vascular contractile status. In addition to the classic Ca²⁺-dependent MLC kinase (MLCK), another unidentified kinase(s) also contributes to MLC phosphorylation in living cells. Inhibitor κB kinase 2 (IKK2)-deficient mouse embryonic fibroblasts demonstrate abnormal morphology and migration, suggesting that IKK2 may be involved in MLC phosphorylation.

OBJECTIVE

Therefore, we tested whether IKK2 is an MLCK in living cells and the role of IKK2 in mediating vasoconstriction and blood pressure regulation.

METHODS AND RESULTS

In the present study, we showed that recombinant IKK2-phosphorylated MLC and intact myosin in vitro, and the kinetic parameters were comparable with those of the classic MLCK. Overexpression of IKK2 increased cellular MLC phosphorylation level, and pharmacological inhibition of IKK2 markedly decreased vascular smooth muscle cell MLC phosphorylation, suggesting that IKK2 is an MLCK in living cells. IKK2 inhibitors dose- and time-dependently attenuated vasoconstriction elicited by diverse agonists, suggesting the physiological importance of IKK2 as an MLCK. Vascular smooth muscle cell-specific IKK2-deficient mice had decreased aortic contractile responses, and reduced hypertensive responses to several vasoconstrictors, compared with wild-type mice, confirming the physiological importance of IKK2 as an MLCK.

CONCLUSIONS

Our data provide a novel mechanism whereby IKK2 regulates MLC phosphorylation as an MLCK and, thus, vascular function and blood pressure.

Pharmacological evidence for Orai channel activation as a source of cardiac abnormal automaticity

Calcium transport through plasma membrane voltage-independent calcium channels is vital for signaling events in non-excitable and excitable cells. Following up on our earlier work, we tested the hypothesis that this type of calcium transport can disrupt myocardial electromechanical stability. Our Western and immunofluorescence analyses show that left atrial and ventricular myocytes express the Orai1 and the Orai3 calcium channels. Adding the Orai activator 2-aminoethoxydiphenyl borate (2-APB) to the superfusate of rat left atria causes these non-automatic muscles to contract spontaneously and persistently at rates of up to 10 Hz, and to produce normal action potentials from normal resting potentials, all in the absence of external stimulation. 2-APB likewise induces such automatic activity in superfused rat left ventricular papillary muscles, and the EC(50)s at which 2-APB induces this activity in both muscles are similar to the concentrations which activate Orais. Importantly, the voltage-independent calcium channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy]ethyl-1H-imidazole (SKF-96365) suppresses this automaticity with an IC(50) of 11 ± 0.6 μM in left atria and 6 ± 1.6 μM in papillary muscles. 1-(5-Iodonaphthalene-1-sulfonyl)-hexahydro-1,4-diazepine (ML-7), a second voltage-independent calcium channel inhibitor, and two calmodulin inhibitors also prevent 2-APB automaticity while two calmodulin-dependent protein kinase II inhibitors do not. Thus an activator of the Orai calcium channels provokes a novel type of high frequency automaticity in non-automatic heart muscle.

Pharmacology of receptor operated calcium entry in human neutrophils

In neutrophils, increases in intracellular calcium [Ca(2+)](i) provide a crucial link between inflammatory mediators and inflammatory responses. The modulation of [Ca(2+)](i) fluxes in non-excitable cells such as neutrophils has been studied for more than 25 years yet remains to be resolved. In these cells, the Ca(2+) influx can occur through at least two mechanisms, as follows: one dependent on the state of filling of the endoplasmic reticulum Ca(2+) stores, termed store operated calcium entry (SOCE), and the other less studied mechanism in neutrophils which is not dependent on the state of the Ca(2+) stores but is regulated by receptor occupation, termed receptor operated calcium entry (ROCE). Over the past ten years, the molecular components of SOCE have been extensively characterized, but in neutrophils, the molecular components of ROCE have only recently been explored. In this review, we discuss recent research findings that have demonstrated an important role for ROCE in human neutrophils. In addition, an overview of pharmacological approaches used to discriminate between ROCE and SOCE will be discussed. The elucidation of the molecular components of ROCE may well provide important pharmacological targets for the development of novel anti-inflammatory drugs.

Abl tyrosine kinase phosphorylates nonmuscle Myosin light chain kinase to regulate endothelial barrier function

This study identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy and examined their influence on nmMLCK function and human lung endothelial barrier regulation. The data indicate an essential role for Abl kinase in vascular barrier regulation via phosphorylation of nmMLCK and the actin-binding protein cortactin.

Nonmuscle myosin light chain kinase (nmMLCK), a multi-functional cytoskeletal protein critical to vascular homeostasis, is highly regulated by tyrosine phosphorylation. We identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy analysis (including Y²³¹, Y⁴⁶⁴, Y⁵⁵⁶, Y⁸⁴⁶) and examined their influence on nmMLCK function and human lung endothelial cell (EC) barrier regulation. Tyrosine phosphorylation of nmMLCK increased kinase activity, reversed nmMLCK-mediated inhibition of Arp2/3-mediated actin polymerization, and enhanced binding to the critical actin-binding phosphotyrosine protein, cortactin. EC challenge with sphingosine 1-phosphate (S1P), a potent barrier-enhancing agonist, resulted in c-Abl and phosphorylated nmMLCK recruitment into caveolin-enriched microdomains, rapid increases in Abl kinase activity, and spatial targeting of c-Abl to barrier-promoting cortical actin structures. Conversely, reduced c-Abl expression in EC (siRNA) markedly attenuated S1P-mediated cortical actin formation, reduced the EC modulus of elasticity (assessed by atomic force microscopy), reduced nmMLCK and cortactin tyrosine phosphorylation, and attenuated S1P-mediated barrier enhancement. These studies indicate an essential role for Abl kinase in vascular barrier regulation via posttranslational modification of nmMLCK and strongly support c-Abl-cortactin-nmMLCK interaction as a novel determinant of cortical actin-based cytoskeletal rearrangement critical to S1P-mediated EC barrier enhancement.

Actions of calcium influx blockers in human neutrophils support a role for receptor-operated calcium entry

The action of two potent store operated Ca2+ entry (SOCE) inhibitors, ML-9 and GdCl3 on Ca2+ fluxes induced by the pro-inflammatory agonists FMLP, PAF, LTB(4) as well as the receptor-independent stimulus thapsigargin has not been documented in human neutrophils. In this study, ML-9 enhanced both release and subsequent Ca2+ influx in response to agonists whereas it enhanced Ca2+ release by thapsigargin, but inhibited Ca2+ influx. In contrast, 1muM GdCl3 completely inhibited Ca2+ influx in response to thapsigargin, but only partially blocked Ca2+ influx after agonist stimulation. These results strongly suggest a major role for receptor-operated Ca2+ influx in human neutrophils.

