Relationship of calcium transients to calcium currents and charge movements in myotubes expressing skeletal and cardiac dihydropyridine receptors

Advances in CaV1.1 gating: New insights into permeation and voltage-sensing mechanisms

The CaV1.1 voltage-gated Ca2+ channel carries L-type Ca2+ current and is the voltage-sensor for excitation-contraction (EC) coupling in skeletal muscle. Significant breakthroughs in the EC coupling field have often been close on the heels of technological advancement. In particular, CaV1.1 was the first voltage-gated Ca2+ channel to be cloned, the first ion channel to have its gating current measured and the first ion channel to have an effectively null animal model. Though these innovations have provided invaluable information regarding how CaV1.1 detects changes in membrane potential and transmits intra- and inter-molecular signals which cause opening of the channel pore and support Ca2+ release from the sarcoplasmic reticulum remain elusive. Here, we review current perspectives on this topic including the recent application of functional site-directed fluorometry.

Healing the Broken Hearts: A Glimpse on Next Generation Therapeutics

Cardiovascular diseases are the leading cause of death worldwide, accounting for 32% of deaths globally and thus representing almost 18 million people according to WHO. Myocardial infarction, the most prevalent adult cardiovascular pathology, affects over half a million people in the USA according to the last records of the AHA. However, not only adult cardiovascular diseases are the most frequent diseases in adulthood, but congenital heart diseases also affect 0.8–1.2% of all births, accounting for mild developmental defects such as atrial septal defects to life-threatening pathologies such as tetralogy of Fallot or permanent common trunk that, if not surgically corrected in early postnatal days, they are incompatible with life. Therefore, both congenital and adult cardiovascular diseases represent an enormous social and economic burden that invariably demands continuous efforts to understand the causes of such cardiovascular defects and develop innovative strategies to correct and/or palliate them. In the next paragraphs, we aim to briefly account for our current understanding of the cellular bases of both congenital and adult cardiovascular diseases, providing a perspective of the plausible lines of action that might eventually result in increasing our understanding of cardiovascular diseases. This analysis will come out with the building blocks for designing novel and innovative therapeutic approaches to healing the broken hearts.

The role of action potential changes in depolarization-induced failure of excitation contraction coupling in mouse skeletal muscle

Excitation-contraction coupling (ECC) is the process by which electrical excitation of muscle is converted into force generation. Depolarization of skeletal muscle resting potential contributes to failure of ECC in diseases such as periodic paralysis, intensive care unit acquired weakness and possibly fatigue of muscle during vigorous exercise. When extracellular K⁺ is raised to depolarize the resting potential, failure of ECC occurs suddenly, over a narrow range of resting potentials. Simultaneous imaging of Ca²⁺ transients and recording of action potentials (APs) demonstrated failure to generate Ca²⁺ transients when APs peaked at potentials more negative than -30mV. An AP property that closely correlated with failure of the Ca²⁺ transient was the integral of AP voltage with respect to time. Simultaneous recording of Ca²⁺ transients and APs with electrodes separated by 1.6mm revealed AP conduction fails when APs peak below -21mV. We hypothesize propagation of APs and generation of Ca²⁺ transients are governed by distinct AP properties: AP conduction is governed by AP peak, whereas Ca²⁺ release from the sarcoplasmic reticulum is governed by AP integral. The reason distinct AP properties may govern distinct steps of ECC is the kinetics of the ion channels involved. Na channels, which govern propagation, have rapid kinetics and are insensitive to AP width (and thus AP integral) whereas Ca²⁺ release is governed by gating charge movement of Cav1.1 channels, which have slower kinetics such that Ca²⁺ release is sensitive to AP integral. The quantitative relationships established between resting potential, AP properties, AP conduction and Ca²⁺ transients provide the foundation for future studies of failure of ECC induced by depolarization of the resting potential.

Into the spotlight: RGK proteins in skeletal muscle

The RGK (Rad, Rem, Rem2 and Gem/Kir) family of small GTPases are potent endogenous inhibitors of voltage-gated Ca²⁺ channels (VGCCs). While the impact of RGK proteins on cardiac physiology has been investigated extensively, much less is known regarding their influence on skeletal muscle biology. Thus, the purpose of this article is to establish a basis for future investigation into the role of RGK proteins in regulating the skeletal muscle excitation-contraction (EC) coupling complex via modulation of the L-type Ca_V1.1 VGCC. The pathological consequences of elevated muscle RGK protein expression in Type II Diabetes, Amyotrophic Lateral Sclerosis (ALS), Duchenne's Muscular Dystrophy and traumatic nerve injury are also discussed.

Correcting the R165K substitution in the first voltage-sensor of CaV1.1 right-shifts the voltage-dependence of skeletal muscle calcium channel activation

The voltage-gated calcium channel CaV1.1a primarily functions as voltage-sensor in skeletal muscle excitation-contraction (EC) coupling. In embryonic muscle the splice variant CaV1.1e, which lacks exon 29, additionally function as a genuine L-type calcium channel. Because previous work in most laboratories used a CaV1.1 expression plasmid containing a single amino acid substitution (R165K) of a critical gating charge in the first voltage-sensing domain (VSD), we corrected this substitution and analyzed its effects on the gating properties of the L-type calcium currents in dysgenic myotubes. Reverting K165 to R right-shifted the voltage-dependence of activation by ~12 mV in both CaV1.1 splice variants without changing their current amplitudes or kinetics. This demonstrates the exquisite sensitivity of the voltage-sensor function to changes in the specific amino acid side chains independent of their charge. Our results further indicate the cooperativity of VSDs I and IV in determining the voltage-sensitivity of CaV1.1 channel gating.

The voltage sensor of excitation-contraction coupling in mammals: Inactivation and interaction with Ca2

In excitation–contraction coupling, voltage-sensing modules (VSMs) of Ca_V1.1 Ca²⁺ channels simultaneously gate the associated pore and Ca²⁺ release channels in the sarcoplasmic reticulum. Ferreira Gregorio et al. find that VSMs adopt two inactivated states, and the degree of inactivation is dependent on external Ca²⁺ and the mouse strain used.

In skeletal muscle, the four-helix voltage-sensing modules (VSMs) of Ca_V1.1 calcium channels simultaneously gate two Ca²⁺ pathways: the Ca_V1.1 pore itself and the RyR1 calcium release channel in the sarcoplasmic reticulum. Here, to gain insight into the mechanism by which VSMs gate RyR1, we quantify intramembrane charge movement associated with VSM activation (sensing current) and gated Ca²⁺ release flux in single muscle cells of mice and rats. As found for most four-helix VSMs, upon sustained depolarization, rodent VSMs lose the ability to activate Ca²⁺ release channels opening; their properties change from a functionally capable mode, in which the mobile sensor charge is called charge 1, to an inactivated mode, charge 2, with a voltage dependence shifted toward more negative voltages. We find that charge 2 is promoted and Ca²⁺ release inactivated when resting, well-polarized muscle cells are exposed to low extracellular [Ca²⁺] and that the opposite occurs in high [Ca²⁺]. It follows that murine VSMs are partly inactivated at rest, which establishes the reduced availability of voltage sensing as a pathogenic mechanism in disorders of calcemia. We additionally find that the degree of resting inactivation is significantly different in two mouse strains, which underscores the variability of voltage sensor properties and their vulnerability to environmental conditions. Our studies reveal that the resting and activated states of VSMs are equally favored by extracellular Ca²⁺. Promotion by an extracellular species of two states of the VSM that differ in the conformation of the activation gate requires the existence of a second gate, inactivation, topologically extracellular and therefore accessible from outside regardless of the activation state.

How and why are calcium currents curtailed in the skeletal muscle voltage-gated calcium channels?

Voltage‐gated calcium channels represent the sole mechanism converting electrical signals of excitable cells into cellular functions such as contraction, secretion and gene regulation. Specific voltage‐sensing domains detect changes in membrane potential and control channel gating. Calcium ions entering through the channel function as second messengers regulating cell functions, with the exception of skeletal muscle, where Ca_V1.1 essentially does not function as a channel but activates calcium release from intracellular stores. It has long been known that calcium currents are dispensable for skeletal muscle contraction. However, the questions as to how and why the channel function of Ca_V1.1 is curtailed remained obscure until the recent discovery of a developmental Ca_V1.1 splice variant with normal channel functions. This discovery provided new means to study the molecular mechanisms regulating the channel gating and led to the understanding that in skeletal muscle, calcium currents need to be restricted to allow proper regulation of fibre type specification and to prevent mitochondrial damage.

