Mechanism of Ca(v)1.2 channel modulation by the amino terminus of cardiac beta2-subunits

Inactivation induced by pathogenic Cav1.3 L-type Ca2+-channel variants enhances sensitivity for dihydropyridine Ca2+ channel blockers

BACKGROUND AND PURPOSE

Pathogenic gain-of-function mutations in Cav1.3 L-type voltage-gated Ca2+-channels (CACNA1D) cause neurodevelopmental disorders with or without endocrine symptoms. We aimed to confirm a pathogenic gain-of function phenotype of CACNA1D de novo missense mutations A749T and L271H, and investigated the molecular mechanism causing their enhanced sensitivity for the Ca2+-channel blocker isradipine, a potential therapeutic for affected patients.

EXPERIMENTAL APPROACH

Wildtype and mutant channels were expressed in tsA-201 cells and their gating analysed using whole-cell and single-channel patch-clamp recordings. The voltage-dependence of isradipine action was quantified using protocols inducing variable fractions of inactivated channels. The molecular basis for altered channel gating in the mutants was investigated using in silico modelling and molecular dynamics simulations.

KEY RESULTS

Both mutations were confirmed pathogenic due to characteristic shifts of voltage-dependent activation and inactivation towards negative potentials (~20 mV). At negative holding potentials both mutations showed significantly higher isradipine sensitivity compared to wildtype. The affinity for wildtype and mutant channels increased with channel inactivation as predicted by the modulated receptor hypothesis (30- to 40-fold). The IC50 was indistinguishable for wildtype and mutants when >50% of channels were inactivated.

CONCLUSIONS AND IMPLICATIONS

Mutations A749T and L271H induce pathogenic gating changes. Like wildtype, isradipine inhibition is strongly voltage-dependent. Our data explains their apparent higher drug sensitivity at a given negative voltage by the availability of more inactivated channels due to their more negative inactivation voltage range. Low nanomolar isradipine concentrations will only inhibit Cav1.3 channels in neurons during prolonged depolarized states without selectivity for mutant channels.

CaM-dependent modulation of human CaV1.3 whole-cell and single-channel currents by C-terminal CaMKII phosphorylation site S1475

Phosphorylation enables rapid modulation of voltage-gated calcium channels (VGCC) in physiological and pathophysiological conditions. How phosphorylation modulates human CaV1.3 VGCC, however, is largely unexplored. We characterized modulation of CaV1.3 gating via S1475, the human equivalent of a phosphorylation site identified in the rat. S1475 is highly conserved in CaV1.3 but absent from all other high-voltage activating calcium channel types co-expressed with CaV1.3 in similar tissues. Further, it is located in the C-terminal EF-hand motif, which binds calmodulin (CaM). This is involved in calcium-dependent channel inactivation (CDI). We used amino acid exchanges that mimic either sustained phosphorylation (S1475D) or phosphorylation resistance (S1475A). Whole-cell and single-channel recordings of phosphorylation state imitating CaV1.3 variants in transiently transfected HEK-293 cells revealed functional relevance of S1475 in human CaV1.3. We obtained three main findings: (1) CaV1.3_S1475D, imitating sustained phosphorylation, displayed decreased current density, reduced CDI and (in-) activation kinetics shifted to more depolarized voltages compared with both wildtype CaV1.3 and the phosphorylation-resistant CaV1.3_S1475A variant. Corresponding to the decreased current density, we find a reduced open probability of CaV1.3_S1475D at the single-channel level. (2) Using CaM overexpression or depletion, we find that CaM is necessary for modulating CaV1.3 through S1475. (3) CaMKII activation led to CaV1.3_WT-current properties similar to those of CaV1.3_S1475D, but did not affect CaV1.3_S1475A, confirming that CaMKII modulates human CaV1.3 via S1475. Given the physiological and pathophysiological importance of CaV1.3, our findings on the S1475-mediated interplay of phosphorylation, CaM interaction and CDI provide hints for approaches on specific CaV1.3 modulation under physiological and pathophysiological conditions. KEY POINTS: Phosphorylation modulates activity of voltage-gated L-type calcium channels for specific cellular needs but is largely unexplored for human CaV1.3 channels. Here we report that S1475, a CaMKII phosphorylation site identified in rats, is functionally relevant in human CaV1.3. Imitating phosphorylation states at S1475 alters current density and inactivation in a calmodulin-dependent manner. In wildtype CaV1.3 but not in the phosphorylation-resistant variant S1475A, CaMKII activation elicits effects similar to constitutively mimicking phosphorylation at S1475. Our findings provide novel insights on the interplay of modulatory mechanisms of human CaV1.3 channels, and present a possible target for CaV1.3-specific gating modulation in physiological and pathophysiological conditions.

