Myosin regulatory light chain modulates the Ca2+ dependence of the kinetics of tension development in skeletal muscle fibers

Identifying the Related Genes of Muscle Growth and Exploring the Functions by Compensatory Growth in Mandarin Fish (Siniperca chuatsi)

How organisms display many different biochemical, physiological processes through genes expression and regulatory mechanisms affecting muscle growth is a central issue in growth and development. In Siniperca chuatsi, the growth-related genes and underlying relevant mechanisms are poorly understood, especially for difference of body sizes and compensatory growth performance. Muscle from 3-month old individuals of different sizes was used for transcriptome analysis. Results showed that 8,942 different expression genes (DEGs) were identified after calculating the RPKM. The DEGs involved in GH-IGF pathways, protein synthesis, ribosome synthesis and energy metabolisms, which were expressed significantly higher in small individuals (S) than large fish (L). In repletion feeding and compensatory growth experiments, eight more significant DEGs were used for further research (GHR2, IGFR1, 4ebp, Mhc, Mlc, Myf6, MyoD, troponin). When food was plentiful, eight genes participated in and promoted growth and muscle synthesis, respectively. Starvation can be shown to inhibit the expression of Mhc, Mlc and troponin, and high expression of GHR2, IGFR1, and 4ebp inhibited growth. Fasting promoted the metabolic actions of GHR2, IGFR1, and 4ebp rather than the growth-promoting actions. MyoD can sense and regulate the hunger, which also worked with Mhc and Mlc to accelerate the compensatory growth of S. chuatsi. This study is helpful to understand the regulation mechanisms of muscle growth-related genes. The elected genes will contribute to the selective breeding in future as candidate genes.

Diversification of the muscle proteome through alternative splicing

Background

Skeletal muscles express a highly specialized proteome that allows the metabolism of energy sources to mediate myofiber contraction. This muscle-specific proteome is partially derived through the muscle-specific transcription of a subset of genes. Surprisingly, RNA sequencing technologies have also revealed a significant role for muscle-specific alternative splicing in generating protein isoforms that give specialized function to the muscle proteome.

Main body

In this review, we discuss the current knowledge with respect to the mechanisms that allow pre-mRNA transcripts to undergo muscle-specific alternative splicing while identifying some of the key trans-acting splicing factors essential to the process. The importance of specific splicing events to specialized muscle function is presented along with examples in which dysregulated splicing contributes to myopathies. Though there is now an appreciation that alternative splicing is a major contributor to proteome diversification, the emergence of improved “targeted” proteomic methodologies for detection of specific protein isoforms will soon allow us to better appreciate the extent to which alternative splicing modifies the activity of proteins (and their ability to interact with other proteins) in the skeletal muscle. In addition, we highlight a continued need to better explore the signaling pathways that contribute to the temporal control of trans-acting splicing factor activity to ensure specific protein isoforms are expressed in the proper cellular context.

Conclusions

An understanding of the signal-dependent and signal-independent events driving muscle-specific alternative splicing has the potential to provide us with novel therapeutic strategies to treat different myopathies.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0152-3) contains supplementary material, which is available to authorized users.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Central and peripheral fatigability in boys and men during maximal contraction

PURPOSE

The purpose of this study was to examine central and peripheral factors of fatigability that could explain the differences in fatigability between adults and prepubertal boys after maximal sustained isometric contraction.

METHODS

A total of 11 untrained adult men and 10 prepubescent boys volunteered to participate in this study. The level of voluntary activation was assessed before and after fatigue by means of the twitch interpolation technique as well as peak twitch torque, maximum rate of torque development and maximum M-wave (Mmax) area of the soleus and medial gastrocnemius. The fatigue-inducing protocol consisted of a sustained maximal voluntary contraction (MVC) of the ankle's plantar flexor at 100% of MVC until the task could no longer be sustained at 50% of MVC.

RESULTS

During the fatigue-inducing protocol, boys were fatigued less, showing longer endurance limit and delayed torque and agonist EMG decrease. After fatigue, the level of activation decreased to a similar extent in both groups, and boys were less affected regarding their peak twitch torque and rate of torque development, whereas no differentiation between the groups was observed regarding the decrease in Mmax area of the examined muscles.

CONCLUSIONS

The results obtained provide evidence that the greater fatigability resistance in prepubertal children during sustained maximal contractions is mainly explained by peripheral rather than central factors.

