The effect of phosphorylation of gizzard myosin on actin activation

Role of ZIP kinase in development of myofibroblast differentiation from HPMCs

During the development of pleural fibrosis, pleural mesothelial cells (PMCs) undergo phenotypic switching from differentiated mesothelial cells to mesenchymal cells (MesoMT). Here, we investigated how external stimuli such as TGF-β induce HPMC-derived myofibroblast differentiation to facilitate the development of pleural fibrosis. TGF-β significantly increased di-phosphorylation but not mono-phosphorylation of myosin II regulatory light chain (RLC) in HPMCs. An increase in RLC di-phosphorylation was also found at the pleural layer of our carbon black bleomycin (CBB) pleural fibrosis mouse model, where it showed filamentous localization that coincided with alpha smooth muscle actin (αSMA) in the cells in the pleura. Among the protein kinases that can phosphorylate myosin II RLC, ZIPK (zipper-interacting kinase) protein expression was significantly augmented after TGF-β stimulation. Furthermore, ZIPK gene silencing attenuated RLC di-phosphorylation, suggesting that ZIPK is responsible for di-phosphorylation of myosin II in HPMCs. Although TGF-β significantly increased the expression of ZIP kinase protein, the change in ZIP kinase mRNA was marginal, suggesting a posttranscriptional mechanism for the regulation of ZIP kinase expression by TGF-β. ZIPK gene knockdown (KD) also significantly reduced TGF-β-induced upregulation of αSMA expression. This finding suggests that siZIPK attenuates myofibroblast differentiation of HPMCs. siZIPK diminished TGF-β-induced contractility of HPMCs consistent with siZIPK-induced decrease in the di-phosphorylation of myosin II RLC. The present results implicate ZIPK in the regulation of the contractility of HPMC-derived myofibroblasts, phenotype switching, and myofibroblast differentiation of HPMCs.NEW & NOTEWORTHY Here, we highlight that ZIP kinase is responsible for di-phosphorylation of myosin light chain, which facilitates stress fiber formation and actomyosin-based cell contraction during mesothelial to mesenchymal transition in human pleural mesothelial cells. This transition has a significant impact on tissue remodeling and subsequent stiffness of the pleura. This study provides insight into a new therapeutic strategy for the treatment of pleural fibrosis.

Skeletal and cardiac muscle calcium transport regulation in health and disease

In healthy muscle, the rapid release of calcium ions (Ca2+) with excitation-contraction (E-C) coupling, results in elevations in Ca2+ concentrations which can exceed 10-fold that of resting values. The sizable transient changes in Ca2+ concentrations are necessary for the activation of signaling pathways, which rely on Ca2+ as a second messenger, including those involved with force generation, fiber type distribution and hypertrophy. However, prolonged elevations in intracellular Ca2+ can result in the unwanted activation of Ca2+ signaling pathways that cause muscle damage, dysfunction, and disease. Muscle employs several calcium handling and calcium transport proteins that function to rapidly return Ca2+ concentrations back to resting levels following contraction. This review will detail our current understanding of calcium handling during the decay phase of intracellular calcium transients in healthy skeletal and cardiac muscle. We will also discuss how impairments in Ca2+ transport can occur and how mishandling of Ca2+ can lead to the pathogenesis and/or progression of skeletal muscle myopathies and cardiomyopathies.

Signaling pathways underlying changes in the contractility of the stomach fundus smooth muscle in diabetic rats

Dysfunction of gastrointestinal (GI) motility is a common complication in patients with diabetes mellitus (DM). Studies related to changes in fundus contraction induced by inhibitors in DM are not well known. Therefore, this study aimed to investigate the signaling pathways involved in the changes in the contraction of fundus smooth muscle obtained from control and DM rats. DM was induced by injecting streptozotocin (65 mg/kg) into Sprague-Dawley rats. The rats were sacrificed after 14 days. Fundus smooth muscle contraction was stimulated using electrical field stimulation (amplitude, 50 V; duration, 1 min; frequency, 2-20 Hz) and acetylcholine (0.1 mM). The inhibitor-mediated cell membrane was pre-treated with atropine, verapamil, methysergide, ketanserin, ondansetron, and GR 113808. Inhibitors related to intracellular signaling, such as U73122, chelerythrine, L-NNA, were also used. ML-9 and Y-27632 were identified as inhibitors of factors of myosin light chain (MLC). The contractility was observed to be lower in the DM group than in the control group. Further, the activities of phospholipase C (PLC), protein kinase C (PKC), and myosin light chain kinase (MLCK) were decreased in the DM group. DM reduced the activity of PLC, PKC, and MLCK, which resulted in a decrease in the contractility of the fundus smooth muscle. Therefore, our results present the mechanism of this DM-mediated GI disorder.

