Phospholamban binds with differential affinity to calcium pump conformers

SERCA2a dysfunction in the pathophysiology of heart failure with preserved ejection fraction: a direct role is yet to be established

With rising incidence, mortality and limited therapeutic options, heart failure with preserved ejection fraction (HFpEF) remains one of the most important topics in cardiovascular medicine today. Characterised by left ventricular diastolic dysfunction partially due to impaired Ca2+ homeostasis, one ion channel in particular, SarcoEndoplasmic Reticulum Ca2+-ATPase (SERCA2a), may play a significant role in its pathophysiology. A better understanding of the complex mechanisms interplaying to contribute to SERCA2a dysfunction will help develop treatments targeting it and thus address the growing clinical challenge HFpEF poses. This review examines the conflicting evidence present for changes in SERCA2a expression and activity in HFpEF, explores potential underlying mechanisms, and finally evaluates the drug and gene therapy trials targeting SERCA2a in heart failure. Recent positive results from trials involving widely used anti-diabetic agents such as sodium-glucose co-transporter protein 2 inhibitors (SGLT2i) and glucagon-like peptide-1 (GLP-1) agonists offer advancement in HFpEF management. The potential interplay between these agents and SERCA2a regulation presents a novel angle that could open new avenues for modulating diastolic function; however, the mechanistic research in this emerging field is limited. Overall, the direct role of SERCA2a dysfunction in HFpEF remains undetermined, highlighting the need for well-designed pre-clinical studies and robust clinical trials.

Dilated cardiomyopathy variant R14del increases phospholamban pentamer stability, blunting dynamic regulation of calcium

The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is a membrane transporter that creates and maintains intracellular Ca2+ stores. In the heart, SERCA is regulated by an inhibitory interaction with the monomeric form of the transmembrane micropeptide phospholamban (PLB). PLB also forms avid homo-pentamers, and the dynamic exchange of PLB between pentamers and SERCA is an important determinant of cardiac responsiveness to exercise. Here, we investigated two naturally occurring pathogenic variants of PLB: a cysteine substitution of Arg9 (R9C) and an in-frame deletion of Arg14 (R14del). Both variants are associated with dilated cardiomyopathy. We previously showed that the R9C mutation causes disulfide crosslinking and hyperstabilization of pentamers. While the pathogenic mechanism of R14del is unclear, we hypothesized this mutation may also alter pentamer stability. Immunoblots revealed a significantly increased pentamer: monomer ratio for R14del-PLB compared to WT-PLB. We quantified homo-oligomerization and SERCA-binding in live cells using fluorescence resonance energy transfer (FRET) microscopy. R14del-PLB showed an increased affinity for homo-oligomerization and decreased binding affinity for SERCA compared to WT. The data suggest that, like R9C, the R14del mutation stabilizes PLB in pentamers, decreasing its ability to regulate SERCA. The R14del mutation reduced the rate of PLB unbinding from pentamers after transient elevations of Ca2+, limiting the recovery of PLB-SERCA complexes. A computational model predicted that hyperstabilization of PLB pentamers by R14del impairs the ability of cardiac Ca2+ handling to respond to changing heart rates between rest and exercise. We postulate that impaired responsiveness to physiological stress contributes to arrhythmogenesis in human carriers of the R14del mutation.

Another-regulin regulates cardiomyocyte calcium handling via integration of neuroendocrine signaling with SERCA2a activity

Calcium (Ca2+) dysregulation is a hallmark feature of cardiovascular disease. Intracellular Ca2+ regulation is essential for proper heart function and is controlled by the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA2a). Another-regulin (ALN) is a newly discovered cardiomyocyte-expressed SERCA2a inhibitor, suggesting cardiomyocyte Ca2+-handling is more complex than previously appreciated. To study the role of ALN in cardiomyocytes, we generated ALN null mice (knockout, KO) and found that cardiomyocytes from these animals displayed enhanced Ca2+ cycling and contractility compared to wildtype (WT) mice, indicating enhanced SERCA2a activity. In vitro and in vivo studies show that ALN is post-translationally modified via phosphorylation on Serine 19 (S19), suggesting this contributes to its ability to regulate SERCA2a. Immunoprecipitation and FRET analysis of ALN-WT, phospho-deficient ALN (S19A), or phosphomimetic ALN (S19D) revealed that S19 phosphorylation alters the SERCA2a-ALN interaction, leading to relief of its inhibitory effects. Adeno-associated virus mediated delivery of ALN-WT or phospho-mutant ALN-S19A/D in ALN KO mice showed that cardiomyocyte-specific expression of phospho-deficient ALN-S19A resulted in increased SERCA2a inhibition characterized by reduced rates of cytoplasmic Ca2+ clearance compared to ALN-WT and ALN-S19D expressing cells, further supporting a role for this phosphorylation event in controlling SERCA2a-regulation by ALN. Levels of ALN phosphorylation were markedly increased in cardiomyocytes in response to Gαq agonists (angiotensin II, endothelin-1, phenylephrine) and Gαq-mediated phosphorylation of ALN translated to increased Ca2+ cycling in cardiomyocytes from WT but not ALN KO mice. Collectively, these results indicate that ALN uniquely regulates Ca2+ handling in cardiomyocytes via integration of neuroendocrine signaling with SERCA2a activity.

Pathological mutations in the phospholamban cytoplasmic region affect its topology and dynamics modulating the extent of SERCA inhibition

Phospholamban (PLN) is a 52 amino acid regulin that allosterically modulates the activity of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in the heart muscle. In its unphosphorylated form, PLN binds SERCA within its transmembrane (TM) domains, approximately 20 Å away from the Ca2+ binding site, reducing SERCA's apparent Ca2+ affinity (pKCa) and decreasing cardiac contractility. During the enzymatic cycle, the inhibitory TM domain of PLN remains anchored to SERCA, whereas its cytoplasmic region transiently binds the ATPase's headpiece. Phosphorylation of PLN at Ser16 by protein kinase A increases the affinity of its cytoplasmic domain to SERCA, weakening the TM interactions with the ATPase, reversing its inhibitory function, and augmenting muscle contractility. How the structural changes caused by pathological mutations in the PLN cytoplasmic region are transmitted to its inhibitory TM domain is still unclear. Using solid-state NMR spectroscopy and activity assays, we analyzed the structural and functional effects of a series of mutations and their phosphorylated forms located in the PLN cytoplasmic region and linked to dilated cardiomyopathy. We found that these missense mutations affect the overall topology and dynamics of PLN and ultimately modulate its inhibitory potency. Also, the changes in the TM tilt angle and cytoplasmic dynamics of PLN caused by these mutations correlate well with the extent of SERCA inhibition. Our study unveils new molecular determinants for designing variants of PLN that outcompete endogenous PLN to regulate SERCA in a tunable manner.

