A beta-subunit normalizes the electrophysiological properties of a cloned N-type Ca2+ channel alpha 1-subunit

The α2δ-like Protein Cachd1 Increases N-type Calcium Currents and Cell Surface Expression and Competes with α2δ-1

Voltage-gated calcium channel auxiliary α2δ subunits are important for channel trafficking and function. Here, we compare the effects of α2δ-1 and an α2δ-like protein called Cachd1 on neuronal N-type (Ca_V2.2) channels, which are important in neurotransmission. Previous structural studies show the α2δ-1 VWA domain interacting with the first loop in Ca_V1.1 domain-I via its metal ion-dependent adhesion site (MIDAS) motif and additional Cache domain interactions. Cachd1 has a disrupted MIDAS motif. However, Cachd1 increases Ca_V2.2 currents substantially (although less than α2δ-1) and increases Ca_V2.2 cell surface expression by reducing endocytosis. Although the effects of α2δ-1 are abolished by mutation of Asp122 in Ca_V2.2 domain-I, which mediates interaction with its VWA domain, the Cachd1 responses are unaffected. Furthermore, Cachd1 co-immunoprecipitates with Ca_V2.2 and inhibits co-immunoprecipitation of α2δ-1 by Ca_V2.2. Cachd1 also competes with α2δ-1 for effects on trafficking. Thus, Cachd1 influences both Ca_V2.2 trafficking and function and can inhibit responses to α2δ-1.

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Cachd1 enhances Ca_V2.2 currents and increases Ca_V2.2 surface expression

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Effects of Cachd1 are not prevented by mutation in Ca_V2.2 VWA interaction site

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The effects of α₂δ-1 are prevented by the same mutation in Ca_V2.2

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Cachd1 competes with α₂δ-1 for its effects on Ca_V2.2

Trafficking of neuronal calcium channels

Neuronal voltage-gated calcium channels (VGCCs) serve complex yet essential physiological functions via their pivotal role in translating electrical signals into intracellular calcium elevations and associated downstream signalling pathways. There are a number of regulatory mechanisms to ensure a dynamic control of the number of channels embedded in the plasma membrane, whereas alteration of the surface expression of VGCCs has been linked to various disease conditions. Here, we provide an overview of the mechanisms that control the trafficking of VGCCs to and from the plasma membrane, and discuss their implication in pathophysiological conditions and their potential as therapeutic targets.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Regulation of high-voltage-activated Ca2+ channel function, trafficking, and membrane stability by auxiliary subunits

Voltage-gated Ca²⁺ (Ca_V) channels mediate Ca²⁺ ions influx into cells in response to depolarization of the plasma membrane. They are responsible for initiation of excitation-contraction and excitation-secretion coupling, and the Ca²⁺ that enters cells through this pathway is also important in the regulation of protein phosphorylation, gene transcription, and many other intracellular events. Initial electrophysiological studies divided Ca_V channels into low-voltage-activated (LVA) and high-voltage-activated (HVA) channels. The HVA Ca_V channels were further subdivided into L, N, P/Q, and R-types which are oligomeric protein complexes composed of an ion-conducting Ca_Vα₁ subunit and auxiliary Ca_Vα₂δ, Ca_Vβ, and Ca_Vγ subunits. The functional consequences of the auxiliary subunits include altered functional and pharmacological properties of the channels as well as increased current densities. The latter observation suggests an important role of the auxiliary subunits in membrane trafficking of the Ca_Vα₁ subunit. This includes the mechanisms by which Ca_V channels are targeted to the plasma membrane and to appropriate regions within a given cell. Likewise, the auxiliary subunits seem to participate in the mechanisms that remove Ca_V channels from the plasma membrane for recycling and/or degradation. Diverse studies have provided important clues to the molecular mechanisms involved in the regulation of Ca_V channels by the auxiliary subunits, and the roles that these proteins could possibly play in channel targeting and membrane Stabilization.

A macromolecular trafficking complex composed of β₂-adrenergic receptors, A-Kinase Anchoring Proteins and L-type calcium channels

Abstract Sympathetic modulation of cardiac L-type calcium channels is an important mechanism for regulating heart rate and cardiac contractility. At the molecular level, activation of β-adrenergic receptors (βAR) increases calcium influx into cardiac myocytes by activating protein kinase A (PKA), leading to subsequent phosphorylation of L-type calcium channels. In the case of the β2AR, this process is facilitated by the presence of A-Kinase Anchoring Proteins (AKAPs) that serve as scaffolding proteins for the L-type calcium channel and the β2AR complex. Our work has shown that, in addition to facilitating PKA phosphorylation of the channel, AKAPs also promote an increase in the Cav1.2 channel surface expression. Here we review the molecular mechanisms of β2AR/AKAP/L-type channel interactions and trafficking.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

Trafficking and stability of voltage-gated calcium channels

Voltage-gated calcium channels are important mediators of calcium influx into electrically excitable cells. The amount of calcium entering through this family of channel proteins is not only determined by the functional properties of channels embedded in the plasma membrane but also by the numbers of channels that are expressed at the cell surface. The trafficking of channels is controlled by numerous processes, including co-assembly with ancillary calcium channel subunits, ubiquitin ligases, and interactions with other membrane proteins such as G protein coupled receptors. Here we provide an overview about the current state of knowledge of calcium channel trafficking to the cell membrane, and of the mechanisms regulating the stability and internalization of this important ion channel family.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

Presynaptic calcium channels: structure, regulators, and blockers

The central and peripheral nervous systems express multiple types of ligand and voltage-gated calcium channels (VGCCs), each with specific physiological roles and pharmacological and electrophysiological properties. The members of the Ca(v)2 calcium channel family are located predominantly at presynaptic nerve terminals, where they are responsible for controlling evoked neurotransmitter release. The activity of these channels is subject to modulation by a number of different means, including alternate splicing, ancillary subunit associations, peptide and small organic blockers, G-protein-coupled receptors (GPCRs), protein kinases, synaptic proteins, and calcium-binding proteins. These multiple and complex modes of calcium channel regulation allow neurons to maintain the specific, physiological window of cytoplasmic calcium concentrations which is required for optimal neurotransmission and proper synaptic function. Moreover, these varying means of channel regulation provide insight into potential therapeutic targets for the treatment of pathological conditions that arise from disturbances in calcium channel signaling. Indeed, considerable efforts are presently underway to identify and develop specific presynaptic calcium channel blockers that can be used as analgesics.

