The cytoplasmic loops between domains II and III and domains III and IV in the skeletal muscle dihydropyridine receptor bind to a contiguous site in the skeletal muscle ryanodine receptor

CaV1.1 Calcium Channel Signaling Complexes in Excitation-Contraction Coupling: Insights from Channelopathies

In skeletal muscle, excitation-contraction (EC) coupling relies on the mechanical coupling between two ion channels: the L-type voltage-gated calcium channel (CaV1.1), located in the sarcolemma and functioning as the voltage sensor of EC coupling, and the ryanodine receptor 1 (RyR1), located on the sarcoplasmic reticulum serving as the calcium release channel. To this day, the molecular mechanism by which these two ion channels are linked remains elusive. However, recently, skeletal muscle EC coupling could be reconstituted in heterologous cells, revealing that only four proteins are essential for this process: CaV1.1, RyR1, and the cytosolic proteins CaVβ1a and STAC3. Due to the crucial role of these proteins in skeletal muscle EC coupling, any mutation that affects any one of these proteins can have devastating consequences, resulting in congenital myopathies and other pathologies.Here, we summarize the current knowledge concerning these four essential proteins and discuss the pathophysiology of the CaV1.1, RyR1, and STAC3-related skeletal muscle diseases with an emphasis on the molecular mechanisms. Being part of the same signalosome, mutations in different proteins often result in congenital myopathies with similar symptoms or even in the same disease.

Distinct Components of Retrograde Ca(V)1.1-RyR1 Coupling Revealed by a Lethal Mutation in RyR1

The molecular basis for excitation-contraction coupling in skeletal muscle is generally thought to involve conformational coupling between the L-type voltage-gated Ca(2+) channel (CaV1.1) and the type 1 ryanodine receptor (RyR1). This coupling is bidirectional; in addition to the orthograde signal from CaV1.1 to RyR1 that triggers Ca(2+) release from the sarcoplasmic reticulum, retrograde signaling from RyR1 to CaV1.1 results in increased amplitude and slowed activation kinetics of macroscopic L-type Ca(2+) current. Orthograde coupling was previously shown to be ablated by a glycine for glutamate substitution at RyR1 position 4242. In this study, we investigated whether the RyR1-E4242G mutation affects retrograde coupling. L-type current in myotubes homozygous for RyR1-E4242G was substantially reduced in amplitude (∼80%) relative to that observed in myotubes from normal control (wild-type and/or heterozygous) myotubes. Analysis of intramembrane gating charge movements and ionic tail current amplitudes indicated that the reduction in current amplitude during step depolarizations was a consequence of both decreased CaV1.1 membrane expression (∼50%) and reduced channel Po (∼55%). In contrast, activation kinetics of the L-type current in RyR1-E4242G myotubes resembled those of normal myotubes, unlike dyspedic (RyR1 null) myotubes in which the L-type currents have markedly accelerated activation kinetics. Exogenous expression of wild-type RyR1 partially restored L-type current density. From these observations, we conclude that mutating residue E4242 affects RyR1 structures critical for retrograde communication with CaV1.1. Moreover, we propose that retrograde coupling has two distinct and separable components that are dependent on different structural elements of RyR1.

Intramolecular ex vivo Fluorescence Resonance Energy Transfer (FRET) of Dihydropyridine Receptor (DHPR) β1a Subunit Reveals Conformational Change Induced by RYR1 in Mouse Skeletal Myotubes

The dihydropyridine receptor (DHPR) β_1a subunit is essential for skeletal muscle excitation-contraction coupling, but the structural organization of β_1a as part of the macromolecular DHPR-ryanodine receptor type I (RyR1) complex is still debatable. We used fluorescence resonance energy transfer (FRET) to probe proximity relationships within the β_1a subunit in cultured skeletal myotubes lacking or expressing RyR1. The fluorescein biarsenical reagent FlAsH was used as the FRET acceptor, which exhibits fluorescence upon binding to specific tetracysteine motifs, and enhanced cyan fluorescent protein (CFP) was used as the FRET donor. Ten β_1a reporter constructs were generated by inserting the CCPGCC FlAsH binding motif into five positions probing the five domains of β_1a with either carboxyl or amino terminal fused CFP. FRET efficiency was largest when CCPGCC was positioned next to CFP, and significant intramolecular FRET was observed for all constructs suggesting that in situ the β_1a subunit has a relatively compact conformation in which the carboxyl and amino termini are not extended. Comparison of the FRET efficiency in wild type to that in dyspedic (lacking RyR1) myotubes revealed that in only one construct (H458 CCPGCC β_1a -CFP) FRET efficiency was specifically altered by the presence of RyR1. The present study reveals that the C-terminal of the β_1a subunit changes conformation in the presence of RyR1 consistent with an interaction between the C-terminal of β_1a and RyR1 in resting myotubes.

Three-dimensional localization of the α and β subunits and of the II-III loop in the skeletal muscle L-type Ca2+ channel

The L-type Ca(2+) channel (dihydropyridine receptor (DHPR) in skeletal muscle acts as the voltage sensor for excitation-contraction coupling. To better resolve the spatial organization of the DHPR subunits (α(1s) or Ca(V)1.1, α(2), β(1a), δ1, and γ), we created transgenic mice expressing a recombinant β(1a) subunit with YFP and a biotin acceptor domain attached to its N- and C- termini, respectively. DHPR complexes were purified from skeletal muscle, negatively stained, imaged by electron microscopy, and subjected to single-particle image analysis. The resulting 19.1-Å resolution, three-dimensional reconstruction shows a main body of 17 × 11 × 8 nm with five corners along its perimeter. Two protrusions emerge from either face of the main body: the larger one attributed to the α(2)-δ1 subunit that forms a flexible hook-shaped feature and a smaller protrusion on the opposite side that corresponds to the II-III loop of Ca(V)1.1 as revealed by antibody labeling. Novel features discernible in the electron density accommodate the atomic coordinates of a voltage-gated sodium channel and of the β subunit in a single docking possibility that defines the α1-β interaction. The β subunit appears more closely associated to the membrane than expected, which may better account for both its role in localizing the α(1s) subunit to the membrane and its suggested role in excitation-contraction coupling.

Early effect of tacrolimus in improving excitation-contraction coupling in myasthenia gravis

OBJECTIVES

Tacrolimus (FK506) is a macrolide T-cell immunomodulator used to treat myasthenia gravis (MG). Besides immunosuppression, tacrolimus has been reported to have the potential to increase muscle strength by enhancing ryanodine receptor (RyR) function. However, few attempts have been made to demonstrate the early effect of tacrolimus as an RyR enhancer in clinical investigation.

METHODS

In 20 MG patients, masseteric compound muscle action potential (CMAP) and mandibular movement-related potentials (MRPs) were recorded simultaneously after stimulating the trigeminal motor nerve with a needle electrode. The excitation-contraction (E-C) coupling time (ECCT) was calculated by the latency difference between CMAP and MRP. Bite force was measured using a pressure-sensitive sheet. Serial assessments of % decrement in masseteric repetitive nerve stimulation (RNS), ECCT and bite force were performed before and within 4 weeks of tacrolimus (3 mg day(-1)) treatment. The median (mean, range) interval of assessment was 2 (2.4, 1-4) weeks. We also measured serum antibodies against RyR, acetylcholine receptor and muscle-specific receptor tyrosine kinase.

