Purification and characterization of ryanodine receptor 3 from mammalian tissue

Transcriptome analysis of mRNA and miRNA in the development of LeiZhou goat muscles

The progression of muscle development is a pivotal aspect of animal ontogenesis, where miRNA and mRNA exert substantial influence as prominent players. It is important to understand the molecular mechanisms involved in skeletal muscle development to enhance the quality and yield of meat produced by Leizhou goats. We employed RNA sequencing (RNA-SEQ) technology to generate miRNA-mRNA profiles in Leizhou goats, capturing their developmental progression at 0, 3, and 6 months of age. A total of 977 mRNAs and 174 miRNAs were found to be differentially expressed based on our analysis. Metabolic pathways, calcium signaling pathways, and amino acid synthesis and metabolism were found to be significantly enriched among the differentially expressed mRNA in the enrichment analysis. Meanwhile, we found that among these differentially expressed mRNA, some may be related to muscle development, such as MYL10, RYR3, and CSRP3. Additionally,, we identified five muscle-specific miRNAs (miR-127-3p, miR-133a-3p, miR-193b-3p, miR-365-3p, and miR-381) that consistently exhibited high expression levels across all three stages. These miRNAs work with their target genes (FHL3, SESN1, PACSIN3, LMCD1) to regulate muscle development. Taken together, our findings suggest that several miRNAs and mRNAs are involved in regulating muscle development and cell growth in goats. By uncovering the molecular mechanisms involved in muscle growth and development, these findings contribute valuable knowledge that can inform breeding strategies aimed at enhancing meat yield and quality in Leizhou goats.

The Genomics of Arthrogryposis, a Complex Trait: Candidate Genes and Further Evidence for Oligogenic Inheritance

Arthrogryposis is a clinical finding that is present either as a feature of a neuromuscular condition or as part of a systemic disease in over 400 Mendelian conditions. The underlying molecular etiology remains largely unknown because of genetic and phenotypic heterogeneity. We applied exome sequencing (ES) in a cohort of 89 families with the clinical sign of arthrogryposis. Additional molecular techniques including array comparative genomic hybridization (aCGH) and Droplet Digital PCR (ddPCR) were performed on individuals who were found to have pathogenic copy number variants (CNVs) and mosaicism, respectively. A molecular diagnosis was established in 65.2% (58/89) of families. Eleven out of 58 families (19.0%) showed evidence for potential involvement of pathogenic variation at more than one locus, probably driven by absence of heterozygosity (AOH) burden due to identity-by-descent (IBD). RYR3, MYOM2, ERGIC1, SPTBN4, and ABCA7 represent genes, identified in two or more families, for which mutations are probably causative for arthrogryposis. We also provide evidence for the involvement of CNVs in the etiology of arthrogryposis and for the idea that both mono-allelic and bi-allelic variants in the same gene cause either similar or distinct syndromes. We were able to identify the molecular etiology in nine out of 20 families who underwent reanalysis. In summary, our data from family-based ES further delineate the molecular etiology of arthrogryposis, yielded several candidate disease-associated genes, and provide evidence for mutational burden in a biological pathway or network. Our study also highlights the importance of reanalysis of individuals with unsolved diagnoses in conjunction with sequencing extended family members.

The cell-wide web coordinates cellular processes by directing site-specific Ca2+ flux across cytoplasmic nanocourses

Ca²⁺ coordinates diverse cellular processes, yet how function-specific signals arise is enigmatic. We describe a cell-wide network of distinct cytoplasmic nanocourses with the nucleus at its centre, demarcated by sarcoplasmic reticulum (SR) junctions (≤400 nm across) that restrict Ca²⁺ diffusion and by nanocourse-specific Ca²⁺-pumps that facilitate signal segregation. Ryanodine receptor subtype 1 (RyR1) supports relaxation of arterial myocytes by unloading Ca²⁺ into peripheral nanocourses delimited by plasmalemma-SR junctions, fed by sarco/endoplasmic reticulum Ca²⁺ ATPase 2b (SERCA2b). Conversely, stimulus-specified increases in Ca²⁺ flux through RyR2/3 clusters selects for rapid propagation of Ca²⁺ signals throughout deeper extraperinuclear nanocourses and thus myocyte contraction. Nuclear envelope invaginations incorporating SERCA1 in their outer nuclear membranes demarcate further diverse networks of cytoplasmic nanocourses that receive Ca²⁺ signals through discrete RyR1 clusters, impacting gene expression through epigenetic marks segregated by their associated invaginations. Critically, this circuit is not hardwired and remodels for different outputs during cell proliferation.

Although calcium signals are known to be critical for many cellular processes, how signaling elicits specific functions remains unclear. In visually striking work, Duan et al. reveal that networks of cytoplasmic nanocourses orchestrate cell activity by directing site-specific calcium signals.

Ryanodine receptor type 3 (RYR3) as a novel gene associated with a myopathy with nemaline bodies

BACKGROUND AND PURPOSE

Nemaline myopathy (NEM) has been associated with mutations in 12 genes to date. However, for some patients diagnosed with NEM, definitive mutations are not identified in the known genes, suggesting that there are other genes involved. This study describes compound heterozygosity for rare variants in ryanodine receptor type 3 (RYR3) gene in one such patient.

METHODS AND RESULTS

Clinical examination of the patient at 22 years of age revealed a long narrow face, high arched palate and bilateral facial weakness. She had proximal weakness in all four limbs, mild scapular winging but no scoliosis. Muscle biopsy revealed wide variation in fibre size with type 1 fibre predominance and atrophy. Abundant nemaline bodies were located in perinuclear and subsarcolemmal areas, and within the cytoplasm. No likely pathogenic mutations in known NEM genes were identified. Copy number variation in known NEM genes was excluded by NEM-targeted comparative genomic hybridization array. Next-generation sequencing revealed compound heterozygous missense variants in the RYR3 gene. RYR3 transcripts are expressed in human fetal and adult skeletal muscle as well as in human brain and cauda equina samples. Immunofluorescence of human skeletal muscle revealed a 'single-row' appearance of RYR3, interspersed between the 'double rows' of ryanodine receptor type 1 (RYR1) at each A-I junction.

CONCLUSION

The results suggest that variants in RYR3 may cause a recessive muscle disease with pathological features including nemaline bodies. We characterize the expression pattern of RYR3 in human skeletal muscle and brain, and the subcellular localization of RYR1 and RYR3 in human skeletal muscle.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Cardiac expression of ryanodine receptor subtype 3; a strategic component in the intracellular Ca2+ release system of Purkinje fibers in large mammalian heart

BACKGROUND

Three distinct Ca²⁺ release channels were identified in dog P-cells: the ryanodine receptor subtype 2 (RyR2) was detected throughout the cell, while the ryanodine receptor subtype 3 (RyR3) and inositol phosphate sensitive Ca²⁺ release channel (InsP3R) were found in the cell periphery. How each of these channels contributes to the Ca²⁺ cycling of P-cells is unclear. Recent modeling of Ca²⁺ mobilization in P-cells suggested that Ca²⁺ sensitivity of Ca²⁺induced Ca²⁺release (CICR) was larger at the P-cell periphery. Our study examined whether this numerically predicted region of Ca²⁺ release exists in live P-cells. We compared the regional Ca²⁺ dynamics with the arrangement of intracellular Ca²⁺ release (CR) channels.

