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

Modeling the mechanism of Ca2+ release in skeletal muscle by DHPRs easing inhibition at RyR I1-sites

Ca2+ release from the sarcoplasmic reticulum (SR) plays a central role in excitation-contraction coupling (ECC) in skeletal muscles. However, the mechanism by which activation of the voltage-sensors/dihydropyridine receptors (DHPRs) in the membrane of the transverse tubular system leads to activation of the Ca2+-release channels/ryanodine receptors (RyRs) in the SR is not fully understood. Recent observations showing that a very small Ca2+ leak through RyR1s in mammalian skeletal muscle can markedly raise the background [Ca2+] in the junctional space (JS) above the Ca2+ level in the bulk of the cytosol indicate that there is a diffusional barrier between the JS and the cytosol at large. Here, I use a mathematical model to explore the hypothesis that a sudden rise in Ca2+ leak through DHPR-coupled RyR1s, caused by reduced inhibition at the RyR1 Ca2+/Mg2+ inhibitory I1-sites when the associated DHPRs are activated, is sufficient to enable synchronized responses that trigger a regenerative rise of Ca2+ release that remains under voltage control. In this way, the characteristic response to Ca2+ of RyR channels is key not only for the Ca2+ release mechanism in cardiac muscle and other tissues, but also for the DHPR-dependent Ca2+ release in skeletal muscle.

Quantal Properties of Voltage-Dependent Ca2+ Release in Frog Skeletal Muscle Persist After Reduction of [Ca2+] in the Sarcoplasmic Reticulum

In skeletal muscle, the Ca2+ release flux elicited by a voltage clamp pulse rises to an early peak that inactivates rapidly to a much lower steady level. Using a double pulse protocol the fast inactivation follows an arithmetic rule: if the conditioning depolarization is less than or equal to the test depolarization, then decay (peak minus steady level) in the conditioning release is approximately equal to suppression (unconditioned minus conditioned peak) of the test release. This is due to quantal activation by voltage, analogous to the quantal activation of IP3 receptor channels. Two mechanisms are possible. One is the existence of subsets of channels with different sensitivities to voltage. The other is that the clusters of Ca2+-gated Ryanodine Receptor (RyR) β in the parajunctional terminal cisternae might constitute the quantal units. These Ca2+-gated channels are activated by the release of Ca2+ through the voltage-gated RyR α channels. If the RyR β were at the basis of quantal release, it should be modified by strong inhibition of the primary voltage-gated release. This was attained in two ways, by sarcoplasmic reticulum (SR) Ca2+ depletion and by voltage-dependent inactivation. Both procedures reduced global Ca2+ release flux, but SR Ca2+ depletion reduced the single RyR current as well. The effect of both interventions on the quantal properties of Ca2+ release in frog skeletal muscle fibers were studied under voltage clamp. The quantal properties of release were preserved regardless of the inhibitory maneuver applied. These findings put a limit on the role of the Ca2+-activated component of release in generating quantal activation.

Elementary calcium release events in the skeletal muscle cells of the honey bee Apis mellifera

Calcium sparks are involved in major physiological and pathological processes in vertebrate muscles but have never been characterized in invertebrates. Here, dynamic confocal imaging on intact skeletal muscle cells isolated enzymatically from the adult honey bee legs allowed the first spatio-temporal characterization of subcellular calcium release events (CREs) in an insect species. The frequency of CREs, measured in x–y time lapse series, was higher than frequencies usually described in vertebrates. Honey bee CREs had a larger spatial spread at half maximum than their vertebrate counterparts and a slightly ellipsoidal shape, two characteristics that may be related to ultrastructural features specific to invertebrate cells. In line-scan experiments, the histogram of CREs’ duration followed a bimodal distribution, supporting the existence of both sparks and embers. Unlike in vertebrates, embers and sparks had similar amplitudes, a difference that could be related to genomic differences and/or excitation–contraction coupling specificities in honey bee skeletal muscle fibres. The first characterization of CREs from an arthropod which shows strong genomic, ultrastructural and physiological differences with vertebrates may help in improving the research field of sparkology and more generally the knowledge in invertebrates cell Ca²⁺ homeostasis, eventually leading to a better understanding of their roles and regulations in muscles but also the myotoxicity of new insecticides targeting ryanodine receptors.

High Time Resolution Analysis of Voltage-Dependent and Voltage-Independent Calcium Sparks in Frog Skeletal Muscle Fibers

In amphibian skeletal muscle calcium (Ca2+) sparks occur both as voltage-dependent and voltage-independent ligand-activated release events. However, whether their properties and their origin show similarities are still in debate. Elevated K+, constant Cl- content solutions were used to initiate small depolarizations of the resting membrane potential to activate dihydropyridine receptors (DHPR) and caffeine to open ryanodine receptors (RyR) on intact fibers. The properties of Ca2+ sparks observed under control conditions were compared to those measured on depolarized cells and those after caffeine treatment. Calcium sparks were recorded on intact frog skeletal muscle fibers using high time resolution confocal microscopy (x-y scan: 30 Hz). Sparks were elicited by 1 mmol/l caffeine or subthreshold depolarization to different membrane potentials. Both treatments increased the frequency of sparks and altered their morphology. Images were analyzed by custom-made computer programs. Both the amplitude (in ΔF/F0; 0.259 ± 0.001 vs. 0.164 ± 0.001; n = 24942 and 43326, respectively; mean ± SE, p < 0.001) and the full width at half maximum (FWHM, in μm; parallel with fiber axis: 2.34 ± 0.01 vs. 1.92 ± 0.01, p < 0.001; perpendicular to fiber axis: 2.08 ± 0.01 vs. 1.68 ± 0.01, p < 0.001) of sparks was significantly greater after caffeine treatment than on depolarized cells. 9.8% of the sparks detected on depolarized fibers and about one third of the caffeine activated sparks (29.7%) overlapped with another one on the previous frame on x-y scans. Centre of overlapping sparks travelled significantly longer distances between consecutive frames after caffeine treatment then after depolarization (in μm; 1.66 ± 0.01 vs. 0.95 ± 0.01, p < 0.001). Our results suggest that the two types of ryanodine receptors, the junctional RyRs controlled by DHPRs and the parajunctional RyRs are activated independently, using alternate ways, with the possibility of cooperation between neighboring release channels.

