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

Role of putative voltage-sensor countercharge D4 in regulating gating properties of CaV1.2 and CaV1.3 calcium channels

Voltage-dependent calcium channels (Ca_V) activate over a wide range of membrane potentials, and the voltage-dependence of activation of specific channel isoforms is exquisitely tuned to their diverse functions in excitable cells. Alternative splicing further adds to the stunning diversity of gating properties. For example, developmentally regulated insertion of an alternatively spliced exon 29 in the fourth voltage-sensing domain (VSD IV) of Ca_V1.1 right-shifts voltage-dependence of activation by 30 mV and decreases the current amplitude several-fold. Previously we demonstrated that this regulation of gating properties depends on interactions between positive gating charges (R1, R2) and a negative countercharge (D4) in VSD IV of Ca_V1.1. Here we investigated whether this molecular mechanism plays a similar role in the VSD IV of Ca_V1.3 and in VSDs II and IV of Ca_V1.2 by introducing charge-neutralizing mutations (D4N or E4Q) in the corresponding positions of Ca_V1.3 and in two splice variants of Ca_V1.2. In both channels the D4N (VSD IV) mutation resulted in a  ̴5 mV right-shift of the voltage-dependence of activation and in a reduction of current density to about half of that in controls. However in Ca_V1.2 the effects were independent of alternative splicing, indicating that the two modulatory processes operate by distinct mechanisms. Together with our previous findings these results suggest that molecular interactions engaging D4 in VSD IV contribute to voltage-sensing in all examined Ca_V1 channels, however its striking role in regulating the gating properties by alternative splicing appears to be a unique property of the skeletal muscle Ca_V1.1 channel.

STAC proteins: The missing link in skeletal muscle EC coupling and new regulators of calcium channel function

Excitation-contraction coupling is the signaling process by which action potentials control calcium release and consequently the force of muscle contraction. Until recently, three triad proteins were known to be essential for skeletal muscle EC coupling: the voltage-gated calcium channel CaV1.1 acting as voltage sensor, the SR calcium release channel RyR1 representing the only relevant calcium source, and the auxiliary CaV β1a subunit. Whether CaV1.1 and RyR1 are directly coupled or whether their interaction is mediated by another triad protein is still unknown. The recent identification of the adaptor protein STAC3 as fourth essential component of skeletal muscle EC coupling prompted vigorous research to reveal its role in this signaling process. Accumulating evidence supports its possible involvement in linking CaV1.1 and RyR1 in skeletal muscle EC coupling, but also indicates a second, much broader role of STAC proteins in the regulation of calcium/calmodulin-dependent feedback regulation of L-type calcium channels.

STAC3 incorporation into skeletal muscle triads occurs independent of the dihydropyridine receptor

Excitation‐contraction (EC) coupling in skeletal muscles operates through a physical interaction between the dihydropyridine receptor (DHPR), acting as a voltage sensor, and the ryanodine receptor (RyR1), acting as a calcium release channel. Recently, the adaptor protein SH3 and cysteine‐rich containing protein 3 (STAC3) has been identified as a myopathy disease gene and as an additional essential EC coupling component. STAC3 interacts with DHPR sequences including the critical EC coupling domain and has been proposed to function in linking the DHPR and RyR1. However, we and others demonstrated that incorporation of recombinant STAC3 into skeletal muscle triads critically depends only on the DHPR but not the RyR1. On the contrary, here, we provide evidence that endogenous STAC3 incorporates into triads in the absence of the DHPR in myotubes and muscle fibers of dysgenic mice. This finding demonstrates that STAC3 interacts with additional triad proteins and is consistent with its proposed role in directly or indirectly linking the DHPR with the RyR1.

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.

