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

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

The mitochondrial permeability transition pore: an evolving concept critical for cell life and death

In this review, we summarize current knowledge of perhaps one of the most intriguing phenomena in cell biology: the mitochondrial permeability transition pore (mPTP). This phenomenon, which was initially observed as a sudden loss of inner mitochondrial membrane impermeability caused by excessive calcium, has been studied for almost 50 years, and still no definitive answer has been provided regarding its mechanisms. From its initial consideration as an in vitro artifact to the current notion that the mPTP is a phenomenon with physiological and pathological implications, a long road has been travelled. We here summarize the role of mitochondria in cytosolic calcium control and the evolving concepts regarding the mitochondrial permeability transition (mPT) and the mPTP. We show how the evolving mPTP models and mechanisms, which involve many proposed mitochondrial protein components, have arisen from methodological advances and more complex biological models. We describe how scientific progress and methodological advances have allowed milestone discoveries on mPTP regulation and composition and its recognition as a valid target for drug development and a critical component of mitochondrial biology.

Altered Organelle Calcium Transport in Ovarian Physiology and Cancer

Calcium levels have a huge impact on the physiology of the female reproductive system, in particular, of the ovaries. Cytosolic calcium levels are influenced by regulatory proteins (i.e., ion channels and pumps) localized in the plasmalemma and/or in the endomembranes of membrane-bound organelles. Imbalances between plasma membrane and organelle-based mechanisms for calcium regulation in different ovarian cell subtypes are contributing to ovarian pathologies, including ovarian cancer. In this review, we focused our attention on altered calcium transport and its role as a contributor to tumor progression in ovarian cancer. The most important proteins described as contributing to ovarian cancer progression are inositol trisphosphate receptors, ryanodine receptors, transient receptor potential channels, calcium ATPases, hormone receptors, G-protein-coupled receptors, and/or mitochondrial calcium uniporters. The involvement of mitochondrial and/or endoplasmic reticulum calcium imbalance in the development of resistance to chemotherapeutic drugs in ovarian cancer is also discussed, since Ca²⁺ channels and/or pumps are nowadays regarded as potential therapeutic targets and are even correlated with prognosis.

Thioredoxin silencing-induced cardiac supercontraction occurs through endoplasmic reticulum stress and calcium overload in chicken

The thioredoxin (Txn) system is the most crucial antioxidant defense mechanism in the myocardium, and hampering the Txn system may compromise cell survival. Calcium (Ca) imbalance is associated with a variety of cardiomyopathies, and dysregulation of Ca2+ homeostasis is often considered a critical starting point for heart disease. However, the roles of Txn and the Txn system in maintaining Ca2+ homeostasis in cardiomyocytes have been infrequently reported. Here, we examined the expression of genes associated with Ca2+ channels using a model of Txn suppression in cardiomyocyte cultures (siRNA and Txn inhibitor) and report that Txn knockdown can cause Ca2+ overload in the myocardial cytoplasm and release of endoplasmic reticulum (ER) Ca2+, which induces ER stress. Our results showed that Txn knockdown could lead to cytosolic Ca2+ overload through upregulated gene expression of Ca2+ channel-related genes in the cytoplasmic and ER membranes. Furthermore, we find that excessive Ca2+ concentrations in the cytoplasm may increase myocardial contraction, and heat shock proteins may play a protective role throughout the process. Our present study reveals a novel model of regulation for low Txn expression in myocardial injury.

Multivariate semi-Markov analysis of burst properties of multiconductance single ion channels

Patch clamp recordings from ion channels often show periods of repetitive activity, known as bursts, which are noticeably separated from each other by periods of inactivity. Depending on the type of channel, such recordings may exhibit (conductance) levels between the closed (zero) level and the fully open level. Properties of bursts are less subject to problems that arise from recording than are properties for individual sojourns at different levels, and study of bursting behaviour provides important information about the finer structure of the underlying channel gating process. For a general finite state space continuous-time Markov chain model allowing one or more nonzero conductance levels, the present paper establishes results about the semi-Markov structure of a single burst and of a sequence of bursts, and uses this in a unified approach to properties of both theoretical and empirical bursts. The distribution and moments of particular burst properties, including the total charge transfer, the total sojourn time and the total number of visits to specified conductance levels during a burst, are derived. Various extensions are also described.

