Enhanced skeletal muscle contraction with myosin light chain phosphorylation by a calmodulin-sensing kinase

Potentiation in mouse lumbrical muscle without myosin light chain phosphorylation: is resting calcium responsible?

The increase in isometric twitch force observed in fast-twitch rodent muscles during or after activity, known universally as potentiation, is normally associated with myosin regulatory light chain (RLC) phosphorylation. Interestingly, fast muscles from mice devoid of detectable skeletal myosin light chain kinase (skMLCK) retain a reduced ability to potentiate twitch force, indicating the presence of a secondary origin for this characteristic feature of the fast muscle phenotype. The purpose of this study was to assess changes in intracellular cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) after a potentiating stimulus in mouse lumbrical muscle (37°C). Lumbricals were loaded with the Ca²⁺-sensitive fluorescent indicators fura-2 or furaptra to detect changes in resting and peak, respectively, intracellular Ca²⁺ levels caused by 2.5 s of 20-Hz stimulation. Although this protocol produced an immediate increase in twitch force of 17 ± 3% (all data are n = 10) (P < 0.01), this potentiation dissipated quickly and was absent 30 s afterward. Fura-2 fluorescence signals at rest were increased by 11.1 ± 1.3% (P < 0.01) during potentiation, indicating a significant increase in resting [Ca²⁺]_i. Interestingly, furaptra signals showed no change to either the amplitude or the duration of the intracellular Ca²⁺ transients (ICTs) that triggered potentiated twitches during this time (P < 0.50). Immunofluorescence work showed that 77% of lumbrical fibers expressed myosin heavy chain isoform IIx and/or IIb, but with low expression of skMLCK and high expression of myosin phosphatase targeting subunit 2. As a result, lumbrical muscles displayed no detectable RLC phosphorylation either at rest or after stimulation. We conclude that stimulation-induced elevations in resting [Ca²⁺]_i, in the absence of change in the ICT, are responsible for a small-magnitude, short-lived potentiation of isometric twitch force. If operative in other fast-twitch muscles, this mechanism may complement the potentiating influence of myosin RLC phosphorylation.

The effect of myosin RLC phosphorylation in normal and cardiomyopathic mouse hearts

Phosphorylation of the myosin regulatory light chain (RLC) by Ca(2+)-calmodulin-activated myosin light chain kinase (MLCK) is known to be essential for the inotropic function of the heart. In this study, we have examined the effects of MLCK-phosphorylation of transgenic (Tg) mouse cardiac muscle preparations expressing the D166V (aspartic acid to valine)-RLC mutation, identified to cause familial hypertrophic cardiomyopathy with malignant outcomes. Our previous work with Tg-D166V mice demonstrated a large increase in the Ca(2+) sensitivity of contraction, reduced maximal ATPase and force and a decreased level of endogenous RLC phosphorylation. Based on studies demonstrating the beneficial and/or protective effects of cardiac myosin phosphorylation for heart function, we hypothesized that an ex vivo phosphorylation of Tg-D166V cardiac muscle may rescue the detrimental contractile phenotypes observed earlier at the level of single myosin molecules and in Tg-D166V papillary muscle fibres. We showed that MLCK-induced phosphorylation of Tg-D166V cardiac myofibrils and muscle fibres was able to increase the reduced myofibrillar ATPase and reverse an abnormally increased Ca(2+) sensitivity of force to the level observed for Tg-wild-type (WT) muscle. However, in contrast to Tg-WT, which displayed a phosphorylation-induced increase in steady-state force, the maximal tension in Tg-D166V papillary muscle fibres decreased upon phosphorylation. With the exception of force generation data, our results support the notion that RLC phosphorylation works as a rescue mechanism alleviating detrimental functional effects of a disease causing mutation. Further studies are necessary to elucidate the mechanism of this unexpected phosphorylation-induced decrease in maximal tension in Tg-D166V-skinned muscle fibres.

Protein phosphatase 1 regulatory subunit 12A and catalytic subunit δ, new members in the phosphatidylinositide 3 kinase insulin-signaling pathway