METHODS

Gene expression of CR channels was measured by qPCR in Purkinje fibers and myocardium of adult Yucatan pig hearts. We characterized the CR channels protein expression in isolated P-cells by immuno-fluorescence, laser scanning confocal microscopy, and 3D reconstruction. The spontaneous Ca²⁺ activity and electrically-evoked Ca²⁺ mobilization were imaged by 2D spinning disk confocal microscopy. Functional regions of P-cell were differentiated by the characteristics of local Ca²⁺ events. We used the Ca²⁺ propagation velocities as indicators of channel Ca²⁺ sensitivity.

RESULTS

RyR2 gene expression was identical in Purkinje fibers and myocardium (6 hearts) while RyR3 and InsP₃R gene expressions were, respectively, 100 and 16 times larger in the Purkinje fibers. Specific fluorescent immuno-staining of Ca²⁺ release channels revealed an intermediate layer of RyR3 expression between a near-membrane InsP3R-region and a central RyR2-region. We found that cell periphery produced two distinct forms of spontaneous Ca²⁺-transients: (1) large asymmetrical Ca²⁺ sparks under the membrane, and (2) typical Ca²⁺-wavelets propagating exclusively around the core of the cell. Larger cell-wide Ca²⁺ waves (CWWs) appeared occasionally traveling in the longitudinal direction through the core of Pcells. Large sparks arose in a micrometric space overlapping the InsP3R expression. The InsP3R antagonists 2-aminoethoxydiphenyl borate (2-APB; 3μM) and xestospongin C (XeC; 50μM) dramatically reduced their frequency. The Ca²⁺ wavelets propagated in a 5-10μm thick layered space which matched the intermediate zone of RyR3 expression. The wavelet incidence was unchanged by 2-APB or XeC, but was reduced by 60% in presence of the RyR3 antagonist dantrolene (10μM). The velocity of wavelets was two times larger (86±16μm/s; n=14) compared to CWWs' (46±10μm/s; n=11; P<0.05). Electric stimulation triggered a uniform and large elevation of Ca²⁺ concentration under the membrane which preceded the propagation of Ca²⁺ into the interior of the cell. Elevated Ca_i propagated at 150μm/s (147±34μm/s; n=5) through the region equivalent to the zone of RyR3 expression. This velocity dropped by 50% (75±24μm/s; n=5) in the central region wherein predominant RyR2 expression was detected.

CONCLUSION

We identified two layers of distinct Ca²⁺ release channels in the periphery of Pcell: an outer layer of InsP₃Rs under the membrane and an inner layer of RyR3s. The propagation of Ca²⁺ events in these layers revealed that Ca²⁺ sensitivity of Ca²⁺ release was larger in the RyR3 layer compared to that of other sub-cellular regions. We propose that RyR3 expression in P-cells plays a role in the stability of electric function of Purkinje fibers.

Structural mapping of divergent regions in the type 1 ryanodine receptor using fluorescence resonance energy transfer

Ryanodine receptors (RyRs) release Ca(2+) to initiate striated muscle contraction. Three highly divergent regions (DRs) in the RyR protein sequence (DR1, DR2, and DR3) may confer isoform-specific functional properties to the RyRs. We used cell-based fluorescence resonance energy transfer (FRET) measurements to localize these DRs to the cryoelectron microscopic (cryo-EM) map of the skeletal muscle RyR isoform (RyR1). FRET donors were targeted to RyR1 using five different FKBP12.6 variants labeled with Alexa Fluor 488. FRET was then measured to the FRET acceptors, Cy3NTA or Cy5NTA, targeted to decahistidine tags introduced within the DRs. DR2 and DR3 were localized to separate positions within the "clamp" region of the RyR1 cryo-EM map, which is presumed to interface with Cav1.1. DR1 was localized to the "handle" region, near the regulatory calmodulin-binding site on the RyR. These localizations provide insights into the roles of DRs in RyR allosteric regulation during excitation contraction coupling.

Association of the RYR3 gene polymorphisms with atherosclerosis in elderly Japanese population

BACKGROUND

The Ryanodine receptor 3 gene (RYR3) encodes an intracellular calcium channel that mediates the efflux of Ca2+ from intracellular stores. Two single-nucleotide polymorphisms (SNPs) in the RYR3 gene have been shown to associate with stroke (rs877087) and carotid intima-media thickness (rs2229116) in two independent genome-wide association studies (GWAS) in Caucasian. We investigated the effect of these two SNPs as well as the 31.1 kilobases spanning region on atherosclerosis in Japanese population.

METHODS

Atherosclerotic severity was assessed by carotid artery (n = 1374) and pathological atherosclerosis index (PAI) (n = 1262), which is a macroscopic examination of the luminal surfaces of 8 systemic arteries in consecutive autopsy samples. 4 tag SNPs in the 31.1 Kb region, rs877087, rs2132207, rs658750 and rs2229116, were genotyped and haplotypes were inferred to study the association with atherosclerotic indices.

RESULTS

rs877087 and rs2229116 were associated with PAI (OR = 2.07 [1.04-4.12] (95% CI), p = 0.038; and OR = 1.38 [1.02-1.86], p = 0.035, respectively). rs2229116 was also associated with common carotid atherosclerosis (OR = 1.45 [1.13-1.86], p = 0.003). The risk allele of rs2229116 was opposite from the original report. The haplotype block of this 31.1 Kb region was different between Caucasian and Japanese. Haplotype analysis revealed that only TAGG haplotype was associated with PAI (OR = 0.67 [0.48-0.94], p = 0.020) and atherosclerosis of common carotid artery (OR = 0.75 [0.58-0.98], p = 0.034).

CONCLUSION

rs877087 and rs2229116 of RYR3 gene are associated with atherosclerosis severity in Japanese. The functional difference caused by rs2229116 needs to be investigated.

