Dihydropyridine receptor alpha subunits in normal and dysgenic muscle in vitro: expression of alpha 1 is required for proper targeting and distribution of alpha 2

Cored in the act: the use of models to understand core myopathies

The core myopathies are a group of congenital myopathies with variable clinical expression - ranging from early-onset skeletal-muscle weakness to later-onset disease of variable severity - that are identified by characteristic 'core-like' lesions in myofibers and the presence of hypothonia and slowly or rather non-progressive muscle weakness. The genetic causes are diverse; central core disease is most often caused by mutations in ryanodine receptor 1 (RYR1), whereas multi-minicore disease is linked to pathogenic variants of several genes, including selenoprotein N (SELENON), RYR1 and titin (TTN). Understanding the mechanisms that drive core development and muscle weakness remains challenging due to the diversity of the excitation-contraction coupling (ECC) proteins involved and the differential effects of mutations across proteins. Because of this, the use of representative models expressing a mature ECC apparatus is crucial. Animal models have facilitated the identification of disease progression mechanisms for some mutations and have provided evidence to help explain genotype-phenotype correlations. However, many unanswered questions remain about the common and divergent pathological mechanisms that drive disease progression, and these mechanisms need to be understood in order to identify therapeutic targets. Several new transgenic animals have been described recently, expanding the spectrum of core myopathy models, including mice with patient-specific mutations. Furthermore, recent developments in 3D tissue engineering are expected to enable the study of core myopathy disease progression and the effects of potential therapeutic interventions in the context of human cells. In this Review, we summarize the current landscape of core myopathy models, and assess the hurdles and opportunities of future modeling strategies.

STAC3 variants cause a congenital myopathy with distinctive dysmorphic features and malignant hyperthermia susceptibility

SH3 and cysteine-rich domain-containing protein 3 (STAC3) is an essential component of the skeletal muscle excitation-contraction coupling (ECC) machinery, though its role and function are not yet completely understood. Here, we report 18 patients carrying a homozygous p.(Trp284Ser) STAC3 variant in addition to a patient compound heterozygous for the p.(Trp284Ser) and a novel splice site change (c.997-1G > T). Clinical severity ranged from prenatal onset with severe features at birth, to a milder and slowly progressive congenital myopathy phenotype. A malignant hyperthermia (MH)-like reaction had occurred in several patients. The functional analysis demonstrated impaired ECC. In particular, KCl-induced membrane depolarization resulted in significantly reduced sarcoplasmic reticulum Ca²⁺ release. Co-immunoprecipitation of STAC3 with Ca_V 1.1 in patients and control muscle samples showed that the protein interaction between STAC3 and Ca_V 1.1 was not significantly affected by the STAC3 variants. This study demonstrates that STAC3 gene analysis should be included in the diagnostic work up of patients of any ethnicity presenting with congenital myopathy, in particular if a history of MH-like episodes is reported. While the precise pathomechanism remains to be elucidated, our functional characterization of STAC3 variants revealed that defective ECC is not a result of Ca_V 1.1 sarcolemma mislocalization or impaired STAC3-Ca_V 1.1 interaction.

ER-plasma membrane junctions: Why and how do we study them?

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are membrane microdomains important for communication between the ER and the PM. ER-PM junctions were first reported in muscle cells in 1957, but mostly ignored in non-excitable cells due to their scarcity and lack of functional significance. In 2005, the discovery of stromal interaction molecule 1 (STIM1) mediating a universal Ca²⁺ feedback mechanism at ER-PM junctions in mammalian cells led to a resurgence of research interests toward ER-PM junctions. In the past decade, several major advancements have been made in this emerging topic in cell biology, including the generation of tools for labeling ER-PM junctions and the unraveling of mechanisms underlying regulation and functions of ER-PM junctions. This review summarizes early studies, recently developed tools, and current advances in the characterization and understanding of ER-PM junctions. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

In Vitro Differentiation of Mature Myofibers for Live Imaging

Skeletal muscles are composed of myofibers, the biggest cells in the mammalian body and one of the few syncytia. How the complex and evolutionarily conserved structures that compose it are assembled remains under investigation. Their size and physiological features often constrain manipulation and imaging applications. The culture of immortalized cell lines is widely used, but it can only replicate the early steps of differentiation. Here, we describe a protocol that enables easy genetic manipulation of myofibers originating from primary mouse myoblasts. After one week of differentiation, the myofibers display contractility, aligned sarcomeres and triads, as well as peripheral nuclei. The entire differentiation process can be followed by live imaging or immunofluorescence. This system combines the advantages of the existing ex vivo and in vitro protocols. The possibility of easy and efficient transfection as well as the ease of access to all differentiation stages broadens the potential applications. Myofibers can subsequently be used not only to address relevant developmental and cell biology questions, but also to reproduce muscle disease phenotypes for clinical applications.

N-WASP is required for Amphiphysin-2/BIN1-dependent nuclear positioning and triad organization in skeletal muscle and is involved in the pathophysiology of centronuclear myopathy

Mutations in amphiphysin-2/BIN1, dynamin 2, and myotubularin are associated with centronuclear myopathy (CNM), a muscle disorder characterized by myofibers with atypical central nuclear positioning and abnormal triads. Mis-splicing of amphiphysin-2/BIN1 is also associated with myotonic dystrophy that shares histopathological hallmarks with CNM. How amphiphysin-2 orchestrates nuclear positioning and triad organization and how CNM-associated mutations lead to muscle dysfunction remains elusive. We find that N-WASP interacts with amphiphysin-2 in myofibers and that this interaction and N-WASP distribution are disrupted by amphiphysin-2 CNM mutations. We establish that N-WASP functions downstream of amphiphysin-2 to drive peripheral nuclear positioning and triad organization during myofiber formation. Peripheral nuclear positioning requires microtubule/Map7/Kif5b-dependent distribution of nuclei along the myofiber and is driven by actin and nesprins. In adult myofibers, N-WASP and amphiphysin-2 are only involved in the maintenance of triad organization but not in the maintenance of peripheral nuclear positioning. Importantly, we confirmed that N-WASP distribution is disrupted in CNM and myotonic dystrophy patients. Our results support a role for N-WASP in amphiphysin-2-dependent nuclear positioning and triad organization and in CNM and myotonic dystrophy pathophysiology.

Commitment of Satellite Cells Expressing the Calcium Channel α2δ1 Subunit to the Muscle Lineage

Satellite cells can maintain or repair muscle because they possess stem cell properties, making them a valuable option for cell therapy. However, cell transplants into skeletal muscle of patients with muscular dystrophy are limited by donor cell attachment, migration, and survival in the host tissue. Cells used for therapy are selected based on specific markers present in the plasma membrane. Although many markers have been identified, there is a need to find a marker that is expressed at different states in satellite cells, activated, quiescent, or differentiated cell. Furthermore, the marker has to be present in human tissue. Recently we reported that the plasma membrane α2δ1 protein is involved in cell attachment and migration in myoblasts. The α2δ1 subunit forms a part of the L-type voltage-dependent calcium channel in adult skeletal muscle. We found that the α2δ1 subunit is expressed in the majority of newly isolated satellite cells and that it appears earlier than the α1 subunits and at higher levels than the β or γ subunits. We also found that those cells that expressed α2δ1 would differentiate into muscle cells. This evidence indicates that the α2δ1 may be used as a marker of satellite cells that will differentiate into muscle.

