Evidence for two concentration-dependent processes for beta-subunit effects on alpha1B calcium channels

Functional modularity of the beta-subunit of voltage-gated Ca2+ channels

The beta-subunit of voltage-gated Ca(2+) channels plays a dual role in chaperoning the channels to the plasma membrane and modulating their gating. It contains five distinct modular domains/regions, including the variable N- and C-terminus, a conserved Src homology 3 (SH3) domain, a conserved guanylate kinase (GK) domain, and a connecting variable and flexible HOOK region. Recent crystallographic studies revealed a highly conserved interaction between the GK domain and alpha interaction domain (AID), the high-affinity binding site in the pore-forming alpha(1) subunit. Here we show that the AID-GK domain interaction is necessary for beta-subunit-stimulated Ca(2+) channel surface expression and that the GK domain alone can carry out this function. We also examined the role of each region of all four beta-subunit subfamilies in modulating P/Q-type Ca(2+) channel gating and demonstrate that the beta-subunit functions modularly. Our results support a model that the conserved AID-GK domain interaction anchors the beta-subunit to the alpha(1) subunit, enabling alpha(1)-beta pair-specific low-affinity interactions involving the N-terminus and the HOOK region, which confer on each of the four beta-subunit subfamilies its distinctive modulatory properties.

Dose-dependent and isoform-specific modulation of Ca2+ channels by RGK GTPases

Although inhibition of voltage-gated calcium channels by RGK GTPases (RGKs) represents an important mode of regulation to control Ca²⁺ influx in excitable cells, their exact mechanism of inhibition remains controversial. This has prevented an understanding of how RGK regulation can be significant in a physiological context. Here we show that RGKs—Gem, Rem, and Rem2—decreased Ca_V1.2 Ca²⁺ current amplitude in a dose-dependent manner. Moreover, Rem2, but not Rem or Gem, produced dose-dependent alterations on gating kinetics, uncovering a new mode by which certain RGKs can precisely modulate Ca²⁺ currents and affect Ca²⁺ influx during action potentials. To explore how RGKs influence gating kinetics, we separated the roles mediated by the Ca²⁺ channel accessory β subunit's interaction with its high affinity binding site in the pore-forming α_1C subunit (AID) from its other putative contact sites by utilizing an α_1C•β3 concatemer in which the AID was mutated to prevent β subunit interaction. This mutant concatemer generated currents with all the hallmarks of β subunit modulation, demonstrating that AID-β–independent interactions are sufficient for β subunit modulation. Using this construct we found that although inhibition of current amplitude was still partially sensitive to RGKs, Rem2 no longer altered gating kinetics, implicating different determinants for this specific mode of Rem2-mediated regulation. Together, these results offer new insights into the molecular mechanism of RGK-mediated Ca²⁺ channel current modulation.

Dominant-negative suppression of Cav2.1 currents by alpha(1)2.1 truncations requires the conserved interaction domain for beta subunits

Episodic ataxia type 2 (EA2) is an autosomal dominant disorder arising from CACNA1A mutations, which commonly predict heterozygous expression of Ca(v)2.1 calcium channels with truncated alpha(1)2.1 pore subunits. We hypothesized that alpha(1)2.1 truncations in EA2 exert dominant-negative effects on the function of wild-type subunits. Wild-type and truncated alpha(1)2.1 subunits with fluorescent protein tags were transiently co-expressed in cells stably expressing Ca(v) auxiliary beta subunits, which facilitate alpha1 subunit functional expression through high-affinity interactions with the alpha interaction domain (AID). Co-expression of wild-type subunits with truncations often resulted in severely reduced whole-cell currents compared to expression of wild-type subunits alone. Cellular image analyses revealed that current suppression was not due to reduced wild-type expression levels. Instead, the current suppression depended on truncations terminating distal to the AID. Moreover, only AID-bearing alpha(1)2.1 proteins co-immunoprecipitated with Ca(v) beta subunits. These results indicate that Ca(v) beta subunits may play a prominent role in EA2 disease pathogenesis.

Up-regulated l-type high voltage-gated calcium channels cause increase in diazepam binding inhibitor induced by sustained morphine exposure in mouse cerebrocortical neurons

Mechanisms of increase in diazepam binding inhibitor (DBI) mRNA expression in mouse cerebrocortical neurons after sustained morphine exposure were investigated. Increases in DBI and its mRNA expressions induced by sustained morphine (0.3 microM) exposure for 3 days were completely abolished by naloxone and nifedipine, but not by omega-agatoxin VIA and omega-conotoxin GIVA. Increase in [(3)H]diltiazem binding to the particulate fractions from the morphine-treated neurons was due to increased B(max) value with no changes in K(d) value. Western blot analysis on L-type high voltage-gated calcium channel (HVCC) subunits revealed the increased expressions of alpha1C, alpha1D, and alpha2/delta1 subunits and decreased of beta4 subunit expression, whereas expression of N- and P/Q-type HVCC subunits was not changed. These results indicate that morphine-induced increase in DBI mRNA expression is mediated via increased Ca(2+) entry through up-regulated L-type HVCCs.

