Central Core Disease Research Articles

G.P.7.11 Central core and cap disease in a case of congenital myopathy

G.P.7.11 Central core and cap disease in a case of congenital myopathy

G.P.1.02 Phenotypic spectrum of core-rod myopathy caused by dominant or recessive RYR1 mutations

G.P.1.01 Phenotypic variations of central core disease O. Paciello , A. Tammaro , L. Passamano , M. Scutifero , E. Picillo , A. Di Martino , S. Papparella , L. Politano University of Naples Federico II, Department of Veterinary Pathology, Naples, Italy, Cardarelli Hospital, Biotechnology Center for Malignant Hyperthermia, Naples, Italy, 3 Second University of Naples, Exp. Medicine – Cardiomyology and Medical Genetics, Naples, Italy, University of Naples Federico II, Department of Veterinary Myology, Naples, Italy

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

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

Mutation screening of the RYR1-cDNA from peripheral B-lymphocytes in 15 Swedish malignant hyperthermia index cases

Malignant hyperthermia (MH), linked to the ryanodine receptor 1 gene (RYR1) on chromosome 19, is a potentially lethal pharmacogenetic disorder which may lead to a disturbance of intracellular calcium homeostasis when susceptible individuals are exposed to halogenated anaesthetics, suxamethonium, or both. Central core disease (CCD) is a rare dominantly inherited congenital myopathy allelic to MH-susceptibility. In this study, 14 unrelated MH-susceptible probands and one CCD patient from Sweden were screened for mutations in the RYR1. Since the RYR1 is also expressed in B-lymphocytes, RYR1-cDNA was transcribed from total RNA extracted from white blood cells. We detected two known RYR1 mutations and two previously described unclassified sequence variants. In addition, six novel sequence variants were detected. All mutations or sequence variants were verified on genomic DNA. Seven of the probands did not show any candidate mutation, although the total coding region of RYR1 was sequenced. Segregation data in in vitro contracture tested family members of three probands support a causative role of three of the novel sequence variants. Our study contributes to the genetic aetiology of MH in Sweden, but also raises questions about the involvement of genes other than RYR1 since nearly half of the probands did not show any sequence variants in the total coding region of the RYR1.

A Structural Model of the Pore-Forming Region of the Skeletal Muscle Ryanodine Receptor (RyR1)

Ryanodine receptors (RyRs) are ion channels that regulate muscle contraction by releasing calcium ions from intracellular stores into the cytoplasm. Mutations in skeletal muscle RyR (RyR1) give rise to congenital diseases such as central core disease. The absence of high-resolution structures of RyR1 has limited our understanding of channel function and disease mechanisms at the molecular level. Here, we report a structural model of the pore-forming region of RyR1. Molecular dynamics simulations show high ion binding to putative pore residues D4899, E4900, D4938, and D4945, which are experimentally known to be critical for channel conductance and selectivity. We also observe preferential localization of Ca2+ over K+ in the selectivity filter of RyR1. Simulations of RyR1-D4899Q mutant show a loss of preference to Ca2+ in the selectivity filter as seen experimentally. Electrophysiological experiments on a central core disease mutant, RyR1-G4898R, show constitutively open channels that conduct K+ but not Ca2+. Our simulations with G4898R likewise show a decrease in the preference of Ca2+ over K+ in the selectivity filter. Together, the computational and experimental results shed light on ion conductance and selectivity of RyR1 at an atomistic level.

Late-onset axial myopathy with cores due to a novel heterozygous dominant mutation in the skeletal muscle ryanodine receptor ( RYR1) gene

Mutations in the skeletal muscle ryanodine receptor ( RYR1) gene have been associated with a wide range of phenotypes including the malignant hyperthermia (MH) susceptibility trait, Central Core Disease (CCD) and other congenital myopathies characterized by early onset and predominant proximal weakness. We report a patient presenting at 77 years with a predominant axial myopathy associated with prominent involvement of spine extensors, confirmed on MRI and muscle biopsy, compatible with a core myopathy. RYR1 mutational analysis revealed a novel heterozygous missense mutation (c.119G>T; p.Gly40Val) affecting the RYR1 N-terminus, previously predominantly associated with MH susceptibility. This case expands the spectrum of RYR1-related phenotypes and suggests that MH-related RYR1 mutations may give rise to overt neuromuscular symptoms later in life, with clinical features not typically found in CCD due to C-terminal hotspot mutations. Late-onset congenital myopathies may be under-recognised and diagnosis requires a high degree of clinical suspicion.