The dynamics of Ca2+ within the sarcoplasmic reticulum of frog skeletal muscle. A simulation study

In skeletal muscle, Ca²⁺ release from the sarcoplasmic reticulum (SR) triggers contraction. In this study we develop a two compartment model to account for the Ca²⁺ dynamics in frog skeletal muscle fibers. The two compartments in the model correspond to the SR and the cytoplasm, where the myofibrils are placed. We use a detailed model for the several Ca²⁺ binding proteins in the cytoplasm in line with previous models. As a new feature, Ca²⁺ binding sites within the SR, attributed to calsequestrin, are modeled based on experimentally obtained properties. The intra SR Ca²⁺ buffer shows cooperativity, well represented by a Hill equation with parameters that depend on the initial [Ca²⁺] in the SR ([Ca²⁺]_SR). The number of total sites as well as the [Ca²⁺]_SR of half saturation are reduced as the resting [Ca²⁺]_SR is reduced, on the other hand the Hill number is not changed. The buffer power remained roughly constant. The release process is activated by a voltage dependent mechanism that increases the Ca²⁺ permeability of the SR. We use the permeability time course and amplitude experimentally obtained during a voltage clamp pulse to drive the simulations. This model successfully reproduces the SR and cytoplasmic transients observed. Additionally, we simulate [Ca²⁺] _SR transients in the case of high concentration of extrinsic Ca²⁺ buffers added to the cytoplasm to explore what properties of the permeability are necessary to account for the experimentally observed [Ca²⁺]_SR transients. The main novelty of the model, the intra SR Ca²⁺ buffer, is crucial for reproducing the experimental observations and it would be of use in future theoretical studies of excitation contraction coupling in skeletal muscle.

Interactions among ryanodine receptor isotypes contribute to muscle fiber type development and function

Mutations affecting ryanodine receptor (RyR) calcium release channels commonly underlie congenital myopathies. Although these channels are known principally for their essential roles in muscle contractility, mutations in the human RYR1 gene result in a broad spectrum of phenotypes, including muscle weakness, altered proportions of fiber types, anomalous muscle fibers with cores or centrally placed nuclei, and dysmorphic craniofacial features. Currently, it is unknown which phenotypes directly reflect requirements for RyRs and which result secondarily to aberrant muscle function. To identify biological processes requiring RyR function, skeletal muscle development was analyzed in zebrafish embryos harboring protein-null mutations. RyR channels contribute to both muscle fiber development and function. Loss of some RyRs had modest effects, altering muscle fiber-type specification in the embryo without compromising viability. In addition, each RyR-encoding gene contributed to normal swimming behavior and muscle function. The RyR channels do not function in a simple additive manner. For example, although isoform RyR1a is sufficient for muscle contraction in the absence of RyR1b, RyR1a normally attenuates the activity of the co-expressed RyR1b channel in slow muscle. RyR3 also acts to modify the functions of other RyR channels. Furthermore, diminished RyR-dependent contractility affects both muscle fiber maturation and craniofacial development. These findings help to explain some of the heterogeneity of phenotypes that accompany RyR1 mutations in humans.

Extraocular muscle function is impaired in ryr3 -/- mice

Extraocular muscles are specialized skeletal muscles expressing a particular set of proteins involved in calcium homeostasis, including RYR3. Eckhardt et al. investigate extraocular muscle function in ryr3^−/− mice and show that ablation of RYR3 significantly impacts vision.

Calcium is an ubiquitous second messenger mediating numerous physiological processes, including muscle contraction and neuronal excitability. Ca²⁺ is stored in the ER/SR and is released into the cytoplasm via the opening of intracellular inositol trisphosphate receptor and ryanodine receptor calcium channels. Whereas in skeletal muscle, isoform 1 of the RYR is the main channel mediating calcium release from the SR leading to muscle contraction, the function of ubiquitously expressed ryanodine receptor 3 (RYR3) is far from clear; it is not known whether RYR3 plays a role in excitation–contraction coupling. We recently reported that human extraocular muscles express high levels of RYR3, suggesting that such muscles may be useful to study the function of this isoform of the Ca²⁺ channel. In the present investigation, we characterize the visual function of ryr3^−/− mice. We observe that ablation of RYR3 affects both mechanical properties and calcium homeostasis in extraocular muscles. These changes significantly impact vision. Our results reveal for the first time an important role for RYR3 in extraocular muscle function.

Ca2+-induced sarcoplasmic reticulum Ca2+ release in myotubularin-deficient muscle fibers

Skeletal muscle deficiency in the 3-phosphoinositide (PtdInsP) phosphatase myotubularin (MTM1) causes myotubular myopathy which is associated with severe depression of voltage-activated sarcoplasmic reticulum Ca²⁺ release through ryanodine receptors. In the present study we aimed at further understanding how Ca²⁺ release is altered in MTM1-deficient muscle fibers, at rest and during activation. While in wild-type muscle fibers, SR Ca²⁺ release exhibits fast stereotyped kinetics of activation and decay throughout the voltage range of activation, Ca²⁺ release in MTM1-deficient muscle fibers exhibits slow and unconventional kinetics at intermediate voltages, suggestive of partial loss of the normal control of ryanodine receptor Ca²⁺ channel activity. In addition, the diseased muscle fibers at rest exhibit spontaneous elementary Ca²⁺ release events at a frequency 30 times greater than that of control fibers. Eighty percent of the events have spatiotemporal properties of archetypal Ca²⁺ sparks while the rest take either the form of lower amplitude, longer duration Ca²⁺ release events or of a combination thereof. The events occur at preferred locations in the fibers, indicating spatially uneven distribution of the parameters determining spontaneous ryanodine receptor 1 opening. Spatially large Ca²⁺ release sources were obviously involved in some of these events, suggesting that opening of ryanodine receptors in one cluster can activate opening of ryanodine receptors in a neighboring one. Overall results demonstrate that opening of Ca²⁺-activated ryanodine receptors is promoted both at rest and during excitation-contraction coupling in MTM1-deficient muscle fibers. Because access to this activation mode is denied to ryanodine receptors in healthy skeletal muscle, this may play an important role in the associated disease situation.

Critical review: Cardiac telocytes vs cardiac lymphatic endothelial cells

The study of cardiac interstitial Cajal-like cells (ICLCs) began in 2005 and continued until 2010, when these cells were renamed as telocytes (TCs). Since then, numerous papers on cardiac ICLCs and TCs have been published. However, in the initial descriptions upon which further research was based, lymphatic endothelial cells (LECs) and initial lymphatics were not considered. No specific antibodies for LECs (such as podoplanin or LYVE-1) were used in cardiac TC studies, although ultrastructurally, LECs and TCs have similar morphological traits, including the lack of a basal lamina. When tissues are longitudinally cut, migrating LECs involved in adult lymphangiogenesis have an ICLC or TC morphology, both in light and transmission electron microscopy. In this paper, we present evidence that at least some cardiac TCs are actually LECs. Therefore, a clear-cut distinction should be made between TCs and LECs, at both the molecular and the ultrastructural levels, in order to avoid obtaining invalid data.

Calcium-induced release of calcium in muscle: 50 years of work and the emerging consensus

Ryanodine-sensitive intracellular Ca2+ channels (RyRs) open upon binding Ca2+ at cytosolic-facing sites. This results in concerted, self-reinforcing opening of RyRs clustered in specialized regions on the membranes of Ca2+ storage organelles (endoplasmic reticulum and sarcoplasmic reticulum), a process that produces Ca2+-induced Ca2+ release (CICR). The process is optimized to achieve large but brief and localized increases in cytosolic Ca2+ concentration, a feature now believed to be critical for encoding the multiplicity of signals conveyed by this ion. In this paper, I trace the path of research that led to a consensus on the physiological significance of CICR in skeletal muscle, beginning with its discovery. I focus on the approaches that were developed to quantify the contribution of CICR to the Ca2+ increase that results in contraction, as opposed to the flux activated directly by membrane depolarization (depolarization-induced Ca2+ release [DICR]). Although the emerging consensus is that CICR plays an important role alongside DICR in most taxa, its contribution in most mammalian muscles appears to be limited to embryogenesis. Finally, I survey the relevance of CICR, confirmed or plausible, to pathogenesis as well as the multiple questions about activation of release channels that remain unanswered after 50 years.