STIM and Orai: the long-awaited constituents of store-operated calcium entry

Rapid changes in cytosolic Ca(2+) concentrations [Ca(2+)](i) are the most commonly used signals in biology to regulate a whole host of cellular functions including contraction, secretion and gene activation. A widely utilized form of Ca(2+) influx is termed store-operated Ca(2+) entry (SOCE) owing to its control by the Ca(2+) content of the endoplasmic reticulum (ER). The underlying molecular mechanism of SOCE has eluded identification until recently when two groups of proteins, the ER Ca(2+) sensors stromal interaction molecule (STIM)1 and STIM2 and the plasma-membrane channels Orai1, Orai2 and Orai3, have been identified. These landmark discoveries have enabled impressive progress in clarifying how these proteins work in concert and what developmental and cellular processes require their participation most. As we begin to better understand the biology of the STIM and Orai proteins, the attention to the pharmacological tools to influence their functions quickly follow suit. Here, we briefly summarize recent developments in this exciting area of Ca(2+) signaling.

Calcineurin is critical for sodium-induced neointimal formation in normotensive and hypertensive rats

It is well known that excessive intake of sodium chloride (sodium) is a risk factor for cardiovascular disease because it raises blood pressure. However, sodium loading reportedly promotes cardiovascular disease independently of its effect on blood pressure. To examine the mechanisms by which sodium loading promotes vascular inflammation independently of its effect on blood pressure, we examined the role of calcineurin in sodium loading-induced vascular inflammation using a wire injury model of the rat femoral artery. Calcineurin mRNA expression in the wire-injured femoral artery was significantly higher in sodium-loaded normotensive rats, such as Wistar-Kyoto (WKY) rats, than that in control WKY rats. Neointimal formation was also significantly enhanced in sodium-loaded WKY rats compared with control WKY rats. Gene transfer of an adenovirus expressing a dominant negative mutant of calcineurin (AdCalAΔC92Q) significantly suppressed neointimal formation in sodium-loaded WKY rats to a level similar to that observed in control WKY rats. Calcineurin expression and neointimal formation were more significantly enhanced in hypertensive rats, such as spontaneously hypertensive rats (SHRs), than those in control WKY rats. AdCalAΔC92Q infection significantly suppressed neointimal formation in SHRs to a level similar to that observed in control WKY rats. These results suggest that sodium loading promotes neointimal formation, even in normotensive rats, and that hypertension further stimulates neointimal formation. These results also suggest that calcineurin plays a pivotal role in this process.

Myosin light chain kinase-independent inhibition by ML-9 of murine TRPC6 channels expressed in HEK293 cells

BACKGROUND AND PURPOSE

Myosin light chain kinase (MLCK) plays a pivotal role in regulation of cellular functions, the evidence often relying on the effects of extracelluarly administered drugs such as ML-9. Here we report that this compound exerts non-specific inhibitory actions on the TRPC6 channel, a transient receptor potential (TRP) protein.

EXPERIMENTAL APPROACH

Macroscopic and single channel currents were recorded from transfected HEK293 cells by patch-clamp techniques.

KEY RESULTS

Cationic currents elicited by carbachol (CCh; 100 microM) in HEK293 cells overexpressing murine TRPC6 (I(TRPC6)) were dose-dependently inhibited by externally applied ML-9 (IC(50)=7.8 microM). This inhibition was voltage-dependent and occurred as fast as external Na(+) removal. Another MLCK inhibitor, wortmannin (3 microM), and MLCK inhibitory peptides MLCK-IP(11-19) (10 microM) and -IP(480-501) (1 microM) showed little effects on I(TRPC6) density and the inhibitory efficacy of ML-9. The extent of the inhibition also unchanged with co-expression of wild-type or a dominant negative mutant of MLCK. Inhibitory effects of ML-9 on I(TRPC6) remained unaffected whether TRPC6 was activated constitutively or by a diacylglycerol analogue OAG (100 microM). Similar rapid inhibition was also observed with a ML-9 relative, ML-7. Intracellular perfusion of ML-9 via patch pipette, dose-dependently suppressed I(TRPC6). In inside-out patch configuration, bath application of ML-9 (and ML-7) rapidly diminished approximately 35pS single TRPC6 channel activities. Contrarily, currents due to TRPC7 expression were rapidly enhanced by externally applied ML-9 and ML-7, which was not prevented by MLCK inhibitory peptides.

CONCLUSION AND IMPLICATIONS

These results strongly suggest that ML compounds inhibit TRPC6 channels via a mechanism independent of inhibition of MLCK activity.

Ca2+ signaling in hypoxic pulmonary vasoconstriction: effects of myosin light chain and Rho kinase antagonists

Antagonists of myosin light chain (MLC) kinase (MLCK) and Rho kinase (ROK) are thought to inhibit hypoxic pulmonary vasoconstriction (HPV) by decreasing the concentration of phosphorylated MLC at any intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMC); however, these antagonists can also decrease [Ca(2+)](i). To determine whether MLCK and ROK antagonists alter Ca(2+) signaling in HPV, we measured the effects of ML-9, ML-7, Y-27632, and HA-1077 on [Ca(2+)](i), Ca(2+) entry, and Ca(2+) release in rat distal PASMC exposed to hypoxia or depolarizing concentrations of KCl. We performed parallel experiments in isolated rat lungs to confirm the inhibitory effects of these agents on pulmonary vasoconstriction. Our results demonstrate that MLCK and ROK antagonists caused concentration-dependent inhibition of hypoxia-induced increases in [Ca(2+)](i) in PASMC and HPV in isolated lungs and suggest that this inhibition was due to blockade of Ca(2+) release from the sarcoplasmic reticulum and Ca(2+) entry through store- and voltage-operated Ca(2+) channels in PASMC. Thus MLCK and ROK antagonists might block HPV by inhibiting Ca(2+) signaling, as well as the actin-myosin interaction, in PASMC. If effects on Ca(2+) signaling were due to decreased phosphorylated myosin light chain concentration, their diversity suggests that MLCK and ROK antagonists may have acted by inhibiting myosin motors and/or altering the cytoskeleton in a manner that prevented achievement of required spatial relationships among the cellular components of the response.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

The role of endothelial calcium and nitric oxide in the localisation of atherosclerosis

