Alternative splicing in intracellular loop connecting domains II and III of the alpha 1 subunit of Cav1.2 Ca2+ channels predicts two-domain polypeptides with unique C-terminal tails

Hyperinsulinemic Hypoglycemia Associated with a CaV1.2 Variant with Mixed Gain- and Loss-of-Function Effects

The voltage-dependent L-type calcium channel isoform CaV1.2 is critically involved in many physiological processes, e.g., in cardiac action potential formation, electromechanical coupling and regulation of insulin secretion by beta cells. Gain-of-function mutations in the calcium voltage-gated channel subunit alpha 1 C (CACNA1C) gene, encoding the CaV1.2 α1-subunit, cause Timothy syndrome (TS), a multisystemic disorder that includes autism spectrum disorders and long QT (LQT) syndrome. Strikingly, TS patients frequently suffer from hypoglycemia of yet unproven origin. Using next-generation sequencing, we identified a novel heterozygous CACNA1C mutation in a patient with congenital hyperinsulinism (CHI) and associated hypoglycemic episodes. We characterized the electrophysiological phenotype of the mutated channel using voltage-clamp recordings and in silico action potential modeling experiments. The identified CaV1.2L566P mutation causes a mixed electrophysiological phenotype of gain- and loss-of-function effects. In silico action potential modeling supports that this mixed electrophysiological phenotype leads to a tissue-specific impact on beta cells compared to cardiomyocytes. Thus, CACNA1C variants may be associated with non-syndromic hyperinsulinemic hypoglycemia without long-QT syndrome, explained by very specific electrophysiological properties of the mutated channel. We discuss different biochemical characteristics and clinical impacts of hypoglycemia in the context of CACNA1C variants and show that these may be associated with significant morbidity for Timothy Syndrome patients. Our findings underline that the potential of hypoglycemia warrants careful attention in patients with CACNA1C variants, and such variants should be included in the differential diagnosis of non-syndromic congenital hyperinsulinism.

The life cycle of voltage-gated Ca2+ channels in neurons: an update on the trafficking of neuronal calcium channels

Neuronal voltage-gated Ca²⁺ (Ca_V) channels play a critical role in cellular excitability, synaptic transmission, excitation-transcription coupling and activation of intracellular signaling pathways. Ca_V channels are multiprotein complexes and their functional expression in the plasma membrane involves finely tuned mechanisms, including forward trafficking from the endoplasmic reticulum (ER) to the plasma membrane, endocytosis and recycling. Whether genetic or acquired, alterations and defects in the trafficking of neuronal Ca_V channels can have severe physiological consequences. In this review, we address the current evidence concerning the regulatory mechanisms which underlie precise control of neuronal Ca_V channel trafficking and we discuss their potential as therapeutic targets.

A CaV2.1 N-terminal fragment relieves the dominant-negative inhibition by an Episodic ataxia 2 mutant

Episodic ataxia 2 (EA2) is an autosomal dominant disorder caused by mutations in the gene CACNA1A that encodes the pore-forming Ca_V2.1 calcium channel subunit. The majority of EA2 mutations reported so far are nonsense or deletion/insertion mutations predicted to form truncated proteins. Heterologous expression of wild-type Ca_V2.1, together with truncated constructs that mimic EA2 mutants, significantly suppressed wild-type calcium channel function, indicating that the truncated protein produces a dominant-negative effect (Jouvenceau et al., 2001; Page et al., 2004). A similar finding has been shown for Ca_V2.2 (Raghib et al., 2001). We show here that a highly conserved sequence in the cytoplasmic N-terminus is involved in this process, for both Ca_V2.1 and Ca_V2.2 channels. Additionally, we were able to interfere with the suppressive effect of an EA2 construct by mutating key N-terminal residues within it. We postulate that the N-terminus of the truncated channel plays an essential part in its interaction with the full-length Ca_V2.1, which prevents the correct folding of the wild-type channel. In agreement with this, we were able to disrupt the interaction between EA2 and the full length channel by co-expressing a free N-terminal peptide.