SERCA2 phosphorylation at serine 663 is a key regulator of Ca2+ homeostasis in heart diseases

Despite advances in cardioprotection, new therapeutic strategies capable of preventing ischemia-reperfusion injury of patients are still needed. Here, we discover that sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA2) phosphorylation at serine 663 is a clinical and pathophysiological event of cardiac function. Indeed, the phosphorylation level of SERCA2 at serine 663 is increased in ischemic hearts of patients and mouse. Analyses on different human cell lines indicate that preventing serine 663 phosphorylation significantly increases SERCA2 activity and protects against cell death, by counteracting cytosolic and mitochondrial Ca2+ overload. By identifying the phosphorylation level of SERCA2 at serine 663 as an essential regulator of SERCA2 activity, Ca2+ homeostasis and infarct size, these data contribute to a more comprehensive understanding of the excitation/contraction coupling of cardiomyocytes and establish the pathophysiological role and the therapeutic potential of SERCA2 modulation in acute myocardial infarction, based on the hotspot phosphorylation level of SERCA2 at serine 663 residue.

Dilated cardiomyopathy variant R14del increases phospholamban pentamer stability, blunting dynamic regulation of cardiac calcium handling

The sarco(endo)plasmic reticulum Ca 2+ ATPase (SERCA) is a membrane transporter that creates and maintains intracellular Ca 2+ stores. In the heart, SERCA is regulated by an inhibitory interaction with the monomeric form of the transmembrane micropeptide phospholamban (PLB). PLB also forms avid homo-pentamers, and dynamic exchange of PLB between pentamers and the regulatory complex with SERCA is an important determinant of cardiac responsiveness to exercise. Here, we investigated two naturally occurring pathogenic mutations of PLB, a cysteine substitution of arginine 9 (R9C) and an in-frame deletion of arginine 14 (R14del). Both mutations are associated with dilated cardiomyopathy. We previously showed that the R9C mutation causes disulfide crosslinking and hyperstabilization of pentamers. While the pathogenic mechanism of R14del is unclear, we hypothesized that this mutation may also alter PLB homo-oligomerization and disrupt the PLB-SERCA regulatory interaction. SDS-PAGE revealed a significantly increased pentamer:monomer ratio for R14del-PLB when compared to WT-PLB. In addition, we quantified homo-oligomerization and SERCA-binding in live cells using fluorescence resonance energy transfer (FRET) microscopy. R14del-PLB showed an increased affinity for homo-oligomerization and decreased binding affinity for SERCA compared to WT, suggesting that, like R9C, the R14del mutation stabilizes PLB in its pentameric form, decreasing its ability to regulate SERCA. Moreover, the R14del mutation reduces the rate of PLB unbinding from the pentamer after a transient Ca 2+ elevation, limiting the rate of re-binding to SERCA. A computational model predicted that hyperstabilization of PLB pentamers by R14del impairs the ability of cardiac Ca 2+ handling to respond to changing heart rates between rest and exercise. We postulate that impaired responsiveness to physiological stress contributes to arrhythmogenesis in human carriers of the R14del mutation.

Micropeptide hetero-oligomerization adds complexity to the calcium pump regulatory network

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) is an ion transporter that creates and maintains intracellular calcium stores. SERCA is inhibited or stimulated by several membrane micropeptides including another-regulin, dwarf open reading frame, endoregulin, phospholamban (PLB), and sarcolipin. We previously showed that these micropeptides assemble into homo-oligomeric complexes with varying affinity. Here, we tested whether different micropeptides can interact with each other, hypothesizing that coassembly into hetero-oligomers may affect micropeptide bioavailability to regulate SERCA. We quantified the relative binding affinity of each combination of candidates using automated fluorescence resonance energy transfer microscopy. All pairs were capable of interacting with good affinity, similar to the affinity of micropeptide self-binding (homo-oligomerization). Testing each pair at a 1:5 ratio and a reciprocal 5:1 ratio, we noted that the affinity of hetero-oligomerization of some micropeptides depended on whether they were the minority or majority species. In particular, sarcolipin was able to join oligomers when it was the minority species but did not readily accommodate other micropeptides in the reciprocal experiment when it was expressed in fivefold excess. The opposite was observed for endoregulin. PLB was a universal partner for all other micropeptides tested, forming avid hetero-oligomers whether it was the minority or majority species. Increasing expression of SERCA decreased PLB-dwarf open reading frame hetero-oligomerization, suggesting that SERCA-micropeptide interactions compete with micropeptide-micropeptide interactions. Thus, micropeptides populate a regulatory network of diverse protein assemblies. The data suggest that the complexity of this interactome increases exponentially with the number of micropeptides that are coexpressed in a particular tissue.

Spatial and Functional Crosstalk between the Mitochondrial Na+-Ca2+ Exchanger NCLX and the Sarcoplasmic Reticulum Ca2+ Pump SERCA in Cardiomyocytes

The mitochondrial Na⁺-Ca²⁺ exchanger, NCLX, was reported to supply Ca²⁺ to sarcoplasmic reticulum (SR)/endoplasmic reticulum, thereby modulating various cellular functions such as the rhythmicity of cardiomyocytes, and cellular Ca²⁺ signaling upon antigen receptor stimulation and chemotaxis in B lymphocytes; however, there is little information on the spatial relationships of NCLX with SR Ca²⁺ handling proteins, and their physiological impact. Here we examined the issue, focusing on the interaction of NCLX with an SR Ca²⁺ pump SERCA in cardiomyocytes. A bimolecular fluorescence complementation assay using HEK293 cells revealed that the exogenously expressed NCLX was localized in close proximity to four exogenously expressed SERCA isoforms. Immunofluorescence analyses of isolated ventricular myocytes showed that the NCLX was localized to the edges of the mitochondria, forming a striped pattern. The co-localization coefficients in the super-resolution images were higher for NCLX–SERCA2, than for NCLX–ryanodine receptor and NCLX–Na⁺/K⁺ ATPase α-1 subunit, confirming the close localization of endogenous NCLX and SERCA2 in cardiomyocytes. The mathematical model implemented with the spatial and functional coupling of NCLX and SERCA well reproduced the NCLX inhibition-mediated modulations of SR Ca²⁺ reuptake in HL-1 cardiomyocytes. Taken together, these results indicated that NCLX and SERCA are spatially and functionally coupled in cardiomyocytes.

Inhibitory and stimulatory micropeptides preferentially bind to different conformations of the cardiac calcium pump

The ATP-dependent ion pump sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) sequesters Ca²⁺ in the endoplasmic reticulum to establish a reservoir for cell signaling. Because of its central importance in physiology, the activity of this transporter is tightly controlled via direct interactions with tissue-specific regulatory micropeptides that tune SERCA function to match changing physiological conditions. In the heart, the micropeptide phospholamban (PLB) inhibits SERCA, while dwarf open reading frame (DWORF) stimulates SERCA. These competing interactions determine cardiac performance by modulating the amplitude of Ca²⁺ signals that drive the contraction/relaxation cycle. We hypothesized that the functions of these peptides may relate to their reciprocal preferences for SERCA binding; SERCA binds PLB more avidly at low cytoplasmic [Ca²⁺] but binds DWORF better when [Ca²⁺] is high. In the present study, we demonstrated this opposing Ca²⁺ sensitivity is due to preferential binding of DWORF and PLB to different intermediate states that SERCA samples during the Ca²⁺ transport cycle. We show PLB binds best to the SERCA E1-ATP state, which prevails at low [Ca²⁺]. In contrast, DWORF binds most avidly to E1P and E2P states that are more populated when Ca²⁺ is elevated. Moreover, FRET microscopy revealed dynamic shifts in SERCA–micropeptide binding equilibria during cellular Ca²⁺ elevations. A computational model showed that DWORF exaggerates changes in PLB–SERCA binding during the cardiac cycle. These results suggest a mechanistic basis for inhibitory versus stimulatory micropeptide function, as well as a new role for DWORF as a modulator of dynamic oscillations of PLB–SERCA regulatory interactions.

