Cyclic AMP-dependent protein kinase phosphorylates residues in the C-terminal domain of the cardiac L-type calcium channel alpha1 subunit

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.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

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.

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Autoinhibitory control of the CaV1.2 channel by its proteolytically processed distal C-terminal domain

Voltage-gated Ca(2+) channels of the Ca(V)1 family initiate excitation-contraction coupling in cardiac, smooth, and skeletal muscle and are primary targets for regulation by the sympathetic nervous system in the 'fight-or-flight' response. In the heart, activation of beta-adrenergic receptors greatly increases the L-type Ca(2+) current through Ca(V)1.2 channels, which requires phosphorylation by cyclic AMP-dependent protein kinase (PKA) anchored via an A-kinase anchoring protein (AKAP15). Surprisingly, the site of interaction of PKA and AKAP15 lies in the distal C-terminus, which is cleaved from the remainder of the channel by in vivo proteolytic processing. Here we report that the proteolytically cleaved distal C-terminal domain forms a specific molecular complex with the truncated alpha(1) subunit and serves as a potent autoinhibitory domain. Formation of the autoinhibitory complex greatly reduces the coupling efficiency of voltage sensing to channel opening and shifts the voltage dependence of activation to more positive membrane potentials. Ab initio structural modelling and site-directed mutagenesis revealed a binding interaction between a pair of arginine residues in a predicted alpha-helix in the proximal C-terminal domain and a set of three negatively charged amino acid residues in a predicted helix-loop-helix bundle in the distal C-terminal domain. Disruption of this interaction by mutation abolished the inhibitory effects of the distal C-terminus on Ca(V)1.2 channel function. These results provide the first functional characterization of this autoinhibitory complex, which may be a major form of the Ca(V)1 family Ca(2+) channels in cardiac and skeletal muscle cells, and reveal a unique ion channel regulatory mechanism in which proteolytic processing produces a more effective autoinhibitor of Ca(V)1.2 channel function.

Channel phosphorylation and modulation of L-type Ca2+ currents by cytosolic Mg2+ concentration

Previous studies have shown that inhibition of L-type Ca(2+) current (I(Ca)) by cytosolic free Mg(2+) concentration ([Mg(2+)](i)) is profoundly affected by activation of cAMP-dependent protein kinase pathways. To investigate the mechanism underlying this counterregulation of I(Ca), rat cardiac myocytes and tsA201 cells expressing L-type Ca(2+) channels were whole cell voltage-clamped with patch pipettes in which [Mg(2+)] ([Mg(2+)](p)) was buffered by citrate and ATP. In tsA201 cells expressing wild-type Ca(2+) channels (alpha(1C)/beta(2A)/alpha(2)delta), increasing [Mg(2+)](p) from 0.2 mM to 1.8 mM decreased peak I(Ca) by 76 +/- 4.5% (n = 7). Mg(2+)-dependent modulation of I(Ca) was also observed in cells loaded with ATP-gamma-S. With 0.2 mM [Mg(2+)](p), manipulating phosphorylation conditions by pipette application of protein kinase A (PKA) or phosphatase 2A (PP(2A)) produced large changes in I(Ca) amplitude; however, with 1.8 mM [Mg(2+)](p), these same manipulations had no significant effect on I(Ca). With mutant channels lacking principal PKA phosphorylation sites (alpha(1C/S1928A)/beta(2A/S478A/S479A)/alpha(2)delta), increasing [Mg(2+)](p) had only small effects on I(Ca). However, when channel open probability was increased by alpha(1C)-subunit truncation (alpha(1CDelta1905)/beta(2A/S478A/S479A)/alpha(2)delta), increasing [Mg(2+)](p) greatly reduced peak I(Ca). Correspondingly, in myocytes voltage-clamped with pipette PP(2A) to minimize channel phosphorylation, increasing [Mg(2+)](p) produced a much larger reduction in I(Ca) when channel opening was promoted with BAY K8644. These data suggest that, around its physiological concentration range, cytosolic Mg(2+) modulates the extent to which channel phosphorylation regulates I(Ca). This modulation does not necessarily involve changes in channel phosphorylation per se, but more generally appears to depend on the kinetics of gating induced by channel phosphorylation.

