Expression of calcium channel subunits in the normal and diseased human myocardium

Pediatric Dilated Cardiomyopathy-Associated LRRC10 (Leucine-Rich Repeat-Containing 10) Variant Reveals LRRC10 as an Auxiliary Subunit of Cardiac L-Type Ca2+ Channels

BACKGROUND

Genetic causes of dilated cardiomyopathy (DCM) are incompletely understood. LRRC10 (leucine-rich repeat-containing 10) is a cardiac-specific protein of unknown function. Heterozygous mutations in LRRC10 have been suggested to cause DCM, and deletion of Lrrc10 in mice results in DCM.

METHODS AND RESULTS

Whole-exome sequencing was carried out on a patient who presented at 6 weeks of age with DCM and her unaffected parents, filtering for rare, deleterious, recessive, and de novo variants. Whole-exome sequencing followed by trio-based filtering identified a homozygous recessive variant in LRRC10, I195T. Coexpression of I195T LRRC10 with the L-type Ca2+ channel (Cav1.2, β2CN2, and α2δ subunits) in HEK293 cells resulted in a significant ≈0.5-fold decrease in ICa,L at 0 mV, in contrast to the ≈1.4-fold increase in ICa,L by coexpression of LRRC10 (n=9-12, P<0.05). Coexpression of LRRC10 or I195T LRRC10 did not alter the surface membrane expression of Cav1.2. LRRC10 coexpression with Cav1.2 in the absence of auxiliary β2CN2 and α2δ subunits revealed coassociation of Cav1.2 and LRRC10 and a hyperpolarizing shift in the voltage dependence of activation (n=6-9, P<0.05). Ventricular myocytes from Lrrc10-/- mice had significantly smaller ICa,L, and coimmunoprecipitation experiments confirmed association between LRRC10 and the Cav1.2 subunit in mouse hearts.

CONCLUSIONS

Examination of a patient with DCM revealed homozygosity for a previously unreported LRRC10 variant: I195T. Wild-type and I195T LRRC10 function as cardiac-specific subunits of L-type Ca2+ channels and exert dramatically different effects on channel gating, providing a potential link to DCM.

Transmural gradients in ion channel and auxiliary subunit expression

Evolution has acted to shape the action potential in different regions of the heart in order to produce a maximally stable and efficient pump. This has been achieved by creating regional differences in ion channel expression levels within the heart as well as differences between equivalent cardiac tissues in different species. These region- and species-dependent differences in channel expression are established by regulatory evolution, evolution of the regulatory mechanisms that control channel expression levels. Ion channel auxiliary subunits are obvious targets for regulatory evolution, in order to change channel expression levels and/or modify channel function. This review focuses on the transmural gradients of ion channel expression in the heart and the role that regulation of auxiliary subunit expression plays in generating and shaping these gradients.

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

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

Role of the cytoskeleton in communication between L-type Ca(2+) channels and mitochondria

The L-type Ca(2+) channel is the main route for Ca(2+) entry into cardiac myocytes, which is essential for the maintenance of cardiac excitation and contraction. Alterations in L-type Ca(2+) channel activity and Ca(2+) homeostasis have been implicated in the development of cardiomyopathies. Cardiac excitation and contraction is fuelled by ATP, synthesized predominantly by the mitochondria via the Ca(2+)-dependent process oxidative phosphorylation. Mitochondrial reactive oxygen species (ROS) are by-products of oxidative phosphorylation and are associated with the development of cardiac pathology. The cytoskeleton plays a role in the communication of signals from the plasma membrane to intracellular organelles. There is good evidence that both L-type Ca(2+) channel activity and mitochondrial function can be modulated by changes in the cytoskeletal network. Activation of the L-type Ca(2+) channel can regulate mitochondrial function through cytoskeletal proteins as a result of transmission of movement from the β(2)-subunit of the channel that occurs during activation and inactivation of the channel. An association between cytoskeletal proteins and the mitochondrial voltage-dependent anion channel (VDAC) may play a role in this response. The L-type Ca(2+) channel is the initiator of contraction in cardiac muscle and the VDAC is responsible for regulating mitochondrial ATP/ADP trafficking. This article presents evidence that a functional coupling between L-type Ca(2+) channels and mitochondria may assist in meeting myocardial energy demand on a beat-to-beat basis.

