Deletion of the C-terminal phosphorylation sites in the cardiac β-subunit does not affect the basic β-adrenergic response of the heart and the Ca(v)1.2 channel

Regulation of Cardiac Cav1.2 Channels by Calmodulin

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

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.

Post-Translational Modification of Cav1.2 and its Role in Neurodegenerative Diseases

Cav1.2 plays an essential role in learning and memory, drug addiction, and neuronal development. Intracellular calcium homeostasis is disrupted in neurodegenerative diseases because of abnormal Cav1.2 channel activity and modification of downstream Ca²⁺ signaling pathways. Multiple post-translational modifications of Cav1.2 have been observed and seem to be closely related to the pathogenesis of neurodegenerative diseases. The specific molecular mechanisms by which Cav1.2 channel activity is regulated remain incompletely understood. Dihydropyridines (DHPs), which are commonly used for hypertension and myocardial ischemia, have been repurposed to treat PD and AD and show protective effects. However, further studies are needed to improve delivery strategies and drug selectivity. Better knowledge of channel modulation and more specific methods for altering Cav1.2 channel function may lead to better therapeutic strategies for neurodegenerative diseases.

Use of Proximity Labeling in Cardiovascular Research

Protein-protein interactions are of paramount importance in regulating normal cardiac physiology. Methodologies to elucidate these interactions in vivo have been limited. Recently, proximity-dependent biotinylation, with the use of BioID, TurboID, and ascorbate peroxidase, has been developed to uncover cellular neighborhoods and novel protein-protein interactions. These cutting-edge techniques have enabled the identification of subcellular localizations of specific proteins and the neighbors or interacting proteins within these subcellular regions. In contrast to classic methods such as affinity purification and subcellular fractionation, these techniques add covalently bound tags in living cells, such that spatial relationships and interaction networks are not disrupted. Recently, these methodologies have been used to identify novel protein-protein interactions relevant to the cardiovascular system. In this review, we discuss the development and current use of proximity biotin-labeling for cardiovascular research.

Reconstitution of β-adrenergic regulation of CaV1.2: Rad-dependent and Rad-independent protein kinase A mechanisms

L-type voltage-gated Ca_V1.2 channels crucially regulate cardiac muscle contraction. Activation of β-adrenergic receptors (β-AR) augments contraction via protein kinase A (PKA)-induced increase of calcium influx through Ca_V1.2 channels. To date, the full β-AR cascade has never been heterologously reconstituted. A recent study identified Rad, a Ca_V1.2 inhibitory protein, as essential for PKA regulation of Ca_V1.2. We corroborated this finding and reconstituted the complete pathway with agonist activation of β1-AR or β2-AR in Xenopus oocytes. We found, and distinguished between, two distinct pathways of PKA modulation of Ca_V1.2: Rad dependent (∼80% of total) and Rad independent. The reconstituted system reproduces the known features of β-AR regulation in cardiomyocytes and reveals several aspects: the differential regulation of posttranslationally modified Ca_V1.2 variants and the distinct features of β1-AR versus β2-AR activity. This system allows for the addressing of central unresolved issues in the β-AR-Ca_V1.2 cascade and will facilitate the development of therapies for catecholamine-induced cardiac pathologies.

