Calcium transport across the sarcoplasmic reticulum: structure and function of Ca2+-ATPase and the ryanodine receptor

Regulation of gene expression during ontogeny of physiological function in the brackishwater amphipod Gammarus chevreuxi

Embryonic development is a complex process involving the co-ordinated onset and integration of multiple morphological features and physiological functions. While the molecular basis of morphological development in embryos is relatively well known for traditional model species, the molecular underpinning of the development of physiological functions is not. Here, we used global gene expression profiling to investigate the transcriptional changes associated with the development of morphological and physiological function in the amphipod crustacean Gammarus chevreuxi. We compared the transcriptomes at three timepoints during the latter half of development, characterised by different stages of the development of heart form and function: 10 days post fertilisation (dpf, Early: no heart structure visible), 15 dpf (Middle: heart present but not fully functional), and 18 dpf (Late: regular heartbeat). Gene expression profiles differed markedly between developmental stages, likely representing a change in the activity of different processes throughout the latter period of G. chevreuxi embryonic development. Differentially expressed genes belonged to one of three distinct clusters based on their expression patterns across development. One of these clusters, which included key genes relating to cardiac contractile machinery and calcium handling, displayed a pattern of sequential up-regulation throughout the developmental period studied. Further analyses of these transcripts could reveal genes that may influence the onset of a regular heartbeat. We also identified morphological and physiological processes that may occur alongside heart development, such as development of digestive caeca and the cuticle. Elucidating the mechanisms underpinning morphological and physiological development of non-model organisms will support improved understanding of conserved mechanisms, addressing the current phylogenetic gap between relatively well known model species.

Redox signaling in heart failure and therapeutic implications

Heart failure is a growing health burden worldwide characterized by alterations in excitation-contraction coupling, cardiac energetic deficit and oxidative stress. While current treatments are mostly limited to antagonization of neuroendocrine activation, more recent data suggest that also targeting metabolism may provide substantial prognostic benefit. However, although in a broad spectrum of preclinical models, oxidative stress plays a causal role for the development and progression of heart failure, no treatment that targets reactive oxygen species (ROS) directly has entered the clinical arena yet. In the heart, ROS derive from various sources, such as NADPH oxidases, xanthine oxidase, uncoupled nitric oxide synthase and mitochondria. While mitochondria are the primary source of ROS in the heart, communication between different ROS sources may be relevant for physiological signalling events as well as pathologically elevated ROS that deteriorate excitation-contraction coupling, induce hypertrophy and/or trigger cell death. Here, we review the sources of ROS in the heart, the modes of pathological activation of ROS formation as well as therapeutic approaches that may target ROS specifically in mitochondria.

Insecticidal action of the botanical insecticide wilforine on Mythimna separata (Walker) related with the changes of ryanodine receptor expression

The detailed molecular mechanism of wilforine, a novel botanical insecticidal component, remains unclear, except for the knowledge that it affects the calcium signaling pathway. The aim of the current study was to examine the underlying molecular mechanism of wilforine in Mythimna separata (Walker) by transcriptome and RNA interference (RNAi), with chlorantraniliprole as control. RNA sequencing showed that the relative expression of genes related to the calcium signaling pathway and muscle contraction in M. separata treated with wilforine significantly changed and was further validated by qRT-PCR. Interestingly, the expression level of the ryanodine receptor (MsRyR) gene was downregulated by wilforine at relatively high concentrations and long treatment time, contrary to that observed using chlorantraniliprole. Furthermore, a putative MsRyR was cloned using a 16,258-bp contiguous sequence containing a 308-bp 5'-untranslated region and 578-bp 3'-untranslated region by RT-PCR and RACE. The results of the RNAi experiment showed that injection of dsMsRyR significantly reduced MsRyR mRNA levels, and growth and development were inhibited. Importantly, silencing of the MsRyR gene resulted in decreased susceptibility to both wilforine and chlorantraniliprole. Together with the results of our previous studies on toxic symptoms and muscle tissue lesions between wilforine and chlorantraniliprole, we propose that RyR Ca²⁺ release channel dysfunction is closely related with significant lethal mechanisms of wilforine.

4-Pyridinio-1,4-Dihydropyridines as Calcium Ion Transport Modulators: Antagonist, Agonist, and Dual Action

A set of six new 4-pyridinio-1,4-dihydropyridine (1,4-DHP) compounds has been synthesized. The calcium channel modulating activity of these compounds was evaluated in an aorta vascular smooth muscle cell line (A7R5), in an isolated rat aortic ring model, and in human neuroblastoma cell lines (SH-SY5Y). The antagonistic effect of these 1,4-DHP was tested by modulating the impact of carbachol-dependent mobilization of intracellular Ca²⁺ in SH-SY5Y cells. The intracellular free Ca²⁺ concentration was measured in confluent monolayers of SH-SY5Y cells and A7R5 cells with the Ca²⁺-sensitive fluorescent indicator Fluo-4 NW. Only four compounds showed calcium channel blocking activity in SH-SY5Y and A7R5 cells as well as in the aortic ring model. Among them, compound 3 was the most active calcium channel antagonist, which had 3 times higher activity on carbachol-activated SH-SY5Y cells than amlodipine. Two of the compounds were inactive. Compound 4 had 9 times higher calcium agonist activity than the classic DHP calcium agonist Bay K8644. The intracellular mechanism for the action of compound 4 using inhibitor analysis was elucidated. Nicotinic as well as muscarinic receptors were not involved. Sarcoplasmic reticulum (ER) Ca²⁺ (SERCA) stores were not affected. Ryanodine receptors (RyRs), another class of intracellular Ca²⁺ releasing channels, participated in the agonist response evoked by compound 4. The electrooxidation data suggest that the studied compounds could serve as antioxidants in OS.

Effects of natural polyphenol-rich pomegranate juice supplementation on plasma ion and lipid profiles following resistance exercise: a placebo-controlled trial

Background

Pomegranate juice (POMj) contains abundant soluble polyphenolic antioxidant compounds and is recommended for its cardioprotective/atheroprotective properties. However, very few studies have investigated the efficacy of POMj supplementation to alter physiological responses during intensive physical exercise. This placebo-controlled study aimed to examine whether supplementation with natural polyphenol-rich-POMj could influence the ionic or lipid responses to an intensive resistance training session in elite athletes.

Methods

Nine elite weightlifters (21 ± 1 years) performed two Olympic-weightlifting sessions after ingesting placebo and POMj supplements. Venous blood samples were collected at rest and 3 min after each session for assessment of plasma sodium ([Na⁺]), potassium ([K⁺]), chloride ([Cl^-]), calcium ([Ca²⁺]), triglyceride ([TG]) and high-density lipoprotein ([HDL-C]), low-density lipoprotein ([HDL-C]) and total ([TC]) cholesterol concentrations.