Skeletal muscle insulin resistance is an early abnormality in individuals with metabolic syndrome and type 2 diabetes (T2D). Insulin receptor substrate-1 (IRS1) plays a key role in insulin signaling, the function of which is regulated by both phosphorylation and dephosphorylation of tyrosine and serine/threonine residues. Numerous studies have focused on kinases in IRS1 phosphorylation and insulin resistance; however, the mechanism for serine/threonine phosphatase action in insulin signaling is largely unknown. Recently, we identified protein phosphatase 1 (PP1) regulatory subunit 12A (PPP1R12A) as a novel endogenous insulin-stimulated interaction partner of IRS1 in L6 myotubes. The current study was undertaken to better understand PPP1R12A's role in insulin signaling. Insulin stimulation promoted an interaction between the IRS1/p85 complex and PPP1R12A; however, p85 and PPP1R12A did not interact independent of IRS1. Moreover, kinase inhibition experiments indicated that insulin-induced interaction between IRS1 and PPP1R12A was reduced by treatment with inhibitors of phosphatidylinositide 3 kinase, PDK1, Akt, and mTOR/raptor but not MAPK. Furthermore, a novel insulin-stimulated IRS1 interaction partner, PP1 catalytic subunit (PP1cδ), was identified, and its interaction with IRS1 was also disrupted by inhibitors of Akt and mTOR/raptor. These results indicate that PPP1R12A and PP1cδ are new members of the insulin-stimulated IRS1 signaling complex, and the interaction of PPP1R12A and PP1cδ with IRS1 is dependent on Akt and mTOR/raptor activation. These findings provide evidence for the involvement of a particular PP1 complex, PPP1R12A/PP1cδ, in insulin signaling and may lead to a better understanding of dysregulated IRS1 phosphorylation in insulin resistance and T2D.

Systems biology of skeletal muscle: fiber type as an organizing principle

Skeletal muscle force generation and contraction are fundamental to countless aspects of human life. The complexity of skeletal muscle physiology is simplified by fiber type classification where differences are observed from neuromuscular transmission to release of intracellular Ca(2+) from the sarcoplasmic reticulum and the resulting recruitment and cycling of cross-bridges. This review uses fiber type classification as an organizing and simplifying principle to explore the complex interactions between the major proteins involved in muscle force generation and contraction.

Site-specific phosphorylation of protein phosphatase 1 regulatory subunit 12A stimulated or suppressed by insulin

Protein phosphatase 1 (PP1) is one of the major phosphatases responsible for protein dephosphorylation in eukaryotes. So far, only few specific phosphorylation sites of PP1 regulatory subunit 12A (PPP1R12A) have been shown to regulate the PP1 activity. The effect of insulin on PPP1R12A phosphorylation is largely unknown. Utilizing a mass spectrometry based phosphorylation identification and quantification approach, we identified 21 PPP1R12A phosphorylation sites (7 novel sites, including Ser20, Thr22, Thr453, Ser478, Thr671, Ser678, and Ser680) and quantified 16 of them under basal and insulin stimulated conditions in hamster ovary cells overexpressing the insulin receptor (CHO/IR), an insulin sensitive cell model. Insulin stimulated the phosphorylation of PPP1R12A significantly at Ser477, Ser478, Ser507, Ser668, and Ser695, while simultaneously suppressing the phosphorylation of PPP1R12A at Ser509 (more than 2-fold increase or decrease compared to basal). Our data demonstrate that PPP1R12A undergoes insulin stimulated/suppressed phosphorylation, suggesting that PPP1R12A phosphorylation may play a role in insulin signal transduction. The novel PPP1R12A phosphorylation sites as well as the new insulin-responsive phosphorylation sites of PPP1R12A in CHO/IR cells provide targets for investigation of the regulation of PPP1R12A and the PPP1R12A-PP1cδ complex in insulin action and other signaling pathways in other cell models, animal models, and humans.

Postactivation potentiation and muscular endurance training

INTRODUCTION

The aim of this study was to investigate muscle twitch force potentiation after voluntary conditioning contractions (CC) of various intensities and the CC duration necessary to achieve maximal potentiation before and after muscular endurance training.

METHODS

Fourteen healthy men and women (23.6 ± 0.96 years of age) performed repeated CCs of 25%, 50%, and 100% maximal voluntary contraction of the adductor pollicis muscle until maximal potentiation. CCs were followed by electrically evoked twitches. The training group performed a fatigue task and endurance trained for 8 weeks.

RESULTS

Endurance time increased by 79.8 ± 22.5% posttraining. Potentiation occurred after all CC intensities and was greater after training. The CC duration needed to achieve maximal potentiation decreased as CC intensity increased. Potentiation was greater during the fatigue task after compared to before training and was correlated with endurance time.

CONCLUSION

An increase in muscle force potentiation may function as a mechanism to prolong muscular endurance.

The effect of work cycle frequency on the potentiation of dynamic force in mouse fast twitch skeletal muscle