Cesarean Section in a Complicated Case of Central Core Disease

Cesarean Section in a Complicated Case of Central Core Disease

Central core disease and susceptibility to malignant hyperthermia in a single family

Sirs: Central core disease (CCD) is a rare congenital myopathy whose phenotype ranges from asymptomatic to severe muscle weakness. Histologically, type I skeletal muscle fibres predominate and show amorphous myofibrillar cores lacking mitochondria and oxidative enzyme activity. Clinical manifestations include hypotonia, delayed motor milestones, proximal muscle weakness, and skeletal anomalies [3, 5]. CCD is usually autosomal dominant with variable penetrance, but can be autosomal recessive [4]. CCD is usually associated with mutations in the ryanodine receptor gene (RYR1), which encodes a homotetrameric Ca-release channel (RyR1) of the sarcoplasmic reticulum crucially involved in excitationcontraction coupling [1, 9]. RYR1 mutations are also associated with malignant hyperthermia susceptibility (MH) and certain forms of the usually recessive multiminicore disease (MmD). About 41% of known RYR1 mutations produce MH, are located in the N-terminal (cytoplasmic) or central region (transmembrane) of the homotetramer, and recur sporadically. CCD-producing mutations are mainly confined to a few families and localize in the C-terminal (also cytoplasmic) domain [5, 7, 8, 10]. We describe two sisters with CCD and their asymptomatic consanguineous parents. The daughters were homozygous and the parents heterozygous for an RYR1 mutation. A 36-year-old woman and her 31-year-old sister were referred to us in 2007 for genetic counselling. We had seen them during childhood for early onset hypotonia and muscle weakness, with delayed motor milestones, and mild deterioration of motor abilities as childhood progressed. There was no family history of neuromuscular disease, but the parents were first cousins (Fig. 1a). Quadriceps needle biopsies from each sister had shown variable fibre size and central nuclei. Oxidative enzyme staining had shown one or more large cores in most fibres (Fig. 2) and type I fibre predominance. CCD was diagnosed. In 2007 the sisters had closely similar phenotypes: elongated face, congenital hip dislocation, foot deformities (repaired surgically during childhood), and thoraco-lumbar scoliosis. There was mild facial muscle weakness, scapular winging, normal arm muscle strength, but evident weakness of lower limb muscles; external ophthalmoplegia was absent. Neither sister could run; they could rise from the floor with difficulty using Gower’s manoeuvre, and climb stairs using a handrail. Serum creatine kinase and respiratory function were normal. Electromyography showed myopathic findings. The parents were asymptomatic with normal neurological examination; they refused muscle biopsy. After receiving written informed consent from each family member, genomic DNA from peripheral blood was screened for RYR1 mutations by PCR and sequencing. The sisters (II1 and II2) were homozygous for c.11708G[A mutation in exon 85 resulting in R3903Q substitution (arginine to glutamine); the parents (I1 and I2) were heterozygous for the same mutation (Fig. 1b). Galli et al. [2] described the same heterozygous mutation associated with MH. Our data suggest that this mutation causes classic CCD only when inherited homozygously, as in our sisters, since the heterozygous parents L. Colleoni G. Melli L. Morandi P. Cudia S. Romaggi R. Mantegazza P. Bernasconi (&) Neurology IV, Neuromuscular Diseases and Neuroimmunology, Neurological Institute Foundation ‘‘Carlo Besta’’, Via Celoria 11, 20133 Milan, Italy e-mail: pbernasconi@istituto-besta.it

α‐Skeletal muscle actin mutants causing different congenital myopathies induce similar cytoskeletal defects in cell line cultures

