High molecular weight proteins in cardiac and skeletal muscle junctional sarcoplasmic reticulum vesicles bind calmodulin, are phosphorylated, and are degraded by Ca2+-activated protease

Activation of Ca2+ transport in cardiac microsomes enriches functional sets of ER and SR proteins

The importance of sarcoplasmic reticulum (SR) Ca2+-handling in heart has led to detailed understanding of Ca2+-release and re-uptake protein complexes, while less is known about other endoplasmic reticulum (ER) functions in the heart. To more fully understand cardiac SR and ER functions, we analyzed cardiac microsomes based on their increased density through the actions of the SR Ca2+-ATPase (SERCA) and the ryanodine receptor that are highly active in cardiomyocytes. Crude cardiac microsomal vesicles loaded with Ca oxalate produced two higher density subfractions, MedSR and HighSR. Proteins from 20.0 μg of MV, MedSR, and HighSR protein were fractionated using SDS-PAGE, then trypsinized from 20 separate gel pieces, and analyzed by LC-MS/MS to determine protein content. From 62,000 individual peptide spectra obtained, we identified 1105 different proteins, of which 354 were enriched ≥ 2.0-fold in SR fractions compared to the crude membrane preparation. Previously studied SR proteins were all enriched, as were proteins associated with canonical ER functions. Contractile, mitochondrial, and sarcolemmal proteins were not enriched. Comparing the levels of SERCA-positive SR proteins in MedSR versus HighSR vesicles produced a range of SR subfraction enrichments signifying differing levels of Ca2+ leak co-localized in the same membrane patch. All known junctional SR proteins were more enriched in MedSR, while canonical ER proteins were more enriched in HighSR membrane. Proteins constituting other putative ER/SR subdomains also exhibited average Esub enrichment values (mean ± S.D.) that spanned the range of possible Esub values, suggesting that functional sets of proteins are localized to the same areas of the ER/SR membrane. We conclude that active Ca2+ loading of cardiac microsomes, reflecting the combined activities of Ca2+ uptake by SERCA, and Ca2+ leak by RyR, permits evaluation of multiple functional ER/SR subdomains. Sets of proteins from these subdomains exhibited similar enrichment patterns across membrane subfractions, reflecting the relative levels of SERCA and RyR present within individual patches of cardiac ER and SR.

Activation of Ca transport in cardiac microsomes enriches functional sets of ER and SR proteins

The importance of sarcoplasmic reticulum (SR) Ca-handling in heart has led to detailed understanding of Ca-release and re-uptake protein complexes, while less is known about other endoplasmic reticulum (ER) functions in the heart. To more fully understand cardiac SR and ER functions, we analyzed cardiac microsomes based on their increased density through the actions of the SR Ca-ATPase (SERCA) and the ryanodine receptor that are highly active in cardiomyocytes. Crude cardiac microsomal vesicles loaded with Ca oxalate produced two higher density subfractions, MedSR and HighSR. Analyses of protein enrichments from the 3 membrane preparations (crude microsomes, MedSR, and HighSR), showed that only a third of microsomal proteins in heart, or 354 proteins, were enriched ≥2.0-fold in SR. Previously studied SR proteins were all enriched, as were proteins associated with canonical ER functions. Contractile, mitochondrial, and sarcolemmal proteins were not enriched. Comparing the levels of SERCA-positive SR proteins in MedSR versus HighSR vesicles produced a range of SR subfraction enrichments signifying differing levels of Ca leak (ryanodine receptor) co-localized in the same membrane patch. All known junctional SR proteins were more enriched in MedSR, while canonical ER proteins were more enriched in HighSR membrane. Proteins from other putative ER/SR subdomains also showed characteristic distributions among SR subpopulations. We conclude that active Ca loading of cardiac microsomes, reflecting the combined activities of Ca uptake by SERCA, and Ca leak by RyR, permits evaluation of multiple functional ER/SR subdomains. Sets of proteins from these subdomains exhibited similar enrichment patterns across membrane subfractions, reflecting the relative levels of SERCA and RyR present within individual patches of cardiac ER and SR.

Resolved Structural States of Calmodulin in Regulation of Skeletal Muscle Calcium Release

Calmodulin (CaM) is proposed to modulate activity of the skeletal muscle sarcoplasmic reticulum (SR) calcium release channel (ryanodine receptor, RyR1 isoform) via a mechanism dependent on the conformation of RyR1-bound CaM. However, the correlation between CaM structure and functional regulation of RyR in physiologically relevant conditions is largely unknown. Here, we have used time-resolved fluorescence resonance energy transfer (TR-FRET) to study structural changes in CaM that may play a role in the regulation of RyR1. We covalently labeled each lobe of CaM (N and C) with fluorescent probes and used intramolecular TR-FRET to assess interlobe distances when CaM is bound to RyR1 in SR membranes, purified RyR1, or a peptide corresponding to the CaM-binding domain of RyR (RyRp). TR-FRET resolved an equilibrium between two distinct structural states (conformations) of CaM, each characterized by an interlobe distance and Gaussian distribution width (disorder). In isolated CaM, at low Ca²⁺, the two conformations of CaM are resolved, centered at 5 nm (closed) and 7 nm (open). At high Ca²⁺, the equilibrium shifts to favor the open conformation. In the presence of RyRp at high Ca²⁺, the closed conformation shifts to a more compact conformation and is the major component. When CaM is bound to full-length RyR1, either purified or in SR membranes, strikingly different results were obtained: 1) the two conformations are resolved and more ordered, 2) the open state is the major component, and 3) Ca²⁺ stabilized the closed conformation by a factor of two. We conclude that the Ca²⁺-dependent structural distribution of CaM bound to RyR1 is distinct from that of CaM bound to RyRp. We propose that the function of RyR1 is tuned to the Ca²⁺-dependent structural dynamics of bound CaM.

STIM2 regulates both intracellular Ca2+ distribution and Ca2+ movement in skeletal myotubes

Stromal interaction molecule 1 (STIM1) along with Orai1 mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism in various tissues including skeletal muscle. However, the role(s) of STIM2, a homolog of STIM1, in skeletal muscle has not been well addressed. The present study, first, was focused on searching for STIM2-binding proteins from among proteins mediating skeletal muscle functions. This study used a binding assay, quadrupole time-of-flight mass spectrometry, and co-immunoprecipitation assay with bona-fide STIM2- and SERCA1a-expressing rabbit skeletal muscle. The region for amino acids from 453 to 729 of STIM2 binds to sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a). Next, oxalate-supported ⁴⁵Ca²⁺-uptake experiments and various single-myotube Ca²⁺ imaging experiments using STIM2-knockdown mouse primary skeletal myotubes have suggested that STIM2 attenuates SERCA1a activity during skeletal muscle contraction, which contributes to the intracellular Ca²⁺ distribution between the cytosol and the SR at rest. In addition, STIM2 regulates Ca²⁺ movement through RyR1 during skeletal muscle contraction as well as SOCE. Therefore, via regulation of SERCA1a activity, STIM2 regulates both intracellular Ca²⁺ distribution and Ca²⁺ movement in skeletal muscle, which makes it both similar to, yet different from, STIM1.