The purpose of this study was to test the hypothesis that the potentiation of concentric twitch force during work cycles is dependent upon both the speed and direction of length change. Concentric and eccentric forces were elicited by stimulating muscles during the shortening and lengthening phases, respectively, of work cycles. Work cycle frequency was varied in order to vary the speed of muscle shortening and/or lengthening; all forces were measured as the muscle passed though optimal length (Lo). Both concentric and eccentric force were assessed before (unpotentiated control) and after (potentiated) the application of a tetanic conditioning protocol known to potentiate twitch force output. The influence of the conditioning protocol on relative concentric force was speed dependent, with forces increased to 1.19±0.01, 1.25±0.01 and 1.30±0.01 of controls at 1.5, 3.3 and 6.9 Hz, respectively (all data N=9–10 with P&lt;0.05). In contrast, the conditioning protocol had only a limited effect on eccentric force at these frequencies (range: 1.06±0.01 to 0.96±0.03). The effect of the conditioning protocol on concentric work (force × distance) was also speed dependent, being decreased at 1.5 Hz (0.84±0.01) and increased at 3.3 and 6.9 Hz (1.05±0.01 and 1.39±0.01, respectively). In contrast, eccentric work was not increased at any frequency (range: 0.88±0.02 to 0.99±0.01). Thus, our results reveal a hysteresis-like influence of activity-dependent potentiation such that concentric force and/or work were increased but eccentric force and/or work were not. These outcomes may have implications for skeletal muscle locomotor function in vivo.

A molecular model of phosphorylation-based activation and potentiation of tarantula muscle thick filaments

Myosin filaments from many muscles are activated by phosphorylation of their regulatory light chains (RLCs). To elucidate the structural mechanism of activation, we have studied RLC phosphorylation in tarantula thick filaments, whose high-resolution structure is known. In the relaxed state, tarantula RLCs are ~50% non-phosphorylated and 50% mono-phosphorylated, while on activation, mono-phosphorylation increases, and some RLCs become bi-phosphorylated. Mass spectrometry shows that relaxed-state mono-phosphorylation occurs on Ser35, while Ca(2+)-activated phosphorylation is on Ser45, both located near the RLC N-terminus. The sequences around these serines suggest that they are the targets for protein kinase C and myosin light chain kinase (MLCK), respectively. The atomic model of the tarantula filament shows that the two myosin heads ("free" and "blocked") are in different environments, with only the free head serines readily accessible to kinases. Thus, protein kinase C Ser35 mono-phosphorylation in relaxed filaments would occur only on the free heads. Structural considerations suggest that these heads are less strongly bound to the filament backbone and may oscillate occasionally between attached and detached states ("swaying" heads). These heads would be available for immediate actin interaction upon Ca(2)(+) activation of the thin filaments. Once MLCK becomes activated, it phosphorylates free heads on Ser45. These heads become fully mobile, exposing blocked head Ser45 to MLCK. This would release the blocked heads, allowing their interaction with actin. On this model, twitch force would be produced by rapid interaction of swaying free heads with activated thin filaments, while prolonged exposure to Ca(2+) on tetanus would recruit new MLCK-activated heads, resulting in force potentiation.

Myosin light chain kinase and the role of myosin light chain phosphorylation in skeletal muscle

Skeletal muscle myosin light chain kinase (skMLCK) is a dedicated Ca(2+)/calmodulin-dependent serine-threonine protein kinase that phosphorylates the regulatory light chain (RLC) of sarcomeric myosin. It is expressed from the MYLK2 gene specifically in skeletal muscle fibers with most abundance in fast contracting muscles. Biochemically, activation occurs with Ca(2+) binding to calmodulin forming a (Ca(2+))(4)•calmodulin complex sufficient for activation with a diffusion limited, stoichiometric binding and displacement of a regulatory segment from skMLCK catalytic core. The N-terminal sequence of RLC then extends through the exposed catalytic cleft for Ser15 phosphorylation. Removal of Ca(2+) results in the slow dissociation of calmodulin and inactivation of skMLCK. Combined biochemical properties provide unique features for the physiological responsiveness of RLC phosphorylation, including (1) rapid activation of MLCK by Ca(2+)/calmodulin, (2) limiting kinase activity so phosphorylation is slower than contraction, (3) slow MLCK inactivation after relaxation and (4) much greater kinase activity relative to myosin light chain phosphatase (MLCP). SkMLCK phosphorylation of myosin RLC modulates mechanical aspects of vertebrate skeletal muscle function. In permeabilized skeletal muscle fibers, phosphorylation-mediated alterations in myosin structure increase the rate of force-generation by myosin cross bridges to increase Ca(2+)-sensitivity of the contractile apparatus. Stimulation-induced increases in RLC phosphorylation in intact muscle produces isometric and concentric force potentiation to enhance dynamic aspects of muscle work and power in unfatigued or fatigued muscle. Moreover, RLC phosphorylation-mediated enhancements may interact with neural strategies for human skeletal muscle activation to ameliorate either central or peripheral aspects of fatigue.