Direct interaction of CaVβ with actin up-regulates L-type calcium currents in HL-1 cardiomyocytes

Expression of the β-subunit (CaVβ) is required for normal function of cardiac L-type calcium channels, and its up-regulation is associated with heart failure. CaVβ binds to the α1 pore-forming subunit of L-type channels and augments calcium current density by facilitating channel opening and increasing the number of channels in the plasma membrane, by a poorly understood mechanism. Actin, a key component of the intracellular trafficking machinery, interacts with Src homology 3 domains in different proteins. Although CaVβ encompasses a highly conserved Src homology 3 domain, association with actin has not yet been explored. Here, using co-sedimentation assays and FRET experiments, we uncover a direct interaction between CaVβ and actin filaments. Consistently, single-molecule localization analysis reveals streaklike structures composed by CaVβ2 that distribute over several micrometers along actin filaments in HL-1 cardiomyocytes. Overexpression of CaVβ2-N3 in HL-1 cells induces an increase in L-type current without altering voltage-dependent activation, thus reflecting an increased number of channels in the plasma membrane. CaVβ mediated L-type up-regulation, and CaVβ-actin association is prevented by disruption of the actin cytoskeleton with cytochalasin D. Our study reveals for the first time an interacting partner of CaVβ that is directly involved in vesicular trafficking. We propose a model in which CaVβ promotes anterograde trafficking of the L-type channels by anchoring them to actin filaments in their itinerary to the plasma membrane.

Structure-function of proteins interacting with the α1 pore-forming subunit of high-voltage-activated calcium channels

Openings of high-voltage-activated (HVA) calcium channels lead to a transient increase in calcium concentration that in turn activate a plethora of cellular functions, including muscle contraction, secretion and gene transcription. To coordinate all these responses calcium channels form supramolecular assemblies containing effectors and regulatory proteins that couple calcium influx to the downstream signal cascades and to feedback elements. According to the original biochemical characterization of skeletal muscle Dihydropyridine receptors, HVA calcium channels are multi-subunit protein complexes consisting of a pore-forming subunit (α1) associated with four additional polypeptide chains β, α2, δ, and γ, often referred to as accessory subunits. Twenty-five years after the first purification of a high-voltage calcium channel, the concept of a flexible stoichiometry to expand the repertoire of mechanisms that regulate calcium channel influx has emerged. Several other proteins have been identified that associate directly with the α1-subunit, including calmodulin and multiple members of the small and large GTPase family. Some of these proteins only interact with a subset of α1-subunits and during specific stages of biogenesis. More strikingly, most of the α1-subunit interacting proteins, such as the β-subunit and small GTPases, regulate both gating and trafficking through a variety of mechanisms. Modulation of channel activity covers almost all biophysical properties of the channel. Likewise, regulation of the number of channels in the plasma membrane is performed by altering the release of the α1-subunit from the endoplasmic reticulum, by reducing its degradation or enhancing its recycling back to the cell surface. In this review, we discuss the structural basis, interplay and functional role of selected proteins that interact with the central pore-forming subunit of HVA calcium channels.

Efficient experimental design and analysis of real-time PCR assays

Real-time polymerase chain reaction (qPCR) is currently the standard for gene quantification studies and has been extensively used in large-scale basic and clinical research. The operational costs and technical errors can become a significant issue due to the large number of sample reactions. In this paper, we present an experimental design strategy and an analysis procedure that are more efficient requiring fewer sample reactions than the traditional approach. We verified mathematically and experimentally the new design on a well-characterized model, to evaluate the gene expression levels of CACNA1C and CACNA1G in hypertrophic ventricular myocytes induced by phenylephrine treatment.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Alpha1A-adrenergic receptor-directed autoimmunity induces left ventricular damage and diastolic dysfunction in rats

BACKGROUND

Agonistic autoantibodies to the alpha(1)-adrenergic receptor occur in nearly half of patients with refractory hypertension; however, their relevance is uncertain.