Type 2 ryanodine receptors are highly sensitive to alcohol

Exposure to ethanol levels reached in circulation during alcohol intoxication (>10mM) constricts cerebral arteries in rats and humans. Remarkably, targets and mechanisms underlying this action remain largely unidentified. Artery diameter is regulated by myocyte Ca(2+) sparks, a vasodilatory signal contributed to by type 2 ryanodine receptors (RyR2). Using laser confocal microscopy in rat cerebral arteries and bilayer electrophysiology we unveil that ethanol inhibits both Ca(2+) spark and RyR2 activity with IC50<20 mM, placing RyR2 among the ion channels that are most sensitive to ethanol. Alcohol directly targets RyR2 and its lipid microenvironment, leading to stabilization of RyR2 closed states.

Genotype-phenotype correlations in recessive RYR1-related myopathies

Background

RYR1 mutations are typically associated with core myopathies and are the most common overall cause of congenital myopathy. Dominant mutations are most often associated with central core disease and malignant hyperthermia, and genotype-phenotype patterns have emerged from the study of these mutations that have contributed to the understanding of disease pathogenesis. The recent availability of genetic testing for the entire RYR1 coding sequence has led to a dramatic expansion in the identification of recessive mutations in core myopathies and other congenital myopathies. To date, no clear patterns have been identified in these recessive mutations, though no systematic examination has yet been performed.

Methods

In this study, we investigated genotype-phenotype correlations in a large combined cohort of unpublished (n = 14) and previously reported (n = 92) recessive RYR1 cases.

Results

Overall examination of this cohort revealed nearly 50% of cases to be non-core myopathy related. Our most significant finding was that hypomorphic mutations (mutations expected to diminish RyR1 expression) were enriched in patients with severe clinical phenotypes. We also determined that hypomorphic mutations were more likely to be encountered in non-central core myopathies. With analysis of the location of non-hypomorphic mutations, we found that missense mutations were generally enriched in the MH/CCD hotspots and specifically enriched in the selectivity filter of the channel pore.

Conclusions

These results support a hypothesis that loss of protein function is a key predictive disease parameter. In addition, they suggest that decreased RyR1 expression may dictate non-core related pathology though, data on protein expression was limited and should be confirmed in a larger cohort. Lastly, the results implicate abnormal ion conductance through the channel pore in the pathogenesis in recessive core myopathies. Overall, our findings represent a comprehensive analysis of genotype-phenotype associations in recessive RYR1-myopathies.

Subtype identification and functional characterization of ryanodine receptors in rat cerebral artery myocytes

Ryanodine receptors (RyRs) regulate contractility in resistance-size cerebral artery smooth muscle, yet their molecular identity, subcellular location, and phenotype in this tissue remain unknown. Following rat resistance-size cerebral artery myocyte sarcoplasmic reticulum (SR) purification and incorporation into POPE-POPS-POPC (5:3:2; wt/wt) bilayers, unitary conductances of 110 +/- 8, 334 +/- 15, and 441 +/- 27 pS in symmetric 300 mM Cs(+) were usually detected. The most frequent (34/40 bilayers) conductance (334 pS) decreased to Collapse

Key Words

• calcium-release channels
• vascular smooth muscle

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MESH Headings

• Animals
• Cells, Cultured
• Cerebral Arteries/chemistry
• Cerebral Arteries/cytology
• Cerebral Arteries/physiology
• Female
• Male
• Muscle Cells/chemistry
• Muscle Cells/physiology
• Protein Subunits/analysis
• Protein Subunits/physiology
• Rats
• Rats, Sprague-Dawley
• Ryanodine Receptor Calcium Release Channel/analysis
• Ryanodine Receptor Calcium Release Channel/physiology

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Grants

• AA11560 NIAAA NIH HHS
• R01 HL094378 NHLBI NIH HHS
• HL77424 NHLBI NIH HHS
• HL67061 NHLBI NIH HHS
• HL94378 NHLBI NIH HHS

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Affiliation(s)

Thirumalini Vaithianathan Department Pharmacology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA
Damodaran Narayanan
Maria T Asuncion-Chin
Loice H Jeyakumar
Jianxi Liu
Sidney Fleischer
Jonathan H Jaggar
Alejandro M Dopico

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Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

