Intracellular Ca(2+) release channels in evolution

Identification of calcium metabolism related score associated with the poor outcome in papillary thyroid carcinoma

Introduction

Papillary thyroid carcinoma is a type of thyroid cancer that exhibits significant variability in prognosis. Extensive research indicates that the impaired signaling of 1,25(OH)2D3-VDR may be a crucial factor in the development and progression of PTC.

Methods

To investigate this further, Integrated analysis mRNA expression information from The Cancer Genome Atlas and GEO, we compared gene expression in cancer and normal tissues and identified differentially expressed genes (DEGs). Through this analysis, we identified DEGs and calculated risk estimates for seven genetic markers.

Results

Subsequently, we constructed predictive models using LASSO-Cox regression to test the predictive value of these markers. Our results revealed that 64 calcium metabolism-related genes showed significant differences between tumor and normal tissues. Ten of the identified DEGs were significantly associated with overall survival, indicating their potential role in disease progression. Using the average risk score for the seven genetic markers, we divided patients into high- and low-risk groups. We found that patients in the low-risk group had significantly better overall survival than those in the high-risk group, highlighting the importance of these genetic markers in predicting prognosis. Further analysis using Cox regression demonstrated that the risk levels had independent predictive power. Additionally, we conducted functional analysis of the identified genetic markers, which showed significant differences in immune status between the two patient groups. We also investigated the effect of these calcium metabolism-related genes on thyroid cancer biological functions, immune microenvironment, and drug resistance.

Discussion

Our findings provide evidence of a novel genetic signature associated with calcium metabolism, which can predict prognosis in patients with PTC. These results may have significant implications for the development of new diagnostic and therapeutic approaches to improve outcomes for PTC patients.

A Comparative Perspective on Functionally-Related, Intracellular Calcium Channels: The Insect Ryanodine and Inositol 1,4,5-Trisphosphate Receptors

Calcium (Ca²⁺) homeostasis is vital for insect development and metabolism, and the endoplasmic reticulum (ER) is a major intracellular reservoir for Ca²⁺. The inositol 1,4,5- triphosphate receptor (IP₃R) and ryanodine receptor (RyR) are large homotetrameric channels associated with the ER and serve as two major actors in ER-derived Ca²⁺ supply. Most of the knowledge on these receptors derives from mammalian systems that possess three genes for each receptor. These studies have inspired work on synonymous receptors in insects, which encode a single IP₃R and RyR. In the current review, we focus on a fundamental, common question: “why do insect cells possess two Ca²⁺ channel receptors in the ER?”. Through a comparative approach, this review covers the discovery of RyRs and IP₃Rs, examines their structures/functions, the pathways that they interact with, and their potential as target sites in pest control. Although insects RyRs and IP₃Rs share structural similarities, they are phylogenetically distinct, have their own structural organization, regulatory mechanisms, and expression patterns, which explains their functional distinction. Nevertheless, both have great potential as target sites in pest control, with RyRs currently being targeted by commercial insecticide, the diamides.

NFIB induces functional astrocytes from human pluripotent stem cell-derived neural precursor cells mimicking in vivo astrogliogenesis

The ability to generate astrocytes from human pluripotent stem cells (hPSCs) offers a promising cellular model to study the development and physiology of human astrocytes. The extant methods for generating functional astrocytes required long culture periods and there remained much ambiguity on whether such paradigms follow the innate developmental program. In this report, we provided an efficient and rapid method for generating physiologically functional astrocytes from hPSCs. Overexpressing the nuclear factor IB in hPSC-derived neural precursor cells induced a highly enriched astrocyte population in 2 weeks. RNA sequencing and functional analyses demonstrated progressive transcriptomic and physiological changes in the cells, resembling in vivo astrocyte development. Further analyses substantiated previous results and established the MAPK pathway necessary for astrocyte differentiation. Hence, this differentiation paradigm provides a prospective in vitro model for human astrogliogenesis studies and the pathophysiology of neurological diseases concerning astrocytes.

The effect of edelfosine on GRA1 and MIC3 expressions in acute toxoplasmosis

Phosphoinositide-dependent phospholipase-C (PI-PLC) triggers the calcium signaling pathway which plays an important role in dense granule and microneme secretion and pathogenesis of Toxoplasma gondii (T. gondii). There are limited data about the effects of phospholipid analogues against T. gondii. The current study assessed the effect of edelfosine, as a phospholipid analogue, on GRA1 and MIC3 expressions using in vitro and in vivo models of acute toxoplasmosis. Infected Vero cells were treated by edelfosine in two subgroups: 24 h following the cell infection and treatment at the same time of cell infection. Animal study was performed on forty mice in four groups including non-infected, infected untreated, infected edelfosine-treated, and infected pyrimethamine-treated. Gene and protein expression analyses were done using quantitative real-time PCR and western blot, respectively. Edelfosine significantly reduced the GRA1 (P < 0.01) and MIC3 (P < 0.01) mRNA and protein expressions in 24 h following the cell infection and at the same time of cell infection groups. In vivo study showed that the edelfosine significantly reduced the GRA1 expression in eye, and MIC3 expression in brain and liver. Moreover, the edelfosine-treated infected mice had significant higher survival rate compared with uninfected mice. The reducing effect of edelfosine on GRA1 and MIC3 mRNA and protein levels 24 h following the cell infection was more than treatment at the same time of cell infection group. Moreover, the effect of edelfosine on GRA1 and MIC3 expression in animal tissues was variable. These data showed that the edelfosine may decrease the T. gondii excretory/secretory antigens through inhibition of PI-PLC.

Calcium signalling in early divergence of Metazoa: mechanisms involved in the control of muscle-like cell contraction in Hydra plagiodesmica

Our laboratory has previously examined the effect of neuropeptides on the activity of the hypostome of the hydra Hydra plagiodesmica Dioni, 1968 (Cnidaria: Hydrozoa). These results showed that the hypostome, a structure extruded during feeding, responds to myoregulatory peptides and that this mechanism might be regulated by changes in the cytosolic levels of calcium (Ca2+). We analyse now the ways in which Ca2+ modulates hypostome activity during feeding. The use of calcium chelators confirms that Ca2+ is relevant in inducing hypostome extrusion. The assay of compounds that modulate the activity of Ca2+ channels in the endoplasmic reticulum suggests that, beyond the extracellular influx of calcium, intracellular sources of the ion are involved and might include both ryanodine receptors (RyR) and the inositol 1,4,5-trisphosphate receptor (IP3R). Bioinformatic searches based on sequences of RyR and IP3R of humans (Homo sapiens Linnaeus, 1758) show that IP3Rs are present in all groups analysed, including Fungi and Choanoflagellata. Although H. plagiodesmica responds to caffeine and ryanodine, which are known to modulate RyRs, this family of receptors seems not to be predicted in Cnidaria, suggesting that this phylum either lacks these kinds of channels or that they possess a different structure compared with those possessed by other Metazoa.