Beta-adrenergic stimulation of L-type Ca2+ channels in cardiac myocytes requires the distal carboxyl terminus of alpha1C but not serine 1928

Beta-adrenoceptor stimulation robustly increases cardiac L-type Ca2+ current (ICaL); yet the molecular mechanism of this effect is still not well understood. Previous reports have shown in vitro phosphorylation of a consensus protein kinase A site at serine 1928 on the carboxyl terminus of the alpha1C subunit; however, the functional role of this site has not been investigated in cardiac myocytes. Here, we examine the effects of truncating the distal carboxyl terminus of the alpha1C subunit at amino acid residue 1905 or mutating the putative protein kinase A site at serine 1928 to alanine in adult guinea pig myocytes, using novel dihydropyridine-insensitive alpha1C adenoviruses, coexpressed with beta2 subunits. Expression of alpha1C truncated at 1905 dramatically attenuated the increase of peak ICaL induced by isoproterenol. However, the point mutation S1928A did not significantly attenuate the beta-adrenergic response. The findings indicate that the distal carboxyl-terminus of alpha1C plays an important role in beta-adrenergic upregulation of cardiac L-type Ca2+ channels, but that phosphorylation of serine 1928 is not required for this effect.

Leukemia Inhibitory Factor Activates Cardiac L-Type Ca 2+ Channels via Phosphorylation of Serine 1829 in the Rabbit Ca v 1.2 Subunit

We have previously reported that leukemia inhibitory factor (LIF) gradually increased cardiac L-type Ca 2+ channel current ( I CaL ), which peaked at 15 minutes in both adult and neonatal rat cardiomyocytes, and this increase was blocked by the mitogen-activated protein kinase kinase inhibitor PD98059. This study investigated the molecular basis of LIF-induced augmentation of I CaL in rodent cardiomyocytes. LIF induced phosphorylation of a serine residue in the α 1c subunit (Ca v 1.2) of L-type Ca 2+ channels in cultured rat cardiomyocytes, and this phosphorylation was inhibited by PD98059. When constructs encoding either a wild-type or a carboxyl-terminal–truncated rabbit Ca v 1.2 subunit were transfected into HEK293 cells, LIF induced phosphorylation of the resultant wild-type protein but not the mutant protein. Cotransfection of constitutively active mitogen-activated protein kinase kinase also resulted in phosphorylation of the Ca v 1.2 subunit in the absence of LIF stimulation. In in-gel kinase assays, extracellular signal–regulated kinase phosphorylated a glutathione S -transferase fusion protein of the carboxyl-terminal region of Ca v 1.2 (residues 1700 through 1923), which contains the consensus sequence Pro-Leu-Ser-Pro. A point mutation within this consensus sequence, which results in a substitution of alanine for serine at residue 1829 (S1829A), was sufficient to abolish the LIF-induced phosphorylation. LIF increased I CaL in HEK cells transfected with wild-type Ca v 1.2 but not with the mutated version. These results provide direct evidence that LIF phosphorylates the serine residue at position 1829 of the Ca v 1.2 subunit via the actions of extracellular signal–regulated kinase and that this phosphorylation increases I CaL in cardiomyocytes.

Modulation of cardiac Ca(V)1.2 channels by dihydropyridine and phosphatase inhibitor requires Ser-1142 in the domain III pore loop

Dihydropyridine-sensitive, voltage-activated calcium channels respond to membrane depolarization with two distinct modes of activity: short bursts of very short openings (mode 1) or repetitive openings of much longer duration (mode 2). Here we show that both the dihydropyridine, BayK8644 (BayK), and the inhibitor of SerThr protein phosphatases, okadaic acid, have identical effects on the gating of the recombinant cardiac calcium channel, Ca(V)1.2 (alpha(1)C). Each produced identical mode 2 gating in cell-attached patches, and each prevented rundown of channel activity when the membrane patch was excised into ATP-free solutions. These effects required Ser or Thr at position 1142 in the domain III pore loop between transmembrane segments S5 and S6, where dihydropyridines bind to the channel. Mutation of Ser-1142 to Ala or Cys produced channels with very low activity that could not be modulated by either BayK or okadaic acid. A molecular model of Ca(V)1.2 indicates that Ser-1142 is unlikely to be phosphorylated, and thus we conclude that BayK binding stabilizes mode 2 gating allosterically by either protecting a phospho Ser/Thr on the alpha(1)C subunit or mimicking phosphorylation at that site.