Antisense oligonucleotide against the Ca channel beta subunit decreases L-type Ca current in rat ventricular myocytes

The role of endogenous beta subunits of the L-type Ca channel in native cardiac ventricular myocytes is unclear. We have therefore investigated the effect of inhibiting beta subunit expression in rat myocytes, by culturing isolated myocytes for 24 h with either antisense oligonucleotide against the beta subunit or with scrambled oligonucleotide (control). Alpha1 subunit expression and distribution were then determined by immunolabeling, and L-type Ca current measured using the whole cell patch-clamp technique. Cells treated with antisense showed increased perinuclear staining for alpha1, decreased Ca current amplitude and a small rightward shift of the activation curve and the I-V relation, with no significant effect on inactivation. These data suggest that endogenous beta subunits in native cardiac myocytes help to traffic the alpha1 subunit to the cell membrane and thus play a major role in determining Ca current amplitude.

Regulation of maximal open probability is a separable function of Ca(v)beta subunit in L-type Ca2+ channel, dependent on NH2 terminus of alpha1C (Ca(v)1.2alpha)

β subunits (Ca_vβ) increase macroscopic currents of voltage-dependent Ca²⁺ channels (VDCC) by increasing surface expression and modulating their gating, causing a leftward shift in conductance–voltage (G-V) curve and increasing the maximal open probability, P_o,max. In L-type Ca_v1.2 channels, the Ca_vβ-induced increase in macroscopic current crucially depends on the initial segment of the cytosolic NH₂ terminus (NT) of the Ca_v1.2α (α_1C) subunit. This segment, which we term the “NT inhibitory (NTI) module,” potently inhibits long-NT (cardiac) isoform of α_1C that features an initial segment of 46 amino acid residues (aa); removal of NTI module greatly increases macroscopic currents. It is not known whether an NTI module exists in the short-NT (smooth muscle/brain type) α_1C isoform with a 16-aa initial segment. We addressed this question, and the molecular mechanism of NTI module action, by expressing subunits of Ca_v1.2 in Xenopus oocytes. NT deletions and chimeras identified aa 1–20 of the long-NT as necessary and sufficient to perform NTI module functions. Coexpression of β_2b subunit reproducibly modulated function and surface expression of α_1C, despite the presence of measurable amounts of an endogenous Ca_vβ in Xenopus oocytes. Coexpressed β_2b increased surface expression of α_1C approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by ∼14 mV, and increased P_o,max 2.8–3.8-fold. Neither the surface expression of the channel without Ca_vβ nor β_2b-induced increase in surface expression or the shift in G-V curve depended on the presence of the NTI module. In contrast, the increase in P_o,max was completely absent in the short-NT isoform and in mutants of long-NT α_1C lacking the NTI module. We conclude that regulation of P_o,max is a discrete, separable function of Ca_vβ. In Ca_v1.2, this action of Ca_vβ depends on NT of α_1C and is α_1C isoform specific.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

The importance of occupancy rather than affinity of CaV(beta) subunits for the calcium channel I-II linker in relation to calcium channel function

The Ca(V)beta subunits of voltage-gated calcium channels regulate the trafficking and biophysical properties of these channels. We have taken advantage of mutations in the tyrosine residue within the alpha interaction domain (AID) in the I-II linker of Ca(V)2.2 which reduce, but do not abolish, the binding of beta1b to the AID of Ca(V)2.2. We have found that the mutation Y388S decreased the affinity of Ca(V)beta1b binding to the Ca(V)2.2 I-II linker from 14 to 329 nm. However, the Y388S mutation had no effect on current density and cell surface expression of Ca(V)2.2/alpha2delta-2/beta1b channels expressed in human embryonic kidney tsA-201 cells, when equivalent proportions of cDNA were used. Furthermore, despite the 24-fold reduced affinity of Ca(V)beta1b for the Y388S I-II linker of Ca(V)2.2, all the key features of modulation as well as trafficking by Ca(V)beta subunits remained intact. This is in contrast to the much more marked effect of the W391A mutation, which abolished interaction with the Ca(V)2.2 I-II linker, and very markedly affected the trafficking of the channels. However, using the Xenopus oocyte expression system, where expression levels can be accurately titrated, when Ca(V)beta1b cDNA was diluted 50-fold, all evidence of interaction with Ca(V)2.2 Y388S was lost, although wild-type Ca(V)2.2 was still normally modulated by the reduced concentration of beta1b. These results indicate that high affinity interaction with the alpha1 subunit is not necessary for any of the modulatory effects of Ca(V)beta subunits, but occupancy of the interaction site is important, and this will occur, despite the reduced affinity, if the Ca(V)beta subunit is present in sufficient excess.

Alternative splicing of the voltage-gated Ca2+ channel beta4 subunit creates a uniquely folded N-terminal protein binding domain with cell-specific expression in the cerebellar cortex

Ca2+ channel beta subunits regulate cell-surface expression and gating of voltage-dependent Ca2+ channel alpha1 subunits. Based on primary sequence comparisons, beta subunits are predicted to be modular structures composed of five domains (A-E) that are related to the large family of membrane-associated guanylate kinase proteins. The crystal structure of the beta subunit core B-D domains has been reported recently; however, little is known about the structures of the A and E domains. The N-terminal A domain differs among the four subtypes of Ca2+ channel beta subunits (beta1-beta4) primarily as the result of two duplications of an ancestral gene containing multiple alternatively spliced exons. At least nine A domain sequences can be generated by alternative splicing. In this report, we focus on one A domain sequence, the highly conserved beta4a A domain. We solved its three-dimensional structure and show that it is expressed in punctate structures throughout the molecular layer of the cerebellar cortex. We also demonstrate that it does not participate directly in Cav2.1 Ca2+ channel gating but serves as a binding site in protein-protein interactions with synaptotagmin I and the LC2 domain of microtubule-associated protein 1A. With respect to beta4 subunits, the interactions are specific for the beta4a splice variant, because they do not occur with the beta4b A domain. These results have strong bearing on our current understanding of the structure of alternatively spliced Ca2+ channel beta subunits and the cell-specific roles they play in the CNS.

