RyR2 Binding of an Antiarrhythmic Cyclic Depsipeptide Mapped Using Confocal Fluorescence Lifetime Detection of FRET

Hyperactivity of cardiac sarcoplasmic reticulum (SR) ryanodine receptor (RyR2) Ca2+-release channels contributes to heart failure and arrhythmias. Reducing the RyR2 activity, particularly during cardiac relaxation (diastole), is a desirable therapeutic goal. We previously reported that the unnatural enantiomer (ent) of an insect-RyR activator, verticilide, inhibits porcine and mouse RyR2 at diastolic (nanomolar) Ca2+ and has in vivo efficacy against atrial and ventricular arrhythmia. To determine the ent-verticilide structural mode of action on RyR2 and guide its further development via medicinal chemistry structure-activity relationship studies, here, we used fluorescence lifetime (FLT)-measurements of Förster resonance energy transfer (FRET) in HEK293 cells expressing human RyR2. For these studies, we used an RyR-specific FRET molecular-toolkit and computational methods for trilateration (i.e., using distances to locate a point of interest). Multiexponential analysis of FLT-FRET measurements between four donor-labeled FKBP12.6 variants and acceptor-labeled ent-verticilide yielded distance relationships placing the acceptor probe at two candidate loci within the RyR2 cryo-EM map. One locus is within the Ry12 domain (at the corner periphery of the RyR2 tetrameric complex). The other locus is sandwiched at the interface between helical domain 1 and the SPRY3 domain. These findings document RyR2-target engagement by ent-verticilide, reveal new insight into the mechanism of action of this new class of RyR2-targeting drug candidate, and can serve as input in future computational determinations of the ent-verticilide binding site on RyR2 that will inform structure-activity studies for lead optimization.

Mapping the ryanodine receptor FK506-binding protein subunit using fluorescence resonance energy transfer

The 12-kDa FK506-binding proteins (FKBP12 and FKBP12.6) are regulatory subunits of ryanodine receptor (RyR) Ca(2+) release channels. To investigate the structural basis of FKBP interactions with the RyR1 and RyR2 isoforms, we used site-directed fluorescent labeling of FKBP12.6, ligand binding measurements, and fluorescence resonance energy transfer (FRET). Single-cysteine substitutions were introduced at five positions distributed over the surface of FKBP12.6. Fluorescent labeling at position 14, 32, 49, or 85 did not affect high affinity binding to the RyR1. By comparison, fluorescent labeling at position 41 reduced the affinity of FKBP12.6 binding by 10-fold. Each of the five fluorescent FKBPs retained the ability to inhibit [(3)H]ryanodine binding to the RyR1, although the maximal extent of inhibition was reduced by half when the label was attached at position 32. The orientation of FKBP12.6 bound to the RyR1 and RyR2 was examined by measuring FRET from the different labeling positions on FKBP12.6 to an acceptor attached within the RyR calmodulin subunit. FRET was dependent on the position of fluorophore attachment on FKBP12.6; however, for any given position, the distance separating donors and acceptors bound to RyR1 versus RyR2 did not differ significantly. Our results show that FKBP12.6 binds to RyR1 and RyR2 in the same orientation and suggest new insights into the discrete structural domains responsible for channel binding and inhibition. FRET mapping of RyR-bound FKBP12.6 is consistent with the predictions of a previous cryoelectron microscopy study and strongly supports the proposed structural model.

Ca2+-calmodulin increases RyR2 open probability yet reduces ryanoid association with RyR2

We have shown that physiological levels of Ca(2+)-calmodulin (Ca(2+)CaM; 50-100 nM) activate cardiac ryanodine receptors (RyR2) incorporated into bilayers and increase the frequency of Ca(2+) sparks and waves in cardiac cells. In contrast, it is well known that Ca(2+)CaM inhibits [(3)H]ryanodine binding to cardiac sarcoplasmic reticulum. Since the [(3)H]ryanodine binding technique does not reflect the effects of Ca(2+)CaM on RyR2 open probability (Po), we have investigated, using the reversible ryanoid, ryanodol, whether Ca(2+)CaM can directly influence the binding of ryanoids to single RyR2 channels independently of Po. We demonstrate that Ca(2+)CaM reduces the rate of ryanodol association to RyR2 without affecting the rate of dissociation. We also find that ryanodol-bound channels fluctuate between at least two distinct subconductance states, M(1) and M(2), in a voltage-dependent manner. Ca(2+)CaM significantly alters the equilibrium between these two states. The results suggest that Ca(2+)CaM binding to RyR2 causes a conformation change to regions of the channel that include the ryanoid binding site, thereby leading to a decrease in ryanoid association rate and modulation of gating within the ryanoid/RyR2 bound state. Our data provide a possible explanation for why the effects of Ca(2+)CaM at the single-channel level are not mirrored by [(3)H]ryanodine binding studies.

Doping control analysis of emerging drugs in human plasma - identification of GW501516, S-107, JTV-519, and S-40503

An important aspect of preventive doping research is the rapid implementation of tests for emerging drugs with potential for misuse into routine doping control assays. New therapeutics of different classes such as PPARdelta-agonists (e.g. GW501516), ryanodine-calstabin-complex stabilizers (e.g. S-107 and JTV-519), and selective androgen receptor modulators (SARMs, e.g. S-40503) are currently used for the treatment of particular medical conditions such as metabolic syndrome, cardiac arrhythmia, debilitating diseases and osteoporosis, respectively. Due to their being at an early stage of clinical trials and the limited availability of data on the metabolism and possible renal elimination of the active drugs, the development of protocols for doping control analyses of plasma specimens could be an option for the detection of the circulating agents. The mass spectrometric fragmentation of four emerging drug candidates (GW501516, S-107, JTV-519, and S-40503) was elucidated by positive electrospray ionization and collision-induced dissociation using a high resolution/high accuracy mass spectrometer. A screening and confirmation procedure was established based on liquid chromatography/tandem mass spectrometry requiring a volume of 100 microL of plasma. Proteins were precipitated using acetonitrile, the specimens were centrifuged and the supernatant analyzed using a triple-quadrupole mass spectrometer employing multiple reaction monitoring of diagnostic ion transitions. The method was validated with regard to specificity, limits of detection (0.4-8.3 ng/mL), recoveries (72-98%), intraday and interday precisions (12-21%), and ion suppression/enhancement effects.

Quantification of the effects of a ryanodine receptor channel mutation on interaction with a ryanoid

Understanding the nature of the interaction of the plant alkaloid ryanodine with its receptor channel (RyR) is important to aid interpretation of physiological studies and provide structure-function information about RyR. We present here the first quantitative description of the relative single-channel kinetic effects of a single-point mutation in RyR2. We exploit the well-characterized ryanoid 8beta-amino-9alpha-hydroxyryanodine that displays reversible kinetics with RyR2. We explicitly show that the effect of the Q4863A mutation is to increase the apparent dissociation constant by increasing the apparent dissociation rate of the ryanoid. The voltage-dependence of the interaction displays no change. We infer that Q4863 is not involved with the voltage-drop but is able to influence ryanoid-bound structural changes. We discuss structural mechanisms by which this mutation could affect ryanoid interaction.