Results

Plasma [K⁺] and [TG] were lowered post-exercise compared to resting values in the PLA condition (p = 0.03 for K⁺ and p = 0.02 for TG) with no pre-to-post exercise differences in the other plasma ion and lipid markers (p > 0.05). Compared to rest, plasma [Na⁺] and [Cl^-] were increased (p = 0.04, %change = 4.10% for Na⁺ and p = 0.02, %change = 4.44% for Cl^-), but there were no differences in the other plasma ion or lipid markers post-exercise after POMj supplementation (p > 0.05). Post-exercise plasma [Na⁺], [Cl^-], and [HDL-C] were greater following POMj supplementation compared to PLA (p = 0.01 for Cl- and HDL-C, p = 0.02 for Na+, and p = 0.04 for TC), with no between-supplement post-exercise differences in the other ion and lipid markers (p > 0.05).

Conclusion

In conclusion, supplementation with POMj has the potential to attenuate the acute imbalance of plasma [K⁺] and to improve blood lipid responses (i.e., HDL-C) following resistance exercises in elite weightlifters. However, further large research in both athletic and non-athletic populations is needed to corroborate these preliminary observations and to elucidate the potential underlying mechanisms and translational potential of our novel observations.

Trial registration

Name of the registry:ClinicalTrials.gov PRSThe registration number:NCT02697903.Date of Registry: 03/03/2016 'Retrospectively registered'.The registration title: Pomegranate Improve Biological Recovery Kinetics in Elite Weightlifter.

Graphical abstract

Supramolecular and Liquid Crystalline Contributions to the Assembly of Myofibril

We compare steps observed during the fibrillogenesis of myofibrils with the sequence of steps predictable by a recent analysis of the structurization and functioning of striated muscles. The predicted assembly steps are based solely on fundamental equilibrium processes, particularly supramolecular interactions and liquid crystalline alignment of the rigid thick and thin filaments hosted within the sarcomer. Satisfactory agreement is obtained between several of the observed and the predicted fibrillogenesis steps. In several cases, however, the actual steps appear to be more complex than expected, evidencing the occurrence of transport and kinetic pathways that may assist the attainment of the equilibrium structure. The memory of the order of a precursor mesophase is imprinted during the remodeling of the surfaces at which the two sets of filaments are anchored. The relevance of the present analysis to the functioning of the myofibril is considered.

Comparison of Activity, Expression, and S-Nitrosylation of Calcium Transfer Proteins between Pale, Soft, and Exudative and Red, Firm, and Non-exudative Pork during Post-Mortem Aging

The research was performed to investigate the difference of activity, expression, and S-nitrosylation of calcium transfer proteins between pale, soft, and exudative (PSE) and red, firm, and non-exudative (RFN) pork. Seven PSE and seven RFN pork longissimus thoracis (LT) muscles were chosen according to pH and L* at 1 h post-mortem and identified by drip loss at 24 h. The nitric oxide synthase (NOS) activity and neuronal nitric oxide synthase (nNOS) expression showed a significant difference between two groups ( p < 0.05). PSE meat had a considerably higher sarcoplasmic calcium concentration compared to RFN meat at 1 h post-mortem aging ( p < 0.05). In PSE meat, the expression of ryanodine receptor 1 (RyR1) and sarcoplasmic reticulum calcium ATPase 1 (SERCA1) was lower than that in RFN meat, while the relative S-nitrosylation level of RyR1 and SERCA1 was higher ( p < 0.05). In addition, a lower activity of SERCA was detected in PSE meat compared to RFN meat ( p < 0.05). Those results indicate that S-nitrosylation of RyR1 and SERCA1 can putatively play a crucial part in regulating calcium homeostasis. A high level of RyR1 and SERCA1 S-nitrosylation can induce the imbalance of calcium in cytoplasm, leading to accelerated pH decline and the development of PSE meat.

Myofibers

Muscle tissue is a highly specialized type of tissue, made up of cells that have as their fundamental properties excitability and contractility. The cellular elements that make up this type of tissue are called muscle fibers, or myofibers, because of the elongated shape they have. Contractility is due to the presence of myofibrils in the muscle fiber cytoplasm, as large cellular assemblies. Also, myofibers are responsible for the force that the muscle generates which represents a countless aspect of human life. Movements due to muscles are based on the ability of muscle fibers to use the chemical energy procured in metabolic processes, to shorten and then to return to the original dimensions. We describe in detail the levels of organization for the myofiber, and we correlate the structural aspects with the functional ones, beginning with neuromuscular transmission down to the biochemical reactions achieved in the sarcoplasmic reticulum by the release of Ca²⁺ and the cycling of crossbridges. Furthermore, we are reviewing the types of muscle contractions and the fiber-type classification.

Activation of histamine H3 receptor decreased cytoplasmic Ca(2+) imaging during electrical stimulation in the skeletal myotubes

Histamine is a neurotransmitter and chemical mediator in multiple physiological processes. Histamine H3 receptor is expressed in the nervous system, heart, and gastrointestinal tract; however, little is known about H3 receptor in skeletal muscle. The aim of this study was to investigate the role of H3 receptor in skeletal myotubes. The expression of H3 receptor and myosin heavy chain (MHC), a late myogenesis marker, was assessed by real-time PCR and immunostaining in C2C12 skeletal myogenesis and adult mid-urethral skeletal muscle tissues. H3 receptor mRNA showed a significant increase upon differentiation of C2C12 into myotubes: 1-, 26-, 91-, and 182-fold at days 0, 2, 4, and 6, respectively. H3 receptor immunostaining in differentiated C2C12 cells and adult skeletal muscles was positive and correlated with that of MHC. The functional role of H3receptor in differentiated myotubes was assessed using an H3 receptor agonist, (R)-a-methylhistamine ((R)-α-MeHA). Ca(2+) imaging, stimulated by electric pacing, was decreased by 55% after the treatment of mature C2C12 myotubes with 1μM (R)-α-MeHA for 10min and 20min, while treatment with 100nm (R)-α-MeHA for 5min caused 45% inhibition. These results suggested that H3 receptor may participate in the maintenance of the relaxed state and prevention of over-contraction in mature differentiated myotubes. The elucidation of the role of H3R in skeletal myogenesis and adult skeletal muscle may open a new direction in the treatment of skeletal muscle disorders, such as muscle weakness, atrophy, and myotonia in motion systems or peri-urethral skeletal muscle tissues.