METHODS/PRINCIPAL FINDINGS

We immunized Lewis rats with the second extracellular-loop peptides of the human alpha(1A)-adrenergic receptor and maintained them for one year. Alpha(1A)-adrenergic antibodies (alpha(1A)-AR-AB) were monitored with a neonatal cardiomyocyte contraction assay by ELISA, and by ERK1/2 phosphorylation in human alpha(1A)-adrenergic receptor transfected Chinese hamster ovary cells. The rats were followed with radiotelemetric blood pressure measurements and echocardiography. At 12 months, the left ventricles of immunized rats had greater wall thickness than control rats. The fractional shortening and dp/dt(max) demonstrated preserved systolic function. A decreased E/A ratio in immunized rats indicated a diastolic dysfunction. Invasive hemodynamics revealed increased left ventricular end-diastolic pressures and decreased dp/dt(min). Mean diameter of cardiomyocytes showed hypertrophy in immunized rats. Long-term blood pressure values and heart rates were not different. Genes encoding sarcomeric proteins, collagens, extracellular matrix proteins, calcium regulating proteins, and proteins of energy metabolism in immunized rat hearts were upregulated, compared to controls. Furthermore, fibrosis was present in immunized hearts, but not in control hearts. A subset of immunized and control rats was infused with angiotensin (Ang) II. The stressor raised blood pressure to a greater degree and led to more cardiac fibrosis in immunized, than in control rats.

CONCLUSIONS/SIGNIFICANCE

We show that alpha(1A)-AR-AB cause diastolic dysfunction independent of hypertension, and can increase the sensitivity to Ang II. We suggest that alpha(1A)-AR-AB could contribute to cardiovascular endorgan damage.

PIKfyve regulates CaV1.2 degradation and prevents excitotoxic cell death

Voltage-gated Ca(2+) channels (VGCCs) play a key role in neuronal signaling but can also contribute to cellular dysfunction and death under pathological conditions such as stroke and neurodegenerative diseases. We report that activation of N-methyl-D-aspartic acid receptors causes internalization and degradation of Ca(V)1.2 channels, resulting in decreased Ca(2+) entry and reduced toxicity. Ca(V)1.2 internalization and degradation requires binding to phosphatidylinositol 3-phosphate 5-kinase (PIKfyve), a lipid kinase which generates phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P(2)) and regulates endosome and lysosome function. Sustained activation of glutamate receptors recruits PIKfyve to Ca(V)1.2 channels, increases cellular levels of PtdIns(3,5)P(2), and promotes targeting of Ca(V)1.2 to lysosomes. Knockdown of PIKfyve prevents Ca(V)1.2 degradation and increases neuronal susceptibility to excitotoxicity. These experiments identify a novel mechanism by which neurons are protected from excitotoxicity and provide a possible explanation for neuronal death in diseases caused by mutations that affect PtdIns(3,5)P(2) regulation.

Potential relevance of alpha(1)-adrenergic receptor autoantibodies in refractory hypertension

Background

Agonistic autoantibodies directed at the α₁-adrenergic receptor (α₁-AAB) have been described in patients with hypertension. We implied earlier that α₁-AAB might have a mechanistic role and could represent a therapeutic target.