Latrunculin A depolarizes starfish oocytes

Depolymerization of the actin cytoskeleton may liberate Ca2+ from InsP3-sensitive stores in some cell types, including starfish oocytes, while inhibiting Ca2+ influx in others. However, no information is available on the modulation of membrane potential (V(m)) by actin. The present study was aimed to ascertain whether the widely employed actin depolymerizing drug, latrunculin A (Lat A), affects V(m) in mature oocytes of the starfish Astropecten aranciacus. Lat A induced a membrane depolarization which was mimicked by cytochalasin D, another popular actin disruptor, and prevented by jasplakinolide, a stabilizer of the actin network. Lat A-elicited depolarization consisted in a positive shift in V(m) which reached the threshold of activation of voltage-gated Ca2+ channels (VGCC), thus triggering an action potential. Lat A-promoted depolarization lacked the action potential in Ca2+-free sea water, while it was abolished upon removal of external Na+. Moreover, membrane depolarization was prevented by pre-injection of BAPTA and heparin, but not ryanodine. These data indicate that Lat A induces a membrane depolarization by releasing Ca2+ from InsP3Rs. The Ca2+ signal in turn activates a Ca2+-dependent Na+ entry, which causes the positive shift in V(m) and stimulates the VGCC.

K201 (JTV519) suppresses spontaneous Ca2+ release and [3H]ryanodine binding to RyR2 irrespective of FKBP12.6 association

K201 (JTV519), a benzothiazepine derivative, has been shown to possess anti-arrhythmic and cardioprotective properties, but the mechanism of its action is both complex and controversial. It is believed to stabilize the closed state of the RyR2 (cardiac ryanodine receptor) by increasing its affinity for the FKBP12.6 (12.6 kDa FK506 binding protein) [Wehrens, Lehnart, Reiken, Deng, Vest, Cervantes, Coromilas, Landry and Marks (2004) Science 304, 292-296]. In the present study, we investigated the effect of K201 on spontaneous Ca2+ release induced by Ca2+ overload in rat ventricular myocytes and in HEK-293 cells (human embryonic kidney cells) expressing RyR2 and the role of FKBP12.6 in the action of K201. We found that K201 abolished spontaneous Ca2+ release in cardiac myocytes in a concentration-dependent manner. Treating ventricular myocytes with FK506 to dissociate FKBP12.6 from RyR2 did not affect the suppression of spontaneous Ca2+ release by K201. Similarly, K201 was able to suppress spontaneous Ca2+ release in FK506-treated HEK-293 cells co-expressing RyR2 and FKBP12.6. Furthermore, K201 suppressed spontaneous Ca2+ release in HEK-293 cells expressing RyR2 alone and in cells co-expressing RyR2 and FKBP12.6 with the same potency. In addition, K201 inhibited [3H]ryanodine binding to RyR2-wt (wild-type) and an RyR2 mutant linked to ventricular tachycardia and sudden death, N4104K, in the absence of FKBP12.6. These observations demonstrate that FKBP12.6 is not involved in the inhibitory action of K201 on spontaneous Ca2+ release. Our results also suggest that suppression of spontaneous Ca2+ release and the activity of RyR2 contributes, at least in part, to the anti-arrhythmic properties of K201.

Dynamics of a three-variable nonlinear model of vasomotion: comparison of theory and experiment

The effects of pharmacological interventions that modulate Ca(2+) homeodynamics and membrane potential in rat isolated cerebral vessels during vasomotion (i.e., rhythmic fluctuations in arterial diameter) were simulated by a third-order system of nonlinear differential equations. Independent control variables employed in the model were [Ca(2+)] in the cytosol, [Ca(2+)] in intracellular stores, and smooth muscle membrane potential. Interactions between ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores and transmembrane ion fluxes via K(+) channels, Cl(-) channels, and voltage-operated Ca(2+) channels were studied by comparing simulations of oscillatory behavior with experimental measurements of membrane potential, intracellular free [Ca(2+)] and vessel diameter during a range of pharmacological interventions. The main conclusion of the study is that a general model of vasomotion that predicts experimental data can be constructed by a low-order system that incorporates nonlinear interactions between dynamical control variables.

Two rings of negative charges in the cytosolic vestibule of type-1 ryanodine receptor modulate ion fluxes

The tetrameric ryanodine receptor calcium release channels (RyRs) are cation-selective channels that have pore architecture similar to that of K+ channels. We recently identified, in close proximity to the selectivity filter motif GGGIG, a conserved lumenal DE motif that has a critical role in RyR ion permeation and selectivity. Here, we substituted three aspartate residues (D4938, D4945, D4953) with asparagine and four glutamate residues (E4942, E4948, E4952, E4955) with glutamine hypothesized to line the cytosolic vestibule of the skeletal muscle RyR (RyR1). Mutant single channel properties were determined using the planar lipid bilayer method. Two mutants (D4938N, D4945N) showed a reduced K+ ion conductance, with D4938N also exhibiting a reduced selectivity for Ca2+ compared to K+. The cytosolic location of D4938 and D4945 was confirmed using the polycation neomycin. Both D4938N and D4945N exhibited an attenuated block by neomycin to a greater extent from the cytosolic than lumenal side. By comparison, charge neutralization of lumenal loop residues (D4899Q, E4900N) eliminated the block from the lumenal but not the cytosolic side. The results suggest that, in addition to negatively charged residues on the lumenal side, rings of four negative charges formed by D4938 and D4945 in the cytosolic vestibule determine RyR ion fluxes.

A Ca2+-binding domain in RyR1 that interacts with the calmodulin binding site and modulates channel activity

A fragment of RyR1 (amino acids 4064-4210) is predicted to fold to at least one lobe of calmodulin and to bind Ca(2+). This fragment of RyR1 (R4064-4210) was subcloned, expressed, refolded, and purified. Consistent with the predicted folding pattern, R4064-4210 was found to bind two molecules of Ca(2+) and undergo a structural change upon binding Ca(2+) that exposes hydrophobic amino acids. R4064-4210 also binds to RyR1, the L-type Ca(2+) channel (Cav(1.1)), and several synthetic calmodulin binding peptides. Both R4064-4210 and a peptide representing the calmodulin-binding region of RyR1 (R3614-3643) alter the Ca(2+) dependence of ((3)H)ryanodine binding to RyR1, suggesting that they may both be interfering with an intramolecular interaction between amino acids 4064-4210 and amino acids 3614-3643 in the native RyR1 to alter or regulate the response of the channel to changes in Ca(2+) concentration. The finding that a domain within RyR1 binds Ca(2+) and interacts with calmodulin-binding motifs may provide insights into the mechanism for calcium- and calmodulin-dependent regulation of this channel and perhaps for its regulation by the L-type Ca(2+) channel.

