Personal recollections on the discovery of the ryanodine receptors of muscle

Molecular characterization and expression patterns of a ryanodine receptor in soybean looper, Chrysodeixis includens

Diamide insecticides, such as chlorantraniliprole, have been widely used to control insect pests by targeting the insect ryanodine receptor (RyR). Due to the efficacious insecticidal activity of diamides, as well as an increasing number of resistance cases, the molecular structure of RyR has been studied in many economically important insects. However, no research has been conducted on diamide resistance and RyR in the soybean looper, Chrysodeixis includens, a significant crop pest. In this study, we found moderate resistance to chlorantraniliprole in a field population from Puerto Rico and sequenced the full-length cDNA of the C. includens RyR gene, which encodes a 5124 amino acid-long protein. Genomic analysis revealed that the CincRyR gene consists of 113 exons, one of the largest exon numbers reported for RyR. Alternative splicing sites were detected in the cytosolic region. The protein sequence showed high similarity to other noctuid RyRs. Conserved structural features included the selectivity filter motif critical for ryanodine binding and ion conduction, as well as various domains involved in ion transport. Two mutation sites associated with diamide resistance in other insects were screened but not found in the Puerto Rico field populations or in the susceptible lab strain. Gene expression analysis indicated high expression of RyR in the third instar larval stage, particularly in muscle-containing tissues. Furthermore, exposure to a sublethal dose of chlorantraniliprole reduced RyR expression levels after 96 h. This study provides a molecular basis for understanding RyR structure and sheds light on potential mechanisms of diamide resistance in C. includens.

Communications between Mitochondria and Endoplasmic Reticulum in the Regulation of Metabolic Homeostasis

Mitochondria associated membranes (MAM), which are the contact sites between endoplasmic reticulum (ER) and mitochondria, have emerged as an important hub for signaling molecules to integrate the cellular and organelle homeostasis, thus facilitating the adaptation of energy metabolism to nutrient status. This review explores the dynamic structural and functional features of the MAM and summarizes the various abnormalities leading to the impaired insulin sensitivity and metabolic diseases.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

Endoplasmic Reticulum-Mitochondrial Contactology: Structure and Signaling Functions

Interorganellar contacts are increasingly recognized as central to the control of cellular behavior. These contacts, which typically involve a small fraction of the endomembrane surface, are local communication hubs that resemble synapses. We propose the term contactology to denote the analysis of interorganellar contacts. Endoplasmic reticulum (ER) contacts with mitochondria were recognized several decades ago; major roles in ion and lipid transfer, signaling, and membrane dynamics have been established, while others continue to emerge. The functional diversity of ER-mitochondrial (ER-mito) contacts is mirrored in their structural heterogeneity, with subspecialization likely supported by multiple, different linker-forming protein structures. The nanoscale size of the contacts has made studying their structure, function, and dynamics difficult. This review focuses on the structure of the ER-mito contacts, methods for studying them, and the roles of contacts in Ca²⁺ and reactive oxygen species (ROS) signaling.

Structural basis for conductance through TRIC cation channels

Mammalian TRICs function as K⁺-permeable cation channels that provide counter ions for Ca²⁺ handling in intracellular stores. Here we describe the structures of two prokaryotic homologues, archaeal SaTRIC and bacterial CpTRIC, showing that TRIC channels are symmetrical trimers with transmembrane pores through each protomer. Each pore holds a string of water molecules centred at kinked helices in two inverted-repeat triple-helix bundles (THBs). The pores are locked in a closed state by a hydrogen bond network at the C terminus of the THBs, which is lost when the pores assume an open conformation. The transition between the open and close states seems to be mediated by cation binding to conserved residues along the three-fold axis. Electrophysiology and mutagenesis studies show that prokaryotic TRICs have similar functional properties to those of mammalian TRICs and implicate the three-fold axis in the allosteric regulation of the channel.

Trimeric intracellular cation channels (TRICs) elicit K⁺ currents to counteract luminal negative potential during Ca²⁺ release from intracellular stores. Here the authors present structures of prokaryotic TRICs in their open and closed states, obtaining molecular insight into TRICs' function.

