Rectification of skeletal muscle ryanodine receptor mediated by FK506 binding protein

The structural basis of ryanodine receptor ion channel function

Large-conductance Ca²⁺ release channels known as ryanodine receptors (RyRs) mediate the release of Ca²⁺ from an intracellular membrane compartment, the endo/sarcoplasmic reticulum. There are three mammalian RyR isoforms: RyR1 is present in skeletal muscle; RyR2 is in heart muscle; and RyR3 is expressed at low levels in many tissues including brain, smooth muscle, and slow-twitch skeletal muscle. RyRs form large protein complexes comprising four 560-kD RyR subunits, four ∼12-kD FK506-binding proteins, and various accessory proteins including calmodulin, protein kinases, and protein phosphatases. RyRs share ∼70% sequence identity, with the greatest sequence similarity in the C-terminal region that forms the transmembrane, ion-conducting domain comprising ∼500 amino acids. The remaining ∼4,500 amino acids form the large regulatory cytoplasmic "foot" structure. Experimental evidence for Ca²⁺, ATP, phosphorylation, and redox-sensitive sites in the cytoplasmic structure have been described. Exogenous effectors include the two Ca²⁺ releasing agents caffeine and ryanodine. Recent work describing the near atomic structures of mammalian skeletal and cardiac muscle RyRs provides a structural basis for the regulation of the RyRs by their multiple effectors.

Stabilization of the skeletal muscle ryanodine receptor ion channel-FKBP12 complex by the 1,4-benzothiazepine derivative S107

Activation of the skeletal muscle ryanodine receptor (RyR1) complex results in the rapid release of Ca²⁺ from the sarcoplasmic reticulum and muscle contraction. Dissociation of the small FK506 binding protein 12 subunit (FKBP12) increases RyR1 activity and impairs muscle function. The 1,4-benzothiazepine derivative JTV519, and the more specific derivative S107 (2,3,4,5,-tetrahydro-7-methoxy-4-methyl-1,4-benzothiazepine), are thought to improve skeletal muscle function by stabilizing the RyR1-FKBP12 complex. Here, we report a high degree of nonspecific and specific low affinity [³H]S107 binding to SR vesicles. SR vesicles enriched in RyR1 bound ∼48 [³H]S107 per RyR1 tetramer with EC₅₀ ∼52 µM and Hillslope ∼2. The effects of S107 and FKBP12 on RyR1 were examined under conditions that altered the redox state of RyR1. S107 increased FKBP12 binding to RyR1 in SR vesicles in the presence of reduced glutathione and the NO-donor NOC12, with no effect in the presence of oxidized glutathione. Addition of 0.15 µM FKBP12 to SR vesicles prevented FKBP12 dissociation; however, in the presence of oxidized glutathione and NOC12, FKBP12 dissociation was observed in skeletal muscle homogenates that contained 0.43 µM myoplasmic FKBP12 and was attenuated by S107. In single channel measurements with FKBP12-depleted RyR1s, in the absence and presence of NOC12, S107 augmented the FKBP12-mediated decrease in channel activity. The data suggest that S107 can reverse the harmful effects of redox active species on SR Ca²⁺ release in skeletal muscle by binding to RyR1 low affinity sites.

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.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Single-channel characterization of the rabbit recombinant RyR2 reveals a novel inactivation property of physiological concentrations of ATP

Ryanodine receptor 2 (RyR2) cDNA has been available for more than 15 years; however, due to the complex nature of ligand gating in this channel, many aspects of recombinant RyR2 function have been unresearched. We established a stable, inducible HEK 293 cell line expressing full-length rabbit RyR2 cDNA and assessed the single-channel properties of the recombinant RyR2, with particular reference to ligand regulation with Ca2+ as the permeant ion. We found that the single-channel conductances of recombinant RyR2 and RyR2 isolated from cardiac muscle are essentially identical, as is irreversible modification by ryanodine. Although it is known that RyR2 expressed in HEK 293 cells is not associated with FKBP12.6, we demonstrate that these channels do not exhibit any discernable disorganized gating characteristics or subconductance states. We also show that the gating of recombinant RyR2 is indistinguishable from that of channels isolated from cardiac muscle when activated by cytosolic Ca2+, caffeine or suramin. The mechanisms underlying ATP activation are also similar; however, the experiments highlighted a novel effect of ATP at physiologically relevant concentrations of 5-10 mM. With Ca2+ as permeant ion, 5-10 mM ATP consistently inactivated recombinant channels (15/16 experiments). Such inactivation was rarely observed with native RyR2 isolated from cardiac muscle (1 in 16 experiments). However, if the channels were purified, inactivation by ATP was then revealed in all experiments. This action of ATP may be relevant for inactivation of sarcoplasmic reticulum Ca2+ release during cardiac excitation-contraction coupling or may represent unnatural behavior that is revealed when RyR2 is purified or expressed in noncardiac systems.

