Different phosphorylated forms of myosin in contracting tracheal smooth muscle

Zipper-interacting protein kinase induces Ca(2+)-free smooth muscle contraction via myosin light chain phosphorylation

The inhibition of myosin phosphatase evokes smooth muscle contraction in the absence of Ca(2+), yet the underlying mechanisms are not understood. To this end, we have cloned smooth muscle zipper-interacting protein (ZIP) kinase cDNA. ZIP kinase is present in various smooth muscle tissues including arteries. Triton X-100 skinning did not diminish ZIP kinase content, suggesting that ZIP kinase associates with the filamentous component in smooth muscle. Smooth muscle ZIP kinase phosphorylated smooth muscle myosin as well as the isolated 20-kDa myosin light chain in a Ca(2+)/calmodulin-independent manner. ZIP kinase phosphorylated myosin light chain at both Ser(19) and Thr(18) residues with the same rate constant. The actin-activated ATPase activity of myosin increased significantly following ZIP kinase-induced phosphorylation. Introduction of ZIP kinase into Triton X-100-permeabilized rabbit mesenteric artery provoked a Ca(2+)-free contraction. A protein phosphatase inhibitor, microcystin LR, also induced contraction in the absence of Ca(2+), which was accompanied by an increase in both mono- and diphosphorylation of myosin light chain. The observed sensitivity of the microcystin-induced contraction to various protein kinase inhibitors was identical to the sensitivity of isolated ZIP kinase to these inhibitors. These results suggest that ZIP kinase is responsible for Ca(2+) independent myosin phosphorylation and contraction in smooth muscle.

Myosin light chain kinase- and PKC-dependent contraction of LES and esophageal smooth muscle

In smooth muscle cells enzymatically isolated from circular muscle of the esophagus (ESO) and lower esophageal sphincter (LES), ACh-induced contraction and myosin light chain (MLC) phosphorylation were similar. Contraction and phosphorylation induced by purified MLC kinase (MLCK) were significantly greater in LES than ESO. ACh-induced contraction and MLC phosphorylation were inhibited by calmodulin and MLCK inhibitors in LES and by protein kinase C (PKC) inhibitors in ESO. Contraction of LES and ESO induced by the PKC agonist 1,2-dioctanoylglycerol (DG) was unaffected by MLCK inhibitors. Caldesmon and calponin concentration-dependently inhibited ACh-induced contraction of ESO and not LES. In ESO, caldesmon antagonist GS17C reversed caldesmon- but not calponin-induced ACh inhibition. GS17C caused contraction of permeabilized ESO but had much less effect on LES. GS17C-induced contraction was not affected by MLCK inhibitors, suggesting that MLCK may not regulate caldesmon-mediated contraction. DG-induced contraction of ESO and LES was inhibited by caldesmon and calponinin, suggesting that these proteins may regulate PKC-dependent contraction. We conclude that calmodulin and MLCK play a role in ACh-induced LES contraction, whereas the classical MLCK may not be the major kinase responsible for contraction and phosphorylation of MLC in ESO. ESO contraction is PKC dependent. Caldesmon and/or calponin may play a role in PKC-dependent contraction.

Relationship between myosin phosphorylation and contractile capability of canine airway smooth muscle

To better understand excitation-contraction coupling in smooth muscle, myosin phosphorylation and force-velocity properties of canine tracheal muscle were compared during the rise and early plateau of force in electrically stimulated tetani. Velocity reached a peak of approximately 1.5 times plateau value when force had risen to approximately 45% of its maximum value and then declined progressively. Except early in the tetanus, when phosphorylation rose rapidly, maximum power and phosphorylation had nearly parallel time courses, reaching peaks of 1.2-1.3 times reference at 6-8 s before declining to the plateau level at approximately 12 s. Force, velocity, maximum power, and phosphorylation fell somewhat during the plateau, with the closest correlation between phosphorylation and power. These results suggest that 1) early velocity slowing is not associated with light chain dephosphorylation and 2) maximum power, which we use to signal changes in activation, is closely correlated with the degree of light chain phosphorylation, at least when phosphorylation level is not changing rapidly. Dissociation of these two properties would be expected early in the tetanus if phosphorylation precedes mechanical activity.

Calyculin-A, an inhibitor for protein phosphatases, induces cortical contraction in unfertilized sea urchin eggs

When an unfertilized sea urchin egg was exposed to calyculin-A (CL-A), an inhibitor of protein phosphatases, for a short period and then lysed, the cortex contracted to exclude cytoplasm and became a cup-shaped mass. We call the contracted cortex "actin cup" since actin filaments were major structural components. Electron microscopic observation revealed that the cup consisted of inner electron-dense layer, middle microfilamentous layer, and outermost granular region. Microfilaments were heavily accumulated in the inner electron-dense layer. The middle layer also contained numerous microfilaments, which were determined to be actin filaments by myosin S1 decoration, and they were aligned so that their barbed ends directed toward the outermost region. Myosin II, Arp2, Arp3, and spectrin were concentrated in the actin cup. Immuno-electron microscopy revealed that myosin II was localized to the electron-dense layer. We further found that the cortical tension of the egg increased just after application of CL-A and reached maximum within 10 min. Cytochalasin B or butanedione monoxime blocked the contraction, which suggested that both actin filaments and myosin ATPase activity were required for the contraction. Myosin regulatory light chain (MRLC) in the actin cup was shown to be phosphorylated at the activation sites Ser-19 and Thr-18, by immunoblotting with anti-phosphoepitope antibodies. The phosphorylation of MRLC was also confirmed by a (32)P in vivo labeling experiment. The CL-A-induced cortical contraction may be a good model system for studying the mechanism of cytokinesis.

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.

Rho-Rho kinase is involved in smooth muscle cell migration through myosin light chain phosphorylation-dependent and independent pathways

Although Rho, a small GTPase, has been demonstrated to play an important role in the smooth muscle contraction and relaxation, little is known about the involvement of Rho protein in smooth muscle cell (SMC) migration. In this study the role of Rho-Rho kinase pathway was examined in SMC migration induced by platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA). C3 transferase, a specific inhibitor of Rho, blocked SMC migration induced by PDGF and LPA. Y-27632, a specific inhibitor of Rho kinase, a direct target molecule of Rho, inhibited PDGF and LPA-induced SMC migration in a concentration dependent manner. Although rapid increase in myosin light chain (MLC) phosphorylation in SMC treated with LPA was observed, no enhanced MLC phosphorylation was detected in response to PDGF. Y-27632 suppressed LPA-induced as well as basal level of MLC phosphorylation. ML-9, a specific inhibitor of myosin light chain kinase (MLCK), inhibited PDGF and LPA-induced SMC migration without the suppression of MLC phosphorylation at 5 min incubation, suggesting that MLCK may contribute to SMC migration via mechanism other than MLC phosphorylation. These results suggest that Rho-Rho kinase pathway is implicated in SMC migration and that different signaling pathways downstream of Rho-Rho kinase may be involved in LPA and PDGF-induced SMC migration. MLC phosphorylation via Rho-Rho kinase pathway appears to be implicated in LPA-dependent SMC migration. Whereas PDGF-mediated SMC migration is independent of increased MLC phosphorylation and other target molecules downstream of Rho-Rho kinase seem to be involved.

Ca2+-induced contraction of cat esophageal circular smooth muscle cells

ACh-induced contraction of esophageal circular muscle (ESO) depends on Ca2+ influx and activation of protein kinase Cepsilon (PKCepsilon). PKCepsilon, however, is known to be Ca2+ independent. To determine where Ca2+ is needed in this PKCepsilon-mediated contractile pathway, we examined successive steps in Ca2+-induced contraction of ESO muscle cells permeabilized by saponin. Ca2+ (0.2-1.0 microM) produced a concentration-dependent contraction that was antagonized by antibodies against PKCepsilon (but not by PKCbetaII or PKCgamma antibodies), by a calmodulin inhibitor, by MLCK inhibitors, or by GDPbetas. Addition of 1 microM Ca2+ to permeable cells caused myosin light chain (MLC) phosphorylation, which was inhibited by the PKC inhibitor chelerythrine, by D609 [phosphatidylcholine-specific phospholipase C inhibitor], and by propranolol (phosphatidic acid phosphohydrolase inhibitor). Ca2+-induced contraction and diacylglycerol (DAG) production were reduced by D609 and by propranolol, alone or in combination. In addition, contraction was reduced by AACOCF(3) (cytosolic phospholipase A(2) inhibitor). These data suggest that Ca2+ may directly activate phospholipases, producing DAG and arachidonic acid (AA), and PKCepsilon, which may indirectly cause phosphorylation of MLC. In addition, direct G protein activation by GTPgammaS augmented Ca2+-induced contraction and caused dose-dependent production of DAG, which was antagonized by D609 and propranolol. We conclude that agonist (ACh)-induced contraction may be mediated by activation of phospholipase through two distinct mechanisms (increased intracellular Ca2+ and G protein activation), producing DAG and AA, and activating PKCepsilon-dependent mechanisms to cause contraction.

