Reduced effects of BAY K 8644 on L-type Ca2+ current in failing human cardiac myocytes are related to abnormal adrenergic regulation

Interaction of the Joining Region in Junctophilin-2 With the L-Type Ca2+ Channel Is Pivotal for Cardiac Dyad Assembly and Intracellular Ca2+ Dynamics

RATIONALE

Ca2+-induced Ca2+ release (CICR) in normal hearts requires close approximation of L-type calcium channels (LTCCs) within the transverse tubules (T-tubules) and RyR (ryanodine receptors) within the junctional sarcoplasmic reticulum. CICR is disrupted in cardiac hypertrophy and heart failure, which is associated with loss of T-tubules and disruption of cardiac dyads. In these conditions, LTCCs are redistributed from the T-tubules to disrupt CICR. The molecular mechanism responsible for LTCCs recruitment to and from the T-tubules is not well known. JPH (junctophilin) 2 enables close association between T-tubules and the junctional sarcoplasmic reticulum to ensure efficient CICR. JPH2 has a so-called joining region that is located near domains that interact with T-tubular plasma membrane, where LTCCs are housed. The idea that this joining region directly interacts with LTCCs and contributes to LTCC recruitment to T-tubules is unknown.

OBJECTIVE

To determine if the joining region in JPH2 recruits LTCCs to T-tubules through direct molecular interaction in cardiomyocytes to enable efficient CICR.

METHODS AND RESULTS

Modified abundance of JPH2 and redistribution of LTCC were studied in left ventricular hypertrophy in vivo and in cultured adult feline and rat ventricular myocytes. Protein-protein interaction studies showed that the joining region in JPH2 interacts with LTCC-α1C subunit and causes LTCCs distribution to the dyads, where they colocalize with RyRs. A JPH2 with induced mutations in the joining region (mutPG1JPH2) caused T-tubule remodeling and dyad loss, showing that an interaction between LTCC and JPH2 is crucial for T-tubule stabilization. mutPG1JPH2 caused asynchronous Ca2+-release with impaired excitation-contraction coupling after β-adrenergic stimulation. The disturbed Ca2+ regulation in mutPG1JPH2 overexpressing myocytes caused calcium/calmodulin-dependent kinase II activation and altered myocyte bioenergetics.

CONCLUSIONS

The interaction between LTCC and the joining region in JPH2 facilitates dyad assembly and maintains normal CICR in cardiomyocytes.

Nanoscale regulation of L-type calcium channels differentiates between ischemic and dilated cardiomyopathies

BACKGROUND

Subcellular localization and function of L-type calcium channels (LTCCs) play an important role in regulating contraction of cardiomyocytes. Understanding how this is affected by the disruption of transverse tubules during heart failure could lead to new insights into the disease.

METHODS

Cardiomyocytes were isolated from healthy donor hearts, as well as from patients with cardiomyopathies and with left ventricular assist devices. Scanning ion conductance and confocal microscopy was used to study membrane structures in the cells. Super-resolution scanning patch-clamp was used to examine LTCC function in different microdomains. Computational modeling predicted the impact of these changes to arrhythmogenesis at the whole-heart level.

FINDINGS

We showed that loss of structural organization in failing myocytes leads to re-distribution of functional LTCCs from the T-tubules to the sarcolemma. In ischemic cardiomyopathy, the increased LTCC open probability in the T-tubules depends on the phosphorylation by protein kinase A, whereas in dilated cardiomyopathy, the increased LTCC opening probability in the sarcolemma results from enhanced phosphorylation by calcium-calmodulin kinase II. LVAD implantation corrected LTCCs pathophysiological activity, although it did not improve their distribution. Using computational modeling in a 3D anatomically-realistic human ventricular model, we showed how LTCC location and activity can trigger heart rhythm disorders of different severity.

INTERPRETATION

Our findings demonstrate that LTCC redistribution and function differentiate between disease aetiologies. The subcellular changes observed in specific microdomains could be the consequence of the action of distinct protein kinases.

FUNDING

This work was supported by NIH grant (ROI-HL 126802 to NT-JG) and British Heart Foundation (grant RG/17/13/33173 to JG, project grant PG/16/17/32069 to RAC). Funders had no role in study design, data collection, data analysis, interpretation, writing of the report.