Myosin light-chain phosphorylation and potentiation of dynamic function in mouse fast muscle

The intent of this study was to determine if the stimulation-induced increase or "potentiation" of dynamic function of mouse extensor digitorum longus muscle (in vitro 25°C) during work cycles is graded to myosin regulatory light-chain (RLC) phosphorylation. To do this, concentric force and muscle work output during sinusoidal length changes were determined before (unpotentiated) and after (potentiated) the application of conditioning stimuli (CS) producing incremental elevations in RLC phosphorylation from rest. Sine wave excursion was from 1.09 to 0.91 of L (o) with a period of 142 ms; stimulating muscles to twitch and generate force during these cycles produced plots of force × displacement termed work loops. Stimulation at 2.5-, 5.0-, and 100-Hz elevated RLC phosphorylation from 0.16±0.02 (rest) to 0.29±0.03, 0.45±0.02 and 0.56±0.02 mol phos per mole RLC, respectively (n= 6-7, P<0.05). These CS potentiated mean concentric force (at all lengths) to 1.14±0.02, 1.26±0.04 and 1.41±0.06 of pre-stimulus, control levels (all n= 5-7, P<0.05) while work was increased to 1.07±0.02, 1.17±0.02 and 1.34±0.03 of controls, respectively. In a No CS condition that did not elevate RLC phosphorylation, neither mean concentric force nor work was altered. Thus, strong correlations between RLC phosphorylation and mean concentric force and work support the hypothesis that this molecular mechanism modulates muscle power output. No length-dependence for concentric force potentiation was observed in any condition, an outcome suggesting that interactions between instantaneous variations in muscle length and shortening velocity during work cycles modulates the potentiation response.

Potentiation of the triceps brachii during voluntary submaximal contractions

INTRODUCTION

The effects of postactivation potentiation (PAP) on evoked contractions are well understood, but less is known about the effect of PAP on voluntary submaximal contractions. Using a measure of neuromuscular efficiency (NME) [NME = (mV EMG / Nm torque)] we explored the effects of PAP in the triceps brachii at two muscle lengths.

METHODS

Evoked twitch and NME were compared at short (40° elbow flexion) and long (120°) muscle lengths. At each length, 12 subjects performed a contraction of 25% maximum voluntary contraction (MVC) torque before and after a potentiating MVC.

RESULTS

Twitch torque potentiated more at short length (216.9 ± 169.3%) than at long length (77.3 ± 32.6%), but PAP moderately improved NME only at short length (12.2 ± 8.7%).

CONCLUSIONS

The greater capacity for PAP at the short length is reflected by greater NME. Compared with evoked responses, the relatively small change in NME suggests a different and more modest role of PAP during voluntary submaximal contractions.

Decreased motor unit discharge rate in the potentiated human tibialis anterior muscle

AIM

The purpose of this study was to examine the influence of post-activation potentiation (PAP), the transient increase in low-frequency isometric force observed after muscle activity, on motor unit discharge rates measured during submaximal contractions.

METHODS

A quadrifilar needle electrode was inserted into the tibialis anterior muscle to determine discharge rate of individual motor units while monopolar electrodes were used to monitor the root-mean-square (RMS) and mean power frequency (MPF) of the surface EMG signal. Control (unpotentiated) and experimental (potentiated) measures were obtained during a 5 s voluntary contraction at 50% of maximal. In between these measures, subjects performed a 10 s maximal voluntary contraction (MVC) to induce PAP.

RESULTS

All subjects data are reported as means ± SEM (n = 10). Compared to baseline values measured prior to the MVC, isometric twitch force measured immediately after the MVC was increased by 260 ± 16% (day 3). Motor unit discharge rate in the potentiated tibialis anterior muscle decreased by approx. 10%, from 20.3 ± 0.8 (before) to 18.3 ± 0.99 pps (P = 0.01) (after). Moreover, the MPF was decreased by approx. 9% (from 58.1 ± 2.84 to 53.6 ± 2.85 Hz; P = 0.01) in the potentiated tibialis anterior. On the other hand, consistent with the absence of fatigue during the MVC, the RMS signal was not altered in the potentiated tibialis anterior (0.29 ± 0.03 vs. 0.33 ± 0.04 mV; P = 0.07).

CONCLUSION

Motor unit discharge rates determined during a brief, submaximal contraction were decreased in the potentiated human tibialis anterior muscle.

Label-free proteomic identification of endogenous, insulin-stimulated interaction partners of insulin receptor substrate-1

Protein-protein interactions are key to most cellular processes. Tandem mass spectrometry (MS/MS)-based proteomics combined with co-immunoprecipitation (CO-IP) has emerged as a powerful approach for studying protein complexes. However, a majority of systematic proteomics studies on protein-protein interactions involve the use of protein overexpression and/or epitope-tagged bait proteins, which might affect binding stoichiometry and lead to higher false positives. Here, we report an application of a straightforward, label-free CO-IP-MS/MS method, without the use of protein overexpression or protein tags, to the investigation of changes in the abundance of endogenous proteins associated with a bait protein, which is in this case insulin receptor substrate-1 (IRS-1), under basal and insulin stimulated conditions. IRS-1 plays a central role in the insulin signaling cascade. Defects in the protein-protein interactions involving IRS-1 may lead to the development of insulin resistance and type 2 diabetes. HPLC-ESI-MS/MS analyses identified eleven novel endogenous insulin-stimulated IRS-1 interaction partners in L6 myotubes reproducibly, including proteins play an important role in protein dephosphorylation [protein phosphatase 1 regulatory subunit 12A, (PPP1R12A)], muscle contraction and actin cytoskeleton rearrangement, endoplasmic reticulum stress, and protein folding, as well as protein synthesis. This novel application of label-free CO-IP-MS/MS quantification to assess endogenous interaction partners of a specific protein will prove useful for understanding how various cell stimuli regulate insulin signal transduction.

