Triadins are not triad-specific proteins: two new skeletal muscle triadins possibly involved in the architecture of sarcoplasmic reticulum

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Generation of a Triadin KnockOut Syndrome Zebrafish Model

Different forms of sudden cardiac death have been described, including a recently identified form of genetic arrhythmogenic disorder, named “Triadin KnockOut Syndrome” (TKOS). TKOS is associated with recessive mutations in the TRDN gene, encoding for TRIADIN, but the pathogenic mechanism underlying the malignant phenotype has yet to be completely defined. Moreover, patients with TKOS are often refractory to conventional treatment, substantiating the need to identify new therapeutic strategies in order to prevent or treat cardiac events. The zebrafish (Danio rerio) heart is highly comparable to the human heart in terms of functions, signal pathways and ion channels, representing a good model to study cardiac disorders. In this work, we generated the first zebrafish model for trdn loss-of-function, by means of trdn morpholino injections, and characterized its phenotype. Although we did not observe any gross cardiac morphological defect between trdn loss-of-function embryos and controls, we found altered cardiac rhythm that was recovered by the administration of arrhythmic drugs. Our model will provide a suitable platform to study the effect of TRDN mutations and to perform drug screening to identify new pharmacological strategies for patients carrying TRDN mutations.

Dynamics of triadin, a muscle-specific triad protein, within sarcoplasmic reticulum subdomains

In skeletal muscle, proteins of the calcium release complex responsible for the excitation-contraction (EC) coupling are exclusively localized in specific reticulum–plasma membrane (ER-PM) contact points named triads. The CRC protein triadin (T95) is localized in the sarcoplasmic reticulum (SR) subdomain of triads where it forms large multimers. However, the mechanisms leading to the steady-state accumulation of T95 in these specific areas of SR are largely unknown. To visualize T95 dynamics, fluorescent chimeras were expressed in triadin knockout myotubes, and their mobility was compared with the mobility of Sec61β, a membrane protein of the SR unrelated to the EC coupling process. At all stages of skeletal muscle cells differentiation, we show a permanent flux of T95 diffusing in the SR membrane. Moreover, we find evidence that a longer residence time in the ER-PM contact point is due to the transmembrane domain of T95 resulting in an overall triad localization.

Interplay between Triadin and Calsequestrin in the Pathogenesis of CPVT in the Mouse

Recessive forms of catecholaminergic polymorphic ventricular tachycardia (CPVT) are induced by mutations in genes encoding triadin or calsequestrin, two proteins that belong to the Ca²⁺ release complex, responsible for intracellular Ca²⁺ release triggering cardiac contractions. To better understand the mechanisms of triadin-induced CPVT and to assay multiple therapeutic interventions, we used a triadin knockout mouse model presenting a CPVT-like phenotype associated with a decrease in calsequestrin protein level. We assessed different approaches to rescue protein expression and to correct intracellular Ca²⁺ release and cardiac function: pharmacological treatment with kifunensine or a viral gene transfer-based approach, using adeno-associated virus serotype 2/9 (AAV2/9) encoding the triadin or calsequestrin. We observed that the levels of triadin and calsequestrin are intimately linked, and that reduction of both proteins contributes to the CPVT phenotype. Different combinations of triadin and calsequestrin expression level were obtained using these therapeutic approaches. A full expression of each is not necessary to correct the phenotype; a fine-tuning of the relative re-expression of both triadin and calsequestrin is required to correct the CPVT phenotype and rescue the cardiac function. AAV-mediated gene delivery of calsequestrin or triadin and treatment with kifunensine are potential treatments for recessive forms of CPVT due to triadin mutations.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

Nanospan, an alternatively spliced isoform of sarcospan, localizes to the sarcoplasmic reticulum in skeletal muscle and is absent in limb girdle muscular dystrophy 2F

Background

Sarcospan (SSPN) is a transmembrane protein that interacts with the sarcoglycans (SGs) to form a tight subcomplex within the dystrophin-glycoprotein complex that spans the sarcolemma and interacts with laminin in the extracellular matrix. Overexpression of SSPN ameliorates Duchenne muscular dystrophy in murine models.

Methods

Standard cloning approaches were used to identify nanospan, and nanospan-specific polyclonal antibodies were generated and validated. Biochemical isolation of skeletal muscle membranes and two-photon laser scanning microscopy were used to analyze nanospan localization in muscle from multiple murine models. Duchenne muscular dystrophy biopsies were analyzed by immunoblot analysis of protein lysates as well as indirect immunofluorescence analysis of muscle cryosections.