SLMAP-3 is downregulated in human dilated ventricles and its overexpression promotes cardiomyocyte response to adrenergic stimuli by increasing intracellular calcium

Structural dilation of cardiomyocytes (CMs) imposes a decline in cardiac performance that precipitates cardiac failure and sudden death. Since membrane proteins are implicated in dilated cardiomyopathy and heart failure, we evaluated the expression of the sarcolemmal membrane-associated protein (SLMAP) in dilated cardiomyopathy and its effect on CM contraction. We found that all 3 SLMAP isoforms (SLMAP-1, -2, and -3) are expressed in CMs and are downregulated in human dilated ventricles. Knockdown of SLMAPs in cultured CMs transduced with recombinant adeno-associated viral particles releasing SLMAP-shRNA precipitated reduced spontaneous contractile rate that was not fully recovered in SLMAP-depleted CMs challenged with isoproterenol (ISO), thus phenotypically mimicking heart failure performance. Interestingly, the overexpression of the SLMAP-3 full-length isoform induced a positive chronotropic effect in CMs that was more pronounced in response to ISO insult (vs. ISO-treated naïve CMs). Confocal live imaging showed that H9c2 cardiac myoblasts overexpressing SLMAP-3 exhibit a higher intracellular calcium transient peak when treated with ISO (vs. ISO-treated cells carrying a control adeno-associated viral particle). Proteomics revealed that SLMAP-3 interacts with the regulator of CM contraction, striatin. Collectively, our data demonstrate that SLMAP-3 is a novel regulator of CM contraction rate and their response to adrenergic stimuli. Loss of SLMAPs phenotypically mimics cardiac failure and crystallizes SLMAPs as predictive of dilated cardiomyopathy and heart failure.

Degradation of T-Tubular Microdomains and Altered cAMP Compartmentation Lead to Emergence of Arrhythmogenic Triggers in Heart Failure Myocytes: An in silico Study

Heart failure (HF) is one of the most common causes of morbidity and mortality worldwide. Although many patients suffering from HF die from sudden cardiac death caused by arrhythmias, the mechanism linking HF remodeling to an increased arrhythmogenic propensity remains incomplete. HF is typically characterized by a progressive loss of transverse tubule (T-tubule) domains, which leads to an altered distribution of L-type calcium channels (LTCCs). Microdomain degradation also causes the disruption of the β₂ adrenergic receptor (β₂AR) and phosphodiesterase (PDE) signaling localization, normally confined to the dyadic space. The goal of this study was to analyze how these subcellular changes affect the function of LTCCs and lead to the emergence of ventricular cell-level triggers of arrhythmias. To accomplish this, we developed a novel computational model of a human ventricular HF myocyte in which LTCCs were divided into six different populations, based on their location and signaling environment they experience. To do so, we included T-tubular microdomain remodeling which led to a subset of LTCCs to be redistributed from the T-tubular to the surface membrane and allowed for different levels of phosphorylation of LTCCs by PKA, based on the presence of β₂ARs and PDEs. The model was used to study the behavior of the LTCC current (I_CaL) under basal and sympathetic stimulation and its effect on cellular action potential. Our results showed that channels redistributed from the T-tubular membrane to the bulk of the sarcolemma displayed an altered function in their new, non-native signaling domain. Incomplete calcium dependent inactivation, which resulted in a longer-lasting and larger-in-magnitude LTCC current, was observed when we decoupled LTCCs from ryanodine receptors and removed them from the dyadic space. The magnitude of the LTCC current, especially in the surface sarcolemma, was also increased via phosphorylation by the redistributed β₂ARs and PDEs. These changes in LTCC current led to the development of early afterdepolarizations. Thus, our study shows that altered LTCC function is a potential cause for the emergence of cell-level triggers of arrhythmia, and that β₂ARs and PDEs present useful therapeutic targets for treatment of HF and prevention of sudden cardiac death.

Microdomain-Specific Modulation of L-Type Calcium Channels Leads to Triggered Ventricular Arrhythmia in Heart Failure

Supplemental Digital Content is available in the text.

Rationale:

Disruption in subcellular targeting of Ca²⁺ signaling complexes secondary to changes in cardiac myocyte structure may contribute to the pathophysiology of a variety of cardiac diseases, including heart failure (HF) and certain arrhythmias.

Objective:

To explore microdomain-targeted remodeling of ventricular L-type Ca²⁺ channels (LTCCs) in HF.

