Cardiac-specific overexpression of the alpha(1) subunit of the L-type voltage-dependent Ca(2+) channel in transgenic mice. Loss of isoproterenol-induced contraction

Opposite effects of Gαi2 or Gαi3 deficiency on reduced basal density and attenuated β-adrenergic response of ventricular Ca2+ currents in myocytes of mice overexpressing the cardiac β1-adrenoceptor

Ca2+ currents (ICaL) carried by ventricular L-type Ca2+ channels (LTCC) are altered in failing hearts, and increased LTCC activity is discussed as a cause of cardiomyopathy. We have shown that lack of the inhibitory G-protein isoform Gαi3 improves cardiac outcome and survival in a murine heart-failure model of cardiac β1-adrenoceptor (β1-AR) overexpression (β1-tg), while lack of the Gαi2 isoform was detrimental in the same heart-failure model. Given the potential role of LTCC and their modulation by β-adrenergic signalling, we now analysed ventricular ICaL in β1-tg mice and in β1-tg mice lacking either Gαi2 or Gαi3. Using the patch-clamp technique, we recorded whole-cell ICaL in ventricular myocytes freshly isolated from adult mice. Compared to age-matched wild-type littermates, basal ICaL was reduced in myocytes from β1-tg mice both under basal conditions (- 8.1 ± 1.6 vs. - 5.5 ± 1.5 pA/pF) and upon β-adrenergic stimulation with 1 µM isoproterenol (- 14.3 ± 5.6 vs. - 7.4 ± 1.9 pA/pF). Lack of Gαi3 normalised basal ICaL to nearly wild-type levels (- 7.5 ± 1.6 pA/pF), while β-adrenergic response remained attenuated (- 9.5 ± 3.6 pA/pF). In contrast, the absence of Gαi2 did not restore basal ICaL (- 5.7 ± 1.8 pA/pF), but restored the β-adrenergic response of ICaL, with the difference from basal current even exceeding that in wild-type mice (- 12.2 ± 2.9 pA/pF).We propose that by restoring basal ICaL, Gαi3 deficiency might contribute to the restoration of contractility in β1-tg mice, while maintaining attenuation of the ICaL response upon β-adrenergic stimulation protects against deleterious effects mediated by enhanced β-AR signalling. In contrast, restored and even enhanced ICaL response to β-adrenergic stimulation might contribute to detrimental effects of Gαi2 deficiency observed in β1-tg mice previously.

Protective effects of Gαi3 deficiency in a murine heart-failure model of β1-adrenoceptor overexpression

We have shown that in murine cardiomyopathy caused by overexpression of the β1-adrenoceptor, Gαi2-deficiency is detrimental. Given the growing evidence for isoform-specific Gαi-functions, we now examined the consequences of Gαi3 deficiency in the same heart-failure model. Mice overexpressing cardiac β1-adrenoceptors with (β1-tg) or without Gαi3-expression (β1-tg/Gαi3-/-) were compared to C57BL/6 wildtypes and global Gαi3-knockouts (Gαi3-/-). The life span of β1-tg mice was significantly shortened but improved when Gαi3 was lacking (95% CI: 592-655 vs. 644-747 days). At 300 days of age, left-ventricular function and survival rate were similar in all groups. At 550 days of age, β1-tg but not β1-tg/Gαi3-/- mice displayed impaired ejection fraction (35 ± 18% vs. 52 ± 16%) compared to wildtype (59 ± 4%) and Gαi3-/- mice (60 ± 5%). Diastolic dysfunction of β1-tg mice was prevented by Gαi3 deficiency, too. The increase of ANP mRNA levels and ventricular fibrosis observed in β1-tg hearts was significantly attenuated in β1-tg/Gαi3-/- mice. Transcript levels of phospholamban, ryanodine receptor 2, and cardiac troponin I were similar in all groups. However, Western blots and phospho-proteomic analyses showed that in β1-tg, but not β1-tg/Gαi3-/- ventricles, phospholamban protein was reduced while its phosphorylation increased. Here, we show that in mice overexpressing the cardiac β1-adrenoceptor, Gαi3 deficiency slows or even prevents cardiomyopathy and increases shortened life span. Previously, we found Gαi2 deficiency to aggravate cardiac dysfunction and mortality in the same heart-failure model. Our findings indicate isoform-specific interventions into Gi-dependent signaling to be promising cardio-protective strategies.

Shexiang Baoxin Pills Could Alleviate Isoproterenol-Induced Heart Failure Probably through its Inhibition of CaV1.2 Calcium Channel Currents

Heart failure (HF) affects millions of patients in the world. Shexiang Baoxin Pills (SXB) are extensively applied to treat coronary artery diseases and HF in Chinese hospitals. However, there are still no explanations for why SXB protects against HF. To assess the protective role, we created the HF model in rats by isoproterenol (ISO) subcutaneous injection, 85 milligrams per kilogram body weight for seven days. Four groups were implemented: CON (control), ISO (HF disease group), CAP (captopril, positive drug treatment), and SXB groups. Echocardiography was used to evaluate rats’ HF in vivo. The human CaV1.2 (hCaV1.2) channel currents were detected in tsA-201 cells by patch clamp technique. Five different concentrations of SXB (5, 10, 30, 50, and 100 mg/L) were chosen in this study. The results showed that SXB increased cardiac systolic function and inhibited rats’ cardiac hypertrophy and myocardial fibrosis induced by ISO. Subsequently, it was found that SXB was inhibited by the peak amplitudes of hCaV1.2 channel current ( $P<0.01$ ). The SXB half inhibitory dosage was 9.09 mg/L. The steady-state activation curve was 22.8 mV depolarization shifted; while the inactivation curve and the recovery from inactivation were not affected significantly. In conclusion, these results indicated that SXB inhibited ISO-induced HF in rats and inhibited the hCaV1.2 channel current. The present study paved the way for SXB to protect itself from HF.

Adrenergic Regulation of Calcium Channels in the Heart

Each heartbeat is initiated by the action potential, an electrical signal that depolarizes the plasma membrane and activates a cycle of calcium influx via voltage-gated calcium channels, calcium release via ryanodine receptors, and calcium reuptake and efflux via calcium-ATPase pumps and sodium-calcium exchangers. Agonists of the sympathetic nervous system bind to adrenergic receptors in cardiomyocytes, which, via cascading signal transduction pathways and protein kinase A (PKA), increase the heart rate (chronotropy), the strength of myocardial contraction (inotropy), and the rate of myocardial relaxation (lusitropy). These effects correlate with increased intracellular concentration of calcium, which is required for the augmentation of cardiomyocyte contraction. Despite extensive investigations, the molecular mechanisms underlying sympathetic nervous system regulation of calcium influx in cardiomyocytes have remained elusive over the last 40 years. Recent studies have uncovered the mechanisms underlying this fundamental biologic process, namely that PKA phosphorylates a calcium channel inhibitor, Rad, thereby releasing inhibition and increasing calcium influx. Here, we describe an updated model for how signals from adrenergic agonists are transduced to stimulate calcium influx and contractility in the heart.

