Functional properties of the ryanodine receptor type 3 (RyR3) Ca2+ release channel

Type 1 and type 3 ryanodine receptors generate different Ca(2+) release event activity in both intact and permeabilized myotubes

In this investigation we use a "dyspedic" myogenic cell line, which does not express any ryanodine receptor (RyR) isoform, to examine the local Ca(2+) release behavior of RyR3 and RyR1 in a homologous cellular system. Expression of RyR3 restored caffeine-sensitive, global Ca(2+) release and causes the appearance of relatively frequent, spontaneous, spatially localized elevations of [Ca(2+)], as well as occasional spontaneous, propagating Ca(2+) release, in both intact and saponin-permeabilized myotubes. Intact myotubes expressing RyR3 did not, however, respond to K(+) depolarization. Expression of RyR1 restored depolarization-induced global Ca(2+) release in intact myotubes and caffeine-induced global release in both intact and permeabilized myotubes. Both intact and permeabilized RyR1-expressing myotubes exhibited relatively infrequent spontaneous Ca(2+) release events. In intact myotubes, the frequency of occurrence and properties of these RyR1-induced events were not altered by partial K(+) depolarization or by application of nifedipine, suggesting that these RyR1 events are independent of the voltage sensor. The events seen in RyR1-expressing myotubes were spatially more extensive than those seen in RyR3-expressing myotubes; however, when analysis was limited to spatially restricted "Ca(2+) spark"-like events, events in RyR3-expressing myotubes were larger in amplitude and duration compared with those in RyR1. Thus, in this skeletal muscle context, differences exist in the spatiotemporal properties and frequency of occurrence of spontaneous release events generated by RyR1 and RyR3. These differences underscore functional differences between the Ca(2+) release behavior of RyR1 and RyR3 in this homologous expression system.

Regulation of calcium sparks and spontaneous transient outward currents by RyR3 in arterial vascular smooth muscle cells

Intracellular Ca(2+) levels control both contraction and relaxation in vascular smooth muscle cells (VSMCs). Ca(2+)-dependent relaxation is mediated by discretely localized Ca(2+) release events through ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). These local increases in Ca(2+) concentration, termed sparks, stimulate nearby Ca(2+)-activated K(+) (BK) channels causing BK currents (spontaneous transient outward currents or STOCs). STOCs are hyperpolarizing currents that oppose vasoconstriction. Several RyR isoforms are coexpressed in VSMCs; however, their role in Ca(2+) spark generation is unknown. To provide molecular information on RyR cluster function and assembly, we examined Ca(2+) sparks and STOCs in RyR3-deficient freshly isolated myocytes of resistance-sized cerebral arteries from knockout mice and compared them to Ca(2+) sparks in cells from wild-type mice. We used RT-PCR to identify RyR1, RyR2, and RyR3 mRNA in cerebral arteries. Ca(2+) sparks in RyR3-deficient cells were similar in peak amplitude (measured as F/F(0)), width at half-maximal amplitude, and duration compared with wild-type cell Ca(2+) sparks. However, the frequency of STOCs (between -60 mV and -20 mV) was significantly higher in RyR3-deficient cells than in wild-type cells. Ca(2+) sparks and STOCs in both RyR3-deficient and wild-type cells were inhibited by ryanodine (10 micromol/L), external Ca(2+) removal, and depletion of SR Ca(2+) stores by caffeine (1 mmol/L). Isolated, pressurized cerebral arteries of RyR3-deficient mice developed reduced myogenic tone. Our results suggest that RyR3 is part of the SR Ca(2+) spark release unit and plays a specific molecular role in the regulation of STOCs frequency in mouse cerebral artery VSMCs after decreased arterial tone.

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

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

Ca2+ release induced by cyclic ADP ribose in mice lacking type 3 ryanodine receptor

The action of cyclic-ADP-ribose was studied on calcium release from sarcoplasmic reticulum of skeletal muscles of neonatal and adult wild-type and RyR3-deficient mice. cADPR increased calcium efflux from microsomes, enhanced caffeine-induced calcium release, and, in 20% of the tests, triggered calcium release in single muscle fibers. These responses occurred only in the diaphragm of adult RyR3-deficient mice. cADPR action was abolished by ryanodine, ruthenium red, and 8-brome-cADPR. These results strongly favor a specific action of cADPR on RyR1. The responsiveness of RyR1 appears in adult muscles when RyR3 is lacking.

RyR3 amplifies RyR1-mediated Ca(2+)-induced Ca(2+) release in neonatal mammalian skeletal muscle

The neonatal mammalian skeletal muscle contains both type 1 and type 3 ryanodine receptors (RyR1 and RyR3) located in the sarcoplasmic reticulum membrane. An allosteric interaction between RyR1 and dihydropyridine receptors located in the plasma membrane mediates voltage-induced Ca(2+) release (VICR) from the sarcoplasmic reticulum. RyR3, which disappears in adult muscle, is not involved in VICR, and the role of the transiently expressed RyR3 remains elusive. Here we demonstrate that RyR1 participates in both VICR and Ca(2+)-induced Ca(2+) release (CICR) and that RyR3 amplifies RyR1-mediated CICR in neonatal skeletal muscle. Confocal measurements of intracellular Ca(2+) in primary cultured mouse skeletal myotubes reveal active sites of Ca(2+) release caused by peripheral coupling between dihydropyridine receptors and RyR1. In myotubes lacking RyR3, the peripheral VICR component is unaffected, and RyR1s alone are able to support inward CICR propagation in most cells at an average speed of approximately 190 microm/s. With the co-presence of RyR1 and RyR3 in wild-type cells, unmitigated radial CICR propagates at 2,440 microm/s. Because neonatal skeletal muscle lacks a well developed transverse tubule system, the RyR3 reinforcement of CICR seems to ensure a robust, uniform, and synchronous activation of Ca(2+) release throughout the cell body. Such functional interplay between RyR1 and RyR3 can serve important roles in Ca(2+) signaling of cell differentiation and muscle contraction.