Contribution of endoplasmic reticulum Ca2+ regulatory mechanisms to the inflammation-induced increase in the evoked Ca2+ transient in rat cutaneous dorsal root ganglion neurons

Persistent inflammation results in an increase in the magnitude and duration of high K(+)-evoked Ca(2+) transients in putative nociceptive cutaneous dorsal root ganglion (DRG) neurons. The purpose of the present study was to determine whether recruitment of Ca(2+)-induced Ca(2+) release (CICR) contributes to these inflammation-induced changes. Acutely dissociated, retrogradely labeled cutaneous DRG neurons from naïve and complete Freund's adjuvant inflamed adult male Sprague-Dawley rats were studied with ratiometric microfluorimetry. Ryanodine only attenuated the duration but not magnitude of the high K(+)-evoked Ca(2+) transient in neurons from inflamed rats. However, there was no significant impact of inflammation on the potency or efficacy of ryanodine-induced block of the caffeine-evoked Ca(2+) transient, or the impact of sarco-endoplasmic reticulum ATPase (SERCA) inhibition on the high K(+)-evoked Ca(2+) transient. Furthermore, while there was no change in the magnitude, an inflammation-induced increase in the duration of the caffeine-evoked Ca(2+) transient was only observed with a prolonged caffeine application. In contrast to the high K(+)-evoked Ca(2+) transient, there was no evidence of direct mitochondrial involvement or that of the Ca(2+) extrusion mechanism, the Na(+)/Ca(2+) exchanger, on the caffeine-evoked Ca(2+) transient, and block of SERCA only increased the duration of this transient. These results indicate the presence of Ca(2+) regulatory domains in cutaneous nociceptive DRG neurons within which cytosolic Ca(2+) increased via influx and release are highly segregated. Furthermore, our results suggest that changes in neither CICR machinery nor the coupling between Ca(2+) influx and CICR are primarily responsible for the inflammation-induced changes in the evoked Ca(2+) transient.

Two ryanodine receptor isoforms in nonmammalian vertebrate skeletal muscle: possible roles in excitation-contraction coupling and other processes

The ryanodine receptor (RyR) is a Ca(2+) release channel in the sarcoplasmic reticulum in vertebrate skeletal muscle and plays an important role in excitation-contraction (E-C) coupling. Whereas mammalian skeletal muscle predominantly expresses a single RyR isoform, RyR1, skeletal muscle of many nonmammalian vertebrates expresses equal amounts of two distinct isoforms, α-RyR and β-RyR, which are homologues of mammalian RyR1 and RyR3, respectively. In this review we describe our current understanding of the functions of these two RyR isoforms in nonmammalian vertebrate skeletal muscle. The Ca(2+) release via the RyR channel can be gated by two distinct modes: depolarization-induced Ca(2+) release (DICR) and Ca(2+)-induced Ca(2+) release (CICR). In frog muscle, α-RyR acts as the DICR channel, whereas β-RyR as the CICR channel. However, several lines of evidence suggest that CICR by β-RyR may make only a minor contribution to Ca(2+) release during E-C coupling. Comparison of frog and mammalian RyR isoforms highlights the marked differences in the patterns of Ca(2+) release mediated by RyR1 and RyR3 homologues. Interestingly, common features in the Ca(2+) release patterns are noticed between β-RyR and RyR1. We will discuss possible roles and significance of the two RyR isoforms in E-C coupling and other processes in nonmammalian vertebrate skeletal muscle.

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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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-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Modulation of calcium signalling by mitochondria

In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca(2+) signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species - which in turn modulate components of the Ca(2+) signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca(2+) pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca(2+) via the mitochondrial Ca(2+) uniporter or transporting Ca(2+) from the interior of the organelle into the cytosol by means of Na+/Ca(2+) or H+/Ca(2+) exchangers. Considerable progress in understanding the relationship between Ca(2+) signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.

Ischemia enhances activation by Ca2+ and redox modification of ryanodine receptor channels from rat brain cortex

Cerebral ischemia stimulates Ca2+ influx and thus increases neuronal intracellular free [Ca2+]. Using a rat model of cerebral ischemia without recirculation, we tested whether ischemia enhances the activation by Ca2+ of ryanodine receptor (RyR) channels, a requisite feature of RyR-mediated Ca2+-induced Ca2+ release (CICR). To this aim, we evaluated how single RyR channels from endoplasmic reticulum vesicles, fused into planar lipid bilayers, responded to cytoplasmic [Ca2+] changes. Endoplasmic reticulum vesicles were isolated from the cortex of rat brains incubated without blood flow for 5 min at 37 degrees C (ischemic) or at 4 degrees C (control). Ischemic brains displayed increased oxidative intracellular conditions, as evidenced by a lower ratio (approximately 130:1) of reduced/oxidized glutathione than controls (approximately 200:1). Single RyR channels from ischemic or control brains displayed the same three responses to Ca2+ reported previously, characterized by low, moderate, or high maximal activity. Relative to controls, RyR channels from ischemic brains displayed with increased frequency the high activity response and with lower frequency the low activity response. Both control and ischemic cortical vesicles contained the RyR2 and RyR3 isoforms in a 3:1 proportion, with undetectable amounts of RyR1. Ischemia reduced [3H]ryanodine binding and total RyR protein content by 35%, and increased at least twofold endogenous RyR2 S-nitrosylation and S-glutathionylation without affecting the corresponding RyR3 endogenous levels. In vitro RyR S-glutathionylation but not S-nitrosylation favored the emergence of high activity channels. We propose that ischemia, by enhancing RyR2 S-glutathionylation, allows RyR2 to sustain CICR; the resulting amplification of Ca2+ entry signals may contribute to cortical neuronal death.

Lysosomes co-localize with ryanodine receptor subtype 3 to form a trigger zone for calcium signalling by NAADP in rat pulmonary arterial smooth muscle

In arterial myocytes the Ca(2+) mobilizing messenger NAADP evokes spatially restricted Ca(2+) bursts from a lysosome-related store that are subsequently amplified into global Ca(2+) waves by Ca(2+)-induced Ca(2+)-release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs). Lysosomes facilitate this process by forming clusters that co-localize with a subpopulation of RyRs on the SR. We determine here whether RyR subtypes 1, 2 or 3 selectively co-localize with lysosomal clusters in pulmonary arterial myocytes using affinity purified specific antibodies. The density of: (1) alphalgP120 labelling, a lysosome-specific protein, in the perinuclear region of the cell (within 1.5mum of the nucleus) was approximately 4-fold greater than in the sub-plasmalemmal (within 1.5mum of the plasma membrane) and approximately 2-fold greater than in the extra-perinuclear (remainder) regions; (2) RyR3 labelling within the perinuclear region was approximately 4- and approximately 14-fold greater than that in the extra-perinuclear and sub-plasmalemmal regions, and approximately 2-fold greater than that for either RyR1 or RyR2; (3) despite there being no difference in the overall densities of fluorescent labelling of lysosomes and RyR subtypes between cells, co-localization with alphalgp120 labelling within the perinuclear region was approximately 2-fold greater for RyR3 than for RyR2 or RyR1; (4) co-localization between alphalgp120 and each RyR subtype declined markedly outside the perinuclear region. Furthermore, selective block of RyR3 and RyR1 with dantrolene (30muM) abolished global Ca(2+) waves but not Ca(2+) bursts in response to intracellular dialysis of NAADP (10nM). We conclude that a subpopulation of lysosomes cluster in the perinuclear region of the cell and form junctions with SR containing a high density of RyR3 to comprise a trigger zone for Ca(2+) signalling by NAADP.