Three-dimensional visualization of FKBP12.6 binding to an open conformation of cardiac ryanodine receptor

The cardiac isoform of the ryanodine receptor (RyR2) from dog binds predominantly a 12.6-kDa isoform of the FK506-binding protein (FKBP12.6), whereas RyR2 from other species binds both FKBP12.6 and the closely related isoform FKBP12. The role played by FKBP12.6 in modulating calcium release by RyR2 is unclear at present. We have used cryoelectron microscopy and three-dimensional (3D) reconstruction techniques to determine the binding position of FKBP12.6 on the surface of canine RyR2. Buffer conditions that should favor the "open" state of RyR2 were used. Quantitative comparison of 3D reconstructions of RyR2 in the presence and absence of FKBP12.6 reveals that FKBP12.6 binds along the sides of the square-shaped cytoplasmic region of the receptor, adjacent to domain 9, which forms part of the four clamp (corner-forming) structures. The location of the FKBP12.6 binding site on "open" RyR2 appears similar, but slightly displaced (by 1-2 nm) from that found previously for FKBP12 binding to the skeletal muscle ryanodine receptor that was in the buffer that favors the "closed" state. The conformation of RyR2 containing bound FKBP12.6 differs considerably from that depleted of FKBP12.6, particularly in the transmembrane region and in the clamp structures. The x-ray structure of FKBP12.6 was docked into the region of the 3D reconstruction that is attributable to bound FKBP12.6, to show the relative orientations of amino acid residues (Gln-31, Asn-32, Phe-59) that have been implicated as being critical in interactions with RyR2. A thorough understanding of the structural basis of RyR2-FKBP12.6 interaction should aid in understanding the roles that have been proposed for FKBP12.6 in heart failure and in certain forms of sudden cardiac death.

Coupled calcium release channels and their regulation by luminal and cytosolic ions

Contraction in skeletal and cardiac muscle occurs when Ca(2+) is released from the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) Ca(2+) release channels. Several isoforms of the RyR exist throughout the animal kingdom, which are modulated by ATP, Ca(2+) and Mg(2+) in the cytoplasm and by Ca(2+) in the lumen of the SR. This review brings to light recent findings on their mechanisms of action in the mammalian isoforms RyR-1 and RyR-2 with an emphasis on RyR-1 from skeletal muscle. Cytoplasmic Mg(2+) is a potent RyR antagonist that binds to two classes of cytoplasmic site, identified as low-affinity, non-specific inhibition sites and high-affinity Ca(2+) activation sites (A-sites). Mg(2+) inhibition at the A-sites is very sensitive to the cytoplasmic and luminal milieu. Cytoplasmic Ca(2+), Mg(2+) and monovalent cations compete for the A-sites. In isolated RyRs, luminal Ca(2+) alters the Mg(2+) affinity of the A-site by an allosteric mechanism mediated by luminal sites. However, in close-packed RyR arrays luminal Ca(2+) can also compete with cytoplasmic ions for the A-site. Activation of RyRs by luminal Ca(2+) has been attributed to either Ca(2+) feedthrough to A-sites or to Ca(2+) regulatory sites on the luminal side of the RyR. As yet there is no consensus on just how luminal Ca(2+) alters RyR activation. Recent evidence indicates that both mechanisms operate and are likely to be important. Allosteric regulation of A-site Mg(2+) affinity could trigger Ca(2+) release, which is reinforced by Ca(2+) feedthrough.

Probing the role of negatively charged amino acid residues in ion permeation of skeletal muscle ryanodine receptor

Sequence comparison suggests that the ryanodine receptors (RyRs) have pore architecture similar to that of the bacterial K+ channel KcsA. The lumenal loop linking the two most C-terminal transmembrane spanning segments in the RyRs has a predicted pore helix and an amino acid motif (GGGIG) similar to the selectivity filter (TVGYG) of KcsA identified by x-ray analysis. The RyRs have many negatively charged amino acid residues in the two regions linking the GGGIG motif and predicted pore helix with the two most C-terminal transmembrane spanning segments. We tested the role of these residues by generating single-site mutants, focusing on amino acid residues conserved among the mammalian RyRs. Replacement of two acidic residues immediately after the GGGIG motif in skeletal muscle ryanodine receptor (RyR1-D4899 and -E4900) with asparagine and glutamine profoundly affected ion permeation and selectivity. By comparison, mutagenesis of aspartate and glutamate residues in the putative linker regions showed a K+ conductance and selectivity for Ca2+ compared to K+ (P(Ca)/P(K)) close to wild-type. The results show that the negatively charged carboxyl oxygens of D4899 and E4900 side chains are major determinants of RyR ion conductance and selectivity.

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.

Mutational analysis of putative calcium binding motifs within the skeletal ryanodine receptor isoform, RyR1

The functional relevance of putative Ca(2+) binding motifs previously identified with Ca(2+) overlay binding analysis within the skeletal muscle ryanodine receptor isoform (RyR1) was examined using mutational analysis. EF hands between amino acid positions 4081 and 4092 (EF1) and 4116 and 4127 (EF2) were scrambled singly or in combination within the full-length rabbit RyR1 cDNA. These cDNAs were expressed in 1B5 RyR-deficient myotubes and channel function assessed using Ca(2+)-imaging techniques, [(3)H]ryanodine binding measurements, and single channel experiments. In intact myotubes, these mutations did not affect functional responses to either depolarization or RyR agonists (caffeine, 4-chloro-m-cresol) compared with wtRyR1. However, in [(3)H]ryanodine binding measurements, both Ca(2+) activation and inhibition of the EF1 mutant was significantly altered compared with wtRyR1. No high affinity [(3)H]ryanodine binding was observed in membranes expressing the EF2 mutation, although in single channel measurements, the EF2-disrupted channel could be activated by micromolar Ca(2+) concentrations. In addition, micromolar levels of ryanodine placed these channels into the classical half-conductance state, thus indicating that occupancy of high affinity ryanodine binding sites is not required for ryanodine-induced subconductance states in RyR1. Disruption of three additional putative RyR1 calcium binding motifs located between amino acid positions 4254 and 4265 (EF3), 4407 and 4418 (EF4), or 4490 and 4502 (EF5) either singly or in combination (EF3-5) did not affect functional responses in 1B5 myotubes except that the EC(50) for caffeine activation for the EF3 construct was significantly increased compared with wtRyR1. However, in [(3)H]ryanodine binding experiments, the Ca(2+)-dependent activation and inactivation of mutated RyRs containing EF3, EF4, or EF5 was unaffected when compared with wtRyR1.