The Ca2+ influx through the mammalian skeletal muscle dihydropyridine receptor is irrelevant for muscle performance

Skeletal muscle excitation-contraction (EC) coupling is initiated by sarcolemmal depolarization, which is translated into a conformational change of the dihydropyridine receptor (DHPR), which in turn activates sarcoplasmic reticulum (SR) Ca²⁺ release to trigger muscle contraction. During EC coupling, the mammalian DHPR embraces functional duality, as voltage sensor and L-type Ca²⁺ channel. Although its unique role as voltage sensor for conformational EC coupling is firmly established, the conventional function as Ca²⁺ channel is still enigmatic. Here we show that Ca²⁺ influx via DHPR is not necessary for muscle performance by generating a knock-in mouse where DHPR-mediated Ca²⁺ influx is eliminated. Homozygous knock-in mice display SR Ca²⁺ release, locomotor activity, motor coordination, muscle strength and susceptibility to fatigue comparable to wild-type controls, without any compensatory regulation of multiple key proteins of the EC coupling machinery and Ca²⁺ homeostasis. These findings support the hypothesis that the DHPR-mediated Ca²⁺ influx in mammalian skeletal muscle is an evolutionary remnant.In mammalian skeletal muscle, the DHPR functions as a voltage sensor to trigger muscle contraction and as a Ca²⁺ channel. Here the authors show that mice where Ca²⁺ influx through the DHPR is eliminated display no difference in skeletal muscle function, suggesting that the Ca²⁺ influx through this channel is vestigial.

Temporal expression of calcium channel subunits in satellite cells and bone marrow mesenchymal cells

Bone marrow-derived mesenchymal stem cells (MSC) can be differentiated into myocytes, as well as adipocytes, chondrocytes, and osteocytes in culture. Calcium channels mediate excitation-contraction coupling and are essential for the function of muscle. However, little is known about the expression of calcium channel subunits and calcium handling in stem cells. We examined whether the expression of calcium channel subunits in MSC is similar to that of skeletal muscle satellite cells and if their levels of expression are modified after treatment with bone morphogenetic protein-4 (BMP4). We found that during myogenic differentiation, MSC first express the α2δ1 subunit and the cardiac channel subunit Cav1.2. In contrast to the α2δ1 subunit levels, the Cav1.2 subunit decreases rapidly with time. The skeletal channel subunit Cav1.1 is detected at day 3 but its expression increases considerably, resembling more closely the expression of the subunits in satellite cells. Treatment of MSC with BMP4 caused a significant increase in expression of Cav1.2, a delay in expression of Cav1.1, and a reduction in the duration of calcium transients when extracellular calcium was removed. Calcium currents and transients followed a pattern related to the expression of the cardiac (Cav1.2) or skeletal (Cav1.1) α1subunits. These results indicate that differentiation of untreated MSC resembles differentiation of skeletal muscle and that BMP4 reduces skeletal muscle calcium channel expression and promotes the expression of cardiac calcium channels during myogenic differentiation.

Rem uncouples excitation-contraction coupling in adult skeletal muscle fibers

The RGK protein Rem uncouples the voltage sensors of Ca_V1.1 from RYR1-mediated sarcoplasmic reticulum Ca²⁺ release via its ability to interact with the auxiliary β_1a subunit.

In skeletal muscle, excitation–contraction (EC) coupling requires depolarization-induced conformational rearrangements in L-type Ca²⁺ channel (Ca_V1.1) to be communicated to the type 1 ryanodine-sensitive Ca²⁺ release channel (RYR1) of the sarcoplasmic reticulum (SR) via transient protein–protein interactions. Although the molecular mechanism that underlies conformational coupling between Ca_V1.1 and RYR1 has been investigated intensely for more than 25 years, the question of whether such signaling occurs via a direct interaction between the principal, voltage-sensing α_1S subunit of Ca_V1.1 and RYR1 or through an intermediary protein persists. A substantial body of evidence supports the idea that the auxiliary β_1a subunit of Ca_V1.1 is a conduit for this intermolecular communication. However, a direct role for β_1a has been difficult to test because β_1a serves two other functions that are prerequisite for conformational coupling between Ca_V1.1 and RYR1. Specifically, β_1a promotes efficient membrane expression of Ca_V1.1 and facilitates the tetradic ultrastructural arrangement of Ca_V1.1 channels within plasma membrane–SR junctions. In this paper, we demonstrate that overexpression of the RGK protein Rem, an established β subunit–interacting protein, in adult mouse flexor digitorum brevis fibers markedly reduces voltage-induced myoplasmic Ca²⁺ transients without greatly affecting Ca_V1.1 targeting, intramembrane gating charge movement, or releasable SR Ca²⁺ store content. In contrast, a β_1a-binding–deficient Rem triple mutant (R200A/L227A/H229A) has little effect on myoplasmic Ca²⁺ release in response to membrane depolarization. Thus, Rem effectively uncouples the voltage sensors of Ca_V1.1 from RYR1-mediated SR Ca²⁺ release via its ability to interact with β_1a. Our findings reveal Rem-expressing adult muscle as an experimental system that may prove useful in the definition of the precise role of the β_1a subunit in skeletal-type EC coupling.

Ca(2+) permeation and/or binding to CaV1.1 fine-tunes skeletal muscle Ca(2+) signaling to sustain muscle function

Background

Ca²⁺ influx through Ca_V1.1 is not required for skeletal muscle excitation-contraction coupling, but whether Ca²⁺ permeation through Ca_V1.1 during sustained muscle activity plays a functional role in mammalian skeletal muscle has not been assessed.

Methods

We generated a mouse with a Ca²⁺ binding and/or permeation defect in the voltage-dependent Ca²⁺ channel, Ca_V1.1, and used Ca²⁺ imaging, western blotting, immunohistochemistry, proximity ligation assays, SUnSET analysis of protein synthesis, and Ca²⁺ imaging techniques to define pathways modulated by Ca²⁺ binding and/or permeation of Ca_V1.1. We also assessed fiber type distributions, cross-sectional area, and force frequency and fatigue in isolated muscles.

Results

Using mice with a pore mutation in Ca_V1.1 required for Ca²⁺ binding and/or permeation (E1014K, EK), we demonstrate that Ca_V1.1 opening is coupled to CaMKII activation and refilling of sarcoplasmic reticulum Ca²⁺ stores during sustained activity. Decreases in these Ca²⁺-dependent enzyme activities alter downstream signaling pathways (Ras/Erk/mTORC1) that lead to decreased muscle protein synthesis. The physiological consequences of the permeation and/or Ca²⁺ binding defect in Ca_V1.1 are increased fatigue, decreased fiber size, and increased Type IIb fibers.

Conclusions

While not essential for excitation-contraction coupling, Ca²⁺ binding and/or permeation via the Ca_V1.1 pore plays an important modulatory role in muscle performance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-014-0027-1) contains supplementary material, which is available to authorized users.

3D Structure of the Dihydropyridine Receptor of Skeletal Muscle

Excitation contraction coupling, the rapid and massive Ca²⁺ release under control of an action potential that triggers muscle contraction, takes places at specialized regions of the cell called triad junctions. There, a highly ordered supramolecular complex between the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR1) mediates the quasi-instantaneous conversion from T-tubule depolarization into Ca²⁺ release from the sarcoplasmic reticulum (SR). The DHPR has several key modules required for EC coupling: the voltage sensors and II-III loop in the alpha1s subunit, and the beta subunit. To gain insight into their molecular organization, this review examines the most updated 3D structure of the DHPR as obtained by transmission electron microscopy and image reconstruction. Although structure determination of a heteromeric membrane protein such as the DHPR is challenging, novel technical advances in protein expression and 3D labeling facilitated this task. The 3D structure of the DHPR complex consists of a main body with five irregular corners around its perimeter encompassing the transmembrane alpha 1s subunit besides the intracellular beta subunit, an extended extracellular alpha 2 subunit, and a bulky intracellular II-III loop. The structural definition attained at 19 Å resolution enabled docking of the atomic coordinates of structural homologs of the alpha1s and beta subunits. These structural features, together with their relative location with respect to the RyR1, are discussed in the context of the functional data.

Amino acid residues 489-503 of dihydropyridine receptor (DHPR) β1a subunit are critical for structural communication between the skeletal muscle DHPR complex and type 1 ryanodine receptor

The β1a subunit is a cytoplasmic component of the dihydropyridine receptor (DHPR) complex that plays an essential role in skeletal muscle excitation-contraction (EC) coupling. Here we investigate the role of the C-terminal end of this auxiliary subunit in the functional and structural communication between the DHPR and the Ca(2+) release channel (RyR1). Progressive truncation of the β1a C terminus showed that deletion of amino acid residues Gln(489) to Trp(503) resulted in a loss of depolarization-induced Ca(2+) release, a severe reduction of L-type Ca(2+) currents, and a lack of tetrad formation as evaluated by freeze-fracture analysis. However, deletion of this domain did not affect expression/targeting or density (Qmax) of the DHPR-α1S subunit to the plasma membrane. Within this motif, triple alanine substitution of residues Leu(496), Leu(500), and Trp(503), which are thought to mediate direct β1a-RyR1 interactions, weakened EC coupling but did not replicate the truncated phenotype. Therefore, these data demonstrate that an amino acid segment encompassing sequence (489)QVQVLTSLRRNLSFW(503) of β1a contains critical determinant(s) for the physical link of DHPR and RyR1, further confirming a direct correspondence between DHPR positioning and DHPR/RyR functional interactions. In addition, our data strongly suggest that the motif Leu(496)-Leu(500)-Trp(503) within the β1a C-terminal tail plays a nonessential role in the bidirectional DHPR/RyR1 signaling that supports skeletal-type EC coupling.