Evidence for a new allele at the SERCA1 locus affecting pork meat quality in part through the imbalance of Ca2+ homeostasis

Sarcoendoplasmic reticulum Ca2+-ATPase 1 (SERCA1) as a Ca2+ release channel plays a key role in the relaxation of skeletal muscle through pumping cytosolic Ca2+ into the SR (sarcoplasmic reticulum). In this study, a novel single nucleotide polymorphism (SNP) in exon 8 (C > T) was detected by tetra-primer ARMS-PCR and the tissue expression pattern of SERCA1 was analyzed in eleven tissues. A model of primary skeletal muscle cells in vitro exposed to dexamethasone (DEX, a synthetic corticosteroid) was also employed to determine whether stress hormones cause an increase in intracellular Ca2+ concentration that is associated with alteration in SERCA1 and in turn subsequently affect meat quality. The results showed that the CC genotype has lower content intramuscular fat and higher water than pig carrying the genotype CT and CC. In addition, the additive effects were both significantly (P < 0.05) and allele T seemed to be associate with increase in intramuscular fat, while decrease in water content. Accompanied with previous studies, the high abundance of porcine SERCA1 was found in skeletal muscle tissue. DEX markedly down-regulated the expression of SERCA1, leading to Ca2+ overload. Furthermore, the imbalance of Ca2+ homeostasis up-regulated the transcription level of Calpain1. Taken together, we demonstrated a novel mechanism that the changes in expression of SERCA1 potential disturb the normal Ca2+ channel as well as the balance of Ca2+ homeostasis and which in turn finally activated Ca2+-dependent proteases such as Calpain1 which could affect meat quality.

Solid-state NMR measurements of the kinetics of the interaction between phospholamban and Ca2 + -ATPase in lipid bilayers

Phospholamban (PLB) is a small transmembrane protein that regulates calcium transport across the sarcoplasmic reticulum (SR) of cardiac cells via a reversible inhibitory interaction with Ca2+-ATPase. In this work solid-state NMR methods have been used to investigate the dynamics of the inhibitory association between PLB and Ca2+-ATPase. Skeletal muscle Ca2+-ATPase was incorporated into phosphatidylcholine membranes together with a ten-fold excess of a null-cysteine mutant of PLB labelled with 13C at Leu-44 in the transmembrane domain ([alpha-13C-L44]AAA-PLB). In these membranes the PLB variant was found to partially inhibit Ca2+-ATPase by reducing the affinity of the enzyme for calcium. Cross-polarization magic angle spinning (CP-MAS) 13C NMR spectra of the membranes exhibited a signature peak from [alpha-13C-L44]AAA-PLB at 56 ppm. Changes in the intensity of the peak were observed at different temperatures, which was diagnostic of direct interaction between [alpha-13C-L44]AAA-PLB and Ca2+-ATPase. Measurements of dipolar couplings between the 13C label and neighbouring protons were analysed to show that the mean residency time for the association of AAA-PLB with Ca2+-ATPase was on the order of 2.5 ms at temperatures between 0 degrees C and 30 degrees C. This new NMR approach will be useful for examining how the association of the two proteins is affected by physiological stimuli such as kinases and the elevation of calcium concentration.

Expression of SERCA isoform with faster Ca2+ transport properties improves postischemic cardiac function and Ca2+ handling and decreases myocardial infarction

Myocardial ischemia-reperfusion (I/R) injury is associated with contractile dysfunction, arrhythmias, and myocyte death. Intracellular Ca(2+) overload with reduced activity of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) is a critical mechanism of this injury. Although upregulation of SERCA function is well documented to improve postischemic cardiac function, there are conflicting reports where pharmacological inhibition of SERCA improved postischemic function. SERCA2a is the primary cardiac isoform regulating intracellular Ca(2+) homeostasis; however, SERCA1a has been shown to substitute SERCA2a with faster Ca(2+) transport kinetics. Therefore, to further address this issue and to evaluate whether SERCA1a expression could improve postischemic cardiac function and myocardial salvage, in vitro and in vivo myocardial I/R studies were performed on SERCA1a transgenic (SERCA1a(+/+)) and nontransgenic (NTG) mice. Langendorff-perfused hearts were subjected to 30 min of global ischemia followed by reperfusion. Baseline preischemic coronary flow and left ventricular developed pressure were significantly greater in SERCA1a(+/+) mice compared with NTG mice. Independent of reperfusion-induced oxidative stress, SERCA1a(+/+) hearts demonstrated greatly improved postischemic (45 min) contractile recovery with less persistent arrhythmias compared with NTG hearts. Morphometry showed better-preserved myocardial structure with less infarction, and electron microscopy demonstrated better-preserved myofibrillar and mitochondrial ultrastructure in SERCA1a(+/+) hearts. Importantly, intraischemic Ca(2+) levels were significantly lower in SERCA1a(+/+) hearts. The cardioprotective effect of SERCA1a was also observed during in vivo regional I/R with reduced myocardial infarct size after 24 h of reperfusion. Thus SERCA1a(+/+) hearts were markedly protected against I/R injury, suggesting that expression of SERCA 1a isoform reduces postischemic Ca(2+) overload and thus provides potent myocardial protection.

Solid-state NMR and functional measurements indicate that the conserved tyrosine residues of sarcolipin are involved directly in the inhibition of SERCA1

The transmembrane protein sarcolipin regulates calcium storage in the sarcoplasmic reticulum of skeletal and cardiac muscle cells by modulating the activity of sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs). The highly conserved C-terminal region ((27)RSYQY-COOH) of sarcolipin helps to target the protein to the sarcoplasmic reticulum membrane and may also participate in the regulatory interaction between sarcolipin and SERCA. Here we used solid-state NMR measurements of local protein dynamics to illuminate the direct interaction between the Tyr(29) and Tyr(31) side groups of sarcolipin and skeletal muscle Ca(2+)-ATPase (SERCA1a) embedded in dioleoylphosphatidylcholine bilayers. Further solid-state NMR experiments together with functional measurements on SERCA1a in the presence of NAc-RSYQY, a peptide representing the conserved region of sarcolipin, suggest that the peptide binds to the same site as the parent protein at the luminal face of SERCA1a, where it reduces V(max) for calcium transport and inhibits ATP hydrolysis with an IC(50) of approximately 200 microM. The inhibitory effect of NAc-RSYQY is remarkably sequence-specific, with the native aromatic residues being essential for optimal inhibitory activity. This combination of physical and functional measurements highlights the importance of aromatic and polar residues in the C-terminal region of sarcolipin for regulating calcium cycling and muscle contractility.

Molecular and Electrophysiological Bases of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disorder characterized by adrenergically mediated polymorphic ventricular tachyarrhythmias. Genetic investigations have identified two variants of the disease: an autosomal dominant form associated with mutations in the gene encoding the cardiac ryanodine receptor (RyR2) and a recessive form associated with homozygous mutations in the gene encoding the cardiac isoform of calsequestrin (CASQ2). Functional characterization of mutations identified in the RyR2 and CASQ2 genes has demonstrated that CPVT are caused by derangements of the control of intracellular calcium. Investigations in a knock-in mouse model have shown that CPVT arrhythmias are initiated by delayed afterdepolarizations and triggered activity. In the present article, we review clinical and molecular understanding of CPVT and discuss the most recent approaches to develop novel therapeutic strategies for the disease.

