Simultaneous recording of intramembrane charge movement components and calcium release in wild-type and S100A1-/- muscle fibres

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

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.

Attenuated Ca(2+) release in a mouse model of limb girdle muscular dystrophy 2A

Background

Mutations in CAPN3 cause limb girdle muscular dystrophy type 2A (LGMD2A), a progressive muscle wasting disease. CAPN3 is a non-lysosomal, Ca-dependent, muscle-specific proteinase. Ablation of CAPN3 (calpain-3 knockout (C3KO) mice) leads to reduced ryanodine receptor (RyR1) expression and abnormal Ca2+/calmodulin-dependent protein kinase II (Ca-CaMKII)-mediated signaling. We previously reported that Ca²⁺ release measured by fura2-FF imaging in response to single action potential stimulation was reduced in old C3KO mice; however, the use of field stimulation prevented investigation of the mechanisms underlying this impairment. Furthermore, our prior studies were conducted on older animals, whose muscles showed advanced muscular dystrophy, which prevented us from establishing whether impaired Ca²⁺ handling is an early feature of disease. In the current study, we sought to overcome these matters by studying single fibers isolated from young wild-type (WT) and C3KO mice using a low affinity calcium dye and high intracellular ethylene glycol-bis(2-aminoethylether)-n,n,n′,n′-tetraacetic acid (EGTA) to measure Ca²⁺ fluxes. Muscles were subjected to both current and voltage clamp conditions.

Methods

Standard and confocal fluorescence microscopy was used to study Ca²⁺ release in single fibers enzymatically isolated from hind limb muscles of wild-type and C3KO mice. Two microelectrode amplifier and experiments were performed under current or voltage clamp conditions. Calcium concentration changes were detected with an impermeant low affinity dye in the presence of high EGTA intracellular concentrations, and fluxes were calculated with a single compartment model. Standard Western blotting analysis was used to measure the concentration of RyR1 and the α subunit of the dihydropyridine (αDHPR) receptors. Data are presented as mean ± SEM and compared with the Student’s test with significance set at p < 0.05.

Results

We found that the peak value of Ca²⁺ fluxes elicited by single action potentials was significantly reduced by 15–20 % in C3KO fibers, but the kinetics was unaltered. Ca²⁺ release elicited by tetanic stimulation was also impaired in C3KO fibers. Confocal studies confirmed that Ca²⁺ release was similarly reduced in all triads of C3KO mice. Voltage clamp experiments revealed a normal voltage dependence of Ca²⁺ release in C3KO mice but reduced peak Ca²⁺ fluxes as with action potential stimulation. These findings concur with biochemical observations of reduced RyR1 and αDHPR levels in C3KO muscles and reduced mechanical output. Confocal studies revealed a similar decrease in Ca²⁺ release at all triads consistent with a homogenous reduction of functional voltage activated Ca²⁺ release sites.

Conclusions

Overall, these results suggest that decreased Ca²⁺ release is an early defect in calpainopathy and may contribute to the observed reduction of CaMKII activation in C3KO mice.

Alternating bipolar field stimulation identifies muscle fibers with defective excitability but maintained local Ca(2+) signals and contraction

Background

Most cultured enzymatically dissociated adult myofibers exhibit spatially uniform (UNI) contractile responses and Ca²⁺ transients over the entire myofiber in response to electric field stimuli of either polarity applied via bipolar electrodes. However, some myofibers only exhibit contraction and Ca²⁺ transients at alternating (ALT) ends in response to alternating polarity field stimulation. Here, we present for the first time the methodology for identification of ALT myofibers in primary cultures and isolated muscles, as well as a study of their electrophysiological properties.

Results

We used high-speed confocal microscopic Ca²⁺ imaging, electric field stimulation, microelectrode recordings, immunostaining, and confocal microscopy to characterize the properties of action potential-induced Ca²⁺ transients, contractility, resting membrane potential, and staining of T-tubule voltage-gated Na⁺ channel distribution applied to cultured adult myofibers. Here, we show for the first time, with high temporal and spatial resolution, that normal control myofibers with UNI responses can be converted to ALT response myofibers by TTX addition or by removal of Na⁺ from the bathing medium, with reappearance of the UNI response on return of Na⁺. Our results suggest disrupted excitability as the cause of ALT behavior and indicate that the ALT response is due to local depolarization-induced Ca²⁺ release, whereas the UNI response is triggered by action potential propagation over the entire myofiber. Consistent with this interpretation, local depolarizing monopolar stimuli give uniform (propagated) responses in UNI myofibers, but only local responses at the electrode in ALT myofibers. The ALT responses in electrically inexcitable myofibers are consistent with expectations of current spread between bipolar stimulating electrodes, entering (hyperpolarizing) one end of a myofiber and leaving (depolarizing) the other end of the myofiber. ALT responses were also detected in some myofibers within intact isolated whole muscles from wild-type and MDX mice, demonstrating that ALT responses can be present before enzymatic dissociation.

