Phosphorylation of a light chain component of myosin from smooth muscle

Vascular calcium signalling and ageing

Changes in cellular Ca²⁺ levels have major influences on vascular function and blood pressure regulation. Vascular smooth muscle cells (SMCs) and endothelial cells (ECs) orchestrate vascular activity in distinct ways, often involving highly specific fluctuations in Ca²⁺ signalling. Ageing is a major risk factor for cardiovascular diseases, but the impact of ageing per se on vascular Ca²⁺ signalling has received insufficient attention. We reviewed the literature for age-related changes in Ca²⁺ signalling in relation to vascular structure and function. Vascular tone dysregulation in several vascular beds has been linked to abnormal expression or activity of SMC voltage-gated Ca²⁺ channels, Ca²⁺ -activated K⁺ channels or TRPC6 channels. Some of these effects were linked to altered caveolae density, microRNA expression or 20-HETE abundance. Intracellular store Ca²⁺ handling was suppressed in ageing mainly via reduced expression of intracellular Ca²⁺ release channels, and Ca²⁺ reuptake or efflux pumps. An increase in mitochondrial Ca²⁺ uptake, leading to oxidative stress, could also play a role in SMC hypercontractility and structural remodelling in ageing. In ECs, ageing entailed diverse effects on spontaneous and evoked Ca²⁺ transients, as well as structural changes at the EC-SMC interface. The concerted effects of altered Ca²⁺ signalling on myogenic tone, endothelium-dependent vasodilatation, and vascular structure are likely to contribute to blood pressure dysregulation and blood flow distribution deficits in critical organs. With the increase in the world's ageing population, future studies should be directed at solving specific ageing-induced Ca²⁺ signalling deficits to combat the imminent accelerated vascular ageing and increased risk of cardiovascular diseases.

Smooth muscle function and myosin polymerization

Smooth muscle is able to function over a much broader length range than striated muscle. The ability to maintain contractility after a large length change is thought to be due to an adaptive process involving restructuring of the contractile apparatus to maximize overlap between the contractile filaments. The molecular mechanism for the length-adaptive behavior is largely unknown. In smooth muscle adapted to different lengths we quantified myosin monomers, basal and activation-induced myosin light chain (MLC) phosphorylation, shortening-velocity, power-output and active force. The muscle was able to generate a constant maximal force over a 2-fold length range when it was allowed to go through isometric contraction/relaxation cycles after each length change (length adaptation). In the relaxed state myosin monomer concentration and basal MLC phosphorylation decreased linearly, while in the activated state activation-induced MLC phosphorylation and shortening-velocity/power-output increased linearly with muscle length. The results suggest that recruitment of myosin monomers and oligomers into the actin filament lattice (where they form force-generating filaments) occurs during muscle adaptation to longer length with the opposite occurring during adaptation to shorter length.

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

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

Development and maintenance of force and stiffness in airway smooth muscle

Airway smooth muscle (ASM) plays a central role in the excessive narrowing of the airway that characterizes the primary functional impairment in asthma. This phenomenon is known as airway hyper-responsiveness (AHR). Emerging evidence suggests that the development and maintenance of ASM force involves dynamic reorganization of the subcellular filament network in both the cytoskeleton and the contractile apparatus. In this review, evidence is presented to support the view that regulation of ASM contraction extends beyond the classical actomyosin interaction and involves processes within the cytoskeleton and at the interfaces between the cytoskeleton, the contractile apparatus, and the extracellular matrix. These processes are initiated when the muscle is activated, and collectively they cause the cytoskeleton and the contractile apparatus to undergo structural transformation, resulting in a more connected and solid state that allows force generated by the contractile apparatus to be transmitted to the extracellular domain. Solidification of the cytoskeleton also serves to stiffen the muscle and hence the airway. Oscillatory strain from tidal breathing and deep inspiration is believed to be the counter balance that prevents hypercontraction and stiffening of ASM in vivo. Dysregulation of this balance could lead to AHR seen in asthma.