Obligatory role of diastolic voltage oscillations in sino-atrial node discharge

The role of diastolic voltage oscillations in the initiation and maintenance of pacemaker discharge was studied in guinea pig-isolated sino-atrial (SA) node by means of a microelecrode technique. When [K(+)](o) is suitably increased, the maximum diastolic potential decreases and all action potentials (APs) assume the characteristics of dominant pacemakers (slow responses with U-shaped diastolic depolarization). Subsequently, as the slope and amplitude of diastolic depolarization (DD) decreases, the threshold is missed, unmasking the fused oscillatory potentials V(os) and ThV(os). As high [K(+)](o) perfusion continues, the oscillatory potentials become separated, V(os) following the AP and ThV(os) appearing later on, when DD enters a less negative voltage range (oscillatory zone). ThV(os) grow in amplitude and attain the threshold, thereby insuring a slow discharge. If [K(+)](o) is further increased, the smaller ThV(os) miss the threshold and SA node becomes quiescent. On reducing high [K(+)](o), ThV(os) re-appear, increase in size and initiate spontaneous discharge. As they occur progressively earlier during DD, ThV(os) eventually fuse with V(os): at that stage, DD appears to continue directly into the upstroke (U-shaped DD) and the oscillations are no longer seen. During recovery in Tyrode solution, size and slope of V(os) and of ThV(os) further increase and cause a faster discharge. When APs assume a subsidiary configuration, their DD (no longer U-shaped) abruptly terminates into the upstroke. In high [K(+)](o), increasing [Ca(2+)](o) or applying a fast drive increase the size and slope of V(os) and of ThV(os), which in turn restore or accelerate discharge. In contrast, low [Ca(2+)](o) abolishes V(os) and ThV(os) and causes SA node arrest. Low [Ni(2+)] (35.5 microM) increases the rate whereas high [Ni(2+)] (0.73 mM) stops the SA node. Ryanodine eliminates V(os) and ThV(os) and markedly slows or stops discharge. Thus, ThV(os) and V(os) are separate voltage oscillations that play an obligatory role in the initiation and maintenance of SA node discharge, V(os) by steepening early DD and ThV(os) by attaining the threshold in the dominant pacemaker range, either by gradually increasing during late DD at slow rates or by fusing with V(os) at fast rates. Both V(os) and ThV(os) are Ca(2+) dependent, but apparently in different ways.

Diketopyridylryanodine has three concentration-dependent effects on the cardiac calcium-release channel/ryanodine receptor

By interacting with more than one site, ryanoids induce multiple effects on calcium-release channels. To date, the kinetics of interaction of only one of these sites has been characterized. Using C(4),C(12)-diketopyridylryanodine in both [(3)H]ryanodine binding and single channel experiments we characterized another site on the cardiac ryanodine receptor (RyR2) with which ryanoids interact. Competitive binding of this ryanoid to RyR2 implied a minimal two-site binding model. At the single channel level, C(4),C(12)-diketopyridylryanodine induced three distinct effects. At nanomolar concentrations, it increased channel open probability severalfold without inducing a subconductance. This effect was independent of membrane holding potential. As for other ryanoids, low micromolar concentrations of C(4),C(12)-diketopyridylryanodine readily induced a subconductance state. The major subconductance had a current amplitude of 52% of fully open, it was reversible, and its time to induction and duration were voltage- and concentration-dependent, affording Hill slopes of >2. At higher micromolar concentrations C(4),C(12)-diketopyridylryanodine induced long lasting, yet reversible shut states. Using a pharmacological strategy we have discerned an additional ryanoid-binding site on RyR2 that triggers an increase in channel activity. This site likely resides outside the strict confines of the transmembrane conducting pathway.

Quantitative relationships between ryanoids, receptor affinity and channel conductance

The review examines the relationship between the structure of several ryanodine analogs and (A) binding, (B) channel conductance, and (C) ligand binding kinetics. Comparative molecular field analysis (CoMFA) and comparative molecular similarity analysis (CoMSIA) are used to quantitatively assign structural correlations. Hydrogen bond donating (but not accepting) ability was found to be highly correlated with ligand affinity. Analysis of the correlation between hydrophobicity and ligand affinity indicates that, in general, deviation from the amphipathic nature of ryanodine weakens binding. Affinities and binding kinetics obtained in vivo are comparable to those obtained in the less-than-physiological in vitro conditions. Therefore, the structure-activity relationships surveyed are relevant to the living cell. The review presents arguments favoring the propositions that (A) the pyrrole is a major factor orienting the ligand in the receptor binding site and (B) that ryanoids alter ryanodine receptor function through allosteric mechanisms.

A novel and rapid approach to isolating functional ryanodine receptors

Conventional methods of isolating and reconstituting ryanodine receptors (RyRs) from native membranes into proteoliposomes take a minimum of 2 days to complete. We have developed an alternative strategy that can be used to isolate and reconstitute functional RyRs in just 3 h with a similar degree of purification. RyRs isolated by this method display characteristic functional behaviour as assessed by radioligand binding and single channel analyses.

Imperatoxin A (IpTx(a)) from Pandinus imperator stimulates [(3)H]ryanodine binding to RyR3 channels

The effect of imperatoxin A (IpTx(a)) on the ryanodine receptor type 3 (RyR3) was studied. IpTx(a) stimulates [(3)H]ryanodine binding to RyR3-containing microsomes, but this effect requires toxin concentrations higher than those required to stimulate RyR1 channels. The effect of IpTx(a) on RyR3 channels was observed at calcium concentrations in the range 0.1 microM to 10 mM. By contrast, RyR2 channels were not significantly affected by IpTx(a) in the same calcium ranges. Single channel current measurements indicated that IpTx(a) induced subconductance state in RyR3 channels that was similar to those observed with RyR1 and RyR2 channels. These results indicate that IpTx(a) is capable of inducing similar subconductance states in all three RyR isoforms, while stimulation of [(3)H]ryanodine binding by this toxin results in isoform-specific responses, with RyR1 being the most sensitive channel, RyR3 displaying an intermediate response and RyR2 the least responsive ones.

Inhibition of ryanodine binding to sarcoplasmic reticulum vesicles of cardiac muscle by Zn(2+) ions

Using the assay of [(3)H]ryanodine binding to the sarcoplasmic reticulum, the effect of Zn(2+) on ryanodine receptors (RyRs) of cardiac muscle was investigated. There was no obvious change in the binding at [Zn(2+)](f) of less than 0.2 microM. However, a decrease of the binding became significant with raising [Zn(2+)](f) to 0.5 microM. The inhibitory effect of Zn(2+) was [Zn(2+)](f)-dependent, with IC(50/ZnI) of 2.1+/-0.4 microM (mean+/-S.D.). Scatchard analysis indicates that both an increase of K(d) and a decrease of B(max) were responsible for Zn(2+)-induced decrease of the binding. The Hill coefficient for this inhibitory effect of Zn(2+) was between 0.8 and 1.2. The interactions of the effects of Zn(2+) and various modulators of RyR indicate that the inhibitory effect of Zn(2+) was mostly mediated through inhibiting Ca(2+) activation sites (CaA) on RyR. Since the [Zn(2+)](f) dependence was not clearly changed by [Ca(2+)](f), the inhibitory effect of Zn(2+) may not be due to competition of Zn(2+) with Ca(2+) for CaA and probably is indirect. The inhibitory effect of Zn(2+) could not be antagonized by 2 mM dithiothreitol, a thiol-reducing agent, suggesting that the binding of Zn(2+) ions to RyRs of cardiac muscle is not accompanied by obvious change of redox state of the RyRs. In comparison with that seen in skeletal muscle [3], the effects of Zn(2+) on ryanodine binding to the sarcoplasmic reticulum of cardiac muscle show several distinct differences. It is indicated that the effect of Zn(2+) on RyRs may be isoform-dependent. The physiological significance of the effects of Zn(2+) is discussed.