Selective posttranslational inhibition of CaVβ1-associated voltage-dependent calcium channels with a functionalized nanobody

Ca²⁺ influx through high-voltage-activated calcium channels (HVACCs) controls diverse cellular functions. A critical feature enabling a singular signal, Ca²⁺ influx, to mediate disparate functions is diversity of HVACC pore-forming α₁ and auxiliary Ca_Vβ₁-Ca_Vβ₄ subunits. Selective Ca_Vα₁ blockers have enabled deciphering their unique physiological roles. By contrast, the capacity to post-translationally inhibit HVACCs based on Ca_Vβ isoform is non-existent. Conventional gene knockout/shRNA approaches do not adequately address this deficit owing to subunit reshuffling and partially overlapping functions of Ca_Vβ isoforms. Here, we identify a nanobody (nb.E8) that selectively binds Ca_Vβ₁ SH3 domain and inhibits Ca_Vβ₁-associated HVACCs by reducing channel surface density, decreasing open probability, and speeding inactivation. Functionalizing nb.E8 with Nedd4L HECT domain yielded Chisel-1 which eliminated current through Ca_Vβ₁-reconstituted Ca_V1/Ca_V2 and native Ca_V1.1 channels in skeletal muscle, strongly suppressed depolarization-evoked Ca²⁺ influx and excitation-transcription coupling in hippocampal neurons, but was inert against Ca_Vβ₂-associated Ca_V1.2 in cardiomyocytes. The results introduce an original method for probing distinctive functions of ion channel auxiliary subunit isoforms, reveal additional dimensions of Ca_Vβ₁ signaling in neurons, and describe a genetically-encoded HVACC inhibitor with unique properties.

Inhibitory effects on L- and N-type calcium channels by a novel CaVβ1 variant identified in a patient with autism spectrum disorder

Voltage-gated calcium channel (VGCC) subunits have been genetically associated with autism spectrum disorders (ASD). The properties of the pore-forming VGCC subunit are modulated by auxiliary β-subunits, which exist in four isoforms (Ca_Vβ_1-4). Our previous findings suggested that activation of L-type VGCCs is a common feature of Ca_Vβ₂ subunit mutations found in ASD patients. In the current study, we functionally characterized a novel Ca_Vβ_1b variant (p.R296C) identified in an ASD patient. We used whole-cell and single-channel patch clamp to study the effect of Ca_Vβ_{1b_R296C} on the function of L- and N-type VGCCs. Furthermore, we used co-immunoprecipitation followed by Western blot to evaluate the interaction of the Ca_Vβ_1b-subunits with the RGK-protein Gem. Our data obtained at both, whole-cell and single-channel levels, show that compared to a wild-type Ca_Vβ_1b, the Ca_Vβ_{1b_R296C} variant inhibits L- and N-type VGCCs. Interaction with and modulation by the RGK-protein Gem seems to be intact. Our findings indicate functional effects of the Ca_Vβ_{1b_R296C} variant differing from that attributed to Ca_Vβ₂ variants found in ASD patients. Further studies have to detail the effects on different VGCC subtypes and on VGCC expression.

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

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

AHNAK: The quiet giant in calcium homeostasis

The phosphoprotein AHNAK is a large, ubiquitously expressed scaffolding protein involved in mediating a host of protein-protein interactions. This enables AHNAK to participate in various multi-protein complexes thereby orchestrating a range of diverse biological processes, including tumour suppression, immune regulation and cell architecture maintenance. A less studied but nonetheless equally important function occurs in calcium homeostasis. It does so by largely interacting with the L-type voltage-gated calcium channel (LVGCC) present in the plasma membrane of excitable cells such as muscles and neurons. Several studies have characterized the underlying basis of AHNAK's functional role in calcium channel modulation, which has led to a greater understanding of this cellular process and its associated pathologies. In this article we review and examine recent advances relating to the physiological aspects of AHNAK in calcium regulation. Specifically, we will provide a broad overview of AHNAK including its structural makeup and its interaction with several isoforms of LVGCC, and how these molecular interactions regulate calcium modulation across various tissues and their implication in muscle and neuronal function.

α-Actinin-1 promotes activity of the L-type Ca2+ channel Cav 1.2

The L-type Ca²⁺ channel Ca_V 1.2 governs gene expression, cardiac contraction, and neuronal activity. Binding of α-actinin to the IQ motif of Ca_V 1.2 supports its surface localization and postsynaptic targeting in neurons. We report a bi-functional mechanism that restricts Ca_V 1.2 activity to its target sites. We solved separate NMR structures of the IQ motif (residues 1,646-1,664) bound to α-actinin-1 and to apo-calmodulin (apoCaM). The Ca_V 1.2 K1647A and Y1649A mutations, which impair α-actinin-1 but not apoCaM binding, but not the F1658A and K1662E mutations, which impair apoCaM but not α-actinin-1 binding, decreased single-channel open probability, gating charge movement, and its coupling to channel opening. Thus, α-actinin recruits Ca_V 1.2 to defined surface regions and simultaneously boosts its open probability so that Ca_V 1.2 is mostly active when appropriately localized.