Purification of myosin light chain kinase from Limulus muscle

It has previously been shown that the regulatory light chains of myosin from Limulus, the horseshoe crab, can be phosphorylated either by purified turkey gizzard smooth muscle myosin light chain (MLC) kinase or by a crude kinase fraction prepared from Limulus muscle [Sellers, J. R. (1981) J. Biol. Chem. 256, 9274-9278]. This phosphorylation was shown to be associated with a 20-fold increase in the actin-activated MgATPase activity of the myosin. We have now purified the Ca2+-calmodulin-dependent MLC kinase from Limulus muscle to near homogeneity by using a combination of low ionic strength extraction, ammonium sulfate fractionation, and chromatography on Sephacryl S-300 and DEAE-Sephacel. The final purification was achieved by affinity chromatography on a calmodulin-Sepharose 4B column. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed 95% of the protein to be comprised of a doublet with Mr = 39000 and 37000. Electrophoresis of the kinase fraction under nondenaturing conditions resulted in a partial separation of the two major bands and demonstrated that each had catalytic activity. An SDS-polyacrylamide gel overlayed with 125I-calmodulin demonstrated that both the Mr 39K and the Mr 37K proteins bind calmodulin. Neither of the bands could be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase. With Limulus myosin light chains as a substrate, the Vmax was 15.4 mumol min-1 mg-1, and the Km was 15.6 microM. The KD for calmodulin was determined to be 6 nM. The enzyme did not phosphorylate histones, casein, actin, or tropomyosin.

Peptide modulators of myosin light chain kinase affect smooth muscle cell contraction

To examine the importance of myosin light chain kinase (MLCK) in the initiation of contraction in smooth muscle, we used a constitutively active form of MLCK (IMLCK) and two specific peptide inhibitors of MLCK to study the activation of skinned single smooth muscle cells. Although unregulated by Ca-calmodulin, IMLCK, in vitro, was found to have biochemical properties like those of MLCK. Upon photolysis of caged ATP, IMLCK caused Ca-free shortening of skinned cells similar in time course and extent to that induced by Ca2+. Two peptide probes, RS-20 and SM-1, patterned after the Ca-calmodulin binding site and a pseudosubstrate inhibitory site, respectively, of the native MLCK molecule, were shown to specifically inhibit MLCK in in vitro experiments. Both peptides dose dependently inhibited Ca-induced shortening of skinned single cells. These results indicate that MLCK plays an essential role in the activation process in the smooth muscle cell in that activation of this enzyme is both necessary and sufficient for the initiation of contraction.

Myosin dephosphorylation during rapid relaxation of hog carotid artery smooth muscle

Changes in myosin light chain phosphorylation were measured during histamine-induced rhythmic contractions of hog carotid artery smooth muscle strips. Histamine made the muscle strips contract spontaneously every 1-5 min, and this allowed measurement of the time course of phosphorylation in relation to force development under conditions where diffusion of the agonist through tissue would not complicate the interpretation of the data. In the absence of histamine, phosphorylation was low [0.12 +/- 0.04 mol P/mol of the 20,000-Da light chain (LC 20)]. Phosphorylation was slightly (but not significantly) higher in the presence of 10 microM histamine in the relaxed state between contractions (0.20 +/- 0.03 mol P/mol LC 20). In muscle strips frozen during force development, when force had reached half of its peak value, phosphorylation was 0.38 +/- 0.06 mol P/mol LC 20. The highest levels of phosphorylation (0.49 +/- 0.04 mol P/mol LC 20) were found in strips frozen at the peak of the rhythmic contractions. Strips frozen when force had declined to half of the peak force showed low levels of phosphorylation (0.17 +/- 0.07 mol P/mol LC 20), indicating that the myosin light chain phosphatase activity was quite high. Mathematical modeling of the kinase and phosphatase reactions suggested that the apparent first-order phosphatase rate constant was at least 0.08 s-1 under these conditions. To obtain a better estimate of this rate constant, a second series of phosphorylation measurements were made early in the relaxation phase of the rhythmic contractions. The highest phosphatase rate constant obtained from these measurements was 0.23 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of actin microfilament integrity in living nonmuscle cells by the cAMP-dependent protein kinase and the myosin light chain kinase

Microinjection of the catalytic subunit of cAMP-dependent protein kinase (A-kinase) into living fibroblasts or the treatment of these cells with agents that elevate the intracellular cAMP level caused marked alterations in cell morphology including a rounded phenotype and a complete loss of actin microfilament bundles. These effects were transient and fully reversible. Two-dimensional gel electrophoresis was used to analyze the changes in phosphoproteins from cells injected with A-kinase. These experiments showed that accompanying the disassembly of actin microfilaments, phosphorylation of myosin light chain kinase (MLCK) increased and concomitantly, the phosphorylation of myosin P-light chain decreased. Moreover, inhibiting MLCK activity via microinjection of affinity-purified antibodies specific to native MLCK caused a complete loss of microfilament bundle integrity and a decrease in myosin P-light chain phosphorylation, similar to that seen after injection of A-kinase. These data support the idea that A-kinase may regulate microfilament integrity through the phosphorylation and inhibition of MLCK activity in nonmuscle cells.

