Molecular structures involved in L-type calcium channel inactivation. Role of the carboxyl-terminal region encoded by exons 40-42 in alpha1C subunit in the kinetics and Ca2+ dependence of inactivation

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Calcium and activity-dependent signaling in the developing cerebral cortex

Calcium influx can be stimulated by various intra- and extracellular signals to set coordinated gene expression programs into motion. As such, the precise regulation of intracellular calcium represents a nexus between environmental cues and intrinsic genetic programs. Mounting genetic evidence points to a role for the deregulation of intracellular calcium signaling in neuropsychiatric disorders of developmental origin. These findings have prompted renewed enthusiasm for understanding the roles of calcium during normal and dysfunctional prenatal development. In this Review, we describe the fundamental mechanisms through which calcium is spatiotemporally regulated and directs early neurodevelopmental events. We also discuss unanswered questions about intracellular calcium regulation during the emergence of neurodevelopmental disease, and provide evidence that disruption of cell-specific calcium homeostasis and/or redeployment of developmental calcium signaling mechanisms may contribute to adult neurological disorders. We propose that understanding the normal developmental events that build the nervous system will rely on gaining insights into cell type-specific calcium signaling mechanisms. Such an understanding will enable therapeutic strategies targeting calcium-dependent mechanisms to mitigate disease.

Alternative Splicing at N Terminus and Domain I Modulates CaV1.2 Inactivation and Surface Expression

The Ca_V1.2 L-type calcium channel is a key conduit for Ca²⁺ influx to initiate excitation-contraction coupling for contraction of the heart and vasoconstriction of the arteries and for altering membrane excitability in neurons. Its α_1C pore-forming subunit is known to undergo extensive alternative splicing to produce many Ca_V1.2 isoforms that differ in their electrophysiological and pharmacological properties. Here, we examined the structure-function relationship of human Ca_V1.2 with respect to the inclusion or exclusion of mutually exclusive exons of the N-terminus exons 1/1a and IS6 segment exons 8/8a. These exons showed tissue selectivity in their expression patterns: heart variant 1a/8a, one smooth-muscle variant 1/8, and a brain isoform 1/8a. Overall, the 1/8a, when coexpressed with Ca_Vβ_2a, displayed a significant and distinct shift in voltage-dependent activation and inactivation and inactivation kinetics as compared to the other three splice variants. Further analysis showed a clear additive effect of the hyperpolarization shift in V_1/2inact of Ca_V1.2 channels containing exon 1 in combination with 8a. However, this additive effect was less distinct for V_1/2act. However, the measured effects were β-subunit-dependent when comparing Ca_Vβ_2a with Ca_Vβ₃ coexpression. Notably, calcium-dependent inactivation mediated by local Ca²⁺-sensing via the N-lobe of calmodulin was significantly enhanced in exon-1-containing Ca_V1.2 as compared to exon-1a-containing Ca_V1.2 channels. At the cellular level, the current densities of the 1/8a or 1/8 variants were significantly larger than the 1a/8a and 1a/8 variants when coexpressed either with Ca_Vβ_2a or Ca_Vβ₃ subunit. This finding correlated well with a higher channel surface expression for the exon 1-Ca_V1.2 isoform that we quantified by protein surface-expression levels or by gating currents. Our data also provided a deeper molecular understanding of the altered biophysical properties of alternatively spliced human Ca_V1.2 channels by directly comparing unitary single-channel events with macroscopic whole-cell currents.

Neural differentiation, selection and transcriptomic profiling of human neuromesodermal progenitor-like cells in vitro

Robust protocols for directed differentiation of human pluripotent cells are required to determine whether mechanisms operating in model organisms are relevant to our own development. Recent work in vertebrate embryos has identified neuromesodermal progenitors as a bipotent cell population that contributes to paraxial mesoderm and spinal cord. However, precise protocols for in vitro differentiation of human spinal cord progenitors are lacking. Informed by signalling in amniote embryos, we show here that transient dual-SMAD inhibition, together with retinoic acid (dSMADi-RA), provides rapid and reproducible induction of human spinal cord progenitors from neuromesodermal progenitor-like cells. Using CRISPR-Cas9 to engineer human embryonic stem cells with a GFP-reporter for neuromesodermal progenitor-associated gene Nkx1.2 we facilitate selection of this cell population. RNA-sequencing was then used to identify human and conserved neuromesodermal progenitor transcriptional signatures, to validate this differentiation protocol and to reveal new pathways/processes in human neural differentiation. This optimised protocol, novel reporter line and transcriptomic data are useful resources with which to dissect molecular mechanisms regulating human spinal cord generation and allow the scaling-up of distinct cell populations for global analyses, including proteomic, biochemical and chromatin interrogation.

Differential Somatic Ca2+ Channel Profile in Midbrain Dopaminergic Neurons

Dopaminergic (DA) neurons located in the ventral midbrain continuously generate a slow endogenous pacemaker activity, the mechanism of which is still debated. It has been suggested that, in the substantia nigra pars compacta (SNc), the pacemaking relies more on Ca(2+) channels and that the density of L-type Ca(2+) channels is higher in these DA neurons than in those located in the ventral tegmental area (VTA). This might lead to a higher Ca(2+) load in SNc DA neurons and explain their higher susceptibility to degeneration. However, direct evidence for this hypothesis is lacking. We found that the L-type current and channel density are indeed higher in the somata of rat SNc DA neurons and that this current undergoes less inactivation in this region. Nonstationary fluctuation analysis measurements showed a much higher number of L-type channels in the soma of SNc DA neurons, as well as a smaller single-channel conductance, pointing to a possible different molecular identity of L-type channels in DA neurons from the two areas. A major consequence of this is that pacemaking and, even more so, bursting are associated with a larger Ca(2+) entry through L-type channels in SNc DA neurons than in their VTA counterparts. Our results establish a molecular and functional difference between two populations of midbrain DA neurons that may contribute to their differential sensitivity to neurodegeneration.

SIGNIFICANCE STATEMENT

Dopamine neurons from the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) are involved in various brain functions, such as movement initiation and goal directed behavior, respectively. This work shows that, although both neurons fire in a similar regular and slow pacemaker mode, this firing activity is supported by different calcium channel landscapes. Indeed, the L-type calcium current is larger in the soma of dopamine neurons of the SNc, leading to a higher charge transfer through L-type channels during pacemaking and bursting. Therefore, these neurons may be physiologically exposed to a larger stress than their neighbors from the VTA.

Calcium Channel CaVα₁ Splice Isoforms - Tissue Specificity and Drug Action

Voltage-gated calcium ion channels are essential for numerous biological functions of excitable cells and there is wide spread appreciation of their importance as drug targets in the treatment of many disorders including those of cardiovascular and nervous systems. Each Cacna1 gene has the potential to generate a number of structurally, functionally, and in some cases pharmacologically unique CaVα1 subunits through alternative pre-mRNA splicing and the use of alternate promoters. Analyses of rapidly emerging deep sequencing data for a range of human tissue transcriptomes contain information to quantify tissue-specific and alternative exon usage patterns for Cacna1 genes. Cellspecific actions of nuclear DNA and RNA binding proteins control the use of alternate promoters and the selection of alternate exons during pre-mRNA splicing, and they determine the spectrum of protein isoforms expressed within different types of cells. Amino acid compositions within discrete protein domains can differ substantially among CaV isoforms expressed in different tissues, and such differences may be greater than those that exist across CaV channel homologs of closely related species. Here we highlight examples of CaV isoforms that have unique expression patterns and that exhibit different pharmacological sensitivities. Knowledge of expression patterns of CaV isoforms in different human tissues, cell populations, ages, and disease states should inform strategies aimed at developing the next generation of CaV channel inhibitors and agonists with improved tissue-specificity.