The relationship between form and function throughout the history of excitation-contraction coupling

Franzini-Armstrong reviews the development of the excitation–contraction coupling field over time.

The concept of excitation–contraction coupling is almost as old as Journal of General Physiology. It was understood as early as the 1940s that a series of stereotyped events is responsible for the rapid contraction response of muscle fibers to an initial electrical event at the surface. These early developments, now lost in what seems to be the far past for most young investigators, have provided an endless source of experimental approaches. In this Milestone in Physiology, I describe in detail the experiments and concepts that introduced and established the field of excitation–contraction coupling in skeletal muscle. More recent advances are presented in an abbreviated form, as readers are likely to be familiar with recent work in the field.

Fundamental constraints in synchronous muscle limit superfast motor control in vertebrates

Superfast muscles (SFMs) are extremely fast synchronous muscles capable of contraction rates up to 250 Hz, enabling precise motor execution at the millisecond time scale. SFM phenotypes have been discovered in most major vertebrate lineages, but it remains unknown whether all SFMs share excitation-contraction coupling pathway adaptations for speed, and if SFMs arose once, or from independent evolutionary events. Here, we demonstrate that to achieve rapid actomyosin crossbridge kinetics bat and songbird SFM express myosin heavy chain genes that are evolutionarily and ontologically distinct. Furthermore, we show that all known SFMs share multiple functional adaptations that minimize excitation-contraction coupling transduction times. Our results suggest that SFM evolved independently in sound-producing organs in ray-finned fish, birds, and mammals, and that SFM phenotypes operate at a maximum operational speed set by fundamental constraints in synchronous muscle. Consequentially, these constraints set a fundamental limit to the maximum speed of fine motor control.

Across animals, different muscle types have evolved to perform vastly different tasks at different speeds. For example, tortoise leg muscles move slowly over several seconds, while the flight muscles of a hummingbird move quickly dozens of times per second. The speed record holders among vertebrates are the so-called superfast muscles, which can move up to 250 times per second. Superfast muscles power the alarming rattle of rattlesnakes, courtship calls in fish, rapid echolocation calls in bats and the elaborate vocal gymnastics of songbirds. Thus these extreme muscles are all around us and are always involved in sound production.

Did superfast muscles evolve from a common ancestor? And how do different superfast muscles achieve their extreme behavior? To answer these questions, Mead et al. studied the systems known to limit contraction speed in all currently known superfast muscles found in rattlesnakes, toadfish, bats and songbirds. This revealed that all the muscles share certain specific adaptations that allow superfast contractions. Furthermore, the three fastest examples – toadfish, songbird and bat – have nearly identical maximum speeds. Although this appears to support the idea that the adaptations all evolved from a shared ancestor, Mead et al. found evidence that suggests otherwise. Each of the three superfast muscles are powered by a different motor protein, which argues strongly in favor of the muscles evolving independently. The existence of such similar mechanisms and performance in independently evolved muscles raises the possibility that the fastest contraction rates measured by Mead et al. represent a maximum speed limit for all vertebrate muscles.

Any technical failure in a racecar most likely will slow it down, while the same failure in a robustly engineered family car may not be so noticeable. Similarly in superfast muscle many cellular and molecular systems need to perform maximally. Therefore by understanding how these extreme muscles work, we also gain a better understanding of how normal muscles contract.

The structural basis of ryanodine receptor ion channel function

Large-conductance Ca²⁺ release channels known as ryanodine receptors (RyRs) mediate the release of Ca²⁺ from an intracellular membrane compartment, the endo/sarcoplasmic reticulum. There are three mammalian RyR isoforms: RyR1 is present in skeletal muscle; RyR2 is in heart muscle; and RyR3 is expressed at low levels in many tissues including brain, smooth muscle, and slow-twitch skeletal muscle. RyRs form large protein complexes comprising four 560-kD RyR subunits, four ∼12-kD FK506-binding proteins, and various accessory proteins including calmodulin, protein kinases, and protein phosphatases. RyRs share ∼70% sequence identity, with the greatest sequence similarity in the C-terminal region that forms the transmembrane, ion-conducting domain comprising ∼500 amino acids. The remaining ∼4,500 amino acids form the large regulatory cytoplasmic "foot" structure. Experimental evidence for Ca²⁺, ATP, phosphorylation, and redox-sensitive sites in the cytoplasmic structure have been described. Exogenous effectors include the two Ca²⁺ releasing agents caffeine and ryanodine. Recent work describing the near atomic structures of mammalian skeletal and cardiac muscle RyRs provides a structural basis for the regulation of the RyRs by their multiple effectors.

Differential effects of contractile potentiators on action potential-induced Ca2+ transients of frog and mouse skeletal muscle fibres

Muscle fibres, isolated from frog tibialis anterior and mouse flexor digitorum brevis (FDB) were loaded with the fast dye MagFluo-4 to study the effects of potentiators caffeine, nitrate, Zn²⁺ and perchlorate on Ca²⁺ transients elicited by single action potentials. Overall, the potentiators doubled the transients amplitude and prolonged by about 1.5-fold their decay time. In contrast, as shown here for the first time, nitrate and Zn²⁺, but not caffeine, activated a late, secondary component of the transient rising phase of frog but not mouse, fibres. The rise time was increased from 1.9 ms in normal solution (NR) to 3.3 ms (nitrate) and 4.4 ms (Zn²⁺). In NR, a single exponential, fitted the rising phase of calcium transients of frog (τ₁ = 0.47 ms) and mouse (τ₁ = 0.28 ms). In nitrate and Zn²⁺ only frog transients showed a secondary exponential component, τ₂ = 0.72 ms (nitrate) and 0.94 ms, (Zn²⁺⁾. We suggest that nitrate and Zn²⁺ activate a late slower component of the ΔF/F signals of frog but not of mouse fibres, possibly promoting Ca²⁺ induced Ca²⁺ release at level of the RyR3, that in frog muscle fibres are localized in the para-junctional region of the triads and are absent in mouse FDB muscle fibres.

Colocalization properties of elementary Ca(2+) release signals with structures specific to the contractile filaments and the tubular system of intact mouse skeletal muscle fibers

Ca(2+) regulates several important intracellular processes. We combined second harmonic generation (SHG) and two photon excited fluorescence microscopy (2PFM) to simultaneously record the SHG signal of the myosin filaments and localized elementary Ca(2+) release signals (LCSs). We found LCSs associated with Y-shaped structures of the myosin filament pattern (YMs), so called verniers, in intact mouse skeletal muscle fibers under hypertonic treatment. Ion channels crucial for the Ca(2+) regulation are located in the tubular system, a system that is important for Ca(2+) regulation and excitation-contraction coupling. We investigated the tubular system of intact, living mouse skeletal muscle fibers using 2PFM and the fluorescent Ca(2+) indicator Fluo-4 dissolved in the external solution or the membrane dye di-8-ANEPPS. We simultaneously measured the SHG signal from the myosin filaments of the skeletal muscle fibers. We found that at least a subset of the YMs observed in SHG images are closely juxtaposed with Y-shaped structures of the transverse tubules (YTs). The distances of corresponding YMs and YTs yield values between 1.3 μm and 4.1 μm including pixel uncertainty with a mean distance of 2.52±0.10 μm (S.E.M., n=41). Additionally, we observed that some of the linear-shaped areas in the tubular system are colocalized with linear-shaped areas in the SHG images.