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

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

Functional groups of ryanodine receptors in rat ventricular cells

Ryanodine receptors (RyR2s) are ion channels in the sarcoplasmic reticulum (SR) that are responsible for Ca2+ release in rat ventricular myocytes. Localization of RyR2s is therefore crucial for our understanding of contraction and other Ca2+-dependent intracellular processes. Recent results (e.g. circular waves and Ca2+ sparks in perinuclear area) raised questions about the classical views of RyR2 distribution and organization within ventricular cells. A Ca2+ spark is a fluorescent signal reflecting the activation of a small group of RyR2s. Frequency and spatio-temporal characteristics of Ca2+ sparks depend on the state of cytoplasmic and intraluminal macromolecular complexes regulating cardiac RyR2 function. We employed electron microscopy, confocal imaging of spontaneous Ca2+ sparks and immunofluorescence to visualize the distribution of RyR2s in ventricular myocytes and to evaluate the local involvement of the macromolecular complexes in regulation of functional activity of the RyR2 group. An electron microscopy study revealed that the axial tubules of the transverse-axial tubular system probably do not have junctions with the network SR (nSR). The nSR was found to be wrapped around intermyofibrillar mitochondria and contained structures similar to feet of the junctional cleft. Treatment of ventricular myocytes with antibodies against RyR2 showed that in addition to the junctional SR, a small number of RyR2s can be localized at the middle of the sarcomere and in the zone of perinuclear mitochondria. Recordings of spontaneous Ca2+ sparks showed the existence of functional groups of RyR2s in these intracellular compartments. We found that within the sarcomere about 20% of Ca2+ sparks were not colocalized with the zone of the junctional or corbular SR (Z-line zone). The spatio-temporal characteristics of sparks found in the Z-line and A-band zones were very similar, whereas sparks from the zone of the perinuclear mitochondria were about 25% longer. Analysis of the initiation sites of Ca2+ sparks within the same junctional SR cluster suggested that 18-25 RyR2s are in the functional group producing a spark. Because of the similarity of the spatio-temporal characteristics of sarcomeric sparks and ultrastructural characteristics of nSR, we suggest that the functional groups of RyR2s in the middle of the sarcomere are macromolecular complexes of approximately 20 RyR2s with regulatory proteins. Our data allowed us to conclude that a significant number of functional RyR2s is located in the middle of the sarcomere and in the zone of perinuclear mitochondria. These RyR2s could contribute to excitation-contraction coupling, mitochondrial and nuclear signalling, and Ca2+-dependent gene regulation, but their existence raises many additional questions.

Intracellular calcium release and cardiac disease

Intracellular calcium release channels are present on sarcoplasmic and endoplasmic reticuli (SR, ER) of all cell types. There are two classes of these channels: ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (IP3R). RyRs are required for excitation-contraction (EC) coupling in striated (cardiac and skeletal) muscles. RyRs are made up of macromolecular signaling complexes that contain large cytoplasmic domains, which serve as scaffolds for proteins that regulate the function of the channel. These regulatory proteins include calstabin1/calstabin2 (FKBP12/FKBP12.6), a 12/12.6 kDa subunit that stabilizes the closed state of the channel and prevents aberrant calcium leak from the SR. Kinases and phosphatases are targeted to RyR2 channels and modulate RyR2 function in response to extracellular signals. In the classic fight or flight stress response, phosphorylation of RyR channels by protein kinase A reduces the affinity for calstabin and activates the channels leading to increased SR calcium release. In heart failure, a cardiac insult causes a mismatch between blood supply and metabolic demands of organs. The chronically activated fight or flight response leads to leaky channels, altered calcium signaling, and contractile dysfunction and cardiac arrhythmias.

Calstabin deficiency, ryanodine receptors, and sudden cardiac death

Altered cardiac ryanodine receptor (RyR2) function has an important role in heart failure and genetic forms of arrhythmias. RyR2 constitutes the major intracellular Ca2+ release channel in the cardiac sarcoplasmic reticulum (SR). The peptidyl-prolyl isomerase calstabin2 (FKBP12.6) is a component of the RyR2 macromolecular signaling complex. Calstabin2 binding to RyR2 is regulated by PKA phosphorylation of Ser2809 in RyR2. PKA phosphorylation of RyR2 decreases the binding affinity for calstabin2 and increases RyR2 open probability and sensitivity to Ca2+-dependent activation. In heart failure, a majority of studies have found that RyR2 becomes chronically PKA hyper-phosphorylated which depletes calstabin2 from the channel complex. Calstabin2 dissociation causes a diastolic SR Ca2+ leak contributing to depressed intracellular Ca2+ cycling and decreased cardiac contractility. Missense mutations linked to genetic forms of exercise-induced arrhythmias and sudden cardiac death also cause decreased calstabin2-binding affinity and leaky RyR2 channels. We review the importance of calstabin2 for RyR2 function and excitation-contraction coupling, and discuss new observations that implicate dysregulation of calstabin2 binding as a central mechanism for abnormal calcium cycling in heart failure and triggered arrhythmias.