Protein partners of the calcium channel β subunit highlight new cellular functions

Calcium plays a key role in cell signalling by its intervention in a wide range of physiological processes. Its entry into cells occurs mainly via voltage-gated calcium channels (VGCC), which are found not only in the plasma membrane of excitable cells but also in cells insensitive to electrical signals. VGCC are composed of different subunits, α1, β, α2δ and γ, among which the cytosolic β subunit (Cavβ) controls the trafficking of the channel to the plasma membrane, its regulation and its gating properties. For many years, these were the main functions associated with Cavβ. However, a growing number of proteins have been found to interact with Cavβ, emphasizing the multifunctional role of this versatile protein. Interestingly, some of the newly assigned functions of Cavβ are independent of its role in the regulation of VGCC, and thus further increase its functional roles. Based on the identity of Cavβ protein partners, this review emphasizes the diverse cellular functions of Cavβ and summarizes both past findings as well as recent progress in the understanding of VGCC.

Cavβ2 transcription start site variants modulate calcium handling in newborn rat cardiomyocytes

In the heart, the main pathway for calcium influx is mediated by L-type calcium channels, a multi-subunit complex composed of the pore-forming subunit CaV1.2 and the auxiliary subunits CaVα2δ1 and CaVβ2. To date, five distinct CaVβ2 transcriptional start site (TSS) variants (CaVβ2a-e) varying only in the composition and length of the N-terminal domain have been described, each of them granting distinct biophysical properties to the L-type current. However, the physiological role of these variants in Ca(2+) handling in the native tissue has not been explored. Our results show that four of these variants are present in neonatal rat cardiomyocytes. The contribution of those CaVβ2 TSS variants on endogenous L-type current and Ca(2+) handling was explored by adenoviral-mediated overexpression of each CaVβ2 variant in cultured newborn rat cardiomyocytes. As expected, all CaVβ2 TSS variants increased L-type current density and produced distinctive changes on L-type calcium channel (LTCC) current activation and inactivation kinetics. The characteristics of the induced calcium transients were dependent on the TSS variant overexpressed. Moreover, the amplitude of the calcium transients varied depending on the subunit involved, being higher in cardiomyocytes transduced with CaVβ2a and smaller in CaVβ2d. Interestingly, the contribution of Ca(2+) influx and Ca(2+) release on total calcium transients, as well as the sarcoplasmic calcium content, was found to be TSS-variant-dependent. Remarkably, determination of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) messenger RNA (mRNA) abundance and cell size change indicates that CaVβ2 TSS variants modulate the cardiomyocyte hypertrophic state. In summary, we demonstrate that expression of individual CaVβ2 TSS variants regulates calcium handling in cardiomyocytes and, consequently, has significant repercussion in the development of hypertrophy.

Rare mutations of CACNB2 found in autism spectrum disease-affected families alter calcium channel function

Autism Spectrum Disorders (ASD) are complex neurodevelopmental diseases clinically defined by dysfunction of social interaction. Dysregulation of cellular calcium homeostasis might be involved in ASD pathogenesis, and genes coding for the L-type calcium channel subunits Ca_V1.2 (CACNA1C) and Ca_Vβ2 (CACNB2) were recently identified as risk loci for psychiatric diseases. Here, we present three rare missense mutations of CACNB2 (G167S, S197F, and F240L) found in ASD-affected families, two of them described here for the first time (G167S and F240L). All these mutations affect highly conserved regions while being absent in a sample of ethnically matched controls. We suggest the mutations to be of physiological relevance since they modulate whole-cell Ba²⁺ currents through calcium channels when expressed in a recombinant system (HEK-293 cells). Two mutations displayed significantly decelerated time-dependent inactivation as well as increased sensitivity of voltage-dependent inactivation. In contrast, the third mutation (F240L) showed significantly accelerated time-dependent inactivation. By altering the kinetic parameters, the mutations are reminiscent of the CACNA1C mutation causing Timothy Syndrome, a Mendelian disease presenting with ASD. In conclusion, the results of our first-time biophysical characterization of these three rare CACNB2 missense mutations identified in ASD patients support the hypothesis that calcium channel dysfunction may contribute to autism.

