Regulation of RYR1 activity by Ca(2+) and calmodulin

Calmodulin: a highly conserved and ubiquitous Ca2+ sensor

Calcium ions (Ca2+) play critical roles in various biological phenomena. The free Ca2+ concentration in the cytoplasm of a resting cell is at the 10-7 M level, whereas that outside the cell is 10-3 M, creating a 10,000-fold gradient of Ca2+ concentrations across the cell membrane, separating the intracellular and extracellular solutions.1),2) When a cell is activated by external stimuli, the intracellular Ca2+ concentration increases to levels of 10-6-10-5 M through Ca2+ entry from the extracellular solution via plasma membrane Ca2+ channels and/or Ca2+ release from intracellular stores. This transient increase in Ca2+ functions as an important signal mediated by Ca2+ sensors. Thus, Ca2+ signals are transmitted to intracellular loci such as distinct, localized targets of Ca2+ sensors. Among numerous Ca2+ sensors present in cells, calmodulin is a highly conserved and ubiquitous Ca2+ sensor.3).

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

RyR1-related myopathy mutations in ATP and calcium binding sites impair channel regulation

The type 1 ryanodine receptor (RyR1) is an intracellular calcium (Ca²⁺) release channel on the sarcoplasmic/endoplasmic reticulum that is required for skeletal muscle contraction. RyR1 channel activity is modulated by ligands, including the activators Ca²⁺ and ATP. Patients with inherited mutations in RyR1 may exhibit muscle weakness as part of a heterogeneous, complex disorder known as RYR1-related myopathy (RYR1-RM) or more recently termed RYR1-related disorders (RYR1-RD). Guided by high-resolution structures of skeletal muscle RyR1, obtained using cryogenic electron microscopy, we introduced mutations into putative Ca²⁺ and ATP binding sites and studied the function of the resulting mutant channels. These mutations confirmed the functional significance of the Ca²⁺ and ATP binding sites identified by structural studies based on the effects on channel regulation. Under normal conditions, Ca²⁺ activates RyR1 at low concentrations (µM) and inhibits it at high concentrations (mM). Mutations in the Ca²⁺-binding site impaired both activating and inhibitory regulation of the channel, suggesting a single site for both high and low affinity Ca²⁺-dependent regulation of RyR1 function. Mutation of residues that interact with the adenine ring of ATP abrogated ATP binding to the channel, whereas mutating residues that interact with the triphosphate tail only affected the degree of activation. In addition, patients with mutations at the Ca²⁺ or ATP binding sites suffer from muscle weakness, therefore impaired RyR1 channel regulation by either Ca²⁺ or ATP may contribute to the pathophysiology of RYR1-RM in some patients.

Calmodulin Binding Proteins and Alzheimer's Disease: Biomarkers, Regulatory Enzymes and Receptors That Are Regulated by Calmodulin

The integral role of calmodulin in the amyloid pathway and neurofibrillary tangle formation in Alzheimer’s disease was first established leading to the “Calmodulin Hypothesis”. Continued research has extended our insight into the central function of the small calcium sensor and effector calmodulin and its target proteins in a multitude of other events associated with the onset and progression of this devastating neurodegenerative disease. Calmodulin’s involvement in the contrasting roles of calcium/CaM-dependent kinase II (CaMKII) and calcineurin (CaN) in long term potentiation and depression, respectively, and memory impairment and neurodegeneration are updated. The functions of the proposed neuronal biomarker neurogranin, a calmodulin binding protein also involved in long term potentiation and depression, is detailed. In addition, new discoveries into calmodulin’s role in regulating glutamate receptors (mGluR, NMDAR) are overviewed. The interplay between calmodulin and amyloid beta in the regulation of PMCA and ryanodine receptors are prime examples of how the buildup of classic biomarkers can underly the signs and symptoms of Alzheimer’s. The role of calmodulin in the function of stromal interaction molecule 2 (STIM2) and adenosine A2A receptor, two other proteins linked to neurodegenerative events, is discussed. Prior to concluding, an analysis of how targeting calmodulin and its binding proteins are viable routes for Alzheimer’s therapy is presented. In total, calmodulin and its binding proteins are further revealed to be central to the onset and progression of Alzheimer’s disease.

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

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

Calcium Mechanisms in Limb-Girdle Muscular Dystrophy with CAPN3 Mutations

Limb-girdle muscular dystrophy recessive 1 (LGMDR1), previously known as LGMD2A, is a rare disease caused by mutations in the CAPN3 gene. It is characterized by progressive weakness of shoulder, pelvic, and proximal limb muscles that usually appears in children and young adults and results in loss of ambulation within 20 years after disease onset in most patients. The pathophysiological mechanisms involved in LGMDR1 remain mostly unknown, and to date, there is no effective treatment for this disease. Here, we review clinical and experimental evidence suggesting that dysregulation of Ca2+ homeostasis in the skeletal muscle is a significant underlying event in this muscular dystrophy. We also review and discuss specific clinical features of LGMDR1, CAPN3 functions, novel putative targets for therapeutic strategies, and current approaches aiming to treat LGMDR1. These novel approaches may be clinically relevant not only for LGMDR1 but also for other muscular dystrophies with secondary calpainopathy or with abnormal Ca2+ homeostasis, such as LGMD2B/LGMDR2 or sporadic inclusion body myositis.

Modulation of cardiac ryanodine receptor 2 by calmodulin

The high-conductance intracellular calcium (Ca²⁺) channel RyR2 is essential for the coupling of excitation and contraction in cardiac muscle. Among various modulators, calmodulin (CaM) regulates RyR2 in a Ca²⁺-dependent manner. Here we reveal the regulatory mechanism by which porcine RyR2 is modulated by human CaM through the structural determination of RyR2 under eight conditions. Apo-CaM and Ca²⁺-CaM bind to distinct but overlapping sites in an elongated cleft formed by the handle, helical and central domains. The shift in CaM-binding sites on RyR2 is controlled by Ca²⁺ binding to CaM, rather than to RyR2. Ca²⁺-CaM induces rotations and intradomain shifts of individual central domains, resulting in pore closure of the PCB95 and Ca²⁺-activated channel. By contrast, the pore of the ATP, caffeine and Ca²⁺-activated channel remains open in the presence of Ca²⁺-CaM, which suggests that Ca²⁺-CaM is one of the many competing modulators of RyR2 gating.