Methods and Results:

Super-resolution scanning patch-clamp, confocal and fluorescence microscopy were used to explore the distribution of single LTCCs in different membrane microdomains of nonfailing and failing human and rat ventricular myocytes. Disruption of membrane structure in both species led to the redistribution of functional LTCCs from their canonical location in transversal tubules (T-tubules) to the non-native crest of the sarcolemma, where their open probability was dramatically increased (0.034±0.011 versus 0.154±0.027, P<0.001). High open probability was linked to enhance calcium–calmodulin kinase II–mediated phosphorylation in non-native microdomains and resulted in an elevated I_Ca,L window current, which contributed to the development of early afterdepolarizations. A novel model of LTCC function in HF was developed; after its validation with experimental data, the model was used to ascertain how HF-induced T-tubule loss led to altered LTCC function and early afterdepolarizations. The HF myocyte model was then implemented in a 3-dimensional left ventricle model, demonstrating that such early afterdepolarizations can propagate and initiate reentrant arrhythmias.

Conclusions:

Microdomain-targeted remodeling of LTCC properties is an important event in pathways that may contribute to ventricular arrhythmogenesis in the settings of HF-associated remodeling. This extends beyond the classical concept of electric remodeling in HF and adds a new dimension to cardiovascular disease.

Myoscape controls cardiac calcium cycling and contractility via regulation of L-type calcium channel surface expression

Calcium signalling plays a critical role in the pathogenesis of heart failure. Here we describe a cardiac protein named Myoscape/FAM40B/STRIP2, which directly interacts with the L-type calcium channel. Knockdown of Myoscape in cardiomyocytes decreases calcium transients associated with smaller Ca(2+) amplitudes and a lower diastolic Ca(2+) content. Likewise, L-type calcium channel currents are significantly diminished on Myoscape ablation, and downregulation of Myoscape significantly reduces contractility of cardiomyocytes. Conversely, overexpression of Myoscape increases global Ca(2+) transients and enhances L-type Ca(2+) channel currents, and is sufficient to restore decreased currents in failing cardiomyocytes. In vivo, both Myoscape-depleted morphant zebrafish and Myoscape knockout (KO) mice display impairment of cardiac function progressing to advanced heart failure. Mechanistically, Myoscape-deficient mice show reduced L-type Ca(2+)currents, cell capacity and calcium current densities as a result of diminished LTCC surface expression. Finally, Myoscape expression is reduced in hearts from patients suffering of terminal heart failure, implying a role in human disease.

Myoscape controls cardiac calcium cycling and contractility via regulation of L-type calcium channel surface expression

Calcium signalling plays a critical role in the pathogenesis of heart failure. Here we describe a cardiac protein named Myoscape/FAM40B/STRIP2, which directly interacts with the L-type calcium channel. Knockdown of Myoscape in cardiomyocytes decreases calcium transients associated with smaller Ca²⁺ amplitudes and a lower diastolic Ca²⁺ content. Likewise, L-type calcium channel currents are significantly diminished on Myoscape ablation, and downregulation of Myoscape significantly reduces contractility of cardiomyocytes. Conversely, overexpression of Myoscape increases global Ca²⁺ transients and enhances L-type Ca²⁺ channel currents, and is sufficient to restore decreased currents in failing cardiomyocytes. In vivo, both Myoscape-depleted morphant zebrafish and Myoscape knockout (KO) mice display impairment of cardiac function progressing to advanced heart failure. Mechanistically, Myoscape-deficient mice show reduced L-type Ca²⁺currents, cell capacity and calcium current densities as a result of diminished LTCC surface expression. Finally, Myoscape expression is reduced in hearts from patients suffering of terminal heart failure, implying a role in human disease.

Heart failure is a major public health issue but due to our poor disease understanding the current therapies are symptomatic. Here the authors identify Myoscape as a novel cardiac protein regulating membrane localization of the L-type calcium channel and heart's contractile force, thus promising new therapeutic avenues for heart failure.

Excitation-contraction coupling of human induced pluripotent stem cell-derived cardiomyocytes

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) hold enormous potential in many fields of cardiovascular research. Overcoming many of the limitations of their embryonic counterparts, the application of iPSC-CMs ranges from facilitating investigation of familial cardiac disease and pharmacological toxicity screening to personalized medicine and autologous cardiac cell therapies. The main factor preventing the full realization of this potential is the limited maturity of iPSC-CMs, which display a number of substantial differences in comparison to adult cardiomyocytes. Excitation–contraction (EC) coupling, a fundamental property of cardiomyocytes, is often described in iPSC-CMs as being more analogous to neonatal than adult cardiomyocytes. With Ca²⁺ handling linked, directly or indirectly, to almost all other properties of cardiomyocytes, a solid understanding of this process will be crucial to fully realizing the potential of this technology. Here, we discuss the implications of differences in EC coupling when considering the potential applications of human iPSC-CMs in a number of areas as well as detailing the current understanding of this fundamental process in these cells.

