Expression of the alpha(2)delta subunit interferes with prepulse facilitation in cardiac L-type calcium channels

Deletion of the α2δ-1 calcium channel subunit increases excitability of mouse chromaffin cells

High voltage-gated Ca2+ channels (HVCCs) shape the electrical activity and control hormone release in most endocrine cells. HVCCs are multi-subunit protein complexes formed by the pore-forming α1 and the auxiliary β, α2δ and γ subunits. Four genes code for the α2δ isoforms. At the mRNA level, mouse chromaffin cells (MCCs) express predominantly the CACNA2D1 gene coding for the α2δ-1 isoform. Here we show that α2δ-1 deletion led to ∼60% reduced HVCC Ca2+ influx with slower inactivation kinetics. Pharmacological dissection showed that HVCC composition remained similar in α2δ-1-/- MCCs compared to wild-type (WT), demonstrating that α2δ-1 exerts similar functional effects on all HVCC isoforms. Consistent with reduced HVCC Ca2+ influx, α2δ-1-/- MCCs showed reduced spontaneous electrical activity with action potentials (APs) having a shorter half-maximal duration caused by faster rising and decay slopes. However, the induced electrical activity showed opposite effects with α2δ-1-/- MCCs displaying significantly higher AP frequency in the tonic firing mode as well as an increase in the number of cells firing AP bursts compared to WT. This gain-of-function phenotype was caused by reduced functional activation of Ca2+-dependent K+ currents. Additionally, despite the reduced HVCC Ca2+ influx, the intracellular Ca2+ transients and vesicle exocytosis or endocytosis were unaltered in α2δ-1-/- MCCs compared to WT during sustained stimulation. In conclusion, our study shows that α2δ-1 genetic deletion reduces Ca2+ influx in cultured MCCs but leads to a paradoxical increase in catecholamine secretion due to increased excitability. KEY POINTS: Deletion of the α2δ-1 high voltage-gated Ca2+ channel (HVCC) subunit reduces mouse chromaffin cell (MCC) Ca2+ influx by ∼60% but causes a paradoxical increase in induced excitability. MCC intracellular Ca2+ transients are unaffected by the reduced HVCC Ca2+ influx. Deletion of α2δ-1 reduces the immediately releasable pool vesicle exocytosis but has no effect on catecholamine (CA) release in response to sustained stimuli. The increased electrical activity and CA release from MCCs might contribute to the previously reported cardiovascular phenotype of patients carrying α2δ-1 loss-of-function mutations.

Role of High Voltage-Gated Ca2+ Channel Subunits in Pancreatic β-Cell Insulin Release. From Structure to Function

The pancreatic islets of Langerhans secrete several hormones critical for glucose homeostasis. The β-cells, the major cellular component of the pancreatic islets, secrete insulin, the only hormone capable of lowering the plasma glucose concentration. The counter-regulatory hormone glucagon is secreted by the α-cells while δ-cells secrete somatostatin that via paracrine mechanisms regulates the α- and β-cell activity. These three peptide hormones are packed into secretory granules that are released through exocytosis following a local increase in intracellular Ca2+ concentration. The high voltage-gated Ca2+ channels (HVCCs) occupy a central role in pancreatic hormone release both as a source of Ca2+ required for excitation-secretion coupling as well as a scaffold for the release machinery. HVCCs are multi-protein complexes composed of the main pore-forming transmembrane α1 and the auxiliary intracellular β, extracellular α2δ, and transmembrane γ subunits. Here, we review the current understanding regarding the role of all HVCC subunits expressed in pancreatic β-cell on electrical activity, excitation-secretion coupling, and β-cell mass. The evidence we review was obtained from many seminal studies employing pharmacological approaches as well as genetically modified mouse models. The significance for diabetes in humans is discussed in the context of genetic variations in the genes encoding for the HVCC subunits.