Interaction via a key tryptophan in the I-II linker of N-type calcium channels is required for beta1 but not for palmitoylated beta2, implicating an additional binding site in the regulation of channel voltage-dependent properties

The CaVbeta subunits of voltage-gated calcium channels regulate these channels in several ways. Here we investigate the role of these auxiliary subunits in the expression of functional N-type channels at the plasma membrane and in the modulation by G-protein-coupled receptors of this neuronal channel. To do so, we mutated tryptophan 391 to an alanine within the alpha-interacting domain (AID) in the I-II linker of CaV2.2. We showed that the mutation W391 virtually abolishes the binding of CaVbeta1b and CaVbeta2a to the CaV2.2 I-II linker and strongly reduced current density and cell surface expression of both CaV2.2/alpha2delta-2/beta1b and/beta2a channels. When associated with CaVbeta1b, the W391A mutation also prevented the CaVbeta1b-mediated hyperpolarization of CaV2.2 channel activation and steady-state inactivation. However, the mutated CaV2.2W391A/beta1b channels were still inhibited to a similar extent by activation of the D2 dopamine receptor with the agonist quinpirole. Nevertheless, key hallmarks of G-protein modulation of N-type currents, such as slowed activation kinetics and prepulse facilitation, were not observed for the mutated channel. In contrast, CaVbeta2a was still able to completely modulate the biophysical properties of CaV2.2W391A channel and allow voltage-dependent G-protein modulation of CaV2.2W391A. Additional data suggest that the concentration of CaVbeta2a in the proximity of the channel is enhanced independently of its binding to the AID by its palmitoylation. This is essentially sufficient for all of the functional effects of CaVbeta2a, which may occur via a second lower-affinity binding site, except trafficking the channel to the plasma membrane, which requires interaction with the AID region.

A CaVbeta SH3/guanylate kinase domain interaction regulates multiple properties of voltage-gated Ca2+ channels

Auxiliary Ca²⁺ channel β subunits (Ca_Vβ) regulate cellular Ca²⁺ signaling by trafficking pore-forming α₁ subunits to the membrane and normalizing channel gating. These effects are mediated through a characteristic src homology 3/guanylate kinase (SH3–GK) structural module, a design feature shared in common with the membrane-associated guanylate kinase (MAGUK) family of scaffold proteins. However, the mechanisms by which the Ca_Vβ SH3–GK module regulates multiple Ca²⁺ channel functions are not well understood. Here, using a split-domain approach, we investigated the role of the interrelationship between Ca_Vβ SH3 and GK domains in defining channel properties. The studies build upon a previously identified split-domain pair that displays a trans SH3–GK interaction, and fully reconstitutes Ca_Vβ effects on channel trafficking, activation gating, and increased open probability (P_o). Here, by varying the precise locations used to separate SH3 and GK domains and monitoring subsequent SH3–GK interactions by fluorescence resonance energy transfer (FRET), we identified a particular split-domain pair that displayed a subtly altered configuration of the trans SH3–GK interaction. Remarkably, this pair discriminated between Ca_Vβ trafficking and gating properties: α_1C targeting to the membrane was fully reconstituted, whereas shifts in activation gating and increased P_o functions were selectively lost. A more extreme case, in which the trans SH3–GK interaction was selectively ablated, yielded a split-domain pair that could reconstitute neither the trafficking nor gating-modulation functions, even though both moieties could independently engage their respective binding sites on the α_1C (Ca_V1.2) subunit. The results reveal that Ca_Vβ SH3 and GK domains function codependently to tune Ca²⁺ channel trafficking and gating properties, and suggest new paradigms for physiological and therapeutic regulation of Ca²⁺ channel activity.

Short-term regulation of excitation-contraction coupling by the beta1a subunit in adult mouse skeletal muscle

The beta1a subunit of the skeletal muscle voltage-gated Ca2+ channel plays a fundamental role in the targeting of the channel to the tubular system as well as in channel function. To determine whether this cytosolic auxiliary subunit is also a regulatory protein of Ca2+ release from the sarcoplasmic reticulum in vivo, we pressure-injected the beta1a subunit into intact adult mouse muscle fibers and recorded, with Fluo-3 AM, the intracellular Ca2+ signal induced by the action potential. We found that the beta1a subunit significantly increased, within minutes, the amplitude of Ca2+ release without major changes in its time course. beta1a subunits with the carboxy-terminus region deleted did not show an effect on Ca2+ release. The possibility that potentiation of Ca2+ release is due to a direct interaction between the beta1a subunit and the ryanodine receptor was ruled out by bilayer experiments of RyR1 single-channel currents and also by Ca2+ flux experiments. Our data suggest that the beta1a subunit is capable of regulating E-C coupling in the short term and that the integrity of the carboxy-terminus region is essential for its modulatory effect.