L-type channel inactivation balances the increased peak calcium current due to absence of Rad in cardiomyocytes

The L-type Ca²⁺ channel (LTCC) provides trigger calcium to initiate cardiac contraction in a graded fashion that is regulated by L-type calcium current (I_Ca,L) amplitude and kinetics. Inactivation of LTCC is controlled to fine-tune calcium flux and is governed by voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). Rad is a monomeric G protein that regulates I_Ca,L and has recently been shown to be critical to β-adrenergic receptor (β-AR) modulation of I_Ca,L. Our previous work showed that cardiomyocyte-specific Rad knockout (cRadKO) resulted in elevated systolic function, underpinned by an increase in peak I_Ca,L, but without pathological remodeling. Here, we sought to test whether Rad-depleted LTCC contributes to the fight-or-flight response independently of β-AR function, resulting in I_Ca,L kinetic modifications to homeostatically balance cardiomyocyte function. We recorded whole-cell I_Ca,L from ventricular cardiomyocytes from inducible cRadKO and control (CTRL) mice. The kinetics of I_Ca,L stimulated with isoproterenol in CTRL cardiomyocytes were indistinguishable from those of unstimulated cRadKO cardiomyocytes. CDI and VDI are both enhanced in cRadKO cardiomyocytes without differences in action potential duration or QT interval. To confirm that Rad loss modulates LTCC independently of β-AR stimulation, we crossed a β₁,β₂-AR double-knockout mouse with cRadKO, resulting in a Rad-inducible triple-knockout mouse. Deletion of Rad in cardiomyocytes that do not express β₁,β₂-AR still yielded modulated I_Ca,L and elevated basal heart function. Thus, in the absence of Rad, increased Ca²⁺ influx is homeostatically balanced by accelerated CDI and VDI. Our results indicate that the absence of Rad can modulate the LTCC without contribution of β₁,β₂-AR signaling and that Rad deletion supersedes β-AR signaling to the LTCC to enhance in vivo heart function.

The quest to identify the mechanism underlying adrenergic regulation of cardiac Ca2+ channels

Activation of protein kinase A by cyclic AMP results in a multi-fold upregulation of CaV1.2 currents in the heart, as originally reported in the 1970's and 1980's. Despite considerable interest and much investment, the molecular mechanisms responsible for this signature modulation remained stubbornly elusive for over 40 years. A key manifestation of this lack of understanding is that while this regulation is readily apparent in heart cells, it has not been possible to reconstitute it in heterologous expression systems. In this review, we describe the efforts of many investigators over the past decades to identify the mechanisms responsible for the β-adrenergic mediated activation of voltage-gated Ca2+ channels in the heart and other tissues.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

Myocardial-restricted ablation of the GTPase RAD results in a pro-adaptive heart response in mice

Existing therapies to improve heart function target β-adrenergic receptor (β-AR) signaling and Ca2+ handling and often lead to adverse outcomes. This underscores an unmet need for positive inotropes that improve heart function without any adverse effects. The GTPase Ras associated with diabetes (RAD) regulates L-type Ca2+ channel (LTCC) current (ICa,L). Global RAD-knockout mice (gRAD-/-) have elevated Ca2+ handling and increased cardiac hypertrophy, but RAD is expressed also in noncardiac tissues, suggesting the possibility that pathological remodeling is due also to noncardiac effects. Here, we engineered a myocardial-restricted inducible RAD-knockout mouse (RADΔ/Δ). Using an array of methods and techniques, including single-cell electrophysiological and calcium transient recordings, echocardiography, and radiotelemetry monitoring, we found that RAD deficiency results in a sustained increase of inotropy without structural or functional remodeling of the heart. ICa,L was significantly increased, with RAD loss conferring a β-AR-modulated phenotype on basal ICa,L Cardiomyocytes from RADΔ/Δ hearts exhibited enhanced cytosolic Ca2+ handling, increased contractile function, elevated sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2a) expression, and faster lusitropy. These results argue that myocardial RAD ablation promotes a beneficial elevation in Ca2+ dynamics, which would obviate a need for increased β-AR signaling to improve cardiac function.

β-adrenergic-mediated dynamic augmentation of sarcolemmal CaV 1.2 clustering and co-operativity in ventricular myocytes

Key points

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Abstract

Voltage‐dependent L‐type Ca_V1.2 channels play an indispensable role in cardiac excitation–contraction coupling. Activation of the β‐adrenergic receptor (βAR)/cAMP/protein kinase A (PKA) signalling pathway leads to enhanced Ca_V1.2 activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. Ca_V1.2 channels exhibit a clustered distribution along the T‐tubule sarcolemma of ventricular myocytes where nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by dynamic, physical, allosteric interactions between adjacent channel C‐terminal tails. This amplifies Ca²⁺ influx and augments myocyte Ca²⁺ transient and contraction amplitudes. We investigated whether βAR signalling could alter Ca_V1.2 channel clustering to facilitate co‐operative channel interactions and elevate Ca²⁺ influx in ventricular myocytes. Bimolecular fluorescence complementation experiments reveal that the βAR agonist, isoproterenol (ISO), promotes enhanced Ca_V1.2–Ca_V1.2 physical interactions. Super‐resolution nanoscopy and dynamic channel tracking indicate that these interactions are expedited by enhanced spatial proximity between channels, resulting in the appearance of Ca_V1.2 ‘super‐clusters’ along the z‐lines of ISO‐stimulated cardiomyocytes. The mechanism that leads to super‐cluster formation involves rapid, dynamic augmentation of sarcolemmal Ca_V1.2 channel abundance after ISO application. Optical and electrophysiological single channel recordings confirm that these newly inserted channels are functional and contribute to overt co‐operative gating behaviour of Ca_V1.2 channels in ISO stimulated myocytes. The results of the present study reveal a new facet of βAR‐mediated regulation of Ca_V1.2 channels in the heart and support the novel concept that a pre‐synthesized pool of sub‐sarcolemmal Ca_V1.2 channel‐containing vesicles/endosomes resides in cardiomyocytes and can be mobilized to the sarcolemma to tune excitation–contraction coupling to meet metabolic and/or haemodynamic demands.

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Cardiac CaV1.2 channels require β subunits for β-adrenergic-mediated modulation but not trafficking

Ca2+ channel β-subunit interactions with pore-forming α-subunits are long-thought to be obligatory for channel trafficking to the cell surface and for tuning of basal biophysical properties in many tissues. Unexpectedly, we demonstrate that transgenic expression of mutant α1C subunits lacking capacity to bind CaVβ can traffic to the sarcolemma in adult cardiomyocytes in vivo and sustain normal excitation-contraction coupling. However, these β-less Ca2+ channels cannot be stimulated by β-adrenergic pathway agonists, and thus adrenergic augmentation of contractility is markedly impaired in isolated cardiomyocytes and in hearts. Similarly, viral-mediated expression of a β-subunit-sequestering peptide sharply curtailed β-adrenergic stimulation of WT Ca2+ channels, identifying an approach to specifically modulate β-adrenergic regulation of cardiac contractility. Our data demonstrate that β subunits are required for β-adrenergic regulation of CaV1.2 channels and positive inotropy in the heart, but are dispensable for CaV1.2 trafficking to the adult cardiomyocyte cell surface, and for basal function and excitation-contraction coupling.

Exclusion of alternative exon 33 of CaV1.2 calcium channels in heart is proarrhythmogenic

Alternative splicing changes the Ca_V1.2 calcium channel electrophysiological property, but the in vivo significance of such altered channel function is lacking. Structure-function studies of heterologously expressed Ca_V1.2 channels could not recapitulate channel function in the native milieu of the cardiomyocyte. To address this gap in knowledge, we investigated the role of alternative exon 33 of the Ca_V1.2 calcium channel in heart function. Exclusion of exon 33 in Ca_V1.2 channels has been reported to shift the activation potential -10.4 mV to the hyperpolarized direction, and increased expression of Ca_V1.2_Δ33 channels was observed in rat myocardial infarcted hearts. However, how a change in Ca_V1.2 channel electrophysiological property, due to alternative splicing, might affect cardiac function in vivo is unknown. To address these questions, we generated mCacna1c exon 33^-/--null mice. These mice contained Ca_V1.2_Δ33 channels with a gain-of-function that included conduction of larger currents that reflects a shift in voltage dependence and a modest increase in single-channel open probability. This altered channel property underscored the development of ventricular arrhythmia, which is reflected in significantly more deaths of exon 33^-/- mice from β-adrenergic stimulation. In vivo telemetric recordings also confirmed increased frequencies in premature ventricular contractions, tachycardia, and lengthened QT interval. Taken together, the significant decrease or absence of exon 33-containing Ca_V1.2 channels is potentially proarrhythmic in the heart. Of clinical relevance, human ischemic and dilated cardiomyopathy hearts showed increased inclusion of exon 33. However, the possible role that inclusion of exon 33 in Ca_V1.2 channels may play in the pathogenesis of human heart failure remains unclear.