Signaling to myosin regulatory light chain in sarcomeres

Myosin regulatory light chain (RLC) phosphorylation in skeletal and cardiac muscles modulates Ca(2+)-dependent troponin regulation of contraction. RLC is phosphorylated by a dedicated Ca(2+)-dependent myosin light chain kinase in fast skeletal muscle, where biochemical properties of RLC kinase and phosphatase converge to provide a biochemical memory for RLC phosphorylation and post-activation potentiation of force development. The recent identification of cardiac-specific myosin light chain kinase necessary for basal RLC phosphorylation and another potential RLC kinase (zipper-interacting protein kinase) provides opportunities for new approaches to study signaling pathways related to the physiological function of RLC phosphorylation and its importance in cardiac muscle disease.

Cardiac myosin light chain kinase is necessary for myosin regulatory light chain phosphorylation and cardiac performance in vivo

In contrast to studies on skeletal and smooth muscles, the identity of kinases in the heart that are important physiologically for direct phosphorylation of myosin regulatory light chain (RLC) is not known. A Ca(2+)/calmodulin-activated myosin light chain kinase is expressed only in cardiac muscle (cMLCK), similar to the tissue-specific expression of skeletal muscle MLCK and in contrast to the ubiquitous expression of smooth muscle MLCK. We have ablated cMLCK expression in male mice to provide insights into its role in RLC phosphorylation in normally contracting myocardium. The extent of RLC phosphorylation was dependent on the extent of cMLCK expression in both ventricular and atrial muscles. Attenuation of RLC phosphorylation led to ventricular myocyte hypertrophy with histological evidence of necrosis and fibrosis. Echocardiography showed increases in left ventricular mass as well as end-diastolic and end-systolic dimensions. Cardiac performance measured as fractional shortening decreased proportionally with decreased cMLCK expression culminating in heart failure in the setting of no RLC phosphorylation. Hearts from female mice showed similar responses with loss of cMLCK associated with diminished RLC phosphorylation and cardiac hypertrophy. Isoproterenol infusion elicited hypertrophic cardiac responses in wild type mice. In mice lacking cMLCK, the hypertrophic hearts showed no additional increases in size with the isoproterenol treatment, suggesting a lack of RLC phosphorylation blunted the stress response. Thus, cMLCK appears to be the predominant protein kinase that maintains basal RLC phosphorylation that is required for normal physiological cardiac performance in vivo.

Fatigue-induced reductions of torque and shortening velocity are muscle dependent

INTRODUCTION

In addition to torque (force) loss during dynamic fatiguing contractions, an important aspect of fatigue is the reduction in shortening velocity. Relatively few reports have studied changes in shortening velocity in response to fatigue, and inconsistencies in the results might depend on the muscle tested. To better understand fatigue-related changes in shortening velocity in different muscles, we compared the fatigue response of two muscles in humans with different physiological properties using the same criterion for task failure.

METHODS

During separate sessions for the triceps brachii and soleus, 10 young males performed repetitive shortening contractions with a moderate load (50% of maximum voluntary isometric contraction (MVC) torque) until they achieved a 50% reduction in loaded shortening velocity at task failure.

RESULTS

In the unfatigued condition, these muscles expressed different physiological features with similar power output but approximately 1.5 times slower shortening velocity, approximately 1.4 times slower normalized evoked 50-Hz maximum rate of relaxation, and approximately 47% lower postactivation potentiation in the soleus compared with the triceps brachii. During the fatiguing task, significantly less total work was performed in the soleus ( approximately 1986 J) compared with the triceps brachii ( approximately 3940 J), suggesting greater endurance in the triceps brachii. Despite similar relative reductions in shortening velocity in both muscles at task failure, the soleus had a smaller reduction in MVC torque than the triceps brachii at task failure. Furthermore, in both muscles, there was a greater reduction at task failure and a faster restoration of shortening velocity compared with MVC torque.

CONCLUSION

Our findings suggest that fatigue-related mechanisms that reduce torque are not the same as those that reduce shortening velocity.