Results

Nanospan is an alternatively spliced isoform of sarcospan. While SSPN has four transmembrane domains and is a core component of the sarcolemmal dystrophin-glycoprotein complex, nanospan is a type II transmembrane protein that does not associate with the dystrophin-glycoprotein complex. We demonstrate that nanospan is enriched in the sarcoplasmic reticulum (SR) fractions and is not present in the T-tubules. SR fractions contain membranes from three distinct structural regions: a region flanking the T-tubules (triadic SR), a SR region across the Z-line (ZSR), and a longitudinal SR region across the M-line (LSR). Analysis of isolated murine muscles reveals that nanospan is mostly associated with the ZSR and triadic SR, and only minimally with the LSR. Furthermore, nanospan is absent from the SR of δ-SG-null (Sgcd^−/−) skeletal muscle, a murine model for limb girdle muscular dystrophy 2F. Analysis of skeletal muscle biopsies from Duchenne muscular dystrophy patients reveals that nanospan is preferentially expressed in type I (slow) fibers in both control and Duchenne samples. Furthermore, nanospan is significantly reduced in Duchenne biopsies.

Conclusions

Alternative splicing of proteins from the SG-SSPN complex produces δ-SG3, microspan, and nanospan that localize to the ZSR and the triadic SR, where they may play a role in regulating resting calcium levels as supported by previous studies (Estrada et al., Biochem Biophys Res Commun 340:865–71, 2006). Thus, alternative splicing of SSPN mRNA generates three protein isoforms (SSPN, microspan, and nanospan) that differ in the number of transmembrane domains affecting subcellular membrane association into distinct protein complexes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-017-0127-9) contains supplementary material, which is available to authorized users.

Three residues in the luminal domain of triadin impact on Trisk 95 activation of skeletal muscle ryanodine receptors

Triadin isoforms, splice variants of one gene, maintain healthy Ca²⁺ homeostasis in skeletal muscle by subserving several functions including an influence on Ca²⁺ release through the ligand-gated ryanodine receptor (RyR1) ion channels. The predominant triadin isoform in skeletal muscle, Trisk 95, activates RyR1 in vitro via binding to previously unidentified amino acids between residues 200 and 232. Here, we identify three amino acids that influence Trisk 95 binding to RyR1 and ion channel activation, using peptides encompassing residues 200-232. Selective alanine substitutions show that K²¹⁸, K²²⁰, and K²²⁴ together facilitate normal Trisk 95 binding to RyR1 and channel activation. Neither RyR1 binding nor activation are altered by alanine substitution of K²²⁰ alone or of K²¹⁸ and K²²⁴. Therefore K²¹⁸, K²²⁰, and K²²⁴ contribute to a robust binding and activation site that is disrupted only when the charge on all three residues is neutralized. We suggest that charged pair interactions between acidic RyR1 residues D⁴⁸⁷⁸, D⁴⁹⁰⁷, and E⁴⁹⁰⁸ and Trisk 95 residues K²¹⁸, K²²⁰, and K²²⁴ facilitate Trisk 95 binding to RyR1 and channel activation. Since K²¹⁸, K²²⁰, and K²²⁴ are also required for CSQ binding to RyRs (Kobayashi et al. 17, J Biol Chem 275, 17639-17646), the results suggest that Trisk 95 may not simultaneously bind to RyR1 and CSQ, contrary to the widely held belief that triadin monomers form a quaternary complex with junctin, CSQ and RyR1. Therefore, the in vivo role of triadin monomers in modulating RyR1 activity is likely unrelated to CSQ.

Triadin and CLIMP-63 form a link between triads and microtubules in muscle cells

In skeletal muscle, the triad is a structure comprising a transverse (T)-tubule and sarcoplasmic reticulum (SR) cisternae. Triads constitute the basis of excitation-contraction coupling as the cradle of the Ca²⁺ release complex. We have shown previously that triadin, a member of this complex, has shaping properties on reticulum membrane and is indirectly involved in a link between triads and microtubules. We have identified here that CLIMP-63 (also known as CKAP4), as the partner of triadin, is responsible for this association of triads and microtubules. Triadin and CLIMP-63 interact through their respective luminal domains and the shaping properties of triadin depend on the capacity of CLIMP-63 to bind microtubules with its cytosolic portion. In skeletal muscle, CLIMP-63 is localized in the SR, including triads, and is associated with the Ca²⁺ release complex through its interaction with triadin. Knockout of triadin in muscles results in the delocalization of CLIMP-63 from triads, its dissociation from the Ca²⁺ release complex and a disorganization of the microtubule network. Our results suggest that the association of triadin and CLIMP-63 could be involved in the shaping of SR terminal cisternae and in the guidance of microtubules close to the triads.

Triadin regulation of the ryanodine receptor complex

The calcium release complex is the major player in excitation-contraction coupling, both in cardiac and skeletal muscle. The core of the complex is the ryanodine receptor, and triadin is a regulating protein. Nevertheless, the precise function of triadin is only partially understood. Besides its function in the anchoring of calsequestrin at the triad/dyad, our recent results allow us to propose hypotheses on new triadin scaffolding functions, based on the studies performed using different models, from triadin knockout mice to human patients, and expression in non-muscle cells, taking into account the presence of multiple triadin isoforms.