Rad-deletion Phenocopies Tonic Sympathetic Stimulation of the Heart

Sympathetic stimulation modulates L-type calcium channel (LTCC) gating to contribute to increased systolic heart function. Rad is a monomeric G-protein that interacts with LTCC. Genetic deletion of Rad (Rad^-/-) renders LTCC in a sympathomimetic state. The study goal was to use a clinically inspired pharmacological stress echocardiography test, including analysis of global strain, to determine whether Rad^-/- confers tonic positive inotropic heart function. Sarcomere dynamics and strain showed partial parallel isoproterenol (ISO) responsiveness for wild-type (WT) and for Rad^-/-. Rad^-/- basal inotropy was elevated compared to WT but was less responsiveness to ISO. Rad protein levels were lower in human patients with end-stage non-ischemic heart failure. These results show that Rad reduction provides a stable inotropic response rooted in sarcomere level function. Thus, reduced Rad levels in heart failure patients may be a compensatory response to need for increased output in the setting of HF. Rad deletion suggests a future therapeutic direction for inotropic support.

Loss of Rad-GTPase produces a novel adaptive cardiac phenotype resistant to systolic decline with aging

Rad-GTPase is a regulator of L-type calcium current (LTCC), with increased calcium current observed in Rad knockout models. While mouse models that result in elevated LTCC have been associated with heart failure, our laboratory and others observe a hypercontractile phenotype with enhanced calcium homeostasis in Rad(-/-). It is currently unclear whether this observation represents an early time point in a decompensatory progression towards heart failure or whether Rad loss drives a novel phenotype with stable enhanced function. We test the hypothesis that Rad(-/-) drives a stable nonfailing hypercontractile phenotype in adult hearts, and we examine compensatory regulation of sarcoplasmic reticulum (SR) loading and protein changes. Heart function was measured in vivo with echocardiography. In vivo heart function was significantly improved in adult Rad(-/-) hearts compared with wild type. Heart wall dimensions were significantly increased, while heart size was decreased, and cardiac output was not changed. Cardiac function was maintained through 18 mo of age with no decompensation. SR releasable Ca(2+) was increased in isolated Rad(-/-) ventricular myocytes. Higher Ca(2+) load was accompanied by sarco/endoplasmic reticulum Ca(2+) ATPase 2a (SERCA2a) protein elevation as determined by immunoblotting and a rightward shift in the thapsigargan inhibitor-response curve. Rad(-/-) promotes morphological changes accompanied by a stable increase in contractility with aging and preserved cardiac output. The Rad(-/-) phenotype is marked by enhanced systolic and diastolic function with increased SR uptake, which is consistent with a model that does not progress into heart failure.

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.

β-adrenergic regulation of the L-type Ca2+ channel does not require phosphorylation of α1C Ser1700

RATIONALE

Sympathetic nervous system triggered activation of protein kinase A, which phosphorylates several targets within cardiomyocytes, augments inotropy, chronotropy, and lusitropy. An important target of β-adrenergic stimulation is the sarcolemmal L-type Ca(2+) channel, CaV1.2, which plays a key role in cardiac excitation-contraction coupling. The molecular mechanisms of β-adrenergic regulation of CaV1.2 in cardiomyocytes, however, are incompletely known. Recently, it has been postulated that proteolytic cleavage at Ala(1800) and protein kinase A phosphorylation of Ser(1700) are required for β-adrenergic modulation of CaV1.2.

OBJECTIVE

To assess the role of Ala(1800) in the cleavage of α1C and the role of Ser(1700) and Thr(1704) in mediating the adrenergic regulation of CaV1.2 in the heart.

METHODS AND RESULTS

Using a transgenic approach that enables selective and inducible expression in mice of FLAG-epitope-tagged, dihydropyridine-resistant CaV1.2 channels harboring mutations at key regulatory sites, we show that adrenergic regulation of CaV1.2 current and fractional shortening of cardiomyocytes do not require phosphorylation of either Ser(1700) or Thr(1704) of the α1C subunit. The presence of Ala(1800) and the (1798)NNAN(1801) motif in α1C is not required for proteolytic cleavage of the α1C C-terminus, and deletion of these residues did not perturb adrenergic modulation of CaV1.2 current.

CONCLUSIONS

These results show that protein kinase A phosphorylation of α1C Ser(1700) does not have a major role in the sympathetic stimulation of Ca(2+) current and contraction in the adult murine heart. Moreover, this new transgenic approach enables functional and reproducible screening of α1C mutants in freshly isolated adult cardiomyocytes in a reliable, timely, cost-effective manner.

Identification of PDZ Domain Containing Proteins Interacting with 1.2 and PMCA4b

PDZ (PSD-95/Disc large/Zonula occludens-1) protein interaction domains bind to cytoplasmic protein C-termini of transmembrane proteins. In order to identify new interaction partners of the voltage-gated L-type Ca2+ channel 1.2 and the plasma membrane Ca2+ ATPase 4b (PMCA4b), we used PDZ domain arrays probing for 124 PDZ domains. We confirmed this by GST pull-downs and immunoprecipitations. In PDZ arrays, strongest interactions with 1.2 and PMCA4b were found for the PDZ domains of SAP-102, MAST-205, MAGI-1, MAGI-2, MAGI-3, and ZO-1. We observed binding of the 1.2 C-terminus to PDZ domains of NHERF1/2, Mint-2, and CASK. PMCA4b was observed to interact with Mint-2 and its known interactions with Chapsyn-110 and CASK were confirmed. Furthermore, we validated interaction of 1.2 and PMCA4b with NHERF1/2, CASK, MAST-205 and MAGI-3 via immunoprecipitation. We also verified the interaction of 1.2 and nNOS and hypothesized that nNOS overexpression might reduce Ca2+ influx through 1.2. To address this, we measured Ca2+ currents in HEK 293 cells co-expressing 1.2 and nNOS and observed reduced voltage-dependent 1.2 activation. Taken together, we conclude that 1.2 and PMCA4b bind promiscuously to various PDZ domains, and that our data provides the basis for further investigation of the physiological consequences of these interactions.

Modulation of SR Ca2+ release by the triadin-to-calsequestrin ratio in ventricular myocytes

Calsequestrin (CSQ) is a Ca(2+) storage protein that interacts with triadin (TRN), the ryanodine receptor (RyR), and junctin (JUN) to form a macromolecular tetrameric Ca(2+) signaling complex in the cardiac junctional sarcoplasmic reticulum (SR). Heart-specific overexpression of CSQ in transgenic mice (TG(CSQ)) was associated with heart failure, attenuation of SR Ca(2+) release, and downregulation of associated junctional SR proteins, e.g., TRN. Hence, we tested whether co-overexpression of CSQ and TRN in mouse hearts (TG(CxT)) could be beneficial for impaired intracellular Ca(2+) signaling and contractile function. Indeed, the depressed intracellular Ca(2+) concentration ([Ca](i)) peak amplitude in TG(CSQ) was normalized by co-overexpression in TG(CxT) myocytes. This effect was associated with changes in the expression of cardiac Ca(2+) regulatory proteins. For example, the protein level of the L-type Ca(2+) channel Ca(v)1.2 was higher in TG(CxT) compared with TG(CSQ). Sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a (SERCA2a) expression was reduced in TG(CxT) compared with TG(CSQ), whereas JUN expression and [(3)H]ryanodine binding were lower in both TG(CxT) and TG(CSQ) compared with wild-type hearts. As a result of these expressional changes, the SR Ca(2+) load was higher in both TG(CxT) and TG(CSQ) myocytes. In contrast to the improved cellular Ca(2+), transient co-overexpression of CSQ and TRN resulted in a reduced survival rate, an increased cardiac fibrosis, and a decreased basal contractility in catheterized mice, working heart preparations, and isolated myocytes. Echocardiographic and hemodynamic measurements revealed a depressed cardiac performance after isoproterenol application in TG(CxT) compared with TG(CSQ). Our results suggest that co-overexpression of CSQ and TRN led to a normalization of the SR Ca(2+) release compared with TG(CSQ) mice but a depressed contractile function and survival rate probably due to cardiac fibrosis, a lower SERCA2a expression, and a blunted response to β-adrenergic stimulation. Thus the TRN-to-CSQ ratio is a critical modulator of the SR Ca(2+) signaling.