Nitric oxide contributes to vascular smooth muscle relaxation in contracting fast-twitch muscles

During skeletal muscle contraction, NO derived from neuronal nitric oxide synthase (nNOS) in skeletal muscle fibers or from endothelial cells (eNOS) may relax vascular smooth muscle contributing to functional hyperemia. To examine the relative importance of these pathways, smooth muscle myosin regulatory light chain (smRLC) phosphorylation was assessed as an index of vascular tone in isolated extensor digitorum longus (EDL) muscles from C57, nNOS(-/-), and eNOS(-/-) mice. The smRLC phosphorylation (in mol phosphate per mol smRLC) in C57 resting muscles (0.12 +/- 0.04) was increased 3.7-fold (0.44 +/- 0.03) by phenylephrine (PE). Reversal of this increase with electrical stimulation (to 0.19 +/- 0.03; P < 0.05) was partially blocked by N(omega)-nitro-l-arginine (NLA). In nNOS(-/-) EDL, the PE-induced increase in smRLC phosphorylation (0.10 +/- 0.02 to 0.49 +/- 0.04) was partially decreased by stimulation (0.25 +/- 0.04). In eNOS(-/-) EDL, the control value for smRLC was increased (0.24 +/- 0.04), and PE-induced smRLC phosphorylation (0.36 +/- 0.06) was decreased by stimulation even in the presence of NLA (to 0.20 +/- 0.02; P < 0.05). These results suggest that in addition to NO-independent mechanisms, NO derived from both nNOS and eNOS plays a role in the integrative vascular response of contracting skeletal muscle.

Thrombin-mediated permeability of human microvascular pulmonary endothelial cells is calcium dependent

BACKGROUND

In response to inflammation, endothelial cytoskeleton rearrangement, cell contraction, and intercellular gap formation contribute to a loss of capillary barrier integrity and resultant interstitial edema formation. The intracellular signals controlling these events are thought to be dependent on intracellular calcium concentration ([Ca2+]i). We hypothesized that, in human pulmonary microvascular endothelial cells, a thrombin-induced increase in permeability to albumin would be dependent on Ca2+i and subsequent actin cytoskeleton rearrangements.

METHODS

Human lung microvascular endothelial cells, grown on 0.4 micromol/L pore membranes, were activated with 10 nmol/L human thrombin in Hank's balanced salt solution/0.5% fetal bovine serum. Select cultures were pretreated (45 minutes) with 4 micromol Fura-2/AM to chelate Ca2+i. Permeability was assessed as diffusion of bovine serum albumin/biotin across the monolayer. Similarly treated cells were stained with rhodamine-phalloidin to demonstrate actin cytoskeletal morphology. Separately, cells loaded 2 micromol Fura-2/AM were assessed at OD340/380nm after thrombin exposure to detect free Ca2+i.

RESULTS

Intracellular Ca2+ levels increased 15-fold (2 seconds) and fell to baseline (10 minutes) after thrombin. Permeability increased 10-fold (30 minutes), and a shift from cortical to actin stress fiber morphology was observed. Chelation of Ca2+i diminished permeability to baseline and reduced the percentage of cells exhibiting stress fiber formation.

CONCLUSION

Thrombin stimulates pulmonary capillary leak by affecting the barrier function of activated pulmonary endothelial cells. These data demonstrate a thrombin-stimulated increase in monolayer permeability, and cytoskeletal F-actin stress fibers were, in part, regulated by endothelial Ca2+i. This early, transient rise in Ca2+i likely activates downstream pathways that more directly affect the intracellular endothelial structural changes that control vascular integrity.

Leukotriene C(4) enhances the contraction of porcine tracheal smooth muscle through the activation of Y-27632, a rho kinase inhibitor, sensitive pathway

1. An unsaturated fatty acid, leukotriene C(4) (LTC(4)), has a potent contractile effect on human airway smooth muscle, and has been implicated in the pathogenesis of human asthma. Using front-surface fluorometry with fura-PE3, the effect of LTC(4) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension were investigated in porcine tracheal smooth muscle strips. 2. The application of LTC(4) induced little or no contraction despite a small and transient increase in [Ca(2+)](i). In the presence of LTC(4), however, the contractions evoked by high K(+) depolarization or a low concentration of carbachol (CCh) were markedly enhanced without inducing any changes in the [Ca(2+)](i) levels, thus indicating that LTC(4) increases the Ca(2+) responsiveness of the contractile apparatus. This LTC(4)-induced increase in Ca(2+) responsiveness could partly be reproduced in the permeabilized preparation of tracheal smooth muscle strips. 3. The LTC(4)-induced enhancement of contraction was accompanied by an increase in myosin light chain (MLC) phosphorylation and was blocked by a rho kinase inhibitor (Y-27632), but not by either a PKC inhibitor (calphostin C) or a tyrosine kinase inhibitor (genistein). 4. These results indicated that, in porcine tracheal smooth muscle, LTC(4) enhances the contraction by increasing the Ca(2+) responsiveness of the contractile apparatus in a MLC phosphorylation dependent manner, possibly through the activation of the rho-rho kinase pathway.

Alteration of the [Ca(2+)](i)-force relationship during the vasorelaxation induced by a Ca(2+) channel blocker SR33805 in the porcine coronary artery

The mechanism of vasorelaxation induced by SR33805 was investigated by simultaneously monitoring the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and force, and by determining level of myosin light chain (MLC) phosphorylation in the medial strip of the porcine coronary artery. SR33805 inhibited the sustained increases in [Ca(2+)](i) and force (IC(50); 3.2+/-1.0 and 49.4+/-27.5 nM, respectively) induced by 118 mM K(+)-depolarization. There was about a 10 fold difference in the inhibitory potency between [Ca(2+)](i) and force. SR33805 completely inhibited the [Ca(2+)](i) elevation induced by a thromboxane A(2) analogue, U46619 and histamine, at concentrations (1 microM) higher than those required for the complete inhibition of K(+)-depolarization induced [Ca(2+)](i) elevation. SR33805 had no effect on the [Ca(2+)](i) elevation induced by histamine or caffeine in the absence of extracellular Ca(2+). SR33805 caused a leftward shift of the [Ca(2+)](i)-force relationship of the contraction induced by cumulative application of extracellular Ca(2+) during 118 mM K(+)-depolarization. The relationship between [Ca(2+)](i) and MLC phosphorylation also shifted to the left by SR33805, while the relationship between MLC phosphorylation and force remained unaffected. In conclusion, SR33805 caused an apparent leftward shift of the [Ca(2+)](i)-force relationship, accompanied by a greater degree of MLC phosphorylation for a given level of [Ca(2+)](i). The mechanism of this leftward shift, however, still remains to be elucidated.

Transepithelial resistance can be regulated by the intestinal brush-border Na(+)/H(+) exchanger NHE3

Initiation of intestinal Na(+)-glucose cotransport results in transient cell swelling and sustained increases in tight junction permeability. Since Na(+)/H(+) exchange has been implicated in volume regulation after physiological cell swelling, we hypothesized that Na(+)/H(+) exchange might also be required for Na(+)-glucose cotransport-dependent tight junction regulation. In Caco-2 monolayers with active Na(+)-glucose cotransport, inhibition of Na(+)/H(+) exchange with 200 microM 5-(N,N-dimethyl)- amiloride induced 36 +/- 2% increases in transepithelial resistance (TER). Evaluation using multiple Na(+)/H(+) exchange inhibitors showed that inhibition of the Na(+)/H(+) exchanger 3 (NHE3) isoform was most closely related to TER increases. TER increases due to NHE3 inhibition were related to cytoplasmic acidification because cytoplasmic alkalinization with 5 mM NH(4)Cl prevented both cytoplasmic acidification and TER increases. However, NHE3 inhibition did not affect TER when Na(+)-glucose cotransport was inhibited. Myosin II regulatory light chain (MLC) phosphorylation decreased up to 43 +/- 5% after inhibition of Na(+)/H(+) exchange, similar to previous studies that associate decreased MLC phosphorylation with increased TER after inhibition of Na(+)-glucose cotransport. However, NHE3 inhibitors did not diminish Na(+)-glucose cotransport. These data demonstrate that inhibition of NHE3 results in decreased MLC phosphorylation and increased TER and suggest that NHE3 may participate in the signaling pathway of Na(+)-glucose cotransport-dependent tight junction regulation.

