Changes in the organization of excitation-contraction coupling structures in failing human heart

Background

The cardiac myocyte t-tubular system ensures rapid, uniform cell activation and several experimental lines of evidence suggest changes in the t-tubular system and associated excitation-contraction coupling proteins may occur in heart failure.

Methods and Results

The organization of t-tubules, L-type calcium channels (DHPRs), ryanodine receptors (RyRs) and contractile machinery were examined in fixed ventricular tissue samples from both normal and failing hearts (idiopathic (non-ischemic) dilated cardiomyopathy) using high resolution fluorescent imaging. Wheat germ agglutinin (WGA), Na-Ca exchanger, DHPR and caveolin-3 labels revealed a shift from a predominantly transverse orientation to oblique and axial directions in failing myocytes. In failure, dilation of peripheral t-tubules occurred and a change in the extent of protein glycosylation was evident. There was no change in the fractional area occupied by myofilaments (labeled with phalloidin) but there was a small reduction in the number of RyR clusters per unit area. The general relationship between DHPRs and RyR was not changed and RyR labeling overlapped with 51±3% of DHPR labeling in normal hearts. In longitudinal (but not transverse) sections there was an ∼30% reduction in the degree of colocalization between DHPRs and RyRs as measured by Pearson''s correlation coefficient in failing hearts.

Conclusions

The results show that extensive remodelling of the t-tubular network and associated excitation-contraction coupling proteins occurs in failing human heart. These changes may contribute to abnormal calcium handling in heart failure. The general organization of the t-system and changes observed in failure samples have subtle differences to some animal models although the general direction of changes are generally similar.     

Subcellular distribution of ryanodine receptors in the cardiac muscle of carp (Cyprinus carpio)

We examined the subcellular localization of ryanodine receptors (RyR) in the cardiac muscle of carp using biochemical, immunohistochemical, and electron microscopic methods and compared it with those of rats and guinea pigs. To achieve this goal, an anti-RyR antibody was newly raised against a synthetic peptide corresponding to an amino acid sequence that was conserved among all sequenced RyRs. Western blot analysis using this antibody detected a single RyR band following the SDS-PAGE of sarcoplasmic reticulum (SR) membranes from carp atrium and ventricle as well as from mammalian hearts and skeletal muscles. The carp heart band had slightly greater mobility than those of mammalian hearts. Although immunohistochemical staining showed evident striations corresponding to the Z lines in longitudinal sections of mammalian hearts, clusters of punctate staining, in contrast, were distributed ubiquitously throughout carp atrium and ventricle. Electron microscopic images of the carp myocardium showed that the SR was observed largely as the subsarcolemmal cisternae and the reticular SR, suggesting that the RyR is localized in the junctional and corbular SR.     

Nanoscale Distribution of Ryanodine Receptors and Caveolin-3 in Mouse Ventricular Myocytes: Dilation of T-Tubules near Junctions