Cross talk between Ca2+ and redox signalling cascades in muscle and neurons through the combined activation of ryanodine receptors/Ca2+ release channels

Calcium release mediated by the ryanodine receptors (RyR) Ca2+ release channels is required for muscle contraction and contributes to neuronal plasticity. In particular, Ca2+ activation of RyR-mediated Ca2+ release can amplify and propagate Ca2+ signals initially generated by Ca2+ entry into cells. Redox modulation of RyR function by a variety of non-physiological or endogenous redox molecules has been reported. The effects of RyR redox modification on Ca2+ release in skeletal muscle as well as the activation of signalling cascades and transcription factors in neurons will be reviewed here. Specifically, the different effects of S-nitrosylation or S-glutathionylation of RyR cysteines by endogenous redox-active agents on the properties of skeletal muscle RyRs will be discussed. Results will be presented indicating that these cysteine modifications change the activity of skeletal muscle RyRs, modify their behaviour towards both activators and inhibitors and affect their interactions with FKBP12 and calmodulin. In the hippocampus, sequential activation of ERK1/2 and CREB is a requisite for Ca2+-dependent gene expression associated with long-lasting synaptic plasticity. The effects of reactive oxygen/nitrogen species on RyR channels from neurons and RyR-mediated sequential activation of neuronal ERK1/2 and CREB produced by hydrogen peroxide and other stimuli will be discussed as well.

Structural Characterization of the RyR1–FKBP12 Interaction

The 12 kDa FK506-binding protein (FKBP12) constitutively binds to the calcium release channel RyR1. Removal of FKBP12 using FK506 or rapamycin causes an increased open probability and an increase in the frequency of sub-conductance states in RyR1. Using cryo-electron microscopy and single-particle image processing, we have determined the 3D difference map of FKBP12 associated with RyR1 at 16 A resolution that can be fitted with the atomic model of FKBP12 in a unique orientation. This has allowed us to better define the surfaces of close apposition between FKBP12 and RyR1. Our results shed light on the role of several FKBP12 residues that had been found critical for the specificity of the RyR1-FKBP12 interaction. As predicted from previous immunoprecipitation studies, our results suggest that Gln3 participates directly in this interaction. The orientation of RyR1-bound FKBP12, with part of its FK506 binding site facing towards RyR1, allows us to propose how FK506 is involved in the dissociation of FKBP12 from RyR1.

Ryanodine receptor binding to FKBP12 is modulated by channel activation state

Ryanodine receptor (RyR) Ca2+ release channels undergo a conformational change between the open and closed states. Its protein modulator, FK506 binding protein 12 (FKBP12), stabilises the channel gating between the four subunits that surround a central Ca2+-conducting pore. To understand the interdependence of RyR and FKBP12 binding, physiological and pharmacological agents were used to modulate the RyR open/closed state. ELISA sandwich binding assays showed that FKBP12 binding was dependent on the free Ca2+ and was lower at 1-10 μM free Ca2+ compared with 1 mM EGTA and 1 mM Ca2+, and this effect was enhanced by the inclusion of 1 mM ATP. Ruthenium red increased the binding of FKBP12. [3H]Ryanodine binding confirmed that 1 mM EGTA, 1 mM Ca2+ and 1 μM ruthenium red closed the channel, whereas 1 μM free Ca2+, 1 μM free Ca2+ + 1 mM ATP, or 10 mM caffeine opened it. These binding conditions were used in surface plasmon resonance studies to measure equilibrium binding kinetics. The affinity constant KA was significantly greater for the closed than the open channel, a change mediated by a decreased dissociation rate constant, kd. The results show that surface plasmon resonance is a powerful technique that can measure differences in RyR1 equilibrium binding kinetics with FKBP12.

Effects of S-glutathionylation and S-nitrosylation on calmodulin binding to triads and FKBP12 binding to type 1 calcium release channels

This study shows that the combination of glutathione (GSH) plus hydrogen peroxide (H2O2) promotes the S-glutathionylation of ryanodine receptor type 1 (RyR1) Ca2+ release channels, and confirms their joint S-glutathionylation and S-nitrosylation by S-nitrosoglutathione (GSNO). In addition, we show that 35S-labeled 12-kDa FK506-binding protein ([35S]FKBP12) bound with a Kd of 13.1 nM to RyR1 present in triads or heavy sarcoplasmic reticulum vesicles; RyR1 S-nitrosylation by NOR-3 or GSNO, but not S-glutathionylation, specifically increased by four- to fivefold this Kd value. RyR1 redox modifications also increased the Kd of [35S]calmodulin binding to triads without affecting Bmax. RyR1 S-glutathionylation (induced by GSH plus H2O2) or RyR1 S-nitrosylation (produced by NOR-3) increased by approximately six- or twofold, respectively, the Kd of apocalmodulin (apoCaM) or Ca2+-calmodulin (CaCaM) binding to triads. Likewise, the combined S-glutathionylation and S-nitrosylation of RyR1 induced by GSNO increased by fourfold the Kd of CaCaM binding to triads and abolished apoCaM binding. As both FKBP12 and CaCaM inhibit RyR1, decreased FKBP12 binding to RyR1 and/or decreased CaCaM binding to either RyR1 or dihydropyridine receptor in triad preparations may cause the reported enhanced activation of Ca2+-induced Ca2+ release kinetics mediated by S-glutathionylation/S-nitrosylation. We discuss possible consequences of these redox modifications on RyR1-mediated Ca2+ release in physiological or pathological conditions.