Methodology/Principal Findings

To pursue the issue, we performed clinical and basic studies. We observed that 41 of 81 patients with refractory hypertension had α₁-AAB; after immunoadsorption blood pressure was significantly reduced in these patients. Rabbits were immunized to generate α₁-adrenergic receptor antibodies (α₁-AB). Patient α₁-AAB and rabbit α₁-AB were purified using affinity chromatography and characterized both by epitope mapping and surface plasmon resonance measurements. Neonatal rat cardiomyocytes, rat vascular smooth muscle cells (VSMC), and Chinese hamster ovary cells transfected with the human α_1A-adrenergic receptor were incubated with patient α₁-AAB and rabbit α₁-AB and the activation of signal transduction pathways was investigated by Western blot, confocal laser scanning microscopy, and gene expression. We found that phospholipase A2 group IIA (PLA2-IIA) and L-type calcium channel (Cacna1c) genes were upregulated in cardiomyocytes and VSMC after stimulation with both purified antibodies. We showed that patient α₁-AAB and rabbit α₁-AB result in protein kinase C alpha activation and transient extracellular-related kinase (EKR1/2) phosphorylation. Finally, we showed that the antibodies exert acute effects on intracellular Ca²⁺ in cardiomyocytes and induce mesentery artery segment contraction.

Conclusions/Significance

Patient α₁-AAB and rabbit α₁-AB can induce signaling pathways important for hypertension and cardiac remodeling. Our data provide evidence for a potential clinical relevance for α₁-AAB in hypertensive patients, and the notion of immunity as a possible cause of hypertension.

The calcium channel beta2 (CACNB2) subunit repertoire in teleosts

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Increased expression of the auxiliary beta(2)-subunit of ventricular L-type Ca(2)+ channels leads to single-channel activity characteristic of heart failure

Background

Increased activity of single ventricular L-type Ca²⁺-channels (L-VDCC) is a hallmark in human heart failure. Recent findings suggest differential modulation by several auxiliary β-subunits as a possible explanation.

Methods and Results

By molecular and functional analyses of human and murine ventricles, we find that enhanced L-VDCC activity is accompanied by altered expression pattern of auxiliary L-VDCC β-subunit gene products. In HEK293-cells we show differential modulation of single L-VDCC activity by coexpression of several human cardiac β-subunits: Unlike β₁ or β₃ isoforms, β_2a and β_2b induce a high-activity channel behavior typical of failing myocytes. In accordance, β₂-subunit mRNA and protein are up-regulated in failing human myocardium. In a model of heart failure we find that mice overexpressing the human cardiac Ca_V1.2 also reveal increased single-channel activity and sarcolemmal β₂ expression when entering into the maladaptive stage of heart failure. Interestingly, these animals, when still young and non-failing (“Adaptive Phase”), reveal the opposite phenotype, viz: reduced single-channel activity accompanied by lowered β₂ expression. Additional evidence for the cause-effect relationship between β₂-subunit expression and single L-VDCC activity is provided by newly engineered, double-transgenic mice bearing both constitutive Ca_V1.2 and inducible β₂ cardiac overexpression. Here in non-failing hearts induction of β₂-subunit overexpression mimicked the increase of single L-VDCC activity observed in murine and human chronic heart failure.

Conclusions

Our study presents evidence of the pathobiochemical relevance of β₂-subunits for the electrophysiological phenotype of cardiac L-VDCC and thus provides an explanation for the single L-VDCC gating observed in human and murine heart failure.

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

L-type calcium channels are composed of a pore, alpha1c (Ca(V)1.2), and accessory beta- and alpha2delta-subunits. The beta-subunit core structure was recently resolved at high resolution, providing important information on many functional aspects of channel modulation. In this study we reveal differential novel effects of five beta2-subunits isoforms expressed in human heart (beta(2a-e)) on the single L-type calcium channel current. These splice variants differ only by amino-terminal length and amino acid composition. Single-channel modulation by beta2-subunit isoforms was investigated in HEK293 cells expressing the recombinant L-type ion conducting pore. All beta2-subunits increased open probability, availability, and peak current with a highly consistent rank order (beta2a approximately = beta2b > beta2e approximately = beta2c > beta2d). We show graded modulation of some transition rates within and between deep-closed and inactivated states. The extent of modulation correlates strongly with the length of amino-terminal domains. Two mutant beta2-subunits that imitate the natural span related to length confirm this conclusion. The data show that the length of amino termini is a relevant physiological mechanism for channel closure and inactivation, and that natural alternative splicing exploits this principle for modulation of the gating properties of calcium channels.

