Evidence of a role for calmodulin in the regulation of calcium release from skeletal muscle sarcoplasmic reticulum

Allosteric mechanism of water-channel gating by Ca2+-calmodulin

Calmodulin (CaM) is a universal regulatory protein that communicates the presence of calcium to its molecular targets and correspondingly modulates their function. This key signaling protein is important for controlling the activity of hundreds of membrane channels and transporters. However, our understanding of the structural mechanisms driving CaM regulation of full-length membrane proteins has remained elusive. In this study, we determined the pseudo-atomic structure of full-length mammalian aquaporin-0 (AQP0, Bos Taurus) in complex with CaM using electron microscopy to understand how this signaling protein modulates water channel function. Molecular dynamics and functional mutation studies reveal how CaM binding inhibits AQP0 water permeability by allosterically closing the cytoplasmic gate of AQP0. Our mechanistic model provides new insight, only possible in the context of the fully assembled channel, into how CaM regulates multimeric channels by facilitating cooperativity between adjacent subunits.

S100A1 and calmodulin regulation of ryanodine receptor in striated muscle

The release of Ca2+ ions from the sarcoplasmic reticulum through ryanodine receptor calcium release channels represents the critical step linking electrical excitation to muscular contraction in the heart and skeletal muscle (excitation-contraction coupling). Two small Ca2+ binding proteins, S100A1 and calmodulin, have been demonstrated to bind and regulate ryanodine receptor in vitro. This review focuses on recent work that has revealed new information about the endogenous roles of S100A1 and calmodulin in regulating skeletal muscle excitation-contraction coupling. S100A1 and calmodulin bind to an overlapping domain on the ryanodine receptor type 1 to tune the Ca2+ release process, and thereby regulate skeletal muscle function. We also discuss past, current and future work surrounding the regulation of ryanodine receptors by calmodulin and S100A1 in both cardiac and skeletal muscle, and the implications for excitation-contraction coupling.

Site-specific modification of calmodulin Ca²(+) affinity tunes the skeletal muscle ryanodine receptor activation profile

The skeletal muscle isoform of the ryanodine receptor Ca²(+)-release channel (RyR1) is regulated by Ca²(+) and CaM (calmodulin). CaM shifts the biphasic Ca²(+)-dependence of RyR1 activation leftward, effectively increasing channel opening at low Ca²(+) and decreasing channel opening at high Ca²(+). The conversion of CaM from a RyR1 activator into an inhibitor is due to the binding of Ca²(+) to CaM; however, which of CaM's four Ca²(+)-binding sites serves as the switch for this conversion is unclear. We engineered a series of mutant CaMs designed to individually increase the Ca²(+) affinity of each of CaM's EF-hands by increasing the number of acidic residues in Ca²(+)-chelating positions. Domain-specific Ca²(+) affinities of each CaM variant were determined by equilibrium fluorescence titration. Mutations in sites I (T26D) or II (N60D) in CaM's N-terminal domain had little effect on CaM Ca²(+) affinity and regulation of RyR1. However, the site III mutation N97D increased the Ca²(+)-binding affinity of CaM's C-terminal domain and caused CaM to inhibit RyR1 at a lower Ca²(+) concentration than wild-type CaM. Conversely, the site IV mutation Q135D decreased the Ca²(+)-binding affinity of CaM's C-terminal domain and caused CaM to inhibit RyR1 at higher Ca²(+) concentrations. These results support the hypothesis that Ca²(+) binding to CaM's C-terminal acts as the switch converting CaM from a RyR1 activator into a channel inhibitor. These results indicate further that targeting CaM's Ca²(+) affinity may be a valid strategy to tune the activation profile of CaM-regulated ion channels.

Regulation of sarcoplasmic reticulum Ca2+ reuptake in porcine airway smooth muscle

Regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in airway smooth muscle (ASM) during agonist stimulation involves sarcoplasmic reticulum (SR) Ca(2+) release and reuptake. The sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) is key to replenishment of SR Ca(2+) stores. We examined regulation of SERCA in porcine ASM: our hypothesis was that the regulatory protein phospholamban (PLN) and the calmodulin (CaM)-CaM kinase (CaMKII) pathway (both of which are known to regulate SERCA in cardiac muscle) play a role. In porcine ASM microsomes, we examined the expression and extent of PLN phosphorylation after pharmacological inhibition of CaM (with W-7) vs. CaMKII (with KN-62/KN-93) and found that PLN is phosphorylated by CaMKII. In parallel experiments using enzymatically dissociated single ASM cells loaded with the Ca(2+) indicator fluo 3 and imaged using fluorescence microscopy, we measured the effects of PLN small interfering RNA, W-7, and KN-62 on [Ca(2+)](i) responses to ACh and direct SR stimulation. PLN small interfering RNA slowed the rate of fall of [Ca(2+)](i) transients to 1 microM ACh, as did W-7 and KN-62. The two inhibitors additionally slowed reuptake in the absence of PLN. In other cells, preexposure to W-7 or KN-62 did not prevent initiation of ACh-induced [Ca(2+)](i) oscillations (which were previously shown to result from repetitive SR Ca(2+) release/reuptake). However, when ACh-induced [Ca(2+)](i) oscillations reached steady state, subsequent exposure to W7 or KN-62 decreased oscillation frequency and amplitude and slowed the fall time of [Ca(2+)](i) transients, suggesting SERCA inhibition. Exposure to W-7 completely abolished ongoing ACh-induced [Ca(2+)](i) oscillations in some cells. Preexposure to W-7 or KN-62 did not affect caffeine-induced SR Ca(2+) release, indicating that ryanodine receptor channels were not directly inhibited. These data indicate that, in porcine ASM, the CaM-CaMKII pathway regulates SR Ca(2+) reuptake, potentially through altered PLN phosphorylation.

Complex of calmodulin with a ryanodine receptor target reveals a novel, flexible binding mode

Calmodulin regulates ryanodine receptor-mediated Ca(2+) release through a conserved binding site. The crystal structure of Ca(2+)-calmodulin bound to this conserved site reveals that calmodulin recognizes two hydrophobic anchor residues at a novel "1-17" spacing that brings the calmodulin lobes close together but prevents them from contacting one another. NMR residual dipolar couplings demonstrate that the detailed structure of each lobe is preserved in solution but also show that the lobes experience domain motions within the complex. FRET measurements confirm the close approach of the lobes in binding the 1-17 target and show that calmodulin binds with one lobe to a peptide lacking the second anchor. We suggest that calmodulin regulates the Ca(2+) channel by switching between the contiguous binding mode seen in our crystal structure and a state where one lobe of calmodulin contacts the conserved binding site while the other interacts with a noncontiguous site on the channel.

Structural Characterization of the RyR1–FKBP12 Interaction

The 12 kDa FK506-binding protein (FKBP12) constitutively binds to the calcium release channel RyR1. Removal of FKBP12 using FK506 or rapamycin causes an increased open probability and an increase in the frequency of sub-conductance states in RyR1. Using cryo-electron microscopy and single-particle image processing, we have determined the 3D difference map of FKBP12 associated with RyR1 at 16 A resolution that can be fitted with the atomic model of FKBP12 in a unique orientation. This has allowed us to better define the surfaces of close apposition between FKBP12 and RyR1. Our results shed light on the role of several FKBP12 residues that had been found critical for the specificity of the RyR1-FKBP12 interaction. As predicted from previous immunoprecipitation studies, our results suggest that Gln3 participates directly in this interaction. The orientation of RyR1-bound FKBP12, with part of its FK506 binding site facing towards RyR1, allows us to propose how FK506 is involved in the dissociation of FKBP12 from RyR1.