Mitsugumin 53 regulates extracellular Ca2+ entry and intracellular Ca2+ release via Orai1 and RyR1 in skeletal muscle

Mitsugumin 53 (MG53) participates in the membrane repair of various cells, and skeletal muscle is the major tissue that expresses MG53. Except for the regulatory effects of MG53 on SERCA1a, the role(s) of MG53 in the unique functions of skeletal muscle such as muscle contraction have not been well examined. Here, a new MG53-interacting protein, Orai1, is identified in skeletal muscle. To examine the functional relevance of the MG53-Orai1 interaction, MG53 was over-expressed in mouse primary or C2C12 skeletal myotubes and the functional properties of the myotubes were examined using cell physiological and biochemical approaches. The PRY-SPRY region of MG53 binds to Orai1, and MG53 and Orai1 are co-localized in the plasma membrane of skeletal myotubes. MG53-Orai1 interaction enhances extracellular Ca²⁺ entry via a store-operated Ca²⁺ entry (SOCE) mechanism in skeletal myotubes. Interestingly, skeletal myotubes over-expressing MG53 or PRY-SPRY display a reduced intracellular Ca²⁺ release in response to K⁺-membrane depolarization or caffeine stimulation, suggesting a reduction in RyR1 channel activity. Expressions of TRPC3, TRPC4, and calmodulin 1 are increased in the myotubes, and MG53 directly binds to TRPC3, which suggests a possibility that TRPC3 also participates in the enhanced extracellular Ca²⁺ entry. Thus, MG53 could participate in regulating extracellular Ca²⁺ entry via Orai1 during SOCE and also intracellular Ca²⁺ release via RyR1 during skeletal muscle contraction.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

A review of the pharmacology and clinical uses of pimobendan

OBJECTIVE

To review the pharmacology, research developments, and clinical uses of pimobendan

DATA SOURCES

Original research articles and clinical studies from 1984 to August 2011.

VETERINARY DATA SYNTHESIS

Pimobendan is approved for use in dogs for the treatment of congestive heart failure (CHF) secondary to chronic valvular heart disease (CVHD) and dilated cardiomyopathy (DCM). Expert-based veterinary guidelines recommend the use of pimobendan in the management of acute, hospital-based therapy for patients with CHF attributable to CVHD.

CONCLUSIONS

The use of pimobendan, an inodilator with phosphodiesterase 3 (PDE3) inhibitory and calcium-sensitizing properties, is regarded as a component of the standard of care in the management of dogs with CHF secondary to both DCM and CVHD. Further studies are warranted to confirm the safety and efficacy of pimobendan for the off-label use of this drug in asymptomatic CVHD, pulmonary arterial hypertension, asymptomatic myocardial diseases, CHF from all other causes and in cats with CHF.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Thermodynamics of calmodulin binding to cardiac and skeletal muscle ryanodine receptor ion channels

The skeletal muscle (RyR1) and cardiac muscle (RyR2) ryanodine receptor calcium release channels contain a single, conserved calmodulin (CaM) binding domain, yet are differentially regulated by CaM. Here, we report that high-affinity [(35)S]CaM binding to RyR1 is driven by favorable enthalpic and entropic contributions at Ca(2+) concentrations from <0.01 to 100 microM. At 0.15 microM Ca(2+), [(35)S]CaM bound to RyR2 with decreased affinity and binding enthalpy compared with RyR1. The rates of [(35)S]CaM dissociation from RyR1 increased as the temperature was raised, whereas at 0.15 microM Ca(2+) the rate from RyR2 was little affected. The results suggest major differences in the energetics of CaM binding to and dissociation from RyR1 and RyR2.

Inhibitors of cyclic nucleotide phosphodiesterase 3 and 5 as therapeutic agents in heart failure

Cyclic nucleotide phosphodiesterases (PDE) 3 and 5 regulate cAMP and cGMP signalling in cardiac and smooth muscle myocytes. Important advances in the understanding of the roles of these enzymes have recently been made. PDE3 inhibitors have inotropic and vasodilatory properties, and although they acutely improve haemodynamics in patients with heart failure, they do not improve long-term morbidity and mortality. Although combination therapy with beta-adrenergic receptor antagonists or selective inhibition of specific PDE3 isoforms might result in a more favourable long-term outcome, more clinical data are needed to test this proposition. The role of PDE5 inhibitors in the treatment of cardiac disease is evolving. PDE5 inhibitors cause pulmonary and systemic vasodilation. How these drugs will compare with other vasodilators in terms of long-term outcomes in patients with heart failure is unknown. Recent studies also suggest that PDE5 inhibitors may have antihypertropic effects, exerted through increased myocardial cGMP signalling, that could be of additional benefit in patients with heart failure.

The Foot Structure from the Type 1 Ryanodine Receptor Is Required for Functional Coupling to Store-operated Channels

In the present study we have explored structural determinants of the functional interaction between skeletal muscle ryanodine receptor (RyR1) and transient receptor potential channel 1 (TRPC1) channels expressed in Chinese hamster ovary cells. We have illustrated a functional interaction between TRPC1 channels and RyR1 for the regulation of store-operated calcium entry (SOCE) initiated after releasing calcium from a caffeine-sensitive intracellular calcium pool. RNA interference experiments directed to reduce the amount of TRPC1 protein indicate that RyR1 associates to at least two different types of store-operated channels (SOCs), one dependent and one independent of TRPC1. In contrast, bradykinin-induced SOCE is completely dependent on the presence of TRPC1 protein, as we have previously illustrated. Removing the foot structure from RyR1 results in normal caffeine-induced release of calcium from internal stores but abolishes the activation of SOCE, indicating that this structure is require for functional coupling to SOCs. The footless RyR1 protein shows a different cellular localization when compared with wild type RyR1. The later protein shows a higher percentage of colocalization with FM-464, a marker of plasma membrane. The implications of the foot structure for the functional and physical coupling to TRPC and SOCs is discussed.