Physiological and pharmacological modulation of the embryonic skeletal muscle calcium channel splice variant CaV1.1e

CaV1.1e is the voltage-gated calcium channel splice variant of embryonic skeletal muscle. It differs from the adult CaV1.1a splice variant by the exclusion of exon 29 coding for 19 amino acids in the extracellular loop connecting transmembrane domains IVS3 and IVS4. Like the adult splice variant CaV1.1a, the embryonic CaV1.1e variant functions as voltage sensor in excitation-contraction coupling, but unlike CaV1.1a it also conducts sizable calcium currents. Consequently, physiological or pharmacological modulation of calcium currents may have a greater impact in CaV1.1e expressing muscle cells. Here, we analyzed the effects of L-type current modulators on whole-cell current properties in dysgenic (CaV1.1-null) myotubes reconstituted with either CaV1.1a or CaV1.1e. Furthermore, we examined the physiological current modulation by interactions with the ryanodine receptor using a chimeric CaV1.1e construct in which the cytoplasmic II-III loop, essential for skeletal muscle excitation-contraction coupling, has been replaced with the corresponding but nonfunctional loop from the Musca channel. Whereas the equivalent substitution in CaV1.1a had abolished the calcium currents, substitution of the II-III loop in CaV1.1e did not significantly reduce current amplitudes. This indicates that CaV1.1e is not subject to retrograde coupling with the ryanodine receptor and that the retrograde coupling mechanism in CaV1.1a operates by counteracting the limiting effects of exon 29 inclusion on the current amplitude. Pharmacologically, CaV1.1e behaves like other L-type calcium channels. Its currents are substantially increased by the calcium channel agonist Bay K 8644 and inhibited by the calcium channel blocker nifedipine in a dose-dependent manner. With an IC50 of 0.37 μM for current inhibition by nifedipine, CaV1.1e is a potential drug target for the treatment of myotonic dystrophy. It might block the excessive calcium influx resulting from the aberrant expression of the embryonic splice variant CaV1.1e in the skeletal muscles of myotonic dystrophy patients.

The juvenile myoclonic epilepsy mutant of the calcium channel β(4) subunit displays normal nuclear targeting in nerve and muscle cells

Voltage-gated calcium channels regulate gene expression by controlling calcium entry through the plasma membrane and by direct interactions of channel fragments and auxiliary β subunits with promoters and the epigenetic machinery in the nucleus. Mutations of the calcium channel β₄ subunit gene (CACNB4) cause juvenile myoclonic epilepsy in humans and ataxia and epileptic seizures in mice. Recently a model has been proposed according to which failed nuclear translocation of the truncated β₄ subunit R482X mutation resulted in altered transcriptional regulation and consequently in neurological disease. Here we examined the nuclear targeting properties of the truncated β_4b(1–481) subunit in tsA-201 cells, skeletal myotubes, and in hippocampal neurons. Contrary to expectation, nuclear targeting of β_4b(1–481) was not reduced compared with full-length β_4b in any one of the three cell systems. These findings oppose an essential role of the β₄ distal C-terminus in nuclear targeting and challenge the idea that the nuclear function of calcium channel β₄ subunits is critically involved in the etiology of epilepsy and ataxia in patients and mouse models with mutations in the CACNB4 gene.

Stable incorporation versus dynamic exchange of β subunits in a native Ca2+ channel complex

Voltage-gated Ca(2+) channels are multi-subunit membrane proteins that transduce depolarization into cellular functions such as excitation-contraction coupling in muscle or neurotransmitter release in neurons. The auxiliary β subunits function in membrane targeting of the channel and modulation of its gating properties. However, whether β subunits can reversibly interact with, and thus differentially modulate, channels in the membrane is still unresolved. In the present study we applied fluorescence recovery after photobleaching (FRAP) of GFP-tagged α1 and β subunits expressed in dysgenic myotubes to study the relative dynamics of these Ca(2+) channel subunits for the first time in a native functional signaling complex. Identical fluorescence recovery rates of both subunits indicate stable interactions, distinct recovery rates indicate dynamic interactions. Whereas the skeletal muscle β1a isoform formed stable complexes with CaV1.1 and CaV1.2, the non-skeletal muscle β2a and β4b isoforms dynamically interacted with both α1 subunits. Neither replacing the I-II loop of CaV1.1 with that of CaV2.1, nor deletions in the proximal I-II loop, known to change the orientation of β relative to the α1 subunit, altered the specific dynamic properties of the β subunits. In contrast, a single residue substitution in the α interaction pocket of β1aM293A increased the FRAP rate threefold. Taken together, these findings indicate that in skeletal muscle triads the homologous β1a subunit forms a stable complex, whereas the heterologous β2a and β4b subunits form dynamic complexes with the Ca(2+) channel. The distinct binding properties are not determined by differences in the I-II loop sequences of the α1 subunits, but are intrinsic properties of the β subunit isoforms.