Moderate intensity continuous training reverses the detrimental effects of ovariectomy on RyR1 phosphorylation in rat skeletal muscle

High 17β-Estradiol (E2) concentrations in isolated ventricular myocytes as well as a lack of ovarian hormones in cardiac muscle of ovariectomized (OVX) rodents has been shown to lead to arrhythmogenic effects by inducing post-translational modifications, including phosphorylation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel ryanodine receptor-2 (RyR2). The effects of estrogens on the phosphorylation status of the RyR1 in skeletal muscle have not been investigated before. Furthermore, while high intensity exercise has been shown to increase RyR phosphorylation, there is no data on the effects of moderate intensity continuous training (MICT). The aims of the study were to investigate the effects of a 3-day treatment with low (1 nM, moderate (5 nM) and high (10 nM, 100 nM) E2 concentrations on RyR1 mRNA and protein expression and phosphorylation status (pRyRSer²⁸⁴⁴) in cultured C2C12 myotubes and to study the effects of OVX on RyR1 expression and phosphorylation in rat skeletal muscle in combination with 3 weeks of MICT. Treatment with low, physiological E2 concentrations reduced dihydropyridine receptor (DHPR) and RyR1 mRNA content in C2C12 myotubes compared to untreated control cells, whereas RyR1 protein phosphorylation (pRyRSer²⁸⁴⁴) was significantly increased after treatment with high, non-physiological E2 concentrations (p ≤ 0.05). RyR1 protein content (p ≤ 0.05) and pRyRSer²⁸⁴⁴ (p ≤ 0.05) were significantly elevated in skeletal muscle of OVX vs. sham-operated rats. Importantly, pRyRSer²⁸⁴⁴ levels were similar to sham-operated controls in OVX rats after MICT (OVX vs. OVX + MICT, p ≤ 0.05). Our results indicate, that one of the actions of estrogens is to alter skeletal muscle Ca²⁺ homeostasis by modulating the expression and phosphorylation of the RyR1 in skeletal muscle. Notably, regular MICT was able to counteract RyR1 phosphorylation in skeletal muscle of OVX rats.

Mutation-linked, excessively tight interaction between the calmodulin binding domain and the C-terminal domain of the cardiac ryanodine receptor as a novel cause of catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Ryanodine receptor (RyR2) is known to be a causal gene of catecholaminergic polymorphic ventricular tachycardia (CPVT), an important inherited disease. Some of the human CPVT-associated mutations have been found in a domain (4026-4172) that has EF hand motifs, the so-called calmodulin (CaM)-like domain (CaMLD).

OBJECTIVE

The purpose of this study was to investigate the underlying mechanism by which CPVT is induced by a mutation at CaMLD.

METHODS

A new N4103K/+ knock-in (KI) mice model was generated.

RESULTS

Sustained ventricular tachycardia was frequently observed after infusion of caffeine plus epinephrine in KI mice. Endogenous CaM bound to RyR2 decreased even at baseline in isolated KI cardiomyocytes. Ca²⁺ spark frequency (CaSpF) was much higher in KI cells than in wild-type cells. Addition of GSH-CaM (higher affinity CaM to RyR2) significantly decreased CaSpF. In response to isoproterenol, spontaneous Ca²⁺ transient (SCaT) was frequently observed in intact KI cells. Incorporation of GSH-CaM into intact KI cells using a protein delivery kit decreased SCaT significantly. An assay using a quartz crystal microbalance technique revealed that mutated CaMLD peptide showed higher binding affinity to CaM binding domain (CaMBD) peptide.

CONCLUSION

In the N4103K mutant, CaM binding affinity to RyR2 was significantly reduced regardless of beta-adrenergic stimulation. We found that this was caused by an abnormally tight interaction between CaMBD and mutated CaM-like domain (N4103K-CaMBD). Thus, CaMBD-CaMLD interaction may be a novel therapeutic target for treatment of lethal arrhythmia.

Are Antioxidants a Potential Therapy for FSHD? A Review of the Literature

Facioscapulohumeral muscular dystrophy (FSHD) is an inherited myopathy affecting approximately 1 in 7500 individuals worldwide. It is a progressive disease characterised by skeletal muscle weakness and wasting. A genetic mutation on the 4q35 chromosome results in the expression of the double homeobox 4 gene (DUX4) which drives oxidative stress, inflammation, toxicity, and atrophy within the skeletal muscle. FSHD is characterised by oxidative stress, and there is currently no cure and a lack of therapies for the disease. Antioxidants have been researched for many years, with investigators aiming to use antioxidants therapeutically for oxidative stress-associated diseases. This has included both natural and synthetic antioxidants. The use of antioxidants in preclinical or clinical models has been largely successful with a plethora of research reporting positive results. However, when translated to clinical trials, the use of antioxidants as a therapeutic intervention for a variety of disease has been largely unsuccessful. Moreover, specifically focusing on FSHD, limited research has been conducted on the use of antioxidants as a therapy in either preclinical or clinical models. This review summarises the current state of antioxidant use in the treatment of FSHD and discusses their potential avenue for therapeutic use for FSHD patients.

Calcium triggers reversal of calmodulin on nested anti-parallel sites in the IQ motif of the neuronal voltage-dependent sodium channel NaV1.2

Several members of the voltage-gated sodium channel family are regulated by calmodulin (CaM) and ionic calcium. The neuronal voltage-gated sodium channel NaV1.2 contains binding sites for both apo (calcium-depleted) and calcium-saturated CaM. We have determined equilibrium dissociation constants for rat NaV1.2 IQ motif [IQRAYRRYLLK] binding to apo CaM (~3nM) and (Ca2+)4-CaM (~85nM), showing that apo CaM binding is favored by 30-fold. For both apo and (Ca2+)4-CaM, NMR demonstrated that NaV1.2 IQ motif peptide (NaV1.2IQp) exclusively made contacts with C-domain residues of CaM (CaMC). To understand how calcium triggers conformational change at the CaM-IQ interface, we determined a solution structure (2M5E.pdb) of (Ca2+)2-CaMC bound to NaV1.2IQp. The polarity of (Ca2+)2-CaMC relative to the IQ motif was opposite to that seen in apo CaMC-Nav1.2IQp (2KXW), revealing that CaMC recognizes nested, anti-parallel sites in Nav1.2IQp. Reversal of CaM may require transient release from the IQ motif during calcium binding, and facilitate a re-orientation of CaMN allowing interactions with non-IQ NaV1.2 residues or auxiliary regulatory proteins interacting in the vicinity of the IQ motif.