Role of MARCKS in regulating endothelial cell proliferation

Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific protein kinase C (PKC) substrate, mediates PKC signaling through its phosphorylation and subsequent modification of its association with filamentous actin (F-actin) and calmodulin (CaM). PKC has long been implicated in cell proliferation, and recent studies have suggested that MARCKS may function as a cell growth suppressor. Therefore, in the present study, we investigated MARCKS protein expression, distribution, and phosphorylation in preconfluent and confluent bovine pulmonary microvascular endothelial cells (BPMEC) in the presence or absence of the vascular endothelial growth factor (VEGF). In addition, we examined functional alterations of MARCKS in these cells by studying the association of MARCKS with F-actin and CaM-dependent myosin light chain (MLC) phosphorylation. Our results indicate that MARCKS protein is downregulated during BPMEC proliferation. Decreased MARCKS association with F-actin, increased actin polymerization, and CaM-dependent MLC phosphorylation appear to mediate cell shape changes and motility during BPMEC growth. In contrast, VEGF stimulated MARCKS phosphorylation without alteration of protein expression during BPMEC proliferation, which may result in reduced interaction between MARCKS and actin or CaM, leading to actin reorganization and MLC phosphorylation. Our data suggest a regulatory role of MARCKS during endothelial cell proliferation.

Inhibitory effects of amlexanox on carbachol-induced contractions of rabbit ciliary muscle and guinea-pig taenia caecum

Instillation of amlexanox, an anti-allergic drug, over a long period improves myopia in some allergy patients and in monkeys. The relaxing effect of amlexanox on persistent contraction of ciliary muscle may be involved in the improvement of myopia. In this study, the mechanism of the noncompetitive inhibition of carbachol-induced contractions by amlexanox (1-100 microM) was investigated in isolated smooth muscle preparations of the rabbit ciliary body and guinea-pig taenia caecum. In ciliary muscles, amlexanox (100 microM) inhibited both the phasic and tonic components of carbachol-induced contractions even in the presence of cyclopiazonic acid (10 microM) where the function of the sarcoplasmic reticulum was impaired, while diltiazem (3.2, 32 microM) did not. In taenia caecum, diltiazem (3.2 microM) slightly inhibited the phasic component and abolished the tonic component of carbachol-induced contractions. Amlexanox also abolished the tonic component, but it did not decrease the 45Ca2+ uptake into taenia caecum smooth muscle cells induced by carbachol. Amlexanox did not increase the cyclic adenosine monophosphate (cyclicAMP) content of ciliary muscles in the presence of 3-isobutyl-1-methylxanthine (10 microM), while forskolin (1 microM) did. Gel-shift assay showed that the inhibition of carbachol-induced contractions by amlexanox was accompanied by a decrease in phosphorylation of the 20-kDa myosin light chain in taenia caecum tissue preparations. Amlexanox had no effect on calmodulin activity, whereas it inhibited phosphorylation of the myosin light chain by purified myosin light-chain kinase from chicken gizzard. These results suggested that amlexanox may not affect either Ca2+ mobilization or calmodulin activity, although it inhibits myosin light-chain kinase, which may inhibit carbachol-induced contraction.

Endotoxin causes phosphorylation of MARCKS in pulmonary vascular endothelial cells

Protein kinase C (PKC) has been implicated in lipopolysaccharide (LPS)-induced endothelial cell (EC) monolayer permeability. Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific PKC substrate, appears to mediate PKC signaling by PKC-dependent phosphorylation of MARCKS and subsequent modification of the association of MARCKS with filamentous actin and calmodulin (CaM). Therefore, in the present study, we investigated LPS-induced MARCKS phosphorylation in bovine pulmonary artery EC (BPAEC). LPS potentiated MARCKS phosphorylation in BPAEC in a time- and dose-dependent manner. The PKC inhibitor, calphostin C, significantly decreased LPS-induced phosphorylation of MARCKS. In addition, downregulation of PKC with phorbol 12-myristate 13-acetate (PMA) did not affect the LPS-induced MARCKS phosphorylation, suggesting that LPS and PMA activate different isoforms of PKC. Pretreatment with SB203580, a specific inhibitor of p38 MAP kinase, or genistein, a tyrosine kinase inhibitor, prevented LPS-induced MARCKS phosphorylation. Phosphorylation at appropriate sites will induce translocation of MARCKS from the cell membrane to the cytosol. However, LPS, in contrast to PMA, did not generate MARCKS translocation in BPAEC, suggesting that MARCKS translocation may not play a role in LPS-induced actin rearrangement and EC permeability. LPS also enhanced both thrombin- and PMA-induced phosphorylation of MARCKS, suggesting that LPS was able to prime these signaling pathways in BPAEC. Because the CaM-dependent phosphorylation of myosin light chains (MLC) results in EC contraction, we studied the effect of LPS on MLC phosphorylation in BPAEC. LPS induced diphosphorylation of MLC in a time-dependent manner, which occurred at lower doses of LPS, than those required to induce MARCKS phosphorylation. In addition, there was no synergism between LPS and thrombin in the induction of MLC phosphorylation. These data indicate that MLC phosphorylation is independent of MARCKS phosphorylation. In conclusion, LPS stimulated MARCKS phosphorylation in BPAEC. This phosphorylation appears to involve activation of PKC, p38 MAP kinase, and tyrosine kinases. Further studies are needed to explore the role of MARCKS phosphorylation in LPS-induced actin rearrangement and EC permeability.

Activation of Rho signaling contributes to lysophosphatidic acid-induced contraction of intact ileal smooth muscle of guinea-pig

To elucidate the possible role of Rho A/Rho-kinase on lysophosphatidic acid (LPA)-induced contraction in intact guinea-pig ileal smooth muscle, we examined effects of pretreatment with a specific inhibitor of Rho-kinase (Y-27632) on the LPA-induced contraction and MLC20 phosphorylation. In addition, we investigated whether LPA actually elicits an activation of Rho A by studying subcellular distribution of Rho A in unstimulated and stimulated smooth muscles by LPA. LPA induced a less intense, but sustained, contraction compared with ACh, and was accompanied by significant increases in MLC20 phosphorylation. The effects of LPA on tension and MLC20 phosphorylation were inhibited by Y-27632. The ACh-induced contraction, but not increases in MLC20 phosphorylation, was partially inhibited by Y-27632. High K+-induced contraction was unaffected by the inhibitor. LPA stimulated translocation of Rho A from the cytosol to the membrane fraction of the muscle. Translocation of Rho A was also induced by ACh and high K+. These results suggest that LPA-induced contraction of intact ileal smooth muscle is dominated through activation of Rho A and Rho-kinase and subsequent increases in MLC20 phosphorylation.Key words: lysophosphatidic acid, Rho, Rho-kinase, ileal smooth muscle.

ATP induces dephosphorylation of myosin light chain in endothelial cells

In cultured porcine aortic endothelial monolayers, the effect of ATP on myosin light chain (MLC) phosphorylation, which controls the endothelial contractile machinery, was studied. ATP (10 microM) reduced MLC phosphorylation but increased cytosolic Ca(2+) concentration ([Ca(2+)](i)). Inhibition of the ATP-evoked [Ca(2+)](i) rise by xestospongin C (10 microM), an inhibitor of the inositol trisphosphate-dependent Ca(2+) release from endoplasmic reticulum, did not affect the ATP-induced dephosphorylation of MLC. MLC dephosphorylation was prevented in the presence of calyculin A (10 nM), an inhibitor of protein phosphatases PP-1 and PP-2A. Thus ATP activates MLC dephosphorylation in a Ca(2+)-independent manner. In the presence of calyculin A, MLC phosphorylation was incremented after addition of ATP, an effect that could be abolished when cells were loaded with the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid acetoxymethyl ester (10 microM). Thus ATP also activates a Ca(2+)-dependent kinase acting on MLC. In summary, ATP simultaneously stimulates a functional antagonism toward both phosphorylation and dephosphorylation of MLC in which the dephosphorylation prevails. In endothelial cells, ATP is the first physiological mediator identified to activate MLC dephosphorylation by a Ca(2+)-independent mechanism.