Alternative splicing generates a novel truncated Cav1.2 channel in neonatal rat heart

L-type Cav1.2 Ca(2+) channel undergoes extensive alternative splicing, generating functionally different channels. Alternatively spliced Cav1.2 Ca(2+) channels have been found to be expressed in a tissue-specific manner or under pathological conditions. To provide a more comprehensive understanding of alternative splicing in Cav1.2 channel, we systematically investigated the splicing patterns in the neonatal and adult rat hearts. The neonatal heart expresses a novel 104-bp exon 33L at the IVS3-4 linker that is generated by the use of an alternative acceptor site. Inclusion of exon 33L causes frameshift and C-terminal truncation. Whole-cell electrophysiological recordings of Cav1.233L channels expressed in HEK 293 cells did not detect any current. However, when co-expressed with wild type Cav1.2 channels, Cav1.233L channels reduced the current density and altered the electrophysiological properties of the wild type Cav1.2 channels. Interestingly, the truncated 3.5-domain Cav1.233L channels also yielded a dominant negative effect on Cav1.3 channels, but not on Cav3.2 channels, suggesting that Cavβ subunits is required for Cav1.233L regulation. A biochemical study provided evidence that Cav1.233L channels enhanced protein degradation of wild type channels via the ubiquitin-proteasome system. Although the physiological significance of the Cav1.233L channels in neonatal heart is still unknown, our report demonstrates the ability of this novel truncated channel to modulate the activity of the functional Cav1.2 channels. Moreover, the human Cav1.2 channel also contains exon 33L that is developmentally regulated in heart. Unexpectedly, human exon 33L has a one-nucleotide insertion that allowed in-frame translation of a full Cav1.2 channel. An electrophysiological study showed that human Cav1.233L channel is a functional channel but conducts Ca(2+) ions at a much lower level.

A naturally occurring truncated Cav1.2 α1-subunit inhibits Ca2+ current in A7r5 cells

Alternative splicing of the voltage-gated Ca(2+) (CaV) α1-subunit adds to the functional diversity of Ca(2+) channels. A variant with a 73-nt deletion in exon 15 of the Cav1.2 α1-subunit (Cav1.2Δ73) produced by alternative splicing that predicts a truncated protein has been described, but its function, if any, is unknown. We sought to determine if, by analogy to other truncated CaV α1-subunits, Cav1.2Δ73 acts as an inhibitor of wild-type Cav1.2 currents. HEK-293 cells were transfected with Cav1.2Δ73 in a pIRES vector with CD8 or in pcDNA3.1 with a V5/his COOH-terminal tag plus β2 and α2δ1 accessory subunits and pEGFP. Production of Cav1.2Δ73 protein was confirmed by Western blotting and immunofluorescence. Voltage-clamp studies revealed the absence of functional channels in transfected cells. In contrast, cells transfected with full-length Cav1.2 plus accessory subunits and pEGFP exhibited robust Ca(2+) currents. A7r5 cells exhibited endogenous Cav1.2-based currents that were greatly reduced (>80%) without a change in voltage-dependent activation when transfected with Cav1.2Δ73-IRES-CD8 compared with empty vector or pIRES-CD8 controls. Transfection of A7r5 cells with an analogous Cav2.3Δ73-IRES-CD8 had no effect on Ca(2+) currents. Immunofluorescence showed intracellular, but not plasma membrane, localization of Cav1.2Δ73-V5/his, as well as colocalization with an endoplasmic reticulum marker, ER Organelle Lights. Expression of Cav1.2Δ73 α1-subunits in A7r5 cells inhibits endogenous Cav1.2 currents. The fact that this variant arises naturally by alternative splicing raises the possibility that it may represent a physiological mechanism to modulate Cav1.2 functional activity.

Understanding alternative splicing of Cav1.2 calcium channels for a new approach towards individualized medicine

Calcium channel blockers (CCBs) are widely used to treat cardiovascular diseases such as hypertension, angina pectoris, hypertrophic cardiomyopathy, and supraventricular tachycardia. CCBs selectively inhibit the inward flow of calcium ions through voltage-gated calcium channels, particularly Ca_v1.2, that are expressed in the cardiovascular system. Changes to the molecular structure of Ca_v1.2 channels could affect sensitivity of the channels to blockade by CCBs. Recently, extensive alternative splicing was found in Ca_v1.2 channels that generated wide phenotypic variations. Cardiac and smooth muscles express slightly different, but functionally important Ca_v1.2 splice variants. Alternative splicing could also modulate the gating properties of the channels and giving rise to different responses to inhibition by CCBs. Importantly, alternative splicing of Ca_v1.2 channels may play an important role to influence the outcome of many cardiovascular disorders. Therefore, the understanding of how alternative splicing impacts Ca_v1.2 channels pharmacology in various diseases and different organs may provide the possibility for individualized therapy with minimal side effects.