Voltage-gated calcium channels function as Ca2+-activated signaling receptors

Voltage-gated calcium channels (VGCCs) are transmembrane cell surface proteins responsible for multifunctional signals. In response to voltage, VGCCs trigger synaptic transmission, drive muscle contraction, and regulate gene expression. Voltage perturbations open VGCCs enabling Ca(2+) binding to the low affinity Ca(2+) binding site of the channel pore. Subsequent to permeation, Ca(2+) targets selective proteins to activate diverse signaling pathways. It is becoming apparent that the Ca(2+)-bound channel triggers secretion in excitable cells and drives contraction in cardiomyocytes prior to Ca(2+) permeation. Here, I highlight recent data implicating receptor-like function of the Ca(2+)-bound channel in converting external Ca(2+) into an intracellular signal. The two sequential mechanistic perspectives of VGCC function are discussed in the context of the prevailing and long-standing current models of depolarization-evoked secretion and cardiac contraction.

Impaired gating of an L-Type Ca(2+) channel carrying a mutation linked to malignant hyperthermia

Recently, we characterized the functional properties of a mutant skeletal muscle L-type Ca(2+) channel (CaV1.1 R174W) linked to the pharmacogenetic disorder malignant hyperthermia. Although the R174W mutation neutralizes the innermost basic amino acid in the voltage-sensing S4 helix of the first conserved membrane repeat of CaV1.1, the ability of the mutant channel to engage excitation-contraction coupling was largely unaffected by the introduction of the bulky tryptophan residue. In stark contrast, the mutation ablated the ability of CaV1.1 to produce L-type current under our standard recording conditions. In this study, we have investigated the mechanism of channel dysfunction more extensively. We found that CaV1.1 R174W will open and conduct Ca(2+) in response to strong or prolonged depolarizations in the presence of the 1,4-dihydropyridine receptor agonist ±Bay K 8644. From these results, we have concluded that the R174W mutation impedes entry into both mode 1(low Po) and mode 2 (high Po) gating states and that these gating impairments can be partially overcome by maneuvers that promote entry into mode 2.

Fibroblast growth factor homologous factors modulate cardiac calcium channels

RATIONALE

Fibroblast growth factor (FGF) homologous factors (FHFs; FGF11-14) are intracellular modulators of voltage-gated Na+ channels, but their cellular distribution in cardiomyocytes indicated that they performed other functions.

OBJECTIVE

We aimed to uncover novel roles for FHFs in cardiomyocytes, starting with a proteomic approach to identify novel interacting proteins.

METHODS AND RESULTS

Affinity purification of FGF13 from rodent ventricular lysates followed by mass spectroscopy revealed an interaction with junctophilin-2, a protein that organizes the close apposition of the L-type Ca2+ channel CaV1.2 and the ryanodine receptor 2 in the dyad. Immunocytochemical analysis revealed that overall T-tubule structure and localization of ryanodine receptor 2 were unaffected by FGF13 knockdown in adult ventricular cardiomyocytes but localization of CaV1.2 was affected. FGF13 knockdown decreased CaV1.2 current density and reduced the amount of CaV1.2 at the surface as a result of aberrant localization of the channels. CaV1.2 current density and channel localization were rescued by expression of an shRNA-insensitive FGF13, indicating a specific role for FGF13. Consistent with these newly discovered effects on CaV1.2, we demonstrated that FGF13 also regulated Ca(2+)-induced Ca2+ release, indicated by a smaller Ca2+ transient after FGF13 knockdown. Furthermore, FGF13 knockdown caused a profound decrease in the cardiac action potential half-width.

CONCLUSIONS

This study demonstrates that FHFs not only are potent modulators of voltage-gated Na+ channels but also affect Ca2+ channels and their function. We predict that FHF loss-of-function mutations would adversely affect currents through both Na+ and Ca2+ channels, suggesting that FHFs may be arrhythmogenic loci, leading to arrhythmias through a novel, dual-ion channel mechanism.

Ca(V)1.1: The atypical prototypical voltage-gated Ca²⁺ channel

Ca(V)1.1 is the prototype for the other nine known Ca(V) channel isoforms, yet it has functional properties that make it truly atypical of this group. Specifically, Ca(V)1.1 is expressed solely in skeletal muscle where it serves multiple purposes; it is the voltage sensor for excitation-contraction coupling and it is an L-type Ca²⁺ channel which contributes to a form of activity-dependent Ca²⁺ entry that has been termed Excitation-coupled Ca²⁺ entry. The ability of Ca(V)1.1 to serve as voltage-sensor for excitation-contraction coupling appears to be unique among Ca(V) channels, whereas the physiological role of its more conventional function as a Ca²⁺ channel has been a matter of uncertainty for nearly 50 years. In this chapter, we discuss how Ca(V)1.1 supports excitation-contraction coupling, the possible relevance of Ca²⁺ entry through Ca(V)1.1 and how alterations of Ca(V)1.1 function can have pathophysiological consequences. This article is part of a Special Issue entitled: Calcium channels.

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.

Skeletal myoblasts for cardiac repair

Stem cells provide an alternative curative intervention for the infarcted heart by compensating for the cardiomyocyte loss subsequent to myocardial injury. The presence of resident stem and progenitor cell populations in the heart, and nuclear reprogramming of somatic cells with genetic induction of pluripotency markers are the emerging new developments in stem cell-based regenerative medicine. However, until safety and feasibility of these cells are established by extensive experimentation in in vitro and in vivo experimental models, skeletal muscle-derived myoblasts, and bone marrow cells remain the most well-studied donor cell types for myocardial regeneration and repair. This article provides a critical review of skeletal myoblasts as donor cells for transplantation in the light of published experimental and clinical data, and indepth discussion of the advantages and disadvantages of skeletal myoblast-based therapeutic intervention for augmentation of myocardial function in the infarcted heart. Furthermore, strategies to overcome the problems of arrhythmogenicity and failure of the transplanted skeletal myoblasts to integrate with the host cardiomyocytes are discussed.

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

Non-Ca2+-conducting Ca2+ channels in fish skeletal muscle excitation-contraction coupling

During skeletal muscle excitation-contraction (EC) coupling, membrane depolarizations activate the sarcolemmal voltage-gated L-type Ca(2+) channel (Ca(V)1.1). Ca(V)1.1 in turn triggers opening of the sarcoplasmic Ca(2+) release channel (RyR1) via interchannel protein-protein interaction to release Ca(2+) for myofibril contraction. Simultaneously to this EC coupling process, a small and slowly activating Ca(2+) inward current through Ca(V)1.1 is found in mammalian skeletal myotubes. The role of this Ca(2+) influx, which is not immediately required for EC coupling, is still enigmatic. Interestingly, whole-cell patch clamp experiments on freshly dissociated skeletal muscle myotubes from zebrafish larvae revealed the lack of such Ca(2+) currents. We identified two distinct isoforms of the pore-forming Ca(V)1.1alpha(1S) subunit in zebrafish that are differentially expressed in superficial slow and deep fast musculature. Both do not conduct Ca(2+) but merely act as voltage sensors to trigger opening of two likewise tissue-specific isoforms of RyR1. We further show that non-Ca(2+) conductivity of both Ca(V)1.1alpha(1S) isoforms is a common trait of all higher teleosts. This non-Ca(2+) conductivity of Ca(V)1.1 positions teleosts at the most-derived position of an evolutionary trajectory. Though EC coupling in early chordate muscles is activated by the influx of extracellular Ca(2+), it evolved toward Ca(V)1.1-RyR1 protein-protein interaction with a relatively small and slow influx of external Ca(2+) in tetrapods. Finally, the Ca(V)1.1 Ca(2+) influx was completely eliminated in higher teleost fishes.