Myosin phosphorylation and force potentiation in skeletal muscle: evidence from animal models

The contractile performance of mammalian fast twitch skeletal muscle is history dependent. The effect of previous or ongoing contractile activity to potentiate force, i.e. increase isometric twitch force, is a fundamental property of fast skeletal muscle. The precise manifestation of force potentiation is dependent upon a variety of factors with two general types being identified; staircase potentiation referring to the progressive increase in isometric twitch force observed during low frequency stimulation while posttetanic potentiation refers to the step-like increase in isometric twitch force observed following a brief higher frequency (i.e. tetanic) stimulation. Classic studies established that the magnitude and duration of potentiation depends on a number of factors including muscle fiber type, species, temperature, sarcomere length and stimulation paradigm. In addition to isometric twitch force, more recent work has shown that potentiation also influences dynamic (i.e. concentric and/or isotonic) force, work and power at a range of stimulus frequencies in situ or in vitro, an effect that may translate to enhanced physiological function in vivo. Early studies performed on both intact and permeabilized models established that the primary mechanism for this modulation of performance was phosphorylation of myosin, a modification that increased the Ca(2+) sensitivity of contraction. More recent work from a variety of muscle models indicates, however, the presence of a secondary mechanism for potentiation that may involve altered Ca(2+) handling. The primary purpose of this review is to highlight these recent findings relative to the physiological utility of force potentiation in vivo.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Characterization and ontogenetic expression analysis of the myosin light chains from the fast white muscle of mandarin fish Siniperca chuatsi

Three full-length complementary DNA (cDNA) clones were isolated encoding the skeletal myosin light chain 1 (MLC1; 1237 bp), myosin light chain 2 (MLC2; 1206 bp) and myosin light chain 3 (MLC3; 1079 bp) from the fast white muscle cDNA library of mandarin fish Siniperca chuatsi. The sequence analysis indicated that MLC1 and MLC3 were not produced from differentially spliced messenger RNAs (mRNA) as reported in birds and rodents but were encoded by different genes. The MLC2 encodes 170 amino acids, which include four EF-hand (helix-loop-helix) structures. The primary structures of the Ca(2+)-binding domain were well conserved among the MLC2s of seven other fish species. The ontogenetic expression analysis by real-time PCR showed that the three light-chain mRNAs were first detected in the gastrula stage, and their expression increased from the tail bud stage to the larval stage. All three MLC mRNAs showed longitudinal expression variation in the fast white muscle of S. chuatsi, especially MLC1 which was highly expressed at the posterior area. Taken together, the study provides a better understanding about the MLC gene structure and their expression pattern in muscle development of S. chuatsi.

Acceleration of stretch activation in murine myocardium due to phosphorylation of myosin regulatory light chain

The regulatory light chains (RLCs) of vertebrate muscle myosins bind to the neck region of the heavy chain domain and are thought to play important structural roles in force transmission between the cross-bridge head and thick filament backbone. In vertebrate striated muscles, the RLCs are reversibly phosphorylated by a specific myosin light chain kinase (MLCK), and while phosphorylation has been shown to accelerate the kinetics of force development in skeletal muscle, the effects of RLC phosphorylation in cardiac muscle are not well understood. Here, we assessed the effects of RLC phosphorylation on force, and the kinetics of force development in myocardium was isolated in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC, subsequently skinned, and then treated with MLCK to phosphorylate RLC. Since RLC phosphorylation may be an important determinant of stretch activation in myocardium, we recorded the force responses of skinned myocardium to sudden stretches of 1% of muscle length both before and after treatment with MLCK. MLCK increased RLC phosphorylation, increased the Ca(2+) sensitivity of isometric force, reduced the steepness of the force-pCa relationship, and increased both Ca(2+)-activated and Ca(2+)-independent force. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force, i.e., stretch activation, to levels greater than pre-stretch force. MLCK had profound effects on the stretch activation responses during maximal and submaximal activations: the amplitude and rate of force decay after stretch were significantly reduced, and the rate of delayed force recovery was accelerated and its amplitude reduced. These data show that RLC phosphorylation increases force and the rate of cross-bridge recruitment in murine myocardium, which would increase power generation in vivo and thereby enhance systolic function.

Age-related changes in twitch properties of plantar flexor muscles in prepubertal children

The twitch of the triceps surae muscle (TS), which characterizes the contractile properties independently of volition, differs in amplitude, but not in time course, when evoked in pre or postpubertal children. The aim of the present study was to compare the TS twitch contractile properties in prepubertal children (7 to 11 y). M-wave and twitch were recorded at rest by supramaximal electrical stimulations of the posterior tibial nerve. Twitches were characterized by peak torque (Pt), contraction time (CT), half relaxation time (HRT), and rate of torque development (dPt/dt). Electromechanical delay (EMD) was quantified with regard to the TS M-wave onset. Pt values increased significantly with the age of the prepubertal children but remained lower than that for adult subjects. CT and HRT values did not change with age. Thus, dPt/dt increased significantly between the 7-year-old and the 11-year-old children but remained significantly lower than that for adults. Despite EMD values decreased with age, they remained significantly higher than those of adult subjects. These results confirmed the link between growth processes and the increase in twitch torque for prepubertal children within a limited range of age. However, the time-course characteristics were not affected by age. The increase in dPt/dt and the decrease in the EMD could be used as indirect indicators of changes in contractile kinetics and in musculo-tendinous stiffness with the age of the prepubertal children. The age-related relationships established by this study will serve as reference values for clinical testing of the TS performances in relation to muscle disease or disuse.