Phosphorylation sites in the Hook domain of CaVβ subunits differentially modulate CaV1.2 channel function

Regulation of L-type calcium current is critical for the development, function, and regulation of many cell types. Ca(V)1.2 channels that conduct L-type calcium currents are regulated by many protein kinases, but the sites of action of these kinases remain unknown in most cases. We combined mass spectrometry (LC-MS/MS) and whole-cell patch clamp techniques in order to identify sites of phosphorylation of Ca(V)β subunits in vivo and test the impact of mutations of those sites on Ca(V)1.2 channel function in vitro. Using the Ca(V)1.1 channel purified from rabbit skeletal muscle as a substrate for phosphoproteomic analysis, we found that Ser(193) and Thr(205) in the HOOK domain of Ca(V)β1a subunits were both phosphorylated in vivo. Ser(193) is located in a potential consensus sequence for casein kinase II, but it was not phosphorylated in vitro by that kinase. In contrast, Thr(205) is located in a consensus sequence for cAMP-dependent phosphorylation, and it was robustly phosphorylated in vitro by PKA. These two sites are conserved in multiple Ca(V)β subunit isoforms, including the principal Ca(V)β subunit of cardiac Ca(V)1.2 channels, Ca(V)β2b. In order to assess potential modulatory effects of phosphorylation at these sites separately from the effects of phosphorylation of the α11.2 subunit, we inserted phosphomimetic or phosphoinhibitory mutations in Ca(V)β2b and analyzed their effects on Ca(V)1.2 channel function in transfected nonmuscle cells. The phosphomimetic mutation Ca(V)β2b(S152E) decreased peak channel currents and shifted the voltage dependence of both activation and inactivation to more positive membrane potentials. The phosphoinhibitory mutation Ca(V)β2b(S152A) had opposite effects. There were no differences in peak Ca(V)1.2 currents or voltage dependence between the phosphomimetic mutation Ca(V)β2b(T164D) and the phosphoinhibitory mutation Ca(V)β2b(T164A). However, calcium-dependent inactivation was significantly increased for the phosphomimetic mutation Ca(V)β2b(T164D). This effect was subunit-specific, as the corresponding mutation in the palmitoylated isoform, Ca(V)β2a, had no effect. Overall, our data identify two conserved sites of phosphorylation of the Hook domain of Ca(V)β subunits in vivo and reveal differential modulatory effects of phosphomimetic mutations in these sites. These results reveal a new dimension of regulation of Ca(V)1.2 channels through phosphorylation of the Hook domains of their β subunits.

L-type Ca2+ channel responses to bay k 8644 in stem cell-derived cardiomyocytes are unusually dependent on holding potential and charge carrier

Human stem cell-derived cardiomyocytes provide a cellular model for the study of electrophysiology in the human heart and are finding a niche in the field of safety pharmacology for predicting proarrhythmia. The cardiac L-type Ca2+ channel is an important target for some of these safety studies. However, the pharmacology of this channel in these cells is altered compared to native cardiac tissue, specifically in its sensitivity to the Ca2+ channel activator S-(-)-Bay K 8644. Using patch clamp electrophysiology, we examined the effects of S-(-)-Bay K 8644 in three separate stem cell-derived cardiomyocyte cell lines under various conditions in an effort to detect more typical responses to the drug. S-(-)-Bay K 8644 failed to produce characteristically large increases in current when cells were held at -40 mV and Ca2+ was used as the charge carrier, although high-affinity binding and the effects of the antagonist isomer, R-(+)-Bay K 8644, were intact. Dephosphorylation of the channel with acetylcholine failed to restore the sensitivity of the channel to the drug. Only when the holding potential was shifted to a more hyperpolarized (-60 mV) level, and external Ca2+ was replaced by Ba2+, could large increases in current amplitude be observed. Even under these conditions, increases in current amplitude varied dramatically between different cell lines and channel kinetics following drug addition were generally atypical. The results indicate that the pharmacology of S-(-)-Bay K 8644 in stem cell-derived cardiomyocytes varies by cell type, is unusually dependent on holding potential and charge carrier, and is different from that observed in primary human heart cells.