Regulation of extracellular calcium-activated cation currents by cAMP in parathyroid cells

Parathyroid cells express Ca2+-sensing receptors that couple changes in the extracellular Ca2+ concentration ([Ca2+]o) to increases in the intracellular free Ca2+ concentration ([Ca2+]i) and to the suppression of parathyroid hormone secretion. Using whole cell patch clamping, we previously identified voltage-independent Ca2+-conducting currents in bovine parathyroid cells that increased with rising [Ca2+]o and were blocked by Cd2+ and nifedipine. Because cAMP-dependent phosphorylation regulates dihydropyridine-sensitive Ca2+ channels in other systems, we tested whether cAMP modulates these currents. At 0.7 mM Ca2+, nonselective Ca2+-conducting currents were suppressed by 30-50% when the recording pipette was perfused with cAMP. High-[Ca2+]o-induced increases in membrane currents were also abrogated. The effects of cAMP were reversible and dose dependent (3 x 10(-9) to 3 x 10(-3) M) and required ATP in the pipette solution. Perfusion of the cell interior with the catalytic subunit of protein kinase A mimicked the effects of cAMP, as did perfusion of the bath with the adenylate cyclase activator forskolin. These findings support the idea that cAMP-dependent phosphorylation suppresses high-[Ca2+]o-induced cation currents and may play a role in regulating ion fluxes in parathyroid cells.

cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels

The involvement of cAMP-dependent phosphorylation sites in establishing the basal activity of cardiac L-type Ca2+ channels was studied in HEK 293 cells transiently cotransfected with mutants of the human cardiac alpha1 and accessory subunits. Systematic individual or combined elimination of high consensus protein kinase A (PKA) sites, by serine to alanine substitutions at the amino and carboxyl termini of the alpha1 subunit, resulted in Ca2+ channel currents indistinguishable from those of wild type channels. Dihydropyridine (DHP)-binding characteristics were also unaltered. To explore the possible involvement of nonconsensus sites, deletion mutants were used. Carboxyl-terminal truncations of the alpha1 subunit distal to residue 1597 resulted in increased channel expression and current amplitudes. Modulation of PKA activity in cells transfected with the wild type channel or any of the mutants did not alter Ca2+ channel functions suggesting that cardiac Ca2+ channels expressed in these cells behave, in terms of lack of PKA control, like Ca2+ channels of smooth muscle cells.

Inhibition of cloned human L-type cardiac calcium channels by 2,3-butanedione monoxime does not require PKA-dependent phosphorylation sites

The oxime derivative 2,3-butanedione monoxime (BDM) is used as an inorganic phosphatase to probe the phosphorylation state of many cellular proteins including the L-type calcium channel in various tissues. We used BDM further to shed light on the controversy surrounding direct phosphorylation of the L-type Ca2+ channel. We employed a recombinant system that utilizes HEK 293 cells expressing wild type and mutant human heart calcium channels. BDM reversibly reduced the calcium channel current induced by expression of the wild type channel in a concentration-dependent manner with an apparent IC50 value of 15.3 mM. Deletion of part of the carboxyl terminus of the alpha 1 subunit, which contains one putative protein kinase A site, or mutating all of the protein kinase A consensus sites of the pore forming subunit, did not significantly change the apparent IC50 value or alter in any other way the blocking effect of BDM on the expressed currents. Our data suggest that BDM produces reversible modifications of the cardiac calcium channel protein leading to an expected reduction in the amplitude of the expressed currents, but the site of action must be different from that of the consensus sites for protein kinase A dependent phosphorylation.