Effects of inserting fluorescent proteins into the alpha1S II-III loop: insights into excitation-contraction coupling

In skeletal muscle, intermolecular communication between the 1,4-dihydropyridine receptor (DHPR) and RYR1 is bidirectional: orthograde coupling (skeletal excitation-contraction coupling) is observed as depolarization-induced Ca(2+) release via RYR1, and retrograde coupling is manifested by increased L-type Ca(2+) current via DHPR. A critical domain (residues 720-765) of the DHPR alpha(1S) II-III loop plays an important but poorly understood role in bidirectional coupling with RYR1. In this study, we examine the consequences of fluorescent protein insertion into different positions within the alpha(1S) II-III loop. In four constructs, a cyan fluorescent protein (CFP)-yellow fluorescent protein (YFP) tandem was introduced in place of residues 672-685 (the peptide A region). All four constructs supported efficient bidirectional coupling as determined by the measurement of L-type current and myoplasmic Ca(2+) transients. In contrast, insertion of a CFP-YFP tandem within the N-terminal portion of the critical domain (between residues 726 and 727) abolished bidirectional signaling. Bidirectional coupling was partially preserved when only a single YFP was inserted between residues 726 and 727. However, insertion of YFP near the C-terminal boundary of the critical domain (between residues 760 and 761) or in the conserved C-terminal portion of the alpha(1S) II-III loop (between residues 785 and 786) eliminated bidirectional coupling. None of the fluorescent protein insertions, even those that interfered with signaling, significantly altered membrane expression or targeting. Thus, bidirectional signaling is ablated by insertions at two different sites in the C-terminal portion of the alpha(1S) II-III loop. Significantly, our results indicate that the conserved portion of the alpha(1S) II-III loop C terminal to the critical domain plays an important role in bidirectional coupling either by conveying conformational changes to the critical domain from other regions of the DHPR or by serving as a site of interaction with other junctional proteins such as RYR1.

The cardiac alpha(1C) subunit can support excitation-triggered Ca2+ entry in dysgenic and dyspedic myotubes

Depolarization-induced entry of divalent ions into skeletal muscle has been attributed to a process termed Excitation-Coupled Ca(2+) Entry (ECCE), which is hypothesized to require the interaction of the ryanodine receptor (RyR1), the L-type Ca(2+) channel (DHPR) and another unidentified cation channel. Thus, ECCE is absent in myotubes lacking either the DHPR (dysgenic) or RyR1 (dyspedic). Furthermore, ECCE, as measured by Mn(2+) quench of Fura-2, is reconstituted by expression of a mutant DHPR alpha(1S) subunit (SkEIIIK) thought to be impermeable to divalent cations. Previously, we showed that the bulk of depolarization-induced Ca(2+) entry could be explained by the skeletal L-type current. Accordingly, one would predict that any Ca(2+) current similar to the endogenous current would restore such entry and that this entry would not require coupling to either the DHPR or RyR1. Here, we show that expression of the cardiac alpha(1C) subunit in either dysgenic or dyspedic myotubes does result in Ca(2+) entry similar to that ascribed to ECCE. We also demonstrate that, when potentiated by strong depolarization and Bay K 8644, SkEIIIK supports entry of Mn(2+). These results strongly support the idea that the L-type channel is the major route of Ca(2+) entry in response to repetitive or prolonged depolarization of skeletal muscle.

Proper restoration of excitation-contraction coupling in the dihydropyridine receptor beta1-null zebrafish relaxed is an exclusive function of the beta1a subunit

The paralyzed zebrafish strain relaxed carries a null mutation for the skeletal muscle dihydropyridine receptor (DHPR) beta(1a) subunit. Lack of beta(1a) results in (i) reduced membrane expression of the pore forming DHPR alpha(1S) subunit, (ii) elimination of alpha(1S) charge movement, and (iii) impediment of arrangement of the DHPRs in groups of four (tetrads) opposing the ryanodine receptor (RyR1), a structural prerequisite for skeletal muscle-type excitation-contraction (EC) coupling. In this study we used relaxed larvae and isolated myotubes as expression systems to discriminate specific functions of beta(1a) from rather general functions of beta isoforms. Zebrafish and mammalian beta(1a) subunits quantitatively restored alpha(1S) triad targeting and charge movement as well as intracellular Ca(2+) release, allowed arrangement of DHPRs in tetrads, and most strikingly recovered a fully motile phenotype in relaxed larvae. Interestingly, the cardiac/neuronal beta(2a) as the phylogenetically closest, and the ancestral housefly beta(M) as the most distant isoform to beta(1a) also completely recovered alpha(1S) triad expression and charge movement. However, both revealed drastically impaired intracellular Ca(2+) transients and very limited tetrad formation compared with beta(1a). Consequently, larval motility was either only partially restored (beta(2a)-injected larvae) or not restored at all (beta(M)). Thus, our results indicate that triad expression and facilitation of 1,4-dihydropyridine receptor (DHPR) charge movement are common features of all tested beta subunits, whereas the efficient arrangement of DHPRs in tetrads and thus intact DHPR-RyR1 coupling is only promoted by the beta(1a) isoform. Consequently, we postulate a model that presents beta(1a) as an allosteric modifier of alpha(1S) conformation enabling skeletal muscle-type EC coupling.

The alpha(1S) III-IV loop influences 1,4-dihydropyridine receptor gating but is not directly involved in excitation-contraction coupling interactions with the type 1 ryanodine receptor

In skeletal muscle, coupling between the 1,4-dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) underlies excitation-contraction (EC) coupling. The III-IV loop of the DHPR alpha(1S) subunit binds to a segment of RyR1 in vitro, and mutations in the III-IV loop alter the voltage dependence of EC coupling, raising the possibility that this loop is directly involved in signal transmission from the DHPR to RyR1. To clarify the role of the alpha(1S) III-IV loop in EC coupling, we examined the functional properties of a chimera (GFP-alpha(1S)[III-IVa]) in which the III-IV loop of the divergent alpha(1A) isoform replaced that of alpha(1S). Dysgenic myotubes expressing GFP-alpha(1S)[III-IVa] yielded myoplasmic Ca(2+) transients that activated at approximately 10 mV more hyperpolarized potentials and that were approximately 65% smaller than those of GFP-alpha(1S). A similar reduction was observed in voltage-dependent charge movements for GFP-alpha(1S)[III-IVa], indicating that the chimeric channels trafficked less well to the membrane but that those that were in the membrane functioned as efficiently in EC coupling as GFP-alpha(1S). Relative to GFP-alpha(1S), L-type currents mediated by GFP-alpha(1S)[III-IVa] were approximately 40% smaller and activated at approximately 5 mV more hyperpolarized potentials. The altered gating of GFP-alpha(1S)[III-IVa] was accentuated by exposure to +/-Bay K 8644, which caused a much larger hyperpolarizing shift in activation compared with its effect on GFP-alpha(1S). Taken together, our observations indicate that the alpha(1S) III-IV loop is not directly involved in EC coupling but does influence DHPR gating transitions important both for EC coupling and activation of L-type conductance.

The calcium channel alpha2/delta1 subunit is involved in extracellular signalling

The alpha2/delta1 subunit forms part of the dihydropyridine receptor, an essential protein complex for excitation-contraction (EC) coupling in skeletal muscle. Because of the lack of a viable knock-out animal, little is known regarding the role of the alpha2/delta1 subunit in EC coupling or in other cell functions. Interestingly, the alpha2/delta1 appears before the alpha1 subunit in development and contains extracellular conserved domains known to be important in cell signalling and inter-protein interactions. These facts raise the possibility that the alpha2/delta1 subunit performs vital functions not associated with EC coupling. Here, we tested the hypothesis that the alpha2/delta1 subunit is important for interactions of muscle cells with their environment. Using confocal microscopy, we followed the immunolocalization of alpha2/delta1 and alpha1 subunits with age. We found that in 2-day-old myotubes, the alpha2/delta1 subunit concentrated towards the ends of the cells, while the alpha1 subunit clustered near the centre. As myotubes aged (6-12 days), the alpha2/delta1 became evenly distributed along the myotubes and co-localized with alpha1. When the expression of alpha2/delta1 was blocked with siRNA, migration, attachment and spreading of myoblasts were impaired while the L-type calcium current remained unaffected. The results suggest a previously unidentified role of the alpha2/delta1 subunit in skeletal muscle and support the involvement of this protein in extracellular signalling. This new role of the alpha2/delta1 subunit may be crucial for muscle development, muscle repair and at times in which myoblast attachment and migration are fundamental.

Accessibility of targeted DHPR sites to streptavidin and functional effects of binding on EC coupling

In skeletal muscle, the dihydropyridine receptor (DHPR) in the plasma membrane (PM) serves as a Ca(2+) channel and as the voltage sensor for excitation-contraction (EC coupling), triggering Ca(2+) release via the type 1 ryanodine receptor (RyR1) in the sarcoplasmic reticulum (SR) membrane. In addition to being functionally linked, these two proteins are also structurally linked to one another, but the identity of these links remains unknown. As an approach to address this issue, we have expressed DHPR alpha(1S) or beta(1a) subunits, with a biotin acceptor domain fused to targeted sites, in myotubes null for the corresponding, endogenous DHPR subunit. After saponin permeabilization, the approximately 60-kD streptavidin molecule had access to the beta(1a) N and C termini and to the alpha(1S) N terminus and proximal II-III loop (residues 671-686). Steptavidin also had access to these sites after injection into living myotubes. However, sites of the alpha(1S) C terminus were either inaccessible or conditionally accessible in saponin- permeabilized myotubes, suggesting that these C-terminal regions may exist in conformations that are occluded by other proteins in PM/SR junction (e.g., RyR1). The binding of injected streptavidin to the beta(1a) N or C terminus, or to the alpha(1S) N terminus, had no effect on electrically evoked contractions. By contrast, binding of streptavidin to the proximal alpha(1S) II-III loop abolished such contractions, without affecting agonist-induced Ca(2+) release via RyR1. Moreover, the block of EC coupling did not appear to result from global distortion of the DHPR and supports the hypothesis that conformational changes of the alpha(1S) II-III loop are necessary for EC coupling in skeletal muscle.