Rad-GTPase contributes to heart rate via L-type calcium channel regulation

Sinoatrial node cardiomyocytes (SANcm) possess automatic, rhythmic electrical activity. SAN rate is influenced by autonomic nervous system input, including sympathetic nerve increases of heart rate (HR) via activation of β-adrenergic receptor signaling cascade (β-AR). L-type calcium channel (LTCC) activity contributes to membrane depolarization and is a central target of β-AR signaling. Recent studies revealed that the small G-protein Rad plays a central role in β-adrenergic receptor directed modulation of LTCC. These studies have identified a conserved mechanism in which β-AR stimulation results in PKA-dependent Rad phosphorylation: depletion of Rad from the LTCC complex, which is proposed to relieve the constitutive inhibition of CaV1.2 imposed by Rad association. Here, using a transgenic mouse model permitting conditional cardiomyocyte selective Rad ablation, we examine the contribution of Rad to the control of SANcm LTCC current (ICa,L) and sinus rhythm. Single cell analysis from a recent published database indicates that Rad is expressed in SANcm, and we show that SANcm ICa,L was significantly increased in dispersed SANcm following Rad silencing compared to those from CTRL hearts. Moreover, cRadKO SANcm ICa,L was not further increased with β-AR agonists. We also evaluated heart rhythm in vivo using radiotelemetered ECG recordings in ambulating mice. In vivo, intrinsic HR is significantly elevated in cRadKO. During the sleep phase cRadKO also show elevated HR, and during the active phase there is no significant difference. Rad-deletion had no significant effect on heart rate variability. These results are consistent with Rad governing LTCC function under relatively low sympathetic drive conditions to contribute to slower HR during the diurnal sleep phase HR. In the absence of Rad, the tonic modulated SANcm ICa,L promotes elevated sinus HR. Future novel therapeutics for bradycardia targeting Rad - LTCC can thus elevate HR while retaining βAR responsiveness.

Requirement of the Ca2+ channel β2 subunit for sympathetic PKA phosphorylation

Facilitation of cardiac function in response to signals from the sympathetic nervous system is initiated by the phosphorylation of L-type voltage-dependent Ca²⁺ channels (VDCCs) by protein kinase A (PKA), which in turn is activated by β-adrenoceptors. Among the five subunits (α₁, β, α₂/δ, and γ) of VDCCs, the α₁ subunit and the family of β subunits are substrates for PKA-catalyzed phosphorylation; however, the subunit responsible for β-adrenergic augmentation of Ca²⁺ channel function has yet to be specifically identified. Here we show that the VDCC β₂ subunit is required for PKA phosphorylation upon sympathetic acceleration. In mice with β₂ subunit-null mutations, cardiac muscle contraction in response to isoproterenol was reduced and there was no significant increase in Ca²⁺ channel currents upon PKA-dependent phosphorylation. These findings indicate that within the sympathetic nervous system the β₂ subunit of VDCCs is required for physiological PKA-dependent channel phosphorylation.

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.

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.

New aspects in cardiac L-type Ca2+ channel regulation

Cardiac excitation-contraction coupling is initiated with the influx of Ca2+ ions across the plasma membrane through voltage-gated L-type calcium channels. This process is tightly regulated by modulation of the channel open probability and channel localization. Protein kinase A (PKA) is found in close association with the channel and is one of the main regulators of its function. Whether this kinase is modulating the channel open probability by phosphorylation of key residues or via alternative mechanisms is unclear. This review summarizes recent findings regarding the PKA-mediated channel modulation and will highlight recently discovered regulatory mechanisms that are independent of PKA activity and involve protein-protein interactions and channel localization.

Mechanism of adrenergic CaV1.2 stimulation revealed by proximity proteomics

Increased cardiac contractility during the fight-or-flight response is caused by β-adrenergic augmentation of CaV1.2 voltage-gated calcium channels1-4. However, this augmentation persists in transgenic murine hearts expressing mutant CaV1.2 α1C and β subunits that can no longer be phosphorylated by protein kinase A-an essential downstream mediator of β-adrenergic signalling-suggesting that non-channel factors are also required. Here we identify the mechanism by which β-adrenergic agonists stimulate voltage-gated calcium channels. We express α1C or β2B subunits conjugated to ascorbate peroxidase5 in mouse hearts, and use multiplexed quantitative proteomics6,7 to track hundreds of proteins in the proximity of CaV1.2. We observe that the calcium-channel inhibitor Rad8,9, a monomeric G protein, is enriched in the CaV1.2 microenvironment but is depleted during β-adrenergic stimulation. Phosphorylation by protein kinase A of specific serine residues on Rad decreases its affinity for β subunits and relieves constitutive inhibition of CaV1.2, observed as an increase in channel open probability. Expression of Rad or its homologue Rem in HEK293T cells also imparts stimulation of CaV1.3 and CaV2.2 by protein kinase A, revealing an evolutionarily conserved mechanism that confers adrenergic modulation upon voltage-gated calcium channels.