The proximal C-terminus of α1C subunits is necessary for junctional membrane targeting of cardiac L-type calcium channels

In cardiac myocytes, LTCCs (L-type calcium channels) form a functional signalling complex with ryanodine receptors at the JM (junctional membrane). Although the specific localization of LTCCs to the JM is critical for excitation–contraction coupling, their targeting mechanism is unclear. Transient transfection of GFP (green fluorescent protein)–α1S or GFP–α1C, but not P/Q-type calcium channel α1A, in dysgenic (α1S-null) GLT myotubes results in correct targeting of these LTCCs to the JMs and restoration of action-potential-induced Ca2+ transients. To identify the sequences of α1C responsible for JM targeting, we generated a range of α1C–α1A chimaeras, deletion mutants and alanine substitution mutants and studied their targeting properties in GLT myotubes. The results revealed that amino acids L1681QAGLRTL1688 and P1693EIRRAIS1700, predicted to form two adjacent α-helices in the proximal C-terminus, are necessary for the JM targeting of α1C. The efficiency of restoration of action-potential-induced Ca2+ transients in GLT myotubes was significantly decreased by mutations in the targeting motif. JM targeting was not disrupted by the distal C-terminus of α1C which binds to the second α-helix. Therefore we have identified a new structural motif in the C-terminus of α1C that mediates the targeting of cardiac LTCCs to JMs independently of the interaction between proximal and distal C-termini of α1C.

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.

Surface traffic of dendritic CaV1.2 calcium channels in hippocampal neurons

In neurons L-type calcium currents function in gene regulation and synaptic plasticity, while excessive calcium influx leads to excitotoxicity and neurodegeneration. The major neuronal Ca(V)1.2 L-type channels are localized in clusters in dendritic shafts and spines. Whereas Ca(V)1.2 clusters remain stable during NMDA-induced synaptic depression, L-type calcium currents are rapidly downregulated during strong excitatory stimulation. Here we used fluorescence recovery after photobleaching (FRAP), live cell-labeling protocols, and single particle tracking (SPT) to analyze the turnover and surface traffic of Ca(V)1.2 in dendrites of mature cultured mouse and rat hippocampal neurons, respectively. FRAP analysis of channels extracellularly tagged with superecliptic pHluorin (Ca(V)1.2-SEP) demonstrated ∼20% recovery within 2 min without reappearance of clusters. Pulse-chase labeling showed that membrane-expressed Ca(V)1.2-HA is not internalized within1 h, while blocking dynamin-dependent endocytosis resulted in increased cluster density after 30 min. Together, these results suggest a turnover rate of clustered Ca(V)1.2s on the hour time scale. Direct recording of the lateral movement in the membrane using SPT demonstrated that dendritic Ca(V)1.2s show highly confined mobility with diffusion coefficients of ∼0.005 μm² s⁻¹. Consistent with the mobile Ca(V)1.2 fraction observed in FRAP, a ∼30% subpopulation of channels reversibly exchanged between confined and diffusive states. Remarkably, high potassium depolarization did not alter the recovery rates in FRAP or the diffusion coefficients in SPT analyses. Thus, an equilibrium of clustered and dynamic Ca(V)1.2s maintains stable calcium channel complexes involved in activity-dependent cell signaling, whereas the minor mobile channel pool in mature neurons allows limited capacity for short-term adaptations.