Conclusions

We suggest that checking for ALT myofiber responsiveness by looking at the end of a myofiber during alternating polarity stimuli provides a test for compromised excitability of myofibers, and could be used to identify inexcitable, damaged or diseased myofibers by ALT behavior in healthy and diseased muscle.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0076-8) contains supplementary material, which is available to authorized users.

β1a490-508, a 19-residue peptide from C-terminal tail of Cav1.1 β1a subunit, potentiates voltage-dependent calcium release in adult skeletal muscle fibers

The α1 and β1a subunits of the skeletal muscle calcium channel, Cav1.1, as well as the Ca(2+) release channel, ryanodine receptor (RyR1), are essential for excitation-contraction coupling. RyR1 channel activity is modulated by the β1a subunit and this effect can be mimicked by a peptide (β1a490-524) corresponding to the 35-residue C-terminal tail of the β1a subunit. Protein-protein interaction assays confirmed a high-affinity interaction between the C-terminal tail of the β1a and RyR1. Based on previous results using overlapping peptides tested on isolated RyR1, we hypothesized that a 19-amino-acid residue peptide (β1a490-508) is sufficient to reproduce activating effects of β1a490-524. Here we examined the effects of β1a490-508 on Ca(2+) release and Ca(2+) currents in adult skeletal muscle fibers subjected to voltage-clamp and on RyR1 channel activity after incorporating sarcoplasmic reticulum vesicles into lipid bilayers. β1a490-508 (25 nM) increased the peak Ca(2+) release flux by 49% in muscle fibers. Considerably fewer activating effects were observed using 6.25, 100, and 400 nM of β1a490-508 in fibers. β1a490-508 also increased RyR1 channel activity in bilayers and Cav1.1 currents in fibers. A scrambled form of β1a490-508 peptide was used as negative control and produced negligible effects on Ca(2+) release flux and RyR1 activity. Our results show that the β1a490-508 peptide contains molecular components sufficient to modulate excitation-contraction coupling in adult muscle fibers.

Establishment of a human skeletal muscle-derived cell line: biochemical, cellular and electrophysiological characterization

Excitation-contraction coupling is the physiological mechanism occurring in muscle cells whereby an electrical signal sensed by the dihydropyridine receptor located on the transverse tubules is transformed into a chemical gradient (Ca2+ increase) by activation of the ryanodine receptor located on the sarcoplasmic reticulum membrane. In the present study, we characterized for the first time the excitation-contraction coupling machinery of an immortalized human skeletal muscle cell line. Intracellular Ca2+ measurements showed a normal response to pharmacological activation of the ryanodine receptor, whereas 3D-SIM (super-resolution structured illumination microscopy) revealed a low level of structural organization of ryanodine receptors and dihydropyridine receptors. Interestingly, the expression levels of several transcripts of proteins involved in Ca2+ homoeostasis and differentiation indicate that the cell line has a phenotype closer to that of slow-twitch than fast-twitch muscles. These results point to the potential application of such human muscle-derived cell lines to the study of neuromuscular disorders; in addition, they may serve as a platform for the development of therapeutic strategies aimed at correcting defects in Ca2+ homoeostasis due to mutations in genes involved in Ca2+ regulation.