We conducted super-resolution light microscopy (LM) imaging of the distribution of ryanodine receptors (RyRs) and caveolin-3 (CAV3) in mouse ventricular myocytes. Quantitative analysis of data at the surface sarcolemma showed that 4.8% of RyR labeling colocalized with CAV3 whereas 3.5% of CAV3 was in areas with RyR labeling. These values increased to 9.2 and 9.0%, respectively, in the interior of myocytes where CAV3 was widely expressed in the t-system but reduced in regions associated with junctional couplings. Electron microscopic (EM) tomography independently showed only few couplings with caveolae and little evidence for caveolar shapes on the t-system. Unexpectedly, both super-resolution LM and three-dimensional EM data (including serial block-face scanning EM) revealed significant increases in local t-system diameters in many regions associated with junctions. We suggest that this regional specialization helps reduce ionic accumulation and depletion in t-system lumen during excitation-contraction coupling to ensure effective local Ca²⁺ release. Our data demonstrate that super-resolution LM and volume EM techniques complementarily enhance information on subcellular structure at the nanoscale.The contraction of cardiac ventricular myocytes depends on the rapid cell-wide transient increase in intracellular [Ca²⁺] upon depolarization of the cell-membrane potential. The cardiac ryanodine receptor (RyR) (1), which is the intracellular Ca²⁺ release channel in the sarcoplasmic reticulum (SR), plays a central role in shaping Ca²⁺ transients. RyRs form clusters of various sizes (2,3) with the majority located within junctions between the SR and the surface membrane and its cytoplasmic extension, the transverse tubular (t-) system. It has been suggested that some RyR clusters are associated with caveolae, a specialized signaling microdomain of the surface membrane. Previous studies were complicated by the limited resolution of optical imaging methods of ∼250 nm, much larger than the nanometer scale of RyRs and caveolae. Accordingly, these studies report varying colocalization between RyRs and caveolin-3 (CAV3), a caveolar marker also expressed in the t-system (4,5).In this work, we investigated the relative distribution of CAV3 and RyRs in mouse ventricular myocytes both in the cytosol and near the cell surface with super-resolution fluorescence microscopy that achieves a resolution approaching 30 nm. Our data revealed unexpected local t-system swellings near junctional couplings, which was supported by two different three-dimensional electron microscopy (EM) modalities with <10-nm resolution: EM tomography and serial block-face scanning EM (SBFSEM).Super-resolution images of CAV3 and RyR labeling at the surface sarcolemma of mouse myocytes showed little overlap, suggesting that few RyRs were in couplings with caveolae (Fig. 1 A, for detailed methods, see the Supporting Material). Only ∼4.8% of RyR labeling was associated with CAV3 positive areas and ∼3.5% of CAV3 associated with RyR positive areas (n = 6 cells from three animals, Fig. 1 B, see also Table S1 in the Supporting Material), broadly consistent with previous data in rats (6). To support this finding, EM tomography was applied to mouse ventricular tissue that included a part of the surface sarcolemma, to our knowledge for the first time. Segmentation of peripheral couplings (containing RyR foot structures) and surface caveolae (∼60 nm in diameter and often interconnected) confirmed that the great majority of peripheral couplings were in regions devoid of caveolae (Fig. 1 C). A few junctional couplings containing feet were between caveolae and subsarcolemmal SR (Fig. 1 D, see also Fig. S1 and Movie S1 in the Supporting Material). We conducted a similar analysis in the cytosol where CAV3 expression occurs in the t-system (5) and RyRs are abundant in dyadic junctions between the t-system and SR terminal cisterns.Open in a separate windowFigure 1Colocalization of CAV3 and RyRs at the surface sarcolemma. (A) Super-resolution micrograph of the distribution of CAV3 (green) and RyRs (red) at the surface of a mouse cardiac myocyte. (B) Analysis of the association of CAV3 with RyRs. The fraction of RyR labeling within CAV3 positive areas was ∼4.8% (front data) whereas ∼3.5% of CAV3 was found in RyR-positive membrane areas. (C) Segmented EM tomogram containing a patch of surface sarcolemma (light blue) and associated caveolae (green) as well as peripheral couplings (red). (D) Detailed view of a region with abundant caveolae. (Arrows) Couplings with caveolae.As shown in Fig. 2 A, the spatial distribution of CAV3 and RyR clusters in super-resolution micrographs taken several microns below the surface sarcolemma is consistent with this view. The association of the two labels is slightly increased (as compared to the surface), according to distance analysis with 9% of CAV3 and 9.2% of RyR labeling associating with each other (Fig. 2 B, n = 6 cells from three animals). The similarity of manually traced t-system in EM tomograms (Fig. 2 C) and super-resolved CAV3 labeling suggested that CAV3 is widely distributed in the t-system except for regions where dyadic membrane junctions occur as CAV3 labeling was much weaker in regions with strong RyR labeling. It was notable that the t-system diameter appeared to increase at regions of strong RyR labeling (Fig. 2 D), broadly consistent with the behavior seen in tomograms (Fig. 2 C). This was confirmed by a quantitative analysis of t-tubule diameters in dyadic versus extradyadic regions on the basis of CAV3 and RyR labeling, with full-width at quarter-maximum mean diameters increasing from ∼150 nm distal to dyads, to ∼190 nm (using CAV3 signal only) or ∼280 nm (using CAV3 and RyR signal) near dyads (Fig. 2, G and H, see also Methods in the Supporting Material). The combined RyR and CAV3 signals seemed to be a better representation of the entire t-system lumen near junctions (see Fig. S2).Open in a separate windowFigure 2Distribution of CAV3 and RyRs in the cell interior. (A) Super-resolution micrograph of CAV3 (green) and RyR (red) distribution at t-system. (Arrow) Direction of longitudinal cell axis. (B) Distance analysis of the CAV3 and RyR association (N = 6 cells per group). (C) Segmented EM tomogram of a similar region with three-dimensional mesh models of t-system membrane (green) and dyadic couplings (red). (D) This image illustrates the tracing (white path) of t-tubules. The label distribution was extracted and linearized along the path (E) to calculate a mask that shows the full width at quarter-maximum diameter along tubules, CAV3 (green) and RyR (red) (F). (G) Histograms of local diameters extracted from traced t-tubules. (H) Mean diameters in junctional (dyad) and nonjunctional (ex-dyad) regions. See main text and the Supporting Material for details. ^**p < 0.01.Taken together, super-resolution imaging and EM tomography strongly support the presence of local t-system dilations in regions where the t-system opposes SR at dyads and such t-system bulges are connected by narrower tubule segments. Further support was provided by SBFSEM, another volume EM technique to study larger cell volumes (albeit at the expense of a slightly lower resolution). SBFSEM clearly showed local t-system dilations were regularly involved in the architecture of most (but not all) dyads (Fig. 3, see also Fig. S3 and Movie S2), as also observed in full three-dimensional super-resolution images (see Fig. S3 C).Open in a separate windowFigure 3Segmented SBFSEM data showing t-system dilations near dyadic junctions. (A) The overview shows t-system membranes (green) and jSR (red) in a mouse myocyte. (B, enlarged inset from panel A) Thin connecting tubules (arrows) and regular swellings in junctional regions at z-lines.Our data identify local dilations of the t-system associated with dyads in mouse cardiac myocytes. Frequent tubule distensions had been observed especially at the intersections of transverse and axial tubules (7), and constrictions were seen in rabbit myocytes although their relationship to dyads was unknown (8). The increased local t-system lumen near junctions may help reduce the predicted ionic accumulation/depletion during excitation-contraction coupling (9). Alternatively, it might simply be secondary to increasing local membrane area and allow the formation of large area junctions that harbor many RyRs. In connection with this point, it would be interesting to investigate the t-system near junctions in species that have larger average tubule diameters (e.g., human and rabbit (10)), or if this architecture changes in mouse heart failure models where t-tubule diameters are often increased.Most peripheral couplings were in regions void of surface caveolae, although a small number of RyR clusters were in junctional couplings between subsarcolemmal SR and caveolae as shown both by the low colocalization between CAV3 and RyRs as well as direct evidence from EM tomography. Similarly, a relatively small fraction of CAV3 colocalized with RyR clusters in the t-system although CAV3 was expressed widely in the t-system. A structural role of CAV3 in the t-system is still unclear—t-tubules in tomogram data did not reveal distinct caveolae shapes on the t-system membrane (see Fig. S4), although this might change in pathology (11). In any case, the t-system exhibits high curvature orthogonal to the tubule axis, which may be supported by CAV3 oligomerization. In addition, the presence of CAV3 in the t-system may be important for regulating other signaling systems (e.g., adrenergic signaling).Finally, our data demonstrate that complementary data from optical super-resolution and three-dimensional EM images assists data interpretation and reliability. We suggest that truly correlative optical and EM imaging approaches should provide further information and improve our knowledge of the basis of cardiac excitation-contraction coupling.     