RESULTS

Bite force increased after tacrolimus treatment accompanying clinical improvement assessed by Myasthenia Gravis Foundation of America classification, but the bite force difference did not reach statistical significance. Wilcoxon matched-pairs signed-ranks test detected a significant ECCT shortening in 12 patients assessed after 1-2 weeks of tacrolimus treatment as well as in eight patients assessed after 3-4 weeks. In contrast, masseteric CMAP and % decrement showed no significant changes after short-term tacrolimus treatment.

CONCLUSIONS

Tacrolimus induces ECCT shortening accompanying clinical improvement despite no improvement in % decrement within 2 weeks.

SIGNIFICANCE

This early effect of tacrolimus may imply a pharmacological enhancement of RyR function to improve E-C coupling in MG.

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

In vivo expression of G-protein beta1gamma2 dimer in adult mouse skeletal muscle alters L-type calcium current and excitation-contraction coupling

A number of G-protein-coupled receptors are expressed in skeletal muscle but their roles in muscle physiology and downstream effector systems remain poorly investigated. Here we explored the functional importance of the G-protein betagamma (Gbetagamma) signalling pathway on voltage-controlled Ca(2+) homeostasis in single isolated adult skeletal muscle fibres. A GFP-tagged Gbeta(1)gamma(2) dimer was expressed in vivo in mice muscle fibres. The GFP fluorescence pattern was consistent with a Gbeta(1)gamma(2) dimer localization in the transverse-tubule membrane. Membrane current and indo-1 fluorescence measurements performed under voltage-clamp conditions reveal a drastic reduction of both L-type Ca(2+) current density and of peak amplitude of the voltage-activated Ca(2+) transient in Gbeta(1)gamma(2)-expressing fibres. These effects were not observed upon expression of Gbeta(2)gamma(2), Gbeta(3)gamma(2) or Gbeta(4)gamma(2). Our data suggest that the G-protein beta(1)gamma(2) dimer may play an important regulatory role in skeletal muscle excitation-contraction coupling.

Effects of inserting fluorescent proteins into the alpha1S II-III loop: insights into excitation-contraction coupling

In skeletal muscle, intermolecular communication between the 1,4-dihydropyridine receptor (DHPR) and RYR1 is bidirectional: orthograde coupling (skeletal excitation-contraction coupling) is observed as depolarization-induced Ca(2+) release via RYR1, and retrograde coupling is manifested by increased L-type Ca(2+) current via DHPR. A critical domain (residues 720-765) of the DHPR alpha(1S) II-III loop plays an important but poorly understood role in bidirectional coupling with RYR1. In this study, we examine the consequences of fluorescent protein insertion into different positions within the alpha(1S) II-III loop. In four constructs, a cyan fluorescent protein (CFP)-yellow fluorescent protein (YFP) tandem was introduced in place of residues 672-685 (the peptide A region). All four constructs supported efficient bidirectional coupling as determined by the measurement of L-type current and myoplasmic Ca(2+) transients. In contrast, insertion of a CFP-YFP tandem within the N-terminal portion of the critical domain (between residues 726 and 727) abolished bidirectional signaling. Bidirectional coupling was partially preserved when only a single YFP was inserted between residues 726 and 727. However, insertion of YFP near the C-terminal boundary of the critical domain (between residues 760 and 761) or in the conserved C-terminal portion of the alpha(1S) II-III loop (between residues 785 and 786) eliminated bidirectional coupling. None of the fluorescent protein insertions, even those that interfered with signaling, significantly altered membrane expression or targeting. Thus, bidirectional signaling is ablated by insertions at two different sites in the C-terminal portion of the alpha(1S) II-III loop. Significantly, our results indicate that the conserved portion of the alpha(1S) II-III loop C terminal to the critical domain plays an important role in bidirectional coupling either by conveying conformational changes to the critical domain from other regions of the DHPR or by serving as a site of interaction with other junctional proteins such as RYR1.

[Molecular architecture of the sarcoplasmic reticulum and its role in the ECC]

The sarcoplasmic reticulum (SR) plays a fundamental role in excitation-contraction coupling, which propagates the electric signal conversion along the muscle fiber's plasmic membrane to a mechanical event manifested as a muscle contraction. It plays a crucial role in calcium homeostasis and intracellular calcium storage control (storage, liberation and uptake) necessary for fiber muscle contraction and then relaxation. These functions take place at the triad, made up of individualized SR subdomains where the protein-specific organization provides efficient and fast coupling. Ryanodine receptors (RyR) and dihydropyridine receptors (DHPR) mainly act in calcium exchanges in the SR. This particular structural and molecular architecture must be correlated to its functional specificity.

[Malignant hyperthermia in children]

INTRODUCTION

Malignant hyperthermia is a rare disease which is mainly an inherited autosomal dominant trait. It is characteristic for muscle rigidity, metabolism or respiratory acidosis, high values of serum creatine kinase. Then appears myoglobinuria which can lead to tubular necrosis and acute renal failure.

CASE OUTLINE

The male child, ten years old, hospitalized because of the high temperature, exhaustion and cough. On the second day of hospitalization, he has hyperthermia (39.8 degrees C), dyspnoea, tachypnoea, was somnolent, occasionally raving, exhausted with the pains in the muscles which were rigid and painfully sensitive. During the night, the urine was dark red, but the diuresis was well. The next day, laboratory analyses showed high values of aspartate aminotransferase (4263 IU/l), alanine aminotransferase (1311 IU/l), lactate dehydrogenase (11787 IU/l), while the values of serum creatine kinase were so high that they could not be registered. The urine analysis showed the negative result on gall colours and haematuria and positive on myoglobin. During the following days, the patient had normal skin temperature, the pains in the muscles were gradually weakened and the urine cleared up. On the third day, the value of creatine kinase was measured and it was 178700 IU/l. During the next two weeks, the clinical finding was gradually normalized while the laboratory values of serum enzyme were gradually normalized only in twenty days.

CONCLUSION

Malignant hyperthermia is a serious clinical syndrome which can be found with, until then, a clinically healthy child (without chronic myopathy) and it appears with the high temperature or during the surgical interventions with anaesthetics, which is especially dangerous.