Show me the pathway! Regulation of paracellular permeability by Na(+)-glucose cotransport

The physiological impact of Na(+)-nutrient cotransport-dependent regulation of intestinal tight junction permeability has been controversial. Nonetheless, increased permeability of small intestinal mucosae and enterocyte tight junctions as a consequence of Na(+)-nutrient cotransport has been documented by a significant number of in vivo and in vitro studies. Some details of the intracellular signaling events that regulate this process have been described recently. The aims of this article are to: (i) review studies of tight junction regulation and paracellular nutrient absorption in mammalian intestine, (ii) identify potential applications of tight junction regulation, and (iii) summarize recent progress in defining molecular mechanisms that lead to altered tight junction permeability.

Kinetics of smooth muscle heavy meromyosin with one thiophosphorylated head

Actin-activated MgATPase of smooth muscle heavy meromyosin is activated by thiophosphorylation of two regulatory light chains, one on each head domain. To understand cooperativity between heads, we examined the kinetics of heavy meromyosin (HMM) with one thiophosphorylated head. Proteolytic gizzard heavy meromyosin regulatory light chains were partially exchanged with recombinant thiophosphorylated His-tagged light chains, and HMM with one thiophosphorylated head was isolated by nickel-affinity chromatography. In vitro motility was observed. By steady-state kinetic analysis, one-head thiophosphorylated heavy meromyosin had a similar K(m) value for actin but a V(max) value of approximately 50% of the fully thiophosphorylated molecule. However, single turnover analysis, which is not sensitive to small amounts of active heads, showed that one-head thiophosphorylated heavy meromyosin was 46-120 times more active than unphosphorylated HMM but only 7-19% as active as the fully thiophosphorylated molecule. Discrepancy between the single turnover and steady-state values could be explained by a small fraction of rigor heads. These rigor heads would have a large effect on the steady-state kinetics of one-head thiophosphorylated HMM. In summary, thiophosphorylation of one head leads to a molecule with unique intermediate kinetics suggesting that thiophosphorylation of one head cooperatively alters the kinetics of the partner head and vice versa.

Maturational differences between vascular and bladder smooth muscle during ovine development

Maturation rates of vascular and visceral smooth muscle (SM) during ovine development were compared by quantifying contractile protein, myosin heavy chain (MHC) isoform contents, and contractile properties of aortas and bladders from female fetal (n = 19) and postnatal (n = 21) sheep. Actin, myosin, and protein contents rose progressively throughout development in both tissues (P </= 0.003); however, expression patterns differed. During the last trimester, i. e., 101-130 days (term approximately 145 days), bladder actin and MHC contents were approximately twofold greater (P < 0.04) than those in the aorta. Although the fractional content of 204-kDa SM1 MHC in the bladder decreased from 74 +/- 3% at midgestation to 48 +/- 2% 3 mo postnatal, the aorta exhibited an increase from 30 +/- 2% to 65 +/- 2%. Bladder MHC (MHC-B) migrating at 200 kDa contained only SM2 throughout development. In contrast, 200-kDa MHC in the aorta was predominantly nonmuscle MHC-B at midgestation, which was gradually replaced by SM2 as development progressed. Along with its early expression of SM2, bladder muscle obtained maximal stress generating capacity (1.7 x 10(5) N/m(2)) by term gestation, whereas the aorta exhibited no contractions until after birth. We conclude that whereas aortic SM maturation is delayed until after birth, bladder SM matures biochemically and functionally during prenatal development, thus supporting early requirements for micturition.

Thrombin-induced phosphorylation of MARCKS does not alter its interactions with calmodulin or actin

Myristoylated alanine-rich C kinase substrate (MARCKS) is a calmodulin (CaM)- and actin-binding protein and prominent protein kinase C (PKC) substrate. In vitro phosphorylation of MARCKS by PKC has been shown to induce the release of both CaM and actin, leading to the suggestion that MARCKS may regulate CaM availability during agonist-induced signalling. In support of this hypothesis we previously demonstrated that thrombin-induced MARCKS phosphorylation in endothelial cells (EC) parallels activation of myosin light chain kinase, a CaM-dependent enzyme. To test this theory further, we transfected CHO cells, which normally do not express significant levels of MARCKS, with a MARCKS cDNA. The thrombin-stimulated phosphorylation of myosin light chains and the sensitivity to CaM antagonists in the MARCKS overexpressing cells was the same as that in control CHO cells. MARCKS associated with the actin cytoskeleton in EC was markedly increased upon treatment with the PKC activator, PMA, but only modestly enhanced by thrombin treatment. Similarly, colocalisation of MARCKS with actin was enhanced when the EC were challenged with PMA but not thrombin. These data may be partially explained by PKC-independent phosphorylation of MARCKS in response to thrombin stimulation.

Increased myosin light chain phosphorylation is not required for growth factor stimulation of collagen matrix contraction

Previous research suggested the possibility that contraction of floating collagen matrices by human fibroblasts required increased myosin light chain (MLC) phosphorylation. In the current studies, we show that increased MLC phosphorylation was neither necessary for platelet-derived growth factor (PDGF)-dependent matrix contraction nor sufficient for lysophosphatidic acid (LPA)-dependent contraction. In contrast, increased MLC phosphorylation did appear to be coupled to the formation of stress fibers by cells spreading in monolayer culture. Signal transduction pathways required for PDGF- and LPA-dependent matrix contraction involved phosphatidylinositol 3-kinase and the G(i) class of heterotrimeric G proteins, respectively. Our results indicate that PDGF- and LPA-dependent contraction of floating collagen matrices can be uncoupled from an increase in MLC phosphorylation.

PKC-dependent regulation of transepithelial resistance: roles of MLC and MLC kinase

The mechanisms by which protein kinase C (PKC) activation results in increased transepithelial resistance (TER) are unknown [G. Hecht, B. Robinson, and A. Koutsouris. Am. J. Physiol. 266 (Gastrointest. Liver Physiol. 29): G214-G221, 1994]. We have previously shown that phosphorylation of the regulatory light chain of myosin II (MLC) is associated with decreases in TER and have suggested that contraction of the perijunctional actomyosin ring (PAMR) increases tight junction (TJ) permeability [J. R. Turner, B. K. Rill, S. L. Carlson, D. Carnes, R. Kerner, R. J. Mrsny, and J. L. Madara. Am. J. Physiol. 273 (Cell Physiol. 42): C1378-C1385, 1997]. We therefore hypothesized that PKC activation alters TER via relaxation of the PAMR. Activation of PKC by the phorbol ester phorbol 12-myristate 13-acetate (PMA) resulted in a progressive dose-dependent increase in TER that was apparent within 15 min (111% of controls) and maximal within 2 h (142% of controls). Similar increases were induced by a diacylglycerol analog, and the effects of both PMA and the diacylglycerol analog were prevented by the PKC inhibitor bisindolylmaleimide I. PMA treatment caused progressive decreases in MLC phosphorylation, by 12% at 15 min and 41% at 2 h. Phosphorylation of MLC kinase (MLCK) increased by 64% within 15 min of PMA treatment and was stable over 2 h (51% greater than that of controls). Thus increases in MLCK phosphorylation preceded decreases in MLC phosphorylation. These data suggest that PKC regulates TER via decreased phosphorylation of MLC, possibly due to inhibitory phosphorylation of MLCK. The decreased phosphorylation of MLC likely reduces PAMR tension, leading to decreased TJ permeability.

Phosphorylation-dependent structural changes in the regulatory light chain domain of smooth muscle heavy meromyosin

Smooth muscle heavy meromyosin, a double-headed proteolytic fragment of myosin lacking the COOH-terminal two-thirds of the tail, has been shown previously to be regulated by phosphorylation. To examine phosphorylation-dependent structural changes near the head-tail junction, we prepared five well regulated heavy meromyosins containing single-cysteine mutants of the human smooth muscle regulatory light chain labeled with the photocross-linking reagent, benzophenone-iodoacetamide. For those mutants that generated cross-links, only one type of cross-linked species was observed, a regulatory light chain dimer. Irradiated mutants fell into two classes. First, for Q15C, A23C, and wild type (Cys-108), a regulatory light chain dimer was formed for dephosphorylated but not thiophosphorylated heavy meromyosin. These data provide direct chemical evidence that in the dephosphorylated state, Gln-15, Ala-23, and Cys-108 on one head are positioned near (within 8.9 A) the regulatory light chain of the partner head and that thiophosphorylation abolishes proximity. This behavior was also observed for the Q15C mutant on a truncated heavy meromyosin lacking both catalytic domains. For the actin-heavy meromyosin complex, cross-links were formed in both de- and thiophosphorylated states. S59C and T134C mutants were in a second mutant class, where regulatory light chain dimers were not detected in dephosphorylated or thiophosphorylated heavy meromyosin, suggesting positions outside the region of interaction of the regulatory light chains.