Incidence of "quasi-ditags" in catalogs generated by Serial Analysis of Gene Expression (SAGE)

Background

Serial Analysis of Gene Expression (SAGE) is a functional genomic technique that quantitatively analyzes the cellular transcriptome. The analysis of SAGE libraries relies on the identification of ditags from sequencing files; however, the software used to examine SAGE libraries cannot distinguish between authentic versus false ditags ("quasi-ditags").

Results

We provide examples of quasi-ditags that originate from cloning and sequencing artifacts (i.e. genomic contamination or random combinations of nucleotides) that are included in SAGE libraries. We have employed a mathematical model to predict the frequency of quasi-ditags in random nucleotide sequences, and our data show that clones containing less than or equal to 2 ditags (which include chromosomal cloning artifacts) should be excluded from the analysis of SAGE catalogs.

Conclusions

Cloning and sequencing artifacts contaminating SAGE libraries could be eliminated using simple pre-screening procedure to increase the reliability of the data.

Imaging microdomain Ca2+ in muscle cells

Ca2+ ions passing through a single or a cluster of Ca2+-permeable channels create microscopic, short-lived Ca2+ gradients that constitute the building blocks of cellular Ca2+ signaling. Over the last decade, imaging microdomain Ca2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca2+ dynamics and has reshaped our understanding of Ca2+ signaling mechanisms. Major advances include the visualization of "Ca2+ sparks" as the elementary events of Ca2+ release from the sarcoplasmic reticulum (SR), "Ca2+ sparklets" produced by openings of single Ca2+-permeable channels, miniature Ca2+ transients in single mitochondria ("marks"), and SR luminal Ca2+ depletion transients ("scraps"). As a model system, a cardiac myocyte contains a 3-dimensional grid of 104 spark ignition sites, stochastic activation of which summates into global Ca2+ transients. Tracking intermolecular coupling between single L-type Ca2+ channels and Ca2+ sparks has provided direct evidence validating the local control theory of Ca2+-induced Ca2+ release in the heart. In vascular smooth muscle myocytes, Ca2+ can paradoxically signal both vessel constriction (by global Ca2+ transients) and relaxation (by subsurface Ca2+ sparks). These findings shed new light on the origin of Ca2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca2+ channels in intact cells.

Caffeine sensitivity of native RyR channels from normal and malignant hyperthermic pigs: effects of a DHPR II-III loop peptide

Enhanced sensitivity to caffeine is part of the standard tests for susceptibility to malignant hyperthermia (MH) in humans and pigs. The caffeine sensitivity of skeletal muscle contraction and Ca(2+) release from the sarcoplasmic reticulum is enhanced, but surprisingly, the caffeine sensitivity of purified porcine ryanodine receptor Ca(2+)-release channels (RyRs) is not affected by the MH mutation (Arg(615)Cys). In contrast, we show here that native malignant hyperthermic pig RyRs (incorporated into lipid bilayers with RyR-associated lipids and proteins) were activated by caffeine at 100- to 1000-fold lower concentrations than native normal pig RyRs. In addition, the results show that the mutant ryanodine receptor channels were less sensitive to high-affinity activation by a peptide (C(S)) that corresponds to a part of the II-III loop of the skeletal dihydropyridine receptor (DHPR). Furthermore, subactivating concentrations of peptide C(S) enhanced the response of normal pig and rabbit RyRs to caffeine. In contrast, the caffeine sensitivity of MH RyRs was not enhanced by the peptide. These novel results showed that in MH-susceptible pig muscles 1). the caffeine sensitivity of native RyRs was enhanced, 2). the sensitivity of RyRs to a skeletal II-III loop peptide was depressed, and 3). an interaction between the caffeine and peptide C(S) activation mechanisms seen in normal RyRs was lost.

Vesl/Homer proteins regulate ryanodine receptor type 2 function and intracellular calcium signaling