Atractylodin Induces Myosin Light Chain Phosphorylation and Promotes Gastric Emptying through Ghrelin Receptor

Atractylodin is one of the main constituents in the rhizomes of Atractylodes lancea Thunb., being capable of treating cancer cachexia-anorexia and age-related diseases as an agonist of growth hormone secretagogue receptor (GHSR). GHSR was herein expressed in human gastric smooth muscle cells (HGSMCs) and activated by ghrelin receptor agonist L-692,585. Like L-692,585, atractylodin also increased Ca²⁺ and enhanced the phosphorylation of myosin light chain (MLC) through GHSR in HGSMCs. In addition, atractylodin promoted gastric emptying and MLC phosphorylation in the gastric antrum of mice also through GHSR. Collectively, atractylodin can activate GHSR in gastric smooth muscle, as a potential target in clinical practice.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Pennogenin tetraglycoside induces rat myometrial contraction and MLC20 phosphorylation via PLC-IP(3) and RhoA/Rho kinase signaling pathways

BACKGROUND

Total steroidal saponins extracted from the rhizome of Paris polyphylla Sm. var. yunnanensis (TSSPs) have been widely used in China for the treatment of abnormal uterine bleeding. We previously studied the main active constituents of TSSPs and their structure-activity relationships with respect to rat myometrial contractions. Tg (pennogenin tetraglycoside) was identified as one of the active ingredients in TSSPs able to induce rat myometrial contractions. However, the mechanisms underlying the pharmacological actions on uterine activity have not been described clearly.

METHODS

Here Tg was screened for effects on contractile activity in isolated uterine strips from estrogen-primed rats and on MLC20 phosphorylation and related signaling pathways in cultured rat myometrial cells as determined by Western blot. Intracellular calcium ([Ca(2+)](i)) was monitored under a confocal microscope using Fluo-4 AM-loaded myometrial cells.

RESULTS

Tg dose-dependently stimulated rat myometrial contractions as well as MLC20 phosphorylation in vitro, which could be completely suppressed by an inhibitor of myosin light chain kinase (MLCK). Use of Ca(2+) channel blockers and kinase inhibitors demonstrated that Tg-induced myometrial contractions are mediated by activation of the phospholipase C (PLC)-inositol triphosphate (IP3) signaling pathway, resulting in increased MLC20 phosphorylation. Furthermore, Y27632, a specific inhibitor of Rho kinase (ROK), notably suppressed Tg-stimulated myometrial contractions and decreased MLC20 phosphorylation.

CONCLUSIONS

These data provide evidence that rat myometrial contractility induced by Tg results from enhanced MLC20 phosphorylation, while both PLC-IP3 and RhoA/ROK signaling pathways mediate the process. These mechanisms may be responsible for the therapeutic effects of TSSPs on abnormal uterine bleeding.

Deletion of SM-B, the high ATPase isoform of myosin, upregulates the PKC-mediated signal transduction pathway in murine urinary bladder smooth muscle

Detrusor smooth muscle (DSM) hypertrophy induced by partial bladder outlet obstruction (PBOO) is associated with changes in the NH2-terminal myosin heavy chain isoform from predominantly SM-B to SM-A, alteration in the Ca2+ sensitization pathway, and the contractile characteristics from phasic to tonic in rabbits. We utilized the SM-B knockout (KO) mouse to determine whether a shift from SM-B to SM-A without PBOO is associated with changes in the signal transduction pathway mediated via PKC and CPI-17, which keeps the myosin phosphorylation (MLC20) level high by inhibiting the myosin phosphatase. DSM strips from SM-B KO mice generated more force in response to electrical field stimulation, KCl, carbachol, and phorbol 12,13-dibutyrate than that of age-matched wild-type mice. There was no difference in the ED50 for carbachol but the maximum response was greater for the SM-B KO mice. DSM from SM-B KO mice revealed increased mass and hypertrophy. The KO mice also showed an overexpression of PKC-alpha, increased levels of phospho-CPI-17, and an elevated level of IP3 and DAG upon stimulation with carbachol. Two-dimensional gel electrophoresis revealed an increased level of MLC20 phosphorylation in response to carbachol. Together, these changes may be responsible for the higher level of force generation and maintenance by the DSM from the SM-B KO bladders. In conclusion, our data show that ablation of SM-B is associated with alteration of PKC-mediated signal transduction and CPI-17-mediated Ca2+ sensitization pathway that regulate smooth muscle contraction. Interestingly, similar changes are also present in PBOO-induced DSM compensatory response in the rabbit model in which SM-B is downregulated.