Interaction of FKBP12.6 with the cardiac ryanodine receptor C-terminal domain

The ryanodine receptor-calcium release channel complex (RyR) plays a pivotal role in excitation-contraction coupling in skeletal and cardiac muscle. RyR channel activity is modulated by interaction with FK506-binding protein (FKBP), and disruption of the RyR-FKBP association has been implicated in cardiomyopathy, cardiac hypertrophy, and heart failure. Evidence for an interaction between RyR and FKBP is well documented, both in skeletal muscle (RyR1-FKBP12) and in cardiac muscle (RyR2-FKBP12.6), however definition of the FKBP-binding site remains elusive. Early reports proposed interaction of a short RyR central domain with FKBP12/12.6, however this site has been questioned, and recently an alternative FKBP12.6 interaction site has been identified within the N-terminal half of RyR2. In this study, we report evidence for the human RyR2 C-terminal domain as a novel FKBP12.6-binding site. Using competition binding assays, we find that short C-terminal RyR2 fragments can displace bound FKBP12.6 from the native RyR2, although they are unable to exclusively support interaction with FKBP12.6. However, expression of a large RyR2 C-terminal construct in mammalian cells encompassing the pore-forming transmembrane domains exhibits rapamycin-sensitive binding specifically to FKBP12.6 but not to FKBP12. We also obtained some evidence for involvement of the RyR2 N-terminal, but not the central domain, in FKBP12.6 interaction. Our studies suggest that a novel interaction site for FKBP12.6 may be present at the RyR2 C terminus, proximal to the channel pore, a sterically appropriate location that would enable this protein to play a central role in the modulation of this critical ion channel.

Ca(2+)-dependent interaction between FKBP12 and calcineurin regulates activity of the Ca(2+) release channel in skeletal muscle

Calcineurin is a Ca(2+) and calmodulin-dependent protein phosphatase with diverse cellular functions. Here we examined the physical and functional interactions between calcineurin and ryanodine receptor (RyR) in a C2C12 cell line derived from mouse skeletal muscle. Coimmunoprecipitation experiments revealed that the association between RyR and calcineurin exhibits a strong Ca(2+) dependence. This association involves a Ca(2+) dependent interaction between calcineurin and FK506-binding protein (FKBP12), an accessory subunit of RyR. Pretreatment with cyclosporin A, an inhibitor of calcineurin, enhanced the caffeine-induced Ca(2+) release (CICR) in C2C12 cells. This effect was similar to those of FK506 and rapamycin, two drugs known to cause dissociation of FKBP12 from RyR. Overexpression of a constitutively active form of calcineurin in C2C12 cells, DeltaCnA(391-521) (deletion of the last 131 amino acids from calcineurin), resulted in a decrease in CICR. This decrease in CICR activity was partially recovered by pretreatment with cyclosporin A. Furthermore, overexpression of an endogenous calcineurin inhibitor (cain) or an inactive form of calcineurin (DeltaCnA(H101Q)) in C2C12 cells resulted in up-regulation of CICR. Taken together, our data suggest that a trimeric-interaction among calcineurin, FKBP12, and RyR is important for the regulation of the RyR channel activity and may play an important role in the Ca(2+) signaling of muscle contraction and relaxation.

Washing out of lipophilic compounds induces a transient increase in the passive Ca(2+) leak in permeabilized A7r5 cells

We have investigated how the immunosuppressant drug FK506 affected the basal Ca(2+) leak in permeabilized A7r5 cells. Non-mitochondrial Ca(2+) stores loaded to steady state with Ca(2+) slowly lost their accumulated Ca(2+) during incubation in a Ca(2+)-free efflux medium. FK506 up to 100 microM had no effect on the basal Ca(2+) leak. In contrast, the rate of Ca(2+) release proceeded much faster immediately after washing out FK506. The increase in rate of Ca(2+) release after washing out of this compound depended on both its initial concentration and on the time of pre-incubation. A similar effect was also observed after removing another immunosuppressant drug (rapamycin) and after removing the inositol 1,4,5-trisphosphate receptor inhibitor xestospongin C. Since all these substances have a high octanol/H(2)O partition coefficient and accumulate in the endoplasmic reticulum membrane, we suggest that the transient increase in the basal Ca(2+) leak is due to the sudden removal of these lipophilic substances from the membrane.