Subcellular Propagation of Cardiomyocyte β-Adrenergic Activation of Calcium Uptake Involves Internal β-Receptors and AKAP7

β-adrenergic receptor (β-AR) signaling in cardiac myocytes is central to cardiac function, but spatiotemporal activation within myocytes is unresolved. In rabbit ventricular myocytes, β-AR agonists or high extracellular [Ca] were applied locally at one end, to measure β-AR signal propagation as Ca-transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca uptake. High local [Ca]_o, increased CaT amplitude under the pipette faster than did ISO, but was also more spatially restricted. Local isoproterenol (ISO) or norepinephrine (NE) increased CaT amplitude and SR Ca uptake, that spread along the myocyte to the unexposed end. Thus, local [Ca]_i decline kinetics reflect spatio-temporal progression of β-AR end-effects in myocytes. To test whether intracellular β-ARs contribute to this response, we used β-AR-blockers that are membrane permeant (propranolol) or not (sotalol). Propranolol completely blocked NE-dependent CaT effects. However, blocking surface β-ARs only (sotalol) suppressed only ∼50% of the NE-induced increase in CaT peak and rate of [Ca]_i decline, but these changes spread more gradually than NE alone. We also tested whether A-kinase anchoring protein 7γ (AKAP7γ; that interacts with phospholamban) is mobile, such that it might contribute to intracellular spatial propagation of β-AR signaling. We found AKAP7γ to be highly mobile using fluorescence recovery after photobleach of GFP tagged AKAP7γ, and that PKA activation accelerated AKAP7γ-GFP wash-out upon myocyte saponin-permeabilization, suggesting increased AKAP7γ mobility. We conclude that local β-AR activation can activate SR Ca uptake at remote myocyte sites, and that intracellular β-AR and AKAP7γ mobility may play a role in this spread of activation.

Fluorescence lifetime imaging microscopy reveals sodium pump dimers in live cells

The sodium-potassium ATPase (Na/K-ATPase, NKA) establishes ion gradients that facilitate many physiological functions including action potentials and secondary transport processes. NKA comprises a catalytic subunit (alpha) that interacts closely with an essential subunit (beta) and regulatory transmembrane micropeptides called FXYD proteins. In the heart, a key modulatory partner is the FXYD protein phospholemman (PLM, FXYD1), but the stoichiometry of the alpha-beta-PLM regulatory complex is unknown. Here, we used fluorescence lifetime imaging and spectroscopy to investigate the structure, stoichiometry, and affinity of the NKA-regulatory complex. We observed a concentration-dependent binding of the subunits of NKA-PLM regulatory complex, with avid association of the alpha subunit with the essential beta subunit as well as lower affinity alpha-alpha and alpha-PLM interactions. These data provide the first evidence that, in intact live cells, the regulatory complex is composed of two alpha subunits associated with two beta subunits, decorated with two PLM regulatory subunits. Docking and molecular dynamics (MD) simulations generated a structural model of the complex that is consistent with our experimental observations. We propose that alpha-alpha subunit interactions support conformational coupling of the catalytic subunits, which may enhance NKA turnover rate. These observations provide insight into the pathophysiology of heart failure, wherein low NKA expression may be insufficient to support formation of the complete regulatory complex with the stoichiometry (alpha-beta-PLM)2.

Nothing Regular about the Regulins: Distinct Functional Properties of SERCA Transmembrane Peptide Regulatory Subunits

The sarco-endoplasmic reticulum calcium ATPase (SERCA) is responsible for maintaining calcium homeostasis in all eukaryotic cells by actively transporting calcium from the cytosol into the sarco-endoplasmic reticulum (SR/ER) lumen. Calcium is an important signaling ion, and the activity of SERCA is critical for a variety of cellular processes such as muscle contraction, neuronal activity, and energy metabolism. SERCA is regulated by several small transmembrane peptide subunits that are collectively known as the “regulins”. Phospholamban (PLN) and sarcolipin (SLN) are the original and most extensively studied members of the regulin family. PLN and SLN inhibit the calcium transport properties of SERCA and they are required for the proper functioning of cardiac and skeletal muscles, respectively. Myoregulin (MLN), dwarf open reading frame (DWORF), endoregulin (ELN), and another-regulin (ALN) are newly discovered tissue-specific regulators of SERCA. Herein, we compare the functional properties of the regulin family of SERCA transmembrane peptide subunits and consider their regulatory mechanisms in the context of the physiological and pathophysiological roles of these peptides. We present new functional data for human MLN, ELN, and ALN, demonstrating that they are inhibitors of SERCA with distinct functional consequences. Molecular modeling and molecular dynamics simulations of SERCA in complex with the transmembrane domains of MLN and ALN provide insights into how differential binding to the so-called inhibitory groove of SERCA—formed by transmembrane helices M2, M6, and M9—can result in distinct functional outcomes.

Dwarf open reading frame (DWORF) is a direct activator of the sarcoplasmic reticulum calcium pump SERCA

The sarco-plasmic reticulum calcium pump (SERCA) plays a critical role in the contraction-relaxation cycle of muscle. In cardiac muscle, SERCA is regulated by the inhibitor phospholamban. A new regulator, dwarf open reading frame (DWORF), has been reported to displace phospholamban from SERCA. Here, we show that DWORF is a direct activator of SERCA, increasing its turnover rate in the absence of phospholamban. Measurement of in-cell calcium dynamics supports this observation and demonstrates that DWORF increases SERCA-dependent calcium reuptake. These functional observations reveal opposing effects of DWORF activation and phospholamban inhibition of SERCA. To gain mechanistic insight into SERCA activation, fluorescence resonance energy transfer experiments revealed that DWORF has a higher affinity for SERCA in the presence of calcium. Molecular modeling and molecular dynamics simulations provide a model for DWORF activation of SERCA, where DWORF modulates the membrane bilayer and stabilizes the conformations of SERCA that predominate during elevated cytosolic calcium.

Structural basis for allosteric control of the SERCA-Phospholamban membrane complex by Ca2+ and phosphorylation

Phospholamban (PLN) is a mini-membrane protein that directly controls the cardiac Ca2+-transport response to β-adrenergic stimulation, thus modulating cardiac output during the fight-or-flight response. In the sarcoplasmic reticulum membrane, PLN binds to the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), keeping this enzyme's function within a narrow physiological window. PLN phosphorylation by cAMP-dependent protein kinase A or increase in Ca2+ concentration reverses the inhibitory effects through an unknown mechanism. Using oriented-sample solid-state NMR spectroscopy and replica-averaged NMR-restrained structural refinement, we reveal that phosphorylation of PLN's cytoplasmic regulatory domain signals the disruption of several inhibitory contacts at the transmembrane binding interface of the SERCA-PLN complex that are propagated to the enzyme's active site, augmenting Ca2+ transport. Our findings address long-standing questions about SERCA regulation, epitomizing a signal transduction mechanism operated by posttranslationally modified bitopic membrane proteins.