Silence of ryanodine receptor gene decreases susceptibility to chlorantraniliprole in the oriental armyworm, Mythimna separata Walker

The ryanodine receptors of insects are the main target sites of diamide insecticides, which show highly selective insecticidal activity relative to toxicity in mammals and provide a novel option for managing lepidopteran pests. The oriental armyworm, Mythimna separata (Walker), is a destructive pest of agricultural crops, and great efforts have been undertaken to control this pest including repeated insecticide applications. In this study, full-length cDNA of a ryanodine receptor gene from M. separata (MsRyR) was cloned and characterized. The cDNA of MsRyR had a 15,372 bp open reading frame and encoded 5124 amino acids (GenBank ID: MG712298). MsRyR shares 78-97% identity with RyR isoforms of other insects, and <50% identities with Homo sapiens RyRs 1-3. Temporal and spatial expression analysis detected MsRyR at all developmental stages and in all tissues. The highest relative levels of MsRyR were detected in the second instar and head. Exposure to chlorantraniliprole after 24 h significantly increased the expression levels of whole body MsRyR mRNA. In addition, dietary ingestion of dsMsRyR significantly reduced the mRNA level of MsRyR and greatly decreased chlorantraniliprole-induced mortality. Our results revealed that the MsRyR could be the molecular target of chlorantraniliprole, and provided the basis for further understanding the resistance mechanism of chlorantraniliprole.

An analysis of variability in genome organisation of intracellular calcium release channels across insect orders

Using publicly available genomic data, combined with RT-PCR validation, we explore structural genomic variation for two major ion channels across insect classes. We have manually curated ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP₃R) ORFs and their corresponding genomic structures from 26 different insects covering major insect orders. We found that, despite high protein identity for both RyRs (>75%) and IP₃Rs (~67%), the overall complexity of the gene structure varies greatly between different insect orders with the simplest genes (fewest introns) found in Diptera and the most complex in Lepidoptera. Analysis of intron conservation patterns indicated that the majority of conserved introns are found close to the 5' end of the channels and in RyR around the highly conserved mutually exclusive splice site. Of the two channels the IP₃Rs appear to have a less well conserved organisation with a greater overall number of unique introns seen between insect orders. We experimentally validated two of the manually curated ORFs for IP₃Rs and confirmed an atypical (3799aa) IP₃R receptor in Myzus persicae, which is approximately 1000 amino acids larger than previously reported for IP₃Rs.

Molecular cloning, mRNA expression and alternative splicing of a ryanodine receptor gene from the citrus whitefly, Dialeurodes citri (Ashmead)

Insect ryanodine receptors are the main targets of diamide insecticides that have highly selective insecticidal activity but are less toxic to mammals. Therefore, these insecticides are ideal for pest control. Ryanodine receptors (RyRs) play a critical role in Ca²⁺ signaling in muscle and non-muscle cells. In this study, we cloned the complete cDNA (DcRyR) of the RyR from the citrus whitefly, Dialeurodes citri, a serious pest of citrus orchards in China. The open reading frame of RyR is 15,378bp long and encodes a protein with 5126 amino acids with a computed molecular weight of 579.523kDa. DcRyR shows a high amino acid sequence identity to RyRs from other insects (76%-95%) and low identity to those from nematodes and mammals (44%-52%). DcRyR shares many features of insect and vertebrate RyRs, including a MIR domain, two RIH domains, three SPRY domains, four copies of RyR repeat domain, RIH-associated domain at the N-terminus, two consensus calcium-binding EF-hands and six transmembrane domains at the C-terminus. The expression of DcRyR mRNA was the highest in the nymphs and lowest in eggs; DcRyR mRNA was 1.85-fold higher in the nymphs than in the eggs. Among the tissues, DcRyR mRNA expression was 4.18- and 4.02-fold higher in the adult head and thorax than in the abdomen. DcRyR had three alternative splice sites and the splice variants showed body part-specific expression and were developmentally regulated. These results may help investigate target-based resistance to diamide insecticides in D. citri.

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.

Vesicle-Mediated Steroid Hormone Secretion in Drosophila melanogaster

Steroid hormones are a large family of cholesterol derivatives regulating development and physiology in both the animal and plant kingdoms, but little is known concerning mechanisms of their secretion from steroidogenic tissues. Here, we present evidence that in Drosophila, endocrine release of the steroid hormone ecdysone is mediated through a regulated vesicular trafficking mechanism. Inhibition of calcium signaling in the steroidogenic prothoracic gland results in the accumulation of unreleased ecdysone, and the knockdown of calcium-mediated vesicle exocytosis components in the gland caused developmental defects due to deficiency of ecdysone. Accumulation of synaptotagmin-labeled vesicles in the gland is observed when calcium signaling is disrupted, and these vesicles contain an ABC transporter that functions as an ecdysone pump to fill vesicles. We propose that trafficking of steroid hormones out of endocrine cells is not always through a simple diffusion mechanism as presently thought, but instead can involve a regulated vesicle-mediated release process.

Molecular Characterization, mRNA Expression and Alternative Splicing of Ryanodine Receptor Gene in the Brown Citrus Aphid, Toxoptera citricida (Kirkaldy)

Ryanodine receptors (RyRs) play a critical role in regulating the release of intracellular calcium, which enables them to be effectively targeted by the two novel classes of insecticides, phthalic acid diamides and anthranilic diamides. However, less information is available about this target site in insects, although the sequence and structure information of target molecules are essential for designing new control agents of high selectivity and efficiency, as well as low non-target toxicity. Here, we provided sufficient information about the coding sequence and molecular structures of RyR in T. citricida (TciRyR), an economically important pest. The full-length TciRyR cDNA was characterized with an open reading frame of 15,306 nucleotides, encoding 5101 amino acid residues. TciRyR was predicted to embrace all the hallmarks of ryanodine receptor, typically as the conserved C-terminal domain with consensus calcium-biding EF-hands (calcium-binding motif) and six transmembrane domains, as well as a large N-terminal domain. qPCR analysis revealed that the highest mRNA expression levels of TciRyR were observed in the adults, especially in the heads. Alternative splicing in TciRyR was evidenced by an alternatively spliced exon, resulting from intron retention, which was different from the case of RyR in Myzus persicae characterized with no alternative splicing events. Diagnostic PCR analysis indicated that the splicing of this exon was not only regulated in a body-specific manner but also in a stage-dependent manner. Taken together, these results provide useful information for new insecticide design and further insights into the molecular basis of insecticide action.