A mathematical model of endothelial cell calcium signalling and nitric oxide synthesis under flow conditions is presented. The model is coupled to two important environmental stimuli for endothelial cells: the frictional shear stress exerted on the cell membrane by the blood flow; and the binding of adenosine triphosphate in the bloodstream to cell surface receptors. These stimuli are closely linked to haemodynamic flow conditions and are, in general, spatially varying, allowing the cellular response in different regions of the endothelium to be evaluated. This is used to indicate which areas of the artery wall experience reduced bioavailability of nitric oxide, which is a major factor in the onset of atherosclerosis. The model thus directly addresses the key issue of the causative link, and its underlying biochemical mechanisms, between incidence of atherosclerosis and regions of low wall shear stress (WSS). Model results show that intracellular levels of free calcium and endothelial nitric oxide synthase are lower in endothelial cells adjacent to a region of recirculating flow than in cells adjacent to regions of fully developed arterial flow. This will lead to deficient levels of nitric oxide in the recirculation zone and hence a potentially elevated risk of developing atherosclerotic plaque. This is consistent with the observed spatial correlation between atherosclerosis and regions of disturbed blood flow and low WSS, and provides a mechanism for the localisation of the disease to sites such as arterial bifurcations and bends.

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

Cilostazol suppresses adhesion of human neutrophils to HUVECs stimulated by FMLP and its mechanisms

The interaction between neutrophils and endothelial cells (ECs) is of great importance in many physiological and pathological progresses. Although cilostazol (CLZ), a novel selective phosphodiesterase (PDE) type 3 inhibitor, has been proved to be useful in vasodilatation and inhibition of platelet aggregation, its effect on adhesion is not clearly known. In this study, we examined the effects and investigated the mechanisms of cilostazol on neutrophil adhesion to human umbilical endothelial cells (HUVECs) triggered by N-formyl-methionyl-leucyl-phenylal-anine (FMLP), a chemotactic peptide. The soluble vascular cell adhesive molecule-1 (sVCAM-1) release from FMLP (10 microM)-stimulated HUVECs was determined by ELISA kits. Fluo-2, a fluorescent indicator, was used to investigate intracellular free calcium concentration ([Ca2+]i) in HUVECs. HL-60 cells were induced to be neutrophilic by DMSO and loaded with Fluo-3, another fluorescent indicator, to detect [Ca2+]i, and CLA was used as a chemiluminescent indicator to determine superoxide production in neutrophilic cells. The result showed that CLZ (1-100 microM) significantly inhibited neutrophil adhesion to FMLP-stimulated HUVECs. In HUVECs, CLZ obviously downregulated sVCAM-1 level, while it had no meaningful influence [Ca2)]i. But in neutrophils, FMLP-activated superoxide generation and [Ca2+]i increase were found being inhibited by exposure to CLZ . Furthermore, we also demonstrated that Ca2+ increase was preceded to the superoxide generation in neutrophils. The results suggest that CLZ involves in adhesion reactions between neutrophil and ECs, partly via VCAM-1 expression in ECs, and decreasing [Ca2+]i induced activation of neutrophils, which means a lot to prevent atherosclerosis and other cardiovascular diseases.

Ca2+-calmodulin-dependent myosin light chain kinase is essential for activation of TRPC5 channels expressed in HEK293 cells

Mammalian homologues of Drosophila transient receptor potential (TRP) proteins are responsible for receptor-activated Ca(2+) influx in vertebrate cells. We previously reported the involvement of intracellular Ca(2+) in the receptor-mediated activation of mammalian canonical transient receptor potential 5 (TRPC5) channels. Here we investigated the role of calmodulin, an important sensor of changes in intracellular Ca(2+), and its downstream cascades in the activation of recombinant TRPC5 channels in human embryonic kidney (HEK) 293 cells. Ca(2+) entry through TRPC5 channels, induced upon stimulation of the G-protein-coupled ATP receptor, was abolished by treatment with W-13, an inhibitor of calmodulin. ML-9 and wortmannin, inhibitors of Ca(2+)-calmodulin-dependent myosin light chain kinase (MLCK), and the expression of a dominant-negative mutant of MLCK inhibited the TRPC5 channel activity, revealing an essential role of MLCK in maintaining TRPC5 channel activity. It is important to note that ML-9 impaired the plasma membrane localization of TRPC5 channels. Furthermore, TRPC5 channel activity measured using the whole-cell patch-clamp technique was inhibited by ML-9, whereas TRPC5 channel activity observed in the cell-excised, inside-out patch was unaffected by ML-9. An antibody that recognizes phosphorylated myosin light chain (MLC) revealed that the basal level of phosphorylated MLC under unstimulated conditions was reduced by ML-9 in HEK293 cells. These findings strongly suggest that intracellular Ca(2+)-calmodulin constitutively activates MLCK, thereby maintaining TRPC5 channel activity through the promotion of plasma membrane TRPC5 channel distribution under the control of phosphorylation/dephosphorylation equilibrium of MLC.

Atherosclerosis and calcium signalling in endothelial cells

The link between atherosclerosis and regions of disturbed flow and low wall shear stress is now firmly established, but the causal mechanisms underlying the link are not yet understood. It is now recognised that the endothelium is not simply a passive barrier between the blood and the vessel wall, but plays an active role in maintaining vascular homeostasis and participates in the onset of atherosclerosis. Calcium signalling is one of the principal intracellular signalling mechanisms by which endothelial cells (EC) respond to external stimuli, such as fluid shear stress and ligand binding. Previous studies have separately modelled mass transport of chemical species in the bloodstream and calcium dynamics in EC via the inositol trisphosphate (IP(3)) signalling pathway. We review existing models of these two phenomena, before going on to integrate the two components to provide an inclusive new model for the calcium response of the endothelium in an arbitrary vessel geometry. This enables the combined effects of fluid flow and biochemical stimulation on EC to be investigated and is the first time spatially varying, physiological fluid flow-related environmental factors have been combined with intracellular signalling in a mathematical model. Model results show that low endothelial calcium levels in the area of disturbed flow at an arterial widening may be one contributing factor to the onset of vascular disease.