Gene splicing of an invertebrate beta subunit (LCavβ) in the N-terminal and HOOK domains and its regulation of LCav1 and LCav2 calcium channels

The accessory beta subunit (Ca(v)β) of calcium channels first appear in the same genome as Ca(v)1 L-type calcium channels in single-celled coanoflagellates. The complexity of this relationship expanded in vertebrates to include four different possible Ca(v)β subunits (β1, β2, β3, β4) which associate with four Ca(v)1 channel isoforms (Ca(v)1.1 to Ca(v)1.4) and three Ca(v)2 channel isoforms (Ca(v)2.1 to Ca(v)2.3). Here we assess the fundamentally-shared features of the Ca(v)β subunit in an invertebrate model (pond snail Lymnaea stagnalis) that bears only three homologous genes: (LCa(v)1, LCa(v)2, and LCa(v)β). Invertebrate Ca(v)β subunits (in flatworms, snails, squid and honeybees) slow the inactivation kinetics of Ca(v)2 channels, and they do so with variable N-termini and lacking the canonical palmitoylation residues of the vertebrate β2a subunit. Alternative splicing of exon 7 of the HOOK domain is a primary determinant of a slow inactivation kinetics imparted by the invertebrate LCa(v)β subunit. LCa(v)β will also slow the inactivation kinetics of LCa(v)3 T-type channels, but this is likely not physiologically relevant in vivo. Variable N-termini have little influence on the voltage-dependent inactivation kinetics of differing invertebrate Ca(v)β subunits, but the expression pattern of N-terminal splice isoforms appears to be highly tissue specific. Molluscan LCa(v)β subunits have an N-terminal "A" isoform (coded by exons: 1a and 1b) that structurally resembles the muscle specific variant of vertebrate β1a subunit, and has a broad mRNA expression profile in brain, heart, muscle and glands. A more variable "B" N-terminus (exon 2) in the exon position of mammalian β3 and has a more brain-centric mRNA expression pattern. Lastly, we suggest that the facilitation of closed-state inactivation (e.g. observed in Ca(v)2.2 and Ca(v)β3 subunit combinations) is a specialization in vertebrates, because neither snail subunit (LCa(v)2 nor LCa(v)β) appears to be compatible with this observed property.

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

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

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

Molecular Determinants of Cav1.2 Calcium Channel Inactivation

Voltage-gated L-type Ca_v1.2 calcium channels couple membrane depolarization to transient increase in cytoplasmic free Ca²⁺ concentration that initiates a number of essential cellular functions including cardiac and vascular muscle contraction, gene expression, neuronal plasticity, and exocytosis. Inactivation or spontaneous termination of the calcium current through Ca_v1.2 is a critical step in regulation of these processes. The pathophysiological significance of this process is manifested in hypertension, heart failure, arrhythmia, and a number of other diseases where acceleration of the calcium current decay should present a benefit function. The central issue of this paper is the inactivation of the Ca_v1.2 calcium channel mediated by multiple determinants.

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.

Single-channel monitoring of reversible L-type Ca(2+) channel Ca(V)α(1)-Ca(V)β subunit interaction

Voltage-dependent Ca(2+) channels are heteromultimers of Ca(V)α(1) (pore), Ca(V)β- and Ca(V)α(2)δ-subunits. The stoichiometry of this complex, and whether it is dynamically regulated in intact cells, remains controversial. Fortunately, Ca(V)β-isoforms affect gating differentially, and we chose two extremes (Ca(V)β(1a) and Ca(V)β(2b)) regarding single-channel open probability to address this question. HEK293α(1C) cells expressing the Ca(V)1.2 subunit were transiently transfected with Ca(V)α(2)δ1 alone or with Ca(V)β(1a), Ca(V)β(2b), or (2:1 or 1:1 plasmid ratio) combinations. Both Ca(V)β-subunits increased whole-cell current and shifted the voltage dependence of activation and inactivation to hyperpolarization. Time-dependent inactivation was accelerated by Ca(V)β(1a)-subunits but not by Ca(V)β(2b)-subunits. Mixtures induced intermediate phenotypes. Single channels sometimes switched between periods of low and high open probability. To validate such slow gating behavior, data were segmented in clusters of statistically similar open probability. With Ca(V)β(1a)-subunits alone, channels mostly stayed in clusters (or regimes of alike clusters) of low open probability. Increasing Ca(V)β(2b)-subunits (co-)expressed (1:2, 1:1 ratio or alone) progressively enhanced the frequency and total duration of high open probability clusters and regimes. Our analysis was validated by the inactivation behavior of segmented ensemble averages. Hence, a phenotype consistent with mutually exclusive and dynamically competing binding of different Ca(V)β-subunits is demonstrated in intact cells.

Posttranscriptional regulation of the β2-subunit of cardiac L-type Ca2+ channels by MicroRNAs during long-term exposure to isoproterenol in rats

INTRODUCTION AND METHODS

The effects of long-term β-adrenergic administration on the expression levels of the cardiac L-type Ca channel β2 subunit, which regulates channel trafficking and function, were characterized in adult rats.