Dynamics-Driven Allostery Underlies Ca2+-Mediated Release of SERCA Inhibition by Phospholamban

Sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and phospholamban (PLB) are essential for intracellular Ca2+ transport in myocytes. Ca2+-dependent activation of SERCA-PLB provides a control function that regulates cytosolic and SR Ca2+ levels. Although experimental and computational studies alone have led to a greater insight into SERCA-PLB regulation, the structural mechanisms for Ca2+ binding reversing inhibition of the complex remain poorly understood. Therefore, we have performed atomistic simulations totaling 32.7 μs and cell-based intramolecular fluorescence resonance energy transfer (FRET) experiments to determine structural changes of PLB-bound SERCA in response to binding of a single Ca2+ ion. Complementary MD simulations and FRET experiments showed that open-to-closed transitions in the structure of the headpiece underlie PLB inhibition of SERCA, and binding of a single Ca2+ ion is sufficient to shift the protein population toward a structurally closed structure of the complex. Closure is accompanied by functional interactions between the N-domain β5-β6 loop and the A-domain and the displacement of the catalytic phosphorylation domain toward a competent structure. We propose that reversal of SERCA-PLB inhibition is achieved by stringing together its controlling modules (A-domain and loop Nβ5-β6) with catalytic elements (P-domain) to regulate function during intracellular Ca2+ signaling. We conclude that binding of a single Ca2+ is a critical mediator of allosteric signaling that dictates structural changes and motions that relieve SERCA inhibition by PLB. Understanding allosteric regulation is of paramount importance to guide therapeutic modulation of SERCA and other evolutionarily related ion-motive ATPases.

Glutamate binding triggers monomerization of unliganded mGluR2 dimers

The Metabotropic glutamate receptor 2 (mGluR2) is involved in several neurological and psychiatric disorders and is an attractive drug target. It is believed to form a strict dimer and the dimeric assembly is necessary for glutamate induced activation. Although many studies have focused on glutamate induced conformational changes, the dimerization propensity of mGluR2 with and without glutamate has never been investigated. Also, the role of the unstructured loop in dimerization of mGluR2 is not clear. Here, using Forster Resonance Energy Transfer (FRET) based assay in live cells we show that mGluR2 does not form a "strict dimer" rather it exists in a dynamic monomer-dimer equilibrium. The unstructured loop moderately destabilizes the dimers. Furthermore, binding of glutamate to mGluR2 induces conformational change that promotes monomerization of mGluR2. In the absence of an unstructured loop, mGluR2 neither undergoes conformational change nor monomerizes upon binding to glutamate.

Atomistic Structure and Dynamics of the Ca2+-ATPase Bound to Phosphorylated Phospholamban

Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) and phospholamban (PLB) are essential components of the cardiac Ca²⁺ transport machinery. PLB phosphorylation at residue Ser16 (pSer16) enhances SERCA activity in the heart via an unknown structural mechanism. Here, we report a fully atomistic model of SERCA bound to phosphorylated PLB and study its structural dynamics on the microsecond time scale using all-atom molecular dynamics simulations in an explicit lipid bilayer and water environment. The unstructured N-terminal phosphorylation domain of PLB samples different orientations and covers a broad area of the cytosolic domain of SERCA but forms a stable complex mediated by pSer16 interactions with a binding site formed by SERCA residues Arg324/Lys328. PLB phosphorylation does not affect the interaction between the transmembrane regions of the two proteins; however, pSer16 stabilizes a disordered structure of the N-terminal phosphorylation domain that releases key inhibitory contacts between SERCA and PLB. We found that PLB phosphorylation is sufficient to guide the structural transitions of the cytosolic headpiece that are required to produce a competent structure of SERCA. We conclude that PLB phosphorylation serves as an allosteric molecular switch that releases inhibitory contacts and strings together the catalytic elements required for SERCA activation. This atomistic model represents a vivid atomic-resolution visualization of SERCA bound to phosphorylated PLB and provides previously inaccessible insights into the structural mechanism by which PLB phosphorylation releases SERCA inhibition in the heart.

Protein docking and steered molecular dynamics suggest alternative phospholamban-binding sites on the SERCA calcium transporter

The transport activity of the sarco(endo)plasmic reticulum calcium ATPase (SERCA) in cardiac myocytes is modulated by an inhibitory interaction with a transmembrane peptide, phospholamban (PLB). Previous biochemical studies have revealed that PLB interacts with a specific inhibitory site on SERCA, and low-resolution structural evidence suggests that PLB interacts with distinct alternative sites on SERCA. High-resolution details of the structural determinants of SERCA regulation have been elusive because of the dynamic nature of the regulatory complex. In this study, we used computational approaches to develop a structural model of SERCA-PLB interactions to gain a mechanistic understanding of PLB-mediated SERCA transport regulation. We combined steered molecular dynamics and membrane protein-protein docking experiments to achieve both a global search and all-atom force calculations to determine the relative affinities of PLB for candidate sites on SERCA. We modeled the binding of PLB to several SERCA conformations, representing different enzymatic states sampled during the calcium transport catalytic cycle. The results of the steered molecular dynamics and docking experiments indicated that the canonical PLB-binding site (comprising transmembrane helices M2, M4, and M9) is the preferred site. This preference was even more stringent for a superinhibitory PLB variant. Interestingly, PLB-binding specificity became more ambivalent for other SERCA conformers. These results provide evidence for polymorphic PLB interactions with novel sites on M3 and with the outside of the SERCA helix M9. Our findings are compatible with previous physical measurements that suggest that PLB interacts with multiple binding sites, conferring dynamic responsiveness to changing physiological conditions.

Live-Cell Cardiac-Specific High-Throughput Screening Platform for Drug-Like Molecules that Enhance Ca2+ Transport

We engineered a concatenated fluorescent biosensor and dual-wavelength fluorescence lifetime (FLT) detection, to perform high-throughput screening (HTS) in living cells for discovery of potential heart-failure drugs. Heart failure is correlated with insufficient activity of the sarcoplasmic reticulum Ca-pump (SERCA2a), often due to excessive inhibition by phospholamban (PLB), a small transmembrane protein. We sought to discover small molecules that restore SERCA2a activity by disrupting this inhibitory interaction between PLB and SERCA2a. Our approach was to fluorescently tag the two proteins and measure fluorescence resonance energy transfer (FRET) to detect changes in binding or structure of the complex. To optimize sensitivity to these changes, we engineered a biosensor that concatenates the two fluorescently labeled proteins on a single polypeptide chain. This SERCA2a-PLB FRET biosensor construct is functionally active and effective for HTS. By implementing 2-wavelength FLT detection at extremely high speed during primary HTS, we culled fluorescent compounds as false-positive Hits. In pilot screens, we identified Hits that alter the SERCA2a-PLB interaction, and a newly developed secondary calcium uptake assay revealed both activators and inhibitors of Ca-transport. We are implementing this approach for large-scale screens to discover new drug-like modulators of SERCA2a-PLB interactions for heart failure therapeutic development.