Prolactin stimulates prostate cell proliferation by increasing endoplasmic reticulum content due to SERCA 2b over-expression

Prolactin (PRL) has been shown to be involved in the differentiation and proliferation of numerous tissues, including the prostate gland. Moreover, variations in [Ca2+]ER (calcium concentration within the endoplasmic reticulum) may play a role in cell growth. However, few studies have focused on the regulation of calcium homoeostasis by prolactin. The present study evaluates the regulation of calcium pools as well as the possible role of [Ca2+]ER variations as a signal for growth modulation by PRL. We show that PRL stimulates the proliferation of normal SV40 immortalized epithelial prostate (PNT1A) cells with a maximum effect at a dose of 100 ng/ml. We also show that 100 ng/ml PRL increases the [Ca2+]ER when measured either by indirect quantification with Fura-2AM after application of 1 mM thapsigargin or by direct quantification with Mag-Fura-2AM within the endoplas-mic reticulum. Western blot analysis shows that the SERCA 2b (sarcoendoplasmic calcium ATPase 2b) is over-expressed in PNT1A cells treated with 100 ng/ml PRL for 24 h. A small inter-fering RNA SERCA 2a/b, used to down-regulate endogenous SERCA 2b expression, reduced both PNT1A cell proliferation and [Ca2+]ER. We thus identify [Ca2+]ER and SERCA 2b as protagonists in PRL-induced proliferation.

Pharmacologic modulation of skeletal muscle metabolism: a microdialysis study

Microdialysis is a valuable tool to measure tissue responses. We hypothesized that skeletal muscle metabolism can be modulated by microdialysis applied drugs which alter cytosolic calcium concentration. With approval of the local animal care committee, the hind limbs of sacrificed male Sprague Dawley rats were perfused either with Ringer's solution or with dantrolene 1 microM at 30 ml hr(-1) and 21 degrees. Microdialysis probes in both hind limbs were perfused at 1 microl min(-1) either with sorbitol 80 mM, calcium 20 mM, 40 mM, 80 mM, caffeine 40 mM, 80 mM, and halothane 10 vol% respectively, and at the contralateral adductor muscle with Ringer as control. Lactate was measured spectrophotometrically in the dialysate at 15 min. intervals. Lactate levels as measured by intramuscular microdialysis were not influenced by intramuscular application of sorbitol 80 mM compared to control measurements with Ringer's solution. Local application of calcium 20 mM, 40 mM, 80 mM, caffeine 40 mM, 80 mM, and halothane 10 vol% via microdialysis increased lactate concentrations, while organ perfusion by dantrolene 1 microM reduced the caffeine-induced lactate increase. Modulation of intramuscular lactate metabolism by exogenous compounds via microdialysis probes generates new insights in skeletal muscle metabolism.

Genetic bases of arrhythmogenic right ventricular Cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a heart muscle disease in which the pathological substrate is a fibro-fatty replacement of the right ventricular myocardium. The major clinical features are different types of arrhythmias with a left branch block pattern. ARVC shows autosomal dominant inheritance with incomplete penetrance. Recessive forms were also described, although in association with skin disorders.

Ten genetic loci have been discovered so far and mutations were reported in five different genes. ARVD1 was associated with regulatory mutations of transforming growth factor beta-3 (TGFβ3), whereas ARVD2, characterized by effort-induced polymorphic arrhythmias, was associated with mutations in cardiac ryanodine receptor-2 (RYR2). All other mutations identified to date have been detected in genes encoding desmosomal proteins: plakoglobin (JUP) which causes Naxos disease (a recessive form of ARVC associated with palmoplantar keratosis and woolly hair); desmoplakin (DSP) which causes the autosomal dominant ARVD8 and plakophilin-2 (PKP2) involved in ARVD9. Desmosomes are important cell-to-cell adhesion junctions predominantly found in epidermis and heart; they are believed to couple cytoskeletal elements to plasma membrane in cell-to-cell or cell-to-substrate adhesions.

Targeted Overexpression of Sarcolipin in the Mouse Heart Decreases Sarcoplasmic Reticulum Calcium Transport and Cardiac Contractility

The role of sarcolipin (SLN) in cardiac physiology was critically evaluated by generating a transgenic (TG) mouse model in which the SLN to sarco(endoplasmic)reticulum (SR) Ca(2+) ATPase (SERCA) ratio was increased in the ventricle. Overexpression of SLN decreases SR calcium transport function and results in decreased calcium transient amplitude and rate of relaxation. SLN TG hearts exhibit a significant decrease in rates of contraction and relaxation when assessed by ex vivo work-performing heart preparations. Similar results were also observed with muscle preparations and myocytes from SLN TG ventricles. Interestingly, the inhibitory effect of SLN was partially relieved upon high dose of isoproterenol treatment and stimulation at high frequency. Biochemical analyses show that an increase in SLN level does not affect PLB levels, monomer to pentamer ratio, or its phosphorylation status. No compensatory changes were seen in the expression of other calcium-handling proteins. These studies suggest that the SLN effect on SERCA pump is direct and is not mediated through increased monomerization of PLB or by a change in PLB phosphorylation status. We conclude that SLN is a novel regulator of SERCA pump activity, and its inhibitory effect can be reversed by beta-adrenergic agonists.

Physiological characterization of 45Ca2+ and 65Zn2+ transport by lobster hepatopancreatic endoplasmic reticulum