RESULTS

Systemic administration of isoproterenol (150 mg·kg·h) for 2 d led to a 50% increase in the ventricular wet weight-to-body weight ratio (mg/g) and of more than two-fold in the expression of actin protein. In contrast, β2 subunit protein levels decreased (down to 49%), while mRNA levels remained unchanged. Furthermore, levels of microRNAs (miRs), including miR-21 and miR-132, were upregulated (7.2 and 7.9 fold, respectively). Transfection of these miRs into HEK293 cells attenuated expression of a luciferase reporter gene controlled by a conserved 3'-untranslated region (UTR) of the β2 subunit (down to 67% and 56%, respectively). Systemic administration of isoproterenol also led to briefer intracellular Ca transients during action potentials measured in isolated cardiomyocytes (down to 65%).

CONCLUSION

These results suggest that cardiac L-type Ca channel β2 subunit protein expression may be downregulated by miRs in response to long-term activation of β-adrenergic signaling, possibly as an adaptive response in cardiac hypertrophy and sustained β-adrenergic states.

Homodimerization of the Src homology 3 domain of the calcium channel β-subunit drives dynamin-dependent endocytosis

Voltage-dependent calcium channels constitute the main entry pathway for calcium into excitable cells. They are heteromultimers formed by an α(1) pore-forming subunit (Ca(V)α(1)) and accessory subunits. To achieve a precise coordination of calcium signals, the expression and activity of these channels is tightly controlled. The accessory β-subunit (Ca(V)β), a membrane associated guanylate kinase containing one guanylate kinase (β-GK) and one Src homology 3 (β-SH3) domain, has antagonistic effects on calcium currents by regulating different aspects of channel function. Although β-GK binds to a conserved site within the α(1)-pore-forming subunit and facilitates channel opening, β-SH3 binds to dynamin and promotes endocytosis. Here, we investigated the molecular switch underlying the functional duality of this modular protein. We show that β-SH3 homodimerizes through a single disulfide bond. Substitution of the only cysteine residue abolishes dimerization and impairs internalization of L-type Ca(V)1.2 channels expressed in Xenopus oocytes while preserving dynamin binding. Covalent linkage of the β-SH3 dimerization-deficient mutant yields a concatamer that binds to dynamin and restores endocytosis. Moreover, using FRET analysis, we show in living cells that Ca(V)β form oligomers and that this interaction is reduced by Ca(V)α(1). Association of Ca(V)β with a polypeptide encoding the binding motif in Ca(V)α(1) inhibited endocytosis. Together, these findings reveal that β-SH3 dimerization is crucial for endocytosis and suggest that channel activation and internalization are two mutually exclusive functions of Ca(V)β. We propose that a change in the oligomeric state of Ca(V)β is the functional switch between channel activator and channel internalizer.

CaBP1 regulates voltage-dependent inactivation and activation of Ca(V)1.2 (L-type) calcium channels

CaBP1 is a Ca(2+)-binding protein that regulates the gating of voltage-gated (Ca(V)) Ca(2+) channels. In the Ca(V)1.2 channel α(1)-subunit (α(1C)), CaBP1 interacts with cytosolic N- and C-terminal domains and blunts Ca(2+)-dependent inactivation. To clarify the role of the α(1C) N-terminal domain in CaBP1 regulation, we compared the effects of CaBP1 on two alternatively spliced variants of α(1C) containing a long or short N-terminal domain. In both isoforms, CaBP1 inhibited Ca(2+)-dependent inactivation but also caused a depolarizing shift in voltage-dependent activation and enhanced voltage-dependent inactivation (VDI). In binding assays, CaBP1 interacted with the distal third of the N-terminal domain in a Ca(2+)-independent manner. This segment is distinct from the previously identified calmodulin-binding site in the N terminus. However, deletion of a segment in the proximal N-terminal domain of both α(1C) isoforms, which spared the CaBP1-binding site, inhibited the effect of CaBP1 on VDI. This result suggests a modular organization of the α(1C) N-terminal domain, with separate determinants for CaBP1 binding and transduction of the effect on VDI. Our findings expand the diversity and mechanisms of Ca(V) channel regulation by CaBP1 and define a novel modulatory function for the initial segment of the N terminus of α(1C).

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.

The N terminus of a schistosome beta subunit regulates inactivation and current density of a Cav2 channel