Response to caffeine and ryanodine receptor isoforms in mouse skeletal muscles

The response to caffeine was studied in mouse muscles [diaphragm, soleus, and extensor digitorum longus (EDL)] with different ryanodine receptor isoform (RyR1, RyR3) composition and in single permeabilized muscle fibers dissected from diaphragm of wild-type (WT) and RyR3-deficient (RyR3-/-) mice at 1, 15, 30, and 60 postnatal days (PND). The caffeine response decreased during development, and, in adult mice, was greater in diaphragm, lower in EDL, and intermediate in soleus. This suggests a direct relation between response to caffeine and RyR3 expression. The lack of RyR3 reduced caffeine response in young, but not in adult mice, and did not abolish the age-dependent variation and the intermuscle differences. In diaphragm single fibers, the response to caffeine increased during development and was reduced in fibers lacking RyR3 both at 15 and 60 PND. A population of fibers highly responsive to caffeine was present in adult WT and disappeared in RyR3-/-. The results confirm the contribution of RyR3 to calcium release for contractile response and clarify the contribution of RyR3 to developmental changes and intermuscle differences.

Functional overlap of IP(3)- and cADP-ribose-sensitive calcium stores in guinea pig myenteric neurons

In myenteric neurons two different receptor subtypes govern the intracellular Ca(2+) stores: the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) and the ryanodine receptor (RyR). Their degree of functional overlap was determined by examining Ca(2+) release in these cells through both superfusion techniques and intracellular microinjection. Microinjection of IP(3) (50 microM) and cADP-ribose (cADPr, 50 microM), specific ligands for the IP(3)R and RyR, respectively, demonstrated mobilization of intracellular Ca(2+) stores. Perfusion with cinnarizine (50 microM) or dantrolene (10 microM), antagonists of the IP(3)R and RyR, respectively, eliminated the Ca(2+) response to microinjected IP(3) and cADPr. Superfusion of the neurons with 100 microM ATP, an IP(3)-mediated Ca(2+)-mobilizing agonist, caused intracellular Ca(2+) increments, which were antagonized by cinnarizine, and the RyR antagonists dantrolene, procaine (5 mM), and ryanodine (1 microM). Caffeine (10 mM) was applied repetitively in Ca(2+)-free conditions to deplete RyR-sensitive stores; subsequent perfusion with ATP demonstrated a Ca(2+) response. Conversely, caffeine caused a Ca(2+) response after repetitive ATP exposures. The internal Ca(2+) stores of myenteric neurons are governed by two receptor subtypes, IP(3)R and RyR, which share partial functional overlap.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

Physiological functions of cyclic ADP-ribose and NAADP as calcium messengers

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two Ca(2+) messengers derived from NAD and NADP, respectively. Although NAADP is a linear molecule, structurally distinct from the cyclic cADPR, it is synthesized by similar enzymes, ADP-ribosyl cyclase and its homolog, CD38. The crystal structure of the cyclase has been solved and its active site identified. These two novel nucleotides have now been shown to be involved in a wide range of cellular functions including: cell cycle regulation in Euglena, a protist; gene expression in plants; and in animal systems, from fertilization to neurotransmitter release and long-term depression in brain. A battery of pharmacological reagents have been developed, providing valuable tools for elucidating the physiological functions of these two novel Ca(2+) messengers. This article reviews these recent results and explores the implications of the existence of multiple Ca(2+) messengers and Ca(2+) stores in cells.

Cyclic ADP-ribose as a second messenger revisited from a new aspect of signal transduction from receptors to ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR), an endogenous modulator of ryanodine receptor Ca(2+)-releasing channels, is found in various tissues. Cytosolic injection of cADPR induces an elevation of intracellular Ca(2+) concentrations or potentiates Ca(2+) increases. cADPR facilitates neurotransmitter or insulin release and modifies ionic currents. cADPR is synthesized by ADP-ribosyl cyclase and is metabolized by cADPR hydrolase. ADP-ribosyl cyclase activity is up-regulated by nitric oxide/cyclic GMP-dependent phosphorylation or receptor stimulation via G-proteins within membranes. These findings suggest that cADPR is a second messenger in cellular Ca(2+) signaling. However, many intriguing issues remain to be addressed before this identity is confirmed.