Stress responses from the endoplasmic reticulum in cancer

The endoplasmic reticulum (ER) is a dynamic organelle that is essential for multiple cellular functions. During cellular stress conditions, including nutrient deprivation and dysregulation of protein synthesis, unfolded/misfolded proteins accumulate in the ER lumen, resulting in activation of the unfolded protein response (UPR). The UPR also contributes to the regulation of various intracellular signaling pathways such as calcium signaling and lipid signaling. More recently, the mitochondria-associated ER membrane (MAM), which is a site of close contact between the ER and mitochondria, has been shown to function as a platform for various intracellular stress responses including apoptotic signaling, inflammatory signaling, the autophagic response, and the UPR. Interestingly, in cancer, these signaling pathways from the ER are often dysregulated, contributing to cancer cell metabolism. Thus, the signaling pathway from the ER may be a novel therapeutic target for various cancers. In this review, we discuss recent research on the roles of stress responses from the ER, including the MAM.

Comparative characterization of two intracellular Ca²⁺-release channels from the red flour beetle, Tribolium castaneum

Ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP₃Rs) are members of a family of tetrameric intracellular Ca²⁺-release channels (CRCs). While it is well known in mammals that RyRs and IP₃Rs modulate multiple physiological processes, the roles of these two CRCs in the development and physiology of insects remain poorly understood. In this study, we cloned and functionally characterized RyR and IP₃R cDNAs (named TcRyR and TcIP₃R) from the red flour beetle, Tribolium castaneum. The composite TcRyR gene contains an ORF of 15,285 bp encoding a protein of 5,094 amino acid residues. The TcIP₃R contains an 8,175 bp ORF encoding a protein of 2,724 amino acids. Expression analysis of TcRyR and TcIP₃R revealed significant differences in mRNA expression levels among T. castaneum during different developmental stages. When the transcript levels of TcRyR were suppressed by RNA interference (RNAi), an abnormal folding of the adult hind wings was observed, while the RNAi-mediated knockdown of TcIP₃R resulted in defective larval–pupal and pupal–adult metamorphosis. These results suggested that TcRyR is required for muscle excitation-contraction (E-C) coupling in T. castaneum, and that calcium release via IP₃R might play an important role in regulating ecdysone synthesis and release during molting and metamorphosis in insects.

N-terminus oligomerization is conserved in intracellular calcium release channels

Oligomerization of all three mammalian ryanodine receptor isoforms, a structural requirement for normal intracellular Ca²⁺ release channel function, is displayed by the discrete N-terminal domain which assembles into homo- and hetero-tetramers. This is demonstrated in yeast, mammalian cells and native tissue by complementary yeast two-hybrid, chemical cross-linking and co-immunoprecipitation assays. The IP₃ (inositol 1,4,5-trisphosphate) receptor N-terminus (residues 1–667) similarly exhibits tetrameric association as indicated by chemical cross-linking and co-immunoprecipitation assays. The presence of either a 15-residue splice insertion or of the cognate ligand IP₃ did not affect tetramerization of the IP₃ receptor N-terminus. Thus N-terminus tetramerization appears to be an essential intrinsic property that is conserved in both the ryanodine receptor and IP₃ receptor families of mammalian intracellular Ca²⁺ release channels.

Intracellular Ca²⁺ channels are of paramount importance for numerous cellular processes. In the present paper we report on a novel N-terminus intersubunit interaction, an essential structure–function parameter, which is conserved in both families of intracellular Ca²⁺ channels.

Molecular cloning and characterization of a ryanodine receptor gene in brown planthopper (BPH), Nilaparvata lugens (Stål)

BACKGROUND

Ryanodine receptors (RyRs) are a distinct class of intracellular calcium (Ca(2+)) release channel. The recent discovery of diamide insecticides has prompted studies on insect RyRs. However, information about the structure and function of insect RyRs is still limited. In this study, we isolated and characterized a full-length RyR cDNA (named NlRyR) from the brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae), a serious rice pest throughout Asia.

RESULTS

The composite NlRyR gene contains an open reading frame of 15 423 bp encoding a protein of 5140 amino acid residues, which shares high sequence identity (78-81%) with other insect homologues, except for two regions (IDR1: 4379-4732; IDR2: 1307-1529) with markedly low identity (44-48 and 38-41%, respectively). All hallmarks of the RyR proteins are conserved in the NlRyR protein, including the RyR domain as well as mannosyltransferase, IP3 R and RyR (pfam02815) (MIR) and RyR and IP3 R homology (pfam01365) (RIH) domains. Expression analysis of NlRyR revealed significant differences in mRNA expression levels among N. lugens developmental stages. Furthermore, three alternative splicing sites were identified in NlRyR, one of which forms the mutually exclusive exons A/B and is conserved in various insect species. Diagnostic PCR assays showed that the splice variant containing exon A was predominantly detected in all developmental stages.

CONCLUSION

NlRyR may play an important role in the control of developmental processes of N. lugens. Alternative splicing may generate the functional diversity of NlRyR. The results provided the basis for further structural and functional characterization of NlRyR.

Molecular characteristics, mRNA expression, and alternative splicing of a ryanodine receptor gene in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Ryanodine receptors (RyRs) are a distinct class of ligand-gated channels controlling the release of calcium from intracellular stores. The emergence of diamide insecticides, which selectively target insect RyRs, has promoted the study of insect RyRs. In the present study, the full-length RyR cDNA (BdRyR) was cloned and characterized from the oriental fruit fly, Bactrocera dorsalis (Hendel), a serious pest of fruits and vegetables throughout East Asia and the Pacific Rim. The cDNA of BdRyR contains a 15,420-bp open reading frame encoding 5,140 amino acids with a predicted molecular weight of 582.4 kDa and an isoelectric point of 5.38. BdRyR shows a high level of amino acid sequence identity (78 to 97%) to other insect RyR isoforms. All common structural features of the RyRs are present in the BdRyR, including a well-conserved C-terminal domain containing consensus calcium-binding EF-hands and six transmembrane domains, and a large N-terminal domain. Quantitative real-time PCR analyses revealed that BdRyR was expressed at the lowest and highest levels in egg and adult, respectively, and that the BdRyR expression levels in the third instar larva, pupa and adult were 166.99-, 157.56- and 808.56-fold higher, respectively, than that in the egg. Among different adult body parts, the highest expression level was observed in the thorax compared with the head and abdomen. In addition, four alternative splice sites were identified in the BdRyR gene, with the first, ASI, being located in the central part of the predicted second spore lysis A/RyR domain. Diagnostic PCR analyses revealed that alternative splice variants were generated not only in a tissue-specific manner but also in a developmentally regulated manner. These results lay the foundation for further understanding the structural and functional properties of BdRyR, and the molecular mechanisms for target site resistance in B. dorsalis.