Direct imaging electron microscopy (EM) methods in modern structural biology: overview and comparison with X-ray crystallography and single-particle cryo-EM reconstruction in the studies of large macromolecules

Determining the structure of macromolecules is important for understanding their function. The fine structure of large macromolecules is currently studied primarily by X-ray crystallography and single-particle cryo-electron microscopy (EM) reconstruction. Before the development of these techniques, macromolecular structure was often examined by negative-staining, rotary-shadowing and freeze-etching EM, which are categorised here as 'direct imaging EM methods'. In this review, the results are summarised by each of the above techniques and compared with respect to four macromolecules: the ryanodine receptor, cadherin, rhodopsin and the ribosome-translocon complex (RTC). The results of structural analysis of the ryanodine receptor and cadherin are consistent between each technique. The results obtained for rhodopsin vary to some extent within each technique and between the different techniques. Finally, the results for RTC are inconsistent between direct imaging EM and other analytical techniques, especially with respect to the space within RTC, the reasons for which are discussed. Then, the role of direct imaging EM methods in modern structural biology is discussed. Direct imaging methods should support and verify the results obtained by other analytical methods capable of solving three-dimensional molecular architecture, and they should still be used as a primary tool for studying macromolecule structure in vivo.

Eudistomin D and penaresin derivatives as modulators of ryanodine receptor channels and sarcoplasmic reticulum Ca2+ ATPase in striated muscle

Eudistomin D (EuD) and penaresin (Pen) derivatives are bioactive alkaloids from marine sponges found to induce Ca(2+) release from striated muscle sarcoplasmic reticulum (SR). Although these alkaloids are believed to affect ryanodine receptor (RyR) gating in a "caffeine-like" manner, no single-channel study confirmed this assumption. Here, EuD and MBED (9-methyl-7-bromoeudistomin D) were contrasted against caffeine on their ability to modulate the SR Ca(2+) loading/leak from cardiac and skeletal muscle SR microsomes as well as the function of RyRs in planar bilayers. The effects of these alkaloids on [(3)H]ryanodine binding and SR Ca(2+) ATPase (SERCA) activity were also tested. MBED (1-5 μM) fully mimicked maximal activating effects of caffeine (20 mM) on SR Ca(2+) leak. At the single-channel level, MBED mimicked the agonistic action of caffeine on cardiac RyR gating (i.e., stabilized long openings characteristic of "high-open-probability" mode). EuD was a partial agonist at the maximal doses tested. The tested Pen derivatives displayed mild to no agonism on RyRs, SR Ca(2+) leak, or [(3)H]ryanodine binding studies. Unlike caffeine, EuD and some Pen derivatives significantly inhibited SERCA at concentrations required to modulate RyRs. Instead, MBED's affinity for RyRs (EC50 ∼ 0.5 μM) was much larger than for SERCA (IC50 > 285 μM). In conclusion, MBED is a potent RyR agonist and, potentially, a better choice than caffeine for microsomal and cell studies due to its reported lack of effects on adenosine receptors and phosphodiesterases. As a high-affinity caffeine-like probe, MBED could also help identify the caffeine-binding site in RyRs.

Ryanodine receptors as leak channels

Ryanodine receptors are Ca(2+) release channels of internal stores. This review focuses on those situations and conditions that transform RyRs from a finely regulated ion channel to an unregulated Ca(2+) leak channel and the pathological consequences of this alteration. In skeletal muscle, mutations in either CaV1.1 channel or RyR1 results in a leaky behavior of the latter. In heart cells, RyR2 functions normally as a Ca(2+) leak channel during diastole within certain limits, the enhancement of this activity leads to arrhythmogenic situations that are tackled with different pharmacological strategies. In smooth muscle, RyRs are involved more in reducing excitability than in stimulating contraction so the leak activity of RyRs in the form of Ca(2+) sparks, locally activates Ca(2+)-dependent potassium channels to reduce excitability. In neurons the enhanced activity of RyRs is associated with the development of different neurodegenerative disorders such as Alzheimer and Huntington diseases. It appears then that the activity of RyRs as leak channels can have both physiological and pathological consequences depending on the cell type and the metabolic condition.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