Central core disease (CCD), congenital fibre type disproportion (CFTD), and nemaline myopathy (NM) are earlyonset clinically heterogeneous congenital myopathies, characterized by generalized muscle weakness and hypotonia. All three diseases are associated with alpha-skeletal muscle actin mutations. We biochemically characterized the CCD and CFTD causing actin mutants and show that all mutants fold correctly and are stable. Expression studies in fibroblasts, myoblasts, and myotubes show that these mutants incorporate in filamentous structures. However they do not intercalate between the nascent z-lines in differentiating muscle cell cultures. We also show that the distribution of mitochondria and of the ryanodine receptors, and calcium release properties from ryanodine receptors, are unchanged in myotubes expressing the CCD causing mutants. CFTD causing mutants induce partly similar phenotypes as NM associated ones, such as rods and thickened actin fibers in cell culture. Our results suggest that molecular mechanisms behind CFTD and NM may be partly related.

Functional characterization of ryanodine receptor (RYR1) sequence variants using a metabolic assay in immortalized B-lymphocytes

Mutations in the RYR1 gene are linked to malignant hyperthermia (MH), central core disease and multi-minicore disease. We screened by DHPLC the RYR1 gene in 24 subjects for mutations, and characterized functional alterations caused by some RYR1 variants. Three novel sequence variants and twenty novel polymorphisms were identified. Immortalized lymphoblastoid cell lines from patients with RYR1 variants and from controls were stimulated with 4-chloro-m-cresol (4-CmC) and the rate of extracellular acidification was recorded. We demonstrate that the increased acidification rate of lymphoblastoid cells in response to 4-CmC is mainly due to RYR1 activation. Cells expressing RYR1 variants in the N-terminal and in the central region of the protein (p.Arg530His, p.Arg2163Pro, p.Asn2342Ser, p.Glu2371Gly and p.Arg2454His) displayed higher activity compared with controls; this could account for the MH-susceptible phenotype. Cell lines harboring RYR1(Cys4664Arg) were significantly less activated by 4-CmC. This result indicates that the p.Cys4664Arg variant causes a leaky channel and depletion of intracellular stores. The functional changes detected corroborate the variants analyzed as disease-causing alterations and the acidification rate measurements as a means to monitor Ca(2+)-induced metabolic changes in cells harboring mutant RYR1 channels.

Understanding The Molecular Defects Of Human MH And CCD

Malignant Hyperthermia (MH) and Central Core Disease (CCD) are skeletal muscle disorders linked to mutations in the skeletal muscle ryanodine receptor (RyR1). Based on their phenotypes, disease-causing RyR1 mutations can be separated into three major groups: MH-only, both MH and CCD (MH/CCD), and CCD-only. The molecular basis for these different genotype-phenotype relationships is largely undefined. We have recently demonstrated that the porcine MH mutation, R615C, increases the sensitivity of the RyR1 channel to luminal Ca2+ activation and reduces the threshold for spontaneous Ca2+ release during store Ca2+ overload, also known as store-overload-induced Ca2+ release (SOICR). To investigate whether human MH and CCD mutations also alter the luminal Ca2+ activation of RyR1 and SOICR, we have generated a number of MH-only, MH/CCD, and CCD-only mutations located in the NH2 terminal, central, and COOH-terminal regions, and established stable, inducible HEK293 cell lines expressing these mutants. Using single cell Ca2+ imaging, we found that MH-only and MH/CCD mutations enhance the propensity for SOICR. On the other hand, some CCD-only mutations suppress or abolish SOICR, but retain caffeine-induced Ca2+ release, while other CCD-only mutants display little or no ryanodine- or caffeine-sensitive channel activity. Single channel studies reveal that, like the R615C MH mutation, MH/CCD mutations markedly sensitize the RyR1 channel to activation by luminal Ca2+. To assess their impact in the context of muscle cells, we are currently establishing stable, inducible mouse skeletal muscle C2C12 cell lines expressing MH and CCD RyR1 mutants. Further studies will address the question of whether altered luminal Ca2+ activation of RyR1 underlies a common defect of human MH and CCD mutations.