The properties of ryanodine-sensitive Ca(2+) release in mouse gastric smooth muscle cells

1. Under voltage-clamped conditions, gastric smooth muscle cells of BALB/c mice developed spontaneous (STOCs) and caffeine- (I(CAF)) and carbachol-induced (I(CCh)) transient outward currents. 2. In fura-2 microscopic measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)), caffeine and carbachol (CCh) provoked similar transient [Ca(2+)](i) elevations. 3. Both I(CCh) and CCh-induced [Ca(2+)](i) elevation of single smooth muscle cells occurred in an 'all-or-nothing' fashion in contrast to the reproducible caffeine responses. 4. On the basis of the suppression of STOCs and I(CAF) by nicardipine, tetraethylammonium and iberiotoxin, but not by charybdotoxin nor apamin, it was suggested that both currents were generated by large conductance type Ca(2+)-activated K(+) channels. 5. In measurements of isometric tension, caffeine produced relaxation of gastric smooth muscle strips in a concentration-dependent manner (0.1 -- 3 mM). The concentration-dependent relaxation with caffeine was mimicked by dibutyryl cyclic AMP which produced potentiation of contraction triggered by 50 mM KCL. 6. At caffeine concentrations >3 mM, a transient contraction followed by relaxation was provoked as the quasi maximal response to caffeine. In the quasi maximal response, caffeine acted as a potent relaxant in smooth muscle strips precontracted with 50 mM KCl or 3 microM CCh. 7. The relaxation with caffeine was significantly accelerated in those strips precontracted with KCl or CCh. All these results suggest that ryanodine-sensitive Ca(2+) release, which is triggered by caffeine, is an important modifier of Ca(2+) homeostasis in the cytoplasm and the contractility of gastric smooth muscle cells of mice.

Adp-ribosyl cyclase and cyclic ADP-ribose hydrolase act as a redox sensor. a primary role for cyclic ADP-ribose in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is unique to pulmonary arteries and serves to match lung perfusion to ventilation. However, in disease states this process can promote hypoxic pulmonary hypertension. Hypoxic pulmonary vasoconstriction is associated with increased NADH levels in pulmonary artery smooth muscle and with intracellular Ca(2+) release from ryanodine-sensitive stores. Because cyclic ADP-ribose (cADPR) regulates ryanodine receptors and is synthesized from beta-NAD(+), we investigated the regulation by beta-NADH of cADPR synthesis and metabolism and the role of cADPR in hypoxic pulmonary vasoconstriction. Significantly higher rates of cADPR synthesis occurred in smooth muscle homogenates of pulmonary arteries, compared with homogenates of systemic arteries. When the beta-NAD(+):beta-NADH ratio was reduced, the net amount of cADPR accumulated increased. This was due, at least in part, to the inhibition of cADPR hydrolase by beta-NADH. Furthermore, hypoxia induced a 10-fold increase in cADPR levels in pulmonary artery smooth muscle, and a membrane-permeant cADPR antagonist, 8-bromo-cADPR, abolished hypoxic pulmonary vasoconstriction in pulmonary artery rings. We propose that the cellular redox state may be coupled via an increase in beta-NADH levels to enhanced cADPR synthesis, activation of ryanodine receptors, and sarcoplasmic reticulum Ca(2+) release. This redox-sensing pathway may offer new therapeutic targets for hypoxic pulmonary hypertension.

Contribution of ryanodine receptor subtype 3 to ca2+ responses in Ca2+-overloaded cultured rat portal vein myocytes

Using an antisense strategy, we have previously shown that in vascular myocytes, subtypes 1 and 2 of ryanodine receptors (RYRs) are required for Ca(2+) release during Ca(2+) sparks and global Ca(2+) responses, evoked by activation of voltage-gated Ca(2+) channels, whereas RYR subtype 3 (RYR3) has no contribution. Here, we investigated the effects of increased Ca(2+) loading of the sarcoplasmic reticulum (SR) on the RYR-mediated Ca(2+) responses and the role of the RYR3 by injecting antisense oligonucleotides targeting the RYR subtypes. RYR3 expression was demonstrated by immunodetection in both freshly dissociated and cultured rat portal vein myocytes. Confocal Ca(2+) measurements revealed that the number of cells showing spontaneous Ca(2+) sparks was strongly increased by superfusing the vascular myocytes in 10 mm Ca(2+)-containing solution. These Ca(2+) sparks were blocked after inhibition of RYR1 or RYR2 by treatment with antisense oligolucleotides but not after inhibition of RYR3. In contrast, inhibition of RYR3 reduced the global Ca(2+) responses induced by caffeine and phenylephrine, indicating that RYR3 participated together with RYR1 and RYR2 to these Ca(2+) responses in Ca(2+)-overloaded myocytes. Ca(2+) transients evoked by photolysis of caged Ca(2+) with increasing flash intensities were also reduced after inhibition of RYR3 and revealed that the [Ca(2+)](i) sensitivity of RYR3 would be similar to that of RYR1 and RYR2. Our results show that, under conditions of increased SR Ca(2+) loading, the RYR3 becomes activable by caffeine and local increases in [Ca(2+)](i).

Functional properties of the native type 3 ryanodine receptor Ca2+-release channel from canine diaphragm

mRNA and protein analyses have previously shown that the diaphragm expresses two ryanodine receptor isoforms: RyR1 and RyR3.RyR1 is the main Ca2+-releasing pathway in this muscle type. We now report the conducting, gating, and immunological properties of the native and purified forms of the less abundant RyR3 channel. The conductance of this native Ca2+-release channel was 330 pS in 50 mM/250 mM trans/cis CsCH3SO3. It was activated by Ca2+ concentrations of 1-1000 µM, and did not inactivate at mM concentrations of Ca2+. Both isoforms were purified by either a sucrose density gradient or immunoprecipitation as > 450 kDa proteins on SDS-PAGE. Western blot analysis confirmed the presence of RyR1 and RyR3, which displayed conductances of 740 ± 30 and 800 ± 25 pS, respectively, in 250 mM KCl. We thus provide evidence that one form of the diaphragm SR Ca2+-release channels may be classified as RyR3, with gating properties different from those of the well-characterized RyR1 and RyR2 isoforms.Key words: diaphragm, calcium channel, ryanodine receptors, skeletal muscles, excitation-contraction coupling.