Molecular characterization of a voltage-gated calcium channel and its potential role in the acaricidal action of scopoletin against Tetranychus cinnabarinus

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is a polyphagous agricultural pest with an extensive host plant range. Scopoletin is a promising acaricidal compound whose acaricidal mechanism may occur by disrupting intracellular Ca²⁺ homeostasis and calcium signaling pathways. However, the underlying mechanism of scopoletin for specific target locations of T. cinnabarinus remains unclear. In this study, a full-length cDNA of the L-type voltage-gated calcium channel (TcLTCC) subunit gene from T. cinnabarinus was cloned and characterized. The expression pattern of the TcLTCC gene in all developmental stages of T. cinnabarinus was analyzed. The gene was highly expressed in larval and nymphal stages and was significantly upregulated after treatment with scopoletin. Knocking down the TcLTCC transcript reduced the sensitivity of T. cinnabarinus to scopoletin. Homology modeling and molecular docking were also conducted. The interaction between scopoletin and TcLTCC showed that scopoletin inserted into the cavity bound to the site of the TcLTCC protein by the driving force of hydrogen bonding. This study provides insights into the mechanism by which scopoletin interacts with TcLTCC. Results can improve the understanding of the toxicity of scopoletin to T. cinnabarinus and provide valuable information for the design of new LTCC inhibitors.

Reduced myocyte complex N-glycosylation causes dilated cardiomyopathy

Protein glycosylation is an essential posttranslational modification that affects a myriad of physiologic processes. Humans with genetic defects in glycosylation, which result in truncated glycans, often present with significant cardiac deficits. Acquired heart diseases and their associated risk factors were also linked to aberrant glycosylation, highlighting its importance in human cardiac disease. In both cases, the link between causation and corollary remains enigmatic. The glycosyltransferase gene, mannosyl (α-1,3-)-glycoprotein β-1,2- N-acetylglucosaminyltransferase (Mgat1), whose product, N-acetylglucosaminyltransferase 1 (GlcNAcT1) is necessary for the formation of hybrid and complex N-glycan structures in the medial Golgi, was shown to be at reduced levels in human end-stage cardiomyopathy, thus making Mgat1 an attractive target for investigating the role of hybrid/complex N-glycosylation in cardiac pathogenesis. Here, we created a cardiomyocyte-specific Mgat1 knockout (KO) mouse to establish a model useful in exploring the relationship between hybrid/complex N-glycosylation and cardiac function and disease. Biochemical and glycomic analyses showed that Mgat1KO cardiomyocytes produce predominately truncated N-glycan structures. All Mgat1KO mice died significantly younger than control mice and demonstrated chamber dilation and systolic dysfunction resembling human dilated cardiomyopathy (DCM). Data also indicate that a cardiomyocyte L-type voltage-gated Ca²⁺ channel (Ca_v) subunit (α2δ1) is a GlcNAcT1 target, and Mgat1KO Ca_v activity is shifted to more-depolarized membrane potentials. Consistently, Mgat1KO cardiomyocyte Ca²⁺ handling is altered and contraction is dyssynchronous compared with controls. The data demonstrate that reduced hybrid/complex N-glycosylation contributes to aberrant cardiac function at whole-heart and myocyte levels drawing a direct link between altered glycosylation and heart disease. Thus, the Mgat1KO provides a model for investigating the relationship between systemic reductions in glycosylation and cardiac disease, showing that clinically relevant changes in cardiomyocyte hybrid/complex N-glycosylation are sufficient to cause DCM and early death.-Ednie, A. R., Deng, W., Yip, K.-P., Bennett, E. S. Reduced myocyte complex N-glycosylation causes dilated cardiomyopathy.

Stroke-Like Episodes and Cerebellar Syndrome in Phosphomannomutase Deficiency (PMM2-CDG): Evidence for Hypoglycosylation-Driven Channelopathy