Localized nuclear and perinuclear Ca(2+) signals in intact mouse skeletal muscle fibers

Nuclear Ca²⁺ is important for the regulation of several nuclear processes such as gene expression. Localized Ca²⁺ signals (LCSs) in skeletal muscle fibers of mice have been mainly studied as Ca²⁺ release events from the sarcoplasmic reticulum. Their location with regard to cell nuclei has not been investigated. Our study is based on the hypothesis that LCSs associated with nuclei are present in skeletal muscle fibers of adult mice. Therefore, we carried out experiments addressing this question and we found novel Ca²⁺ signals associated with nuclei of skeletal muscle fibers (with possibly attached satellite cells). We measured localized nuclear and perinuclear Ca²⁺ signals (NLCSs and PLCSs) alongside cytosolic localized Ca²⁺ signals (CLCSs) during a hypertonic treatment. We also observed NLCSs under isotonic conditions. The NLCSs and PLCSs are Ca²⁺ signals in the range of micrometer [FWHM (full width at half maximum): 2.75 ± 0.27 μm (NLCSs) and 2.55 ± 0.17 μm (PLCSs), S.E.M.]. Additionally, global nuclear Ca²⁺ signals (NGCSs) were observed. To investigate which type of Ca²⁺ channels contribute to the Ca²⁺ signals associated with nuclei in skeletal muscle fibers, we performed measurements with the RyR blocker dantrolene, the DHPR blocker nifedipine or the IP₃R blocker Xestospongin C. We observed Ca²⁺ signals associated with nuclei in the presence of each blocker. Nifedipine and dantrolene had an inhibitory effect on the fraction of fibers with PLCSs. The situation for the fraction of fibers with NLCSs is more complex indicating that RyR is less important for the generation of NLCSs compared to the generation of PLCSs. The fraction of fibers with NLCSs and PLCSs is not reduced in the presence of Xestospongin C. The localized perinuclear and intranuclear Ca²⁺ signals may be a powerful tool for the cell to regulate adaptive processes as gene expression. The intranuclear Ca²⁺ signals may be particularly interesting in this respect.

Structural and functional properties of ryanodine receptor type 3 in zebrafish tail muscle

The ryanodine receptor (RyR)1 isoform of the sarcoplasmic reticulum (SR) Ca(2+) release channel is an essential component of all skeletal muscle fibers. RyR1s are detectable as "junctional feet" (JF) in the gap between the SR and the plasmalemma or T-tubules, and they are required for excitation-contraction (EC) coupling and differentiation. A second isoform, RyR3, does not sustain EC coupling and differentiation in the absence of RyR1 and is expressed at highly variable levels. Anatomically, RyR3 expression correlates with the presence of parajunctional feet (PJF), which are located on the sides of the SR junctional cisternae in an arrangement found only in fibers expressing RyR3. In frog muscle fibers, the presence of RyR3 and PJF correlates with the occurrence of Ca(2+) sparks, which are elementary SR Ca(2+) release events of the EC coupling machinery. Here, we explored the structural and functional roles of RyR3 by injecting zebrafish (Danio rerio) one-cell stage embryos with a morpholino designed to specifically silence RyR3 expression. In zebrafish larvae at 72 h postfertilization, fast-twitch fibers from wild-type (WT) tail muscles had abundant PJF. Silencing resulted in a drop of the PJF/JF ratio, from 0.79 in WT fibers to 0.03 in the morphants. The frequency with which Ca(2+) sparks were detected dropped correspondingly, from 0.083 to 0.001 sarcomere(-1) s(-1). The few Ca(2+) sparks detected in morphant fibers were smaller in amplitude, duration, and spatial extent compared with those in WT fibers. Despite the almost complete disappearance of PJF and Ca(2+) sparks in morphant fibers, these fibers looked structurally normal and the swimming behavior of the larvae was not affected. This paper provides important evidence that RyR3 is the main constituent of the PJF and is the main contributor to the SR Ca(2+) flux underlying Ca(2+) sparks detected in fully differentiated frog and fish fibers.

Malignant hyperthermia and the clinical significance of type-1 ryanodine receptor gene (RYR1) variants: proceedings of the 2013 MHAUS Scientific Conference

The Malignant Hyperthermia Association of the United States and the Department of Anesthesia at the University of Toronto sponsored a Scientific Conference on November 1-2, 2013 in Toronto, ON, Canada. The multidisciplinary group of experts, including clinicians, geneticists, and physiologists involved in research related to malignant hyperthermia (MH), shared new insights into the pathophysiology of diseases linked to the type-1 ryanodine receptor gene (RYR1) as well as the relationship between MH and "awake MH" conditions, such as exertional rhabdomyolysis and exertional heat illness. In addition, the molecular genetics of MH and clinical issues related to the diagnosis and management of disorders linked to RYR1 were presented. The conference also honoured Dr. David H. MacLennan for his contributions to our understanding of the genetics, pathogenesis, and treatment of MH and other RYR1-related myopathies. This report represents a summary of the proceedings of this conference.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder

Ca²⁺ movements during intercall intervals in superfast toadfish swimbladder muscle fibers depend on the accumulation of Ca²⁺ on parvalbumin and a consequent slow rate of Ca²⁺ pumping.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80–100-Hz calls lasting ∼500 ms interleaved with silent periods (intercall intervals) lasting ∼10 s. To understand the intracellular movements of Ca²⁺ during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca²⁺ indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling. The fluo-4 fluorescence signal was measured during and after the stimulus trains; the signal was also simulated with a kinetic model of the underlying myoplasmic Ca²⁺ movements, including the binding and transport of Ca²⁺ by the sarcoplasmic reticulum (SR) Ca²⁺ pumps. The estimated total amount of Ca²⁺ released from the SR during a first stimulus train is ∼6.5 mM (concentration referred to the myoplasmic water volume). At 40 ms after cessation of stimulation, the myoplasmic free Ca²⁺ concentration ([Ca²⁺]) is below the threshold for force generation (∼3 µM), yet the estimated concentration of released Ca²⁺ remaining in the myoplasm (Δ[Ca_M]) is large, ∼5 mM, with ∼80% bound to parvalbumin. At 10 s after stimulation, [Ca²⁺] is ∼90 nM (three times the assumed resting level) and Δ[Ca_M] is ∼1.3 mM, with 97% bound to parvalbumin. Ca²⁺ movements during the intercall interval thus appear to be strongly influenced by (a) the accumulation of Ca²⁺ on parvalbumin and (b) the slow rate of Ca²⁺ pumping that ensues when parvalbumin lowers [Ca²⁺] near the resting level. With repetitive stimulus trains initiated at 10-s intervals, Ca²⁺ release and pumping come quickly into balance as a result of the stability (negative feedback) supplied by the increased rate of Ca²⁺ pumping at higher [Ca²⁺].