Protease treatment of cerebellar purkinje cells renders omega-agatoxin IVA-sensitive Ca2+ channels insensitive to inhibition by omega-conotoxin GVIA

The identification of currents carried by N- and P-type Ca(2+) channels in the nervous system relies on the use of omega-conotoxin (CTx) GVIA and omega-agatoxin (Aga) IVA. The peptide omega-Aga-IVA inhibits P-type currents at nanomolar concentrations and N-type currents at micromolar concentrations. omega-CTx-GVIA blocks N-type currents, but there have been no reports that it can also inhibit P-type currents. To assess the effects of omega-CTx-GVIA on P-type channels, we made patch-clamp recordings from the soma of Purkinje cells in cerebellar slices of mature [postnatal days (P) 40-50, P40-50] and immature (P13-20) rats, in which P-type channels carry most of the Ca(2+) channel current (>/=85%). These showed that micromolar concentrations of omega-CTx-GVIA inhibited the current in P40-50 cells (66%, 3 microM; 78%, 10 microM) and in P13-20 Purkinje cells (86%, 3 muM; 89%, 10 microM). The inhibition appeared to be reversible, in contrast to the known irreversible inhibition of N-type current. Exposure of slices from young animals to the enzyme commonly used to dissociate Purkinje cells, protease XXIII, abolished the inhibition by omega-CTx-GVIA but not by omega-Aga-IVA (84%, 30 nM). Our finding that micromolar concentrations of omega-CTx-GVIA inhibit P-type currents suggests that specific block of N-type current requires the use of submicromolar concentrations. The protease-induced removal of block by omega-CTx-GVIA but not by omega-Aga-IVA indicates a selective proteolytic action at site(s) on P-type channels with which omega-CTx-GVIA interacts. It also suggests that Ca(2+) channel pharmacology in neurons dissociated using protease may not predict that in neurons not exposed to the enzyme.

Kinetics of internalization and degradation of N-type voltage-gated calcium channels: Role of the α2/δ subunit

The contribution of voltage-gated calcium channels to excitable cell function depends, critically, upon the mechanisms that control their expression at the cell surface. While co-assembly of the pore forming alpha(1) and auxiliary beta subunits enhances channel surface expression, the levels are still only 30-40% of those seen with the core alpha(1B)/beta(1b)/alpha(2)delta calcium channel complex. To rationalize this observation, it has been suggested that the alpha(2)/delta subunit might stabilize calcium channel expression at the cell surface. To test this notion, we have resolved the effect of the alpha(2)/delta subunit on the rates of binding, internalization and degradation of defined N-type calcium channel surface complexes expressed in HEK293 cells, through pulse-labeling with the selective, cell impermeable, radioligand [(125)I]-omega-CgTx. Through detailed kinetic and sensitivity analysis we show that alpha(1B)/beta(1b)/alpha(2)delta complexes are internalized slowly (k(int) 0.4/h), whereupon, most become degraded (k(deg) 0.02/h). In contrast, alpha(1B)/beta(1b) complexes are internalized more rapidly (k(int) 0.8/h), following which they are either quickly degraded (k(deg) 0.1/h) or are sequestered slowly (k(tra) 0.1/h) to a pool that is metabolically stable within the time-frame of our experiments (24h). In neither case did we find evidence for recycling via the cell surface. Thus, our data argue for a novel mechanism where complexes lacking an alpha(2)/delta subunit are cleared from the cell surface and are rapidly degraded or stored, possibly for further attempts at complexation as new alpha(2)/delta subunits become available. The slower rate of internalization of complexes containing the alpha(2)/delta subunit rationalizes the stabilizing effect this subunit has upon calcium channel surface expression and suggests a mechanism by which alpha(2)delta mutations may cause severe neurological deficits.

Contribution of the kinetics of G protein dissociation to the characteristic modifications of N-type calcium channel activity

Direct G protein inhibition of N-type calcium channels is recognized by characteristic biophysical modifications. In this study, we quantify and simulate the importance of G protein dissociation on the phenotype of G protein-regulated whole-cell currents. Based on the observation that the voltage-dependence of the time constant of recovery from G protein inhibition is correlated with the voltage-dependence of channel opening, we depict all G protein effects by a simple kinetic scheme. All landmark modifications in calcium currents, except inhibition, can be successfully described using three simple biophysical parameters (extent of block, extent of recovery, and time constant of recovery). Modifications of these parameters by auxiliary beta subunits are at the origin of differences in N-type channel regulation by G proteins. The simulation data illustrate that channel reluctance can occur as the result of an experimental bias linked to the variable extent of G protein dissociation when peak currents are measured at various membrane potentials. To produce alterations in channel kinetics, the two most important parameters are the extents of initial block and recovery. These data emphasize the contribution of the degree and kinetics of G protein dissociation in the modification of N-type currents.

Pharmacological characterization of recombinant N-type calcium channel (Cav2.2) mediated calcium mobilization using FLIPR

The N-type voltage-gated calcium channel (Ca(v)2.2) functions in neurons to regulate neurotransmitter release. It comprises a clinically relevant target for chronic pain. We have validated a calcium mobilization approach to assessing Ca(v)2.2 pharmacology in two stable Ca(v)2.2 cell lines: alpha1(B), alpha2delta, beta(3)-HEK-293 and alpha1(B), beta(3)-HEK-293. Ca(v)2.2 channels were opened by addition of KCl and Ca(2+) mobilization was measured by Fluo-4 fluorescence on a fluorescence imaging plate reader (FLIPR(96)). Ca(v)2.2 expression and biophysics were confirmed by patch-clamp electrophysiology (EP). Both cell lines responded to KCl with adequate signal-to-background. Signals from both cell lines were inhibited by omega-conotoxin (ctx)-MVIIa and omega-conotoxin (ctx)-GVIa with IC(50) values of 1.8 and 1nM, respectively, for the three-subunit stable, and 0.9 and 0.6nM, respectively, for the two-subunit stable. Other known Ca(v)2.2 blockers were characterized including cadmium, flunarizine, fluspirilene, and mibefradil. IC(50) values correlated with literature EP-derived values. Novel Ca(v)2.2 pharmacology was identified in classes of compounds with other primary pharmacological activities, including Na(+) channel inhibitors and antidepressants. Novel Na(+) channel compounds with high potency at Ca(v)2.2 were identified in the phenoxyphenyl pyridine, phenoxyphenyl pyrazole, and other classes. The highest potency at Ca(v)2.2 tricyclic antidepressant identified was desipramine.

Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

The pore-forming alpha subunits of voltage-gated calcium channels contain the essential biophysical machinery that underlies calcium influx in response to cell depolarization. In combination with requisite auxiliary subunits, these pore subunits form calcium channel complexes that are pivotal to the physiology and pharmacology of diverse cells ranging from sperm to neurons. Not surprisingly, mutations in the pore subunits generate diverse pathologies, termed channelopathies, that range from failures in excitation-contraction coupling to night blindness. Over the last decade, major insights into the mechanisms of pathogenesis have been derived from animals showing spontaneous or induced mutations. In parallel, there has been considerable growth in our understanding of the workings of voltage-gated ion channels from a structure-function, regulation and cell biology perspective. Here we document our current understanding of the mutations underlying channelopathies involving the voltage-gated calcium channel alpha subunits in humans and other species.