A CaV1.1 Ca2+ channel splice variant with high conductance and voltage-sensitivity alters EC coupling in developing skeletal muscle

The Ca(2+) channel alpha(1S) subunit (Ca(V)1.1) is the voltage sensor in skeletal muscle excitation-contraction (EC) coupling. Upon membrane depolarization, this sensor rapidly triggers Ca(2+) release from internal stores and conducts a slowly activating Ca(2+) current. However, this Ca(2+) current is not essential for skeletal muscle EC coupling. Here, we identified a Ca(V)1.1 splice variant with greatly distinct current properties. The variant of the CACNA1S gene lacking exon 29 was expressed at low levels in differentiated human and mouse muscle, and up to 80% in myotubes. To test its biophysical properties, we deleted exon 29 in a green fluorescent protein (GFP)-tagged alpha(1S) subunit and expressed it in dysgenic (alpha(1S)-null) myotubes. GFP-alpha(1S)Delta 29 was correctly targeted into triads and supported skeletal muscle EC coupling. However, the Ca(2+) currents through GFP-alpha(1S)Delta 29 showed a 30-mV left-shifted voltage dependence of activation and a substantially increased open probability, giving rise to an eightfold increased current density. This robust Ca(2+) influx contributed substantially to the depolarization-induced Ca(2+) transient that triggers contraction. Moreover, deletion of exon 29 accelerated current kinetics independent of the auxiliary alpha(2)delta-1 subunit. Thus, characterizing the Ca(V)1.1 Delta 29 splice variant revealed the structural bases underlying the specific gating properties of skeletal muscle Ca(2+) channels, and it suggests the existence of a distinct mode of EC coupling in developing muscle.

Activity and calcium regulate nuclear targeting of the calcium channel beta4b subunit in nerve and muscle cells

Auxiliary beta subunits are critical determinants of membrane expression and gating properties of voltage-gated calcium channels. Mutations in the beta(4) subunit gene cause ataxia and epilepsy. However, the specific function of beta(4) in neurons and its causal relation to neurological diseases are unknown. Here we report the localization of the beta(4) subunit in the nuclei of cerebellar granule and Purkinje cells. beta(4b) was the only beta isoform showing nuclear targeting when expressed in neurons and skeletal myotubes. Its specific nuclear targeting property was mapped to an N-terminal double-arginine motif, which was necessary and sufficient for targeting beta subunits into the nucleus. Spontaneous electrical activity and calcium influx negatively regulated beta(4b) nuclear localization by a CRM-1-dependent nuclear export mechanism. The activity-dependent shuttling of beta(4b) into and out of the nucleus indicates a specific role of this beta subunit in neurons, in communicating the activity of calcium channels to the nucleus.

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.

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.

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

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

Expression of L-type calcium channel alpha(1)-1.2 and alpha(1)-1.3 subunits on rat sacral motoneurons following chronic spinal cord injury

In the presence of the monoamines serotonin and norepinephrine, motoneurons readily generate large persistent inward currents (PICs). The resulting plateau potentials amplify and sustain motor output. Monoaminergic input to the cord originates in the brainstem and the sharp reduction in monoamine levels that occurs following acute spinal cord injury greatly decreases motoneuron excitability. However, recent studies in the adult sacral cord of the rat have shown that motoneurons reacquire the ability to generate PICs and plateau potentials within 1-2 months following spinal transection. Ca(v)1.3 L-type calcium channels are involved in generating PICs in both healthy and injured animals. Additionally, expression of Ca(v)1.2 and Ca(v)1.3 L-type calcium channels is altered in several pathological conditions. Therefore, in this paper we analyzed the expression of L-type calcium channel alpha(1) subunits within the motoneuron pool following a complete transection of the spinal cord at the level of the sacral vertebra (S)2 segment. The analysis was done both caudally (S4 segment) and rostrally [thoracic vertebra (T)6 segment] from the injury site. The S4 segment was significantly reduced in diameter when compared with control animals, and this reduction was more evident in the white matter. Ca(v)1.2 alpha(1) subunit expression significantly increased (26%) in the motoneuron pool located caudally but not rostrally from the injury site. In contrast, the expression of Ca(v)1.3 alpha(1) subunit remained unchanged in both S4 and T6 segments. The differential expression of the two alpha(1) subunits in spinal injury suggests that Ca(v)1.2 and Ca(v)1.3 channels have different functions in neuronal adaptation following spinal cord injury.