The Brugada syndrome mutation A39V does not affect surface expression of neuronal rat Cav1.2 channels

Background

A loss of function of the L-type calcium channel, Cav1.2, results in a cardiac specific disease known as Brugada syndrome. Although many Brugada syndrome channelopathies reduce channel function, one point mutation in the N-terminus of Cav1.2 (A39V) has been shown to elicit disease a phenotype because of a loss of surface trafficking of the channel. This lack of cell membrane expression could not be rescued by the trafficking chaperone Cavβ.

Findings

We report that despite the striking loss of trafficking described previously in the cardiac Cav1.2 channel, the A39V mutation while in the background of the brain isoform traffics and functions normally. We detected no differences in biophysical properties between wild type Cav1.2 and A39V-Cav1.2 in the presence of either a cardiac (Cavβ2b), or a neuronal beta subunit (Cavβ1b). In addition, the A39V-Cav1.2 mutant showed a normal Cavβ2b mediated increase in surface expression in tsA-201 cells.

Conclusions

The Brugada syndrome mutation A39V when introduced into rat brain Cav1.2 does not trigger the loss-of-trafficking phenotype seen in a previous study on the human heart isoform of the channel.

Voltage sensor of ion channels and enzymes

Placed in the cell membrane (a two-dimensional environment), ion channels and enzymes are able to sense voltage. How these proteins are able to detect the difference in the voltage across membranes has attracted much attention, and at times, heated debate during the last few years. Sodium, Ca2+ and K+ voltage-dependent channels have a conserved positively charged transmembrane (S4) segment that moves in response to changes in membrane voltage. In voltage-dependent channels, S4 forms part of a domain that crystallizes as a well-defined structure consisting of the first four transmembrane (S1-S4) segments of the channel-forming protein, which is defined as the voltage sensor domain (VSD). The VSD is tied to a pore domain and VSD movements are allosterically coupled to the pore opening to various degrees, depending on the type of channel. How many charges are moved during channel activation, how much they move, and which are the molecular determinants that mediate the electromechanical coupling between the VSD and the pore domains are some of the questions that we discuss here. The VSD can function, however, as a bona fide proton channel itself, and, furthermore, the VSD can also be a functional part of a voltage-dependent phosphatase.

Ion channels in key marine invertebrates; their diversity and potential for applications in biotechnology

Of the intra-membrane proteins, the class that comprises voltage and ligand-gated ion channels represents the major substrate whereby signals pass between and within cells in all organisms. It has been presumed that vertebrate and particularly mammalian ion channels represent the apex of evolutionary complexity and diversity and much effort has been focused on understanding their function. However, the recent availability of cheap high throughput genome sequencing has massively broadened and deepened the quality of information across phylogeny and is radically changing this view. Here we review current knowledge on such channels in key marine invertebrates where physiological evidence is backed up by molecular sequences and expression/functional studies. As marine invertebrates represent a much greater range of phyla than terrestrial vertebrates and invertebrates together, we argue that these animals represent a highly divergent, though relatively underused source of channel novelty. As ion channels are exquisitely selective sensors for voltage and ligands, their potential and actual applications in biotechnology are manifold.

Alternative splicing of Cav1.2 channel exons in smooth muscle cells of resistance-size arteries generates currents with unique electrophysiological properties

Voltage-dependent calcium (Ca(2+), Ca(V)1.2) channels are the primary Ca(2+) entry pathway in smooth muscle cells of resistance-size (myogenic) arteries, but their molecular identity remains unclear. Here we identified and quantified Ca(V)1.2 alpha(1)-subunit splice variation in myocytes of rat resistance-size (100-200 microm diameter) cerebral arteries. Full-length clones containing either exon 1b or the recently identified exon 1c exhibited additional primary splice variation at exons 9*, 21/22, 31/32, and +/- 33. Real-time PCR confirmed the findings from full-length clones and indicated that the major Ca(V)1.2 variant contained exons 1c, 8, 21, and 32+33, with approximately 57% containing 9*. Exon 9* was more prevalent in clones containing 1c (72%) than in those containing 1b (33%), suggesting exon-selective combinatorial splicing. To examine the functional significance of this splicing profile, membrane currents produced by each of the four exon 1b/c/ +/- 9* variants were characterized following transfection in HEK293 cells. Exon 1c and 9* caused similar hyperpolarizing shifts in both current-voltage relationships and voltage-dependent activation of currents. Furthermore, exon 9* induced a hyperpolarizing shift only in the voltage-dependent activation of channels containing exon 1b, but not in those containing exon 1c. In contrast, exon 1b, 1c, or +9* did not alter voltage-dependent inactivation. In summary, we have identified the Ca(V)1.2 alpha(1)-subunit splice variant population that is expressed in myocytes of resistance-size arteries and the unique electrophysiological properties of recombinant channels formed by exon 1 and 9* variation. The predominance of exon 1c and 9* in smooth muscle cell Ca(V)1.2 channels causes a hyperpolarizing shift in the voltage sensitivity of currents toward the physiological arterial voltage range.