Roles and Regulation of Voltage-gated Calcium Channels in Arrhythmias

Calcium flowing through voltage-dependent calcium channels into cardiomyocytes mediates excitation–contraction coupling, controls action-potential duration and automaticity in nodal cells, and regulates gene expression. Proper surface targeting and basal and hormonal regulation of calcium channels are vital for normal cardiac physiology. In this review, we discuss the roles of voltage-gated calcium channels in the heart and the mechanisms by which these channels are regulated by physiological signaling pathways in health and disease.

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.

The L-type calcium channel current modulation mechanism: the plot thickens and fogs

Stressful situations provoke the fight-or-flight response, incurring rapid elevation of cardiac output via activation of protein kinase A (PKA). In this issue of the JCI, Yang et al. focus on the L-type calcium channel complex (LTCC), and their findings require reexamination of dogmatic principles. LTCC phosphorylation sites identified and studied to date are dispensable for PKA modulation of LTCC; however, a CaVβ2-CaV1.2 calcium channel interaction is now shown to be required. Yang et al. suggest a new hypothesis that LTCC modulation involves rearrangement of auxiliary proteins within the LTCC. However, we still do not know the targets of PKA that mediate LTCC modulation.

Caveolae-Specific CaMKII Signaling in the Regulation of Voltage-Dependent Calcium Channel and Cardiac Hypertrophy

Cardiac hypertrophy is a major risk for the progression of heart failure; however, the underlying molecular mechanisms contributing to this process remain elusive. The caveolae microdomain plays pivotal roles in various cellular processes such as lipid homeostasis, signal transduction, and endocytosis, and also serves as a signaling platform. Although the caveolae microdomain has been postulated to have a major contribution to the development of cardiac pathologies, including cardiac hypertrophy, recent evidence has placed this role into question. Lack of direct evidence and appropriate methods for determining activation of caveolae-specific signaling has thus far limited the ability to obtain a definite answer to the question. In this review, we focus on the potential physiological and pathological roles of the multifunctional kinase Ca²⁺/calmodulin-dependent kinase II and voltage-dependent L-type calcium channel in the caveolae, toward gaining a better understanding of the contribution of caveolae-based signaling in cardiac hypertrophy.

Regulation of microdomain voltage-gated L-type calcium channels in cardiac health and disease

Cav1.2 channels in the heart mediate excitation-contraction (E-C) coupling; tune cardiac excitability; and regulate gene expression. In ventricular myocytes, Ca_V1.2 channels are predominantly located in t-tubules where they are in proximity to ryanodine receptors to trigger cardiac E-C coupling. A subset of ventricular Ca_V1.2 channels existing on the surface sarcolemma, including in caveolae, have less well-defined functions. Cardiac Ca_V1.2 channels are famously up-regulated by protein kinase A as a component of the 'fight-or-flight' response. The molecular details of how this kinase regulates cardiac Ca_V1.2 channels are controversial and under intensive study. Here, we critically review recent work addressing the putative functions of microdomain cardiac Ca_V1.2 channels, and their regulation by distinct kinases in health and disease.

Protein kinase C enhances plasma membrane expression of cardiac L-type calcium channel, CaV1.2

L-type-voltage-dependent Ca²⁺ channels (L-VDCCs; Ca_V1.2, α_1C), crucial in cardiovascular physiology and pathology, are modulated via activation of G-protein-coupled receptors and subsequently protein kinase C (PKC). Despite extensive study, key aspects of the mechanisms leading to PKC-induced Ca²⁺ current increase are unresolved. A notable residue, Ser1928, located in the distal C-terminus (dCT) of α_1C was shown to be phosphorylated by PKC. Ca_V1.2 undergoes posttranslational modifications yielding full-length and proteolytically cleaved CT-truncated forms. We have previously shown that, in Xenopus oocytes, activation of PKC enhances α_1C macroscopic currents. This increase depended on the isoform of α_1C expressed. Only isoforms containing the cardiac, long N-terminus (L-NT), were upregulated by PKC. Ser1928 was also crucial for the full effect of PKC. Here we report that, in Xenopus oocytes, following PKC activation the amount of α_1C protein expressed in the plasma membrane (PM) increases within minutes. The increase in PM content is greater with full-length α_1C than in dCT-truncated α_1C, and requires Ser1928. The same was observed in HL-1 cells, a mouse atrium cell line natively expressing cardiac α_1C, which undergoes the proteolytic cleavage of the dCT, thus providing a native setting for exploring the effects of PKC in cardiomyocytes. Interestingly, activation of PKC preferentially increased the PM levels of full-length, L-NT α_1C. Our findings suggest that part of PKC regulation of Ca_V1.2 in the heart involves changes in channel's cellular fate. The mechanism of this PKC regulation appears to involve the C-terminus of α_1C, possibly corroborating the previously proposed role of NT-CT interactions within α_1C.