Sarcomeric determinants of striated muscle relaxation kinetics

Ca2+ is the primary regulator of force generation by cross-bridges in striated muscle activation and relaxation. Relaxation is as necessary as contraction and, while the kinetics of Ca2+-induced force development have been investigated extensively, those of force relaxation have been both studied and understood less well. Knowledge of the molecular mechanisms underlying relaxation kinetics is of special importance for understanding diastolic function and dysfunction of the heart. A number of experimental models, from whole muscle organs and intact muscle fibres down to single myofibrils, have been used to explore the cascade of kinetic events leading to mechanical relaxation. By using isolated myofibrils and fast solution switching techniques we can distinguish the sarcomeric mechanisms of relaxation from those of myoplasmic Ca2+ removal. There is strong evidence that cross-bridge mechanics and kinetics are major determinants of the time course of striated muscle relaxation whilst thin filament inactivation kinetics and cooperative activation of thin filament by cycling, force-generating cross-bridges do not significantly limit the relaxation rate. Results in myofibrils can be explained well by a simple two-state model of the cross-bridge cycle in which the apparent rate of the force generating transition is modulated by fast, Ca2+-dependent equilibration between off- and on-states of actin. Inter-sarcomere dynamics during the final rapid phase of full force relaxation are responsible for deviations from this simple model.

Relaxation kinetics following sudden Ca(2+) reduction in single myofibrils from skeletal muscle

To investigate the roles of cross-bridge dissociation and cross-bridge-induced thin filament activation in the time course of muscle relaxation, we initiated force relaxation in single myofibrils from skeletal muscles by rapidly (approximately 10 ms) switching from high to low [Ca(2+)] solutions. Full force decay from maximal activation occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere "give" and dominated by inter-sarcomere dynamics (see the companion paper, Stehle, R., M. Krueger, and G. Pfitzer. 2002. Biophys. J. 83:2152-2161), while the former occurs under nearly isometric conditions and is sensitive to mechanical perturbations. Decreasing the Ca(2+)-activated force preceding the start of relaxation does not increase the rate of the slow isometric phase, suggesting that cycling force-generating cross-bridges do not significantly sustain activation during relaxation. This conclusion is strengthened by the finding that the rate of isometric relaxation from maximum force to any given Ca(2+)-activated force level is similar to that of Ca(2+)-activation from rest to that given force. It is likely, therefore, that the slow rate of force decay in full relaxation simply reflects the rate at which cross-bridges leave force-generating states. Because increasing [P(i)] accelerates relaxation while increasing [MgADP] slows relaxation, both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

Myosin light chain 2 modulates MgADP-induced contraction in rabbit skeletal and bovine cardiac skinned muscle

Skinned skeletal and cardiac muscle fibres can be activated by MgADP in the presence of MgATP without Ca2+; the isometric tension is developed in a sigmoidal manner with the addition of MgADP under relaxing conditions. The critical concentrations of MgADP for this MgADP-induced contraction are about 7.5 and 2.6 mM for skeletal and cardiac muscle fibres, respectively. To investigate whether muscle regulatory proteins, myosin light chain 2 (LC2) and troponin C (TnC), play a part in the MgADP-induced contraction, these proteins were partly extracted by treatment with trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CDTA), a chelater of divalent cations, and the MgADP-tension relationship was examined in rabbit psoas and bovine cardiac skinned fibres. We found that the sigmoidal MgADP-tension relationship became hyperbolic after a partial extraction of LC2 (about 30 %) and TnC (about 70 %). Reconstitution with LC2 restored the sigmoidal MgADP-tension relationship of control fibres almost fully in both skeletal and cardiac fibres, whereas reconstitution with TnC alone had no effect. Furthermore, cardiac fibres reconstituted with skeletal LC2 exhibited an MgADP-tension relationship intermediate between skeletal and cardiac fibres. The partial extraction of LC2 and TnC resulted in a reduction of the inhibitory effect of inorganic phosphate (P(i)) on the MgADP-activated tension. Reconstitution with LC2 restored the original P(i)-tension relationship, whereas reconstitution with TnC had no effect. In other words, extraction of LC2 apparently increased the affinity of myosin for MgADP but decreased the affinity for P(i). These results demonstrate that LC2 modulates MgADP-induced activation of actomyosin interaction.