The β subunit of high voltage-activated Ca(2+) (Ca(v)) channels targets the pore-forming α(1) subunit to the plasma membrane and tunes the biophysical phenotype of the Ca(v) channel complex. We used a combination of molecular biology and whole-cell patch clamp to investigate the functional role of a long N-terminal polyacidic motif (NPAM) in a Ca(v)β subunit of the human parasite Schistosoma mansoni (β(Sm)), a motif that does not occur in other known Ca(v)β subunits. When expressed in human embryonic kidney cells stably expressing Ca(v)2.3, β(Sm) accelerates Ca(2+)/calmodulin-independent inactivation of Ca(v)2.3. Deleting the first 44 amino acids of β(Sm), a region that includes NPAM, significantly slows the predominant time constant of inactivation (τ(fast)) under conditions that prevent Ca(2+)/CaM-dependent inactivation (β(Sm): τ(fast) = 66 ms; β(SmΔ2-44): τ(fast) = 111 ms, p < 0.01). Interestingly, deleting the amino acids that are N-terminal to NPAM (2-24 or 2-17) results in faster inactivation than with an intact N terminus (τ(fast) = 42 ms with β(SmΔ2-17); τ(fast) = 40 ms with β(SmΔ2-24), p < 0.01). This suggests that NPAM is the structural determinant for accelerating Ca(2+)/calmodulin-independent inactivation. We also created three chimeric subunits that contain the first 44 amino acids of β(Sm) attached to mammalian β(1b), β(2a), and β(3) subunits. For any given mammalian β subunit, inactivation was faster if it contained the N terminus of β(Sm) than if it did not. Co-expression of the mammalian α(2)δ-1 subunit resulted in doubling of the inactivation rate, but the effects of NPAM persisted. Thus, it appears that the schistosome Ca(v) channel complex has acquired a new function that likely contributes to reducing the amount of Ca(2+) that enters the cells in vivo. This feature is of potential interest as a target for new antihelminthics.

Oligomerization of Cavbeta subunits is an essential correlate of Ca2+ channel activity

Voltage-gated calcium channels conduct Ca(2+) ions in response to membrane depolarization. The resulting transient increase in cytoplasmic free calcium concentration is a critical trigger for the initiation of such vital responses as muscle contraction and transcription. L-type Ca(v)1.2 calcium channels are complexes of the pore-forming α(1C) subunit associated with cytosolic Ca(v)β subunits. All major Ca(v)βs share a highly homologous membrane associated guanylate kinase-like (MAGUK) domain that binds to α(1C) at the α-interaction domain (AID), a short motif in the linker between transmembrane repeats I and II. In this study we show that Ca(v)β subunits form multimolecular homo- and heterooligomeric complexes in human vascular smooth muscle cells expressing native calcium channels and in Cos7 cells expressing recombinant Ca(v)1.2 channel subunits. Ca(v)βs oligomerize at the α(1C) subunits residing in the plasma membrane and bind to the AID. However, Ca(v)β oligomerization occurs independently on the association with α(1C). Molecular structures responsible for Ca(v)β oligomerization reside in 3 regions of the guanylate kinase subdomain of MAGUK. An augmentation of Ca(v)β homooligomerization significantly increases the calcium current density, while heterooligomerization may also change the voltage-dependence and inactivation kinetics of the channel. Thus, oligomerization of Ca(v)β subunits represents a novel and essential aspect of calcium channel regulation.

Anion-sensitive regions of L-type CaV1.2 calcium channels expressed in HEK293 cells

L-type calcium currents (I_Ca) are influenced by changes in extracellular chloride, but sites of anion effects have not been identified. Our experiments showed that CaV1.2 currents expressed in HEK293 cells are strongly inhibited by replacing extracellular chloride with gluconate or perchlorate. Variance-mean analysis of I_Ca and cell-attached patch single channel recordings indicate that gluconate-induced inhibition is due to intracellular anion effects on Ca²⁺ channel open probability, not conductance. Inhibition of CaV1.2 currents produced by replacing chloride with gluconate was reduced from ∼75%–80% to ∼50% by omitting β subunits but unaffected by omitting α₂δ subunits. Similarly, gluconate inhibition was reduced to ∼50% by deleting an α1 subunit N-terminal region of 15 residues critical for β subunit interactions regulating open probability. Omitting β subunits with this mutant α1 subunit did not further diminish inhibition. Gluconate inhibition was unchanged with expression of different β subunits. Truncating the C terminus at AA1665 reduced gluconate inhibition from ∼75%–80% to ∼50% whereas truncating it at AA1700 had no effect. Neutralizing arginines at AA1696 and 1697 by replacement with glutamines reduced gluconate inhibition to ∼60% indicating these residues are particularly important for anion effects. Expressing CaV1.2 channels that lacked both N and C termini reduced gluconate inhibition to ∼25% consistent with additive interactions between the two tail regions. Our results suggest that modest changes in intracellular anion concentration can produce significant effects on CaV1.2 currents mediated by changes in channel open probability involving β subunit interactions with the N terminus and a short C terminal region.