Proteolytic cleavage and PKA phosphorylation of α1C subunit are not required for adrenergic regulation of CaV1.2 in the heart

Calcium influx through the voltage-dependent L-type calcium channel (Ca_V1.2) rapidly increases in the heart during "fight or flight" through activation of the β-adrenergic and protein kinase A (PKA) signaling pathway. The precise molecular mechanisms of β-adrenergic activation of cardiac Ca_V1.2, however, are incompletely known, but are presumed to require phosphorylation of residues in α_1C and C-terminal proteolytic cleavage of the α_1C subunit. We generated transgenic mice expressing an α_1C with alanine substitutions of all conserved serine or threonine, which is predicted to be a potential PKA phosphorylation site by at least one prediction tool, while sparing the residues previously shown to be phosphorylated but shown individually not to be required for β-adrenergic regulation of Ca_V1.2 current (17-mutant). A second line included these 17 putative sites plus the five previously identified phosphoregulatory sites (22-mutant), thus allowing us to query whether regulation requires their contribution in combination. We determined that acute β-adrenergic regulation does not require any combination of potential PKA phosphorylation sites conserved in human, guinea pig, rabbit, rat, and mouse α_1C subunits. We separately generated transgenic mice with inducible expression of proteolytic-resistant α_1C Prevention of C-terminal cleavage did not alter β-adrenergic stimulation of Ca_V1.2 in the heart. These studies definitively rule out a role for all conserved consensus PKA phosphorylation sites in α_1C in β-adrenergic stimulation of Ca_V1.2, and show that phosphoregulatory sites on α_1C are not redundant and do not each fractionally contribute to the net stimulatory effect of β-adrenergic stimulation. Further, proteolytic cleavage of α_1C is not required for β-adrenergic stimulation of Ca_V1.2.

Beta-adrenergic regulation of the heart expressing the Ser1700A/Thr1704A mutated Cav1.2 channel

Beta-adrenergic stimulation of the heart increases I_Ca. PKA dependent phosphorylation of several amino acids (among them Ser 1700 and Thr 1704 in the carboxy-terminus of the Cav1.2 α₁c subunit) has been implicated as decisive for the β-adrenergic up-regulation of cardiac I_Ca. Mutation of Ser 1700 and Thr 1704 to alanine results in the Cav1.2PKA_P2^-/- mice. Cav1.2PKA_P2^-/- mice display reduced cardiac L-type current. Fractional shortening and ejection fraction in the intact animal and I_Ca in isolated cardiomyocytes (CM) are stimulated by isoproterenol. Cardiac specific expression of the mutated Cav1.2PKA_P2^-/- gene reduces Cav1.2 α₁c protein concentration, I_Ca, and the β-adrenergic stimulation of L-type I_Ca in CMs. Single channels were not detected on the CM surface of the cCav1.2PKA_P2^-/- hearts. This outcome supports the notion that S1700/1704 is essential for expression of the Cav1.2 channel and that isoproterenol stimulates I_Ca in Cav1.2PKA_P2^-/- CMs.