HSV-1 amplicon vectors are a highly efficient gene delivery system for skeletal muscle myoblasts and myotubes

Analysis of RyR1 structure function in muscle cells is made difficult by the low (<5%) transfection efficiencies of myoblasts or myotubes using calcium phosphate or cationic lipid techniques. We inserted the full-length 15.3-kb RyR1 cDNA into a herpes simplex virus type 1 (HSV-1) amplicon vector, pHSVPrPUC between the ori/IE 4/5 promoter sequence and the HSV-1 DNA cleavage/packaging signal (pac). pHSVGN and pHSVGRyR1, two amplicons that expressed green fluorescent protein, were used for fluorescence-activated cell sorter analysis of transduction efficiency. All amplicons were packaged into HSV-1 virus particles using a helper virus-free packaging system and yielded 10(6) transducing vector units/ml. HSVRyR1, HSVGRyR1, and HSVGN virions efficiently transduced mouse myoblasts and myotubes, expressing the desired product in 70-90% of the cells at multiplicity of infection 5. The transduced cells appeared healthy and RyR1 produced by this method was targeted properly and restored skeletal excitation-contraction coupling in dyspedic myotubes. The myotubes produced sufficient protein to allow single-channel analyses from as few as 10 100-mm dishes. In most cases this method could preclude the need for permanent transfectants for the study of RyR1 structure function.

How many cysteine residues regulate ryanodine receptor channel activity?

RyRs contain 80-100 cysteine residues per subunit, of which approximately 25% are free for covalent modification, while the remainder are either modified or form intraprotein disulfides. Oxidizing and nitrosylating reagents have several effects on single RyR channel activity, which depend on the type of modifying reagent, the isoform of the RyR, and ligands bound to the channel. We present evidence here for four major classes of functional cysteine residues associated with RyR channels, i.e., two classes with free -SH groups that either activate or inhibit channels when covalently modified and two classes, with endogenous modification, that either inhibit or activate. Single-channel characteristics provide evidence for four discrete responses within the first activating class, two responses within the second inhibiting class and two types of response within the third endogenously modified class. All but one of these changes in channel properties depend on residues located on the cytoplasmic or membrane-associated domains of the RyR; the remaining response is confined to the luminal domain. If it is assumed that each type of response depends on a separate subclass of cysteine residue and that each subclass contains a minimum of one cysteine per subunit, our results suggest that there are at least nine cysteine residues per subunit with functional connections to the gating mechanism of RyR channels. These cysteine residues may be selectively modified under physiological and pathological conditions to regulate Ca2+ release from the sarcoplasmic reticulum and contraction.

Oxidation and reduction of pig skeletal muscle ryanodine receptors

Time-dependent effects of cysteine modification were compared in skeletal ryanodine receptors (RyRs) from normal pigs and RyR(MH) (Arg(615) to Cys(615)) from pigs susceptible to malignant hyperthermia, using the oxidizing reagents 4,4'-dithiodipyridine (4, 4'-DTDP) and 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB) or the reducing agent dithiothreitol (DTT). Normal and RyR(MH) channels responded similarly to all reagents. DTNB (1 mM), either cytoplasmic (cis) or luminal (trans), or 1 mM 4,4'-DTDP (cis) activated RyRs, introducing an additional long open time constant. 4,4'-DTDP (cis), but not DTNB, inhibited channels after >5 min. Activation and inhibition were relieved by DTT (1-10 mM). DTT (10 mM, cytoplasmic or luminal), without oxidants, activated RyRs, and activation reversed with 1 mM DTNB. Control RyR activity was maintained with 1 mM DTNB and 10 mM DTT present on the same or opposite sides of the bilayer. We suggest that 1) 4,4'-DTDP and DTNB covalently modify RyRs by oxidizing activating or inhibiting thiol groups; 2) a modified thiol depresses mammalian skeletal RyR activity under control conditions; 3) both the activating thiols and the modified thiols, accessible from either cytoplasm or lumen, reside in the transmembrane region; 4) some cardiac sulfhydryls are unavailable in skeletal RyRs; and 5) Cys(615) in RyR(MH) is functionally unimportant in redox cycling.