Rescue and worsening of congenital heart block-associated electrocardiographic abnormalities in two transgenic mice

INTRODUCTION

Congenital heart block (CHB) is a passively acquired autoimmune disease considered to be due to the transfer of maternal autoantibodies, anti-SSA/Ro -SSB/La, to the fetus resulting in atrioventricular (AV) block and sinus bradycardia. We previously established a murine model for CHB where pups born to immunized wild-type (WT) mothers exhibited electrocardiographic abnormalities similar to those seen in CHB and demonstrated inhibition of L-type Ca channels (LTCCs) by maternal antibodies. Here, we hypothesize that overexpression of LTCC should rescue, whereas knockout of LTCC should worsen the electrocardiographic abnormalities in mice.

METHODS AND RESULTS

Transgenic (TG) mice were immunized with SSA/Ro and SSB/La antigens. Pups born to immunized WT mothers had significantly greater sinus bradycardia and AV block compared to pups from nonimmunized WT. TG pups overexpressing LTCC had significantly less sinus bradycardia and AV block compared to their non-TG littermates and to pups born to immunized WT mothers. All LTCC knockout pups born to immunized mothers had sinus bradycardia, advanced degree of AV block, and decreased fetal parity. No sinus bradycardia or AV block were manifested in pups from control nonimmunized WT mothers. IgG from mothers with CHB children, but not normal IgG, completely inhibited intracellular Ca transient ([Ca](i)T) amplitude.

CONCLUSIONS

Cardiac-specific overexpression of LTCC significantly reduced the incidence of AV block and sinus bradycardia in pups exposed to anti-SSA/Ro -SSB/La autoantibodies, whereas exposure of LTCC knockout pups to these autoantibodies significantly worsened the electrocardiographic abnormalities. These findings support the hypothesis that maternal antibodies inhibit LTCC and [Ca](i)T thus contributing to the development of CHB. Altogether, the results are relevant to the development of novel therapies for CHB.

Robust L-type calcium current expression following heterozygous knockout of the Cav1.2 gene in adult mouse heart

Mechanisms that contribute to maintaining expression of functional ion channels at relatively constant levels following perturbations of channel biosynthesis are likely to contribute significantly to the stability of electrophysiological systems in some pathological conditions. In order to examine the robustness of L-type calcium current expression, the response to changes in Ca²⁺ channel Cav1.2 gene dosage was studied in adult mice. Using a cardiac-specific inducible Cre recombinase system, Cav1.2 mRNA was reduced to 11 ± 1% of control values in homozygous floxed mice and the mice died rapidly (11.9 ± 3 days) after induction of gene deletion. In these homozygous knockout mice, echocardiographic analysis showed that myocardial contractility was reduced to 14 ± 1% of control values shortly before death. For these mice, no effective compensatory changes in ion channel gene expression were triggered following deletion of both Cav1.2 alleles, despite the dramatic decay in cardiac function. In contrast to the homozygote knockout mice, following knockout of only one Cav1.2 allele, cardiac function remained unchanged, as did survival.Cav1.2mRNAexpression in the left ventricle of heterozygous knockout mice was reduced to 58 ± 3% of control values and there was a 21 ± 2% reduction in Cav1.2 protein expression. There was no significant reduction in L-type Ca²⁺ current density in these mice. The results are consistent with a model of L-type calcium channel biosynthesis in which there are one or more saturated steps, which act to buffer changes in both total Cav1.2 protein and L-type current expression.

Cardiac phenotype induced by a dysfunctional α 1C transgene: a general problem for the transgenic approach

Based on stable integration of recombinant DNA into a host genome, transgenic technology has become an important genetic engineering methodology. An organism whose genetic characteristics have been altered by the insertion of foreign DNA is supposed to exhibit a new phenotype associated with the function of the transgene. However, successful insertion may not be sufficient to achieve specific modification of function. In this study we describe a strain of transgenic mouse, G7-882, generated by incorporation into the mouse genome of human CaV 1.2 α(1C) cDNA deprived of 3'-UTR to exclude transcription. We found that, in response to chronic infusion of isoproterenol, G7-882 develops dilated cardiomyopathy, a misleading "transgenic artifact" compatible with the expected function of the incorporated "correct" transgene. Specifically, using magnetic resonance imaging (MRI), we found that chronic β-adrenergic stimulation of G7-882 mice caused left ventricular hypertrophy and aggravated development of dilated cardiomyopathy, although no significant changes in the kinetics, density and voltage dependence of the calcium current were observed in G7-882 cardiomyocytes as compared to cells from wild type mice. This result illustrates the possibility that even when a functional transgene is expressed, an observed change in phenotype may be due to the artifact of "incidental incorporation" leading to misleading conclusions. To exclude this possibility and thus provide a robust tool for exploring biological function, the new transgenic phenotype must be replicated in several independently generated transgenic strains.

Role of calcium channels in congenital heart block

Congenital heart block (CHB) is a conduction abnormality that affects hearts of foetuses and/or newborn to mothers with autoantibodies reactive with the intracellular soluble ribonucleoproteins 48-kD La, 52-kD Ro and 60-kD Ro. CHB carries substantial mortality and morbidity, with more than 60% of affected children requiring lifelong pacemakers. Several hypotheses have been proposed to explain the pathogenesis of CHB. These can be grouped under three main hypotheses: Apoptosis, Serotoninergic and Ca channel hypothesis. Here, we discuss these hypotheses and provide recent scientific thinking that will most likely dominate the future of this field of research.

Rad As a Novel Regulator of Excitation–Contraction Coupling and β-Adrenergic Signaling in Heart

Rationale : Rad (Ras associated with diabetes) GTPase, a monomeric small G protein, binds to Ca v β subunit of the L-type Ca 2+ channel (LCC) and thereby regulates LCC trafficking and activity. Emerging evidence suggests that Rad is an important player in cardiac arrhythmogenesis and hypertrophic remodeling. However, whether and how Rad involves in the regulation of excitation–contraction (EC) coupling is unknown.

Objective : This study aimed to investigate possible role of Rad in cardiac EC coupling and β-adrenergic receptor (βAR) inotropic mechanism.

Methods and Results : Adenoviral overexpression of Rad by 3-fold in rat cardiomyocytes suppressed LCC current ( I Ca ), [Ca 2+ ] i transients, and contractility by 60%, 42%, and 38%, respectively, whereas the “gain” function of EC coupling was significantly increased, due perhaps to reduced “redundancy” of LCC in triggering sarcoplasmic reticulum release. Conversely, ≈70% Rad knockdown by RNA interference increased I Ca (50%), [Ca 2+ ] i transients (52%) and contractility (58%) without altering EC coupling efficiency; and the dominant negative mutant RadS105N exerted a similar effect on I Ca . Rad upregulation caused depolarizing shift of LCC activation and hastened time-dependent LCC inactivation; Rad downregulation, however, failed to alter these attributes. The Na + /Ca 2+ exchange activity, sarcoplasmic reticulum Ca 2+ content, properties of Ca 2+ sparks and propensity for Ca 2+ waves all remained unperturbed regardless of Rad manipulation. Rad overexpression, but not knockdown, negated βAR effects on I Ca and Ca 2+ transients.