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Comparison of myoplasmic calcium movements during excitation-contraction coupling in frog twitch and mouse fast-twitch muscle fibers

Single twitch fibers from frog leg muscles were isolated by dissection and micro-injected with furaptra, a rapidly responding fluorescent Ca(2+) indicator. Indicator resting fluorescence (FR) and the change evoked by an action potential (ΔF) were measured at long sarcomere length (16°C); ΔF/FR was scaled to units of ΔfCaD, the change in fraction of the indicator in the Ca(2+)-bound form. ΔfCaD was simulated with a multicompartment model of the underlying myoplasmic Ca(2+) movements, and the results were compared with previous measurements and analyses in mouse fast-twitch fibers. In frog fibers, sarcoplasmic reticulum (SR) Ca(2+) release evoked by an action potential appears to be the sum of two components. The time course of the first component is similar to that of the entire Ca(2+) release waveform in mouse fibers, whereas that of the second component is severalfold slower; the fractional release amounts are ~0.8 (first component) and ~0.2 (second component). Similar results were obtained in frog simulations with a modified model that permitted competition between Mg(2+) and Ca(2+) for occupancy of the regulatory sites on troponin. An anatomical basis for two release components in frog fibers is the presence of both junctional and parajunctional SR Ca(2+) release channels (ryanodine receptors [RyRs]), whereas mouse fibers (usually) have only junctional RyRs. Also, frog fibers have two RyR isoforms, RyRα and RyRβ, whereas the mouse fibers (usually) have only one, RyR1. Our simulations suggest that the second release component in frog fibers functions to supply extra Ca(2+) to activate troponin, which, in mouse fibers, is not needed because of the more favorable location of their triadic junctions (near the middle of the thin filament). We speculate that, in general, parajunctional RyRs permit increased myofilament activation in fibers whose triadic junctions are located at the z-line.

Developmental profile of localized spontaneous Ca(2+) release events in the dendrites of rat hippocampal pyramidal neurons

Recent experiments demonstrate that localized spontaneous Ca(2+) release events can be detected in the dendrites of pyramidal cells in the hippocampus and other neurons (J. Neurosci. 29 (2009) 7833-7845). These events have some properties that resemble ryanodine receptor mediated "sparks" in myocytes, and some that resemble IP(3) receptor mediated "puffs" in oocytes. They can be detected in the dendrites of rats of all tested ages between P3 and P80 (with sparser sampling in older rats), suggesting that they serve a general signaling function and are not just important in development. However, in younger rats the amplitudes of the events are larger than the amplitudes in older animals and almost as large as the amplitudes of Ca(2+) signals from backpropagating action potentials (bAPs). The rise time of the event signal is fast at all ages and is comparable to the rise time of the bAP fluorescence signal at the same dendritic location. The decay time is slower in younger animals, primarily because of weaker Ca(2+) extrusion mechanisms at that age. Diffusion away from a brief localized source is the major determinant of decay at all ages. A simple computational model closely simulates these events with extrusion rate the only age dependent variable.

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

The contribution of Ca2+-induced Ca2+ release (CICR) to trigger muscle contraction is controversial. It was studied on isolated muscle fibres using synthetic localized increases in Ca2+ concentration, SLICs, generated by two-photon photorelease from nitrodibenzofuran (NDBF)-EGTA just outside the permeabilized plasma membrane. SLICs provided a way to increase cytosolic [Ca2+] rapidly and reversibly, up to 8 μM, levels similar to those reached during physiological activity. They improve over previous paradigms in rate of rise, locality and reproducibility. Use of NDBF-EGTA allowed for the separate modification of resting [Ca2+], trigger [Ca2+] and resting [Mg2+]. In frog muscle, SLICs elicited propagated responses that had the characteristics of CICR. The threshold [Ca2+] for triggering a response was 0.5 μM or less. As this value is much lower than concentrations prevailing near channels during normal activity, the result supports participation of CICR in the physiological control of contraction in amphibian muscle. As SLICs were applied outside cells, the primary stimulus was Ca2+, rather than the radiation or subproducts of photorelease. Therefore the responses qualify as ‘classic' CICR. By contrast, mouse muscle fibres did not respond unless channel-opening drugs were present at substantial concentrations, an observation contrary to the physiological involvement of CICR in mammalian excitation–contraction coupling. In mouse muscle, the propagating wave had a substantially lower release flux, which together with a much higher threshold justified the absence of response when drugs were not present. The differences in flux and threshold may be ascribed to the absence of ryanodine receptor 3 (RyR3) isoforms in adult mammalian muscle.

Influence of intracellular Ca2+ release modulating drugs on bupivacaine infiltration anesthesia in mice

The endoplasmic reticulum inside neurons can provide enormous amounts of releasable Ca2+ to increase cytosolic Ca2+ levels through the activation of endoplasmic membrane ion channels. Ryanodine (RyR) channels release Ca2+ into the cytosol when activated by Ca2+ influx through voltage-gated channels, or by cyclicADP ribose. Inositol tris-phosphate (IP3) channels are stimulated by phospolipid metabolism and the release of IP3. The hypothesis was tested that drugs that bind RyR or IP3 channels would affect the anesthetic potency of bupivacaine. The radiant heat tail-flick test was used to assess for anesthesia following subcutaneous infiltration of bupivacaine and Ca2+ modulating drugs in the tails of mice. No musculature is contained in the tail that could result in motor block. The RyR channel agonists 4-chloro-m-cresol and poly-L-lysine significantly reduced the anesthetic potency of bupivacaine. The plant alkaloid ryanodine elicited a bi-phasic effect, with low concentrations blocking bupivacaine anesthesia, and a high concentration enhancing anesthesia. Alternatively, the RyR channel antagonist dantrolene sodium dose-dependently increased bupivacaine's potency. However, the IP3 channel drugs were inactive. The IP3 agonist adenophostin A failed to affect bupivacaine anesthesia. Furthermore, bupivacaine was unaffected by the IP3 channel antagonists xestospongin C or low molecular weight heparin. Our results indicate that only the RyR channel drugs modulated the anesthetic effects of bupivacaine. Electrophysiological and molecular studies of sensory dorsal root ganglia neurons, the source of Adelta and C-fiber nociceptors, have demonstrated the presence of RyR3 Ca2+ release channels. This provides the first evidence that RyR channels might affect bupivacaine anesthesia in some fashion.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

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.

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

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

Frog alpha- and beta-ryanodine receptors provide distinct intracellular Ca2+ signals in a myogenic cell line

Background

In frog skeletal muscle, two ryanodine receptor (RyR) isoforms, α-RyR and β-RyR, are expressed in nearly equal amounts. However, the roles and significance of the two isoforms in excitation-contraction (E-C) coupling remains to be elucidated.