[TRIC(trimeric intracellular cation)-A channels contribute to blood pressure maintenance in vascular smooth muscle]

TRIC (trimeric intracellular cation) channel subtypes, namely TRIC-A and TRIC-B, are intracellular monovalent cation channels postulated to mediate counter-ion movements facilitating physiological Ca²⁺ release from intracellular stores. Tric-a-knockout mice developed hypertension during the daytime due to enhanced myogenic tone in resistance arteries. There are two Ca²⁺ release mechanisms in vascular smooth muscle cells (VSMCs); incidental opening of ryanodine receptors (RyRs) generates local Ca²⁺ sparks to induce hyperpolarization, while agonist-induced activation of inositol trisphosphate receptors (IP₃Rs) evokes global Ca²⁺ transients causing contraction. Tric-a gene ablation inhibited RyR-mediated hyperpolarization signaling to stimulate voltage-dependent Ca²⁺ influx, and adversely enhanced IP₃R-mediated Ca²⁺ transients by overloading Ca²⁺ stores in VSMCs. Therefore, TRIC-A channels contribute to maintaining blood pressure in vascular smooth muscles.

Coupled gating of skeletal muscle ryanodine receptors is modulated by Ca2+, Mg2+, and ATP

Coupled gating (synchronous openings and closures) of groups of skeletal muscle ryanodine receptors (RyR1), which mimics RyR1-mediated Ca(2+) release underlying Ca(2+) sparks, was first described by Marx et al. (Marx SO, Ondrias K, Marks AR. Science 281: 818-821, 1998). The nature of the RyR1-RyR1 interactions for coupled gating still needs to be characterized. Consequently, we defined planar lipid bilayer conditions where ∼25% of multichannel reconstitutions contain mixtures of coupled and independently gating RyR1. In ∼10% of the cases, all RyRs (2-10 channels; most frequently 3-4) gated in coupled fashion, allowing for quantification. Our results indicated that coupling required cytosolic solutions containing ATP/Mg(2+) and high (50 mM) luminal Ca(2+) (Ca(lum)) or Sr(2+) solutions. Bursts of coupled activity (events) started and ended abruptly, with all channels activating/deactivating within ∼300 μs. Coupled RyR1 were heterogeneous, where highly active RyR1 ("drivers") seemed open during the entire coupled event (P(o) = 1), while other RyR1s ("followers") displayed abundant flickering and smaller amplitude. Drivers mean open time increased with cytosolic Ca(2+) (Ca(cyt)) or caffeine, whereas followers flicker frequency was Ca(cyt) independent and more sensitive to inhibition by cytosolic Mg(2+). Coupled events were insensitive to varying lumen-to-cytosol Ca(2+) fluxes from ∼1 to 8 pA, which does not corroborate coupling of neighboring RyR1 by local Ca(2+)-induced Ca(2+) release. However, coupling requires specific Ca(lum) sites, as it was lost when Ca(lum) was replaced by luminal Ba(2+) or Mg(2+). In summary, coupled events reveal complex interactions among heterogeneous RyR1, differentially modulated by cytosolic ATP/Mg(2+), Ca(cyt), and Ca(lum,) which under cell-like ionic conditions may parallel synchronous RyR1 gating during Ca(2+) sparks.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

TRIC-A channels in vascular smooth muscle contribute to blood pressure maintenance