A single CaVbeta can reconstitute both trafficking and macroscopic conductance of voltage-dependent calcium channels

Voltage-dependent calcium-channel beta subunits (Ca(V)beta) strongly modulate pore-forming alpha(1) subunits by trafficking channel complexes to the plasma membrane and enhancing channel open probability (P(o)). Despite their central role, it is unclear whether binding of a single Ca(V)beta, or multiple Ca(V)betas, to an alpha(1) subunit governs the two distinct functions. Conventional experiments utilizing coexpression of alpha(1) and Ca(V)beta subunits have been unable to resolve the ambiguity due to difficulties in establishing their stoichiometry in functional channels. Here, we unambiguously establish a 1: 1 stoichiometry by covalently linking Ca(V)beta(2b) to the carboxyl terminus of alpha(1C) (Ca(V)1.2), creating alpha(1C).beta(2b). Recombinant L-type channels reconstituted in HEK 293 cells with alpha(1C).beta(2b) supported whole-cell currents to the same extent as channels reconstituted via coexpression of the individual subunits. Analysis of gating charge showed alpha(1C).beta(2b) fully restored channel trafficking to the plasma membrane. Co-transfecting Ca(V)beta(2a) with alpha(1C).beta(2b) had little further impact on function. To rule out the possibility that fused Ca(V)beta(2b) was interacting in trans with neighbouring alpha(1) molecules, alpha(1C).beta(2b) was cotransfected with alpha(1B) (Ca(V)2.2), and pharmacological block with nimodipine showed an absence of alpha(1B) trafficking. These results establish that association of a single Ca(V)beta with a pore-forming alpha(1) subunit captures the functional essence of HVA calcium channels, and introduce alpha(1)-Ca(V)beta fusion proteins as a powerful new tool to probe structure-function mechanisms.

Transfer of β subunit regulation from high to low voltage-gated Ca2+ channels

High voltage-activated Ca(2+) channel expression and gating is controlled by their beta subunits. Although the sites of interaction are known at the atomic level, how beta modulates gating remains to be determined. Using a chimeric approach, beta subunit regulation was conferred to a low voltage-activated channel. Regulation was dependent on a rigid linker connecting the alpha(1) interaction domain to IS6. Chimeric channels also revealed a role for IS6 in channel gating. Taken together, these results support a direct coupling model where beta subunits alter movements in IS6 that occur as the channel transits between closed, open, and inactivated states.

Alone at last! New functions for Ca2+ channel beta subunits?

New functions for voltage-gated Ca(2+) channel auxiliary beta subunits have recently been identified. These functions appear to be regulated by the beta subunit alone, independently of any effects on the Ca(2+) influx; hence, the beta subunit may not be truly "auxiliary" and may play more fundamental roles in Ca(2+) homeostasis or gene regulation. These new findings raise important questions and open new, exciting research avenues.

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

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

Ca2+ channel beta-subunits: structural insights AID our understanding

It has taken 17 years from the first identification of a voltage-gated Ca2+ channel (CaV) beta-subunit as a band on a gel following purification of skeletal muscle dihydropyridine (DHP) receptors in 1987 to the publication of key information on the structures of Ca2+ channel beta-subunits. Three recent X-ray crystallographic studies have now solved the structures of the core domains of three Ca2+ channel beta-subunits. In this article, the properties of these cytoplasmic auxiliary subunits will first be summarized. Then the CaVbeta structures and the information they provide regarding how these proteins interact with the CaValpha1 subunit will be discussed and the possible implications of these new data for G-protein modulation of Ca2+ channels will be examined.

The C-terminal Residues in the Alpha-interacting Domain (AID) Helix Anchor CaVβ Subunit Interaction and Modulation of CaV2.3 Channels

The alpha-interacting domain (AID) in the I-II linker of high voltage-activated (HVA) Ca(2+) channel alpha1 subunits binds with high affinity to Ca(V)beta auxiliary subunits. The recently solved crystal structures of the AID-Ca(V)beta complex in Ca(V)1.1/1.2 have revealed that this interaction occurs through a set of six mostly invariant residues Glu/Asp(6), Leu(7), Gly(9), Tyr(10), Trp(13), and Ile(14) (where the superscript refers to the position of the residue starting with the QQ signature doublet) distributed among three alpha-helical turns in the proximal section of the I-II linker. We show herein that alanine mutations of N-terminal AID residues Gln(1), Gln(2), Ile(3), Glu(4), Glu(6), Leu(7), and Gly(9) in Ca(V)2.3 did not abolish [(35)S]Ca(V)beta 1b or [(35)S]Ca(V)beta 3 subunit overlay binding to fusion proteins nor did they prevent the typical modulation of whole cell currents by Ca(V)beta 3. Mutations of the invariant Tyr(10) with either hydrophobic (Ala), aromatic (Phe), or positively charged (Arg, Lys) residues yielded Ca(V)beta 3-responsive whole cell currents, whereas mutations with negatively charged residues (Asp, Glu) disrupted Ca(V)beta 3 binding and modulation. In contrast, modulation and binding by Ca(V)beta 3 was significantly weakened in I14A (neutral and hydrophobic) and I14S (neutral and polar) mutants and eradicated in negatively charged I14D and I14E or positively charged I14R and I14K mutants. Ca(V)beta 3-induced modulation was only preserved with the conserved I14L mutation. Molecular replacement analyses carried out using a three-dimensional homology model of the AID helix from Ca(V)2.3 suggests that a high degree of hydrophobicity and a restrained binding pocket could account for the strict structural specificity of the interaction site found at position Ile(14). Altogether these results indicate that the C-terminal residues Trp(13) (1) and Ile(14) anchor Ca(V)beta subunit functional modulation of HVA Ca(2+) channels.