Calcium-dependent energetics of calmodulin domain interactions with regulatory regions of the Ryanodine Receptor Type 1 (RyR1)

Calmodulin (CaM) allosterically regulates the homo-tetrameric human Ryanodine Receptor Type 1 (hRyR1): apo CaM activates the channel, while (Ca(2+))4-CaM inhibits it. CaM-binding RyR1 residues 1975-1999 and 3614-3643 were proposed to allow CaM to bridge adjacent RyR1 subunits. Fluorescence anisotropy titrations monitored the binding of CaM and its domains to peptides encompassing hRyR(11975-1999) or hRyR1(3614-3643). Both CaM and its C-domain associated in a calcium-independent manner with hRyR1(3614-3643) while N-domain required calcium and bound ~250-fold more weakly. Association with hRyR1(11975-1999) was weak. Both hRyR1 peptides increased the calcium-binding affinity of both CaM domains, while maintaining differences between them. These energetics support the CaM C-domain association with hRyR1(3614-3643) at low calcium, positioning CaM to respond to calcium efflux. However, the CaM N-domain affinity for hRyR(11975-1999) alone was insufficient to support CaM bridging adjacent RyR1 subunits. Other proteins or elements of the hRyR1 structure must contribute to the energetics of CaM-mediated regulation.

Gap junction regulation by calmodulin

Intracellular Ca(2+) activated calmodulin (CaM) inhibits gap junction channels in the low nanomolar to high micromolar range of [Ca(2+)]i. This regulation plays an essential role in numerous cellular processes that include hearing, lens transparency, and synchronized contractions of the heart. Previous studies have indicated that gap junction mediated cell-to-cell communication was inhibited by CaM antagonists. More recent evidence indicates a direct role of CaM in regulating several members of the connexin family. Since the intracellular loop and carboxyl termini of connexins are largely "invisible" in electron microscopy and X-ray crystallographic structures due to disorder in these domains, peptide models encompassing the putative CaM binding sites of several intracellular domains of connexins have been used to identify the Ca(2+)-dependent CaM binding sites of these proteins. This approach has been used to determine the CaM binding affinities of peptides derived from a number of different connexin-subfamilies.

Two potential calmodulin-binding sequences in the ryanodine receptor contribute to a mobile, intra-subunit calmodulin-binding domain

Calmodulin (CaM), a 16 kDa ubiquitous calcium-sensing protein, is known to bind tightly to the calcium release channel/ryanodine receptor (RyR), and modulate RyR function. CaM binding studies using RyR fragments or synthetic peptides have revealed the presence of multiple, potential CaM-binding regions in the primary sequence of RyR. In the present study, we inserted GFP into two of these proposed CaM-binding sequences and mapped them onto the three-dimensional structure of intact cardiac RyR2 by cryo-electron microscopy. Interestingly, we found that the two potential CaM-binding regions encompassing, Arg3595 and Lys4269, respectively, are in close proximity and are adjacent to the previously mapped CaM-binding sites. To monitor the conformational dynamics of these CaM-binding regions, we generated a fluorescence resonance energy transfer (FRET) pair, a dual CFP- and YFP-labeled RyR2 (RyR2R3595-CFP/K4269-YFP) with CFP inserted after Arg3595 and YFP inserted after Lys4269. We transfected HEK293 cells with the RyR2R3595-CFP/K4269-YFP cDNA, and examined their FRET signal in live cells. We detected significant FRET signals in transfected cells that are sensitive to the channel activator caffeine, suggesting that caffeine is able to induce conformational changes in these CaM-binding regions. Importantly, no significant FRET signals were detected in cells co-transfected with cDNAs encoding the single CFP (RyR2R3595-CFP) and single YFP (RyR2K4269-YFP) insertions, indicating that the FRET signal stemmed from the interaction between R3595-CFP and K4269-YFP that are in the same RyR subunit. These observations suggest that multiple regions in the RyR2 sequence may contribute to an intra-subunit CaM-binding pocket that undergoes conformational changes during channel gating.

Skeletal muscle-specific T-tubule protein STAC3 mediates voltage-induced Ca2+ release and contractility

Excitation-contraction (EC) coupling comprises events in muscle that convert electrical signals to Ca(2+) transients, which then trigger contraction of the sarcomere. Defects in these processes cause a spectrum of muscle diseases. We report that STAC3, a skeletal muscle-specific protein that localizes to T tubules, is essential for coupling membrane depolarization to Ca(2+) release from the sarcoplasmic reticulum (SR). Consequently, homozygous deletion of src homology 3 and cysteine rich domain 3 (Stac3) in mice results in complete paralysis and perinatal lethality with a range of musculoskeletal defects that reflect a blockade of EC coupling. Muscle contractility and Ca(2+) release from the SR of cultured myotubes from Stac3 mutant mice could be restored by application of 4-chloro-m-cresol, a ryanodine receptor agonist, indicating that the sarcomeres, SR Ca(2+) store, and ryanodine receptors are functional in Stac3 mutant skeletal muscle. These findings reveal a previously uncharacterized, but required, component of the EC coupling machinery of skeletal muscle and introduce a candidate for consideration in myopathic disorders.

Ryanodine receptor calcium release channels: lessons from structure-function studies

Ryanodine receptors (RyRs) are the largest known ion channels. They are Ca(2+) release channels found primarily on the sarcoplasmic reticulum of myocytes. Several hundred mutations in RyRs are associated with skeletal or cardiomyocyte disease in humans. Many of these mutations can now be mapped onto the high resolution structures of individual RyR domains and on full-length tetrameric cryo-electron microscopy structures. A closely related Ca(2+) release channel, the inositol 1,4,5-trisphospate receptor (IP3 R), shows a conserved structural architecture at the N-terminus, suggesting that both channels evolved from an ancestral unicellular RyR/IP3 R. The functional insights provided by recent structural studies for both channels will aid in the development of rationale treatments for a myriad of Ca(2+)-signaled malignancies.

The calmodulin regulator protein, PEP-19, sensitizes ATP-induced Ca2+ release

PEP-19 is a small, intrinsically disordered protein that binds to the C-domain of calmodulin (CaM) via an IQ motif and tunes its Ca(2+) binding properties via an acidic sequence. We show here that the acidic sequence of PEP-19 has intrinsic Ca(2+) binding activity, which may modulate Ca(2+) binding to CaM by stabilizing an initial Ca(2+)-CaM complex or by electrostatically steering Ca(2+) to and from CaM. Because PEP-19 is expressed in cells that exhibit highly active Ca(2+) dynamics, we tested the hypothesis that it influences ligand-dependent Ca(2+) release. We show that PEP-19 increases the sensitivity of HeLa cells to ATP-induced Ca(2+) release to greatly increase the percentage of cells responding to sub-saturating doses of ATP and increases the frequency of Ca(2+) oscillations. Mutations in the acidic sequence of PEP-19 that inhibit or prevent it from modulating Ca(2+) binding to CaM greatly inhibit its effect on ATP-induced Ca(2+) release. Thus, this cellular effect of PEP-19 does not depend simply on binding to CaM via the IQ motif but requires its acidic metal binding domain. Tuning the activities of Ca(2+) mobilization pathways places PEP-19 at the top of CaM signaling cascades, with great potential to exert broad effects on downstream CaM targets, thus expanding the biological significance of this small regulator of CaM signaling.