Regulation of Ca2+-independent smooth muscle contraction by alternative staurosporine-sensitive kinase

It is well known that inhibition of myosin phosphatase induces smooth muscle contraction in the absence of Ca2+. We characterized the kinase(s) which plays a role in Ca2+-independent, microcystin-LR-induced contraction in permeabilized smooth muscle of the rabbit portal vein. Assessments of various protein kinase inhibitors revealed this kinase(s) (1) was sensitive to staurosporine (1 microM), but resistant to other agents including wortmannin (10 microM), Y-27632 ((R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide+ ++, 100 microM). HA1077 (1-(5-isoquinolinylsulfonyl)-homopiperazine, 100 microM), H-7 (1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, 100 microM), and calphostin C (100 microM), and (2) induced phosphorylation of 20 kDa myosin light chain at serine-19. We concluded that other kinases exist which phosphorylate myosin light chain at serine-19 and induce Ca2+-independent smooth muscle contraction, distinct from Rho-associated kinase, myosin light chain kinase, and protein kinase C.

NH2-terminal fragments of the 130 kDa subunit of myosin phosphatase increase the Ca2+ sensitivity of porcine renal artery

1. The effects of the NH2-terminal fragments of M130, a 130 kDa regulatory subunit of smooth muscle myosin phosphatase, on contraction and myosin light chain phosphorylation were investigated in Triton X-100-permeabilized porcine renal artery. 2. Incubation of the permeabilized fibres with M1301-633 (a fragment containing amino acid residues 1-633) or M13044-633 enhanced the Ca2+-induced contraction and shifted the [Ca2+]i-force relationship to the left (EC50 of Ca2+: 330 nM, control, without fragment; 145 nM, M1301-633; 163 nM, M13044-633). Pre-incubation for 1-3 h was needed for these long constructs. 3. M1301-374, M130304-511 and M130297-374, i.e. relatively short constructs compared with M1301-633 and M13044-633, also induced leftward shifts of the [Ca2+]i-force relationship (EC50 of Ca2+: 65 nM, 72 nM and 180 nM, respectively). However, these required no pre-incubation. 4. Deletion of residues 304-374 from the most potent construct, M1301-374, abolished the Ca2+-sensitizing effect. 5. Wortmannin inhibited the enhancement of contraction induced by M130 fragments when added before contraction was initiated and partially inhibited the effects when added after steady-state contraction. 6. M1301-374 slowed the rate of relaxation in Ca2+-free medium. The time for 50 % relaxation with this fragment was 510 +/- 51 s, compared with 274 +/- 14 s for control. 7. The levels of myosin light chain phosphorylation (22.4 %) and force (34. 5 %) obtained with 300 nM Ca2+ were increased by 3 microM M1301-374 to 35.7 and 92.2 %, respectively. However, M1301-374 had no effect on the phosphorylation-force relationship. 8. In conclusion, the NH2-terminal M130 fragments containing residues 304-374 inhibited myosin phosphatase, increased myosin light chain phosphorylation and increased the Ca2+ sensitivity of the contractile apparatus in permeabilized porcine renal artery.

Properties of filament-bound myosin light chain kinase

Myosin light chain kinase binds to actin-containing filaments from cells with a greater affinity than to F-actin. However, it is not known if this binding in cells is regulated by Ca2+/calmodulin as it is with F-actin. Therefore, the binding properties of the kinase to stress fibers were examined in smooth muscle-derived A7r5 cells. Full-length myosin light chain kinase or a truncation mutant lacking residues 2-142 was expressed as chimeras containing green fluorescent protein at the C terminus. In intact cells, the full-length kinase bound to stress fibers, whereas the truncated kinase showed diffuse fluorescence in the cytoplasm. After permeabilization with saponin, the fluorescence from the truncated kinase disappeared, whereas the fluorescence of the full-length kinase was retained on stress fibers. Measurements of fluorescence intensities and fluorescence recovery after photobleaching of the full-length myosin light chain kinase in saponin-permeable cells showed that Ca2+/calmodulin did not dissociate the kinase from these filaments. However, the filament-bound kinase was sufficient for Ca2+-dependent phosphorylation of myosin regulatory light chain and contraction of stress fibers. Thus, dissociation of myosin light chain kinase from actin-containing thin filaments is not necessary for phosphorylation of myosin light chain in thick filaments. We note that the distance between the N terminus and the catalytic core of the kinase is sufficient to span the distance between thin and thick filaments.

Cooperativity between two heads of dictyostelium myosin II in in vitro motility and ATP hydrolysis

To elucidate the significance of the two-headed structure of myosin II, we have engineered and characterized recombinant single-headed myosin II. A tail segment of a myosin II heavy chain fused with a His-tag was expressed in wild-type Dictyostelium cells. Single-headed myosin, which consists of a full length myosin heavy chain and a tagged tail, was isolated on the basis of the affinities for Nickel agarose and actin. Actin sliding velocity by the single-headed myosin was about half of the two-headed, whereas the minimum density of the heads to support continuous movement was twofold higher. Actin-activated MgATPase activity of the single-headed myosin in solution in the presence of 24 microM actin was less than half of the two headed. This decrease is primarily because of fourfold-elevated Kapp for actin and secondary to 40% lower Vmax. These results suggest that the two heads of a Dictyostelium myosin II molecule act cooperatively on an actin filament. We propose a mechanism by which two heads move actin efficiently based on the cooperativity.

Myosin light chain phosphorylation and contractile proteins in a canine two-hemorrhage model of subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage (SAH) impairs both contraction and relaxation response in cerebral arteries. We tested the hypothesis that cerebral vasospasm might be ATP-independent contraction, such as latch state, and protein synthesis might be substantially downregulated due to ATP consumption after long-lasting contraction.

METHODS

Chronic cerebral vasospasm was induced in the canine 2-hemorrhage model of SAH. The normal and spastic basilar arteries were stabilized in Krebs-Henseleit solution, and contraction was induced by 30 micromol/L prostaglandin F2alpha (PGF2alpha) in vitro and in vivo. Before and at 15 minutes and 1 hour after the treatment with PGF2alpha, the levels of phosphorylated 20-kDa myosin light chain (MLC20) were measured. The time course of expression of contraction proteins actin and MLC20, and contraction-inhibiting proteins h-caldesmon and calponin was determined by immunoblotting techniques.

RESULTS

A significant vasospasm occurred in the basilar artery during days 4 to 21, most prominently on days 7 and 14. There were no significant differences in the baseline levels of phosphorylated MLC20 between normal and spastic basilar arteries. The increase in MLC20 phosphorylation by PGF2alpha was significantly attenuated in the spastic basilar artery in vitro and in vivo (P<0.05). The immunoreactivity for actin, h-caldesmon, and calponin in the spastic basilar arteries was progressively decreased until day 14 and returned to the normal level on day 21. In contrast, protein levels of MLC20 did not significantly change during days 0 to 21.

CONCLUSIONS

Chronic cerebral vasospasm closely resembles the latch state, and temporary deficiencies of contractile proteins may result from increased destruction and inhibition of protein synthesis.