A new phosphorylation site in cardiac L-type Ca2+ channels (Cav1.2) responsible for its cAMP-mediated modulation

Cardiac L-type Ca(2+) channels are modulated by phosphorylation by protein kinase A (PKA). To explore the PKA-targeted phosphorylation site(s), five potential phosphorylation sites in the carboxyl (COOH) terminal region of the α1C-subunit of the guinea pig Cav1.2 Ca(2+) channel were mutated by replacing serine (S) or threonine (T) residues with alanine (A): S1574A (C1 site), S1626A (C2), S1699A (C3), T1908A, (C4), S1927A (C5), and their various combinations. The wild-type Ca(2+) channel activity was enhanced three- to fourfold by the adenylyl cyclase activator forskolin (Fsk, 5 μM), and that of mutants at C3, C4, C5, and combination of these sites was also significantly increased by Fsk. However, Fsk did not modulate the activity of the C1 and C2 mutants and mutants of combined sites involving the C1 site. Three peptides of the COOH-terminal tail of α1C, termed CT1 [corresponding to amino acids (aa) 1509-1789, containing sites C1-3], CT2 (aa 1778-2003, containing sites C4 and C5), and CT3 (aa 1942-2169), were constructed, and their phosphorylation by PKA was examined. CT1 and CT2, but not CT3, were phosphorylated in vitro by PKA. Three CT1 mutants at two sites of C1-C3 were also phosphorylated by PKA, but the mutant at all three sites was not. The CT2 mutant at the C4 site was phosphorylated by PKA, but the mutant at C5 sites was not. These results suggest that Ser(1574) (C1 site) may be a potential site for the channel modulation mediated by PKA.

The effect of trichostatin-A and tumor necrosis factor on expression of splice variants of the MaxiK and L-type channels in human myometrium

The onset of human parturition is associated with up-regulation of pro-inflammatory cytokines including tumor necrosis factor (TNF) as well as changes in ion flux, principally Ca²⁺ and K⁺, across the myometrial myocytes membrane. Elevation of intra-cellular Ca²⁺ from the sarcoplasmic reticulum opens L-type Ca²⁺ channels (LTCCs); in turn this increased calcium level activates MaxiK channels leading to relaxation. While the nature of how this cross-talk is governed remains unclear, our previous work demonstrated that the pro-inflammatory cytokine, TNF, and the histone deacetylase inhibitor, Trichostatin-A (TSA), exerted opposing effects on the expression of the pro-quiescent Gαs gene in human myometrial cells. Consequently, in this study we demonstrate that the different channel splice variants for both MaxiK and LTCC are expressed in primary myometrial myocytes. MaxiK mRNA expression was sensitive to TSA stimulation, this causing repression of the M1, M3, and M4 splice variants. A small but not statistically significantly increase in MaxiK expression was also seen in response to TNF. In contrast to this, expression of LTCC splice variants was seen to be influenced by both TNF and TSA. TNF induced overall increase in total LTCC expression while TSA stimulated a dual effect: causing induction of LTCC exon 8 expression but repressing expression of other LTCC splice variants including that encoding exons 30, 31, 33, and 34, exons 30–34 and exons 40–43. The significance of these observations is discussed herein.

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.

1,4-Dihydropyridine Calcium Channel Blockers: Homology Modeling of the Receptor and Assessment of Structure Activity Relationship

1,4-Dihydropyridine (DHP), an important class of calcium antagonist, inhibits the influx of extracellular Ca+2 through L-type voltage-dependent calcium channels. Three-dimensional (3D) structure of calcium channel as a receptor for 1,4-dihydropyridine is a step in understanding its mode of action. Protein structure prediction and modeling tools are becoming integral parts of the standard toolkit in biological and biomedical research. So, homology modeling (HM) of calcium channel alpha-1C subunit as DHP receptor model was achieved. The 3D structure of potassium channel was used as template for HM process. The resulted dihydropyridine receptor model was checked by different means to assure stereochemical quality and structural integrity of the model. This model was achieved in an attempt to understand the mode of action of DHP calcium channel antagonist and in further computer-aided drug design (CADD) analysis. Also the structure-activity relationship (SAR) of DHPs as antihypertensive and antianginal agents was reviewed, summarized, and discussed.

Ca2+ signaling in human induced pluripotent stem cell-derived cardiomyocytes (iPS-CM) from normal and catecholaminergic polymorphic ventricular tachycardia (CPVT)-afflicted subjects

Derivation of cardiomyocytes from induced pluripotent stem cells (iPS-CMs) allowed us to probe the Ca(2+)-signaling parameters of human iPS-CMs from healthy- and catecholaminergic polymorphic ventricular tachycardia (CPVT1)-afflicted individuals carrying a novel point mutation p.F2483I in ryanodine receptors (RyR2). iPS-CMs were dissociated on day 30-40 of differentiation and patch-clamped within 3-6 days. Calcium currents (ICa) averaged ∼8pA/pF in control and mutant iPS-CMs. ICa-induced Ca(2+)-transients in control and mutant cells had bell-shaped voltage-dependence similar to that of ICa, consistent with Ca(2+)-induced Ca(2+)-release (CICR) mechanism. The ratio of ICa-activated to caffeine-triggered Ca(2+)-transients was ∼0.3 in both cell types. Caffeine-induced Ca(2+)-transients generated significantly smaller Na(+)-Ca(2+) exchanger current (INCX) in mutant cells, reflecting their smaller Ca(2+)-stores. The gain of CICR was voltage-dependent as in adult cardiomyocytes. Adrenergic agonists enhanced ICa, but differentially altered the CICR gain, diastolic Ca(2+), and Ca(2+)-sparks in mutant cells. The mutant cells, when Ca(2+)-overloaded, showed longer and wandering Ca(2+)-sparks that activated adjoining release sites, had larger CICR gain at -30mV yet smaller Ca(2+)-stores. We conclude that control and mutant iPS-CMs express the adult cardiomyocyte Ca(2+)-signaling phenotype. RyR2 F2483I mutant myocytes have aberrant unitary Ca(2+)-signaling, smaller Ca(2+)-stores, higher CICR gains, and sensitized adrenergic regulation, consistent with functionally altered Ca(2+)-release profile of CPVT syndrome.

The voltage-dependent L-type Ca2+ (CaV1.2) channel C-terminus fragment is a bi-modal vasodilator

Voltage-dependent L-type Ca(2+) channels (CaV1.2) are the primary Ca(2+) entry pathway in vascular smooth muscle cells (myocytes). CaV1.2 channels control systemic blood pressure and organ blood flow and are pathologically altered in vascular diseases, which modifies vessel contractility. The CaV1.2 distal C-terminus is susceptible to proteolytic cleavage, which yields a truncated CaV1.2 subunit and a cleaved C-terminal fragment (CCt). Previous studies in cardiac myocytes and neurons have identified CCt as both a transcription factor and CaV1.2 channel inhibitor, with different signalling mechanisms proposed to underlie some of these effects. CCt existence and physiological functions in arterial myocytes are unclear, but important to study given the functional significance of CaV1.2 channels. Here, we show that CCt exists in myocytes of both rat and human resistance-size cerebral arteries, where it locates to both the nucleus and plasma membrane. Recombinant CCt expression in arterial myocytes inhibited CaV1.2 transcription and reduced CaV1.2 protein. CCt induced a depolarizing shift in the voltage dependence of both CaV1.2 current activation and inactivation, and reduced non-inactivating current in myocytes. Recombinant truncated CCt lacking a putative nuclear localization sequence (92CCt) did not locate to the nucleus and had no effect on arterial CaV1.2 transcription or protein. However, 92CCt shifted the voltage dependence of CaV1.2 activation and inactivation similarly to CCt. CCt and 92CCt both inhibited pressure- and depolarization-induced vasoconstriction, although CCt was a far more effective vasodilator. These data demonstrate that endogenous CCt exists and reduces both CaV1.2 channel expression and voltage sensitivity in arterial myocytes. Thus, CCt is a bi-modal vasodilator.