Ryanodine receptor calcium release channels: an evolutionary perspective

Ryanodine receptors (RyRs), along with the related inositol 1,4,5-trisphosphate receptors (IP(3)Rs), mediate the release of Ca(2+) from intracellular organelles of eukaryotes. As discussed in other chapters, such increases in intracellular Ca(2+) levels act a fundamental second messenger, regulating a diverse array of cellular processes. For over two decades, it has been reported that vertebrates express multiple RYR genes, whereas non-vertebrate multicellular organisms possess a single homologue within their genomes. Recently, the existence of RyR-like channels in unicellular organisms has also been reported. This chapter exploits recent expansions in available genome data to generate an overview of the expression of RyR-like genes in organisms representing a broad range of viral, archaeal, bacterial and eukaryotic taxa. Analyses of the multidomain structures and phylogenetic relationships of these proteins has lead to a model in which, early during eukaryotic evolution, IP(3)R-like ancestral Ca(2+) release channels were converted to RyR proteins via the addition of promiscuous protein domains, possibly via horizontal gene transfer mechanisms.

Ryanodine receptors, a family of intracellular calcium ion channels, are expressed throughout early vertebrate development

BACKGROUND

Calcium signals ([Ca2+]i) direct many aspects of embryo development but their regulation is not well characterised. Ryanodine receptors (RyRs) are a family of intracellular Ca2+ release channels that control the flux of Ca2+ from internal stores into the cytosol. RyRs are primarily known for their role in excitation-contraction coupling in adult striated muscle and ryr gene mutations are implicated in several human diseases. Current evidence suggests that RyRs do not have a major role to play prior to organogenesis but regulate tissue differentiation.

FINDINGS

The sequences of the five zebrafish ryr genes were confirmed, their evolutionary relationship established and the primary sequences compared to other vertebrates, including humans. RyRs are differentially expressed in slow (ryr1a), fast (ryr3) and both types (ryr1b) of developing skeletal muscle. There are two ryr2 genes (ryr2a and ryr2b) which are expressed exclusively in developing CNS and cardiac tissue, respectively. In addition, ryr3 and ryr2a mRNA is detectable in the initial stages of development, prior to embryonic axis formation.

CONCLUSIONS

Our work reveals that zebrafish ryr genes are differentially expressed throughout the developing embryo from cleavage onwards. The data suggests that RyR-regulated Ca2+ signals are associated with several aspects of embryonic development, from organogenesis through to the differentiation of the musculoskeletal, cardiovascular and nervous system. These studies will facilitate further work to explore the developmental function of RyRs in each of these tissue types.

A model of the physiological basis of a multivariate phenotype that is mediated by Ca(2+) signaling and controlled by ryanodine receptor composition

Calcium-signals occur in a wide variety of tissue types - from skeletal, smooth and cardiac muscle to pancreatic and brain tissues. Ca(2+) signals regulate diverse processes including muscle contraction, hormone secretion, neural communication and gene expression. Together these different tissues and processes form the basis of a multivariate trait. Calcium signals are characterized by Ca(2+) transients, which are sharp increases in Ca(2+) concentration over a short period of time. In this paper we derive and analyze a model of Ca(2+) transients for skeletal muscle, neurons and cardiac tissue based on underlying biophysical principles. Tissue differentiation in our model and in nature comes about by varying the ryanodine receptor (RyR) channel composition of tissues. In vertebrates, there are typically three types of RyR channels (labeled RyR1, RyR2 and RyR3 in mammals and α-RyR, cardiac-RyR and β-RyR in birds, amphibians and fish). Different compositions of these three RyR channels generate different Ca(2+) transient properties. There are four Ca(2+) transient properties that we measure: maximum amplitude, duration, half duration (D(50)) and integrated concentration. In agreement with experimental work, our results find that the addition of RyR3 amplifies Ca(2+) transients in skeletal muscle. An important consequence of shared molecular components between tissue types in a multivariate setting is that the shared components cause individual traits of a multivariate trait to be correlated in function. Here we show how correlations in Ca(2+) transient properties between tissues can be predicted using an underlying biophysical model.

Ca2+ signaling tools acquired from prostasomes are required for progesterone-induced sperm motility

Progesterone-induced calcium ion (Ca2+) signals in the neck region of sperm play a pivotal role in promoting sperm motility. Here, we show that a long-lasting Ca2+ signal required for sperm motility in response to progesterone depends on their pH-dependent fusion with prostasomes, which are small vesicles secreted by the prostate. We found that prostasome fusion led to the transfer of progesterone receptors, cyclic adenosine diphosphoribose (cADPR)-synthesizing enzymes, ryanodine receptors (RyRs), and other Ca2+ signaling tools from prostasomes to the sperm neck. Progesterone-induced sperm motility relied on cADPR-mediated Ca2+ mobilization through RyR located on acidic Ca2+ stores, followed by Ca2+ entry through store-operated channels. Treatment of prostasome-fused sperm with a cADPR antagonist or fusion with prostasomes in which type 2 RyR was depleted resulted in low fertilization rates, reduced sperm motility, or both. Thus, we conclude that sperm motility depends on the acquisition of Ca2+ signaling tools from prostasomes.

A calcium-induced calcium release mechanism supports luteinizing hormone-induced testosterone secretion in mouse Leydig cells

Leydig cells are responsible for the synthesis and secretion of testosterone, processes controlled by luteinizing hormone (LH). Binding of LH to a G protein-coupled receptor in the plasma membrane results in an increase in cAMP and in intracellular Ca2+ concentration ([Ca2+]i). Here we show, using immunofluorescence, that Leydig cells express ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs). Measurements of intracellular calcium changes using the fluorescent calcium-sensitive dye fluo-3 and confocal microscopy show that both types of receptors are involved in a calcium-induced calcium release (CICR) mechanism, which amplifies the initial Ca2+ influx through plasma membrane T-type calcium channels (CaV3). The RyRs and IP3Rs are functional, as judged from both their activation by caffeine and IP3 and block by ryanodine and 2-aminoethoxydiphenyl borate (2-APB), respectively. RyRs are the principal players involved in the release of Ca2+ from the endoplasmic reticulum, as evidenced by the fact that global Ca2+ changes evoked by LH are readily blocked by 100 μM ryanodine but not by 2-APB or xestospongin C. Finally, steroid production by Leydig cells is inhibited by ryanodine but not by 2-APB. These results not only broaden our understanding of the role played by calcium in Leydig cells but also show, for the first time, that RyRs have an important role in determining testosterone secretion by the testis.