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

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

Superoxide-induced potentiation in the hippocampus requires activation of ryanodine receptor type 3 and ERK

Reactive oxygen species (ROS) are required for the induction of long-term potentiation (LTP) and behave as signaling molecules via redox modifications of target proteins. In particular, superoxide is necessary for induction of LTP, and application of superoxide to hippocampal slices is sufficient to induce LTP in area CA1. Although a rise in postsynaptic intracellular calcium is necessary for LTP induction, superoxide-induced potentiation does not require calcium flux through N-methyl-d-aspartate (NMDA) receptors. Ryanodine receptors (RyRs) mediate calcium-induced calcium release from intracellular stores and have been shown to modulate LTP. In this study, we investigated the highly redox-sensitive RyRs and L-type calcium channels as calcium sources that might mediate superoxide-induced potentiation. In agreement with previous studies of skeletal and cardiac muscle, we found that superoxide enhances activation of RyRs in the mouse hippocampus. We identified a functional coupling between L-type voltage-gated calcium channels and RyRs and identified RyR3, a subtype enriched in area CA1, as the specific isoform required for superoxide-induced potentiation. Superoxide also enhanced the phosphorylation of extracellular signal-regulated kinase (ERK) in area CA1, and ERK was necessary for superoxide-induced potentiation. Thus superoxide-induced potentiation requires the redox targeting of RyR3 and the subsequent activation of ERK.

Personal recollections on the discovery of the ryanodine receptors of muscle

The intracellular Ca(2+) release channels are indispensable molecular machinery in practically all eukaryotic cells of multicellular animals. They serve a key role in cell signaling by way of Ca(2+) as a second messenger. In response to a signaling event, the channels release Ca(2+) from intracellular stores. The resulting rise in cytoplasmic Ca(2+) concentration triggers the cell to carry out its specialized role, after which the intracellular Ca(2+) concentration must be reduced so that the signaling event can again be repeated. There are two types of intracellular Ca(2+) release channels, i.e., the ryanodine receptors and the inositol triphosphate receptors. My focus in this minireview is to present a personal account, from the vantage point our laboratory, of the discovery, isolation, and characterization of the ryanodine receptors from mammalian muscle. There are three isoforms: ryanodine receptor 1 (RyR1), first isolated from rabbit fast twitch skeletal muscle; ryanodine receptor 2 (RyR2), first isolated from dog heart; and ryanodine receptor 3 (RyR3), first isolated from bovine diaphragm muscle. The ryanodine receptors are the largest channel structures known. The RyR isoforms are very similar albeit with important differences. Natural mutations in humans in these receptors have already been associated with a number of muscle diseases.

RyR1-specific requirement for depolarization-induced Ca2+ sparks in urinary bladder smooth muscle

Ryanodine receptor subtype 1 (RyR1) has been primarily characterized in skeletal muscle but several studies have revealed its expression in smooth muscle. Here, we used Ryr1-null mice to investigate the role of this isoform in Ca2+ signaling in urinary bladder smooth muscle. We show that RyR1 is required for depolarization-induced Ca2+ sparks, whereas RyR2 and RyR3 are sufficient for spontaneous or caffeine-induced Ca2+ sparks. Immunostaining revealed specific subcellular localization of RyR1 in the superficial sarcoplasmic reticulum; by contrast, RyR2 and RyR3 are mainly expressed in the deep sarcoplasmic reticulum. Paradoxically, lack of depolarization-induced Ca2+ sparks in Ryr1–/– myocytes was accompanied by an increased number of cells displaying spontaneous or depolarization-induced Ca2+ waves. Investigation of protein expression showed that FK506-binding protein (FKBP) 12 and FKBP12.6 (both of which are RyR-associated proteins) are downregulated in Ryr1–/– myocytes, whereas expression of RyR2 and RyR3 are unchanged. Moreover, treatment with rapamycin, which uncouples FKBPs from RyR, led to an increase of RyR-dependent Ca2+ signaling in wild-type urinary bladder myocytes but not in Ryr1–/– myocytes.

In conclusion, although decreased amounts of FKBP increase Ca2+ signals in Ryr1–/– urinary bladder myocytes the depolarization-induced Ca2+ sparks are specifically lost, demonstrating that RyR1 is required for depolarization-induced Ca2+ sparks and suggesting that the intracellular localization of RyR1 fine-tunes Ca2+ signals in smooth muscle.

Effects of ATP, Mg2+, and redox agents on the Ca2+ dependence of RyR channels from rat brain cortex

Despite their relevance for neuronal Ca(2+)-induced Ca(2+) release (CICR), activation by Ca(2+) of ryanodine receptor (RyR) channels of brain endoplasmic reticulum at the [ATP], [Mg(2+)], and redox conditions present in neurons has not been reported. Here, we studied the effects of varying cis-(cytoplasmic) free ATP concentration ([ATP]), [Mg(2+)], and RyR redox state on the Ca(2+) dependence of endoplasmic reticulum RyR channels from rat brain cortex. At pCa 4.9 and 0.5 mM adenylylimidodiphosphate (AMP-PNP), increasing free [Mg(2+)] up to 1 mM inhibited vesicular [(3)H]ryanodine binding; incubation with thimerosal or dithiothreitol decreased or enhanced Mg(2+) inhibition, respectively. Single RyR channels incorporated into lipid bilayers displayed three different Ca(2+) dependencies, defined by low, moderate, or high maximal fractional open time (P(o)), that depend on RyR redox state, as we have previously reported. In all cases, cis-ATP addition (3 mM) decreased threshold [Ca(2+)] for activation, increased maximal P(o), and shifted channel inhibition to higher [Ca(2+)]. Conversely, at pCa 4.5 and 3 mM ATP, increasing cis-[Mg(2+)] up to 1 mM inhibited low activity channels more than moderate activity channels but barely modified high activity channels. Addition of 0.5 mM free [ATP] plus 0.8 mM free [Mg(2+)] induced a right shift in Ca(2+) dependence for all channels so that [Ca(2+)] <30 microM activated only high activity channels. These results strongly suggest that channel redox state determines RyR activation by Ca(2+) at physiological [ATP] and [Mg(2+)]. If RyR behave similarly in living neurons, cellular redox state should affect RyR-mediated CICR.