Competitive and non-competitive regulation of calcium-dependent inactivation in CaV1.2 L-type Ca2+ channels by calmodulin and Ca2+-binding protein 1

CaV1.2 interacts with the Ca(2+) sensor proteins, calmodulin (CaM) and calcium-binding protein 1 (CaBP1), via multiple, partially overlapping sites in the main subunit of CaV1.2, α1C. Ca(2+)/CaM mediates a negative feedback regulation of Cav1.2 by incoming Ca(2+) ions (Ca(2+)-dependent inactivation (CDI)). CaBP1 eliminates this action of CaM through a poorly understood mechanism. We examined the hypothesis that CaBP1 acts by competing with CaM for common interaction sites in the α1C- subunit using Förster resonance energy transfer (FRET) and recording of Cav1.2 currents in Xenopus oocytes. FRET detected interactions between fluorescently labeled CaM or CaBP1 with the membrane-attached proximal C terminus (pCT) and the N terminus (NT) of α1C. However, mutual overexpression of CaM and CaBP1 proved inadequate to quantitatively assess competition between these proteins for α1C. Therefore, we utilized titrated injection of purified CaM and CaBP1 to analyze their mutual effects. CaM reduced FRET between CaBP1 and pCT, but not NT, suggesting competition between CaBP1 and CaM for pCT only. Titrated injection of CaBP1 and CaM altered the kinetics of CDI, allowing analysis of their opposite regulation of CaV1.2. The CaBP1-induced slowing of CDI was largely eliminated by CaM, corroborating a competition mechanism, but 15-20% of the effect of CaBP1 was CaM-resistant. Both components of CaBP1 action were present in a truncated α1C where N-terminal CaM- and CaBP1-binding sites have been deleted, suggesting that the NT is not essential for the functional effects of CaBP1. We propose that CaBP1 acts via interaction(s) with the pCT and possibly additional sites in α1C.

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.

Hypoxic regulation of cardiac Ca2+ channel: possible role of haem oxygenase

Acute and chronic hypoxias are common cardiac diseases that lead often to arrhythmia and impaired contractility. At the cellular level it is unclear whether the suppression of cardiac Ca(2+) channels (Ca(V)1.2) results directly from oxygen deprivation on the channel protein or is mediated by intermediary proteins affecting the channel. To address this question we measured the early effects of hypoxia (5-60 s, P(O(2)) < 5 mmHg) on Ca(2+) current (I(Ca)) and tested the involvement of protein kinase A (PKA) phosphorylation, Ca(2+)/calmodulin-mediated signalling and the haem oxygenase (HO) pathway in the hypoxic regulation of Ca(V)1.2 in rat and cat ventricular myocytes and HEK-293 cells. Hypoxic suppression of ICa) and Ca(2+) transients was significant within 5 s and intensified in the following 50 s, and was reversible. Phosphorylation by cAMP or the phosphatase inhibitor okadaic acid desensitized I(Ca) to hypoxia, while PKA inhibition by H-89 restored the sensitivity of I(Ca) to hypoxia. This phosphorylation effect was specific to Ca(2+), but not Ba(2+) or Na(+), permeating through the channel. CaMKII inhibitory peptide and Bay K8644 reversed the phosphorylation-induced desensitization to hypoxia. Mutation of CAM/CaMKII-binding motifs of the α(1c) subunit of Ca(V)1.2 fully desensitized the Ca(2+) channel to hypoxia. Rapid application of HO inhibitors (zinc protoporphyrin (ZnPP) and tin protoporphyrin (SnPP)) suppressed the channel in a manner similar to acute hypoxia such that: (1) I(Ca) and I(Ba) were suppressed within 5 s of ZnPP application; (2) PKA activation and CaMKII inhibitors desensitized I(Ca), but not I(Ba), to ZnPP; and (3) hypoxia failed to further suppress I(Ca) and I(Ba) in ZnPP-treated myocytes. We propose that the binding of HO to the CaM/CaMKII-specific motifs on Ca(2+) channel may mediate the rapid response of the channel to hypoxia.

Calcineurin Controls Voltage-Dependent-Inactivation (VDI) of the Normal and Timothy Cardiac Channels

Ca²⁺-entry in the heart is tightly controlled by Cav1.2 inactivation, which involves Ca²⁺-dependent inactivation (CDI) and voltage-dependent inactivation (VDI) components. Timothy syndrome, a subtype-form of congenital long-QT syndrome, results from a nearly complete elimination of VDI by the G406R mutation in the α₁1.2 subunit of Cav1.2. Here, we show that a single (A1929P) or a double mutation (H1926A-H1927A) within the CaN-binding site at the human C-terminal tail of α₁1.2, accelerate the inactivation rate and enhances VDI of both wt and Timothy channels. These results identify the CaN-binding site as the long-sought VDI-regulatory motif of the cardiac channel. The substantial increase in VDI and the accelerated inactivation caused by the selective inhibitors of CaN, cyclosporine A and FK-506, which act at the same CaN-binding site, further support this conclusion. A reversal of enhanced-sympathetic tone by VDI-enhancing CaN inhibitors could be beneficial for improving Timothy syndrome complications such as long-QT and autism.

Functional characterization of alternative splicing in the C terminus of L-type CaV1.3 channels

Ca(V)1.3 channels are unique among the high voltage-activated Ca(2+) channel family because they activate at the most negative potentials and display very rapid calcium-dependent inactivation. Both properties are of crucial importance in neurons of the suprachiasmatic nucleus and substantia nigra, where the influx of Ca(2+) ions at subthreshold membrane voltages supports pacemaking function. Previously, alternative splicing in the Ca(V)1.3 C terminus gives rise to a long (Ca(V)1.3(42)) and a short form (Ca(V)1.3(42A)), resulting in a pronounced activation at more negative voltages and faster inactivation in the latter. It was further shown that the C-terminal modulator in the Ca(V)1.3(42) isoforms modulates calmodulin binding to the IQ domain. Using splice variant-specific antibodies, we determined that protein localization of both splice variants in different brain regions were similar. Using the transcript-scanning method, we further identified alternative splicing at four loci in the C terminus of Ca(V)1.3 channels. Alternative splicing of exon 41 removes the IQ motif, resulting in a truncated Ca(V)1.3 protein with diminished inactivation. Splicing of exon 43 causes a frameshift and exhibits a robust inactivation of similar intensity to Ca(V)1.3(42A). Alternative splicing of exons 44 and 48 are in-frame, altering interaction of the distal modulator with the IQ domain and tapering inactivation slightly. Thus, alternative splicing in the C terminus of Ca(V)1.3 channels modulates its electrophysiological properties, which could in turn alter neuronal firing properties and functions.

L-type calcium channel auto-regulation of transcription

L-type calcium channels (LTCC) impact the function of nearly all excitable cells. The classical LTCC function is to mediate trans-sarcolemmal Ca(2+) flux. This review focuses on the contribution of a mobile segment of the LTCC that regulates ion channel function, and also serves as a regulator of transcription in the nucleus. Specifically we highlight recent work demonstrating an auto-feedback regulatory pathway whereby the LTCC transcription factor regulates the LTCC. Also discussed is acute and long-term regulation of function by the LTCC-transcription regulator.

