Cloning of the beta(2a) subunit of the voltage-dependent calcium channel from human heart: cooperative effect of alpha(2)/delta and beta(2a) on the membrane expression of the alpha(1C) subunit

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

Stapled Voltage-Gated Calcium Channel (CaV) α-Interaction Domain (AID) Peptides Act As Selective Protein-Protein Interaction Inhibitors of CaV Function

For many voltage-gated ion channels (VGICs), creation of a properly functioning ion channel requires the formation of specific protein–protein interactions between the transmembrane pore-forming subunits and cystoplasmic accessory subunits. Despite the importance of such protein–protein interactions in VGIC function and assembly, their potential as sites for VGIC modulator development has been largely overlooked. Here, we develop meta-xylyl (m-xylyl) stapled peptides that target a prototypic VGIC high affinity protein–protein interaction, the interaction between the voltage-gated calcium channel (Ca_V) pore-forming subunit α-interaction domain (AID) and cytoplasmic β-subunit (Ca_Vβ). We show using circular dichroism spectroscopy, X-ray crystallography, and isothermal titration calorimetry that the m-xylyl staples enhance AID helix formation are structurally compatible with native-like AID:Ca_Vβ interactions and reduce the entropic penalty associated with AID binding to Ca_Vβ. Importantly, electrophysiological studies reveal that stapled AID peptides act as effective inhibitors of the Ca_Vα₁:Ca_Vβ interaction that modulate Ca_V function in an Ca_Vβ isoform-selective manner. Together, our studies provide a proof-of-concept demonstration of the use of protein–protein interaction inhibitors to control VGIC function and point to strategies for improved AID-based Ca_V modulator design.

Differential gene expression of cardiac ion channels in human dilated cardiomyopathy

Background

Dilated cardiomyopathy (DCM) is characterized by idiopathic dilation and systolic contractile dysfunction of the cardiac chambers. The present work aimed to study the alterations in gene expression of ion channels involved in cardiomyocyte function.

Methods and Results

Microarray profiling using the Affymetrix Human Gene® 1.0 ST array was performed using 17 RNA samples, 12 from DCM patients undergoing cardiac transplantation and 5 control donors (CNT). The analysis focused on 7 cardiac ion channel genes, since this category has not been previously studied in human DCM. SCN2B was upregulated, while KCNJ5, KCNJ8, CLIC2, CLCN3, CACNB2, and CACNA1C were downregulated. The RT-qPCR (21 DCM and 8 CNT samples) validated the gene expression of SCN2B (p < 0.0001), KCNJ5 (p < 0.05), KCNJ8 (p < 0.05), CLIC2 (p < 0.05), and CACNB2 (p < 0.05). Furthermore, we performed an IPA analysis and we found a functional relationship between the different ion channels studied in this work.

Conclusion

This study shows a differential expression of ion channel genes involved in cardiac contraction in DCM that might partly underlie the changes in left ventricular function observed in these patients. These results could be the basis for new genetic therapeutic approaches.

Regulation of voltage-dependent calcium channels by RGK proteins

RGK proteins belong to the Ras superfamily of monomeric G-proteins, and currently include four members - Rad, Rem, Rem2, and Gem/Kir. RGK proteins are broadly expressed, and are the most potent known intracellular inhibitors of high-voltage-activated Ca²⁺ (Ca(V)1 and Ca(V)2) channels. Here, we review and discuss the evidence in the literature regarding the functional mechanisms, structural determinants, physiological role, and potential practical applications of RGK-mediated inhibition of Ca(V)1/Ca(V)2 channels. This article is part of a Special Issue entitled: Calcium channels.

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.

Disruption of the IS6-AID linker affects voltage-gated calcium channel inactivation and facilitation

Two processes dominate voltage-gated calcium channel (Ca_V) inactivation: voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). The Ca_Vβ/Ca_Vα₁-I-II loop and Ca²⁺/calmodulin (CaM)/Ca_Vα₁–C-terminal tail complexes have been shown to modulate each, respectively. Nevertheless, how each complex couples to the pore and whether each affects inactivation independently have remained unresolved. Here, we demonstrate that the IS6–α-interaction domain (AID) linker provides a rigid connection between the pore and Ca_Vβ/I-II loop complex by showing that IS6-AID linker polyglycine mutations accelerate Ca_V1.2 (L-type) and Ca_V2.1 (P/Q-type) VDI. Remarkably, mutations that either break the rigid IS6-AID linker connection or disrupt Ca_Vβ/I-II association sharply decelerate CDI and reduce a second Ca²⁺/CaM/Ca_Vα₁–C-terminal–mediated process known as calcium-dependent facilitation. Collectively, the data strongly suggest that components traditionally associated solely with VDI, Ca_Vβ and the IS6-AID linker, are essential for calcium-dependent modulation, and that both Ca_Vβ-dependent and CaM-dependent components couple to the pore by a common mechanism requiring Ca_Vβ and an intact IS6-AID linker.