Conclusion : These results establish Rad as a novel endogenous regulator of cardiac EC coupling and βAR signaling and support a parsimonious model in which Rad buffers Ca v β to modulate LCC activity, EC coupling, and βAR responsiveness.

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

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

Transgenic simulation of human heart failure-like L-type Ca2+-channels: implications for fibrosis and heart rate in mice

AIMS

Cardiac L-type Ca(2+)-currents show distinct alterations in chronic heart failure, including increased single-channel activity and blunted adrenergic stimulation, but minor changes of whole-cell currents. Expression of L-type Ca(2+)-channel beta(2)-subunits is enhanced in human failing hearts. In order to determine whether prolonged alteration of Ca(2+)-channel gating by beta(2)-subunits contributes to heart failure pathogenesis, we generated and characterized transgenic mice with cardiac overexpression of a beta(2a)-subunit or the pore Ca(v)1.2 or both, respectively.

METHODS AND RESULTS

Four weeks induction of cardiac-specific overexpression of rat beta(2a)-subunits shifted steady-state activation and inactivation of whole-cell currents towards more negative potentials, leading to increased Ca(2+)-current density at more negative test potentials. Activity of single Ca(2+)-channels was increased in myocytes isolated from beta(2a)-transgenic mice. Ca(2+)-current stimulation by 8-Br-cAMP and okadaic acid was blunted in beta(2a)-transgenic myocytes. In vivo investigation revealed hypotension and bradycardia upon Ca(v)1.2-transgene expression but not in mice only overexpressing beta(2a). Double-transgenics showed cardiac arrhythmia. Interstitial fibrosis was aggravated by the beta(2a)-transgene compared with Ca(v)1.2-transgene expression alone. Overt cardiac hypertrophy was not observed in any model.

CONCLUSION

Cardiac overexpression of a Ca(2+)-channel beta(2a)-subunit alone is sufficient to induce Ca(2+)-channel properties characteristic of chronic human heart failure. beta(2a)-overexpression by itself did not induce cardiac hypertrophy or contractile dysfunction, but aggravated the development of arrhythmia and fibrosis in Ca(v)1.2-transgenic mice.

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.

Aerobic capacity-dependent differences in cardiac gene expression

Aerobic capacity is a strong predictor of cardiovascular mortality. To determine the relationship between inborn aerobic capacity and cardiac gene expression we examined genome-wide gene expression in hearts of rats artificially selected for high and low running capacity (HCR and LCR, respectively) over 16 generations. The artificial selection of LCR caused accumulation of risk factors of cardiovascular disease similar to the metabolic syndrome seen in human, whereas HCR had markedly better cardiac function. We also studied alterations in gene expression in response to exercise training in these animals. Left ventricle gene expression of both sedentary and exercise-trained HCR and LCR was characterized by microarray and gene ontology analysis. Out of 28,000 screened genes, 1,540 were differentially expressed between sedentary HCR and LCR. Only one gene was found differentially expressed by exercise training, but this gene had unknown name and function. Sedentary HCR expressed higher amounts of genes involved in lipid metabolism, whereas sedentary LCR expressed higher amounts of the genes involved in glucose metabolism. This suggests a switch in cardiac energy substrate utilization from normal mitochondrial fatty acid β-oxidation in HCR to carbohydrate metabolism in LCR, an event that often occurs in diseased hearts. LCR were also associated with pathological growth signaling and cellular stress. Hypoxic conditions seemed to be a common source for several of these observations, triggering hypoxia-induced alterations of transcription. In conclusion, inborn high vs. low aerobic capacity was associated with differences in cardiac energy substrate, growth signaling, and cellular stress.

Endothelial nitric oxide synthase decreases beta-adrenergic responsiveness via inhibition of the L-type Ca2+ current

Signaling via endothelial nitric oxide synthase (NOS3) limits the heart's response to beta-adrenergic (beta-AR) stimulation, which may be protective against arrhythmias. However, mechanistic data are limited. Therefore, we performed simultaneous measurements of action potential (AP, using patch clamp), Ca2+ transients (fluo 4), and myocyte shortening (edge detection). L-type Ca2+ current (ICa) was directly measured by the whole cell ruptured patch-clamp technique. Myocytes were isolated from wild-type (WT) and NOS3 knockout (NOS3-/-) mice. NOS3-/- myocytes exhibited a larger incidence of beta-AR (isoproterenol, 1 microM)-induced early afterdepolarizations (EADs) and spontaneous activity (defined as aftercontractions). We also examined ICa, a major trigger for EADs. NOS3-/- myocytes had a significantly larger beta-AR-stimulated increase in ICa compared with WT myocytes. In addition, NOS3-/- myocytes had a larger response to beta-AR stimulation compared with WT myocytes in Ca2+ transient amplitude, shortening amplitude, and AP duration (APD). We observed similar effects with specific NOS3 inhibition [L-N5-(1-iminoethyl)-ornithine (l-NIO), 10 microM] in WT myocytes as with NOS3 knockout. Specifically, l-NIO further increased isoproterenol-stimulated EADs and aftercontractions. l-NIO also further increased the isoproterenol-stimulated ICa, Ca2+ transient amplitude, shortening amplitude, and APD (all P < 0.05 vs isoproterenol alone). l-NIO had no effect in NOS3-/- myocytes. These results indicate that NOS3 signaling inhibits the beta-AR response by reducing ICa and protects against arrhythmias. This mechanism may play an important role in heart failure, where arrhythmias are increased and NOS3 expression is decreased.

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.

Multiple downstream proarrhythmic targets for calmodulin kinase II: Moving beyond an ion channel-centric focus

The multifunctional Ca(2+) calmodulin-dependent protein kinase II (CaMKII) has emerged as a pro-arrhythmic signaling molecule. CaMKII can participate in arrhythmia signaling by effects on ion channel proteins, intracellular Ca(2+) uptake and release, regulation of cell death, and by activation of hypertrophic signaling pathways. The pleuripotent nature of CaMKII is reminiscent of another serine-threonine kinase, protein kinase A (PKA), which shares many of the same protein targets and is the downstream kinase most associated with beta-adrenergic receptor stimulation. The ability of CaMKII to localize and coordinate activity of multiple protein targets linked to Ca(2+) signaling set CaMKII apart from other "traditional" arrhythmia drug targets, such as ion channel proteins. This review will discuss some of the biology of CaMKII and focus on work that has been done on molecular, cellular, and whole animal models that together build a case for CaMKII as a pro-arrhythmic signal and as a potential therapeutic target for arrhythmias and structural heart disease.

Mouse models to study L-type calcium channel function

Calcium influx through voltage gated L-type Ca2+ channels has evolved as one of the most widely used transmembrane signalling mechanisms in eukaryotic organisms. Although pharmacological inhibitors of L-type Ca2+ channels have an important place in medical therapy, the full therapeutic potential of the 4 L-type Ca2+ channel subtypes has not been explored yet. To dissect the physiological relevance of the L-type Ca2+ channel subtype diversity, gene-targeted mouse models carrying deletions of these channels ("knockout mice") have been generated. This review focuses on recent data from studies in mice lacking the Ca(v)1.2 and Ca(v)1.3 pore subunits, which have elucidated some of the roles of L-type Ca2+ channels as mediators of signalling between cell membrane and intracellular processes like blood pressure regulation, smooth muscle contractility, insulin secretion, cardiac development, and learning and memory.