The calcium channel α2/δ1 subunit interacts with ATP5b in the plasma membrane of developing muscle cells

The α2/δ1 and α(1)1.1 subunits are present at a 1:1 ratio in the dihydropyridine receptor (DHPR) from adult skeletal muscle. In contrast, during early myotube development α2/δ1 is present at higher levels than α(1)1.1 and localizes at the ends of the cells, suggesting that α2/δ1 may have a role independent from DHPRs. We sought to identify binding partners of α2/δ1 at a period when levels of α(1)1.1 are low. Analysis of protein complexes in their native configuration established that α2/δ1 may be associating with ATP5b, a subunit of a mitochondrial ATP synthase complex. This interaction was confirmed with fluorescence resonance energy transfer and coimmunoprecipitation. The association of α2/δ1 and ATP5b occurs in intracellular membranes and at the plasma membrane, where they form a functional signaling complex capable of accelerating the rate of decline of calcium transients. The acceleration of decay was more evident when myotubes were stimulated with a train of pulses. Our data indicate that the α2/δ1 subunit is not only part of the DHPR but that it may interact with other cellular components in developing myotubes, such as the ATP5b in its atypical localization in the plasma membrane.

Alpha2delta1 dihydropyridine receptor subunit is a critical element for excitation-coupled calcium entry but not for formation of tetrads in skeletal myotubes

It has been shown that small interfering RNA (siRNA) partial knockdown of the alpha(2)delta(1) dihydropyridine receptor subunits cause a significant increase in the rate of activation of the L-type Ca(2+) current in myotubes but have little or no effect on skeletal excitation-contraction coupling. This study used permanent siRNA knockdown of alpha(2)delta(1) to address two important unaddressed questions. First, does the alpha(2)delta(1) subunit contribute to the size and/or spacing of tetradic particles? Second, is the alpha(2)delta(1) subunit important for excitation-coupled calcium entry? We found that the size and spacing of tetradic particles is unaffected by siRNA knockdown of alpha(2)delta(1), indicating that the visible particle represents the alpha(1s) subunit. Strikingly, >97% knockdown of alpha(2)delta(1) leads to a complete loss of excitation-coupled calcium entry during KCl depolarization and a more rapid decay of Ca(2+) transients during bouts of repetitive electrical stimulation like those occurring during normal muscle activation in vivo. Thus, we conclude that the alpha(2)delta(1) dihydropyridine receptor subunit is physiologically necessary for sustaining Ca(2+) transients in response to prolonged depolarization or repeated trains of action potentials.

Computer modeling of siRNA knockdown effects indicates an essential role of the Ca2+ channel alpha2delta-1 subunit in cardiac excitation-contraction coupling

L-type Ca(2+) currents determine the shape of cardiac action potentials (AP) and the magnitude of the myoplasmic Ca(2+) signal, which regulates the contraction force. The auxiliary Ca(2+) channel subunits alpha(2)delta-1 and beta(2) are important regulators of membrane expression and current properties of the cardiac Ca(2+) channel (Ca(V)1.2). However, their role in cardiac excitation-contraction coupling is still elusive. Here we addressed this question by combining siRNA knockdown of the alpha(2)delta-1 subunit in a muscle expression system with simulation of APs and Ca(2+) transients by using a quantitative computer model of ventricular myocytes. Reconstitution of dysgenic muscle cells with Ca(V)1.2 (GFP-alpha(1C)) recapitulates key properties of cardiac excitation-contraction coupling. Concomitant depletion of the alpha(2)delta-1 subunit did not perturb membrane expression or targeting of the pore-forming GFP-alpha(1C) subunit into junctions between the outer membrane and the sarcoplasmic reticulum. However, alpha(2)delta-1 depletion shifted the voltage dependence of Ca(2+) current activation by 9 mV to more positive potentials, and it slowed down activation and inactivation kinetics approximately 2-fold. Computer modeling revealed that the altered voltage dependence and current kinetics exert opposing effects on the function of ventricular myocytes that in total cause a 60% prolongation of the AP and a 2-fold increase of the myoplasmic Ca(2+) concentration during each contraction. Thus, the Ca(2+) channel alpha(2)delta-1 subunit is not essential for normal Ca(2+) channel targeting in muscle but is a key determinant of normal excitation and contraction of cardiac muscle cells, and a reduction of alpha(2)delta-1 function is predicted to severely perturb normal heart function.

Caveolin-1(-/-)- and caveolin-2(-/-)-deficient mice both display numerous skeletal muscle abnormalities, with tubular aggregate formation

Here, we examine the role of "non-muscle" caveolins (Cav-1 and Cav-2) in skeletal muscle biology. Our results indicate that skeletal muscle fibers from male Cav-1(-/-) and Cav-2(-/-) mice show striking abnormalities, such as tubular aggregates, mitochondrial proliferation/aggregation, and increased numbers of M-cadherin-positive satellite cells. Notably, these skeletal muscle defects were more pronounced with increasing age. Because Cav-2-deficient mice displayed normal expression levels of Cav-1, whereas Cav-1-null mice exhibited an almost complete deficiency in Cav-2, these skeletal muscle abnormalities seem to be due to loss of Cav-2. Thus, Cav-2(-/-) mice represent a novel animal model-and the first genetically well-defined mouse model-that can be used to study the pathogenesis of tubular aggregate formation, which remains a poorly understood age-related skeletal muscle abnormality. Finally, because Cav-1 and Cav-2 were not expressed within mature skeletal myofibers, our results indicate that development of these abnormalities probably originates in stem/precursor cells, such as satellite cells or myoblasts. Consistent with this hypothesis, skeletal muscle isolated from male Cav-3(-/-) mice did not show any of these abnormalities. As such, this is the first study linking stem cells with the genesis of these intriguing muscle defects.

The beta 1a subunit is essential for the assembly of dihydropyridine-receptor arrays in skeletal muscle

Homozygous zebrafish of the mutant relaxed (red(ts25)) are paralyzed and die within days after hatching. A significant reduction of intramembrane charge movements and the lack of depolarization-induced but not caffeine-induced Ca(2+) transients suggested a defect in the skeletal muscle dihydropyridine receptor (DHPR). Sequencing of DHPR cDNAs indicated that the alpha(1S) subunit is normal, whereas the beta(1a) subunit harbors a single point mutation resulting in a premature stop. Quantitative RT-PCR revealed that the mutated gene is transcribed, but Western blot analysis and immunocytochemistry demonstrated the complete loss of the beta(1a) protein in mutant muscle. Thus, the immotile zebrafish relaxed is a beta(1a)-null mutant. Interestingly, immunocytochemistry showed correct triad targeting of the alpha(1S) subunit in the absence of beta(1a). Freeze-fracture analysis of the DHPR clusters in relaxed myotubes revealed an approximately 2-fold reduction in cluster size with a normal density of DHPR particles within the clusters. Most importantly, DHPR particles in the junctional membranes of the immotile zebrafish mutant relaxed entirely lacked the normal arrangement in arrays of tetrads. Thus, our data indicate that the lack of the beta(1a) subunit does not prevent triad targeting of the DHPR alpha(1S) subunit but precludes the skeletal muscle-specific arrangement of DHPR particles opposite the ryanodine receptor (RyR1). This defect properly explains the complete deficiency of skeletal muscle excitation-contraction coupling in beta(1)-null model organisms.