Differential expression of ryanodine receptors in the rat cochlea

This study examined the localization and functional expression of ryanodine receptors (RyR) within the cochlea using a combination of reverse transcription-polymerase chain reaction, immunolabeling techniques, and confocal Ca2+ imaging. All three RyR isoform mRNA transcripts were detected in the adult rat cochlea. Immunoperoxidase and immunofluorescence labeling showed that the three isoforms were differentially expressed. The most pronounced RyR protein expression, involving all three isoforms, occurred in the cell bodies of the spiral ganglion neurons. RyR3 labeling extended to the synaptic terminals innervating the inner and outer hair cells. RyR2 expression also occurred in the inner hair cells and supporting cells of the organ of Corti, while cells associated with ion homeostasis in the cochlea, such as the interdental cells of the spiral limbus (RyR1), and the epithelial cells of the spiral prominence and basal cells of the stria vascularis (RyR2 and RyR3), were also immunopositive. The functionality of RyR-gated Ca2+ stores in the spiral ganglion neurons was shown by confocal calcium imaging of fluo-4 fluorescence in rat cochlear slices. Caffeine (5 mM) evoked an increase in intracellular Ca2+ concentration in the cell bodies of the spiral ganglion neurons which occurred inthe absence of external Ca2+. Ryanodine (50 nm-1 microM) evoked comparable increases in intracellular Ca2+ concentration. These findings suggest that RyR-mediated Ca2+ release may be involved in auditory neurotransmission, sound transduction, and cochlear electrochemical homeostasis.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

Three-dimensional visualization of FKBP12.6 binding to an open conformation of cardiac ryanodine receptor

The cardiac isoform of the ryanodine receptor (RyR2) from dog binds predominantly a 12.6-kDa isoform of the FK506-binding protein (FKBP12.6), whereas RyR2 from other species binds both FKBP12.6 and the closely related isoform FKBP12. The role played by FKBP12.6 in modulating calcium release by RyR2 is unclear at present. We have used cryoelectron microscopy and three-dimensional (3D) reconstruction techniques to determine the binding position of FKBP12.6 on the surface of canine RyR2. Buffer conditions that should favor the "open" state of RyR2 were used. Quantitative comparison of 3D reconstructions of RyR2 in the presence and absence of FKBP12.6 reveals that FKBP12.6 binds along the sides of the square-shaped cytoplasmic region of the receptor, adjacent to domain 9, which forms part of the four clamp (corner-forming) structures. The location of the FKBP12.6 binding site on "open" RyR2 appears similar, but slightly displaced (by 1-2 nm) from that found previously for FKBP12 binding to the skeletal muscle ryanodine receptor that was in the buffer that favors the "closed" state. The conformation of RyR2 containing bound FKBP12.6 differs considerably from that depleted of FKBP12.6, particularly in the transmembrane region and in the clamp structures. The x-ray structure of FKBP12.6 was docked into the region of the 3D reconstruction that is attributable to bound FKBP12.6, to show the relative orientations of amino acid residues (Gln-31, Asn-32, Phe-59) that have been implicated as being critical in interactions with RyR2. A thorough understanding of the structural basis of RyR2-FKBP12.6 interaction should aid in understanding the roles that have been proposed for FKBP12.6 in heart failure and in certain forms of sudden cardiac death.

Ryanodine receptor subtype 2 encodes Ca2+ oscillations activated by acetylcholine via the M2 muscarinic receptor/cADP-ribose signalling pathway in duodenum myocytes

In this study, we characterized the signalling pathway activated by acetylcholine that encodes Ca2+ oscillations in rat duodenum myocytes. These oscillations were observed in intact myocytes after removal of external Ca2+, in permeabilized cells after abolition of the membrane potential and in the presence of heparin (an inhibitor of inositol 1,4,5-trisphosphate receptors) but were inhibited by ryanodine, indicating that they are dependent on Ca2+ release from intracellular stores through ryanodine receptors. Ca2+ oscillations were selectively inhibited by methoctramine (a M2 muscarinic receptor antagonist). The M2 muscarinic receptor-activated Ca2+ oscillations were inhibited by 8-bromo cyclic adenosine diphosphoribose and inhibitors of adenosine diphosphoribosyl cyclase (ZnCl2 and anti-CD38 antibody). Stimulation of ADP-ribosyl cyclase activity by acetylcholine was evaluated in permeabilized cells by measuring the production of cyclic guanosine diphosphoribose (a fluorescent compound), which resulted from the cyclization of nicotinamide guanine dinucleotide. As duodenum myocytes expressed the three subtypes of ryanodine receptors, an antisense strategy revealed that the ryanodine receptor subtype 2 alone was required to initiate the Ca2+ oscillations induced by acetylcholine and also by cyclic adenosine diphosphoribose and rapamycin (a compound that induced uncoupling between 12/12.6 kDa FK506-binding proteins and ryanodine receptors). Inhibition of cyclic adenosine diphosphoribose-induced Ca2+ oscillations, after rapamycin treatment, confirmed that both compounds interacted with the ryanodine receptor subtype 2. Our findings show for the first time that the M2 muscarinic receptor activation triggered Ca2+ oscillations in duodenum myocytes by activation of the cyclic adenosine diphosphoribose/FK506-binding protein/ryanodine receptor subtype 2 signalling pathway.

Localization of TRPC1 channel in the sinus endothelial cells of rat spleen

The ultrastructural localization of transient receptor potential C1 (TRPC1) channels in the sinus endothelial cells of rat spleen was examined by confocal laser scanning and electron microscopy. In addition, the localization of the closely associated proteins and channels, VE-cadherin, calreticulin, inositol-1,4,5-trisphosphate receptors type 1 (IP3R1), and ryanodine receptor (RyR), was also examined. Immunofluorescence microscopy of tissue cryosections revealed TRPC1 channels to be localized within the cytoplasm, in the superficial layer of the apical and basal parts of the cells, and in the junctional area of the adjacent endothelial cells. The distribution of Ca2+-storing tubulovesicular structures within endothelial cells was established by using tissue sections treated with osmium ferricyanide. Electron microscopy revealed densely stained tubulovesicular structures closely apposed to the plasma membrane and that occasionally ran closely parallel to the plasma membrane and near the caveolae and junctional apparatus. Immunolocalization analysis at the electron microscopy level using immunogold bound to the secondary antibody confirmed that TRPC1 channels were localized in the plasma membrane, caveolae, and vesicular structures in the subplasmalemmal cytoplasm of sinus endothelial cells. Calreticulin was predominantly localized in endoplasmic reticulum. IP3R1 and RyR, considered to be type 3, were colocalized in endoplasmic reticulum in proximity to the plasma membrane and caveolae. Thus, TRPC1 channels in sinus endothelial cells of the spleen might play an important role in controlling blood cell passage through phenomena including cytoskeletal reorganization, cell retraction, and disassembly of adherens junctions.