Proteins within the intracellular calcium store determine cardiac RyR channel activity and cardiac output

SUMMARY

The contractile function of the heart requires the release of Ca(2+) from intracellular Ca(2+) stores in the sarcoplasmic reticulum (SR) of cardiac muscle cells. The efficacy of Ca(2+) release depends on the amount of Ca(2+) loaded into the Ca(2+) store and the way in which this 'Ca(2+) load' influences the activity of the cardiac ryanodine receptor Ca(2+) release channel (RyR2). The effects of the Ca(2+) load on Ca(2+) release through RyR2 are facilitated by: (i) the sensitivity of RyR2 itself to luminal Ca(2+) concentrations; and (ii) interactions between the cardiac Ca(2+) -binding protein calsequestrin (CSQ) 2 and RyR2, transmitted through the 'anchoring' proteins junctin and/or triadin. Mutations in RyR2 are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT) and sudden cardiac death. The tachycardia is associated with changes in the sensitivity of RyR2 to luminal Ca(2+) . Triadin-, junctin- or CSQ-null animals survive, but their longevity and ability to tolerate stress is compromised. These studies reveal the importance of the proteins in normal muscle function, but do not reveal the molecular nature of their functional interactions, which must be defined before changes in the proteins leading to CPVT and heart disease can be understood. Herein, we discuss known interactions between the RyR, triadin, junctin and CSQ with emphasis on the cardiac isoforms of the proteins. Where there is little known about the cardiac isoforms, we discuss evidence from skeletal isoforms.

A skeletal muscle ryanodine receptor interaction domain in triadin

Excitation-contraction coupling in skeletal muscle depends, in part, on a functional interaction between the ligand-gated ryanodine receptor (RyR1) and integral membrane protein Trisk 95, localized to the sarcoplasmic reticulum membrane. Various domains on Trisk 95 can associate with RyR1, yet the domain responsible for regulating RyR1 activity has remained elusive. We explored the hypothesis that a luminal Trisk 95 KEKE motif (residues 200-232), known to promote RyR1 binding, may also form the RyR1 activation domain. Peptides corresponding to Trisk 95 residues 200-232 or 200-231 bound to RyR1 and increased the single channel activity of RyR1 by 1.49 ± 0.11-fold and 1.8 ± 0.15-fold respectively, when added to its luminal side. A similar increase in [(3)H]ryanodine binding, which reflects open probability of the channels, was also observed. This RyR1 activation is similar to activation induced by full length Trisk 95. Circular dichroism showed that both peptides were intrinsically disordered, suggesting a defined secondary structure is not necessary to mediate RyR1 activation. These data for the first time demonstrate that Trisk 95's 200-231 region is responsible for RyR1 activation. Furthermore, it shows that no secondary structure is required to achieve this activation, the Trisk 95 residues themselves are critical for the Trisk 95-RyR1 interaction.

Role of triadin in the organization of reticulum membrane at the muscle triad

The terminal cisternae represent one of the functional domains of the skeletal muscle sarcoplasmic reticulum (SR). They are closely apposed to plasma membrane invaginations, the T-tubules, with which they form structures called triads. In triads, the physical interaction between the T-tubule-anchored voltage-sensing channel DHPR and the SR calcium channel RyR1 is essential because it allows the depolarization-induced calcium release that triggers muscle contraction. This interaction between DHPR and RyR1 is based on the peculiar membrane structures of both T-tubules and SR terminal cisternae. However, little is known about the molecular mechanisms governing the formation of SR terminal cisternae. We have previously shown that ablation of triadins, a family of SR transmembrane proteins that interact with RyR1, induced skeletal muscle weakness in knockout mice as well as a modification of the shape of triads. Here we explore the intrinsic molecular properties of the longest triadin isoform Trisk 95. We show that when ectopically expressed, Trisk 95 can modulate reticulum membrane morphology. The membrane deformations induced by Trisk 95 are accompanied by modifications of the microtubule network organization. We show that multimerization of Trisk 95 by disulfide bridges, together with interaction with microtubules, are responsible for the ability of Trisk 95 to structure reticulum membrane. When domains responsible for these molecular properties are deleted, anchoring of Trisk 95 to the triads in muscle cells is strongly decreased, suggesting that oligomers of Trisk 95 and microtubules contribute to the organization of the SR terminal cisternae in a triad.