A Calmodulin Binding Domain of RyR Increases Activation of Spontaneous Ca2+ Sparks in Frog Skeletal Muscle

The calmodulin C lobe binding region (residues 3614-3643) on the sarcoplasmic reticulum Ca2+ release channel (RyR1) is thought to be a region of contact between subunits within RyR1 homotetramer Ca2+ release channels. To determine whether the 3614-3643 region is a regulatory site/interaction domain within RyR in muscle fibers, we have investigated the effect of a synthetic peptide corresponding to this region (R3614-3643) on Ca2+ sparks in frog skeletal muscle fibers. R3614-3643 (0.2-3.0 microM) promoted the occurrence of Ca2+ sparks in a highly cooperative dose-dependent manner, with a half-maximal activation at 0.47 microM and a maximal increase in frequency of approximately 5-fold. A peptide with a single amino acid substitution within R3614-3643 (L3624D) retained the ability to bind Ca(2+)-free calmodulin but did not increase Ca2+ spark frequency, suggesting that R3614-3643 does not modulate Ca2+ sparks by removal of endogenous calmodulin. Our data support a model in which the calmodulin binding domain of RyR1 modulates channel activity by at least two mechanisms: direct binding of calmodulin as well as interactions with other regions of RyR.

Regulation of ryanodine receptors by calsequestrin: effect of high luminal Ca2+ and phosphorylation

Calsequestrin, the major calcium sequestering protein in the sarcoplasmic reticulum of muscle, forms a quaternary complex with the ryanodine receptor calcium release channel and the intrinsic membrane proteins triadin and junctin. We have investigated the possibility that calsequestrin is a luminal calcium concentration sensor for the ryanodine receptor. We measured the luminal calcium concentration at which calsequestrin dissociates from the ryanodine receptor and the effect of calsequestrin on the response of the ryanodine receptor to changes in luminal calcium. We provide electrophysiological and biochemical evidence that: 1), luminal calcium concentration of >/=4 mM dissociates calsequestrin from junctional face membrane, whereas in the range of 1-3 mM calsequestrin remains attached; 2), the association with calsequestrin inhibits ryanodine receptor activity, but amplifies its response to changes in luminal calcium concentration; and 3), under physiological calcium conditions (1 mM), phosphorylation of calsequestrin does not alter its ability to inhibit native ryanodine receptor activity when the anchoring proteins triadin and junctin are present. These data suggest that the quaternary complex is intact in vivo, and provides further evidence that calsequestrin is involved in the sarcoplasmic reticulum calcium signaling pathway and has a role as a luminal calcium sensor for the ryanodine receptor.

Co-expression of MG29 and ryanodine receptor leads to apoptotic cell death: effect mediated by intracellular Ca2+ release

Perturbation of intracellular Ca2+ homeostasis has been shown to regulate the process of cell proliferation and apoptosis. Our previous studies show that mitsugumin 29 (MG29), a synaptophysin-related protein localized in the triad junction of skeletal muscle, serves an essential role in muscle Ca2+ signaling by regulating the process of store-operated Ca2+ entry. Here we report a functional interaction between MG29 and the ryanodine receptor (RyR)/Ca2+ release channel. The purified MG29 protein enhances activity of the RyR/Ca2+ release channel incorporated into the lipid bilayer membrane. Co-expression of MG29 and RyR in Chinese hamster ovary cells leads to apoptotic cell death resulting from depletion of intracellular Ca2+ stores, despite neither protein expression alone exhibits any significant effect on cell viability. In transient expression studies, the presence of RyR in the endoplasmic reticulum leads to retention of MG29 from the plasma membrane into the intracellular organelles. This functional interaction between MG29 and RyR could have important implications in the Ca2+ signaling processes of muscle cells. Our data also show that perturbation of intracellular Ca2+ homeostasis can serve as a key signal in the initiation of apoptosis.

The calmodulin binding region of the skeletal ryanodine receptor acts as a self-modulatory domain

A synthetic peptide (CaMBP) matching amino acids 3614-3643 of the skeletal ryanodine receptor (RyR1) binds to both Ca2+-free calmodulin (CaM) and Ca2+-bound CaM with nanomolar affinity [J. Biol. Chem. 276 (2001) 2069]. We report here that CaMBP increases [3H]ryanodine binding to RyR1 in a dose- and Ca2+-dependent manner; it also induces Ca2+ release from SR vesicles, and increases open probability (P(o)) of single RyR channels reconstituted in planar lipid bilayers. Further, CaMBP removes CaM associated with SR vesicles and increases [3H]ryanodine binding to purified RyR1, suggesting that its mechanism of action is two-fold: it removes endogenous inhibitors and also interacts directly with complementary regions in RyR1. Remarkably, the N-terminus of CaMBP activates RyRs while the C-terminus of CaMBP inhibits RyR activity, suggesting the presence of two discrete functional subdomains within this region. A ryr1 mutant lacking this region, RyR1-Delta3614-3643, was constructed and expressed in dyspedic myoblasts (RyR1-knockout). The depolarization-, caffeine- and 4-chloro-m-cresol (4-CmC)-induced Ca2+ transients in these cells were dramatically reduced compared with cells expressing wild type RyR1. Deletion of the 3614-3643 region also resulted in profound changes in unitary conductance and channel gating. We thus propose that the RyR1 3614-3643 region acts not only as the CaM binding site, but also as an important modulatory domain for RyR1 function.

Conformational coupling of DHPR and RyR1 in skeletal myotubes is influenced by long-range allosterism: evidence for a negative regulatory module

Four ryanodine receptor type 1 and 2 chimeras (R4, R9, R10, and R16) and their respective wild-type ryanodine receptors (type 1 and 2; wtRyR1 and wtRyR2) were expressed in dyspedic 1B5 to identify possible negative regulatory modules of the Ca2+ release channel that are under the influence of the dihydropyridine receptor (DHPR). Responses of intact 1B5 myotubes expressing each construct to caffeine in the absence or presence of either La3+ and Cd2+ or the organic DHPR blocker nifedipine were determined by imaging single 1B5 myotubes loaded with fluo 4. The presence of La3+ and Cd2+ or nifedipine in the external medium at concentrations known to block Ca2+ entry through the DHPRs significantly decreased the caffeine EC50 of wtRyR1 (2.80 +/- 0.12 to 0.83 +/- 0.09 mM; P < 0.05). On the other hand, DHPR blockade did not significantly alter the caffeine EC50 values of wtRyR2, chimeras R10 and R16, whereas the caffeine EC50 values of chimeras R4 and R9 were significantly increased (1.27 +/- 0.05 to 2.60 +/- 0.16 mM, and 1.15 +/- 0.03 to 2.11 +/- 0.32 mM, respectively; P < 0.05). Despite the fact that all the chimeras form fully functional Ca2+ release channels in situ, sarcoplasmic reticulum (SR) containing R4, R10, and R16 did not possess high-affinity binding of [3H]ryanodine regardless of Ca2+ concentration. These results suggest the presence of an interaction between RyR1 and the DHPR, which is not present in RyR2, that contributes negative control of SR Ca2+ release induced by direct agonists such as caffeine. Although we were unable to define the negative module using RyR1-RyR2 chimeras, they further demonstrated that the RyR is very sensitive to long-range allosterism.

Calmodulin modulates initiation but not termination of spontaneous Ca2+ sparks in frog skeletal muscle

Calmodulin is a ubiquitous Ca(2+) sensing protein that binds to and modulates the sarcoplasmic reticulum Ca(2+) release channel, ryanodine receptor (RYR). Here we assessed the effects of calmodulin on the local Ca(2+) release properties of RYR in permeabilized frog skeletal muscle fibers. Fluorescently labeled recombinant calmodulin in the internal solution localized at the Z-line/triad region. Calmodulin (0.05-5.0 micro M) in the internal solution (free [Ca(2+)](i) approximately 50-100 nM) initiated a highly cooperative dose-dependent increase in Ca(2+) spark frequency, with a half-maximal activation (K) of 1.1 micro M, a Hill coefficient (n) of 4.2 and a fractional maximal increase in frequency (R) of 17-fold. A non-Ca(2+) binding mutant of calmodulin elicited a similar highly cooperative dose-dependent increase in spark frequency (K = 1.0 micro M; n = 3.7; R = 12-fold). Spatiotemporal properties of Ca(2+) sparks were essentially unaffected by either wild-type or mutant calmodulin. An N-terminal extension of calmodulin, (N+3)calmodulin, that binds to but does not activate RYR at nM [Ca(2+)] in sarcoplasmic reticulum vesicles, prevented the calmodulin-induced increase in spark frequency. These data suggest that exogenous Ca(2+)-free calmodulin cooperatively sensitizes the Ca(2+) release channel to open, but that Ca(2+) binding to the added calmodulin does not play a significant role in the termination of Ca(2+) sparks.