TRIC channel subtypes, namely TRIC-A and TRIC-B, are intracellular monovalent cation channels postulated to mediate counter-ion movements facilitating physiological Ca(2+) release from internal stores. Tric-a-knockout mice developed hypertension during the daytime due to enhanced myogenic tone in resistance arteries. There are two Ca(2+) release mechanisms in vascular smooth muscle cells (VSMCs); incidental opening of ryanodine receptors (RyRs) generates local Ca(2+) sparks to induce hyperpolarization, while agonist-induced activation of inositol trisphosphate receptors (IP(3)Rs) evokes global Ca(2+) transients causing contraction. Tric-a gene ablation inhibited RyR-mediated hyperpolarization signaling to stimulate voltage-dependent Ca(2+) influx, and adversely enhanced IP(3)R-mediated Ca(2+) transients by overloading Ca(2+) stores in VSMCs. Moreover, association analysis identified single-nucleotide polymorphisms (SNPs) around the human TRIC-A gene that increase hypertension risk and restrict the efficiency of antihypertensive drugs. Therefore, TRIC-A channels contribute to maintaining blood pressure, while TRIC-A SNPs could provide biomarkers for constitutional diagnosis and personalized medical treatment of essential hypertension.

CGP-37157 inhibits the sarcoplasmic reticulum Ca²+ ATPase and activates ryanodine receptor channels in striated muscle

7-Chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one [CGP-37157 (CGP)], a benzothiazepine derivative of clonazepam, is commonly used as a blocker of the mitochondrial Na+/Ca²+ exchanger. However, evidence suggests that CGP could also affect other targets, such as L-type Ca²+ channels and plasmalemma Na+/Ca²+ exchanger. Here, we tested the possibility of a direct modulation of ryanodine receptor channels (RyRs) and/or sarco/endoplasmic reticulum Ca²+-stimulated ATPase (SERCA) by CGP. In the presence of ruthenium red (inhibitor of RyRs), CGP decreased SERCA-mediated Ca²+ uptake of cardiac and skeletal sarcoplasmic reticulum (SR) microsomes (IC₅₀ values of 6.6 and 9.9 μM, respectively). The CGP effects on SERCA activity correlated with a decreased V(max) of ATPase activity of SERCA-enriched skeletal SR fractions. CGP (≥ 5 μM) also increased RyR-mediated Ca²+ leak from skeletal SR microsomes. Planar bilayer studies confirmed that both cardiac and skeletal RyRs are directly activated by CGP (EC(50) values of 9.4 and 12.0 μM, respectively). In summary, we found that CGP inhibits SERCA and activates RyR channels. Hence, the action of CGP on cellular Ca²+ homeostasis reported in the literature of cardiac, skeletal muscle, and other nonmuscle systems requires further analysis to take into account the contribution of all CGP-sensitive Ca²+ transporters.

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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Chloroform extract of hog barn dust modulates skeletal muscle ryanodine receptor calcium-release channel (RyR1)

Skeletal muscle weakness is a reported ailment in individuals working in commercial hog confinement facilities. To date, specific mechanisms responsible for this symptom remain undefined. The purpose of this study was to assess whether hog barn dust (HBD) contains components that are capable of binding to and modulating the activity of type 1 ryanodine receptor Ca2+-release channel (RyR1), a key regulator of skeletal muscle function. HBD collected from confinement facilities in Nebraska were extracted with chloroform, filtered, and rotary evaporated to dryness. Residues were resuspended in hexane-chloroform (20:1) and precipitates, referred to as HBDorg, were air-dried and studied further. In competition assays, HBDorg dose-dependently displaced [3H]ryanodine from binding sites on RyR1 with an IC50 of 1.5±0.1 microg/ml (Ki=0.4±0.0 microg/ml). In single-channel assays using RyR1 reconstituted into a lipid bilayer, HBDorg exhibited three distinct dose-dependent effects: first it increased the open probability of RyR1 by increasing its gating frequency and dwell time in the open state, then it induced a state of reduced conductance (55% of maximum) that was more likely to occur and persist at positive holding potentials, and finally it irreversibly closed RyR1. In differentiated C2C12 myotubes, addition of HBD triggered a rise in intracellular Ca2+ that was blocked by pretreatment with ryanodine. Since persistent activation and/or closure of RyR1 results in skeletal muscle weakness, these new data suggest that HBD is responsible, at least in part, for the muscle ailment reported by hog confinement workers.