Rod phosphorylation favors folding in a catch muscle myosin

Myosin from a molluscan catch muscle is unusual in being phosphorylated in the rod by an endogenous heavy chain kinase. The overall structure of the molecule resembles that of other muscle myosins, although the tail is somewhat longer (approximately equal to 1700 A). At low ionic strength the unphosphorylated molecules associate in filaments that display a striking axial repeat of 145 A. Phosphorylation of the rod enhances myosin solubility in the range of NaCl between 0.05 and 0.15 M. Depending on the ionic strength and the counterions present, the soluble species corresponds to an antiparallel folded dimer (15 S) or to a folded monomer (10 S). Unphosphorylated myosin can also be partially solubilized into folded monomers by addition of ATP in 0.15 M NaCl. A similar molecular folding has also been observed in smooth muscle and nonmuscle myosins that depends, however, on the state of phosphorylation of the light chains in the myosin head. We discuss these results in relation to possible mechanisms for control of catch contraction.

Myosin rod phosphorylation and the catch state of molluscan muscles

"Catch" is a prolonged state of tension in molluscan smooth muscles shown by mechanical measurements to be associated with the level of protein phosphorylation. Myosin isolated from these muscles is unusual in being phosphorylated in the rod portion by an endogenous kinase, like certain nonmuscle myosins. These findings suggest that the myosin rod is a target for phosphorylation and that this reaction may control the transition from catch to relaxation.

Protein phosphorylation in cultured endothelial cells

We have investigated the protein phosphorylation systems present in cultured bovine aortic and pulmonary artery endothelial cells. The cells contain cyclic AMP-dependent protein kinase, three calcium/calmodulin-dependent protein kinases, protein kinase C, and at least one tyrosine kinase. No cyclic GMP-dependent protein kinase activity was found. The cells also contained numerous substrates for cyclic AMP-dependent protein kinase and protein kinase C. Fewer substrates were found for the calcium/calmodulin-dependent protein kinases. There was little difference between either protein kinase activities or substrates when pulmonary artery endothelium was compared to aortic endothelium grown under similar culture conditions. It is likely that these various protein kinases and their respective substrate proteins are involved in mediating several of the actions of the hormones and drugs which affect the vascular endothelium.

Kinetic model for isometric contraction in smooth muscle on the basis of myosin phosphorylation hypothesis

A kinetic model was proposed to simulate an isometric contraction curve in smooth muscle on the basis of the myosin phosphorylation hypothesis. The Ca2+-calmodulin-dependent activation of myosin light-chain kinase and the phosphorylation-dephosphorylation reaction of myosin were mathematically treated. Solving the kinetic equations at a steady state, we could calculate the relationship between the Ca2+ concentration and the myosin phosphorylation. Assuming that two-head-phosphorylated myosin has an actin-activated Mg2+-ATPase activity and that this state corresponds to an active state, we computed the time courses of the myosin phosphorylation and the active state for various Ca2+ transients. The time course of the active state was converted into that of isometric tension by use of Sandow's model composed of a contractile element and a series elastic component. The model could simulate not only the isometric contraction curves for any given Ca2+ transient but also the following experimental results: the calmodulin-dependent shift of the Ca2+ sensitivity of isometric tension observed in skinned muscle fibers, the disagreement between the Ca2+ sensitivity of myosin phosphorylation and that of isometric tension at a steady state, and the disagreement between the time course of myosin phosphorylation and that of isometric tension development.

Phosphorylation of chicken gizzard myosin and the Ca2+-sensitivity of the actin-activated Mg2+-ATPase

A method is described for the preparation of partially and fully phosphorylated chicken gizzard myosin. When fully phosphorylated it possessed an actin-activated Mg2+-ATPase of similar specific activity to that of mammalian skeletal muscle myosin. The Mg2+-ATPase activity of these preparations was related in a non-linear fashion to increasing phosphorylation of the P light chain. When P light chain phosphorylation occurred during enzymic assay the Mg2+-ATPase activity remained constant. Fully phosphorylated preparations of gizzard myosin possessed an actin-activated Mg2+-ATPase that was not Ca2+-sensitive, whereas the Mg2+-ATPase of partially phosphorylated myosin preparations was Ca2+-sensitive.