The L-type calcium channel in the heart: the beat goes on

Sydney Ringer would be overwhelmed today by the implications of his simple experiment performed over 120 years ago showing that the heart would not beat in the absence of Ca2+. Fascination with the role of Ca2+ has proliferated into all aspects of our understanding of normal cardiac function and the progression of heart disease, including induction of cardiac hypertrophy, heart failure, and sudden death. This review examines the role of Ca2+ and the L-type voltage-dependent Ca2+ channels in cardiac disease.

T-type Ca2+ channels are involved in high glucose-induced rat neonatal cardiomyocyte proliferation

Infants develop hypertrophic cardiomyopathy in approximately 30% of diabetic pregnancies. We have characterized the effects of glucose on voltage-gated T-type Ca2+ channels and intracellular free calcium concentration, [Ca2+]i in neonatal rat cardiomyocytes. We found that T-type Ca2+ channel current density increased significantly in primary culture neonatal cardiac myocytes that were treated with 25 mM glucose for 48 h when compared with those that were treated with 5 mM glucose. High-glucose treatment also caused a higher Ca2+ influx elicited by 50 mM KCl in the myocytes. KCl-induced Ca2+ influx was attenuated when nickel was present. Real-time PCR studies demonstrated that mRNA levels of both alpha1G (Ca(v)3.1) and alpha1H (Ca(v)3.2) T-type Ca2+ channels were elevated after high-glucose treatment. High-glucose also significantly increased ventricular cell proliferation as well as the proportion of cells in the S-phase of the cell cycle; both effects were reversed by nickel or mibefradil. These results indicate that high glucose causes a rise in [Ca2+]i in neonatal cardiac myocytes by a mechanism that is associated with the regulation of the T-type Ca2+ channel activity.

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.

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

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

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

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

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

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

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

A Ca(2+)-dependent transgenic model of cardiac hypertrophy: A role for protein kinase Calpha

BACKGROUND

Calcium imbalances have been implicated as an underlying mechanism of human cardiac dysfunction. The voltage-dependent calcium channel plays a critical role in calcium regulation in the heart. Thus, aberrant calcium signaling arising from this channel could initiate the calcium imbalances observed in heart failure. In the present study, we used a transgenic mouse with an increased number of L-type calcium channels to identify the role of an increased, sustained ingress of calcium as an initiator of hypertrophy.

METHODS AND RESULTS

Whole-heart histology and electrophysiology in isolated cardiomyocytes identified calcium-channel overexpression in the hearts of transgenic mice. Calcium-channel density was increased in 2-, 4-, and 8-month-old transgenic cardiomyocytes. Ventricular fibrosis, damage, and remodeling became more pronounced as the transgenic mice aged. Apoptosis was also present in transgenic hearts at 8 months of age. Increased protein kinase Calpha activation was elevated before the development of hypertrophy and failure.

CONCLUSIONS

Transgenic mice developed hypertrophy and severe cardiomyopathy as a function of age, thus confirming that changes in channel density are sufficient to induce disease. The small, sustained increase in the ingress of Ca(2+) through the calcium channel elevated protein kinase Calpha before the development of hypertrophy, suggesting that protein kinase Calpha plays an important role in triggering hypertrophy.