Structural insights into excitation-contraction coupling by electron cryomicroscopy

In muscle, excitation-contraction coupling is defined as the process linking depolarization of the surface membrane with Ca2+ release from cytoplasmic stores, which activates contraction of striated muscle. This process is primarily controlled by interplay between two Ca2+ channels--the voltage-gated L-type Ca2+ channel (dihydropyridine receptor, DHPR) localized in the t-tubule membrane and the Ca2+-release channel (ryanodine receptor, RyR) of the sarcoplasmic reticulum membrane. The structures of both channels have been extensively studied by several groups using electron cryomicroscopy and single particle reconstruction techniques. The structures of RyR, determined at resolutions of 22-30 A, reveal a characteristic mushroom shape with a bulky cytoplasmic region and the membrane-spanning stem. While the cytoplasmic region exhibits a complex structure comprising a multitude of distinctive domains with numerous intervening cavities, at this resolution no definitive statement can be made about the location of the actual pore within the transmembrane region. Conformational changes associated with functional transitions of the Ca2+ release channel from closed to open states have been characterized. Further experiments determined localization of binding sites for various channel ligands. The structural studies of the DHPR are less developed. Although four 3D maps of the DHPR were reported recently at 24-30 A resolution from studies of frozen-hydrated and negatively stained receptors, there are some discrepancies between reported structures with respect to the overall appearance and dimensions of the channel structure. Future structural studies at higher resolution are needed to refine the structures of both channels and to substantiate a proposed molecular model for their interaction.

Inositol 1,4,5-trisphosphate and ryanodine receptors mobilize calcium from a common functional pool in human U373 MG cells

This investigation concentrates on the change in Ca(2+) concentration ([Ca(2+)]) caused by ryanodine in U373 MG cells. This cell type from a human astrocytoma is a unique cellular model because it only expresses the type 3 ryanodine receptor (RyR3), which is generally the least abundant isoform. In the presence of physiological [Ca(2+)] in the extracellular medium, U373 MG cells are caffeine-insensitive, even after forskolin treatment, and ryanodine-sensitive only when an unusually high concentration (30 microM) is applied. Xestospongin C behaves like thapsigargin and therefore cannot be used as a selective antagonist of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). After ryanodine challenge, addition of an analog of Substance P (SP), which should deplete InsP(3)-sensitive stores, has no effect on [Ca(2+)](i). After thapsigargin treatment, which unmasks the calcium leak from intracellular stores, neither ryanodine nor SP change [Ca(2+)](i), suggesting that thapsigargin completely depletes the ryanodine-sensitive and the InsP(3)-sensitive stores of U373 MG cells. Finally, in experiments monitoring the [Ca(2+)] in intracellular stores, InsP(3) stimulation of permeabilized cells causes a decrease in [Ca(2+)] that is not affected by subsequent ryanodine treatment. Our results support the conclusion that U373 MG cells express both InsP(3)Rs and RyRs that can individually or in combination mobilize only one functional Ca(2+) pool.

Expression levels of RyR1 and RyR3 control resting free Ca2+ in skeletal muscle

To better understand the role of the transient expression of ryanodine receptor (RyR) type 3 (RyR3) on Ca(2+) homeostasis during the development of skeletal muscle, we have analyzed the effect of expression levels of RyR3 and RyR1 on the overall physiology of cultured myotubes and muscle fibers. Dyspedic myotubes were infected with RyR1 or RyR3 containing virions at 0.2, 0.4, 1.0, and 4.0 moieties of infection (MOI), and analysis of their pattern of expression, caffeine sensitivity, and resting free Ca(2+) concentration ([Ca(2+)](r)) was performed. Although increased MOI resulted in increased expression of each receptor isoform, it did not significantly affect the immunopattern of RyRs or the expression levels of calsequestrin, triadin, or FKBP-12. Interestingly, myotubes expressing RyR3 always had significantly higher [Ca(2+)](r) and lower caffeine EC(50) than did cells expressing RyR1. Although some of the increased sensitivity of RyR3 to caffeine could be attributed to the higher [Ca(2+)](r) in RyR3-expressing cells, studies of [(3)H]ryanodine binding demonstrated intrinsic differences in caffeine sensitivity between RyR1 and RyR3. Tibialis anterior (TA) muscle fibers at different stages of postnatal development exhibited a transient increase in [Ca(2+)](r) coordinately with their level of RyR3 expression. Similarly, adult soleus fibers, which also express RyR3, had higher [Ca(2+)](r) than did adult TA fibers, which exclusively express RyR1. These data show that in skeletal muscle, RyR3 increases [Ca(2+)](r) more than RyR1 does at any expression level. These data suggest that the coexpression of RyR1 and RyR3 at different levels may constitute a novel mechanism by which to regulate [Ca(2+)](r) in skeletal muscle.

Ryanodine Receptor Oligomeric Interaction

Specific interactions between adjacent ryanodine receptor (RyR) molecules to form ordered two-dimensional arrays in the membrane have been demonstrated using electron microscopy both in situ, in tissues and cells, and in vitro, with the purified protein. RyR interoligomeric association has also been inferred from observations of simultaneous channel gating during multi-RyR channel recordings in lipid bilayers. In this study, we report experiments designed to identify the region(s) of the RyR molecule, participating in this reciprocal interaction. Using epitope-specific antibodies, we identified a RyR tryptic fragment that specifically bound the intact immobilized RyR. Three overlapping RyR fragments encompassing this epitope, expressed using an in vitro mammalian expression system, were immunoprecipitated by RyR. To refine the binding regions, smaller RyR fragments were expressed as glutathione S-transferase (GST) fusion proteins, and their binding to RyR was monitored using a "sandwich" enzyme-linked immunosorbent assay. Three GST-RyR fusion proteins demonstrated specific binding, dependent upon ionic strength. Binding was greatest at 50-150 mm NaCl for two GST-RyR constructs, and a third GST-RyR construct demonstrated maximum binding between 150 and 450 mm NaCl. The binding at high NaCl concentration suggested involvement of a hydrophobic interaction. In silico analysis of secondary structure showed evidence of coil regions in two of these RyR fragment sequences, which might explain these data. In GST pull-down assays, these same three fragments captured RyR2, and two of them retained RyR1. These results identify a region at the center of the linear RyR (residues 2540-3207 of human RyR2) which is able to bind to the RyR oligomer. This region may constitute a specific subdomain participating in RyR-RyR interaction.