Calsequestrin and the calcium release channel of skeletal and cardiac muscle

Calsequestrin is by far the most abundant Ca(2+)-binding protein in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. It allows the Ca2+ required for contraction to be stored at total concentrations of up to 20mM, while the free Ca2+ concentration remains at approximately 1mM. This storage capacity confers upon muscle the ability to contract frequently with minimal run-down in tension. Calsequestrin is highly acidic, containing up to 50 Ca(2+)-binding sites, which are formed simply by clustering of two or more acidic residues. The Kd for Ca2+ binding is between 1 and 100 microM, depending on the isoform, species and the presence of other cations. Calsequestrin monomers have a molecular mass of approximately 40 kDa and contain approximately 400 residues. The monomer contains three domains each with a compact alpha-helical/beta-sheet thioredoxin fold which is stable in the presence of Ca2+. The protein polymerises when Ca2+ concentrations approach 1mM. The polymer is anchored at one end to ryanodine receptor (RyR) Ca2+ release channels either via the intrinsic membrane proteins triadin and junctin or by binding directly to the RyR. It is becoming clear that calsequestrin has several functions in the lumen of the SR in addition to its well-recognised role as a Ca2+ buffer. Firstly, it is a luminal regulator of RyR activity. When triadin and junctin are present, calsequestrin maximally inhibits the Ca2+ release channel when the free Ca2+ concentration in the SR lumen is 1mM. The inhibition is relieved when the Ca2+ concentration alters, either because of small changes in the conformation of calsequestrin or its dissociation from the junctional face membrane. These changes in calsequestrin's association with the RyR amplify the direct effects of luminal Ca2+ concentration on RyR activity. In addition, calsequestrin activates purified RyRs lacking triadin and junctin. Further roles for calsequestrin are indicated by the kinase activity of the protein, its thioredoxin-like structure and its influence over store operated Ca2+ entry. Clearly, calsequestrin plays a major role in calcium homeostasis that extends well beyond its ability to buffer Ca2+ ions.

Ca2+/calmodulin-dependent protein kinase II phosphorylation regulates the cardiac ryanodine receptor

The cardiac ryanodine receptor (RyR2)/calcium release channel on the sarcoplasmic reticulum is required for muscle excitation-contraction coupling. Using site-directed mutagenesis, we identified the specific Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylation site on recombinant RyR2, distinct from the site for protein kinase A (PKA) that mediates the "fight-or-flight" stress response. CaMKII phosphorylation increased RyR2 Ca2+ sensitivity and open probability. CaMKII was activated at increased heart rates, which may contribute to enhanced Ca2+-induced Ca2+ release. Moreover, rate-dependent CaMKII phosphorylation of RyR2 was defective in heart failure. CaMKII-mediated phosphorylation of RyR2 may contribute to the enhanced contractility observed at higher heart rates. The full text of this article is available online at http://circres.ahajournals.org.

Calcineurin and intracellular Ca2+-release channels: regulation or association?

The Ca(2+)- and calmodulin-dependent phosphatase calcineurin was reported to interact with the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor (RyR) and to modulate their phosphorylation status and activity. However, controversial data on the molecular mechanisms involved and on the functional relevance of calcineurin for these channel-complexes have been described. Hence, we will focus on the functional importance of calcineurin for IP(3)R and RyR function and on the different mechanisms by which Ca(2+)-dependent dephosphorylation can affect the gating of those intracellular Ca(2+)-release channels. Since many studies made use of immunosuppressive drugs that are inhibiting calcineurin activity, we will also have to take the different side effects of these drugs into account for the proper interpretation of the effects of calcineurin on intracellular Ca(2+)-release channels. In addition, it became recently known that various other phosphatases and kinases can associate with these channels, thereby forming macromolecular complexes. The relevance of these enzymes for IP(3)R and RyR functioning will be reviewed since in some cases they could interfere with the effects ascribed to calcineurin. Finally, we will discuss the downstream effects of calcineurin on the regulation of the expression levels of intracellular Ca(2+)-release channels as well as the relation between IP(3)R- and RyR-mediated Ca(2+) release and calcineurin-dependent gene expression.

The calmodulin binding region of the skeletal ryanodine receptor acts as a self-modulatory domain

A synthetic peptide (CaMBP) matching amino acids 3614-3643 of the skeletal ryanodine receptor (RyR1) binds to both Ca2+-free calmodulin (CaM) and Ca2+-bound CaM with nanomolar affinity [J. Biol. Chem. 276 (2001) 2069]. We report here that CaMBP increases [3H]ryanodine binding to RyR1 in a dose- and Ca2+-dependent manner; it also induces Ca2+ release from SR vesicles, and increases open probability (P(o)) of single RyR channels reconstituted in planar lipid bilayers. Further, CaMBP removes CaM associated with SR vesicles and increases [3H]ryanodine binding to purified RyR1, suggesting that its mechanism of action is two-fold: it removes endogenous inhibitors and also interacts directly with complementary regions in RyR1. Remarkably, the N-terminus of CaMBP activates RyRs while the C-terminus of CaMBP inhibits RyR activity, suggesting the presence of two discrete functional subdomains within this region. A ryr1 mutant lacking this region, RyR1-Delta3614-3643, was constructed and expressed in dyspedic myoblasts (RyR1-knockout). The depolarization-, caffeine- and 4-chloro-m-cresol (4-CmC)-induced Ca2+ transients in these cells were dramatically reduced compared with cells expressing wild type RyR1. Deletion of the 3614-3643 region also resulted in profound changes in unitary conductance and channel gating. We thus propose that the RyR1 3614-3643 region acts not only as the CaM binding site, but also as an important modulatory domain for RyR1 function.

PDE3 inhibition in dilated cardiomyopathy: reasons to reconsider

BACKGROUND

PDE3 cyclic nucleotide phosphodiesterases have important roles in regulating cAMP- and cGMP-mediated signaling. Drugs that inhibit these enzymes raise cAMP and cGMP content in cardiac and vascular smooth muscle and increase the phosphorylation of proteins by cAMP- and cGMP-dependent protein kinases (PK-A and PK-G), thereby eliciting inotropic and vasodilatory responses.

METHODS

Although these actions are beneficial acutely in patients with dilated cardiomyopathy, long-term use of these agents was shown in several clinical trials to increase mortality. Several new clinical studies, however, suggest PDE3 inhibitors may be safe and effective when used in conjunction with beta-adrenergic receptor antagonists, whereas new studies at the cellular and molecular levels indicate that there are several isoforms of these enzymes in cardiac and vascular myocytes that are likely to regulate cAMP content in different intracellular compartments.

CONCLUSIONS

Both sets of observations suggest that PDE3 inhibition may be refined to allow more selective effects on phosphorylation of PK-A substrates, possibly allowing the beneficial effects of PDE3 inhibition to be separated from the adverse long-term consequences of their use.

PKA phosphorylation activates the calcium release channel (ryanodine receptor) in skeletal muscle: defective regulation in heart failure

The type 1 ryanodine receptor (RyR1) on the sarcoplasmic reticulum (SR) is the major calcium (Ca2+) release channel required for skeletal muscle excitation-contraction (EC) coupling. RyR1 function is modulated by proteins that bind to its large cytoplasmic scaffold domain, including the FK506 binding protein (FKBP12) and PKA. PKA is activated during sympathetic nervous system (SNS) stimulation. We show that PKA phosphorylation of RyR1 at Ser2843 activates the channel by releasing FKBP12. When FKB12 is bound to RyR1, it inhibits the channel by stabilizing its closed state. RyR1 in skeletal muscle from animals with heart failure (HF), a chronic hyperadrenergic state, were PKA hyperphosphorylated, depleted of FKBP12, and exhibited increased activity, suggesting that the channels are "leaky." RyR1 PKA hyperphosphorylation correlated with impaired SR Ca2+ release and early fatigue in HF skeletal muscle. These findings identify a novel mechanism that regulates RyR1 function via PKA phosphorylation in response to SNS stimulation. PKA hyperphosphorylation of RyR1 may contribute to impaired skeletal muscle function in HF, suggesting that a generalized EC coupling myopathy may play a role in HF.