Targeted disruption of the voltage-dependent calcium channel alpha2/delta-1-subunit

Cardiac L-type voltage-dependent Ca(2+) channels are heteromultimeric polypeptide complexes of alpha(1)-, alpha(2)/delta-, and beta-subunits. The alpha(2)/delta-1-subunit possesses a stereoselective, high-affinity binding site for gabapentin, widely used to treat epilepsy and postherpetic neuralgic pain as well as sleep disorders. Mutations in alpha(2)/delta-subunits of voltage-dependent Ca(2+) channels have been associated with different diseases, including epilepsy. Multiple heterologous coexpression systems have been used to study the effects of the deletion of the alpha(2)/delta-1-subunit, but attempts at a conventional knockout animal model have been ineffective. We report the development of a viable conventional knockout mouse using a construct targeting exon 2 of alpha(2)/delta-1. While the deletion of the subunit is not lethal, these animals lack high-affinity gabapentin binding sites and demonstrate a significantly decreased basal myocardial contractility and relaxation and a decreased L-type Ca(2+) current peak current amplitude. This is a novel model for studying the function of the alpha(2)/delta-1-subunit and will be of importance in the development of new pharmacological therapies.

Alanine-scanning mutagenesis defines a conserved energetic hotspot in the CaValpha1 AID-CaVbeta interaction site that is critical for channel modulation

Voltage-gated calcium channels (CaVs) are large, multisubunit complexes that control cellular calcium entry. CaV pore-forming (CaValpha1) and cytoplasmic (CaVbeta) subunits associate through a high-affinity interaction between the CaValpha1 alpha interaction domain (AID) and CaVbeta alpha binding pocket (ABP). Here we analyze AID-ABP interaction thermodynamics using isothermal titration calorimetry. We find that commensurate with their strong sequence similarity, all CaV1 and CaV2 AID peptides bind CaVbeta with similar nanomolar affinities. Although the AID-ABP interface encompasses 24 side chains, alanine-scanning mutagenesis reveals that the binding energy is focused in two complementary hotspots comprising four deeply conserved residues. Electrophysiological experiments show that hotspot interaction disruption prevents trafficking and functional modulation of CaV1.2 by CaVbeta. Together, the data support the primacy of the AID-ABP interface for CaValpha1-CaVbeta association, underscore the idea that hotspots dominate protein-protein interaction affinities, and uncover a target for strategies to control cellular excitability by blocking CaValpha1-CaVbeta complex formation.

Adenoviral-mediated expression of dihydropyridine-insensitive L-type calcium channels in cardiac ventricular myocytes and fibroblasts

Cardiac voltage-gated Ca2+ channels regulate the intracellular Ca2+ concentration and are therefore essential for muscle contraction, second messenger activation, gene expression and electrical signaling. As a first step in accessing the structural versus functional properties of the L-type Ca2+ channel in the heart, we have expressed a dihydropyridine (DHP)-insensitive CaV1.2 channel in rat ventricular myocytes and fibroblasts. Following isolation and culture, cells were infected with adenovirus expressing either LacZ or a mutant CaV1.2 channel (CaV1.2DHPi) containing the double mutation (T1039Y & Q1043M). This mutation renders the channel insensitive to neutral DHP compounds such as nisoldipine. The whole-cell, L-type Ca2+ current (ICa) measured in control myocytes was inhibited in a concentration-dependent manner by nisoldipine with an IC50 of 66 nM and complete block at 250 nM. In contrast, ICa in cells infected with AdCaV1.2DHPi was inhibited by only 35% by 500 nM nisoldipine but completely blocked by 50 microM diltiazem. In order to study CaV1.2DHPi in isolation, myocytes infected with AdCaV1.2DHPi were incubated with nisoldipine. Under this condition the cells expressed a large ICa (12 pA/pF) and displayed Ca2+ transients during field stimulation. Furthermore, addition of 2 microM forskolin and 100 microM 3-isobutyl-1-methylxanthine (IBMX), to stimulate protein kinase A, strongly increased IBa in the AdCaV1.2DHPi-infected cells. A Cd2+-sensitive IBa was also recorded in cardiac fibroblasts infected with AdCaV1.2DHPi. Thus, expression of CaV1.2DHPi will provide an important tool in studies of cardiac myocyte and fibroblast function.