Viral expression of a SERCA2a-activating PLB mutant improves calcium cycling and synchronicity in dilated cardiomyopathic hiPSC-CMs

There is increasing momentum toward the development of gene therapy for heart failure (HF) that is defined by impaired calcium (Ca²⁺) transport and reduced contractility. We have used FRET (fluorescence resonance energy transfer) between fluorescently-tagged SERCA2a (the cardiac Ca²⁺ pump) and PLB (phospholamban, ventricular peptide inhibitor of SERCA) to test directly the effectiveness of loss-of-inhibition/gain-of-binding (LOI/GOB) PLB mutants (PLB_M) that were engineered to compete with the binding of inhibitory wild-type PLB (PLB_WT). Our therapeutic strategy is to relieve PLB_WT inhibition of SERCA2a by using the reserve adrenergic capacity mediated by PLB to enhance cardiac contractility. Using a FRET assay, we determined that the combination of a LOI PLB mutation (L31A) and a GOB PLB mutation (I40A) results in a novel engineered LOI/GOB PLB_M (L31A/I40A) that effectively competes with PLB_WT binding to cardiac SERCA2a in HEK293-6E cells. We demonstrated that co-expression of PLB_M enhances SERCA Ca-ATPase activity by increasing enzyme Ca²⁺ affinity (1/K_Ca) in PLB_WT-inhibited HEK293 cell homogenates. For an initial assessment of PLB_M physiological effectiveness, we used human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) from a healthy individual. In this system, we observed that adeno-associated virus 2 (rAAV2)-driven expression of PLB_M enhances the amplitude of SR Ca²⁺ release and the rate of SR Ca²⁺ re-uptake. To assess therapeutic potential, we used a hiPSC-CM model of dilated cardiomyopathy (DCM) containing PLB mutation R14del, where we observed that rAAV2-driven expression of PLB_M rescues arrhythmic Ca²⁺ transients and alleviates decreased Ca²⁺ transport. Thus, we propose that PLB_M transgene expression is a promising gene therapy strategy that directly targets the underlying pathophysiology of abnormal Ca²⁺ transport and thus contractility in underlying systolic heart failure.

Newly Discovered Micropeptide Regulators of SERCA Form Oligomers but Bind to the Pump as Monomers

The recently-discovered single-span transmembrane proteins endoregulin (ELN), dwarf open reading frame (DWORF), myoregulin (MLN), and another-regulin (ALN) are reported to bind to the SERCA calcium pump in a manner similar to that of known regulators of SERCA activity, phospholamban (PLB) and sarcolipin (SLN). To determine how micropeptide assembly into oligomers affects the availability of the micropeptide to bind to SERCA in a regulatory complex, we used co-immunoprecipitation and fluorescence resonance energy transfer (FRET) to quantify micropeptide oligomerization and SERCA-binding. Micropeptides formed avid homo-oligomers with high-order stoichiometry (n > 2 protomers per homo-oligomer), but it was the monomeric form of all micropeptides that interacted with SERCA. In view of these two alternative binding interactions, we evaluated the possibility that oligomerization occurs at the expense of SERCA-binding. However, even the most avidly oligomeric micropeptide species still showed robust FRET with SERCA, and there was a surprising positive correlation between oligomerization affinity and SERCA-binding. This comparison of micropeptide family members suggests that the same structural determinants that support oligomerization are also important for binding to SERCA. Moreover, the unique oligomerization/SERCA-binding profile of DWORF is in harmony with its distinct role as a PLB-competing SERCA activator, in contrast to the inhibitory function of the other SERCA-binding micropeptides.

Thermodynamics of Cation Binding to the Sarcoendoplasmic Reticulum Calcium ATPase Pump and Impacts on Enzyme Function

Sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) is a transmembrane pump that plays an important role in transporting calcium into the sarcoplasmic reticulum (SR). While calcium (Ca²⁺) binds SERCA with micromolar affinity, magnesium (Mg²⁺) and potassium (K⁺) also compete with Ca²⁺ binding. However, the molecular bases for these competing ions' influence on the SERCA function and the selectivity of the pump for Ca²⁺ are not well-established. We therefore used in silico methods to resolve molecular determinants of cation binding in the canonical site I and II Ca²⁺ binding sites via (1) triplicate molecular dynamics (MD) simulations of Mg²⁺, Ca²⁺, and K⁺-bound SERCA, (2) mean spherical approximation (MSA) theory to score the affinity and selectivity of cation binding to the MD-resolved structures, and (3) state models of SERCA turnover informed from MSA-derived affinity data. Our key findings are that (a) coordination at sites I and II is optimized for Ca²⁺ and to a lesser extent for Mg²⁺ and K⁺, as determined by MD-derived cation-amino acid oxygen and bound water configurations, (b) the impaired coordination and high desolvation cost for Mg²⁺ precludes favorable Mg²⁺ binding relative to Ca²⁺, while K⁺ has limited capacity to bind site I, and (c) Mg²⁺ most likely acts as inhibitor and K⁺ as intermediate in SERCA's reaction cycle, based on a best-fit state model of SERCA turnover. These findings provide a quantitative basis for SERCA function that leverages molecular-scale thermodynamic data and rationalizes enzyme activity across broad ranges of K⁺, Ca²⁺, and Mg²⁺ concentrations.

The Phospholamban Pentamer Alters Function of the Sarcoplasmic Reticulum Calcium Pump SERCA

The interaction of phospholamban (PLN) with the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) pump is a major regulatory axis in cardiac muscle contractility. The prevailing model involves reversible inhibition of SERCA by monomeric PLN and storage of PLN as an inactive pentamer. However, this paradigm has been challenged by studies demonstrating that PLN remains associated with SERCA and that the PLN pentamer is required for the regulation of cardiac contractility. We have previously used two-dimensional (2D) crystallization and electron microscopy to study the interaction between SERCA and PLN. To further understand this interaction, we compared small helical crystals and large 2D crystals of SERCA in the absence and presence of PLN. In both crystal forms, SERCA molecules are organized into identical antiparallel dimer ribbons. The dimer ribbons pack together with distinct crystal contacts in the helical versus large 2D crystals, which allow PLN differential access to potential sites of interaction with SERCA. Nonetheless, we show that a PLN oligomer interacts with SERCA in a similar manner in both crystal forms. In the 2D crystals, a PLN pentamer interacts with transmembrane segments M3 of SERCA and participates in a crystal contact that bridges neighboring SERCA dimer ribbons. In the helical crystals, an oligomeric form of PLN also interacts with M3 of SERCA, though the PLN oligomer straddles a SERCA-SERCA crystal contact. We conclude that the pentameric form of PLN interacts with M3 of SERCA and that it plays a distinct structural and functional role in SERCA regulation. The interaction of the pentamer places the cytoplasmic domains of PLN at the membrane surface proximal to the calcium entry funnel of SERCA. This interaction may cause localized perturbation of the membrane bilayer as a mechanism for increasing the turnover rate of SERCA.