A new role for type 1 ryanodine receptor

Type 1 ryanodine receptors (RyR1) are the second most common isoform found in neurons. We hypothesize that in nerve terminals from neurohypophysis, L-type Ca2+ channels are coupled to RyR1 in the same way found in EC coupling in skeletal muscle. In these nerve terminals we showed (J.Neuroscience 2006, 26 -7565) that L-type channels are the sensors of membrane potential for Voltage Induced Ca2+ Release (VICaR), independently of their role as Ca2+ current carriers and that RyRs are the effectors through which Ca2+ is released into the cytosol. Here we wished to determine which RyR isoforms are responsible for VICaR. We studied Ca2+ syntillas (scintilla, L., spark in synaptic structure, a nerve terminal) in two different mutant mice with a knock-in mutation in RyR1 in physiological extracellular [Ca2+]. In the first, R163C, which has a gain of function phenotype, described as leaky and causes Central Core Disease and Malignant Hyperthermia in humans, depolarization to -60mV from -80mV caused a global increase in [Ca2+]. This increase was not seen in WT where such depolarization caused only an increase in syntilla frequency. In the second RyR1 mutant, I4898T, which has a loss of function phenotype, described as EC uncoupling, and causes Central Core Disease in humans, there was not only an absence of the global increase in [Ca2+] but also a significant decrease in syntilla frequency at −60mV compared to −80mV. Moreover basal [Ca2+] in the terminals, measured using fura-2, was significantly lower in I4898T. Finally, depolarization to 0mV resulted in a 4 fold decrease in the Ca2+ transient compared to WT. These data shows, for the first time in a preparation other than skeletal muscle, that RyR1 is involved in determining global [Ca2+] both at rest and by a process of VICaR, upon depolarization.

Clues to the Formation of Cores in a Mouse Model of Malignant Hyperthermia

Malignant hyperthermia (MH) and central core disease (CCD) are closely related diseases of skeletal muscle linked to mutations in the ryanodine receptor (RyR1) gene. Heterozygous mice harbouring a human MH mutation (Y522S), which is linked to MH susceptibility (MHS) with cores, were shown to display MH susceptibility that is also associated with heat-induced sudden death and mitochondrial damage (Durham et al., 2008; Cell: 133, 53). Here we show that RyR1Y522S/wt fibers develop amorphous cores which mimic, at least in later stages, the structural alterations observed in muscle biopsies from CCD patients. By examining mice at various ages (2 m - 1 y), we identified early steps in the formation of cores, a feat that has not been possible in human CCD muscle. The earliest and most obvious event in “core” formation is the swelling and partial disruption of mitochondria within discrete regions of the cell. This defect quickly leads to disarray of closely associated sarcoplasmic reticulum (SR) and calcium release units (CRUs), ultimately leading to the eventual complete disorganization of both organelles. The core regions also exhibit clearly delimited myofibril contractures, probably due increased Ca2+ leak from the SR diffusing from adjacent regions that cannot be sequestered. In later stages, amorphous cores, lacking all structural components become more frequent. We suggest that the initial triggering event in core formation is a local imbalance in Ca2+ release due to increased RyR1 leak that initially alters mitochondrial activity and then eventually disrupts both mitochondria and SR structure/function. Loss of mitochondrial ATP production and SR Ca2+ sequestration in these regions leads to local contractures and sarcomeric disruption within the core regions.

Dysfunctional Mitochondria In Hearts From Type 1 Ryanodine Receptor (RyR1) Knockout Mice