Transcriptional regulation of L-type calcium channel expression in cardiac myocytes

The L-type calcium channel is a heteromultimeric protein complex, which is expressed in the cardiac sarcolemma. Although post-translational regulation of its subunits by protein kinase A (PKA) has been widely reported, little is known about molecular processes that regulate expression of calcium channel subunits (alpha(1C), alpha(2)- delta, and beta(2A)subunits). Previous studies from our group demonstrate that the steady-state mRNA level of the alpha(1C)unit is increased by treatment of myocytes with beta -adrenergic agonists. The current study is designed to determine whether the mRNA levels for all subunits of the L-type calcium channel are coordinately controlled by a beta -adrenergic agonist, and whether this occurs predominantly through control of rate of transcription. Nuclear run-on assays were used to determine the transcription initiation rate of these genes in cultured neonatal rat cardiac myocytes. In isoproterenol (10(-7)m)-treated myocytes, transcription of genes encoding the alpha(1C), alpha(2)- delta, and beta(2A)subunits was enhanced. The increases in transcription initiation rate for alpha(1C), alpha(2)- delta, and beta(2A)subunits genes were 404%, 367%, and 240% of control, respectively. Pretreatment with the beta -adrenergic antagonist propranolol (10(-5)m) or PKA inhibitor H-89 (10(-6)m) blocked the effects of isoproterenol, while either drug alone did not affect the gene transcription rate significantly. Steady state mRNA levels of the subunits increased following isoproterenol treatment. These results suggest that beta -adrenergic stimulation and the PKA signaling pathway play an important role in transcriptional regulation of the L-type calcium channel in myocyte. The expression of all the subunits of this ion channel is under coordinate transcriptional control.

Developmental regulation of the L-type calcium channel alpha1C subunit expression in heart

We used Northern analyses, RNase protection assays and immunoblot analyses to examine the relationship among developmental age of the heart, abundance of mRNA and L-type calcium channel alpha1C subunit protein, and to establish the size of the native protein in heart. Northern analysis, RNase protection assays, and immunoblots were used to study RNA and protein from rat heart of various ages. In fetal and adult ventricles there was a predominant 8.3-kb transcript for the alpha1C subunit with no change in transcript size during development. RNase protection assays demonstrated a 2-fold increase in abundance of the DHP receptor message during postnatal development. Immunoblots identified a 240 kD protein, corresponding to the predicted molecular mass of the full length alpha1C subunit. No change in size of protein for the alpha1C subunit was observed at any developmental stage and there was no evidence for a truncated isoform. There was an approximate 2-fold increase in alpha1C subunit protein in ventricular homogenates during postnatal development. Thus, in the developing rat heart, alterations in calcium channel properties during development appear to result neither from alternative splicing that produces a smaller transcript for the alpha1C subunit nor from expression of a truncated protein, but at least in part from transcriptionally-regulated expression of the 240 kDa polypepde.

Expression of Ca(2+) channel subunits during cardiac ontogeny in mice and rats: identification of fetal alpha(1C) and beta subunit isoforms

Functional cardiac L-type calcium channels are composed of the pore-forming alpha(1C) subunit and the regulatory beta(2) and alpha(2)/delta subunits. To investigate possible developmental changes in calcium channel composition, we examined the temporal expression pattern of alpha(1C) and beta(2) subunits during cardiac ontogeny in mice and rats, using sequence-specific antibodies. Fetal and neonatal hearts showed two size forms of alpha(1C) with 250 and 220 kDa. Quantitative immunoblotting revealed that the rat cardiac 250-kDa alpha(1C) subunit increased about 10-fold from fetal days 12-20 and declined during postnatal maturation, while the 220-kDa alpha(1C) decreased to undetectable levels. The expression profile of the 85-kDa beta(2) subunit was completely different: beta(2) was not detected at fetal day 12, rose in the neonatal stage, and persisted during maturation. Additional beta(2)-stained bands of 100 and 90 kDa were detected in fetal and newborn hearts, suggesting the transient expression of beta(2) subunit variants. Furthermore, two fetal proteins with beta(4) immunoreactivity were identified in rat hearts that declined during prenatal development. In the fetal rat heart, beta(4) gene expression was confirmed by RT-PCR. Cardiac and brain beta(4) mRNA shared the 3 prime region, predicting identical primary sequences between amino acid residues 62-519, diverging however, at the 5 prime portion. The data indicate differential developmental changes in the expression of Ca(2+) channel subunits and suggest a role of fetal alpha(1C) and beta isoforms in the assembly of Ca(2+) channels in immature cardiomyocytes.