Regulation of the function of mammalian myosin and its conformational change

It has been known that the phosphorylation of the regulatory light chain, residing at the head/rod junction of the molecule activates the motor activity of smooth muscle and non-muscle conventional myosin (myosin II), and triggers a large conformational change of the molecule from the inhibited folded conformation to the active extended conformation. Recent structural analysis has revealed the structural basis of the inhibition of the motor function of the two heads in the inhibited conformation. On the other hand, recent studies have revealed that a processive unconventional myosin, myosin V, also shows a large change in the conformation from the folded to an extended form and this explains the activation mechanism of myosin V motor activity. These findings suggest the presence of a common scenario for the regulation of motor protein functions.

The latch-bridge hypothesis of smooth muscle contraction

In contrast to striated muscle, both normalized force and shortening velocities are regulated functions of cross-bridge phosphorylation in smooth muscle. Physiologically this is manifested as relatively fast rates of contraction associated with transiently high levels of cross-bridge phosphorylation. In sustained contractions, Ca2+, cross-bridge phosphorylation, and ATP consumption rates fall, a phenomenon termed "latch". This review focuses on the Hai and Murphy (1988a) model that predicted the highly non-linear dependence of force on phosphorylation and a directly proportional dependence of shortening velocity on phosphorylation. This model hypothesized that (i) cross-bridge phosphorylation was obligatory for cross-bridge attachment, but also that (ii) dephosphorylation of an attached cross-bridge reduced its detachment rate. The resulting variety of cross-bridge cycles as predicted by the model could explain the observed dependencies of force and velocity on cross-bridge phosphorylation. New evidence supports modifications for more general applicability. First, myosin light chain phosphatase activity is regulated. Activation of myosin phosphatase is best demonstrated with inhibitory regulatory mechanisms acting via nitric oxide. The second modification of the model incorporates cooperativity in cross-bridge attachment to predict improved data on the dependence of force on phosphorylation. The molecular basis for cooperativity is unknown, but may involve thin filament proteins absent in striated muscle.

Regional variation in myosin isoforms and phosphorylation at the resting tone in urinary bladder smooth muscle

Urinary bladder filling and emptying requires coordinated control of bladder body and urethral smooth muscles. Bladder dome, midbladder, base, and urethra showed significant differences in the percentage of 20-kDa myosin light chain (LC20) phosphorylation (35.45 +/- 4.6, 24.7 +/- 2.2, 13.6+/- 2.1, and 12.8 +/- 2.7%, respectively) in resting muscle. Agonist-mediated force was associated with a rise in LC20 phosphorylation, but the extent of phosphorylation at all levels of force was less for urethral than for bladder body smooth muscle. RT-PCR and quantitative competitive RT-PCR analyses of total RNA from bladder body and urethral smooth muscles revealed only a slight difference in myosin heavy chain mRNA copy number per total RNA, whereas mRNA copy numbers for NH2-terminal isoforms SM-B (inserted) and SM-A (noninserted) in these muscles showed a significant difference (2.28 x 10(8) vs. 1.68 x 10(8) for SM-B and 0.12 x 10(8) vs. 0.42 x 10(8) for SM-A, respectively), which was also evident at the protein level. The ratio of COOH-terminal isoforms SM2:SM1 in the urethra was moderately but significantly lower than that in other regions of the bladder body. A high degree of LC20 phosphorylation and SM-B in the bladder body may help to facilitate fast cross-bridge cycling and force generation required for rapid emptying, whereas a lower level of LC20 phosphorylation and the presence of a higher amount of SM-A in urethral smooth muscle may help to maintain the high basal tone of urethra, required for urinary continence.

Myosin light chain phosphorylation at resting level and the composition of myosin isoforms in the bladder body and urethra