Proper restoration of excitation-contraction coupling in the dihydropyridine receptor beta1-null zebrafish relaxed is an exclusive function of the beta1a subunit

The paralyzed zebrafish strain relaxed carries a null mutation for the skeletal muscle dihydropyridine receptor (DHPR) beta(1a) subunit. Lack of beta(1a) results in (i) reduced membrane expression of the pore forming DHPR alpha(1S) subunit, (ii) elimination of alpha(1S) charge movement, and (iii) impediment of arrangement of the DHPRs in groups of four (tetrads) opposing the ryanodine receptor (RyR1), a structural prerequisite for skeletal muscle-type excitation-contraction (EC) coupling. In this study we used relaxed larvae and isolated myotubes as expression systems to discriminate specific functions of beta(1a) from rather general functions of beta isoforms. Zebrafish and mammalian beta(1a) subunits quantitatively restored alpha(1S) triad targeting and charge movement as well as intracellular Ca(2+) release, allowed arrangement of DHPRs in tetrads, and most strikingly recovered a fully motile phenotype in relaxed larvae. Interestingly, the cardiac/neuronal beta(2a) as the phylogenetically closest, and the ancestral housefly beta(M) as the most distant isoform to beta(1a) also completely recovered alpha(1S) triad expression and charge movement. However, both revealed drastically impaired intracellular Ca(2+) transients and very limited tetrad formation compared with beta(1a). Consequently, larval motility was either only partially restored (beta(2a)-injected larvae) or not restored at all (beta(M)). Thus, our results indicate that triad expression and facilitation of 1,4-dihydropyridine receptor (DHPR) charge movement are common features of all tested beta subunits, whereas the efficient arrangement of DHPRs in tetrads and thus intact DHPR-RyR1 coupling is only promoted by the beta(1a) isoform. Consequently, we postulate a model that presents beta(1a) as an allosteric modifier of alpha(1S) conformation enabling skeletal muscle-type EC coupling.

The alpha(1S) III-IV loop influences 1,4-dihydropyridine receptor gating but is not directly involved in excitation-contraction coupling interactions with the type 1 ryanodine receptor

In skeletal muscle, coupling between the 1,4-dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) underlies excitation-contraction (EC) coupling. The III-IV loop of the DHPR alpha(1S) subunit binds to a segment of RyR1 in vitro, and mutations in the III-IV loop alter the voltage dependence of EC coupling, raising the possibility that this loop is directly involved in signal transmission from the DHPR to RyR1. To clarify the role of the alpha(1S) III-IV loop in EC coupling, we examined the functional properties of a chimera (GFP-alpha(1S)[III-IVa]) in which the III-IV loop of the divergent alpha(1A) isoform replaced that of alpha(1S). Dysgenic myotubes expressing GFP-alpha(1S)[III-IVa] yielded myoplasmic Ca(2+) transients that activated at approximately 10 mV more hyperpolarized potentials and that were approximately 65% smaller than those of GFP-alpha(1S). A similar reduction was observed in voltage-dependent charge movements for GFP-alpha(1S)[III-IVa], indicating that the chimeric channels trafficked less well to the membrane but that those that were in the membrane functioned as efficiently in EC coupling as GFP-alpha(1S). Relative to GFP-alpha(1S), L-type currents mediated by GFP-alpha(1S)[III-IVa] were approximately 40% smaller and activated at approximately 5 mV more hyperpolarized potentials. The altered gating of GFP-alpha(1S)[III-IVa] was accentuated by exposure to +/-Bay K 8644, which caused a much larger hyperpolarizing shift in activation compared with its effect on GFP-alpha(1S). Taken together, our observations indicate that the alpha(1S) III-IV loop is not directly involved in EC coupling but does influence DHPR gating transitions important both for EC coupling and activation of L-type conductance.

A dihydropyridine receptor alpha1s loop region critical for skeletal muscle contraction is intrinsically unstructured and binds to a SPRY domain of the type 1 ryanodine receptor

The II-III loop of the dihydropyridine receptor (DHPR) alpha(1s) subunit is a modulator of the ryanodine receptor (RyR1) Ca(2+) release channel in vitro and is essential for skeletal muscle contraction in vivo. Despite its importance, the structure of this loop has not been reported. We have investigated its structure using a suite of NMR techniques which revealed that the DHPR II-III loop is an intrinsically unstructured protein (IUP) and as such belongs to a burgeoning structural class of functionally important proteins. The loop does not possess a stable tertiary fold: it is highly flexible, with a strong N-terminal helix followed by nascent helical/turn elements and unstructured segments. Its residual structure is loosely globular with the N and C termini in close proximity. The unstructured nature of the II-III loop may allow it to easily modify its interaction with RyR1 following a surface action potential and thus initiate rapid Ca(2+) release and contraction. The in vitro binding partner for the II-III was investigated. The II-III loop interacts with the second of three structurally distinct SPRY domains in RyR1, whose function is unknown. This interaction occurs through two preformed N-terminal alpha-helical regions and a C-terminal hydrophobic element. The A peptide corresponding to the helical N-terminal region is a common probe of RyR function and binds to the same SPRY domain as the full II-III loop. Thus the second SPRY domain is an in vitro binding site for the II-III loop. The possible in vivo role of this region is discussed.

Bridging the myoplasmic gap: recent developments in skeletal muscle excitation–contraction coupling

Conformational coupling between the L-type voltage-gated Ca(2+) channel (or 1,4-dihydropyridine receptor; DHPR) and the ryanodine-sensitive Ca(2+) release channel of the sarcoplasmic reticulum (RyR1) is the mechanistic basis for excitation-contraction (EC) coupling in skeletal muscle. In this article, recent findings regarding the roles of the individual cytoplasmic domains (the amino- and carboxyl-termini, cytoplasmic loops I-II, II-III, and III-IV) of the DHPR alpha(1S) subunit in bi-directional communication with RyR1 will be discussed.

Hemicalcin, a new toxin from the Iranian scorpion Hemiscorpius lepturus which is active on ryanodine-sensitive Ca2+ channels

In the present work, we purified and characterized a novel toxin named hemicalcin from the venom of the Iranian chactoid scorpion Hemiscorpius lepturus where it represents 0.6% of the total protein content. It is a 33-mer basic peptide reticulated by three disulfide bridges, and that shares between 85 and 91% sequence identity with four other toxins, all known or supposed to be active on ryanodine-sensitive calcium channels. Hemicalcin differs from these other toxins by seven amino acids at positions 9 (leucine/arginine), 12 (alanine/glutamic acid), 13 (aspartic acid/asparagine), 14 (lysine/asparagine), 18 (serine/glycine), 26 (threonine/alanine) and 28 (proline/isoleucine/alanine). In spite of these differences, hemicalcin remains active on ryanodine-sensitive Ca2+ channels, since it increases [3H]ryanodine binding on RyR1 (ryanodine receptor type 1) and triggers Ca2+ release from sarcoplasmic vesicles. Bilayer lipid membrane experiments, in which the RyR1 channel is reconstituted and its gating properties are analysed, indicate that hemicalcin promotes an increase in the opening probability at intermediate concentration and induces a long-lasting subconductance level of 38% of the original amplitude at higher concentrations. Mice intracerebroventricular inoculation of 300 ng of hemicalcin induces neurotoxic symptoms in vivo, followed by death. Overall, these data identify a new biologically active toxin that belongs to a family of peptides active on the ryanodine-sensitive channel.

Is excitation–contraction coupling impaired in myasthenia gravis?

OBJECTIVE

To investigate whether excitation-contraction (E-C) coupling of muscle is impaired in patients with myasthenia gravis (MG).

METHODS

In 51 patients with generalized MG and 35 normal subjects, compound muscle action potentials (CMAPs) of the abductor pollicis brevis, and movement-related potentials using an accelerometer placed at the thumb tip were simultaneously recorded after median nerve stimulation at the wrist. The E-C coupling time (ECCT) was estimated by a latency difference between CMAP and movement-related potential. Antibodies against acetylcholine receptor (AChR), ryanodine receptor (RyR), and muscle specific receptor tyrosine kinase (MuSK) were measured by immunoassays.