Ryanoid modification of the cardiac muscle ryanodine receptor channel results in relocation of the tetraethylammonium binding site

The interaction of ryanodine and derivatives of ryanodine with the high affinity binding site on the ryanodine receptor (RyR) channel brings about a characteristic modification of channel function. In all cases, channel open probability increases dramatically and single-channel current amplitude is reduced. The amplitude of the ryanoid-modified conductance state is determined by structural features of the ligand. An investigation of ion handling in the ryanodine-modified conductance state has established that reduced conductance results from changes in both the affinity of the channel for permeant ions and the relative permeability of ions within the channel (Lindsay, A.R.G., A. Tinker, and A.J. Williams. 1994. J. Gen. Physiol. 104:425-447). It has been proposed that these alterations result from a reorganization of channel structure induced by the binding of the ryanoid. The experiments reported here provide direct evidence for ryanoid-induced restructuring of RyR. TEA+ is a concentration- and voltage-dependent blocker of RyR in the absence of ryanoids. We have investigated block of K+ current by TEA+ in the unmodified open state and modified conductance states of RyR induced by 21-amino-9alpha-hydroxyryanodine, 21-azido-9alpha-hydroxyryanodine, ryanodol, and 21-p-nitrobenzoylamino-9alpha-hydroxyryanodine. Analysis of the voltage dependence of block indicates that the interaction of ryanoids with RyR leads to an alteration in this parameter with an apparent relocation of the TEA+ blocking site within the voltage drop across the channel and an alteration in the affinity of the channel for the blocker. The degree of change of these parameters correlates broadly with the change in conductance of permeant cations induced by the ryanoids, indicating that modification of RyR channel structure by ryanoids is likely to underlie both phenomena.

Altered interaction of FKBP12.6 with ryanodine receptor as a cause of abnormal Ca(2+) release in heart failure

OBJECTIVE

Little information is available as to the Ca(2+) release function of the sarcoplasmic reticulum (SR) in heart failure. We assessed whether the alteration in this function in heart failure is related to a change in the role of FK binding protein (FKBP), which is tightly coupled with the cardiac ryanodine receptor (RyR) and recently identified as a modulatory protein acting to stabilize the gating function of RyR.

METHODS

SR vesicles were isolated from dog LV muscles [normal (N), n=6; heart failure induced by 3-weeks pacing (HF), n=6]. The time course of the SR Ca(2+) release was continuously monitored using a stopped-flow apparatus, and [3H]ryanodine-binding and [3H]dihydro-FK506-binding assays were also performed.

RESULTS

FK506, which specifically binds to FKBP12.6 and dissociates it from RyR, decreased the polylysine-induced enhancement of [3H]ryanodine-binding by 38% in N (P<0.05) but it had no effect in HF. In HF, the rate constant for the polylysine-induced Ca(2+) release from the SR was 61% smaller than in N. FK506 decreased the rate constant for the polylysine-induced Ca(2+) release by 67% in N (P<0.05) but had no effect in HF. The [3H]dihydro-FK506-binding assay revealed that the number (B(max)) of FKBPs was decreased by 83% in HF (P<0.05), while the K(d) value was unchanged. FK506 did not significantly change SR Ca(2+.)-ATPase activity in either N or HF.

CONCLUSIONS

In HF, the number of FKBPs showed a tremendous decrease; this may underlie the RyR-channel instability and the impairment of the Ca(2+) release function of RyR seen in the failing heart.

Pseudoreceptor model for ryanodine derivatives at calcium release channels

This paper describes the generation of a pseudoreceptor model for ryanodine receptor (RyR) modulating ryanoids in rabbit skeletal muscle. For this purpose, the molecular modelling software PrGen was applied to correlate experimentally determined and calculated free energies of binding for a set of 15 ryanodine derivatives. The final model indicates a narrow cleft with hydrogen bond donor and acceptor capacities (represented by an Asn) as most crucial for binding the pyrrole carboxylate substituent at C3 of ryanodine. In addition, hydrophobic residues flank the aromatic pyrrole ring (Tyr, Phe, and Ile). Two of those residues (Tyr and Ile) interact with the 2-isopropyl moiety, which seems to contribute to binding. Opposite to the pyrrole locus, a second hydrophobic region (represented by a Leu) restricts ryanodine derivatives in their longitudinal axis and leads to the discrimination of equatorial and axial positioned methyl groups and of polar substituents at C9. Finally, a charged glutamate residue generates strong hydrogen bonding and electrostatic interactions with the hydroxyl groups at C10 and C15. For this binding-site model--composed of six amino acid residues--a correlation for the training set ligands of R = 0.99 (Q2 = 0.975) and a root mean square (rms) deviation of 0.568 kcal/mol for the prediction of the binding energies of four test set ligands was obtained. Based on this pseudoreceptor model the putative topology of the real binding site of ryanoids will be discussed.

Calcium regulation by skeletal muscle membranes of horses with recurrent exertional rhabdomyolysis

OBJECTIVE

To determine whether an alteration in calcium regulation by skeletal muscle sarcoplasmic reticulum, similar to known defects that cause malignant hyperthermia (MH), could be identified in membrane vesicles isolated from the muscles of Thoroughbreds with recurrent exertional rhabdomyolysis (RER).

SAMPLE POPULATION

Muscle biopsy specimens from 6 Thoroughbreds with RER and 6 healthy (control) horses.

PROCEDURES

RER was diagnosed on the basis of a history of > 3 episodes of exertional rhabdomyolysis confirmed by increases in serum creatine kinase (CK) activity. Skeletal muscle membrane vesicles, prepared by differential centrifugation of muscle tissue homogenates obtained from the horses, were characterized for sarcoplasmic reticulum (SR) activities, including the Ca2+ release rate for the ryanodine receptor-Ca2+ release channel, [3H]ryanodine binding activities, and rate of SR Ca2+-ATPase activity and its activation by Ca2+.

RESULTS

Time course of SR Ca2+-induced Ca2+ release and [3H]ryanodine binding to the ryanodine receptor after incubation with varying concentrations of ryanodine, caffeine, and ionized calcium did not differ between muscle membranes obtained from control and RER horses. Furthermore, the maximal rate of SR Ca2+-ATPase activity and its affinity for Ca2+ did not differ between muscle membranes from control horses and horses with RER.

CONCLUSIONS AND CLINICAL RELEVANCE

Despite clinical and physiologic similarities between RER and MH, we concluded that RER in Thoroughbreds does not resemble the SR ryanodine receptor defect responsible for MH and may represent a novel defect in muscle excitation-contraction coupling, calcium regulation, or contractility.