Expression and functional characterization of the cardiac muscle ryanodine receptor Ca(2+) release channel in Chinese hamster ovary cells

To study the function and regulation of the cardiac ryanodine receptor (RyR2) Ca(2+) release channel, we expressed the RyR2 proteins in a Chinese hamster ovary (CHO) cell line, and assayed its function by single channel current recording and confocal imaging of intracellular Ca(2+) ([Ca(2+)](i)). The 16-kb cDNA encoding the full-length RyR2 was introduced into CHO cells using lipofectAmine and electroporation methods. Incorporation of microsomal membrane vesicles isolated from these transfected cells into lipid bilayer membrane resulted in single Ca(2+) release channel activities similar to those of the native Ca(2+) release channels from rabbit cardiac muscle SR membranes, both in terms of gating kinetics, conductance, and ryanodine modification. The expressed RyR2 channels were found to exhibit more frequent transitions to subconductance states than the native RyR2 channels and RyR1 expressed in CHO cells. Caffeine, an exogenous activator of RyR, induced release of [Ca(2+)](i) from these cells. Confocal imaging of cells expressing RyR2 did not detect spontaneous or caffeine-induced local Ca(2+) release events (i.e., "Ca(2+) sparks") typically seen in cardiac muscle. Our data show that the RyR2 expressed in CHO cells forms functional Ca(2+) release channels. Furthermore, the lack of localized Ca(2+) release events in these cells suggests that Ca(2+) sparks observed in cardiac muscle may involve cooperative gating of a group of Ca(2+) release channels and/or their interaction with muscle-specific proteins.

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.

Inactivation of Ca2+ release channels (ryanodine receptors RyR1 and RyR2) with rapid steps in [Ca2+] and voltage

The transient responses of sheep cardiac and rabbit skeletal ryanodine receptors (RyRs) to step changes in membrane potential and cytosolic [Ca2+] were measured. Both cardiac and skeletal RyRs have two voltage-dependent inactivation processes (tau approximately 1-3 s at +40 mV) that operate at opposite voltage extremes. Approximately one-half to two-thirds of RyRs inactivated when the bilayer voltage was stepped either way between positive and negative values. Inactivation was not detected (within 30 s) in RyRs with Po less than 0.2. Inactivation rates increased with intraburst open probability (Po) and in proportion to the probability of a long-lived, RyR open state (P(OL)) RyR inactivation depended on P(OL) and not on the particular activator (Ca2+ (microM), ATP, caffeine, and ryanodine), inhibitor (mM Ca2+ and Mg2+), or gating mode. The activity of one-half to two-thirds of RyRs declined (i.e., the RyRs inactivated) after [Ca2+] steps from subactivating (0.1 microM) to activating (1-100 microM) levels. This was due to the same inactivation mechanism responsible for inactivation after voltage steps. Both forms of inactivation had the same kinetics and similar dependencies on Po and voltage. Moreover, RyRs that failed to inactivate after voltage steps also did not inactivate after [Ca2+] steps. The inactivating response to [Ca2+] steps (0.1-1 microM) was not RyRs "adapting" to steady [Ca2+] after the step, because a subsequent step from 1 to 100 microM failed to reactivate RyRs.

Effect of luminal calcium on Ca2+ release channel activity of sarcoplasmic reticulum in situ

Ca2+ influx into empty SR in the absence of Ca2+ pump activity was determined in skinned frog skeletal muscle fibers and compared with Ca2+ efflux from loaded SR (i.e., Ca2+ release) to deepen our understanding of the properties of the Ca2+ release channel (CRC). Calcium content in SR increased approximately in a first-order kinetics and finally reached the equilibrium level determined by cytoplasmic Ca2+ ([Ca2+]c). Because AMP caused an increase in the rate of Ca2+ influx, and procaine, Mg2+, and high concentrations of Ca2+ caused a characteristic decrease, the major Ca2+ influx pathway was concluded to be the CRC, as is true of Ca2+ release. The apparent rate constant (k(app)) of Ca2+ efflux did not significantly change when the loading level was decreased to one-third. At a given [Ca2+]c, the same equilibrium level of calcium in SR was attained with a similar k(app) by both Ca2+ influx and Ca2+ efflux. The relationship between [Ca2+]c and calcium in SR indicated the Ca2+ binding sites in SR. These results, together with the anticipated effects of these Ca2+ buffer sites on kinetics, are consistent with the idea that luminal Ca2+ inhibits the CRC.

Locations of calmodulin and FK506-binding protein on the three-dimensional architecture of the skeletal muscle ryanodine receptor

Isolated skeletal muscle ryanodine receptors (RyRs) complexed with the modulatory ligands, calmodulin (CaM) or 12-kDa FK506-binding protein (FKBP12), have been characterized by electron cryomicroscopy and three-dimensional reconstruction. RyRs are composed of 4 large subunits (molecular mass 565 kDa) that assemble to form a 4-fold symmetric complex that, architecturally, comprises two major substructures, a large ( approximately 80% of the total mass) cytoplasmic assembly and a smaller transmembrane assembly. Both CaM and FKBP12 bind to the cytoplasmic assembly at sites that are 10 and 12 nm, respectively, from the putative entrance to the transmembrane ion channel. FKBP12 binds along the edge of the square-shaped cytoplasmic assembly near the face that interacts in vivo with the sarcolemma/transverse tubule membrane system, whereas CaM binds within a cleft that faces the junctional face of the sarcoplasmic reticulum membrane at the triad junction. Both ligands interact with a domain that connects directly to a cytoplasmic extension of the transmembrane assembly of the receptor, and thus might cause structural changes in the domain which in turn modulate channel gating.