RESULTS

The mean ECCT was significantly longer in patients with MG (mean+/-SEM; 2.79+/-0.1 ms; p=0.002) than in normal controls (2.52+/-0.1 ms). Among MG patients, the mean ECCT was longer for patients with thymoma than for those without it (P=0.04), and was shorter for patients treated with FK506 (an immunosuppressant and also an enhancer of RyR related Ca(2+) release) than for those not receiving this treatment (p=0.04). ECCT had no significant correlation with anti-AChR, anti-RyR, or anti-MuSK antibodies.

CONCLUSIONS

In MG, E-C coupling appears to be impaired, particularly in patients with thymoma, and FK506 possibly facilitates E-C coupling.

SIGNIFICANCE

The functional implication of impaired E-C coupling is not established, but it may contribute to muscle weakness in patients with MG.

Titin and ryanodine receptor antibodies in myasthenia gravis

Some myasthenia gravis (MG) patients have antibodies against skeletal muscle antigens in addition to the acetylcholine receptor (AChR). Two major antigens for non-AchR antibodies in MG are the Ca(2+) release channel of the sarcoplasmic reticulum, the ryanodine receptor (RyR) and titin, a gigantic filamentous muscle protein essential for muscle structure, function and development. RyR and titin antibodies are found mainly in thymoma MG patients and in a few late-onset MG patients and correlate with a severe MG disease. The presence of titin antibodies, which bind to key regions near the A/I junction and in the central I-band, correlates with myopathy. The immunosuppressant (FK506), which enhances Ca(2+) release from the RyR, seems to have a symptomatic effect on MG patients with RyR antibodies. The RyR antibodies recognize a region near the N-terminus important for channel regulation and inhibit Ca(2+) release in vitro. However, evidence that antibodies against the intracellular antigens RyR and titin are pathogenic in vivo is still missing.

Effects of peptide C corresponding to the Glu724-Pro760 region of the II-III loop of the DHP (dihydropyridine) receptor alpha1 subunit on the domain- switch-mediated activation of RyR1 (ryanodine receptor 1) Ca2+ channels

The Leu720-Leu764 region of the II-III loop of the dihydropyridine receptor is believed to be important for both orthograde and retrograde communications with the RyR (ryanodine receptor), but its actual role has not yet been resolved. Our recent studies suggest that voltage-dependent activation of the RyR channel is mediated by a pair of interacting N-terminal and central domains, designated as the 'domain switch'. To investigate the effect of peptide C (a peptide corresponding to residues Glu724-Pro760) on domain- switch-mediated activation of the RyR, we measured Ca2+ release induced by DP (domain peptide) 1 or DP4 (which activates the RyR by mediation of the domain switch) and followed the Ca2+ release time course using a luminal Ca2+ probe (chlortetracycline) under Ca2+-clamped conditions. Peptide C produced a significant potentiation of the domain-switch-mediated Ca2+ release, provided that the Ca2+ concentration was sufficiently low (e.g. 0.1 microM) and the Ca2+ channel was only partially activated by the domain peptide. However, at micromolar Ca2+ concentrations, peptide C inhibits activation. Covalent cross-linking of fluorescently labelled peptide C to the RyR and screening of the fluorescently labelled tryptic fragments permitted us to localize the peptide-C-binding site to residues 450-1400, which may represent the primary region involved in physical coupling. Based on the above findings, we propose that the physiological role of residues Glu724-Pro760 is to facilitate depolarization-induced and domain-switch-mediated RyR activation at sub- or near-threshold concentrations of cytoplasmic Ca2+ and to suppress activation upon an increase of cytoplasmic Ca2+.

Interaction between the dihydropyridine receptor Ca2+ channel beta-subunit and ryanodine receptor type 1 strengthens excitation-contraction coupling

Previous studies have shown that the skeletal dihydropyridine receptor (DHPR) pore subunit Ca(V)1.1 (alpha1S) physically interacts with ryanodine receptor type 1 (RyR1), and a molecular signal is transmitted from alpha1S to RyR1 to trigger excitation-contraction (EC) coupling. We show that the beta-subunit of the skeletal DHPR also binds RyR1 and participates in this signaling process. A novel binding site for the DHPR beta1a-subunit was mapped to the M(3201) to W(3661) region of RyR1. In vitro binding experiments showed that the strength of the interaction is controlled by K(3495)KKRR_ _R(3502), a cluster of positively charged residues. Phenotypic expression of skeletal-type EC coupling by RyR1 with mutations in the K(3495)KKRR_ _R(3502) cluster was evaluated in dyspedic myotubes. The results indicated that charge neutralization or deletion severely depressed the magnitude of RyR1-mediated Ca(2+) transients coupled to voltage-dependent activation of the DHPR. Meantime the Ca(2+) content of the sarcoplasmic reticulum was not affected, and the amplitude and activation kinetics of the DHPR Ca(2+) currents were slightly affected. The data show that the DHPR beta-subunit, like alpha1S, interacts directly with RyR1 and is critical for the generation of high-speed Ca(2+) signals coupled to membrane depolarization. These findings indicate that EC coupling in skeletal muscle involves the interplay of at least two subunits of the DHPR, namely alpha1S and beta1a, interacting with possibly different domains of RyR1.

The alpha1S N-terminus is not essential for bi-directional coupling with RyR1

The dihydropyridine receptor (DHPR) alpha(1S) II-III loop has been shown to be critical for excitation-contraction (EC) coupling in skeletal muscle, but the importance of other cytoplasmic regions, especially the N-terminus (residues 1-51), remains unclear. In this study, we found that deletion of alpha(1S) residues 2-37 (weakly conserved with N-termini of other L-type Ca(2+) channels) had little effect on the ability of alpha(1S) to serve as a Ca(2+) channel or voltage sensor for EC coupling. Strikingly, deletion of 10 additional residues, which are conserved in L-type channels, resulted in ablation of DHPR function. Specifically, confocal microscopy and measurement of charge movement showed that removal of residues 2-47 resulted in a failure of sarcolemmal insertion. Our results indicate that the weakly conserved, distal alpha(1S) N-terminus is not critical for EC coupling or function as a Ca(2+) channel. However, integrity of the proximal alpha(1S) N-terminus is necessary for sarcolemmal expression of the DHPR.