Bladder filling depends upon the coordinated control of a storage chamber, the bladder body, and its outlet, the bladder base and urethra. Bladder emptying results from development of force in the bladder body and relaxation of the outlet. Muscle strips from bladder body reveal phasic characteristics, whereas the strips from urethral wall are tonic. To determine whether the compositions of myosin heavy chain (MHC) isoforms and the level of myosin light chain (MLC) phosphorylation contribute to the regional variation in the contractile states of the bladder smooth muscle, we analyzed the levels of MLC phosphorylation and the expression of myosin isoforms in smooth muscle tissues from different regions of the urinary bladder. Strips of bladder from the dome, mid body, base of the bladder and urethra were removed and analyzed for the levels of MLC phosphorylation at the resting tone. The expression of MHC isoforms that differ in the C-terminus (SM1 and SM2) and in the N-terminal region (SM-A and SM-B), formed by alternative splicing of the pre-mRNA at either the 3' end or the 5' end, respectively, was analyzed. The expression of these isoforms was characterized at the mRNA and protein levels using reverse transcriptase-polymerase chain reaction (RT-PCR), SDS-PAGE, and Western blotting. The levels of MLC phosphorylation were 35.5 +/- 4.6, 24.7 +/- 2.2, 13.6 +/- 2.1, and 12.8 +/- 2.7 for dome, mid bladder body, base and urethra respectively. Almost 100% of the MHC mRNA in the dome, mid bladder body, and base contains a 7-amino acid insert near the ATP-binding region, whereas the MHC in the urethral smooth muscle is only 81% inserted. Prior studies have shown that inserted myosin has a two-fold higher actin-activated ATPase activity compared to the myosin isoform that lacks the insert, and the maximum velocity of shortening of smooth muscle containing this insert is high compared to muscle that do not contain the insert. The expression of SM1 and SM2 were not significantly different. Our data suggests the presence of a high degree of inserted myosin and LC20 phosphorylation in the bladder dome and mid-body helps to facilitate rapid force development and emptying. Non-inserted myosin and the low level of MLC phosphorylation in the urethra may contribute to slowly or non-cycling myosin cross bridges and the maintenance of a tonic or contracted state during bladder filling.

Characterization of the functional domains on the C-terminal region of caldesmon using full-length and mutant caldesmon molecules

A series of C-terminal deletion mutants of chicken gizzard smooth muscle caldesmon (CaD) were made using a polymerase chain reaction cloning strategy and a baculovirus expression system, and the precise locations of the functional domains of CaD involved in the regulation of actomyosin ATPase and the binding of actin, tropomyosin, and calmodulin were analyzed. Our results reveal a high affinity calmodulin-binding domain that consists of at least three calmodulin-binding determinants localized in residues 690-717, 658-689, and 628-657. The residues between positions 718 and 756 and positions 598 and 627 have no detectable calmodulin-binding site. A high affinity tropomyosin-binding domain is located between residues 718 and 756. The 159 residues at the C terminus of CaD contain multiple actin-binding determinants; the major ones are localized in the regions between residues 718 and 756 and residues 690 and 717. The amino acid residues between positions 718 and 756 contain the major determinant involved in the inhibition of the actin activation of smooth muscle myosin ATPase since CaD-(1-717) caused only 30% of the inhibition produced by the full-length CaD. Further deletion between residues 690 and 717 (CaD-(1-689) revealed a low level (10% of that seen for full-length CaD) of inhibition of the actomyosin ATPase. These data clearly demonstrate that the region of the last 66 amino acid residues at the CaD C terminus contains two or more major actin-binding motifs, one tropomyosin-binding domain, one high affinity calmodulin-binding determinant, and the domain that is responsible for the inhibition of the actin-activated ATPase of myosin.

Phosphorylation of vertebrate nonmuscle and smooth muscle myosin heavy chains and light chains

In this article we review the various amino acids present in vertebrate nonmuscle and smooth muscle myosin that can undergo phosphorylation. The sites for phosphorylation in the 20 kD myosin light chain include serine-19 and threonine-18 which are substrates for myosin light chain kinase and serine-1 and/or -2 and threonine-9 which are substrates for protein kinase C. The sites in vertebrate smooth muscle and nonmuscle myosin heavy chains that can be phosphorylated by protein kinase C and casein kinase II are also summarized. Original data indicating that treatment of human T-lymphocytes (Jurkat cell line) with phorbol 12-myristate 13-acetate results in phosphorylation of both the 20 kD myosin light chain as well as the 200 kD myosin heavy chain is presented. We identified the amino acids phosphorylated in the human T-lymphocytes myosin light chains as serine-1 or serine-2 and in the myosin heavy chains as serine-1917 by 1-dimensional isoelectric focusing of tryptic phosphopeptides. Untreated T-lymphocytes contain phosphate in the serine-19 residue of the myosin light chain, and in a residue tentatively identified as serine-1944 in the myosin heavy chain.

Evidence that unphosphorylated smooth muscle myosin supports smooth muscle contraction

Unphosphorylated smooth muscle myosin filaments do not disassemble in MgATP, provided that the solution is supplemented either by 25% serum albumin or by 6% polyethylene glycol 6000. These filaments are able to support actomyosin retraction but their ATPase activity is not activated by tropomyosin-decorated F-actin.