L-type calcium channel as a cardiac oxygen sensor

Acute oxygen sensing in the heart is thought to occur through redox regulation and phosphorylation of membrane channels. Here we report a novel O2-sensing mechanism involving the C-terminus of the L-type Ca2+ channel and regulated by PKA phosphorylation. In patch-clamped myocytes, oxygen deprivation decreased ICa within 40 s. The suppressive effect of anoxia was relieved by PKA-mediated phosphorylation only when Ca2+ was the charge carrier, whereas phosphorylated IBa remained sensitive to O2 withdrawal. Suppression of Ca2+ release by thapsigargin did not alter the response of ICa to anoxia, suggesting a mandatory role for Ca2+ influx and not Ca2+-induced Ca2+ release (CICR) in O2 regulation of the channel. Consistent with this idea, mutation of 80 amino acids in the Ca2+/CaM-binding domain of the recombinant alpha1C subunit that removes Ca2+ dependent inactivation (CDI) abolished O2 sensitivity of the channel. Our findings suggest that the Ca2+/CaM binding domain of the L-type Ca2+ may represent a molecular site for O2 sensing of the heart.

Calmodulin- and Ca2+-dependent facilitation and inactivation of the Cav1.2 Ca2+ channels in guinea-pig ventricular myocytes

The L-type Ca(2+) channel (Ca(V)1.2) shows clear Ca(2+)-dependent facilitation and inactivation. Here we have examined the effects of calmodulin (CaM) and Ca(2+) on Ca(2+) channel in guinea-pig ventricular myocytes in the inside-out patch mode, where rundown of the channels was controlled. At a free [Ca(2+)] of 0.1 microM, CaM (0.15, 0.7, 1.4, 2.1, 3.5, and 7.0 microM) + ATP (2.4 mM) induced channel activities of 27%, 98%, 142%, 222%, 65%, and 20% relative to the control activity, respectively, showing a bell-shaped relationship. Similar results were observed at a free [Ca(2+)] <0.01 microM or with a Ca(2+)-insensitive mutant, CaM(1234), suggesting that apoCaM may induce facilitation and inactivation of the channel activity. The bell-shaped curve of CaM was shifted to the lower concentration side with increasing [Ca(2+)]. A simple model for CaM- and Ca(2+)-dependent modulations of the channel activity, which involves two CaM-binding sites, was proposed. We suggest that both apoCaM and Ca(2+)/CaM can induce facilitation and inactivation of Ca(V)1.2 Ca(2+) channels and that the basic role of Ca(2+) is to accelerate CaM-dependent facilitation and inactivation.

Highly variable mRNA expression and splicing of L-type voltage-dependent calcium channel alpha subunit 1C in human heart tissues

OBJECTIVES

The voltage-dependent L-type calcium channel alpha-subunit 1c (Cav1.2, CACNA1C) undergoes extensive mRNA splicing, leading to numerous isoforms with different functions. L-type calcium channel blockers are used in the treatment of hypertension and arrhythmias, but response varies between individuals. We have studied the interindividual variability in mRNA expression and splicing of CACNA1C, in 65 heart tissue samples, taken from heart transplant recipients.

METHODS

Splice variants were measured quantitatively by polymerase chain reaction in 12 splicing loci of CACNA1C mRNA. To search for functional cis-acting polymorphisms, we determined allelic expression ratios for total CACNA1C mRNA and several splice variants using marker single nucleotide polymorphisms in exon 4 and exon 30.

RESULTS

Total CACNA1C mRNA levels varied approximately 50-fold. Substantial splicing occurred in six loci generating two or more splice variants, some with known functional differences. Splice patterns varied broadly between individuals. Two heart tissues expressed predominantly the dihydropyridine-sensitive smooth muscle isoform of CACNA1C (containing exon 8), rather than the cardiac isoform (containing exon 8a). Lack of significant allelic expression imbalance, observed with total mRNA and several splice variants, argued against CACNA1C polymorphisms as a cause of variability. Taken together, highly variable splicing can cause profound phenotypic variations of CACNA1C function, potentially associated with disease susceptibility and response to L-type calcium channel blockers.

Sequence differences in the IQ motifs of CaV1.1 and CaV1.2 strongly impact calmodulin binding and calcium-dependent inactivation

The proximal C terminus of the cardiac L-type calcium channel (Ca(V)1.2) contains structural elements important for the binding of calmodulin (CaM) and calcium-dependent inactivation, and exhibits extensive sequence conservation with the corresponding region of the skeletal L-type channel (Ca(V)1.1). However, there are several Ca(V)1.1 residues that are both identical in six species and are non-conservatively changed from the corresponding Ca(V)1.2 residues, including three of the "IQ motif." To investigate the functional significance of these residue differences, we used native gel electrophoresis and expression in intact myotubes to compare the binding of CaM to extended regions (up to 300 residues) of the C termini of Ca(V)1.1 and Ca(V)1.2. We found that in the presence of Ca(2+) (either millimolar or that in resting myotubes), CaM bound strongly to C termini of Ca(V)1.2 but not of Ca(V)1.1. Furthermore, replacement of two residues (Tyr(1657) and Lys(1662)) within the IQ motif of a C-terminal Ca(V)1.2 construct with the divergent residues of Ca(V)1.1 (His(1532) and Met(1537)) led to a weakening of CaM binding (native gels), whereas the reciprocal substitution in Ca(V)1.1 caused a gain of CaM binding. In full-length Ca(V)1.2, substitution of these same two divergent residues with those of Ca(V)1.1 (Y1657H, K1662M) eliminated calcium-dependent inactivation of the heterologously expressed channel. Thus, our results reveal that a conserved difference between the IQ motifs of Ca(V)1.2 and Ca(V)1.1 has a profound effect on both CaM binding and calcium-dependent inactivation.

Calmodulin-dependent gating of Ca(v)1.2 calcium channels in the absence of Ca(v)beta subunits

It is generally accepted that to generate calcium currents in response to depolarization, Ca(v)1.2 calcium channels require association of the pore-forming alpha(1C) subunit with accessory Ca(v)beta and alpha(2)delta subunits. A single calmodulin (CaM) molecule is tethered to the C-terminal alpha(1C)-LA/IQ region and mediates Ca2+-dependent inactivation of the channel. Ca(v)beta subunits are stably associated with the alpha(1C)-interaction domain site of the cytoplasmic linker between internal repeats I and II and also interact dynamically, in a Ca2+-dependent manner, with the alpha(1C)-IQ region. Here, we describe a surprising discovery that coexpression of exogenous CaM (CaM(ex)) with alpha(1C)/alpha(2)delta in COS1 cells in the absence of Ca(v)beta subunits stimulates the plasma membrane targeting of alpha(1C), facilitates calcium channel gating, and supports Ca2+-dependent inactivation. Neither real-time PCR with primers complementary to monkey Ca(v)beta subunits nor coimmunoprecipitation analysis with exogenous alpha(1C) revealed an induction of endogenous Ca(v)beta subunits that could be linked to the effect of CaM(ex). Coexpression of a calcium-insensitive CaM mutant CaM(1234) also facilitated gating of Ca(v)beta-free Ca(v)1.2 channels but did not support Ca2+-dependent inactivation. Our results show there is a functional matchup between CaM(ex) and Ca(v)beta subunits that, in the absence of Ca(v)beta, renders Ca2+ channel gating facilitated by CaM molecules other than the one tethered to LA/IQ to support Ca2+-dependent inactivation. Thus, coexpression of CaM(ex) creates conditions when the channel gating, voltage- and Ca2+-dependent inactivation, and plasma-membrane targeting occur in the absence of Ca(v)beta. We suggest that CaM(ex) affects specific Ca(v)beta-free conformations of the channel that are not available to endogenous CaM.