Multiple loops of the dihydropyridine receptor pore subunit are required for full-scale excitation-contraction coupling in skeletal muscle

Understanding which cytosolic domains of the dihydropyridine receptor participate in excitation-contraction (EC) coupling is critical to validate current structural models. Here we quantified the contribution to skeletal-type EC coupling of the alpha1S (CaV1.1) II-III loop when alone or in combination with the rest of the cytosolic domains of alpha1S. Chimeras consisting of alpha1C (CaV1.2) with alpha1S substitutions at each of the interrepeat loops (I-II, II-III, and III-IV loops) and N- and C-terminal domains were evaluated in dysgenic (alpha1S-null) myotubes for phenotypic expression of skeletal-type EC coupling. Myotubes were voltage-clamped, and Ca2+ transients were measured by confocal line-scan imaging of fluo-4 fluorescence. In agreement with previous results, the alpha1C/alpha1S II-III loop chimera, but none of the other single-loop chimeras, recovered a sigmoidal fluorescence-voltage curve indicative of skeletal-type EC coupling. To quantify Ca2+ transients in the absence of inward Ca2+ current, but without changing the external solution, a mutation, E736K, was introduced into the P-loop of repeat II of alpha1C. The Ca2+ transients expressed by the alpha1C(E736K)/alpha1S II-III loop chimera were approximately 70% smaller than those expressed by the Ca2+-conducting alpha1C/alpha1S II-III variant. The low skeletal-type EC coupling expressed by the alpha1C/alpha1S II-III loop chimera was confirmed in the Ca2+-conducting alpha1C/alpha1S II-III loop variant using Cd2+ (10(-4) M) as the Ca2+ current blocker. In contrast to the behavior of the II-III loop chimera, Ca2+ transients expressed by an alpha1C/alpha1S chimera carrying all tested skeletal alpha1S domains (all alpha1S interrepeat loops, N- and C-terminus) were similar in shape and amplitude to wild-type alpha1S, and did not change in the presence of the E736K mutation or in the presence of 10(-4) M Cd2+. Controls indicated that similar dihydropyridine receptor charge movements were expressed by the non-Ca2+ permeant alpha1S(E1014K) variant, the alpha1C(E736K)/alpha1S II-III loop chimera, and the alpha1C(E736K)/alpha1S chimera carrying all tested alpha1S domains. The data indicate that the functional recovery produced by the alpha1S II-III loop is incomplete and that multiple cytosolic domains of alpha1S are necessary for a quantitative recovery of the EC-coupling phenotype of skeletal myotubes. Thus, despite the importance of the II-III loop there may be other critical determinants in alpha1S that influence the efficiency of EC coupling.

Structural insights into excitation-contraction coupling by electron cryomicroscopy

In muscle, excitation-contraction coupling is defined as the process linking depolarization of the surface membrane with Ca2+ release from cytoplasmic stores, which activates contraction of striated muscle. This process is primarily controlled by interplay between two Ca2+ channels--the voltage-gated L-type Ca2+ channel (dihydropyridine receptor, DHPR) localized in the t-tubule membrane and the Ca2+-release channel (ryanodine receptor, RyR) of the sarcoplasmic reticulum membrane. The structures of both channels have been extensively studied by several groups using electron cryomicroscopy and single particle reconstruction techniques. The structures of RyR, determined at resolutions of 22-30 A, reveal a characteristic mushroom shape with a bulky cytoplasmic region and the membrane-spanning stem. While the cytoplasmic region exhibits a complex structure comprising a multitude of distinctive domains with numerous intervening cavities, at this resolution no definitive statement can be made about the location of the actual pore within the transmembrane region. Conformational changes associated with functional transitions of the Ca2+ release channel from closed to open states have been characterized. Further experiments determined localization of binding sites for various channel ligands. The structural studies of the DHPR are less developed. Although four 3D maps of the DHPR were reported recently at 24-30 A resolution from studies of frozen-hydrated and negatively stained receptors, there are some discrepancies between reported structures with respect to the overall appearance and dimensions of the channel structure. Future structural studies at higher resolution are needed to refine the structures of both channels and to substantiate a proposed molecular model for their interaction.

Involvement of a heptad repeat in the carboxyl terminus of the dihydropyridine receptor beta1a subunit in the mechanism of excitation-contraction coupling in skeletal muscle

Chimeras consisting of the homologous skeletal dihydropyridine receptor (DHPR) beta1a subunit and the heterologous cardiac/brain beta2a subunit were used to determine which regions of beta1a were responsible for the skeletal-type excitation-contraction (EC) coupling phenotype. Chimeras were transiently transfected in beta1 knockout myotubes and then voltage-clamped with simultaneous measurement of confocal fluo-4 fluorescence. All chimeras expressed a similar density of DHPR charge movements, indicating that the membrane density of DHPR voltage sensors was not a confounding factor in these studies. The data indicates that a beta1a-specific domain present in the carboxyl terminus, namely the D5 region comprising the last 47 residues (beta1a 478-524), is essential for expression of skeletal-type EC coupling. Furthermore, the location of beta1aD5 immediately downstream from conserved domain D4 is also critical. In contrast, chimeras in which beta1aD5 was swapped by the D5 region of beta2a expressed Ca(2+) transients triggered by the Ca(2+) current, or none at all. A hydrophobic heptad repeat is present in domain D5 of beta1a (L478, V485, V492). To determine the role of this motif, residues in the heptad repeat were mutated to alanines. The triple mutant beta1a(L478A/V485A/V492A) recovered weak skeletal-type EC coupling (DeltaF/F(max) = 0.4 +/- 0.1 vs. 2.7 +/- 0.5 for wild-type beta1a). However, a triple mutant with alanine substitutions at positions out of phase with the heptad repeat, beta1a(S481A/L488A/S495A), was normal (DeltaF/F(max) = 2.1 +/- 0.4). In summary, the presence of the beta1a-specific D5 domain, in its correct position after conserved domain D4, is essential for skeletal-type EC coupling. Furthermore, a heptad repeat in beta1aD5 controls the EC coupling activity. The carboxyl terminal heptad repeat of beta1a might be involved in protein-protein interactions with ryanodine receptor type 1 required for DHPR to ryanodine receptor type 1 signal transmission.

Maurocalcine and domain A of the II-III loop of the dihydropyridine receptor Cav 1.1 subunit share common binding sites on the skeletal ryanodine receptor

Maurocalcine is a scorpion venom toxin of 33 residues that bears a striking resemblance to the domain A of the dihydropyridine voltage-dependent calcium channel type 1.1 (Cav1.1) subunit. This domain belongs to the II-III loop of Cav1.1, which is implicated in excitation-contraction coupling. Besides the structural homology, maurocalcine also modulates RyR1 channel activity in a manner akin to a synthetic peptide of domain A. Because of these similarities, we hypothesized that maurocalcine and domain A may bind onto an identical region(s) of RyR1. Using a set of RyR1 fragments, we demonstrate that peptide A and maurocalcine bind onto two discrete RyR1 regions: fragments 3 and 7 encompassing residues 1021-1631 and 3201-3661, respectively. The binding onto fragment 7 is of greater importance and was thus further investigated. We found that the amino acid region 3351-3507 of RyR1 (fragment 7.2) is sufficient for these interactions. Proof that peptide A and maurocalcine bind onto the same site is provided by competition experiments in which binding of fragment 7.2 to peptide A is inhibited by preincubation with maurocalcine. Moreover, when expressed in COS-7 cells, RyR1 carrying a deletion of fragment 7 shows a loss of interaction with both peptide A and maurocalcine. At the functional level, this deletion abolishes the maurocalcine induced stimulation of [3H]ryanodine binding onto microsomes of transfected COS-7 cells without affecting the caffeine and ATP responses.