Cellular signaling proteins such as metabotropic glutamate receptors, Shank, and different types of ion channels are physically linked by Vesl (VASP/Ena-related gene up-regulated during seizure and LTP)/Homer proteins [Curr. Opin. Neurobiol. 10 (2000) 370; Trends Neurosci. 23 (2000) 80; J. Cell Sci. 113 (2000) 1851]. Vesl/Homer proteins have also been implicated in differentiation and physiological adaptation processes [Nat. Neurosci. 4 (2001) 499; Nature 411 (2001) 962; Biochem. Biophys. Res. Commun. 279 (2000) 348]. Here we provide evidence that a Vesl/Homer subtype, Vesl-1L/Homer-1c (V-1L), reduces the function of the intracellular calcium channel ryanodine receptor type 2 (RyR2). In contrast, Vesl-1S/Homer-1a (V-1S) had no effect on RyR2 function but reversed the effects of V-1L. In live cells, in calcium release studies and in single-channel electrophysiological recordings of RyR2, V-1L reduced RyR2 activity. Important physiological functions and pharmacological properties of RyR2 are preserved in the presence of V-1L. Our findings demonstrate that a protein-protein interaction between V-1L and RyR2 is not only necessary for organizing the structure of intracellular calcium signaling proteins [Curr. Opin. Neurobiol. 10 (2000) 370; Trends Neurosci. 23(2000)80; J. Cell Sci. 113 (2000) 1851; Nat Neurosci. 4 (2001) 499; Nature 411 (2001) 962; Biochem. Biophys. Res. Commun. 279 (2000) 348; Nature 386 (1997) 284], but that V-1L also directly regulates RyR2 channel activity by changing its biophysical properties. Thereby it may control cellular calcium homeostasis. These observations suggest a novel mechanism for the regulation of RyR2 and calcium-dependent cellular functions.

Dysregulated ryanodine receptors mediate cellular toxicity: restoration of normal phenotype by FKBP12.6

Ca2+ homeostasis is a vital cellular control mechanism in which Ca2+ release from intracellular stores plays a central role. Ryanodine receptor (RyR)-mediated Ca2+ release is a key modulator of Ca2+ homeostasis, and the defective regulation of RyR is pathogenic. However, the molecular events underlying RyR-mediated pathology remain undefined. Cells stably expressing recombinant human RyR2 (Chinese hamster ovary cells, CHOhRyR2) had similar resting cytoplasmic Ca2+ levels ([Ca2+]c) to wild-type CHO cells (CHOWT) but exhibited increased cytoplasmic Ca2+ flux associated with decreased cell viability and proliferation. Intracellular Ca2+ flux increased with human RyR2 (hRyR2) expression levels and determined the extent of phenotypic modulation. Co-expression of FKBP12.6, but not FKBP12, or incubation of cells with ryanodine suppressed intracellular Ca2+ flux and restored normal cell viability and proliferation. Restoration of normal phenotype was independent of the status of resting [Ca2+]c or ER Ca2+ load. Heparin inhibition of endogenous inositol trisphosphate receptors (IP3R) had little effect on intracellular Ca2+ handling or viability. However, purinergic stimulation of endogenous IP3R resulted in apoptotic cell death mediated by hRyR2 suggesting functional interaction occurred between IP3R and hRyR2 Ca2+ release channels. These data demonstrate that defective regulation of RyR causes altered cellular phenotype via profound perturbations in intracellular Ca2+ signaling and highlight a key modulatory role of FKBP12.6 in hRyR2 Ca2+ channel function.

Targeting of protein kinase A by muscle A kinase-anchoring protein (mAKAP) regulates phosphorylation and function of the skeletal muscle ryanodine receptor

Protein kinase A anchoring proteins (AKAPs) tether cAMP-dependent protein kinase (PKA) to specific subcellular locations. The muscle AKAP, mAKAP, co-localizes with the sarcoplasmic reticulum Ca2+ release channel or ryanodine receptor (RyR). The purpose of this study was to determine whether anchoring of PKA by mAKAP regulates RyR function. Either mAKAP or mAKAP-P, which is unable to anchor PKA, was expressed in CHO cells stably expressing the skeletal muscle isoform of RyR (CHO-RyR1). Immunoelectron microscopy showed that mAKAP co-localized with RyR1 in disrupted skeletal muscle. Following the addition of 10 microm forskolin to activate adenylyl cyclase, RyR1 phosphorylation in CHO-RyR1 cells expressing mAKAP increased by 42.4 +/- 6.6% (n = 4) compared with cells expressing mAKAP-P. Forskolin treatment alone did not increase the amplitude of the cytosolic Ca2+ transient in CHO-RyR1 cells expressing mAKAP or mAKAP-P; however, forskolin plus 10 mm caffeine elicited a cytosolic Ca2+ transient, the amplitude of which increased by 22% (p < 0.05) in RyR1/mAKAP-expressing cells compared with RyR1/mAKAP-P-expressing cells. Therefore, localization of PKA by mAKAP at RyR1 increases both PKA-dependent RyR phosphorylation as well as efflux of Ca2+ through the RyR. Therefore, RyR1 function is regulated by mAKAP targeting of PKA, implying an important functional role for PKA phosphorylation of RyR in skeletal muscle.