Molecular characterization of flubendiamide sensitivity in the lepidopterous ryanodine receptor Ca(2+) release channel

Flubendiamide is a benzenedicarboxamide derivative that shows selective insecticidal activity against lepidopterous insects. The specific modulatory effects of flubendiamide on ryanodine binding in insect muscle microsomal membranes suggest that the ryanodine receptor (RyR) Ca(2+) release channel is a primary target of flubendiamide. However, the molecular mechanisms underlying the species-specific action of flubendiamide are unclear. We have cloned cDNA encoding a novel RyR from the lepidopterous silkworm RyR (sRyR) and tested the sensitivity to flubendiamide of the recombinant sRyR in HEK293 cells. Confocal localization studies and Ca(2+) imaging techniques revealed that sRyRs form Ca(2+) release channels in the endoplasmic reticulum. Importantly, flubendiamide induced release of Ca(2+) through the sRyR, but not through the rabbit RyR isoforms. Photoaffinity labeling of sRyR deletion mutants using a photoreactive derivative revealed that flubendiamide is mainly incorporated into the transmembrane domain (amino acids 4111-5084) of the sRyR. The rabbit cardiac muscle isoform RyR2 (rRyR2) and the RyR mutant carrying a replacement of the transmembrane domain (residues 4084-5084) with its counterpart sequence from rRyR2 (residues 3936-4968) were not labeled by the photoreactive compound. This replacement in the sRyR significantly impaired the responses to flubendiamide but only marginally reduced the sensitivity to caffeine, a general RyR activator. Furthermore, deletion of the N-terminal sequence (residues 183-290) abolished the responses of the sRyR to flubendiamide but not the sensitivity to caffeine. Our results suggest that the transmembrane domain plays an important role in the formation of an action site for flubendiamide, while the N-terminus is a structural requirement for flubendiamide-induced activation of the sRyR.

A gene-specific cerebral types 1, 2, and 3 RyR protein knockdown induces an antidepressant-like effect in mice

Elevation of baseline intracellular calcium levels was observed in platelets or lymphoblasts of patients with bipolar affective disorders suggesting an altered intracellular Ca(2+) homeostasis in the pathophysiology of mood disorders. The role of supraspinal endoplasmic ryanodine receptors (RyRs), which allow mobilization of intracellular Ca(2+) stores, in the modulation of depressive states was, then, investigated. Ryanodine and FK506 reduced the immobility time in the mouse forced swimming test showing an antidepressant-like profile comparable with that produced by amitriptyline and clomipramine. We generated types 1, 2, and 3 RyR knockdown mice by using selective antisense oligonucleotides (aODN) to investigate the role of each RyR isoform. A gene-specific cerebral RyR protein level reduction in knockdown animals was demonstrated by immunoblotting, immunoprecipitation, and immunohistochemical experiments. Repeated intracerebroventricular administration of aODNs complementary to the sequence of the types 1, 2, or 3 RyR produced an antidepressant-like response in the forced swimming test. The aODN-induced reduction of immobility time was temporary and reversible and did not impair motor coordination, spontaneous mobility, and exploratory activity. These findings identify cerebral RyRs as critical targets underlying depressive states and should facilitate the comprehension of the pathophysiology of mood disorders and help developing of new therapeutical strategies.

Development of a neurotoxic equivalence scheme of relative potency for assessing the risk of PCB mixtures

PCBs produce adverse effects in humans and animals by several modes of action. The first mode of action is binding of coplanar or mono-ortho-PCBs to the aryl hydrocarbon (Ah) receptor leading to effects associated with the activation of this receptor. The remaining PCB congeners do not activate this receptor and have different modes of action underlying their toxic effects. One mode of action that has been shown for di-ortho-substituted non-coplanar PCBs (PCB congeners with two or more chlorines in the ortho-positions) is the interference with intracellular signaling pathways dependent on Ca(2+) homeostasis and the resulting cellular, organ-level and organismal effects. The ortho-substituted non-coplanar congeners produce other cellular or organ-level effects including changes in protein kinase C translocation, changes in cellular dopamine (DA) uptake, formation of reactive oxygen species, and thyroid effects. Here, we propose a scheme for developing relative potency estimates (REP) for the PCB congeners not considered in the TEF scheme used to assess the toxicity of coplanar and mono-ortho-PCBs and chlorinated dioxins and furans. Because a number of the modes of action listed here for the ortho-substituted non-coplanar PCB congeners have been implicated in the neurotoxic effects of these PCBs congeners, this relative potency scheme is referred to here as the Neurotoxic Equivalent (NEQ) scheme for estimating toxicity of PCB mixtures. The Neurotoxic Equivalent (NEQ) values are developed in a way similar in concept to the derivation of the well-known TEF congener values. Although this scheme is in its infancy and the set of NEQ values are limited by the current data, there are several compelling reasons for proposing such a scheme now. First, it should open discussions as to how different modes of action can be utilized to predict congener potency differences for the effects they produce. Second, consideration and evaluation of the ability of the proposed NEQ scheme to predict the toxicity of PCB mixtures will assist in the identification of the specific modes of action relevant to the effects produced by non-coplanar PCBs. If other modes of action are suggested and subsequently identified, then other schemes of relative potency could be developed specifically for those modes of action, distinct from either the TEF scheme or the NEQ scheme. Knowing these other modes of action and the relative toxicity of the various congeners would advance our understanding of PCB toxicology and thereby ultimately improve our ability to estimate the toxic potency of PCB mixtures for each identified mode of action. Third, a quantitative scheme for assessing the toxicity of the non-coplanar PCB congeners present in a mixture has the potential to improve significantly future risk assessments of PCB mixtures.