Dominant-negative calcium channel suppression by truncated constructs involves a kinase implicated in the unfolded protein response

Expression of the calcium channel Ca(V)2.2 is markedly suppressed by coexpression with truncated constructs of Ca(V)2.2. Furthermore, a two-domain construct of Ca(V)2.1 mimicking an episodic ataxia-2 mutation strongly inhibited Ca(V)2.1 currents. We have now determined the specificity of this effect, identified a potential mechanism, and have shown that such constructs also inhibit endogenous calcium currents when transfected into neuronal cell lines. Suppression of calcium channel expression requires interaction between truncated and full-length channels, because there is inter-subfamily specificity. Although there is marked cross-suppression within the Ca(V)2 calcium channel family, there is no cross-suppression between Ca(V)2 and Ca(V)3 channels. The mechanism involves activation of a component of the unfolded protein response, the endoplasmic reticulum resident RNA-dependent kinase (PERK), because it is inhibited by expression of dominant-negative constructs of this kinase. Activation of PERK has been shown previously to cause translational arrest, which has the potential to result in a generalized effect on protein synthesis. In agreement with this, coexpression of the truncated domain I of Ca(V)2.2, together with full-length Ca(V)2.2, reduced the level not only of Ca(V)2.2 protein but also the coexpressed alpha2delta-2. Thapsigargin, which globally activates the unfolded protein response, very markedly suppressed Ca(V)2.2 currents and also reduced the expression level of both Ca(V)2.2 and alpha2delta-2 protein. We propose that voltage-gated calcium channels represent a class of difficult-to-fold transmembrane proteins, in this case misfolding is induced by interaction with a truncated cognate Ca(V) channel. This may represent a mechanism of pathology in episodic ataxia-2.

Targeting mechanisms of high voltage-activated Ca2+ channels

Functional voltage-dependent Ca2+ channel complexes are assembled by three to four subunits: alpha1, beta, alpha2delta subunits (C. Leveque et al., 1994, J. Biol Chem. 269, 6306-6312; M. W. McEnery et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88, 11095-11099) and at least in muscle cells also y subunits (B. M. Curtis and W. A. Catterall, 1984, Biochemistry 23, 2113-2118). Ca2+ channels mediate the voltage-dependent Ca2+ influx in subcellular compartments, triggering such diverse processes as neurotransmitter release, dendritic action potentials, excitation-contraction, and excitation-transcription coupling. The targeting of biophysically defined Ca2+ channel complexes to the correct subcellular structures is, thus, critical to proper cell and physiological functioning. Despite their importance, surprisingly little is known about the targeting mechanisms by which Ca2+ channel complexes are transported to their site of function. Here we summarize what we know about the targeting of Ca2+ channel complexes through the cell to the plasma membrane and subcellular structures.

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.

Structural basis of the alpha1-beta subunit interaction of voltage-gated Ca2+ channels

High-voltage-activated Ca2+ channels are essential for diverse biological processes. They are composed of four or five subunits, including alpha1, alpha2-delta, beta and gamma (ref. 1). Their expression and function are critically dependent on the beta-subunit, which transports alpha1 to the surface membrane and regulates diverse channel properties. It is believed that the beta-subunit interacts with alpha1 primarily through the beta-interaction domain (BID), which binds directly to the alpha-interaction domain (AID) of alpha1; however, the molecular mechanism of the alpha1-beta interaction is largely unclear. Here we report the crystal structures of the conserved core region of beta3, alone and in complex with AID, and of beta4 alone. The structures show that the beta-subunit core contains two interacting domains: a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain. The AID binds to a hydrophobic groove in the GK domain through extensive interactions, conferring extremely high affinity between alpha1 and beta-subunits. The BID is essential both for the structural integrity of and for bridging the SH3 and GK domains, but it does not participate directly in binding alpha1. The presence of multiple protein-interacting modules in the beta-subunit opens a new dimension to its function as a multi-functional protein.

Structural Analysis of the Voltage-Dependent Calcium Channel β Subunit Functional Core and Its Complex with the α1 Interaction Domain

Voltage-dependent calcium channels (VDCC) are multiprotein assemblies that regulate the entry of extracellular calcium into electrically excitable cells and serve as signal transduction centers. The alpha1 subunit forms the membrane pore while the intracellular beta subunit is responsible for trafficking of the channel to the plasma membrane and modulation of its electrophysiological properties. Crystallographic analyses of a beta subunit functional core alone and in complex with a alpha1 interaction domain (AID) peptide, the primary binding site of beta to the alpha1 subunit, reveal that beta represents a novel member of the MAGUK protein family. The findings illustrate how the guanylate kinase fold has been fashioned into a protein-protein interaction module by alteration of one of its substrate sites. Combined results indicate that the AID peptide undergoes a helical transition in binding to beta. We outline the mechanistic implications for understanding the beta subunit's broad regulatory role of the VDCC, particularly via the AID.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

Calcium channel blockers in the treatment of disease

Over the last 20 years the combination of patch clamp and molecular biology techniques have resulted in an explosion in our knowledge of the different calcium channel types and their roles in physiology. A crucial component to this advance has been the discovery of specific blockers for different calcium channel types. These blockers have not only permitted researchers to assign specific functions to different channel types, but their specificity allowed them to be used to treat diseases that resulted from enhanced calcium channel function. In some cases, the enhanced calcium channel function is secondary to other problems leading to increased cellular excitability. The specificity of calcium channel blockers, however, has provided a means to treat symptoms of the pathophysiology until more effective treatments become available to address the underlying problems. The goal of this review is to introduce the various types of calcium channels, their primary physiologic roles, drugs that block these channels, and diseases that are currently being treated with these drugs.