Absence of triadin, a protein of the calcium release complex, is responsible for cardiac arrhythmia with sudden death in human

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disease so far related to mutations in the cardiac ryanodine receptor (RYR2) or the cardiac calsequestrin (CASQ2) genes. Because mutations in RYR2 or in CASQ2 are not retrieved in all CPVT cases, we searched for mutations in the physiological protein partners of RyR2 and CSQ2 in a large cohort of CPVT patients with no detected mutation in these two genes. Based on a candidate gene approach, we focused our investigations on triadin and junctin, two proteins that link RyR2 and CSQ2. Mutations in the triadin (TRDN) and in the junctin (ASPH) genes were searched in a cohort of 97 CPVT patients. We identified three mutations in triadin which cosegregated with the disease on a recessive mode of transmission in two families, but no mutation was found in junctin. Two TRDN mutations, a 4 bp deletion and a nonsense mutation, resulted in premature stop codons; the third mutation, a p.T59R missense mutation, was further studied. Expression of the p.T59R mutant in COS-7 cells resulted in intracellular retention and degradation of the mutant protein. This was confirmed after in vivo expression of the mutant triadin in triadin knock-out mice by viral transduction. In this work, we identified TRDN as a new gene responsible for an autosomal recessive form of CPVT. The mutations identified in the two families lead to the absence of the protein, thereby demonstrating the importance of triadin for the normal function of the cardiac calcium release complex in humans.

Sequential stages in the age-dependent gradual formation and accumulation of tubular aggregates in fast twitch muscle fibers: SERCA and calsequestrin involvement

Tubular aggregates (TAs), ordered arrays of elongated sarcoplasmic reticulum (SR) tubules, are present in skeletal muscle from patients with myopathies and are also experimentally induced by extreme anoxia. In wild-type mice TAs develop in a clear age-, sex- (male), and fiber type- (fast twitch) dependence. However, the events preceding the appearance of TAs have not been explored. We investigated the sequential stages leading to the initial appearance and maturation of TAs in EDL from male mice. TAs' formation requires two temporally distinct steps that operate via different mechanisms. Initially (before 1 year of age), the SR Ca(2+) binding protein calsequestrin (CASQ) accumulates specifically at the I band level causing swelling of free SR cisternae. In the second stage, the enlarged SR sacs at the I band level extend into multiple, longitudinally oriented tubules with a full complement of sarco(endo)plasmic reticulum Ca(2+) ATPases (SERCA) in the membrane and CASQ in the lumen. Tubules gradually acquire a regular cylindrical shape and uniform size apparently in concert with partial crystallization of SERCA. Multiple, small TAs associate to form fewer mature TAs of very large size. Interestingly, in fibers from CASQ1-knockout mice abnormal aggregates of SR tubules have different conformation and never develop into ordered aggregates of straight cylinders, possibly due to lack of CASQ accumulation. We conclude that TAs do not arise abruptly but are the final result of a gradually changing SR architecture and we suggest that the crystalline ATPase within the aggregates may be inactive.

Calsequestrin (CASQ1) rescues function and structure of calcium release units in skeletal muscles of CASQ1-null mice

Amplitude of Ca(2+) transients, ultrastructure of Ca(2+) release units, and molecular composition of sarcoplasmic reticulum (SR) are altered in fast-twitch skeletal muscles of calsequestrin-1 (CASQ1)-null mice. To determine whether such changes are directly caused by CASQ1 ablation or are instead the result of adaptive mechanisms, here we assessed ability of CASQ1 in rescuing the null phenotype. In vivo reintroduction of CASQ1 was carried out by cDNA electro transfer in flexor digitorum brevis muscle of the mouse. Exogenous CASQ1 was found to be correctly targeted to the junctional SR (jSR), as judged by immunofluorescence and confocal microscopy; terminal cisternae (TC) lumen was filled with electron dense material and its width was significantly increased, as judged by electron microscopy; peak amplitude of Ca(2+) transients was significantly increased compared with null muscle fibers transfected only with green fluorescent protein (control); and finally, transfected fibers were able to sustain cytosolic Ca(2+) concentration during prolonged tetanic stimulation. Only the expression of TC proteins, such as calsequestrin 2, sarcalumenin, and triadin, was not rescued as judged by Western blot. Thus our results support the view that CASQ1 plays a key role in both Ca(2+) homeostasis and TC structure.

On the footsteps of Triadin and its role in skeletal muscle

Calcium is a crucial element for striated muscle function. As such, myoplasmic free Ca²⁺ concentration is delicately regulated through the concerted action of multiple Ca²⁺ pathways that relay excitation of the plasma membrane to the intracellular contractile machinery. In skeletal muscle, one of these major Ca²⁺ pathways is Ca²⁺ release from intracellular Ca²⁺ stores through type-1 ryanodine receptor/Ca²⁺ release channels (RyR1), which positions RyR1 in a strategic cross point to regulate Ca²⁺ homeostasis. This major Ca²⁺ traffic point appears to be highly sensitive to the intracellular environment, which senses through a plethora of chemical and protein-protein interactions. Among these modulators, perhaps one of the most elusive is Triadin, a muscle-specific protein that is involved in many crucial aspect of muscle function. This family of proteins mediates complex interactions with various Ca²⁺ modulators and seems poised to be a relevant modulator of Ca²⁺ signaling in cardiac and skeletal muscles. The purpose of this review is to examine the most recent evidence and current understanding of the role of Triadin in muscle function, in general, with particular emphasis on its contribution to Ca²⁺ homeostasis.