In situ modulation of the human cardiac ryanodine receptor (hRyR2) by FKBP12.6

The ryanodine receptor complex (RyR), a large oligomeric assembly that functions as a Ca(2+)-release channel in the sarcoplasmic reticulum (SR)/endoplasmic reticulum (ER), comprises four RyR subunits and four FK506-binding proteins (FKBP). The precise mode of interaction and modulation of the cardiac RyR (RyR2) channel by FKBP12/FKBP12.6 remains to be fully defined. We have generated a series of Chinese-hamster ovary (CHO) cell lines stably expressing discrete levels of recombinant human RyR2 (hRyR2) (CHO(hRyR2)). Confocal microscopy of CHO(hRyR2) cells co-expressing either FKBP12 or FKBP12.6 demonstrated that FKBP12.6 was sequestered from the cytoplasm to ER membranes as the cellular levels of hRyR2 increased. There was negligible hRyR2-induced subcellular redistribution of FKBP12. The magnitude of Ca(2+) release in CHO(hRyR2) cells in response to stimulation by 4-chloro- m -cresol was in direct proportion to the expression levels of hRyR2. However, in CHO(hRyR2) cells co-expressing FKBP12.6, Ca(2+) release triggered by the addition of 4-chloro- m -cresol was markedly decreased. In contrast, co-expression of FKBP12 did not affect agonist-induced Ca(2+) release in CHO(hRyR2) cells. Resting cytoplasmic [Ca(2+)] in CHO(hRyR2) remained unaltered after co-expression of FKBP12 or FKBP12.6, but estimation of the ER Ca(2+) load status showed that co-expression of FKBP12.6, but not FKBP12, promoted superfilling of the ER Ca(2+) store which could not be released by RyR2 after agonist activation. The effects of FKBP12.6 on hRyR2-mediated intracellular Ca(2+) handling could be antagonized using rapamycin (5 microM). These results suggest that FKBP12.6 associates with hRyR2 in situ to modulate precisely the functionality of hRyR2 Ca(2+)-release channel.

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

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

Spatial characteristics of sarcoplasmic reticulum Ca2+ release events triggered by L-type Ca2+ current and Na+ current in guinea-pig cardiac myocytes

Ca2+ signals in cardiac muscle cells are composed of spatially limited elementary events termed Ca2+ sparks. Several studies have also indicated that Ca2+ signals smaller than Ca2+ sparks can be elicited. These signals have been termed Ca2+ quarks and were proposed to result from the opening of a single Ca2+ release channel of the sarcoplasmic reticulum. We used laser-scanning confocal microscopy to examine the subcellular properties of Na+ current (I(Na))- and L-type Ca2+ current (I(Ca,L))-induced Ca2+ transients in voltage-clamped ventricular myocytes isolated from guinea-pigs. Both currents, I(Na) and I(Ca,L), evoked substantial, global Ca2+ transients. To examine the spatiotemporal properties of such Ca2+ signals, we performed power spectral analysis of these Ca2+ transients and found that both lacked spatial frequency components characteristic for Ca2+ sparks. The application of 10 microM verapamil to partially block L-type Ca2+ current reduced the corresponding Ca2+ transients down to individual Ca2+ sparks. In contrast, I(Na)-induced Ca2+ responses were still spatially homogeneous and lacked Ca2+ sparks even for small current amplitudes. By using high resistance patch pipettes (> 4 MOmega) to exaggerate the loss of voltage control during I(Na), Ca2+ sparks appeared superimposed on a homogeneous Ca2+ release component and were exclusively triggered during the flow of I(Na). In the presence of 10 microM ryanodine both I(Ca,L) and I(Na) elicited small, residual Ca2+ transients that were spatially homogeneous but displayed distinctively different temporal profiles. We conclude that I(Na) is indeed able to cause Ca2+ release in guinea-pig ventricular myocytes. In contrast to I(Ca,L)-induced Ca2+ transients, which are built up from the recruitment of individual Ca2+ sparks, the I(Na)-evoked cellular responses were always homogeneous, indicating that their underlying elementary Ca2+ release event is distinct from the Ca2+ spark. Thus, I(Na)-induced Ca2+ transients are composed of smaller Ca2+ signals, most likely Ca2+ quarks.