Identification of functionally segregated sarcoplasmic reticulum calcium stores in pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle, Ca(2+) release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) may induce constriction and dilation in a manner that is not mutually exclusive. We show here that the targeting of different sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA) and RyR subtypes to discrete SR regions explains this paradox. Western blots identified protein bands for SERCA2a and SERCA2b, whereas immunofluorescence labeling of isolated pulmonary arterial smooth muscle cells revealed striking differences in the spatial distribution of SERCA2a and SERCA2b and RyR1, RyR2, and RyR3, respectively. Almost all SERCA2a and RyR3 labeling was restricted to a region within 1.5 microm of the nucleus. In marked contrast, SERCA2b labeling was primarily found within 1.5 microm of the plasma membrane, where labeling for RyR1 was maximal. The majority of labeling for RyR2 lay in between these two regions of the cell. Application of the vasoconstrictor endothelin-1 induced global Ca(2+) waves in pulmonary arterial smooth muscle cells, which were markedly attenuated upon depletion of SR Ca(2+) stores by preincubation of cells with the SERCA inhibitor thapsigargin but remained unaffected after preincubation of cells with a second SERCA antagonist, cyclopiazonic acid. We conclude that functionally segregated SR Ca(2+) stores exist within pulmonary arterial smooth muscle cells. One sits proximal to the plasma membrane, receives Ca(2+) via SERCA2b, and likely releases Ca(2+) via RyR1 to mediate vasodilation. The other is located centrally, receives Ca(2+) via SERCA2a, and likely releases Ca(2+) via RyR3 and RyR2 to initiate vasoconstriction.

Calcium signaling via two-pore channels: local or global, that is the question

Recently, we identified, for the first time, two-pore channels (TPCs, TPCN for gene name) as a novel family of nicotinic acid adenine dinucleotide phosphate (NAADP)-gated, endolysosome-targeted calcium release channels. Significantly, three subtypes of TPCs have been characterized, TPC1-3, with each being targeted to discrete acidic calcium stores, namely lysosomes (TPC2) and endosomes (TPC1 and TPC3). That TPCs act as NAADP-gated calcium release channels is clear, given that NAADP binds to high- and low-affinity sites associated with TPC2 and thereby induces calcium release and homologous desensitization, as observed in the case of endogenous NAADP receptors. Moreover, NAADP-evoked calcium signals via TPC2 are ablated by short hairpin RNA knockdown of TPC2 and by depletion of acidic calcium stores with bafilomycin. Importantly, however, NAADP-evoked calcium signals were biphasic in nature, with an initial phase of calcium release from lysosomes via TPC2, being subsequently amplified by calcium-induced calcium release (CICR) from the endoplasmic reticulum (ER). In marked contrast, calcium release via endosome-targeted TPC1 induced only spatially restricted calcium signals that were not amplified by CICR from the ER. These findings provide new insights into the mechanisms that cells may utilize to "filter" calcium signals via junctional complexes to determine whether a given signal remains local or is converted into a propagating global signal. Essentially, endosomes and lysosomes represent vesicular calcium stores, quite unlike the ER network, and TPCs do not themselves support CICR or, therefore, propagating regenerative calcium waves. Thus "quantal" vesicular calcium release via TPCs must subsequently recruit inositol 1,4,5-trisphoshpate receptors and/or ryanodine receptors on the ER by CICR to evoke a propagating calcium wave. This may call for a revision of current views on the mechanisms of intracellular calcium signaling. The purpose of this review is, therefore, to provide an appropriate framework for future studies in this area.

Crystal structure of type I ryanodine receptor amino-terminal beta-trefoil domain reveals a disease-associated mutation "hot spot" loop