Differential contribution of skeletal and cardiac II-III loop sequences to the assembly of dihydropyridine-receptor arrays in skeletal muscle

The plasmalemmal dihydropyridine receptor (DHPR) is the voltage sensor in skeletal muscle excitation-contraction (e-c) coupling. It activates calcium release from the sarcoplasmic reticulum via protein-protein interactions with the ryanodine receptor (RyR). To enable this interaction, DHPRs are arranged in arrays of tetrads opposite RyRs. In the DHPR alpha(1S) subunit, the cytoplasmic loop connecting repeats II and III is a major determinant of skeletal-type e-c coupling. Whether the essential II-III loop sequence (L720-L764) also determines the skeletal-specific arrangement of DHPRs was examined in dysgenic (alpha(1S)-null) myotubes reconstituted with distinct alpha(1) subunit isoforms and II-III loop chimeras. Parallel immunofluorescence and freeze-fracture analysis showed that alpha(1S) and chimeras containing L720-L764, all of which restored skeletal-type e-c coupling, displayed the skeletal arrangement of DHPRs in arrays of tetrads. Conversely, alpha(1C) and those chimeras with a cardiac II-III loop and cardiac e-c coupling properties were targeted into junctional membranes but failed to form tetrads. However, an alpha(1S)-based chimera with the heterologous Musca II-III loop produced tetrads but did not reconstitute skeletal muscle e-c coupling. These findings suggest an inhibitory role in tetrad formation of the cardiac II-III loop and that the organization of DHPRs in tetrads vis-a-vis the RyR is necessary but not sufficient for skeletal-type e-c coupling.

Clinical review: Treatment of heat stroke: should dantrolene be considered?

Rapid and efficient cooling is the most important therapeutic objective in patients with heat stroke (HS). This article reviews the mechanism of action and rationale for the use of dantrolene as a potential supportive cooling method in the treatment of HS. Relevant studies were included to support discussion of the role of dantrolene for the treatment of HS. In some studies dantrolene was shown to accelerate cooling rate when administered after the development of exertional HS. Dantrolene was also found to be effective in reducing the extent of HS signs when given as pretreatment in an animal model. Accumulated data do not support the routine use of dantrolene as an adjuvant cooling technique in HS, but administration of this drug in severe cases, or in which no improvement is observed, appears rational. Further trials are needed in order to assess the true effectiveness of dantrolene in HS.

Mapping sites of potential proximity between the dihydropyridine receptor and RyR1 in muscle using a cyan fluorescent protein-yellow fluorescent protein tandem as a fluorescence resonance energy transfer probe

Excitation-contraction coupling in skeletal muscle involves conformational coupling between the dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) at junctions between the plasma membrane and sarcoplasmic reticulum. In an attempt to find which regions of these proteins are in close proximity to one another, we have constructed a tandem of cyan and yellow fluorescent proteins (CFP and YFP, respectively) linked by a 23-residue spacer, and measured the fluorescence resonance energy transfer (FRET) of the tandem either in free solution or after attachment to sites of the alpha1S and beta1a subunits of the DHPR. For all of the sites examined, attachment of the CFP-YFP tandem did not impair function of the DHPR as a Ca2+ channel or voltage sensor for excitation-contraction coupling. The free tandem displayed a 27.5% FRET efficiency, which decreased significantly after attachment to the DHPR subunits. At several sites examined for both alpha1S (N-terminal, proximal II-III loop of a two fragment construct) and beta1a (C-terminal), the FRET efficiency was similar after expression in either dysgenic (alpha1S-null) or dyspedic (RyR1-null) myotubes. However, compared with dysgenic myotubes, the FRET efficiency in dyspedic myotubes increased from 9.9 to 16.7% for CFP-YFP attached to the N-terminal of beta1a, and from 9.5 to 16.8% for CFP-YFP at the C-terminal of alpha1S. Thus, the tandem reporter suggests that the C terminus of alpha1S and the N terminus of beta1a may be in close proximity to the ryanodine receptor.

Functional analysis of the R1086H malignant hyperthermia mutation in the DHPR reveals an unexpected influence of the III-IV loop on skeletal muscle EC coupling

Malignant hyperthermia (MH) is an inherited pharmacogenetic disorder caused by mutations in the skeletal muscle ryanodine receptor (RyR1) and the dihydropyridine receptor (DHPR) alpha(1S)-subunit. We characterized the effects of an MH mutation in the DHPR cytoplasmic III-IV loop of alpha(1S) (R1086H) on DHPR-RyR1 coupling after reconstitution in dysgenic (alpha(1S) null) myotubes. Compared with wild-type alpha(1S), caffeine-activated Ca(2+) release occurred at approximately fivefold lower concentrations in nonexpressing and R1086H-expressing myotubes. Although maximal voltage-gated Ca(2+) release was similar in alpha(1S)- and R1086H-expressing myotubes, the voltage dependence of Ca(2+) release was shifted approximately 5 mV to more negative potentials in R1086H-expressing myotubes. Our results demonstrate that alpha(1S) functions as a negative allosteric modulator of release channel activation by caffeine/voltage and that the R1086H MH mutation in the intracellular III-IV linker disrupts this negative regulatory influence. Moreover, a low caffeine concentration (2 mM) caused a similar shift in voltage dependence of Ca(2+) release in alpha(1S)- and R1086H-expressing myotubes. Compared with alpha(1S)-expressing myotubes, maximal L channel conductance (G(max)) was reduced in R1086H-expressing myotubes (alpha(1S) 130 +/- 10.2, R1086H 88 +/- 6.8 nS/nF; P < 0.05). The decrease in G(max) did not result from a change in retrograde coupling with RyR1 as maximal conductance-charge movement ratio (G(max)/Q(max)) was similar in alpha(1S)- and R1086H-expressing myotubes and a similar decrease in G(max) was observed for an analogous mutation engineered into the cardiac L channel (R1217H). In addition, both R1086H and R1217H DHPRs targeted normally and colocalized with RyR1 in sarcoplasmic reticulum (SR)-sarcolemmal junctions. These results indicate that the R1086H MH mutation in alpha(1S) enhances RyR1 sensitivity to activation by both endogenous (voltage sensor) and exogenous (caffeine) activators.

Molecular basis of the high-affinity activation of type 1 ryanodine receptors by imperatoxin A

Both imperatoxin A (IpTx(a)), a 33-residue peptide toxin from scorpion venom, and peptide A, derived from the II-III loop of dihydropyridine receptor (DHPR), interact specifically with the skeletal ryanodine receptor (RyR1), which is a Ca(2+)-release channel in the sarcoplasmic reticulum, but with considerably different affinities. IpTx(a) activates RyR1 with nanomolar affinity, whereas peptide A activates RyR1 at micromolar concentrations. To investigate the molecular basis for high-affinity activation of RyR1 by IpTx(a), we have determined the NMR solution structure of IpTx(a), and identified its functional surface by using alanine-scanning analogues. A detailed comparison of the functional surface profiles for two peptide activators revealed that IpTx(a) exhibits a large functional surface area (approx. 1900 A(2), where 1 A=0.1 nm), based on a short double-stranded antiparallel beta-sheet structure, while peptide A bears a much smaller functional surface area (approx. 800 A(2)), with the five consecutive basic residues (Arg(681), Lys(682), Arg(683), Arg(684) and Lys(685)) being clustered at the C-terminal end of the alpha-helix. The functional surface of IpTx(a) is composed of six essential residues (Leu(7), Lys(22), Arg(23), Arg(24), Arg(31) and Arg(33)) and several other important residues (His(6), Lys(8), Arg(9), Lys(11), Lys(19), Lys(20), Gly(25), Thr(26), Asn(27) and Lys(30)), indicating that amino acid residues involved in RyR1 activation make up over the half of the toxin molecule with the exception of cysteine residues. Taken together, these results suggest that the site where peptide A binds to RyR1 belongs to a subset of macrosites capable of being occupied by IpTx(a), resulting in differing the affinity and the mode of activation.