Enhanced binding of calmodulin to the ryanodine receptor corrects contractile dysfunction in failing hearts

AIMS

The channel function of the cardiac ryanodine receptor (RyR2) is modulated by calmodulin (CaM). However, the involvement of CaM in aberrant Ca(2+) release in diseased hearts remains unclear. Here, we investigated the pathogenic role of defective CaM binding to the RyR2 in the channel dysfunction associated with heart failure.

METHODS AND RESULTS

The involvement of CaM in aberrant Ca(2+) release was assessed in normal and pacing-induced failing canine hearts. The apparent affinity of CaM for RyR2 was considerably lower in failing sarcoplasmic reticulum (SR) compared with normal SR. Thus, the amount of CaM bound to RyR2 was markedly decreased in failing myocytes. Expression of the CaM isoform Gly-Ser-His-CaM (GSH-CaM), which has much higher binding affinity than wild-type CaM for RyR1, restored normal CaM binding to RyR2 in both SR and myocytes of failing hearts. The Ca(2+) spark frequency (SpF) was markedly higher and the SR Ca(2+) content was lower in failing myocytes compared with normal myocytes. The incorporation of GSH-CaM into the failing myocytes corrected the aberrant SpF and SR Ca(2+) content to normal levels.

CONCLUSION

Reduced CaM binding to RyR2 seems to play a critical role in the pathogenesis of aberrant Ca(2+) release in failing hearts. Correction of the reduced CaM binding to RyR2 stabilizes the RyR2 channel function and thereby restores normal Ca(2+) handling and contractile function to failing hearts.

Calmodulin-binding locations on the skeletal and cardiac ryanodine receptors

Ryanodine receptor types 1 (RyR1) and 2 (RyR2) are calcium release channels that are highly enriched in skeletal and cardiac muscle, respectively, where they play an essential role in excitation-contraction coupling. Apocalmodulin (apo-CaM) weakly activates RyR1 but inhibits RyR2, whereas Ca(2+)-calmodulin inhibits both isoforms. Previous cryo-EM studies showed distinctly different binding locations on RyR1 for the two states of CaM. However, recent studies employing FRET appear to challenge these findings. Here, using cryo-EM, we have determined that a CaM mutant that is incapable of binding calcium binds to RyR1 at the apo site, regardless of the calcium concentration. We have also re-determined the location of RyR1-bound Ca(2+)-CaM using uniform experimental conditions. Our results show the existence of the two overlapping but distinct binding sites for CaM in RyR1 and imply that the binding location switch is due to Ca(2+) binding to CaM, as opposed to direct effects of Ca(2+) on RyR1. We also discuss explanations that could resolve the apparent conflict between the cryo-EM and FRET results. Interestingly, apo-CaM binds to RyR2 at a similar binding location to that of Ca(2+)-CaM on RyR1, in seeming agreement with the inhibitory effects of these two forms of CaM on their respective receptors.

Physiology and pathology of calcium signaling in the brain

Calcium (Ca(2+)) plays fundamental and diversified roles in neuronal plasticity. As second messenger of many signaling pathways, Ca(2+) as been shown to regulate neuronal gene expression, energy production, membrane excitability, synaptogenesis, synaptic transmission, and other processes underlying learning and memory and cell survival. The flexibility of Ca(2+) signaling is achieved by modifying cytosolic Ca(2+) concentrations via regulated opening of plasma membrane and subcellular Ca(2+) sensitive channels. The spatiotemporal patterns of intracellular Ca(2+) signals, and the ultimate cellular biological outcome, are also dependent upon termination mechanism, such as Ca(2+) buffering, extracellular extrusion, and intra-organelle sequestration. Because of the central role played by Ca(2+) in neuronal physiology, it is not surprising that even modest impairments of Ca(2+) homeostasis result in profound functional alterations. Despite their heterogeneous etiology neurodegenerative disorders, as well as the healthy aging process, are all characterized by disruption of Ca(2+) homeostasis and signaling. In this review we provide an overview of the main types of neuronal Ca(2+) channels and their role in neuronal plasticity. We will also discuss the participation of Ca(2+) signaling in neuronal aging and degeneration.

The genetics of calcium signaling in Drosophila melanogaster

BACKGROUND

Genetic screens for behavioral and physiological defects in Drosophila melanogaster, helped identify several components of calcium signaling of which some, like the Trps, were novel. For genes initially identified in vertebrates, reverse genetic methods have allowed functional studies at the cellular and systemic levels.

SCOPE OF REVIEW

The aim of this review is to explain how various genetic methods available in Drosophila have been used to place different arms of Ca2+ signaling in the context of organismal development, physiology and behavior.

MAJOR CONCLUSION

Mutants generated in genes encoding a range of Ca2+ transport systems, binding proteins and enzymes affect multiple aspects of neuronal and muscle physiology. Some also affect the maintenance of ionic balance and excretion from malpighian tubules and innate immune responses in macrophages. Aspects of neuronal physiology affected include synaptic growth and plasticity, sensory transduction, flight circuit development and function. Genetic interaction screens have shown that mechanisms of maintaining Ca2+ homeostasis in Drosophila are cell specific and require a synergistic interplay between different intracellular and plasma membrane Ca2+ signaling molecules.

GENERAL SIGNIFICANCE

Insights gained through genetic studies of conserved Ca2+ signaling pathways have helped understand multiple aspects of fly physiology. The similarities between mutant phenotypes of Ca2+ signaling genes in Drosophila with certain human disease conditions, especially where homologous genes are causative factors, are likely to aid in the discovery of underlying disease mechanisms and help develop novel therapeutic strategies. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

S100A1 and calmodulin regulation of ryanodine receptor in striated muscle

The release of Ca2+ ions from the sarcoplasmic reticulum through ryanodine receptor calcium release channels represents the critical step linking electrical excitation to muscular contraction in the heart and skeletal muscle (excitation-contraction coupling). Two small Ca2+ binding proteins, S100A1 and calmodulin, have been demonstrated to bind and regulate ryanodine receptor in vitro. This review focuses on recent work that has revealed new information about the endogenous roles of S100A1 and calmodulin in regulating skeletal muscle excitation-contraction coupling. S100A1 and calmodulin bind to an overlapping domain on the ryanodine receptor type 1 to tune the Ca2+ release process, and thereby regulate skeletal muscle function. We also discuss past, current and future work surrounding the regulation of ryanodine receptors by calmodulin and S100A1 in both cardiac and skeletal muscle, and the implications for excitation-contraction coupling.