Stroke-like episodes (SLE) occur in phosphomannomutase deficiency (PMM2-CDG), and may complicate the course of channelopathies related to Familial Hemiplegic Migraine (FHM) caused by mutations in CACNA1A (encoding Ca_V2.1 channel). The underlying pathomechanisms are unknown. We analyze clinical variables to detect risk factors for SLE in a series of 43 PMM2-CDG patients. We explore the hypothesis of abnormal Ca_V2.1 function due to aberrant N-glycosylation as a potential novel pathomechanism of SLE and ataxia in PMM2-CDG by using whole-cell patch-clamp, N-glycosylation blockade and mutagenesis. Nine SLE were identified. Neuroimages showed no signs of stroke. Comparison of characteristics between SLE positive versus negative patients' group showed no differences. Acute and chronic phenotypes of patients with PMM2-CDG or CACNA1A channelopathies show similarities. Hypoglycosylation of both Ca_V2.1 subunits (α_1A and α_2α) induced gain-of-function effects on channel gating that mirrored those reported for pathogenic CACNA1A mutations linked to FHM and ataxia. Unoccupied N-glycosylation site N283 at α_1A contributes to a gain-of-function by lessening Ca_V2.1 inactivation. Hypoglycosylation of the α₂δ subunit also participates in the gain-of-function effect by promoting voltage-dependent opening of the Ca_V2.1 channel. Ca_V2.1 hypoglycosylation may cause ataxia and SLEs in PMM2-CDG patients. Aberrant Ca_V2.1 N-glycosylation as a novel pathomechanism in PMM2-CDG opens new therapeutic possibilities.

The α2δ-1 subunit remodels CaV1.2 voltage sensors and allows Ca2+ influx at physiological membrane potentials

Voltage-sensing domains (VSDs) in voltage-gated calcium channels sense the potential difference across membranes and interact with the pore to open it. Savalli et al. find that the accessory subunit α₂δ-1 increases the sensitivity of VSDs I–III and also their efficiency of coupling to the pore.

Excitation-evoked calcium influx across cellular membranes is strictly controlled by voltage-gated calcium channels (Ca_V), which possess four distinct voltage-sensing domains (VSDs) that direct the opening of a central pore. The energetic interactions between the VSDs and the pore are critical for tuning the channel’s voltage dependence. The accessory α₂δ-1 subunit is known to facilitate Ca_V1.2 voltage-dependent activation, but the underlying mechanism is unknown. In this study, using voltage clamp fluorometry, we track the activation of the four individual VSDs in a human L-type Ca_V1.2 channel consisting of α_1C and β₃ subunits. We find that, without α₂δ-1, the channel complex displays a right-shifted voltage dependence such that currents mainly develop at nonphysiological membrane potentials because of very weak VSD–pore interactions. The presence of α₂δ-1 facilitates channel activation by increasing the voltage sensitivity (i.e., the effective charge) of VSDs I–III. Moreover, the α₂δ-1 subunit also makes VSDs I–III more efficient at opening the channel by increasing the coupling energy between VSDs II and III and the pore, thus allowing Ca influx within the range of physiological membrane potentials.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

Functional characterization of CaVα2δ mutations associated with sudden cardiac death

L-type Ca(2+) channels play a critical role in cardiac rhythmicity. These ion channels are oligomeric complexes formed by the pore-forming CaVα1 with the auxiliary CaVβ and CaVα2δ subunits. CaVα2δ increases the peak current density and improves the voltage-dependent activation gating of CaV1.2 channels without increasing the surface expression of the CaVα1 subunit. The functional impact of genetic variants of CACNA2D1 (the gene encoding for CaVα2δ), associated with shorter repolarization QT intervals (the time interval between the Q and the T waves on the cardiac electrocardiogram), was investigated after recombinant expression of the full complement of L-type CaV1.2 subunits in human embryonic kidney 293 cells. By performing side-by-side high resolution flow cytometry assays and whole-cell patch clamp recordings, we revealed that the surface density of the CaVα2δ wild-type protein correlates with the peak current density. Furthermore, the cell surface density of CaVα2δ mutants S755T, Q917H, and S956T was not significantly different from the cell surface density of the CaVα2δ wild-type protein expressed under the same conditions. In contrast, the cell surface expression of CaVα2δ D550Y, CaVα2δ S709N, and the double mutant D550Y/Q917H was reduced, respectively, by ≈30-33% for the single mutants and by 60% for the latter. The cell surface density of D550Y/Q917H was more significantly impaired than protein stability, suggesting that surface trafficking of CaVα2δ was disrupted by the double mutation. Co-expression with D550Y/Q917H significantly decreased CaV1.2 currents as compared with results obtained with CaVα2δ wild type. It is concluded that D550Y/Q917H reduced inward Ca(2+) currents through a defect in the cell surface trafficking of CaVα2δ. Altogether, our results provide novel insight in the molecular mechanism underlying the modulation of CaV1.2 currents by CaVα2δ.