Caldesmon and the structure of smooth muscle thin filaments: electron microscopy of isolated thin filaments

Native and synthetic vertebrate smooth muscle thin filaments have been examined by electron microscopy in order to determine the arrangement of the regulatory protein caldesmon. In synthetic filaments of actin-caldesmon, long slender molecules were sometimes seen running along the thin filament, suggesting that caldesmon can associate with actin along its length, while at other times lateral projections were observed. In native filaments, containing actin, caldesmon and tropomyosin, we found no evidence for lateral projections extending from the filaments, suggesting that caldesmon does not act as a crosslinking protein in vivo. In contrast, elongated molecules were clearly seen following the long pitch actin helices. We suggest that these may represent an association of caldesmon and tropomyosin. Antibodies developed against an N-terminal fragment of caldesmon caused thin filaments to aggregate laterally into arrays displaying approximately 35-38 nm repeats; thin filament aggregates with this periodicity were obtained previously (Lehman et al., 1989) using antibodies to the C-terminal segment of caldesmon. These results suggest that both ends of caldesmon are closely associated with the shaft of the thin filament, supporting a model in which the elongated caldesmon molecule runs along the filament, possibly interacting with tropomyosin, following the long pitch actin helices.

Caldesmon and the structure of smooth muscle thin filaments: immunolocalization of caldesmon on thin filaments

Antibodies reacting with chicken gizzard caldesmon were used to determine the distribution of caldesmon on smooth muscle thin filaments. Antibodies developed against both the intact caldesmon molecule and a 40 kilodalton proteolytic fragment cause thin filaments to aggregate laterally. Aggregates produced with the latter antibody display regular periodic labelling with a repeat of approximately 38 nm, a distribution characteristic of proteins associated with tropomyosin on thin filaments. The stoichiometry of caldesmon on thin filaments has been critically reevaluated and alternative models of caldesmon distribution on thin filaments are proposed.

Effects of ATP reduction on the pattern of force development and myosin light chain phosphorylation in intact arterial smooth muscle

The effect of reduction of ATP content on phosphorylation of the 20 kDa light chain of myosin (MLC) and force development in intact carotid arterial smooth muscle was investigated. With reduction of ATP to 23% of control by treatment with 2-deoxyglucose there was reduction in basal, in peak and 30 min MLC phosphorylation during contraction (P less than 0.001). The rate of force development was reduced, but maximal force was the same as control. By treatment with 50 microM iodoacetate, the resting ATP content was unchanged but fell to 22% after 30 min contraction. Basal MLC phosphorylation was the same as control, but peak (P less than 0.001) and 30 min phosphorylation were lower (P less than 0.005), even though the rate and magnitude of force development were greater. The results indicate that neither rate nor magnitude of force development correlate with MLC phosphorylation. Basal and initial MLC phosphorylation may play a cooperative role in contractile function.

Organization of the hTMnm gene. Implications for the evolution of muscle and non-muscle tropomyosins

We have isolated clones of human genomic DNA which contain the structural elements of the hTMnm gene. In non-muscle tissue this gene produces a 2.5 kb (1 kb = 10(3) bases or base-pairs) mRNA encoding TM30nm, a 248 amino acid cytoskeletal tropomyosin. In muscle, alternative splicing of this gene results in the expression of a 1.3 kb mRNA encoding a 285 amino acid skeletal muscle alpha-tropomyosin. The hTMnm gene spans at least 42 kb of DNA and consists of 13 exons, only five of which are common to both the 2.5 kb and 1.3 kb transcripts. The boundaries of the exons giving rise to the muscle-specific isoform are identical to the base to those of other genes encoding muscle tropomyosins. A comparison of the structures of exons encoding the amino-terminal sequences of the muscle and non-muscle isoforms suggests that the hTMnm gene has evolved by a specific pattern of exon duplication with alternative splicing.

Modulation of monomer-polymer equilibrium of phosphorylated smooth muscle myosin: effects on actin activation

Actin activation of the adenosinetriphosphatase (ATPase) of phosphorylated gizzard myosin at low (2 mM) free Mg2+ concentration and 50 mM total ionic strength continues to increase on raising the free Ca2+ concentration near pCa 3. Similar levels of activity can be obtained by increasing the free Mg2+ concentration to a higher (in excess of 4 mM free) concentration. In the presence of micromolar concentrations of free Ca2+ and low free Mg2+ concentration, the actin-activated adenosine 5'-triphosphate (ATP) hydrolysis exhibits an initial rapid rate which progressively slows to a final, lower but more linear rate. In the presence of high divalent cation concentrations, the fast rate of ATP hydrolysis is maintained during the entire ATPase assay. The ionic conditions which favor the slow rate of ATP hydrolysis are correlated with increased proportions of folded myosin monomers while higher rates of ATP hydrolysis are correlated with increased levels of aggregated myosin. Elevating the thin filament proteins to saturating concentrations does not abolish the change in ATPase rate or the final distribution of myosin aggregates and monomers; however, the stability of the myosin aggregates is enhanced by the presence of thin filament proteins in low divalent cation conditions. The nonlinear profile of the actin-activated ATP hydrolysis in low divalent cation concentrations is eliminated by utilizing nonfilamentous, phosphorylated heavy meromyosin. The data presented indicate that Ca2+ and Mg2+ alter monomer-polymer equilibrium of stably phosphorylated myosin. The alteration of monomer-polymer equilibrium by Ca2+ at low Mg2+ concentration modulates ATPase rates.