The crustacean hepatopancreas is an epithelial-lined, multifunctional organ that, among other activities, regulates the flow of calcium into and out of the animal's body throughout the life cycle. Transepithelial calcium flow across this epithelial cell layer occurs by the combination of calcium channels and cation exchangers at the apical pole of the cell and by an ATP-dependent, calcium ATPase in conjunction with a calcium channel and an Na+/Ca2+ antiporter in the basolateral cell region. The roles of intracellular organelles such as mitochondria, lysosomes, and endoplasmic reticulum (ER) in transepithelial calcium transport or in transient calcium sequestration are unclear, but may be involved in transferring cytosolic calcium from one cell pole to the other. The ER membrane has a complement of ATP-dependent calcium ATPases (SERCA) and calcium channels that regulate the uptake and possible transfer of calcium through this organelle during periods of intense calcium fluxes across the epithelium as a whole. This investigation characterized the mechanisms of calcium transport by lobster hepatopancreatic ER vesicles and the effects of drugs and heavy metals on them. Kinetic constants for 45Ca2+ influx under control conditions were K(n) (m)=10.38+/-1.01 microM, J(max)=14.75+/-1.27 pmol/mg protein x sec, and n=2.53+/-0.46. The Hill coefficient for 45Ca2+ influx under control conditions, approximating 2, suggests that approximately two calcium ions were transported for each transport cycle in the absence of ATP or the inhibitors. Addition of 1 mM ATP to the incubation medium significantly (P<0.01) elevated the rate of 45Ca2+ influx at all calcium activities used and retained the sigmoidal nature of the transport relationship. The kinetic constants for 45Ca2+ influx in the presence of 1 mM ATP were K(n) (m)=12.76+/-0.91 microM, J(max)=25.46+/-1.45 pmol/mg protein x sec, and n=1.95+/-0.15. Kinetic analyses of ER 65Zn2+ influx resulted in a sigmoidal relationship between transport rate and zinc activity under control conditions (K(n) (m)=38.63+/-0.52 microM, J(max)=19.35+/-0.17 pmol/mg protein x sec, n=1.81+/-0.03). The Addition of 1 mM ATP enhanced 65Zn2+ influx at each zinc activity, but maintained the overall sigmoidal nature of the kinetic relationship. The kinetic constants for zinc influx in the presence of 1 mM ATP were K(n) (m)=34.59+/-2.31 microM, J(max)=26.09+/-1.17 pmol/mg protein x sec, and n=1.96+/-0.17. Both sigmoidal and ATP-dependent calcium and zinc influxes by ER vesicles were reduced in the presence of thapsigargin and vanadate. This investigation found that lobster hepatopancreatic ER exhibited a thapsigargin- and vanadate-inhibited, SERCA-like, calcium ATPase. This transporter displayed cooperative calcium transport kinetics (Hill coefficient, n approximately 2.0) and was inhibited by the heavy metals zinc and copper, suggesting that the metals may reduce the binding and transport of calcium when they are present in the cytosol.

The cardiac ryanodine receptor (RyR2) and its role in heart disease

The cardiac ryanodine receptor has become a subject of increasing interest as its role in the etiology of cardiac disease is becoming more apparent. In this article, we review the current knowledge of the structure and function of the cardiac ryanodine receptor and its implications in cardiac pathophysiology. Cardiac ryanodine receptors function by regulating calcium release from the sarcoplasmic reticulum in cardiomyocytes, thereby playing an integral role in excitation-contraction coupling. In heart failure, the myocardium remains in a chronic hyperadrenergic state. This leads to protein kinase A hyperphosphorylation of ryanodine receptors within cardiomyocytes, ultimately leading to calcium leakage from the sarcoplasmic reticulum into the cytosol and thus impairing excitation-contraction coupling. These mechanisms could partially explain the pathophysiology underlying the reduced cardiac output seen in heart failure. Beta-adrenergic blockade appears to correct the abnormality and reestablishes normal ryanodine receptor function. These calcium leaks can also generate delayed afterdepolarizations, which can lead to fatal arrhythmias. Two genetic diseases have been linked to mutations in the cardiac ryanodine receptor: arrhythmogenic right ventricular dysplasia type 2 and catecholaminergic polymorphic ventricular tachycardia or familial polymorphic ventricular tachycardia. As our understanding of this receptor and its modulators deepens, the possibility of clinical application draws near.

Normal mode-based fitting of atomic structure into electron density maps: application to sarcoplasmic reticulum Ca-ATPase

A method for the flexible docking of high-resolution atomic structures into lower resolution densities derived from electron microscopy is presented. The atomic structure is deformed by an iterative process using combinations of normal modes to obtain the best fit of the electron microscopical density. The quality of the computed structures has been evaluated by several techniques borrowed from crystallography. Two atomic structures of the SERCA1 Ca-ATPase corresponding to different conformations were used as a starting point to fit the electron density corresponding to a different conformation. The fitted models have been compared to published models obtained by rigid domain docking, and their relation to the known crystallographic structures are explored by normal mode analysis. We find that only a few number of modes contribute significantly to the transition. The associated motions involve almost exclusively rotation and translation of the cytoplasmic domains as well as displacement of cytoplasmic loops. We suggest that the movements of the cytoplasmic domains are driven by the conformational change that occurs between nonphosphorylated and phosphorylated intermediate, the latter being mimicked by the presence of vanadate at the phosphorylation site in the electron microscopy structure.

NMR solution structure and topological orientation of monomeric phospholamban in dodecylphosphocholine micelles

Phospholamban is an integral membrane protein that regulates the contractility of cardiac muscle by maintaining cardiomyocyte calcium homeostasis. Abnormalities in association of protein kinase A with PLB have recently been linked to human heart failure, where a single mutation is responsible for dilated cardiomyopathy. To date, a high-resolution structure of phospholamban in a lipid environment has been elusive. Here, we describe the first structure of recombinant, monomeric, biologically active phospholamban in lipid-mimicking dodecylphosphocholine micelles as determined by multidimensional NMR experiments. The overall structure of phospholamban is "L-shaped" with the hydrophobic domain approximately perpendicular to the cytoplasmic portion. This is in agreement with our previously published solid-state NMR data. In addition, there are two striking discrepancies between our structure and those reported previously for synthetic phospholamban in organic solvents: a), in our structure, the orientation of the cytoplasmic helix is consistent with the amphipathic nature of these residues; and b), within the hydrophobic helix, residues are positioned on two discrete faces of the helix as consistent with their functional roles ascribed by mutagenesis. This topology renders the two phosphorylation sites, Ser-16 and Thr-17, more accessible to kinases.

A missense mutation in CASQ2 is associated with autosomal recessive catecholamine-induced polymorphic ventricular tachycardia in Bedouin families from Israel

Catecholamine-induced polymorphic ventricular tachycardia (CPVT) is characterized by episodes of syncope, seizures or sudden death, in response to physical activity or emotional stress, and affects mainly young children with morphologically normal hearts. We have recently described an autosomal recessive form of the disorder in seven families from a Bedouin tribe in the north of Israel, and mapped the disease-causing gene to chromosome 1p13-1p21. Direct sequencing of the calsequestrin 2 (CASQ2), a candidate gene from within the linkage interval, revealed a negatively charged aspartic acid change to a positively charged histidine at position 307 of the protein. CASQ2 serves as the major calcium reservoir within cardiac myocytes. This mutation occurs in a highly conserved residue of the protein. The implication of the calcium release cascade in this disease, may lead to a better understanding of the pathophysiologic events underlying ventricular tachycardia, and to the use of drugs directly involved in intracellular calcium control for the treatment of the CPVT patients.