Pathogenesis of myositis and myasthenia associated with titin and ryanodine receptor antibodies

Some myasthenia gravis (MG) patients have antibodies against skeletal muscle antigens in addition to the acetylcholine receptor (AChR). Two major antigens for these antibodies are the Ca(2+) release channel of the sarcoplasmic reticulum, the ryanodine receptor (RyR), and titin, a gigantic filamentous muscle protein essential for muscle structure, function, and development. RyR and titin antibodies are found in MG patients with a thymoma and in a proportion of late-onset MG, and they correlate with severe MG disease. The RyR antibodies recognize a region near the N-terminus important for channel regulation. They inhibit Ca(2+) release from sarcoplasmic reticulum in vitro. There is electrophysiological evidence for a disordered excitation-contraction coupling in MG patients. The presence of titin antibodies, which bind to key regions near the A/I junction and in the central I-band, correlates with myopathy in MG patients. However, so far, there is no direct evidence that antibodies against the intracellular antigens RyR and titin are pathogenic in vivo.

Structural requirements of the dihydropyridine receptor alpha1S II-III loop for skeletal-type excitation-contraction coupling

Residues Leu720-Leu764 within the II-III loop of the skeletal muscle dihydropyridine receptor (DHPR) alpha1S subunit represent a critical domain for the orthograde excitation-contraction coupling as well as for retrograde DHPR L-current-enhancing coupling with the ryanodine receptor (RyR1). To better understand the molecular mechanism underlying this bidirectional DHPR-RyR1 signaling interaction, we analyzed the critical domain to the single amino acid level. To this end, constructs based on the highly dissimilar housefly DHPR II-III loop in an otherwise skeletal DHPR as an interaction-inert sequence background were expressed in dysgenic (alpha1S-null) myotubes for simultaneous recordings of depolarization-induced intracellular Ca2+ transients (orthograde coupling) and whole-cell Ca2+ currents (retrograde coupling). In the minimal skeletal II-III loop sequence (Asp734-Asp748 required for full bidirectional coupling, eight amino acids heterologous between skeletal and cardiac DHPR were exchanged for the corresponding cardiac residues. Four of these skeletal-specific residues (Ala739, Phe741, Pro742, and Asp744) turned out to be essential for orthograde and two of them (Ala739 and Phe741) for retrograde coupling, indicating that orthograde coupling does not necessarily correlate with retrograde signaling. Secondary structure predictions of the critical domain show that an alpha-helical (cardiac sequence-type) conformation of a cluster of negatively charged residues (Asp744-Glu751 of alpha1S) corresponds with significantly reduced Ca2+ transients. Conversely, a predicted random coil structure (skeletal sequence-type) seems to be prerequisite for the restoration of skeletal-type excitation-contraction coupling. Thus, not only the primary but also the secondary structure of the critical domain is an essential determinant of the tissue-specific mode of EC coupling.

Amino acids 1-1,680 of ryanodine receptor type 1 hold critical determinants of skeletal type for excitation-contraction coupling. Role of divergence domain D2

To identify domains of the ryanodine receptor (RyR1) that are functionally relevant for excitation-contraction (EC) coupling in vivo, we have studied the ability of RyR1/RyR3 chimera to rescue skeletal EC coupling in dyspedic myotubes. In this work we show that chimeric receptors containing amino acids 1-1,680 of RyR1 were able to render depolarization-induced Ca2+ release to RyR3. Within this region, residues 1,272-1,455, containing divergent domain D2 of RyR1, proved to be a critical element because the absence of this region selectively abolished depolarization-evoked Ca2+ transients without affecting chemically induced activation. Although the D2 domain by itself failed to restore skeletal EC coupling to RyR3, the addition of the D2 region resulted in a dramatic enhancement of EC coupling restored by an RyR3 chimera containing amino acids 1,681-3,770 of RyR1. These results suggest that although the D2 domain of RyR1 plays a key role during EC coupling, additional region(s) from the N-terminal end of RyR1 as well as previously identified regions of the central portion of the receptor are needed in order to allow normal EC coupling.

RyR1/RyR3 chimeras reveal that multiple domains of RyR1 are involved in skeletal-type E-C coupling

Skeletal-type E-C coupling is thought to require a direct interaction between RyR1 and the alpha(1S)-DHPR. Most available evidence suggests that the cytoplasmic II-III loop of the dihydropyridine receptor (DHPR) is the primary source of the orthograde signal. However, identification of the region(s) of RyR1 involved in bidirectional signaling with the alpha(1S)-DHPR remains elusive. To identify these regions we have designed a series of chimeric RyR cDNAs in which different segments of RyR1 were inserted into the corresponding region of RyR3 and expressed in dyspedic 1B5 myotubes. RyR3 provides a preferable background than RyR2 for defining domains essential for E-C coupling because it possesses less sequence homology to RyR1 than the RyR2 backbone used in previous studies. Our data show that two regions of RyR1 (chimera Ch-10 aa 1681-2641 and Ch-9 aa 2642-3770), were independently able to restore skeletal-type E-C coupling to RyR3. These two regions were further mapped and the critical RyR1 residues were 1924-2446 (Ch-21) and 2644-3223 (Ch-19). These results both support and refine the previous hypothesis that multiple domains of RyR1 combine to functionally interact with the DHPR during E-C coupling.

Ryanodine receptor antibodies in myasthenia gravis: epitope mapping and effect on calcium release in vitro

Patients with myasthenia gravis can have antibodies against skeletal muscle ryanodine receptor (Ry1), the sarcoplasmic reticulum calcium-release channel, which plays a crucial role in excitation-contraction coupling. We have screened a panel of overlapping Ry1 fusion proteins with Ry1 antibody-containing myasthenia gravis sera to identify the main immunogenic region. The pc2 Ry1 fusion protein representing a Ry1 region close to the N-terminus (residues 799-1172) was identified as the main immunogenic region for the antibodies. The binding kinetics of the Ry1 antibodies to the pc2 Ry1 fusion protein were tested using an optical biosensor. Ry1 antibodies in the IgG fraction from sera of patients with myasthenia gravis bound with high affinity and with a stoichiometry of 1:1. The functional effect of these Ry1 antibodies was tested in an in vitro Ca2+-release assay. The Ry1 antibodies induced a twofold increase of the half-maximal concentration for 4-Cl-m-cresol-induced Ca2+ release from terminal cisternae vesicles but had no effect on V(max). The effect on 4-Cl-m-cresol-induced Ca2+ release was specific, as preincubation of the active IgG fraction with the pc2 Ry1 fusion protein abolished the inhibition. These data suggest that the Ry1 sequence defined by residues 799-1172 is involved in the regulation of Ry1 function, and that this regulation could be functionally affected in vivo in patients with myasthenia gravis.