RyR1 and RyR3 isoforms provide distinct intracellular Ca2+ signals in HEK 293 cells

Ryanodine receptors (RyRs) are expressed on the endoplasmic reticulum of many cells, where they form intracellular Ca2+-release channels that participate in the generation of intracellular Ca2+ signals. Here we report studies on the intracellular localisation and functional properties of transfected RyR1 or RyR3 channels in HEK 293 cells. Immunofluorescence studies indicated that both RyR1 and RyR3 did not form clusters but were homogeneously distributed throughout the endoplasmic reticulum. Ca2+ release experiments showed that transfected RyR1 and RyR3 channels responded to caffeine, although with different sensitivity, generating a global release of Ca2+ from the entire endoplasmic reticulum. However, video imaging and confocal microscopy analysis revealed that, in RyR3-expressing cells, local spontaneous Ca2+ release events were observed. No such spontaneous activity was observed in RyR1-expressing cells or in control cells. Interestingly, the spontaneous release events observed in RyR3-expressing cells were restricted to one or two regions of the endoplasmic reticulum, suggesting the formation of a further subcellular organisation of RyR3 in Ca2+ release units. These results demonstrate that different RyR isoforms can engage in the generation of distinct intracellular Ca2+ signals in HEK 293 cells.

The transmembrane segment of ryanodine receptor contains an intracellular membrane retention signal for Ca(2+) release channel

The ryanodine receptor (RyR) is a large homotetrameric protein with a hydrophobic domain at the C-terminal end that resides in the endoplasmic reticulum (ER) or sarcoplasmic reticulum membrane and forms the conduction pore of a Ca(2+) release channel. Our previous studies showed that RyR expressed in heterologous cells localized to the ER membrane. Confocal microscopic imaging indicated that the ER retention signal is likely present within the C-terminal portion of RyR, a region that contains four putative transmembrane segments. To identify the amino acid sequence responsible for ER retention of RyR, we expressed fusion proteins containing intercellular adhesion molecule (ICAM), various fragments of RyR, and green fluorescent protein (GFP) in Chinese hamster ovary and COS-7 cells. ICAM is a plasma membrane-resident glycoprotein and serves as a reporter for protein trafficking to the cell surface membrane. Imaging analyses indicated that ICAM-GFP fusion proteins with RyR sequence preceding the four transmembrane segments, ICAM-RyR-(3661-3993)-GFP, and with RyR sequence corresponding to transmembrane segments 1, 2, and 3, ICAM-RyR-(4558-4671)-GFP and ICAM-RyR-(4830-4919)-GFP, were localized to the plasma membrane; fusion proteins containing the fourth transmembrane segment of RyR, ICAM-RyR-(4913-4943)-GFP, were retained in the ER. Biochemical assay showed that ICAM-RyR-GFP fusion proteins that target to the plasma membrane are fully glycosylated, and those retained in the intracellular membrane are core-glycosylated. Together our data indicate that amino acids 4918-4943 of RyR contain the signal sequence for ER retention of the Ca(2+) release channel.

Multivariate semi-Markov analysis of burst properties of multiconductance single ion channels

Patch clamp recordings from ion channels often show periods of repetitive activity, known as bursts, which are noticeably separated from each other by periods of inactivity. Depending on the type of channel, such recordings may exhibit (conductance) levels between the closed (zero) level and the fully open level. Properties of bursts are less subject to problems that arise from recording than are properties for individual sojourns at different levels, and study of bursting behaviour provides important information about the finer structure of the underlying channel gating process. For a general finite state space continuous-time Markov chain model allowing one or more nonzero conductance levels, the present paper establishes results about the semi-Markov structure of a single burst and of a sequence of bursts, and uses this in a unified approach to properties of both theoretical and empirical bursts. The distribution and moments of particular burst properties, including the total charge transfer, the total sojourn time and the total number of visits to specified conductance levels during a burst, are derived. Various extensions are also described.

Molecular genetics of Ca(2+) stores and intracellular Ca(2+) signalling

An increasing number of studies based on recombinant cells and on mouse models that express an altered repertoire of some of the key components of the intracellular Ca(2+) release stores are becoming available as a result of molecular genetics techniques. Information from these studies, together with results from studies of human diseases caused by mutations in genes that encode proteins of the intracellular Ca(2+) stores, are providing a significant advancement in understanding the interactive nature of the molecular machinery that underlies intracellular Ca(2+) signalling and how the different components of the Ca(2+) stores contribute to the regulation of cellular functions.