Molecular basis of calmodulin binding to cardiac muscle Ca(2+) release channel (ryanodine receptor)

Calmodulin (CaM) is a ubiquitous Ca2+-binding protein that regulates the ryanodine receptors (RyRs) by direct binding. CaM inhibits the skeletal muscle ryanodine receptor (RyR1) and cardiac muscle receptor (RyR2) at >1 microm Ca2+ but activates RyR1 and inhibits RyR2 at <1 microm Ca2+. Here we tested whether CaM regulates RyR2 by binding to a highly conserved site identified previously in RyR1. Deletion of RyR2 amino acid residues 3583-3603 resulted in background [35S]CaM binding levels. In single channel measurements, deletion of the putative CaM binding site eliminated CaM inhibition of RyR2 at Ca2+ concentrations below and above 1 microm. Five RyR2 single or double mutants in the CaM binding region (W3587A, L3591D, F3603A, W3587A/L3591D, L3591D/F3603A) eliminated or greatly reduced [35S]CaM binding and inhibition of single channel activities by CaM depending on the Ca2+ concentration. An RyR2 mutant, which assessed the effects of 4 amino acid residues that differ between RyR1 and RyR2 in or flanking the CaM binding domain, bound [35S]CaM and was inhibited by CaM, essentially identical to wild type (WT)-RyR2. Three RyR1 mutants (W3620A, L3624D, F3636A) showed responses to CaM that differed from corresponding mutations in RyR2. The results indicate that CaM regulates RyR1 and RyR2 by binding to a single, highly conserved CaM binding site and that other RyR type-specific sites are likely responsible for the differential functional regulation of RyR1 and RyR2 by CaM.

Calcium, calmodulin, and calcium-calmodulin kinase II: heartbeat to heartbeat and beyond

Calcium (Ca) is the key regulator of cardiac contraction during excitation-contraction (E-C) coupling. However, differences exist between the amount of Ca being transported into the myocytes upon electrical stimulation as compared to Ca released from the sarcoplasmic reticulum (SR). Moreover, alterations in E-C coupling occur in cardiac hypertrophy and heart failure. In addition to the direct effects of Ca on the myofilaments, Ca plays a pivotal role in activation of a number of Ca-dependent proteins or second messengers, which can modulate E-C coupling. Of these proteins, calmodulin (CaM) and Ca-CaM-dependent kinase II (CaMKII) are of special interest in the heart because of their role of modulating Ca influx, SR Ca release, and SR Ca uptake during E-C coupling. Indeed, CaM and CaMKII may be associated with some ion channels and Ca transporters and both can modulate acute cellular Ca handling. In addition to the changes in Ca, CaM and CaMKII signals from beat-to-beat, changes may occur on a longer time scale. These may occur over seconds to minutes involving phosphorylation/dephosphorylation reactions, and even a longer time frame in altering gene transcription (excitation-transcription (E-T) coupling) in hypertrophic signaling and heart failure. Here we review the classical role of Ca in E-C coupling and extend this view to the role of the Ca-dependent proteins CaM and CaMKII in modulating E-C coupling and their contribution to E-T coupling.

Calmodulin as an ion channel subunit

A surprising variety of ion channels found in a wide range of species from Homo to Paramecium use calmodulin (CaM) as their constitutive or dissociable Ca(2+)-sensing subunits. The list includes voltage-gated Ca(2+) channels, various Ca(2+)- or ligand-gated channels, Trp family channels, and even the Ca(2+)-induced Ca(2+) release channels from organelles. Our understanding of CaM chemistry and its relation to enzymes has been instructive in channel research, yet the intense study of CaM regulation of ion channels has also revealed unexpected CaM chemistry. The findings on CaM channel interactions have indicated the existence of secondary interaction sites in addition to the primary CaM-binding peptides and the functional differences between the N- and C-lobes of CaM. The study of CaM in channel biology will figure into our understanding on how this uniform, universal, vital, and ubiquitous Ca(2+) decoder coordinates the myriad local and global cell physiological transients.

Apocalmodulin and Ca2+-calmodulin bind to neighboring locations on the ryanodine receptor

Calmodulin (CaM) binds to the ryanodine receptor/calcium release channel of skeletal muscle (RyR1), both in the absence and presence of Ca(2+), and regulates the activity of the channel activity by activating and inhibiting it, respectively. Using cryo-electron microscopy and three-dimensional reconstruction, we found that one apoCaM binds per RyR1 subunit along the sides of the cytoplasmic assembly of the receptor. This location is distinct from but close to the location found for Ca(2+)-CaM, providing a structural basis for efficient switching of CaM between these two positions with the oscillating intracellular Ca(2+) concentration that generates muscle relaxation/contraction cycles. The locations of apoCaM and Ca(2+)-CaM at a critical region for RYR1-dihydropyridine receptor interaction are suggestive of a direct role for CaM in the mechanism of excitation-contraction coupling.

Identification of apocalmodulin and Ca2+-calmodulin regulatory domain in skeletal muscle Ca2+ release channel, ryanodine receptor

Fusion proteins and full-length mutants were generated to identify the Ca(2+)-free (apoCaM) and Ca(2+)-bound (CaCaM) calmodulin binding sites of the skeletal muscle Ca(2+) release channel/ryanodine receptor (RyR1). [(35)S]Calmodulin (CaM) overlays of fusion proteins revealed one potential Ca(2+)-dependent (aa 3553-3662) and one Ca(2+)-independent (aa 4302-4430) CaM binding domain. W3620A or L3624D substitutions almost abolished completely, whereas V3619A or L3624A substitutions reduced [(35)S]CaM binding to fusion protein (aa 3553-3662). Three full-length RyR1 single-site mutants (V3619A,W3620A,L3624D) and one deletion mutant (Delta4274-4535) were generated and expressed in human embryonic kidney 293 cells. L3624D exhibited greatly reduced [(35)S]CaM binding affinity as indicated by a lack of noticeable binding of apoCaM and CaCaM (nanomolar) and the requirement of CaCaM (micromolar) for the inhibition of RyR1 activity. W3620A bound CaM (nanomolar) only in the absence of Ca(2+) and did not show inhibition of RyR1 activity by 3 microm CaCaM. V3619A and the deletion mutant bound apoCaM and CaCaM at levels compared with wild type. V3619A activity was inhibited by CaM with IC(50) approximately 200 nm, as compared with IC(50) approximately 50 nm for wild type and the deletion mutant. [(35)S]CaM binding experiments with sarcoplasmic reticulum vesicles suggested that apoCaM and CaCaM bind to the same region of the native RyR1 channel complex. These results indicate that the intact RyR1 has a single CaM binding domain that is shared by apoCaM and CaCaM.

Calmodulin binding and inhibition of cardiac muscle calcium release channel (ryanodine receptor)

Metabolically (35)S-labeled calmodulin (CaM) was used to determine the CaM binding properties of the cardiac ryanodine receptor (RyR2) and to identify potential channel domains for CaM binding. In addition, regulation of RyR2 by CaM was assessed in [(3)H]ryanodine binding and single-channel measurements. Cardiac sarcoplasmic reticulum vesicles bound approximately four CaM molecules per RyR2 tetramer in the absence of Ca(2+); in the presence of 100 microm Ca(2+), the vesicles bound 7.5 CaM molecules per tetramer. Purified RyR2 bound approximately four [(35)S]CaM molecules per RyR tetramer, both in the presence and absence of Ca(2+). At least four CaM binding domains were identified in [(35)S]CaM overlays of fusion proteins spanning the full-length RyR2. The affinity (but not the stoichiometry) of CaM binding was altered by redox state as controlled by the presence of either GSH or GSSG. Inhibition of RyR2 activity by CaM was influenced by Ca(2+) concentration, redox state, and other channel modulators. Parallel experiments with the skeletal muscle isoform showed major differences in the CaM binding properties and regulation by CaM of the skeletal and cardiac ryanodine receptors.