Caffeine-induced release of intracellular Ca2+ from Chinese hamster ovary cells expressing skeletal muscle ryanodine receptor. Effects on full-length and carboxyl-terminal portion of Ca2+ release channels

The ryanodine receptor (RyR)/Ca2+ release channel is an essential component of excitation-contraction coupling in striated muscle cells. To study the function and regulation of the Ca2+ release channel, we tested the effect of caffeine on the full-length and carboxyl-terminal portion of skeletal muscle RyR expressed in a Chinese hamster ovary (CHO) cell line. Caffeine induced openings of the full length RyR channels in a concentration-dependent manner, but it had no effect on the carboxyl-terminal RyR channels. CHO cells expressing the carboxyl-terminal RyR proteins displayed spontaneous changes of intracellular [Ca2+]. Unlike the native RyR channels in muscle cells, which display localized Ca2+ release events (i.e., "Ca2+ sparks" in cardiac muscle and "local release events" in skeletal muscle), CHO cells expressing the full length RyR proteins did not exhibit detectable spontaneous or caffeine-induced local Ca2+ release events. Our data suggest that the binding site for caffeine is likely to reside within the amino-terminal portion of RyR, and the localized Ca2+ release events observed in muscle cells may involve gating of a group of Ca2+ release channels and/or interaction of RyR with muscle-specific proteins.

Single-channel properties of the recombinant skeletal muscle Ca2+ release channel (ryanodine receptor)

We report transient expression of a full-length cDNA encoding the Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum (ryanodine receptor) in HEK-293 cells. The single-channel properties of the 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate-solubilized and sucrose gradient-purified recombinant Ca2+ release channels were investigated by using single-channel recordings in planar lipid bilayers. The recombinant Ca2+ release channel exhibited a K+ conductance of 780 pS when symmetrical 250 mM KCl was used as the conducting ion and a Ca2+ conductance of 116 pS in 50 mM luminal Ca2+. Opening events of the recombinant channels were brief, with an open time constant of approximately 0.22 ms. The recombinant Ca2+ release channel was more permeable to Ca2+ than to K+, with a pCa2+/pK+ ratio of 6.8. The response of the recombinant Ca2+ release channel to various concentrations of Ca2+ was biphasic, with the channel being activated by micromolar Ca2+ and inhibited by millimolar Ca2+. The recombinant channels were activated by ATP and caffeine, inhibited by Mg2+ and ruthenium red, and modified by ryanodine. Most recombinant channels were asymmetrically blocked, conducting current unidirectionally from the luminal to the cytoplasmic side of the channel. These data demonstrate that the properties of recombinant Ca2+ release channel expressed in HEK-293 cells are very similar, if not identical, to those of the native channel.

Reduced inhibitory effect of Mg2+ on ryanodine receptor-Ca2+ release channels in malignant hyperthermia

Malignant hyperthermia (MH) is a potentially fatal, inherited skeletal muscle disorder in humans and pigs that is caused by abnormal regulation of Ca2+ release from the sarcoplasmic reticulum (SR). MH in pigs is associated with a single mutation (Arg615Cys) in the SR ryanodine receptor (RyR) Ca2+ release channel. The way in which this mutation leads to excessive Ca2+ release is not known and is examined here. Single RyR channels from normal and MH-susceptible (MHS) pigs were examined in artificial lipid bilayers. High cytoplasmic (cis) concentrations of either Ca2+ or Mg2+ (>100 microM) inhibited channel opening less in MHS RyRs than in normal RyRs. This difference was more prominent at lower ionic strength (100 mM versus 250 mM). In 100 mM cis Cs+, half-maximum inhibition of activity occurred at approximately 100 microM Mg2+ in normal RyRs and at approximately 300 microM Mg2+ in MHS RyRs, with an average Hill coefficient of approximately 2 in both cases. The level of Mg2+ inhibition was not appreciably different in the presence of either 1 or 50 microM activating Ca2+, showing that it was not substantially influenced by competition between Mg2+ and Ca2+ for the Ca2+ activation site. Even though the absolute inhibitory levels varied widely between channels and conditions, the inhibitory effects of Ca2+ and Mg2+ were virtually identical for the same conditions in any given channel, indicating that the two cations act at the same low-affinity inhibitory site. It seems likely that at the cytoplasmic [Mg2+] in vivo (approximately 1 mM), this Ca2+/Mg2+-inhibitory site will be close to fully saturated with Mg2+ in normal RyRs, but less fully saturated in MHS RyRs. Therefore MHS RyRs should be more sensitive to any activating stimulus, which would readily account for the development of an MH episode.