Calcium channel inactivation: possible role in signal transduction and Ca2+ signaling

Voltage gated Ca2+ channels are major routes for the entry of intracellular Ca2+ coupled to membrane depolarization that appear to vary greatly with respect to their voltage dependence and kinetics. Such variability maybe in part related to the attached signaling properties of the channel, in addition to the transport of calcium. In the present review we consider the possible role of calcium-dependent inactivation of Cav1.2 in Ca2+ signal transduction and signaling of calcium release from the cardiac sarcoplasmic reticulum. We explore the specific roles of Ca2+-sensing calmodulin-binding domains of the C-terminal tail (LA and K) of the channel in mediating Ca2+-induced Ca2+ release and signal transduction. Our experiments point to an intriguing possibility that the C-terminal tail of Cav1.2 may translocate the Ca2+ signal as a part of inactivation mechanism and the corresponding voltage-gated rearrangement of the C-terminus. We show how a dynamic and transient regulation, in a Ca2+-dependent manner, defines molecular events including Ca2+ release and signaling of cAMP-responsive element-binding protein (CREB)-dependent transcription. We propose that such Ca2+-dependent C-tail translocation that also initiates the channel inactivation, may have evolved specifically for the Cav1.2 channel.

Modulation of voltage- and Ca2+-dependent gating of CaV1.3 L-type calcium channels by alternative splicing of a C-terminal regulatory domain

Low voltage activation of Ca(V)1.3 L-type Ca(2+) channels controls excitability in sensory cells and central neurons as well as sinoatrial node pacemaking. Ca(V)1.3-mediated pacemaking determines neuronal vulnerability of dopaminergic striatal neurons affected in Parkinson disease. We have previously found that in Ca(V)1.4 L-type Ca(2+) channels, activation, voltage, and calcium-dependent inactivation are controlled by an intrinsic distal C-terminal modulator. Because alternative splicing in the Ca(V)1.3 alpha1 subunit C terminus gives rise to a long (Ca(V)1.3(42)) and a short form (Ca(V)1.3(42A)), we investigated if a C-terminal modulatory mechanism also controls Ca(V)1.3 gating. The biophysical properties of both splice variants were compared after heterologous expression together with beta3 and alpha2delta1 subunits in HEK-293 cells. Activation of calcium current through Ca(V)1.3(42A) channels was more pronounced at negative voltages, and inactivation was faster because of enhanced calcium-dependent inactivation. By investigating several Ca(V)1.3 channel truncations, we restricted the modulator activity to the last 116 amino acids of the C terminus. The resulting Ca(V)1.3(DeltaC116) channels showed gating properties similar to Ca(V)1.3(42A) that were reverted by co-expression of the corresponding C-terminal peptide C(116). Fluorescence resonance energy transfer experiments confirmed an intramolecular protein interaction in the C terminus of Ca(V)1.3 channels that also modulates calmodulin binding. These experiments revealed a novel mechanism of channel modulation enabling cells to tightly control Ca(V)1.3 channel activity by alternative splicing. The absence of the C-terminal modulator in short splice forms facilitates Ca(V)1.3 channel activation at lower voltages expected to favor Ca(V)1.3 activity at threshold voltages as required for modulation of neuronal firing behavior and sinoatrial node pacemaking.

β2-Adrenergic receptor agonists stimulate L-type calcium current independent of PKA in newborn rabbit ventricular myocytes

Selective stimulation of β2-adrenergic receptors (ARs) in newborn rabbit ventricular myocardium invokes a positive inotropic effect that is lost during postnatal maturation. The underlying mechanisms for this age-related stimulatory response remain unresolved. We examined the effects of β2-AR stimulation on L-type Ca2+ current ( ICa,L) during postnatal development. ICa,L was measured (37°C; either Ca2+ or Ba2+ as the charge carrier) using the whole-cell patch-clamp technique in newborn (1 to 5 days old) and adult rabbit ventricular myocytes. Ca2+ transients were measured concomitantly by dialyzing the cell with indo-1. Activation of β2-ARs (with either 100 nM zinterol or 1 μM isoproterenol in the presence of the β1-AR antagonist, CGP20712A) stimulated ICa,L twofold in newborns but not in adults. The β2-AR-mediated increase in Ca2+ transient amplitude in newborns was due exclusively to the augmentation of ICa,L. Zinterol increased the rate of inactivation of ICa,L and increased the Ca2+ flux integral. The β2-AR inverse agonist, ICI-118551 (500 nM), but not the β1-AR antagonist, CGP20712A (500 nM), blocked the response to zinterol. Unexpectedly, the PKA blockers, H-89 (10 μM), PKI 6-22 amide (10 μM), and Rp-cAMP (100 μM), all failed to prevent the response to zinterol but completely blocked responses to selective β1-AR stimulation of ICa,L in newborns. Our results demonstrate that in addition to the conventional β1-AR/cAMP/PKA pathway, newborn rabbit myocardium exhibits a novel β2-AR-mediated, PKA-insensitive pathway that stimulates ICa,L. This striking developmental difference plays a major role in the age-related differences in inotropic responses to β2-AR agonists.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Alternative splicing of the Ca(v)1.3 channel IQ domain, a molecular switch for Ca2+-dependent inactivation within auditory hair cells

Native Ca(V)1.3 channels within cochlear hair cells exhibit a surprising lack of Ca2+-dependent inactivation (CDI), given that heterologously expressed Ca(V)1.3 channels show marked CDI. To determine whether alternative splicing at the C terminus of the Ca(V)1.3 gene may produce a hair cell splice variant with weak CDI, we transcript-scanned mRNA obtained from rat cochlea. We found that the alternate use of exon 41 acceptor sites generated a splice variant that lost the calmodulin-binding IQ motif of the C terminus. These Ca(V)1.3(IQdelta) ("IQ deleted") channels exhibited a lack of CDI, which was independent of the type of coexpressed beta-subunits. Ca(V)1.3(IQdelta) channel immunoreactivity was preferentially localized to cochlear outer hair cells (OHCs), whereas that of Ca(V)1.3(IQfull) channels (IQ-possessing) labeled inner hair cells (IHCs). The preferential expression of Ca(V)1.3(IQdelta) within OHCs suggests that these channels may play a role in processes such as electromotility or activity-dependent gene transcription rather than neurotransmitter release, which is performed predominantly by IHCs in the cochlea.

Atherosclerosis-related molecular alteration of the human CaV1.2 calcium channel alpha1C subunit

Atherosclerosis is an inflammatory process characterized by proliferation and dedifferentiation of vascular smooth muscle cells (VSMC). Ca(v)1.2 calcium channels may have a role in atherosclerosis because they are essential for Ca(2+)-signal transduction in VSMC. The pore-forming Ca(v)1.2alpha1 subunit of the channel is subject to alternative splicing. Here, we investigated whether the Ca(v)1.2alpha1 splice variants are affected by atherosclerosis. VSMC were isolated by laser-capture microdissection from frozen sections of adjacent regions of arteries affected and not affected by atherosclerosis. In VSMC from nonatherosclerotic regions, RT-PCR analysis revealed an extended repertoire of Ca(v)1.2alpha1 transcripts characterized by the presence of exons 21 and 41A. In VSMC affected by atherosclerosis, expression of the Ca(v)1.2alpha1 transcript was reduced and the Ca(v)1.2alpha1 splice variants were replaced with the unique exon-22 isoform lacking exon 41A. Molecular remodeling of the Ca(v)1.2alpha1 subunits associated with atherosclerosis caused changes in electrophysiological properties of the channels, including the kinetics and voltage-dependence of inactivation, recovery from inactivation, and rundown of the Ca(2+) current. Consistent with the pathophysiological state of VSMC in atherosclerosis, cell culture data pointed to a potentially important association of the exon-22 isoform of Ca(v)1.2alpha1 with proliferation of VSMC. Our findings are consistent with a hypothesis that localized changes in cytokine expression generated by inflammation in atherosclerosis affect alternative splicing of the Ca(v)1.2alpha1 gene in the human artery that causes molecular and electrophysiological remodeling of Ca(v)1.2 calcium channels and possibly affects VSMC proliferation.