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.

L-type calcium channel alpha-subunit and protein kinase inhibitors modulate Rem-mediated regulation of current

Cardiac voltage-gated L-type Ca channels (Ca(V)) are multiprotein complexes, including accessory subunits such as Ca(V)beta2 that increase current expression. Recently, members of the Rad and Gem/Kir-related family of small GTPases have been shown to decrease current, although the mechanism remains poorly defined. In this study, we evaluated the contribution of the L-type Ca channel alpha-subunit (Ca(V)1.2) to Ca(V)beta2-Rem inhibition of Ca channel current. Specifically, we addressed whether protein kinase A (PKA) modulation of the Ca channel modifies Ca(V)beta2-Rem inhibition of Ca channel current. We first tested the effect of Rem on Ca(V)1.2 in human embryonic kidney 293 (HEK-293) cells using the whole cell patch-clamp configuration. Rem coexpression with Ca(V)1.2 reduces Ba current expression under basal conditions, and Ca(V)beta2a coexpression enhances Rem block of Ca(V)1.2 current. Surprisingly, PKA inhibition by 133 nM H-89 or 50 microM Rp-cAMP-S partially relieved the Rem-mediated inhibition of current activity both with and without Ca(V)beta2a. To test whether the H-89 action was a consequence of the phosphorylation status of Ca(V)1.2, we examined Rem regulation of the PKA-insensitive Ca(V)1.2 serine 1928 (S1928) to alanine mutation (Ca(V)1.2-S1928A). Ca(V)1.2-S1928A current was not inhibited by Rem and when coexpression with Ca(V)beta2a was not completely blocked by Rem coexpression, suggesting that the phosphorylation of S1928 contributes to Rem-mediated Ca channel modulation. As a model for native Ca channel complexes, we tested the ability of Rem overexpression in HIT-T15 cells and embryonic ventricular myocytes to interfere with native current. We find that native current is also sensitive to Rem block and that H-89 pretreatment relieves the ability of Rem to regulate Ca current. We conclude that Rem is capable of regulating L-type current, that release of Rem block is modulated by cellular kinase pathways, and that the Ca(V)1.2 COOH terminus contributes to Rem-dependent channel inhibition.

Molecular and Electrophysiological Differences in the L-Type Ca2+ Channel of the Atrium and Ventricle of Rat Hearts

BACKGROUND

Many pathological conditions induce electrical remodeling, possibly through intracellular Ca2+ overload, but the currently available L-type Ca2+ channel blockers may be detrimental because of their global negative inotropic effects.

METHODS AND RESULTS

To determine whether the L-type Ca2+ channel is identical throughout the heart, the distribution of the mRNAs and proteins comprising the L-type Ca2+ channel and its electrophysiological properties were analyzed in rat atria and ventricles. The mRNA of alpha2delta-2 (Cacna2d2) was more abundantly expressed in the atrium (approximately 5-fold) than in the ventricle. In contrast, alpha1C (Cacna1c) (Cav1.2) mRNA was significantly less abundant in the atrium. The level of the alpha1C (Cacna1c) (Cav1.2) protein was decreased (approximately 0.5-fold) and that of alpha2 delta-1 (Cacna2d1) was increased (approximately 2-fold) in the atrium compared with the ventricle. Although the peak ICa,L density showed no significant differences, voltage dependence of inactivation and activation of the current showed a more depolarized shift in the atrium than in the ventricle.

CONCLUSION

These results indicate that in the rat heart the L-type Ca2+ channel differs between the atrium and ventricle with regard to gene expression and electrophysiological properties.

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.

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.

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.