Role of endoplasmic reticulum calcium content in prostate cancer cell growth regulation by IGF and TNFalpha

Variations in calcium concentration within the endoplasmic reticulum ([Ca(2+)](ER)) may play a role in cell growth. This study evaluates the regulation of calcium pools by growth modulators of prostate cancer (PC) cells, the insulin growth factor (IGF), and the tumor necrosis growth factor-alpha (TNFalpha) as well as evaluating the possible role of [Ca(2+)](ER) variations as signals for growth modulation. We show that IGF (5 ng/ml), which increases cell growth, induces an increase in [Ca(2+)](ER) whereas TNFalpha (1 ng/ml) which reduces cell proliferation and induces apoptosis, reduces [Ca(2+)](ER). IGF-induced [Ca(2+)](ER) increase is correlated to an overexpression of the sarcoendoplasmic calcium-ATPase 2B (SERCA2b), whereas TNFalpha-induced [Ca(2+)](ER) decrease is associated to a reduction in SERCA2b expression. Pretreatment with epidermal growth factors (EGF) or IGF does not prevent TNFalpha from affecting the induction of apoptosis, [Ca(2+)](ER) reduction and SERCA2b downregulation. Reduction in [Ca(2+)](ER) induced by thapsigargin (TG) (from 1 pM to 1 microM, 48 h) reduces LNCaP growth in a dose dependent manner and induces apoptosis when cells are treated with 1 microM TG. We also show that a transient TG application (1 pM, 1 nM, 1 microM 15 min) is insufficient to induce a long lasting decrease in [Ca(2+)](ER), since [Ca(2+)](ER) remains identical to the control for 48 h following TG application. These treatments (1 pM and 1 nM, 15 min) do not modify cell growth. However, TG (1 microM, 15 min) induces apoptosis. We thus identify [Ca(2+)](ER) and SERCA2b as a central targets for causing LNCaP PC cell life or death induced by growth modulators. Furthermore our results indicate that calcium pool contents can regulate cell growth.

What we don't know about the structure of ryanodine receptor calcium release channels

1. The ryanodine receptor (RyR) is the Ca2+ release channel in the sarcoplamic reticulum of skeletal and cardiac muscle and is essential for respiration and heart beat. The RyR channel releases Ca2+ from intracellular stores in a variety of other cell types, where it normally coexists with the inositiol 1,4,5-trisphosphate receptor (IP3R). The RyR and IP3R, forming a superfamily of homotetrameric ligand-gated intracellular Ca2+ channels, serve discrete functions: they can be located in independent Ca2+ stores with different activation mechanisms and can be coupled to different signalling pathways. 2. Although functional characteristics of the RyR have been investigated intensely, there remain major gaps in our knowledge about the structure of the protein, its ion-conducting pore, its ligand-binding sites and sites supporting the many protein/protein interactions that underlie the in vivo function of the channel. 3. Of particular importance are the transmembrane segments that form the membrane-spanning domain of the protein and the pore, define the conductance and selectivity of the channel and dictate the cytoplasmic and luminal domains and the overall protein structure. Hydropathy profiles predict between four and 12 transmembrane segments. One popular model shows four transmembrane segments in the C-terminal one-tenth of the protein. However, there is substantial evidence for a larger number of membrane-spanning segments located in both the C-terminal and central parts of the protein. 4. A model of the RyR pore based on the Streptomyces lividans KcsA channel structure is presented. Protein/protein interactions between the RyR and other regulatory proteins, as well as within the RyR subunit, are discussed.

Use of continuous-elution gel electrophoresis as a preparative tool for blot overlay analysis

Blot overlay techniques have long been used to directly visualize protein-protein interactions within membrane complexes. However, this approach is often hampered by the limited quantities of purified membrane proteins available for conjugation with marker molecules. Here we applied continuous-elution gel electrophoresis as a preparative alternative to isolate sufficient amounts of a homogeneous protein sample to be used as a peroxidase-labeled probe in blot overlays. Microsomal muscle proteins ranging from approximately 20 to 600 kDa were electrophoretically separated and various marker proteins present in eluted fractions were identified by immunoblotting. Since the supramolecular structure of calsequestrin has recently been determined, this terminal cisternae protein was isolated as a model protein for studying protein-protein interactions. In blot overlay assays, peroxidase-conjugated calsequestrin specifically bound to the ryanodine receptor, triadin, calsequestrin itself, and junctin, illustrating that the biological binding affinities are retained in electrophoretically prepared muscle proteins. Potential applications for differential blot overlay approaches and for analyzing pathophysiological preparations from dystrophic muscle were evaluated. Since continuous-elution gel electrophoresis can separate a wide range of differently sized proteins from subcellular fractions, our report indicates that this technique can be utilized for the rapid identification of protein-protein interactions in future high-throughput analyses of subproteomes.

Topology of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum (RyR1)

To define the topology of the skeletal muscle ryanodine receptor (RyR1), enhanced GFP (EGFP) was fused in-frame to the C terminus of RyR1, replacing a series of C-terminal deletions that started near the beginning or the end of predicted transmembrane helices M1-M10. The constructs were expressed in HEK-293 (human embryonic kidney cell line 293) or mouse embryonic fibroblast (MEF) cells, and confocal microscopy of intact and saponin-permeabilized cells was used to determine the subcellular location of the truncated fusion proteins. The fusion protein truncated after M3 exhibited uniform cytoplasmic fluorescence, which was lost after permeabilization, indicating that proposed M', M", M1, M2, and M3 sequences are not membrane-associated. The fusion protein truncated at the end of the M4-M5 loop and containing M4 was membrane-associated. All longer truncated fusion proteins were also associated with intracellular membranes. Mapping by protease digestion and extraction of isolated microsomes demonstrated that EGFP positioned after either M5, the N-terminal half of M7 (M7a), or M8 was located in the lumen, and that EGFP positioned after either M4, M6, the C-terminal half of M7 (M7b), or M10 was located in the cytoplasm. These results indicate that RyR1 contains eight transmembrane helices, organized as four hairpin loops. The first hairpin is likely to be made up of M4a-M4b. However, it could be made up from M3-M4, which might form a hairpin loop even though M3 alone is not membrane-associated. The other three hairpin loops are formed from M5-M6, M7a-M7b, and M8-M10. M9 is not a transmembrane helix, but it might form a selectivity filter between M8 and M10.

The FAD- and O(2)-dependent reaction cycle of Ero1-mediated oxidative protein folding in the endoplasmic reticulum

The endoplasmic reticulum (ER) supports disulfide formation through an essential protein relay involving Ero1p and protein disulfide isomerase (PDI). We find that in addition to having a tightly associated flavin adenine dinucleotide (FAD) moiety, yeast Ero1p is highly responsive to small changes in physiological levels of free FAD. This sensitivity underlies the dependence of oxidative protein folding on cellular FAD levels. FAD is synthesized in the cytosol but can readily enter the ER lumen and promote Ero1p-catalyzed oxidation. Ero1p then uses molecular oxygen as its preferred terminal electron acceptor. Thus Ero1p directly couples disulfide formation to the consumption of molecular oxygen, but its activity is modulated by free lumenal FAD levels, potentially linking disulfide formation to a cell's nutritional or metabolic status.

Solid-state NMR and rigid body molecular dynamics to determine domain orientations of monomeric phospholamban

Solid-state NMR spectroscopy, in conjunction with rigid body molecular dynamics calculations, shows that monomeric phospholamban in lipid bilayers has two distinct helical domains, with an interhelical angle within 60-100 degrees, ruling out the possibility of a continuous alpha-helical structure for this protein.