Ca(2+)-dependent components of inactivation of unitary cardiac L-type Ca(2+) channels

A Ca(2+) ion-dependent inactivation (CDI) of L-type Ca(2+) channels (LCC) is vital in limiting and shaping local Ca(2+) ion signalling in a variety of excitable cell types. However, under physiological conditions the unitary LCC properties that underlie macroscopic inactivation are unclear. Towards this end, we have probed the gating kinetics of individual cardiac LCCs recorded with a physiological Ca(2+) ion concentration (2 mM) permeating the channel, and in the absence of channel agonists. Upon depolarization the ensemble-averaged LCC current decayed with a fast and a slow exponential component. We analysed the unitary behaviour responsible for this biphasic decay by means of a novel kinetic dissection of LCC gating parameters. We found that inactivation was caused by a rapid decrease in the frequency of LCC reopening, and a slower decline in mean open time of the LCC. In contrast, with barium ions permeating the channel ensemble-averaged currents displayed only a single, slow exponential decay and little time dependence of the LCC open time. Our results demonstrate that the fast and slow phases of macroscopic inactivation reflect the distinct time courses for the decline in the frequency of LCC reopening and the open dwell time, both of which are modulated by Ca(2+) influx. Analysis of the evolution of CDI in individual LCC episodes was employed to examine the stochastic nature of the underlying molecular switch, and revealed that influx on the order of a thousand Ca(2+) ions may be sufficient to trigger CDI. This is the first study to characterize both the unitary kinetics and the stoichiometry of CDI of LCCs with a physiological Ca(2+) concentration. These novel findings may provide a basis for understanding the mechanisms regulating unitary LCC gating, which is a pivotal element in the local control of Ca(2+)-dependent signalling processes.

Structural and biophysical determinants of single Ca(V)3.1 and Ca(V)3.2 T-type calcium channel inhibition by N(2)O

We investigated the biophysical mechanism of inhibition of recombinant T-type calcium channels Ca(V)3.1 and Ca(V)3.2 by nitrous oxide (N(2)O). To identify functionally important channel structures, chimeras with reciprocal exchange of the N-terminal domains I and II and C-terminal domains III and IV were examined. In whole-cell recordings N(2)O significantly inhibited Ca(V)3.2, and - less pronounced - Ca(V)3.1. A Ca(V)3.2-prevalent inhibition of peak currents was also detected in cell-attached multi-channel patches. In cell-attached patches containing < or = 3 channels N(2)O reduced average peak current of Ca(V)3.2 by decreasing open probability and open time duration. Effects on Ca(V)3.1 were smaller and mediated by a reduced fraction of sweeps containing channel activity. Without drug, single Ca(V)3.1 channels were significantly less active than Ca(V)3.2. Chimeras revealed that domains III and IV control basal gating properties. Domains I and II, in particular a histidine residue within Ca(V)3.2 (H191), are responsible for the subtype-prevalent N(2)O inhibition. Our study demonstrates the biophysical (open times, open probability) and structural (domains I and II) basis of action of N(2)O on Ca(V)3.2. Such a fingerprint of single channels can help identifying the molecular nature of native channels. This is exemplified by a characterization of single channels expressed in human hMTC cells as functional homologues of recombinant Ca(V)3.1.

Diversity and developmental expression of L-type calcium channel beta2 proteins and their influence on calcium current in murine heart

By now, little is known on L-type calcium channel (LTCC) subunits expressed in mouse heart. We show that CaVbeta2 proteins are the major CaVbeta components of the LTCC in embryonic and adult mouse heart, but that in embryonic heart CaVbeta3 proteins are also detectable. At least two CaVbeta2 variants of approximately 68 and approximately 72 kDa are expressed. To identify the underlying CaVbeta2 variants, cDNA libraries were constructed from poly(A)(+) RNA isolated from hearts of 7-day-old and adult mice. Screening identified 60 independent CaVbeta2 cDNA clones coding for four types of CaVbeta2 proteins only differing in their 5' sequences. CaVbeta2-N1, -N4, and -N5 but not -N3 were identified in isolated cardiomyocytes by RT-PCR and were sufficient to reconstitute the CaVbeta2 protein pattern in vitro. Significant L-type Ca(2+) currents (I(Ca)) were recorded in HEK293 cells after co-expression of CaV1.2 and CaVbeta2. Current kinetics were determined by the type of CaVbeta2 protein, with the approximately 72-kDa CaVbeta2a-N1 shifting the activation of I(Ca) significantly to depolarizing potentials compared with the other CaVbeta2 variants. Inactivation of I(Ca) was accelerated by CaVbeta2a-N1 and -N4, which also lead to slower activation compared with CaVbeta2a-N3 and -N5. In summary, this study reveals the molecular LTCC composition in mouse heart and indicates that expression of various CaVbeta2 proteins may be used to adapt the properties of LTCCs to changing myocardial requirements during development and that CaVbeta2a-N1-induced changes of I(Ca) kinetics might be essential in embryonic heart.

Effect of Ca(v)beta subunits on structural organization of Ca(v)1.2 calcium channels

BACKGROUND

Voltage-gated Ca(v)1.2 calcium channels play a crucial role in Ca(2+) signaling. The pore-forming alpha(1C) subunit is regulated by accessory Ca(v)beta subunits, cytoplasmic proteins of various size encoded by four different genes (Ca(v)beta(1)-beta(4)) and expressed in a tissue-specific manner.