Inhibitors of cyclic nucleotide phosphodiesterase PDE3 as adjunct therapy for dilated cardiomyopathy

PDE3 cyclic nucleotide phosphodiesterases are important in cyclic AMP (cAMP) and possibly cyclic GMP-mediated signalling in cardiac and vascular smooth muscle myocytes. Drugs that inhibit these enzymes have inotropic and vasodilatory actions that have proven useful in the short-term treatment of contractile failure and pulmonary hypertension in dilated cardiomyopathy (both ischaemic and idiopathic). With long-term usage, however, these drugs appear to increase mortality in treated patients through an as yet undetermined mechanism that is in some way attributable to an increase in intracellular cAMP content in cardiac myocytes. Several recent clinical trials have raised the possibility that these drugs may be used to advantage in dilated cardiomyopathy when they are administered in combination with beta-adrenoceptor antagonists, which act to lower intracellular cAMP content. In this review, the relevant basic and clinical data are examined and the possible justification for the combination of two therapies with seemingly opposite effects on intracellular cAMP content is considered.

Mass spectrometry of cardiac calsequestrin characterizes microheterogeneity unique to heart and indicative of complex intracellular transit

Cardiac calsequestrin concentrates in junctional sarcoplasmic reticulum in heart and skeletal muscle cells by an undefined mechanism. During transit through the secretory pathway, it undergoes an as yet uncharacterized glycosylation and acquires phosphate on CK2-sensitive sites. In this study, we have shown that active calsequestrin phosphorylation occurred in nonmuscle cells as well as muscle cells, reflecting a widespread cellular process. To characterize this post-translational modification and resolve individual molecular mass species, we subjected purified calsequestrin to mass spectrometry using electrospray ionization. Mass spectra showed that calsequestrin glycan structure in nonmuscle cells was that expected for an endoplasmic reticulum-localized glycoprotein and showed that each glycoform existed as four mass peaks representing molecules that also had 0-3 phosphorylation sites occupied. In heart, mass peaks indicated carbohydrate modifications characteristic of transit through Golgi compartments. Phosphorylation did not occur on every glycoform present, suggesting a far more complex movement of calsequestrin molecules in heart cells. Significant amounts of calsequestrin contained glycan with only a single mannose residue, indicative of a novel post-endoplasmic reticulum mannosidase activity. In conclusion, glyco- and phosphoforms of calsequestrin chart a complex cellular transport in heart, with calsequestrin following trafficking pathways not present or not accessible to the same molecules in nonmuscle.

Lack of effect of cADP-ribose and NAADP on the activity of skeletal muscle and heart ryanodine receptors

The calcium release channels/ryanodine receptors (RyRs) are potential/putative targets of cADPR (cyclic ADP-ribose) action in many tissue systems. In striated muscles, where RyRs predominate, cADPR action on these channels is controversial. Here cADPR modulation of cardiac and skeletal muscle RyR channels was tested. We considered factors reported as necessary for cADPR action, such as the presence of calmodulin and/or FK binding proteins (FKBPs). We found: 1) The RyR channel isoforms were insensitive to cADPR (or its metabolite NAADP [nicotinic acid adenine dinucleotide phosphate]) under all conditions examined, as studied by: 1a) single channel recordings in planar lipid bilayers; 1b) macroscopic behavior of the RyRs in sarcoplasmic reticulum (SR) microsomes (including crude microsome preparations likely to retain putative cADPR cofactors) at room temperature and at 37 degrees C (net energized Ca2+ uptake or passive Ca2+ leak); 2) [32P]cADPR did not bind significantly to SR microsomes; 3) cADPR did not affect FKBP association to SR membranes. We conclude that cADPR does not interact directly with RyRs or RyR-associated SR proteins. Our results under in vitro conditions suggest that c ADPR effects on Ca2+ signaling observed in vivo in mammalian striated muscle cells may reflect indirect modulation of RyRs or RyR-independent Ca2+ release systems.

Calreticulin and calsequestrin are differentially distributed in canine heart

Cardiac sarcoplasmic reticulum (SR) consists of three continuous yet distinct regions: longitudinal, corbular, and junctional. Ca(2+)uptake, catalyzed by the Ca(2+)-dependent ATPase, is thought to occur throughout the SR whereas Ca(2+)release occurs in the region of the junctional SR. In the SR, Ca(2+)is stored in a complex with Ca(2+)-binding proteins such as calsequestrin. In the present study, the distribution of the high-affinity calcium-binding protein, calreticulin, in canine cardiac SR was determined and compared with the distribution of other SR proteins. Three types of distribution were observed. Many proteins, including the Ca(2+)-ATPase and two mannose-containing glycoproteins (52 and 131 kDa), were present in all subfractions. These proteins were absent or depleted from the sarcolemma-enriched fraction. The ryanodine receptor/Ca(2+)release channel and calsequestrin were localized to the junctional SR. Calreticulin immunoreactivity was detected in fractions containing longitudinal SR but not in fractions enriched in sarcolemma or junctional SR. Hence calreticulin is present in a unique vesicular fraction and the Ca(2+)-binding proteins calreticulin and calsequestrin display different patterns of distribution in canine heart.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Therapeutic potential of cyclic nucleotide phosphodiesterase inhibitors in heart failure

There are several reasons to believe that agents that augment cAMP-mediated signalling in cardiac myocytes should have beneficial effects in patients with heart failure. However, clinical trials of first-generation cyclic nucleotide phosphodiesterase (PDE3) inhibitors, which raise cAMP content by blocking its hydrolysis, have shown that chronic administration of these drugs affect survival adversely. The problem may be the non-selective activation of a broad spectrum of cAMP-regulated cellular responses these agents elicit. More selective (or alternatively selective) cyclic nucleotide PDE inhibitors might improve results by evoking a more restricted set of cellular responses.

Postulated role of interdomain interaction within the ryanodine receptor in Ca(2+) channel regulation

Localized distribution of malignant hyperthermia (MH) and central core disease (CCD) mutations in N-terminal and central domains of the ryanodine receptor suggests that the interaction between these domains may be involved in Ca(2+) channel regulation. To test this hypothesis, we investigated the effects of a new synthetic domain peptide DP4 corresponding to the Leu(2442)-Pro(2477) region of the central domain. DP4 enhanced ryanodine binding and induced a rapid Ca(2+) release. The concentration for half-maximal activation by agonists was considerably reduced in the presence of DP4. These effects of DP4 are analogous to the functional modifications of the ryanodine receptor caused by MH/CCD mutations (viz. hyperactivation of the channel and hypersensitization of the channel to agonists). Replacement of Arg of DP4 with Cys, mimicking the in vivo Arg(2458)-to-Cys(2458) mutation, abolished the activating effects of DP4. An N-terminal domain peptide DP1 (El-Hayek, R., Saiki, Y., Yamamoto, T., and Ikemoto, N. (1999) J. Biol. Chem. 274, 33341-33347) shows similar activation/sensitization effects. The addition of both DP4 and DP1 produced mutual interference of their activating functions. We tentatively propose that contact between the two (N-terminal and central) domains closes the channel, whereas removal of the contact by these domain peptides or by MH/CCD mutations de-blocks the channel, resulting in hyperactivation/hyper-sensitization effects.