Differential Ca(2+) sensitivity of skeletal and cardiac muscle ryanodine receptors in the presence of calmodulin

Calmodulin (CaM) activates the skeletal muscle ryanodine receptor Ca(2+) release channel (RyR1) in the presence of nanomolar Ca(2+) concentrations. However, the role of CaM activation in the mechanisms that control Ca(2+) release from the sarcoplasmic reticulum (SR) in skeletal muscle and in the heart remains unclear. In media that contained 100 nM Ca(2+), the rate of (45)Ca(2+) release from porcine skeletal muscle SR vesicles was increased approximately threefold in the presence of CaM (1 microM). In contrast, cardiac SR vesicle (45)Ca(2+) release was unaffected by CaM, suggesting that CaM activated the skeletal RyR1 but not the cardiac RyR2 channel isoform. The activation of RyR1 by CaM was associated with an approximately sixfold increase in the Ca(2+) sensitivity of [(3)H]ryanodine binding to skeletal muscle SR, whereas the Ca(2+) sensitivity of cardiac SR [(3)H]ryanodine binding was similar in the absence and presence of CaM. Cross-linking experiments identified both RyR1 and RyR2 as predominant CaM binding proteins in skeletal and cardiac SR, respectively, and [(35)S]CaM binding determinations further indicated comparable CaM binding to the two isoforms in the presence of micromolar Ca(2+). In nanomolar Ca(2+), however, the affinity and stoichiometry of RyR2 [(35)S]CaM binding was reduced compared with that of RyR1. Together, our results indicate that CaM activates RyR1 by increasing the Ca(2+) sensitivity of the channel, and further suggest differences in CaM's functional interactions with the RyR1 and RyR2 isoforms that may potentially contribute to differences in the Ca(2+) dependence of channel activation in skeletal and cardiac muscle.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Calmodulin and cyclic ADP-ribose interaction in Ca2+ signaling related to cardiac sarcoplasmic reticulum: superoxide anion radical-triggered Ca2+ release

Reactive oxygen species (ROS) are often shown to damage cellular functions. The targets of oxidative damage depend on the nature of ROS produced and the site of generation. In contrast, ROS can also regulate signal transduction. In this case, ROS may either induce or enhance events, which lead to forward directions of cellular signaling. The consequences of regulation of signal transduction can be observed in physiological processes such as muscle contraction. Here, we discuss the concentration-dependent effects of superoxide anion radical (*O2-) on Ca2+ release from the cardiac sarcoplasmic reticulum (SR). Recent studies suggest that the ADP-ribosyl cyclase pathway, through its production of cyclic adenosine 5'-diphosphoribose (cADPR), may control Ca2+ mobilization in cardiac muscle cells. *O2- has dual effects that are concentration dependent. At low concentrations (nearly nanomolar levels), *O2- induces Ca2+ release by stimulating synthesis of cADPR, which requires calmodulin for sensitization of ryanodine-sensitive Ca2+-release channels (RyRC). At these low concentrations, *O2- is responsible for regulation of cellular signal transduction. At higher concentrations (micromolar levels), *O2- produces a loss in the function of calmodulin that is to inhibit RyRC. This results in an increase in Ca2+ release, which is linked to cell injury. The difference in the functions of low and high concentrations of *O2- may result in two distinct physiological roles in cardiac muscle Ca2+ signaling.

Structure of the skeletal muscle calcium release channel activated with Ca2+ and AMP-PCP

The functional state of the skeletal muscle Ca2+ release channel is modulated by a number of endogenous molecules during excitation-contraction. Using electron cryomicroscopy and angular reconstitution techniques, we determined the three-dimensional (3D) structure of the skeletal muscle Ca2+ release channel activated by a nonhydrolyzable analog of ATP in the presence of Ca2+. These ligands together produce almost maximum activation of the channel and drive the channel population toward a predominately open state. The resulting 30-A 3D reconstruction reveals long-range conformational changes in the cytoplasmic region that might affect the interaction of the Ca2+ release channel with the t-tubule voltage sensor. In addition, a central opening and mass movements, detected in the transmembrane domain of both the Ca(2+)- and the Ca2+/nucleotide-activated channels, suggest a mechanism for channel opening similar to opening-closing of the iris in a camera diaphragm.

The Ca(2+)-ATPase of the sarcoplasmic reticulum in skeletal and cardiac muscle. An overview from the very beginning to more recent prospects

The discovery of the ATP-driven calcium pump in the sarcoplasmic reticulum membranes reaches back to the postwar (World War II) years and would not be possible without the generous support by the American scientific community. It was this community that in pre- and postwar years gave shelter to many European scientists, which in return stimulated scientific development in the United States. These pre- and postwar relations helped to establish the calcium pump as a physiologically relevant mechanism in all kinds of cells. The pump and its counterpart, the calcium release channel, proved to be controlled by various intrinsic mechanisms. Rising hydrogen concentrations as occurring in ischemic muscles switch off pump activity and counteract allosterically caffeine-induced calcium release (CICR). Rising phosphate or the presence of other calcium-precipitating anions, on the other hand, prevents pump inhibition by intraluminal calcium precipitation, which, simultaneously, can increase the quantity of releasable calcium. The inactivation of CICR by removing medium chloride must be considered as a hint of additional mechanisms by which calcium-dependent activity regulation can be modified.

Caffeine- and noradrenaline-induced contractions of human vas deferens: contrasting effects of procaine, ryanodine and W-7

1. The effects of ryanodine, procaine, and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on noradrenaline (NA)- and caffeine-induced contractions of human vas deferens were investigated. 2. In the presence of nifedipine (1 microM), NA ( 100 microM) evoked biphasic contractions. Caffeine (20 mM) evoked repeatable tonic contractions. 3. Ryanodine (30 microM) inhibited the initial but not the secondary component of NA contractions. Procaine (1 and 10 mM) inhibited both components. Contractions induced by caffeine were unaffected by ryanodine or procaine. 4. The calmodulin antagonist W-7 (100 microM) reduced, in a reversible manner, both components of NA-induced response. Caffeine-induced contractions were also reduced in most preparations (8 of 11). In all preparations, contractions induced by caffeine were markedly inhibited after the washout of W-7. Higher doses of W-7 (300 microM) induced an increase in basal tension. 5. These results indicate that NA contracts the longitudinal muscle of human vas deferens by a ryanodine-sensitive calcium-induced calcium release (CICR) mechanism and, in addition, a ryanodine-insensitive pathway: both are sensitive to procaine. In contrast, contraction induced by caffeine is mediated by a pathway that is atypically insensitive to either ryanodine or procaine. The sensitivity of NA- and caffeine-induced contraction to W-7 suggests a role for calcium and its interaction with calmodulin in the response to both agents. The paradoxical action of W-7 is discussed.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Detection and functional characterization of ryanodine receptors from sea urchin eggs

1. Immunoblot analysis, [3H]ryanodine binding, and planar lipid bilayer techniques were used to identify and characterize the functional properties of ryanodine receptors (RyRs) from Lytechinus pictus and Strongylocentrotus purpuratus sea urchin eggs. 2. An antibody against mammalian skeletal RyRs identified an approximately 400 kDa band in the cortical microsomes of sea urchin eggs while a cardiac-specific RyR antibody failed to recognize this protein. [3H]Ryanodine binding to cortical microsomes revealed the presence of a high-affinity (Kd = 13 nM), saturable (maximal density of receptor sites, Bmax = 1.56 pmol (mg protein)-1) binding site that exhibited a biphasic response to Ca2+. 3. Upon reconstitution of cortical microsomes into lipid bilayers, only sparse and unstable openings of a high-conductance cation channel were detected. Addition of crude sea urchin egg homogenate to the cytosolic (cis side) of the channel increased the frequency of openings and stabilized channel activity. The homogenate-activated channels were Ca2+ sensitive, selective for Ca2+ over Cs+, and driven by ryanodine into a long-lived subconductance state that represented approximately 40 % of the full conductance level. Homogenate dialysed in membranes with a molecular weight cut-off <= 2000 lacked the capacity to increase the frequency of RyR openings and to stabilize channel activity. 4. Direct application of cyclic adenosine diphosphoribose (cADPR) or photolysis of NPE-cADPR ('caged' cADPR) by ultraviolet laser pulses produced transient activation of sea urchin egg RyRs. Calmodulin (CaM) failed to activate reconstituted RyRs; however, channel activity was inhibited by the CaM blocker trifluoroperazine, suggesting that CaM was necessary but not sufficient to sustain RyR activity. 5. These findings suggest that a functional Ca2+ release unit in sea urchin eggs is a complex of several molecules, one of which corresponds to a protein functionally similar to mammalian RyRs.