Molecular heterogeneity of calcium channel beta-subunits in canine and human heart: evidence for differential subcellular localization

Multiple Ca2+ channel beta-subunit (Ca(v)beta) isoforms are known to differentially regulate the functional properties and membrane trafficking of high-voltage-activated Ca2+ channels, but the precise isoform expression pattern of Ca(v)beta subunits in ventricular muscle has not been fully characterized. Using sequence data from the Human Genome Project to define the intron/exon structure of the four known Ca(v)beta genes, we designed a systematic RT-PCR strategy to screen human and canine left ventricular myocardial samples for all known Ca(v)beta isoforms. A total of 18 different Ca(v)beta isoforms were detected in both canine and human ventricles including splice variants from all four Ca(v)beta genes. Six of these isoforms have not previously been described. Western blots of ventricular membrane fractions and immunocytochemistry demonstrated that all four Ca(v)beta subunit genes are expressed at the protein level, and the Ca(v)beta subunits show differential subcellular localization with Ca(v)beta1b, Ca(v)beta2, and Ca(v)beta3 predominantly localized to the T-tubule sarcolemma, whereas Ca(v)beta1a and Ca(v)beta4 are more prevalent in the surface sarcolemma. Coexpression of the novel Ca(v)beta2c subunits (Ca(v)beta(2cN1), Ca(v)beta(2cN2), Ca(v)beta(2cN4)) with the pore-forming alpha1C (Ca(v)1.2) and Ca(v)alpha2delta subunits in HEK 293 cells resulted in a marked increase in ionic current and Ca(v)beta2c isoform-specific modulation of voltage-dependent activation. These results demonstrate a previously unappreciated heterogeneity of Ca(v)beta subunit isoforms in ventricular myocytes and suggest the presence of different subcellular populations of Ca2+ channels with distinct functional properties.

Cardiac L-type calcium channel beta-subunits expressed in human heart have differential effects on single channel characteristics

l-Type calcium channels are multiprotein complexes composed of pore-forming (CaV1.2) and modulatory auxiliary alpha2delta- and beta-subunits. We demonstrate expression of two different isoforms for the beta2-subunit (beta2a, beta2b) and the beta3-subunit (beta3a, beta3trunc) in human non-failing and failing ischemic myocardium. Quantitatively, in the left ventricle expression of beta2b transcripts prevails in the order of > beta3 >> beta2a. The expressed cardiac full-length beta3-subunit is identical to the beta3a-isoform, and beta3trunc results from deletion of exon 6 (20 nn) entailing a reading frameshift and translation stop at nucleotide position 495. In failing ischemic myocardium beta3trunc expression increases whereas overall beta3 expression remains unchanged. Heterologous coexpression studies demonstrated that beta2 induced larger currents through rabbit and human cardiac CaV1.2 pore subunits than beta3 isoforms. All beta-subunits increased channel availability at single channel level, but beta2 exerted an additional, marked stimulation of rapid gating (open and closed times, first latency), leading to higher peak current values. We conclude that cardiac beta-subunit isoforms differentially modulate calcium inward currents because of regulatory effects within the channel protein complex. Moreover, differences in the various beta-subunit gene products present in human heart might account for altered single channel behavior found in human heart failure.

Distinctive modulatory effects of five human auxiliary beta2 subunit splice variants on L-type calcium channel gating

Sequence analysis of the human genome permitted cloning of five Ca(2+)-channel beta(2) splice variants (beta(2a)-beta(2e)) that differed only in their proximal amino-termini. The functional consequences of such beta(2)-subunit diversity were explored in recombinant L-type channels reconstituted in HEK 293 cells. Beta(2a) and beta(2e) targeted autonomously to the plasma membrane, whereas beta(2b)-beta(2d) localized to the cytosol when expressed in HEK 293 cells. The pattern of modulation of L-type channel voltage-dependent inactivation gating correlated with the subcellular localization of the component beta(2) variant-membrane-bound beta(2a) and beta(2e) subunits conferred slow(er) channel inactivation kinetics and displayed a smaller fraction of channels recovering from inactivation with fast kinetics, compared to beta(2b)-beta(2d) channels. The varying effects of beta(2) subunits on inactivation gating were accounted for by a quantitative model in which L-type channels reversibly distributed between fast and slow forms of voltage-dependent inactivation-membrane-bound beta(2) subunits substantially decreased the steady-state fraction of fast inactivating channels. Finally, the beta(2) variants also had distinctive effects on L-type channel steady-state activation gating, as revealed by differences in the waveforms of tail-activation (G-V) curves, and conferred differing degrees of prepulse facilitation to the channel. Our results predict important physiological consequences arising from subtle changes in Ca(2+)-channel beta(2)-subunit structure due to alternative splicing and emphasize the utility of splice variants in probing structure-function mechanisms.