Selective insect antifeedant and toxic action of ryanoid diterpenes

In this work, we have studied the antifeedant and insecticidal effects of several natural ryanoid diterpenes. These compounds can be classified in two groups according to their chemical structures: ryanodol/isoryanodol-type (nonalkaloidal type) and ryanodine-type (alkaloidal type) ryanoids. The nonalkaloidal ryanoids were isolated from Persea indica (Lauraceae) while the alkaloidal ryanoids (ryanodines and spiganthines) were isolated from Spigelia anthelmia (Loganiaceae). The effects of these compounds on the feeding behavior and performance (with and without piperonyl butoxide pretreatment) of Spodoptera littoralis larvae and Leptinotarsa decemlineata adults indicate that some strongly deterred these insects, L. decemlineata being less sensitive than S. littoralis. Their antifeedant effects did not parallel their toxic action. Additionally, more than 60% of the nonalkaloidal ryanoids were antifeedants and/or toxic in contrast to 30% of active alkaloidal ones, supporting the hypothesis of a ryanodol-specific mode of action in insects.

Structure-function relationships among ryanodine derivatives. Pyridyl ryanodine definitively separates activation potency from high affinity

Ryanodine derivatives are differentially effective on the two limbs of the ryanodine concentration-effect curve. This study comparing ryanodine, ryanodol, and pyridyl ryanodine and nine C10Oeq esters of them focuses on structure-function relations underlying their differential effectiveness. Ryanodol and pyridyl ryanodine had significantly lower affinities than ryanodine, but their EC50act values (concentration of ryanoid that induces one-half of full efficacy), potencies, and efficacies were not diminished in like fashion. Ryanodine and ryanodol were partial agonists, whereas pyridyl ryanodine was a full agonist, having a diminished deactivation potency. C10Oeq esterifications enhanced affinities and efficacies of the base ryanoids. The C10-Oeq ester derivatives of ryanodine and pyridyl ryanodine, but not those of ryanodol, lost their capacity to deactivate RyR1s. Thus, affinity differences among ryanoids clearly do not predicate functional differences as regards activation of Ca2+ release channels. The pyrrole carboxylate on the C3 of ryanodine is dispensable to ryanoid activation of Ca2+ release channels. Ryanodol lacks this ring, but it nevertheless effects substantial activation. Moreover, its C10-Oeq esters display full efficacy. The increased ability of all the C10-Oeq derivatives to release Ca2+ from the vesicles strengthens their role in directly impeding deactivation of RyR1, perhaps by interaction with some component within the transmembrane ionic flux pathway.

Structural components of ryanodine responsible for modulation of sarcoplasmic reticulum calcium channel function

Comparative molecular field analysis (CoMFA) was used to analyze the relationship between the structure of a group of ryanoids and the modulation of the calcium channel function of the ryanodine receptor. The conductance properties of ryanodine receptors purified from sheep heart were measured using the planar, lipid bilayer technique. The magnitude of the ryanoid-induced fractional conductance was strongly correlated to specific structural loci on the ligand. Briefly, electrostatic effects were more prominent than steric effects. The 10-position of the ryanoid had the greatest influence on fractional conductance. Different regions of the ligand have opposing effects on fractional conductance. For example, steric bulk at the 10-position is correlated with decreased fractional conductance, whereas steric bulk at the 2-position (isopropyl position) is correlated with increased fractional conductance. In contrast to fractional conductance, the 3-position (the pyrrole locus) had the greatest influence on ligand binding, whereas the 10-position had comparatively little influence on binding. Two possible models of ryanodine action, a direct (or channel plug) mechanism and an allosteric mechanism, were examined in light of the CoMFA. Taken together, the data do not appear to be consistent with direct interaction between ryanodine and the translocating ion. The data appear to be more consistent with an allosteric mechanism. It is suggested the ryanoids act by inducing or stabilizing a conformational change in the ryanodine receptor that results in the observed alterations in cation conductance.

The pharmacology of ryanodine and related compounds

The goal of this review has been to describe the current state of the pharmacology of ryanodine and related compounds relative to the vertebrate RyRs. Resolution of questions concerning the molecular properties of RyR channel function and the contributions made by the RyR isoforms to cellular signaling in a variety of tissues will require the production of new pharmacological agents directed against these proteins. Novel naturally occurring ryanodine congeners have been identified, and significant advances have been made in developing chemical approaches that permit the structure of ryanodine to be derivatized in selective ways. Moreover, several of these changes have yielded compounds that differ in their binding affinities and in their abilities to modify the properties of the RyR channels. These advances give substance to the possibility of designing the required pharmacological agents based on rational design changes of the structure ryanodine.

Two structural configurations of the skeletal muscle calcium release channel

Here we present the determination of the three-dimensional structure of the skeletal muscle Ca2+-release channel in an open state using electron cryomicroscopy and angular reconstitution. In contrast to our reconstruction of the channel in its closed state, the density map of the channel driven towards its open state, by the presence of Ca2+ and ryanodine, features a central opening in the transmembrane region-the likely passageway for Ca2+ ions from the sarcoplasmic reticulum to the cytosol. The opening of the channel is associated with a 4 degree rotation of its transmembrane region with respect to its cytoplasmic region, and with significant mass translocations within the entire cytoplasmic region of the channel tetramer.

Electrophysiological effects of ryanodine derivatives on the sheep cardiac sarcoplasmic reticulum calcium-release channel

We have examined the effects of a number of derivatives of ryanodine on K+ conduction in the Ca2+ release channel purified from sheep cardiac sarcoplasmic reticulum (SR). In a fashion comparable to that of ryanodine, the addition of nanomolar to micromolar quantities to the cytoplasmic face (the exact amount depending on the derivative) causes the channel to enter a state of reduced conductance that has a high open probability. However, the amplitude of that reduced conductance state varies between the different derivatives. In symmetrical 210 mM K+, ryanodine leads to a conductance state with an amplitude of 56.8 +/- 0.5% of control, ryanodol leads to a level of 69.4 +/- 0.6%, ester A ryanodine modifies to one of 61.5 +/- 1.4%, 9,21-dehydroryanodine to one of 58.3 +/- 0.3%, 9 beta,21beta-epoxyryanodine to one of 56.8 +/- 0.8%, 9-hydroxy-21-azidoryanodine to one of 56.3 +/- 0.4%, 10-pyrroleryanodol to one of 52.2 +/- 1.0%, 3-epiryanodine to one of 42.9 +/- 0.7%, CBZ glycyl ryanodine to one of 29.4 +/- 1.0%, 21-p-nitrobenzoyl-amino-9-hydroxyryanodine to one of 26.1 +/- 0.5%, beta-alanyl ryanodine to one of 14.3 +/- 0.5%, and guanidino-propionyl ryanodine to one of 5.8 +/- 0.1% (chord conductance at +60 mV, +/- SEM). For the majority of the derivatives the effect is irreversible within the lifetime of a single-channel experiment (up to 1 h). However, for four of the derivatives, typified by ryanodol, the effect is reversible, with dwell times in the substate lasting tens of seconds to minutes. The effect caused by ryanodol is dependent on transmembrane voltage, with modification more likely to occur and lasting longer at +60 than at -60 mV holding potential. The addition of concentrations of ryanodol insufficient to cause modification does not lead to an increase in single-channel open probability, such as has been reported for ryanodine. At concentrations of > or = 500 mu M, ryanodine after initial rapid modification of the channel leads to irreversible closure, generally within a minute. In contrast, comparable concentrations of beta-alanyl ryanodine do not cause such a phenomenon after modification, even after prolonged periods of recording (>5 min). The implications of these results for the site(s) of interaction with the channel protein and mechanism of the action of ryanodine are discussed. Changes in the structure of ryanodine can lead to specific changes in the electrophysiological consequences of the interaction of the alkaloid with the sheep cardiac SR Ca2+ release channel.