Phosphorylation of the regulatory light chains of myosin affects Ca2+ sensitivity of skeletal muscle contraction

The role of phosphorylation of the myosin regulatory light chains (RLC) is well established in smooth muscle contraction, but in striated (skeletal and cardiac) muscle its role is still controversial. We have studied the effects of RLC phosphorylation in reconstituted myosin and in skinned skeletal muscle fibers where Ca2+ sensitivity and the kinetics of steady-state force development were measured. Skeletal muscle myosin reconstituted with phosphorylated RLC produced a much higher Ca2+ sensitivity of thin filament-regulated ATPase activity than nonphosphorylated RLC (change in -log of the Ca2+ concentration producing half-maximal activation = approximately 0.25). The same was true for the Ca2+ sensitivity of force in skinned skeletal muscle fibers, which increased on reconstitution of the fibers with the phosphorylated RLC. In addition, we have shown that the level of endogenous RLC phosphorylation is a crucial determinant of the Ca2+ sensitivity of force development. Studies of the effects of RLC phosphorylation on the kinetics of force activation with the caged Ca2+, DM-nitrophen, showed a slight increase in the rates of force development with low statistical significance. However, an increase from 69 to 84% of the initial steady-state force was observed when nonphosphorylated RLC-reconstituted fibers were subsequently phosphorylated with exogenous myosin light chain kinase. In conclusion, our results suggest that, although Ca2+ binding to the troponin-tropomyosin complex is the primary regulator of skeletal muscle contraction, RLC play an important modulatory role in this process.

Nonuniform distribution of myosin light chains within the thick filaments of lobster slow muscle: Immunocytochemical study

The in situ distribution of the alpha and beta myosin light chains was investigated at the subsarcomeric and subfilament levels in individual fibers of the superficial flexor muscle (SFM) of the lobster, Homarus americanus. Polyclonal antibodies were produced against the two classes of myosin light chains and used for subsequent immunolocalization on thin sections of sarcomeres and on isolated filaments from both the medial and lateral fiber bundles of the SFM. The beta myosin light chains were uniformly distributed within the crossbridge region of sarcomeres of both medial and lateral bundles. The alpha myosin light chains were uniformly distributed within the crossbridge region of sarcomeres from the medial bundle, but were nonuniformly distributed over the crossbridge region of lateral bundle sarcomeres. In the latter, the number of alpha myosin light chains was highest toward the center of the thick filaments, diminishing towards the ends. Similar distributions of alpha light chains were found in isolated myosin filaments. These data demonstrate that heterogeneity in protein composition extends to the level of the myosin filament and suggest that the myosin filament substructure in lobster may be different than that found in vertebrate skeletal muscle.

Invited Review: plasticity and energetic demands of contraction in skeletal and cardiac muscle

Numerous studies have explored the energetic properties of skeletal and cardiac muscle fibers. In this mini-review, we specifically explore the interactions between actin and myosin during cross-bridge cycling and provide a conceptual framework for the chemomechanical transduction that drives muscle fiber energetic demands. Because the myosin heavy chain (MHC) is the site of ATP hydrolysis and actin binding, we focus on the mechanical and energetic properties of different MHC isoforms. Based on the conceptual framework that is provided, we discuss possible sites where muscle remodeling may impact the energetic demands of contraction in skeletal and cardiac muscle.

Contractile response of skeletal muscle to low peroxide concentrations: myofibrillar calcium sensitivity as a likely target for redox-modulation

Endogenous peroxides and related reactive oxygen species may influence various steps in the contractile process. Single mouse skeletal muscle fibers were used to study the effects of hydrogen peroxide (H2O2) and t-butyl hydroperoxide (t-BOOH) on force and myoplasmic Ca2+ concentration ([Ca2+]i). Both peroxides (1010 to 105 M) decreased tetanic [Ca2+]i and increased force during submaximal tetani. Catalase (1 kU/ml) blocked the effect of H2O2, but not of t-BOOH. The decrease in tetanic [Ca2+]i was constant, while the effect on force was biphasic: A transitory increase was followed by a steady decline to the initial level. Myofibrillar Ca2+ sensitivity remained increased during incubation with either peroxide. Only the highest peroxide concentration (10 mM) increased resting [Ca2+]i and slowed the return of [Ca2+]i to its resting level after a contraction, evidence of impaired sarcoplasmic reticulum Ca2+ re-uptake. The peroxides increased maximal force production and the rate of force redevelopment, and decreased maximum shortening velocity. N-ethylmaleimide (25 mM, thiol-alkylating agent) prevented the response to 1 mM H2O2. These results show that myofibrillar Ca2+ sensitivity and cross-bridge kinetics are influenced by H2O2 and t-BOOH concentrations that approach those found physiologically, and these findings indicate a role for endogenous oxidants in the regulation of skeletal muscle function.