Sarcoplasmic reticulum adenosine triphosphatase overexpression in the L-type Ca2+ channel mouse results in cardiomyopathy and Ca2+ -induced arrhythmogenesis

BACKGROUND

Overexpression of the L-type voltage-dependent calcium channel alpha(1C)-subunit (L-VDCC OE) in transgenic mice results in adaptive hypertrophy followed by a maladaptive phase associated with a decrease in sarcoplasmic reticulum adenosine triphosphatase (SERCA)2a expression at 8 to 10 months of age. Overexpressing SERCA to manipulate calcium (Ca(2+)) cycling and prevent pathologic phenotypes in some models of heart failure has been proven to be a promising genetic strategy.

OBJECTIVE

In this study we investigated whether genetic manipulation that increases Ca(2+) uptake into the sarcoplasmic reticulum by overexpressing SERCA1a (skeletal muscle specific) into the L-VDCC OE background could restore or further deteriorate Ca(2+) cycling, contractile dysfunction, and electrical remodeling in the heart failure phenotype.

RESULTS

We found that the survival rate of L-VDCC OE/SERCA1a OE double transgenic mice decreased by 50%. L-VDCC OE/SERCA1a OE mice displayed an accelerated phenotype of severe dilation of both ventricles associated with deteriorated left ventricular function. Voltage clamp experiments revealed enhanced increased inward Ca(2+) current density and decreased the transient outward potassium current. Action potential duration in double transgenic ventricular myocytes was prolonged, and isoproterenol induced early after depolarization. These mice demonstrated a high incidence of spontaneous left ventricular arrhythmia. Expression of the proarrhythmic signaling protein Ca(2+)/calmodulin-dependent kinase II (CaMKII) was increased while connexin43 expression was decreased, defining an important putative mechanism in the electrophysiologic disturbances and mortality.

CONCLUSIONS

Despite previous reports of improved cardiac function in heart failure models after SERCA intervention, our results advocate the need to elucidate the involvement of augmented Ca(2+) cycling in arrhythmogenesis.

Ahnak is critical for cardiac Ca(v)1.2 calcium channel function and its β‐adrenergic regulation

Defective L-type Ca2+ channel (I(CaL)) regulation is one major cause for contractile dysfunction in the heart. The I(CaL) is enhanced by sympathetic nervous stimulation: via the activation of beta-adrenergic receptors, PKA phosphorylates the alpha1C(Ca(V)1.2)- and beta2-channel subunits and ahnak, an associated 5643-amino acid (aa) protein. In this study, we examined the role of a naturally occurring, genetic variant Ile5236Thr-ahnak on I(CaL). Binding experiments with ahnak fragments (wild-type, Ile5236Thr mutated) and patch clamp recordings revealed that Ile5236Thr-ahnak critically affected both beta2 subunit interaction and I(CaL) regulation. Binding affinity between ahnak-C1 (aa 4646-5288) and beta2 subunit decreased by approximately 50% after PKA phosphorylation or in the presence of Ile5236Thr-ahnak peptide. On native cardiomyocytes, intracellular application of this mutated ahnak peptide mimicked the PKA-effects on I(CaL) increasing the amplitude by approximately 60% and slowing its inactivation together with a leftward shift of its voltage dependency. Both mutated Ile5236Thr-peptide and Ile5236Thr-fragment (aa 5215-5288) prevented specifically the further up-regulation of I(CaL) by isoprenaline. Hence, we suggest the ahnak-C1 domain serves as physiological brake on I(CaL). Relief from this inhibition is proposed as common pathway used by sympathetic signaling and Ile5236Thr-ahnak fragments to increase I(CaL). This genetic ahnak variant might cause individual differences in I(CaL) regulation upon physiological challenges or therapeutic interventions.

Short-term regulation of excitation-contraction coupling by the beta1a subunit in adult mouse skeletal muscle

The beta1a subunit of the skeletal muscle voltage-gated Ca2+ channel plays a fundamental role in the targeting of the channel to the tubular system as well as in channel function. To determine whether this cytosolic auxiliary subunit is also a regulatory protein of Ca2+ release from the sarcoplasmic reticulum in vivo, we pressure-injected the beta1a subunit into intact adult mouse muscle fibers and recorded, with Fluo-3 AM, the intracellular Ca2+ signal induced by the action potential. We found that the beta1a subunit significantly increased, within minutes, the amplitude of Ca2+ release without major changes in its time course. beta1a subunits with the carboxy-terminus region deleted did not show an effect on Ca2+ release. The possibility that potentiation of Ca2+ release is due to a direct interaction between the beta1a subunit and the ryanodine receptor was ruled out by bilayer experiments of RyR1 single-channel currents and also by Ca2+ flux experiments. Our data suggest that the beta1a subunit is capable of regulating E-C coupling in the short term and that the integrity of the carboxy-terminus region is essential for its modulatory effect.

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.

Calcium-calcineurin signaling in the regulation of cardiac hypertrophy

Cardiac hypertrophy is a leading predicator of progressive heart disease that often leads to heart failure and a loss of cardiac contractile performance associated with profound alterations in intracellular calcium handling. Recent investigation has centered on identifying the molecular signaling pathways that regulate cardiac myocyte hypertrophy, as well as the mechanisms whereby alterations in calcium handling are associated with progressive heart failure. One potential focal regulator of cardiomyocyte hypertrophy that also responds to altered calcium handling is the calmodulin-activated serine/threonine protein phosphatase calcineurin (PP2B). Once activated by increases in calcium, calcineurin mediates the hypertrophic response through its downstream transcriptional effector nuclear factor of activated T cells (NFAT), which is directly dephosphorylated by calcineurin resulting in nuclear translocation. While previous studies have convincingly demonstrated the sufficiency of calcineurin to mediate cardiac hypertrophy and progressive heart failure, its necessity remains an area of ongoing investigation. Here we weigh an increasing body of literature that suggests a causal link between calcineurin signaling and the cardiac hypertrophic response and heart failure through the use of pharmacologic inhibitors (cyclosporine A and FK506) and genetic approaches. We will also discuss the manner in which calcineurin-NFAT signaling is negatively regulated in the heart through a diverse array of kinases and inhibitory proteins. Finally, we will discuss emerging theories as to the mechanisms whereby alterations in intracellular calcium handling might stimulate calcineurin within the context of a contractile cell continually experiencing calcium flux.

Mineralocorticoid Receptor Antagonism Prevents the Electrical Remodeling That Precedes Cellular Hypertrophy After Myocardial Infarction

Background— Cardiac hypertrophy underlies arrhythmias and sudden death, for which mineralocorticoid receptor (MR) activity has recently been implicated. We sought to establish the sequence of ionic events that link the initiating insult and MR to hypertrophy development.

Methods and Results— Using whole-cell, patch-clamp and quantitative reverse transcription–polymerase chain reaction techniques on right ventricular myocytes of a myocardial infarction (MI) rat model, we examined the cellular response over time. One week after MI, no sign of cellular hypertrophy was found, but action potential duration (APD) was lengthened. Both an increase in Ca 2+ current ( I Ca ) and a decrease in K + transient outward current ( I to ) underlay this effect. Consistently, the relative expression of mRNA coding for the Ca 2+ channel α1C subunit (Ca v 1.2) increased, and that of the K + channel K v 4.2 subunit decreased. Three weeks after MI, AP prolongation endured, whereas cellular hypertrophy developed. I Ca density, Ca v 1.2, and K v 4.2 mRNA levels regained control levels, but I to density remained reduced. Long-term treatment with RU28318, an MR antagonist, prevented this electrical remodeling. In a different etiologic model of abdominal aortic constriction, we confirmed that APD prolongation and modifications of ionic currents precede cellular hypertrophy.

Conclusions— Electrical remodeling, which is triggered at least in part by MR activation, is an initial, early cellular response to hypertrophic insults.