Three-dimensional distribution of cardiac Na+-Ca2+ exchanger and ryanodine receptor during development

Mechanisms of cardiac excitation-contraction coupling in neonates are still not clearly defined. Previous work in neonates shows reverse-mode Na(+)-Ca(2+) exchange to be the primary route of Ca(2+) entry during systole and the neonatal sarcoplasmic reticulum to have similar capability as that of adult in storing and releasing Ca(2+). We investigated Na(+)-Ca(2+) exchanger (NCX) and ryanodine receptor (RyR) distribution in developing ventricular myocytes using immunofluorescence, confocal microscopy, and digital image analysis. In neonates, both NCX and RyR clusters on the surface of the cell displayed a short longitudinal periodicity of approximately 0.7 microm. However, by adulthood, both proteins were also found in the interior. In the adult, clusters of NCX on the surface of the cell retained the approximately 0.7-microm periodicity whereas clusters of RyR adopted a longer longitudinal periodicity of approximately 2.0 microm. This suggests that neonatal myocytes also have a peri-M-line RyR distribution that is absent in adult myocytes. NCX and RyR colocalized voxel density was maximal in neonates and declined significantly with ontogeny. We conclude in newborns, Ca(2+) influx via NCX could potentially activate the dense network of peripheral Ca(2+) stores via peripheral couplings, evoking Ca(2+)-induced Ca(2+) release.     

Nanoscale organization of junctophilin-2 and ryanodine receptors within peripheral couplings of rat ventricular cardiomyocytes

The peripheral distributions of the cardiac ryanodine receptor (RyR) and a junctional protein, junctophilin-2 (JPH2), were examined using single fluorophore localization-based super-resolution microscopy in rat ventricular myocytes. JPH2 was strongly associated with RyR clusters. Estimates of the colocalizing fraction of JPH labeling with RyR was ~90% within 30 nm of RyR clusters. This is comparable to fractions estimated from confocal data (~87%). Similarly, most RyRs were associated with JPH2 labeling in super-resolution images (~81% within 30 nm of JPH2 clusters). The shape of associated RyR clusters and JPH2 clusters were very similar, but not identical, suggesting that JPH2 is dispersed throughout RyR clusters and that the packing of JPH2 into junctions and the assembly of RyR clusters are tightly linked.     