C-terminal modulator controls Ca2+-dependent gating of Cav1.4 L-type Ca2+ channels

Tonic neurotransmitter release at sensory cell ribbon synapses is mediated by calcium (Ca2+) influx through L-type voltage-gated Ca2+ channels. This tonic release requires the channels to inactivate slower than in other tissues. Ca(v)1.4 L-type voltage-gated Ca2+ channels (LTCCs) are found at high densities in photoreceptor terminals, and alpha1 subunit mutations cause human congenital stationary night blindness type-2 (CSNB2). Ca(v)1.4 voltage-dependent inactivation is slow and Ca2+-dependent inactivation (CDI) is absent. We show that removal of the last 55 or 122 (C122) C-terminal amino acid residues of the human alpha1 subunit restores calmodulin-dependent CDI and shifts voltage of half-maximal activation to more negative potentials. The C terminus must therefore form part of a mechanism that prevents calmodulin-dependent CDI of Ca(v)1.4 and controls voltage-dependent activation. Fluorescence resonance energy transfer experiments in living cells revealed binding of C122 to C-terminal motifs mediating CDI in other Ca2+ channels. The absence of this modulatory mechanism in the CSNB2 truncation mutant K1591X underlines its importance for normal retinal function in humans.

Alternative splicing in the C-terminus of CaV2.2 controls expression and gating of N-type calcium channels

N-type Ca(V)2.2 calcium channels localize to presynaptic nerve terminals of nociceptors where they control neurotransmitter release. Nociceptive neurons express a unique set of ion channels and receptors important for optimizing their role in transmission of noxious stimuli. Included among these is a structurally and functionally distinct N-type calcium channel splice isoform, Ca(V)2.2e[37a], expressed in a subset of nociceptors and with limited expression in other parts of the nervous system. Ca(V)2.2[e37a] arises from the mutually exclusive replacement of e37a for e37b in the C-terminus of Ca(V)2.2 mRNA. N-type current densities in nociceptors that express a combination of Ca(V)2.2e[37a] and Ca(V)2.2e[37b] mRNAs are significantly larger compared to cells that express only Ca(V)2.2e[37b]. Here we show that e37a supports increased expression of functional N-type channels and an increase in channel open time as compared to Ca(V)2.2 channels that contain e37b. To understand how e37a affects N-type currents we compared macroscopic and single-channel ionic currents as well as gating currents in tsA201 cells expressing Ca(V)2.2e[37a] and Ca(V)2.2e[37b]. When activated, Ca(V)2.2e[37a] channels remain open for longer and are expressed at higher density than Ca(V)2.2e[37b] channels. These unique features of the Ca(V)2.2e[37a] isoform combine to augment substantially the amount of calcium that enters cells in response to action potentials. Our studies of the e37a/e37b splice site reveal a multifunctional domain in the C-terminus of Ca(V)2.2 that regulates the overall activity of N-type calcium channels in nociceptors.

A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel

The inactivation of voltage-gated L-type Ca(2+) channels (Ca(V)1) regulates Ca(2+) entry and controls intracellular Ca(2+) levels that are essential for cellular activity. The molecular entities implicated in L-channel (Ca(V)1.2) inactivation are not fully identified. Here we show for the first time the functional impact of one of the two highly conserved clusters of six negatively charged glutamates and aspartate (802-807; poly ED motif) at the II-III loop of the alpha 1 subunits of rabbit of Ca(v)1.2, alpha(1)1.2 and alpha(1)1.2 DeltaN60-Delta1733) on voltage-dependent inactivation. Mutation of the poly ED motif to alanine or glutamine/asparagine greatly enhanced voltage-dependent inactivation, shifting the voltage dependence to negative potentials by >50 mV and conferring a neuronal like inactivation kinetics onto Ca(V)1.2. The large shift in the midpoint of inactivation of the steady-state inactivation kinetics was observed also in Ca(2+) or Ba(2+) and was not altered by the beta2A subunit. Missing from the fast inactivating neuronal P/Q (Ca(V)2.1)-, N (Ca(V)2.2)- or R (Ca(V)2.3)-type channels and modulating Ca(V)1.2 inactivation kinetics, the poly ED motif is likely to be a specific L-type Ca(2+) channels inactivating domain. Our results fit a model in which the poly ED either by itself or as part of a larger inactivating motif acts as Ca(V)1.2 specific built-in "stopper." In this model, Ca(V)1 accomplishes a large Ca(2+) influx during depolarization, possibly by the poly ED hindering occlusion at the pore. Furthermore, the selective designed poly ED perhaps clarifies major inactivation differences between L- and non-L-type calcium channels.

Analysis of the modal hypothesis of Ca2+-dependent inactivation of L-type Ca2+ channels

A kinetic model of Ca2+-dependent inactivation (CDI) of L-type Ca2+ channels was developed. The model is based on the hypothesis that postulates the existence of four short-lived modes with lifetimes of a few hundreds of milliseconds. Our findings suggest that the transitions between the modes is primarily determined by the binding of Ca2+ to two intracellular allosteric sites located in different motifs of the CI region, which have greatly differing binding rates for Ca2+ (different k(on)). The slow-binding site is controlled by local Ca2+ near a single open channel that is consistent with the "domain" CDI model, and Ca2+ binding to the fast-binding site(s) depends on Ca2+ arising from distant sources that is consistent with the "shell" CDI model. The model helps to explain numerous experimental findings that are poorly understood so far.

Regulation of voltage-gated Ca2+ channels by calmodulin

Calmodulin, a highly versatile and ubiquitously expressed Ca2+ sensor, regulates the function of many enzymes and ion channels. Both Ca2+-dependent inactivation and Ca2+-dependent facilitation of the voltage-gated Ca2+ channels Cav1.2 and Cav2.1 are regulated through an interaction with Ca2+-bound calmodulin. This review addresses the functional regulation of Cav1.2 and Cav2.1 by calmodulin and discusses how Ca2+ binding to a single calmodulin molecule can regulate opposing functions of the voltage-gated Ca2+ channels.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

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.

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.