Modifications of intracellular Ca2+ signalling during nerve growth factor-induced neuronal differentiation of rat adrenal chromaffin cells

Postnatal sympathetic neurons (SNs) and chromaffin cells (CCs) derive from neural crest precursors. CCs can differentiate in vitro into SN-like cells after nerve growth factor (NGF) exposure. This study examines changes of intracellular Ca2+ homeostasis and dynamics of CCs under conditions that promote a neuronal phenotype. Spontaneous Ca2+ fluctuations, a frequent observation in early cultures of CCs, diminished after > 10 days in vitro in control cells and ceased in NGF-treated ones. At the same time, Ca2+ rises resulting from entry upon membrane depolarization, gradually increased both their size and peak d[Ca2+]i/dt, resembling those recorded in SNs. Concomitantly, caffeine-induced Ca2+ rises, resulting from Ca2+ release from intracellular stores, increased their size and their peak d[Ca2+]i/dt by > 1000%, and developed transient and sustained release components, similar to those of SNs. The transient component, linked to regenerative Ca2+ release, appeared after > 10 days of NGF treatment, suggesting a delayed steep enhancement of Ca2+-induced Ca2+ release (CICR). Immunostaining showed that proteins coded by the three known isoforms of ryanodine receptors (RyRs) are present in CCs, but that only RyR2 increased significantly after NGF treatment. Since the transient release component increased more steeply than RyR2 immunostaining, we suggest that the development of robust CICR requires both an increased expression of RyRs and more efficient functional coupling among them. NGF-induced transdifferentiation of chromaffin cells involves the enhancement of both voltage-gated Ca2+ influx and Ca2+ release from intracellular stores. These modifications are likely to complement the extensive morphological and functional reorganization required for the replacement of the endocrine phenotype with the neuronal one.

Molecular cloning of cDNA encoding a drosophila ryanodine receptor and functional studies of the carboxyl-terminal calcium release channel

Ryanodine is a plant alkaloid that was originally used as an insecticide. To study the function and regulation of the ryanodine receptor (RyR) from insect cells, we have cloned the entire cDNA sequence of RyR from the fruit fly Drosophila melanogaster. The primary sequence of the Drosophila RyR contains 5134 amino acids, which shares approximately 45% identity with RyRs from mammalian cells, with a large cytoplasmic domain at the amino-terminal end and a small transmembrane domain at the carboxyl-terminal end. To characterize the Ca(2+) release channel activity of the cloned Drosophila RyR, we expressed both full-length and a deletion mutant of Drosophila RyR lacking amino acids 277-3650 (Drosophila RyR-C) in Chinese hamster ovary cells. For subcellular localization of the expressed Drosophila RyR and Drosophila RyR-C proteins, green fluorescent protein (GFP)-Drosophila RyR and GFP-Drosophila RyR-C fusion constructs were generated. Confocal microscopic imaging identified GFP-Drosophila RyR and GFP-Drosophila RyR-C on the endoplasmic reticulum membranes of transfected cells. Upon reconstitution into the lipid bilayer membrane, Drosophila RyR-C formed a large conductance cation-selective channel, which was sensitive to modulation by ryanodine. Opening of the Drosophila RyR-C channel required the presence of microM concentration of Ca(2+) in the cytosolic solution, but the channel was insensitive to inhibition by Ca(2+) at concentrations as high as 20 mM. Our data are consistent with our previous observation with the mammalian RyR that the conduction pore of the calcium release channel resides within the carboxyl-terminal end of the protein and further demonstrate that structural and functional features are essentially shared by mammalian and insect RyRs.

A negatively charged region of the skeletal muscle ryanodine receptor is involved in Ca(2+)-dependent regulation of the Ca(2+) release channel

The ryanodine receptor/Ca(2+) release channels from skeletal (RyR1) and cardiac (RyR2) muscle cells exhibit different inactivation profiles by cytosolic Ca(2+). D3 is one of the divergent regions between RyR1 (amino acids (aa) 1872-1923) and RyR2 (aa 1852-1890) and may contain putative binding site(s) for Ca(2+)-dependent inactivation of RyR. To test this possibility, we have deleted the D3 region from RyR1 (DeltaD3-RyR1), residues 1038-3355 from RyR2 (Delta(1038-3355)-RyR2) and inserted the skeletal D3 into Delta(1038-3355)-RyR2 to generate sD3-RyR2. The channels formed by DeltaD3-RyR1 and Delta(1038-3355)-RyR2 are resistant to inactivation by mM [Ca(2+)], whereas the chimeric sD3-RyR2 channel exhibits significant inactivation at mM [Ca(2+)]. The DeltaD3-RyR1 channel retains its sensitivity to activation by caffeine, but is resistant to inactivation by Mg(2+). The data suggest that the skeletal D3 region is involved in the Ca(2+)-dependent regulation of the RyR1 channel.