Expression pattern of voltage-dependent calcium channel alpha1 and beta subunits in adrenal gland of N-type Ca2+ channel alpha1B subunit gene-deficient mice

The Ca2+ channel alpha1B subunit is a pore-forming component capable of generating N-type Ca2+ channel activity. Although N-type Ca2+ channel plays a role in a variety of neuronal functions, alpha1B-deficient mice exhibit normal life span without apparent abnormalities of behavior, histology or plasma norepinephrine level, presumably owing to compensation by some other Ca2+ channel alpha1 or beta subunit. In this study, we studied the levels of alpha1A, alpha1C, alpha1D, C1E, beta1, beta2, beta3 and beta4 mRNAs in adrenal gland of alpha1B-deficient mice. The alpha1A mRNA in homozygous mice was expressed at higher level than in wild or heterozygous mice, but no difference in the expression levels of alpha1c, alpha1D, alpha1E, beta1, beta2, beta3 and beta4 was found among wild, heterozygous and homozygous mice. The protein level of alpha1A in homozygous mice was also expressed at higher level than in wild or heterozygous mice. To examine whether increased expression is induced by cis-regulatory element within 5'-upstream region of alpha1A gene, we examined lacZ expression in alpha1B-deficient x alpha1A6.3-lacZ mice (carrying a 6.3-kb 5'-upstream fragment of alpha1A gene fused to E. coli lacZ reporter gene), which express lacZ in medullar chromaffin cells, but not in cortex. The levels of lacZ expression in homozygous alpha1B-deficient x alpha1A6.3-lacZ mice were higher than in wild or heterozygous mice. Therefore, a possible explanation of the normal behavior and plasma norepinephrine level of alpha1B-deficient mice is that compensation by alpha1A subunit occurs and that 6.3-kb 5'-upstream region of alpha1A gene contains enhancer cis-element(s) for compensation in adrenal medulla chromaffin cells.

Voltage-gated sodium and calcium channels in nerve, muscle, and heart

Voltage-gated ion channels are membrane proteins which underlie rapid electrical signals among neurons and the spread of excitation in skeletal muscle and heart. We outline some recent advances in the study of voltage-sensitive sodium and calcium channels. Investigations are providing insight into the changes in molecular conformation associated with open-closed gating of the channels, the mechanisms by which they allow only specific ion species to pass through and carry an electric current, and the pathological consequences of small perturbations in channel structure which result from genetic mutations. Determination of three-dimensional structures, coupled with molecular manipulations by site-directed mutagenesis, and parallel electrophysiological analyses of currents through the ion channels, are providing an understanding of the roles and function of these channels at an unprecedented level of molecular detail. Crucial to these advances are studies of bacterial homologues of ion channels from man and other eukaryotes, and the use of naturally occurring peptide toxins which target different ion channel types with exquisite specificity.

Inhibition of High Voltage-Activated Calcium Channels by Spider Toxin PnTx3-6

Animal peptide toxins have become powerful tools to study structure-function relationships and physiological roles of voltage-activated Ca(2+) channels. In the present study, we investigated the effects of PnTx3-6, a neurotoxin purified from the venom of the spider Phoneutria nigriventer on cloned mammalian Ca(2+) channels expressed in human embryonic kidney 293 cells and endogenous Ca(2+) channels in N18 neuroblastoma cells. Whole-cell patch-clamp measurements indicate that PnTx3-6 reversibly inhibited L-(alpha(1C)/Ca(v)1.2), N-(alpha(1B)/Ca(v)2.2), P/Q-(alpha(1A)/Ca(v)2.1), and R-(alpha(1E)/Ca(v)2.3) type channels with varying potency (alpha(1B) > alpha(1E) > alpha(1A) > alpha(1C)) and IC(50) values of 122, 136, 263, and 607 nM, respectively. Inhibition occurred without alteration of the kinetics or the voltage dependence of the exogenously expressed Ca(2+) channels. In N18 cells, PnTx3-6 exhibited highest potency against N-type (conotoxin-GVIA-sensitive) current. In contrast to its effects on high voltage-activated Ca(2+) channels subtypes, application of 1 microM PnTx3-6 did not affect alpha(1G)/Ca(v)3.1 T-type Ca(2+) channels. Based on our study, we suggest that PnTx3-6 acts as a omega-toxin that targets high voltage-activated Ca(2+) channels, with a preference for the Ca(v)2 subfamily (N-, P/Q-, and R-types).

Chronic ethanol exposure induces an N-type calcium channel splice variant with altered channel kinetics

Chronic ethanol exposure increases the density of N-type calcium channels in brain. We report that ethanol increases levels of mRNA for a splice variant of the N channel specific subunit alpha1 2.2 that lacks exon 31a. Whole cell recordings demonstrated an increase in N-type current with a faster activation rate and a shift in activation to more negative potentials after chronic alcohol exposure, consistent with increased abundance of channels containing this variant. These results identify a novel mechanism whereby chronic ethanol exposure can increase neuronal excitability by altering levels of channel splice variants.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

The α2δ Auxiliary Subunit Reduces Affinity of ω-Conotoxins for Recombinant N-type (Cav2.2) Calcium Channels

The omega-conotoxins from fish-hunting cone snails are potent inhibitors of voltage-gated calcium channels. The omega-conotoxins MVIIA and CVID are selective N-type calcium channel inhibitors with potential in the treatment of chronic pain. The beta and alpha(2)delta-1 auxiliary subunits influence the expression and characteristics of the alpha(1B) subunit of N-type channels and are differentially regulated in disease states, including pain. In this study, we examined the influence of these auxiliary subunits on the ability of the omega-conotoxins GVIA, MVIIA, CVID and analogues to inhibit peripheral and central forms of the rat N-type channels. Although the beta3 subunit had little influence on the on- and off-rates of omega-conotoxins, coexpression of alpha(2)delta with alpha(1B) significantly reduced on-rates and equilibrium inhibition at both the central and peripheral isoforms of the N-type channels. The alpha(2)delta also enhanced the selectivity of MVIIA, but not CVID, for the central isoform. Similar but less pronounced trends were also observed for N-type channels expressed in human embryonic kidney cells. The influence of alpha(2)delta was not affected by oocyte deglycosylation. The extent of recovery from the omega-conotoxin block was least for GVIA, intermediate for MVIIA, and almost complete for CVID. Application of a hyperpolarizing holding potential (-120 mV) did not significantly enhance the extent of CVID recovery. Interestingly, [R10K]MVIIA and [O10K]GVIA had greater recovery from the block, whereas [K10R]CVID had reduced recovery from the block, indicating that position 10 had an important influence on the extent of omega-conotoxin reversibility. Recovery from CVID block was reduced in the presence of alpha(2)delta in human embryonic kidney cells and in oocytes expressing alpha(1B-b). These results may have implications for the antinociceptive properties of omega-conotoxins, given that the alpha(2)delta subunit is up-regulated in certain pain states.

Uncoupling of calcium channel alpha1 and beta subunits in developing neurons

Calcium channel beta subunits are key modulators of calcium channel function and membrane targeting of the pore-forming alpha1 subunit. Here we show that an invertebrate (Lymnaea stagnalis) homolog of P/Q- and N-type calcium channels (LCav2), although colocalized with beta subunits in synapses of mature neurons, is physically uncoupled from the beta subunits in the leading edge of growth cones of outgrowing neurons. Moreover, LCav2 channels that mediate transmitter release in mature synapses also participate in neuronal outgrowth in growth cones. The differential association of beta subunits with synaptic calcium channels and those expressed in emergent neuronal growth suggests that beta subunits may play a role in the transformation of Cav2 calcium channel function in immature neurons and mature synapses.