Bridging the myoplasmic gap: recent developments in skeletal muscle excitation–contraction coupling

Conformational coupling between the L-type voltage-gated Ca(2+) channel (or 1,4-dihydropyridine receptor; DHPR) and the ryanodine-sensitive Ca(2+) release channel of the sarcoplasmic reticulum (RyR1) is the mechanistic basis for excitation-contraction (EC) coupling in skeletal muscle. In this article, recent findings regarding the roles of the individual cytoplasmic domains (the amino- and carboxyl-termini, cytoplasmic loops I-II, II-III, and III-IV) of the DHPR alpha(1S) subunit in bi-directional communication with RyR1 will be discussed.

Isoform switching in myofibrillar and excitation-contraction coupling proteins contributes to diminished contractile function in regenerating rat soleus muscle

Postnatal development of skeletal muscle occurs through the progressive transformation of diverse biochemical, metabolic, morphological, and functional characteristics from the embryonic to the adult phenotype. Since muscle regeneration recapitulates postnatal development of muscle fiber, it offers an appropriate experimental model to investigate the existing relationships between diverse muscle functions and the expression of key protein isoforms, particularly at the single-fiber level. This study was carried out in regenerating soleus muscle 14 days after injury. At this intermediate stage, the regenerating muscle exhibited a recovery of mass greater than its force generation capacity. The lower specific tension of regenerating muscle suggested intrinsic defective excitation-contraction coupling and/or contractility processes. The presence of developmental isoforms of both the voltage-gated Ca2+channel (α1C) and of ryanodine receptor 3, paralleled by an abnormal caffeine contracture development, confirms the immature excitation-contraction coupling of the regenerating muscle. The defective Ca2+handling could also be confirmed by the lower sarcoplasmic reticulum caffeine sensitivity of regenerating single fibers. Also, regenerating single fibers revealed a lower maximal specific tension, which was associated with the residual presence of embryonic myosin heavy chains. Moreover, the fibers showed a reduced Ca2+sensitivity of myofibrillar proteins, particularly those simultaneously expressing the slow and fast isoforms of troponin C. The present results indicate that the expression of developmental proteins determines the incomplete functional recovery of regenerating soleus.

Changes in the functioning of the electromechanical connection during tetanic contraction

The functioning of the electromechanical connection during tetanic contraction in frog skeletal muscle was studied. Analysis using caffeine, calcium-free medium, the ryanodine receptor blocker dantrolene, and the Ca-ATPase inhibitor thapsigargin showed that the initial increase in tetanus, as in twitch contractions, did not require the presence of calcium ions in the surrounding medium, which is in agreement with published data. Contraction was accompanied by activation of potential-dependent release of calcium from the sarcoplasmic reticulum. In contrast, the secondary rise phase and/or the duration of the tetanus plateau were critically dependent on the present of Ca2+ in the surrounding medium. Given that contraction in this situation was inhibited by dantrolene, activation of prolonged contraction was also mediated by calcium released from the sarcoplasmic reticulum, though ryanodine receptors were now activated not by changes in the membrane potential but by the influx of external calcium. Thus, external calcium plays a significant role in the formation of prolonged contractile responses, providing for longer-lasting maintenance of power in contracted muscles.

Reconstitution of local Ca2+ signaling between cardiac L-type Ca2+ channels and ryanodine receptors: insights into regulation by FKBP12.6

Ca+-induced Ca2+ release (CICR) in the heart involves local Ca2+ signaling between sarcolemmal L-type Ca2+ channels (dihydropyridine receptors, DHPRs) and type 2 ryanodine receptors (RyR2s) in the sarcoplasmic reticulum (SR). We reconstituted cardiac-like CICR by expressing a cardiac dihydropyridine-insensitive (T1066Y/Q1070M) α1-subunit (α1CYM) and RyR2 in myotubes derived from RyR1-knockout (dyspedic) mice. Myotubes expressing α1CYM and RyR2 were vesiculated and exhibited spontaneous Ca2+ oscillations that resulted in chaotic and uncontrolled contractions. Coexpression of FKBP12.6 (but not FKBP12.0) with α1CYM and RyR2 eliminated vesiculations and reduced the percentage of myotubes exhibiting uncontrolled global Ca2+ oscillations (63% and 13% of cells exhibited oscillations in the absence and presence of FKBP12.6, respectively). α1CYM/RyR2/FKBP12.6-expressing myotubes exhibited robust and rapid electrically evoked Ca2+ transients that required extracellular Ca2+. Depolarization-induced Ca2+ release in α1CYM/RyR2/FKBP12.6-expressing myotubes exhibited a bell-shaped voltage dependence that was fourfold larger than that of myotubes expressing α1CYM alone (maximal fluorescence change was 2.10 ± 0.39 and 0.54 ± 0.07, respectively), despite similar Ca2+ current densities. In addition, the gain of CICR in α1CYM/RyR2/FKBP12.6-expressing myotubes exhibited a nonlinear voltage dependence, being considerably larger at threshold potentials. We used this molecular model of local α1C-RyR2 signaling to assess the ability of FKBP12.6 to inhibit spontaneous Ca2+ release via a phosphomimetic mutation in RyR2 (S2808D). Electrically evoked Ca2+ release and the incidence of spontaneous Ca2+ oscillations did not differ in wild-type RyR2- and S2808D-expressing myotubes over a wide range of FKBP12.6 expression. Thus a negative charge at S2808 does not alter in situ regulation of RyR2 by FKBP12.6.

Multiple loops of the dihydropyridine receptor pore subunit are required for full-scale excitation-contraction coupling in skeletal muscle

Understanding which cytosolic domains of the dihydropyridine receptor participate in excitation-contraction (EC) coupling is critical to validate current structural models. Here we quantified the contribution to skeletal-type EC coupling of the alpha1S (CaV1.1) II-III loop when alone or in combination with the rest of the cytosolic domains of alpha1S. Chimeras consisting of alpha1C (CaV1.2) with alpha1S substitutions at each of the interrepeat loops (I-II, II-III, and III-IV loops) and N- and C-terminal domains were evaluated in dysgenic (alpha1S-null) myotubes for phenotypic expression of skeletal-type EC coupling. Myotubes were voltage-clamped, and Ca2+ transients were measured by confocal line-scan imaging of fluo-4 fluorescence. In agreement with previous results, the alpha1C/alpha1S II-III loop chimera, but none of the other single-loop chimeras, recovered a sigmoidal fluorescence-voltage curve indicative of skeletal-type EC coupling. To quantify Ca2+ transients in the absence of inward Ca2+ current, but without changing the external solution, a mutation, E736K, was introduced into the P-loop of repeat II of alpha1C. The Ca2+ transients expressed by the alpha1C(E736K)/alpha1S II-III loop chimera were approximately 70% smaller than those expressed by the Ca2+-conducting alpha1C/alpha1S II-III variant. The low skeletal-type EC coupling expressed by the alpha1C/alpha1S II-III loop chimera was confirmed in the Ca2+-conducting alpha1C/alpha1S II-III loop variant using Cd2+ (10(-4) M) as the Ca2+ current blocker. In contrast to the behavior of the II-III loop chimera, Ca2+ transients expressed by an alpha1C/alpha1S chimera carrying all tested skeletal alpha1S domains (all alpha1S interrepeat loops, N- and C-terminus) were similar in shape and amplitude to wild-type alpha1S, and did not change in the presence of the E736K mutation or in the presence of 10(-4) M Cd2+. Controls indicated that similar dihydropyridine receptor charge movements were expressed by the non-Ca2+ permeant alpha1S(E1014K) variant, the alpha1C(E736K)/alpha1S II-III loop chimera, and the alpha1C(E736K)/alpha1S chimera carrying all tested alpha1S domains. The data indicate that the functional recovery produced by the alpha1S II-III loop is incomplete and that multiple cytosolic domains of alpha1S are necessary for a quantitative recovery of the EC-coupling phenotype of skeletal myotubes. Thus, despite the importance of the II-III loop there may be other critical determinants in alpha1S that influence the efficiency of EC coupling.