Elevated nuclear Foxo1 suppresses excitability of skeletal muscle fibers

Forkhead box O 1 (Foxo1) controls the expression of proteins that carry out processes leading to skeletal muscle atrophy, making Foxo1 of therapeutic interest in conditions of muscle wasting. The transcription of Foxo1-regulated proteins is dependent on the translocation of Foxo1 to the nucleus, which can be repressed by insulin-like growth factor-1 (IGF-1) treatment. The role of Foxo1 in muscle atrophy has been explored at length, but whether Foxo1 nuclear activity affects skeletal muscle excitation-contraction (EC) coupling has not yet been examined. Here, we use cultured adult mouse skeletal muscle fibers to investigate the effects of Foxo1 overexpression on EC coupling. Fibers expressing Foxo1-green fluorescent protein (GFP) exhibit an inability to contract, impaired propagation of action potentials, and ablation of calcium transients in response to electrical stimulation compared with fibers expressing GFP alone. Evaluation of the transverse (T)-tubule system morphology, the membranous system involved in the radial propagation of the action potential, revealed an intact T-tubule network in fibers overexpressing Foxo1-GFP. Interestingly, long-term IGF-1 treatment of Foxo1-GFP fibers, which maintains Foxo1-GFP outside the nucleus, prevented the loss of normal calcium transients, indicating that Foxo1 translocation and the atrogenes it regulates affect the expression of proteins involved in the generation and/or propagation of action potentials. A reduction in the sodium channel Nav1.4 expression in fibers overexpressing Foxo1-GFP was also observed in the absence of IGF-1. We conclude that increased nuclear activity of Foxo1 prevents the normal muscle responses to electrical stimulation and that this indicates a novel capability of Foxo1 to disable the functional activity of skeletal muscle.

A calcium channel mutant mouse model of hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HypoPP) is a familial skeletal muscle disorder that presents with recurrent episodes of severe weakness lasting hours to days associated with reduced serum potassium (K+). HypoPP is genetically heterogeneous, with missense mutations of a calcium channel (Ca(V)1.1) or a sodium channel (Na(V)1.4) accounting for 60% and 20% of cases, respectively. The mechanistic link between Ca(V)1.1 mutations and the ictal loss of muscle excitability during an attack of weakness in HypoPP is unknown. To address this question, we developed a mouse model for HypoPP with a targeted Ca(V)1.1 R528H mutation. The Ca(V)1.1 R528H mice had a HypoPP phenotype for which low K+ challenge produced a paradoxical depolarization of the resting potential, loss of muscle excitability, and weakness. A vacuolar myopathy with dilated transverse tubules and disruption of the triad junctions impaired Ca2+ release and likely contributed to the mild permanent weakness. Fibers from the Ca(V)1.1 R528H mouse had a small anomalous inward current at the resting potential, similar to our observations in the Na(V)1.4 R669H HypoPP mouse model. This "gating pore current" may be a common mechanism for paradoxical depolarization and susceptibility to HypoPP arising from missense mutations in the S4 voltage sensor of either calcium or sodium channels.

NOX2-dependent ROS is required for HDAC5 nuclear efflux and contributes to HDAC4 nuclear efflux during intense repetitive activity of fast skeletal muscle fibers

Reactive oxygen species (ROS) have been linked to oxidation and nuclear efflux of class IIa histone deacetylase 4 (HDAC4) in cardiac muscle. Here we use HDAC-GFP fusion proteins expressed in isolated adult mouse flexor digitorum brevis muscle fibers to study ROS mediation of HDAC localization in skeletal muscle. H(2)O(2) causes nuclear efflux of HDAC4-GFP or HDAC5-GFP, which is blocked by the ROS scavenger N-acetyl-l-cysteine (NAC). Repetitive stimulation with 100-ms trains at 50 Hz, 2/s ("50-Hz trains") increased ROS production and caused HDAC4-GFP or HDAC5-GFP nuclear efflux. During 50-Hz trains, HDAC5-GFP nuclear efflux was completely blocked by NAC, but HDAC4-GFP nuclear efflux was only partially blocked by NAC and partially blocked by the calcium-dependent protein kinase (CaMK) inhibitor KN-62. Thus, during intense activity both ROS and CaMK play roles in nuclear efflux of HDAC4, but only ROS mediates HDAC5 nuclear efflux. The 10-Hz continuous stimulation did not increase the rate of ROS production and did not cause HDAC5-GFP nuclear efflux but promoted HDAC4-GFP nuclear efflux that was sensitive to KN-62 but not NAC and thus mediated by CaMK but not by ROS. Fibers from NOX2 knockout mice lacked ROS production and ROS-dependent nuclear efflux of HDAC5-GFP or HDAC4-GFP during 50-Hz trains but had unmodified Ca(2+) transients. Our results demonstrate that ROS generated by NOX2 could play important roles in muscle remodeling due to intense muscle activity and that the nuclear effluxes of HDAC4 and HDAC5 are differentially regulated by Ca(2+) and ROS during muscle activity.