Cross-bridge interaction kinetics in rat myocardium are accelerated by strong binding of myosin to the thin filament

To determine the ability of strong-binding myosin cross-bridges to activate the myocardial thin filament, we examined the Ca2+ dependence of force and cross-bridge interaction kinetics at 15 degrees C in the absence and presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1) in chemically skinned myocardium from adult rats. Relative to control conditions, application of 6 microM NEM-S1 significantly increased Ca2+-independent tension, measured at pCa 9.0, from 0.8 +/- 0.3 to 3.7 +/- 0.8 mN mm-2. Furthermore, NEM-S1 potentiated submaximal Ca2+-activated forces and thereby increased the Ca2+ sensitivity of force, i.e. the [Ca2+] required for half-maximal activation (pCa50) increased from pCa 5.85 +/- 0.05 to 5.95 +/- 0.04 (change in pCa50 (dpCa50) = 0.11 +/- 0.02). The augmentation of submaximal force by NEM-S1 was accompanied by a marked reduction in the steepness of the force-pCa relationship for forces less than 0.50 Po (maximum Ca2+-activated force), i.e. the Hill coefficient (n2) decreased from 4.72 +/- 0.38 to 1.54 +/- 0.07. In the absence of NEM-S1, the rate of force redevelopment (ktr) was found to increase from 1.11 +/- 0.21 s-1 at submaximal [Ca2+] (pCa 6.0) to 9.28 +/- 0.41 s-1 during maximal Ca2+ activation (pCa 4.5). Addition of NEM-S1 reduced the Ca2+ dependence of ktr by eliciting maximal values at low levels of Ca2+, i.e. ktr was 9.38 +/- 0.30 s-1 at pCa 6.6 compared to 9.23 +/- 0.27 s-1 at pCa 4. At intermediate levels of Ca2+, ktr was less than maximal but was still greater than values obtained at the same pCa in the absence of NEM-S1. NEM-S1 dramatically reduced both the extent and rate of relaxation from steady-state submaximal force following flash photolysis of the caged Ca2+ chelator diazo-2. These data demonstrate that strongly bound myosin cross-bridges increase the level of thin filament activation in myocardium, which is manifested by an increase in the rate of cross-bridge attachment, potentiation of force at low levels of free Ca2+, and slowed rates of relaxation.

Polyamines decrease Ca(2+) sensitivity of tension and increase rates of activation in skinned cardiac myocytes

Owing in part to their interactions with membrane proteins, polyamines (e.g., spermine, spermidine, and putrescine) have been identified as potential modulators of membrane excitability and Ca(2+) homeostasis in cardiac myocytes. To investigate whether polyamines also affect cardiac myofilament proteins, we assessed the effects of polyamines on contractility using rat myocytes and trabeculae that had been permeabilized with Triton X-100. Spermine, spermidine, and putrescine reversibly increased the [Ca(2+)] required for half-maximal tension (i.e., right-shifted tension pCa curves), with the following order of efficacy: spermine (+4) > spermidine (+3) > putrescine (+2). However, synthetic analogs that differed from spermine in charge distribution were not as effective as spermine in altering isometric tension. None of the polyamines had a significant effect on maximal tension, except at high concentrations. After flash photolysis of DM-Nitrophen (a caged Ca(2+) chelator), spermine accelerated the rate of tension development at low and intermediate but not high [Ca(2+)]. These results indicate that polyamines, especially spermine, interact with myofilament proteins to reduce apparent Ca(2+) binding affinity and speed cross-bridge cycling kinetics at submaximal [Ca(2+)].

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Magnesium binding to DM-nitrophen and its effect on the photorelease of calcium

The effect of Mg(2+) on the process of Ca(2+) release from the caged Ca(2+) compound DM-nitrophen (NP) was studied in vitro by steady light UV photolysis of NP in the presence of Ca(2+) and Mg(2+). Ca(2+) release during photolysis and its relaxation/recovery after photolysis were monitored with the Ca(2+)-sensitive dye fura-2. Mg(2+) speeds the photorelease of Ca(2+) during photolysis and slows the relaxation of Ca(2+) to new steady-state levels after photolysis. Within the context of a model describing NP photolysis, we determined the on and off rates of Mg(2+) binding to unphotolyzed NP (k(on) = 6.0 x 10(4) M(-1) s(-1); k(off) = 1.5 x 10(-1) s(-1)). Furthermore, to fully account for the slow postphotolysis kinetics of Ca(2+) in the presence of Mg(2+) we were forced to add an additional photoproduct to the standard model of NP photolysis. The additional photoproduct is calculated to have a Ca(2+) affinity of 13.3 microM and is hypothesized to be produced by the photolysis of free or Mg(2+)-bound NP; photolysis of Ca(2+)-bound NP produces the previously documented 3 mM Ca(2+) affinity photoproduct.