Targeting protein-protein interactions for therapeutic discovery via FRET-based high-throughput screening in living cells

We have developed a structure-based high-throughput screening (HTS) method, using time-resolved fluorescence resonance energy transfer (TR-FRET) that is sensitive to protein-protein interactions in living cells. The membrane protein complex between the cardiac sarcoplasmic reticulum Ca-ATPase (SERCA2a) and phospholamban (PLB), its Ca-dependent regulator, is a validated therapeutic target for reversing cardiac contractile dysfunction caused by aberrant calcium handling. However, efforts to develop compounds with SERCA2a-PLB specificity have yet to yield an effective drug. We co-expressed GFP-SERCA2a (donor) in the endoplasmic reticulum membrane of HEK293 cells with RFP-PLB (acceptor), and measured FRET using a fluorescence lifetime microplate reader. We screened a small-molecule library and identified 21 compounds (Hits) that changed FRET by >3SD. 10 of these Hits reproducibly alter SERCA2a-PLB structure and function. One compound increases SERCA2a calcium affinity in cardiac membranes but not in skeletal, suggesting that the compound is acting specifically on the SERCA2a-PLB complex, as needed for a drug to mitigate deficient calcium transport in heart failure. The excellent assay quality and correlation between structural and functional assays validate this method for large-scale HTS campaigns. This approach offers a powerful pathway to drug discovery for a wide range of protein-protein interaction targets that were previously considered “undruggable”.

Redistribution of SERCA calcium pump conformers during intracellular calcium signaling

The conformational changes of a calcium transport ATPase were investigated with molecular dynamics (MD) simulations as well as fluorescence resonance energy transfer (FRET) measurements to determine the significance of a discrete structural element for regulation of the conformational dynamics of the transport cycle. Previous MD simulations indicated that a loop in the cytosolic domain of the SERCA calcium transporter facilitates an open-to-closed structural transition. To investigate the significance of this structural element, we performed additional MD simulations and new biophysical measurements of SERCA structure and function. Rationally designed in silico mutations of three acidic residues of the loop decreased SERCA domain-domain contacts and increased domain-domain separation distances. Principal component analysis of MD simulations suggested decreased sampling of compact conformations upon N-loop mutagenesis. Deficits in headpiece structural dynamics were also detected by measuring intramolecular FRET of a Cer-YFP-SERCA construct (2-color SERCA). Compared with WT, the mutated 2-color SERCA shows a partial FRET response to calcium, whereas retaining full responsiveness to the inhibitor thapsigargin. Functional measurements showed that the mutated transporter still hydrolyzes ATP and transports calcium, but that maximal enzyme activity is reduced while maintaining similar calcium affinity. In live cells, calcium elevations resulted in concomitant FRET changes as the population of WT 2-color SERCA molecules redistributed among intermediates of the transport cycle. Our results provide novel insights on how the population of SERCA pumps responds to dynamic changes in intracellular calcium.

Structural basis for relief of phospholamban-mediated inhibition of the sarcoplasmic reticulum Ca2+-ATPase at saturating Ca2+ conditions

Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) is critical for cardiac Ca²⁺ transport. Reversal of phospholamban (PLB)-mediated SERCA inhibition by saturating Ca²⁺ conditions operates as a physiological rheostat to reactivate SERCA function in the absence of PLB phosphorylation. Here, we performed extensive atomistic molecular dynamics simulations to probe the structural mechanism of this process. Simulation of the inhibitory complex at superphysiological Ca²⁺ concentrations ([Ca²⁺] = 10 mm) revealed that Ca²⁺ ions interact primarily with SERCA and the lipid headgroups, but not with PLB's cytosolic domain or the cytosolic side of the SERCA-PLB interface. At this [Ca²⁺], a single Ca²⁺ ion was translocated from the cytosol to the transmembrane transport sites. We used this Ca²⁺-bound complex as an initial structure to simulate the effects of saturating Ca²⁺ at physiological conditions ([Ca²⁺]_total ≈ 400 μm). At these conditions, ∼30% of the Ca²⁺-bound complexes exhibited structural features consistent with an inhibited state. However, in ∼70% of the Ca²⁺-bound complexes, Ca²⁺ moved to transport site I, recruited Glu⁷⁷¹ and Asp⁸⁰⁰, and disrupted key inhibitory contacts involving the conserved PLB residue Asn³⁴ Structural analysis showed that Ca²⁺ induces only local changes in interresidue inhibitory interactions, but does not induce repositioning or changes in PLB structural dynamics. Upon relief of SERCA inhibition, Ca²⁺ binding produced a site I configuration sufficient for subsequent SERCA activation. We propose that at saturating [Ca²⁺] and in the absence of PLB phosphorylation, binding of a single Ca²⁺ ion in the transport sites rapidly shifts the equilibrium toward a noninhibited SERCA-PLB complex.

Conformational memory in the association of the transmembrane protein phospholamban with the sarcoplasmic reticulum calcium pump SERCA

The sarcoplasmic reticulum Ca²⁺-ATPase SERCA promotes muscle relaxation by pumping calcium ions from the cytoplasm into the sarcoplasmic reticulum. SERCA activity is regulated by a variety of small transmembrane peptides, most notably by phospholamban in cardiac muscle and sarcolipin in skeletal muscle. However, how phospholamban and sarcolipin regulate SERCA is not fully understood. In the present study, we evaluated the effects of phospholamban and sarcolipin on calcium translocation and ATP hydrolysis by SERCA under conditions that mimic environments in sarcoplasmic reticulum membranes. For pre-steady-state current measurements, proteoliposomes containing SERCA and phospholamban or sarcolipin were adsorbed to a solid-supported membrane and activated by substrate concentration jumps. We observed that phospholamban altered ATP-dependent calcium translocation by SERCA within the first transport cycle, whereas sarcolipin did not. Using pre-steady-state charge (calcium) translocation and steady-state ATPase activity under substrate conditions (various calcium and/or ATP concentrations) promoting particular conformational states of SERCA, we found that the effect of phospholamban on SERCA depends on substrate preincubation conditions. Our results also indicated that phospholamban can establish an inhibitory interaction with multiple SERCA conformational states with distinct effects on SERCA's kinetic properties. Moreover, we noted multiple modes of interaction between SERCA and phospholamban and observed that once a particular mode of association is engaged it persists throughout the SERCA transport cycle and multiple turnover events. These observations are consistent with conformational memory in the interaction between SERCA and phospholamban, thus providing insights into the physiological role of phospholamban and its regulatory effect on SERCA transport activity.

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.

L30A Mutation of Phospholemman Mimics Effects of Cardiac Glycosides in Isolated Cardiomyocytes

To determine if mutations introduced into phospholemman (PLM) could increase the level of PLM-Na,K-ATPase (NKA) binding, we performed scanning mutagenesis of the transmembrane domain of PLM and measured Förster resonance energy transfer (FRET) between each mutant and NKA. We observed an increased level of binding to NKA for several PLM mutants compared to that of the wild type (WT), including L27A, L30A, and I32A. In isolated cardiomyocytes, overexpression of WT PLM increased the amplitude of the Ca²⁺ transient compared to the GFP control. The Ca²⁺ transient amplitude was further increased by L30A PLM overexpression. The L30A mutation also delayed Ca²⁺ extrusion and increased the duration of cardiomyocyte contraction. This mimics aspects of the effect of cardiac glycosides, which are known to increase contractility through inhibition of NKA. No significant differences between WT and L30A PLM-expressing myocytes were observed after treatment with isoproterenol, suggesting that the superinhibitory effects of L30A are reversible with β-adrenergic stimulation. We also observed a decrease in the extent of PLM tetramerization with L30A compared to WT using FRET, suggesting that L30 is an important residue for mediating PLM-PLM binding. Molecular dynamics simulations revealed that the potential energy of the L30A tetramer is greater than that of the WT, and that the transmembrane α helix is distorted by the mutation. The results identify PLM residue L30 as an important determinant of PLM tetramerization and of functional inhibition of NKA by PLM.