Intracellular Ca(2+) release mechanisms: multiple pathways having multiple functions within the same cell type?

The elevation of the cytosolic and nuclear Ca(2+) concentration is a fundamental signal transduction mechanism in almost all eukaryotic cells. Interestingly, three Ca(2+)-mobilising second messengers, D-myo-inositol 1,4,5-trisphosphate (InsP(3)), cyclic adenosine diphosphoribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP(+)) were identified in a phylogenetically wide range of different organisms. Moreover, in an as yet very limited number of cell types, sea urchin eggs, mouse pancreatic acinar cells, and human Jurkat T-lymphocytes, all three Ca(2+)-mobilising ligands have been shown to be involved in the generation of Ca(2+) signals. This situation raises the question why during evolution all three messengers have been conserved in the same cell type. From a theoretical point of view the following points may be considered: (i) redundant mechanisms ensuring intact Ca(2+) signalling even if one system does not work, (ii) the need for subcellularly localised Ca(2+) elevations to obtain a certain physiological response of the cell, and (iii) tight control of a physiological response of the cell by a temporal sequence of Ca(2+) signalling events. These theoretical considerations are compared to the current knowledge regarding the three messengers in sea urchin eggs, mouse pancreatic acinar cells, and human Jurkat T lymphocytes.

Functional visualization of the separate but interacting calcium stores sensitive to NAADP and cyclic ADP-ribose

Cells possess multiple Ca(2+) stores and their selective mobilization provides the spatial-temporal Ca(2+) signals crucial in regulating diverse cellular functions. Except for the inositol trisphosphate (IP(3))-sensitive Ca(2+) stores, the identities and the mechanisms of how these internal stores are mobilized are largely unknown. In this study, we describe two Ca(2+) stores, one of which is regulated by cyclic ADP-ribose (cADPR) and the other by nicotinic acid adenine dinucleotide phosphate (NAADP). We took advantage of the large size of the sea urchin egg and stratified its organelles by centrifugation. Using photolysis to produce either uniform or localized increases of cADPR and NAADP from their respective caged analogs, the two separate stores could be visually identified by Ca(2+) imaging and shown to be segregated to the opposite poles of the eggs. The cADPR-pole also contained the IP(3)-sensitive Ca(2+) stores, the egg nucleus and the endoplasmic reticulum (ER); the latter was visualized using Bodipy-thapsigargin. On the other hand, the mitochondria, as visualized by rhodamine 123, were segregated to the opposite pole together with the NAADP-sensitive calcium stores. Fertilization of the stratified eggs elicited a Ca(2+) wave starting at the cADPR-pole and propagating toward the NAADP-pole. These results provide the first direct and visual evidence that the NAADP-sensitive Ca(2+) stores are novel and distinct from the ER. During fertilization, communicating signals appear to be transmitted from the ER to NAADP-sensitive Ca(2+) stores, leading to their activation.

Divergent functional properties of ryanodine receptor types 1 and 3 expressed in a myogenic cell line

Of the three known ryanodine receptor (RyR) isoforms expressed in muscle, RyR1 and RyR2 have well-defined roles in contraction. However, studies on mammalian RyR3 have been difficult because of low expression levels relative to RyR1 or RyR2. Using the herpes simplex virus 1 (HSV-1) helper-free amplicon system, we expressed either RyR1 or RyR3 in 1B5 RyR-deficient myotubes. Western blot analysis revealed that RyR1- or RyR3-transduced cells expressed the appropriate RyR isoform of the correct molecular mass. Although RyR1 channels exhibited the expected unitary conductance for Cs(+) in bilayer lipid membranes, 74 of 88 RyR3 channels exhibited pronounced subconductance behavior. Western blot analysis with an FKBP12/12.6-selective antibody reveals that differences in gating behavior exhibited by RyR1 and RyR3 may be, in part, the result of lower affinity of RyR3 for FKBP12. In calcium imaging studies, RyR1 restored skeletal-type excitation-contraction coupling, whereas RyR3 did not. Although RyR3-expressing myotubes were more sensitive to caffeine than those expressing RyR1, they were much less sensitive to 4-chloro-m-cresol (CMC). In RyR1-expressing cells, regenerative calcium oscillations were observed in response to caffeine and CMC but were never seen in RyR3-expressing 1B5 cells. In [(3)H]ryanodine binding studies, only RyR1 exhibited sensitivity to CMC, but both RyR isoforms responded to caffeine. These functional differences between RyR1 and RyR3 expressed in a mammalian muscle context may reflect differences in association with accessory proteins, especially FKBP12, as well as structural differences in modulator binding sites.

Cyclic ADP ribose as a calcium-mobilizing messenger

This Perspective by Galione and Churchill is one in a series on intracellular calcium release mechanisms. The authors review the evidence for cyclic adenosine diphosphate ribose (cADPR) being a second messenger involved in regulating intracellular calcium. In addition, the physiological stimuli and responses mediated by cADPR are discussed. The Perspective is accompanied by a movie showing a calcium wave triggered by cADPR.