Relationship between length-tension relation and 20,000 dalton myosin light chain phosphorylation in guinea-pig taenia caeci

1. Relationship between length-tension relation and phosphorylation of 20,000 dalton myosin light chain (LC20) in guinea-pig taenia caeci was investigated. 2. At in situ length (Lb), a good linear correlation was obtained between isometric tension and LC20 phosphorylation in high-K+-stimulated muscle. 3. In 100 mM K+-stimulated muscle, the active tension decreased at muscle lengths other than Lb, but no significant decrease in degree of LC20 phosphorylation was observed. 4. These results suggest that in guinea-pig taenia caeci, the major portion of the decrease in active tension at muscle lengths other than Lb is not due to a decrease in degree of activation.

Biochemical regulation of airway smooth muscle tone: current knowledge and therapeutic implications

Evidence collected during the last decade indicates that the molecular processes responsible for smooth muscle contraction are fundamentally different from those responsible for skeletal muscle contraction. Furthermore, because of the diverse functional roles of various smooth muscles, it would not be surprising if significant differences in regulatory processes also exist among different smooth muscles. Such diversity may already be exemplified by differences in cross-bridge kinetics and sources of activator Ca2+. Additional unique regulatory features of various smooth muscle types will undoubtedly be uncovered by further research. A convincing body of data suggests that activation of the adenylate cyclase/protein kinase cascade is responsible for the bronchodilation produced by beta-adrenoceptor agonists. Although the exact mechanism by which the activation of cAMP-dependent protein kinase induces relaxation is not clear, the phosphorylation of multiple substrates may be involved. Phosphorylation of these substrates can promote relaxation by decreasing the myoplasmic Ca2+ concentration, decreasing the Ca2+ sensitivity of the contractile apparatus, or both. Thus, because beta-adrenoceptor agonists act as physiologic antagonists of broncho-constriction, they should relax airway smooth muscle regardless of the mediator(s) responsible for the bronchospasm. Perhaps this is the major reason that the beta-adrenoceptor agonists have become the premier class of drugs used in the treatment of bronchial asthma. As useful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: cardiovascular and skeletal muscle side effects, an inherent subsensitivity of the asthmatic patient population to beta-adrenoceptor agonists, the development of tolerance and a loss of efficacy during severe asthmatic episodes. The fact that these drawbacks are probably shared by all sympathomimetic bronchodilators suggests that little therapeutic advantage will be gained by developing new beta-adrenoceptor agonists. The task of developing novel bronchodilators will be facilitated by an understanding at the molecular level of the diversity among smooth muscles and the processes that regulate smooth muscle tone. Hopefully, such knowledge will lead to a new generation of highly effective, tissue-selective bronchodilators with significant therapeutic advantages over those currently available.

A simple and rapid preparation of fully phosphorylated and fully dephosphorylated skeletal muscle myosin. Application to the preparation of a phosphorylated LC2-modified artificial isozyme

Fast skeletal myosin LC2 is phosphorylated on ser-15 by a specific myosin light chain kinase (MLCK) in the presence of Ca2+ and calmodulin, and dephosphorylated by a muscle phosphate in the presence of Mg2+. Fully dephosphorylated myosin is obtained by dialysis of muscle crude extract (0.06 M NaCl, 0.01 M Tris-HCl, pH 7.5, 50 microM EGTA); fully phosphorylated myosin is obtained by addition of Ca2+ (0.2 mM), Mg2+ (10 mM) and ATP (3 mM) and 5 min incubation at 28 degrees C. The following reaction characteristics were noted. The crude extract is a very efficient phosphorylating complex and can be diluted to phosphorylate or dephosphorylate purified myosin. Phosphorylation and dephosphorylation appear monophasic, showing no evidence of negative cooperativity in this particular type of myosin and medium. Phosphorylation is 24 times slower in the presence of 0.45 M KCl, 5 mM pyrophosphate. Thiophosphorylated myosin is slowly dephosphorylated by phosphatase. At the crude myosin stage the dephosphorylation reaction is efficiently inhibited (at 0-4 degrees C) by the presence of 70 mM NaF. Myosin-[(T)-LC2'] (a myosin species in which LC2 has been selectively modified by trypsin) is an interesting species refractory to phosphorylation. The myosin-[(T)-LC2'] isozyme can be obtained fully phosphorylated by phosphorylation of myosin followed by limited tryptic proteolysis as described earlier. Urea-PAGE as used separates LC2, phosphoryl-LC2, LC2' and phosphoryl-LC2' effectively and in this order. Through this procedure the (de)-phosphorylating complex is ipso facto specific to the myosin species considered; the method avoids lengthy preparations of purified proteins and is easy, rapid and efficient.