Auxiliary subunit regulation of high-voltage activated calcium channels expressed in mammalian cells

The effects of auxiliary calcium channel subunits on the expression and functional properties of high-voltage activated (HVA) calcium channels have been studied extensively in the Xenopus oocyte expression system, but are less completely characterized in a mammalian cellular environment. Here, we provide the first systematic analysis of the effects of calcium channel beta and alpha(2)-delta subunits on expression levels and biophysical properties of three different types (Ca(v)1.2, Ca(v)2.1 and Ca(v)2.3) of HVA calcium channels expressed in tsA-201 cells. Our data show that Ca(v)1.2 and Ca(v)2.3 channels yield significant barium current in the absence of any auxiliary subunits. Although calcium channel beta subunits were in principle capable of increasing whole cell conductance, this effect was dependent on the type of calcium channel alpha(1) subunit, and beta(3) subunits altogether failed to enhance current amplitude irrespective of channel subtype. Moreover, the alpha(2)-delta subunit alone is capable of increasing current amplitude of each channel type examined, and at least for members of the Ca(v)2 channel family, appears to act synergistically with beta subunits. In general agreement with previous studies, channel activation and inactivation gating was regulated both by beta and by alpha(2)-delta subunits. However, whereas pronounced regulation of inactivation characteristics was seen with the majority of the auxiliary subunits, effects on voltage dependence of activation were only small (< 5 mV). Overall, through a systematic approach, we have elucidated a previously underestimated role of the alpha(2)-delta(1) subunit with regard to current enhancement and kinetics. Moreover, the effects of each auxiliary subunit on whole cell conductance and channel gating appear to be specifically tailored to subsets of calcium channel subtypes.

Increased expression of P/Q-type Ca2+ channel α1A subunit mRNA in cerebellum of N-type Ca2+ channel α1B subunit gene-deficient mice

The Ca(2+) channel alpha(1B) subunit is a pore-forming component capable of generating N-type Ca(2+) channel activity. Although the N-type Ca(2+) channel plays a role in a variety of neuronal functions, alpha(1B)-deficient mice show normal behavior, presumably owing to compensation by the other Ca(2+) channels. In this study, we examined the mRNA expression of the P/Q-type Ca(2+) channel alpha(1A) subunit in cerebellum of alpha(1B)-deficient mice. The alpha(1A) subunit mRNA in homozygous alpha(1B)-deficient mice was expressed at a significantly higher level than in wild or heterozygous mice. To examine whether the increased expression is induced by a cis-regulatory element within the 5'-upstream region of the alpha(1A) subunit gene, we examined lacZ expression in alpha(1B)-deficient x alpha(1A)3.0-lacZ mice (carrying a 3.0-kb 5'-upstream fragment of the alpha(1A) subunit gene fused to Escherichia coli lacZ reporter gene), which express lacZ in granule but not in Purkinje cells, and in alpha(1B)-deficient x alpha(1A)6.3-lacZ mice (carrying a 6.3-kb 5'-upstream region fused to lacZ gene), which express lacZ in Purkinje but not in granule cells. The levels of lacZ expression in homozygous alpha(1B)-deficient x alpha(1A)3.0-lacZ mice were significantly higher than in wild or heterozygous mice, but no difference in lacZ expression level was found among wild, heterozygous and homozygous alpha(1B)-deficient x alpha(1A)6.3-lacZ mice. Therefore, a possible explanation of the normal behavior of alpha(1B)-deficient mice is that compensation by alpha(1A) subunit gene occurs and that the 3.0-kb 5'-upstream region of alpha(1A) subunit gene contains an enhancer cis-element(s) for compensation in cerebellar granule cells.

Differential modulation of cardiac Ca2+ channel gating by beta-subunits

To investigate the mechanisms that increase ionic currents when Ca(2+) channels' alpha(1) subunits are co-expressed with the beta-subunits, we compared channel activity of Ca(V)1.2 (alpha(1C)) co-expressed with beta(1a) and beta(2a) in Xenopus oocytes. Normalized by charge movement, ionic currents were near threefold larger with beta(2a) than with beta(1a). At the single-channel level, the open probability (P(o)) was over threefold larger with beta(2a), and traces with high P(o) were more frequent. Among traces with P(o) > 0.1, the mean duration of burst of openings (MBD) were nearly twice as long for alpha(1C)beta(2a) (15.1 +/- 0.7 ms) than for alpha(1C)beta(1a) (8.4 +/- 0.5 ms). Contribution of endogenous beta(3xo) was ruled out by comparing MBDs with alpha(1C)-cRNA alone (4.7 +/- 0.1 ms) with beta(3xo) (14.3 +/- 1.1 ms), and with beta(1b) (8.2 +/- 0.5 ms). Open-channel current amplitude distributions were indistinguishable for alpha(1C)beta(1a) and alpha(1C)beta(2a), indicating that opening and closing kinetics are similar with both subunits. Simulations with constant opening and closing rates reproduced the microscopic kinetics accurately, and therefore we conclude that the conformational change-limiting MBD is differentially regulated by the beta-subunits and contributes to the larger ionic currents associated with beta(2a), whereas closing and opening rates do not change, which should reflect the activity of a separate gate.

Extracellular ATP-induced calcium channel inhibition mediated by P1/P2Y purinoceptors in hamster submandibular ganglion neurons

1. The presence and profile of purinoceptors in neurons of the hamster submandibular ganglion (SMG) have been studied using the whole-cell configuration of the patch-clamp technique. 2. Extracellular application of adenosine 5'-triphosphate (ATP) reversibly inhibited voltage-dependent Ca(2+) channel (VDCC) currents (I(Ca)) via G(i/o)-protein in a voltage-dependent manner. 3. Extracellular application of uridine 5'-triphosphate (UTP), 2-methylthioATP (2-MeSATP), alpha,beta-methylene ATP (alpha,beta-MeATP) and adenosine 5'-diphosphate (ADP) also inhibited I(Ca). The rank order of potency was ATP=UTP>ADP>2-MeSATP=alpha,beta-MeATP. 4. The P2 purinoceptor antagonists, suramin and pyridoxal-5-phosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS), partially antagonized the ATP-induced inhibition of I(Ca), while coapplication of suramin and the P1 purinoceptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), virtually abolished I(Ca) inhibition. DPCPX alone partially antagonized I(Ca) inhibition. 5. Suramin antagonized the UTP-induced inhibition of I(Ca), while DPCPX had no effect. 6. Extracellular application of adenosine (ADO) also inhibited I(Ca) in a voltage-dependent manner via G(i/o)-protein activation. 7. Mainly N- and P/Q-type VDCCs were inhibited by both ATP and ADO via G(i/o)-protein betagamma subunits in seemingly convergence pathways.