Signaling from beta-adrenoceptor to L-type calcium channel: identification of a novel cardiac protein kinase A target possessing similarities to AHNAK

A novel calcium channel-associated protein of approximately 700 kDa has been identified in mammalian cardiomyocytes that undergoes substantial cAMP-dependent protein kinase (PKA) phosphorylation. It was therefore designated as phosphoprotein 700 (pp700). The pp700 interacts specifically with the beta(2) subunit of cardiac L-type calcium channels as revealed by coprecipitation experiments using affinity-purified antibodies against different calcium channel subunits. It is surprising that amino acid sequence analysis of pig pp700 revealed homology to AHNAK-encoded protein, which was originally identified in human cell lines of neural crest origin as 700-kDa phosphoprotein. Cardiac AHNAK expression was assessed on mRNA level by reverse transcriptase-polymerase chain reaction. Sequence-directed antibodies raised against human AHNAK recognized pp700 in immunoblotting and immunoprecipitation experiments, confirming the homology between both proteins. Anti-AHNAK antibodies labeled preferentially the plasma membrane of cardiomyocytes in cryosections of rat cardiac tissue and isolated cardiomyocytes. Sarcolemmal pp700/AHNAK localization was not influenced by stimulation of either the PKA or the protein kinase C pathway. In back-phosphorylation studies with cardiac biopsies, we identified distinct pp700 pools. The membrane-associated fraction of pp700 underwent substantial in vivo phosphorylation on beta-adrenergic receptor stimulation by isoproterenol, whereas the cytoplasmic fraction of pp700 was not accessible to endogenous PKA. It is important that in vivo phosphorylation occurred in that pp700 fraction which coprecipitated with the calcium channel beta subunit. We hypothesize that both phosphorylation of pp700 and its coupling to the beta subunit play a physiological role in cardiac beta-adrenergic signal transduction. Haase, H., Podzuweit, T., Lutsch, G., Hohaus, A., Kostka, S., Lindschau, C., Kott, M., Kraft, R., Morano, I. Signaling from beta-adrenoceptor to L-type calcium channel: identification of a novel cardiac protein kinase A target that has similarities to AHNAK.

Subunit expression of the cardiac L-type calcium channel is differentially regulated in diastolic heart failure of the cardiac allograft

BACKGROUND

Left ventricular diastolic dysfunction is a major cause of cardiac allograft failure. Multimeric L-type calcium channels (alpha1-, alpha2/delta-, and beta-subunits) are essential for excitation/contraction coupling in the heart. Their gene expression was studied in allografts that developed diastolic heart failure.

METHODS AND RESULTS

mRNA levels of calcium channel subunits were measured by competitive reverse transcriptase-polymerase chain reaction in microbiopsy samples from the interventricular septum. Size and tissue variabilities between biopsy samples were assessed by determination of cardiac calsequestrin mRNA levels. In the cardiac allografts studied, mRNA levels in microbiopsy samples were considered to represent left ventricular gene expression, because septal and left ventricular gene expression in Northern blots was equivalent, and left ventricles contracted homogeneously. Biopsy samples (n=72) were taken from allografts with normal left ventricular end-diastolic pressure (LVEDP; 8 to 13 mm Hg; n=30), moderately elevated LVEDP (14 to 18 mm Hg; n=26), and elevated LVEDP (19 to 28 mm Hg; n=16). Increased LVEDP was related to slowed diastolic relaxation determined by the time constant tau (r2=0.86), whereas systolic performance (dP/dt; ejection fraction) was preserved. With increasing LVEDP, mRNA levels of the pore-forming alpha1c-subunit (n=15) and of the regulatory alpha2/delta-subunit (n=17) remained unchanged but decreased exponentially (r2=-0.83) for the regulatory beta-subunit (n=40). Compared with cardiac allografts with normal LVEDP (n=15), beta-subunit mRNA level was reduced by 75% at elevated LVEDP (n=9; P=0.012). In an explanted, diastolically failing cardiac allograft, beta-subunit expression was reduced correspondingly by 72% and 76% on the mRNA level in septal and left ventricular myocardium and by 80% on the protein level.