The role of auxiliary dihydropyridine receptor subunits in muscle

The skeletal muscle dihydropyridine receptor is a slowly-activating calcium channel that functions as the voltage sensor in excitation-contraction coupling. In addition to the pore-forming alpha(1S) subunit it contains the transmembrane alpha(2)delta-1 and gamma(1) subunits and the cytoplasmic beta(1a) subunit. Although the roles of the auxiliary subunits in calcium channel function have been intensively studied in heterologous expression systems, their functions in excitation-contraction coupling has only recently been elucidated in muscle cells of various null-mutant animal models. In this article we will briefly outline the current state of these investigations.

The Ca2+ Channel α2δ-1 Subunit Determines Ca2+ Current Kinetics in Skeletal Muscle but Not Targeting of α1S or Excitation-Contraction Coupling

Auxiliary channel subunits regulate membrane expression and modulate current properties of voltage-activated Ca(2+) channels and thus are involved in numerous important cell functions, including muscle contraction. Whereas the importance of the alpha(1S), beta(1a), and gamma Ca(2+) channel subunits in skeletal muscle has been determined by using null-mutant mice, the role of the alpha(2)delta-1 subunit in skeletal muscle is still elusive. We addressed this question by small interfering RNA silencing of alpha(2)delta-1 in reconstituted dysgenic (alpha(1S)-null) myotubes and in BC3H1 skeletal muscle cells. Immunofluorescence labeling of the alpha(1S) and alpha(2)delta-1 subunits and whole cell patch clamp recordings demonstrated that triad targeting and functional expression of the skeletal muscle Ca(2+) channel were not compromised by the depletion of the alpha(2)delta-1 subunit. The amplitudes and voltage dependences of L-type Ca(2+) currents and of the depolarization-induced Ca(2+) transients were identical in control and in alpha(2)delta-1-depleted muscle cells. However, alpha(2)delta-1 depletion significantly accelerated the current kinetics, most likely by the conversion of slowly activating into fast activating Ca(2+) channels. Reverse transcription-PCR analysis indicated that alpha(2)delta-1 is the exclusive isoform expressed in differentiated BC3H1 cells and that depletion of alpha(2)delta-1 was not compensated by the up-regulation of any other alpha(2)delta isoform. Thus, in skeletal muscle the Ca(2+) channel alpha(2)delta-1 subunit functions as a major determinant of the characteristic slow L-type Ca(2+) current kinetics. However, this subunit is not essential for targeting of Ca(2+) channels or for their primary physiological role in activating skeletal muscle excitation-contraction coupling.

Evidence for conformational coupling between two calcium channels

Ryanodine receptor 1 (RyR1, the sarcoplasmic reticulum Ca(2+) release channel) and alpha(1S)dihydropyridine receptor (DHPR, the surface membrane voltage sensor) of skeletal muscle belong to separate membrane systems but are functionally and structurally linked. Four alpha(1S)DHPRs associated with the four identical subunits of a RyR form a tetrad. We treated skeletal muscle cell lines with ryanodine, at concentrations that block RyRs, and determined whether this treatment affects the distance between DHPRs in the tetrad. We find a substantial ( approximately 2-nm) shift in the alpha(1S)DHPR positions, indicating that ryanodine induces large conformational changes in the RyR1 cytoplasmic domain and that the alpha(1S)DHPR-RyR complex acts as a unit.

Insulin-like growth factor-induced transcriptional activity of the skeletal alpha-actin gene is regulated by signaling mechanisms linked to voltage-gated calcium channels during myoblast differentiation

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal alpha-actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal alpha-actin gene expression in C2C12 myoblasts. IGF-I increased skeletal alpha-actin promoter activity by 107% compared with the control condition. Ni(+) [T-type voltage-gated Ca(2+) channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal alpha-actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-I-induced activation of the skeletal alpha-actin promoter by 29-48%. IGF-I failed to increase skeletal alpha-actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mm K(+) and 30 mm K(+)+IGF-I increased skeletal alpha-actin promoter activity by 162% and 76% compared with non-IGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K(+)+IGF-I-induced activation of the skeletal alpha-actin promoter. Constitutively active calcineurin increased skeletal alpha-actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni(+)-inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni(+). IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni(+). These data suggest that IGF-I-induced activation of the skeletal alpha-actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes.

Gamma 1 subunit interactions within the skeletal muscle L-type voltage-gated calcium channels

Voltage-gated calcium channels mediate excitationcontraction coupling in the skeletal muscle. Their molecular composition, similar to neuronal channels, includes the pore-forming alpha(1) and auxiliary alpha(2)delta, beta, and gamma subunits. The gamma subunits are the least characterized, and their subunit interactions are unclear. The physiological importance of the neuronal gamma is emphasized by epileptic stargazer mice that lack gamma(2). In this study, we examined the molecular basis of interaction between skeletal gamma(1) and the calcium channel. Our data show that the alpha(1)1.1, beta(1a), and alpha(2)delta subunits are still associated in gamma(1) null mice. Reexpression of gamma(1) and gamma(2) showed that gamma(1), but not gamma(2), incorporates into gamma(1) null channels. By using chimeric constructs, we demonstrate that the first half of the gamma(1) subunit, including the first two transmembrane domains, is important for subunit interaction. Interestingly, this chimera also restores calcium conductance in gamma(1) null myotubes, indicating that the domain mediates both subunit interaction and current modulation. To determine the subunit of the channel that interacts with gamma(1), we examined the channel in muscular dysgenesis mice. Cosedimentation experiments showed that gamma(1) and alpha(2)delta are not associated. Moreover, alpha(1)1.1 and gamma(1) subunits form a complex in transiently transfected cells, indicating direct interaction between the gamma(1) and alpha(1)1.1 subunits. Our data demonstrate that the first half of gamma(1) subunit is required for association with the channel through alpha(1)1.1. Because subunit interactions are conserved, these studies have broad implications for gamma heterogeneity, function and subunit association with voltage-gated calcium channels.

The structure of Ca(2+) release units in arthropod body muscle indicates an indirect mechanism for excitation-contraction coupling

The relative disposition of ryanodine receptors (RyRs) and L-type Ca(2+) channels was examined in body muscles from three arthropods. In all muscles the disposition of ryanodine receptors in the junctional gap between apposed SR and T tubule elements is highly ordered. By contrast, the junctional membrane of the T tubule is occupied by distinctive large particles that are clustered within the small junctional domain, but show no order in their arrangement. We propose that the large particles of the junctional T tubules represent L-type Ca(2+) channels involved in excitation-contraction (e-c) coupling, based on their similarity in size and location with the L-type Ca(2+) channels or dihydropyridine receptors (DHPRs) of skeletal and cardiac muscle. The random arrangement of DHPRs in arthropod body muscles indicates that there is no close link between them and RyRs. This matches the architecture of vertebrate cardiac muscle and is in keeping with the similarity in e-c coupling mechanisms in cardiac and invertebrate striated muscles.