Comparative genomic and phylogenetic analyses of calcium ATPases and calcium-regulated proteins in the apicomplexa

The phylum Apicomplexa comprises a large group of early branching eukaryotes that includes a number of human and animal parasites. Calcium controls a number of vital processes in apicomplexans including protein secretion, motility, and differentiation. Despite the importance of calcium as a second messenger, very little is known about the systems that control homeostasis or that regulate calcium signaling in parasites. The recent completion of many apicomplexan genomes provides new opportunity to define calcium response pathways in this group of parasites in comparison to model organisms. Whole-genome comparison between the apicomplexans Plasmodium spp., Cryptosporidium spp., and Toxoplasma gondii revealed the presence of several P-Type Ca2+ transporting ATPases including a single endoplasmic reticulum (ER)-type sarcoplasmic-endoplasmic reticulum Ca2+ ATPase, several Golgi-like Ca2+ ATPases, and a single Ca2+/H+ exchanger. Only T. gondii showed evidence of plasma membrane-type Ca2+ ATPases or voltage-gated calcium channels. Despite pharmacological evidence for IP3 and ryanodine-mediated calcium release, animal-type calcium channels were not readily identified in parasites, indicating they are more similar to plants. Downstream of calcium release, a variety of EF-hand-containing proteins regulate calcium responses. Our analyses detected a single conserved calmodulin (CaM) homologue, 3 distinct centrin (CETN)-caltractin-like proteins, one of which is shared with ciliates, and a variety of deep-branching, CaM-CETN-like proteins. Apicomplexans were also found to contain a wide array of calcium-dependent protein kinases (CDPKs), which are commonly found in plants. Toxoplasma gondii contains more than 20 CDPK or CDPK-related kinases, which likely regulate a variety of responses including secretion, motility, and differentiation. Genomic and phylogenetic comparisons revealed that apicomplexans contain a variety of unusual calcium response pathways that are distinct from those seen in vertebrates. Notably, plant-like pathways for calcium release channels and calcium-dependent kinases are found in apicomplexans. The experimental flexibility of T. gondii should allow direct experimental manipulation of these pathways to validate their biological roles. The central importance of calcium in signaling and development, and the novel characteristics of many of these systems, indicates that parasite calcium pathways may be exploited as new therapeutic targets for intervention.

Evidence that the cADPR signalling pathway controls calcium-mediated microneme secretion in Toxoplasma gondii

The protozoan parasite Toxoplasma gondii relies on calcium-mediated exocytosis to secrete adhesins on to its surface where they can engage host cell receptors. Increases in intracellular calcium occur in response to Ins(1,4,5)P3 and caffeine, an agonist of ryanodine-responsive calcium-release channels. We examined lysates and microsomes of T. gondii and detected evidence of cADPR (cyclic ADP ribose) cyclase and hydrolase activities, the two enzymes that control the second messenger cADPR, which causes calcium release from RyR (ryanodine receptor). We also detected endogenous levels of cADPR in extracts of T. gondii. Furthermore, T. gondii microsomes that were loaded with 45Ca2+ released calcium when treated with cADPR, and the RyR antagonists 8-bromo-cADPR and Ruthenium Red blocked this response. Although T. gondii microsomes also responded to Ins(1,4,5)P3, the inhibition profiles of these calcium-release channels were mutually exclusive. The RyR antagonists 8-bromo-cADPR and dantrolene inhibited protein secretion and motility in live parasites. These results indicate that RyR calcium-release channels that respond to the second-messenger cADPR play an important role in regulating intracellular Ca2+, and hence host cell invasion, in protozoan parasites.

Mechanisms of Ca2+ liberation at fertilization

The mechanisms underlying the Ca2+ release at fertilization of several animal organisms are reported. Four main classical theories are described, i.e., that of Ca2+ release following simple sperm contact and a G protein stimulation; that of simple sperm contact followed by a tyrosine kinase receptor activation; that of the necessity of introduction by sperm into the egg of molecules for Ca2+ release; and that the molecule introduced into the marine eggs for Ca2+ release is the same Ca2+. Two other mechanisms for Ca2+ release are also illustrated: that of ryanodine receptor stimulation and that of NAADP formation.

Intracellular calcium release and cardiac disease

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

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Role of calcium during Toxoplasma gondii invasion and egress

Calcium is a ubiquitous signalling molecule involved in a large number of cellular processes in eukaryotic cells. In the obligate intracellular parasite, Toxoplasma gondii, for example, a rise in calcium concentration is associated with significant morphological changes, secretion of proteins involved in host cell invasion and rapid egress from the host cell. Recent findings indicate that calcium released from the parasite's intracellular pools is necessary and sufficient to induce some of the events critical for invasion and egress. In addition, ethanol, a powerful inducer of invasion-related events, is shown here to also induce rapid egress from the host cell, indicating that a common mechanism for calcium release might be involved during both invasion and egress.

Distinct Roles of Inositol 1,4,5-Trisphosphate Receptor Types 1 and 3 in Ca2+ Signaling

Three subtypes of inositol 1,4,5-trisphosphate receptor (IP(3)R1, IP(3)R2, and IP(3)R3) Ca(2+) release channel share basic properties but differ in terms of regulation. To what extent they contribute to complex Ca(2+) signaling, such as Ca(2+) oscillations, remains largely unknown. Here we show that HeLa cells express comparable amounts of IP(3)R1 and IP(3)R3, but knockdown by RNA interference of each subtype results in dramatically distinct Ca(2+) signaling patterns. Knockdown of IP(3)R1 significantly decreases total Ca(2+) signals and terminates Ca(2+) oscillations. Conversely, knockdown of IP(3)R3 leads to more robust and long lasting Ca(2+) oscillations than in controls. Effects of IP(3)R3 knockdown are surprisingly similar in COS-7 cells that predominantly (>90% of total IP(3)R) express IP(3)R3, suggesting that IP(3)R3 functions as an anti-Ca(2+)-oscillatory unit without contributing to peak amplitude of Ca(2+) signals, irrespective of its relative expression level. Therefore, differential expression of the IP(3)R subtype is critical for various forms of Ca(2+) signaling, and, particularly, IP(3)R1 and IP(3)R3 have opposite roles in generating Ca(2+) oscillations.

Evidence for the transport of glutathione through ryanodine receptor channel type 1

In the present study, we have investigated the role of RyR1 (ryanodine receptor calcium channel type 1) in glutathione (GSH) transport through the sarcoplasmic reticulum (SR) membrane of skeletal muscles. Lanthanum chloride, a prototypic blocker of cation channels, inhibited the influx and efflux of GSH in SR vesicles. Using a rapid-filtration-based assay and lanthanum chloride as a transport blocker, an uptake of radiolabelled GSH into SR vesicles was observed. Pretreatment of SR vesicles with the RyR1 antagonists Ruthenium Red and ryanodine as well as with lanthanum chloride blocked the GSH uptake. An SR-like GSH uptake appeared in microsomes obtained from an HEK-293 (human embryonic kidney 293) cell line after transfection of RyR1. These observations strongly suggest that RyR1 mediates GSH transport through the SR membranes of skeletal muscles.

