Phospholamban modulates the functional coupling between nucleotide domains in Ca-ATPase oligomeric complexes in cardiac sarcoplasmic reticulum

Calcium Handling in Inherited Cardiac Diseases: A Focus on Catecholaminergic Polymorphic Ventricular Tachycardia and Hypertrophic Cardiomyopathy

Calcium (Ca2+) is the major mediator of cardiac contractile function. It plays a key role in regulating excitation-contraction coupling and modulating the systolic and diastolic phases. Defective handling of intracellular Ca2+ can cause different types of cardiac dysfunction. Thus, the remodeling of Ca2+ handling has been proposed to be a part of the pathological mechanism leading to electrical and structural heart diseases. Indeed, to ensure appropriate electrical cardiac conduction and contraction, Ca2+ levels are regulated by several Ca2+-related proteins. This review focuses on the genetic etiology of cardiac diseases related to calcium mishandling. We will approach the subject by focalizing on two clinical entities: catecholaminergic polymorphic ventricular tachycardia (CPVT) as a cardiac channelopathy and hypertrophic cardiomyopathy (HCM) as a primary cardiomyopathy. Further, this review will illustrate the fact that despite the genetic and allelic heterogeneity of cardiac defects, calcium-handling perturbations are the common pathophysiological mechanism. The newly identified calcium-related genes and the genetic overlap between the associated heart diseases are also discussed in this review.

Natural Polyphenols as SERCA Activators: Role in the Endoplasmic Reticulum Stress-Related Diseases

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a key protein responsible for transporting Ca²⁺ ions from the cytosol into the lumen of the sarco/endoplasmic reticulum (SR/ER), thus maintaining Ca²⁺ homeostasis within cells. Accumulating evidence suggests that impaired SERCA function is associated with disruption of intracellular Ca²⁺ homeostasis and induction of ER stress, leading to different chronic pathological conditions. Therefore, appropriate strategies to control Ca²⁺ homeostasis via modulation of either SERCA pump activity/expression or relevant signaling pathways may represent a useful approach to combat pathological states associated with ER stress. Natural dietary polyphenolic compounds, such as resveratrol, gingerol, ellagic acid, luteolin, or green tea polyphenols, with a number of health-promoting properties, have been described either to increase SERCA activity/expression directly or to affect Ca²⁺ signaling pathways. In this review, potential Ca²⁺-mediated effects of the most studied polyphenols on SERCA pumps or related Ca²⁺ signaling pathways are summarized, and relevant mechanisms of their action on Ca²⁺ regulation with respect to various ER stress-related states are depicted. All data were collected using scientific search tools (i.e., Science Direct, PubMed, Scopus, and Google Scholar).

The RalGAPα1-RalA signal module protects cardiac function through regulating calcium homeostasis

Sarcoplasmic/endoplasmic reticulum calcium ATPase SERCA2 mediates calcium re-uptake from the cytosol into sarcoplasmic reticulum, and its dysfunction is a hallmark of heart failure. Multiple factors have been identified to modulate SERCA2 activity, however, its regulation is still not fully understood. Here we identify a Ral-GTPase activating protein RalGAPα1 as a critical regulator of SERCA2 in cardiomyocytes through its downstream target RalA. RalGAPα1 is induced by pressure overload, and its deficiency causes cardiac dysfunction and exacerbates pressure overload-induced heart failure. Mechanistically, RalGAPα1 regulates SERCA2 through direct interaction and its target RalA. Deletion of RalGAPα1 decreases SERCA2 activity and prolongs calcium re-uptake into sarcoplasmic reticulum. GDP-bound RalA, but not GTP-bound RalA, binds to SERCA2 and activates the pump for sarcoplasmic reticulum calcium re-uptake. Overexpression of a GDP-bound RalAS28N mutant in the heart preserves cardiac function in a mouse model of heart failure. Our findings have therapeutic implications for treatment of heart failure.