Pyruvate restores β-adrenergic sensitivity of L-type Ca(2+) channels in failing rat heart: role of protein phosphatase

Oxidative stress plays a major role in the pathogenesis of heart failure, where the contractile response to β-adrenergic stimulation is profoundly depressed. This condition involves L-type Ca(2+) channels, but the mechanisms underlying their impaired adrenergic regulation are unclear. Thus the present study explored the basis for impaired adrenergic control of Ca(2+) channels in a rat infarction model of heart failure. Patch-clamp recordings of L-type Ca(2+) current (I(Ca,L)) from ventricular myocytes isolated from infarcted hearts showed a blunted response to intracellular cAMP that was reversed by treatment with exogenous pyruvate. Biochemical studies showed that basal and cAMP-stimulated protein kinase A activities were similar in infarcted and sham-operated hearts, whereas molecular analysis also found that binding of protein kinase A to the α(1C) subunit of voltage-gated Ca(2+) channel isoform 1.2 was not different between groups. By contrast, protein phosphatase 2A (PP2A) activity and binding to α(1C) were significantly less in infarcted hearts. The PP2A inhibitor okadaic acid markedly increased I(Ca,L) in sham-operated myocytes, but this response was significantly less in myocytes from infarcted hearts. However, pyruvate normalized I(Ca,L) stimulation by okadaic acid, and this effect was blocked by inhibitors of thioredoxin reductase, implicating a functional role for the redox-active thioredoxin system. Our data suggest that blunted β-adrenergic stimulation of I(CaL) in failing hearts results from hyperphosphorylation of Ca(2+) channels secondary to oxidation-induced impairment of PP2A function. We propose that the redox state of Ca(2+) channels or PP2A is controlled by the thioredoxin system which plays a key role in Ca(2+) channel remodeling of the failing heart.

β2-adrenoceptor transfection enhances contractile reserve of isolated rat ventricular myocytes exposed to chronic isoprenaline stimulation by improving β1-adrenoceptor responsiveness

CONTEXT

Heart failure (HF) is a progressive deterioration in heart function associated with overactivity of the sympathetic nervous system. Elevated sympathetic nervous system activity down regulates the β-adrenergic signal system, suppressing β-adrenoceptors (β-ARs)-mediated contractile support in the failing heart.

OBJECTIVE

We investigated the effects of β(2)-AR gene transfer on shortening amplitude of isolated ventricular myocytes under chronic exposure to isoprenaline (ISO), and further determine the contributions of β(1)-AR and β(2)-AR to the contraction.

MATERIALS AND METHODS

Cardiomyocytes were isolated from adult rat hearts and then transfected with β(2)-AR gene using an adenovirus vector. Four hours after the infection, cardiomyocytes were treated with ISO for another 24 hours to imitate high levels of circulating catecholamines in HF. Western blotting was performed to measure myocardial protein expression of β(2)-AR. Video-based edge-detection system was used to evaluate basal and ISO-stimulated shortening amplitudes of cardiomyocytes.

RESULTS

β(2)-AR gene transfer increased β(2)-AR protein content. Chronic ISO stimulation produced a negative inotropic response, whereas acute ISO stimulation showed a positive inotropic response. β(2)-AR gene transfer had no significant effects on shortening amplitude of cardiomyocytes under normal conditions, but enhanced the blunted contraction of cardiomyocytes under pathological conditions induced by chronic ISO stimulation, and the effect was inhibited by β(1)-AR antagonist, CGP 20712A, instead of β(2)-AR antagonist, ICI 118,551.

DISCUSSION AND CONCLUSIONS

We conclude that β(2)-AR gene transfer in isolated ventricular myocytes under chronic ISO stimulation improves cellular contraction, and the beneficial effects might be mediated by improving β(1)-adrenoceptor responsiveness.