Single-channel gating and regulation of human L-type calcium channels in cardiomyocytes of transgenic mice

Overexpression of human cardiac L-type Ca(2+) channel pores (hCa(v)1.2) in mice causes heart failure. Earlier studies showed Ca(v)1.2-mRNA increase by 2.8-fold, but whole-cell current density enhancement by </=1.5-fold only. Three possible explanations were examined: (1) poor translation of hCa(v)1.2 and of its accessory subunits, (2) altered sarcolemmal insertion of functional channels, and (3) lower single-channel activity of overexpressed channels. Western blots revealed a 2.7-fold increase of Ca(v)1.2 protein in transgenic myocytes, but less enhanced expression of beta(1a) and beta(1b) subunits. beta(2) and alpha(2)/delta were significantly lowered. Density of functional channels was increased by 3.0-fold. Single-channel gating was impaired in transgenic cardiomyocytes: open probability and ensemble average currents were reduced by 60%. Furthermore, channels of transgenic myocytes were not stimulated by 8-Br-cAMP, in contrast to wild-types. Expression of malcomposed, dysfunctional L-type Ca(2+) channels in murine cardiomyocytes overexpressing hCa(v)1.2 explains the moderate enhancement of whole-cell currents and illustrates compensatory mechanisms in a transgenic disease model.

Intracellular Ca(2+) regulates responsiveness of cardiac L-type Ca(2+) current to protein kinase A: role of calmodulin

The goal of this study was to determine whether the protein kinase A (PKA) responsiveness of the cardiac L-type Ca(2+) current (ICa) is affected during transient increases in intracellular Ca(2+) concentration. Ventricular myocytes were isolated from 3- to 4-day-old neonatal rats and cultured on aligned collagen thin gels. When measured in 1 or 2 mM Ca(2+) external solution, the aligned myocytes displayed a large ICa that was weakly regulated (20% increase) during stimulation of PKA by 2 microM forskolin. In contrast, application of forskolin caused a 100% increase in ICa when the external Ca(2+) concentration was reduced to 0.5 mM or replaced with Ba(2+). This Ca(2+)-dependent inhibition was also observed when the cells were treated with 1 microM isoproterenol, 100 microM 3-isobutyl-1-methylxanthine, or 500 microM 8-bromo-cAMP. The responsiveness of ICa to PKA was restored during intracellular dialysis with a calmodulin (CaM) inhibitory peptide but not during treatment with inhibitors of protein kinase C, Ca(2+)/CaM-dependent protein kinase, or calcineurin. Adenoviral-mediated expression of a CaM molecule with mutations in all four Ca(2+)-binding sites also increased the PKA sensitivity of ICa. Finally, adult mouse ventricular myocytes displayed a greater response to forskolin and cAMP in external Ba(2+). Thus Ca(2+) entering the myocyte through the voltage-gated Ca(2+) channel regulates the PKA responsiveness of ICa.

Electrical remodeling in hearts from a calcium-dependent mouse model of hypertrophy and failure: complex nature of K+ current changes and action potential duration

OBJECTIVES

This study was designed to identify possible electrical remodeling (ER) in transgenic (Tg) mice with over-expressed L-type Ca(2+) channels. Transient outward K(+) current (I(to)) and action potential duration (APD) were studied in 2-, 4-, 8-, and 9- to 12-month-old mice to determine linkage to ventricular remodeling (VR), ER, and heart failure (HF).

BACKGROUND

Prolongation of APD and reduction in current density of I(to) are thought to be hallmarks of VR and HF. Mechanisms are not understood.

METHODS

Patch-clamp, perfused hearts, echocardiography, and Western blots were employed using 2-, 4-, 8-, and 9- to 12-month-old Tg mice.

RESULTS

Transgenic mice developed slow VR statistically manifesting at four months and continuing through death at 12 to 14 months, despite a slight up-regulation of I(to). A slight decrease or no change in APD was observed up to eight months; however, at 9 to 12 months, a small increase in APD was detected. Early afterdepolarizations were observed after application of 4-aminopyridine in Tg mice. No change was detected in protein of Kv4.3 and Kv4.2 up to eight months. At 9 to 12 months, Tg mice showed a slight decrease (41.4 +/- 6.9%, p < 0.05) in Kv4.2, consistent with a decrease in I(to). Surprisingly, Kv1.4 (the "fetal" K(+)-channel form) was up-regulated, and restitution of I(to) was slowed. Echocardiography revealed cardiac enlargement with impaired chamber function in hearts that were taken from the older animals.

CONCLUSIONS

Contrary to accepted dogma, APD and I(to) in a mouse model of hypertrophy and HF are not hallmarks of pathophysiology. We suggest that [Ca(2+)](i) (i.e., [Ca(2+)] concentration) is the primary factor in triggering cardiac enlargement and arrhythmogenesis.

Stretch-modulation of second messengers: effects on cardiomyocyte ion transport

In cardiomyocytes, mechanical stress induces a variety of hypertrophic responses including an increase in protein synthesis and a reprogramming of gene expression. Recently, the calcium signaling has been reported to play an important role in the development of cardiac hypertrophy. In this article, we report on the role of the calcium signaling in stretch-induced gene expression in cardiomyocytes. Stretching of cultured cardiomyocytes up-regulates the expression of brain natriuretic peptide (BNP). Intracellular calcium-elevating agents such as the calcium ionophore A23187, the calcium channel agonist BayK8644 and the sarcoplasmic reticulum calcium-ATPase inhibitor thapsigargin up-regulate BNP gene expression. Conversely, stretch-induced BNP gene expression is suppressed by EGTA, stretch-activated ion channel inhibitors, voltage-dependent calcium channel antagonists, and long-time exposure to thapsigargin. Furthermore, stretch increases the activity of calcium-dependent effectors such as calcineurin and calmodulin-dependent kinase II, and inhibitors of calcineurin and calmodulin-dependent kinase II significantly attenuated stretch-induced hypertrophy and BNP expression. These results suggest that calcineurin and calmodulin-dependent kinase II are activated by calcium influx and subsequent calcium-induced calcium release, and play an important role in stretch-induced gene expression during the development of cardiac hypertrophy.

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

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

Inhibition of calcineurin and sarcolemmal Ca2+ influx protects cardiac morphology and ventricular function in K(v)4.2N transgenic mice

BACKGROUND

Cardiac-targeted expression of truncated K(v)4.2 subunit (K(v)4.2N) reduces transient outward current (I(to)) density, prolongs action potentials (APs), and enhances contractility in 3- to 4-week-old transgenic mice. By 13 to 15 weeks of age, these mice develop severely impaired cardiac function and signs of heart failure. In this study, we examined whether augmented contractility in K(v)4.2N mice results from elevations in intracellular calcium ([Ca2+]i) secondary to AP prolongation and investigated the putative roles of calcineurin activation in heart disease development of K(v)4.2N mice.

METHODS AND RESULTS

At 3 to 4 weeks of age, L-type Ca2+ influx and peak [Ca2+]i were significantly elevated in K(v)4.2N myocytes compared with control because of AP prolongation. Cardiac calcineurin activity was also significantly elevated in K(v)4.2N mice by 5 weeks of age relative to controls and increased progressively as heart disease developed. This was associated with activation of protein kinase C (PKC)-alpha and PKC-theta but not PKC-epsilon, as well as increases in beta-myosin heavy chain (beta-MHC) and reductions in sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA)-2a expression. Treatment with either cyclosporin A or verapamil prevented increases in heart weight to body weight ratios, interstitial fibrosis, impaired contractility, PKC activation, and changes in the expression patterns of beta-MHC and SERCA2a.

CONCLUSIONS

Our results demonstrate that AP prolongation caused by I(to) reduction results in enhanced Ca2+ cycling and hypercontractility in mice and suggests that elevations in [Ca2+]i via I(Ca,L) and activation of calcineurin play a central role in disease development after I(to) reduction using the K(v)4.2N construct.