Dimerization of SERCA2a Enhances Transport Rate and Improves Energetic Efficiency in Living Cells

The type 2a sarco/endoplasmic reticulum (ER) Ca²⁺-ATPase (SERCA2a) plays a key role in intracellular Ca²⁺ regulation in the heart. We have previously shown evidence of stable homodimers of SERCA2a in heterologous cells and cardiomyocytes. However, the functional significance of the pump dimerization remains unclear. Here, we analyzed how SERCA2a dimerization affects ER Ca²⁺ transport. Fluorescence resonance energy transfer experiments in HEK293 cells transfected with fluorescently labeled SERCA2a revealed increasing dimerization of Ca²⁺ pumps with increasing expression level. This concentration-dependent dimerization provided means of comparison of the functional characteristics of monomeric and dimeric pumps. SERCA-mediated Ca²⁺ uptake was measured with the ER-targeted Ca²⁺ sensor R-CEPIA1er in cells cotransfected with SERCA2a and ryanodine receptor. For each individual cell, the maximal ER Ca²⁺ uptake rate and the maximal Ca²⁺ load, together with the pump expression level, were analyzed. This analysis revealed that the ER Ca²⁺ uptake rate increased as a function of SERCA2a expression, with a particularly steep, nonlinear increase at high expression levels. Interestingly, the maximal ER Ca²⁺ load also increased with an increase in the pump expression level, suggesting improved catalytic efficiency of the dimeric species. Reciprocally, thapsigargin inhibition of a fraction of the population of SERCA2a reduced not only the maximal ER Ca²⁺ uptake rate but also the maximal Ca²⁺ load. These data suggest that SERCA2a dimerization regulates Ca²⁺ transport by improving both the SERCA2a turnover rate and catalytic efficacy. Analysis of ER Ca²⁺ uptake in cells cotransfected with human wild-type SERCA2a (SERCA2a^WT) and SERCA2a mutants with different catalytic activity revealed that an intact catalytic cycle in both protomers is required for enhancing the efficacy of Ca²⁺ transport by a dimer. The data are consistent with the hypothesis of functional coupling of two SERCA2a protomers in a dimer that reduces the energy barrier of rate-limiting steps of the catalytic cycle of Ca²⁺ transport.

SPEG Controls Calcium Reuptake Into the Sarcoplasmic Reticulum Through Regulating SERCA2a by Its Second Kinase-Domain

RATIONALE

SPEG (Striated muscle preferentially expressed protein kinase) has 2 kinase-domains and is critical for cardiac development and function. However, it is not clear how these 2 kinase-domains function to maintain cardiac performance.

OBJECTIVE

To determine the molecular functions of the 2 kinase-domains of SPEG.

METHODS AND RESULTS

A proteomics approach identified SERCA2a (sarcoplasmic/endoplasmic reticulum calcium ATPase 2a) as a protein interacting with the second kinase-domain but not the first kinase-domain of SPEG. Furthermore, the second kinase-domain of SPEG could phosphorylate Thr⁴⁸⁴ on SERCA2a, promote its oligomerization and increase calcium reuptake into the sarcoplasmic/endoplasmic reticulum in culture cells and primary neonatal rat cardiomyocytes. Phosphorylation of SERCA2a by SPEG enhanced its calcium-transporting activity without affecting its ATPase activity. Depletion of Speg in neonatal rat cardiomyocytes inhibited SERCA2a-Thr⁴⁸⁴ phosphorylation and sarcoplasmic reticulum calcium reuptake. Moreover, overexpression of SERCA2a^Thr484Ala mutant protein also slowed sarcoplasmic reticulum calcium reuptake in neonatal rat cardiomyocytes. In contrast, domain mapping and phosphorylation analysis revealed that the first kinase-domain of SPEG interacted and phosphorylated its recently identified substrate JPH2 (junctophilin-2). An inducible heart-specific Speg knockout mouse model was generated to further study this SPEG-SERCA2a signal nexus in vivo. Inducible deletion of Speg decreased SERCA2a-Thr⁴⁸⁴ phosphorylation and its oligomerization in the heart. Importantly, inducible deletion of Speg inhibited SERCA2a calcium-transporting activity and impaired calcium reuptake into the sarcoplasmic reticulum in cardiomyocytes, which preceded morphological and functional alterations of the heart and eventually led to heart failure in adult mice.