The L-type calcium channel alpha 1C subunit gene undergoes extensive, uncoordinated alternative splicing

The alpha1C subunit is the pore-forming protein for the L-type calcium channel. Previous studies indicate that there is possible tissue-specific alternative splicing of this gene. In this study we cloned the entire open reading frame of the alpha1C subunit cDNA from adult rat cardiac myocytes in a single piece (6.64 kb). Using 75 positive clones that were identified by restriction enzyme mapping, we tested the alternative splicing patterns of the Ca(v) 1.2 gene that encodes the alpha1C subunit protein and focused on five loci: IS6, post-IS6, IIIS2, IVS3, and the c-terminus. The results indicate that: (1) alternative splicing occurs in most of the loci, giving rise to two or three different isoforms at those sites; (2) there is a predominant form for each splicing site, (3) there does not appear to be consistent coordination of splicing at multiple loci of this gene. Alternative splicing is not tissue-specific in most regions.

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.

The impact of splice isoforms on voltage-gated calcium channel alpha1 subunits

Semi-conserved exon boundaries in members of the CACNA1 gene family result in recurring pre-mRNA splicing patterns. The resulting variations in the encoded pore-forming subunit of the voltage-gated calcium channel affect functionally significant regions, such as the vicinity of the voltage-sensing S4 segments or the intracellular loops that are important for protein interaction. In addition to generating functional diversity, RNA splicing regulates the quantitative expression of other splice isoforms of the same gene by producing transcripts with premature stop codons which encode two-domain or three-domain channels. An overview of some of the known splice isoforms of the alpha(1) calcium channel subunits and their significance is given.

Smooth muscle uses another promoter to express primarily a form of human Cav1.2 L-type calcium channel different from the principal heart form

Several different first exons and amino termini have been reported for the cardiac Ca channel known as alpha(1C) or Ca(V)1.2. The aim of this study was to investigate whether the expression of this channel is regulated by different promoters in smooth muscle cells and in heart in humans. Ribonuclease protection assay (RPA) indicates that the longer first exon 1a is found in certain human smooth muscle-containing tissues, notably bladder and fetal aorta, but that it is not expressed to any significant degree in lung or intestine. On the other hand, all four smooth muscle-containing tissues examined strongly express transcripts containing exon 1b, first reported cloned from human fibroblast cells. In addition, primary cultures of human colonic myocytes and coronary artery smooth muscle cells express predominantly transcripts containing exon 1b. The promoter immediately upstream of exon 1b was cloned, and it displays functional promoter activity when luciferase-expressing constructs were transfected into three different cultured smooth muscle cells: primary human coronary artery smooth muscles cells, primary human colonocytes, and the fetal rat aorta-derived A7r5 cell line. These results indicate that expression in smooth muscle is primarily driven by a promoter different from that which drives expression in cardiac myocytes.

Systematic identification of splice variants in human P/Q-type channel alpha1(2.1) subunits: implications for current density and Ca2+-dependent inactivation

P/Q-type (Ca(v)2.1) calcium channels support a host of Ca2+-driven neuronal functions in the mammalian brain. Alternative splicing of the main alpha1A (alpha1(2.1)) subunit of these channels may thereby represent a rich strategy for tuning the functional profile of diverse neurobiological processes. Here, we applied a recently developed "transcript-scanning" method for systematic determination of splice variant transcripts of the human alpha1(2.1) gene. This screen identified seven loci of variation, which together have never been fully defined in humans. Genomic sequence analysis clarified the splicing mechanisms underlying the observed variation. Electrophysiological characterization and a novel analytical paradigm, termed strength-current analysis, revealed that one focus of variation, involving combinatorial inclusion and exclusion of exons 43 and 44, exerted a primary effect on current amplitude and a corollary effect on Ca2+-dependent channel inactivation. These findings significantly expand the anticipated scope of functional diversity produced by splice variation of P/Q-type channels.