Mechanisms of Ca2+ sensitization of force production by noradrenaline in rat mesenteric small arteries

1. Mechanisms of Ca2+ sensitization of force production by noradrenaline were investigated by measuring contractile responses, intracellular Ca2+ concentration ([Ca2+]i) and phosphorylation of the myosin light chain (MLC) in intact and alpha-toxin-permeabilized rat mesenteric small arteries. 2. The effects of noradrenaline were investigated at constant membrane potential by comparing fully depolarized intact arteries in the absence and presence of noradrenaline. Contractile responses to K-PSS (125 mM K+) and NA-K-PSS (K-PSS + 10 microM noradrenaline) were titrated to 30 and 75%, respectively, of control force, by adjusting extracellular Ca2+ ([Ca2+]o). At both force levels, [Ca2+]i was substantially lower with NA-K-PSS than with K-PSS. With K-PSS, the proportion of MLC phosphorylated (approximately 30%) was similar at 30 and 75% of control force; with NA-K-PSS, MLC phosphorylation was greater at the higher force level (40 vs. 34%). 3. In alpha-toxin-permeabilized arteries, the force response to 1 microM Ca2+ was increased by 10 microM noradrenaline, and MLC phosphorylation was increased from 35 to 45%. The protein kinase C (PKC) inhibitor calphostin C (100 nM) abolished the noradrenaline-induced increase in MLC phosphorylation and contractile response, without affecting the contraction in response to Ca2+. Treatment with ATP gamma S in the presence of the MLC kinase inhibitor ML-9 increased the sensitivity to Ca2+ and abolished the response to noradrenaline. 4. The present results show that that in rat mesenteric small arteries noradrenaline-induced Ca2+ sensitization is associated with an increased proportion of phosphorylated MLC. The results are consistent with a decreased MLC phosphatase activity mediated through PKC. Furthermore, while MLC phosphorylation is a requirement for force production, the results show that other factors are also involved in force regulation.

Protein kinase C activation during Ca2+-independent vascular smooth muscle contraction

The cellular signaling mechanisms that modulate the sustained vascular smooth muscle contractions that occur in vasospasm are not known. We and others have hypothesized that a kinase cascade involving protein kinase C (PKC) modulates sustained vascular smooth muscle contraction. The purpose of this investigation was to develop a model in which the traditional contractile pathways involving myosin light chain phosphorylation are not activated and determine if the PKC pathway is activated under these conditions. The phosphorylation of caldesmon, myosin light chain (MLC20), and the specific PKC substrate, MARCKS (myristoylated, alanine-rich C-kinase substrate) was measured in bovine carotid arterial smoothmuscle (BCASM) stimulated with phorbol 12,13-dibutyrate (PDBu) under Ca2+-containing and Ca2+-free conditions. PDBu stimulation led to increases in caldesmon and MARCKS phosphorylation to the same degree in the presence or absence of Ca2+. PDBu stimulation but did not lead to increases in MLC20 phosphorylation over basal levels in Ca2+-free conditions. Immunoblot analysis of BCASM using PKC isoform-specific antibodies demonstrated the presence of one "Ca2+- dependent" PKC isoform: alpha, and two of the "Ca2+-independent" isoforms: epsilon and zeta. These data suggest that Ca2+-independent isoforms of PKC may play a role in the sustained phase of BCASM contractions through a kinase cascade that involves caldesmon and MARCKS phosphorylation but not MLC20 phosphorylation.

Skeletal muscle contractions stimulate cGMP formation and attenuate vascular smooth muscle myosin phosphorylation via nitric oxide

Nitric oxide generated by neuronal nitric oxide synthase in contracting skeletal muscle fibers may regulate vascular relaxation via a cGMP-mediated pathway. Neuronal nitric oxide synthase content is greatly reduced in skeletal muscles from mdx mice. cGMP formation increased in contracting extensor digitorum longus muscles in vitro from C57 control, but not mdx mice. The increase in cGMP content was abolished with NG-nitro-L-arginine. Sodium nitroprusside treatment increased cGMP levels in muscles from both C57 and mdx mice. Skeletal muscle contractions also inhibited phenylephrine-induced phosphorylation of smooth muscle myosin regulatory light chain. Arteriolar dilation was attenuated in contracting muscles from mdx but not C57 mice. NO generated in contracting skeletal muscle may contribute to vasodilation in response to exercise.

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

Cytosolic Ca2+ ([Ca2+]i) plays an important role in endothelial cell signaling. Although it has been suggested that the influx of Ca2+ can be triggered by depletion of intracellular Ca2+ stores, the mechanism (or mechanisms) underlying this phenomenon needs further elaboration. In the present study, involvement of myosin light-chain kinase (MLCK) in the regulation of Ca2+ signaling was investigated in agonist- and fluid flow-stimulated endothelial cells loaded with Ca2+-sensitive dyes. Bradykinin (BK) and thapsigargin caused an increase in [Ca2+]i followed by a sustained rise due to Ca2+ influx from extracellular space and shifted total myosin light-chain (MLC) from the unphosphorylated to the diphosphorylated form. ML-9 (100 microM), an inhibitor of MLCK, abolished Ca2+ influx and prevented MLC diphosphorylation in BK- and thapsigargin-treated cells, but did not affect Ca2+ mobilization from internal stores. Fluid flow stimulation (shear stress=5 dynes/cm2) increased [Ca2+]i and enhanced MLC phosphorylation. ML-9 also inhibited Ca2+ response and MLC phosphorylation in fluid flow-stimulated cells. The Ca2+ influx in response to BK was linearly correlated with the diphosphorylation of MLC in ML-9 treated cells. Effects of ML-5 and ML-7, analogs of ML-9, to inhibit Ca2+ influx paralleled their potencies to inhibit MLCK activity. These findings demonstrate that MLCK plays an essential role in regulating the plasmalemmal Ca2+ influx in agonist- and fluid flow-stimulated endothelial cells. This study is the first to report the close relationship between Ca2+ influx and MLC diphosphorylation.

Interaction of actin and ADP with the head domain of smooth muscle myosin: implications for strain-dependent ADP release in smooth muscle

Transient kinetic methods were used to study interactions between actin, MgADP, and smooth muscle (chicken gizzard) myosin subfragment 1 (smS1). The equilibrium dissociation constant (Kd) of actin for smS1 was 3.5 nM, tighter than that of skeletal S1 (skS1). Actin binding to smS1 was weakened 5-fold by saturation with ADP compared to 30-60-fold for skS1. The Kd of ADP for smS1 was increased from 1.2 to 5 microM by actin, whereas for skS1 values increased from 2 to 100 microM. Thus, coupling between ADP and actin binding is weaker for smS1. Previous studies show that release of ADP from actin.smS1.ADP produces a tilt of the regulatory domain [Whittaker, M., Wilson-Kubalek, E. M., Smith, J. E., Faust, L., Milligan, R. A., and Sweeney, H. L. (1995) Nature 378, 748-751]. This result was confirmed by independent structural methods; tilting was absent for skS1, and the Kd for ADP was in agreement with the values measured here [Gollub, J., Cremo, C. R., and Cooke, R. (1996) Nat. Struct. Biol. 3, 796-802; Poole, K. I. V., Lorenz, M., Ellison, P., Evans, G., Rosenbaum, G., Boesecke, P., Holmes, K. C., and Cremo, C. R. (1997) J. Muscle Res. Cell Motility 18, 264]. We discuss tilting upon ADP release with respect to our measurements, previous measurements with skS1, and nucleotide concentrations in smooth muscle. We propose that these data suggest a strain-dependent ADP release mechanism that may be accentuated in smooth muscles.

Myosin light chain phosphorylation and Mn2+ -dependent norepinephrine-induced contractions in guinea-pig vas deferens

We have reported that norepinephrine but not K+ induced a sustained and dose-dependent contraction without extracellular Ca2+ and Mn2+ in Ca2+-depleted Mn2+-loaded vas deferens from the guinea-pig. In the present study, we determined the phosphorylation of the 20-kDa myosin light chain and examined the effects of inhibitors of calmodulin and myosin light chain kinase on the Mn2+-dependent norepinephrine-induced contraction in order to evaluate the contribution of phosphorylation to this contraction. W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-homopiperazine] and wortmannin inhibited this contraction. However, the Mn2+-dependent norepinephrine-induced contraction developed without a significant increase in the phosphorylation of the 20-kDa myosin light chain. In beta-escin-permeabilized preparations, Mn2+ induced contractions that were inhibited by ML-9. These results suggest that the activation of myosin light chain kinase is essential for the development of Mn2+-dependent norepinephrine-induced contractions and that the phosphorylation of myosin light chain may act as a trigger for these contractions.

Myosin light-chain phosphorylation controls insulin secretion at a proximal step in the secretory cascade

The aim of this study was to investigate how insulin secretion is controlled by phosphorylation of the myosin light chain (MLC). Ca2+-evoked insulin release from pancreatic islets permeabilized with streptolysin O was inhibited by different monoclonal antibodies against myosin light-chain kinase (MLCK) to an extent parallel to their inhibition of purified MLCK. Anti-MLCK antibody also inhibited insulin release caused by the stable GTP analog guanosine 5'-O-(3-thiodiphosphate), even at a substimulatory concentration (0.1 microM) of Ca2+. Free Ca2+ increased MLC peptide phosphorylation by beta-cell extracts in vitro. In contrast to the phosphorylation by purified MLCK or by calmodulin (CaM) kinase II, the activity partially remained with the beta-cell under nonstimulatory Ca2+ (0.1 microM) conditions. The MLCK inhibitor ML-9 inhibited the activity in the beta-cell with both substimulatory and stimulatory Ca2+, whereas KN-62, an inhibitor of CaM kinase II, only exerted an influence in the latter case. ML-9 decreased intracellular granule movement in MIN6 cells under basal and acetylcholine-stimulated conditions. We propose that MLC phosphorylation may modulate translocation of secretory granules, resulting in enhanced insulin secretion.