Calcium-induced calcium release in smooth muscle: the case for loose coupling

This article reviews the key experiments demonstrating calcium-induced calcium release (CICR) in smooth muscle and contrasts the biophysical and molecular features of coupling between the sarcolemmal (L-type Ca(2+) channel) and sarcoplasmic reticulum (ryanodine receptor) Ca(2+) channels in smooth and cardiac muscle. Loose coupling refers to the coupling process in smooth muscle in which gating of ryanodine receptors is non-obligate and may occur with a variable delay following opening of the sarcolemmal Ca(2+) channels. These features have been observed in the earliest studies of CICR in smooth muscle and are in marked contrast to cardiac CICR, where a close coupling between T-tubular and SR membranes results in tight coupling between the gating events. The relationship between this "loose coupling" and distinct subcellular release sites within smooth muscle cells, termed frequent discharge sites, is discussed.

Biochemical characterization, distribution and phylogenetic analysis of Drosophila melanogaster ryanodine and IP3 receptors, and thapsigargin-sensitive Ca2+ ATPase

We characterized the biochemistry, distribution and phylogeny of Drosophila ryanodine (RyR) and inositol triphosphate (IP3R) receptors and the endoplasmic reticulum Ca2+-ATPase (SERCA) by using binding and enzymatic assays, confocal microscopy and amino acid sequence analysis. [3H]-ryanodine binding in total membranes was enhanced by AMP-PCP, caffeine and xanthine, whereas Mg2+, Ruthenium Red and dantrolene were inhibitors. [3H]-ryanodine binding showed a bell-shaped curve with increasing free [Ca2+], without complete inhibition at millimolar levels of [Ca2+]. [3H]-IP3 binding was inhibited by heparin, 2-APB and xestospongin C. Microsomal Ca2+-ATPase activity was inhibited by thapsigargin. Confocal microscopy demonstrated abundant expression of ryanodine and inositol triphosphate receptors and abundant Ca2+-ATPase in Drosophila embryos and adults. Ryanodine receptor was expressed mainly in the digestive tract and parts of the nervous system. Maximum parsimony and Neighbour Joining were used to generate a phylogenetic classification of Drosophila ryanodine and insitol triphosphate receptors and Ca2+-ATPase based on 48 invertebrate and vertebrate complete sequences. The consensus trees indicated that Drosophila proteins grouped with proteins from other invertebrates, separately from vertebrate counterparts. Despite evolutionary distances, our functional results demonstrate that Drosophila ryanodine and inositol triphosphate receptors and Ca2+-ATPase are reasonably similar to vertebrate counterparts. Our protein expression data are consistent with the known functions of these proteins in the Drosophila digestive tract and nervous system. Overall, results show Drosophila as a valuable tool for intracellular Ca2+ dynamics studies in eukaryotes.

Glutathione transport in the endo/sarcoplasmic reticulum

Glutathione transport through the endo/sarcoplasmic reticulum (ER/SR) membrane might play a role in the maintenance of the thiol redox potential difference between the lumen and the cytosol. The transport of glutathione (both GSH and glutathione disulfide, GSSG) is entirely different in the ER and SR membranes. The transport measurements based on either rapid filtration or light scattering techniques revealed that the SR membrane transports glutathione much faster than the hepatic ER membrane or microsomal membranes prepared from heart or brain. The fastest transport has been measured in the membrane of muscle terminal cisternae, which is enriched in ryanodine receptor type 1 (RyR1). All the studied membranes have been found to be equally impermeable to various hydrophilic substances of similar size to glutathione, thus the glutathione transport in muscle microsomes and terminal cysternae as well as the correlation between the rate of glutathione transport and the abundance of RyR1 are specific. In both muscle microsomes and terminal cysternae, glutathione influx can be either inhibited or activated by antagonists and agonists of the ryanodine receptor, respectively, while these agents do not influence the transport of other small permeant molecules. These findings strongly suggest that the ryanodine receptor channel activity is directly associated with glutathione transport activity in the skeletal muscle sarcoplasmic reticulum membrane.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

Molecular genetics of ryanodine receptors Ca2+-release channels

The family of ryanodine receptor (RyR) genes encodes three highly related Ca(2+)-release channels: RyR1, RyR2 and RyR3. RyRs are known as the Ca(2+)-release channels that participate to the mechanism of excitation-contraction coupling in striated muscles, but they are also expressed in many other cell types. Actually, in several cells two or three RyR isoforms can be co-expressed and interactive feedbacks among them may be important for generation of intracellular Ca(2+) signals and regulation of specific cellular functions. Important developments have been obtained in understanding the biochemical complexity underlying the process of Ca(2+) release through RyRs. The 3-D structure of these large molecules has been obtained and some regulatory regions have been mapped within these 3-D reconstructions. Recent studies have clarified the role of protein kinases and phosphatases that, by physically interacting with RyRs, appear to play a role in the regulation of these Ca(2+)-release channels. These and other recent advancements in understanding RyR biology will be the object of this review.

Homer proteins and InsP(3) receptors co-localise in the longitudinal sarcoplasmic reticulum of skeletal muscle fibres

Striated muscle represents one of the best models for studies on Ca(2+) signalling. However, although much is known on the localisation and molecular interactions of the ryanodine receptors (RyRs), far less is known on the localisation and on the molecular interactions of the inositol trisphosphate receptors (InsP(3)Rs) in striated muscle cells. Recently, members of the Homer protein family have been shown to cluster type 1 metabotropic glutamate receptors (mGluR1) in the plasma membrane and to interact with InsP(3)R in the endoplasmic reticulum of neurons. Thus, these scaffolding proteins are good candidates for organising plasma membrane receptors and intracellular effector proteins in signalosomes involved in intracellular Ca(2+) signalling. Homer proteins are also expressed in skeletal muscle, and the type 1 ryanodine receptor (RyR1) contains a specific Homer-binding motif. We report here on the relative sub-cellular localisation of InsP(3)Rs and Homer proteins in skeletal muscle cells with respect to the localisation of RyRs. Immunofluorescence analysis showed that both Homer and InsP(3)R proteins present a staining pattern indicative of a localisation at the Z-line, clearly distinct from that of RyR1. Consistent herewith, in sub-cellular fractionation experiments, Homer proteins and InsP(3)R were both found in the fractions enriched in longitudinal sarcoplasmic reticulum (LSR) but not in fractions of terminal cisternae that are enriched in RyRs. Thus, in skeletal muscle, Homer proteins may play a role in the organisation of a second Ca(2+) signalling compartment containing the InsP(3)R, but are apparently not involved in the organisation of RyRs at triads.