Phosphorylation of myosin in non-muscle and smooth muscle cells. Possible rules and evolutionary trends

Reversible phosphorylation of myosin subunits is observed in almost all eukaryotic cells. The data concerning sites and effects of phosphorylation on actin-activated ATPase activity of myosin and on its filament formation are described. These observations are discussed in terms of possible evolutionary trends and rules which may govern the process of myosin phosphorylation.

Modulation of smooth muscle actomyosin ATPase by thin filament associated proteins

Caldesmon binds equally to both gizzard actin and actin containing stoichiometric amounts of bound tropomyosin. The binding of caldesmon to actin inhibits the actin-activation of the Mg-ATPase activity of phosphorylated myosin only when the actin contains bound tropomyosin. The reversal of this inhibition requires Ca2+-calmodulin; but it occurs without complete release of bound caldesmon. Although phosphorylation of the caldesmon occurs during the ATPase assay, a direct correlation between caldesmon phosphorylation and the release of the inhibited actomyosin ATPase is not consistently observed.

Myosin light chain phosphorylation and evidence for latchbridge formation in norepinephrine stimulated canine veins

The role of 20000 dalton myosin light chain phosphorylation in mediating venous smooth muscle contraction was studied in isolated preparations of canine jugular and femoral vein. One min 10(-5) M norepinephrine-induced contraction was accompanied by significant increases in phosphorylation (jugular - 21 to 46%; femoral - 19 to 54%) which were reversed within 10 min after agonist washout. During 40 min stimulation, phosphorylation and isometric force redevelopment rates declined to near basal levels while force was maintained. These findings implicate light chain phosphorylation as a prerequisite for initial tension development by crossbridge cycling in venous smooth muscle. However, long term tension can be maintained through a process similar to the latchbridge state in tracheal and arterial smooth muscle.

Caldesmon association with smooth muscle thin filaments isolated in the presence and absence of calcium

Thin filaments isolated from chicken gizzard smooth muscle in either the presence or the absence of Ca2+ possess identical caldesmon contents. Hence, a 'flip-flop' mechanism, involving Ca2+-dependent association and dissociation of caldesmon and thin filaments, does not appear to operate in vivo and is an unlikely model for caldesmon function.

The effect of leiotonin fraction on stably phosphorylated smooth muscle myosin

This study was designed to determine the effect of leiotonin on the actin-activation once the myosin is stably phosphorylated. Gizzard myosin was stably phosphorylated by ATP-gamma-S using the gizzard light chain kinase. Addition of leiotonin preparation to phosphorylated myosin reconstituted with actin and tropomyosin did not alter the ATPase activity. Furthermore, leiotonin did not confer the calcium sensitivity of the ATPase activity. These experiments show that the actin-activated ATPase activity of stably phosphorylated gizzard myosin is not altered by leiotonin.

Effects of Ca2+ and Mg2+ on the actomyosin adenosine-5'-triphosphatase of stably phosphorylated gizzard myosin

There are conflicting reports on the effect of Ca2+ on actin activation of myosin adenosine-triphosphatase (ATPase) once the light chain is fully phosphorylated by a calcium calmodulin dependent kinase. Using thiophosphorylated gizzard myosin, Sherry et al. [Sherry, J. M. F., Gorecka, A., Aksoy, M. O., Dabrowska, R., & Hartshorne, D. J. (1978) Biochemistry 17, 4417-4418] observed that the actin activation of ATPase was not inhibited by the removal of Ca2+. Hence, it was suggested that the regulation of actomyosin ATPase activity of gizzard myosin by calcium occurs only via phosphorylation. In the present study, phosphorylated and thiophosphorylated myosins were prepared free of kinase and phosphatase activity; hence, the ATPase activity could be measured at various concentrations of Ca2+ and Mg2+ without affecting the level of phosphorylation. The ATPase activity of myosin was activated either by skeletal muscle or by gizzard actin at various concentrations of Mg2+ and either at pCa 5 or at pCa 8. The activation was sensitive to Ca2+ at low Mg2+ concentrations with both actins. Tropomyosin potentiated the actin-activated ATPase activity at all Mg2+ and Ca2+ concentrations. The calcium sensitivity of phosphorylated and thiophosphorylated myosin reconstituted with actin and tropomyosin was most pronounced at a free Mg2+ concentration of about 3 mM. The binding of 125I-tropomyosin to actin showed that the calcium sensitivity of ATPase observed at low Mg2+ concentration is not due to a calcium-mediated binding of tropomyosin to F-actin. The actin activation of both myosins was insensitive to Ca2+ when the Mg2+ concentration was increased above 5 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of alpha-tropomyosin sequences from smooth and striated muscle