Activation and deactivation of sarcoplasmic reticulum calcium release channels: molecular dissection of mechanisms via novel semi-synthetic ryanoids

The plant alkaloids ryanodine and dehydroryanodine are high affinity, biphasic modulators of the intracellularly located, calcium-regulated calcium release channels of a variety of cell types. To date, little is certain about the molecular basis of the interactions that prompt low concentrations of ryanodine (nanomolar to low micromolar) to activate (open) the channels and higher concentrations to deactivate (functionally close) the sarcoplasmic reticulum calcium release channel. In the present study, we approached this question using novel, semi-synthetic C10-Oeq ester derivatives of ryanodine and dehydroryanodine as molecular probes of the ryanodine binding sites on the calcium release channel. Binding affinities of these C10-Oeq ester derivatives of ryanodine and dehydroryanodine with acidic, basic and neutral side chains (Kd values > 53.9 nM, Kd values 0.3-0.7 nM and Kd values 1.3-20.4 nM, compared with 2.3 and 2.8 nM for ryanodine and dehydroryanodine, respectively) were evaluated for their ability to modulate the patency of the sarcoplasmic reticulum calcium release channel. With the exception of only two derivatives tested to date, all the semi-synthetic C10-Oeq esters selectively activate the Ca2+ release channel. That is, they produce no functional closure of the sarcoplasmic reticulum calcium release channels at the highest concentration that could be tested. Half-maximal concentrations for activation (EC50act values) ranged from 0.87-4.2 microM, compared with an EC50act of 1.3 microM for ryanodine. Using a low concentration (0.5 nM) of a high specific activity, radioiodinated derivative of ryanodine, C10-Oeq N-(4-azido-5-125iodo salicyloyl) glycyl ryanodine (1400 Ci/mmol) as the radioligand in displacement binding affinity assays, two distinct, sequential ryanodine binding isotherms were demonstrated within the normal 0-300 nM ryanodine sigmoidal displacement curve. A high affinity site had an IC50 of 0.5 nM (Kd = 0.26 +/- 0.02 nM). Above this concentration, an apparent plateau occurred between 3 and 6 nM ryanodine, and at higher concentrations a lower affinity site was revealed that demonstrated an IC50 of about 25 nM (Kd = 11.7 +/- 1.2 nM). Scatchard analysis from direct binding of C10-Oeq N-(4-azido-5-125iodo salicyloyl) glycyl ryanodine to junctional sarcoplasmic reticulum vesicles also suggests the presence of more than one class of binding sites within the nanomolar concentration range. The high affinity site demonstrated a Bmax of 3 pmol/mg protein. We were unable to saturate the lower affinity binding sites with this ligand. To evaluate the functional effects occurring among sarcoplasmic reticulum calcium release channel monomers as a consequence of ryanodine's binding, we utilized a photo-activatable derivative of ryanodine, C10-Oeq N-(4-azido salicyloyl) glycyl ryanodine that demonstrates channel modulating characteristics similar to ryanodine. Covalently labeling the sarcoplasmic reticulum calcium-release channels with this ligand, followed by measurements of rates of calcium efflux and SDS-PAGE of the labeled protein, revealed that deactivation of the sarcoplasmic reticulum calcium release channels of skeletal muscle by this ryanoid occurred at concentrations which apparently produce virtually irreversibly interactions between receptor monomers. This 'polymerization' was indicated by the progressive appearance of two higher molecular weight protein bands on SDS-PAGE, concomitant with progressive decreases in the ryanodine receptor monomer band that runs at an apparent molecular mass of 365 kDa. In summary, we have prepared and utilized novel C10-Oeq ester derivatives of ryanodine and dehydroryanodine in studies aimed at better understanding the molecular basis for the complex biphasic actions of ryanodine on the sarcoplasmic reticulum calcium release channels from rabbit skeletal muscle cells. The described studies presage correlations that may be useful in furthering our understa

Spiganthine, the cardioactive principle of Spigelia anthelmia

Spiganthine [1] was isolated as the main cardioactive principle from medicinally used extracts of Spigelia anthelmia. Its structure was established by spectroscopic methods. The biological effect of spiganthine is characterized by a delay in contraction development of the heart muscle.

Structural determinants of high-affinity binding of ryanoids to the vertebrate skeletal muscle ryanodine receptor: a comparative molecular field analysis

Ryanodine binds to specific membrane proteins, altering the calcium permeability of intracellular membranes. In this study 19 ryanoids were isolated or synthesized and the structures correlated to the strength of binding to vertebrate skeletal muscle ryanodine receptors. Global minima were determined by employment of molecular mechanics and dynamics augmented by systematic searching of conformational space. Overall, steric and electrostatic factors contribute about equally to the differences in the experimentally determined dissociation constants. The dominant electrostatic interaction is localized to a hydroxyl group in an apolar region of the molecule. The pyrrole and isopropyl groups located together at one pole of the molecule have the greatest effect on steric interactions between ligand and receptor. We suggest ryanodine binds to the receptor with the pyrrole and isopropyl groups buried deep inside a cleft in the protein. This arrangement places special importance on the conformation of the pyrrole and isopropyl groups. In contrast, the opposite pole appears to be positioned at the entrance of the binding pocket because bulky adducts placed in the 9 position of ryanodine alter binding minimally. For example, a fluorescent ryanodine adduct was synthesized which has a dissociation constant close to that of ryanodine. Detailed examination reveals subtle interactions between ryanoid and receptor. In many cases, the major factors altering the strength of binding were found to be conformational alterations in the molecule remote from the site of covalent modification.

Photoaffinity labeling of the ryanodine receptor/Ca2+ release channel with an azido derivative of ryanodine

Ryanodine receptors/Ca2+ release channels play an important role in regulating the intracellular free calcium concentrations in both muscle and nonmuscle cells. Ryanodine, a neutral plant alkaloid, specifically binds to and modulates these Ca2+ release channels. In the work described here, we characterize the interaction of a tritium-labeled, photoactivable derivative of ryanodine (3H-labeled 10-O-[3-(4-azidobenzamido)propionyl]ryanodine ([3H]ABRy)) with the ryanodine receptor of skeletal, cardiac, and brain membranes. Scatchard analysis demonstrates that this ligand binds to a single class of high affinity sites in skeletal muscle triads. Furthermore, competition binding assays of [3H]ryanodine with skeletal, cardiac, and brain membranes in the presence of increasing concentrations of unlabeled ABRy illustrate that this azido derivative of ryanodine is able to specifically displace [3H]ryanodine from its binding site(s). Analysis of the effects of Ca2+, ATP, and KCl on [3H]ABRy binding in triad membranes shows a similar modulation of binding to that seen in these membranes with [3H]ryanodine. Photoaffinity labeling of triads with [3H]ABRy resulted in specific and covalent incorporation of [3H]ABRy into a 565-kDa protein that was shown to be the skeletal muscle ryanodine receptor. Digestion of the labeled ryanodine receptor revealed a [3H]ABRy-labeled 76-kDa tryptic fragment that was identified with an antibody directed against the COOH-terminal of the receptor. These results demonstrate that the 76-kDa COOH-terminal tryptic fragment contains the high affinity binding site for ryanodine.