Posttetanic potentiation in mdx muscle

X-linked muscular dystrophy of the mouse (mdx) has been reported to progressively remodel skeletal muscle to preferentially reduce fast fiber composition. Despite this, mdx muscle displays normal levels of posttetanic potentiation (PTP). Since PTP may primarily depend on phosphorylation of the myosin regulatory light chain (RLC) in fast muscle fibers, maintenance of PTP with mdx disease progression is paradoxical and may represent an adaptation of the diseased muscle. This study assesses the role of RLC phosphorylation during PTP of mdx muscle. Extensor digitorum longus muscles were isolated from mdx and from C57BL/10 (control) mice at ~50 (young) and ~300 (adult) days and stimulated in vitro (25°C) to induce PTP. During potentiation, muscles were harvested for subsequent determination of RLC phosphorylation levels. Immunofluorescence was used to assess muscle fiber type composition and no age effects were found. The magnitude of PTP was higher (P < 0.05) in mdx than control muscles at both young (mdx: 21.9 ± 1.6%; control: 17.7 ± 1.2%) and adult (mdx: 30.4 ± 1.8%; control: 23.2 ± 2.2%) ages. However, RLC phosphate content was similar between all groups both at rest and following stimulation. Our results are consistent with a model where the sensitivity of mdx muscle to RLC phosphorylation-induced force potentiation is increased by disease- and age-dependent alterations in excitation-contraction coupling noted for mdx and aging muscle.

In vivo phosphoproteome of human skeletal muscle revealed by phosphopeptide enrichment and HPLC-ESI-MS/MS

Protein phosphorylation plays an essential role in signal transduction pathways that regulate substrate and energy metabolism, contractile function, and muscle mass in human skeletal muscle. Abnormal phosphorylation of signaling enzymes has been identified in insulin-resistant muscle using phosphoepitope-specific antibodies, but its role in other skeletal muscle disorders remains largely unknown. This may be in part due to insufficient knowledge of relevant targets. Here, we therefore present the first large-scale in vivo phosphoproteomic study of human skeletal muscle from 3 lean, healthy volunteers. Trypsin digestion of 3-5 mg human skeletal muscle protein was followed by phosphopeptide enrichment using SCX and TiO(2). The resulting phosphopeptides were analyzed by HPLC-ESI-MS/MS. Using this unbiased approach, we identified 306 distinct in vivo phosphorylation sites in 127 proteins, including 240 phosphoserines, 53 phosphothreonines, and 13 phosphotyrosines in at least 2 out of 3 subjects. In addition, 61 ambiguous phosphorylation sites were identified in at least 2 out of 3 subjects. The majority of phosphoproteins detected are involved in sarcomeric function, excitation-contraction coupling (the Ca(2+)-cycle), glycolysis, and glycogen metabolism. Of particular interest, we identified multiple novel phosphorylation sites on several sarcomeric Z-disk proteins known to be involved in signaling and muscle disorders. These results provide numerous new targets for the investigation of human skeletal muscle phosphoproteins in health and disease and demonstrate feasibility of phosphoproteomics research of human skeletal muscle in vivo.

Cardiac myosin is a substrate for zipper-interacting protein kinase (ZIPK)

Zipper-interacting protein kinase (ZIPK) is a member of the death-associated protein kinase family associated with apoptosis in nonmuscle cells where it phosphorylates myosin regulatory light chain (RLC) to promote membrane blebbing. ZIPK mRNA and protein are abundant in heart tissue and isolated ventricular neonatal rat cardiac myocytes. An unbiased substrate search performed with purified ZIPK on heart homogenates led to the discovery of a prominent 20-kDa protein substrate identified as RLC of ventricular myosin. Biochemical analyses showed ZIPK phosphorylated cardiac RLC at Ser-15 with a V(max) value 2-fold greater than the value for smooth/nonmuscle RLC; cardiac RLC is a favorable biochemical substrate. Knockdown of ZIPK in cardiac myocytes by small interfering RNA significantly decreased the extent of RLC Ser-15 phosphorylation. Thus, ZIPK may act as a cardiac RLC kinase and thereby affect contractility.

Causes of fatigue in slow-twitch rat skeletal muscle during dynamic activity

Skeletal muscle fatigue is most often studied in vitro at room temperature and is classically defined as a decline in maximum force production or power output, exclusively linked to repeated isometric contractions. However, most muscles shorten during normal use, and we propose that both the functional correlate of fatigue, as well as the fatigue mechanism, will be different during dynamic contractions compared with static contractions. Under isoflurane anesthesia, fatigue was induced in rat soleus muscles in situ by isotonic shortening contractions at 37 degrees C. Muscles were stimulated repeatedly for 1 s at 30 Hz every 2 s for a total of 15 min. The muscles were allowed to shorten isotonically against a load corresponding to one-third of maximal isometric force. Maximal unloaded shortening velocity (V(0)), maximum force production (F(max)), and isometric relaxation rate (-dF/dt) was reduced after 100 s but returned to almost initial values at the end of the stimulation protocol. Likewise, ATP and creatine phosphate (CrP) were reduced after 100 s, but the level of CrP was partially restored to initial values after 15 min. The rate of isometric force development, the velocity of shortening, and isotonic shortening were also reduced at 100 s, but in striking contrast, did not recover during the remainder of the stimulation protocol. The regulatory myosin light chain (MLC2s) was dephosphorylated after 100 s and did not recover. Although metabolic changes may account for the changes of F(max), -dF/dt, and V(0), dephosphorylation of MLC2s may be involved in the fatigue seen as sustained slower contraction velocities and decreased muscle shortening.