CONCLUSIONS

Our data demonstrate that the 2 kinase-domains of SPEG may play distinct roles to regulate cardiac function. The second kinase-domain of SPEG is a critical regulator for SERCA2a. Our findings suggest that SPEG may serve as a new target to modulate SERCA2a activation for treatment of heart diseases with impaired calcium homeostasis.

Cardiac Calcium ATPase Dimerization Measured by Cross-Linking and Fluorescence Energy Transfer

The cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA) establishes the intracellular calcium gradient across the sarcoplasmic reticulum membrane. It has been proposed that SERCA forms homooligomers that increase the catalytic rate of calcium transport. We investigated SERCA dimerization in rabbit left ventricular myocytes using a photoactivatable cross-linker. Western blotting of cross-linked SERCA revealed higher-molecular-weight species consistent with SERCA oligomerization. Fluorescence resonance energy transfer measurements in cells transiently transfected with fluorescently labeled SERCA2a revealed that SERCA readily forms homodimers. These dimers formed in the absence or presence of the SERCA regulatory partner, phospholamban (PLB) and were unaltered by PLB phosphorylation or changes in calcium or ATP. Fluorescence lifetime data are compatible with a model in which PLB interacts with a SERCA homodimer in a stoichiometry of 1:2. Together, these results suggest that SERCA forms constitutive homodimers in live cells and that dimer formation is not modulated by SERCA conformational poise, PLB binding, or PLB phosphorylation.

HNO enhances SERCA2a activity and cardiomyocyte function by promoting redox-dependent phospholamban oligomerization

AIMS

Nitroxyl (HNO) interacts with thiols to act as a redox-sensitive modulator of protein function. It enhances sarcoplasmic reticular Ca(2+) uptake and myofilament Ca(2+) sensitivity, improving cardiac contractility. This activity has led to clinical testing of HNO donors for heart failure. Here we tested whether HNO alters the inhibitory interaction between phospholamban (PLN) and the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) in a redox-dependent manner, improving Ca(2+) handling in isolated myocytes/hearts.

RESULTS

Ventriculocytes, sarcoplasmic reticulum (SR) vesicles, and whole hearts were isolated from control (wildtype [WT]) or PLN knockout (pln(-/-)) mice. Compared to WT, pln(-/-) myocytes displayed enhanced resting sarcomere shortening, peak Ca(2+) transient, and blunted β-adrenergic responsiveness. HNO stimulated shortening, relaxation, and Ca(2+) transient in WT cardiomyocytes, and evoked positive inotropy/lusitropy in intact hearts. These changes were markedly blunted in pln(-/-) cells/hearts. HNO enhanced SR Ca(2+) uptake in WT but not pln(-/-) SR-vesicles. Spectroscopic studies in insect cell microsomes expressing SERCA2a±PLN showed that HNO increased Ca(2+)-dependent SERCA2a conformational flexibility but only when PLN was present. In cardiomyocytes, HNO achieved this effect by stabilizing PLN in an oligomeric disulfide bond-dependent configuration, decreasing the amount of free inhibitory monomeric PLN available.

INNOVATION

HNO-dependent redox changes in myocyte PLN oligomerization relieve PLN inhibition of SERCA2a.

CONCLUSIONS

PLN plays a central role in HNO-induced enhancement of SERCA2a activity, leading to increased inotropy/lusitropy in intact myocytes and hearts. PLN remains physically associated with SERCA2a; however, less monomeric PLN is available resulting in decreased inhibition of the enzyme. These findings offer new avenues to improve Ca(2+) handling in failing hearts.