IP3 receptor function and localization in myotubes: an unexplored Ca2+ signaling pathway in skeletal muscle

We present evidence for an unexplored inositol 1,4,5-trisphosphate-mediated Ca2+ signaling pathway in skeletal muscle. RT-PCR methods confirm expression of all three known isotypes of the inositol trisphosphate receptor in cultured rodent muscle. Confocal microscopy of cultured mouse muscle, doubly labeled for inositol receptor type 1 and proteins of known distribution, reveals that the receptors are localized to the I band of the sarcoplasmic reticulum, and this staining is continuous with staining of the nuclear envelope region. These results suggest that the receptors are positioned to mediate a slowly propagating Ca2+ wave that follows the fast Ca2+ transient upon K+ depolarization. This slow wave, imaged using fluo-3, resulted in an increase in nucleoplasmic Ca2+ lasting tens of seconds, but not contraction; the slow wave was blocked by both the inositol trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and the phospholipase C inhibitor U-73122. To test the hypothesis that these slow Ca2+ signals are involved in signal cascades leading to regulation of gene expression, we assayed for early effects of K+ depolarization on mitogen-activated protein kinases, specifically extracellular-signal related kinases 1 and 2 and the transcription factor cAMP response element-binding protein (CREB). Within 30-60 seconds following depolarization, phosphorylation of both the kinases and CREB was evident and could be inhibited by 2-aminoethoxydiphenyl borate. These results suggest a signaling system mediated by Ca2+ and inositol trisphosphate that could regulate gene expression in muscle cells.

The triad targeting signal of the skeletal muscle calcium channel is localized in the COOH terminus of the alpha(1S) subunit

The specific localization of L-type Ca(2+) channels in skeletal muscle triads is critical for their normal function in excitation-contraction (EC) coupling. Reconstitution of dysgenic myotubes with the skeletal muscle Ca(2+) channel alpha(1S) subunit restores Ca(2+) currents, EC coupling, and the normal localization of alpha(1S) in the triads. In contrast, expression of the neuronal alpha(1A) subunit gives rise to robust Ca(2+) currents but not to triad localization. To identify regions in the primary structure of alpha(1S) involved in the targeting of the Ca(2+) channel into the triads, chimeras of alpha(1S) and alpha(1A) were constructed, expressed in dysgenic myotubes, and their subcellular distribution was analyzed with double immunofluorescence labeling of the alpha(1S)/alpha(1A) chimeras and the ryanodine receptor. Whereas chimeras containing the COOH terminus of alpha(1A) were not incorporated into triads, chimeras containing the COOH terminus of alpha(1S) were correctly targeted. Mapping of the COOH terminus revealed a triad-targeting signal contained in the 55 amino-acid sequence (1607-1661) proximal to the putative clipping site of alpha(1S). Transferring this triad targeting signal to alpha(1A) was sufficient for targeting and clustering the neuronal isoform into skeletal muscle triads and caused a marked restoration of Ca(2+)-dependent EC coupling.

IP(3) receptors, IP(3) transients, and nucleus-associated Ca(2+) signals in cultured skeletal muscle

Inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)R) and ryanodine receptors (RyR) were localized in cultured rodent muscle fractions by binding of radiolabeled ligands (IP(3) and ryanodine), and IP(3)R were visualized in situ by fluorescence immunocytological techniques. Also explored was the effect of K(+) depolarization on IP(3) mass and Ca(2+) transients studied using a radio-receptor displacement assay and fluorescence imaging of intracellular fluo 3. RyR were located in a microsomal fraction; IP(3)R were preferentially found in the nuclear fraction. Fluorescence associated with anti-IP(3)R antibody was found in the region of the nuclear envelope and in a striated pattern in the sarcoplasmic areas. An increase in external K(+) affected membrane potential and produced an IP(3) transient. Rat myotubes displayed a fast-propagating Ca(2+) signal, corresponding to the excitation-contraction coupling transient and a much slower Ca(2+) wave. Both signals were triggered by high external K(+) and were independent of external Ca(2+). Slow waves were associated with cell nuclei and were propagated leaving "glowing" nuclei behind. Different roles are proposed for at least two types of Ca(2+) release channels, each mediating an intracellular signal in cultured skeletal muscle.

The sarcoplasmic reticulum and the control of muscle contraction

Activation of muscle contraction is a rapid event that is initiated by electrical activity in the surface membrane and transverse (T) tubules. This is followed by release of calcium from the inner membrane system, the sarcoplasmic reticulum (SR). Using electron microscopy (EM), K. R. Porter and his laboratory defined the SR, the unique junctions between SR and T tubules, and the continuity between T tubules and surface membrane. Current research in this area centers on the interaction between T tubules and SR. This is mediated by 2 well-identified calcium channels: the dihydropyridine receptors (DHPRs) that act as voltage sensors in the T tubules, and the ryanodine receptors (RyRs) or calcium release channels of the SR. The relative positions of these 2 molecules differ significantly in skeletal and cardiac muscle, and this correlates well with known functional differences in the control of contraction. Molecular biology experiments combined with EM indicate that DHPRs are linked to RyRs in skeletal but probably not in cardiac muscle.

Structural requirement of the calcium-channel subunit alpha2delta for gabapentin binding

Gabapentin [Neurontin, 1-(aminomethyl)cyclohexaneacetic acid] is a novel anticonvulsant drug with a high binding affinity for the Ca(2+)-channel subunit alpha(2)delta. In this study, the gabapentin-binding properties of wild-type and mutated porcine brain alpha(2)delta proteins were investigated. Removal of the disulphide bonds between the alpha(2) and the delta subunits did not result in a significant loss of gabapentin binding, suggesting that the disulphide linkage between the two subunits is not required for binding. Singly expressed alpha(2) protein remained membrane associated. However, alpha(2) alone was unable to bind gabapentin, unless the cells were concurrently transfected with the expression vector for delta, suggesting that both alpha(2) and delta are required for gabapentin binding. Using internal deletion mutagenesis, we mapped two regions [amino acid residues 339-365 (DeltaF) and 875-905 (DeltaJ)] within the alpha(2) subunit that are not required for gabapentin binding. Further, deletion of three other individual regions [amino acid residues 206-222 (DeltaD), 516-537 (DeltaH) and 583-603 (DeltaI)] within the alpha(2) subunit disrupted gabapentin binding, suggesting the structural importance of these regions. Using alanine to replace four to six amino acid residues in each of these regions abolished gabapentin binding. These results demonstrate that region D, between the N-terminal end and the first putative transmembrane domain of alpha(2), and regions H and I, between the putative splicing acceptor sites (Gln(511) and Ser(601)), may play important roles in maintaining the structural integrity for gabapentin binding. Further single amino acid replacement mutagenesis within these regions identified Arg(217) as critical for gabapentin binding.