Voltage-gated rearrangements associated with differential beta-subunit modulation of the L-type Ca(2+) channel inactivation

Auxiliary beta-subunits bound to the cytoplasmic alpha(1)-interaction domain of the pore-forming alpha(1C)-subunit are important modulators of voltage-gated Ca(2+) channels. The underlying mechanisms are not yet well understood. We investigated correlations between differential modulation of inactivation by beta(1a)- and beta(2)- subunits and structural responses of the channel to transition into distinct functional states. The NH(2)-termini of the alpha(1C)- and beta-subunits were fused with cyan or yellow fluorescent proteins, and functionally coexpressed in COS1 cells. Fluorescence resonance energy transfer (FRET) between them or with membrane-trapped probes was measured in live cells under voltage clamp. It was found that in the resting state, the tagged NH(2)-termini of the alpha(1C)- and beta-subunit fluorophores are separated. Voltage-dependent inactivation generates strong FRET between alpha(1C) and beta(1a) suggesting mutual reorientation of the NH(2)-termini, but their distance vis-à-vis the plasma membrane is not appreciably changed. These voltage-gated rearrangements were substantially reduced when the beta(1a)-subunit was replaced by beta(2). Differential beta-subunit modulation of inactivation and of FRET between alpha(1C) and beta were eliminated by inhibition of the slow inactivation. Thus, differential beta-subunit modulation of inactivation correlates with the voltage-gated motion between the NH(2)-termini of alpha(1C)- and beta-subunits and targets the mechanism of slow voltage-dependent inactivation.

Sites on Calmodulin That Interact with the C-terminal Tail of Cav1.2 Channel

Two fragments of the C-terminal tail of the alpha(1) subunit (CT1, amino acids 1538-1692 and CT2, amino acids 1596-1692) of human cardiac L-type calcium channel (Ca(V)1.2) have been expressed, refolded, and purified. A single Ca(2+)-calmodulin binds to each fragment, and this interaction with Ca(2+)-calmodulin is required for proper folding of the fragment. Ca(2+)-calmodulin, bound to these fragments, is in a more extended conformation than calmodulin bound to a synthetic peptide representing the IQ motif, suggesting that either the conformation of the IQ sequence is different in the context of the longer fragment, or other sequences within CT2 contribute to the binding of calmodulin. NMR amide chemical shift perturbation mapping shows the backbone conformation of calmodulin is nearly identical when bound to CT1 and CT2, suggesting that amino acids 1538-1595 do not contribute to or alter calmodulin binding to amino acids 1596-1692 of Ca(V)1.2. The interaction with CT2 produces the greatest changes in the backbone amides of hydrophobic residues in the N-lobe and hydrophilic residues in the C-lobe of calmodulin and has a greater effect on residues located in Ca(2+) binding loops I and II in the N-lobe relative to loops III and IV in the C-lobe. In conclusion, Ca(2+)-calmodulin assumes a novel conformation when part of a complex with the C-terminal tail of the Ca(V)1.2 alpha(1) subunit that is not duplicated by synthetic peptides corresponding to the putative binding motifs.

Neurotransmitter modulation of extracellular H+ fluxes from isolated retinal horizontal cells of the skate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular H(+) fluxes from horizontal cells isolated from the skate retina. A standing H(+) flux was detected from quiescent cells, indicating a higher concentration of free hydrogen ions near the extracellular surface of the cell as compared to the surrounding solution. The standing H(+) flux was reduced by removal of extracellular sodium or application of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting activity of a Na(+)-H(+) exchanger. Glutamate decreased H(+) flux, lowering the concentration of free hydrogen ions around the cell. AMPA/kainate receptor agonists mimicked the response, and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) eliminated the effects of glutamate and kainate. Metabotropic glutamate agonists were without effect. Glutamate-induced alterations in H(+) flux required extracellular calcium, and were abolished when cells were bathed in an alkaline Ringer solution. Increasing intracellular calcium by photolysis of the caged calcium compound NP-EGTA also altered extracellular H(+) flux. Immunocytochemical localization of the plasmalemma Ca(2+)-H(+)-ATPase (PMCA pump) revealed intense labelling within the outer plexiform layer and on isolated horizontal cells. Our results suggest that glutamate modulation of H(+) flux arises from calcium entry into cells with subsequent activation of the plasmalemma Ca(2+)-H(+)-ATPase. These neurotransmitter-induced changes in extracellular pH have the potential to play a modulatory role in synaptic processing in the outer retina. However, our findings argue against the hypothesis that hydrogen ions released by horizontal cells normally act as the inhibitory feedback neurotransmitter onto photoreceptor synaptic terminals to create the surround portion of the centre-surround receptive fields of retinal neurones.

New Short Splice Variants of the Human Cardiac Cavβ2 Subunit

Two new short splice variants of the Ca2+ channel beta2 subunit were cloned from human heart poly(A)(+) mRNA. The 410-amino acid beta2f subunit is encoded by exons 1A, 2A, 3, 4, 12, 13, and 14 of the human Cavbeta2 gene and lacks the protein kinase A phosphorylation site, the beta-interaction domain (De Waard, M., Pragnell, M., and Campbell, K. P. (1994) Neuron 13, 495-503), 40% of the beta-SH3 domain, and 73% of the guanylate kinase domain of the putative membrane-associated guanylate kinases module (McGee, A. W., Nunziato, D. A., Maltez, J. M., Prehoda, K. E., Pitt, G. S., and Bredt, D. S. (2004) Neuron 42, 89-99), and helix alpha3 of the alpha1-subunit binding pocket (Van Petegem F., Clark, K. A., Chatelain, F. C., and Minor, D. L., Jr. (2004) Nature 429, 671-675). The beta2g transcript has two potential initiation codons. With the second ATG codon, it generates the 164-amino acid beta2Deltag subunit encoded essentially by the distal part of exon 14, and thus beta2Deltag completely lacks any of the above motifs. Immunoprecipitation analysis confirmed stable association of beta2f and beta2Deltag with the alpha1C subunit. The plasma membrane localization of beta2f and beta2Deltag was substantially increased by co-expression of the alpha1C,77 and alpha2delta subunits. In COS1 cells, beta2f and beta2Deltag increased plasma membrane targeting of the pore-forming alpha1C subunit and differentially facilitated (beta2f > beta2Deltag) the voltage gating of otherwise silent Cav1.2 channels. We conclude that it is unlikely that the beta-interaction domain, membrane-associated guanylate kinases module, and the alpha1-subunit binding pocket helix alpha3 are essential for the interaction of the alpha1C and beta2 subunits and suggest that in addition to the alpha1-subunit binding pocket helices alpha5 and alpha8, a yet unresolved C-terminal beta2 region plays a crucial role.

Smooth muscle-selective alternatively spliced exon generates functional variation in Cav1.2 calcium channels

Voltage-gated calcium channels play a major role in many important processes including muscle contraction, neurotransmission, excitation-transcription coupling, and hormone secretion. To date, 10 calcium channel alpha(1)-subunits have been reported, of which four code for L-type calcium channels. In our previous work, we uncovered by transcript-scanning the presence of 19 alternatively spliced exons in the L-type Ca(v)1.2 alpha(1)-subunit. Here, we report the smooth muscle-selective expression of alternatively spliced exon 9(*) in Ca(v)1.2 channels found on arterial smooth muscle. Specific polyclonal antibody against exon 9(*) localized the intense expression of 9(*)-containing Ca(v)1.2 channels on the smooth muscle wall of arteries, but the expression on cardiac muscle was low. Whole-cell patch clamp recordings of the 9(*)-containing Ca(v)1.2 channels in HEK293 cells demonstrated -9 and -11-mV hyperpolarized shift in voltage-dependent activation and current-voltage relationships, respectively. The steady-state inactivation property and sensitivity to blockade by nifedipine of the +/-exon 9(*) splice variants were, however, not significantly different. Such cell-selective expression of an alternatively spliced exon strongly indicates the customization and fine tuning of calcium channel functions through alternative splicing of the pore-forming alpha(1)-subunit. The generation of proteomic variations by alternative splicing of the calcium channel Ca(v)1.2 alpha(1)-subunit can potentially provide a flexible mechanism for muscle or neuronal cells to respond to various physiological signals or to diseases.