Nucleotide activation of the Ca-ATPase

We have used fluorescence spectroscopy, molecular modeling, and limited proteolysis to examine structural dynamics of the sarcoplasmic reticulum Ca-ATPase (SERCA). The Ca-ATPase in sarcoplasmic reticulum vesicles from fast twitch muscle (SERCA1a isoform) was selectively labeled with fluorescein isothiocyanate (FITC), a probe that specifically reacts with Lys-515 in the nucleotide-binding site. Conformation-specific proteolysis demonstrated that FITC labeling does not induce closure of the cytoplasmic headpiece, thereby assigning FITC-SERCA as a nucleotide-free enzyme. We used enzyme reverse mode to synthesize FITC monophosphate (FMP) on SERCA, producing a phosphorylated pseudosubstrate tethered to the nucleotide-binding site of a Ca(2+)-free enzyme (E2 state to prevent FMP hydrolysis). Conformation-specific proteolysis demonstrated that FMP formation induces SERCA headpiece closure similar to ATP binding, presumably due to the high energy phosphoryl group on the fluorescent probe (ATP·E2 analog). Subnanosecond-resolved detection of fluorescence lifetime, anisotropy, and quenching was used to characterize FMP-SERCA (ATP·E2 state) versus FITC-SERCA in Ca(2+)-free, Ca(2+)-bound, and actively cycling phosphoenzyme states (E2, E1, and EP). Time-resolved spectroscopy revealed that FMP-SERCA exhibits increased probe dynamics but decreased probe accessibility compared with FITC-SERCA, indicating that ATP exhibits enhanced dynamics within a closed cytoplasmic headpiece. Molecular modeling was used to calculate the solvent-accessible surface area of FITC and FMP bound to SERCA crystal structures, revealing a positive correlation of solvent-accessible surface area with quenching but not anisotropy. Thus, headpiece closure is coupled to substrate binding but not active site dynamics. We propose that dynamics in the nucleotide-binding site of SERCA is important for Ca(2+) binding (distal allostery) and phosphoenzyme formation (direct activation).

Role of conformational sampling of Ser16 and Thr17-phosphorylated phospholamban in interactions with SERCA

Phosphorylation of phospholamban (PLB) at Ser16 and/ or Thr17 is believed to release its inhibitory effect on sarcoplasmic reticulum calcium ATPase. Ser16 phosphorylation of PLB has been suggested to cause a conformational change that alters the interaction between the enzyme and protein. Using computer simulations, the conformational sampling of Ser16 phosphorylated PLB in implicit membrane environment is compared here with the unphosphorylated PLB system to investigate these conformational changes. The results suggest that conformational changes in the cytoplasmic domain of PLB upon phosphorylation at Ser16 increase the likelihood of unfavorable interactions with SERCA in the E2 state prompting a conformational switch of SERCA from E2 to E1. Phosphorylation of PLB at Thr17 on the other hand does not appear to affect interactions with SERCA significantly suggesting that the mechanism of releasing the inhibitory effect is different between Thr17 phosphorylated and Ser16 phosphorylated PLB.

A phospholamban-tethered cardiac Ca2+ pump reveals stoichiometry and dynamic interactions between the two proteins

To study PLB (phospholamban) inhibition of the cardiac Ca2+ pump [SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a)], a fusion protein (SER-20G-PLB) was engineered by tethering SERCA2a with PLB through a 20-glycine residue chain, allowing the PLB tether to either bind to or dissociate from the inhibition site on SERCA2a. When expressed in insect cells, SER-20G-PLB produced active Ca2+ uptake, which was stimulated by the anti-PLB antibody, both similar to that which occurred with the control sample co-expressing WT (wild-type)-SERCA2a and WT-PLB. The KCa values of Ca2+-dependent ATPase were similar for SER-20G-PLB (0.29±0.02 μM) and for the control sample (0.30±0.02 μM), both greater than 0.17±0.01 μM for WT-SERCA2a expressed alone. Thus SER-20G-PLB retains a fully active Ca2+ pump, but its apparent Ca2+ affinity was decreased intrinsically by tethered PLB at a 1:1 molar stoichiometry. Like WT-PLB, SER-20G-PLB ran as both monomers and homo-pentamers on SDS/PAGE. As Ca2+ concentrations increase from 0 to the micromolar range, the proportion of non-inhibiting pentamers increased from 32% to 52%, suggesting that Ca2+ activation of the pump completely dissociates the PLB tether from the inhibition site on SERCA2a, with concurrent association of PLB pentamers. Collectively, the regulation of SERCA2a is achieved through the Ca2+-dependent equilibria involving PLB association and dissociation from SERCA2a, and assembling and disassembling of SER-20G-PLB pentamers.