Correct targeting of dihydropyridine receptors and triadin in dyspedic mouse skeletal muscle in vivo

Excitation-contraction coupling in skeletal muscle involves junctions (triads and dyads) between sarcoplasmic reticulum (SR) and transverse (T) -tubules. Two proteins of the junctional SR, ryanodine receptors (RyRs) and triadin and one protein of T tubules, dihydropyridine receptors (DHPRs) are located at these junctions. We studied the targeting of DHPRs and triadin to T-tubules and SR in skeletal muscles of dyspedic mouse embryos lacking RyR1. In normal differentiating muscle fibers DHPRs, triadin and RyRs are located in intensely immunolabeled foci that are randomly distributed across the fiber. Correlation with electron microscopy and with previous studies indicates that the foci represent the location of triads and dyads. In dyspedic fibers DHPRs and triadin antibodies stain internal foci of the two proteins; RyR antibodies are completely negative. The appearance and location of the foci in dyspedic fibers is similar to that of normal muscle, but their fluorescent intensity is weaker. The SR Ca-ATPase has more diffuse distribution than triadin in both normal and dyspedic fibers. These observations indicate that an interaction with RyRs is not necessary for the appropriate targeting of DHPRs or triadin to junctional domains of T tubules and SR respectively.

Direct monitoring of the calcium concentration in the sarcoplasmic and endoplasmic reticulum of skeletal muscle myotubes

Direct monitoring of the free Ca2+ concentration in the sarcoplasmic reticulum (SR) was carried out in rat skeletal myotubes transfected with a specifically targeted aequorin chimera (srAEQ). Myotubes were also transfected with a chimeric aequorin (erAEQ) that we have demonstrated previously is retained in the endoplasmic reticulum (ER). Immunolocalization analysis showed that although both recombinant proteins are distributed in an endomembrane network identifiable with immature SR, the erAEQ protein was retained also in the perinuclear membrane. The difficulty of measuring [Ca2+] in 100-1000 microM range was overcome with the use of the synthetic coelenterazine analogue, coelenterazine n. We demonstrate that the steady state levels of [Ca2+] measured with srAEQ is around 300 microM, whereas that measured with erAEQ is significantly lower, i.e. around 200 microM. The effects of caffeine, high KCl, and nicotinic receptor stimulation, in the presence or absence of external calcium or after blockade of the Ca-ATPase, were investigated with both chimeras. The kinetics of [Ca2+] changes revealed by the erAEQ were similar, but not identical, neither quantitatively nor qualitatively, to those monitored with the srAEQ, indicating that at this stage of muscle development, differences exist between SR and ER in their mechanisms of Ca2+ handling. The functional implications of these findings are discussed.

Molecular origin of the L-type Ca2+ current of skeletal muscle myotubes selectively deficient in dihydropyridine receptor beta1a subunit

The origin of Ibetanull, the Ca2+ current of myotubes from mice lacking the skeletal dihydropyridine receptor (DHPR) beta1a subunit, was investigated. The density of Ibetanull was similar to that of Idys, the Ca2+ current of myotubes from dysgenic mice lacking the skeletal DHPR alpha1S subunit (-0.6 +/- 0.1 and -0.7 +/- 0.1 pA/pF, respectively). However, Ibetanull activated at significantly more positive potentials. The midpoints of the GCa-V curves were 16.3 +/- 1.1 mV and 11.7 +/- 1.0 mV for Ibetanull and Idys, respectively. Ibetanull activated significantly more slowly than Idys. At +30 mV, the activation time constant for Ibetanull was 26 +/- 3 ms, and that for Idys was 7 +/- 1 ms. The unitary current of normal L-type and beta1-null Ca2+ channels estimated from the mean variance relationship at +20 mV in 10 mM external Ca2+ was 22 +/- 4 fA and 43 +/- 7 fA, respectively. Both values were significantly smaller than the single-channel current estimated for dysgenic Ca2+ channels, which was 84 +/- 9 fA under the same conditions. Ibetanull and Idys have different gating and permeation characteristics, suggesting that the bulk of the DHPR alpha1 subunits underlying these currents are different. Ibetanull is suggested to originate primarily from Ca2+ channels with a DHPR alpha1S subunit. Dysgenic Ca2+ channels may be a minor component of this current. The expression of DHPR alpha1S in beta1-null myotubes and its absence in dysgenic myotubes was confirmed by immunofluorescence labeling of cells.

Differences in both inositol 1,4,5-trisphosphate mass and inositol 1,4,5-trisphosphate receptors between normal and dystrophic skeletal muscle cell lines

Human normal (RCMH) and Duchenne muscular dystrophy (RCDMD) cell lines, as well as newly developed normal and dystrophic murine cell lines, were used for the study of both changes in inositol 1,4,5-trisphosphate (IP3) mass and IP3 binding to receptors. Basal levels of IP3 were increased two- to threefold in dystrophic human and murine cell lines compared to normal cell lines. Potassium depolarization induced a time-dependent IP3 rise in normal human cells and cells of the myogenic mouse cell line (129CB3), which returned to their basal levels after 60 s. However, in the human dystrophic cell line (RCDMD), IP3 levels remained high up to 200 s after potassium depolarization. Expression of IP3 receptors was studied measuring specific binding of 3H-IP3 in the murine cell lines (normal 129CB3 and dystrophic mdx XLT 4-2). All the cell lines bind 3H-IP3 with relatively high affinity (Kd: between 40 and 100 nmol/L). IP3 receptors are concentrated in the nuclear fraction, and their density is significantly higher in dystrophic cells compared to normal. These findings together with high basal levels of IP3 mass suggest a possible role for this system in the deficiency of intracellular calcium regulation in Duchenne muscular dystrophy.

Association of calcium channel alpha1S and beta1a subunits is required for the targeting of beta1a but not of alpha1S into skeletal muscle triads

The skeletal muscle L-type Ca2+ channel is a complex of five subunits that is specifically localized in the triad. Its primary function is the rapid activation of Ca2+ release from cytoplasmic stores in a process called excitation-contraction coupling. To study the role of alpha1S-beta1a interactions in the incorporation of the functional channel complex into the triad, alpha1S and beta1a [or a beta1a-green fluorescent protein (GFP) fusion protein] were expressed alone and in combination in myotubes of the dysgenic cell line GLT. betaGFP expressed in dysgenic myotubes that lack the skeletal muscle alpha1S subunit was diffusely distributed in the cytoplasm. On coexpression with the alpha1S subunit betaGFP distribution became clustered and colocalized with alpha1S immunofluorescence. Based on the colocalization of betaGFP and alpha1S with the ryanodine receptor the clusters were identified as T-tubule/sarcoplasmic reticulum junctions. Expression of alpha1S with and without beta1a restored Ca2+ currents and depolarization-induced Ca2+ release. The translocation of betaGFP from the cytoplasm into the junctions failed when betaGFP was coexpressed with alpha1S mutants in which the beta interaction domain had been altered (alpha1S-Y366S) or deleted (alpha1S-Delta351-380). Although alpha1S-Y366S did not associate with betaGFP it was incorporated into the junctions, and it restored Ca2+ currents and depolarization-induced Ca2+ release. Thus, beta1a requires the association with the beta interaction domain in the I-II cytoplasmic loop of alpha1S for its own incorporation into triad junctions, but stable alpha1S-beta1a association is not necessary for the targeting of alpha1S into the triads or for its normal function in Ca2+ conductance and excitation-contraction coupling.