Multiple calcium stores: separate but interacting

Multiple mechanisms exist for increasing the concentration of intracellular calcium. This Perspective by Lee is one in a series on intracellular calcium release mechanisms and focuses on the calcium store operated by nicotinic acid adenine dinucleotide phosphate (NAADP). The characterization of the NAADP-operated calcium store as separate from the inositol trisphosphate (IP(3))-operated and cyclic ADP-ribose (cADPR)-operated calcium stores is discussed. Lee also addresses the role of NAADP in regulating intracellular calcium fluctuations during fertilization and hormonal activation of pancreatic acinar cells.

Ca(2+) signals mediated by Ins(1,4,5)P(3)-gated channels in rat ureteric myocytes

Localized Ca(2+)-release signals (puffs) and propagated Ca(2+) waves were characterized in rat ureteric myocytes by confocal microscopy. Ca(2+) puffs were evoked by photorelease of low concentrations of Ins(1,4,5)P(3) from a caged precursor and by low concentrations of acetylcholine; they were also observed spontaneously in Ca(2+)-overloaded myocytes. Ca(2+) puffs showed some variability in amplitude, time course and spatial spread, suggesting that Ins(1,4,5)P(3)-gated channels exist in clusters containing variable numbers of channels and that within these clusters a variable number of channels can be recruited. Immunodetection of Ins(1,4,5)P(3) receptors revealed the existence of several spots of fluorescence in the confocal cell sections, supporting the existence of clusters of Ins(1,4,5)P(3) receptors. Strong Ins(1,4,5)P(3) photorelease and high concentrations of acetylcholine induced Ca(2+) waves that originated from an initiation site and propagated in the whole cell by spatial recruitment of neighbouring Ca(2+)-release sites. Both Ca(2+) puffs and Ca(2+) waves were blocked selectively by intracellular applications of heparin and an anti-Ins(1,4,5)P(3)-receptor antibody, but were unaffected by ryanodine and intracellular application of an anti-ryanodine receptor antibody. mRNAs encoding for the three subtypes of Ins(1,4,5)P(3) receptor and subtype 3 of ryanodine receptor were detected in these myocytes, and the maximal binding capacity of [(3)H]Ins(1,4,5)P(3) was 10- to 12-fold higher than that of [(3)H]ryanodine. These results suggest that Ins(1,4,5)P(3)-gated channels mediate a continuum of Ca(2+) signalling in smooth-muscle cells expressing a high level of Ins(1,4,5)P(3) receptors and no subtypes 1 and 2 of ryanodine receptors.

Ca2+-induced Ca2+ release supports the relay mode of activity in thalamocortical cells

Ca2+ ions play an important role during rhythmic bursting of thalamocortical neurons within sleep. The function of Ca2+ during the tonic relay mode of these neurons during wakefulness is less clear. Here, we report that tonic activity in thalamocortical cells results in an increase in the intracellular Ca2+ concentration and subsequent release of Ca2+ from intracellular stores mediated via ryanodine receptors (RyRs). Blockade of Ca2+ release shifted the regular firing of single action potentials toward the generation of spike clusters. Regular spike firing and intracellular Ca2+ release thus appear to be functionally coupled in a positive feedback manner, thereby supporting the relay mode of thalamocortical cells during wakefulness. Regulatory influences may be coupled to this system via the cyclic ADP ribose pathway.

ATP-induced activation of expressed RyR3 at low free calcium

Ca(2+) channel properties of the mink ryanodine receptor type 3 (RyR3), expressed in HEK293 cells, were studied in planar lipid bilayers to which RyR3 rich membrane fragments had been fused. RyR3 channels were not active at resting levels of Ca(2+)(free) but were gated by an additional 1 mM ATP, exhibiting long open times. The second major finding was the absence of channel inactivation at millimolar Ca(2+)(free). Insertion of a myc tag at the N-terminus of RyR3 did not affect the channel properties. As to skeletal muscle, the observed type 3 channel properties appear physiologically meaningful by assisting type 1 channels in calcium release.

Comparison of Ca(2+) sparks produced independently by two ryanodine receptor isoforms (type 1 or type 3)

The molecular determinants of a Ca(2+) spark, those events that determine the sudden opening and closing of a small number of ryanodine receptor (RyR) channels limiting Ca(2+) release to a few milliseconds, are unknown. As a first step we investigated which of two RyR isoforms present in mammalian embryonic skeletal muscle, RyR type 1(RyR-1) or RyR type 3 (RyR-3) has the ability to generate Ca(2+) sparks. Their separate contributions were investigated in intercostal muscle cells of RyR-1 null and RyR-3 null mouse embryos. A comparison of Ca(2+) spark parameters of RyR-1 null versus RyR-3 null cells measured at rest with fluo-3 showed that neither the peak fluorescence intensity (DeltaF/F(o) = 1.25 +/- 0.7 vs. 1.55 +/- 0.6), spatial width at half-max intensity (FWHM = 2.7 +/- 1.2 vs. 2.6 +/- 0.6 microm), nor the duration at half-max intensity (FTHM = 45 +/- 49 vs. 43 +/- 25 ms) was significantly different. Sensitivity to caffeine (0.1 mM) was remarkably different, with sparks in RyR-1 null myotubes becoming brighter and longer in duration, whereas those in RyR-3 null cells remained unchanged. Controls performed in double RyR-1/RyR-3 null cells obtained by mice breeding showed that sparks were not observed in the absence of both isoforms in >150 cells imaged. In conclusion, 1) RyR-1 and RyR-3 appear to be the only intracellular Ca(2+) channels that participate in Ca(2+) spark activity in embryonic skeletal muscle; 2) except in their responsiveness to caffeine, both isoforms have the ability to produce Ca(2+) sparks with nearly identical properties, so it is rather unlikely that a single RyR isoform, when others are also present, would be responsible for Ca(2+) sparks; and 3) because RyR-1 null cells are excitation-contraction (EC) uncoupled and RyR-3 null cells exhibit a normal phenotype, Ca(2+) sparks result from the inherent activity of small clusters of RyRs regardless of the participation of these RyRs in EC coupling.