Differential effects of maurocalcine on Ca2+ release events and depolarization-induced Ca2+ release in rat skeletal muscle

Maurocalcine (MCa), a 33 amino acid toxin obtained from scorpion venom, has been shown to interact with the isolated skeletal-type ryanodine receptor (RyR1) and to strongly modify its calcium channel gating. In this study, we explored the effects of MCa on RyR1 in situ to establish whether the functional interaction of RyR1 with the voltage-sensing dihydropyridine receptor (DHPR) would modify the ability of MCa to interact with RyR1. In developing skeletal muscle cells the addition of MCa into the external medium induced a calcium transient resulting from RyR1 activation and strongly inhibited the effect of the RyR1 agonist chloro-m-cresol. In contrast, MCa failed to affect the depolarization-induced Ca(2+) release. In intact adult fibres MCa did not induce any change in the cytosolic Ca(2+) concentration. However, when the surface membrane was permeabilized and calcium release events were readily observable, MCa had a time-dependent dual effect: it first increased event frequency, from 0.060 +/- 0.002 to 0.150 +/- 0.007 sarcomere(-1) s(-1), and reduced the amplitude of individual events without modifying their spatial distribution. Later on it induced the appearance of long-lasting events resembling the embers observed in control conditions but having a substantially longer duration. We propose that the functional coupling of DHPRs and RyR1s within a Ca(2+) release unit prevents MCa from either reaching its binding site or from being able to modify the gating not only of the RyR1s physically coupled to DHPRs but all RyR1s within the Ca(2+) release unit.

Reprogramming autologous skeletal myoblasts to express cardiomyogenic function. Challenges and possible approaches

Cell transplantation therapy is emerging as a promising mode of treatment following myocardial infarction. Of the various cell types that can potentially be used for transplantation, autologous skeletal myoblasts appear particularly attractive, because this would avoid issues of immunogenicity, tumorigenesis, ethics and donor availability. Additionally, skeletal myoblasts display much higher levels of ischemic tolerance and graft survival compared to other cell types. There is some evidence for improvement in heart function with skeletal myoblast transplantation. However, histological analysis revealed that transplanted myoblasts do not transdifferentiate into functional cardiomyocytes in situ. This is evident by the lack of expression of cardiac-specific antigens, and the absence of intercalated disc formation. Instead, there is differentiation into myotubes that are not electromechanically coupled to neighboring cardiomyocytes. This could in turn limit the clinical efficacy of treatment. This review would therefore examine the various challenges faced in attempting to reprogram autologous skeletal myoblast to express cardiomyogenic function, together with the various possible strategies that could be employed to achieve this objective.

Involvement of a heptad repeat in the carboxyl terminus of the dihydropyridine receptor beta1a subunit in the mechanism of excitation-contraction coupling in skeletal muscle

Chimeras consisting of the homologous skeletal dihydropyridine receptor (DHPR) beta1a subunit and the heterologous cardiac/brain beta2a subunit were used to determine which regions of beta1a were responsible for the skeletal-type excitation-contraction (EC) coupling phenotype. Chimeras were transiently transfected in beta1 knockout myotubes and then voltage-clamped with simultaneous measurement of confocal fluo-4 fluorescence. All chimeras expressed a similar density of DHPR charge movements, indicating that the membrane density of DHPR voltage sensors was not a confounding factor in these studies. The data indicates that a beta1a-specific domain present in the carboxyl terminus, namely the D5 region comprising the last 47 residues (beta1a 478-524), is essential for expression of skeletal-type EC coupling. Furthermore, the location of beta1aD5 immediately downstream from conserved domain D4 is also critical. In contrast, chimeras in which beta1aD5 was swapped by the D5 region of beta2a expressed Ca(2+) transients triggered by the Ca(2+) current, or none at all. A hydrophobic heptad repeat is present in domain D5 of beta1a (L478, V485, V492). To determine the role of this motif, residues in the heptad repeat were mutated to alanines. The triple mutant beta1a(L478A/V485A/V492A) recovered weak skeletal-type EC coupling (DeltaF/F(max) = 0.4 +/- 0.1 vs. 2.7 +/- 0.5 for wild-type beta1a). However, a triple mutant with alanine substitutions at positions out of phase with the heptad repeat, beta1a(S481A/L488A/S495A), was normal (DeltaF/F(max) = 2.1 +/- 0.4). In summary, the presence of the beta1a-specific D5 domain, in its correct position after conserved domain D4, is essential for skeletal-type EC coupling. Furthermore, a heptad repeat in beta1aD5 controls the EC coupling activity. The carboxyl terminal heptad repeat of beta1a might be involved in protein-protein interactions with ryanodine receptor type 1 required for DHPR to ryanodine receptor type 1 signal transmission.

Functional interaction of CaV channel isoforms with ryanodine receptors studied in dysgenic myotubes

The L-type Ca(2+) channels Ca(V)1.1 (alpha(1S)) and Ca(V)1.2 (alpha(1C)) share properties of targeting but differ by their mode of coupling to ryanodine receptors in muscle cells. The brain isoform Ca(V)2.1 (alpha(1A)) lacks ryanodine receptor targeting. We studied these three isoforms in myotubes of the alpha(1S)-deficient skeletal muscle cell line GLT under voltage-clamp conditions and estimated the flux of Ca(2+) (Ca(2+) input flux) resulting from Ca(2+) entry and release. Surprisingly, amplitude and kinetics of the input flux were similar for alpha(1C) and alpha(1A) despite a previously reported strong difference in responsiveness to extracellular stimulation. The kinetic flux characteristics of alpha(1C) and alpha(1A) resembled those in alpha(1S)-expressing cells but the contribution of Ca(2+) entry was much larger. alpha(1C) but not alpha(1A)-expressing cells revealed a distinct transient flux component sensitive to sarcoplasmic reticulum depletion by 30 microM cyclopiazonic acid and 10 mM caffeine. This component likely results from synchronized Ca(2+)-induced Ca(2+) release that is absent in alpha(1A)-expressing myotubes. In cells expressing an alpha(1A)-derivative (alpha(1)Aas(1592-clip)) containing the putative targeting sequence of alpha(1S), a similar transient component was noticeable. Yet, it was considerably smaller than in alpha(1C), indicating that the local Ca(2+) entry produced by the chimera is less effective in triggering Ca(2+) release despite similar global Ca(2+) inward current density.

External Ca(2+)-dependent excitation--contraction coupling in a population of ageing mouse skeletal muscle fibres

In the present work, we investigate whether changes in excitation-contraction (EC) coupling mode occur in skeletal muscles from ageing mammals by examining the dependence of EC coupling on extracellular Ca(2+). Single intact muscle fibres from flexor digitorum brevis muscles from young (2-6 months) and old (23-30 months) mice were subjected to tetanic contractile protocols in the presence and absence of external Ca(2+). Contractile experiments in the absence of external Ca(2+) show that about half of muscle fibres from old mice are dependent upon external Ca(2+) for maintaining maximal tetanic force output, while young fibres are not. Decreased force in the absence of external Ca(2+) was not due to changes in charge movement as revealed by whole-cell patch-clamp experiments. Ca(2+) transients, measured by fluo-4 fluorescence, declined in voltage-clamped fibres from old mice in the absence of external Ca(2+). Similarly, Ca(2+) transients declined in parallel with tetanic contractile force in single intact fibres. Examination of inward Ca(2+) current and of mRNA and protein assays suggest that these changes in EC coupling mode are not due to shifts in dihydropyridine receptor (DHPR) and/or ryanodine receptor (RyR) isoforms. These results indicate that a change in EC coupling mode occurs in a population of fibres in ageing skeletal muscle, and is responsible for the age-related dependence on extracellular Ca(2+).

Metabolic biotinylation as a probe of supramolecular structure of the triad junction in skeletal muscle

Excitation-contraction coupling in skeletal muscle involves conformational coupling between dihydropyridine receptors (DHPRs) in the plasma membrane and ryanodine receptors (RyRs) in the sarcoplasmic reticulum. However, it remains uncertain what regions, if any, of the two proteins interact with one another. Toward this end, it would be valuable to know the spatial interrelationships of DHPRs and RyRs within plasma membrane/sarcoplasmic reticulum junctions. Here we describe a new approach based on metabolic incorporation of biotin into targeted sites of the DHPR. To accomplish this, cDNAs were constructed with a biotin acceptor domain (BAD) fused to selected sites of the DHPR, with fluorescent protein (XFP) attached at a second site. All of the BAD-tagged constructs properly targeted to junctions (as indicted by small puncta of XFP) and were functional for excitation-contraction coupling. To determine whether the introduced BAD was biotinylated and accessible to avidin (approximately 60 kDa), myotubes were fixed, permeablized, and exposed to fluorescently labeled avidin. Upon expression in beta1-null or dysgenic (alpha1S-null) myotubes, punctate avidin fluorescence co-localized with the XFP puncta for BAD attached to the beta1a N- or C-terminals, or the alpha1S N-terminal or II-III loop. However, BAD fused to the alpha1S C-terminal was inaccessible to avidin in dysgenic myotubes (containing RyR1). In contrast, this site was accessible to avidin when the identical construct was expressed in dyspedic myotubes lacking RyR1. These results indicate that avidin has access to a number of sites of the DHPR within fully assembled (RyR1-containing) junctions, but not to the alpha1S C-terminal, which appears to be occluded by the presence of RyR1.