Mutations in the genome of the cardiac or skeletal ryanodine receptor subtypes (RyR2 and RyR1, respectively) cause diseases like catecholaminergic polymorphic ventricular tachycardia, malignant hyperthermia, or central core disease. The objective of this study is to investigate the consequences of a dysfunctional ryanodine receptor type 1 (RyR1) on the functionality of heart mitochondria.In neonatal control mice, cardiac mitochondrial oxygen consumption was 20.68 ± 2.11 nmol/min/mg protein in the presence of 2 mM succinate or 5 mM malate and 5 mM glutamate. Addition of 1 mM ADP, to induce state 3 respiration, increased oxygen consumption significantly to 52.37 ± 2.14 nmol/min/mg (n=5). On the contrary, cardiac mitochondria from RyR1 knockout mice showed no increase of oxygen consumption upon addition of 1 mM ADP or 10 μM dinitrophenol. The respiratory control index of RyR1 knockout mice was 1.16 ± 0.48 (n=4) compared to 2.42 ± 0.38 in control mice (n=5).Atractyloside, a specific inhibitor of the adenine nucleotide translocase (ANT) did not inhibit oxygen consumption in cardiac tissue homogenates from RyR1 knockout mice. Consistent with this finding is that ANT protein was not detected using specific antibodies in heart mitochondria from RyR1 knockout mice (n=45), while VDAC and cytochrome c protein levels were not different. Lastly, expression mitochondrial creatine kinase was higher in RyR1 knockout mice than in control animals (n=8). However, mitochondrial contact sites, consisting of VDAC, the creatine kinase octamer and ANT, were absent in heart mitochondria from RyR1 knockout mice.These results suggest that RyR1 knockout mice have dysfunctional mitochondria and are unable to consume oxygen for ATP production. These findings are consistent with the notion that mitochondrial RyR1 plays a central role in Ca2+ regulated ATP production in the heart.Supported by HL-33333.

Ryanodine Receptor Pore Structure and Function

Ryanodine Receptors (RyRs) are ion channels that regulate muscle contraction by releasing calcium ions from intracellular stores into the cytoplasm. Mutations in skeletal muscle RyR (RyR1) give rise to congenital diseases such as the Central Core Disease. The absence of high-resolution structures of RyR1 has limited our understanding of channel function and disease mechanisms at the molecular level. Here, we report a structural model of the pore-forming region of RyR1 and electrophysiological studies on a Central Core Disease mutant RyR1-G4898R. Molecular dynamics simulations on the structural model show preferential localization of Ca2+ over K+ in the selectivity filter. We observe high ion binding to the residues D4899, E4900, D4938 and D4945 along the pore, which are experimentally known to be critical for channel function and selectivity. Furthermore, simulations on the mutant RyR1-D4899Q show a loss of preference to Ca2+ in the selectivity filter as seen experimentally. Electrophysiological experiments on RyR1-G4898R show constitutively open channels that conduct K+ but not Ca2+. Our simulations with G4898R likewise show a decrease in preference of Ca2+ over K+ in the selectivity filter. Together, the computational and experimental results shed light on the functioning of the RyR1 pore at an atomistic level.

S-nitrosylated Cysteins In The Y522S Ca2+ Release Channel RyR1

The Y522S mutation, linked to malignant hyperthermia (MH) and central core diseases in humans, causes a leaky Ca2+ release channel (RyR1), when engineered to mice (RyR1Y522S/wt). The elevated levels of resting Ca2+ in mice harboring the Y522S mutation drive an increased generation of reactive nitrogen species (RNS). S-nitrosylation of the mutant RyR1 increases its temperature sensitivity, resulting in uncontrolled muscle contractions during heat stress, and decreases its sensitivity to Ca2+ inhibition further promoting SR Ca2+ leak, leading to a feedforward cyclic mechanism that continuously increases the temperature sensitivity of RyR1 and RNS production [1].We have used selective isotope coded affinity tag labeling and mass spectroscopy, to identify the cysteines that are endogenously S-nitrosylated in the mutant Ca2+ release channel RyR1 and responsible for its temperature sensitivity.This work was supported by grants from NIH (AR050503 and AR053349) and MDA to SLH[1]. RyR1 S-Nitrosylation Underlies Environmental Heat Stroke and Sudden Death in Y522S RyR1 Knock in Mice, W.J. Durham, P. Aracena-Parks, C. Long, A.E. Rossi, S. A. Goonasekera, S. Boncompagni, D. L. Galvan, C.P. Gilman, M.R. Baker, N. Shirokova, F. Protasi, R. Dirksen, and S.L. Hamilton. Cell, (2008), 133, 53–65.