Protein kinase A regulates C-terminally truncated CaV 1.2 in Xenopus oocytes: roles of N- and C-termini of the α1C subunit

KEY POINTS

β-Adrenergic stimulation enhances Ca²⁺ entry via L-type Ca_V 1.2 channels, causing stronger contraction of cardiac muscle cells. The signalling pathway involves activation of protein kinase A (PKA), but the molecular details of PKA regulation of Ca_V 1.2 remain controversial despite extensive research. We show that PKA regulation of Ca_V 1.2 can be reconstituted in Xenopus oocytes when the distal C-terminus (dCT) of the main subunit, α_1C , is truncated. The PKA upregulation of Ca_V 1.2 does not require key factors previously implicated in this mechanism: the clipped dCT, the A kinase-anchoring protein 15 (AKAP15), the phosphorylation sites S1700, T1704 and S1928, or the β subunit of Ca_V 1.2. The gating element within the initial segment of the N-terminus of the cardiac isoform of α_1C is essential for the PKA effect. We propose that the regulation described here is one of two or several mechanisms that jointly mediate the PKA regulation of Ca_V 1.2 in the heart.

ABSTRACT

β-Adrenergic stimulation enhances Ca²⁺ currents via L-type, voltage-gated Ca_V 1.2 channels, strengthening cardiac contraction. The signalling via β-adrenergic receptors (β-ARs) involves elevation of cyclic AMP (cAMP) levels and activation of protein kinase A (PKA). However, how PKA affects the channel remains controversial. Recent studies in heterologous systems and genetically engineered mice stress the importance of the post-translational proteolytic truncation of the distal C-terminus (dCT) of the main (α_1C ) subunit. Here, we successfully reconstituted the cAMP/PKA regulation of the dCT-truncated Ca_V 1.2 in Xenopus oocytes, which previously failed with the non-truncated α_1C . cAMP and the purified catalytic subunit of PKA, PKA-CS, injected into intact oocytes, enhanced Ca_V 1.2 currents by ∼40% (rabbit α_1C ) to ∼130% (mouse α_1C ). PKA blockers were used to confirm specificity and the need for dissociation of the PKA holoenzyme. The regulation persisted in the absence of the clipped dCT (as a separate protein), the A kinase-anchoring protein AKAP15, and the phosphorylation sites S1700 and T1704, previously proposed as essential for the PKA effect. The Ca_V β_2b subunit was not involved, as suggested by extensive mutagenesis. Using deletion/chimeric mutagenesis, we have identified the initial segment of the cardiac long-N-terminal isoform of α_1C as a previously unrecognized essential element involved in PKA regulation. We propose that the observed regulation, that exclusively involves the α_1C subunit, is one of several mechanisms underlying the overall PKA action on Ca_V 1.2 in the heart. We hypothesize that PKA is acting on Ca_V 1.2, in part, by affecting a structural 'scaffold' comprising the interacting cytosolic N- and C-termini of α_1C .

Caveolae-specific activation loop between CaMKII and L-type Ca2+ channel aggravates cardiac hypertrophy in α1-adrenergic stimulation

Activation of CaMKII induces a myriad of biological processes and plays dominant roles in cardiac hypertrophy. Caveolar microdomain contains many calcium/calmodulin-dependent kinase II (CaMKII) targets, including L-type Ca²⁺ channel (LTCC) complex, and serves as a signaling platform. The location of CaMKII activation is thought to be critical; however, the roles of CaMKII in caveolae are still elusive due to lack of methodology for the assessment of caveolae-specific activation. Our aim was to develop a novel tool for the specific analysis of CaMKII activation in caveolae and to determine the functional role of caveolar CaMKII in cardiac hypertrophy. To assess the caveolae-specific activation of CaMKII, we generated a fusion protein composed of phospholamban and caveolin-3 (cPLN-Cav3) and GFP fusion protein with caveolin-binding domain fused to CaMKII inhibitory peptide (CBD-GFP-AIP), which inhibits CaMKII activation specifically in caveolae. Caveolae-specific activation of CaMKII was detected using phosphospecific antibody for PLN (Thr¹⁷). Furthermore, adenoviral overexpression of LTCC β_2a-subunit (β_2a) in NRCMs showed its constitutive phosphorylation by CaMKII, which induces hypertrophy, and that both phosphorylation and hypertrophy are abolished by CBD-GFP-AIP expression, indicating that β_2a phosphorylation occurs specifically in caveolae. Finally, β_2a phosphorylation was observed after phenylephrine stimulation in β_2a-overexpressing mice, and attenuation of cardiac hypertrophy after chronic phenylephrine stimulation was observed in nonphosphorylated mutant of β_2a-overexpressing mice. We developed novel tools for the evaluation and inhibition of caveolae-specific activation of CaMKII. We demonstrated that phosphorylated β_2a dominantly localizes to caveolae and induces cardiac hypertrophy after α₁-adrenergic stimulation in mice.NEW & NOTEWORTHY While signaling in caveolae is thought to be important in cardiac hypertrophy, direct evidence is missing due to lack of tools to assess caveolae-specific signaling. This is the first study to demonstrate caveolae-specific activation of CaMKII signaling in cardiac hypertrophy induced by α₁-adrenergic stimulation using an originally developed tool.