S100 calcium binding proteins and ion channels

S100 Ca(2+)-binding proteins have been associated with a multitude of intracellular Ca(2+)-dependent functions including regulation of the cell cycle, cell differentiation, cell motility and apoptosis, modulation of membrane-cytoskeletal interactions, transduction of intracellular Ca(2+) signals, and in mediating learning and memory. S100 proteins are fine tuned to read the intracellular free Ca(2+) concentration and affect protein phosphorylation, which makes them candidates to modulate certain ion channels and neuronal electrical behavior. Certain S100s are secreted from cells and are found in extracellular fluids where they exert unique extracellular functions. In addition to their neurotrophic activity, some S100 proteins modulate neuronal electrical discharge activity and appear to act directly on ion channels. The first reports regarding these effects suggested S100-mediated alterations in Ca(2+) fluxes, K(+) currents, and neuronal discharge activity. Recent reports revealed direct and indirect interactions with Ca(2+), K(+), Cl(-), and ligand activated channels. This review focuses on studies of the physical and functional interactions of S100 proteins and ion channels.

Voltage clamp methods for the study of membrane currents and SR Ca(2+) release in adult skeletal muscle fibres

Skeletal muscle excitation-contraction (E-C)(1) coupling is a process composed of multiple sequential stages, by which an action potential triggers sarcoplasmic reticulum (SR)(2) Ca(2+) release and subsequent contractile activation. The various steps in the E-C coupling process in skeletal muscle can be studied using different techniques. The simultaneous recordings of sarcolemmal electrical signals and the accompanying elevation in myoplasmic Ca(2+), due to depolarization-initiated SR Ca(2+) release in skeletal muscle fibres, have been useful to obtain a better understanding of muscle function. In studying the origin and mechanism of voltage dependency of E-C coupling a variety of different techniques have been used to control the voltage in adult skeletal fibres. Pioneering work in muscles isolated from amphibians or crustaceans used microelectrodes or 'high resistance gap' techniques to manipulate the voltage in the muscle fibres. The development of the patch clamp technique and its variant, the whole-cell clamp configuration that facilitates the manipulation of the intracellular environment, allowed the use of the voltage clamp techniques in different cell types, including skeletal muscle fibres. The aim of this article is to present an historical perspective of the voltage clamp methods used to study skeletal muscle E-C coupling as well as to describe the current status of using the whole-cell patch clamp technique in studies in which the electrical and Ca(2+) signalling properties of mouse skeletal muscle membranes are being investigated.

Mice null for calsequestrin 1 exhibit deficits in functional performance and sarcoplasmic reticulum calcium handling

In skeletal muscle, the release of calcium (Ca²⁺) by ryanodine sensitive sarcoplasmic reticulum (SR) Ca²⁺ release channels (i.e., ryanodine receptors; RyR1s) is the primary determinant of contractile filament activation. Much attention has been focused on calsequestrin (CASQ1) and its role in SR Ca²⁺ buffering as well as its potential for modulating RyR1, the L-type Ca²⁺ channel (dihydropyridine receptor, DHPR) and other sarcolemmal channels through sensing luminal [Ca²⁺]. The genetic ablation of CASQ1 expression results in significant alterations in SR Ca²⁺ content and SR Ca²⁺ release especially during prolonged activation. While these findings predict a significant loss-of-function phenotype in vivo, little information on functional status of CASQ1 null mice is available. We examined fast muscle in vivo and in vitro and identified significant deficits in functional performance that indicate an inability to sustain contractile activation. In single CASQ1 null skeletal myofibers we demonstrate a decrease in voltage dependent RyR Ca²⁺ release with single action potentials and a collapse of the Ca²⁺ release with repetitive trains. Under voltage clamp, SR Ca²⁺ release flux and total SR Ca²⁺ release are significantly reduced in CASQ1 null myofibers. The decrease in peak Ca²⁺ release flux appears to be solely due to elimination of the slowly decaying component of SR Ca²⁺ release, whereas the rapidly decaying component of SR Ca²⁺ release is not altered in either amplitude or time course in CASQ1 null fibers. Finally, intra-SR [Ca²⁺] during ligand and voltage activation of RyR1 revealed a significant decrease in the SR[Ca²⁺]_free in intact CASQ1 null fibers and a increase in the release and uptake kinetics consistent with a depletion of intra-SR Ca²⁺ buffering capacity. Taken together we have revealed that the genetic ablation of CASQ1 expression results in significant functional deficits consistent with a decrease in the slowly decaying component of SR Ca²⁺ release.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