Deficits in Ca2+ Release and in vivo Muscle Strength in Heterozygous I4895T RyR1 Knock-In Mice

The mutation from isoleucine to threonine of the skeletal isoform of the ryanodine receptor (RyR1) at residue 4898 results in severe Central Core Disease (CCD). Under homozygous expression (IT/IT), we reported a lack of Ca2+ release in response to electrical and pharmacological activation despite SR Ca2+ store content indistinguishable from control. Here we used heterozygous knock-in mice for the I4895T (IT/+; analogous to human I4898T) RyR1 mutation to determine the effects of the mutation on muscle strength and Ca2+ handling in flexor digitorum brevis (FDB) and interosseous muscle fibers.We compared in vivo muscle strength of wild-type (WT) and IT/+ mice. IT/+ mice exhibited significant weakness in both upper body and grip strength assays (4-paw peak grip force: 2400 ± 70 mN, n=8 and 2040 ± 80 mN, n=14 in WT and IT/+ mice, respectively). We also determined the magnitude of action potential- and ligand-evoked Ca2+ release in single intact FDB fibers using Ca2+ fluorometry. The magnitude of both electrically- and ligand-evoked Ca2+ release was significantly reduced in IT/+ fibers. Moreover, the maximum rate of change in mag-fluo-4 fluorescence during the rising phase of the electrically-evoked Ca2+ transient was significantly reduced in IT/+ fibers (WT 0.17 ± 0.01 ΔF/F/ms vs IT/+ 0.11 ± 0.01 ΔF/F/ms, n= 53, 56, respectively). Finally, the frequency (1.9 ± 0.5 and 0.8 ± 0.3 events/scan) and Ca2+ spark mass (5.9 ± 0.3 and 4.6 ± 0.2 μm3) of local Ca2+ release induced by osmotic shock (440 mOsm with sucrose, 750 lines/sec) were reduced in acutely dissociated IT/+ interosseous fibers compared to that of WT fibers. Together, these findings are consistent with the hypothesis that the IT mutation in the putative RyR1 selectivity filter significantly reduces Ca2+ flux through the channel.

Ryanodine Receptor Structure: Progress and Challenges

Ryanodine Receptor Structure: Progress and Challenges

Joint hypermobility as a distinctive feature in the differential diagnosis of myopathies

Congenital and adult-onset inherited myopathies represent a wide spectrum of syndromes. Classification is based upon clinical features and biochemical and genetic defects. Joint hypermobility is one of the distinctive clinical features that has often been underrecognized so far. We therefore present an overview of myopathies associated with joint hypermobility: Ullrich congenital muscular dystrophy, Bethlem myopathy, congenital muscular dystrophy with joint hyperlaxity, multi-minicore disease, central core disease, and limb girdle muscular dystrophy 2E with joint hyperlaxity and contractures. We shortly discuss a second group of disorders characterised by both muscular features and joint hypermobility: the inherited disorders of connective tissue Ehlers-Danlos syndrome and Marfan syndrome. Furthermore, we will briefly discuss the extent and pattern of joint hypermobility in these myopathies and connective tissue disorders and propose two grading scales commonly used to score the severity of joint hypermobility. We will conclude focusing on the various molecules involved in these disorders and on their role and interactions in muscle and tendon, with a view to further elucidate the pathophysiology of combined hypermobility and myopathy. Hopefully, this review will contribute to enhanced recognition of joint hypermobility and thus be of aid in differential diagnosis.

Ryanodine Receptor Activation by Cav1.2 Is Involved in Dendritic Cell Major Histocompatibility Complex Class II Surface Expression

Dendritic cells express the skeletal muscle ryanodine receptor (RyR1), yet little is known concerning its physiological role and activation mechanism. Here we show that dendritic cells also express the Ca(v)1.2 subunit of the L-type Ca(2+) channel and that release of intracellular Ca(2+) via RyR1 depends on the presence of extracellular Ca(2+) and is sensitive to ryanodine and nifedipine. Interestingly, RyR1 activation causes a very rapid increase in expression of major histocompatibility complex II molecules on the surface of dendritic cells, an effect that is also observed upon incubation of mouse BM12 dendritic cells with transgenic T cells whose T cell receptor is specific for the I-A(bm12) protein. Based on the present results, we suggest that activation of the RyR1 signaling cascade may be important in the early stages of infection, providing the immune system with a rapid mechanism to initiate an early response, facilitating the presentation of antigens to T cells by dendritic cells before their full maturation.