Abnormal cardiac structure and function in mice expressing nonphosphorylatable cardiac regulatory myosin light chain 2

A role for myosin phosphorylation in modulating normal cardiac function has long been suspected, and we hypothesized that changing the phosphorylation status of a cardiac myosin light chain might alter cardiac function in the whole animal. To test this directly, transgenic mice were created in which three potentially phosphorylatable serines in the ventricular isoform of the regulatory myosin light chain were mutated to alanines. Lines were obtained in which replacement of the endogenous species in the ventricle with the nonphosphorylatable, transgenically encoded protein was essentially complete. The mice show a spectrum of cardiovascular changes. As previously observed in skeletal muscle, Ca(2+) sensitivity of force development was dependent upon the phosphorylation status of the regulatory light chain. Structural abnormalities were detected by both gross histology and transmission electron microscopic analyses. Mature animals showed both atrial hypertrophy and dilatation. Echocardiographic analysis revealed that as a result of chamber enlargement, severe tricuspid valve insufficiency resulted in a detectable regurgitation jet. We conclude that regulated phosphorylation of the regulatory myosin light chains appears to play an important role in maintaining normal cardiac function over the lifetime of the animal.

Aging-dependent depression in the kinetics of force development in rat skinned myocardium

Normal aging of the rodent heart results in prominent prolongation of the twitch. We tested the hypothesis that increased expression of beta-myosin heavy chain (MHC), as occurs in the normal aging process in the rodent heart, contributes to the prolongation of the twitch by depressing the kinetics of cross-bridge interaction. Using 3-, 9-, 21-, and 33-mo-old male Fischer 344 x Brown Norway F1 hybrid rats, we examined both the rate of tension development (kCa) and unloaded shortening velocity in chemically skinned myocardium. Although kCa in all four age groups was dependent on the level of Ca2+ activation, both submaximal and maximal kCa were significantly slower in 9-, 21-, and 33-mo-old rats relative to 3-mo-old rats. Furthermore, unloaded shortening velocity was significantly reduced in 9-, 21-, and 33-mo-old rats compared with 3-mo-old rats. Collectively, these data strongly suggest that the aging-related increase in beta-MHC expression results in a progressive slowing of cross-bridge interaction kinetics in skinned myocardium, which most likely contributes to the overall aging-dependent reduction in myocardial functional capacity.

Regulation of skeletal muscle tension redevelopment by troponin C constructs with different Ca2+ affinities

In maximally activated skinned fibers, the rate of tension redevelopment (ktr) following a rapid release and restretch is determined by the maximal rate of cross-bridge cycling. During submaximal Ca2+ activations, however, ktr regulation varies with thin filament dynamics. Thus, decreasing the rate of Ca2+ dissociation from TnC produces a higher ktr value at a given tension level (P), especially in the [Ca2+] range that yields less than 50% of maximal tension (Po). In this study, native rabbit TnC was replaced with chicken recombinant TnC, either wild-type (rTnC) or mutant (NHdel), with decreased Ca2+ affinity and an increased Ca2+ dissociation rate (koff). Despite marked differences in Ca2+ sensitivity (>0.5 DeltapCa50), fibers reconstituted with either of the recombinant proteins exhibited similar ktr versus tension profiles, with ktr low (1-2 s-1) and constant up to approximately 50% Po, then rising sharply to a maximum (16 +/- 0.8 s-1) in fully activated fibers. This behavior is predicted by a four-state model based on coupling between cross-bridge cycling and thin filament regulation, where Ca2+ directly affects only individual thin filament regulatory units. These data and model simulations confirm that the range of ktr values obtained with varying Ca2+ can be regulated by a rate-limiting thin filament process.

Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium

1. The effects of ventricular myosin heavy chain (MHC) composition on the kinetics of activation and relaxation were examined in both chemically skinned and intact myocardial preparations from adult rats. Thyroid deficiency was induced to alter ventricular MHC isoform expression from approximately 80% alpha-MHC/20% beta-MHC in euthyroid rats to 100% beta-MHC, without altering the expression of thin-filament-associated regulatory proteins. 2. In single skinned myocytes, increased expression of beta-MHC did not significantly affect either maximal Ca2+-activated tension (P0) or the Ca2+ sensitivity of tension (pCa50). However, unloaded shortening velocity (V0) decreased by 80% due to increased beta-MHC expression. 3. The kinetics of activation and relaxation were examined in skinned multicellular preparations using the caged Ca2+ compound DM-nitrophen and caged Ca2+ chelator diazo-2, respectively. Myocardium expressing 100% beta-MHC exhibited apparent rates of submaximal and maximal tension development (kCa) that were 60% lower than in control myocardium, and a 2-fold increase in the half-time for relaxation from steady-state submaximal force. 4. The time courses of cell shortening and intracellular Ca2+ transients were assessed in living, electrically paced myocytes, both with and without beta-adrenergic stimulation (70 nM isoproterenol (isoprenaline)). Thyroid deficiency had no affect on either the extent of myocyte shortening or the resting or peak fura-2 fluorescence ratios. However, induction of beta-MHC expression by thyroid deficiency was associated with increased half-times for myocyte shortening and relengthening and increased half-time for the decay of the fura-2 fluorescence ratio. Qualitatively similar results were obtained in both the absence and the presence of beta-adrenergic stimulation although the beta-agonist accelerated the kinetics of the twitch and the Ca2+ transient. 5. Collectively, these data provide evidence that increased beta-MHC expression contributes significantly to the observed depression of contractile function in thyroid deficient myocardium by slowing the rates of both force development and force relaxation.