Identification of the calcium channel alpha 1E (Ca(v)2.3) isoform expressed in atrial myocytes

Antisense oligonucleotides targeting the calcium channel alpha 1E (Ca(v)2.3) subunit significantly inhibit the insulin-like growth factor-1 (IGF-1)-stimulated increase in low voltage-activated (LVA) (T-type) calcium current in cultured rat atrial myocytes [Proc. Natl. Acad. Sci. U.S.A. 94(1997) 14936]. As part of a continuing effort to understand the regulation of LVA current expression in the heart, we have identified the specific alpha 1E isoform that is expressed in atrial tissue. Through reverse transcription-polymerase chain reaction (RT-PCR), nine overlapping partial clones spanning the entire coding region of the cardiac alpha 1E mRNA were obtained. The predominate isoform in atrial tissue was identified and found to be highly homologous to the alpha 1E isoform previously isolated from kidney and the islets of Langerhans [Eur. J. Biochem. 257(1998) 274]. The expression of alpha 1E in the heart occurs specifically in cardiac myocytes and not in smooth muscle or fibroblasts as demonstrated by RT-PCR performed on isolated atrial myocytes and by in situ hybridization.

Ca2+ channels and epilepsy

The epilepsies encompass diverse seizure disorders afflicting as many as 50 million people worldwide. Many forms of epilepsy are intractable to current therapies and there is a pressing need to develop agents and strategies to not only suppress seizures, but also cure epilepsy. Recent insights from molecular genetics and pharmacology now point to an important role for voltage-dependent calcium channels in epilepsy. In this article, I first provide an introduction to the classification of the epilepsies and an overview of neuronal Ca(2+) channels. Next, I attempt to review the evidence for a role of Ca(2+) channels in epilepsy and the insights gained from genetics and pharmacology. Lastly, I describe new avenues for how such information might be exploited in the development of therapeutic reagents.

Use of a purified and functional recombinant calcium-channel beta4 subunit in surface-plasmon resonance studies

Native high-voltage-gated calcium channels are multi-subunit complexes comprising a pore-forming subunit Ca(v) and at least two auxiliary subunits alpha(2)delta and beta. The beta subunit facilitates cell-surface expression of the channel and contributes significantly to its biophysical properties. In spite of its importance, detailed structural and functional studies are hampered by the limited availability of native beta subunit. Here, we report the purification of a recombinant calcium-channel beta(4) subunit from bacterial extracts by using a polyhistidine tag. The purified protein is fully functional since it binds on the alpha1 interaction domain, its main Ca(v)-binding site, and regulates the activity of P/Q calcium channel expressed in Xenopus oocytes in a similar way to the beta(4) subunit produced by cRNA injection. We took advantage of the functionality of the purified material to (i) develop an efficient surface-plasmon resonance assay of the interaction between two calcium channel subunits and (ii) measure, for the first time, the affinity of the recombinant His-beta(4) subunit for the full-length Ca(v)2.1 channel. The availability of this purified material and the development of a surface-plasmon resonance assay opens two immediate research perspectives: (i) drug screening programmes applied to the Ca(v)/beta interaction and (ii) crystallographic studies of the calcium-channel beta(4) subunit.

The interaction between the I-II loop and the III-IV loop of Cav2.1 contributes to voltage-dependent inactivation in a beta -dependent manner

We have investigated the molecular mechanisms whereby the I-II loop controls voltage-dependent inactivation in P/Q calcium channels. We demonstrate that the I-II loop is localized in a central position to control calcium channel activity through the interaction with several cytoplasmic sequences; including the III-IV loop. Several experiments reveal the crucial role of the interaction between the I-II loop and the III-IV loop in channel inactivation. First, point mutations of two amino acid residues of the I-II loop of Ca(v)2.1 (Arg-387 or Glu-388) facilitate voltage-dependent inactivation. Second, overexpression of the III-IV loop, or injection of a peptide derived from this loop, produces a similar inactivation behavior than the mutated channels. Third, the III-IV peptide has no effect on channels mutated in the I-II loop. Thus, both point mutations and overexpression of the III-IV loop appear to act similarly on inactivation, by competing off the native interaction between the I-II and the III-IV loops of Ca(v)2.1. As they are known to affect inactivation, we also analyzed the effects of beta subunits on these interactions. In experiments in which the beta(4) subunit is co-expressed, the III-IV peptide is no longer able to regulate channel inactivation. We conclude that (i) the contribution of the I-II loop to inactivation is partly mediated by an interaction with the III-IV loop and (ii) the beta subunits partially control inactivation by modifying this interaction. These data provide novel insights into the mechanisms whereby the beta subunit, the I-II loop, and the III-IV loop altogether can contribute to regulate inactivation in high voltage-activated calcium channels.

Time- and subunit-dependent differential mRNA expression of L-type Ca2+ channel during progression of right ventricular hypertrophy

To clarify the molecular basis for changes in L-type calcium channel (VLCC) density in ventricular hypertrophy, we analyzed the mRNA expression of all the subunits including the main subunit alpha1c and auxiliary subunits (alpha2delta, beta2 and beta3) composing VLCC in rat right ventricular hypertrophy (RVH) induced by monocrotaline injection. To test the hypothesis that the expression of each subunit might change differently during progression of RVH, leading to an altered electrophysiologic outcome for VLCC, we investigated the ratio of the mRNA level of each auxiliary subunit to the main subunit. After monocrotaline injection, alpha1c mRNA showed a transient decrease on the 14th day and thereafter significantly increased to reach approximately 1.8 fold that of the control level on the 21st day. The auxiliary subunit alpha2delta mRNA showed a pattern similar to that of alpha1c. The beta3 mRNA increased rapidly after monocrotaline injection and increased approximately 4.1 fold. On the other hand, beta2 mRNA showed no significant changes. Accordingly, only the mRNA ratio of beta3 to alpha1c showed a significant increase among the auxiliary subunits after the monocrotaline injection. The ratio increased to a maximum of approximately 5.7 fold on the 14th day and thereafter decreased. These results suggest that VLCC density may be modified not only by alpha1c but also by its auxiliary subunit expression in ventricular hypertrophy, and provide a clue for understanding the controversial electrophysiologic results on VLCC density in hypertrophied hearts.

The transmembrane domain of syntaxin 1A negatively regulates voltage-sensitive Ca(2+) channels

Syntaxin 1A has a pronounced inhibitory effect on the activation kinetics and current amplitude of voltage-gated Ca(2+) channels. This study explores the molecular basis of syntaxin interaction with N- and Lc-type Ca(2+) channels by way of functional assays of channel gating in a Xenopus oocytes expression system. A chimera of syntaxin 1A and syntaxin 2 in which the transmembrane domain of syntaxin 2 replaced the transmembrane of syntaxin 1A (Sx1-2), significantly reduced the rate of activation of N- and Lc-channels. This shows a similar effect to that demonstrated by syntaxin 1A, though the current was not inhibited. The major sequence differences at the transmembrane of the syntaxin isoforms are that the two highly conserved cysteines Cys 271 and Cys 272 in syntaxin 1A correspond to the valines Val 272 and Val 273 in syntaxin 2 transmembrane. Mutating either cysteines in Sx1-1 (syntaxin 1A) to valines, did not affect modulation of the channel while a double mutant C271/272V was unable to regulate inward current. Transfer of these two cysteines to the transmembrane of syntaxin 2 by mutating Val 272 and Val 273 to Cys 272 and Cys 273 led to channel inhibition. When cleaved by botulinum toxin, the syntaxin 1A fragments, amino acids 1-253 and 254-288, which includes the transmembrane domain, were both unable to inhibit current amplitude but retained the ability to modify the activation kinetics of the channel. A full-length syntaxin 1A and the integrity of the two cysteines within the transmembrane are crucial for coordinating Ca(2+) entry through the N- and Lc-channels. These results suggest that upon membrane depolarization, the voltage-gated N- and Lc-type Ca(2+)-channels signal the exocytotic machinery by interacting with syntaxin 1A at the transmembrane and the cytosolic domains. Cleavage with botulinum toxin disrupts the coupling of the N- and Lc-type channels with syntaxin 1A and abolishes exocytosis, supporting the hypothesis that these channels actively participate in Ca(2+) regulated secretion.