Vectorial Ca2+ release via ryanodine receptors contributes to Ca2+ extrusion from freshly isolated rabbit aortic endothelial cells

In this study, we identified ryanodine receptors (RyRs) as a component of a cytosolic Ca(2+) removal pathway in freshly isolated rabbit aortic endothelial cells. In an earlier article, we reported that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and Na(+)/Ca(2+) exchanger (NCX) function in series to extrude cytosolic Ca(2+) to the extracellular space. Here we employed caffeine and ryanodine as modulators of RyR and showed that they act as the linkage between SERCA and NCX in removing Ca(2+) from the cytoplasm. Our data indicate that both 15 mM caffeine and 1 microM ryanodine facilitated Ca(2+) extrusion by activating RyRs while 100 microM ryanodine had the opposite effect by blocking RyRs. A further attempt to investigate RyR pharmacology revealed that in the absence of extracellular Ca(2+), ryanodine at 1 microM, but not 100 microM, stimulated Ca(2+) loss from the endoplasmic reticulum (ER). Blockade of RyR had no effect on the Ca(2+) removal rate when NCX had been previously blocked. In addition, the localization of RyR was determined using confocal microscopy of BODIPY TR-X fluorescent staining. Taken together, our findings suggest that in freshly isolated endothelial cells Ca(2+) is removed in part by transport through SERCA, RyR, and eventually NCX, and that RyR and NCX are in close functional proximity near the plasma membrane. After blockade of this component, Ca(2+) extrusion could be further inhibited by carboxyeosin, indicating a parallel contribution by the plasmalemmal Ca(2+)-ATPase (PMCA).     

Size Matters: Ryanodine Receptor Cluster Size Heterogeneity Potentiates Calcium Waves

Ryanodine receptors (RyRs) mediate calcium (Ca)-induced Ca release and intracellular Ca homeostasis. In a cardiac myocyte, RyRs group into clusters of variable size from a few to several hundred RyRs, creating a spatially nonuniform intracellular distribution. It is unclear how heterogeneity of RyR cluster size alters spontaneous sarcoplasmic reticulum (SR) Ca releases (Ca sparks) and arrhythmogenic Ca waves. Here, we tested the impact of heterogeneous RyR cluster size on the initiation of Ca waves. Experimentally, we measured RyR cluster sizes at Ca spark sites in rat ventricular myocytes and further tested functional impacts using a physiologically detailed computational model with spatial and stochastic intracellular Ca dynamics. We found that the spark frequency and amplitude increase nonlinearly with the size of RyR clusters. Larger RyR clusters have lower SR Ca release threshold for local Ca spark initiation and exhibit steeper SR Ca release versus SR Ca load relationship. However, larger RyR clusters tend to lower SR Ca load because of the higher Ca leak rate. Conversely, smaller clusters have a higher threshold and a lower leak, which tends to increase SR Ca load. At the myocyte level, homogeneously large or small RyR clusters limit Ca waves (because of low load for large clusters but low excitability for small clusters). Mixtures of large and small RyR clusters potentiates Ca waves because the enhanced SR Ca load driven by smaller clusters enables Ca wave initiation and propagation from larger RyR clusters. Our study suggests that a spatially heterogeneous distribution of RyR cluster size under pathological conditions may potentiate Ca waves and thus afterdepolarizations and triggered arrhythmias.     

Hierarchical clustering of ryanodine receptors enables emergence of a calcium clock in sinoatrial node cells

The sinoatrial node, whose cells (sinoatrial node cells [SANCs]) generate rhythmic action potentials, is the primary pacemaker of the heart. During diastole, calcium released from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) interacts with membrane currents to control the rate of the heartbeat. This “calcium clock” takes the form of stochastic, partially periodic, localized calcium release (LCR) events that propagate, wave-like, for limited distances. The detailed mechanisms controlling the calcium clock are not understood. We constructed a computational model of SANCs, including three-dimensional diffusion and buffering of calcium in the cytosol and SR; explicit, stochastic gating of individual RyRs and L-type calcium channels; and a full complement of voltage- and calcium-dependent membrane currents. We did not include an anatomical submembrane space or inactivation of RyRs, the two heuristic components that have been used in prior models but are not observed experimentally. When RyRs were distributed in discrete clusters separated by >1 µm, only isolated sparks were produced in this model and LCR events did not form. However, immunofluorescent staining of SANCs for RyR revealed the presence of bridging RyR groups between large clusters, forming an irregular network. Incorporation of this architecture into the model led to the generation of propagating LCR events. Partial periodicity emerged from the interaction of LCR events, as observed experimentally. This calcium clock becomes entrained with membrane currents to accelerate the beating rate, which therefore was controlled by the activity of the SERCA pump, RyR sensitivity, and L-type current amplitude, all of which are targets of β-adrenergic–mediated phosphorylation. Unexpectedly, simulations revealed the existence of a pathological mode at high RyR sensitivity to calcium, in which the calcium clock loses synchronization with the membrane, resulting in a paradoxical decrease in beating rate in response to β-adrenergic stimulation. The model indicates that the hierarchical clustering of surface RyRs in SANCs may be a crucial adaptive mechanism. Pathological desynchronization of the clocks may explain sinus node dysfunction in heart failure and RyR mutations.     