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.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Differential contribution of skeletal and cardiac II-III loop sequences to the assembly of dihydropyridine-receptor arrays in skeletal muscle

The plasmalemmal dihydropyridine receptor (DHPR) is the voltage sensor in skeletal muscle excitation-contraction (e-c) coupling. It activates calcium release from the sarcoplasmic reticulum via protein-protein interactions with the ryanodine receptor (RyR). To enable this interaction, DHPRs are arranged in arrays of tetrads opposite RyRs. In the DHPR alpha(1S) subunit, the cytoplasmic loop connecting repeats II and III is a major determinant of skeletal-type e-c coupling. Whether the essential II-III loop sequence (L720-L764) also determines the skeletal-specific arrangement of DHPRs was examined in dysgenic (alpha(1S)-null) myotubes reconstituted with distinct alpha(1) subunit isoforms and II-III loop chimeras. Parallel immunofluorescence and freeze-fracture analysis showed that alpha(1S) and chimeras containing L720-L764, all of which restored skeletal-type e-c coupling, displayed the skeletal arrangement of DHPRs in arrays of tetrads. Conversely, alpha(1C) and those chimeras with a cardiac II-III loop and cardiac e-c coupling properties were targeted into junctional membranes but failed to form tetrads. However, an alpha(1S)-based chimera with the heterologous Musca II-III loop produced tetrads but did not reconstitute skeletal muscle e-c coupling. These findings suggest an inhibitory role in tetrad formation of the cardiac II-III loop and that the organization of DHPRs in tetrads vis-a-vis the RyR is necessary but not sufficient for skeletal-type e-c coupling.

The monoclonal antibody mAB 1A binds to the excitation--contraction coupling domain in the II-III loop of the skeletal muscle calcium channel alpha(1S) subunit

Interactions of the II-III loop of the voltage-gated Ca(2+) channel alpha(1S) subunit with the Ca(2+) release channel (RyR1) are essential for skeletal-type excitation-contraction (EC) coupling. Here, we characterized the binding site of the monoclonal alpha(1S) antibody mAB 1A and used it to probe the structure of the II-III loop in chimeras with different EC coupling properties. Phage-display epitope mapping of mAB 1A revealed a minimal consensus binding sequence X-P-X-X-D-X-P. Immunofluorescence labeling of (1S), alpha(1C), alpha(1D), and of II-III loop chimeras expressed in dysgenic myotubes established that mAB 1A reacted specifically with amino acids 737-744 in the II-III loop of alpha(1S), which is within the domain (D734-L764) critical for bidirectional coupling with RyR1. Comparing mAB 1A immunoreactivity with known structural and functional properties of II-III loop chimeras in which the non-conserved skeletal residues were systematically mutated to their cardiac counterparts indicated a correlation of mAB 1A immunoreactivity and skeletal-type EC coupling.

Functional analysis of the R1086H malignant hyperthermia mutation in the DHPR reveals an unexpected influence of the III-IV loop on skeletal muscle EC coupling