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

Effects of conformational peptide probe DP4 on bidirectional signaling between DHPR and RyR1 calcium channels in voltage-clamped skeletal muscle fibers

In skeletal muscle, excitation-contraction coupling involves the activation of dihydropyridine receptors (DHPR) and type-1 ryanodine receptors (RyR1) to produce depolarization-dependent sarcoplasmic reticulum Ca²⁺ release via orthograde signaling. Another form of DHPR-RyR1 communication is retrograde signaling, in which RyRs modulate the gating of DHPR. DP4 (domain peptide 4), is a peptide corresponding to residues Leu²⁴⁴²-Pro²⁴⁷⁷ of the central domain of the RyR1 that produces RyR1 channel destabilization. Here we explore the effects of DP4 on orthograde excitation-contraction coupling and retrograde RyR1-DHPR signaling in isolated murine muscle fibers. Intracellular dialysis of DP4 increased the peak amplitude of Ca²⁺ release during step depolarizations by 64% without affecting its voltage-dependence or kinetics, and also caused a similar increase in Ca²⁺ release during an action potential waveform. DP4 did not modify either the amplitude or the voltage-dependence of the intramembrane charge movement. However, DP4 augmented DHPR Ca²⁺ current density without affecting its voltage-dependence. Our results demonstrate that the conformational changes induced by DP4 regulate both orthograde E-C coupling and retrograde RyR1-DHPR signaling.

S100A1 and calmodulin regulation of ryanodine receptor in striated muscle

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

Stimulation of Ca²+ signals in neurons by electrically coupled electrolyte-oxide-semiconductor capacitors

Electrolyte-oxide-semiconductor capacitors (EOSCs) are a class of microtransducers for extracellular electrical stimulation that have been successfully employed to activate voltage-dependent sodium channels at the neuronal soma to generate action potentials in vitro. In the present work, we report on their use to control Ca²+ signalling in cultured mammalian cells, including neurons. Evidence is provided that EOSC stimulation with voltage waveforms in the microsecond or nanosecond range activates two distinct Ca²+ pathways, either by triggering Ca²+ entry through the plasma membrane or its release from intracellular stores. Ca²+ signals were activated in non-neuronal and neuronal cell lines, CHO-K1 and SH-SY5Y. On this basis, stimulation was tailored to rat and bovine neurons to mimic physiological somatic Ca²+ transients evoked by glutamate. Being minimally invasive and easy to use, the new method represents a versatile complement to standard electrophysiology and imaging techniques for the investigation of Ca²+ signalling in dissociated primary neurons and cell lines.

Modulation of sarcoplasmic reticulum Ca2+ release in skeletal muscle expressing ryanodine receptor impaired in regulation by calmodulin and S100A1

In vitro, calmodulin (CaM) and S100A1 activate the skeletal muscle ryanodine receptor ion channel (RyR1) at submicromolar Ca(2+) concentrations, whereas at micromolar Ca(2+) concentrations, CaM inhibits RyR1. One amino acid substitution (RyR1-L3625D) has previously been demonstrated to impair CaM binding and regulation of RyR1. Here we show that the RyR1-L3625D substitution also abolishes S100A1 binding. To determine the physiological relevance of these findings, mutant mice were generated with the RyR1-L3625D substitution in exon 74, which encodes the CaM and S100A1 binding domain of RyR1. Homozygous mutant mice (Ryr1(D/D)) were viable and appeared normal. However, single RyR1 channel recordings from Ryr1(D/D) mice exhibited impaired activation by CaM and S100A1 and impaired CaCaM inhibition. Isolated flexor digitorum brevis muscle fibers from Ryr1(D/D) mice had depressed Ca(2+) transients when stimulated by a single action potential. However, during repetitive stimulation, the mutant fibers demonstrated greater relative summation of the Ca(2+) transients. Consistently, in vivo stimulation of tibialis anterior muscles in Ryr1(D/D) mice demonstrated reduced twitch force in response to a single action potential, but greater summation of force during high-frequency stimulation. During repetitive stimulation, Ryr1(D/D) fibers exhibited slowed inactivation of sarcoplasmic reticulum Ca(2+) release flux, consistent with increased summation of the Ca(2+) transient and contractile force. Peak Ca(2+) release flux was suppressed at all voltages in voltage-clamped Ryr1(D/D) fibers. The results suggest that the RyR1-L3625D mutation removes both an early activating effect of S100A1 and CaM and delayed suppressing effect of CaCaM on RyR1 Ca(2+) release, providing new insights into CaM and S100A1 regulation of skeletal muscle excitation-contraction coupling.