Counteracting Protein Kinase Activity in the Heart: The Multiple Roles of Protein Phosphatases

Decades of cardiovascular research have shown that variable and flexible levels of protein phosphorylation are necessary to maintain cardiac function. A delicate balance between phosphorylated and dephosphorylated states of proteins is guaranteed by a complex interplay of protein kinases (PKs) and phosphatases. Serine/threonine phosphatases, in particular members of the protein phosphatase (PP) family govern dephosphorylation of the majority of these cardiac proteins. Recent findings have however shown that PPs do not only dephosphorylate previously phosphorylated proteins as a passive control mechanism but are capable to actively control PK activity via different direct and indirect signaling pathways. These control mechanisms can take place on (epi-)genetic, (post-)transcriptional, and (post-)translational levels. In addition PPs themselves are targets of a plethora of proteinaceous interaction partner regulating their endogenous activity, thus adding another level of complexity and feedback control toward this system. Finally, novel approaches are underway to achieve spatiotemporal pharmacologic control of PPs which in turn can be used to fine-tune misleaded PK activity in heart disease. Taken together, this review comprehensively summarizes the major aspects of PP-mediated PK regulation and discusses the subsequent consequences of deregulated PP activity for cardiovascular diseases in depth.

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

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

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.

Surface L-type Ca2+ channel expression levels are increased in aged hippocampus

Age-related increase in L-type Ca²⁺ channel (LTCC) expression in hippocampal pyramidal neurons has been hypothesized to underlie the increased Ca²⁺ influx and subsequent reduced intrinsic neuronal excitability of these neurons that lead to age-related cognitive deficits. Here, using specific antibodies against Ca_v1.2 and Ca_v1.3 subunits of LTCCs, we systematically re-examined the expression of these proteins in the hippocampus from young (3 to 4 month old) and aged (30 to 32 month old) F344xBN rats. Western blot analysis of the total expression levels revealed significant reductions in both Ca_v1.2 and Ca_v1.3 subunits from all three major hippocampal regions of aged rats. Despite the decreases in total expression levels, surface biotinylation experiments revealed significantly higher proportion of expression on the plasma membrane of Ca_v1.2 in the CA1 and CA3 regions and of Ca_v1.3 in the CA3 region from aged rats. Furthermore, the surface biotinylation results were supported by immunohistochemical analysis that revealed significant increases in Ca_v1.2 immunoreactivity in the CA1 and CA3 regions of aged hippocampal pyramidal neurons. In addition, we found a significant increase in the level of phosphorylated Ca_v1.2 on the plasma membrane in the dentate gyrus of aged rats. Taken together, our present findings strongly suggest that age-related cognitive deficits cannot be attributed to a global change in L-type channel expression nor to the level of phosphorylation of Ca_v1.2 on the plasma membrane of hippocampal neurons. Rather, increased expression and density of LTCCs on the plasma membrane may underlie the age-related increase in L-type Ca²⁺ channel activity in CA1 pyramidal neurons.