Allosteric modulation of Ca2+ channels by G proteins, voltage-dependent facilitation, protein kinase C, and Ca(v)beta subunits

N-type and P/Q-type Ca(2+) channels are inhibited by neurotransmitters acting through G protein-coupled receptors in a membrane-delimited pathway involving Gbetagamma subunits. Inhibition is caused by a shift from an easily activated "willing" (W) state to a more-difficult-to-activate "reluctant" (R) state. This inhibition can be reversed by strong depolarization, resulting in prepulse facilitation, or by protein kinase C (PKC) phosphorylation. Comparison of regulation of N-type Ca(2+) channels containing Cav2.2a alpha(1) subunits and P/Q-type Ca(2+) channels containing Ca(v)2.1 alpha(1) subunits revealed substantial differences. In the absence of G protein modulation, Ca(v)2.1 channels containing Ca(v)beta subunits were tonically in the W state, whereas Ca(v)2.1 channels without beta subunits and Ca(v)2.2a channels with beta subunits were tonically in the R state. Both Ca(v)2.1 and Ca(v)2.2a channels could be shifted back toward the W state by strong depolarization or PKC phosphorylation. Our results show that the R state and its modulation by prepulse facilitation, PKC phosphorylation, and Ca(v)beta subunits are intrinsic properties of the Ca(2+) channel itself in the absence of G protein modulation. A common allosteric model of G protein modulation of Ca(2+)-channel activity incorporating an intrinsic equilibrium between the W and R states of the alpha(1) subunits and modulation of that equilibrium by G proteins, Ca(v)beta subunits, membrane depolarization, and phosphorylation by PKC accommodates our findings. Such regulation will modulate transmission at synapses that use N-type and P/Q-type Ca(2+) channels to initiate neurotransmitter release.

Control of gating mode by a single amino acid residue in transmembrane segment IS3 of the N-type Ca2+ channel

N-type Ca(2+) channels can be inhibited by neurotransmitter-induced release of G protein betagamma subunits. Two isoforms of Ca(v)2.2 alpha1 subunits of N-type calcium channels from rat brain (Ca(v)2.2a and Ca(v)2.2b; initially termed rbB-I and rbB-II) have different functional properties. Unmodulated Ca(v)2.2b channels are in an easily activated "willing" (W) state with fast activation kinetics and no prepulse facilitation. Activating G proteins shifts Ca(v)2.2b channels to a difficult to activate "reluctant" (R) state with slow activation kinetics; they can be returned to the W state by strong depolarization resulting in prepulse facilitation. This contrasts with Ca(v)2.2a channels, which are tonically in the R state and exhibit strong prepulse facilitation. Activating or inhibiting G proteins has no effect. Thus, the R state of Ca(v)2.2a and its reversal by prepulse facilitation are intrinsic to the channel and independent of G protein modulation. Mutating G177 in segment IS3 of Ca(v)2.2b to E as in Ca(v)2.2a converts Ca(v)2.2b tonically to the R state, insensitive to further G protein modulation. The converse substitution in Ca(v)2.2a, E177G, converts it to the W state and restores G protein modulation. We propose that negatively charged E177 in IS3 interacts with a positive charge in the IS4 voltage sensor when the channel is closed and produces the R state of Ca(v)2.2a by a voltage sensor-trapping mechanism. G protein betagamma subunits may produce reluctant channels by a similar molecular mechanism.

The [beta]2a subunit is a molecular groom for the Ca2+ channel inactivation gate

Ca(2+) channel inactivation is a key element in controlling the level of Ca(2+) entry through voltage-gated Ca(2+) channels. Interaction between the pore-forming alpha(1) subunit and the auxiliary beta subunit is known to be a strong modulator of voltage-dependent inactivation. Here, we demonstrate that an N-terminal membrane anchoring site (MAS) of the beta(2a) subunit strongly reduces alpha(1A) (Ca(V)2.1) Ca(2+) channel inactivation. This effect can be mimicked by the addition of a transmembrane segment to the N terminus of the beta(2a) subunit. Inhibition of inactivation by beta(2a) also requires a link between MAS and another important molecular determinant, the beta interaction domain (BID). Our data suggest that mobility of the Ca(2+) channel I-II loop is necessary for channel inactivation. Interaction of this loop with other identified intracellular channel domains may constitute the basis of voltage-dependent inactivation. We thus propose a conceptually novel mechanism for slowing of inactivation by the beta(2a) subunit, in which the immobilization of the channel inactivation gate occurs by means of MAS and BID.

G protein modulation of recombinant P/Q-type calcium channels by regulators of G protein signalling proteins

1. Fast synaptic transmission is triggered by the activation of presynaptic Ca2+ channels which can be inhibited by Gbetagamma subunits via G protein-coupled receptors (GPCR). Regulators of G protein signalling (RGS) proteins are GTPase-accelerating proteins (GAPs), which are responsible for >100-fold increases in the GTPase activity of G proteins and might be involved in the regulation of presynaptic Ca2+ channels. In this study we investigated the effects of RGS2 on G protein modulation of recombinant P/Q-type channels expressed in a human embryonic kidney (HEK293) cell line using whole-cell recordings. 2. RGS2 markedly accelerates transmitter-mediated inhibition and recovery from inhibition of Ba2+ currents (IBa) through P/Q-type channels heterologously expressed with the muscarinic acetylcholine receptor M2 (mAChR M2). 3. Both RGS2 and RGS4 modulate the prepulse facilitation properties of P/Q-type Ca2+ channels. G protein reinhibition is accelerated, while release from inhibition is slowed. These kinetics depend on the availability of G protein alpha and betagamma subunits which is altered by RGS proteins. 4. RGS proteins unmask the Ca2+ channel beta subunit modulation of Ca2+ channel G protein inhibition. In the presence of RGS2, P/Q-type channels containing the beta2a and beta3 subunits reveal significantly altered kinetics of G protein modulation and increased facilitation compared to Ca2+ channels coexpressed with the beta1b or beta4 subunit.