Modulation of intracellular Ca2+ release and capacitative Ca2+ entry by CaMKII inhibitors in bovine vascular endothelial cells

The effects of inhibitors of CaMKII on intracellular Ca2+ signaling were examined in single calf pulmonary artery endothelial (CPAE) cells using indo-1 microfluorometry to measure cytoplasmic Ca2+ concentration ([Ca2+]i). The three CaMKII inhibitors, KN-93, KN-62, and autocamtide-2-related inhibitory peptide (AIP), all reduced the plateau phase of the [Ca2+]i transient evoked by stimulation with extracellular ATP. Exposure to KN-93 or AIP alone in the presence of 2 mM extracellular Ca2+ resulted in a dose-dependent increase of [Ca2+]i consisting of a rapid and transient Ca2+ spike followed by a small sustained plateau phase of elevated [Ca2+]i. Exposure to KN-93 in the absence of extracellular Ca2+ caused a transient rise of [Ca2+]i, suggesting that exposure to CaMKII inhibitors directly triggered release of Ca2+ from intracellular endoplasmic reticulum (ER) Ca2+ stores. Repetitive stimulation with KN-93 and ATP, respectively, revealed that both components released Ca2+ largely from the same store. Pretreatment of CPAE cells with the membrane-permeable inositol 1,4,5-trisphosphate (IP3) receptor blocker 2-aminoethoxydiphenyl borate caused a significant inhibition of the KN-93-induced Ca2+ response, suggesting that exposure to KN-93 affects Ca2+ release from an IP3-sensitive store. Depletion of Ca2+ stores by exposure to ATP or to the ER Ca2+ pump inhibitor thapsigargin triggered robust capacitative Ca2+ entry (CCE) signals in CPAE cells that could be blocked effectively with KN-93. The data suggest that in CPAE cells, CaMKII modulates Ca2+ handling at different levels. The use of CaMKII inhibitors revealed that in CPAE cells, the most profound effects of CaMKII are inhibition of release of Ca2+ from intracellular stores and activation of CCE.

Deletion of MLCK210 induces subtle changes in vascular reactivity but does not affect cardiac function

Myosin light chain kinase (MLCK) plays a key role in the regulation of actomyosin contraction in a large variety of cells. Two isoforms have been described: a short isoform, widely expressed in smooth muscle cells; and a long isoform (MLCK210), mainly localized in the endothelium. This study investigated the consequences on different cardiovascular parameters of MLCK210 gene deletion using MLCK210 knockout mice and of pharmacological inhibition of the kinase using a specific MLCK inhibitor. Deletion of MLCK210 did not affect systolic blood pressure and heart rate or echocardiographic measurements. Electrocardiographic analysis showed neither atrio- nor intraventricular conduction or repolarization defects. Ex vivo responses of aortic rings to vasoconstrictor and vasodilator agonists were not modified in MLCK210 null mice. However, deletion of MLCK210 attenuated shear stress-induced dilation and produced changes in the balance of endothelial-relaxing factors of small mesenteric arteries (SMA). In particular, a reduced flow-mediated NO-dependent dilation was observed. However, it was partially compensated by enhanced indomethacin-sensitive dilation. No significant changes were detected in the endothelium-derived hyperpolarizing component of the vasodilator response. The above effects of MLCK210 gene deletion were confirmed in SMA from wild-type mice by the use of the MLCK enzymatic inhibitor MMZ-10-057. In summary, deletion of MLCK210 was not associated with abnormalities of main in vivo cardiovascular parameters in mice. This study demonstrates a role for MLCK210 in the regulation of flow-dependent dilation in SMA.

p21-activated kinase regulates endothelial permeability through modulation of contractility

Endothelial cells lining the vasculature have close cell-cell associations that maintain separation of the blood fluid compartment from surrounding tissues. Permeability is regulated by a variety of growth factors and cytokines and plays a role in numerous physiological and pathological processes. We examined a potential role for the p21-activated kinase (PAK) in the regulation of vascular permeability. In both bovine aortic and human umbilical vein endothelial cells, PAK is phosphorylated on Ser141 during the activation downstream of Rac, and the phosphorylated subfraction translocates to endothelial cell-cell junctions in response to serum, VEGF, bFGF, TNFalpha, histamine, and thrombin. Blocking PAK activation or translocation prevents the increase in permeability across the cell monolayer in response to these factors. Permeability correlates with myosin phosphorylation, formation of actin stress fibers, and the appearance of paracellular pores. Inhibition of myosin phosphorylation blocks the increase in permeability. These data suggest that PAK is a central regulator of endothelial permeability induced by multiple growth factors and cytokines via an effect on cell contractility. PAK may therefore be a suitable drug target for the treatment of pathological conditions where vascular leak is a contributing factor, such as ischemia and inflammation.

Inhibition of pancreatic adenocarcinoma cellular invasiveness by blebbistatin: a novel myosin II inhibitor

Blebbistatin is a novel 1-phenyl-2-pyrrolidinone derivative capable of inhibiting non-muscle myosin II activity with a high degree of specificity. We examined the effects of blebbistatin on pancreatic adenocarcinoma cellular migration, invasion, adhesion, and spreading. Blebbistatin dose-dependently inhibited cellular migration and invasiveness, quantified by modified Boyden chamber assay. Matrix metalloproteinase 2 and 9 activities were unaffected by blebbistatin and cellular proliferation was inhibited only by concentrations of blebbistatin exceeding those required to inhibit myosin II activity and to interfere with migration and invasion. While blebbistatin treatment did not affect cell adhesion to the extracellular matrix component fibronectin, it markedly impaired cell spreading on this substrate. Cell surface expression of the archetypal fibronectin receptor (alpha(5)beta(1) integrin) was unaffected by blebbistatin. Our observations illustrate the critical role of non-muscle myosin II in pancreatic adenocarcinoma cellular invasiveness and extracellular matrix interaction and suggest that therapeutic strategies targeting myosin II warrant further investigation.

ML-9, a myosin light chain kinase inhibitor, reduces intracellular Ca2+ concentration in guinea pig trachealis

We investigated the effects of ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine], a myosin light chain kinase (MLCK) inhibitor, on intracellular Ca2+ concentration ([Ca2+]i), contraction induced by high K+ and an agonist, and capacitative Ca2+ entry in fura-2-loaded guinea pig tracheal smooth muscle. ML-9 inhibited both the increase in [Ca2+]i and the contraction induced by 60 mM K+, 1 microM methacholine or 1 microM thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. However, another MLCK inhibitor, wortmannin (3 microM), inhibited the contraction elicited by these stimuli without affecting [Ca2+]i. Under the condition that the thapsigargin-induced contraction was fully suppressed by 3 microM wortmannin, 30 microM ML-9 caused a further decrease in [Ca2+]i. The inhibitory effects of ML-9 on [Ca2+]i and the contraction elicited by methacholine were similar to those of SKF-96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride), a Ca2+ channel blocker. These results indicate that ML-9 acts as a potent inhibitor of Ca2+-permeable channels independently of MLCK inhibition in tracheal smooth muscle.