Methodology/Principal Findings

In this study, we expressed either or both α-RyR and β-RyR in 1B5 RyR-deficient myotubes using the herpes simplex virus 1 helper-free amplicon system. Immunological characterizations revealed that α-RyR and β-RyR are appropriately expressed and targeted at the junctions in 1B5 myotubes. In Ca²⁺ imaging studies, each isoform exhibited caffeine-induced Ca²⁺ transients, an indicative of Ca²⁺-induced Ca²⁺ release (CICR). However, the fashion of Ca²⁺ release events was fundamentally different: α-RyR mediated graded and sustained Ca²⁺ release observed uniformly throughout the cytoplasm, whereas β-RyR supported all-or-none type regenerative Ca²⁺ oscillations and waves. α-RyR but not β-RyR exhibited Ca²⁺ transients triggered by membrane depolarization with high [K⁺]_o that were nifedipine-sensitive, indicating that only α-RyR mediates depolarization-induced Ca²⁺ release. Myotubes co-expressing α-RyR and β-RyR demonstrated high [K⁺]_o-induced Ca²⁺ transients which were indistinguishable from those with myotubes expressing α-RyR alone. Furthermore, procaine did not affect the peak height of high [K⁺]_o-induced Ca²⁺ transients, suggesting minor amplification of Ca²⁺ release by β-RyR via CICR in 1B5 myotubes.

Conclusions/Significance

These findings suggest that α-RyR and β-RyR provide distinct intracellular Ca²⁺ signals in a myogenic cell line. These distinct properties may also occur in frog skeletal muscle and will be important for E-C coupling.

A reappraisal of the Ca2+ dependence of fast inactivation of Ca2+ release in frog skeletal muscle

Two procedures to inhibit Ca(2+) release designed to differentiate between local and common pool mechanisms for the Ca(2+) dependent, fast inactivation of Ca(2+) release in skeletal muscle of the frog were compared. Inhibition by voltage dependent inactivation of Ca(2+) release, without modification of the single channel current of the Ryanodine Receptor (RyR) and the [Ca(2+)] close to the open pore, produced a reduction in the rate of inactivation linearly related to the reduction in the peak of Ca(2+) release flux. Linear fits in the individual fibers were performed, giving average values (+/-SEM, N = 8) of the best fit parameters of 5.75 x 10(-3) +/- 7.35 x 10(-4 )microM(-1) for the slope and 0.07 +/- 0.015 ms(-1) for the ordinate intercept. Inhibition of Ca(2+) release by reducing the Ca content of the sarcoplasmic reticulum (SR) involves reduction of the Ca(2+) current through the single RyR. The reduction in rate of inactivation also followed linearly the reduction in Ca(2+) peak release flux. The average values (+/-SEM) of the best fit parameters of linear fits were 14 x 10(-3) +/- 3.76 x 10(-3 )microM(-1) and 0.019 +/- 0.006 ms(-1) (N = 7) for slope and ordinate intercept respectively. The differences between both parameters were statistically significant (by t test, at P = 0.05). The extent of inactivation, measured by the peak/final Ca(2+) release flux ratio, was differentially affected by the two procedures. Inhibition by voltage dependent inactivation, despite slowing down the fast inactivation, increased the peak/final Ca(2+) release flux ratio. In contrast, depletion of the SR reticulum reduced it. If the fast inactivation is driven by the high [Ca(2+)] attained locally, close to the open pore of the RyR, the inhibition of Ca(2+) release due to voltage dependent inactivation should not modify the rate of inactivation while inhibition by SR Ca(2+) depletion should reduce it. A process driven by [Ca(2+)] in a common pool should depend on the overall Ca(2+) release independently of how it was modified. In this case both inhibitory procedures should reduce the inactivation rate similarly. Our findings are generally consistent with a common pool process. The differences between the two protocols could be understood if the organization of RyR in junctional and parajunctional release units is considered.

Optical single-channel resolution imaging of the ryanodine receptor distribution in rat cardiac myocytes

We have applied an optical super-resolution technique based on single-molecule localization to examine the peripheral distribution of a cardiac signaling protein, the ryanodine receptor (RyR), in rat ventricular myocytes. RyRs form clusters with a mean size of approximately 14 RyRs per cluster, which is almost an order of magnitude smaller than previously estimated. Clusters were typically not circular (as previously assumed) but elongated with an average aspect ratio of 1.9. Edge-to-edge distances between adjacent RyR clusters were often <50 nm, suggesting that peripheral RyR clusters may exhibit strong intercluster signaling. The wide variation of cluster size, which follows a near-exponential distribution, is compatible with a stochastic cluster assembly process. We suggest that calcium sparks may be the result of the concerted activation of several RyR clusters forming a functional "supercluster" whose gating is controlled by both cytosolic and sarcoplasmic reticulum luminal calcium levels.

Calcium-Induced Calcium Release in Skeletal Muscle

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

Sarcoplasmic reticulum-mitochondrial through-space coupling in skeletal muscle

The skeletal muscle contractile machine is fueled by both calcium and ATP. Calcium ions activate the contractile machinery by binding to troponin C and relieving troponin-tropomyosin inhibition of actinomyosin interaction. ATP binding to myosin during the contractile cycle results in myosin detachment from actin, and energy liberated from subsequent ATP hydrolysis is then used to drive the next contractile cycle. ATP is also used to lower myoplasmic calcium levels during muscle relaxation. Thus, muscle contractility is intimately linked to the proper control of sarcomeric Ca2+ delivery and (or) removal and ATP generation and (or) utilization. In skeletal muscle, the sarcoplasmic reticulum (SR) is the primary regulator of calcium storage, release, and reuptake, while glycolysis and the mitochondria are responsible for cellular ATP production. However, the SR and mitochondrial function in muscle are not independent, as calcium uptake into the mitochondria increases ATP production by stimulating oxidative phosphorylation and mitochondrial ATP production, and production and (or) detoxification of reactive oxygen and nitrogen species (ROS/RNS), in turn, modulates SR calcium release and reuptake. Close spatial Ca2+/ATP/ROS/RNS communication between the SR and mitochondria is facilitated by the structural attachment of mitochondria to the calcium release unit (CRU) by 10 nm of electron-dense tethers. The resultant anchoring of mitochondria to the CRU provides a structural basis for maintaining bidirectional SR-mitochondrial through-space communication during vigorous contraction. This review will consider the degree to which this structural link enables privileged or microdomain communication between the SR and mitochondria in skeletal muscle.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

[Molecular architecture of the sarcoplasmic reticulum and its role in the ECC]

The sarcoplasmic reticulum (SR) plays a fundamental role in excitation-contraction coupling, which propagates the electric signal conversion along the muscle fiber's plasmic membrane to a mechanical event manifested as a muscle contraction. It plays a crucial role in calcium homeostasis and intracellular calcium storage control (storage, liberation and uptake) necessary for fiber muscle contraction and then relaxation. These functions take place at the triad, made up of individualized SR subdomains where the protein-specific organization provides efficient and fast coupling. Ryanodine receptors (RyR) and dihydropyridine receptors (DHPR) mainly act in calcium exchanges in the SR. This particular structural and molecular architecture must be correlated to its functional specificity.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Calcium-dependent inactivation terminates calcium release in skeletal muscle of amphibians