Functional behaviour of the ryanodine receptor/Ca(2+)-release channel in vesiculated derivatives of the junctional membrane of terminal cisternae of rabbit fast muscle sarcoplasmic reticulum

We have devised a novel procedure, employing Chaps rather than Triton [Costello B., Chadwick C., Saito A., Chu A., Maurer A., Fleischer S. J Cell Biol 1986; 103: 741-753], for obtaining vesiculated derivatives of the junctional face membrane (JFM) domain of isolated terminal cisternae (TC) from fast skeletal muscle of the rabbit. Enriched JFM is minimally contaminated with junctional transverse tubules. The characteristic ultrastructural features and the most essential features of TC function relating to this membrane domain-i.e. both the Ca(2+)-release system and the Ca2+ and calmodulin (CaM)-dependent protein kinase (CaM I PK) system-appear to be retained in enriched JFM. We show that our isolation procedure, yielding up to a 2.5-fold enrichment in ryanodine receptor (RyR) protein and in the maximum number of high affinity [3H]-ryanodine binding sites, does not alter the assembly for integral proteins associated with the receptor in its native membrane environment, i.e. FKBP-12, triadin and the structurally related protein junction [Jones L.R., Zhang L., Sanborn K., Jorgensen A., Kelley J. J Biol Chem 1995; 270: 30787-30796] having, in common, the property to bind calsequestrin (CS) in overlays in the presence of EGTA. The substrate specificity of endogenous CaM I PK is also the same as that of parent TC vesicles. Phosphorylation of mainly triadin and of a high M(r) polypeptide, and not of the RyR, is the most remarkable common property. Retention of peripheral proteins, like CS and histidine-rich Ca(2+)-binding protein, although not that endogenous CaM, and of a unique set of CaM-binding proteins, unlike that of junctional SR-specific integral proteins, is shown to be influenced by the concentration of Ca2+ during incubation of TC vesicles with Chaps. Characterization of RyR functional behaviour with [3H]-ryanodine has indicated extensive similarities between the enriched JFM and parent TC vessicles, as far as the characteristic bell shaped Ca(2+)-dependence of [3H]-ryanodine binding and the dose-dependent sensitization to Ca2+ by caffeine, reflecting the inherent properties of SR Ca(2+)-release channel, as well as concerning the stimulation of [3H]-ryanodine binding by increasing concentrations of KCl. Stabilizing the RyR in a maximally active state by optimizing concentrations of KCl (1 M), at also optimal concentrations of Ca2+ (pCa 4), rendered the receptor less sensitive to inhibition by 1 microM CaM, to a greater extent in the case of enriched JFM. That was not accounted for by any significant difference in the IC50 concentrations of CaM varying between 40 nM to approximately 80 nM, at low-intermediate and at high KCl concentrations, respectively. Additional results with enriched JFM using doxorubicin, a pharmacological Ca2+ channel allosteric modifier, strengthen the hypothesis that the conformational state at which RyR is stabilized, according to the experimental assay conditions for [3H]-ryanodine binding, directly influences CaM-sensitivity.

Gating of the skeletal calcium release channel by ATP is inhibited by protein phosphatase 1 but not by Mg2+

We have previously found that dephosphorylation/phosphorylation of the calcium release channel (CRC) of skeletal muscle confers channel sensitivity/insensitivity to the block by physiological [Mg2+] (approximately 1 mM). These studies have now been extended to modulation by ATP. Terminal cisternae vesicles of sarcoplasmic reticulum were incorporated into planar lipid bilayers. CRC gating by ATP (0.67 mM), in the absence of Ca2+ (< 1nM), was studied by treatment with protein kinase A (PKA) or phosphatase 1 (PPT1) and assayed in the presence and absence of free Mg2+ (1 mM). PPT1, PKA, and Mg2+ were directly applied to the bilayer using the microsyringe method, which controls the environment of the CRC in the bilayer for phosphorylation/ dephosphorylation cycles and for assays. PKA treated channels were activated by ATP to high open probabilities, while PPT1 treated channels were not activatable by ATP. Opening and closing of channels during cycles of PKA and PPT1 applications, respectively, provided evidence that the change of CRC activity is due to cyclic phosphorylation/dephosphorylation. Free Mg2+ (1 mM) did not block channels activated by ATP. The new finding is that channel gating by ATP can be controlled by the state of phosphorylation without inhibition by free Mg2+.

Glucose-independent inhibition of yeast plasma-membrane H+-ATPase by calmodulin antagonists

Glucose metabolism causes activation of the yeast plasma-membrane H+-ATPase. The molecular mechanism of this regulation is not known, but it is probably mediated by phosphorylation of the enzyme. The involvement in this process of several kinases has been suggested but their actual role has not been proved. The physiological role of a calmodulin-dependent protein kinase in glucose-induced activation was investigated by studying the effect of specific calmodulin antagonists on the glucose-induced ATPase kinetic changes in wild-type and two mutant strains affected in the glucose regulation of the enzyme. Preincubation of the cells with calmidazolium or compound 48/80 impeded the increase in ATPase activity by reducing the Vmax of the enzyme without modifying the apparent affinity for ATP in the three strains. In one mutant, pma1-T912A, the putative calmodulin-dependent protein kinase-phosphorylatable Thr-912 was eliminated, and in the other, pma1-P536L, H+-ATPase was constitutively activated, suggesting that the antagonistic effect was not mediated by a calmodulin-dependent protein kinase and not related to glucose regulation. This was corroborated when the in vitro effect of the calmodulin antagonists on H+-ATPase activity was tested. Purified plasma membranes from glucose-starved or glucose-fermenting cells from both pma1-P890X, another constitutively activated ATPase mutant, and wild-type strains were preincubated with calmidazolium or melittin. In all cases, ATP hydrolysis was inhibited with an IC50 of approximately 1 microM. This inhibition was reversed by calmodulin. Analysis of the calmodulin-binding protein pattern in the plasma-membrane fraction eliminates ATPase as the calmodulin target protein. We conclude that H+-ATPase inhibition by calmodulin antagonists is mediated by an as yet unidentified calmodulin-dependent membrane protein.

Functional characterization of a distinct ryanodine receptor mutation in human malignant hyperthermia-susceptible muscle

Malignant hyperthermia is an inherited autosomal disorder of skeletal muscle in which certain volatile anesthetics and depolarizing muscle relaxants trigger an abnormally high release of Ca2+ from the intracellular Ca2+ store, the sarcoplasmic reticulum. In about 50% of cases, malignant hyperthermia susceptibility is linked to the gene encoding the skeletal muscle ryanodine receptor/Ca2+ release channel (RYR1). To date, eight point mutations have been identified in human RYR1. Although these mutations are thought to lead to an increased caffeine and halothane sensitivity in the contractile response of skeletal muscle, their functional consequences have not been investigated on the molecular level. In the present study, we provide the first functional characterization of a point mutation located in the central part of RYR1, Gly2434 --> Arg. Using high affinity [3H]ryanodine binding as the experimental approach, we show that this mutation enhances the sensitivity of RYR1 to activating concentrations of Ca2+ and to the exogenous and diagnostically used ligands caffeine and 4-chloro-m-cresol. In parallel, the sensitivity to inhibiting concentrations of Ca2+ and calmodulin was reduced, transferring the mutant Ca2+ release channel into a hyperexcitable state.