Effects of cytochrome P450 inhibitors on agonist-induced Ca2+ responses and production of NO and PGI2 in vascular endothelial cells

Production of endothelium-dependent vascular relaxing factors, such as nitric oxide (NO) and prostaglandin I2 (PGI2), and endothelium-derived hyperpolarizing factor (EDHF), is regulated in part by changes in intracellular Ca2+ concentration ([Ca2+]i) in vascular endothelial cells (ECs). Cytochrome P450 (CYP), shown to mediate endothelium-dependent hyperpolarization via epoxyeicosatrienoic acids, is one of the candidates for EDHF. In this study we tested the hypotheses that CYP might be involved in EC Ca2+ signaling and that CYP activity might be linked with production of vasodilating factors other than EDHF. To this end, structurally different CYP inhibitors including SKF 525A, econazole and miconazole were tested on primary cultured porcine aortic endothelial cells. Intracellular Ca2+ concentration was measured using the fluorescent Ca2+ indicator fura-2/AM. Bradykinin (BK, 10 nM) and thapsigargin (TG 1 microM) provoked large biphasic increases in [Ca2+], which consist of Ca2+ release from intracellular stores and transplasmalemmal Ca2+ entry. SKF 525A dose-dependently (30-100 microM) inhibited BK- and TG-stimulated Ca2+ entry, but not intracellular Ca2+ store release. Econazole (10 microM) and miconazole (10 microM) had the same effect as SKF 525A on the Ca2+ entry. SKF 525A also dose-dependently inhibited BK-stimulated production of NO and PGI2, assessed by measuring cGMP and 6-keto-PGF(1alpha) concentration. These data suggest that, in addition to its regulation of EDHF production, CYP also contributes to the regulation of other endothelium-dependent vasorelaxing factors by modifying EC Ca2+ signaling.

On the endothelial cell I(SOC)

Ca2+ store depletion activates both Ca2+ selective and non-selective currents in endothelial cells. Recently, considerable progress has been made in understanding the molecular make-up and regulation of an endothelial cell thapsigargin-activated Ca2+ selective current, I(SOC). Indeed, I(SOC) is a relatively small inward Ca2+ current that exhibits an approximate +40mV reversal potential and is strongly inwardly rectifying. This current is sensitive to organization of the actin-based cytoskeleton. Transient receptor potential (TRP) proteins 1 and 4 (TRPC1 and TRPC4, respectively) each contribute to the molecular basis of I(SOC), although it is TRPC4 that appears to be tethered to the cytoskeleton through a dynamic interaction with protein 4.1. Activation of I(SOC) requires association between protein 4.1 and the actin-based cytoskeleton (mediated through spectrin), suggesting protein 4.1 mediates the physical communication between Ca2+ store depletion and channel activation. Thus, at present findings indicate a TRPC4-protein 4.1 physical linkage regulates I(SOC) activation following Ca2+ store depletion.

Effect of SNI-2011 on amylase secretion from parotid tissue in rats and in neuronal nitric oxide synthase knockout mice

The effect of (+/-)cis-2-methylspilo(1,3-oxathiolane-5,3')quinuclidine (SNI-2011) on the secretory pathway of amylase in parotid tissues was investigated. SNI-2011-induced exocytosis was inhibited by a cell-permeable Ca(2+) chelator or inhibitors of calmodulin kinase II, neuronal nitric oxide synthase (nNOS), soluble guanyl cyclase, cyclic GMP-dependent protein kinase (PKG), and myosin light chain kinase, suggesting that these enzymes were coupled with the exocytosis. Stimulation with SNI-2011 of isolated rat parotid acinar cells loaded with 4,5-diaminofluorescein/diacetate (DAF-2/DA) induced a fast increase in DAF fluorescence corresponding to an increase in the NO production. SNI-2011-induced amylase secretion from parotid tissues in nNOS knockout mice has not been observed yet in spite of the expression of muscarinic M(3) receptors and the maintenance of secretory response to isoproterenol in the tissues. These results indicate the implication of the activation of Ca(2+)- and calmodulin-dependent enzymes and NOS-PKG signaling pathway in SNI-2011-induced amylase secretion from parotid acinar cells.

Adhesion-dependent cell mechanosensitivity

The conversion of physical signals, such as contractile forces or external mechanical perturbations, into chemical signaling events is a fundamental cellular process that occurs at cell-extracellular matrix contacts, known as focal adhesions. At these sites, transmembrane integrin receptors are associated via their cytoplasmic domains with the actin cytoskeleton. This interaction with actin is mediated by a submembrane plaque, consisting of numerous cytoskeletal and signaling molecules. Application of intrinsic or external forces to these structures dramatically affects their assembly and triggers adhesion-mediated signaling. In this review, we discuss the structure-function relationships of focal adhesions and the possible mode of action of the putative mechanosensor associated with them. We also discuss the general phenomenon of mechanosensitivity, and the approaches used to measure local forces at adhesion sites, the cytoskeleton-mediated regulation of local contractility, and the nature of the signaling networks that both affect contractility and are affected by it.

Role of integrins in endothelial mechanosensing of shear stress

The focal pattern of atherosclerotic lesions in arterial vessels suggests that local blood flow patterns are important factors in atherosclerosis. Although disturbed flows in the branches and curved regions are proatherogenic, laminar flows in the straight parts are atheroprotective. Results from in vitro studies on cultured vascular endothelial cells with the use of flow channels suggest that integrins and the associated RhoA small GTPase play important roles in the mechanotransduction mechanism by which shear stress is converted to cascades of molecular signaling to modulate gene expression. By interacting dynamically with extracellular matrix proteins, the mechanosensitive integrins activate RhoA and many signaling molecules in the focal adhesions and cytoplasm. Through such mechanotransduction mechanisms, laminar shear stress upregulates genes involved in antiapoptosis, cell cycle arrest, morphological remodeling, and NO production, thus contributing to the atheroprotective effects. This review summarizes some of the recent findings relevant to these mechanotransduction mechanisms. These studies show that integrins play an important role in mechanosensing in addition to their involvement in cell attachment and migration.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

The muscarinic acetylcholine receptor-stimulated increase in aquaporin-5 levels in the apical plasma membrane in rat parotid acinar cells is coupled with activation of nitric oxide/cGMP signal transduction