Differential effects of Ca2+ channel beta1a and beta2a subunits on complex formation with alpha1S and on current expression in tsA201 cells

To study the interactions of the alpha1S subunit of the skeletal muscle L-type Ca2+ channel with the skeletal beta1a and the cardiac beta2a, these subunits were expressed alone or in combination in tsA201 cells. Immunofluorescence- and green fluorescent protein-labeling showed that, when expressed alone, beta1a was diffusely distributed throughout the cytoplasm, beta2a was localized in the plasma membrane, and alpha1S was concentrated in a tubular/reticular membrane system, presumably the endoplasmic reticulum (ER). Upon coexpression with alpha1S, beta1a became colocalized with alpha1S in the ER. Upon coexpression with beta2a, alpha1S redistributed to the plasma membrane, where it aggregated in large clusters. Coexpression of alpha1S with beta1a but not with beta2a increased the frequency at which cells expressed L-type currents. A point mutation (alpha1S-Y366S) or deletion (alpha1S-Delta351-380) in the beta interaction domain of alpha1S blocked both translocation of beta1a to the ER and beta2a-induced translocation of the alpha1S mutants to the plasma membrane. However, the point mutation did not interfere with beta1a-induced current stimulation. Thus, beta1a and beta2a are differentially distributed in tsA201 cells and upon coexpression with alpha1S, form alpha1S. beta complexes in different cellular compartments. Complex formation but not current stimulation requires the intact beta interaction domain in the I-II cytoplasmic loop of alpha1S.

Formation of junctions involved in excitation-contraction coupling in skeletal and cardiac muscle

During excitation-contraction (e-c) coupling of striated muscle, depolarization of the surface membrane is converted into Ca2+ release from internal stores. This process occurs at intracellular junctions characterized by a specialized composition and structural organization of membrane proteins. The coordinated arrangement of the two key junctional components--the dihydropyridine receptor (DHPR) in the surface membrane and the ryanodine receptor (RyR) in the sarcoplasmic reticulum--is essential for their normal, tissue-specific function in e-c coupling. The mechanisms involved in the formation of the junctions and a potential participation of DHPRs and RyRs in this process have been subject of intensive studies over the past 5 years. In this review we discuss recent advances in understanding the organization of these molecules in skeletal and cardiac muscle, as well as their concurrent and independent assembly during development of normal and mutant muscle. From this information we derive a model for the assembly of the junctions and the establishment of the precise structural relationship between DHPRs and RyRs that underlies their interaction in e-c coupling.

Formation of triads without the dihydropyridine receptor alpha subunits in cell lines from dysgenic skeletal muscle

Muscular dysgenesis (mdg/mdg), a mutation of the skeletal muscle dihydropyridine receptor (DHPR) alpha 1 subunit, has served as a model to study the functions of the DHPR in excitation-contraction coupling and its role in triad formation. We have investigated the question of whether the lack of the DHPR in dysgenic skeletal muscle results in a failure of triad formation, using cell lines (GLT and NLT) derived from dysgenic (mdg/mdg) and normal (+/+) muscle, respectively. The lines were generated by transfection of myoblasts with a plasmid encoding a Large T antigen. Both cell lines express muscle-specific proteins and begin organization of sarcomeres as demonstrated by immunocytochemistry. Similar to primary cultures, dysgenic (GLT) myoblasts show a higher incidence of cell fusion than their normal counterparts (NLT). NLT myotubes develop spontaneous contractile activity, and fluorescent Ca2+ recordings show Ca2+ release in response to depolarization. In contrast, GLTs show neither spontaneous nor depolarization-induced Ca2+ transients, but do release Ca2+ from the sarcoplasmic reticulum (SR) in response to caffeine. Despite normal transverse tubule (T-tubule) formation, GLT myotubes lack the alpha 1 subunit of the skeletal muscle DHPR, and the alpha 2 subunit is mistargeted. Nevertheless, the ryanodine receptor (RyR) frequently develops its normal, clustered organization in the absence of both DHPR alpha subunits in the T-tubules. In EM, these RyR clusters correspond to T-tubule/SR junctions with regularly spaced feet. These findings provide conclusive evidence that interactions between the DHPR and RyR are not involved in the formation of triad junctions or in the normal organization of the RyR in the junctional SR.

The alpha 2/delta subunit of voltage sensitive Ca2+ channels is a single transmembrane extracellular protein which is involved in regulated secretion

The membrane topology of alpha 2/delta subunit was investigated utilizing electrophysiological functional assay and specific anti-alpha 2 antibodies. (a) cRNA encoding a deleted alpha 2/delta subunit was coinjected with alpha 1C subunit of the L-type calcium channel into Xenopus oocytes. The truncated form, lacking the third putative TM domain (alpha 2/delta delta TMIII), failed to amplify the expressed inward currents, normally induced by alpha 1C coinjected with intact alpha 2/delta subunit. Western blot analysis of alpha 2/delta delta TMIII shows the appearance of a degraded alpha 2 protein and no expression of the full-size two-TM truncated-protein. The improper processing of alpha 2/delta delta TMIII suggests that the alpha 2/delta is a single TM domain protein and the TM region is positioned at the delta subunit. (b) External application of anti-alpha 2 antibodies, prepared for an epitope within the alternatively spliced and 'intracellular' region, inhibits depolarization induced secretion in PC12, further supporting an external location of the alpha 2 subunit and establishing delta subunit as the only membrane anchor for the extracellular alpha 2 subunit.

Properties of calcium currents and contraction in cultured rat diaphragm muscle

The characterization of calcium currents and contraction simultaneously measured in cultured rat diaphragm muscle cells was carried out in the present study. Whole-cell patch-clamp experiments were designed to further elucidate the mechanism of excitation-contraction (E-C) coupling in diaphragm which, though generally considered a skeletal-type muscle, has been reported to exhibit properties indicative of a cardiac-like E-C coupling mechanism. Normalized current/voltage (I/V) curves for two concentrations of external calcium (2.5 and 5 mM) were obtained from diaphragm myoballs. Both curves showed peaks corresponding to the activation of a T-type calcium current and a dihydropyridine-sensitive L-type calcium current. The normalized curve for the voltage dependence of the activation of contraction in diaphragm myoballs followed a typical Boltzmann-type relationship to the peak of contraction. Thereafter, the curve declined in a manner that was more pronounced in diaphragm compared to that measured in additional experiments using cultured rat limb muscle myoballs. This effect could be interpreted in terms of a more pronounced participation of the L-type current in E-C coupling in cultured diaphragm muscle. An increased likelihood of cultured diaphragm muscle to undergo depletion of sarcoplasmic reticular calcium stores during repetitive stimulation, or a heightened propensity for the voltage sensor for E-C coupling in diaphragm to enter the inactive state could also explain this effect. Maximal contractile activity was only slightly affected when the L-type current was blocked by externally applied cadmium (2 mM) or cobalt (3 mM), suggesting that a pronounced calcium-current-dependent component of contraction is unlikely in cultured diaphragm muscle. These results show that T- and L-type calcium channels are expressed in cultured rat diaphragm muscle cells and that, in contrast to cardiac muscle, the entry of calcium ions via L-type voltage-dependent calcium channels is not a prerequisite for contraction. Differences in the voltage sensitivity of contraction, observed at depolarized membrane potentials in cultured rat diaphragm and limb muscle cells, suggest that the voltage sensor for E-C coupling in diaphragm might more readily enter an inactivated configuration - possibly by a mechanism which is dependent on the concentration of external calcium.