Decreased expression of ryanodine receptors alters calcium-induced calcium release mechanism in mdx duodenal myocytes

It is generally believed that alterations of calcium homeostasis play a key role in skeletal muscle atrophy and degeneration observed in Duchenne's muscular dystrophy and mdx mice. Mechanical activity is also impaired in gastrointestinal muscles, but the cellular and molecular mechanisms of this pathological state have not yet been investigated. We showed, in mdx duodenal myocytes, that both caffeine- and depolarization-induced calcium responses were inhibited, whereas acetylcholine- and thapsigargin-induced calcium responses were not significantly affected compared with control mice. Calcium-induced calcium release efficiency was impaired in mdx duodenal myocytes depending only on inhibition of ryanodine receptor expression. Duodenal myocytes expressed both type 2 and type 3 ryanodine receptors and were unable to produce calcium sparks. In control and mdx duodenal myocytes, both caffeine- and depolarization-induced calcium responses were dose-dependently and specifically inhibited with the anti-type 2 ryanodine receptor antibody. A strong inhibition of type 2 ryanodine receptor in mdx duodenal myocytes was observed on the mRNA as well as on the protein level. Taken together, our results suggest that inhibition of type 2 ryanodine receptor expression in mdx duodenal myocytes may account for the decreased calcium release from the sarcoplasmic reticulum and reduced mechanical activity.

RyR1 exhibits lower gain of CICR activity than RyR3 in the SR: evidence for selective stabilization of RyR1 channel

We showed that frog alpha-ryanodine receptor (alpha-RyR) had a lower gain of Ca(2+)-induced Ca(2+) release (CICR) activity than beta-RyR in sarcoplasmic reticulum (SR) vesicles, indicating selective "stabilization" of the former isoform (Murayama T and Ogawa Y. J Biol Chem 276: 2953-2960, 2001). To know whether this is also the case with mammalian RyR1, we determined [(3)H]ryanodine binding of RyR1 and RyR3 in bovine diaphragm SR vesicles. The value of [(3)H]ryanodine binding (B) was normalized by the number of maximal binding sites (B(max)), whereby the specific activity of each isoform was expressed. This B/B(max) expression demonstrated that ryanodine binding of individual channels for RyR1 was <15% that for RyR3. Responses to Ca(2+), Mg(2+), adenine nucleotides, and caffeine were not substantially different between in situ and purified isoforms. These results suggest that the gain of CICR activity of RyR1 is markedly lower than that of RyR3 in mammalian skeletal muscle, indicating selective stabilization of RyR1 as is true of frog alpha-RyR. The stabilization was partly eliminated by FK506 and partly by solubilization of the vesicles with CHAPS, each of which was additive to the other. In contrast, high salt, which greatly enhances [(3)H]ryanodine binding, caused only a minor effect on the stabilization of RyR1. None of the T-tubule components, coexisting RyR3, or calmodulin was the cause. The CHAPS-sensitive intra- and intermolecular interactions that are common between mammalian and frog skeletal muscles and the isoform-specific inhibition by FKBP12, which is characteristic of mammals, are likely to be the underlying mechanisms.

A direct mass-action mechanism explains capacitative calcium entry in Jurkat and skeletal L6 muscle cells

We examined capacitative calcium entry (CCE) in Jurkat and in L6 skeletal muscle cells. We found that extracellular Ca2+ can enter the endoplasmic reticulum (ER) of both cell types even in the presence of thapsigargin, which blocks entry into the ER from the cytosol through the CaATPase. Moreover, extracellular Ca2+ entry into the ER was evident even when intracellular flow of Ca2+ was in the direction of ER to cytosol due to the presence of caffeine. ER Ca2+ content was assessed by two separate means. First, we used the Mag-Fura fluorescent dye, which is sensitive only to the relatively high concentrations of Ca2+ found in the ER. Second, we transiently expressed an ER-targeted derivative of aequorin, which reports Ca2+ by luminescence. In both cases, the Ca2+ concentration in the ER increased in response to extracellular Ca2+ after the ER had been previously depleted despite blockade by thapsigargin. We found two differences between the Jurkat and L6 cells. L6, but not Jurkat cells, inhibited Ca2+ uptake at very high Ca2+ concentrations. Second, ryanodine receptor blockers inhibited the appearance of cytosolic Ca2+ during CCE if added before Ca2+ in both cases, but the L6 cells were much more sensitive to ryanodine. Both of these can be explained by the known difference in ryanodine receptors between these cell types. These findings imply that the origin of cytosolic Ca2+ during CCE is the ER. Furthermore, kinetic data demonstrated that Ca2+ filled the ER before the cytosol during CCE. Our results suggest a plasma membrane Ca2+ channel and an ER Ca2+ channel joined in tandem, allowing Ca2+ to flow directly from the extracellular space to the ER. This explains CCE; any decrease in ER [Ca2+] relative to extracellular [Ca2+] would provide the gradient for refilling the ER through a mass-action mechanism.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

Three-dimensional reconstruction of the recombinant type 2 ryanodine receptor and localization of its divergent region 1

Isoform 2 of the ryanodine receptor (RyR2) is the major calcium release channel in cardiac muscle. In the present study, two kinds of RyR2 cDNA were constructed, one encoding the wild type mouse RyR2 (RyR2(wt)) and the other encoding modified RyR2, into which was inserted a cDNA encoding green fluorescent protein (GFP). GFP was inserted into the divergent region 1 (DR1) of RyR2, after the Asp-4365 (RyR2(D4365-GFP)). HEK293 cells expressing both RyR2(wt) and RyR2(D4365-GFP) cDNAs showed caffeine- and ryanodine-sensitive calcium release, demonstrating that both wild type and modified RyR2s form functional calcium release channels. Cells expressing the fusion protein, RyR2(D4365-GFP), were readily identified by their fluorescence due to the presence of GFP, indicating that the inserted GFP folded properly. Both expressed RyR2s were purified from cell lysates in a single step by affinity chromatography using a GST-FKBP12.6 as the affinity ligand. Cryoelectron microscopy of purified RyR2s showed structurally intact receptors, and three-dimensional reconstructions were obtained by single particle image processing. The three-dimensional reconstruction of RyR2(wt) appeared very similar to that of the native RyR2 purified from dog heart. The location of the inserted GFP, and consequently of DR1, was mapped on the three-dimensional structure of RyR2 to one of the subunit's characteristic domains, domain 3, also known as the "handle" domain. This study describes the first internal fusion of a protein into a ryanodine receptor, and it demonstrates the potential of this technology for localizing functional and structural domains on the three-dimensional structure of RyR.