Ca2+ regulates the kinetics of tension development in intact cardiac muscle

The goal of this study was to determine whether Ca2+ plays a role in regulating tension development kinetics in intact cardiac muscle. In cardiac muscle, this fundamental issue of Ca2+ regulation has been controversial. The approach was to induce steady-state tetanic contractions of intact right ventricular trabeculae from rat hearts at varying external Ca2+ concentrations ([Ca2+]) at 22 degreesC. During tetani, cross bridges were mechanically disrupted and the kinetics of tension redevelopment were assessed from the rate constant of exponential tension redevelopment (ktr). There was a relationship between ktr and external [Ca2+] that was similar in form to the relationship between tension and [Ca2+]. Thus a close relationship also existed between ktr and tension (r = 0.88; P < 0. 001); whereas at maximal tetanic tension (saturating cytosolic [Ca2+]), ktr was 16.4 +/- 2.2 s-1 (mean +/- SE, n = 7), at zero tension (low cytosolic [Ca2+]), ktr extrapolated to 20% of maximum (3.3 +/- 0.7 s-1). Qualitatively similar results were obtained using different mechanical protocols to disrupt cross bridges. These data demonstrate that tension redevelopment kinetics in intact cardiac muscle are influenced by the level of Ca2+ activation. These findings contrast with the findings of one previous study of intact cardiac muscle. Activation dependence of tension development kinetics may play an important role in determining the rate and extent of myocardial tension rise during the cardiac cycle in vivo.

Fourier transform infrared photolysis studies of caged compounds

Time-resolved FTIR difference spectroscopy is a powerful tool for investigating molecular reaction mechanisms of proteins. In order to detect, beyond the large background absorbance of the protein and the water, absorbance bands of protein groups that undergo reactions, difference spectra have to be performed between a ground state and an activated state of the sample. Because the absorbance changes are small, the reaction has to be started in situ, in the apparatus, and in thin protein films. The use of caged compounds offers an elegant approach to initiate protein reactions with a nanosecond UV laser flash. Here, time-resolved FTIR and FT-Raman photolysis studies of the commonly used caged compounds, caged Pi, caged ATP, caged GTP, and caged calcium are presented. The use of specific isotopic labels allows us to assign the IR bands to specific groups. Because metal ions play an important role in many biological systems, their influence on FTIR spectra of caged compounds is discussed. The results presented should provide a good basis for further FTIR studies on molecular reaction mechanisms of energy or signal transducing proteins. As an example of such investigations, the time-resolved FTIR studies on the GTPase reaction of H-ras p21 using caged GTP is presented.

Phosphorylation of myosin regulatory light chain eliminates force-dependent changes in relaxation rates in skeletal muscle

The rate of relaxation from steady-state force in rabbit psoas fiber bundles was examined before and after phosphorylation of myosin regulatory light chain (RLC). Relaxation was initiated using diazo-2, a photolabile Ca2+ chelator that has low Ca2+ binding affinity (K(Ca) = 4.5 x 10(5) M(-1)) before photolysis and high affinity (K(Ca) = 1.3 x 10(7) M(-1)) after photolysis. Before phosphorylating RLC, the half-times for relaxation initiated from 0.27 +/- 0.02, 0.51 +/- 0.03, and 0.61 +/- 0.03 Po were 90 +/- 6, 140 +/- 6, and 182 +/- 9 ms, respectively. After phosphorylation of RLC, the half-times for relaxation from 0.36 +/- 0.03 Po, 0.59 +/- 0.03 Po, and 0.65 +/- 0.02 Po were 197 +/- 35 ms, 184 +/- 35 ms, and 179 +/- 22 ms. This slowing of relaxation rates from steady-state forces less than 0.50 Po was also observed when bundles of fibers were bathed with N-ethylmaleimide-modified myosin S-1, a strongly binding cross-bridge derivative of S1. These results suggest that phosphorylation of RLC slows relaxation, most likely by slowing the apparent rate of transition of cross-bridges from strongly bound (force-generating) to weakly bound (non-force-generating) states, and reduces or eliminates Ca2+ and cross-bridge activation-dependent changes in relaxation rates.