Requirement of ryanodine receptor subtypes 1 and 2 for Ca(2+)-induced Ca(2+) release in vascular myocytes

While the roles of subtypes 1 and 2 of the ryanodine receptors (RYRs) have been studied in cellular systems expressing specifically one or the other of these subtypes (i.e. skeletal and cardiac muscle), the function of these receptors has not been evaluated in smooth muscles. We have previously reported RYR-mediated elementary (Ca(2+) sparks) and global Ca(2+) responses in rat portal vein myocytes. Here, we investigated the respective roles of all three RYR subtypes expressed in these cells as revealed by reverse transcriptase-polymerase chain reaction. Antisense oligonucleotides targeting each one of the three RYR subtypes were shown to specifically inhibit the expression of the corresponding mRNA and protein without affecting the other RYR subtypes. Confocal Ca(2+) measurements revealed that depolarization-induced Ca(2+) sparks and global Ca(2+) responses were blocked when either RYR1 or RYR2 expression was suppressed. Caffeine-induced Ca(2+) responses were partly inhibited by the same antisense oligonucleotides. Neither the corresponding scrambled oligonucleotides nor the antisense oligonucleotides targeting RYR3 affected depolarization- or caffeine-induced Ca(2+) responses. Our results show that, in vascular myocytes, the two RYR1 and RYR2 subtypes are required for Ca(2+) release during Ca(2+) sparks and global Ca(2+) responses, evoked by activation of voltage-gated Ca(2+) channels.

cADP-ribose potentiates cytosolic Ca2+ elevation and Ca2+ entry via L-type voltage-activated Ca2+ channels in NG108-15 neuronal cells

The effects of cADP-ribose (cADPR), a metabolite of beta-NAD(+), on the elevation of cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) and Ca(2+) influx through voltage-activated Ca(2+) channels (VACCs) were studied in NG108-15 neuroblastomaxglioma hybrid cells. NG108-15 cells were pre-loaded with fura-2 and whole-cell patch-clamped. Application of cADPR through patch pipettes did not by itself trigger any [Ca(2+)](i) rise at the resting membrane potential. A rise in [Ca(2+)](i) was evoked upon sustained membrane depolarization, and was significantly larger in cADPR-infused cells than in non-infused cells. This potentiation in the [Ca(2+)](i) elevation was reproduced by infusion of beta-NAD(+), and was blocked by 8-bromo-cADPR and antagonized by external application of ryanodine or by pretreatment of cells with FK506. Nicotinamide inhibited beta-NAD(+)-induced, but not cADPR-elicited, potentiation. [Ca(2+)](i) increases or Ca(2+) influx, measured by Mn(2+) quenching, elicited by the same protocol of depolarization was blocked completely by nifedipine but not by omega-conotoxin. Ca(2+) influx in cADPR- or beta-NAD(+)-infused cells was steeper and greater than that in control cells, and was inhibited partly by ryanodine. In contrast, ryanodine accelerated Ca(2+) influx in non-infused cells. These results show that cADPR amplifies both depolarization-induced [Ca(2+)](i) increase and Ca(2+) influx through L-type VACCs. These results suggest that cADPR functions on ryanodine receptors as a direct agonist and also interacts with L-type VACCs as an indirect agonist, i.e. via a retrograde signal.

Contractile function is unaltered in diaphragm from mice lacking calcium release channel isoform 3

Skeletal muscle expresses at least two isoforms of the calcium release channel in the sarcoplasmic reticulum (RyR1 and RyR3). Whereas the function of RyR1 is well defined, the physiological significance of RyR3 is unclear. Some authors have suggested that RyR3 participates in excitation-contraction coupling and that RyR3 may specifically confer resistance to fatigue. To test this hypothesis, we measured contractile function of diaphragm strips from adult RyR3-deficient mice (exon 2-targeted mutation) and their heterozygous and wild-type littermates. In unfatigued diaphragm, there were no differences in isometric contractile properties (twitch characteristics, force-frequency relationships, maximal force) among the three groups. Our fatigue protocol (30 Hz, 0.25 duty cycle, 37 degrees C) depressed force to 25% of the initial force; however, lack of RyR3 did not accelerate the decline in force production. The force-frequency relationship was shifted to higher frequencies and was depressed in fatigued diaphragm; lack of RyR3 did not exaggerate these changes. We therefore provide evidence that RyR3 deficiency does not alter contractile function of adult muscle before, during, or after fatigue.