Transcriptional profiling identifies differentially expressed genes in developing turkey skeletal muscle

Background

Skeletal muscle growth and development from embryo to adult consists of a series of carefully regulated changes in gene expression. Understanding these developmental changes in agriculturally important species is essential to the production of high quality meat products. For example, consumer demand for lean, inexpensive meat products has driven the turkey industry to unprecedented production through intensive genetic selection. However, achievements of increased body weight and muscle mass have been countered by an increased incidence of myopathies and meat quality defects. In a previous study, we developed and validated a turkey skeletal muscle-specific microarray as a tool for functional genomics studies. The goals of the current study were to utilize this microarray to elucidate functional pathways of genes responsible for key events in turkey skeletal muscle development and to compare differences in gene expression between two genetic lines of turkeys. To achieve these goals, skeletal muscle samples were collected at three critical stages in muscle development: 18d embryo (hyperplasia), 1d post-hatch (shift from myoblast-mediated growth to satellite cell-modulated growth by hypertrophy), and 16wk (market age) from two genetic lines: a randombred control line (RBC2) maintained without selection pressure, and a line (F) selected from the RBC2 line for increased 16wk body weight. Array hybridizations were performed in two experiments: Experiment 1 directly compared the developmental stages within genetic line, while Experiment 2 directly compared the two lines within each developmental stage.

Results

A total of 3474 genes were differentially expressed (false discovery rate; FDR < 0.001) by overall effect of development, while 16 genes were differentially expressed (FDR < 0.10) by overall effect of genetic line. Ingenuity Pathways Analysis was used to group annotated genes into networks, functions, and canonical pathways. The expression of 28 genes involved in extracellular matrix regulation, cell death/apoptosis, and calcium signaling/muscle function, as well as genes with miscellaneous function was confirmed by qPCR.

Conclusions

The current study identified gene pathways and uncovered novel genes important in turkey muscle growth and development. Future experiments will focus further on several of these candidate genes and the expression and mechanism of action of their protein products.

Site-specific modification of calmodulin Ca²(+) affinity tunes the skeletal muscle ryanodine receptor activation profile

The skeletal muscle isoform of the ryanodine receptor Ca²(+)-release channel (RyR1) is regulated by Ca²(+) and CaM (calmodulin). CaM shifts the biphasic Ca²(+)-dependence of RyR1 activation leftward, effectively increasing channel opening at low Ca²(+) and decreasing channel opening at high Ca²(+). The conversion of CaM from a RyR1 activator into an inhibitor is due to the binding of Ca²(+) to CaM; however, which of CaM's four Ca²(+)-binding sites serves as the switch for this conversion is unclear. We engineered a series of mutant CaMs designed to individually increase the Ca²(+) affinity of each of CaM's EF-hands by increasing the number of acidic residues in Ca²(+)-chelating positions. Domain-specific Ca²(+) affinities of each CaM variant were determined by equilibrium fluorescence titration. Mutations in sites I (T26D) or II (N60D) in CaM's N-terminal domain had little effect on CaM Ca²(+) affinity and regulation of RyR1. However, the site III mutation N97D increased the Ca²(+)-binding affinity of CaM's C-terminal domain and caused CaM to inhibit RyR1 at a lower Ca²(+) concentration than wild-type CaM. Conversely, the site IV mutation Q135D decreased the Ca²(+)-binding affinity of CaM's C-terminal domain and caused CaM to inhibit RyR1 at higher Ca²(+) concentrations. These results support the hypothesis that Ca²(+) binding to CaM's C-terminal acts as the switch converting CaM from a RyR1 activator into a channel inhibitor. These results indicate further that targeting CaM's Ca²(+) affinity may be a valid strategy to tune the activation profile of CaM-regulated ion channels.

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 channelopathies

Ryanodine receptors (RyR) are intracellular Ca2+-permeable channels that provide the sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure-function relationships of these ion channels.

Intracellular translocation of calmodulin and Ca2+/calmodulin-dependent protein kinase II during the development of hypertrophy in neonatal cardiomyocytes

We have recently shown that stimulation of cultured neonatal cardiomyocytes with endothelin-1 (ET-1) first produces conformational disorder within the ryanodine receptor (RyR2) and diastolic Ca(2+) leak from the sarcoplasmic reticulum (SR), then develops hypertrophy (HT) in the cardiomyocytes (Hamada et al., 2009 [3]). The present paper addresses the following question. By what mechanism does crosstalk between defective operation of RyR2 and activation of the HT gene program occur? Here we show that the immuno-stain of calmodulin (CaM) is localized chiefly in the cytoplasmic area in the control cells; whereas, in the ET-1-treated/hypertrophied cells, major immuno-staining is localized in the nuclear region. In addition, fluorescently labeled CaM that has been introduced into the cardiomyocytes using the BioPORTER system moves from the cytoplasm to the nucleus with the development of HT. The immuno-confocal imaging of Ca(2+)/CaM-dependent protein kinase II (CaMKII) also shows cytoplasm-to-nucleus shift of the immuno-staining pattern in the hypertrophied cells. In an early phase of hypertrophic growth, the frequency of spontaneous Ca(2+) transients increases, which accompanies with cytoplasm-to-nucleus translocation of CaM. In a later phase of hypertrophic growth, further increase in the frequency of spontaneous Ca(2+) transients results in the appearance of trains of Ca(2+) spikes, which accompanies with nuclear translocation of CaMKII. The cardio-protective reagent dantrolene (the reagent that corrects the de-stabilized inter-domain interaction within the RyR2 to a normal mode) ameliorates aberrant intracellular Ca(2+) events and prevents nuclear translocation of both CaM and CaMKII, then prevents the development of HT. These results suggest that translocation of CaM and CaMKII from the cytoplasm to the nucleus serves as messengers to transmit the pathogenic signal elicited in the surface membrane and in the RyR2 to the nuclear transcriptional sites to activate HT program.

Dissociation of calmodulin from cardiac ryanodine receptor causes aberrant Ca(2+) release in heart failure

AIMS

Calmodulin (CaM) is well known to modulate the channel function of the cardiac ryanodine receptor (RyR2). However, the possible role of CaM on the aberrant Ca(2+) release in diseased hearts remains unclear. In this study, we investigated the state of RyR2-bound CaM and channel dysfunctions in pacing-induced failing hearts.