Structure-function of proteins interacting with the α1 pore-forming subunit of high-voltage-activated calcium channels

Openings of high-voltage-activated (HVA) calcium channels lead to a transient increase in calcium concentration that in turn activate a plethora of cellular functions, including muscle contraction, secretion and gene transcription. To coordinate all these responses calcium channels form supramolecular assemblies containing effectors and regulatory proteins that couple calcium influx to the downstream signal cascades and to feedback elements. According to the original biochemical characterization of skeletal muscle Dihydropyridine receptors, HVA calcium channels are multi-subunit protein complexes consisting of a pore-forming subunit (α1) associated with four additional polypeptide chains β, α2, δ, and γ, often referred to as accessory subunits. Twenty-five years after the first purification of a high-voltage calcium channel, the concept of a flexible stoichiometry to expand the repertoire of mechanisms that regulate calcium channel influx has emerged. Several other proteins have been identified that associate directly with the α1-subunit, including calmodulin and multiple members of the small and large GTPase family. Some of these proteins only interact with a subset of α1-subunits and during specific stages of biogenesis. More strikingly, most of the α1-subunit interacting proteins, such as the β-subunit and small GTPases, regulate both gating and trafficking through a variety of mechanisms. Modulation of channel activity covers almost all biophysical properties of the channel. Likewise, regulation of the number of channels in the plasma membrane is performed by altering the release of the α1-subunit from the endoplasmic reticulum, by reducing its degradation or enhancing its recycling back to the cell surface. In this review, we discuss the structural basis, interplay and functional role of selected proteins that interact with the central pore-forming subunit of HVA calcium channels.

The Cav1.2 N terminus contains a CaM kinase site that modulates channel trafficking and function

The L-type voltage-gated calcium channel Cav1.2 and the calcium-activated CaM kinase cascade both regulate excitation transcription coupling in the brain. CaM kinase is known to associate with the C terminus of Cav1.2 in a region called the PreIQ-IQ domain, which also binds multiple calmodulin molecules. Here we identify and characterize a second CaMKII binding site in the N terminus of Cav1.2 that is formed by a stretch of four amino residues (cysteine-isoleucine-serine-isoleucine) and which regulates channel expression and function. By using live cell imaging of tsA-201 cells we show that GFP fusion constructs of the CaMKII binding region, termed N2B-II co-localize with mCherry-CaMKII. Mutating CISI to AAAA ablates binding to and colocalization with CaMKII. Cav1.2-AAAA channels show reduced cell surface expression in tsA-201 cells, but interestingly, display an increase in channel function that offsets the trafficking deficit. Altogether our data reveal that the proximal N terminus of Cav1.2 contains a CaMKII binding region which contributes to channel surface expression and function.

L-type CaV1.2 calcium channels: from in vitro findings to in vivo function

The L-type Cav1.2 calcium channel is present throughout the animal kingdom and is essential for some aspects of CNS function, cardiac and smooth muscle contractility, neuroendocrine regulation, and multiple other processes. The L-type CaV1.2 channel is built by up to four subunits; all subunits exist in various splice variants that potentially affect the biophysical and biological functions of the channel. Many of the CaV1.2 channel properties have been analyzed in heterologous expression systems including regulation of the L-type CaV1.2 channel by Ca(2+) itself and protein kinases. However, targeted mutations of the calcium channel genes confirmed only some of these in vitro findings. Substitution of the respective serines by alanine showed that β-adrenergic upregulation of the cardiac CaV1.2 channel did not depend on the phosphorylation of the in vitro specified amino acids. Moreover, well-established in vitro phosphorylation sites of the CaVβ2 subunit of the cardiac L-type CaV1.2 channel were found to be irrelevant for the in vivo regulation of the channel. However, the molecular basis of some kinetic properties, such as Ca(2+)-dependent inactivation and facilitation, has been approved by in vivo mutagenesis of the CaV1.2α1 gene. This article summarizes recent findings on the in vivo relevance of well-established in vitro results.