Dynamic Interreceptor Coupling Contributes to the Consistent Open Duration of Ryanodine Receptors

Ca²⁺ spark is the elementary Ca²⁺ signaling event in muscle excitation-contraction coupling. The rise time of Ca²⁺ spark is rather stable under different conditions, suggesting consistent open duration of ryanodine receptors (RyRs) in vivo. It has been proposed that the array-based behavior of RyRs plays an important role in shaping Ca²⁺ spark dynamics, particularly in controlling the open duration of RyR clusters. Therefore, we investigated the possible role of inter-RyR coupling in stabilization of the open time of arrayed RyRs under several potential perturbations, for instance, array size, inter-RyR coupling noise, and up-regulation or down-regulation of the activity of partial RyRs in the array. We found that RyR arrays with dynamic coupling showed consistent open duration against the perturbations, whereas the RyR array with constant coupling did not. On the other hand, the open probability and amplitude of RyR arrays with dynamic interreceptor coupling were sensitive to the perturbations. These two points were consistent with experimental observations, indicating that the RyR array with dynamic coupling could recapitulate in vivo open properties of RyRs. Our findings support the idea that dynamic coupling is a feasible in vivo working mechanism of RyR arrays.     

Sodium-calcium exchanger and lipid rafts in pig coronary artery smooth muscle

Pig coronary artery smooth muscle expresses, among many other proteins, Na+-Ca2+-exchanger NCX1 and sarcoplasmic reticulum Ca2+ pump SERCA2. NCX1 has been proposed to play a role in refilling the sarcoplasmic reticulum Ca2+ pool suggesting a functional linkage between the two proteins. We hypothesized that this functional linkage may require close apposition of SERCA2 and NCX1 involving regions of plasma membrane like lipid rafts. Lipid rafts are specialized membrane microdomains that appear as platforms to co-localize proteins. To determine the distribution of NCX1, SERCA2 and lipid rafts, we isolated microsomes from the smooth muscle tissue, treated them with non-ionic detergent and obtained fractions of different densities by sucrose density gradient centrifugal flotation. We examined the distribution of NCX1; SERCA2; non-lipid raft plasma membrane marker transferrin receptor protein; lipid raft markers caveolin-1, flotillin-2, prion protein, GM1-gangliosides and cholesterol; and cytoskeletal markers clathrin, actin and myosin. Distribution of markers identified two subsets of lipid rafts that differ in their components. One subset is rich in caveolin-1 and flotillin-2 and the other in GM1-gangliosides, prion protein and cholesterol. NCX1 distribution correlated strongly with SERCA2, caveolin-1 and flotillin-2, less strongly with the other membrane markers and negatively with the cytoskeletal markers. These experiments were repeated with a non-detergent method of treating microsomes with sonication at high pH and similar results were obtained. These observations are consistent with the observed functional linkage between NCX1 and SERCA2 and suggest a role for NCX1 in supplying Ca2+ for refilling the sarcoplasmic reticulum.     

Nanoscale organization of ryanodine receptor distribution and phosphorylation pattern determines the dynamics of calcium sparks

Super-resolution imaging techniques have provided a better understanding of the relationship between the nanoscale organization and function of ryanodine receptors (RyRs) in cardiomyocytes. Recent data have indicated that this relationship is disrupted in heart failure (HF), as RyRs are dispersed into smaller and more numerous clusters. However, RyRs are also hyperphosphorylated in this condition, and this is reported to occur preferentially within the cluster centre. Thus, the combined impact of RyR relocalization and sensitization on Ca²⁺ spark generation in failing cardiomyocytes is likely complex and these observations suggest that both the nanoscale organization of RyRs and the pattern of phosphorylated RyRs within clusters could be critical determinants of Ca²⁺ spark dynamics. To test this hypothesis, we used computational modeling to quantify the relationships between RyR cluster geometry, phosphorylation patterns, and sarcoplasmic reticulum (SR) Ca²⁺ release. We found that RyR cluster disruption results in a decrease in spark fidelity and longer sparks with a lower amplitude. Phosphorylation of some RyRs within the cluster can play a compensatory role, recovering healthy spark dynamics. Interestingly, our model predicts that such compensation is critically dependent on the phosphorylation pattern, as phosphorylation localized within the cluster center resulted in longer Ca²⁺ sparks and higher spark fidelity compared to a uniformly distributed phosphorylation pattern. Our results strongly suggest that both the phosphorylation pattern and nanoscale RyR reorganization are critical determinants of Ca²⁺ dynamics in HF.     