In skeletal muscle of amphibians, the cell-wide cytosolic release of calcium that enables contraction in response to an action potential appears to be built of Ca²⁺ sparks. The mechanism that rapidly terminates this release was investigated by studying the termination of Ca²⁺ release underlying sparks. In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca²⁺ release. This relationship included a range of T where a paradoxically decreased with increasing T. Three different methods were used to estimate Ca²⁺ release flux in groups of sparks of different T. Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional. A simple model in which release sources were inactivated by cytosolic Ca²⁺ was able to explain the relationship. The predictive value of the model, evaluated by analyzing the variance of spark amplitude, was found to be high when allowance was made for the out-of-focus error contribution to the total variance. This contribution was estimated using a theory of confocal scanning (Ríos, E., N. Shirokova, W.G. Kirsch, G. Pizarro, M.D. Stern, H. Cheng, and A. González. Biophys. J. 2001. 80:169–183), which was confirmed in the present work by simulated line scanning of simulated sparks. Considering these results and other available evidence it is concluded that Ca²⁺-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca²⁺ release in amphibians. Given the similarities in kinetics of release termination observed in cell-averaged records of amphibian and mammalian muscle, and in spite of differences in activation mechanisms, CDI is likely to play a central role in mammals as well. Trivially, an inverse proportionality between release flux and duration, in sparks or in global release of skeletal muscle, maintains constancy of the amount of released Ca²⁺.

Spontaneous and voltage-activated Ca2+ release in adult mouse skeletal muscle fibres expressing the type 3 ryanodine receptor

The physiological properties and role of the type 3 ryanodine receptor (RyR3), a calcium release channel expressed in a wide variety of cell types, remain mysterious. We forced, in vivo, the expression of RyR3 in adult mouse skeletal muscle fibres using a GFP-RyR3 DNA construct. GFP fluorescence was found within spatially restricted regions of muscle fibres where it exhibited a sarcomere-related banded pattern consistent with a localization within or near the junctional sarcoplasmic reticulum membrane. Immunostaining confirmed the presence of RyR3 together with RyR1 within the GFP-positive areas. In approximately 90% of RyR3-positive fibres microinjected with the calcium indicator fluo-3, we detected repetitive spontaneous transient elevations of intracellular Ca2+ that persisted when fibres were voltage-clamped at -80 mV. These Ca2+ transients remained essentially confined to the RyR3 expression region. They ranged from wide local events to propagating Ca2+ waves and were in some cases associated with local contractile activity. When voltage-clamp depolarizations were applied while fluo-3 or rhod-2 fluorescence was measured within the RyR3-expressing region, no voltage-evoked 'spark-like' elementary Ca2+ release event could be detected. Still global voltage-activated Ca2+ release exhibited a prominent early peak within the RyR3-expressing regions. Measurements were also taken from muscles fibres expressing a GFP-RyR1 construct; positive fibres also yielded a local banded pattern of GFP fluorescence but exhibited no spontaneous Ca2+ release. Results demonstrate that RyR3 is a very potent source of voltage-independent Ca2+ release activity. Conversely we find no evidence that it could contribute to the production of discrete voltage-activated Ca2+ release events in differentiated mammalian skeletal muscle.

Simulation of Ca2+ movements within the sarcomere of fast-twitch mouse fibers stimulated by action potentials

Ca²⁺ release from the sarcoplasmic reticulum (SR) of skeletal muscle takes place at the triadic junctions; following release, Ca²⁺ spreads within the sarcomere by diffusion. Here, we report multicompartment simulations of changes in sarcomeric Ca²⁺ evoked by action potentials (APs) in fast-twitch fibers of adult mice. The simulations include Ca²⁺ complexation reactions with ATP, troponin, parvalbumin, and the SR Ca²⁺ pump, as well as Ca²⁺ transport by the pump. Results are compared with spatially averaged Ca²⁺ transients measured in mouse fibers with furaptra, a low-affinity, rapidly responding Ca²⁺ indicator. The furaptra Δf_CaD signal (change in the fraction of the indicator in the Ca²⁺-bound form) evoked by one AP is well simulated under the assumption that SR Ca²⁺ release has a peak of 200–225 μM/ms and a FDHM of ∼1.6 ms (16°C). Δf_CaD elicited by a five-shock, 67-Hz train of APs is well simulated under the assumption that in response to APs 2–5, Ca²⁺ release decreases progressively from 0.25 to 0.15 times that elicited by the first AP, a reduction likely due to Ca²⁺ inactivation of Ca²⁺ release. Recovery from inactivation was studied with a two-AP protocol; the amplitude of the second release recovered to >0.9 times that of the first with a rate constant of 7 s⁻¹. An obvious feature of Δf_CaD during a five-shock train is a progressive decline in the rate of decay from the individual peaks of Δf_CaD. According to the simulations, this decline is due to a reduction in available Ca²⁺ binding sites on troponin and parvalbumin. The effects of sarcomere length, the location of the triadic junctions, resting [Ca²⁺], the parvalbumin concentration, and possible uptake of Ca²⁺ by mitochondria were also investigated. Overall, the simulations indicate that this reaction-diffusion model, which was originally developed for Ca²⁺ sparks in frog fibers, works well when adapted to mouse fast-twitch fibers stimulated by APs.

Binding property of avian skeletal muscle ryanodine receptor isoforms with dihydropyridine receptor and calmodulin

Ca(2+) release during excitation-contraction coupling in avian skeletal muscle is controlled by two ryanodine receptor isoforms, alphaRYR and betaRYR. Two other proteins, dihydropyridine receptor (DHPR) and calmodulin (CaM), have been shown to play important roles in regulating the RYR channel activity. In the current study, we measured the protein contents of DHPR and RYR in turkey skeletal muscle and obtained a ratio of 1:1 between DHPR and alphaRYR which suggests that only a subpopulation of alphaRYR is interacting with DHPR. Two CaM derivatives, the photoactivable crosslinking probe [(125)I]-Bz-CaM and metabolically labeled probe [(35)S]CaM, were used to study the interaction between CaM and RYR isoforms in turkey skeletal muscle. The alphaRYR and betaRYR displayed a marked difference in their CaM binding behavior. At a Ca(2+) concentration of 200 microM, CaM bound to both isoforms at a ratio of one CaM molecule per one RYR subunit. At a Ca(2+) concentration of <10 nM, CaM bound primarily to alphaRYR and the binding affinity was significantly lower than that at micromolar level of Ca(2+) concentration. Cloning and sequencing of putative CaM binding sites in alphaRYR and betaRYR suggests that differences in primary structures of the CaM binding sites of each RYR isoform may contribute to the differential CaM binding behavior of alphaRYR and betaRYR.