Shaping a new Ca²⁺ conductance to suppress early afterdepolarizations in cardiac myocytes

Sudden cardiac death (SCD) due to ventricular fibrillation (VF) is a major world-wide health problem. A common trigger of VF involves abnormal repolarization of the cardiac action potential causing early afterdepolarizations (EADs). Here we used a hybrid biological-computational approach to investigate the dependence of EADs on the biophysical properties of the L-type Ca(2+) current (I(Ca,L)) and to explore how modifications of these properties could be designed to suppress EADs. EADs were induced in isolated rabbit ventricular myocytes by exposure to 600 μmol l(-1) H(2)O(2) (oxidative stress) or lowering the external [K(+)] from 5.4 to 2.0-2.7 mmol l(-1) (hypokalaemia). The role of I(Ca,L) in EAD formation was directly assessed using the dynamic clamp technique: the paced myocyte's V(m) was input to a myocyte model with tunable biophysical parameters, which computed a virtual I(Ca,L), which was injected into the myocyte in real time. This virtual current replaced the endogenous I(Ca,L), which was suppressed with nifedipine. Injecting a current with the biophysical properties of the native I(Ca,L) restored EAD occurrence in myocytes challenged by H(2)O(2) or hypokalaemia. A mere 5 mV depolarizing shift in the voltage dependence of activation or a hyperpolarizing shift in the steady-state inactivation curve completely abolished EADs in myocytes while maintaining a normal Ca(i) transient. We propose that modifying the biophysical properties of I(Ca,L) has potential as a powerful therapeutic strategy for suppressing EADs and EAD-mediated arrhythmias.

Molecular determinants of the CaVbeta-induced plasma membrane targeting of the CaV1.2 channel

Ca(V)beta subunits modulate cell surface expression and voltage-dependent gating of high voltage-activated (HVA) Ca(V)1 and Ca(V)2 alpha1 subunits. High affinity Ca(V)beta binding onto the so-called alpha interaction domain of the I-II linker of the Ca(V)alpha1 subunit is required for Ca(V)beta modulation of HVA channel gating. It has been suggested, however, that Ca(V)beta-mediated plasma membrane targeting could be uncoupled from Ca(V)beta-mediated modulation of channel gating. In addition to Ca(V)beta, Ca(V)alpha2delta and calmodulin have been proposed to play important roles in HVA channel targeting. Indeed we show that co-expression of Ca(V)alpha2delta caused a 5-fold stimulation of the whole cell currents measured with Ca(V)1.2 and Ca(V)beta3. To gauge the synergetic role of auxiliary subunits in the steady-state plasma membrane expression of Ca(V)1.2, extracellularly tagged Ca(V)1.2 proteins were quantified using fluorescence-activated cell sorting analysis. Co-expression of Ca(V)1.2 with either Ca(V)alpha2delta, calmodulin wild type, or apocalmodulin (alone or in combination) failed to promote the detection of fluorescently labeled Ca(V)1.2 subunits. In contrast, co-expression with Ca(V)beta3 stimulated plasma membrane expression of Ca(V)1.2 by a 10-fold factor. Mutations within the alpha interaction domain of Ca(V)1.2 or within the nucleotide kinase domain of Ca(V)beta3 disrupted the Ca(V)beta3-induced plasma membrane targeting of Ca(V)1.2. Altogether, these data support a model where high affinity binding of Ca(V)beta to the I-II linker of Ca(V)alpha1 largely accounts for Ca(V)beta-induced plasma membrane targeting of Ca(V)1.2.

Functional properties of the CaV1.2 calcium channel activated by calmodulin in the absence of alpha2delta subunits

Voltage-activated CaV1.2 calcium channels require association of the pore-forming alpha1C subunit with accessory CaVbeta and alpha2delta subunits. Binding of a single calmodulin (CaM) to alpha1C supports Ca2+-dependent inactivation (CDI). The human CaV1.2 channel is silent in the absence of CaVbeta and/or alpha2delta. Recently, we found that coexpression of exogenous CaM (CaMex) supports plasma membrane targeting, gating facilitation and CDI of the channel in the absence of CaVbeta. Here we discovered that CaMex and its Ca2+-insensitive mutant (CaM1234) rendered active alpha1C/CaVbeta channel in the absence of alpha2delta. Coexpression of CaMex with alpha1C and beta2d in calcium-channel-free COS-1 cells recovered gating of the channel and supported CDI. Voltage-dependence of activation was shifted by approximately +40 mV to depolarization potentials. The calcium current reached maximum at +40 mV (20 mM Ca2+) and exhibited approximately 3 times slower activation and 5 times slower inactivation kinetics compared to the wild-type channel. Furthermore, both CaMex and CaM1234 accelerated recovery from inactivation and induced facilitation of the calcium current by strong depolarization prepulse, the properties absent from the human vascular/neuronal CaV1.2 channel. The data suggest a previously unknown action of CaM that in the presence of CaVbeta; translates into activation of the alpha2delta-deficient calcium channel and alteration of its properties.