Glucagon-like peptide-1 regulates calcium homeostasis and electrophysiological activities of HL-1 cardiomyocytes

Glucagon like-peptide-1 (GLP-1) is an incretin hormone with antidiabetic effects through stimulating insulin secretion, β cell neogenesis, satiety sensation, and inhibiting glucagon secretion. Administration of GLP-1 provides cardioprotective effects through attenuating cardiac inflammation and insulin resistance. GLP-1 also modulates the heart rate and systolic pressure, which suggests that GLP-1 may have cardiac electrical effects. Therefore, the purposes of this study were to evaluate whether GLP-1 has direct cardiac effects and identify the underlying mechanisms. Patch clamp, confocal microscopy with Fluo-3 fluorescence, and Western blot analyses were used to evaluate the electrophysiological characteristics, calcium homeostasis, and calcium regulatory proteins in HL-1 atrial myocytes with and without GLP-1 (1 and 10nM) incubation for 24h. GLP-1 (1 and 10nM) and control cells had similar action potential durations. However, GLP-1 at 10nM significantly increased calcium transients and sarcoplasmic reticular Ca(2+) contents. Compared to the control, GLP-1 (10nM)-treated cells significantly decreased phosphorylation of the ryanodine receptor at S2814 and total phospholamban, but there were similar protein levels of sarcoplasmic reticular Ca(2+)-ATPase and the sodium-calcium exchanger. Moreover, exendin (9-39) amide (a GLP-1 receptor antagonist, 10nM) attenuated GLP-1-mediated effects on total SR content and phosphorylated ryanodine receptor S2814. This study demonstrates GLP-1 may regulate HL-1 cell arrhythmogenesis through modulating calcium handling proteins.

Atomic-level mechanisms for phospholamban regulation of the calcium pump

We performed protein pKa calculations and molecular dynamics (MD) simulations of the calcium pump (sarcoplasmic reticulum Ca(2+)-ATPase (SERCA)) in complex with phospholamban (PLB). X-ray crystallography studies have suggested that PLB locks SERCA in a low-Ca(2+)-affinity E2 state that is incompatible with metal-ion binding, thereby blocking the conversion toward a high-Ca(2+)-affinity E1 state. Estimation of pKa values of the acidic residues in the transport sites indicates that at normal intracellular pH (7.1-7.2), PLB-bound SERCA populates an E1 state that is deprotonated at residues E309 and D800 yet protonated at residue E771. We performed three independent microsecond-long MD simulations to evaluate the structural dynamics of SERCA-PLB in a solution containing 100 mM K(+) and 3 mM Mg(2+). Principal component analysis showed that PLB-bound SERCA lies exclusively along the structural ensemble of the E1 state. We found that the transport sites of PLB-bound SERCA are completely exposed to the cytosol and that K(+) ions bind transiently (≤5 ns) and nonspecifically (nine different positions) to the two transport sites, with a total occupancy time of K(+) in the transport sites of 80%. We propose that PLB binding to SERCA populates a novel (to our knowledge) E1 intermediate, E1⋅H(+)771. This intermediate serves as a kinetic trap that controls headpiece dynamics and depresses the structural transitions necessary for Ca(2+)-dependent activation of SERCA. We conclude that PLB-mediated regulation of SERCA activity in the heart results from biochemical and structural transitions that occur primarily in the E1 state of the pump.

Phospholamban and sarcolipin: Are they functionally redundant or distinct regulators of the Sarco(Endo)Plasmic Reticulum Calcium ATPase?

In muscle, the Sarco(Endo)plasmic Reticulum Calcium ATPase (SERCA) activity is regulated by two distinct proteins, PLB and SLN, which are highly conserved throughout vertebrate evolution. PLB is predominantly expressed in the cardiac muscle, while SLN is abundant in skeletal muscle. SLN is also found in the cardiac atria and to a lesser extent in the ventricle. PLB regulation of SERCA is central to cardiac function, both at rest and during extreme physiological demand. Compared to PLB, the physiological relevance of SLN remained a mystery until recently and some even thought it was redundant in function. Studies on SLN suggest that it is an uncoupler of the SERCA pump activity and can increase ATP hydrolysis resulting in heat production. Using genetically engineered mouse models for SLN and PLB, we showed that SLN, not PLB, is required for muscle-based thermogenesis. However, the mechanism of how SLN binding to SERCA results in uncoupling SERCA Ca(2+) transport from its ATPase activity remains unclear. In this review, we discuss recent advances in understanding how PLB and SLN differ in their interaction with SERCA. We will also explore whether structural differences in the cytosolic domain of PLB and SLN are the basis for their unique function and physiological roles in cardiac and skeletal muscle.

Rheostatic Regulation of the SERCA/Phospholamban Membrane Protein Complex Using Non-Coding RNA and Single-Stranded DNA oligonucleotides

The membrane protein complex between sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and phospholamban (PLN) is a prime therapeutic target for reversing cardiac contractile dysfunctions caused by calcium mishandling. So far, however, efforts to develop drugs specific for this protein complex have failed. Here, we show that non-coding RNAs and single-stranded DNAs (ssDNAs) interact with and regulate the function of the SERCA/PLN complex in a tunable manner. Both in HEK cells expressing the SERCA/PLN complex, as well as in cardiac sarcoplasmic reticulum preparations, these short oligonucleotides bind and reverse PLN's inhibitory effects on SERCA, increasing the ATPase's apparent Ca(2+) affinity. Solid-state NMR experiments revealed that ssDNA interacts with PLN specifically, shifting the conformational equilibrium of the SERCA/PLN complex from an inhibitory to a non-inhibitory state. Importantly, we achieved rheostatic control of SERCA function by modulating the length of ssDNAs. Since restoration of Ca(2+) flux to physiological levels represents a viable therapeutic avenue for cardiomyopathies, our results suggest that oligonucleotide-based drugs could be used to fine-tune SERCA function to counterbalance the extent of the pathological insults.

STIM1 enhances SR Ca2+ content through binding phospholamban in rat ventricular myocytes

In ventricular myocytes, the physiological function of stromal interaction molecule 1 (STIM1), an endo/sarcoplasmic reticulum (ER/SR) Ca(2+) sensor, is unclear with respect to its cellular localization, its Ca(2+)-dependent mobilization, and its action on Ca(2+) signaling. Confocal microscopy was used to measure Ca(2+) signaling and to track the cellular movement of STIM1 with mCherry and immunofluorescence in freshly isolated adult rat ventricular myocytes and those in short-term primary culture. We found that endogenous STIM1 was expressed at low but measureable levels along the Z-disk, in a pattern of puncta and linear segments consistent with the STIM1 localizing to the junctional SR (jSR). Depleting SR Ca(2+) using thapsigargin (2-10 µM) changed neither the STIM1 distribution pattern nor its mobilization rate, evaluated by diffusion coefficient measurements using fluorescence recovery after photobleaching. Two-dimensional blue native polyacrylamide gel electrophoresis and coimmunoprecipitation showed that STIM1 in the heart exists mainly as a large protein complex, possibly a multimer, which is not altered by SR Ca(2+) depletion. Additionally, we found no store-operated Ca(2+) entry in control or STIM1 overexpressing ventricular myocytes. Nevertheless, STIM1 overexpressing cells show increased SR Ca(2+) content and increased SR Ca(2+) leak. These changes in Ca(2+) signaling in the SR appear to be due to STIM1 binding to phospholamban and thereby indirectly activating SERCA2a (Sarco/endoplasmic reticulum Ca(2+) ATPase). We conclude that STIM1 binding to phospholamban contributes to the regulation of SERCA2a activity in the steady state and rate of SR Ca(2+) leak and that these actions are independent of store-operated Ca(2+) entry, a process that is absent in normal heart cells.