Increased muscle stress-sensitivity induced by selenoprotein N inactivation in mouse: a mammalian model for SEPN1-related myopathy

Selenium is an essential trace element and selenoprotein N (SelN) was the first selenium-containing protein shown to be directly involved in human inherited diseases. Mutations in the SEPN1 gene, encoding SelN, cause a group of muscular disorders characterized by predominant affection of axial muscles. SelN has been shown to participate in calcium and redox homeostasis, but its pathophysiological role in skeletal muscle remains largely unknown. To address SelN function in vivo, we generated a Sepn1-null mouse model by gene targeting. The Sepn1^−/− mice had normal growth and lifespan, and were macroscopically indistinguishable from wild-type littermates. Only minor defects were observed in muscle morphology and contractile properties in SelN-deficient mice in basal conditions. However, when subjected to challenging physical exercise and stress conditions (forced swimming test), Sepn1^−/− mice developed an obvious phenotype, characterized by limited motility and body rigidity during the swimming session, as well as a progressive curvature of the spine and predominant alteration of paravertebral muscles. This induced phenotype recapitulates the distribution of muscle involvement in patients with SEPN1-Related Myopathy, hence positioning this new animal model as a valuable tool to dissect the role of SelN in muscle function and to characterize the pathophysiological process.

Trisk 32 regulates IP(3) receptors in rat skeletal myoblasts

To date, four isoforms of triadins have been identified in rat skeletal muscle. While the function of the 95-kDa isoform in excitation-contraction coupling has been studied in detail, the role of the 32-kDa isoform (Trisk 32) remains elusive. Here, Trisk 32 overexpression was carried out by stable transfection in L6.G8 myoblasts. Co-localization of Trisk 32 and IP(3) receptors (IP(3)R) was demonstrated by immunocytochemistry, and their association was shown by co-immunoprecipitation. Functional effects of Trisk 32 on IP(3)-mediated Ca(2+) release were assessed by measuring changes in [Ca(2+)](i) following the stimulation by bradykinin or vasopressin. The amplitude of the Ca(2+) transients evoked by 20 μM bradykinin was significantly higher in Trisk 32-overexpressing (p < 0.01; 426 ± 84 nM, n = 27) as compared to control cells (76 ± 12 nM, n = 23). The difference remained significant (p < 0.02; 217 ± 41 nM, n = 21, and 97 ± 29 nM, n = 31, respectively) in the absence of extracellular Ca(2+). Similar observations were made when 0.1 μM vasopressin was used to initiate Ca(2+) release. Possible involvement of the ryanodine receptors (RyR) in these processes was excluded, after functional and biochemical experiments. Furthermore, Trisk 32 overexpression had no effect on store-operated Ca(2+) entry, despite a decrease in the expression of STIM1. These results suggest that neither the increased activity of RyR, nor the amplification of SOCE, is responsible for the differences observed in bradykinin- or vasopressin-evoked Ca(2+) transients; rather, they were due to the enhanced activity of IP(3)R. Thus, Trisk 32 not only co-localizes with, but directly contributes to, the regulation of Ca(2+) release via IP(3)R.

Reduced gain of excitation-contraction coupling in triadin-null myotubes is mediated by the disruption of FKBP12/RyR1 interaction

Several studies have suggested that triadin (Tdn) may be a critical component of skeletal EC-coupling. However, using Tdn-null mice we have shown that triadin ablation results in no significant disruption of skeletal EC-coupling. To analyze the role of triadin in EC-coupling signaling here we used whole-cell voltage clamp and simultaneous recording of intracellular Ca²+ release to characterize the retrograde and orthograde signaling between RyR1 and DHPR in cultured myotubes. DHPR Ca²+ currents elicited by depolarization of Wt and Tdn-null myotubes displayed similar current densities and voltage dependence. However, kinetic analysis of the Ca²+ current shows that activation time constant of the slow component was slightly decreased in Tdn-null cells. Voltage-evoked Ca²+ transient of Tdn-null myotubes showed small but significant reduction in peak fluorescence amplitude but no differences in voltage dependence. This difference in Ca²+ amplitude was averted by over-expression of FKBP12.6. Our results show that bi-directional signaling between DHPR and RyR1 is preserved nearly intact in Tdn-null myotubes and that the effect of triadin ablation on Ca²+ transients appears to be secondary to the reduced FKBP12 binding capacity of RyR1 in Tdn-null myotubes. These data suggest that skeletal triadins do not play a direct role in skeletal EC-coupling.