METHODS AND RESULTS

The characteristics of CaM binding to RyR2 and the role of CaM on the aberrant Ca(2+) release were assessed in normal and failing canine hearts. The affinity of CaM binding to RyR2 was lower in failing sarcoplasmic reticulum (SR) than in normal SR. Addition of FK506, which dissociates FKBP12.6 from RyR2, to normal SR reduced the CaM-binding affinity. Dantrolene restored a normal level of the CaM-binding affinity in either FK506-treated (normal) SR or failing SR, suggesting that the defective inter-domain interaction between the N-terminal domain and the central domain of RyR2 (the therapeutic target of dantrolene) is involved in the reduction of the CaM-binding affinity in failing hearts. In saponin-permeabilized cardiomyocytes, the frequency of spontaneous Ca(2+) sparks was much more increased in failing cardiomyocytes than in normal cardiomyocytes, whereas the addition of a high concentration of CaM attenuated the aberrant increase of Ca(2+) sparks.

CONCLUSION

The defective inter-domain interaction between N-terminal and central domains within RyR2 reduces the binding affinity of CaM to RyR2, thereby causing the spontaneous Ca(2+) release events in failing hearts. Correction of the defective CaM binding may be a new strategy to protect against the aberrant Ca(2+) release in heart failure.

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.

The role of CACNA1S in predisposition to malignant hyperthermia

BACKGROUND

Malignant hyperthermia (MH) is an inherited pharmacogenetic disorder of skeletal muscle, characterised by an elevated calcium release from the skeletal muscle sarcoplasmic reticulum. The dihydropyridine receptor (DHPR) plays an essential role in excitation-contraction coupling and calcium homeostasis in skeletal muscle. This study focuses on the gene CACNA1S which encodes the alpha1 subunit of the DHPR, in order to establish whether CACNA1S plays a major role in MH susceptibility in the UK.

METHODS

We investigate the CACNA1S locus in detail in 50 independent MH patients, the largest study to date, to identify novel variants that may predispose to disease and also to characterise the haplotype structure across CACNA1S.

RESULTS

We present CACNA1S cDNA sequencing data from 50 MH patients in whom RYR1 mutations have been excluded, and subsequent mutation screening analysis. Furthermore we present haplotype analysis of unphased CACNA1S SNPs to (1) assess CACNA1S haplotype frequency differences between susceptible MH cases and a European control group and (2) analyse population-based association via clustering of CACNA1S haplotypes based on disease risk.

CONCLUSION

The study identified a single potentially pathogenic change in CACNA1S (p.Arg174Trp), and highlights that the haplotype structure across CACNA1S is diverse, with a high degree of variability.

Identifying genes that interact with Drosophila presenilin and amyloid precursor protein

The gamma-secretase complex is involved in cleaving transmembrane proteins such as Notch and one of the genes targeted in Alzheimer's disease known as amyloid precursor protein (APP). Presenilins function within the catalytic core of gamma-secretase, and mutated forms of presenilins were identified as causative factors in familial Alzheimer's disease. Recent studies show that in addition to Notch and APP, numerous signal transduction pathways are modulated by presenilins, including intracellular calcium signaling. Thus, presenilins appear to have diverse roles. To further understand presenilin function, we searched for Presenilin-interacting genes in Drosophila by performing a genetic modifier screen for enhancers and suppressors of Presenilin-dependent Notch-related phenotypes. We identified 177 modifiers, including known members of the Notch pathway and genes involved in intracellular calcium homeostasis. We further demonstrate that 53 of these modifiers genetically interacted with APP. Characterization of these genes may provide valuable insights into Presenilin function in development and disease.

Determinants in CaV1 channels that regulate the Ca2+ sensitivity of bound calmodulin

Calmodulin binds to IQ motifs in the alpha(1) subunit of Ca(V)1.1 and Ca(V)1.2, but the affinities of calmodulin for the motif and for Ca(2+) are higher when bound to Ca(V)1.2 IQ. The Ca(V)1.1 IQ and Ca(V)1.2 IQ sequences differ by four amino acids. We determined the structure of calmodulin bound to Ca(V)1.1 IQ and compared it with that of calmodulin bound to Ca(V)1.2 IQ. Four methionines in Ca(2+)-calmodulin form a hydrophobic binding pocket for the peptide, but only one of the four nonconserved amino acids (His-1532 of Ca(V)1.1 and Tyr-1675 of Ca(V)1.2) contacts this calmodulin pocket. However, Tyr-1675 in Ca(V)1.2 contributes only modestly to the higher affinity of this peptide for calmodulin; the other three amino acids in Ca(V)1.2 contribute significantly to the difference in the Ca(2+) affinity of the bound calmodulin despite having no direct contact with calmodulin. Those residues appear to allow an interaction with calmodulin with one lobe Ca(2+)-bound and one lobe Ca(2+)-free. Our data also provide evidence for lobe-lobe interactions in calmodulin bound to Ca(V)1.2.

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

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

Role of ryanodine receptor as a Ca²(+) regulatory center in normal and failing hearts

Abnormal Ca²(+) cycling is important in various cardiac diseases. Evidence has accumulated that dysregulation of Ca²(+) release from the ryanodine receptor (RyR2) plays a significant role in cardiac dysfunction. Spontaneous Ca²(+) release through RyR2 during diastole decreases sarcoplasmic reticulum (SR) Ca²(+) content, and also induces delayed after depolarization (DAD) as a substrate for lethal arrhythmia. Several disease-linked mutations in the RyR have been reported in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) or arrythmogenic right ventricular cardiomyopathy type 2 (ARVC2). The unique distribution of these mutation sites has produced the concept that the interaction among the putative regulatory domains within the RyR may play a key role in regulating the channel opening, and that there seems to be a common abnormality in the channel disorder between heart failure and CPVT/ARVC2. We review here the considerable body of evidence regarding defective channel gating of RyR2 in the pathogenesis of heart failure and lethal arrhythmia.