Isoform-dependent formation of heteromeric Ca2+ release channels (ryanodine receptors)

Three ryanodine receptor (RyR) isoforms, RyR1, RyR2, and RyR3, are expressed in mammalian tissues. It is unclear whether RyR isoforms are capable of forming heteromeric channels. To investigate their ability to form heteromeric channels, we co-expressed different RyR isoforms in HEK293 cells and examined their interactions biochemically and functionally. Immunoprecipitation studies revealed that RyR2 is able to interact physically with RyR3 and RyR1 in HEK293 cells and that RyR1 does not interact with RyR3. Co-expression of a ryanodine binding deficient mutant of RyR2, RyR2 (I4827T), with RyR3 (wt) restored [(3)H]ryanodine binding to the mutant. Interactions between RyR isoforms were further assessed by complementation analysis using mutants RyR2 (I4827T), RyR2 (E3987A), RyR3 (I4732T), RyR3 (E3885A), and RyR1 (E4032A), all of which are deficient in caffeine response. Caffeine-induced Ca(2+) release was restored in HEK293 cells co-transfected with mutants RyR2 (I4827T) and RyR3 (E3885A), RyR2 (E3987A) and RyR3 (I4732T), or RyR2 (I4827T) and RyR1 (E4032A), but not with RyR1 (E4032A) and RyR3 (I4732T), indicating that mutants of RyR2 and RyR3, or RyR2 and RyR1, but not RyR1 and RyR3, are able to complement each other. Co-expression of RyR3 (wt) and a pore mutant of RyR2, RyR2 (G4824A), produced regulatable single channels with intermediate unitary conductances. These observations demonstrate that RyR2 is capable of forming functional heteromeric channels with RyR3 and RyR1, whereas RyR1 is incapable of forming heteromeric channels with RyR3.

Ryanodine receptor calcium release channels

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.

Roles of two ryanodine receptor isoforms coexisting in skeletal muscle

Ryanodine receptor (RyR) is a Ca(2)(+) release channel in the sarcoplasmic reticulum and plays an important role in excitation-contraction coupling in skeletal muscle. The Ca(2)(+) release through the RyR channel can be gated by two distinct modes: depolarization-induced Ca(2)(+) release (DICR) and Ca(2)(+)-induced Ca(2)(+) release (CICR). Two different RyR isoforms, RyR1 (or alpha-RyR) and RyR3 (or beta-RyR), have been found to be expressed in skeletal muscle. Most adult mammalian muscles express primarily RyR1, whereas almost equal amounts of the two RyR isoforms exist in many nonmammalian vertebrate muscles. RyR1 is believed to be responsible for both DICR and CICR, whereas RyR3 may function as the CICR channel. Recent findings demonstrate that alpha-RyR is selectively and markedly suppressed in CICR activity in frog skeletal muscle. This selective suppression of RyR1, although to a lesser extent, also was found to occur in mammalian skeletal muscle. This short review describes the biological meanings of this selective suppression and discusses physiological roles and significance of the two RyR isoforms in vertebrate skeletal muscle.

Fine mapping the candidate region for peripheral neuropathy with or without agenesis of the corpus callosum in the French Canadian population

Peripheral neuropathy with or without agenesis of the corpus callosum (ACCPN [MIM 2180000]) is an autosomal recessive disease characterised by progressive sensorimotor neuropathy, mental retardation, dysmorphic features and complete or partial agenesis of the corpus callosum. The ACCPN gene was mapped in 1996 to a 4 cM region on chromosome 15. We have since collected additional French Canadian (FC) families and typed a total of 11 polymorphic markers spanning approximately 18 cM on chromosome 15. Through the use of haplotype analysis we have confirmed the presence of a founder haplotype in the FC population, and identified critical recombinants which reduce the ACCPN candidate interval to a approximately 2 cM or 1000 Kb region flanked by markers D15S1040 and ACTC. Linkage disequilibrium analysis supports the haplotype data, and suggests that the ACCPN gene lies nearest to marker D15S1232.

Characterization of Ca2+ release from heterogeneous Ca2+ stores in sarcoplasmic reticulum isolated from arterial and gastric smooth muscle

Ca2+ transport was investigated in vesicles of sarcoplasmic reticulum subfractionated from bovine main pulmonary artery and porcine gastric antrum using digitonin binding and zonal density gradient centrifugation. Gradient fractions recovered at 15-33% sucrose were studied as the sarcoplasmic reticulum component using Fluo-3 fluorescence or 45Ca2+ Millipore filtration. Thapsigargin blocked active Ca2+ uptake and induced a slow Ca2+ release from actively loaded vesicles. Unidirectional 45Ca2+ efflux from passively loaded vesicles showed multicompartmental kinetics. The time course of an initial fast component could not be quantitatively measured with the sampling method. The slow release had a half-time of several minutes. Both components were inhibited by 20 microM ruthenium red and 10 mM Mg2+. Caffeine, inositol 1,4,5-trisphosphate, ATP, and diltiazem accelerated the slow component. A Ca2+ release component activated by ryanodine or cyclic adenosine diphosphate ribose was resolved with Fluo-3. Comparison of tissue responses showed that the fast Ca2+ release was significantly smaller and more sensitive to inhibition by Mg2+ and ruthenium red in arterial vesicles. They released more Ca2+ in response to inositol 1,4,5-trisphosphate and were more sensitive to activation by cyclic adenosine diphosphate ribose. Ryanodine and caffeine, in contrast, were more effective in gastric antrum. In each tissue, the fraction of the Ca2+ store released by sequential application of caffeine and inositol 1,4,5-trisphosphate depended on the order applied and was additive. The results indicate that sarcoplasmic reticulum purified from arterial and gastric smooth muscle represents vesicle subpopulations that retain functional Ca2+ channels that reflect tissue-specific pharmacological modulation. The relationship of these differences to physiological responses has not been determined.

Type 3 ryanodine receptors of skeletal muscle are segregated in a parajunctional position

A key event in skeletal muscle activation is the rapid release of Ca(2+) from the sarcoplasmic reticulum (SR), the Ca(2+) storage organelle in the muscle cell. The surface membrane/transverse tubules and the SR form functional units (calcium release units containing one or two couplons or junctions), where the voltage-sensing dihydropyridine receptor of the surface membrane interacts with the SR Ca(2+) release channel [ryanodine receptor (RyR)] and depolarization of the cell membrane is converted into Ca(2+) release from the SR. Although RyR1 is the most important isoform in skeletal muscle, some muscles also express high levels of RyR3, an isoform with a wide tissue distribution. The cytoplasmic domains of RyRs are visible in the electron microscope as periodically disposed feet. We find that, in muscles containing only RyR1, feet are exclusively located over the junctional SR surface facing the surface membrane/transverse tubule. In muscles containing RyR1 as well as RyR3, additional feet are located in lateral parajunctional regions immediately adjacent to junctional SR. Biochemical content of RyR3 and content of parajunctional feet are highly correlated in different muscles and the disposition of parajunctional versus junctional feet are notably different. On the basis of these two observations, we postulate that RyR3s are restricted to the parajunctional region, and thus their activation must be indirect and derivative during excitation-contraction coupling.

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.

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.