Ca²⁺ channel activators reveal differential L-type Ca²⁺ channel pharmacology between native and stem cell-derived cardiomyocytes

Human stem cell-derived cardiomyocytes provide new models for studying the ion channel pharmacology of human cardiac cells for both drug discovery and safety pharmacology purposes. However, detailed pharmacological characterization of ion channels in stem cell-derived cardiomyocytes is lacking. Therefore, we used patch-clamp electrophysiology to perform a pharmacological survey of the L-type Ca²⁺ channel in induced pluripotent and embryonic stem cell-derived cardiomyocytes and compared the results with native guinea pig ventricular cells. Six structurally distinct antagonists [nifedipine, verapamil, diltiazem, lidoflazine, bepridil, and 2-[(cis-2-phenylcyclopentyl)imino]-azacyclotridecane hydrochloride (MDL 12330)] and two structurally distinct activators [methyl 2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-1,4-dihydropyridine-3-carboxylate (Bay K8644) and 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176)] were used. The IC₅₀ values for the six antagonists showed little variability between the three cell types. However, whereas Bay K8644 produced robust increases in Ca²⁺ channel current in guinea pig myocytes, it failed to enhance current in the two stem cell lines. Furthermore, Ca²⁺ channel current kinetics after addition of Bay K8644 differed in the stem cell-derived cardiomyocytes compared with native cells. FPL 64176 produced consistently large increases in Ca²⁺ channel current in guinea pig myocytes but had a variable effect on current amplitude in the stem cell-derived myocytes. The effects of FPL 64176 on current kinetics were similar in all three cell types. We conclude that, in the stem cell-derived myocytes tested, L-type Ca²⁺ channel antagonist pharmacology is preserved, but the pharmacology of activators is altered. The results highlight the need for extensive pharmacological characterization of ion channels in stem cell-derived cardiomyocytes because these complex proteins contain multiple sites of drug action.

Ahnak1 is a tuneable modulator of cardiac Ca(v)1.2 calcium channel activity

Ahnak1 has been implicated in the beta-adrenergic regulation of the cardiac L-type Ca(2+) channel current (I (CaL)) by its binding to the regulatory Cavβ(2) subunit. In this study, we addressed the question whether ahnak1/Cavβ(2) interactions are essential or redundant for beta-adrenergic stimulation of I (CaL). Three naturally occurring ahnak1 variants (V5075 M, G5242R, and T5796 M) identified by genetic screening of cardiomyopathy patients did essentially not influence the in vitro Cavβ(2) interaction as assessed by recombinant proteins. But, we observed a robust increase in Cavβ(2) binding by mutating Ala at position 4984 to Pro which creates a PxxP consensus motif in the ahnak1 protein fragment. Surface plasmon resonance measurements revealed that this mutation introduced an additional Cavβ(2) binding site. The functionality of A4984P was supported by the specific action of the Pro-containing ahnak1-derived peptide (P4984) in beta-adrenergic regulation of I (CaL). Patch clamp recordings on cardiomyocytes showed that intracellular perfusion of P4984 markedly reduced I (CaL) response to the beta-adrenergic agonist, isoprenaline, while the Ala-containing counterpart failed to affect I (CaL). Interestingly, I (CaL) of ahnak1-deficient cardiomyocytes was not affected by peptide application. Moreover, I (CaL) of ahnak1-deficient cardiomyocytes showed intact beta-adrenergic responsiveness. Similarly isolated ahnak1-deficient mouse hearts responded normally to adrenergic challenge. Our results indicate that ahnak1 is not essential for beta-adrenergic up-regulation of I (CaL) and cardiac contractility in mice. But, tuning ahnak1/Cavβ(2) interaction provides a tool for modulating the beta-adrenergic response of I (CaL).

GSK-3alpha directly regulates beta-adrenergic signaling and the response of the heart to hemodynamic stress in mice

The glycogen synthase kinase-3 (GSK-3) family of serine/threonine kinases consists of 2 highly related isoforms, alpha and beta. Although GSK-3beta has an important role in cardiac development, much remains unknown about the function of either GSK-3 isoform in the postnatal heart. Herein, we present what we believe to be the first studies defining the role of GSK-3alpha in the mouse heart using gene targeting. Gsk3a(-/-) mice over 2 months of age developed progressive cardiomyocyte and cardiac hypertrophy and contractile dysfunction. Following thoracic aortic constriction in young mice, we observed enhanced hypertrophy that rapidly transitioned to ventricular dilatation and contractile dysfunction. Surprisingly, markedly impaired beta-adrenergic responsiveness was found at both the organ and cellular level. This phenotype was reproduced by acute treatment of WT cardiomyocytes with a small molecule GSK-3 inhibitor, confirming that the response was not due to a chronic adaptation to LV dysfunction. Thus, GSK-3alpha appears to be the central regulator of a striking range of essential processes, including acute and direct positive regulation of beta-adrenergic responsiveness. In the absence of GSK-3alpha, the heart cannot respond effectively to hemodynamic stress and rapidly fails. Our findings identify what we believe to be a new paradigm of regulation of beta-adrenergic signaling and raise concerns given the rapid expansion of drug development targeting GSK-3.