G Protein Modulation of Voltage-Gated Calcium Channels

Calcium influx into any cell requires fine tuning to guarantee the correct balance between activation of calcium-dependent processes, such as muscle contraction and neurotransmitter release, and calcium-induced cell damage. G protein-coupled receptors play a critical role in negative feedback to modulate the activity of the CaV2 subfamily of the voltage-dependent calcium channels, which are largely situated on neuronal and neuro-endocrine cells. The basis for the specificity of the relationships among membrane receptors, G proteins, and effector calcium channels will be discussed, as well as the mechanism by which G protein-mediated inhibition is thought to occur. The inhibition requires free G beta gamma dimers, and the cytoplasmic linker between domains I and II of the CaV2 alpha 1 subunits binds G beta gamma dimers, whereas the intracellular N terminus of CaV2 alpha 1 subunits provides essential determinants for G protein modulation. Evidence suggests a key role for the beta subunits of calcium channels in the process of G protein modulation, and the role of a class of proteins termed "regulators of G protein signaling" will also be described.

CaVβ subunit-mediated up-regulation of CaV2.2 currents triggered by D2 dopamine receptor activation

Voltage-dependent Ca(2+) channels (VDCCs) are subject to modulation by a number of pathways, including membrane-delimited inhibition by heterotrimeric G-proteins and modulation through phosphorylation by diverse kinases. Here we report that in the Xenopus oocyte expression system Ca(V)2.2 channels undergo a sustained, linear and irreversible run-up lasting up to 30 min, which is potentiated during G-protein-mediated inhibition by activation of co-expressed G-protein coupled receptors (GPCRs). This up-regulation is not a result of receptor desensitization, but is associated with a hyperpolarization of the voltage for activation and depends on the presence of accessory subunits such that beta subunits promote, and alpha2delta subunits oppose the current increase. We have investigated the involvement of G-proteins and found that over-expression of Galpha(o) subunits or Galpha-transducin reduced the amount of agonist-mediated up-regulation. However, we have found no evidence for the involvement of any second messenger pathways in the increase of current run-up in the presence of a GPCR agonist. Taken together, our data suggest that the effect reported herein involves an enhancement of the GTPase activity of endogenous Galpha subunits, which is triggered by GPCR activation and mediated by accessory Ca(V)beta subunits. It may involve an increased association of Ca(V)beta subunits with alpha1 subunits in the plasma membrane or trafficking of channels to the plasma membrane.

Competitive and synergistic interactions of G protein beta(2) and Ca(2+) channel beta(1b) subunits with Ca(v)2.1 channels, revealed by mammalian two-hybrid and fluorescence resonance energy transfer measurements

Presynaptic Ca2+ channels are inhibited by metabotropic receptors. A possible mechanism for this inhibition is that G protein betagamma subunits modulate the binding of the Ca2+ channel beta subunit on the Ca2+ channel complex and induce a conformational state from which channel opening is more reluctant. To test this hypothesis, we analyzed the binding of Ca2+ channel beta and G protein beta subunits on the two separate binding sites, i.e. the loopI-II and the C terminus, and on the full-length P/Q-type alpha12.1 subunit by using a modified mammalian two-hybrid system and fluorescence resonance energy transfer (FRET) measurements. Analysis of the interactions on the isolated bindings sites revealed that the Ca2+ channel beta1b subunit induces a strong fluorescent signal when interacting with the loopI-II but not with the C terminus. In contrast, the G protein beta subunit induces FRET signals on both the C terminus and loopI-II. Analysis of the interactions on the full-length channel indicates that Ca2+ channel beta1b and G protein beta subunits bind to the alpha1 subunit at the same time. Coexpression of the G protein increases the FRET signal between alpha1/beta1b FRET pairs but not for alpha1/beta1b FRET pairs where the C terminus was deleted from the alpha1 subunit. The results suggest that the G protein alters the orientation and/or association between the Ca2+ channel beta and alpha12.1 subunits, which involves the C terminus of the alpha1 subunit and may corresponds to a new conformational state of the channel.