The present study investigated the role of nitric oxide (NO)/cGMP signal transduction in the M(3) muscarinic acetylcholine receptor (mAChR)-stimulated increase in aquaporin-5 (AQP5) levels in the apical plasma membrane (APM) of rat parotid glands. Pretreatment of rat parotid tissue with the NO scavenger 2-(4carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium inhibited both acetylcholine (ACh)- and pilocarpine-induced increases in AQP5 in the APM. NO donors [3-morpholinosydnonimine (SIN-1) and (S)-nitroso-N-acetylpenicillamine (SNAP)] mimicked the effects of mAChR agonists. A selective protein kinase G inhibitor [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo-[1,2,3-fg-3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT5823)] and an NO synthase inhibitor (N(6)-imminoethyl-L-lysine) blocked SIN-1- and SNAP-induced increases in AQP5 in the APM. A calmodulin kinase II inhibitor [(8)-5-isoquinolinesulfonic acid, 4-[2-(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-(4-phenyl-1-piperazinyl)-propyl]phenyl ester (KN-62)] decreased the pilocarpine-induced increase of AQP5 in the APM. Using diaminofluorescinein-2 diacetate, enhanced NO synthase activity was detected in isolated parotid acinar cells after ACh-treatment. Treatment with dibutyryl cGMP, but not dibutyryl cAMP, induced an increase in AQP5 levels in the APM. BAPTA-AM inhibited the cGMP-induced increase in AQP5 in the APM. Pretreatment of the tissues with a myosin light chain kinase inhibitor [(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9)] inhibited a mAChR-stimulated increase in AQP5 levels in the APM. Although there was a significant ACh-induced increase in AQP5 in the APM in the absence of extracellular Ca(2+), the maximal effect of ACh on the AQP5 levels in the APM occurred in the presence of extracellular Ca(2+). These results suggest that NO/cGMP signal transduction has a crucial role in Ca(2+) homeostasis in the mAChR-stimulated increase in AQP5 levels in the APM of rat parotid glands.

Shear stress-mediated cytoskeletal remodeling and cortactin translocation in pulmonary endothelial cells

Hemodynamic forces in the form of shear stress (SS) and mechanical strain imposed by circulating blood are recognized factors involved in the control of systemic endothelial cell (EC) cytoskeletal structure and function. However, the effects of acute SS on pulmonary endothelium have not been precisely characterized, nor the mechanism of rapid SS-induced EC cytoskeletal rearrangement understood. We exposed bovine and human pulmonary EC monolayers to laminar SS (10 dynes/cm2) in a parallel plate flow chamber and observed increased actin stress fiber formation 15 min after application of flow. Acute SS-induced pronounced cortical cytoskeletal rearrangement characterized by myosin light chain kinase (MLCK)- and Rho-associated kinase (RhoK)-dependent accumulation of diphosphorylated regulatory myosin light chains (MLC) in the cortical actin ring, junctional protein tyrosine phosphorylation, and transient peripheral translocation of cortactin, an actin-binding protein involved in the regulation of actin polymerization. SS-induced cortactin translocation was independent of Erk-1,2 MAP kinase, p60(Src), MLCK, or RhoK activities, and unaffected by overexpression of a cortactin mutant lacking four major p60(Src) phosphorylation sites. However, both SS-induced transient cortactin translocation and cytoskeletal reorientation in response to sustained (24 h) SS was abolished in cells overexpressing either dominant negative Rac 1 or a dominant negative construct of its downstream target, p21-activated kinase (PAK)-1. Our results suggest a potential role for cortactin in the SS-induced EC cortical cytoskeletal remodeling and demonstrate a novel mechanism of Rac GTPase-dependent regulation of the pulmonary endothelial cytoskeleton by SS.

Insulin inhibits coronary endothelial cell calcium entry and coronary artery relaxation

Hyperinsulinemia is closely related to coronary artery disease. Endothelial cells are important for the control of vascular tone, and dysfunction of endothelial cells has been implicated in coronary artery disease. The direct effects of insulin on coronary endothelial cells are nonetheless unknown. In this study, the acute effects of high-dose insulin were investigated on agonist-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) in porcine coronary endothelial cells and coronary relaxation. Bradykinin (10 n M ) and cyclopiazonic acid (100 microM), an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, provoked large increases in [Ca(2+)](i) in coronary endothelial cells. This increase was dose-dependently inhibited by a 10-min preincubation with high doses of insulin (10, 30, 100 mU/ml). Under Ca(2+)-free conditions, bradykinin and cyclopiazonic acid provoked transient, small increases in [Ca(2+)](i). These increases were not affected by pretreatment with insulin (100 mU/ml). Bradykinin (1, 10, 100, 1,000 n M ) and cyclopiazonic acid (10 microM) significantly relaxed porcine coronary artery rings precontracted with histamine (1 microM). The vasodilator effects of bradykinin and cyclopiazonic acid were dose-dependently inhibited by insulin. These acute effects were not observed at physiologic concentrations. Our data indicate that high-dose insulin inhibits agonist-induced Ca(2+) response in coronary endothelial cells and attenuates agonist-induced coronary vasodilatation. The study suggests that hyperinsulinemia might be associated with coronary artery disease via derangement of endothelial Ca(2+)-dependent functions.

Contractile regulation of the Na(+)-K(+)-2Cl(-) cotransporter in vascular smooth muscle

Vasoconstrictors activate the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 in rat aortic smooth muscle, but the mechanism is unknown. Efflux of (86)Rb(+) from rat aorta in response to phenylephrine (PE) was measured in the absence and presence of bumetanide, a specific inhibitor of NKCC1. Removal of extracellular Ca(2+) completely abolished the activation of NKCC1 by PE. This was not due to inhibition of Ca(2+)-dependent K(+) channels since blocking these channels with Ba(2+) in Ca(2+)-replete solution did not prevent activation of NKCC1 by PE. Stimulation of NKCC1 by PE was inhibited 70% by 75 microM ML-9, 97% by 2 microM wortmannin, and 70% by 2 mM 2,3-butanedione monoxime, each of which inhibited isometric force generation in aortic rings. Bumetanide-insensitive Rb(+) efflux, an indication of Ca(2+)-dependent K(+) channel activity, was reduced by ML-9 but not by the other inhibitors. Stretching of aortic rings on tubing to increase lumen diameter to 120% of normal almost completely blocked the stimulation of NKCC1 by PE without inhibiting the stimulation by hypertonic shrinkage. We conclude that activation of the Na(+)-K(+)-2Cl(-) cotransporter by PE is the direct result of smooth muscle contraction through Ca(2+)-dependent activation of myosin light chain kinase. This indicates that the Na(+)-K(+)-2Cl(-) cotransporter is regulated by the contractile state of vascular smooth muscle.