Experimental diabetes is associated with functional activation of protein kinase C epsilon and phosphorylation of troponin I in the heart, which are prevented by angiotensin II receptor blockade

A cardiomyopathy that is characterized by an impairment in diastolic relaxation and a loss of calcium sensitivity of the isolated myofibril has been described in chronic diabetic animals and humans. To explore a possible role for protein kinase C (PKC)-mediated phosphorylation of myofibrillar proteins in this process, we characterized the subcellular distribution of the major PKC isoforms seen in the adult heart in cardiocytes isolated from diabetic rats and determined patterns of phosphorylation of the major regulatory proteins, including troponin I (TnI). Rats were made diabetic with a single injection of streptozotocin, and myocardiocytes were isolated and studied 3 to 4 weeks later. In nondiabetic animals, 76% of the PKC epsilon isoform was located in the cytosol and 24% was particulate, whereas in diabetic animals, 55% was cytosolic and 45% was particulate (P < .05). PKC delta, the other major PKC isoform seen in adult cardiocytes, did not show a change in subcellular localization. In parallel, TnI phosphorylation was increased 5-fold in cardiocytes isolated from the hearts of diabetic animals relative to control animals (P < .01). The change in PKC epsilon distribution and in TnI phosphorylation in diabetic animals was completely prevented by rendering the animals euglycemic with insulin or by concomitant treatment with a specific angiotensin II type-1 receptor (AT1) antagonist. Since PKC phosphorylation of TnI has been associated with a loss of calcium sensitivity of intact myofibrils, these data suggest that angiotensin II receptor-mediated activation of PKC may play a role in the contractile dysfunction seen in chronic diabetes.

The possible role of myosin A1 light chain in the weakening of actin-myosin interaction

The effects resulting from the removal of the N-terminus of myosin A1 by limited papain cleavage are investigated. The myosin and heavy meromyosin K+-ATPase and Ca2+-ATPase activities, and actin-activated ATPase activity of heavy meromyosin (HMM) and subfragment-1, are studied. Myosin and HMM preparations devoid of the A1 N-terminus exhibits lower Ca2+-ATPase activities at low ionic strength whereas no differences in K+- or Ca2+-ATPase activities are observed at high ionic strength. Direct binding of actin to monomeric myosin under K+-activated ATPase conditions is much more effective for myosin containing a shortened A1 light chain. The kinetic constants K(app) for actin and V(max) are calculated from actin-activation curves for HMM and subfragment-1. The kinetic constants for HMM are determined under conditions assuring saturation of regulatory light chains (RLC) either with Mg2+ or Ca2+. The removal of the A1 N-terminus influences the actin-myosin interaction in a Ca2+- and phosphorylation-dependent manner; in most cases, this leads to an increase in affinity. In the case of subfragment-1, the removal of the N-terminus of A1 led to a decrease in affinity. It is reasonable to assume that the intact A1 light chain may cause weakening of the actin-myosin interaction under certain conditions. This weakening may be regulated by RLC phosphorylation and RLC-bound calcium-for-magnesium exchange. Such an effect requires a structural minimum that is present in HMM but not in subfragment-1. Implications of such a role for the A1 N-terminus in the myosin-actin interaction are discussed.

Altered kinetics of contraction in skeletal muscle fibers containing a mutant myosin regulatory light chain with reduced divalent cation binding

We examined the kinetic properties of rabbit skinned skeletal muscle fibers in which the endogenous myosin regulatory light chain (RLC) was partially replaced with a mutant RLC (D47A) containing a point mutation within the Ca2+/Mg2+ binding site that severely reduced its affinity for divalent cations. We found that when approximately 50% of the endogenous RLC was replaced by the mutant, maximum tension declined to approximately 60% of control and the rate constant of active tension redevelopment (ktr) after mechanical disruption of cross-bridges was reduced to approximately 70% of control. This reduction in ktr was not an indirect effect on kinetics due to a reduced number of strongly bound myosin heads, because when the strongly binding cross-bridge analog N-ethylmaleimide-modified myosin subfragment1 (NEM-S1) was added to the fibers, there was no effect upon maximum ktr. Fiber stiffness declined after D47A exchange in a manner indicative of a decrease in the number of strongly bound cross-bridges, suggesting that the force per cross-bridge was not significantly affected by the presence of D47A RLC. In contrast to the effects on ktr, the rate of tension relaxation in steadily activated fibers after flash photolysis of the Ca2+ chelator diazo-2 increased by nearly twofold after D47A exchange. We conclude that the incorporation of the nondivalent cation-binding mutant of myosin RLC decreases the proportion of cycling cross-bridges in a force-generating state by decreasing the rate of formation of force-generating bridges and increasing the rate of detachment. These results suggest that divalent cation binding to myosin RLC plays an important role in modulating the kinetics of cross-bridge attachment and detachment.