Calmodulin sensitivity of the sarcoplasmic reticulum ryanodine receptor from normal and malignant-hyperthermia-susceptible muscle

Ca2+ release from sarcoplasmic reticulum (SR) of malignant-hyperthermia-susceptible (MHS) muscle is hypersensitive to Ca2+ and caffeine. To determine if an abnormal calmodulin (CaM) regulation of the SR Ca(2+)-release-channel-ryanodine-receptor complex (RYR1) contributes to this hypersensitivity, we investigated the effect of CaM on high-affinity [3H]ryanodine binding to isolated SR vesicles from normal and MHS pig skeletal muscle. CaM modulated [3H]ryanodine binding in a Ca(2+)-dependent manner. In the presence of maximally activating Ca2+ concentrations, CaM inhibited [3H]ryanodine binding with no differences between normal and MHS vesicles. In the absence of Ca2+, however, CaM activated [3H]ryanodine binding with a 2-fold-higher potency in MHS vesicles. Significant differences between normal and MHS tissue were observed for CaM concentrations between 50 nM and 10 microM. A polyclonal antibody raised against the central region of RYR1 specifically inhibited this activating effect of CaM without affecting the inhibition by CaM. This indicates that the central region of RYR1 is a potential binding domain for CaM in the absence of Ca2+. It is suggested that in vivo an enhanced CaM sensitivity of RYR1 might contribute to the abnormal high release of Ca2+ from the SR of MHS muscle.

Adenine nucleotide diphosphates: emerging second messengers acting via intracellular Ca2+ release

Release of Ca2+ from intracellular stores is a widespread mechanism in regulation of cell function. Two hitherto unknown adenine diphosphonucleotides were recently identified, which trigger Ca2+ release from intracellular stores via channels that are distinct from the well-known receptor/channel controlled by inositol 1,4,5,-trisphosphate (IP3): cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Here we review synthesis of cADPR from beta-NAD, its hydrolysis to adenosine diphosphoribose (noncyclic) by cADPR glycohydrolase, as well as our knowledge about the metabolism of NAADP. The Ca2+ release triggered by cADPR, NAADP, or IP3 can be distinguished by the action of inhibitors and by desensitization studies. Evidence now emerges that cADPR synthesis from beta-NAD can be stimulated, at least in some cell types by all-trans-retinoic acid as a first messenger. We then review the properties of cADPR and NAADP as potential second messengers in the intracrine regulation of cell functions. Although their exact role in signaling sequences is not yet known, cADPR and NAADP are likely to play important intracellular regulatory functions, as extensively documented for the process of egg fertilization.

Modulation of the skeletal muscle ryanodine receptor by endogenous phosphorylation of 160/150-kDa proteins of the sarcoplasmic reticulum

This paper demonstrates and characterizes the inhibition of ryanodine binding caused by the phosphorylation of the 160/150-kDa proteins in skeletal muscle sarcoplasmic reticulum (SR). Inhibition of ryanodine binding was obtained by preincubation of SR membranes with ATP + NaF . The inhibition was characterized by the following findings: (a) If ATP was replaced by AdoPP[NH]P, inhibition of ryanodine binding activity was not observed. (b) The inhibitory effect of preincubation with ATP + NaF, like the phosphorylation of 150/160-kDa proteins, was Ca2+ dependent. (c) Inhibition of ryanodine binding, as the protein phosphorylation, was not observed if NaF (> 30 mM) was replaced with okadaic acid. (d) The optimal pH for the inhibition and the phosphorylation was about 7.0. (e) Both the phosphorylation of the 160/150-kDa proteins and inhibition of ryanodine binding were prevented by dichlorobenzimidazole riboside and hemin, inhibitors of casein kinase II. (f) Dephosphorylation of the 160/150-kDa proteins prevented the inhibition of ryanodine binding. (g) The presence of NP-40 during the phosphorylation prevented both the 160/150-kDa phosphorylation and the inhibition of ryanodine binding. Furthermore, a linear relationship was obtained between the degree of ryanodine binding inhibition and the level of phosphorylation of the 160/150-kDa proteins, as controlled by ATP or NaF concentrations. The binding affinity for Ca2+ of the ryanodine receptor (RyR) was modified by phosphorylation of the 160/150-kDa proteins, decreasing by up to 100-fold. The phosphorylation of the SR membranes resulted in an elimination of ryanodine binding sites with slight effect on the ryanodine binding affinity. These results suggest the modulation of the properties of the RyR by phosphorylation/dephosphorylation of the 160/150-kDa proteins. The identification of the phosphorylated 160/150-kDa proteins, their kinase, and the structural interactions between them and the RyR are presented in the accompanying paper.

Effect of calmodulin antagonists on calmodulin-induced biphasic modulation of Ca(2+)-induced Ca2+ release

1. Calmodulin (CaM) has a biphasic effect on Ca(2+)-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR): potentiation and inhibition at low (pCa > 6.0) and high (pCa 5) Ca2+ concentrations, respectively. To characterize the mode of action of CaM, we studied the effect of CaM antagonists on the CICR in skinned muscle fibres of the rabbit. Ca2+ release was measured by microfluorometry with Fura-2. 2. A CaM antagonist, trifluoperazine (TFP), potentiated the CICR in a dose-dependent manner (10-300 microM) at pCa 6, where a simple reversal of the CaM effect would be inhibition of the CICR. Furthermore, 100 microM TFP sensitized the CICR to Ca2+. A similar effect was produced by other CaM antagonists that were tested: chlorpromazine, W-7, mastoparan, and peptide fragment of CaM-binding residues of CaM-dependent protein kinase II. 3. The biphasic effect of CaM on the CICR was observed even in the presence of high concentrations of CaM antagonists or CaM-bindings peptides. 4. From these results we suggest that CaM has a unique mode of action on the CICR which is quite different from the effect of CaM on known enzymes.

Age-related abnormalities in regulation of the ryanodine receptor in rat fast-twitch muscle

The tibialis anterior (TA) muscles of 6-month-old and 24-month-old male Wistar rats, after being characterized, at the fast motor unit level, for twitch properties, were dissected and processed by a procedure [Margreth A., Damiani E., Tobaldin G. Biochem Biophys Res Commun 1993; 197: 1303-1311] aimed at obtaining a representative total membrane fraction comprising 70-80% of the total muscle content of sarcoplasmic reticulum (SR) and transverse tubule (TT) membranes (about 20 mg protein/g). Skeletal muscle membranes were analyzed for protein composition, and the content and functional properties of specific components of the free and junctional subcompartments of the SR and of junctional TT. Our results, while confirming a twitch prolongation in TA of old rats, do not demonstrate any associated age-related change concerning: (a) the overall number and functional properties of Ca2+ pumps, as characterized by kinetic parameters, Ca(2+)-dependency, and the protein isoform specificity of SR Ca(2+)-ATPase; (b) the number of functional junctional SR Ca(2+)-release channels, on the basis of Bmax values for high-affinity binding of [3H]-ryanodine to skeletal muscle membranes at optimal Ca2+; (c) the overall muscle dihydropyridine receptor/ryanodine receptor (RyR) ratio. We conclude from these findings, and the additional negative evidence for changes in membrane density of specific components of junctional SR, including 60 kDa Ca(2+)-calmodulin protein kinase, that this membrane domain, like the Ca(2+)-pump domain of the SR, are in no way basically altered at early stages of the aging process, as investigated here. Because of that, we allege particular significance to the occurrence of age-related, specific abnormalities in regulation of RyR in rat TA. The main supportive evidence is as follows: (a) an increased sensitivity to Ca2+ of the RyR of old muscle, and, more importantly; (b) an increased sensitivity to caffeine of [3H]ryanodine binding to the RyR at optimal Ca2+ and also optimal for the activity of the Ca(2+)-release channel. The results reported here also demonstrate that there are two classes of caffeine sites in rat TA muscle, as defined by differences in EC50 values at resting (pCa 7) and at high Ca2+ (pCa 4-5), that sites involved in stimulation of [3H]-ryanodine binding to the RyR are distinguished by a higher affinity (caffeine below mM), and that only these sites undergo age-related changes. Thus, although the underlying age-related abnormality of the RyR remains to be elucidated, it appears to satisfy the requirement for being regarded as a specific change, which in itself might argue for its being fundamentally related to the twitch prolongation of the muscle.