Tropomyosins are a closely related family of proteins with a dimeric alpha-coiled-coil structure. Skeletal isoforms are composed of two types of subunits, alpha and beta which, in turn, are assorted into two main molecular species alpha alpha and alpha beta. Both isoforms are present in different molar ratios in individual skeletal muscle types. In small mammals, however, only alpha-chain is expressed in cardiac muscle. Tropomyosin, in association with the troponin complex (troponin-I, -T and -C) plays a central role in the Ca2+-dependent regulation of vertebrate striated muscle contraction. On the other hand, despite structural similarities with the striated isoforms, the function of this protein in smooth muscle and non-muscle cells remains unknown, because in these cells contraction is thought to be regulated by myosin-linked processes independently of tropomyosin. Here we report the nucleotide sequences of cloned complementary DNAs for rat striated and smooth muscle alpha-tropomyosin. Comparison of the derived amino-acid sequences reveals the existence of tissue-specific peptides that delimit the putative troponin-I and troponin-T binding domains of tropomyosin. S1-nuclease mapping studies reveal the existence of three distinct alpha-tropomyosin messenger RNA isoforms each encoding a different protein; these isoforms are tissue-specific, developmentally regulated and most probably encoded by the same gene.

Selective purification of the 20,000-Da light chains of smooth muscle myosin

The 20,000-Da light chains of gizzard smooth muscle myosin have been purified to homogeneity. Actomyosin, prepared by MgATP extraction of myofibrils, was denatured in 8 M urea, 1 M guanidine HCl, and 0.05% sodium dodecyl sulfate. Myosin heavy chains were precipitated with ethanol and the light chain enriched fraction was dialyzed and subjected to chromatography on DEAE-Sephacel. Fractions containing the 20,000-Da light chains were further purified by hydrophobic chromatography on phenyl-Sepharose. The 20,000-Da light chains eluted at low ionic strength from the phenyl-Sepharose column were judged to be greater than 95% pure by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and contained only 0.04 mol of phosphate/mol of light chain. The yield of light chains was calculated to be 219 +/- 17 mg/kg of starting gizzard smooth muscle. This method may be useful for preparation of homogeneous 20,000-Da smooth muscle myosin light chains in the quantities necessary for study of contractile systems.

Activation of contraction and ATPase activity in intact and chemically skinned smooth muscle of rat portal vein. Dependence on Ca++ and muscle length

The mechanical manifestations of muscle contraction (force development or shortening) are accompanied by an increased turnover of chemical energy (ATPase activity, JATP). In intact rat portal veins activated by high potassium medium to produce graded contractions at different levels of extracellular calcium, a linear dependence of oxygen consumption on force was found. The slope of the relation (metabolic tension cost) was higher during early stages than during late stages of contraction, possibly reflecting a transient high crossbridge ATP turnover rate. Chemically skinned (Triton X-100) rat portal vein preparations were used to study the energy turnover of the smooth muscle contractile system under constant activation. In these preparations, JATP increased on activation by calcium in the presence of 1 microM calmodulin, remained constant for maintained contractures, and decreased promptly on relaxation. Force declined with each repeated contraction at optimal calcium level (10(-4.5) M), but the relation between force and JATP remained invariant and agreed with that of the intact muscle during the transient of high ATP turnover. Calcium activation in the range 10(-9) to 10(-4.5) M caused a progressively steeper (nonlinear) increase in JATP with force. Length-force relations showed a lower relative force at muscle lengths below L0 in the intact than in the skinned tissue, possibly indicating depression of excitation-contraction coupling. The slope of the relation between JATP and force was lower when length was varied than when calcium was varied. At a length where no external force was produced, the activated muscle had a significantly higher JATP than while relaxed (10(-9) M Ca++).(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the myosin heavy chain. Its effect on actin-activated Mg2+-stimulated ATPase in leukaemic myeloblasts

Myosin purified from a murine myeloid leukaemia cell line (M1) that had been incubated with [32P]orthophosphate incorporated 32P into the heavy, but not the light, chain. When the heavy chain was dephosphorylated by bacterial alkaline phosphatase, myosin that had low actin-activated ATPase activity gained higher activity only in the presence of the light-chain kinase. In the absence of the light-chain kinase, however, the Mg2+-stimulated ATPase activity of myosin was not activated by actin, regardless of phosphatase treatment. These results indicate that the activity of M1 myosin ATPase is regulated by phosphorylation of both the light and heavy chains. A scheme for this regulation by phosphorylation is presented and discussed.