Calmodulin and cyclic ADP-ribose interaction in Ca2+ signaling related to cardiac sarcoplasmic reticulum: superoxide anion radical-triggered Ca2+ release

Reactive oxygen species (ROS) are often shown to damage cellular functions. The targets of oxidative damage depend on the nature of ROS produced and the site of generation. In contrast, ROS can also regulate signal transduction. In this case, ROS may either induce or enhance events, which lead to forward directions of cellular signaling. The consequences of regulation of signal transduction can be observed in physiological processes such as muscle contraction. Here, we discuss the concentration-dependent effects of superoxide anion radical (*O2-) on Ca2+ release from the cardiac sarcoplasmic reticulum (SR). Recent studies suggest that the ADP-ribosyl cyclase pathway, through its production of cyclic adenosine 5'-diphosphoribose (cADPR), may control Ca2+ mobilization in cardiac muscle cells. *O2- has dual effects that are concentration dependent. At low concentrations (nearly nanomolar levels), *O2- induces Ca2+ release by stimulating synthesis of cADPR, which requires calmodulin for sensitization of ryanodine-sensitive Ca2+-release channels (RyRC). At these low concentrations, *O2- is responsible for regulation of cellular signal transduction. At higher concentrations (micromolar levels), *O2- produces a loss in the function of calmodulin that is to inhibit RyRC. This results in an increase in Ca2+ release, which is linked to cell injury. The difference in the functions of low and high concentrations of *O2- may result in two distinct physiological roles in cardiac muscle Ca2+ signaling.

A postulated role of the near amino-terminal domain of the ryanodine receptor in the regulation of the sarcoplasmic reticulum Ca(2+) channel

To test the hypothesis that interactions among several putative domains of the ryanodine receptor (RyR) are involved in the regulation of its Ca(2+) release channel, we synthesized several peptides corresponding to selected NH(2)-terminal regions of the RyR. We then examined their effects on ryanodine binding and Ca(2+) release activities of the sarcoplasmic reticulum isolated from skeletal and cardiac muscle. Peptides 1-2s, 1-2c, and 1 enhanced ryanodine binding to cardiac RyR and induced a rapid Ca(2+) release from cardiac SR in a dose-dependent manner. The order of the potency for the activation of the Ca(2+) release channel was 1-2c > 1 > 1-2s. Interestingly, these peptides produced significant activation of the cardiac RyR at near zero or subactivating [Ca(2+)], indicating that the peptides enhanced the Ca(2+) sensitivity of the channel. Peptides 1-2c, 1-2s, and 1 had virtually no effect on skeletal RyR, although occasional and variable extents of activation were observed in ryanodine binding assays performed at 36 degrees C. Peptide 3 affected neither cardiac nor skeletal RyR. We propose that domains 1 and 1-2 of the RyR, to which these activating peptides correspond, would interact with one or more other domains within the RyR (including presumably the Ca(2+)-binding domain) to regulate the Ca(2+) channel.

Effect of beta-adrenoceptor activation on [Ca2+]i regulation in murine skeletal myotubes

The present study used real-time confocal microscopy to examine the effects of the beta2-adrenoceptor agonist salbutamol on regulation of intracellular Ca2+ concentration ([Ca2+]i) in myotubes derived from neonatal mouse limb muscles. Immunocytochemical staining for ryanodine receptors and skeletal muscle myosin confirmed the presence of sarcomeres. The myotubes displayed both spontaneous and ACh-induced rapid (<2-ms rise time) [Ca2+]i transients. The [Ca2+]i transients were frequency modulated by both low and high concentrations of salbutamol. Exposure to alpha-bungarotoxin and tetrodotoxin inhibited ACh-induced [Ca2+]i transients and the response to low concentrations of salbutamol but not the response to higher concentrations. Preexposure to caffeine inhibited the subsequent [Ca2+]i response to lower concentrations of salbutamol and significantly blunted the response to higher concentrations. Preexposure to salbutamol diminished the [Ca2+]i response to caffeine. Inhibition of dihydropyridine-sensitive Ca2+ channels with nifedipine or PN-200-110 did not prevent [Ca2+]i elevations induced by higher concentrations of salbutamol. The effects of salbutamol were mimicked by the membrane-permeant analog dibutyryl adenosine 3', 5'-cyclic monophosphate. These data indicate that salbutamol effects in skeletal muscle predominantly involve enhanced sarcoplasmic reticulum Ca2+ release.

Oxidation of the skeletal muscle Ca2+ release channel alters calmodulin binding

This study presents evidence for a close relationship between the oxidation state of the skeletal muscle Ca2+ release channel (RyR1) and its ability to bind calmodulin (CaM). CaM enhances the activity of RyR1 in low Ca2+ and inhibits its activity in high Ca2+. Oxidation, which activates the channel, blocks the binding of 125I-labeled CaM at both micromolar and nanomolar Ca2+ concentrations. Conversely, bound CaM slows oxidation-induced cross-linking between subunits of the RyR1 tetramer. Alkylation of hyperreactive sulfhydryls (<3% of the total sulfhydryls) on RyR1 with N-ethylmaleimide completely blocks oxidant-induced intersubunit cross-linking and inhibits Ca2+-free 125I-CaM but not Ca2+/125I-CaM binding. These studies suggest that 1) the sites on RyR1 for binding apocalmodulin have features distinct from those of the Ca2+/CaM site, 2) oxidation may alter the activity of RyR1 in part by altering its interaction with CaM, and 3) CaM may protect RyR1 from oxidative modifications during periods of oxidative stress.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

E-C coupling failure in mouse EDL muscle after in vivo eccentric contractions

The objectives of this research were to determine the contribution of excitation-contraction (E-C) coupling failure to the decrement in maximal isometric tetanic force (Po) in mouse extensor digitorum longus (EDL) muscles after eccentric contractions and to elucidate possible mechanisms. The left anterior crural muscles of female ICR mice (n = 164) were injured in vivo with 150 eccentric contractions. Po, caffeine-, 4-chloro-m-cresol-, and K+-induced contracture forces, sarcoplasmic reticulum (SR) Ca2+ release and uptake rates, and intracellular Ca2+ concentration ([Ca2+]i) were then measured in vitro in injured and contralateral control EDL muscles at various times after injury up to 14 days. On the basis of the disproportional reduction in Po (approximately 51%) compared with caffeine-induced force (approximately 11-21%), we estimate that E-C coupling failure can explain 57-75% of the Po decrement from 0 to 5 days postinjury. Comparable reductions in Po and K+-induced force (51%), and minor reductions (0-6%) in the maximal SR Ca2+ release rate, suggest that the E-C coupling defect site is located at the t tubule-SR interface immediately after injury. Confocal laser scanning microscopy indicated that resting [Ca2+]i was elevated and peak tetanic [Ca2+]i was reduced, whereas peak 4-chloro-m-cresol-induced [Ca2+]i was unchanged immediately after injury. By 3 days postinjury, 4-chloro-m-cresol-induced [Ca2+]i became depressed, probably because of decreased SR Ca2+ release and uptake rates (17-31%). These data indicate that the decrease in Po during the first several days after injury primarily stems from a failure in the E-C coupling process.