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

Regulation of cardiac L-type Ca²⁺ channel CaV1.2 via the β-adrenergic-cAMP-protein kinase A pathway: old dogmas, advances, and new uncertainties

In the heart, adrenergic stimulation activates the β-adrenergic receptors coupled to the heterotrimeric stimulatory Gs protein, followed by subsequent activation of adenylyl cyclase, elevation of cyclic AMP levels, and protein kinase A (PKA) activation. One of the main targets for PKA modulation is the cardiac L-type Ca²⁺ channel (CaV1.2) located in the plasma membrane and along the T-tubules, which mediates Ca²⁺ entry into cardiomyocytes. β-Adrenergic receptor activation increases the Ca²⁺ current via CaV1.2 channels and is responsible for the positive ionotropic effect of adrenergic stimulation. Despite decades of research, the molecular mechanism underlying this modulation has not been fully resolved. On the contrary, initial reports of identification of key components in this modulation were later refuted using advanced model systems, especially transgenic animals. Some of the cardinal debated issues include details of specific subunits and residues in CaV1.2 phosphorylated by PKA, the nature, extent, and role of post-translational processing of CaV1.2, and the role of auxiliary proteins (such as A kinase anchoring proteins) involved in PKA regulation. In addition, the previously proposed crucial role of PKA in modulation of unstimulated Ca²⁺ current in the absence of β-adrenergic receptor stimulation and in voltage-dependent facilitation of CaV1.2 remains uncertain. Full reconstitution of the β-adrenergic receptor signaling pathway in heterologous expression systems remains an unmet challenge. This review summarizes the past and new findings, the mechanisms proposed and later proven, rejected or disputed, and emphasizes the essential issues that remain unresolved.

Manipulating L-type calcium channels in cardiomyocytes using split-intein protein transsplicing

Manipulating expression of large genes (>6 kb) in adult cardiomyocytes is challenging because these cells are only efficiently transduced by viral vectors with a 4-7 kb packaging capacity. This limitation impedes understanding structure-function mechanisms of important proteins in heart. L-type calcium channels (LTCCs) regulate diverse facets of cardiac physiology including excitation-contraction coupling, excitability, and gene expression. Many important questions about how LTCCs mediate such multidimensional signaling are best resolved by manipulating expression of the 6.6 kb pore-forming α1C-subunit in adult cardiomyocytes. Here, we use split-intein-mediated protein transsplicing to reconstitute LTCC α1C-subunit from two distinct halves, overcoming the difficulty of expressing full-length α1C in cardiomyocytes. Split-intein-tagged α1C fragments encoding dihydropyridine-resistant channels were incorporated into adenovirus and reconstituted in cardiomyocytes. Similar to endogenous LTCCs, recombinant channels targeted to dyads, triggered Ca(2+) transients, associated with caveolin-3, and supported β-adrenergic regulation of excitation-contraction coupling. This approach lowers a longstanding technical hurdle to manipulating large proteins in cardiomyocytes.

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.

CaV1.2 signaling complexes in the heart

L-type Ca(2+) channels (LTCCs) are essential for generation of the electrical and mechanical properties of cardiac muscle. Furthermore, regulation of LTCC activity plays a central role in mediating the effects of sympathetic stimulation on the heart. The primary mechanism responsible for this regulation involves β-adrenergic receptor (βAR) stimulation of cAMP production and subsequent activation of protein kinase A (PKA). Although it is well established that PKA-dependent phosphorylation regulates LTCC function, there is still much we do not understand. However, it has recently become clear that the interaction of the various signaling proteins involved is not left to completely stochastic events due to random diffusion. The primary LTCC expressed in cardiac muscle, CaV1.2, forms a supramolecular signaling complex that includes the β2AR, G proteins, adenylyl cyclases, phosphodiesterases, PKA, and protein phosphatases. In some cases, the protein interactions with CaV1.2 appear to be direct, in other cases they involve scaffolding proteins such as A kinase anchoring proteins and caveolin-3. Functional evidence also suggests that the targeting of these signaling proteins to specific membrane domains plays a critical role in maintaining the fidelity of receptor mediated LTCC regulation. This information helps explain the phenomenon of compartmentation, whereby different receptors, all linked to the production of a common diffusible second messenger, can vary in their ability to regulate LTCC activity. The purpose of this review is to examine our current understanding of the signaling complexes involved in cardiac LTCC regulation.