Rho-associated kinase directly induces smooth muscle contraction through myosin light chain phosphorylation

Small GTPase Rho plays pivotal roles in the Ca2+ sensitization of smooth muscle. However, the GTP-bound active form of Rho failed to exert Ca2+-sensitizing effects in extensively Triton X-100-permeabilized smooth muscle preparations, due to the loss of the important diffusible cofactor (Gong, M. C., Iizuka, K., Nixon, G. , Browne, J. P., Hall, A., Eccleston, J. F., Sugai, M., Kobayashi, S. , Somlyo, A. V., and Somlyo, A. P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1340-1345). Here we demonstrate the contractile effects of Rho-associated kinase (Rho-kinase), recently identified as a putative target of Rho, on the Triton X-100-permeabilized smooth muscle of rabbit portal vein. Introduction of the constitutively active form of Rho-kinase into the cytosol of Triton X-100-permeabilized smooth muscle provoked a contraction and a proportional increase in levels of monophosphorylation of myosin light chain in both the presence and the absence of cytosolic Ca2+. These effects of constitutively active Rho-kinase were wortmannin (a potent myosin light chain kinase inhibitor)-insensitive. Immunoblot analysis revealed that the amount of native Rho-kinase was markedly lower in Triton X-100-permeabilized tissue than in intact tissue. Our results demonstrate that Rho-kinase directly modulates smooth muscle contraction through myosin light chain phosphorylation, independently of the Ca2+-calmodulin-dependent myosin light chain kinase pathway.

Myosin isoform heterogeneity in single smooth muscle cells

We review the current understanding of the myosin heavy chain (MHC) isoforms and show that the mRNA levels of smooth muscle (SM)1 and SM2 mimic the expressed levels of SM1 and SM2 protein. The reverse transcriptase-polymerase chain reaction technique has been shown to be sufficiently sensitive to examine SM-MHC expression at the single cell level. Most single smooth muscle cells isolated from adult rabbit carotid express both SM1 and SM2. However, expression of these SM-MHC isoforms at the cellular level is nonuniform and highly variable. This work provides a foundation for future investigations as to the possible unique functional characteristics of the SM-MHC isoforms, SM1 and SM2. This methodology may also prove useful when used with mechanical studies to determine the physiological significance of the alternatively spliced myosin isoforms, including the SM-MHC-head and LC17 isoforms.

Arterial muscle myosin heavy chains and light chains in spontaneous hypertension

Increased maximum velocity of shortening (Vmax), increased shortening ability (delta Lmax) and decreased relaxation rate have been reported for arterial smooth muscle from 16- to 18-week-old spontaneously, hypertensive rats (SHR) compared with age-matched normotensive Wistar-Kyoto rats (WKY). Vmax is dependent on actomyosin ATPase activity, and this activity is in turn dependent on the level of phosphorylation of the 20-kDa myosin light chain (MLC20) normally a function of calcium concentration. In this article, methods are described and data are presented from studies addressing possible intracellular regulatory mechanisms that might lead to the altered contractility of the SHR arterial muscle. In one study, myofibrillar protein was extracted from 16- to 18-week-old SHR and WKY caudal arterial muscle. The Mg(2+)-activated ATPase activity was measured under conditions where the Ca2+ concentration was controlled. In another study, the amount of myosin present and relative proportions of the myosin heavy chain (MHC) isoforms were determined by quantitative SDS-PAGE using heavy molecular weight standards and bovine serum albumin as the standard for concentration. In a third study, MLC20 phosphorylation levels in electrically stimulated arterial muscle were determined by urea glycerol gel electrophoresis and Western blot analyses. The SHR (n = 6) myofibrillar ATPase liberated 0.011 +/- 0.003 mumol Pi/mg myosin/min, which was significantly more than the 0.006 +/- 0.001 mumol Pi/mg myosin/min liberated by the WKY (n = 4) myofibrillar ATPase (P < 0.05). Consistent with the increased ATPase activity, phosphorylation of MLC20 was increased by 2.8 times as much in the SHR compared with the WKY electrically stimulated arterial muscle. However, there was no difference in MHC isoform pattern in the SHR compared with the WKY arterial muscle in contrast to the findings of at least one other laboratory. This discrepancy is discussed. The data reviewed in this article lead to the conclusions that an increased actin-activated myosin ATPase activity and MLC20 phosphorylation are likely responsible for the increased velocity of shortening previously reported in SHR arterial muscle and the increased ATPase activity is not a function of an increased myosin content or of altered MHC isoform pattern in the SHR muscle.

Intramolecular binding contributes to the activation of CDPK, a protein kinase with a calmodulin-like domain

The activity of calmodulin-like domain protein kinase (CDPK) is regulated by the direct binding of Ca2+. Unmodified soybean CDPK alpha and a chimeric enzyme in which the calmodulin-like domain (CLD) was replaced by VU-1 calmodulin had similar values of Vmax(app) (3.19, 3.46, and 3.60, 3.93 mumol/ min/mg, respectively), and each was activated 30-70-fold by Ca2+. To determine if activation results from the binding of the CLD to the autoinhibitory (junction) domain of CDPK alpha in a manner analogous to the activation of calmodulin-dependent enzymes by calmodulin, recombinant CLD and truncation mutants of CDPK alpha were expressed in bacteria and highly purified. In blot overlays, biotinylated CLD bound to mutants containing residues 312-328 of the junction domain. In an electrophoretic mobility shift assay CLD bound synthetic peptides containing residues 318-332 in a calcium-dependent manner, providing direct evidence for binding of CLD to a site in the junction domain. Mutants of CDPK alpha from which all or part of the CLD had been deleted were constitutively inactive. Addition of 20 microM CLD to these mutants in the presence, but not the absence, of calcium stimulated their activities, but to various degrees. His6-CDPK alpha (1-328), which contained none of the CLD, was activated only 5-fold, but the activity of His6-CDPK alpha (1-398), which retained nearly half of the CLD in its sequence, was stimulated 64-fold. The latter activity approached that of unmodified CDPK alpha and was half maximal at a CLD concentration of 7 microM. Our results suggest that binding of CLD to the junction domain contributes to, but is not sufficient for activation. Although calmodulin supported full activity of the chimeric enzyme, its addition to His6-CDPK alpha (1-398) resulted in activity that was only 6% of that of the unmodified enzyme and which was half-maximal at 20 microM Arabidopsis calmodulin. These results support the conclusion that simple binding of the calmodulin-like domain to the junction domain is not sufficient for activation.

Characterization of the motor and enzymatic properties of smooth muscle long S1 and short HMM: role of the two-headed structure on the activity and regulation of the myosin motor

Truncated mutants of smooth muscle myosin containing various lengths of the S2 portion were expressed in Sf9 cells and purified. Truncated myosin having a heavy chain molecular mass of 128 kDa and larger formed a stable dimer, while 108 kDa myosin remained a monomer. On the other hand, 114 and 110 kDa myosins existed as both monomer and dimer. The enzymatic activity and also the in vitro actin sliding activity of these mutant myosins were measured, and the following findings were obtained. (1) Both the actin sliding activity and the actin-activated ATPase activity showed phosphorylation dependence when myosin forms a dimer while the monomeric form was phosphorylation-independent. This indicates that the interaction between the two heads is operating and critical for the regulation. (2) The actin sliding velocity of the dimer form was twice as large as that of the monomer form, while the actin-activated ATPase activity of the two forms was identical, suggesting that the mechano-chemical efficiency is affected by the interaction between the two heads. (3) The depression of the Mg(2+)-ATPase activity of myosin at low ionic strength, characteristic of the 6S-10S transition of smooth muscle myosin, is abolished with the monomer form, suggesting that the association of the two heads is critical for the 6S-10S transition.