Superoxide anion radical-triggered Ca2+ release from cardiac sarcoplasmic reticulum through ryanodine receptor Ca2+ channel

The ryanodine receptor Ca2+ channel (RyRC) constitutes the Ca2+-release pathway in sarcoplasmic reticulum (SR) of cardiac muscle. A direct mechanical and a Ca2+-triggered mechanism (Ca2+-induced Ca2+ release) have been proposed to explain the in situ activation of Ca2+ release in cardiac muscle. A variety of chemical oxidants have been shown to activate RyRC; however, the role of modification induced by oxygen-derived free radicals in pathological states of the muscle remains to be elucidated. It has been hypothesized that oxygen-derived free radicals initiate Ca2+-mediated functional changes in or damage to cardiac muscle by acting on the SR and promoting an increase in Ca2+ release. We confirmed that superoxide anion radical (O2-) generated from hypoxanthine-xanthine oxidase reaction decreases calmodulin content and increases 45Ca2+ efflux from the heavy fraction of canine cardiac SR vesicles; hypoxanthine-xanthine oxidase also decreases Ca2+ free within the intravesicular space of the SR with no effect on Ca2+-ATPase activity. Current fluctuations through single Ca2+-release channels have been monitored after incorporation into planar phospholipid bilayers. We demonstrate that activation of the channel by O2- is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O2--triggered Ca2+ release through the RyRC. For the first time, we show that O2- stimulates Ca2+ release from heavy SR vesicles and suggest the importance of accessory proteins such as calmodulin in modulating the effect of O2-. The decreased calmodulin content induced by oxygen-derived free radicals, especially O2-, is a likely mechanism of accumulation of cytosolic Ca2+ (due to increased Ca2+ release from SR) after reperfusion of the ischemic heart.

Functional interactions between cytoplasmic domains of the skeletal muscle Ca2+ release channel

The skeletal muscle Ca2+ release channel (RYR1), which plays a critical role in excitation-contraction coupling, is a homotetramer with a subunit molecular mass of 565 kDa. Oxidation of the channel increases its activity and produces intersubunit cross-links within the RYR1 tetramer (Aghdasi, B., Zhang, J., Wu, Y., Reid, M. B., and Hamilton, S. L. (1997) J. Biol. Chem. 272, 3739-3748). Alkylation of hyperreactive sulfhydryls on RYR1 with N-ethylmaleimide (NEM) inhibits channel function and blocks the intersubunit cross-linking. We used calpain and tryptic cleavage, two-dimensional SDS-polyacrylamide gel electrophoresis, N-terminal sequencing, sequence-specific antibody Western blotting, and [14C]NEM labeling to identify the domains involved in these effects. Our data are consistent with a model in which 1) diamide, an oxidizing agent, simultaneously produces an intermolecular cross-link between adjacent subunits within the RYR1 tetramer and an intramolecular cross-link within a single subunit; 2) all of the cysteines involved in both cross-links are in either the region between amino acids approximately 2100 and 2843 or the region between amino acids 2844 and 4685; 3) oxidation exposes a new calpain cleavage site in the central domain of the RYR1 (in the region around amino acid 2100); 4) sulfhydryls that react most rapidly with NEM are located in the N-terminal domain (between amino acids 426 and 1396); 5) alkylation of the N-terminal cysteines completely inhibits the formation of both inter- and intrasubunit cross-links. In summary, we present evidence for interactions between the N-terminal region and the putatively cytoplasmic central domains of RYR1 that appear to influence subunit-subunit interactions and channel activity.

Factors influencing [3H]ryanodine binding to the skeletal muscle Ca2+ release channel

Optimal [3H]ryanodine binding to skeletal muscle sarcoplasmic reticulum membranes is dependent on a number of factors such as Ca2+ concentration, ionic strength, and the presence of modulators of the Ca2+ release channel. The rate of association of [3H]-ryanodine with its binding site is slower than a diffusion limited process, and often the binding reaches a peak value which is followed by a slow decline. This phenomenon makes it extremely difficult to determine kinetic constants for [3H]ryanodine binding. The inclusion of bovine serum albumin (BSA) or the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) in the incubation buffer prevents the decrease in [3H]ryanodine binding observed in association studies. BSA or Chaps slows this decline in binding partially by preventing a conversion to a more rapidly dissociating component. Pretreatment of the membranes with Chaps does not prevent the decrease in [3H]ryanodine binding, suggesting that Chaps is not exerting its effect by extracting a lipid or peripheral membrane protein. The decrease in affinity observed in the absence of BSA and Chaps appears to require the occupation of the high-affinity ryanodine binding site. Incubation for extended times in the absence of [3H]ryanodine prior to the initiation of the association produced similar curves to those obtained without preincubation. These combined results suggest that Chaps and BSA stabilize the ryanodine-modified Ca2+ release channel by preventing an alteration in the ryanodine binding site which leads to decreased affinity, thus allowing for a more quantitative interpretation of binding data.

Modulation of the skeletal muscle ryanodine receptor by endogenous phosphorylation of 160/150-kDa proteins of the sarcoplasmic reticulum

This paper demonstrates and characterizes the inhibition of ryanodine binding caused by the phosphorylation of the 160/150-kDa proteins in skeletal muscle sarcoplasmic reticulum (SR). Inhibition of ryanodine binding was obtained by preincubation of SR membranes with ATP + NaF . The inhibition was characterized by the following findings: (a) If ATP was replaced by AdoPP[NH]P, inhibition of ryanodine binding activity was not observed. (b) The inhibitory effect of preincubation with ATP + NaF, like the phosphorylation of 150/160-kDa proteins, was Ca2+ dependent. (c) Inhibition of ryanodine binding, as the protein phosphorylation, was not observed if NaF (> 30 mM) was replaced with okadaic acid. (d) The optimal pH for the inhibition and the phosphorylation was about 7.0. (e) Both the phosphorylation of the 160/150-kDa proteins and inhibition of ryanodine binding were prevented by dichlorobenzimidazole riboside and hemin, inhibitors of casein kinase II. (f) Dephosphorylation of the 160/150-kDa proteins prevented the inhibition of ryanodine binding. (g) The presence of NP-40 during the phosphorylation prevented both the 160/150-kDa phosphorylation and the inhibition of ryanodine binding. Furthermore, a linear relationship was obtained between the degree of ryanodine binding inhibition and the level of phosphorylation of the 160/150-kDa proteins, as controlled by ATP or NaF concentrations. The binding affinity for Ca2+ of the ryanodine receptor (RyR) was modified by phosphorylation of the 160/150-kDa proteins, decreasing by up to 100-fold. The phosphorylation of the SR membranes resulted in an elimination of ryanodine binding sites with slight effect on the ryanodine binding affinity. These results suggest the modulation of the properties of the RyR by phosphorylation/dephosphorylation of the 160/150-kDa proteins. The identification of the phosphorylated 160/150-kDa proteins, their kinase, and the structural interactions between them and the RyR are presented in the accompanying paper.