Modulation of cardiac ryanodine receptors of swine and rabbit by a phosphorylation-dephosphorylation mechanism

1. The regulation of the cardiac Ca2+ release channel-ryanodine receptor (RyR) by exogenous acid phosphatase (AcPh) and purified Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) was studied in swine and rabbit sarcoplasmic reticulum (SR) vesicles using [3H]ryanodine binding and planar bilayer reconstitution experiments. 2. Addition of AcPh (1-20 U ml-1) to a standard incubation medium increased [3H]ryanodine binding in a Ca(2+)-dependent manner. Stimulation was only readily apparent in media containing micromolar Ca2+ concentrations. 3. Scatchard analysis of [3H]ryanodine binding curves revealed that AcPh enhanced binding by increasing the affinity of the receptor for [3H]ryanodine without recruiting additional receptor sites (Kd, 9.8 +/- 0.85 and 3.9 +/- 0.65 nM; Bmax (the maximal receptor density), 1.45 +/- 0.14 and 1.47 +/- 0.12 pmol mg-1 for control and AcPh, respectively). The failure of AcPh to increase Bmax suggested that the number of receptors that were 'dormant' due to phosphorylation in the SR preparation was very small. 4. At the single channel level, AcPh increased the open probability (Po) of RyR channels by increasing the opening rate and inducing the appearance of a longer open state while having no effect on single channel conductance. Thus AcPh acted directly on RyR channels or a closely associated regulatory protein. 5. CaMKII decreased both [3H]ryanodine binding and Po of RyRs when added to medium supplemented with micromolar levels of Ca2+ and calmodulin (CaM). Addition of a synthetic peptide inhibitor of CaMKII, or replacement of ATP with the non-hydrolysable ATP analogue adenylyl[beta, gamma-methylene]-diphosphate (AMP-PCP), prevented CaMKII inhibition of RyRs, suggesting that CaMKII acted specifically through a phosphorylation mechanism. 6. The inhibition of RyR channel activity by CaMKII was reversed by the addition of AcPh. Thus we showed that an in vitro phosphorylation-dephosphorylation mechanism effectively regulates RyRs. 7. The results suggest that intracellular signalling pathways that lead to activation of CaMKII may reduce efflux of Ca2+ from the SR by inhibition of RyR channel activity. The Ca2+ dependence of CaMKII inhibition suggests that the role of the phosphorylation mechanism is to modulate the RyR response to Ca2+.

Enhancing effect of calmodulin on Ca(2+)-induced Ca2+ release in the sarcoplasmic reticulum of rabbit skeletal muscle fibres

1. We analysed the effect of calmodulin on Ca(2+)-induced Ca2+ release (CICR) in the sarcoplasmic reticulum (SR) using chemically skinned fibres of rabbit psoas muscle. Ca2+ release was measured using fura-2 microfluorometry. 2. In saponin-skinned fibres, calmodulin potentiated Ca2+ release at low Ca2+ concentrations (< 3 microM), while it showed an inhibitory effect at high Ca2+ concentrations (3-30 microM). 3. Co-application of ryanodine and calmodulin at 0.3 microM Ca2+, but not ryanodine alone, induced a decline in the Ca2+ uptake capacity of the SR, an effect expected from the open-lock of active CICR channels by ryanodine. Thus, potentiation of Ca2+ release by calmodulin at low Ca2+ concentrations can be regarded as a result of the activation of the ryanodine receptor. 4. Greater concentrations of calmodulin were required for potentiation of CICR at low Ca2+ concentrations (1 microM) than for inhibition at high Ca2+ concentrations (10 microM). 5. In beta-escin-permeabilized fibres in which intrinsic calmodulin was retained, the rates of CICR were similar to those measured in the presence of 1 microM calmodulin in saponin-permeabilized fibres. 6. These results suggest that calmodulin plays an important role in the regulation of CICR channels in intact skeletal muscle fibres.

Effect of Mg2+ and ATP on depolarization-induced Ca2+ release in isolated triads

The effect of different free Mg2+ and ATP concentrations on depolarization-induced Ca2+ release in isolated skeletal muscle triadic vesicles was examined by simultaneously monitoring direct effects on ryanodine receptors from either isolated or coupled terminal cisternae. Free Mg2+ was increased to concentrations of 11-14 microM, 81 microM, 175-181 microM, and 1 mM while total ATP concentration was kept constant or MgATP concentration was kept constant. We observed the following. 1) Increasing MgATP reduces the measurable Ca2+ release from isolated vesicles by stimulating the Ca(2+)-ATPase in the terminal cisternae. 2) Half-maximal inhibition of functionally coupled ryanodine receptors during depolarization-induced Ca2+ release is observed at 1 mM Mg2+, whereas half-maximal inhibition of the nondepolarized ryanodine receptor is seen at 75 microM Mg2+ at the same free ATP and MgATP concentrations. 3) Two separate time constants for Ca2+ release were obtained for nondepolarized ryanodine receptors with free Mg2+ at 14 microM and free ATP at 6.1 mM; this may represent triadic ryanodine receptors vs. isolated terminal cisternae ryanodine receptors.

Calmodulin activation and inhibition of skeletal muscle Ca2+ release channel (ryanodine receptor)

The calmodulin-binding properties of the rabbit skeletal muscle Ca2+ release channel (ryanodine receptor) and the channel's regulation by calmodulin were determined at < or = 0.1 microM and micromolar to millimolar Ca2+ concentrations. [125I]Calmodulin and [3H]ryanodine binding to sarcoplasmic reticulum (SR) vesicles and purified Ca2+ release channel preparations indicated that the large (2200 kDa) Ca2+ release channel complex binds with high affinity (KD = 5-25 nM) 16 calmodulins at < or = 0.1 microM Ca2+ and 4 calmodulins at 100 microM Ca2+. Calmodulin-binding affinity to the channel showed a broad maximum at pH 6.8 and was highest at 0.15 M KCl at both < or = 0.1 MicroM and 100 microM Ca2+. Under condition closely related to those during muscle contraction and relaxation, the half-times of calmodulin dissociation and binding were 50 +/- 20 s and 30 +/- 10 min, respectively. SR vesicle-45Ca2+ flux, single-channel, and [3H]ryanodine bind measurements showed that, at < or = 0.2 microM Ca2+, calmodulin activated the Ca2+ release channel severalfold. Ar micromolar to millimolar Ca2+ concentrations, calmodulin inhibited the Ca(2+)-activated channel severalfold. Hill coefficients of approximately 1.3 suggested no or only weak cooperative activation and inhibition of Ca2+ release channel activity by calmodulin. These results suggest a role for calmodulin in modulating SR Ca2+ release in skeletal muscle at both resting and elevated Ca2+ concentrations.

Calmodulin binding sites of the skeletal, cardiac, and brain ryanodine receptor Ca2+ channels: modulation by the catalytic subunit of cAMP-dependent protein kinase?

In this study, we define calmodulin binding sites of skeletal, cardiac, and brain ryanodine receptor (RYR) Ca2+ channels. Cardiac and brain RYR peptides corresponding to the calmodulin binding sites present in the skeletal RYR [Menegazzi, P., et al. (1994) Biochemistry 33, 9078-9084] were synthesized, and their interaction with calmodulin was monitored by fluorescent techniques. The central portions of the skeletal, cardiac, and brain RYR protomers display one high (CaM1; Kd ranging between 2.7 and 10.2 nM) and one low affinity (CaM2; Kd ranging between 116 and 142 nM) calmodulin binding site. Depending on the RYR model having 4 or 12 transmembrane segments, a third calmodulin binding site (CaM3) was identified a few residues upstream from the putative transmembrane segment M1 or M5. Its affinity for calmodulin varied between the RYR isoforms: the cardiac RYR CaM3 displays a high affinity (9.09 +/- 1.0 nM, n = 5), while the skeletal and brain RYR CaM3 have low affinity, the lowest affinity being displayed by the brain isoform (234 +/- 39 nM, n = 3). The RYRs calmodulin binding site CaM1 encompasses the sequence Arg-His-Arg-Val(Ile)-Ser-Leu, which is phosphorylated in vitro by the catalytic subunit of the cAMP-dependent protein kinase. Phosphorylation of RYR PM1 peptides occurs on the Ser, corresponding to amino acid number 2919, 3020, and 3055 of the brain, cardiac, and skeletal RYR protomers, respectively. We found that phosphorylation of the RYR PM1 peptides was inhibited by calmodulin binding and that the formation of the PM1 peptide-calmodulin complex was inhibited by peptide phosphorylation. These data indicate that the effect of calmodulin binding to RYR CaM1 may be regulated by the phosphorylation state of the Ser residue localized within the sequence Arg-His-Arg-Val(Ile)-Ser-Leu.