The spider toxin omega-Aga IIIA defines a high affinity site on neuronal high voltage-activated calcium channels

The spider toxin omega-agatoxin IIIA (omega-Aga-IIIA) is a potent inhibitor of high voltage-activated calcium currents in the mammalian brain. To establish the biochemical parameters governing its action, we radiolabeled the toxin and examined its binding to native and recombinant calcium channels. In experiments with purified rat synaptosomal membranes, both kinetic and equilibrium data demonstrate one-to-one binding of omega-Aga-IIIA to a single population of high affinity sites, with K(d) = approximately 9 pm and B(max) = approximately 1.4 pmol/mg protein. Partial inhibition of omega-Aga-IIIA binding by omega-conotoxins GVIA, MVIIA, and MVIIC identifies N and P/Q channels as components of this population. omega-Aga-IIIA binds to recombinant alpha(1B) and alpha(1E) calcium channels with a similar high affinity (K(d) = approximately 5-9 pm) in apparent one-to-one fashion. Results from recombinant alpha(1B) binding experiments demonstrate virtually identical B(max) values for omega-Aga-IIIA and omega-conotoxin MVIIA, providing further evidence for a one-to-one stoichiometry of agatoxin binding to calcium channels. The combined evidence suggests that omega-Aga-IIIA defines a unique, high affinity binding site on N-, P/Q-, and R-type calcium channels.

Voltage-gated calcium currents in axotomized adult rat cutaneous afferent neurons

The effect of sciatic nerve injury on the somatic expression of voltage-gated calcium currents in adult rat cutaneous afferent dorsal root ganglion (DRG) neurons identified via retrograde Fluoro-gold labeling was studied using whole cell patch-clamp techniques. Two weeks after a unilateral ligation and transection of the sciatic nerve, the L(4)-L(5) DRG were dissociated and barium currents were recorded from cells 3-10 h later. Cutaneous afferents (35-50 microm diam) were classified as type 1 (possessing only high-voltage-activated currents; HVA) or type 2 (having both high- and low-voltage-activated currents). Axotomy did not change the percentage of neurons exhibiting a type 2 phenotype or the properties of low-threshold T-type current found in type 2 neurons. However, in type 1 neurons the peak density of HVA current available at a holding potential of -60 mV was reduced in axotomized neurons (83.9 +/- 5.6 pA/pF, n = 53) as compared with control cells (108.7 +/- 6.9 pA/pF, n = 58, P < 0.01, unpaired t-test). A similar reduction was observed at more negative holding potentials, suggesting differences in steady-state inactivation are not responsible for the effect. Separation of the type 1 cells into different size classes indicates that the reduction in voltage-gated barium current occurs selectively in the larger (capacitance >80 pF) cutaneous afferents (control: 112.4 +/- 10.6 pA/pF, n = 30; ligated: 72.6 +/- 5.0 pA/pF, n = 36; P < 0.001); no change was observed in cells with capacitances of 45-80 pF. Isolation of the N- and P¿Q-type components of the HVA current in the large neurons using omega-conotoxin GVIA and omega-agatoxin TK suggests a selective reduction in N-type barium current after nerve injury, as the density of omega-CgTx GVIA-sensitive current decreased from 56.9 +/- 6.6 pA/pF in control cells (n = 13) to 31.3 +/- 4.6 pA/pF in the ligated group (n = 12; P < 0.005). The HVA barium current of large cutaneous afferents also demonstrates a depolarizing shift in the voltage dependence of inactivation after axotomy. Injured type 1 cells exhibited faster inactivation kinetics than control neurons, although the rate of recovery from inactivation was similar in the two groups. The present results indicate that nerve injury leads to a reorganization of the HVA calcium current properties in a subset of cutaneous afferent neurons.

Coexpression of cloned alpha(1B), beta(2a), and alpha(2)/delta subunits produces non-inactivating calcium currents similar to those found in bovine chromaffin cells

Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by omega-conotoxin GVIA (omega-CgTx GVIA). In contrast to neuronal N-type calcium currents that inactivate during long depolarizations and that require negative holding potentials to remove inactivation, many chromaffin cells exhibit N-type calcium channel currents that show little inactivation during maintained depolarizations and that exhibit no decrease in channel availability at depolarized holding potentials. N-type calcium channels are thought to be produced by combination of the pore-forming alpha(1B) subunit and accessory beta and alpha(2)/delta subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the alpha(1B) and accessory beta (beta(1b), beta(1c,) beta(2a), beta(2b), and beta(3a)) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopus oocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by omega-CgTx GVIA. Coexpression of bovine alpha(1B) with beta(1b), beta(1c), beta(2b), or beta(3a) produced currents that were holding potential dependent. In contrast, coexpression of bovine alpha(1B) with beta(2a) produced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine alpha(1B), alpha(2)/delta, and beta(2a).

Cloning and functional expression of novel N-type Ca(2+) channel variants

Voltage-dependent Ca(2+) channels are structurally and functionally diverse. As Ca(2+) currents recorded from embryonic chick dorsal root ganglion (DRG) neurons differ significantly from their mammalian counterparts, information on the primary sequence of the chick channels will help define the structural underpinnings of Ca(2+) channel function. Here, we report the cloning and functional expression of full-length Ca(2+) channel alpha(1B) subunit cDNAs derived from chick DRGs. Two variable regions (A and B) have been identified in the cytoplasmic linker between repeats I and II; a third (C) in the carboxyl terminus extends the open reading frame by 525 nucleotides. The A and C inserts are absent, and the B insert is present in all other class B clones reported to date. The unique shorter channels appear to predominate in DRG neurons. Results represent a requisite first step in defining the structural elements that underlie variations in function and modulation of Ca(2+) channels.

Release-independent depression at pyramidal inputs onto specific cell targets: dual recordings in slices of rat cortex

1. Paired intracellular recordings were performed in slices of adult rat neocortex and hippocampus to examine presynaptic depression. A novel form of depression that occurs even in the absence of transmitter release during conditioning activity was observed at a subset of synaptic connections. 2. In each pair studied, a pyramidal neurone was presynaptic and inputs onto a range of morphologically identified postsynaptic target cells were analysed; high probability connections exhibiting the more traditional forms of release-dependent depression, as well as low probability connections exhibiting facilitation, were tested (n = 35). 3. Connections were tested with presynaptic spike pairs and trains of spikes with a range of interspike intervals. Sweeps in which the first action potential elicited no detectable response (apparent failures of transmission) and sweeps in which the first action potential elicited large EPSPs were selected. Second EPSPs that followed apparent failures were then compared with second EPSPs that followed large first EPSPs. 4. Release-independent depression was apparent when second EPSPs at brief interspike intervals (<10-15 ms) were on average smaller than second EPSPs at longer interspike intervals, even following apparent failures and when the second EPSP amplitude at these short intervals was independent of the amplitude of the first EPSP. 5. Release-independent depression appeared selectively expressed. Depressing inputs onto some interneurones, such as CA1 basket-like and bistratified cells, and facilitating inputs onto others, such as some fast spiking neocortical interneurones, exhibited this phenomenon. In contrast, depressing inputs onto 10/10 neocortical pyramids and facilitating inputs onto 7/7 oriens-lacunosum moleculare and 5/5 burst firing, sparsely spiny neocortical interneurones did not.