Caveolin 3 is associated with the calcium release complex and is modified via in vivo triadin modification

The triadin isoforms Trisk 95 and Trisk 51 are both components of the skeletal muscle calcium release complex. To investigate the specific role of Trisk 95 and Trisk 51 isoforms in muscle physiology, we overexpressed Trisk 95 or Trisk 51 using adenovirus-mediated gene transfer in skeletal muscle of newborn mice. Overexpression of either Trisk 95 or Trisk 51 alters the muscle fiber morphology, while leaving unchanged the expression of the ryanodine receptor, the dihydropyridine receptor, and calsequestrin. We also observe an aberrant expression of caveolin 3 in both Trisk 95- and Trisk 51-overexpressing skeletal muscles. Using a biochemical approach, we demonstrate that caveolin 3 is associated with the calcium release complex in skeletal muscle. Taking advantage of muscle and non-muscle cell culture models and triadin null mouse skeletal muscle, we further dissect the molecular organization of the caveolin 3-containing calcium release complex. Our data demonstrate that the association of caveolin 3 with the calcium release complex occurs via a direct interaction with the transmembrane domain of the ryanodine receptor. Taken together, these data suggest that caveolin 3-containing membrane domains and the calcium release complex are functionally linked and that Trisk 95 and Trisk 51 are instrumental to the regulation of this interaction, the integrity of which may be crucial for muscle physiology.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Triadin deletion induces impaired skeletal muscle function

Triadin is a multiple proteins family, some isoforms being involved in muscle excitation-contraction coupling, and some having still unknown functions. To obtain clues on triadin functions, we engineered a triadin knock-out mouse line and characterized the physiological effect of triadin ablation on skeletal muscle function. These mice presented a reduced muscle strength, which seemed not to alter their survival and has been characterized in the present work. We first checked in these mice the expression level of the different proteins involved in calcium homeostasis and observed in fast muscles an increase in expression of dihydropyridine receptor, with a large reduction in calsequestrin expression. Electron microscopy analysis of KO muscles morphology demonstrated the presence of triads in abnormal orientation and a reduction in the sarcoplasmic reticulum terminal cisternae volume. Using calcium imaging on cultured myotubes, we observed a reduction in the total amount of calcium stored in the sarcoplasmic reticulum. Physiological studies have been performed to evaluate the influence of triadin deletion on skeletal muscle function. Muscle strength has been measured both on the whole animal model, using hang test or electrical stimulation combined with NMR analysis and strength measurement, or on isolated muscle using electrical stimulation. All the results obtained demonstrate an important reduction in muscle strength, indicating that triadin plays an essential role in skeletal muscle function and in skeletal muscle structure. These results indicate that triadin alteration leads to the development of a myopathy, which could be studied using this new animal model.

Ca(2+) signaling in striated muscle: the elusive roles of triadin, junctin, and calsequestrin

This review focuses on molecular interactions between calsequestrin, triadin, junctin and the ryanodine receptor in the lumen of the sarcoplasmic reticulum. These interactions modulate changes in Ca(2+) release in response to changes in the Ca(2+) load within the sarcoplasmic reticulum store in striated muscle and are of fundamental importance to Ca(2+) homeostasis, since massive adaptive changes occur when expression of the proteins is manipulated, while mutations in calsequestrin lead to functional changes which can be fatal. We find that calsequestrin plays a different role in the heart and skeletal muscle, enhancing Ca(2+) release in the heart, but depressing Ca(2+) release in skeletal muscle. We also find that triadin and junctin exert independent influences on the ryanodine receptor in skeletal muscle where triadin alone modifies excitation-contraction coupling, while junctin alone supports functional interactions between calsequestrin and the ryanodine receptor.

Junctin and triadin each activate skeletal ryanodine receptors but junctin alone mediates functional interactions with calsequestrin

Normal Ca(2+) signalling in skeletal muscle depends on the membrane associated proteins triadin and junctin and their ability to mediate functional interactions between the Ca(2+) binding protein calsequestrin and the type 1 ryanodine receptor in the lumen of the sarcoplasmic reticulum. This important mechanism conserves intracellular Ca(2+) stores, but is poorly understood. Triadin and junctin share similar structures and are lumped together in models of interactions between skeletal muscle calsequestrin and ryanodine receptors, however their individual roles have not been examined at a molecular level. We show here that purified skeletal ryanodine receptors are similarly activated by purified triadin or purified junctin added to their luminal side, although a lack of competition indicated that the proteins act at independent sites. Surprisingly, triadin and junctin differed markedly in their ability to transmit information between skeletal calsequestrin and ryanodine receptors. Purified calsequestrin inhibited junctin/triadin-associated, or junctin-associated, ryanodine receptors and the calsequestrin re-associated channel complexes were further inhibited when luminal Ca(2+) fell from 1mM to <or=100 microM, as seen with native channels (containing endogenous calsequestrin/triadin/junctin). In contrast, skeletal calsequestrin had no effect on the triadin/ryanodine receptor complex and the channel activity of this complex increased when luminal Ca(2+) fell, as seen with purified channels prior to triadin/calsequestrin re-association. Therefore in this cell free system, junctin alone mediates signals between luminal Ca(2+), skeletal calsequestrin and skeletal ryanodine receptors and may curtail resting Ca(2+) leak from the sarcoplasmic reticulum. We suggest that triadin serves a different function which may dominate during excitation-contraction coupling.