The molecular effects of skeletal muscle myosin regulatory light chain phosphorylation

Phosphorylation of the myosin regulatory light chain (RLC) in skeletal muscle has been proposed to act as a molecular memory of recent activation by increasing the rate of force development, ATPase activity, and isometric force at submaximal activation in fibers. It has been proposed that these effects stem from phosphorylation-induced movement of myosin heads away from the thick filament backbone. In this study, we examined the molecular effects of skeletal muscle myosin RLC phosphorylation using in vitro motility assays. We showed that, independently of the thick filament backbone, the velocity of skeletal muscle myosin is decreased upon phosphorylation due to an increase in the myosin duty cycle. Furthermore, we did not observe a phosphorylation-dependent shift in calcium sensitivity in the absence of the myosin thick filament. These data suggest that phosphorylation-induced movement of myosin heads away from the thick filament backbone explains only part of the observed phosphorylation-induced changes in myosin mechanics. Last, we showed that the duty cycle of skeletal muscle myosin is strain dependent, consistent with the notion that strain slows the rate of ADP release in striated muscle.

Signaling processes for initiating smooth muscle contraction upon neural stimulation

Relationships among biochemical signaling processes involved in Ca2+/calmodulin (CaM)-dependent phosphorylation of smooth muscle myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) were determined. A genetically-encoded biosensor MLCK for measuring Ca(2+)-dependent CaM binding and activation was expressed in smooth muscles of transgenic mice. We performed real-time evaluations of the relationships among [Ca2+](i), MLCK activation, and contraction in urinary bladder smooth muscle strips neurally stimulated for 3 s. Latencies for the onset of [Ca2+](i) and kinase activation were 55 +/- 8 and 65 +/- 6 ms, respectively. Both increased with RLC phosphorylation at 100 ms, whereas force latency was 109 +/- 3 ms. [Ca2+](i), kinase activation, and RLC phosphorylation responses were maximal by 1.2 s, whereas force increased more slowly to a maximal value at 3 s. A delayed temporal response between RLC phosphorylation and force is probably due to mechanical effects associated with elastic elements in the tissue. MLCK activation partially declined at 3 s of stimulation with no change in [Ca2+](i) and also declined more rapidly than [Ca2+](i) during relaxation. The apparent desensitization of MLCK to Ca2+ activation appears to be due to phosphorylation in its calmodulin binding segment. Phosphorylation of two myosin light chain phosphatase regulatory proteins (MYPT1 and CPI-17) or a protein implicated in strengthening membrane adhesion complexes for force transmission (paxillin) did not change during force development. Thus, neural stimulation leads to rapid increases in [Ca2+](i), MLCK activation, and RLC phosphorylation in phasic smooth muscle, showing a tightly coupled Ca2+ signaling complex as an elementary mechanism initiating contraction.

The motor complex of Plasmodium falciparum: phosphorylation by a calcium-dependent protein kinase

Calcium-dependent protein kinases (CDPKs) of Apicomplexan parasites are crucial for the survival of the parasite throughout its life cycle. CDPK1 is expressed in the asexual blood stages of the parasite, particularly late stage schizonts. We have identified two substrates of Plasmodium falciparum CDPK1: myosin A tail domain-interacting protein (MTIP) and glideosome-associated protein 45 (GAP45), both of which are components of the motor complex that generates the force required by the parasite to actively invade host cells. Indirect immunofluorescence shows that CDPK1 localizes to the periphery of P. falciparum merozoites and is therefore suitably located to act on MTIP and GAP45 at the inner membrane complex. A proportion of both GAP45 and MTIP is phosphorylated in schizonts, and we demonstrate that both proteins can be efficiently phosphorylated by CDPK1 in vitro. A primary phosphorylation of MTIP occurs at serine 47, whereas GAP45 is phosphorylated at two sites, one of which could also be detected in phosphopeptides purified from parasite lysates. Both CDPK1 activity and host cell invasion can be inhibited by the kinase inhibitor K252a, suggesting that CDPK1 is a suitable target for antimalarial drug development.