Novel functional properties of Ca(2+) channel beta subunits revealed by their expression in adult rat heart cells

Recombinant adenoviruses were used to overexpress green fluorescent protein (GFP)-fused auxiliary Ca(2+) channel beta subunits (beta(1)-beta(4)) in cultured adult rat heart cells, to explore new dimensions of beta subunit functions in vivo. Distinct beta-GFP subunits distributed differentially between the surface sarcolemma, transverse elements, and nucleus in single heart cells. All beta-GFP subunits increased the native cardiac whole-cell L-type Ca(2+) channel current density, but produced distinctive effects on channel inactivation kinetics. The degree of enhancement of whole-cell current density was non-uniform between beta subunits, with a rank order of potency beta(2a) approximately equal to beta(4) > beta(1b) > beta(3). For each beta subunit, the increase in L-type current density was accompanied by a correlative increase in the maximal gating charge (Q(max)) moved with depolarization. However, beta subunits produced characteristic effects on single L-type channel gating, resulting in divergent effects on channel open probability (P(o)). Quantitative analysis and modelling of single-channel data provided a kinetic signature for each channel type. Spurred on by ambiguities regarding the molecular identity of the actual endogenous cardiac L-type channel beta subunit, we cloned a new rat beta(2) splice variant, beta(2b), from heart using 5' rapid amplification of cDNA ends (RACE) PCR. By contrast with beta(2a), expression of beta(2b) in heart cells yielded channels with a microscopic gating signature virtually identical to that of native unmodified channels. Our results provide novel insights into beta subunit functions that are unattainable in traditional heterologous expression studies, and also provide new perspectives on the molecular identity of the beta subunit component of cardiac L-type Ca(2+) channels. Overall, the work establishes a powerful experimental paradigm to explore novel functions of ion channel subunits in their native environments.

Mice with cardiac-specific sequestration of the beta-subunit of the L-type calcium channel

The beta subunit of the L-type voltage-dependent calcium channel modifies the properties of the channel complex by both allosteric modulation of the alpha1 subunit function and by chaperoning the translocation of the alpha1 subunit to the plasma membrane. The goal of this study was to investigate the functional effect of changing the in vivo stoichiometry between the alpha1 and beta subunits by creating a dominant negative expression system in a transgenic mouse model. The high affinity beta subunit-binding domain of the alpha1 subunit was overexpressed in a cardiac-specific manner to act as a beta subunit trap. We found that the predominant beta isoform was located primarily in the membrane bound fraction of heart protein, whereas the beta1 and beta3 were mostly cytosolic. There was a significant diminution of the amount of beta2 in the membrane fraction of the transgenic animals, resulting in a decrease in contractility of the heart and a decrease in L-type calcium current density in the myocyte. However, there were no distinguishable differences in beta1 and beta3 protein expression levels in the membrane bound fraction between transgenic and non-transgenic animals. Since the beta1 and beta3 isoforms only make up a small portion of the total beta subunit in the heart, slight changes in this fraction are not detectable using Western analysis. In contrast, beta1 and beta3 in skeletal muscle and brain, the predominant isoforms in these tissues, respectively, are membrane bound.

Cloning of a functional splice variant of L-type calcium channel beta 2 subunit from rat heart

L-type Ca(2+) channels are heteromultimeric and finely tuned by auxiliary subunits in different tissues and regions. Among auxiliary subunits, beta subunit has been shown to play important roles in many functional aspects of Ca(2+) channel. Rat heart was reported to specifically express beta(2a) subunit. However, the slow inactivation rates of Ca(2+) currents recorded from recombinant Ca(2+) channels with the beta(2a) subunit, and the reported inability to detect beta(2a) subunit in rabbit heart by reverse transcription-PCR analysis raise the possibility of the existence of other beta subunits. We cloned a splice variant of beta(2) subunit from rat heart, using rapid amplification of cDNA 5' ends. The splice variant is highly similar to human beta(2c) subunit that was cloned from human ventricle. Northern blot analysis detected the rat beta(2c) subunit abundantly in rat heart and brain. The deduced amino acid sequence of the beta(2c) subunit was different from that of the beta(2a) subunit only in the N-terminal region. When the beta(2c) subunit was expressed along with alpha(1c) and alpha(2)delta subunits in baby hamster kidney cells, the inactivation rates were comparable with those from native cardiac myocytes, although those with the beta(2a) subunit were slow. Taken together, these observations suggest that the beta(2c) subunit is a functional beta(2) subunit expressed in heart and that the short N-terminal region plays a major role in modifying inactivation kinetics.