Ca2+-dependent inactivation of Ca2+-induced Ca2+ release in bullfrog sympathetic neurons

We studied inactivation of Ca(2+)-induced Ca(2+) release (CICR) via ryanodine receptors (RyRs) in bullfrog sympathetic neurons. The rate of rise in [Ca(2+)](i) due to CICR evoked by a depolarizing pulse decreased markedly within 10-20 ms to a much slower rate despite persistent Ca(2+) entry and little depletion of Ca(2+) stores. The Ca(2+) entry elicited by the subsequent pulse within 50 ms, during which the [Ca(2+)](i) level remained unchanged, did not generate a distinct [Ca(2+)](i) rise. This mode of [Ca(2+)](i) rise was unaffected by a mitochondrial uncoupler, carbonyl cyanide p-trifluromethoxy-phenylhydrazone (FCCP, 1 microm). Paired pulses of varying interval and duration revealed that recovery from inactivation became distinct >or= 50 ms after depolarization and depended on [Ca(2+)](i). The inactivation was prevented by BAPTA (>or= 100 microm) but not by EGTA (<or= 10 mM), whereas the activation was less affected by BAPTA. When CICR was partially activated, some of the non-activated RyRs were also inactivated directly. Thus, the inactivation in these neurons is induced by Ca(2+) binding to the high-affinity regulatory sites residing very close to Ca(2+) channels and/or RyRs, although the sites for activation are located much closer to those Ca(2+) sources. The rate of [Ca(2+)](i) decay after the pulse decreased with increasing pulse duration longer than 10 ms, and this was abolished by BAPTA. Thus, some mechanism counteracting Ca(2+) clearance is induced after full inactivation and potentiated during the pulse. Possible models for RyR inactivation were proposed and the roles of inactivation in Ca(2+) signalling were discussed.

Interaction of the Lys(3614)-Asn(3643) calmodulin-binding domain with the Cys(4114)-Asn(4142) region of the type 1 ryanodine receptor is involved in the mechanism of Ca2+/agonist-induced channel activation

In the present study we show that the interaction of the CaM (calmodulin)-binding domain (Lys(3614)-Asn(3643)) with the Cys(4114)-Asn(4142) region (a region included in the CaM-like domain) serves as an intrinsic regulator of the RyR1 (type-1 ryanodine receptor). We tested the effects of antibodies raised against the two putative key regions of RyR1 [anti-(Lys(3614)-Asn(3643)) and anti-(Cys(4114)-Asn(4142)) antibodies]. Both antibodies produced significant inhibition of [3H]ryanodine-binding activity of RyR1. This suggests that the inter-domain interaction between the two domains, Lys(3614)-Asn(3643) and Cys(4114)-Asn(4142), activates the channel, and that the binding of antibody to either side of the interacting domain pair interfered with the formation of a 'channel-activation link' between the two regions. In order to spectroscopically monitor the mode of interaction of these domains, the site of inter-domain interaction was fluorescently labelled with MCA [(7-methoxycoumarin-4-yl)acetyl] in a site-directed manner. The accessibility of the bound MCA to a large molecular mass fluorescence quencher, BSA-QSY (namely, the size of a gap between the interacting domains) decreased with an increase of [Ca2+] in a range of 0.03-2.0 microM, as determined by Stern-Volmer fluorescence quenching analysis. The Ca2+-dependent decrease in the quencher accessibility was more pronounced in the presence of 150 microM 4-CmC (4-chlorometacresol), and was reversed by 1 mM Mg2+ (a well-known inhibitor of Ca2+/agonist-induced channel activation). These results suggest that the Lys(3614)-Asn(3643) and Cys(4114)-Asn(4142) regions of RyR1 interact with each other in a Ca2+- and agonist-dependent manner, and this serves as a mechanism of Ca2+- and agonist-dependent activation of the RyR1 Ca2+ channel.

RyR1 S-nitrosylation underlies environmental heat stroke and sudden death in Y522S RyR1 knockin mice

Mice with a malignant hyperthermia mutation (Y522S) in the ryanodine receptor (RyR1) display muscle contractures, rhabdomyolysis, and death in response to elevated environmental temperatures. We demonstrate that this mutation in RyR1 causes Ca(2+) leak, which drives increased generation of reactive nitrogen species (RNS). Subsequent S-nitrosylation of the mutant RyR1 increases its temperature sensitivity for activation, producing muscle contractures upon exposure to elevated temperatures. The Y522S mutation in humans is associated with central core disease. Many mitochondria in the muscle of heterozygous Y522S mice are swollen and misshapen. The mutant muscle displays decreased force production and increased mitochondrial lipid peroxidation with aging. Chronic treatment with N-acetylcysteine protects against mitochondrial oxidative damage and the decline in force generation. We propose a feed-forward cyclic mechanism that increases the temperature sensitivity of RyR1 activation and underlies heat stroke and sudden death. The cycle eventually produces a myopathy with damaged mitochondria.

Calmodulin in adult mammalian skeletal muscle: localization and effect on sarcoplasmic reticulum Ca2+ release

Calmodulin is a ubiquitous Ca2+ binding protein that binds to ryanodine rectors (RyR) and is thought to modulate its activity. Here we evaluated the effects of recombinant calmodulin on the rate of occurrence and spatial properties of Ca2+ sparks as an assay of activation in saponin-permeabilized mouse myofibers. Control myofibers exhibited a time-dependent increase and subsequent decrease in spark frequency. Recombinant wild-type calmodulin prevented the time-dependent appearance of Ca2+ sparks and decreased the derived Ca2+ flux from the sarcoplasmic reticulum during a spark by approximately 37%. A recombinant Ca2+-insensitive form of calmodulin resulted in an instantaneous increase in spark frequency as well as an increase in the derived Ca2+ flux by approximately 24%. Endogenous calmodulin was found to primarily localize to the Z-line. Surprisingly, removal of endogenous calmodulin did not alter the time dependence of Ca2+ spark appearance. These results indicate that calmodulin may not be essential for RyR1-dependent Ca2+ release in adult mammalian skeletal muscle.

Secretagogin expression in tumours of the human brain and its coverings

Secretagogin is a recently described calcium-binding protein, which is expressed in some neurons of the human brain. In this study we systematically investigated secretagogin expression in 245 tumours of the human brain and its coverings using immunohistochemistry. We found focal or widespread secretagogin expression in tumour cells in 1/18 oligoastrocytomas, 1/19 oligodendrogliomas, 2/20 anaplastic oligodendrogliomas, 2/9 ependymomas, 2/11 anaplastic ependymomas, 2/10 glioblastomas, 3/11 gangliogliomas and 1/2 anaplastic gangliogliomas, 10/10 central neurocytomas, 5/10 classic medulloblastomas, 4/5 desmoplastic medulloblastomas, 3/5 large cell/anaplastic medulloblastomas, 3/5 neuroblastomas, 3/10 meningiomas, 2/10 haemangioblastomas, and 13/19 pituitary adenomas. Further, we observed secretagogin expression in endothelial cells in 5/10 meningiomas, 2/5 haemangiopericytomas, and 2/10 haemangioblastomas. We detected no secretagogin expression in fibrillary astrocytoma, pilocytic astrocytoma, DNT, pineocytoma, pineoblastoma, subependymal giant cell astrocytoma (SEGA), atypical teratoid/rhabdoid tumour (AT/RT), or primary central nervous system lymphoma (PCNSL). We conclude that secretagogin is differentially expressed in human neuronal, glial, and embryonal brain tumours, meningial neoplasms and pituitary adenomas. Our findings indicate that secretagogin is involved in the calcium metabolism of tumour cells and endothelial cells in a subset of neoplasms of the brain and its coverings. Anti-secretagogin immunohistochemistry does not seem to be helpful in most differential diagnostic situations in surgical neuropathology.