Decreased (45)Ca(2)(+) uptake in P/Q-type calcium channels in homozygous lethargic (Cacnb4lh) mice is associated with increased beta3 and decreased beta4 calcium channel subunit mRNA expression

The mutated gene in the lethargic (Cacnb4lh) mouse model of absence seizures encodes the beta4 subunit of voltage-gated calcium channels (VGCCs), leading to decreased mRNA expression of a beta4 subunit that is truncated and cannot bind to alpha1 subunits of VGCCs. In this study we accomplished two goals. First, we studied the functional consequence of altered VGCCs by examining the effects of a selective P/Q-type channel antagonist on KCl-induced (45)Ca(2)(+) uptake in brain synaptosomes from Cacnb4lh homozygotes and non-epileptic controls (designated by +/+). We found that depolarization-induced (45)Ca(2)(+) uptake was significantly reduced in the brains of Cacnb4lh homozygotes, and that the reduced uptake was completely accounted for by reduced function of P/Q-type calcium channel. Second, we examined VGCC subunit composition to determine if other subunits were altered in addition to the mutation affecting beta4 subunits in Cacnb4lh homozygotes; when alterations were found, we determined if they were regional or global. We used in situ hybridization histochemistry (ISHH) to analyze the neuro-anatomic distribution of beta4, beta1b, beta2, beta3, alpha1A, alpha1B, alpha1C, alpha1E, and alpha1G subunit mRNAs in brain sections from matched Cacnb4lh homozygotes and +/+ controls. Our results indicated that expression of beta4 subunit mRNA is globally reduced throughout the brains of Cacnb4lh homozygotes, in contrast to a small but significant global increase in the expression of beta3 subunit mRNA. There were no significant differences in expression of the other VGCC subunit mRNAs examined. Together, these findings indicate that a host of changes in VGCC subunit composition accompany reduced function of P/Q-type channels in homozygous lethargic mice.

alpha 1B N-type calcium channel isoforms with distinct biophysical properties

N-type calcium channels both generate the initial calcium signal to trigger neurotransmitter release and also interact with synaptic release proteins at many mammalian central nervous system synapses. Two isoforms of the alpha 1B N-type channel from rat brain (alpha 1B-I and alpha 1B-II) were found to differ in four regions: (1) a glutamate (Glu) to glycine (Gly) substitution in domain I S3; (2) a Gly to Glu substitution in the domain I-II linker; (3) the insertion or deletion of an alanine (Ala) in the domain I-II linker; and (4) the presence or absence of serine/phenylalanine/methionine/glycine (SFMG) in the linker between domain III S3-S4. Comparison of the electrophysiological properties of the alpha 1B-I and alpha 1B-II N-type channels shows that they exhibit distinct kinetics as well as altered current-voltage relations. Utilizing chimeric alpha 1B-I and alpha 1B-II cDNAs, we show that: (1) the Glu 177 to Gly substitution in domain I S3 increases the rate of activation by approximately 15-fold; (2) the presence or absence of Ala 415 in the domain I-II linker alters current-voltage relations by approximately 10 mV but does not affect channel kinetics; (3) the substitution of Gly 387 to Glu in the domain I-II linker also has no effect on kinetics; and (4) the presence or absence of SFMG (1236-1239) in domain III S3-S4 did not significantly affect channel current-voltage relations, kinetics, or steady state inactivation. We conclude that molecularly distinct alpha 1B isoforms are expressed in rat brain and may account for some of the functional diversity of N-type currents in native cells.

Interactions of presynaptic Ca2+ channels and snare proteins in neurotransmitter release

N- and P/Q-type Ca2+ channels are localized in high density in presynaptic nerve terminals and are crucial elements in neuronal excitation-secretion coupling. In addition to mediating Ca2+ entry to initiate transmitter release, they are thought to interact directly with proteins of the synaptic vesicle docking/fusion machinery. These Ca2+ channels can be purified from brain as a complex with SNARE proteins, which are involved in exocytosis. In addition, N-type and P/Q-type Ca2+ channels are colocalized with syntaxin in high-density clusters in nerve terminals. The synaptic protein interaction (synprint) sites in the intracellular loop II-III (LII-III) of both alpha 1B and alpha 1A subunits of N-type and P/Q-type Ca2+ channels bind to syntaxin, SNAP-25, and synaptotagmin. Ca2+ has a biphasic effect on the interactions of N-type Ca2+ channels with SNARE complexes, stimulating optimal binding in the range of 10-30 microM. PKC or CaM KII phosphorylation of the N-type synprint peptide inhibits interactions with SNARE complexes containing syntaxin and SNAP-25. Introduction of the synprint peptides into presynaptic superior cervical ganglion neurons reversibly inhibits EPSPs from synchronous transmitter release by 42%. At physiological Ca2+ concentrations, synprint peptides significantly reduce transmitter release in injected frog neuromuscular junctions in cell culture, consistent with detachment of 70% of the docked vesicles from Ca2+ channels as analyzed by a theoretical model. Together, these studies suggest that presynaptic Ca2+ channels not only provide the Ca2+ signal required by the exocytotic machinery, but also contain structural elements that are integral to vesicle docking, priming, and fusion processes.

Neuronal voltage-activated calcium channels: on the roles of the alpha 1E and beta 3 subunits

Many neurons of the central and peripheral nervous systems display multiple high voltage-activated (HVA) Ca2+ currents, often classified as L-, N-, P-, Q, and R-type. The heterogeneous properties of these channels have been attributed to diversity in their pore-forming alpha 1, subunits, in association with various beta subunits. However, there are large gaps in understanding how individual subunits contribute to Ca2+ channel diversity. Here we describe experiments to investigate the roles of alpha 1E and beta 3 subunits in mammalian neurons. The alpha 1E subunit is the leading candidate to account for the R-type channel, the least understood of the various types of high voltage-activated Ca2+ channels. Incubation with alpha 1E antisense oligonucleotide caused a 53% decrease in the peak R-type current density, while no significant changes in the current expression were seen in sense oligonucleotide-treated cells. The specificity of the alpha 1E antisense oligonucleotides was supported by the lack of change in the amplitude of P/Q current. These results upheld the hypothesis that members of the E class of alpha 1 subunits support the high voltage-activated R-type current in cerebellar granule cells. We studied the role of the Ca2+ channel beta 3 subunit using a gene targeting strategy. In sympathetic beta 3-/- neurons, the L-type current was significantly reduced relative to wild type (wt). In addition, N-type Ca2+ channels made up a smaller proportion of the total Ca2+ current than in wt due to a lower N-type current density in a group of neurons with small total currents. Voltage-dependent activation of P/Q-type Ca2+ channels was described by two Boltzmann components with different voltage dependence. The absence of the beta 3 subunit was associated with a shift in the more depolarized component of the activation along the voltage axis toward more negative potentials. The overall conclusion is that deletion of the beta 3 subunit affects at least three distinct types of HVA Ca2+ channel, but no single type of channel is solely dependent on beta 3.