Contribution of ryanodine receptor type 3 to Ca(2+) sparks in embryonic mouse skeletal muscle

The kinetic behavior of Ca(2+) sparks in knockout mice lacking a specific ryanodine receptor (RyR) isoform should provide molecular information on function and assembly of clusters of RyRs. We examined resting Ca(2+) sparks in RyR type 3-null intercostal myotubes from embryonic day 18 (E18) mice and compared them to Ca(2+) sparks in wild-type (wt) mice of the same age and to Ca(2+) sparks in fast-twitch muscle cells from the foot of wt adult mice. Sparks from RyR type 3-null embryonic cells (368 events) were significantly smaller, briefer, and had a faster time to peak than sparks from wt cells (280 events) of the same age. Sparks in adult cells (220 events) were infrequent, yet they were highly reproducible with population means smaller than those in embryonic RyR type 3-null cells but similar to those reported in adult amphibian skeletal muscle fibers. Three-dimensional representations of the spark peak intensity (DeltaF/Fo) vs. full width at half-maximal intensity (FWHM) vs. full duration at half-maximal intensity (FTHM) showed that wt embryonic sparks were considerably more variable in size and kinetics than sparks in adult muscle. In all cases, tetracaine (0.2 mM) abolished Ca(2+) spark activity, whereas caffeine (0.1 mM) lengthened the spark duration in wt embryonic and adult cells but not in RyR type 3-null cells. These results confirmed that sparks arose from RyRs. The low caffeine sensitivity of RyR type 3-null cells is entirely consistent with observations by other investigators. There are three conclusions from this study: i) RyR type-1 engages in Ca(2+) spark activity in the absence of other RyR isoforms in RyR type 3-null myotubes; ii) Ca(2+) sparks with parameters similar to those reported in adult amphibian skeletal muscle can be detected, albeit at a low frequency, in adult mammalian skeletal muscle cells; and iii) a major contributor to the unusually large Ca(2+) sparks observed in normal (wt) embryonic muscle is RyR type 3. To explain the reduction in the size of sparks in adult compared to embryonic skeletal muscle, we suggest that in embryonic muscle, RyR type 1 and RyR type 3 channels co-contribute to Ca(2+) release during the same spark and that Ca(2+) sparks undergo a maturation process which involves a decrease in RyR type 3.

Type 3 and type 1 ryanodine receptors are localized in triads of the same mammalian skeletal muscle fibers

The type 3 ryanodine receptor (RyR3) is a ubiquitous calcium release channel that has recently been found in mammalian skeletal muscles. However, in contrast to the skeletal muscle isoform (RyR1), neither the subcellular distribution nor the physiological role of RyR3 are known. Here, we used isoform-specific antibodies to localize RyR3 in muscles of normal and RyR knockout mice. In normal hind limb and diaphragm muscles of young mice, RyR3 was expressed in all fibers where it was codistributed with RyR1 and with the skeletal muscle dihydropyridine receptor. This distribution pattern indicates that RyR3 is localized in the triadic junctions between the transverse tubules and the sarcoplasmic reticulum. During development, RyR3 expression declined rapidly in some fibers whereas other fibers maintained expression of RyR3 into adulthood. Comparing the distribution of RyR3-containing fibers with that of known fiber types did not show a direct correlation. Targeted deletion of the RyR1 or RyR3 gene resulted in the expected loss of the targeted isoform, but had no adverse effects on the expression and localization of the respective other RyR isoform. The localization of RyR3 in skeletal muscle triads, together with RyR1, is consistent with an accessory function of RyR3 in skeletal muscle excitation-contraction coupling.

A unified mechanism of enzymatic synthesis of two calcium messengers: cyclic ADP-ribose and NAADP

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize Ca2+ from two different types of intracellular stores and through completely independent mechanisms. The two Ca2+ messengers are also structurally distinct. cADPR is a cyclic nucleotide derived from NAD, while NAADP is a linear metabolite of NADP. Systems responsive to these two novel signaling molecules are widespread among eukaryotes and include protozoan, plant, invertebrate, mammalian as well as human cells. Despite their functional and structural differences, cADPR and NAADP are sibling messengers synthesized by a single enzyme, ADP-ribosyl cyclase. In this article the recent progress in understanding the physiological roles of cADPR and NAADP is briefly reviewed. A unified mechanism of catalysis is also proposed, which takes into consideration the crystallographic structure of ADP-ribosyl cyclase and accounts for its novel multi-functionality.

Cyclic ADP-ribose: a novel Ca2+-mobilising second messenger

Cyclic ADP-ribose (cADPR) was discovered as a potent Ca2+-mobilising natural compound in sea urchin eggs. Recently, cADPR was reported to stimulate Ca2+ signalling in several higher eukaryotic cell systems (e.g., smooth and cardiac muscle cells, neuronal cells, adrenal chromaffin cells, macrophages, pancreatic acinar cells and T-lymphocytes). The following aspects of the role of cADPR as a Ca2+-mobilising second messenger are reviewed: coupling of metabolism of cADPR to stimulation of receptors in the plasma membrane, properties and pharmacology of Ca2+ release by cADPR and the involvement of cADPR in Ca2+ entry.