Radioimmunoassay for the calcium release channel agonist ryanodine

A novel photo-activatable derivative of ryanodine, 9-hydroxy-21-(4-azidobenzoyloxy)-9-epiryanodine, has been synthesized and conjugated to keyhole limpet hemocyanin for the production of antibodies with high affinity and specificity to ryanodine. The anti-ryanodine antibodies reacted specifically on immunoblots with the azido-ryanodine compound covalently conjugated to bovine serum albumin. A radioimmunoassay specific for ryanodine was developed using the anti-ryanodine antibodies, and a dissociation constant for ryanodine of 1 nM was determined. Half-maximal inhibition constants (IC50) for various ryanodine derivatives were found to range between 3.2 and 200 nM. These IC50 values correlated very well with the IC50 values obtained for the compounds binding to the skeletal muscle membrane receptor. These antibodies should be useful for the characterization of the ryanodine binding site on the sarcoplasmic reticulum Ca2+ release channel.

High-purity ryanodine and 9,21-dehydroryanodine for in vitro diagnosis of malignant hyperthermia in man

Susceptibility to malignant hyperthermia (MH) is currently diagnosed by the in vitro contracture test (IVCT) in skeletal muscle. However, this test does not possess absolute specificity. Thus, in addition to the established procedure, the "ryanodine contracture test" has been proposed to improve discrimination between MH-susceptible (MHS) and normal (MHN) patients. In all previous studies, the ryanodine used was a mixture consisting of high-purity ryanodine (HPR) and 9,21-dehydroryanodine (DHR). Therefore, in this study the effects of both substances were investigated in concentrations of 2, 5 and 10 mumol litre-1. With all concentrations, contractures appeared earlier in MHS than in MHN muscles, but these differences were significant at all contracture levels with HPR only. Moreover, with the smallest concentration (2 mumol litre-1), the best discrimination between MHS and MHN was observed. Classification of MH-equivocal patients (MHE) as MHS or MHN seems to be possible with the use of ryanodine-induced contractures. The contracture test with HPR should therefore be added to the established procedure of the IVCT.

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

Bioactive ryanoids from nucleophilic additions to 4,12-seco-4,12-dioxoryanodine

Ryanoids are the most potent inhibitors known for the calcium-release channel (ryanodine receptor), and they are also botanical insecticides. Twenty-two new ryanoids are described in which the C-4, C-12 bond is ruptured or replaced with an oxygen bridge and in which substituents at C-4 and C-12 are modified to have a wide range of polarities. They are obtained by nucleophilic additions to the 4,12-seco-4,12-dioxo compounds or diketones prepared from ryanodine and dehydroryanodine by periodate oxidation. Structures of the new compounds are distinguished by changes in NMR chemical shifts of 13C and 1H nuclei in the regions of C-4 and C-12. The new ryanoids are compared with ryanodine as inhibitors of [3H]ryanodine binding using a rabbit muscle sarcoplasmic reticulum preparation alone or with ATP and a mouse brain receptor with ATP. They are also examined as knockdown agents for houseflies pretreated with a cytochrome P450 oxidase inhibitor to suppress detoxification and then injection with the ryanoid. The diketones have very weak binding activity in the receptor assays and very low toxicity to flies. Activity approaching that of ryanodine in both the receptor and fly assays is obtained for ketals with small groups at C-12 and polar substituents such as OH or NHOH at C-4. The oximes range from low to moderate potency. Addition of thiols to the vinyl group of dehydroryanodine gives three thioethers all of low biological activity. With most ryanoids addition of ATP to the muscle system increases its sensitivity to near that found for the brain receptor with ATP; possible exceptions are compounds with phenyl substituents. Activity at the calcium-release channel generally follows housefly toxicity although the hydrazine and hydroxyamine adducts are much weaker than expected perhaps due to dissociation under the assay conditions.

Immunohistochemical localization of ryanodine binding proteins in the central nervous system of gymnotiform fish

The ryanodine receptor, an integral membrane protein of the sarcoplasmic reticulum in muscle, embodies a high conductance channel permeable to calcium ions. Recent studies have identified ryanodine-binding proteins in avian and mammalian central nervous systems. These neuronal ryanodine receptors appear to function as Ca2+ channels which may gate the release of Ca2+ from caffeine-sensitive intracellular pools in neurons. In the present investigation, we employed monoclonal antibodies against ryanodine-binding proteins of avian muscle cells to the brain of weakly electric gymnotiform fish. Immunoprecipitation and Western blot analysis revealed two isoforms in the fish brain, with molecular weights comparable to those of avian and fish muscle ryanodine-binding proteins. By employing immunohistochemical techniques, we mapped these proteins in fish brain. Ryanodine receptor-like immunoreactivity was found in nerve cell bodies as well as dendrites and axonal processes. The ryanodine-binding protein is distributed throughout the neuraxis in specific cell types of the gymnotiform brain. In the telencephalon, immunoreactive cells were found in the glomerular layer of the olfactory bulb, in the supracommissural subdivision of the ventral telencephalon, and in the intermediate rostral subdivision of the ventral telencephalon. In the diencephalon, immunoreactive cells or fibers were observed in the nucleus prethalamicus and the habenula, within the nucleus at the base of the optic tract and the adjacent dorsal tegmental nucleus, the pretectal nuclei A and B, and the nucleus electrosensorius. In addition, immunopositive cells were seen in several nuclei of the hypothalamus, with the inferior and lateral subdivision of the nucleus recessus lateralis displaying the highest concentration of neurons. In the mesencephalon, the optic tectum contained the greatest number of immunopositive cells. In the rhombencephalon, labelling was seen in the nucleus of the lateral valvula, central gray, lateral tegmental nucleus, in boundary cells of the nucleus praeminentialis, efferent octavolateral nucleus, an area adjacent to the medial edge of the lateral reticular nucleus, nucleus medialis, and electrosensory lateral line lobe. As in avian brain, cerebellar Purkinje cells were positive for ryanodine-binding protein, although only subsets of Purkinje cells were labelled.

Synthesis and biochemical properties of an 125I-labelled ryanodine derivative

The synthesis of a novel radioiodinated ryanodine-O10eq-N-acylamino acylate along with biological data are reported. The affinity of the iodinated product, 7, was comparable to ryanodine, 7.97 nM and 6.47 nM, respectively. Conversion of the non-radioactive iodinated ryanodine analog to the [125I] isotope was accomplished by conversion of 7 to the trimethyltin derivative followed by [125I] exchange using chloramine-T in organic solvent. The radioiodinated ryanodine analog, 9, bound to cardiac membrane preparations in a protein dependent and saturable manner indicating that this analog may represent a useful new tool for the study of ryanodine receptors and that modifications about the C-10 hydroxy group of ryanodine may be carried out without loss in biological activity.