Mechanism of action of Gq to inhibit G beta gamma modulation of CaV2.2 calcium channels: probed by the use of receptor-G alpha tandems

The stable interaction of a G-protein coupled receptor and a particular partner G-protein was made possible by creating tandems between the alpha(2A) adrenergic receptor (alpha(2A)-R) and pertussis toxin-resistant mutants of different G alpha subunits of heterotrimeric G-proteins. Both alpha(2A)-R-G alpha(o) and alpha(2A)-R-G alpha(i) proved able to reconstitute agonist-induced voltage-dependent inhibition of N-type calcium channels (Ca(V)2.2) similar to the wild-type alpha(2A)-R when expressed in COS-7 cells. The interaction of G(q) with the G(i/o) signaling pathways was studied by expressing either G alpha(q) or a chimeric construct based on G alpha(q) containing the last five amino acids of G alpha(z), which is activated by alpha(2A)-R. It was found that G alpha(qz5) activated by the wild-type alpha(2A)-R inhibited Ca(V)2.2 currents in a voltage-independent fashion. Furthermore, G alpha(qz5) counteracted the voltage-dependent inhibition resulting from alpha(2A)-R-G alpha(o) activation. We subsequently investigated the basis for the behavior of G alpha(qz5). Our evidence suggests that this occurs as a result of a downstream effect of activation of G alpha(qz5) because it was blocked by C-terminal construct of phospholipase C beta 1. Furthermore it is likely to occur in part via protein kinase C (PKC) activation, because the PKC activator phorbol dibutyrate mimicked the effects of G alpha(qz5) in alpha(2A)-R-G alpha(o)-transfected cells. Conversely, cells expressing both alpha(2A)-R-G alpha(o) and G alpha(qz5) exhibited a partial restoration of voltage-dependent inhibition in the presence of the PKC inhibitor bisindolylmaleimide I (GF 109203X). The potential sites of phosphorylation are discussed.

The beta1a subunit regulates the functional properties of adult frog and mouse L-type Ca2+ channels of skeletal muscle

The beta(1a) subunit, one of the auxiliary subunits of Ca(V)1.1 channels, was expressed in COS-1 cells, purified by electroelution and electrodialysis techniques and identified by Western blot using monoclonal antibodies. The purified beta(1a) subunit strongly interacted in vitro with the alpha interaction domain (AID) of Ca(V)1.1 channels. The actions of the purified beta(1a) subunit on Ca(V)1.1 channel currents were assessed in whole cell voltage clamp experiments performed in vesicles derived from frog and mouse adult skeletal muscle plasma membranes. L-type inward currents were recorded in solutions containing Ba(2+) (I(Ba)). Values of peak I(Ba) were doubled by the beta(1a) subunit in frog and mouse muscle vesicles and the amplitude of the slow component of tail currents was greatly increased. The actions of the beta(1a) subunit on Ca(V)1.1 channel currents reached a steady state within 20 min. The beta(1a) subunit had no effect on the time courses of activation or inactivation of I(Ba) or shifted the current-voltage relation. Non-linear capacitive currents were recorded in solutions that contained mostly impermeant ions. Charge movement depended on voltage with average Boltzmann parameters: Q(max) = 28.0 +/- 6.6 nC microF(-1), V = -58.0 +/- 2.0 mV and k = 15.3 +/- 1.1 mV (n = 24). In the presence of the beta(1a) subunit, these parameters remained unchanged: Q(max) = 29.8 +/- 3.5 nC microF(-1), V = -54.5 +/- 2.2 mV and k = 16.4 +/- 1.3 mV (n = 21). Overall, the work describes a novel preparation to explore in situ the role of the beta(1a) subunit on the function of adult Ca(V)1.1 channels.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

beta-Subunits: fine tuning of Ca(2+) channel block

Ca(2+) channel blockers such as 1,4-dihydropyridines, phenylalkylamines, diltiazem and mibefradil exert their anti-arrhythmic and anti-hypertensive action by restricting Ca(2+) entry into myocardial cells and smooth muscle cells. Binding sites for these drugs are present on the pore-forming alpha(1)-subunits of voltage-dependent Ca(2+) (Ca(v)) channels. However, striking new data show that auxillary beta-subunits also influence drug sensitivity significantly. These findings are summarized and the underlying molecular mechanisms and their pharmacological relevance are discussed.

Trafficking of L-type calcium channels mediated by the postsynaptic scaffolding protein AKAP79

Accurate calcium signaling requires spatial and temporal coordination of voltage-gated calcium channels (VGCCs) and a variety of signal transduction proteins. Accordingly, regulation of L-type VGCCs involves the assembly of complexes that include the channel subunits, protein kinase A (PKA), protein kinase A anchoring proteins (AKAPs), and beta2-adrenergic receptors, although the molecular details underlying these interactions remain enigmatic. We show here, by combining extracellular epitope splicing into the channel pore-forming subunit and immunoassays with whole cell and single channel electrophysiological recordings, that AKAP79 directly regulates cell surface expression of L-type calcium channels independently of PKA. This regulation involves a short polyproline sequence contained specifically within the II-III cytoplasmic loop of the channel. Thus we propose a novel mechanism whereby AKAP79 and L-type VGCCs function as components of a biosynthetic mechanism that favors membrane incorporation of organized molecular complexes in a manner that is independent of PKA phosphorylation events.

A specific tryptophan in the I-II linker is a key determinant of beta-subunit binding and modulation in Ca(V)2.3 calcium channels