Ca(2+) sparks operated by membrane depolarization require isoform 3 ryanodine receptor channels in skeletal muscle

Stimuli are translated to intracellular calcium signals via opening of inositol trisphosphate receptor and ryanodine receptor (RyR) channels of the sarcoplasmic reticulum or endoplasmic reticulum. In cardiac and skeletal muscle of amphibians the stimulus is depolarization of the transverse tubular membrane, transduced by voltage sensors at tubular-sarcoplasmic reticulum junctions, and the unit signal is the Ca(2+) spark, caused by concerted opening of multiple RyR channels. Mammalian muscles instead lose postnatally the ability to produce sparks, and they also lose RyR3, an isoform abundant in spark-producing skeletal muscles. What does it take for cells to respond to membrane depolarization with Ca(2+) sparks? To answer this question we made skeletal muscles of adult mice expressing exogenous RyR3, demonstrated as immunoreactivity at triad junctions. These muscles showed abundant sparks upon depolarization. Sparks produced thusly were found to amplify the response to depolarization in a manner characteristic of Ca(2+)-induced Ca(2+) release processes. The amplification was particularly effective in responses to brief depolarizations, as in action potentials. We also induced expression of exogenous RyR1 or yellow fluorescent protein-tagged RyR1 in muscles of adult mice. In these, tag fluorescence was present at triad junctions. RyR1-transfected muscle lacked voltage-operated sparks. Therefore, the voltage-operated sparks phenotype is specific to the RyR3 isoform. Because RyR3 does not contact voltage sensors, their opening was probably activated by Ca(2+), secondarily to Ca(2+) release through junctional RyR1. Physiologically voltage-controlled Ca(2+) sparks thus require a voltage sensor, a master junctional RyR1 channel that provides trigger Ca(2+), and a slave parajunctional RyR3 cohort.

Ca2+ sparks and T tubule reorganization in dedifferentiating adult mouse skeletal muscle fibers

Ca(+) sparks are rare in healthy adult mammalian skeletal muscle but may appear when adult fiber integrity is compromised, and occur in embryonic muscle but decline as the animal develops. Here we used cultured adult mouse flexor digitorum brevis muscle fibers to monitor occurrence of Ca(2+) sparks during maintenance of adult fiber morphology and during eventual fiber morphological dedifferentiation after various times in culture. Fibers cultured for up to 3 days retain normal morphology and striated appearance. Ca(2+) sparks were rare in these fibers. At 5-7 days in culture, many of the original muscle fibers exhibit sprouting and loss of striations, as well as the occurrence of spontaneous Ca(2+) sparks. The average rate of occurrence of Ca(2+) sparks is >10-fold higher after 5-7 days in culture than in days 1-3. With the use of fibers cultured for 7 days, application of the Ca(2+) channel blockers Co(2+) or nifedipine almost completely suppressed the occurrence of Ca(2+) sparks, as previously shown in embryonic fibers, suggesting that Ca(2+) sparks may be generated by similar mechanisms in dedifferentiating cultured adult fibers and in embryonic fibers before final differentiation. The sarcomeric disruption observed under transmitted light microscopy in dedifferentiating fibers was accompanied by morphological changes in the transverse (T) tubular system, as observed by fluorescence confocal imaging of both an extracellular marker dye and membrane staining dyes. Changes in T tubule morphology coincided with the appearance of Ca(2+) sparks, suggesting that Ca(2+) sparks may either be a signal for, or the result of, disruption of DHPR-ryanodine receptor 1 coupling.

Calcium release domains in mammalian skeletal muscle studied with two-photon imaging and spot detection techniques

The spatiotemporal characteristics of the Ca(2+) release process in mouse skeletal muscle were investigated in enzymatically dissociated fibers from flexor digitorum brevis (FDB) muscles, using a custom-made two-photon microscope with laser scanning imaging (TPLSM) and spot detection capabilities. A two-microelectrode configuration was used to electrically stimulate the muscle fibers, to record action potentials (APs), and to control their myoplasmic composition. We used 125 muM of the low-affinity Ca(2+) indicator Oregon green 488 BAPTA-5N (OGB-5N), and 5 or 10 mM of the Ca(2+) chelator EGTA (pCa 7) in order to arrest fiber contraction and to constrain changes in the [Ca(2+)] close to the release sites. Image and spot data showed that the resting distribution of OGB-5N fluorescence was homogeneous along the fiber, except for narrow peaks ( approximately 23% above the bulk fluorescence) centered at the Z-lines, as evidenced by their nonoverlapping localization with respect to di-8-ANEPPS staining of the transverse tubules (T-tubules). Using spot detection, localized Ca(2+) transients evoked by AP stimulation were recorded from adjacent longitudinal positions 100 nm apart. The largest and fastest DeltaF/F transients were detected at sites flanking the Z-lines and colocalized with T-tubules; the smallest and slowest were detected at the M-line, whereas transients at the Z-line showed intermediate features. Three-dimensional reconstructions demonstrate the creation of two AP-evoked Ca(2+) release domains per sarcomere, which flank the Z-line and colocalize with T-tubules. In the presence of 10 mM intracellular EGTA, these domains are formed in approximately 1.4 ms and dissipate within approximately 4 ms, after the peak of the AP. Their full-width at half-maximum (FWHM), measured at the time that Ca(2+) transients peaked at T-tubule locations, was 0.62 mum, similar to the 0.61 mum measured for di-8-ANEPPS profiles. Both these values exceed the limit of resolution of the optical system, but their similarity suggests that at high [EGTA] the Ca(2+) domains in adult mammalian muscle fibers are confined to Ca(2+) release sites located at the junctional sarcoplasmic reticulum (SR).

Differential sensitivity to perchlorate and caffeine of tetracaine-resistant Ca2+ release in frog skeletal muscle

In voltage clamped frog skeletal muscle fibres 0.2 mM tetracaine strongly suppresses Ca(2+) release. After this treatment Ca(2+) release flux lacks its characteristic initial peak and the remaining steady component is strongly reduced when compared with the control condition. We studied the effect of two agonists of Ca(2+) release on these tetracaine treated fibres. 8 mM ClO(4)(-) added after tetracaine potentiated release flux from 0.11 +/- 0.03 mM s(-1) to 0.34 +/- 0.07 mM s(-1) (n = 6) although without recovery of the peak at any test voltage. The voltage dependence of the increased release was shifted towards more negative potentials (approximately -10 mV). The effects of ClO(4)(-) on charge movement under these conditions showed the previously described characteristic changes consisting in a left shift of its voltage dependence (approximately -9 mV) together with a slower kinetics, both at the ON and OFF transients. Caffeine at 0.5 mM in the presence of the same concentration of tetracaine failed to potentiate release flux independently of the test voltage applied. When the cut ends of the fibre were exposed to a 10 mM BAPTA intracellular solution, in the absence of tetracaine, the peak was progressively abolished. Under these conditions caffeine potentiated release restoring the peak (from 0.63 +/- 0.12 mM s(-1) to 1.82 +/- 0.23 mM s(-1)) with no effect on charge movement. Taken together the present results suggest that tetracaine is blocking a Ca(2+) sensitive component of release flux. It is speculated that the suppressed release includes a component that is dependent on Ca(2+) and mainly mediated by the activation of the beta ryanodine receptors (the RyR3 equivalent isoform). These receptors are located parajunctionally in the frog and are not interacting with the dihydropyridine receptor.