Abnormal junctions between surface membrane and sarcoplasmic reticulum in skeletal muscle with a mutation targeted to the ryanodine receptor

Junctions that mediate excitation-contraction (e-c) coupling are formed between the sarcoplasmic reticulum (SR) and either the surface membrane or the transverse (T) tubules in normal skeletal muscle. Two structural components of the junctions, the feet of the SR and the tetrads of T tubules, have been identified respectively as ryanodine receptors (RyRs, or SR calcium-release channels), and as groups of four dihydropyridine receptors (DHPRs, or voltage sensors of e-c coupling). A targeted mutation (skrrm1) of the gene for skeletal muscle RyRs in mice results in the absence of e-c coupling in homozygous offspring of transgenic parents. The mutant gene is expected to produce no functional RyRs, and we have named the mutant mice "dyspedic" because they lack feet--the cytoplasmic domain of RyRs anchored in the SR membrane. We have examined the development of junctions in skeletal muscle fibers from normal and dyspedic embryos. Surprisingly, despite the absence of RyRs, junctions are formed in dyspedic myotubes, but the junctional gap between the SR and T tubule is narrow, presumably because the feet are missing. Tetrads are also absent from these junctions. The results confirm the identity of RyRs and feet and a major role for RyRs and tetrads in e-c coupling. Since junctions form in the absence of feet and tetrads, coupling of SR to surface membrane and T tubules appears to be mediated by additional proteins, distinct from either RyRs or DHPRs.

Myofibrillogenesis in rodent skeletal muscle in vitro: two pathways involving thick filament aggregates

Thick filament aggregates play an important role in myofibrillogenesis in rodent skeletal muscle in vitro. This ultrastructural study describes these aggregates, shows their involvement in the process of myofibril formation, and correlates their appearance and function with current models of myofibrillogenesis. Initially, following myoblast fusion in normal mouse skeletal muscle in vitro, abundant stress fiber-like structures (SFLS) are found near the periphery of early myotubes. These undergo internal rearrangements, forming subcortical sarcomeres and early myofibrils. However, additional thick filaments are synthesized, and some join appositionally to the nascent myofibrils, increasing their diameter. More interiorly, this thick filament synthesis accelerates, with filaments aligning into aggregates resembling discrete A-bands, usually with M-lines and M-regions. The ends of these 'A-band' aggregates are infiltrated with ribosomes and capped by flocculent material. Ultimately, aggregates are incorporated into preexisting myofibrils or associate end-to-end to form new, parallel myofibrils, the flocculent material forming putative I-bands with diminished Z-lines and few thin filaments. As differentiation continues, Z-lines and thin filaments appear, forming true myofibrils. Dysgenic mouse skeletal muscle develops similarly, but when this non-contractile cell matures (i.e., generates action potentials), filaments and their organization break down. Cloned myogenic rat L5/A10 cells also follow this developmental pattern, but in mature, contracting myotubes, Z-lines remain irregular and thin filaments are reduced. In all three types of muscle developing in vitro, thick filament aggregates are a common and predominant feature and as such appear to constitute an additional or alternate pathway to previously described models of myofibrillogenesis.

Molecular organization of transverse tubule/sarcoplasmic reticulum junctions during development of excitation-contraction coupling in skeletal muscle

The relationship between the molecular composition and organization of the triad junction and the development of excitation-contraction (E-C) coupling was investigated in cultured skeletal muscle. Action potential-induced calcium transients develop concomitantly with the first expression of the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR), which are colocalized in clusters from the time of their earliest appearance. These DHPR/RyR clusters correspond to junctional domains of the transverse tubules (T-tubules) and sarcoplasmic reticulum (SR), respectively. Thus, at first contact T-tubules and SR form molecularly and structurally specialized membrane domains that support E-C coupling. The earliest T-tubule/SR junctions show structural characteristics of mature triads but are diverse in conformation and typically are formed before the extensive development of myofibrils. Whereas the initial formation of T-tubule/SR junctions is independent of association with myofibrils, the reorganization into proper triads occurs as junctions become associated with the border between the A band and the I band of the sarcomere. This final step in triad formation manifests itself in an increased density and uniformity of junctions in the cytoplasm, which in turn results in increased calcium release and reuptake rates.

Restoration of junctional tetrads in dysgenic myotubes by dihydropyridine receptor cDNA

Excitation-contraction coupling was restored in primary cultures of dysgenic myotubes by transfecting the cells with an expression plasmid encoding the rabbit skeletal muscle dihydropyridine receptor. Dishes containing normal, dysgenic, and transfected myotubes were fixed, freeze-fractured, and replicated for electron microscopy. Numerous small domains in the surface membrane of normal myotubes contain ordered arrays of intramembrane particles in groups of four (tetrads). The disposition of tetrads in the arrays is consistent with alternate positioning of tetrads relative to the underlying feet of the sarcoplasmic reticulum. Dysgenic myotubes have no arrays of tetrads. Some myotubes from successfully transfected cultures have arrays of tetrads with spacings equal to those found in normal myotubes. Thus the dihydropyridine receptor appears to be needed for the formation of tetrads and their association with the sarcoplasmic reticulum feet. This result is consistent with the hypothesis that each tetrad is composed of four dihydropyridine receptors.

Co-localization of 1,4-dihydropyridine receptor alpha 2/delta subunit and N-CAM during early myogenesis in vitro

The surface distribution of the alpha 2/delta subunit of the 1,4-dihydropyridine receptor and its topographical relationship with the neural cell adhesion molecule (N-CAM) were investigated during early myogenesis in vitro, by double immunocytochemical labeling with the monoclonal antibody 3007 and an anti-N-CAM polyclonal antiserum. The monoclonal antibody 3007 has been previously shown to immunoprecipitate dihydropyridine receptor from skeletal muscle T-tubules. In further immunoprecipitation experiments on such preparations and muscle cell cultures, it was demonstrated here that the monoclonal antibody 3007 exclusively recognizes the alpha 2/delta subunit of the 1,4-dihydropyridine receptor. In rabbit muscle cell cultures, the labeling for both alpha 2/delta and N-CAM was first detected on myoblasts, in the form of spots on the membrane and perinuclear patches. Spots of various sizes organized in aggregates were then found on the membrane of myotubes. At fusion (T0), aggregates of N-CAM spots alone were found at the junction between fusing cells. At T6 and later stages, all alpha 2/delta aggregates present on myotubes co-localized with N-CAM, while less than 3% of N-CAM aggregates did not co-localize with alpha 2/delta. A uniform N-CAM staining also made its appearance. At T12, when myotubes showed prominent contractility, alpha 2/delta-N-CAM aggregates diminished in size. Dispersed alpha 2/delta spots of a small regular size spread over the whole surface of the myotubes and alignments of these spots became visible. Corresponding N-CAM spots were now occasionally seen, and uniform N-CAM staining was prominent. These results show that alpha 2/delta and N-CAM are co-localized and that their distributions undergo concomitant changes during early myogenesis until the T-tubule network starts to be organized. This suggest that these two proteins might jointly participate in morphogenetic events preceding the formation of T-tubules.