Endothelial intracellular Ca2+ release following monocyte adhesion is required for the transendothelial migration of monocytes

Although molecular changes accompanying leukocyte extravasation have been investigated intensively, the particular events following leukocyte adhesion and leading to the actual transendothelial migration process remain largely unknown. To characterize intraendothelial signals elicited by leukocyte adhesion and functionally required for their transmigration, we recorded endothelial free cytosolic intracellular Ca(2+)levels ([Ca(2+)]i) during the course of leukocyte adhesion. We show that monocyte and granulocyte adhesion induced Ca(2+)transients in either untreated or TNF-alpha-stimulated microvascular endothelial cells (HMEC-1). The functional significance of these [Ca(2+)]i rises was demonstrated by treating filter-grown endothelial monolayers with BAPTA/AM. This in traendothelial Ca(2+)chelation left monocyte adhesion basically unaffected, but caused a significant and dose-dependent reduction of the transendothelial migration of monocytes. Granulocyte diapedesis, on the other hand, was hardly modified. Thapsigargin-treatment of endothelial cells almost completely inhibited the transmigration of monocytes suggesting that the necessary Ca(2+)transients depended on a release from intracellular Ca(2+)stores. Our results thus show that the transmigration of monocytes through endothelial monolayers of microvascular origin is favoured by an increase of the intraendothelial [Ca(2+)]i induced by leukocyte adhesion to the endothelial cells.

Effects of SK&F 96365 and mefenamic acid on Ca2+ influx in stimulated endothelial cells and on endothelium-derived hyperpolarizing factor-mediated arterial hyperpolarization and relaxation

This study was undertaken to assess how Ca2+ influx into endothelial cells via Ca2+-permeable nonselective cation channels (NSCCs) is important in vascular responses mediated by endothelium-derived hyperpolarizing factor (EDHF). In cultured porcine aortic endothelial cells, the sustained increases in the intracellular Ca2+ concentration ([Ca2+]i) elicited by bradykinin and cyclopiazonic acid, which were strongly dependent on the presence of extracellular Ca2+, were suppressed by the NSCC blockers, SK&F 96365 and mefenamic acid. In porcine coronary artery with intact endothelium, bradykinin elicited a rapid fall in the membrane potential, followed by sustained hyperpolarization with a slow decay. In the presence of SK&F 96365 or mefenamic acid, the peak amplitude was severely reduced and the decay phase of hyperpolarization to bradykinin was greatly accelerated, which was apparently similar to the response obtained in Ca2+-free medium. Cyclopiazonic acid caused sustained hyperpolarization in an extracellular Ca2+-dependent manner, an effect which was markedly diminished by SK&F 96365 and mefenamic acid. In rings of coronary artery precontracted with U46619, bradykinin and cyclopiazonic acid produced endothelium-dependent relaxations even in the presence of N(G)-nitro-L-arginine and indomethacin. SK&F 96365 and mefenamic acid significantly attenuated the relaxant responses. These results indicate that the increase in [Ca2+]i of endothelial cells due to Ca2+ entry via NSCCs plays a crucial role in the maintenance of the EDHF-mediated vascular responses.

Myosin light chain kinase regulates capacitative ca(2+) entry in human monocytes/macrophages

Monocytes/macrophages are present in all stages of atherosclerosis. Although many of their activities depend to various extents on changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), mechanisms regulating [Ca(2+)](i) in these cells remain unclear. We aimed to explore the role of myosin light chain kinase (MLCK) in Ca(2+) signaling in freshly isolated human monocytes/macrophages. Large capacitative Ca(2+) entry (CCE) was observed under fura 2 fluoroscopy in human monocytes/macrophages treated with thapsigargin and cyclopiazonic acid. ML-9 and wortmannin, 2 structurally different inhibitors of MLCK, dose-dependently (1 to 100 micromol/L) prevented CCE and completely did so at 100 micromol/L, whereas inhibitors of tyrosine kinase and protein kinase C had only partial effects. Western blotting showed that thapsigargin significantly caused myosin light chain phosphorylation, which was almost completely blocked by ML-9 (100 micromol/L) and wortmannin (100 micromol/L). ML-9 also dose-dependently (1 to 100 micromol/L) inhibited this phosphorylation, which was well correlated with its inhibition of CCE. Transfection with MLCK antisense completely prevented CCE in response to thapsigargin and cyclopiazonic acid, whereas MLCK sense had no effect. These data strongly indicate that MLCK regulates CCE in human monocytes/macrophages. The study suggests a possible involvement of MLCK in many Ca(2+)-dependent activities of monocytes/macrophages.

Myosin light-chain kinase regulates endothelial calcium entry and endothelium-dependent vasodilation

Activation of smooth muscle myosin light-chain kinase (MLCK) causes contraction. Here we have proven that MLCK controls Ca2+ entry (CE) in endothelial cells (ECs): MLCK antisense oligonucleotides strongly prevented bradykinin (BK)- and thapsigargin (TG)-induced endothelial Ca2+ response, while MLCK sense did not. We also show that the relevant mechanism is not phosphorylation of myosin light-chain (MLC): MLC phosphorylation by BK required CE, but MLC phosphorylation caused by the phosphatase inhibitor calyculin A did not trigger Ca2+ response. Most important, we provide for the first time strong evidence that, in contrast to its role in smooth muscle cells, activation of MLCK in ECs stimulates the production of important endothelium-derived vascular relaxing factors: MLCK antisense and MLCK inhibitors abolished BK- and TG-induced nitric oxide production, and MLCK inhibitors substantially inhibited acetylcholine-stimulated hyperpolarization of smooth muscle cell membrane in rat mesenteric artery. These results indicate that MLCK controls endothelial CE, but not through MLC phosphorylation, and unveils a hitherto unknown physiological function of the enzyme: vasodilation through its action in endothelial cells. The study discovers a counter-balancing role of MLCK in the regulation of vascular tone.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Endothelial [Ca2+]i signaling during transmigration of polymorphonuclear leukocytes