Reduction of I(to) causes hypertrophy in neonatal rat ventricular myocytes

Prolonged action potential duration (APD) and decreased transient outward K+ current (I(to)) as a result of decreased expression of K(v4.2) and K(v4.3) genes are commonly observed in heart disease. We found that treatment of cultured neonatal rat ventricular myocytes with Heteropoda Toxin3, a blocker of cardiac I(to), induced hypertrophy as measured using cell membrane capacitance and (3)H-leucine uptake. To dissect the role of specific I(to)-encoding genes in hypertrophy, I(to) was selectively reduced by overexpressing mutant dominant-negative (DN) transgenes. I(to) amplitude was reduced equally (by about 50%) by overexpression of DN K(v1.4) (K(v1.4)N) or DN K(v4.2) (either K(v4.2)N or K(v4.2)W362F), but only DN K(v4.2) prolonged APD duration (at 1 Hz) and induced myocyte hypertrophy. This hypertrophy was prevented by coexpressing wild-type K(v4.2) channels (K(v4.2)F) with the DN K(v4.2) genes, suggesting the hypertrophy is due to I(to) reduction and not nonspecific effects of transgene overexpression. The hypertrophy caused by reductions of K(v4.x)-based I(to) was associated with increased activity of the calcium-dependent phosphatase, calcineurin, and could be prevented by coinfection with Ad-CAIN, a specific calcineurin inhibitor. The hypertrophy and calcineurin activation induced by K(v4.2)N infection were prevented by blocking Ca2+ entry and excitability with verapamil or high [K+]o. Our studies suggest that reductions of K(v4.2/3)-based I(to) play a role in hypertrophy signaling by activation of calcineurin.

Ca(2+) signaling in cardiac myocytes overexpressing the alpha(1) subunit of L-type Ca(2+) channel

Voltage-gated L-type Ca(2+) channels (LCCs) provide Ca(2+) ingress into cardiac myocytes and play a key role in intracellular Ca(2+) homeostasis and excitation-contraction coupling. We investigated the effects of a constitutive increase of LCC density on Ca(2+) signaling in ventricular myocytes from 4-month-old transgenic (Tg) mice overexpressing the alpha(1) subunit of LCC in the heart. At this age, cells were somewhat hypertrophic as reflected by a 20% increase in cell capacitance relative to those from nontransgenic (Ntg) littermates. Whole cell I(Ca) density in Tg myocytes was elevated by 48% at 0 mV compared with the Ntg group. Single-channel analysis detected an increase in LCC density with similar conductance and gating properties. Although the overexpressed LCCs triggered an augmented SR Ca(2+) release, the "gain" function of EC coupling was uncompromised, and SR Ca(2+) content, diastolic cytosolic Ca(2+), and unitary properties of Ca(2+) sparks were unchanged. Importantly, the enhanced I(Ca) entry and SR Ca(2+) release were associated with an upregulation of the Na(+)-Ca(2+) exchange activity (indexed by the half decay time of caffeine-elicited Ca(2+) transient) by 27% and SR Ca(2+) recycling by approximately 35%. Western analysis detected a 53% increase in the Na(+)-Ca(2+) exchanger expression but no change in the abundance of ryanodine receptor (RyR), SERCA2, and phospholamban. Analysis of I(Ca) kinetics suggested that SR Ca(2+) release-dependent inactivation of LCCs remains intact in Tg cells. Thus, in spite of the modest cardiac hypertrophy, the overexpressed LCCs form functional coupling with RyRs, preserving both orthograde and retrograde Ca(2+) signaling between LCCs and RyRs. These results also suggest that a modest but sustained increase in Ca(2+) influx triggers a coordinated remodeling of Ca(2+) handling to maintain Ca(2+) homeostasis.

Use of transgenic mice to study voltage-dependent Ca2+ channels

During the past decade a great number of genes encoding high- and low-voltage-dependent Ca(2+) channels and their accessory subunits have been cloned. Studies of Ca(2+) channel structure-function relationships and channel regulation using cDNA expression in heterologous expression systems have revealed intricate details of subunit interaction, regulation of channels by protein kinase A (PKA) and protein kinase C (PKC), drug binding sites, mechanisms of drug action, the ion conduction pathway and other aspects of channel function. In recent years, however, we have arrived at the brink of an entirely new strategy to study Ca(2+) channels by overexpressing or knocking out genes encoding these channels in transgenic mice. In this article, various models of gene knockout or gene overexpression will be discussed. This new approach will reveal many secrets regarding Ca(2+) channel regulation and the control of Ca(2+)-dependent cellular processes.

Gender influences on sarcoplasmic reticulum Ca2+-handling in failing human myocardium

Gender has recently been implicated as an important modulator of cardiovascular disease. However, it is not known how gender may specifically influence the Ca2+-handling deficits that characterize the depressed cardiac contractility of human heart failure. To elucidate the contributory role of gender to sarcoplasmic reticulum (SR) Ca2+ cycling alterations, the protein levels of SR Ca2+-ATPase (SERCA), phospholamban, and calsequestrin, as well as the site-specific phospholamban phosphorylation status, were quantified in a mixed gender population of failing (n=14) and donor (n=15) myocardia. The apparent affinity (EC50) and the maximal velocity (Vmax) of SR Ca2+-uptake were also determined to lend functional significance to any observed protein alterations. Phospholamban and calsequestrin levels were not altered; however, SERCA protein levels were significantly reduced in failing hearts. Additionally, phospholamban phosphorylation (serine-16 and threonine-17 sites) and myocardial cAMP content were both attenuated. The alterations in SR protein levels were also accompanied by a decreased V(max)and an increased EC50 (diminished apparent affinity) of SR Ca2+-uptake for Ca2+ in failing myocardia. Myocardial protein levels and Ca2+ uptake parameters were then analyzed with respect to gender, which revealed that the decreases in phosphorylated serine-16 were specific to male failing hearts, reflecting increases in the EC50 values of SR Ca2+-uptake for Ca2+, compared to donor males. These findings suggest that although decreased SERCA protein and phospholamban phosphorylation levels contribute to depressed SR Ca2+-uptake and left ventricular function in heart failure, the specific subcellular alterations which underlie these effects may not be uniform with respect to gender.

The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.

Cardiac hypertrophy and impaired relaxation in transgenic mice overexpressing triadin 1

Triadin 1 is a major transmembrane protein in cardiac junctional sarcoplasmic reticulum (SR), which forms a quaternary complex with the ryanodine receptor (Ca(2+) release channel), junctin, and calsequestrin. To better understand the role of triadin 1 in excitation-contraction coupling in the heart, we generated transgenic mice with targeted overexpression of triadin 1 to mouse atrium and ventricle, employing the alpha-myosin heavy chain promoter to drive protein expression. The protein was overexpressed 5-fold in mouse ventricles, and overexpression was accompanied by cardiac hypertrophy. The levels of two other junctional SR proteins, the ryanodine receptor and junctin, were reduced by 55% and 73%, respectively, in association with triadin 1 overexpression, whereas the levels of calsequestrin, the Ca(2+)-binding protein of junctional SR, and of phospholamban and SERCA2a, Ca(2+)-handling proteins of the free SR, were unchanged. Cardiac myocytes from triadin 1-overexpressing mice exhibited depressed contractility; Ca(2+) transients decayed at a slower rate, and cell shortening and relengthening were diminished. The extent of depression of cell shortening of triadin 1-overexpressing cardiomyocytes was rate-dependent, being more depressed under low stimulation frequencies (0.5 Hz), but reaching comparable levels at higher frequencies of stimulation (5 Hz). Spontaneously beating, isolated work-performing heart preparations overexpressing triadin 1 also relaxed at a slower rate than control hearts, and failed to adapt to increased afterload appropriately. The fast time inactivation constant, tau(1), of the l-type Ca(2+) channel was prolonged in transgenic cardiomyocytes. Our results provide evidence for the coordinated regulation of junctional SR protein expression in heart independent of free SR protein expression, and furthermore suggest an important role for triadin 1 in regulating the contractile properties of the heart during excitation-contraction coupling.