Differential integration of Ca2+-calmodulin signal in intact ventricular myocytes at low and high affinity Ca2+-calmodulin targets

Cardiac myocyte intracellular calcium varies beat-to-beat and calmodulin (CaM) transduces Ca2+ signals to regulate many cellular processes (e.g. via CaM targets such as CaM-dependent kinase and calcineurin). However, little is known about the dynamics of how CaM targets process the Ca2+ signals to generate appropriate biological responses in the heart. We hypothesized that the different affinities of CaM targets for the Ca2+-bound CaM (Ca2+-CaM) shape their actions through dynamic and tonic interactions in response to the repetitive Ca2+ signals in myocytes. To test our hypothesis, we used two fluorescence resonance energy transfer-based biosensors, BsCaM-45 (Kd = approximately 45 nm) and BsCaM-2 (Kd = approximately 2 nm), to monitor the real time Ca2+-CaM dynamics at low and high affinity CaM targets in paced adult ventricular myocytes. Compared with BsCaM-2, BsCaM-45 tracks the beat-to-beat Ca2+-CaM alterations more closely following the Ca2+ oscillations at each myocyte contraction. When pacing frequency is raised from 0.1 to 1.0 Hz, the higher affinity BsCaM-2 demonstrates significant elevation of diastolic Ca2+-CaM binding compared with the lower affinity BsCaM-45. Biochemically detailed computational models of Ca2+-CaM biosensors in beating cardiac myocytes revealed that the different Ca2+-CaM binding affinities of BsCaM-2 and BsCaM-45 are sufficient to predict their differing kinetics and diastolic integration. Thus, data from both experiments and computational modeling suggest that CaM targets with low versus high Ca2+-CaM affinities (like CaM-dependent kinase versus calcineurin) respond differentially to the same Ca2+ signal (phasic versus integrating), presumably tuned appropriately for their respective and distinct Ca2+ signaling pathways.

Myosin regulatory light chain phosphorylation attenuates cardiac hypertrophy

Hyperphosphorylation of myosin regulatory light chain (RLC) in cardiac muscle is proposed to cause compensatory hypertrophy. We therefore investigated potential mechanisms in genetically modified mice. Transgenic (TG) mice were generated to overexpress Ca2+/calmodulin-dependent myosin light chain kinase specifically in cardiomyocytes. Phosphorylation of sarcomeric cardiac RLC and cytoplasmic nonmuscle RLC increased markedly in hearts from TG mice compared with hearts from wild-type (WT) mice. Quantitative measures of RLC phosphorylation revealed no spatial gradients. No significant hypertrophy or structural abnormalities were observed up to 6 months of age in hearts of TG mice compared with WT animals. Hearts and cardiomyocytes from WT animals subjected to voluntary running exercise and isoproterenol treatment showed hypertrophic cardiac responses, but the responses for TG mice were attenuated. Additional biochemical measurements indicated that overexpression of the Ca2+/calmodulin-binding kinase did not perturb other Ca2+/calmodulin-dependent processes involving Ca2+/calmodulin-dependent protein kinase II or the protein phosphatase calcineurin. Thus, increased myosin RLC phosphorylation per se does not cause cardiac hypertrophy and probably inhibits physiological and pathophysiological hypertrophy by contributing to enhanced contractile performance and efficiency.

Hydroxyl radical and glutathione interactions alter calcium sensitivity and maximum force of the contractile apparatus in rat skeletal muscle fibres

Studies on intact muscle fibres indicate that reactive oxygen species (ROS) produced during muscle activity, or applied exogenously, can cause decreased force responses primarily by reducing the Ca(2+) sensitivity of the contractile apparatus. Identification of the molecular basis of this effect is complicated by the fact that studies on skinned muscle fibres in general have not observed reduced contractile Ca(2+) sensitivity when applying ROS, predominantly H(2)O(2). Here, using skinned fibres from rat extensor digitorum longus (EDL) and soleus muscle, it is shown that although H(2)O(2) (> or = 100 microm) has little effect by itself, when added in the presence of myoglobin it causes marked reduction in the Ca(2+) sensitivity of the contractile apparatus, probably due to production of hydroxyl radicals (OH(*)). Maximum force production is also reduced, but only with larger or more prolonged treatments. The effects are not prevented by tempol, a potent superoxide scavenger. Dithiotreitol (DTT) produces little reversal of the sensitivity change if applied afterwards, but it does substantially reverse all the changes if applied before the fibre undergoes an activation sequence. When glutathione (GSH, 5 mM) is present, exposure of EDL fibres to H(2)O(2) and myoglobin causes an increase in Ca(2+) sensitivity, with longer treatments causing a subsequent decrease, whereas in soleus fibres it causes only decreases in sensitivity and maximum force. The increased Ca(2+) sensitivity in EDL fibres is evidently due to the summed actions of (i) a potentiating effect of glutathionylation, which can be reversed by DTT and only occurs in fast-twitch fibres, and (ii) a less reversible reduction in sensitivity. Western blotting showed that reductions in Ca(2+) sensitivity were not due to loss of troponin-C. The present findings help provide a mechanistic basis for diverse findings on the effects of ROS in muscle fibres and implicate OH(*) radicals and glutathione as likely mediators of the effects.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.