Toxoplasma gondii microneme secretion involves intracellular Ca(2+) release from inositol 1,4,5-triphosphate (IP(3))/ryanodine-sensitive stores

Calcium-mediated microneme secretion in Toxoplasma gondii is stimulated by contact with host cells, resulting in the discharge of adhesins that mediate attachment. The intracellular source of calcium and the signaling pathway(s) triggering release have not been characterized, prompting our search for mediators of calcium signaling and microneme secretion in T. gondii. We identified two stimuli of microneme secretion, ryanodine and caffeine, which enhanced release of calcium from parasite intracellular stores. Ethanol, a previously characterized trigger of microneme secretion, stimulated an increase in parasite inositol 1,4,5-triphosphate, implying that this second messenger may mediate intracellular calcium release. Consistent with this observation, xestospongin C, an inositol 1,4,5-triphosphate receptor antagonist, inhibited microneme secretion and blocked parasite attachment and invasion of host cells. Collectively, these results suggest that T. gondii possess an intracellular calcium release channel with properties of the inositol 1,4,5-triphosphate/ryanodine receptor superfamily. Intracellular calcium channels, previously studied almost exclusively in multicellular animals, appear to also be critical to the control of parasite calcium during the initial steps of host cell entry.

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

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

A pivotal role for cADPR-mediated Ca2+ signaling: regulation of endothelin-induced contraction in peritubular smooth muscle cells

cADPR, a potent calcium-mobilizing intracellular messenger synthesized by ADP-ribosyl cyclases regulates openings of ryanodine receptors (RyR). Here we report that in the rat testis, a functional cADPR Ca2+ release system is essential for the contractile response of peritubular smooth muscle cells (PSMC) to endothelin (ET). We previously showed that this potent smooth muscle agonist elicits intracellular Ca2+ release in PSMC and seminiferous tubule contraction via activation of ETA and ETB receptors. ETB-R induces the mobilization of a thapsigargin-sensitive but IP3-independent intracellular Ca2+ pool. Stimulation of permeabilized PSMC with cADPR was found to elicit large Ca2+ releases blocked by either a selective antagonist of cADPR or a RyR blocker, but not by heparin. Western blotting and confocal fluorescence microscopy indicated the specific expression of type 2 RyR in perinuclear localization. ET was found to stimulate the activity of ADP-ribosyl cyclase. Microinjection of the selective cADPR antagonist 8NH2-cADPR completely abolished subsequent stimulation of Ca2+ signaling via ETA and ETB receptors. cADPR therefore appears to have an obligatory role for ETA-R and ETB-R-mediated calcium signaling in PSMC. However, ETB-R seem to be coupled exclusively to cADPR whereas ETA-R activation may be linked to IP3 and cADPR signaling pathways.

Imperatoxin a enhances Ca(2+) release in developing skeletal muscle containing ryanodine receptor type 3

Most adult mammalian skeletal muscles contain only one isoform of ryanodine receptor (RyR1), whereas neonatal muscles contain two isoforms (RyR1 and RyR3). Membrane depolarization fails to evoke calcium release in muscle cells lacking RyR1, demonstrating an essential role for this isoform in excitation-contraction coupling. In contrast, the role of RyR3 is unknown. We studied the participation of RyR3 in calcium release in wild type (containing both RyR1 and RyR3 isoforms) and RyR3-/- (containing only RyR1) myotubes in the presence or absence of imperatoxin A (IpTxa), a high-affinity agonist of ryanodine receptors. IpTxa significantly increased the amplitude and the rate of release only in wild-type myotubes. Calcium currents, recorded simultaneously with the transients, were not altered with IpTxa treatment. [(3)H]ryanodine binding to RyR1 or RyR3 was significantly increased in the presence of IpTxa. Additionally, IpTxa modified the gating and conductance level of single RyR1 or RyR3 channels when studied in lipid bilayers. Our data show that IpTxa can interact with both RyRs and that RyR3 is functional in myotubes and it can amplify the calcium release signal initiated by RyR1, perhaps through a calcium-induced mechanism. In addition, our data indicate that when RyR3-/- myotubes are voltage-clamped, the effect of IpTxa is not detected because RyR1s are under the control of the dihydropyridine receptor.

Identification and function of ryanodine receptor subtype 3 in non-pregnant mouse myometrial cells

Subtype 3 of the ryanodine receptor (RYR3) is a ubiquitous Ca2+ release channel which is predominantly expressed in smooth muscle tissues and certain regions of the brain. We show by reverse transcription-polymerase chain reaction (RT-PCR) that non-pregnant mouse myometrial cells expressed only RYR3 and therefore could be a good model for studying the role of endogenous RYR3. Expression of RYR3 was confirmed by Western blotting and immunostaining. Confocal Ca2+ measurements revealed that in 1.7 mM extracellular Ca2+, neither caffeine nor photolysis of caged Ca2+ were able to trigger any Ca2+ responses, whereas in the same cells oxytocin activated propagated Ca2+ waves. However, under conditions of increased sarcoplasmic reticulum (SR) Ca2+ loading, brought about by superfusing myometrial cells in 10 mM extracellular Ca2+, all the myometrial cells responded to caffeine and photolysis of caged Ca2+, indicating that it was possible to activate RYR3. The caffeine-induced Ca2+ responses were inhibited by intracellular application of an anti-RYR3-specific antibody. Immunodetection of RYR3 with the same antibody revealed a rather homogeneous distribution of fluorescence in confocal cell sections. In agreement with these observations, spontaneous or triggered Ca2+ sparks were not detected. In conclusion, our results suggest that under conditions of increased SR Ca2+ loading, endogenous RYR3 may contribute to the Ca2+ responses of myometrial cells.

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

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

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

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

Ryanodine receptor channel-dependent glutathione transport in the sarcoplasmic reticulum of skeletal muscle

We found that glutathione transport across endo/sarcoplasmic reticulum membranes correlates with the abundance of ryanodine receptor type 1 (RyR1). The transport was the fastest in muscle terminal cisternae, fast in muscle microsomes and slow in liver, heart, and brain microsomes. Glutathione influx could be inhibited by RyR1 blockers and the inhibitory effect was counteracted by RyR1 agonists. The effect of blockers was specific to glutathione, as the transport of other small molecules was not hindered. Therefore, the glutathione transport activity seems to be associated with RyR1 in sarcoplasmic reticulum.

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

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