Evidence for tissue-associated alpha(2) macroglobulin in mouse skeletal muscle

Alpha(2)-Macroglobulin (alpha(2)M), a major serum protease inhibitor, was localized in mouse skeletal muscle by immunoperoxidase histochemistry. In all muscles examined (mm. soleus, plantaris, and extensor digitorum longus) specific immunoreactivity occurred diffusely in extracellular structures (periendomysium, blood vessel wall) as well as inside about a half of the muscle fibers. This localization pattern did not change substantially by extensively perfusing deeply anesthetized mice with phosphate buffered saline (PBS) to remove serum alpha(2)M. In release experiments on fresh (nonfixed) cryostat sections, specific immunoreactivity persisted after an extensive prewash with PBS (up to 5-6 h), but a new specific staining appeared inside those fibers that were originally negative. Western blotting experiments were negative on the soluble fraction of muscle homogenate, thus confirming that the perfusion procedure was effective in removing serum alpha(2)M. By contrast, three specific bands (185, 165, and 35 kDa) appeared in detergent-solubilized extracts (0.3% Triton X-100), indicating the occurrence of tissue-associated alpha(2)M. Confocal immunofluorescence microscopy revealed that the intracellular specific staining was associated to a longitudinal network, probably corresponding to the sarcoplasmic reticulum. A multifunctional role of alpha(2)M in skeletal muscle was hypothesized.

Purification, primary structure, and immunological characterization of the 26-kDa calsequestrin binding protein (junctin) from cardiac junctional sarcoplasmic reticulum

Previously we identified a protein of apparent M(r) = 26,000 as the major calsequestrin binding protein in junctional sarcoplasmic reticulum vesicles isolated from cardiac and skeletal muscle (Mitchell, R. D., Simmerman, H. K. B., and Jones, L. R. (1988) J. Biol. Chem. 263, 1376-1381). Here we describe the purification and primary structure of the 26-kDa calsequestrin binding protein. The protein was purified 164-fold from cardiac microsomes and shown by immunoblotting to be highly enriched in junctional membrane subfractions. It ran as a closely spaced doublet on SDS-polyacrylamide gel electrophoresis and bound 125I-calsequestrin intensely. Cloning of the cDNA predicted a protein of 210 amino acids containing a single transmembrane domain. The protein has a short N-terminal region located in the cytoplasm, and the bulk of the molecule, which is highly charged and basic, projects into the sarcoplasmic reticulum lumen. Significant homologies were found with triadin and aspartyl beta-hydroxylase, suggesting that all three proteins are members of a family of single membrane-spanning endoplasmic reticulum proteins. Immunocytochemical labeling localized the 26-kDa protein to junctional sarcoplasmic reticulum in cardiac and skeletal muscle. The same gene product was expressed in these two tissues. The calsequestrin binding activity of the 26-kDa protein combined with its codistribution with calsequestrin and ryanodine receptors strongly suggests that the protein plays an important role in the organization and/or function of the Ca2+ release complex. Because the 26-kDa calsequestrin binding protein is an integral component of the junctional sarcoplasmic reticulum membrane in cardiac and skeletal muscle, we have named it Junctin.

Calmodulin binding sites of the skeletal, cardiac, and brain ryanodine receptor Ca2+ channels: modulation by the catalytic subunit of cAMP-dependent protein kinase?

In this study, we define calmodulin binding sites of skeletal, cardiac, and brain ryanodine receptor (RYR) Ca2+ channels. Cardiac and brain RYR peptides corresponding to the calmodulin binding sites present in the skeletal RYR [Menegazzi, P., et al. (1994) Biochemistry 33, 9078-9084] were synthesized, and their interaction with calmodulin was monitored by fluorescent techniques. The central portions of the skeletal, cardiac, and brain RYR protomers display one high (CaM1; Kd ranging between 2.7 and 10.2 nM) and one low affinity (CaM2; Kd ranging between 116 and 142 nM) calmodulin binding site. Depending on the RYR model having 4 or 12 transmembrane segments, a third calmodulin binding site (CaM3) was identified a few residues upstream from the putative transmembrane segment M1 or M5. Its affinity for calmodulin varied between the RYR isoforms: the cardiac RYR CaM3 displays a high affinity (9.09 +/- 1.0 nM, n = 5), while the skeletal and brain RYR CaM3 have low affinity, the lowest affinity being displayed by the brain isoform (234 +/- 39 nM, n = 3). The RYRs calmodulin binding site CaM1 encompasses the sequence Arg-His-Arg-Val(Ile)-Ser-Leu, which is phosphorylated in vitro by the catalytic subunit of the cAMP-dependent protein kinase. Phosphorylation of RYR PM1 peptides occurs on the Ser, corresponding to amino acid number 2919, 3020, and 3055 of the brain, cardiac, and skeletal RYR protomers, respectively. We found that phosphorylation of the RYR PM1 peptides was inhibited by calmodulin binding and that the formation of the PM1 peptide-calmodulin complex was inhibited by peptide phosphorylation. These data indicate that the effect of calmodulin binding to RYR CaM1 may be regulated by the phosphorylation state of the Ser residue localized within the sequence Arg-His-Arg-Val(Ile)-Ser-Leu.

Identification and characterization of three calmodulin binding sites of the skeletal muscle ryanodine receptor

In the present study, we have identified calmodulin binding sequences in the skeletal muscle ryanodine receptor Ca2+ release channel. Ligand overlays on RYR fusion proteins indicate that the skeletal muscle RYR contains three calmodulin binding regions defined by residues 2937-3225, 3546-3655, and 4425-4621. The RYR fusion protein PC28 (residues 2937-3225) bound calmodulin in the presence of EGTA and Ca2+, while RYR fusion protein PC26 (residues 3546-3655) exhibited strong calmodulin binding at 10 microM Ca2+. The RYR fusion protein PC15 (residues 4425-4621) did not bind calmodulin in the presence of either EGTA or 10-50 microM Ca2+. In the presence of 100-500 microM Ca2+, the RYR fusion protein PC15 exhibited an affinity for calmodulin of approximately 50 nM. Peptides RYR1 PM2 (residues 3610-3629) and RYR1 PM3 (4534-4552) encompassing putative RYR-calmodulin binding sites were synthesized. The synthetic peptides interacted directly with dansylcalmodulin as demonstrated by their capacity to affect the fluorescence emission of dansylcalmodulin. Missense mutation analysis indicates that the Lys and Arg residues are essential for calmodulin binding to the synthetic peptide RYR1 PM3. The RYR calmodulin binding site defined by peptide PM3 lies in the myoplasmic loop 2, a few residues upstream of the putative transmembrane segment M5; the other two calmodulin binding sites are next to the putative transmembrane segments M' and M''. Thus, the effect of calmodulin on Ca2+ release might involve the regulation of the putative transmembrane segments M5, M', and M''.