Triadin: what possible function 20 years later?

During the last 20 years, the identification of triadin function in cardiac and skeletal muscle has been the focus of numerous studies. First thought of as the missing link between the ryanodine receptor and the dihydropyridine receptor and responsible of skeletal type excitation-contraction coupling, the current hypothesis on triadin function has slowly evolved, and triadin is envisaged now as a regulator of calcium release, both in cardiac and skeletal muscle. Nevertheless, none of the experiments performed up to now has given a clear cut view of what triadin really does in muscle. The problem became more complex with the identification of multiple triadin isoforms, having possibly multiple functions. Using a different approach from what has been done previously, we have obtained new clues about the function of triadin. Our data point to a possible involvement of triadin in reticulum structure, in relation with the microtubule network.

Triadins modulate intracellular Ca(2+) homeostasis but are not essential for excitation-contraction coupling in skeletal muscle

To unmask the role of triadin in skeletal muscle we engineered pan-triadin-null mice by removing the first exon of the triadin gene. This resulted in a total lack of triadin expression in both skeletal and cardiac muscle. Triadin knockout was not embryonic or birth-lethal, and null mice presented no obvious functional phenotype. Western blot analysis of sarcoplasmic reticulum (SR) proteins in skeletal muscle showed that the absence of triadin expression was associated with down-regulation of Junctophilin-1, junctin, and calsequestrin but resulted in no obvious contractile dysfunction. Ca(2+) imaging studies in null lumbricalis muscles and myotubes showed that the lack of triadin did not prevent skeletal excitation-contraction coupling but reduced the amplitude of their Ca(2+) transients. Additionally, null myotubes and adult fibers had significantly increased myoplasmic resting free Ca(2+).[(3)H]Ryanodine binding studies of skeletal muscle SR vesicles detected no differences in Ca(2+) activation or Ca(2+) and Mg(2+) inhibition between wild-type and triadin-null animals. Subtle ultrastructural changes, evidenced by the appearance of longitudinally oriented triads and the presence of calsequestrin in the sacs of the longitudinal SR, were present in fast but not slow twitch-null muscles. Overall, our data support an indirect role for triadin in regulating myoplasmic Ca(2+) homeostasis and organizing the molecular complex of the triad but not in regulating skeletal-type excitation-contraction coupling.

Microdomains of endoplasmic reticulum within the sarcoplasmic reticulum of skeletal myofibers

The relationship between the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of skeletal muscle cells has remained obscure. In this study, we found that ER- and SR-specific membrane proteins exhibited diverse solubility properties when extracted with mild detergents. Accordingly, the major SR-specific protein Ca(2+)-ATPase (SERCA) remained insoluble in Brij 58 and floated in sucrose gradients while typical ER proteins were partially or fully soluble. Sphingomyelinase treatment rendered SERCA soluble in Brij 58. Immunofluorescence staining for resident ER proteins revealed dispersed dots over I bands contrasting the continuous staining pattern of SERCA. Infection of isolated myofibers with enveloped viruses indicated that interfibrillar protein synthesis occurred. Furthermore, we found that GFP-tagged Dad1, able to incorporate into the oligosaccharyltransferase complex, showed the dot-like structures but the fusion protein was also present in membranes over the Z lines. This behaviour mimics that of cargo proteins that accumulated over the Z lines when blocked in the ER. Taken together, the results suggest that resident ER proteins comprised Brij 58-soluble microdomains within the insoluble SR membrane. After synthesis and folding in the ER-microdomains, cargo proteins and non-incorporated GFP-Dad1 diffused into the Z line-flanking compartment which likely represents the ER exit sites.