Cumulative activation of voltage-dependent KVS-1 potassium channels

In this study, we reveal the existence of a novel use-dependent phenomenon in potassium channels, which we refer to as cumulative activation (CA). CA consists of an increase in current amplitude in response to repetitive depolarizing step pulses to the same potential. CA persists for up to 20 s and is similar to a phenomenon called "voltage-dependent facilitation" observed in some calcium channels. The KVS-1 K+ channel, which exhibits CA, is a rapidly activating and inactivating voltage-dependent potassium channel expressed in chemosensory and other neurons of Caenorhabditis elegans. It is unusual in being most closely related to the Shab (Kv2) family of potassium channels, which typically behave like delayed rectifier K+ channels in other species. The magnitude of CA depends on the frequency, voltage, and duration of the depolarizing step pulse. CA also radically changes the activation and inactivation kinetics of the channel, suggesting that the channel may undergo a physical modification in a use-dependent manner; thus, a model that closely simulates the behavior of the channel postulates the existence of two populations of channels, unmodified and modified. Use-dependent changes in the behavior of potassium channels, such as CA observed in KVS-1, could be involved in functional mechanisms of cellular plasticity such as synaptic depression that represent the cellular basis of learning and memory.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

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.

Subcellular distribution of high-voltage-activated calcium channel subtypes in rat globus pallidus neurons

Globus pallidus (GP) neurons receive dense inhibitory synaptic inputs interspersed with sparse excitatory inputs distributed across the entire extent of their somata and dendrites. Yet, despite this predominance of inhibitory influence, GP neurons fire at a high tonic rate, suggesting that intrinsic properties play an important role in determining the physiological characteristics of these neurons. High-voltage-activated (HVA) calcium channels represent an important class of conductances that plays roles in controlling neurotransmitter release, postsynaptic excitability, and intracellular calcium signaling. To better understand the intrinsic properties of GP neurons, we examined the subcellular localization of HVA calcium channels by using immunocytochemistry at the electron microscopic level. Peroxidase labeling with antibodies against P/Q-, N-, and R-type HVA calcium channels demonstrated the presence of these channels in both proximal and distal dendrites of GP neurons. P/Q-, N-, and R-type channels were also found in presynaptic terminals, whereas L-type channels were found exclusively postsynaptically in neuronal elements. Immunogold labeling demonstrated that, although the density of intracellular L-type calcium channel labeling remains constant throughout the proximal-distal extent of the dendritic tree of GP neurons, the density of plasma membrane-bound channels is greater in distal dendrites. The finding of HVA calcium channels distributed throughout the whole dendritic tree of GP neurons indicates that these channels may interact with synaptic inputs to allow rich processing possibilities for GP neuron dendrites. Furthermore, the finding of a greater density of plasma membrane-bound L-type channels in distal dendrites expands the view that L-type channels are important only in somatic and proximal locations.

Neuronal Ca(V)1.3alpha(1) L-type channels activate at relatively hyperpolarized membrane potentials and are incompletely inhibited by dihydropyridines