Human coronavirus 229E binds to CD13 in rafts and enters the cell through caveolae

CD13, a receptor for human coronavirus 229E (HCoV-229E), was identified as a major component of the Triton X-100-resistant membrane microdomain in human fibroblasts. The incubation of living fibroblasts with an anti-CD13 antibody on ice gave punctate labeling that was evenly distributed on the cell surface, but raising the temperature to 37 degrees C before fixation caused aggregation of the labeling. The aggregated labeling of CD13 colocalized with caveolin-1 in most cells. The HCoV-229E virus particle showed a binding and redistribution pattern that was similar to that caused by the anti-CD13 antibody: the virus bound to the cell evenly when incubated on ice but became colocalized with caveolin-1 at 37 degrees C; importantly, the virus also caused sequestration of CD13 to the caveolin-1-positive area. Electron microscopy confirmed that HCoV-229E was localized near or at the orifice of caveolae after incubation at 37 degrees C. The depletion of plasmalemmal cholesterol with methyl beta-cyclodextrin significantly reduced the HCoV-229E redistribution and subsequent infection. A caveolin-1 knockdown by RNA interference also reduced the HCoV-229E infection considerably. The results indicate that HCoV-229E first binds to CD13 in the Triton X-100-resistant microdomain, then clusters CD13 by cross-linking, and thereby reaches the caveolar region before entering cells.     

Multiple ryanodine receptor subtypes and heterogeneous ryanodine receptor-gated Ca2+ stores in pulmonary arterial smooth muscle cells

Ryanodine receptors (RyRs) of pulmonary arterial smooth muscle cells (PASMCs) play important roles in major physiological processes such as hypoxic pulmonary vasoconstriction and perinatal pulmonary vasodilatation. Recent studies show that three subtypes of RyRs are coexpressed and RyR-gated Ca2+ stores are distributed heterogeneously in systemic vascular myocytes. However, the molecular identity and subcellular distribution of RyRs have not been examined in PASMCs. In this study we detected mRNA and proteins of all three subtypes in rat intralobar PASMCs using RT-PCR and Western blot. Quantitative real-time RT-PCR showed that RyR2 mRNA was most abundant, approximately 15-20 times more than the other two subtypes. Confocal fluorescence microscopy revealed that RyRs labeled with BODIPY TR-X ryanodine were localized in the peripheral and perinuclear regions and were colocalized with sarcoplasmic reticulum labeled with Fluo-5N. Immunostaining showed that the subsarcolemmal regions exhibited clear signals of RyR1 and RyR2, whereas the perinuclear compartments contained mainly RyR1 and RyR3. Ca2+ sparks were recorded in both regions, and their activities were enhanced by a subthreshold concentration of caffeine or by endothelin-1, indicating functional RyR-gated Ca2+ stores. Moreover, 18% of the perinuclear sparks were prolonged [full duration/half-maximum (FDHM) = 193.3 +/- 22.6 ms] with noninactivating kinetics, in sharp contrast to the typical fast inactivating Ca2+ sparks (FDHM = 44.6 +/- 3.2 ms) recorded in the same PASMCs. In conclusion, multiple RyR subtypes are expressed differentially in peripheral and perinuclear RyR-gated Ca2+ stores; the molecular complexity and spatial heterogeneity of RyRs may facilitate specific Ca2+ regulation of cellular functions in PASMCs.     