Conducting and voltage-dependent behaviors of the native and purified SR Ca2+-release channels from the canine diaphragm

The ryanodine-sensitive Ca2+-release channel of the canine diaphragm sarcoplasmic reticulum (SR) was characterized using biochemical assays and the planar lipid bilayer technique. Diaphragm SR membranes have a [3H]ryanodine-binding capacity (Bmax) of 1.2 pmol/mg protein and a binding affinity (K(D)) of 6.3 nM. The conductance of the native channel was 330 pS in 50 mM/250 mM trans/cis CsCH3SO3 and was reduced to 71 pS by 10 mM Ca2+ trans. The Ca2+-release channel was purified as a 400 kDa protein on SDS-PAGE and displayed a conductance of 715 pS in 200 mM KCl. The native and purified Ca2+ channels were activated by micromolar Ca2+ and ATP and inhibited by Mg2+, ryanodine and ruthenium red. Although diaphragm muscle contraction was shown to depend on extracellular Ca2+ like cardiac muscles, we provide evidence that the diaphragm SR Ca2+-release channel may be classified as a skeletal ryanodine receptor isoform. First, the IC50 for [3H]ryanodine binding was in the same range as estimated for skeletal SR, with 20 nM. Second, the channel was maximally activated by 10-30 microM cytoplasmic Ca2+ and inhibited at higher concentrations. Third, ryanodine binding to the diaphragm SR was less sensitive to Ca2+ than cardiac SR, with EC50, values of 50 and 1 microM, respectively. Finally, Ca2+-release activity and [3H]ryanodine binding capacity of the diaphragm and skeletal SR were similarly more sensitive to Mg2+ than cardiac SR. Together, these results suggest a predominantly skeletal-type of excitation-contraction coupling in the diaphragm.

Deletion of amino acids 1641-2437 from the foot region of skeletal muscle ryanodine receptor alters the conduction properties of the Ca release channel

The ryanodine receptor (RyR) of skeletal muscle contains two functional domains: a carboxyl-terminal hydrophobic domain that forms the putative conduction pore of the calcium release channel, and a large cytoplasmic domain that corresponds to the "foot structure." To understand the contribution of the foot structure to the function of the calcium release channel, we studied a RyR deletion mutant, delta(1641-2437)-RyR, in which a region that is rich in glutamate and aspartate residues (a.a. 1641-2437) was removed. The wild-type and delta(1641-2437)-RyR proteins were expressed in a Chinese hamster ovary (CHO) cell line, and functions of single calcium release channels were measured in the lipid bilayer membrane. The wild-type RyR forms functional calcium release channels with a linear current-voltage relationship similar to that of the native channel identified in the sarcoplasmic reticulum membrane of skeletal muscle, whereas the channels formed by delta(1641-2437)-RyR exhibit significant inward rectification, i.e., currents moving from cytoplasm into SR lumen were approximately 20% less than that in the opposite direction. As in to the wt-RyR channel, opening of the delta(1641-2437)-RyR channel has a bell-shaped dependence on the cytoplasmic calcium, but the calcium-dependent activation and inactivation processes of the delta(1641-2437)-RyR channel are shifted to higher calcium concentrations. Our data show that deletion of a.a. 1641-2437 from the foot region of the skeletal muscle RyR results in changes in both ion conduction and calcium-dependent regulation of the calcium release channel.

Functional calcium release channel formed by the carboxyl-terminal portion of ryanodine receptor

The ryanodine receptor (RyR) is one of the key proteins involved in excitation-contraction (E-C) coupling in skeletal muscle, where it functions as a Ca2+ release channel in the sarcoplasmic reticulum (SR) membrane. RyR consists of a single polypeptide of approximately 560 kDa normally arranged in a homotetrameric structure, which contains a carboxyl (C)-terminal transmembrane domain and a large amino (N)-terminal cytoplasmic domain. To test whether the carboxyl-terminal portion of RyR is sufficient to form a Ca2+ release channel, we expressed the full-length (RyR-wt) and C-terminal (RyR-C, approximately 130 kDa) RyR proteins in a Chinese hamster ovary (CHO) cell line, and measured their Ca2+ release channel functions in planar lipid bilayer membranes. The single-channel properties of RyR-wt were found to be similar to those of RyR from skeletal muscle SR. The RyR-C protein forms a cation-selective channel that shares some of the channel properties with RyR-wt, including activation by cytoplasmic Ca2+ and regulation by ryanodine. Unlike RyR-wt, which exhibits a linear current-voltage relationship and inactivates at millimolar Ca2+, the channels formed by RyR-C display significant inward rectification and fail to close at high cytoplasmic Ca2+. Our results show that the C-terminal portion of RyR contains structures sufficient to form a functional Ca2+ release channel, but the N-terminal portion of RyR also affects the ion-conduction and calcium-dependent regulation of the Ca2+ release channel.