Phosphorylation of the myofibrillar proteins and the regulation of contractile activity in muscle

Evidence now exists for the phosphorylation of all the major proteins of the myofibril with the exception of troponin C. Although uncertainty exists in most cases about the role of phosphorylation of the myofibrillar proteins, there is substantial evidence that phosphorylation of serine 20 of rabbit cardiac troponin I leads to a lowering of the sensitivity of the actomyosin ATPase to Ca2+. This process is of special importance in the physiological response of the heart to adrenalin. A well defined enzymic system involving a specific kinase and a phosphatase is present in most muscles for the phosphorylation and dephosphorylation of the P light chain (regulatory, L2 or DTNB light chain) of myosin. Myosin light-chain kinase is very active in fast skeletal muscles, and although it is unlikely that phosphorylation followed by dephosphorylation of the P light chain occurs fast enough to be synchronous with the contractile cycle, phosphorylation may have a modulatory role in this tissue. Both post-tetanic potentiation and the reduced actomyosin ATPase turnover rate observed in fast-twitch muscle as a consequence of sustained forceful contraction have been suggested by different investigators to be consequences of P light chain phosphorylation. Nevertheless, unequivocal evidence associating either of these effects with phosphorylation is not yet available. Kinase activity is also high in vertebrate smooth muscle and it has been suggested that phosphorylation of the P light chain is the process that activates the actomyosin ATPase in this tissue. Evidence from a number of studies indicates, however, that regulation of smooth muscle actomyosin ATPase may not be a simple phosphorylation-dephosphorylation process.

Increased synthesis of the phosphorylated form of the myosin light chains in cardiac hypertrophy in the rat

The incorporation rate of [3H]leucine into cardiac myosin subunits was studied in rat hearts undergoing hypertrophy secondary to constriction of the ascending aorta. Cardiac myosin was prepared by a modified Shiverick's method on the second and fourth day after constriction. Myosin light chains were separated by urea and subjected to two-dimensional electrophoresis. Incorporation of [3H]leucine was determined in electrophoretically separated heavy and light chains by the method of Martin et al. (11). It was found that the incorporation rate of [3H]leucine into the phosphorylated form of the myosin light chain 2 is significantly increased in hypertrophic heart as compared to sham animals.

Myosin light chain phosphorylation-dephosphorylation in mammalian skeletal muscle

Phosphorylation of the myosin light chain 2 (LC2) subunit was examined in rat fast-twitch and slow-twitch skeletal muscles in response to repetitive stimulation at 23 and 35 degrees C and on incubation of fast-twitch skeletal muscle with isoproterenol. After a 1-s tetany at 35 degrees C, LC2 phosphate content in extensor digitorum longus muscle increased rapidly and transiently from 0.21 to 0.51 mol phosphate/mol LC2. This pattern of phosphorylation was similar to that observed at 23 degrees C. Increases in LC2 phosphate content were dependent on the frequency and duration of stimulation. In soleus muscle LC2 phosphate content was minimal following a 1-s tetany but increased markedly following more prolonged tetanies. On incubation of extensor digitorum longus muscle with isoproterenol (20 microM), LC2 phosphate content did not change, whereas phosphorylase a levels increased. A positive correlation existed between LC2 phosphate content and potentiation of peak twitch tension in both types of muscles, suggesting a physiological function for LC2 phosphorylation.

Electrophoretic comparison of the proteins of some perch (Perca fluviatilis L.) head muscles

Biochemical analysis of perch body and head muscle myosins and parvalbumins was made by polyacrylamide gel electrophoresis. White and red body muscle myosins were characterized with regard to the light chains. THe existence of slow red-type myosin was demonstrated in the jaw adductor A3 (15%), in the adductor operculi (7%) and in the red part of the levator operculi anterior (34%). The presence of red fibres in these muscles was also reflected in the distribution of the sarcoplasmic calcium-binding proteins, the parvalbumins. These muscles were mainly implicated in slow continuous movements such as respiration. These results were compared to histochemical and functional aspects of perch head muscle differentiation. Close agreement was found between biochemical, histochemical and electromyographical methods of muscle analysis.

Protein phosphorylation: quantitative analysis in vivo and in intact cell systems

Protein phosphorylation-dephosphorylation appears to be an essential component in the regulation of many cellular processes by hormones and drugs. This concept has developed primarily from in vitro biochemical studies in which various purified proteins have been phosphorylated and dephosphorylated by distinct protein kinases and phosphoprotein phosphatases. However, the more difficult, but essential, task of demonstrating the physiological occurrence of these reactions in intact tissue or cell preparations in many cases has not been undertaken in a quantitative manner. There are 4 basic approaches for assessing the extent of protein phosphorylation in vivo and in intact cell systems, each having particular advantages and disadvantages. These are summarized in Table 2. The applicability of any one procedure will be highly dependent upon the protein under investigation. For instance, chemical measurements of total protein-bound phosphate may provide only limited information for proteins which are phosphorylated at multiple sites but could be highly useful for those proteins such as glycogen phosphorylase which are phosphorylated at single sites. The relative ease and the high sensitivity of measuring 32P incorporation into proteins will tempt many investigators to rely heavily on this approach. It is a very powerful procedure, particularly for the initial identification of phosphoproteins, but ultimately quantitative conclusions regarding 32P incorporation must be corroborated by one or more of the other procedures. There is no simple, single experimental approach that may be used under all circumstances, but by integrating these procedures firm conclusions may be drawn regarding the physiological importance of phorphorylation of specific proteins.