Ryanodine receptor calcium release channels

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.

Identification of a dantrolene-binding sequence on the skeletal muscle ryanodine receptor

Dantrolene is a drug that suppresses intracellular Ca(2+) release from sarcoplasmic reticulum (SR) in skeletal muscle and is used as a therapeutic agent in individuals susceptible to malignant hyperthermia. Although its precise mechanism of action has not been elucidated, we have identified the N-terminal region (amino acids 1-1400) of the skeletal muscle isoform of the ryanodine receptor (RyR1), the primary Ca(2+) release channel in SR, as a molecular target for dantrolene using the photoaffinity analog [(3)H]azidodantrolene. Here, we demonstrate that heterologously expressed RyR1 retains its capacity to be specifically labeled with [(3)H]azidodantrolene, indicating that muscle specific factors are not required for this ligand-receptor interaction. Synthetic domain peptides of RyR1 previously shown to affect RyR1 function in vitro and in vivo were exploited as potential drug binding site mimics and used in photoaffinity labeling experiments. Only DP1 and DP1-2s, peptides containing the amino acid sequence corresponding to RyR1 residues 590-609, were specifically labeled by [(3)H]azidodantrolene. A monoclonal anti-RyR1 antibody that recognizes RyR1 and its 1400-amino acid N-terminal fragment recognizes DP1 and DP1-2s in both Western blots and immunoprecipitation assays and specifically inhibits [(3)H]azidodantrolene photolabeling of RyR1 and its N-terminal fragment in SR. Our results indicate that synthetic domain peptides can mimic a native, ligand-binding conformation in vitro and that the dantrolene-binding site and the epitope for the monoclonal antibody on RyR1 are equivalent and composed of amino acids 590-609.

Structure of the voltage-gated L-type Ca2+ channel by electron cryomicroscopy

Voltage-dependent L-type Ca(2+) channels play important functional roles in many excitable cells. We present a three-dimensional structure of an L-type Ca(2+) channel. Electron cryomicroscopy in conjunction with single-particle processing was used to determine a 30-A resolution structure of the channel protein. The asymmetrical channel structure consists of two major regions: a heart-shaped region connected at its widest end with a handle-shaped region. A molecular model is proposed for the arrangement of this skeletal muscle L-type Ca(2+) channel structure with respect to the sarcoplasmic reticulum Ca(2+)-release channel, the physical partner of the L-type channel for signal transduction during the excitation-contraction coupling in muscle.

Interactions between dihydropyridine receptors and ryanodine receptors in striated muscle

Excitation-contraction coupling in both skeletal and cardiac muscle depends on structural and functional interactions between the voltage-sensing dihydropyridine receptor L-type Ca(2+) channels in the surface/transverse tubular membrane and ryanodine receptor Ca(2+) release channels in the sarcoplasmic reticulum membrane. The channels are targeted to either side of a narrow junctional gap that separates the external and internal membrane systems and are arranged so that bi-directional structural and functional coupling can occur between the proteins. There is strong evidence for a physical interaction between the two types of channel protein in skeletal muscle. This evidence is derived from studies of excitation-contraction coupling in intact myocytes and from experiments in isolated systems where fragments of the dihydropyridine receptor can bind to the ryanodine receptors in sarcoplasmic reticulum vesicles or in lipid bilayers and alter channel activity. Although micro-regions that participate in the functional interactions have been identified in each protein, the role of these regions and the molecular nature of the protein-protein interaction remain unknown. The trigger for Ca(2+) release through ryanodine receptors in cardiac muscle is a Ca(2+) influx through the L-type Ca(2+) channel. The Ca(2+) entering through the surface membrane Ca(2+) channels flows directly onto underlying ryanodine receptors and activates the channels. This was thought to be a relatively simple system compared with that in skeletal muscle. However, complexities are emerging and evidence has now been obtained for a bi-directional physical coupling between the proteins in cardiac as well as skeletal muscle. The molecular nature of this coupling remains to be elucidated.

Identification of a region of RyR1 that participates in allosteric coupling with the alpha(1S) (Ca(V)1.1) II-III loop

In skeletal muscle, excitation-contraction (EC) coupling and retrograde signaling are thought to result from direct interactions between the ryanodine receptor (RyR1) and the alpha(1) subunit of the dihydropyridine receptor (alpha(1S)). Previous work has shown that the s53 region of alpha(1S) (residues 720-765 in the II-III loop) and regions R10 (1635-2636) and R9 (2659-3720) of RyR1 are involved in this signaling. Using the yeast two-hybrid system, we here report an interaction between s53 and the sR16 region of RyR1 (1837-2168, within R10), whereas no interaction was seen using upstream residues of the alpha(1S) II-III loop (s31, 666-709). The specificity of the s53-sR16 interaction was tested by using fragments of the cardiac RyR (RyR2) and DHPR (alpha(1C)) that correspond to sR16 and s53, respectively. No interaction was observed for sR16 x c53 (alpha(1C) 850-897), but weak interaction was occasionally observed for s53 x cR16 (RyR2 1817-2142). To test the functional significance of the s53 x sR16 interaction, we expressed in dyspedic myotubes a chimeric RyR (chimeraR16) in which sR16 was substituted for the corresponding region of RyR2. ChimeraR16 was found to mediate weak skeletal-type EC coupling. To test the necessity of sR16 sequence for coupling, we used "chimeraR16-rev," in which sR16 and a small upstream region of RyR1 were replaced by RyR2 sequence. ChimeraR16-rev did not differ from RyR1 in its ability to mediate EC coupling. Thus, interaction between residues 720-765 of alpha(1S) and residues 1837-2168 of RyR1 appears to contribute to but is not essential for EC coupling in skeletal muscle.

T-tubule depolarization-induced local events in the ryanodine receptor, as monitored with the fluorescent conformational probe incorporated by mediation of peptide A

There is a considerable controversy about the postulated role of the Thr(671)-Leu(690) (peptide A) region of the dihydropyridine (DHP) receptor alpha1 II-III loop. Here we report that peptide A introduced the fluorescence probe methyl coumarin acetamido (MCA) in a well defined region of the ryanodine receptor (RyR), A-site, in a specific manner. Depolarization of the T-tubule moiety of the triad induced a rapid increase of the fluorescence intensity of the MCA attached to the A-site. Other RyR agonists, which activate the RyR without mediation of the DHP receptor (e.g. caffeine, polylysine, and peptide A), induced Ca(2+) release without producing such an MCA fluorescence increase. Both magnitudes of the fluorescence change and Ca(2+) release increased with the increase in the degree of T-tubule depolarization. MCA fluorescence increase at the A-site and subsequent sarcoplasmic reticulum Ca(2+) release were blocked by blocking of the DHP receptor-to-RyR communication. These results may be accounted for by two alternative models as follows. (a) Upon T-tubule depolarization a portion of the DHP receptor comes close to the RyR, forming a hydrophobic interface (within such an interface the A-site is located), or (b) T-tubule depolarization may produce a local conformational change in the A-site-containing region of the RyR that is not necessarily within the DHP receptor/RyR junction.