Regulation and function of Ca2+-calmodulin-dependent protein kinase II of fast-twitch rat skeletal muscle

The activation and function of Ca(2+)-calmodulin-dependent kinase II (CaMKII) in contracting rat skeletal muscle was examined. The increase in autonomous activity and phosphorylation at Thr(287) of CaMKII of gastrocnemius muscle in response to contractions in situ was rapid and transient, peaking at 1-3 min, but reversed after 30 min of contractions. There was a positive correlation between CaMKII phosphorylation at Thr(287) and autonomous CaMKII activity. In contrast to the rapid and transient increase in autonomous CaMKII activity, the phosphorylation of the putative CaMKII substrate trisk95/triadin was rapid and sustained during contractions. There were no changes in CaMKII activity and phosphorylation or trisk95 phosphorylation in the resting contralateral muscles during stimulation. When fast-twitch muscles were contracted ex vivo, CaMKII inhibition resulted in a greater magnitude of fatigue as well as blunted CaMKII and trisk95 phosphorylation, identifying trisk95 as a physiological CaMKII substrate. In summary, skeletal muscle CaMKII activation was rapid and sustained during exercise/contraction and is mediated by factors within the contracting muscle, probably through allosteric activation via Ca(2+)-CaM. CaMKII may signal through trisk95 to modulate Ca(2+) release in fast-twitch rat skeletal muscle during exercise/contraction.

Calsequestrin targeting to sarcoplasmic reticulum of skeletal muscle fibers

Calsequestrin (CS) is the low-affinity, high-capacity calcium binding protein segregated to the lumen of terminal cisternae (TC) of the sarcoplasmic reticulum (SR). The physiological role of CS in controlling calcium release from the SR depends on both its intrinsic properties and its localization. The mechanisms of CS targeting were investigated in skeletal muscle fibers and C2C12 myotubes, a model of SR differentiation, with four deletion mutants of epitope (hemagglutinin, HA)-tagged CS: CS-HA24NH2, CS-HA2D, CS-HA3D, and CS-HAHT, a double mutant of the NH2 terminus and domain III. As judged by immunofluorescence of transfected skeletal muscle fibers, only the double CS-HA mutant showed a homogeneous distribution at the sarcomeric I band, i.e., it did not segregate to TC. As shown by subfractionation of microsomes derived from transfected skeletal muscles, CS-HAHT was largely associated to longitudinal SR whereas CS-HA was concentrated in TC. In C2C12 myotubes, as judged by immunofluorescence, not only CS-HAHT but also CS-HA3D and CS-HA2D were not sorted to developing SR. Condensation competence, a property referable to CS oligomerization, was monitored for the several CS-HA mutants in C2C12 myoblasts, and only CS-HA3D was found able to condense. Together, the results indicate that 1) there are at least two targeting sequences at the NH2 terminus and domain III of CS, 2) SR-specific target and structural information is contained in these sequences, 3) heterologous interactions with junctional SR proteins are relevant for segregation, 4) homologous CS-CS interactions are involved in the overall targeting process, and 5) different targeting mechanisms prevail depending on the stage of SR differentiation.

Sarcoplasmic reticulum: The dynamic calcium governor of muscle

The sarcoplasmic reticulum (SR) provides feedback control required to balance the processes of calcium storage, release, and reuptake in skeletal muscle. This balance is achieved through the concerted action of three major classes of SR calcium-regulatory proteins: (1) luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage; (2) SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release; and (3) sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pumps for calcium reuptake. Proper calcium storage, release, and reuptake are essential for normal skeletal muscle function. We review SR structure and function during normal skeletal muscle activity, the proteins that orchestrate calcium storage, release, and reuptake, and how phenotypically distinct muscle diseases (e.g., malignant hyperthermia, central core disease, and Brody disease) can result from subtle alterations in the activity of several key components of the SR calcium-regulatory machinery.

Triadin (Trisk 95) overexpression blocks excitation-contraction coupling in rat skeletal myotubes

To identify the function of triadin in skeletal muscle, adenovirus-mediated overexpression of Trisk 95 or Trisk 51, the two major skeletal muscle isoforms, was induced in rat skeletal muscle primary cultures, and the physiological behavior of the modified cells was analyzed. Overexpression did not modify the expression level of their protein partners ryanodine receptor, dihydropyridine receptor, and the other triadin. Caffeine-induced calcium release was also unaffected by triadin overexpression. Nevertheless, in the absence of extracellular calcium, depolarization-induced calcium release was almost abolished in Trisk 95 overexpressing myotubes (T95 myotubes), and not modified in Trisk 51 overexpressing myotubes (T51 myotubes). This was not because of a modification of dihydropyridine receptors, as depolarization in presence of external calcium still induced a calcium release, and the activation curve of dihydropyridine receptor was unchanged, in both T95 and T51 myotubes. The calcium release complex was also maintained in T95 myotubes as Trisk 95, ryanodine receptor, dihydropyridine receptor, and Trisk 51 were still co-localized. The effect of Trisk 95 overexpression on depolarization-induced calcium release was reversed by a simultaneous infection with an antisense Trisk 95 adenovirus, indicating the specificity of this effect. Thus, the level of Trisk 95 and not Trisk 51 is important on regulating the calcium release complex, and an excess of this protein can lead to an inhibition of the physiological function of the complex.