Vascular endothelium plays an important role in regulating the transendothelial migration of polymorphonuclear leukocytes (PMNs). In this study, the intracellular calcium ion ([Ca2+]i) signaling of endothelial cells (ECs) during PMN transmigration was examined at the single-cell level. Human umbilical vein ECs were cultured on a thin layer of collagen gel. The ECs were labeled with fura-2, immersed in formyl-Met-Leu-Phe, and subsequently perfused with fresh buffer to establish a gradient of chemoattractant across the EC monolayer. The entire process of PMN rolling on, adhering to, and transmigrating across the EC monolayer was recorded under both phase-contrast and fluorescence optics. The data showed the following: (1) At high concentration (approximately 3 × 106/mL), both PMN suspension and its supernatant stimulated frequent EC [Ca2+]i elevations across the monolayer; (2) when used at lower concentration (approximately 5 × 105/mL) to avoid the interference of soluble factors, PMN transmigration, but not rolling or adhesion, was accompanied by EC [Ca2+]i elevation; (3) the latter EC [Ca2+]i elevation occurred simultaneously in ECs adjacent to the transmigration site, but not in those that were not in direct contact with the transmigrating PMNs; (4) this EC [Ca2+]i elevation was an initial and required event for PMN transmigration; and (5) PMNs pretreated with 5,5′-dimethyl-1,2-bis(2-aminophenoxy)ethane-N, N, N′, N′-tetraacetic acid transmigrated with the accompanying EC [Ca2+]i elevation, but they became elongated in the collagen gel. In conclusion, PMNs induce adjacent EC [Ca2+]i signaling, which apparently mediates the “gating” step for their subsequent transmigration.

Inhibitors of myosin light chain kinase and phosphodiesterase reduce ventilator-induced lung injury

Alveolar overdistension due to high peak inflation pressures (PIP) is associated with an increased capillary filtration coefficient (K(fc)). To determine which signal pathways contribute to this injury, we perfused isolated rat lungs with 5% bovine albumin in Krebs solution and measured K(fc) after successive 30-min periods of ventilation with peak inflation pressures (PIP) of 7, 20, 30, and 35 cmH(2)O. In a high-PIP control group, K(fc) increased significantly after ventilation with 30 and 35 cmH(2)O PIP, but significant increases were prevented by treatment with 100 microM trifluoperazine, an inhibitor of Ca(2+)/calmodulin, 500 nM ML-7, an inhibitor of myosin light chain kinase (MLCK), a combination of isoproterenol (20 microM) and rolipram (10 microM) to enhance intracellular cAMP levels, and a dose of KT-5720 (2 microM), which inhibits MLCK and protein kinase C. These studies suggest that the Ca(2+)/calmodulin-MLCK pathway augments capillary fluid leak after a modest high-PIP injury and that this is attenuated by kinase inhibition and increased intracellular cAMP.

Endothelial [Ca2+]i signaling during transmigration of polymorphonuclear leukocytes

Vascular endothelium plays an important role in regulating the transendothelial migration of polymorphonuclear leukocytes (PMNs). In this study, the intracellular calcium ion ([Ca2+]i) signaling of endothelial cells (ECs) during PMN transmigration was examined at the single-cell level. Human umbilical vein ECs were cultured on a thin layer of collagen gel. The ECs were labeled with fura-2, immersed in formyl-Met-Leu-Phe, and subsequently perfused with fresh buffer to establish a gradient of chemoattractant across the EC monolayer. The entire process of PMN rolling on, adhering to, and transmigrating across the EC monolayer was recorded under both phase-contrast and fluorescence optics. The data showed the following: (1) At high concentration (approximately 3 × 106/mL), both PMN suspension and its supernatant stimulated frequent EC [Ca2+]i elevations across the monolayer; (2) when used at lower concentration (approximately 5 × 105/mL) to avoid the interference of soluble factors, PMN transmigration, but not rolling or adhesion, was accompanied by EC [Ca2+]i elevation; (3) the latter EC [Ca2+]i elevation occurred simultaneously in ECs adjacent to the transmigration site, but not in those that were not in direct contact with the transmigrating PMNs; (4) this EC [Ca2+]i elevation was an initial and required event for PMN transmigration; and (5) PMNs pretreated with 5,5′-dimethyl-1,2-bis(2-aminophenoxy)ethane-N, N, N′, N′-tetraacetic acid transmigrated with the accompanying EC [Ca2+]i elevation, but they became elongated in the collagen gel. In conclusion, PMNs induce adjacent EC [Ca2+]i signaling, which apparently mediates the “gating” step for their subsequent transmigration.

Store-operated calcium entry and increased endothelial cell permeability

We hypothesized that myosin light chain kinase (MLCK) links calcium release to activation of store-operated calcium entry, which is important for control of the endothelial cell barrier. Acute inhibition of MLCK caused calcium release from inositol trisphosphate-sensitive calcium stores and prevented subsequent activation of store-operated calcium entry by thapsigargin, suggesting that MLCK serves as an important mechanism linking store depletion to activation of membrane calcium channels. Moreover, in voltage-clamped single rat pulmonary artery endothelial cells, thapsigargin activated an inward calcium current that was abolished by MLCK inhibition. F-actin disruption activated a calcium current, and F-actin stabilization eliminated the thapsigargin-induced current. Thapsigargin increased endothelial cell permeability in the presence, but not in the absence, of extracellular calcium, indicating the importance of calcium entry in decreasing barrier function. Although MLCK inhibition prevented thapsigargin from stimulating calcium entry, it did not prevent thapsigargin from increasing permeability. Rather, inhibition of MLCK activity increased permeability that was especially prominent in low extracellular calcium. In conclusion, MLCK links store depletion to activation of a store-operated calcium entry channel. However, inhibition of calcium entry by MLCK is not sufficient to prevent thapsigargin from increasing endothelial cell permeability.

Midkine inhibits bradykinin-stimulated Ca(2+) signaling and nitric oxide production in endothelial cells

Effects of the heparin-binding growth factor midkine (MK) were investigated on endothelial Ca(2+) signaling and nitric oxide (NO) production. Bradykinin (10 nM) and thapsigargin (1 microM) provoked large Ca(2+) influxes under fura-2/AM fluoroscopy. Pretreatment with human MK dose-dependently (1-500 ng/ml) inhibited the Ca(2+) response to bradykinin but not that to thapsigargin. Anti-MK antibody prevented this effect. In Ca(2+)-free medium, MK greatly inhibited intracellular Ca(2+) store release by bradykinin and not that by thapsigargin, which effect was prevented by the antibody. Bradykinin increased NO production by 6.7-fold, which was inhibited 6, 44, 79, and 90% by MK at 1, 10, 100, and 500 ng/ml, respectively. MK did not affect thapsigargin-induced NO production. Our data clearly indicate that MK inhibits bradykinin-induced Ca(2+) response and NO production from endothelial cells.