Development of the excitation-contraction coupling apparatus in skeletal muscle: peripheral and internal calcium release units are formed sequentially

The development of calcium release units and of transverse tubules has been studied in skeletal muscle fibres from embryonal and newborn chicken. Three constituents of calcium release units: the tetrads, the feet and an internal protein directly associated with junctional surface of the sarcoplasmic reticulum are visualized by various electron microscope techniques. Evidence in the literature indicates that the three components correspond to the voltage sensors, the sarcoplasmic reticulum calcium release channels and the calcium binding protein calsequestrin respectively. We recognize two stages at which important events in membrane morphogenesis occur. The first stage coincides with early myofibrillogenesis (starting at approximately embryonal day E5.5), and it involves the assembly of calcium release units at the periphery of the muscle fibre in which feet and the internal protein are identified. Groups of tetrads also are present at very early stages and their disposition indicates a relation to the feet of peripheral couplings. Thus three major components of the excitation-contraction coupling pathway are in place as soon as myofibrils develop. The density of groups of tetrads in the surface membrane of primary and secondary fibres is similar, despite differences in developmental stages. The second stage involves the formation of a complex transverse tubule network and of internal sarcoplasmic reticulum-transverse tubule junctions, while peripheral couplings disappear. This stage starts abruptly (between E15 and E16) and simultaneously in primary and secondary fibres. It coincides with the myotube-to-myofibre transition. The two stages are separated by a relatively long intervening period (between E9 and E16). During the latter part of this period some primitive transverse tubules appear, and form junctions with the sarcoplasmic reticulum, but they remain strictly located at the periphery of the fibre and are not numerous. Finally, after the second stage there is a prolonged (up to 4 weeks) period of maturation, during which density of free sarcoplasmic reticulum increases, triads acquire a location at the A-I junction and fibre type differences appear. We conclude that a system for calcium uptake, storage and release exists at the periphery of the myotube during early myogenesis. The complexity of the system and its ability to deliver calcium through the entire fibre develop in parallel to the formation of myofibrils.

Triad formation: organization and function of the sarcoplasmic reticulum calcium release channel and triadin in normal and dysgenic muscle in vitro

Excitation-contraction (E-C) coupling is thought to involve close interactions between the calcium release channel (ryanodine receptor; RyR) of the sarcoplasmic reticulum (SR) and the dihydropyridine receptor (DHPR) alpha 1 subunit in the T-tubule membrane. Triadin, a 95-kD protein isolated from heavy SR, binds both the RyR and DHPR and may thus participate in E-C coupling or in interactions responsible for the formation of SR/T-tubule junctions. Immunofluorescence labeling of normal mouse myotubes shows that the RyR and triadin co-aggregate with the DHPR in punctate clusters upon formation of functional junctions. Dysgenic myotubes with a deficiency in the alpha 1 subunit of the DHPR show reduced expression and clustering of RyR and triadin; however, both proteins are still capable of forming clusters and attaining mature cross-striated distributions. Thus, the molecular organization of the RyR and triadin in the terminal cisternae of SR as well as its association with the T-tubules are independent of interactions with the DHPR alpha 1 subunit. Analysis of calcium transients in dysgenic myotubes with fluorescent calcium indicators reveals spontaneous and caffeine-induced calcium release from intracellular stores similar to those of normal muscle; however, depolarization-induced calcium release is absent. Thus, characteristic calcium release properties of the RyR do not require interactions with the DHPR; neither do they require the normal organization of the RyR in the terminal SR cisternae. In hybrids of dysgenic myotubes fused with normal cells, both action potential-induced calcium transients and the normal clustered organization of the RyR are restored in regions expressing the DHPR alpha 1 subunit.

Characterization of spontaneous and action potential-induced calcium transients in developing myotubes in vitro

We have investigated the onset and maturation of action potential- and calcium-induced calcium release from the sarcoplasmic reticulum during the differentiation of excitation-contraction coupling in skeletal muscle. Microfluorometry and video imaging of cultured myotubes loaded with the fluorescent calcium indicator fluo-3 revealed the dynamics, time course, and physiological properties of calcium transients as well as their changes during development. Spontaneous and stimulated contractions in well-differentiated myotubes are accompanied by brief (200-500 ms) increases in the concentration of free cytoplasmic calcium. These transients are modulated by sub-threshold concentrations of caffeine, resulting in a plateau of elevated calcium. Two novel types of calcium transients were observed in non-contracting myotubes. 1) Fast localized transients (FLTs) are radially restricted focal release events that occur spontaneously within the myoplasm at various densities and frequencies. 2) Upon addition of caffeine, propagating calcium waves are generated (35-70 microns/s velocity), which are accompanied by contractures. Aside from caffeine sensitivity, calcium waves and contraction-related sustained release events are similar in amplitude and duration, as well as in their inactivation and refractory properties. Thus, these transients may represent calcium-induced calcium release in quiescent and active myotubes, respectively. Following one calcium-induced calcium release event, myotubes become refractory to new calcium-induced transients; however, action potential-induced transients and FLTs are not blocked. This suggests that these transients occur by distinct release mechanisms and that dual modes of calcium release exist prior to the coupling of calcium release to excitation.

Structural analysis of muscle development: transverse tubules, sarcoplasmic reticulum, and the triad

Increased interest in the mechanism of excitation-contraction (E-C) coupling over the last few years has been accompanied by numerous investigations into the development of the underlying cellular structures. Areas of particular interest include: (1) the compartmentalization and specialization of an external and an internal membrane system, the T-tubules, and the sarcoplasmic reticulum, respectively; (2) interactions between the membrane proteins of both systems upon the formation of a junction, the triad; and (3) membrane-cytoskeletal interactions leading to the orderly arrangement of the triads with respect to the myofibrils. Structural studies using newly available specific molecular probes and a variety of in vivo and in vitro model systems have provided new insights into the cellular and molecular mechanisms involved in the development of the E-C coupling apparatus in skeletal muscle.

Coordinated development of myofibrils, sarcoplasmic reticulum and transverse tubules in normal and dysgenic mouse skeletal muscle, in vivo and in vitro

We studied the development of transverse (T)-tubules and sarcoplasmic reticulum (SR) in relationship to myofibrillogenesis in normal and dysgenic (mdg/mdg) mouse skeletal muscle by immunofluorescent labeling of specific membrane and myofibrillar proteins. At E16 the development of the myofibrils and membranes in dysgenic and normal diaphragm was indistinguishable, including well developed myofibrils, a delicate network of T-tubules, and a prominent SR which was not yet cross-striated. In diaphragms of E18 dysgenic mice, both the number and size of muscle fibers and myofibrillar organization were deficient in comparison to normal diaphragms, as previously reported. T-tubule labeling was abnormal, showing only scattered tubules and fragments. However, many muscle fibers displayed cross striation of sarcomeric proteins and SR comparable to normal muscle. In cultured myotubes, cross-striated organization of sarcomeric proteins proceeded essentially in two stages: first around the Z-line and later in the A-band. Sarcomeric organization of the SR coincided with the first stage, while the appearance of T-tubules in the mature transverse orientation occurred infrequently, only after A-band maturation. In culture, myofibrillar and membrane organization was equivalent in normal and dysgenic muscle at the earlier stage of development, but half as many dysgenic myotubes reached the later stage as compared to normal. We conclude that the mdg mutation has little effect on the initial stage of membrane and myofibril development and that the deficiencies often seen at later stages result indirectly from the previously described absence of dihydropyridine receptor function in the mutant.