RYR1 mutations causing central core disease are associated with more severe malignant hyperthermia in vitro contracture test phenotypes

Malignant hyperthermia (MH) and central core disease (CCD) are autosomal dominant disorders of skeletal muscle. Susceptibility to MH is only apparent after exposure to volatile anesthetics and/or depolarizing muscle relaxants. CCD patients present with diffuse muscular weakness but are also at risk of MH. Mutations in RYR1 (19q13.1), encoding a skeletal muscle calcium release channel (ryanodine receptor), account for the majority of MH and CCD cases. Fifteen RYR1 N-terminal mutations are considered causative of MH susceptibility, five of which are also associated with CCD. In the first extensive UK population survey, eight of 15 mutations were detected in 85 out of 297 (29%) unrelated MH susceptible cases, with G2434R detected in 53 cases (18%). Mutation type was shown to affect significantly MH phenotypes (in vitro contracture test (IVCT) response to caffeine, halothane, and ryanodine). RYR1 mutations associated with both CCD and MH (R163C, R2163H, R2435H) had more severe caffeine and halothane response phenotypes than those associated with MH alone. Mutations near the amino terminal (R163C, G341R) had a relatively greater effect on responses to caffeine than halothane, with a significantly increased caffeine:halothane tension ratio compared to G2434R of the central domain. All phenotypes were more severe in males than females, and were also affected by muscle specimen size and viability. Discordance between RYR1 genotype and IVCT phenotype was observed in seven families (nine individuals), with five false-positives and four false-negatives. This represents the most extensive study of MH patient clinical and genetic data to date and demonstrates that RYR1 mutations involved in CCD are those associated with one end of the spectrum of MH IVCT phenotypes.

Screening for ryanodine receptor type 2 mutations in families with effort-induced polymorphic ventricular arrhythmias and sudden death: early diagnosis of asymptomatic carriers

OBJECTIVES

We sought to establish the role of genetic screening for ryanodine receptor type 2 (RyR2) gene mutations in families with effort-induced polymorphic ventricular arrhythmia (PVA), syncope and juvenile sudden death.

BACKGROUND

The RyR2 mutations have been associated with PVA, syncope and sudden death in response to physical or emotional stress.

METHODS

We studied 81 subjects (39 males and 42 females; mean age 31 +/- 20 years) belonging to eight families with pathogenic RyR2 mutations. All subjects underwent screening for RyR2 mutations, electrocardiography (ECG), 24-h Holter monitoring, signal-averaged electrocardiography (SAECG), two-dimensional echocardiography and exercise stress testing. Electrophysiologic (EP) study was performed in nine patients.

RESULTS

Six different RyR2 mutations were found in eight families. Forty-three family members carried the gene mutation. Of these, 28 (65%) showed effort-induced arrhythmic symptoms or signs and one died suddenly during follow-up. Family history revealed 19 juvenile cases of sudden death during effort or emotion. In two families sharing the same mutation, no subject presented with PVA during the stress test; thus, sudden death and syncope were the only clinical manifestations. The 12-lead ECG was normal in all but two subjects, whereas five patients showed positive late potentials on the SAECG. In 17 (39.5%) of 43 subjects, the two-dimensional echocardiogram revealed localized kinetic abnormalities and mild structural alterations of the right ventricle. The EP study was not able to induce PVA.

CONCLUSIONS

The absence of symptoms and PVA on the stress test in more than one-third of carriers of RyR2 mutations, as well as the lack of PVA inducibility by the EP study, underlies the importance of genetic screening for the early diagnosis of asymptomatic carriers and prevention of sudden death.

Drastic reduction of calsequestrin-like proteins and impaired calcium binding in dystrophic mdx muscle

Although the reduction in dystrophin-associated glycoproteins is the primary pathophysiological consequence of the deficiency in dystrophin, little is known about the secondary abnormalities leading to x-linked muscular dystrophy. As abnormal Ca(2+) handling may be involved in myonecrosis, we investigated the fate of key Ca(2+) regulatory membrane proteins in dystrophic mdx skeletal muscle membranes. Whereas the expression of the ryanodine receptor, the dihydropyridine receptor, the Ca(2+)-ATPase, and calsequestrin was not affected, a drastic decline in calsequestrin-like proteins of 150-220 kDa was observed in dystrophic microsomes using one-dimensional immunoblotting, two-dimensional immunoblotting with isoelectric focusing, diagonal two-dimensional blotting technique, and immunoprecipitation. In analogy, overall Ca(2+) binding was reduced in the sarcoplasmic reticulum of dystrophic muscle. The reduction in Ca(2+) binding proteins might be directly involved in triggering impaired Ca(2+) sequestration within the lumen of the sarcoplasmic reticulum. Thus disturbed sarcolemmal Ca(2+) fluxes seem to influence overall Ca(2+) homeostasis, resulting in distinct changes in the expression profile of a subset of Ca(2+) handling proteins, which might be an important factor in the progressive functional decline of dystrophic muscle fibers.

Structure and orientation of sarcolipin in lipid environments

Sarcolipin (SLN) is a 31 amino acid integral membrane protein that regulates Ca-ATPase activity in skeletal muscle. Here, we report the three-dimensional structure and topology of synthetic SLN in lipid environments, as determined by solution and solid-state NMR spectroscopy. 2D solution NMR experiments were performed on SLN solubilized in sodium dodecyl sulfate (SDS) micelles. We found that SLN adopts a highly defined alpha-helical conformation from F9 through R27, with a backbone RMSD of 0.65 A and a side chain RMSD of 1.66 A. The N-terminus (M1 through L8) and the C-terminus (S28 through Y31) are mostly unstructured. The orientation of the SLN was determined using one-dimensional (15)N NMR solid-state spectroscopy. The protein was incorporated into phospholipid bilayers prepared from a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine. The (15)N chemical shift solid-state spectra from selectively labeled SLN samples indicate that SLN orients perpendicularly to the plane of the membrane bilayers. These results support the proposed mechanism of Ca-ATPase regulation of SLN via protein-protein intramembranous interactions between the highly conserved transmembrane domains of the Ca-ATPase and the conserved transmembrane domain of SLN.