Muscle contraction and relaxation is regulated by transient elevations of myoplasmic Ca(2+). Ca(2+) is released from stores in the lumen of the sarco(endo)plasmic reticulum (SER) to initiate formation of the Ca(2+) transient by activation of a class of Ca(2+) release channels referred to as ryanodine receptors (RyRs) and is pumped back into the SER lumen by Ca(2+)-ATPases (SERCAs) to terminate the Ca(2+) transient. Mutations in the type 1 ryanodine receptor gene, RYR1, are associated with 2 skeletal muscle disorders, malignant hyperthermia (MH), and central core disease (CCD). The evaluation of proposed mechanisms by which RyR1 mutations cause MH and CCD is hindered by the lack of high-resolution structural information. Here, we report the crystal structure of the N-terminal 210 residues of RyR1 (RyR(NTD)) at 2.5 A. The RyR(NTD) structure is similar to that of the suppressor domain of type 1 inositol 1,4,5-trisphosphate receptor (IP(3)Rsup), but lacks most of the long helix-turn-helix segment of the "arm" domain in IP(3)Rsup. The N-terminal beta-trefoil fold, found in both RyR and IP(3)R, is likely to play a critical role in regulatory mechanisms in this channel family. A disease-associated mutation "hot spot" loop was identified between strands 8 and 9 in a highly basic region of RyR1. Biophysical studies showed that 3 MH-associated mutations (C36R, R164C, and R178C) do not adversely affect the global stability or fold of RyR(NTD), supporting previously described mechanisms whereby mutations perturb protein-protein interactions.

SR/ER-mitochondrial local communication: calcium and ROS

Mitochondria form junctions with the sarco/endoplasmic reticulum (SR/ER), which support signal transduction and biosynthetic pathways and affect organellar distribution. Recently, these junctions have received attention because of their pivotal role in mediating calcium signal propagation to the mitochondria, which is important for both ATP production and mitochondrial cell death. Many of the SR/ER-mitochondrial calcium transporters and signaling proteins are sensitive to redox regulation and are directly exposed to the reactive oxygen species (ROS) produced in the mitochondria and SR/ER. Although ROS has been emerging as a novel signaling entity, the redox signaling of the SR/ER-mitochondrial interface is yet to be elucidated. We describe here possible mechanisms of the mutual interaction between local Ca(2+) and ROS signaling in the control of SR/ER-mitochondrial function.

Novel types of Ca2+ release channels participate in the secretory cycle of Paramecium cells

A database search of the Paramecium genome reveals 34 genes related to Ca(2+)-release channels of the inositol-1,4,5-trisphosphate (IP(3)) or ryanodine receptor type (IP(3)R, RyR). Phylogenetic analyses show that these Ca(2+) release channels (CRCs) can be subdivided into six groups (Paramecium tetraurelia CRC-I to CRC-VI), each one with features in part reminiscent of IP(3)Rs and RyRs. We characterize here the P. tetraurelia CRC-IV-1 gene family, whose relationship to IP(3)Rs and RyRs is restricted to their C-terminal channel domain. CRC-IV-1 channels localize to cortical Ca(2+) stores (alveolar sacs) and also to the endoplasmic reticulum. This is in contrast to a recently described true IP(3) channel, a group II member (P. tetraurelia IP(3)R(N)-1), found associated with the contractile vacuole system. Silencing of either one of these CRCs results in reduced exocytosis of dense core vesicles (trichocysts), although for different reasons. Knockdown of P. tetraurelia IP(3)R(N) affects trichocyst biogenesis, while CRC-IV-1 channels are involved in signal transduction since silenced cells show an impaired release of Ca(2+) from cortical stores in response to exocytotic stimuli. Our discovery of a range of CRCs in Paramecium indicates that protozoans already have evolved multiple ways for the use of Ca(2+) as signaling molecule.

Malignant hyperthermia: a pharmacogenetic disorder

Malignant hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics or depolarizing muscle relaxants in predisposed individuals. Exercise or stress-induced MH episodes, in the absence of any obvious pharmacological trigger, have been reported, but these are rare. A considerable effort has taken place over the last two decades to identify mutations associated with MH and characterize their functional effects. A number of different, but complementary systems, have been developed and implemented to this end. The results of such studies have identified commonalities in functional affects of mutations, and also uncovered unexpected complexities in both the structure and function of the skeletal muscle calcium-release channel. The following review is an attempt to provide a summary of the background to current MH research, and highlight some recent advances in our knowledge of the molecular basis of the phenotypic expression of this disorder.