Transcript scanning reveals novel and extensive splice variations in human l-type voltage-gated calcium channel, Cav1.2 alpha1 subunit

The L-type (Cav1.2) voltage-gated calcium channels play critical roles in membrane excitability, gene expression, and muscle contraction. The generation of splice variants by the alternative splicing of the poreforming Cav1.2 alpha1-subunit (alpha(1)1.2) may thereby provide potent means to enrich functional diversity. To date, however, no comprehensive scan of alpha(1)1.2 splice variation has been performed, particularly in the human context. Here we have undertaken such a screen, exploiting recently developed "transcript scanning" methods to probe the human gene. The degree of variation turns out to be surprisingly large; 19 of the 55 exons comprising the human alpha(1)1.2 gene were subjected to alternative splicing. Two of these are previously unrecognized exons and two others were not known to be spliced. Comparisons of fetal and adult heart and brain uncovered a large IVS3-S4 variability resulting from combinatorial utilization of exons 31-33. Electrophysiological characterization of such IVS3-S4 variation revealed unmistakable shifts in the voltage dependence of activation, according to an interesting correlation between increased IVS3-S4 linker length and activation at more depolarized potentials. Steady-state inactivation profiles remained unaltered. This systematic portrait of splice variation furnishes a reference library for comprehending combinatorial arrangements of Cav1.2 splice exons, especially as they impact development, physiology, and disease.

Multiple Structural Elements Contribute to the Slow Kinetics of the Cav3.3 T-type Channel

Molecular cloning and expression studies established the existence of three T-type Ca(2+) channel (Ca(v)3) alpha(1) subunits: Ca(v)3.1 (alpha(1G)), Ca(v)3.2 (alpha(1H)), and Ca(v)3.3 (alpha(1I)). Although all three channels are low voltage-activated, they display considerable differences in their kinetics, with Ca(v)3.1 and Ca(v)3.2 channels activating and inactivating much faster than Ca(v)3.3 channels. The goal of the present study was to determine the structural elements that confer the distinctively slow kinetics of Ca(v)3.3 channels. To address this question, a series of chimeric channels between Ca(v)3.1 and Ca(v)3.3 channels were constructed and expressed in Xenopus oocytes. Kinetic analysis showed that the slow activation and inactivation kinetics of the Ca(v)3.3 channel were not completely abolished by substitution with any one portion of the Ca(v)3.1 channel. Likewise, the Ca(v)3.1 channel failed to acquire the slow kinetics by simply adopting one portion of the Ca(v)3.3 channel. These findings suggest that multiple structural elements contribute to the slow kinetics of Ca(v)3.3 channels.

Repositioning of charged I-II loop amino acid residues within the electric field by beta subunit as a novel working hypothesis for the control of fast P/Q calcium channel inactivation

We have investigated the contribution of the Ca(v)beta subunits to the process of inactivation dependent of the I-II loop of Ca(v)alpha(2.1). Two amino acid residues located in the alpha1 interaction domain (AID) of the I-II loop of Ca(v)alpha(2.1) (Arg(387) and Glu(388)) have been directly implicated in voltage-dependent inactivation of this channel. Various point mutations of these residues disrupt the interaction between the I-II loop and the III-IV loop, and thereby modify the inactivation properties of the channel by accelerating its kinetics and shifting the steady-state inactivation curve towards hyperpolarized potentials. A similar disruption is produced by Ca(v)beta(4) subunit association with the I-II loop. Moreover, in the presence of Ca(v)beta(4) subunit, introducing negatively charged residues at positions 387 or 388 slows inactivation kinetics down, whereas introducing positive charges has the opposite effect. The shift of the steady-state inactivation curve is also amino acid charge-dependent. In contrast, mutation of Arg(387) or Glu(388) does not alter the differential regulation of the different Ca(v)beta isoforms on inactivation. These results suggest that the expression of Ca(v)beta(4) alters the contribution of charged residues at positions 387 and 388 to inactivation. We discuss these results with regard to the actual hypotheses on the mechanisms of calcium channel inactivation. We introduce the working concept that Ca(v)beta-subunits produce a conformational repositioning of charged AID residues within the electric field.

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.

Cav1.4alpha1 subunits can form slowly inactivating dihydropyridine-sensitive L-type Ca2+ channels lacking Ca2+-dependent inactivation

The neuronal L-type calcium channels (LTCCs) Cav1.2alpha1 and Cav1.3alpha1 are functionally distinct. Cav1.3alpha1 activates at lower voltages and inactivates more slowly than Cav1.2alpha1, making it suitable to support sustained L-type Ca2+ inward currents (ICa,L) and serve in pacemaker functions. We compared the biophysical and pharmacological properties of human retinal Cav1.4alpha1 using the whole-cell patch-clamp technique after heterologous expression in tsA-201 cells with other L-type alpha1 subunits. Cav1.4alpha1-mediated inward Ba2+ currents (IBa) required the coexpression of alpha2delta1 and beta3 or beta2a subunits and were detected in a lower proportion of transfected cells than Cav1.3alpha1. IBa activated at more negative voltages (5% activation threshold; -39mV; 15 mm Ba2+) than Cav1.2alpha1 and slightly more positive than Cav1.3alpha1. Voltage-dependent inactivation of IBa was slower than for Cav1.2alpha1 and Cav1.3alpha1( approximately 50% inactivation after 5 sec; alpha2delta1 + beta3 coexpression). Inactivation was not increased with Ca2+ as the charge carrier, indicating the absence of Ca2+-dependent inactivation. Cav1.4alpha1 exhibited voltage-dependent, G-protein-independent facilitation by strong depolarizing pulses. The dihydropyridine (DHP)-antagonist isradipine blocked Cav1.4alpha1 with approximately 15-fold lower sensitivity than Cav1.2alpha1 and in a voltage-dependent manner. Strong stimulation by the DHP BayK 8644 was found despite the substitution of an otherwise L-type channel-specific tyrosine residue in position 1414 (repeat IVS6) by a phenylalanine. Cav1.4alpha1 + alpha2delta1 + beta channel complexes can form LTCCs with intermediate DHP antagonist sensitivity lacking Ca2+-dependent inactivation. Their biophysical properties should enable them to contribute to sustained ICa,L at negative potentials, such as required for tonic neurotransmitter release in sensory cells and plateau potentials in spiking neurons.

Effect of hypoxia on calcium channels depends on extracellular calcium in CA1 hippocampal neurons

Our previous studies have shown that short lasting hypoxia induces an increase of Ca(2+) influx into the cell through high voltage-activated Ca(2+) channels in hippocampal neurons. This effect was abolished by removing of free Ca(2+) from intracellular solution. The aim of this study was to compare hypoxic responses at different extracellular Ca(2+) concentrations ([Ca(2+)](e)) in hippocampal neurons to ascertain whether the hypoxic sensitivity is restricted to Ca(2+) ions. Whole-cell patch-clamp recordings were made from acutely dissociated CA1 hippocampal neurons of rats. Polarographic method for measurements of O(2) partial pressure was used. Here we found that at 2 mM [Ca(2+)](e) the hypoxic effect was significant (up to approximately 94%), whereas [Ca(2+)](e) elevations to 5 and 15 mM resulted in gradual decreasing of the effect. We found, that total Ca(2+) charge carried into the cell under the hypoxia was similar at all [Ca(2+)](e), whereas Ca(2+) charge carried at normoxia was different for different [Ca(2+)](e), being larger at higher [Ca(2+)](e). These data indicated a saturation of the hypoxic effect due to limitation in the channel conductance. Therefore, we suggested that the hypoxic effect can be connected with increase of channel conductance, and the level of channel conductance at normoxia can determine the amplitude of hypoxic effect.