The heart: a novel gonadotrophin-releasing hormone target

Gonadotrophin-releasing hormone (GnRH) is a hypothalamic hormone transported by the hypophyseal portal bloodstream to the pituitary gland, where it binds to GnRH receptors. However, GnRH receptors are expressed in multiple extrapituitary tissues, although their physiological relevance is not fully understood. GnRH agonists are employed extensively in steroid deprivation therapy, especially to suppress testosterone in prostate cancer. Because GnRH agonist treatment is associated with increased coronary heart disease and myocardial infarction, we investigated the impact of GnRH on cardiomyocyte contractile function. Cardiomyocytes were isolated from mouse hearts and mechanical and intracellular Ca(2+) properties were evaluated, including peak shortening amplitude (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90) ), maximal velocity of shortening/relengthening (± dLdt), electrically-stimulated rise in Fura-2 fluorescence intensity (ΔFFI) and Ca(2+) decay. GnRH (1 ng/ml) increased PS, ± dL/dt, resting FFI and ΔFFI, and prolonged TPS, TR(90) and Ca(2+) decay time, whereas 1 pg/ml GnRH affected all these cardiomyocyte variables, except TPS, resting FFI and ΔFFI. A concentration of 1 fg/ml GnRH and the GnRH cleavage product, GnRH-[1-5] (300 pg/ml), had no effect on any cardiomyocyte parameter. The 1 pg/ml GnRH-elicited responses were attenuated by the GnRH receptor antagonist cetrorelix (10 μm), the protein kinase A (PKA) inhibitor H89 (1 μm) but not the protein kinase C inhibitor chelerythrine chloride (1 μm). These data revealed that GnRH is capable of regulating cardiac contractile function via a GnRH receptor/PKA-dependent mechanism. If present in the human heart, dysfunction of such a system may play an important role in cardiac pathology observed in men treated with GnRH agonists for prostate cancer.

Effects of phospholamban transmembrane mutants on the calcium affinity, maximal activity, and cooperativity of the sarcoplasmic reticulum calcium pump

Regulation of the SERCA calcium pump by phospholamban (PLB) is largely due to interactions between their respective transmembrane domains. In spite of numerous mutagenesis and kinetic studies, we still do not have a clear mechanistic picture of how PLB influences the calcium transport cycle of SERCA. Herein, we have created alanine mutants for each residue in the transmembrane domain of PLB, we have co-reconstituted these mutants with SERCA into proteoliposomes, and we have performed kinetic simulations of the calcium-dependent ATPase activity isotherms. The PLB transmembrane mutants had a variable effect on the calcium affinity, maximal activity, and cooperativity of SERCA, such that a range of values was observed. Kinetic simulations using a well-established reaction scheme for SERCA then allowed us to correlate the effects on SERCA activity with changes in the reaction scheme rate constants. Only three steps in the reaction scheme were affected by the presence of PLB, namely, binding of the first calcium ion, a subsequent conformational change in SERCA, and binding of the second calcium ion. The ability of wild-type and mutant forms of PLB to alter the apparent calcium affinity of SERCA correlated with a decreased rate of binding of the second calcium ion. In addition, the ability of wild-type and mutant forms of PLB to alter the maximal activity of SERCA correlated with a change in the forward rate constant for the slow conformational change in SERCA following binding of the first calcium ion.