Expression and subcellular localization of the ryanodine receptor in rat pancreatic acinar cells

The ryanodine receptor (RyR) is the principal Ca2+-release channel in excitable cells, whereas the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) is primarily responsible for Ca2+ release in non-excitable cells, including epithelia. RyR also is expressed in a number of non-excitable cell types, but is thought to serve as an auxiliary or alternative Ca2+-release pathway in those cells. Here we use reverse transcription PCR to show that a polarized epithelium, the pancreatic acinar cell, expresses the type 2, but not the type 1 or 3, isoform of RyR. We furthermore use immunochemistry to demonstrate that the type 2 RyR is distributed throughout the basolateral and, to a lesser extent, the apical region of the acinar cell, but is excluded from the trigger zone, where cytosolic Ca2+ signals originate in this cell type. Since propagation of Ca2+ waves in acinar cells is sensitive to ryanodine, caffeine and Ca2+, these findings suggest that Ca2+ waves in this cell type result from the co-ordinated release of Ca2+, first from InsP3Rs in the trigger zone, then from RyRs elsewhere in the cell. RyR may play a fundamental role in Ca2+ signalling in polarized epithelia, including for Ca2+ signals initiated by InsP3.

Ca2+ sparks in embryonic mouse skeletal muscle selectively deficient in dihydropyridine receptor alpha1S or beta1a subunits

Ca2+ sparks are miniature Ca2+ release events from the sarcoplasmic reticulum of muscle cells. We examined the kinetics of Ca2+ sparks in excitation-contraction uncoupled myotubes from mouse embryos lacking the beta1 subunit and mdg embryos lacking the alpha1S subunit of the dihydropyridine receptor. Ca2+ sparks occurred spontaneously without a preferential location in the myotube. Ca2+ sparks had a broad distribution of spatial and temporal dimensions with means much larger than those reported in adult muscle. In normal myotubes (n = 248 sparks), the peak fluorescence ratio, DeltaF/Fo, was 1.6 +/- 0.6 (mean +/- SD), the full spatial width at half-maximal fluorescence (FWHM) was 3.6 +/- 1.1 micrometer and the full duration of individual sparks, Deltat, was 145 +/- 64 ms. In beta-null myotubes (n = 284 sparks), DeltaF/Fo = 1.9 +/- 0.4, FWHM = 5.1 +/- 1.5 micrometer, and Deltat = 168 +/- 43 ms. In mdg myotubes (n = 426 sparks), DeltaF/Fo = 1 +/- 0.5, the FWHM = 2.5 +/- 1.1 micrometer, and Deltat = 97 +/- 50 ms. Thus, Ca2+ sparks in mdg myotubes were significantly dimmer, smaller, and briefer than Ca2+ sparks in normal or beta-deficient myotubes. In all cell types, the frequency of sparks, DeltaF/Fo, and FWHM were gradually decreased by tetracaine and increased by caffeine. Both results confirmed that Ca2+ sparks of resting embryonic muscle originated from spontaneous openings of ryanodine receptor channels. We conclude that dihydropyridine receptor alpha1S and beta1 subunits participate in the control of Ca2+ sparks in embryonic skeletal muscle. However, excitation-contraction coupling is not essential for Ca2+ spark formation in these cells.

Expression and subcellular localization of the ryanodine receptor in rat pancreatic acinar cells

The ryanodine receptor (RyR) is the principal Ca2+-release channel in excitable cells, whereas the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) is primarily responsible for Ca2+ release in non-excitable cells, including epithelia. RyR also is expressed in a number of non-excitable cell types, but is thought to serve as an auxiliary or alternative Ca2+-release pathway in those cells. Here we use reverse transcription PCR to show that a polarized epithelium, the pancreatic acinar cell, expresses the type 2, but not the type 1 or 3, isoform of RyR. We furthermore use immunochemistry to demonstrate that the type 2 RyR is distributed throughout the basolateral and, to a lesser extent, the apical region of the acinar cell, but is excluded from the trigger zone, where cytosolic Ca2+ signals originate in this cell type. Since propagation of Ca2+ waves in acinar cells is sensitive to ryanodine, caffeine and Ca2+, these findings suggest that Ca2+ waves in this cell type result from the co-ordinated release of Ca2+, first from InsP3Rs in the trigger zone, then from RyRs elsewhere in the cell. RyR may play a fundamental role in Ca2+ signalling in polarized epithelia, including for Ca2+ signals initiated by InsP3.

Voltage-controlled Ca2+ release in normal and ryanodine receptor type 3 (RyR3)-deficient mouse myotubes

1. Primary cultured myotubes were derived from satellite cells of the diaphragm obtained from both normal mice (RyR3+/+) and mice with a targeted mutation eliminating expression of the type 3 isoform of the ryanodine receptor (RyR3-/-). Using the whole-cell patch clamp technique, L-type Ca2+ currents were measured during step depolarizations. Simultaneously, intracellular Ca2+ transients were recorded with the fluorescent indicator dye fura-2. 2. After correction for non-instantaneous binding of Ca2+ to the indicator dye and taking into account the dynamics of Ca2+ binding to intracellular constituents, an estimate of the time course of the Ca2+ release rate from the sarcoplasmic reticulum (SR) was obtained. 3. The calculated SR Ca2+ release flux exhibited a marked peak within less than 12 ms after the onset of the voltage-clamp depolarization and fell rapidly thereafter to a five times lower, almost steady level. It declined rapidly after termination of the depolarization. 4. Signals in normal and RyR3-deficient myotubes showed no significant difference in the activation of Ca2+ conductance and in amplitude, time course and voltage dependence of the Ca2+ efflux from the SR. 5. In conclusion, the characteristics of voltage-controlled Ca2+ release reported here are similar to those of mature mammalian muscle fibres. In contrast to differences observed in the contractile properties of RyR3-deficient muscle fibres, a contribution of RyR3 to excitation-contraction coupling could not be detected in myotubes.