Calmodulin is a selective mediator of Ca(2+)-induced Ca2+ release via the ryanodine receptor-like Ca2+ channel triggered by cyclic ADP-ribose

The ryanodine receptor-like Ca2+ channel (RyRLC) is responsible for Ca2+ wave propagation and Ca2+ oscillations in certain nonmuscle cells by a Ca(2+)-induced Ca2+ release (CICR) mechanism. Cyclic ADP-ribose (cADPR), an enzymatic product derived from NAD+, is the only known endogenous metabolite that acts as an agonist on the RyRLC. However, the mode of action of cADPR is not clear. We have identified calmodulin as a functional mediator of cADPR-triggered CICR through the RyRLC in sea urchin eggs. cADPR-induced Ca2+ release consisted of two phases, an initial rapid release phase and a subsequent slower release. The second phase was selectively potentiated by calmodulin which, in turn, was activated by Ca2+ released during the initial phase. Caffeine enhanced the action of calmodulin. Calmodulin did not play a role in inositol 1,4,5-trisphosphate-induced Ca2+ release. These findings offer insights into the multiple pathways that regulate intracellular Ca2+ signaling.

Phosphorylation modulates the function of the calcium release channel of sarcoplasmic reticulum from cardiac muscle

The cardiac calcium release channel (CRC) of sarcoplasmic reticulum vesicles was incorporated into planar lipid membranes to evaluate modulation of channel activity by phosphorylation and dephosphorylation. For this purpose a microsyringe application directly to the membrane was used to achieve sequential and multiple treatments of channels with highly purified kinases and phosphatases. Cyclic application of protein kinase A (PKA) or Ca2+/calmodulin-dependent protein kinase II (CalPK) and potato acid phosphatase or protein phosphatase 1 revealed a channel block by Mg2+ (-mM), that is referable to dephosphorylated states of the channel, and that the Mg2+ block could be removed by phosphorylation of the CRC by either PKA or CalPK. By contrast, activation of endogenous CalPK (end CalPK) led to channel closure which could be reversed by dephosphorylation using potato acid phosphatase or protein phosphatase 1. Calmodulin by itself (which activates end CalPK in the presence of MgATP) blocks the channel in the dephosphorylated state, which can be overcome by treatment with CalPK but not PKA. Our findings reveal important insights regarding channel regulation of the ryanodine receptor: 1) the calcium release channel must be phosphorylated to be in the active state at conditions approximating physiological Mg2+ concentrations (-mM); and 2) there are multiple sites of phosphorylation on the calcium release channel with different functional consequences, which may be relevant to the regulation of E-C coupling. Phosphorylation of the CRC may be involved in recruitment of active channels, and/or it may be directly involved in each Ca2+ contraction cycle of the heart. For example, Ca2+ release may require phosphorylation of the CRC by protein kinases at sites which overcome the block by Mg2+. Inactivation may involve CRC block by calmodulin and/or phosphorylation by endogenous CalPK at the junctional face membrane.

Tissue-specific and developmentally regulated alternative splicing in mouse skeletal muscle ryanodine receptor mRNA

The ryanodine receptor is a channel for Ca2+ release from intracellular stores. By PCR analysis, we identified two alternatively spliced regions in mRNA of the mouse skeletal muscle ryanodine receptor (sRyR). The splice variants were characterized by the presence or absence of 15 bp (ASI) and 18 bp (ASII) exons. The exclusion of these exons results in the absence of the regions corresponding to Ala3481-Gln3485 and Val3865-Asn3870, respectively, of rabbit sRyR; these amino acid sequences exist in the modulatory region, where sites for phosphorylation and binding of Ca2+, calmodulin and ATP are postulated to be. We also detected sRyR in brain and heart as well as in skeletal muscle, and the splicing patterns were found to be tissue-specific. Only the ASII-lacking isoform was detected in heart, whereas in other tissues the ASII-containing isoform was predominant. The splicing patterns were also found to change during development. In skeletal muscle, the ASI-containing isoform increased gradually from embryo to adult. The ASII-lacking isoform abruptly increased upon birth, but the ASII-containing isoform increased steadily afterwards. In cerebrum, the ratio of the ASII-containing isoform to the ASII-lacking one increased abruptly during embryonic days 14 and 18. These findings suggest that the alternative splicing of ASI and ASII, by affecting the modulatory region, generates functionally different sRyR isoforms in a tissue-specific and developmentally regulated manner.

The S100 protein family: history, function, and expression

The S100 family of calcium binding proteins contains approximately 16 members each of which exhibits a unique pattern of tissue/cell type specific expression. Although the distribution of these proteins is not restricted to the nervous system, the implication of several members of this family in nervous system development, function, and disease has sparked new interest in these proteins. We now know that the original two members of this family, S100A1 and S100B, can regulate a diverse group of cellular functions including cell-cell communication, cell growth, cell structure, energy metabolism, contraction and intracellular signal transduction. Although some members of the family may function extracellularly, most appear to function as intracellular calcium-modulated proteins and couple extracellular stimuli to cellular responses via interaction with other cellular proteins called target proteins. Interaction of these proteins with target proteins appear to involve cysteine residues (one in S100A1 and two in S100B), as well as a stretch of 13 amino acids, in the middle of the molecule called the linker region, which connects the two EF-hand calcium binding domains. In addition to the amino acid sequence and secondary structures of these proteins, the structures of the genes encoding these proteins are highly conserved. Studies on the expression of these proteins have demonstrated that a complex mixture of transcriptional and postranscriptional mechanisms regulate S100 expression. Further analysis of the function and expression of these proteins in both nervous and nonnervous tissues will provide important information regarding the role of altered S100 expression in nervous system development, function and disease.

Localization of calmodulin binding sites on the ryanodine receptor from skeletal muscle by electron microscopy

Calmodulin (CaM) is a regulator of the calcium release channel (ryanodine receptor) of the sarcoplasmic reticulum of skeletal and cardiac muscle. The locations where CaM binds on the surface of the skeletal muscle ryanodine receptor were determined by electron microscopy. Wheat germ CaM was labeled specifically at Cys-27 with a maleimide derivative of a 1.4-nm-diameter gold cluster, and the gold-cluster-labeled CaM was bound to the purified ryanodine receptor. The complexes were imaged in the frozen-hydrated state by cryoelectron microscopy with no stains or fixatives present. In the micrographs, gold clusters were frequently observed near the corners of the square-shaped images of the ryanodine receptors. In some images, all four corners of the receptor were occupied by gold clusters. Image averaging allowed the site of CaM binding to be determined in two dimensions with an estimated precision of 4 nm. No changes were apparent in the quaternary structure of the ryanodine receptor upon binding CaM to the resolution attained, about 3 nm. Side views of the ryanodine receptor, in which the receptor is oriented approximately perpendicular to the much more frequent fourfold symmetric views, were occasionally observed, and showed that the CaM binding site is most likely on the surface of the receptor that faces the cytoplasm. We conclude that the CaM binding site is at least 10 nm from the transmembrane channel of the receptor and, consequently, that long-range conformational changes are involved in the modulation of the calcium channel activity of the receptor by CaM.

Ion channel regulation by calmodulin binding

While many ion channels are modulated by phosphorylation, there is growing evidence that they can also be regulated by Ca(2+)-calmodulin, apparently through direct binding. In some cases, this binding activates channels; in others, it modulates channel activities. These phenomena have been documented in Paramecium, in Drosophila, in vertebrate photoreceptors and olfactory receptors, as well as in ryanodine receptor Ca(2+)-release channels. Furthermore, studies on calmodulin mutants in Paramecium have shown a clear bipartite distribution of two groups of mutations in the calmodulin gene that lead to opposite behavioral and electrophysiological phenotypes. These results indicate that the N-lobe of calmodulin specifically interacts with one class of ion-channel proteins and the C-lobe with another.