METHODS AND RESULTS

Here we investigated the effect of three major Ca(v)beta types, beta(1b), beta(2d) and beta(3), on the structure of Ca(v)1.2 in the plasma membrane of live cells. Total internal reflection fluorescence microscopy showed that the tendency of Ca(v)1.2 to form clusters depends on the type of the Ca(v)beta subunit present. The highest density of Ca(v)1.2 clusters in the plasma membrane and the smallest cluster size were observed with neuronal/cardiac beta(1b) present. Ca(v)1.2 channels containing beta(3), the predominant Ca(v)beta subunit of vascular smooth muscle cells, were organized in a significantly smaller number of larger clusters. The inter- and intramolecular distances between alpha(1C) and Ca(v)beta in the plasma membrane of live cells were measured by three-color FRET microscopy. The results confirm that the proximity of Ca(v)1.2 channels in the plasma membrane depends on the Ca(v)beta type. The presence of different Ca(v)beta subunits does not result in significant differences in the intramolecular distance between the termini of alpha(1C), but significantly affects the distance between the termini of neighbor alpha(1C) subunits, which varies from 67 A with beta(1b) to 79 A with beta(3).

CONCLUSIONS

Thus, our results show that the structural organization of Ca(v)1.2 channels in the plasma membrane depends on the type of Ca(v)beta subunits present.

The calcium channel beta2 (CACNB2) subunit repertoire in teleosts

BACKGROUND

Cardiomyocyte contraction is initiated by influx of extracellular calcium through voltage-gated calcium channels. These oligomeric channels utilize auxiliary beta subunits to chaperone the pore-forming alpha subunit to the plasma membrane, and to modulate channel electrophysiology 1. Several beta subunit family members are detected by RT-PCR in the embryonic heart. Null mutations in mouse beta2, but not in the other three beta family members, are embryonic lethal at E10.5 due to defects in cardiac contractility 2. However, a drawback of the mouse model is that embryonic heart rhythm is difficult to study in live embryos due to their intra-uterine development. Moreover, phenotypes may be obscured by secondary effects of hypoxia. As a first step towards developing a model for contributions of beta subunits to the onset of embryonic heart rhythm, we characterized the structure and expression of beta2 subunits in zebrafish and other teleosts.

RESULTS

Cloning of two zebrafish beta2 subunit genes (beta2.1 and beta2.2) indicated they are membrane-associated guanylate kinase (MAGUK)-family genes. Zebrafish beta2 genes show high conservation with mammals within the SH3 and guanylate kinase domains that comprise the "core" of MAGUK proteins, but beta2.2 is much more divergent in sequence than beta2.1. Alternative splicing occurs at the N-terminus and within the internal HOOK domain. In both beta2 genes, alternative short ATG-containing first exons are separated by some of the largest introns in the genome, suggesting that individual transcript variants could be subject to independent cis-regulatory control. In the Tetraodon nigrovidis and Fugu rubripes genomes, we identified single beta2 subunit gene loci. Comparative analysis of the teleost and human beta2 loci indicates that the short 5' exon sequences are highly conserved. A subset of 5' exons appear to be unique to teleost genomes, while others are shared with mammals. Alternative splicing is temporally and spatially regulated in embryo and adult. Moreover, a different subset of spliced beta2 transcript variants is detected in the embryonic heart compared to the adult.

CONCLUSION

These studies refine our understanding of beta2 subunit diversity arising from alternative splicing, and provide the groundwork for functional analysis of beta2 subunit diversity in the embryonic heart.

Oestrogen directly inhibits the cardiovascular L-type Ca2+ channel Cav1.2

Oestrogen can modify the contractile function of vascular smooth muscle and cardiomyocytes. The negative inotropic actions of oestrogen on the heart and coronary vasculature appear to be mediated by L-type Ca(2+) channel (Ca(v)1.2) inhibition, but the underlying mechanisms remain elusive. We tested the hypothesis that oestrogen directly inhibits the cardiovascular L-type Ca(2+) current, I(CaL). The effect of oestrogen on I(CaL) was measured in Ca(v)1.2-transfected HEK-293 cells using the whole-cell patch-clamp technique. The current revealed typical activation and inactivation profiles of nifedipine- and cadmium-sensitive I(CaL). Oestrogen (50 microM) rapidly reduced I(CaL) by 50% and shifted voltage-dependent activation and availability to more negative potentials. Furthermore, oestrogen blocked the Ca(2+) channel in a rate-dependent way, exhibiting higher efficiency of block at higher stimulation frequencies. Our data suggest that oestrogen inhibits I(CaL) through direct interaction of the steroid with the channel protein.