Ca2+-dependent excitation-contraction coupling triggered by the heterologous cardiac/brain DHPR beta2a-subunit in skeletal myotubes

Molecular determinants essential for skeletal-type excitation-contraction (EC) coupling have been described in the cytosolic loops of the dihydropyridine receptor (DHPR) alpha1S pore subunit and in the carboxyl terminus of the skeletal-specific DHPR beta1a-subunit. It is unknown whether EC coupling domains present in the beta-subunit influence those present in the pore subunit or if they act independent of each other. To address this question, we investigated the EC coupling signal that is generated when the endogenous DHPR pore subunit alpha1S is paired with the heterologous heart/brain DHPR beta2a-subunit. Studies were conducted in primary cultured myotubes from beta1 knockout (KO), ryanodine receptor type 1 (RyR1) KO, ryanodine receptor type 3 (RyR3) KO, and double RyR1/RyR3 KO mice under voltage clamp with simultaneous monitoring of confocal fluo-4 fluorescence. The beta2a-mediated Ca2+ current recovered in beta1 KO myotubes lacking the endogenous DHPR beta1a-subunit verified formation of the alpha1S/beta1a pair. In myotube genotypes which express no or low-density L-type Ca2+ currents, namely beta1 KO and RyR1 KO, beta2a overexpression recovered a wild-type density of nifedipine-sensitive Ca2+ currents with a slow activation kinetics typical of skeletal myotubes. Concurrent with Ca2+ current recovery, there was a drastic reduction of voltage-dependent, skeletal-type EC coupling and emergence of Ca2+ transients triggered by the Ca2+ current. A comparison of beta2a overexpression in RyR3 KO, RyR1 KO, and double RyR1/RyR3 KO myotubes concluded that both RyR1 and RyR3 isoforms participated in Ca2+-dependent Ca2+ release triggered by the beta2a-subunit. In beta1 KO and RyR1 KO myotubes, the Ca2+-dependent EC coupling promoted by beta2a overexpression had the following characteristics: 1), L-type Ca2+ currents had a wild-type density; 2), Ca2+ transients activated much slower than controls overexpressing beta1a, and the rate of fluorescence increase was consistent with the activation kinetics of the Ca2+ current; 3), the voltage dependence of the Ca2+ transient was bell-shaped and the maximum was centered at approximately +30 mV, consistent with the voltage dependence of the Ca2+ current; and 4), Ca2+ currents and Ca2+ transients were fully blocked by nifedipine. The loss in voltage-dependent EC coupling promoted by beta2a was inferred by the drastic reduction in maximal Ca2+ fluorescence at large positive potentials (DeltaF/Fmax) in double dysgenic/beta1 KO myotubes overexpressing the pore mutant alpha1S (E1014K) and beta2a. The data indicate that beta2a, upon interaction with the skeletal pore subunit alpha1S, overrides critical EC coupling determinants present in alpha1S. We propose that the alpha1S/beta pair, and not the alpha1S-subunit alone, controls the EC coupling signal in skeletal muscle.

Determination of depolarisation- and agonist-evoked calcium fluxes on skeletal muscle cells in primary culture

Changes in intracellular calcium concentration ([Ca2+]i) evoked by prolonged depolarisation (120 mM KCl) or by the application of 15 mM caffeine were measured on skeletal muscle cells in primary culture. The extrusion rate (PVmax) of calcium from the myoplasm was determined, which in turn enabled the calculation of the calcium flux (Fl) underlying the measured calcium transients. PVmax was found to increase during differentiation, from 107 +/- 10 microM/s at the early myotube stage to 596 +/- 36 microM/s in secondary myotubes. This was paralleled by a decrease in resting [Ca2+]i from 99 +/- 4 to 51 +/- 2 nM. The depolarisation-evoked Fl rose to peak and then ceased despite the continuous presence of KCl. In contrast, the caffeine-induced Fl showed a peak and a clear steady-level with a peak-to-steady ratio of 5.6 +/- 1.2. Removal of external calcium suppressed the depolarisation--induced flux by 88 +/- 5% indicating that both an influx and a release from the SR underlie the K(+)-evoked calcium transients. Subsequent applications of caffeine resulted in essentially identical fluxes indicating an efficient refilling of the internal stores. Moreover, if a depolarisation-induced calcium transient preceded the second caffeine-evoked release, the latter was significantly larger than the first suggesting that much of the calcium that entered was stored in the SR rather than extruded.

Structural requirements of the dihydropyridine receptor alpha1S II-III loop for skeletal-type excitation-contraction coupling

Residues Leu720-Leu764 within the II-III loop of the skeletal muscle dihydropyridine receptor (DHPR) alpha1S subunit represent a critical domain for the orthograde excitation-contraction coupling as well as for retrograde DHPR L-current-enhancing coupling with the ryanodine receptor (RyR1). To better understand the molecular mechanism underlying this bidirectional DHPR-RyR1 signaling interaction, we analyzed the critical domain to the single amino acid level. To this end, constructs based on the highly dissimilar housefly DHPR II-III loop in an otherwise skeletal DHPR as an interaction-inert sequence background were expressed in dysgenic (alpha1S-null) myotubes for simultaneous recordings of depolarization-induced intracellular Ca2+ transients (orthograde coupling) and whole-cell Ca2+ currents (retrograde coupling). In the minimal skeletal II-III loop sequence (Asp734-Asp748 required for full bidirectional coupling, eight amino acids heterologous between skeletal and cardiac DHPR were exchanged for the corresponding cardiac residues. Four of these skeletal-specific residues (Ala739, Phe741, Pro742, and Asp744) turned out to be essential for orthograde and two of them (Ala739 and Phe741) for retrograde coupling, indicating that orthograde coupling does not necessarily correlate with retrograde signaling. Secondary structure predictions of the critical domain show that an alpha-helical (cardiac sequence-type) conformation of a cluster of negatively charged residues (Asp744-Glu751 of alpha1S) corresponds with significantly reduced Ca2+ transients. Conversely, a predicted random coil structure (skeletal sequence-type) seems to be prerequisite for the restoration of skeletal-type excitation-contraction coupling. Thus, not only the primary but also the secondary structure of the critical domain is an essential determinant of the tissue-specific mode of EC coupling.

Depolarization-induced Ca2+ release in ischemic spinal cord white matter involves L-type Ca2+ channel activation of ryanodine receptors

The mechanisms of Ca(2+) release from intracellular stores in CNS white matter remain undefined. In rat dorsal columns, electrophysiological recordings showed that in vitro ischemia caused severe injury, which persisted after removal of extracellular Ca(2+); Ca(2+) imaging confirmed that an axoplasmic Ca(2+) rise persisted in Ca(2+)-free perfusate. However, depletion of Ca(2+) stores or reduction of ischemic depolarization (low Na(+), TTX) were protective, but only in Ca(2+)-free bath. Ryanodine or blockers of L-type Ca(2+) channel voltage sensors (nimodipine, diltiazem, but not Cd(2+)) were also protective in zero Ca(2+), but their effects were not additive with ryanodine. Immunoprecipitation revealed an association between L-type Ca(2+) channels and RyRs, and immunohistochemistry confirmed colocalization of Ca(2+) channels and RyR clusters on axons. Similar to "excitation-contraction coupling" in skeletal muscle, these results indicate a functional coupling whereby depolarization sensed by L-type Ca(2+) channels activates RyRs, thus releasing damaging amounts of Ca(2+) under pathological conditions in white matter.