Localization of caveolin-1 and c-src in mature and differentiating photoreceptors: raft proteins co-distribute with rhodopsin during development

Numerous biochemical and morphological studies have provided insight into the distribution pattern of caveolin-1 and the presence of membrane rafts in the vertebrate retina. To date however, studies have not addressed the localization profile of raft specific proteins during development. Therefore the purpose of our studies was to follow the localization pattern of caveolin-1, phospho-caveolin-1 and c-src in the developing retina and compare it to that observed in adults. Specific antibodies were used to visualize the distribution of caveolin-1, c-src, a kinase phosphorylating caveolin-1, and phospho-caveolin-1. The labeling pattern of this scaffolded complex was compared to those of rhodopsin and rhodopsin kinase. Samples were analyzed at various time points during postnatal development and compared to adult retinas. The immunocytochemical studies were complemented with immunoblots and immunoprecipitation studies. In the mature retina caveolin-1 and c-src localized mainly to the cell body and IS of photoreceptors, with only very weakly labeled OS. In contrast, phospho-caveolin-1 was only detectable in the OS of photoreceptors. During development we followed the expression and distribution profile of these proteins in a temporal sequence with special attention to the period when OS formation is most robust. Double labeling immunocytochemistry and immunoprecipitation showed rhodopsin to colocalize and co-immunoprecipitate with caveolin-1 and c-src. Individual punctate structures between the outer limiting membrane and the outer plexiform layer were seen at P10 to be labeled by both rhodopsin and caveolin-1 as well as by rhodopsin and c-src, respectively. These studies suggest that membrane raft specific proteins are co-distributed during development, thereby pointing to a role for such complexes in OS formation. In addition, the presence of small punctate structures containing caveolin-1, c-src and rhodopsin raise the possibility that these proteins may transport together to OS during development and that caveolin-1 exists predominantly in a phosphorylated form in the OS.     

Numerical analysis of ryanodine receptor activation by L-type channel activity in the cardiac muscle diad.

Computer simulations were used to examine the response of ryanodine receptors (RyRs) to the sarcolemmal calcium influx via L-type calcium channels (DHPRs). The effects of ryanodine receptor organization, diad geometry, DHPR single-channel current, and DHPR gating were examined. In agreement with experimental findings, the simulations showed that RyRs can respond rapidly (approximately 0.4 ms) to calcium influx via DHPRs. The responsiveness of the RyR depends on the geometrical arrangement between the RyRs and the DHPR in the diad, with wider diads being generally less responsive. When the DHPR single-channel current is small (approximately 25 fA), the organization of RyRs into small clusters results in an improved responsiveness. With experimentally observed DHPR mean open and closed times (0.17 ms and 4 ms, respectively) it is the first opening of the DHPR that is most likely to activate the RyR. A measure of the efficiency (Q) by which DHPR gating evokes sarcoplasmic reticulum release is defined. Q is at maximum for tau approximately 0.3 ms, and we interpret this finding in terms of the "tuning" of DHPR gating to RyR response. If certain cardiac myopathies are associated with a mismatch in the "tuning," then modification of DHPR gating with drugs to "retune" calcium-induced calcium release should be possible.     

Identification of a polymorphic ryanodine receptor gene from Heliothis virescens (Lepidoptera: noctuidae)

cDNAs encoding the C-terminal 1172 amino acids of a ryanodine receptor (RyR) from the lepidopteran pest Heliothis virescens (Hv-RyR) have been cloned and characterised. Sequence comparisons, organisational studies on corresponding genomic regions and a genetic segregation analysis provide evidence for two polymorphic alleles of the Hv-RyR locus.Comparison of the Hv-RyR C-terminal amino acid sequence with equivalent regions of other RyRs reveals a high level of overall amino acid homology (74% identity with D. melanogaster and between 47.9 and 50.1% with vertebrate isoforms). Homologies are however not uniformly distributed, though regions of high and low similarity are consistent with patterns in other RyR isoforms. The structural similarity of Hv-RyR with other RyRs is also indicated by comparison of hydropathy profiles and other previously described functional domains. Such results are consistent with this region of Hv-RyR containing the Ca(2+) channel itself and being intimately involved in RyR regulation. Potential uses of the cDNAs described in the discovery and development of novel ryanodine like insecticides are discussed.     

The Ca 2+ leak paradox and rogue ryanodine receptors: SR Ca 2+ efflux theory and practice