Functional expression of transgenic 1sDHPR channels in adult mammalian skeletal muscle fibres

We investigated the effects of the overexpression of two enhanced green fluorescent protein (EGFP)-tagged α1sDHPR variants on Ca2+ currents (ICa), charge movements (Q) and SR Ca2+ release of muscle fibres isolated from adult mice. Flexor digitorum brevis (FDB)muscles were transfected by in vivo electroporation with plasmids encoding for EGFP-α1sDHPR-wt and EGFP-α1sDHPR-T935Y (an isradipine-insensitive mutant). Two-photon laser scanning microscopy (TPLSM) was used to study the subcellular localization of transgenic proteins, while ICa, Q and Ca2+ release were studied electrophysiologically and optically under voltage-clamp conditions. TPLSM images demonstrated that most of the transgenic α1sDHPR was correctly targeted to the transverse tubular system (TTS). Immunoblotting analysis of crude extracts of transfected fibres demonstrated the synthesis of bona fide transgenic EGFP-α1sDHPR-wt in quantities comparable to that of native α1sDHPR. Though expression of both transgenic variants of the alpha subunit of the dihydropyridine receptor (α1sDHPR) resulted in ∼50% increase in Q, they surprisingly had no effect on the maximal Ca2+ conductance (gCa) nor the SR Ca2+ release. Nonetheless, fibres expressing EGFP-α1sDHPR-T935Y exhibited up to 70% isradipine-insensitive ICa (ICa-ins) with a right-shifted voltage dependence compared to that in control fibres. Interestingly, Qand SRCa2+ release also displayed right-shifted voltage dependence in fibres expressing EGFP-α1sDHPR-T935Y. In contrast, the midpoints of the voltage dependence of gCa, Q and Ca2+ release were not different from those in control fibres and in fibres expressing EGFP-α1sDHPR-wt. Overall, our results suggest that transgenic α1sDHPRs are correctly trafficked and inserted in the TTS membrane, and that a substantial fraction of the mworks as conductive Ca2+ channels capable of physiologically controlling the release of Ca2+ from the SR. A plausible corollary of this work is that the expression of transgenic variants of the α1sDHPR leads to the replacement of native channels interacting with the ryanodine receptor 1 (RyR1), thus demonstrating the feasibility of molecular remodelling of the triads in adult skeletal muscle fibres.

Excitation-contraction coupling and minor triadic proteins in low-frequency fatigue

Low-frequency fatigue (LFF) is characterized by a proportionally greater loss of force at low compared with high activation frequencies and a prolonged recovery. Recent work suggests a calcium-induced uncoupling of excitation-contraction coupling underlies LFF. Here, newly characterized triadic proteins are described, and possible mechanisms by which they may contribute to LFF are suggested.

S100A1 promotes action potential-initiated calcium release flux and force production in skeletal muscle

The role of S100A1 in skeletal muscle is just beginning to be elucidated. We have previously shown that skeletal muscle fibers from S100A1 knockout (KO) mice exhibit decreased action potential (AP)-evoked Ca(2+) transients, and that S100A1 binds competitively with calmodulin to a canonical S100 binding sequence within the calmodulin-binding domain of the skeletal muscle ryanodine receptor. Using voltage clamped fibers, we found that Ca(2+) release was suppressed at all test membrane potentials in S100A1(-/-) fibers. Here we examine the role of S100A1 during physiological AP-induced muscle activity, using an integrative approach spanning AP propagation to muscle force production. With the voltage-sensitive indicator di-8-aminonaphthylethenylpyridinium, we first demonstrate that the AP waveform is not altered in flexor digitorum brevis muscle fibers isolated from S100A1 KO mice. We then use a model for myoplasmic Ca(2+) binding and transport processes to calculate sarcoplasmic reticulum Ca(2+) release flux initiated by APs and demonstrate decreased release flux and greater inactivation of flux in KO fibers. Using in vivo stimulation of tibialis anterior muscles in anesthetized mice, we show that the maximal isometric force response to twitch and tetanic stimulation is decreased in S100A1(-/-) muscles. KO muscles also fatigue more rapidly upon repetitive stimulation than those of wild-type counterparts. We additionally show that fiber diameter, type, and expression of key excitation-contraction coupling proteins are unchanged in S100A1 KO muscle. We conclude that the absence of S100A1 suppresses physiological AP-induced Ca(2+) release flux, resulting in impaired contractile activation and force production in skeletal muscle.