The ancillary beta subunits modulate the activation and inactivation properties of high-voltage activated (HVA) Ca(2+) channels in an isoform-specific manner. The beta subunits bind to a high-affinity interaction site, alpha-interaction domain (AID), located in the I-II linker of HVA alpha1 subunits. Nine residues in the AID motif are absolutely conserved in all HVA channels (QQxExxLxGYxxWIxxxE), but their contribution to beta-subunit binding and modulation remains to be established in Ca(V)2.3. Mutations of W386 to either A, G, Q, R, E, F, or Y in Ca(V)2.3 disrupted [(35)S]beta3-subunit overlay binding to glutathione S-transferase fusion proteins containing the mutated I-II linker, whereas mutations (single or multiple) of nonconserved residues did not affect the protein-protein interaction with beta3. The tryptophan residue at position 386 appears to be an essential determinant as substitutions with hydrophobic (A and G), hydrophilic (Q, R, and E), or aromatic (F and Y) residues yielded the same results. beta-Subunit modulation of W386 (A, G, Q, R, E, F, and Y) and Y383 (A and S) mutants was investigated after heterologous expression in Xenopus oocytes. All mutant channels expressed large inward Ba(2+) currents with typical current-voltage properties. Nonetheless, the typical hallmarks of beta-subunit modulation, namely the increase in peak currents, the hyperpolarization of peak voltages, and the modulation of the kinetics and voltage dependence of inactivation, were eliminated in all W386 mutants, although they were preserved in part in Y383 (A and S) mutants. Altogether these results suggest that W386 is critical for beta-subunit binding and modulation of HVA Ca(2+) channels.

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 novel product of a five-exon stargazin-related gene abolishes Ca(V)2.2 calcium channel expression

We have cloned and characterized a new member of the voltage-dependent Ca(2+) channel gamma subunit family, with a novel gene structure and striking properties. Unlike the genes of other potential gamma subunits identified by their homology to the stargazin gene, CACNG7 is a five-, and not four-exon gene whose mRNA encodes a protein we have designated gamma(7). Expression of human gamma(7) has been localized specifically to brain. N-type current through Ca(V)2.2 channels was almost abolished when co-expressed transiently with gamma(7) in either Xenopus oocytes or COS-7 cells. Furthermore, immunocytochemistry and western blots show that gamma(7) has this effect by causing a large reduction in expression of Ca(V)2.2 rather than by interfering with trafficking or biophysical properties of the channel. No effect of transiently expressed gamma(7) was observed on pre-existing endogenous N-type calcium channels in sympathetic neurones. Low homology to the stargazin-like gamma subunits, different gene structure and the unique functional properties of gamma(7) imply that it represents a distinct subdivision of the family of proteins identified by their structural and sequence homology to stargazin.

Dominant-negative synthesis suppression of voltage-gated calcium channel Cav2.2 induced by truncated constructs

Voltage-gated calcium channel alpha1 subunits consist of four domains (I-IV), each with six transmembrane segments. A number of truncated isoforms have been identified to occur as a result of alternative splicing or mutation. We have examined the functional consequences for expression of full-length Ca(v)2.2 (alpha1B) of its coexpression with truncated constructs of Ca(v)2.2. Domains I-II or domains III-IV, when expressed individually, together with the accessory subunits beta1b and alpha2delta-1, did not form functional channels. When they were coexpressed, low-density whole-cell currents and functional channels with properties similar to wild-type channels were observed. However, when domain I-II, domain III-IV, or domain I alone were coexpressed with full-length Ca(v)2.2, they markedly suppressed its functional expression, although at the single channel level, when channels were recorded, there were no differences in their biophysical properties. Furthermore, when it was coexpressed with either domain I-II or domain I, the fluorescence of green fluorescent protein (GFP)-Ca(v)2.2 and expression of Ca(v)2.2 protein was almost abolished. Suppression does not involve sequestration of the Ca(v)beta subunit, because loss of GFP-Ca(v)2.2 expression also occurred in the absence of beta subunit, and the effect of domain I-II or domain I could not be mimicked by the cytoplasmic I-II loop of Ca(v)2.2. It requires transmembrane segments, because the isolated Ca(v)2.2 N terminus did not have any effect. Our results indicate that the mechanism of suppression of Ca(v)2.2 by truncated constructs containing domain I involves inhibition of channel synthesis, which may represent a role of endogenously expressed truncated Ca(v) isoforms.

Dominant-negative synthesis suppression of voltage-gated calcium channel Cav2.2 induced by truncated constructs

Voltage-gated calcium channel alpha1 subunits consist of four domains (I-IV), each with six transmembrane segments. A number of truncated isoforms have been identified to occur as a result of alternative splicing or mutation. We have examined the functional consequences for expression of full-length Ca(v)2.2 (alpha1B) of its coexpression with truncated constructs of Ca(v)2.2. Domains I-II or domains III-IV, when expressed individually, together with the accessory subunits beta1b and alpha2delta-1, did not form functional channels. When they were coexpressed, low-density whole-cell currents and functional channels with properties similar to wild-type channels were observed. However, when domain I-II, domain III-IV, or domain I alone were coexpressed with full-length Ca(v)2.2, they markedly suppressed its functional expression, although at the single channel level, when channels were recorded, there were no differences in their biophysical properties. Furthermore, when it was coexpressed with either domain I-II or domain I, the fluorescence of green fluorescent protein (GFP)-Ca(v)2.2 and expression of Ca(v)2.2 protein was almost abolished. Suppression does not involve sequestration of the Ca(v)beta subunit, because loss of GFP-Ca(v)2.2 expression also occurred in the absence of beta subunit, and the effect of domain I-II or domain I could not be mimicked by the cytoplasmic I-II loop of Ca(v)2.2. It requires transmembrane segments, because the isolated Ca(v)2.2 N terminus did not have any effect. Our results indicate that the mechanism of suppression of Ca(v)2.2 by truncated constructs containing domain I involves inhibition of channel synthesis, which may represent a role of endogenously expressed truncated Ca(v) isoforms.