Ca²⁺ efflux from the sarcoplasmic reticulum (SR) is routed primarily through SR Ca²⁺ release channels (ryanodine receptors, RyRs). When clusters of RyRs are activated by trigger Ca²⁺ influx through L-type Ca²⁺ channels (dihydropyridine receptors, DHPR), Ca²⁺ sparks are observed. Close spatial coupling between DHPRs and RyR clusters and the relative insensitivity of RyRs to be triggered by Ca²⁺ together ensure the stability of this positive-feedback system of Ca²⁺ amplification. Despite evidence from single channel RyR gating experiments that phosphorylation of RyRs by protein kinase A (PKA) or calcium-calmodulin dependent protein kinase II (CAMK II) causes an increase in the sensitivity of the RyR to be triggered by [Ca²⁺]_i there is little clear evidence to date showing an increase in Ca²⁺ spark rate. Indeed, there is some evidence that the SR Ca²⁺ content may be decreased in hyperadrenergic disease states. The question is whether or not these observations are compatible with each other and with the development of arrhythmogenic extrasystoles that can occur under these conditions. Furthermore, the appearance of an increase in the SR Ca²⁺ “leak” under these conditions is perplexing. These and related complexities are analyzed and discussed in this report. Using simple mathematical modeling discussed in the context of recent experimental findings, a possible resolution to this paradox is proposed. The resolution depends upon two features of SR function that have not been confirmed directly but are broadly consistent with several lines of indirect evidence: (1) the existence of unclustered or “rogue” RyRs that may respond differently to local [Ca²⁺]_i in diastole and during the [Ca²⁺]_i transient; and (2) a decrease in cooperative or coupled gating between clustered RyRs in response to physiologic phosphorylation or hyper-phosphorylation of RyRs in disease states such as heart failure. Taken together, these two features may provide a framework that allows for an improved understanding of cardiac Ca²⁺ signaling.     

Sarcoplasmic Reticulum Structure and Functional Properties that Promote Long-Lasting Calcium Sparks

Calcium (Ca) sparks are the fundamental sarcoplasmic reticulum (SR) Ca release events in cardiac myocytes, and they have a typical duration of 20–40 ms. However, when a fraction of ryanodine receptors (RyRs) are blocked by tetracaine or ruthenium red, Ca sparks lasting hundreds of milliseconds have been observed experimentally. The fundamental mechanism underlying these extremely prolonged Ca sparks is not understood. In this study, we use a physiologically detailed mathematical model of subcellular Ca cycling to examine how Ca spark duration is influenced by the number of functional RyRs in a junctional cluster (which is reduced by tetracaine or ruthenium red) and other SR Ca handling properties. One RyR cluster contains a few to several hundred RyRs, and we use a four-state Markov RyR gating model. Each RyR opens stochastically and is regulated by cytosolic and luminal Ca. We varied the number of functional RyRs in the single cluster, diffusion within the SR network, diffusion between network and junctional SR, cytosolic Ca diffusion, SERCA uptake activity, and RyR open probability. For long-lasting Ca release events, opening events within the cluster must occur continuously because the typical open time of the RyR is only a few milliseconds. We found the following: 1) if the number of RyRs is too small, it is difficult to maintain consecutive openings and stochastic attrition terminates the release; 2) if the number of RyRs is too large, the depletion of Ca from the junctional SR terminates the release; and 3) very long release events require relatively small-sized RyR clusters (reducing flux as seen experimentally with tetracaine) and sufficiently rapid intra-SR Ca diffusion, such that local junctional intra-SR [Ca] can be maintained by intra-SR diffusion and overall SR Ca reuptake.     

Expression and functional activity of ryanodine receptors (RyRs) during skeletal muscle development

Two isoforms of ryanodine receptors are expressed in skeletal muscles, RyR1 and RyR3. We investigated the relative level of expression of RyRs in developing murine skeletal muscles using [3H]ryanodine binding and immunoprecipitation experiments. In the diaphragm RyR3 accounted for 11% of total RyRs in 5-day-old mice and for 3% of total RyRs in 60-day-old mice. In hindlimb muscles, RyR3 accounted for 3% and 1% of total RyRs in 5-day-old and adult mice, respectively. The activity of RyR1 channels in native microsomal vesicles from murine muscles was found to be as low as 35% of that measured after CHAPS exposure, while no inhibition was observed for RyR3. CHAPS sensitivity of recombinant RyR1 and RyR3 expressed in HEK293 cells was also investigated. The activity of recombinant RyR1 but not RyR3 channels was found to be inhibited in native conditions, suggesting that this property may not be dependent on a muscle environment.     

Functional properties of ryanodine receptors from rat dorsal root ganglia

The properties of ryanodine receptors (RyRs) from rat dorsal root ganglia (DRGs) have been studied. The density of RyRs (Bmax) determined by [3H]ryanodine binding was 63 fmol/mg protein with a dissociation constant (Kd) of 1.5 nM. [3H]Ryanodine binding increased with caffeine, decreased with ruthenium red and tetracaine, and was insensitive to millimolar concentrations of Mg2+ or Ca2+. DRG RyRs reconstituted in planar lipid bilayers were Ca2+-dependent and displayed the classical long-lived subconductance state in response to ryanodine; however, unlike cardiac and skeletal RyRs, they lacked Ca2+-dependent inactivation. Antibodies against RyR3, but not against RyR1 or RyR2, detected DRG RyRs. Thus, DRG RyRs are immunologically related to RyR3, but their lack of divalent cation inhibition is unique among RyR subtypes.