Different interactions of cardiac and skeletal muscle ryanodine receptors with FK-506 binding protein isoforms

In the present study, we compare functional consequences of dissociation and reconstitution of binding proteins FKBP12 and FKBP12.6 with ryanodine receptors from cardiac (RyR2) and skeletal muscle (RyR1). The skeletal muscle RyR1 channel became activated on removal of endogenously bound FKBP12, consistent with previous reports. Both FKBP12 and FKBP12.6 rebind to FKBP-depleted RyR1 and restore its quiescent channel behavior by altering ligand sensitivity, as studied by single-channel recordings in planar lipid bilayers, and macroscopic behavior of the channels (ryanodine binding and net energized Ca2- uptake). By contrast, removal of FKBP12.6 from the cardiac RyR2 did not modulate the function of the channel using the same types of assays as for RyR1. FKBP12 or FKBP12.6 had no effect on channel activity of FKBP12.6-depleted cardiac RyR2, although FKBP12.6 rebinds. Our studies reveal important differences between the two ryanodine receptor isoforms with respect to their functional interaction with FKBP12 and FKBP12.6.

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

Ryanodine receptors from rabbit skeletal muscle are reversibly activated by rapamycin

In this report we demonstrate that the immunosuppressive drug, rapamycin, can reversibly activate the skeletal muscle ryanodine receptor calcium release channel (RyR) in terminal cisternae vesicles incorporated into planar lipid bilayers. This reveals a second mechanism of activation of RyRs by rapamycin. Irreversible channel activation and openings to subconductance levels are seen when rapamycin forms a complex with and removes the tightly bound 12 kDa FK506-binding protein (FKBP12) from the RyR. We show here that micromolar rapamycin activates RyRs which were previously 'stripped' of > 95% of their FKBP12s. Rapamycin caused a 6-fold increase in mean current, which was largely reversible, but no increase in the fraction of openings to subconductance levels. Therefore native RyRs, stripped of FKBP12, are directly activated by the macrocyclic lactone, rapamycin.

Subconductance states in single-channel activity of skeletal muscle ryanodine receptors after removal of FKBP12

FKBP12 was removed from ryanodine receptors (RyRs) by incubation of rabbit skeletal muscle terminal cisternae membranes with rapamycin. The extent of FKBP12 removal was estimated by immunostaining Western blots of terminal cisternae proteins. Single FKBP12-depleted RyR channels, incorporated into planar lipid bilayers, were modulated by Ca2+, ATP, ryanodine, and ruthenium red in the cis chamber and opened frequently to the normal maximum conductance of approximately 230 pS and to substate levels of approximately 0.25, approximately 0.5, and approximately 0.75 of the maximum conductance. Substate activity was rarely seen in native RyRs. Ryanodine did not after the number of conductance levels in FKBP12-depleted channels, but, at a membrane potential of +40 mV, reduced both the maximum and the substate conductances by approximately 50%. FKBP12-stripped channels were activated by a 10-fold-lower [Ca2+] and inhibited by a 10-fold-higher [Ca2+], than RyRs from control-incubated and native terminal cisternae vesicles. The open probability (Po) of these FKBP12-deficient channels was greater than that of control channels at 0.1 microM and 1 mM cis Ca2+ but no different at 10 microM cis Ca2+, where channels showed maximal Ca2+ activation. The approximately 0.25 substate was less sensitive than the maximum conductance to inhibition by Ca2+ and was the dominant level in channels inhibited by 1 mM cis Ca2+. The results show that FKBP12 coordinates the gating of channel activity in control and ryanodine-modified RyRs.

Cryoelectron microscopy resolves FK506-binding protein sites on the skeletal muscle ryanodine receptor

A 12-kDa immunophilin (FKBP12) is an integral component of the skeletal muscle ryanodine receptor (RyR). The RyR is a hetero-oligomeric complex with structural formula (FKBP)4(Ryr1)4, where Ryr1 is the 565-kDa product of the Ryr1 gene. To aid in the detection of the immunophilin's location in the receptor, we exchanged the FKBP12 present in RyR-enriched vesicles derived from sarcoplasmic reticulum with an engineered construct of FKBP12 fused to glutathione S-transferase and then isolated the complexes. Cryoelectron microscopy and image averaging of the complexes (in an orientation displaying the RyR's fourfold symmetry) revealed four symmetrically distributed, diffuse density regions that were located just outside the boundary defining the cytoplasmic assembly of the RyR. These regions are attributed to the glutathione transferase portion of the fusion protein because they are absent from receptors lacking the fusion protein. To more precisely define the location of FKBP12, we similarly analyzed complexes of RyR containing FKBP12 itself. Apparently some FKBP is lost during the purification or storage of the RyR because, to detect the receptor-bound immunophilin, it was necessary to add FKBP12 to the purified receptor before electron microscopy. Averaged images of these complexes showed a region of density that had not been observed previously in images of isolated receptors, and its position, along the edges of the transmembrane assembly, agreed with the position of the FKBP12 deduced from the experiments with the fusion protein. The proposed locations for FKBP12 are about 10 nm from the transmembrane baseplate assembly that contains the ion channel of the RyR.