CONCLUSIONS

The downregulated expression of the calcium channel beta-subunit might contribute to altered calcium handling in diastolically failing cardiac allografts.

L-type calcium channel expression depends on the differentiated state of vascular smooth muscle cells

Despite intensive interest in understanding the differentiation of vascular smooth muscle cells (VSMC), no information is available about differential regulation of ion channels in these cells. Since expression of the L-type Ca2+ channel can be influenced by differentiation in other cell types, we tested the hypothesis that the L-type (C class) channel is a specific differentiation marker of VSMC and that expression of these channels depends on the state of cell differentiation. We used rat aortic (A7r5) VSMC, which express functional L-type Ca2+ channels, and induced dedifferentiation by cell culture in different media. Treatment with retinoic acid was used to redifferentiate the VSMC. We characterized the differentiated state of the cells by using immunohistochemistry and Western blot analysis for smooth muscle (SM) alpha-actin and SM-myosin heavy chain (MHC). The number of functional Ca2+ channels was significantly decreased in dedifferentiated VSMC and increased upon differentiation with retinoic acid. Ca2+ channel function was assessed by whole-cell voltage clamp techniques. Using Western blot and dihydropyridine binding analysis, we found that the expression of the Ca2+ channel alpha1 subunit, and to a lesser extent the beta2 subunit, was directly correlated with the expression of SM alpha-actin and SM-MHC. We conclude that expression of L-type Ca2+ channel alpha1 subunits, and thus a functional Ca2+ channel, is highly coordinated with expression of the SM-specific proteins required for specialized smooth muscle cell functions. Furthermore, our results demonstrate that the L-type Ca2+ channel is a novel marker for differentiation of VSMC. The data suggest that regulation of ion channel expression during differentiation may have physiological importance for normal smooth muscle function and may influence VSMC behavior under pathophysiological conditions.

Beta1B subunit of voltage-dependent Ca2+ channels is predominant isoform expressed in human neuroblastoma cell line IMR32

Human neuroblastoma cells (IMR32) respond to treatment with either dibutyryl-cAMP or nerve factor by acquiring a neuronal phenotype which is accompanied by a marked increase in the density of neuronal (N-type) VDCC currents. Using IMR32 cells as a model for neuronal differentiation, we were interested in examining possible changes in the level of expression of the alpha1B subunit of N-type calcium channels as well as beta subunit isoforms. Upon differentiation with dibutyryl-cAMP and 5-bromo-2-deoxyuridine for 16 days, we observed a dramatic increase in alpha1B protein which initiated between day 8 and 10. Day 10 evidenced maximal expression of alpha1B protein, which was followed by an interval of relatively constant expression of alpha1B (day 12 to day 16). Monitoring beta subunit expression using a pan specific anti-beta antibody (Ab CW20), we observed an increase in expression of a single 82 kDa beta subunit. The predominant 82 kDa beta subunit expressed throughout the course of differentiation was identified as the beta1b isoform using a panel of beta subunit specific antibodies. Of significance, neither the beta2 nor beta3 isoforms were detected in full differentiated IMR32 cells. Contrary to a previous report on the absence of neurotypic expression of VDCC beta subunits in a second model for in vitro differentiation, NGF-treated rat pheochromocytoma cells (PC12 cells) [1], we report the regulated expression of the beta1b protein in differentiated IMR32 cells suggesting a cell specific function for this beta subunit which parallels the acquisition of the neuronal phenotype. The restrictive expression of the beta1b in IMR32 cells may reflect a cell-type specific function that extends beyond its role as an auxiliary subunit of VDCC complexes.