Binding property of avian skeletal muscle ryanodine receptor isoforms with dihydropyridine receptor and calmodulin

Ca(2+) release during excitation-contraction coupling in avian skeletal muscle is controlled by two ryanodine receptor isoforms, alphaRYR and betaRYR. Two other proteins, dihydropyridine receptor (DHPR) and calmodulin (CaM), have been shown to play important roles in regulating the RYR channel activity. In the current study, we measured the protein contents of DHPR and RYR in turkey skeletal muscle and obtained a ratio of 1:1 between DHPR and alphaRYR which suggests that only a subpopulation of alphaRYR is interacting with DHPR. Two CaM derivatives, the photoactivable crosslinking probe [(125)I]-Bz-CaM and metabolically labeled probe [(35)S]CaM, were used to study the interaction between CaM and RYR isoforms in turkey skeletal muscle. The alphaRYR and betaRYR displayed a marked difference in their CaM binding behavior. At a Ca(2+) concentration of 200 microM, CaM bound to both isoforms at a ratio of one CaM molecule per one RYR subunit. At a Ca(2+) concentration of <10 nM, CaM bound primarily to alphaRYR and the binding affinity was significantly lower than that at micromolar level of Ca(2+) concentration. Cloning and sequencing of putative CaM binding sites in alphaRYR and betaRYR suggests that differences in primary structures of the CaM binding sites of each RYR isoform may contribute to the differential CaM binding behavior of alphaRYR and betaRYR.

Use-dependent inhibition of the skeletal muscle ryanodine receptor by the suramin analogue NF676

The skeletal muscle Ca2+ release channel, the ryanodine receptor, is activated by the trypanocidal drug suramin via the calmodulin-binding site. As calmodulin activates and inhibits the ryanodine receptor depending on whether Ca2+ is absent or present, suramin analogues were screened for inhibition of the ryanodine receptor. Up to 300 microM, the novel suramin analogue, 4,4'-(carbonyl-bis(imino-4,1-phenylene-(2,5-benzimidazolylene)carbonylimino))-bis-benzenesulfonic acid disodium salt (NF676) was not able to significantly inhibit the basal [3H]ryanodine binding. However, kinetic analysis of the high affinity [3H]ryanodine binding elucidates a time-dependent increment of inhibition by NF676, which is indicative for an open channel blocker. Moreover, the ryanodine receptor was much more sensitive towards inhibition by NF676 when preactivated with caffeine or the nonhydrolysable ATP analogue, adenylyl-imidodiphosphate. Nonetheless, the suramin activated ryanodine receptor was not susceptible towards high-affinity NF676 inhibition, indicating an allosteric hindrance between the binding sites of suramin and NF676. In the line of this finding, NF676 per se was not capable to elute the purified ryanodine receptor from a calmodulin-Sepharose, but it prevented the elution by suramin. Other than suramin, NF676 did not inhibit the Ca2+ ATPase of the sarcoplasmic reticulum. However, suramin-induced Ca2+ release from sarcoplasmic reticulum was completely abrogated by preincubation with NF676. Taken together, we conclude from these data that NF676 represents a novel lead compound as a potent use-dependent blocker of the skeletal muscle ryanodine receptor via an allosteric interaction with the suramin-binding site.

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.

Functional properties of ryanodine receptors carrying three amino acid substitutions identified in patients affected by multi-minicore disease and central core disease, expressed in immortalized lymphocytes

More than 80 mutations in the skeletal muscle ryanodine receptor gene have been found to be associated with autosomal dominant forms of malignant hyperthermia and central core disease, and with recessive forms of multi-minicore disease. Studies on the functional effects of pathogenic dominant mutations have shown that they mostly affect intracellular Ca2+ homoeostasis, either by rendering the channel hypersensitive to activation (malignant hyperthermia) or by altering the amount of Ca2+ released subsequent to physiological or pharmacological activation (central core disease). In the present paper, we show, for the first time, data on the functional effect of two recently identified recessive ryanodine receptor 1 amino acid substitutions, P3527S and V4849I, as well as that of R999H, another substitution that was identified in two siblings that were affected by multi-minicore disease. We studied the intracellular Ca2+ homoeostasis of EBV (Epstein-Barr virus)-transformed lymphoblastoid cells from the affected patients, their healthy relatives and control individuals. Our results show that the P3527S substitution in the homozygous state affected the amount of Ca2+ released after pharmacological activation with 4-chloro-m-cresol and caffeine, but did not affect the size of the thapsigargin-sensitive Ca2+ stores. The other substitutions had no effect on either the size of the intracellular Ca2+ stores, or on the amount of Ca2+ released after ryanodine receptor activation; however, both the P3527S and V4849I substitutions had a small but significant effect on the resting Ca2+ concentration.

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

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

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.

Role of the Met3534-Ala4271 region of the ryanodine receptor in the regulation of Ca2+ release induced by calmodulin binding domain peptide

CaMBP, a peptide corresponding to the 3614-3643 calmodulin (CaM) binding region of the ryanodine receptor (RyR1), is known to activate RyR1 Ca2+ channel. To analyze the mechanism of channel regulation by the CaMBP-RyR1 interaction, we investigated a), CaMBP binding to RyR1, b), induced local conformational changes in the CaMBP binding region of RyR1 using the fluorescent conformational probe badan attached to CaMBP (CaMBP-badan), and c), effects of "a" and "b" on SR Ca2+ release. We also monitored the interaction of CaMBP-badan with CaM and a peptide corresponding to the Met3534-Ala4271 region of RyR1 (R3534-4271) as a control. At lower peptide concentrations (< or =15 microM), CaMBP binding to RyR1 increased the intensity of badan fluorescence emission at a shorter wavelength (the state resembling CaMBP-badan/Ca-CaM) and induced Ca2+ release. Further increase in CaMBP concentration (up to approximately 50 microM) produced more binding of CaMBP accompanied by further increase in the badan fluorescence emission but at a longer wavelength (the state resembling CaMBP-badan/apo-CaM) and inhibited Ca2+ release. Binding of CaMBP-badan to R3534-4271 increased the intensity of badan fluorescence, showing the similar concentration-dependent red-shift of the emission maximum. It is proposed that CaMBP interacts with two classes of binding sites located in the Met3534-Ala4271 region of RyR1, which activate and inhibit the Ca2+ channel, respectively.