An unusual cation channel mediates photic control of locomotion in Drosophila

A unique family of putative ion channels that are related to voltage-gated sodium and calcium channels has been identified in genomic and cDNA studies of metazoans. Aside from evidence for expression of family members in the nervous system, little is known about the operation of the channel or its functional significance. In the present study, this conserved family's sole Drosophila member, a gene known both as CG1517 and as Dmalpha1U, is shown to correspond to the narrow abdomen (na) gene and is the locus of a set of mutations that affect sensitivity to anesthetics. Immunohistochemistry of adult heads reveals that the channel is expressed in the neuropil of the central complex and optic lobe; expression is severely depressed in the mutants. In addition to previously described defects, the mutant phenotype is demonstrated here to include dysfunction in the coupling between light and locomotor behavior. Most dramatically, mutant flies have an inversion of relative locomotor activity in light versus dark. The involvement of the channel in daily rhythms of the fruit fly is especially provocative because the human ortholog lies in a candidate region linked to bipolar disorder, a disease frequently associated with altered diurnal behavior.

Molecular characterization of a two-domain form of the neuronal voltage-gated P/Q-type calcium channel alpha(1)2.1 subunit

We characterized the neuronal two-domain (95kD-alpha(1)2.1) form of the alpha(1)2.1 subunit of the voltage-gated calcium channels using genetic and molecular analysis. The 95kD-alpha(1)2.1 is absent in neuronal preparations from CACNA1A null mouse demonstrating that alpha(1)2.1 and 95kD-alpha(1)2.1 arise from the same gene. A recombinant two-domain form (alpha(1AI-II)) of alpha(1)2.1 associates with the beta subunit and is trafficked to the plasma membrane. Translocation of the alpha(1AI-II) to the plasma membrane requires association with the beta subunit, since a mutation in the alpha(1AI-II) that inhibits beta subunit association reduces membrane trafficking. Though the alpha(1AI-II) protein does not conduct any voltage-gated currents, we have previously shown that it generates a high density of non-linear charge movements [Ahern et al., Proc. Natl. Acad. Sci. USA 98 (2001) 6935-6940]. In this study, we demonstrate that co-expression of the alpha(1AI-II) significantly reduces the current amplitude of alpha(1)2.1/beta(1a)/alpha(2)delta channels, via competition for the beta subunit. Taken together, our results demonstrate a dual functional role for the alpha(1AI-II) protein, both as a voltage sensor and modulator of P/Q-type currents in recombinant systems. These studies suggest an in vivo role for the 95kD-alpha(1)2.1 in altering synaptic activity via protein-protein interactions and/or regulation of P/Q-type currents.

Functional diversity in neuronal voltage-gated calcium channels by alternative splicing of Ca(v)alpha1

Alternative splicing is a critical mechanism used extensively in the mammalian nervous system to increase the level of diversity that can be achieved by a set of genes. This review focuses on recent studies of voltage-gated calcium (Ca) channel Ca(v)alpha1 subunit splice isoforms in neurons. Voltage-gated Ca channels couple changes in neuronal activity to rapid changes in intracellular Ca levels that in turn regulate an astounding range of cellular processes. Only ten genes have been identified that encode Ca(v)alpha1 subunits, an insufficient number to account for the level of functional diversity among voltage-gated Ca channels. The consequences of regulated alternative splicing among the genes that comprise voltage-gated Ca channels permits specialization of channel function, optimizing Ca signaling in different regions of the brain and in different cellular compartments. Although the full extent of alternative splicing is not yet known for any of the major subtypes of voltage-gated Ca channels, it is already clear that it adds a rich layer of structural and functional diversity".

Ontogeny of excitation-contraction coupling in the mammalian heart

The neonate mammalian heart is phenotypically different from the adult heart in many respects. Understanding these phenotypic differences are a fundamental component of understanding the mechanisms of congenital heart disease and its treatment. Differences in excitation-contraction (E-C) coupling of the neonatal heart from that of the adult include less reliance on intercellular sources of Ca(2+) such as that from sarcoplasmic reticulum (SR). Electron micrographs indicate that these immature cardiomyocytes lack transverse tubules and the SR is sparse. This paper focuses on the changes in the phenotype of E-C coupling during ontogeny in the mammalian heart and the molecular mechanisms underlying these changes.