S100A1: a regulator of striated muscle sarcoplasmic reticulum Ca2+ handling, sarcomeric, and mitochondrial function

Calcium (Ca²⁺) signaling plays a key role in a wide range of physiological functions including control of cardiac and skeletal muscle performance. To assure a precise coordination of both temporally and spatially transduction of intracellular Ca²⁺ oscillations to downstream signaling networks and target operations, Ca²⁺ cycling regulation in muscle tissue is conducted by a plethora of diverse molecules. Ca²⁺ S100A1 is a member of the Ca²⁺-binding S100 protein family and represents the most abundant S100 isoform in cardiac and skeletal muscle. Early studies revealed distinct expression patterns of S100A1 in healthy and diseased cardiac tissue from animal models and humans. Further elaborate investigations uncovered S100A1 protein as a basic requirement for striated muscle Ca²⁺ handling integrity. S100A1 is a critical regulator of cardiomyocyte Ca²⁺ cycling and contractile performance. S100A1-mediated inotropy unfolds independent and on top of βAR-stimulated contractility with unchanged βAR downstream signaling. S100A1 has further been detected at different sites within the cardiac sarcomere indicating potential roles in myofilament function. More recently, a study reported a mitochondrial location of S100A1 in cardiomyocytes. Additionally, normalizing the level of S100A1 protein by means of viral cardiac gene transfer in animal heart failure models resulted in a disrupted progression towards cardiac failure and enhanced survival. This brief review is confined to the physiological and pathophysiological relevance of S100A1 in cardiac and skeletal muscle Ca²⁺ handling with a particular focus on its potential as a molecular target for future therapeutic interventions.

Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K⁺ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K⁺ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca²⁺ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} produced an increase in the resting [Ca²⁺]. Ca²⁺ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca²⁺ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca²⁺ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca²⁺ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 10 mM potentiated Ca²⁺ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on Ca²⁺ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} might be mediated by depolarization dependent changes in resting [Ca²⁺] and Ca²⁺ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on force generation.

The Qgamma component of intra-membrane charge movement is present in mammalian muscle fibres, but suppressed in the absence of S100A1

S100A1 is a Ca(2+) binding protein that modulates excitation-contraction (EC) coupling in skeletal and cardiac muscle. S100A1 competes with calmodulin for binding to the skeletal muscle SR Ca(2+) release channel (the ryanodine receptor type 1, RyR1) at a site that also interacts with the C-terminal tail of the voltage sensor of EC coupling, the dihydropyridine receptor. Ablation of S100A1 leads to delayed and decreased action potential evoked Ca(2+) transients, possibly linked to altered voltage sensor activation. Here we investigate the effects of S100A1 on voltage sensor activation in skeletal muscle utilizing whole-cell patch clamp electrophysiology to record intra-membrane charge movement currents in isolated flexor digitorum brevis (FDB) muscle fibres from wild-type and S100A1 knock-out (KO) mice. In contrast to recent reports, we found that FDB fibres exhibit two distinct components of intra-membrane charge movement, an initial rapid component (Q(beta)), and a delayed, steeply voltage dependent 'hump' component (Q(gamma)) previously recorded primarily in amphibian but not mammalian fibres. Surprisingly, we found that Q(gamma) was selectively suppressed in S100A1 KO fibres, while the Q(beta) component of charge movement was unaffected. This result was specific to S100A1 and not a compensatory result of genetic manipulation, as transient intracellular application of S100A1 restored Q(gamma). Furthermore, we found that exposure to the RyR1 inhibitor dantrolene suppressed a similar component of charge movement in FDB fibres. These results shed light on voltage sensor activation in mammalian muscle, and support S100A1 as a positive regulator of the voltage sensor and Ca(2+) release channel in skeletal muscle EC coupling.