Wortmannin inhibits contraction without altering electrical activity in canine gastric smooth muscle

Wortmannin, an inhibitor of myosin light-chain kinase (10-30 microM), completely and irreversibly abolished (in 75% of tissues from canine gastric antrum) phase contractions caused by slow waves with no significant effects on resting membrane potential or the frequency, amplitude, or duration of spontaneous slow waves. Responses to agents that normally cause hyperpolarization (cromakalim, sodium nitroprusside, and forskolin) were unaffected by wortmannin treatment. It was also possible to study the excitatory effects of agents and conditions that normally result in loss of intracellular impalements: 1) elevated extracellular K+ concentrations altered membrane potential close to values predicted by the Nernst equation, and 2) high concentrations of acetylcholine produced depolarization and rapid oscillations in membrane potential coincident with contractile activity. Cholinergic increases in myosin light-chain phosphorylation and contractions were partially blocked by wortmannin. In canine antrum, wortmannin inhibition of contraction was irreversible, although in other tissue types, partial recovery of contractions was observed when wortmannin was removed. Wortmannin can be a useful agent to investigate the electrophysiology of some smooth muscles when movement might lead to recording artifacts or loss of signal.

Regulation of endothelial cell gap formation and barrier dysfunction: role of myosin light chain phosphorylation

Endothelial cell (EC) contraction results in intercellular gap formation and loss of the selective vascular barrier to circulating macromolecules. We tested the hypothesis that phosphorylation of regulatory myosin light chains (MLC) by Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) is critical to EC barrier dysfunction elicited by thrombin. Thrombin stimulated a rapid (< 15 sec) increase in [Ca2+]i which preceded maximal MLC phosphorylation (60 sec) with a 6 to 8-fold increase above constitutive levels of phosphorylated MLC. Dramatic cellular shape changes indicative of contraction and gap formation were observed at 5 min with maximal increases in albumin permeability occurring by 10 min. Neither the Ca2+ ionophore, A23187, nor phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), alone or in combination, produced MLC phosphorylation. The combination was synergistic, however, in stimulating EC contraction/gap formation and barrier dysfunction (3 to 4-fold increase). Down-regulation or inhibition of PKC activity attenuated thrombin-induced MLC phosphorylation (approximately 40% inhibition) and both thrombin- and PMA-induced albumin clearance (approximately 50% inhibition). Agents which augmented [cAMP]i partially blocked thrombin-induced MLC phosphorylation (approximately 50%) and completely inhibited both thrombin- and PMA-induced EC permeability (100% inhibition). Furthermore, cAMP produced significant reduction in the basal levels of constitutive MLC phosphorylation. Finally, MLCK inhibition (with either ML-7 or KT 5926) or Ca2+/calmodulin antagonism (with either trifluoperazine or W-7) attenuated thrombin-induced MLC phosphorylation and barrier dysfunction. These results suggest a model wherein EC contractile events, gap formation and barrier dysfunction occur via MLCK-dependent and independent mechanisms and are significantly modulated by both PKC and cAMP-dependent protein kinase A activities.

Polyacrylamide gel electrophoretic methods in the separation of structural muscle proteins

Polyacrylamide gel electrophoresis plays a major role in analyzing the function of muscle structural proteins. This review describes one- and two-dimensional gel electrophoretic methods for qualitative and quantitative investigation of the muscle proteins, with special emphasis on determination of protein phosphorylation. The electrophoretic studies established the subunit structures of the muscle proteins, characterized their multiple forms, revealed changes in subunit composition or shifts in isoform distribution of specific proteins during development, upon stimulation or denervation of the muscle. Protein phosphorylation during muscle contraction is preferentially studied by two-dimensional gel electrophoresis. The same method demonstrated protein alterations in human neuromuscular diseases.

Selective inhibition of myosin phosphorylation and tension of hyperplastic arteries by the kinase inhibitor HA1077

To examine possible alterations in myosin light chain phosphorylation in hyperplastic arteries, rabbit strips from right hyperplastic and left normal control carotid arteries were used for experiments 6 weeks after the ballooning procedure. When the hyperplastic artery was stimulated with various concentrations of K+ (10, 20, 30, 40 and 60 mM), the maximal tension in response to each concentration was significantly higher (P < 0.05) than that in the control artery. The maximal extent of myosin light chain phosphorylation induced by 60 mM K+ in the hyperplastic artery was also significantly higher than that in the control (55.1 +/- 4.1 vs. 45.1 +/- 3.2%, mean +/- S.D.). However, the [Ca2+]i response to elevated K+ in hyperplastic arteries was much the same as that in control arteries, when measured with fura-PE3. HA1077 (1-5-(isoquinolinesulfonyl)-homopiperazine), a protein kinase inhibitor, was about 3-5 times more effective in inhibiting the tension and myosin light chain phosphorylation induced by 60 mM K+ in the hyperplastic artery than in the control artery. Nifedipine inhibited the tension and myosin light chain phosphorylation to the same extent in control and hyperplastic arteries. Thus, an alteration of the myosin light chain phosphorylation system, but not an alteration of Ca2+ mobilization, may be involved in the enhanced contraction of the hyperplastic artery. The enhanced phosphorylation of myosin light chain may be sensitive to HA1077.

Two heads are required for phosphorylation-dependent regulation of smooth muscle myosin

Recent structural evidence (Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65) suggests that the two heads of skeletal muscle myosin interact when the protein is bound to filamentous actin. Direct chemical cross-linking experiments show that the two heads of smooth muscle myosin interact in the presence of filamentous actin and the absence of ATP (Onishi, H., Maita, T., Matsuda, G., and Fujiwara, K. (1992) Biochemistry 31, 1201-1210). Head-head interactions may be important in the mechanism of phosphorylation-dependent regulation of smooth muscle myosin. To explore the structural elements essential for phosphorylation-dependent regulation, we purified a proteolytic fragment of chicken gizzard myosin containing only one head attached to an intact tail. This molecule contained a partially digested regulatory light chain, which was replaced with exogenously added intact light chain in either the thiophosphorylated or the unphosphorylated state. Control experiments showed that this replacement was nearly quantitative and did not alter the actin-activated ATPase of this myosin. Electron micrographs confirmed that the single-headed preparation contained an intact form of single-headed myosin. The unphosphorylated single-headed myosin hydrolyzed ATP rapidly and moved actin filaments in an in vitro motility assay. Phosphorylation had minimal effects upon these properties. Therefore, we conclude that phosphorylation-dependent regulation in this myosin requires two heads. These findings may have important implications in studies of other regulated motor proteins that contain two motor domains.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Effect of a peptide inhibitor of protein kinase C on G-protein-mediated increase in myofilament Ca(2+)-sensitivity in rabbit arterial skinned muscle

1. To investigate the role of protein kinase C in the increase mediated by guanosine 5'-triphosphate (GTP)-binding proteins (G-proteins) in the sensitivity of the contractile proteins to Ca2+ in vascular smooth muscle, the effect of a novel peptide inhibitor of protein kinase C (PKC19-36) on Ca(2+)-induced contraction and myosin light chain (MLC) phosphorylation was studied in the presence and absence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) in beta-escin-skinned smooth muscle strips of rabbit mesenteric artery. For comparison, the effects were also observed of PKC19-36 on the action of phorbol 12,13-dibutylate (PDBu, an activator of PKC) on the two Ca(2+)-induced responses. 2. In beta-escin-skinned strips treated with ionomycin, Ca2+ (0.1-3 microM) concentration-dependently produced contraction in parallel with an increase in MLC-phosphorylation. GTP gamma S (10 microM) and PDBu (0.1 microM) each shifted both the Ca(2+)-force and Ca(2+)-MLC-phosphorylation relationships to the left without a significant change in either maximum response. The relationship between force and MLC-phosphorylation was not modified by either GTP gamma S or PDBu, indicating that the sensitivity of MLC-phosphorylation to Ca2+ is enhanced by both GTP gamma S and PDBu. 3. PKC19-36 itself modified neither the contraction nor MLC-phosphorylation induced by Ca2+ but it did block the PDBu-induced enhancement of these two Ca(2+)-induced responses. By contrast, PKC19-36 did not modify the GTP gamma S-induced enhancement of the two Ca(2+)-induced responses. Guanosine 5'-O-(2-thiodiphosphate) (GDP Beta S) attenuated the GTP gamma S-induced enhancement of the Ca2+-induced contraction.4. These results suggest that GTP gamma S increases Ca2+-induced MLC-phosphorylation through the activation of a PKC-independent mechanism and thus causes an increase in the sensitivity of the contractile proteins to Ca2+ in Beta-escin-skinned smooth muscle of rabbit mesenteric artery.