Malignant hyperthermia (MH) is an inherited pharmacogenetic disorder caused by mutations in the skeletal muscle ryanodine receptor (RyR1) and the dihydropyridine receptor (DHPR) alpha(1S)-subunit. We characterized the effects of an MH mutation in the DHPR cytoplasmic III-IV loop of alpha(1S) (R1086H) on DHPR-RyR1 coupling after reconstitution in dysgenic (alpha(1S) null) myotubes. Compared with wild-type alpha(1S), caffeine-activated Ca(2+) release occurred at approximately fivefold lower concentrations in nonexpressing and R1086H-expressing myotubes. Although maximal voltage-gated Ca(2+) release was similar in alpha(1S)- and R1086H-expressing myotubes, the voltage dependence of Ca(2+) release was shifted approximately 5 mV to more negative potentials in R1086H-expressing myotubes. Our results demonstrate that alpha(1S) functions as a negative allosteric modulator of release channel activation by caffeine/voltage and that the R1086H MH mutation in the intracellular III-IV linker disrupts this negative regulatory influence. Moreover, a low caffeine concentration (2 mM) caused a similar shift in voltage dependence of Ca(2+) release in alpha(1S)- and R1086H-expressing myotubes. Compared with alpha(1S)-expressing myotubes, maximal L channel conductance (G(max)) was reduced in R1086H-expressing myotubes (alpha(1S) 130 +/- 10.2, R1086H 88 +/- 6.8 nS/nF; P < 0.05). The decrease in G(max) did not result from a change in retrograde coupling with RyR1 as maximal conductance-charge movement ratio (G(max)/Q(max)) was similar in alpha(1S)- and R1086H-expressing myotubes and a similar decrease in G(max) was observed for an analogous mutation engineered into the cardiac L channel (R1217H). In addition, both R1086H and R1217H DHPRs targeted normally and colocalized with RyR1 in sarcoplasmic reticulum (SR)-sarcolemmal junctions. These results indicate that the R1086H MH mutation in alpha(1S) enhances RyR1 sensitivity to activation by both endogenous (voltage sensor) and exogenous (caffeine) activators.

Slow calcium signals after tetanic electrical stimulation in skeletal myotubes

The fluorescent calcium signal from rat myotubes in culture was monitored after field-stimulation with tetanic protocols. After the calcium signal sensitive to ryanodine and associated to the excitation-contraction coupling, a second long-lasting calcium signal refractory to ryanodine was consistently found. The onset kinetics of this slow signal were slightly modified in nominally calcium-free medium, as were both the frequency and number of pulses during tetanus. No signal was detected in the presence of tetrodotoxin. The participation of the dihydropyridine receptor (DHPR) as the voltage sensor for this signal was assessed by treatment with agonist and antagonist dihydropyridines (Bay K 8644 and nifedipine), showing an enhanced and inhibitory response, respectively. In the dysgenic GLT cell line, which lacks the alpha1(S) subunit of the DHPR, the signal was absent. Transfection of these cells with the alpha1(S) subunit restored the slow signal. In myotubes, the inositol 1,4,5-trisphosphate (IP(3)) mass increase induced by a tetanus protocol preceded in time the slow calcium signal. Both an IP(3) receptor blocker and a phospholipase C inhibitor (xestospongin C and U73122, respectively) dramatically inhibit this signal. Long-lasting, IP(3)-generated slow calcium signals appear to be a physiological response to activity-related fluctuations in membrane potential sensed by the DHPR.

Ca2+ influx through alpha1S DHPR may play a role in regulating Ca2+ release from RyR1 in skeletal muscle

Differentiated primary myotubes isolated from wild-type mice exhibit ryanodine-sensitive, spontaneous global Ca2+ oscillations as well as spontaneous depolarizations in the plasma membrane. Immunolabeling of these myotubes showed expression of both alpha1S dihydropyridine receptors (DHPRs) and ryanodine-sensitive Ca2+-release channel 1 (RyR1), the two key proteins in skeletal excitation-contraction (E-C) coupling. Spontaneous global Ca2+ oscillations could be inhibited by addition of 0.1 mM CdCl2/0.5 mM LaCl3 or 5 microM nifedipine to the extracellular bathing solution. After either treatment, Ca2+ oscillations could be restored upon extensive washing. Although exposure to DHPR antagonists completely blocked Ca2+ oscillations, normal orthograde signaling between DHPRs and RyRs, such as that elicited by 80 mM KCl depolarization, was still observed. In addition, we showed that spontaneous Ca2+ oscillations were never present in cultured mdg myotubes, which lack the expression of alpha1SDHPRs. These results suggest that under physiological conditions in conjunction with the mechanical coupling between the alpha1SDHPRs and RyR1, the initiation of Ca2+ oscillations in myotubes may be facilitated, in part, by the Ca2+ influx through the alpha1s-subunit of the DHPR.