Cooperation of two-domain Ca(2+) channel fragments in triad targeting and restoration of excitation- contraction coupling in skeletal muscle

The specific incorporation of the skeletal muscle voltage-dependent Ca(2+) channel in the triad is a prerequisite of normal excitation-contraction (EC) coupling. Sequences involved in membrane expression and in targeting of Ca(2+) channels into skeletal muscle triads have been described in different regions of the alpha(1S) subunit. Here we studied the targeting properties of two-domain alpha(1S) fragments, green fluorescent protein (GFP)-I x II (1-670) and III x IV (691-1873) expressed alone or in combination in dysgenic (alpha(1S)-null) myotubes. Immunofluorescence analysis showed that GFP-I x II or III x IV expressed separately were not targeted into triads. In contrast, on coexpression the two alpha(1S) fragments were colocalized with one another and with the ryanodine receptor in the triads. Coexpression of GFP-I x II and III x IV also fully restored Ca(2+) currents and depolarization-induced Ca(2+) transients, despite the severed connection between the two channel halves and the absence of amino acids 671-690 from either alpha(1S) fragment. Thus, triad targeting, like the rescue of function, requires the cooperation and coassembly of the two complementary channel fragments. Transferring the C terminus of alpha(1S) to the N-terminal two-domain fragment (GFP-I x II x tail), or transferring the I-II connecting loop containing the beta interaction domain to the C-terminal fragment (III x IV x beta in) did not improve the targeting properties of the individually expressed two-domain channel fragments. Thus, the cooperation of GFP-I.II and III.IV in targeting cannot be explained solely by a sequential action of the beta subunit by means of the I-II loop in releasing the channel from the sarcoplasmic reticulum and of the C terminus in triad targeting.

Structure-functional diversity of human L-type Ca2+ channel: perspectives for new pharmacological targets

The L-type Ca2+ channels mediate depolarization-induced influx of Ca2+ into a wide variety of cells and thus play a central role in triggering cardiac and smooth muscle contraction. Because of this role, clinically important classes of 1,4-dihydropyridine, phenylalkylamine, and benzothiazepine Ca2+ channel blockers were developed as powerful medicines to treat hypertension and angina pectoris. Molecular cloning studies revealed that the channel is subject to extensive structure-functional variability due to alternative splicing. In this review, we will focus on a potentially important role of genetically driven variability of Ca2+ channels in expression regulation and mutations, Ca2+-induced inactivation, and modulation of sensitivity to Ca2+ channel blockers with the perspective for new pharmacological targets.

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.

Intramembrane charge movements and excitation- contraction coupling expressed by two-domain fragments of the Ca2+ channel

To investigate the molecular basis of the voltage sensor that triggers excitation-contraction (EC) coupling, the four-domain pore subunit of the dihydropyridine receptor (DHPR) was cut in the cytoplasmic linker between domains II and III. cDNAs for the I-II domain (alpha1S 1-670) and the III-IV domain (alpha1S 701-1873) were expressed in dysgenic alpha1S-null myotubes. Coexpression of the two fragments resulted in complete recovery of DHPR intramembrane charge movement and voltage-evoked Ca(2+) transients. When fragments were expressed separately, EC coupling was not recovered. However, charge movement was detected in the I-II domain expressed alone. Compared with I-II and III-IV together, the charge movement in the I-II domain accounted for about half of the total charge (Q(max) = 3 +/- 0.23 vs. 5.4 +/- 0.76 fC/pF, respectively), and the half-activation potential for charge movement was significantly more negative (V(1/2) = 0.2 +/- 3.5 vs. 22 +/- 3.4 mV, respectively). Thus, interactions between the four internal domains of the pore subunit in the assembled DHPR profoundly affect the voltage dependence of intramembrane charge movement. We also tested a two-domain I-II construct of the neuronal alpha1A Ca(2+) channel. The neuronal I-II domain recovered charge movements like those of the skeletal I-II domain but could not assist the skeletal III-IV domain in the recovery of EC coupling. The results demonstrate that a functional voltage sensor capable of triggering EC coupling in skeletal myotubes can be recovered by the expression of complementary fragments of the DHPR pore subunit. Furthermore, the intrinsic voltage-sensing properties of the alpha1A I-II domain suggest that this hemi-Ca(2+) channel could be relevant to neuronal function.