Calcium-dependent block of ryanodine receptor channel of swine skeletal muscle by direct binding of calmodulin

The interaction of the Ca2+ binding protein calmodulin (CaM) with the ryanodine receptor of the sarcoplasmic reticulum (SR) of pig skeletal muscle was investigated by [3H]-ryanodine binding, planar bilayer recordings, and rapid filtration of 45Ca(2+)-loaded SR. Inhibition of [3H]-ryanodine binding by CAM was phosphorylation-independent, had an IC50 of approximately 0.1 microM and was optimal at 10 microM Ca(2+). CaM also inhibited [3H]-ryanodine binding to CHAPS-solubilized and purified ryanodine receptors, suggesting a direct CaM-ryanodine receptor interaction. In single channel recordings, CaM blocked Ca2+ release channels in a Ca(2+)-dependent manner by decreasing the number of open events per unit time without affecting the mean open time or unitary channel conductance. Rapid filtration of 45Ca2+ passively loaded into SR vesicles showed that CaM blocked Ca2+ release within milliseconds of exposure of SR to a Ca2+ release medium containing 10 microM CaM. In controls, an increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 47 +/- 10% of the 45Ca2+ in 20 ms. CaM reduced the release to 23 +/- 12% in the same period. These results are compatible with a direct mechanism of Ca2+ release channel blockade by CaM and suggest that CaM could play a significant role in the inactivation of SR Ca2+ release during excitation-contraction coupling.

Calmodulin interaction with the skeletal muscle sarcoplasmic reticulum calcium channel protein

Studies were initiated to define the equilibria of calmodulin binding to the skeletal muscle sarcoplasmic reticulum (SR) Ca(2+)-release channel protein in native SR vesicles. Calmodulin affinity-labeling experiments indicated that the major calmodulin receptor in heavy SR preparations was a protein of M(r) > 450,000, corresponding to the Ca(2+)-release channel protein. [3H]Ryanodine-binding assays indicated 10.6 +/- 0.9 pmol of high-affinity ryanodine binding per milligram of SR protein. Wheat germ calmodulin was derivatized with rhodamine-x-maleimide. The affinity and binding capacity of the channel protein in SR vesicles for the derivatized calmodulin (Rh-CaM) were determined by fluorescence anisotropy in the presence of (1) 1 mM EGTA, (2) 0.1 mM CaCl2, and (3) 0.1 mM CaCl2 plus 1 mM MgCl2. In the presence of EGTA, Rh-CaM bound to the channel protein with a Kd of 8.6 +/- 0.8 nM and a Bmax of 229 +/- 7 pmol/mg, suggesting that calmodulin binds to the channel protein at [Ca2+] comparable to that in resting muscle. In the presence of 0.1 mM CaCl2, the binding equilibrium shifted to a two-site ligand-binding model; the high-affinity class of sites had a Bmax1 of 54 +/- 7 pmol/mg and a Kd1 of 4.3 +/- 1.1 nM, while the lower affinity class of sites had a Bmax2 of 166 +/- 28 pmol/mg and a Kd2 of 239 +/- 102 nM. In the presence of both Ca2+ and Mg2+, there was a further change in the Rh-CaM/channel protein interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic potentiation of E-C coupling increases force in rat skeletal muscle

We examined the mechanism(s) which allow terbutaline, a beta 2-adrenergic agonist, to increase isometric force in bundles of normal and denervated rat soleus fibers. Terbutaline (10 mumol/L) potentiated tetanic contractions during exposure to 1 mmol/L ouabain, 10 mumol/L nifedipine, or 0.5 mmol/L iodoacetate. Terbutaline induced equivalent increases in submaximal potassium (K+) contracture and tetanic force: these effects were mimicked by 2 mmol/L dibutyryl-cyclic AMP. Therefore, terbutaline increased force by a cyclic AMP-dependent mechanism other than enhancement of sodium-pump activity, dihydropyridine sensitive Ca2+ currents, glycolysis, or action potentials. Pretreatment with 1 mmol/L caffeine induced submaximal potentiation of peak tetanic force but prevented further potentiation by terbutaline. This suggested that terbutaline did not influence the myofilaments, but acted on the sarcoplasmic reticulum (SR) to increase the myoplasmic Ca2+ concentration and hence force production. We speculate that force is potentiated following beta-adrenoceptor activation by a cyclic AMP-dependent phosphorylation of Ca2+ release channels to facilitate SR calcium release during tetanic stimulation.

Enhancement of Ca2+ release channel activity by phosphorylation of the skeletal muscle ryanodine receptor

The Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum (SR) can be phosphorylated by membrane associated protein kinase(s) utilizing endogenously synthesized or exogenously added ATP. The channel protein has been enriched in non-phosphorylated and phosphorylated form from heavy SR following solubilization with CHAPS (3-[(3-cholamidopropyl)dimethylammonio-1-propane-sulfonate) and ultracentrifugation on a linear sucrose/CHAPS gradient. Reconstitution of the isolated channels into planar bilayers shows that phosphorylation enhances the open probability by increasing the sensitivity towards micromolar Ca2+ and ATP. The phosphorylation induced enhancement of the channel activity can be reversed by purified protein phosphatase 2A.

Changes of tension and [Ca2+]i during beta-adrenoceptor activation of single, intact fibres from mouse skeletal muscle

beta-Adrenergic agonists increase tension production in fast-twitch skeletal muscle, but the underlying mechanism is unknown. In the present study we have exposed intact, single fibres from a mouse muscle to the beta 2-adrenergic agonist terbutaline. Fibres were stimulated to produce 350-ms tetani at 20-100 Hz while measuring the myoplasmic Ca2+ concentration ([Ca2+]i) and tension. The fluorescent indicator Indo-1 was used to measure [Ca2+]i. Application of terbutaline resulted in marked increases of both tetanic [Ca2+]i and tension. Terbutaline had no significant effect on myofibrillar function as judged from normal Ca2+ sensitivity and tension production at saturating [Ca2+]i. The rate of [Ca2+]i and tension decline during relaxation was not affected by terbutaline, thus indicating a normal function of the sarcoplasmic reticulum (SR) Ca2+ pumps. The effect of terbutaline developed gradually over 5-10 min when fibres were stimulated each minute; the full effect of terbutaline was also obtained after a 10-min rest period in terbutaline. The [Ca2+]i at rest was not affected by terbutaline. In conclusion, beta-adrenergic stimulation increases tetanic tension by enhancing SR Ca2+ release.