Phosphorylation of myosin light chains in perfused rat heart. Effect of adrenaline and increased cytoplasmic calcium ions

1. A method was developed for the isolation of essentially pure myosin light chains from perfused rat heart. The phosphorylation of the P-light chains was estimated by hydrolysis and measurement of phosphate released, by electrophoresis in 8 M-urea and by 32P incorporation in perfusion with [32P]Pi. 2. In control perfusions there was 0.5-0.6 mol of phosphate/mol of P-light chain. This was not changed by perfusion with 5 microM-adrenaline for 10-40s. Perfusion for 1 min with medium containing 7.5 mM-CaCl2, or for 30s with medium containing 118 mM-KCl, also did not change the phosphorylation of P-light chains. 3. It is concluded that phosphorylation of P-light chains is not important in mediating the action of inotropic agents in the heart.

The Croonian lecture, 1979: Regulation of muscle contraction

In this lecture I review briefly the history of the recognition of calcium ion as the sole regulatory factor of muscle contraction at the molecular level and how this led to the discovery of the troponin-tropomyosin system, which is the regulatory system of striated muscles of almost all deuterostomias and some protostomias. This is followed by a brief comment on the myosin-linked regulation, which plays a dominating role in many protostomian muscles. The regulatory mechanism in vertebrate smooth muscle is then discussed; the view is advanced that the leiotonin-tropomyosin system may be the only regulatory device for this muscle. Ca-binding components of troponin and smooth muscles of vertebrates are compared with modulator protein, an omnipresent Ca-binding protein of very conservative nature throughout evolution. Finally, the modes of action of Ca ion in different kinds of cell motility are discussed from an evolutionary point of view.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.

Actomyosin from mammary myoepithelial cells and phosphorylation by myosin light chain kinase

The oxytocin-sensitive myoepithelial cells of the mammary gland form a system with characteristics of a potentially useful model for studying the mechanism of action of oxytocin and coupling phenomena of excitation-contraction. Our objectives were to develop a method for isolating mammary actomyosin, to determine the amount of actomyosin in the glands of lactating and nonlactating animals, and to investigate control of contractile protein interaction. Actomyosin in mammary glands represented a substantial portion of the soluble protein in the gland ranging from 9% of the total in lactating to 17% in weaned rats. The isolated actomyosin had a molecular composition like that of actomyosin of smooth muscle and the isolated actomyosin contained a light chain kinase that phosphorylated the 20,000 dalton light chain of myosin (L20). The kinase isolated as a component of actomyosin preparations did not show calcium control, but it did when isolated from mammary cytosol. Strips of involuted mammary tissue from rats developed tension when oxytocin was added to the bathing medium; thus, the myoepithelial cells appeared to retain their sensitivity to oxytocin even in nonlactating animals and may be a useful model for studying the action of oxytocin. We suggest that one of the final steps in the milk-ejection reflex is phosphorylation of myosin causing a contraction of the myoepithelial cells of the mammary gland.

Phosphorylated proteins as physiological effectors

A variety of neurotransmitters, hormones, and other regulatory agents affect the phosphorylation of specific proteins in their target tissues. The types of stimuli that share this common effect on protein phosphorylation include numerous substances that do not act through cyclic AMP. These and other observations suggest that many different classes of regulatory substances achieve certain of their biological effects by altering the phosphorylation of specific proteins.

Purification and properties of myosin light-chain kinase from fast skeletal muscle

1. A procedure is described for the isolation of myosin light-chain kinase from rabbit fast skeletal muscle as a homogeneous protein. 2. Myosin light-chain kinase is a monomeric enzyme of mol.wt. 77000. Under some conditions of storage it is converted into components of mol.wts. about 50000 and 30000 that possess enzymic activity. 3. The enzyme is clearly different in structure and properties from any other protein kinase so far isolated from muscle. 4. The enzyme is highly specific for the P-light chain (18000-20000-dalton light chain) of myosin and requires Ca2+ for activity. 5. The P-light chain is phosphorylated at a similar rate whether isolated or associated with the rest of the myosin molecule. 6. The effects of pH, bivalent cation and other nucleotides on the enzymic activity are described. 7. The role of the phosphorylation of the P-light chain of myosin in muscle function is discussed.