Enhanced basal contractility but reduced excitation-contraction coupling efficiency and beta-adrenergic reserve of hearts with increased Cav1.2 activity

Cardiac remodeling during heart failure development induces a significant increase in the activity of the L-type Ca(2+) channel (Cav1.2). However, the effects of enhanced Cav1.2 activity on myocyte excitation-contraction (E-C) coupling, cardiac contractility, and its regulation by the beta-adrenergic system are not clear. To recapitulate the increased Cav1.2 activity, a double transgenic (DTG) mouse model overexpressing the Cavbeta2a subunit in a cardiac-specific and inducible manner was established. We studied cardiac (in vivo) and myocyte (in vitro) contractility at baseline and upon beta-adrenergic stimulation. E-C coupling efficiency was evaluated in isolated myocytes as well. The following results were found: 1) in DTG myocytes, L-type Ca(2+) current (I(Ca,L)) density, myocyte fractional shortening (FS), peak Ca(2+) transients, and sarcoplasmic reticulum (SR) Ca(2+) content (caffeine-induced Ca(2+) transient peak) were significantly increased (by 100.8%, 48.8%, 49.8%, and 46.8%, respectively); and 2) cardiac contractility evaluated with echocardiography [ejection fraction (EF) and (FS)] and invasive intra-left ventricular pressure (maximum dP/dt and -dP/dt) measurements were significantly greater in DTG mice than in control mice. However, 1) the cardiac contractility (EF, FS, dP/dt, and -dP/dt)-enhancing effect of the beta-adrenergic agonist isoproterenol (2 microg/g body wt ip) was significantly reduced in DTG mice, which could be attributed to the loss of beta-adrenergic stimulation on contraction, Ca(2+) transients, I(Ca,L), and SR Ca(2+) content in DTG myocytes; and 2) E-C couplng efficiency was significantly lower in DTG myocytes. In conclusion, increasing Cav1.2 activity by promoting its high-activity mode enhances cardiac contractility but decreases E-C coupling efficiency and the adrenergic reserve of the heart.

{beta}1-Adrenergic receptor activation induces mouse cardiac myocyte death through both L-type calcium channel-dependent and -independent pathways

Cardiac diseases persistently increase the contractility demands of cardiac myocytes, which require activation of the sympathetic nervous system and subsequent increases in myocyte Ca(2+) transients. Persistent exposure to sympathetic and/or Ca(2+) stress is associated with myocyte death. This study examined the respective roles of persistent beta-adrenergic receptor (beta-AR) agonist exposure and high Ca(2+) concentration in myocyte death. Ventricular myocytes (VMs) were isolated from transgenic (TG) mice with cardiac-specific and inducible expression of the beta(2a)-subunit of the L-type Ca(2+) channel (LTCC). VMs were cultured, and the rate of myocyte death was measured in the presence of isoproterenol (ISO), other modulators of Ca(2+) handling and the beta-adrenergic system, and inhibitors of caspases and reactive oxygen species generation. The rate of myocyte death was greater in TG vs. wild-type myocytes and accelerated by ISO in both groups, although ISO did not increase LTCC current (I(Ca-L)) in TG-VMs. Nifedipine, an LTCC antagonist, only partially prevented myocyte death. These results suggest both LTCC-dependent and -independent mechanisms in ISO induced myocyte death. ISO increased the contractility of wild type and TG-VMs by enhancing sarcoplasmic reticulum function and inhibiting sarco(endo)plasmic reticulum Ca(2+)-ATPase, Na(+)/Ca(2+) exchanger, and CaMKII partially protected myocyte from death induced by both Ca(2+) and ISO. Caspase and reactive oxygen species inhibitors did not, but beta(2)-AR activation did, reduce myocyte death induced by enhanced I(Ca-L) and ISO stimulation. Our results suggest that catecholamines induce myocyte necrosis primarily through beta(1)-AR-mediated increases in I(Ca-L), but other mechanisms are also involved in rodents.

L-type Ca(2+) current in ventricular cardiomyocytes

L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.

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.