L-type calcium channels regulate a diverse array of cellular functions within excitable cells. Of the four molecularly defined subclasses of L-type Ca channels, two are expressed ubiquitously in the mammalian nervous system (Ca(V)1.2alpha(1) and Ca(V)1.3alpha(1)). Despite diversity at the molecular level, neuronal L-type channels are generally assumed to be functionally and pharmacologically similar, i.e., high-voltage activated and highly sensitive to dihydropyridines. We now show that Ca(V)1.3alpha(1) L-type channels activate at membrane potentials approximately 25 mV more hyperpolarized, compared with Ca(V)1.2alpha(1). This unusually negative activation threshold for Ca(V)1.3alpha(1) channels is independent of the specific auxiliary subunits coexpressed, of alternative splicing in domains I-II, IVS3-IVS4, and the C terminus, and of the expression system. The use of high concentrations of extracellular divalent cations has possibly obscured the unique voltage-dependent properties of Ca(V)1.3alpha(1) in certain previous studies. We also demonstrate that Ca(V)1.3alpha(1) channels are pharmacologically distinct from Ca(V)1.2alpha(1). The IC(50) for nimodipine block of Ca(V)1.3alpha(1) L-type calcium channel currents is 2.7 +/- 0.3 microm, a value 20-fold higher than the concentration required to block Ca(V)1.2alpha(1). The relatively low sensitivity of the Ca(V)1.3alpha(1) subunit to inhibition by dihydropyridine is unaffected by alternative splicing in the IVS3-IVS4 linker. Our results suggest that functional and pharmacological criteria used commonly to distinguish among different Ca currents greatly underestimate the biological importance of L-type channels in cells expressing Ca(v)1.3alpha(1).

Neuronal Ca(V)1.3alpha(1) L-type channels activate at relatively hyperpolarized membrane potentials and are incompletely inhibited by dihydropyridines

L-type calcium channels regulate a diverse array of cellular functions within excitable cells. Of the four molecularly defined subclasses of L-type Ca channels, two are expressed ubiquitously in the mammalian nervous system (Ca(V)1.2alpha(1) and Ca(V)1.3alpha(1)). Despite diversity at the molecular level, neuronal L-type channels are generally assumed to be functionally and pharmacologically similar, i.e., high-voltage activated and highly sensitive to dihydropyridines. We now show that Ca(V)1.3alpha(1) L-type channels activate at membrane potentials approximately 25 mV more hyperpolarized, compared with Ca(V)1.2alpha(1). This unusually negative activation threshold for Ca(V)1.3alpha(1) channels is independent of the specific auxiliary subunits coexpressed, of alternative splicing in domains I-II, IVS3-IVS4, and the C terminus, and of the expression system. The use of high concentrations of extracellular divalent cations has possibly obscured the unique voltage-dependent properties of Ca(V)1.3alpha(1) in certain previous studies. We also demonstrate that Ca(V)1.3alpha(1) channels are pharmacologically distinct from Ca(V)1.2alpha(1). The IC(50) for nimodipine block of Ca(V)1.3alpha(1) L-type calcium channel currents is 2.7 +/- 0.3 microm, a value 20-fold higher than the concentration required to block Ca(V)1.2alpha(1). The relatively low sensitivity of the Ca(V)1.3alpha(1) subunit to inhibition by dihydropyridine is unaffected by alternative splicing in the IVS3-IVS4 linker. Our results suggest that functional and pharmacological criteria used commonly to distinguish among different Ca currents greatly underestimate the biological importance of L-type channels in cells expressing Ca(v)1.3alpha(1).

Multiple structural elements contribute to voltage-dependent facilitation of neuronal alpha 1C (CaV1.2) L-type calcium channels

Voltage- and frequency-dependent facilitation of calcium channel activity has been implicated in a number of key physiological processes. Various mechanisms have been proposed to mediate these regulations, including a switch between channel gating modes, voltage-dependent phosphorylation, and a voltage-dependent deinhibition of G-protein block. Studying such modulation on recombinant Ca channels expressed in oocytes, we previously reported that alpha(1C) L-type calcium channel contrast with non-L type Ca channels by its ability to exhibit facilitation by pre-depolarization (Voltage-dependent facilitation of a neuronal alpha(IC) L-type calcium channel, E. Bourinet et al., EMBO Journal, 1994; 13, 5032-5039). To further analyze this effect, we have investigated the molecular determinants which mediate the differences in voltage-dependent facilitation between "facilitable" alpha(1C) and "non facilitable" alpha(1E) calcium channels. We used a series of chimeras which combine the four transmembrane domains of the two channels. Results show that the four domains of alpha(1C) contribute to facilitation, with domain I being most critical. This domain is required but not sufficient alone to generate facilitation. The minimal requirement to observe the effect is the presence of domain I plus one of the three others. We conclude that similarly to activation gating, voltage-dependent facilitation of alpha(1C) is a complex process which involves multiple structural elements were domains I and III play the major role.