Ca2+ pools and cell growth. Evidence for sarcoendoplasmic Ca2+-ATPases 2B involvement in human prostate cancer cell growth control

The present study demonstrates for the first time that intracellular calcium-ATPases and calcium pool content are closely associated with prostate cancer LNCaP cell growth. Cell growth was modulated by changing the amount of epidermal growth factor, serum, and androgene in culture media. Using the microspectrofluorimetric method with Fura-2 and Mag Fura-2 as probes, we show that in these cells, the growth rate is correlated with intracellular calcium pool content. Indeed, an increased growth rate is correlated with an increase in the calcium pool filling state, whereas growth-inhibited cells show a reduced calcium pool load. Using Western blotting and immunocytochemistry, we show that endoplasmic reticulum calcium pump expression is closely linked to LNCaP cell growth, and are a common target of physiological stimuli that control cell growth. Moreover, we clearly demonstrate that inhibition of these pumps, using thapsigargin, inhibits LNCaP cell growth and prevents growth factor from stimulating cell proliferation. Our results thus provide evidence for the essential role of functional endoplasmic reticulum calcium pumps and calcium pool in control of prostate cancer LNCaP cell growth, raising the prospect of new targets for the treatment of prostate cancer.

A missense mutation in a highly conserved region of CASQ2 is associated with autosomal recessive catecholamine-induced polymorphic ventricular tachycardia in Bedouin families from Israel

Catecholamine-induced polymorphic ventricular tachycardia (PVT) is characterized by episodes of syncope, seizures, or sudden death, in response to physical activity or emotional stress. Two modes of inheritance have been described: autosomal dominant and autosomal recessive. Mutations in the ryanodine receptor 2 gene (RYR2), which encodes a cardiac sarcoplasmic reticulum (SR) Ca(2+)-release channel, were recently shown to cause the autosomal dominant form of the disease. In the present report, we describe a missense mutation in a highly conserved region of the calsequestrin 2 gene (CASQ2) as the potential cause of the autosomal recessive form. The CASQ2 protein serves as the major Ca(2+) reservoir within the SR of cardiac myocytes and is part of a protein complex that contains the ryanodine receptor. The mutation, which is in full segregation in seven Bedouin families affected by the disorder, converts a negatively charged aspartic acid into a positively charged histidine, in a highly negatively charged domain, and is likely to exert its deleterious effect by disrupting Ca(2+) binding.

The skeletal muscle channelopathies: basic science, clinical genetics and treatment

The human neurological channelopathies are a rapidly expanding group of mainly genetic conditions that are characterized by dysfunction of membrane-bound glycoproteins (ion channels). The skeletal muscle channelopathies were the first to be characterized in this group. In recent years significant progress has been made in our understanding of the molecular genetic and cellular electrophysiological bases of these disorders. DNA-based diagnosis is now a reality for many of the channelopathies. The advances made have implications for both genetic counselling and for tailoring treatment to specific channelopathies.

What the structure of a calcium pump tells us about its mechanism

The report of the crystal structure of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum in its Ca(2+)-bound form [Toyoshima, Nakasako and Ogawa (2000) Nature (London) 405, 647-655] provides an opportunity to interpret much kinetic and mutagenic data on the ATPase in structural terms. There are no large channels leading from the cytoplasmic surface to the pair of high-affinity Ca(2+) binding sites within the transmembrane region. One possible access pathway involves the charged residues in transmembrane alpha-helix M1, with a Ca(2+) ion passing through the first site to reach the second site. The Ca(2+)-ATPase also contains a pair of binding sites for Ca(2+) that are exposed to the lumen. In the four-site model for transport, phosphorylation of the ATPase leads to transfer of the two bound Ca(2+) ions from the cytoplasmic to the lumenal pair of sites. In the alternating four-site model for transport, phosphorylation leads to release of the bound Ca(2+) ions directly from the cytoplasmic pair of sites, linked to closure of the pair of lumenal binding sites. The lumenal pair of sites could involve a cluster of conserved acidic residues in the loop between M1 and M2. Since there is no obvious pathway from the high-affinity sites to the lumenal surface of the membrane, transport of Ca(2+) ions must involve a significant change in the packing of the transmembrane alpha-helices. The link between the phosphorylation domain and the pair of high-affinity Ca(2+) binding sites is probably provided by two small helices, P1 and P2, in the phosphorylation domain, which contact the loop between transmembrane alpha-helices M6 and M7.

Mutagenic analysis of functional residues in putative substrate-binding site and acidic domains of vacuolar H+-pyrophosphatase

Vacuolar H(+)-translocating inorganic pyrophosphatase (V-PPase) uses PP(i) as an energy donor and requires free Mg(2+) for enzyme activity and stability. To determine the catalytic domain, we analyzed charged residues (Asp(253), Lys(261), Glu(263), Asp(279), Asp(283), Asp(287), Asp(723), Asp(727), and Asp(731)) in the putative PP(i)-binding site and two conserved acidic regions of mung bean V-PPase by site-directed mutagenesis and heterologous expression in yeast. Amino acid substitution of the residues with alanine and conservative residues resulted in a marked decrease in PP(i) hydrolysis activity and a complete loss of H(+) transport activity. The conformational change of V-PPase induced by the binding of the substrate was reflected in the susceptibility to trypsin. Wild-type V-PPase was completely digested by trypsin but not in the presence of Mg-PP(i), while two V-PPase mutants, K261A and E263A, became sensitive to trypsin even in the presence of the substrate. These results suggest that the second acidic region is also implicated in the substrate hydrolysis and that at least two residues, Lys(261) and Glu(263), are essential for the substrate-binding function. From the observation that the conservative mutants K261R and E263D showed partial activity of PP(i) hydrolysis but no proton pump activity, we estimated that two residues, Lys(261) and Glu(263), might be related to the energy conversion from PP(i) hydrolysis to H(+) transport. The importance of two residues, Asp(253) and Glu(263), in the Mg(2+)-binding function was also suggested from the trypsin susceptibility in the presence of Mg(2+). Furthermore, it was found that the two acidic regions include essential common motifs shared among the P-type ATPases.

Mutations of the cardiac ryanodine receptor (RyR2) gene in familial polymorphic ventricular tachycardia

BACKGROUND

Familial polymorphic ventricular tachycardia is an autosomal-dominant, inherited disease with a relatively early onset and a mortality rate of approximately 30% by the age of 30 years. Phenotypically, it is characterized by salvoes of bidirectional and polymorphic ventricular tachycardias in response to vigorous exercise, with no structural evidence of myocardial disease. We previously mapped the causative gene to chromosome 1q42-q43. In the present study, we demonstrate that patients with familial polymorphic ventricular tachycardia have missense mutations in the cardiac sarcoplasmic reticulum calcium release channel (ryanodine receptor type 2 [RyR2]).

METHODS AND RESULTS

In 3 large families studied, 3 different RyR2 mutations (P2328S, Q4201R, V4653F) were detected and shown to fully cosegregate with the characteristic arrhythmic phenotype. These mutations were absent in the nonaffected family members and in 100 healthy controls. In addition to identifying 3 causative mutations, we identified a number of single nucleotide polymorphisms that span the genomic structure of RyR2 and will be useful for candidate-based association studies for other arrhythmic disorders.

CONCLUSIONS

Our data illustrate that mutations of the RyR2 gene cause at least one variety of inherited polymorphic tachycardia. These findings define a new entity of disorders of myocardial calcium signaling.