Cardiac-type EC-coupling in dysgenic myotubes restored with Ca2+ channel subunit isoforms alpha1C and alpha1D does not correlate with current density

Ca(2+)-induced Ca(2+)-release (CICR)-the mechanism of cardiac excitation-contraction (EC) coupling-also contributes to skeletal muscle contraction; however, its properties are still poorly understood. CICR in skeletal muscle can be induced independently of direct, calcium-independent activation of sarcoplasmic reticulum Ca(2+) release, by reconstituting dysgenic myotubes with the cardiac Ca(2+) channel alpha(1C) (Ca(V)1.2) subunit. Ca(2+) influx through alpha(1C) provides the trigger for opening the sarcoplasmic reticulum Ca(2+) release channels. Here we show that also the Ca(2+) channel alpha(1D) isoform (Ca(V)1.3) can restore cardiac-type EC-coupling. GFP-alpha(1D) expressed in dysgenic myotubes is correctly targeted into the triad junctions and generates action potential-induced Ca(2+) transients with the same efficiency as GFP-alpha(1C) despite threefold smaller Ca(2+) currents. In contrast, GFP-alpha(1A), which generates large currents but is not targeted into triads, rarely restores action potential-induced Ca(2+) transients. Thus, cardiac-type EC-coupling in skeletal myotubes depends primarily on the correct targeting of the voltage-gated Ca(2+) channels and less on their current size. Combined patch-clamp/fluo-4 Ca(2+) recordings revealed that the induction of Ca(2+) transients and their maximal amplitudes are independent of the different current densities of GFP-alpha(1C) and GFP-alpha(1D). These properties of cardiac-type EC-coupling in dysgenic myotubes are consistent with a CICR mechanism under the control of local Ca(2+) gradients in the triad junctions.

Ca2+ current and charge movements in skeletal myotubes promoted by the beta-subunit of the dihydropyridine receptor in the absence of ryanodine receptor type 1

The beta-subunit of the dihydropyridine receptor (DHPR) enhances the Ca(2+) channel and voltage-sensing functions of the DHPR. In skeletal myotubes, there is additional modulation of DHPR functions imposed by the presence of ryanodine receptor type-1 (RyR1). Here, we examined the participation of the beta-subunit in the expression of L-type Ca(2+) current and charge movements in RyR1 knock-out (KO), beta1 KO, and double beta1/RyR1 KO myotubes generated by mating heterozygous beta1 KO and RyR1 KO mice. Primary myotube cultures of each genotype were transfected with various beta-isoforms and then whole-cell voltage-clamped for measurements of Ca(2+) and gating currents. Overexpression of the endogenous skeletal beta1a isoform resulted in a low-density Ca(2+) current either in RyR1 KO (36 +/- 9 pS/pF) or in beta1/RyR1 KO (34 +/- 7 pS/pF) myotubes. However, the heterologous beta2a variant with a double cysteine motif in the N-terminus (C3, C4), recovered a Ca(2+) current that was entirely wild-type in density in RyR1 KO (195 +/- 16 pS/pF) and was significantly enhanced in double beta1/RyR1 KO (115 +/- 18 pS/pF) myotubes. Other variants tested from the four beta gene families (beta1a, beta1b, beta1c, beta3, and beta4) were unable to enhance Ca(2+) current expression in RyR1 KO myotubes. In contrast, intramembrane charge movements in beta2a-expressing beta1a/RyR1 KO myotubes were significantly lower than in beta1a-expressing beta1a/RyR1 KO myotubes, and the same tendency was observed in the RyR1 KO myotube. Thus, beta2a had a preferential ability to recover Ca(2+) current, whereas beta1a had a preferential ability to rescue charge movements. Elimination of the double cysteine motif (beta2a C3,4S) eliminated the RyR1-independent Ca(2+) current expression. Furthermore, Ca(2+) current enhancement was observed with a beta2a variant lacking the double cysteine motif and fused to the surface membrane glycoprotein CD8. Thus, tethering the beta2a variant to the myotube surface activated the DHPR Ca(2+) current and bypassed the requirement for RyR1. The data suggest that the Ca(2+) current expressed by the native skeletal DHPR complex has an inherently low density due to inhibitory interactions within the DHPR and that the beta1a-subunit is critically involved in process.

Voltage-activated calcium signals in myotubes loaded with high concentrations of EGTA

In the present study we describe the analysis of optically recorded whole cell Ca(2+) transients elicited by depolarization in cultured skeletal myotubes. Myotubes were obtained from the mouse muscle-derived cell line C2C12 and from mouse satellite cells. The cells were voltage-clamped and perfused with an artificial intracellular solution containing 15 mM EGTA to ensure that the bulk of the Ca(2+) mobilized by depolarization is bound to this extrinsic buffer. The apparent on- and off-rate constants of EGTA and the dissociation rate constant of fura-2 in the cell were estimated by investigating the Ca(2+)-dependence of kinetic components of the fluorescence decay after repolarization. These parameters were used to calculate the time course of the total voltage-controlled flux of Ca(2+) to the myoplasmic space (Ca(2+) input flux). The validity of the procedure was confirmed by model simulations using artificial Ca(2+) input fluxes. Both C2C12 and primary-cultured myotubes showed a very similar phasic-tonic time course of the Ca(2+) input flux. In most measurements, the input flux was considerably larger and showed a different time course than the estimated Ca(2+) flux carried by the L-type Ca(2+) channels, indicating that it consists mainly of voltage-controlled Ca(2+) release from the sarcoplasmic reticulum. In cells with extremely small fluorescence transients, the calculated input fluxes matched the kinetic characteristics of the Ca(2+) inward current, indicating that Ca(2+) release was absent. These measurements served as a control for the fidelity of the fluorimetric flux analysis. The procedures promise a deeper insight into alterations of Ca(2+) release gating in studies employing myotube expression systems for mutant or chimeric protein components of excitation-contraction coupling.

Truncation of the carboxyl terminus of the dihydropyridine receptor beta1a subunit promotes Ca2+ dependent excitation-contraction coupling in skeletal myotubes

We investigated the contribution of the carboxyl terminus region of the beta1a subunit of the skeletal dihydropyridine receptor (DHPR) to the mechanism of excitation-contraction (EC) coupling. cDNA-transfected beta1 KO myotubes were voltage clamped, and Ca(2+) transients were analyzed by confocal fluo-4 fluorescence. A chimera with an amino terminus half of beta2a and a carboxyl terminus half of beta1a (beta2a 1-287/beta1a 325-524) recapitulates skeletal-type EC coupling quantitatively and was used to generate truncated variants lacking 7 to 60 residues from the beta1a-specific carboxyl terminus (Delta7, Delta21, Delta29, Delta35, and Delta60). Ca(2+) transients recovered by the control chimera have a sigmoidal Ca(2+) fluorescence (DeltaF/F) versus voltage curve with saturation at potentials more positive than +30 mV, independent of external Ca(2+) and stimulus duration. In contrast, the amplitude of Ca(2+) transients expressed by the truncated variants varied with the duration of the pulse, and for Delta29, Delta35, and Delta60, also varied with external Ca(2+) concentration. For Delta7 and Delta21, a 50-ms depolarization produced a sigmoidal DeltaF/F versus voltage curve with a lower than control maximum fluorescence. Moreover, for Delta29, Delta35, and Delta60, a 200-ms depolarization increased the maximum fluorescence and changed the shape of the DeltaF/F versus voltage curve, from sigmoidal to bell-shaped, with a maximum at approximately +30 mV. The change in voltage dependence, together with the external Ca(2+) dependence and additional controls with ryanodine, indicated a loss of skeletal-type EC coupling and the emergence of an EC coupling component triggered by the Ca(2+) current. Analyses of d(DeltaF/F)/dt showed that the rate of cytosolic Ca(2+) increase during the Ca(2+) transient was fivefold faster for the control chimera than for the severely truncated variants (Delta29, Delta35, and Delta60) and was consistent with the kinetics of the DHPR Ca(2+) current. In summary, absence of the beta1a-specific carboxyl terminus (last 29 to 60 residues of the control chimera) results in a loss of the fast component of the Ca(2+) transient, bending of the DeltaF/F versus voltage curve, and emergence of EC coupling triggered by the Ca(2+) current. The studies underscore the essential role of the carboxyl terminus region of the DHPR beta1a subunit in fast voltage dependent EC coupling in skeletal myotubes.

Dissociation of charge movement from calcium release and calcium current in skeletal myotubes by gabapentin

The skeletal muscle L-type calcium channel or dihydropyridine receptor (DHPR) plays an integral role in excitation-contraction (E-C) coupling. Its activation initiates three sequential events: charge movement (Q(r)), calcium release, and calcium current (I(Ca,L)). This relationship suggests that changes in Q(r) might affect release and I(Ca,L). Here we studied the effect of gabapentin (GBP) on the three events generated by DHPRs in skeletal myotubes in culture. GBP specifically binds to the alpha(2)/delta(1) subunit of the brain and skeletal muscle DHPR. Myotubes were stimulated with a protocol that included a depolarizing prepulse to inactivate voltage-dependent proteins other than DHPRs. Gabapentin (50 microM) significantly increased Q(r) while decreasing the rate of rise of calcium transients. Gabapentin also reduced the maximum amplitude of the I(Ca,L) (as we previously reported) without modifying the kinetics of activation. Exposure of GBP-treated myotubes to 10 microM nifedipine prevented the increase of Q(r) promoted by this drug, indicating that the extra charge recorded originated from DHPRs. Our data suggest that GBP dissociates the functions of the DHPR from the initial voltage-sensing step and implicates a role for the alpha(2)/delta(1) subunit in E-C coupling.