ATP-Binding Cassette Transporter Structure Changes Detected by Intramolecular Fluorescence Energy Transfer for High-Throughput Screening

Multidrug resistance protein 1 (MRP1) actively transports a wide variety of drugs out of cells. To quantify MRP1 structural dynamics, we engineered a "two-color MRP1" construct by fusing green fluorescent protein (GFP) and TagRFP to MRP1 nucleotide-binding domains NBD1 and NBD2, respectively. The recombinant MRP1 protein expressed and trafficked normally to the plasma membrane. Two-color MRP1 transport activity was normal, as shown by vesicular transport of [(3)H]17β-estradiol-17-β-(D-glucuronide) and doxorubicin efflux in AAV-293 cells. We quantified fluorescence resonance energy transfer (FRET) from GFP to TagRFP as an index of NBD conformational changes. Our results show that ATP binding induces a large-amplitude conformational change that brings the NBDs into closer proximity. FRET was further increased by substrate in the presence of ATP but not by substrate alone. The data suggest that substrate binding is required to achieve a fully closed and compact structure. ATP analogs bind MRP1 with reduced apparent affinity, inducing a partially closed conformation. The results demonstrate the utility of the two-color MRP1 construct for investigating ATP-binding cassette transporter structural dynamics, and it holds great promise for high-throughput screening of chemical libraries for unknown activators, inhibitors, or transportable substrates of MRP1.

Acute inotropic and lusitropic effects of cardiomyopathic R9C mutation of phospholamban

A naturally occurring R9C mutation of phospholamban (PLB) triggers cardiomyopathy and premature death by altering regulation of sarco/endoplasmic reticulum calcium-ATPase (SERCA). The goal of this study was to investigate the acute physiological consequences of the R9C-PLB mutation on cardiomyocyte calcium kinetics and contractility. We measured the physiological consequences of R9C-PLB mutation on calcium transients and sarcomere shortening in adult cardiomyocytes. In contrast to studies of chronic R9C-PLB expression in transgenic mice, we found that acute expression of R9C-PLB exerts a positively inotropic and lusitropic effect in cardiomyocytes. Importantly, R9C-PLB exhibited blunted sensitivity to frequency potentiation and β-adrenergic stimulation, two major physiological mechanisms for the regulation of cardiac performance. To identify the molecular mechanism of R9C pathology, we quantified the effect of R9C on PLB oligomerization and PLB-SERCA binding. FRET measurements in live cells revealed that R9C-PLB exhibited an increased propensity for oligomerization, and this was further increased by oxidative stress. The R9C also decreased PLB binding to SERCA and altered the structure of the PLB-SERCA regulatory complex. The structural change after oxidative modification of R9C-PLB was similar to that observed after PLB phosphorylation. We conclude that R9C mutation of PLB decreases SERCA inhibition by decreasing the amount of the regulatory complex and altering its conformation. This has an acute inotropic/lusitropic effect but yields negative consequences of impaired frequency potentiation and blunted β-adrenergic responsiveness. We envision a self-reinforcing mechanism beginning with phosphomimetic R9C-PLB oxidation and loss of SERCA inhibition, leading to impaired calcium regulation and heart failure.

SERCA2a stimulation by istaroxime: a novel mechanism of action with translational implications

Sarcoplasmic reticular (SR) Ca(2+) -ATPase (SERCA2a) is central to cardiac electrophysiological and mechanical function. It ensures full diastolic relaxation minimizing delayed after-potentials that would otherwise compromise membrane electrophysiological stability, and optimizes SR Ca(2+) refilling and systolic contraction. Previous studies demonstrated that the small molecule agent istaroxime stimulates SERCA2a-ATPase activity, restoring its function in failing hearts, and enhancing indices of mechanical, and SR Ca(2+) release and re-uptake, activity. Ferrandi et al (2013) now elegantly demonstrate its ability to dissociate the phospholamdan (PB) bound to cardiac SERCA2a, thereby removing the inhibitory effect of PB on SERCA2a. This effect was independent of the cAMP/PKA system and modified a specific SERCA2a reaction step. They used SERCA-enriched SR preparations from a rigorously validated and realistic physiological, canine model of cardiac failure with established Na(+) -K(+) -ATPase sensitivity to cardiac glycosides and SR Ca(2+) handling features. These findings potentially translate into a novel management of the major and increasingly important public health challenge of chronic cardiac failure.

LINKED ARTICLE

This article is a commentary on Ferrandi et al., pp. 1849-1861 of volume 169 issue 8. To view this paper visit http://dx.doi.org/10.1111/bph.12278.

FGF-23 dysregulates calcium homeostasis and electrophysiological properties in HL-1 atrial cells

BACKGROUND

Fibroblast growth factor (FGF)-23 is a key regulator of phosphate homeostasis. Higher FGF-23 levels are correlated with poor outcomes in cardiovascular diseases. FGF-23 can produce cardiac hypertrophy and increase intracellular calcium, which can change cardiac electrical activity. However, it is not clear whether FGF-23 possesses arrhythmogenic potential through calcium dysregulation. Therefore, the purposes of this study were to evaluate the electrophysiological effects of FGF-23 and identify the underlying mechanisms.

METHODS

Patch clamp, confocal microscope with Fluo-4 fluorescence, and Western blot analyses were used to evaluate the electrophysiological characteristics, calcium homeostasis and calcium regulatory proteins in HL-1 atrial myocytes with and without FGF-23 (10 and 25 ng/mL) incubation for 24 h.

RESULTS

FGF-23 (25 ng/mL) increased L-type calcium currents, calcium transient and sarcoplasmic reticulum Ca(2+) contents in HL-1 cells. FGF-23 (25 ng/mL)-treated cells (n = 14) had greater incidences (57%, 17% and 15%, P < 0·05) of delayed afterdepolarizations than control (n = 12) and FGF-23 (10 ng/mL)-treated cells (n = 13). Compared with control cells, FGF-23 (25 ng/mL)-treated cells (n = 14) exhibited increased phosphorylation of calcium/calmodulin-dependent protein kinase IIδ and phospholamban (PLB) at threonine 17 but had similar phosphorylation extents of PLB at serine 16, total PLB and sarcoplasmic reticulum Ca(2+) -ATPase protein. Moreover, the FGF receptor inhibitor (PD173074, 10 nM), calmodulin inhibitor (W7, 5 μM) and phospholipase C inhibitor (U73122, 1 μM) attenuated the effects of FGF-23 on calcium/calmodulin-dependent protein kinase II phosphorylation.

CONCLUSIONS

FGF-23 increases HL-1 cells arrhythmogenesis with calcium dysregulation through modulating calcium-handling proteins.

Phospholamban C-terminal residues are critical determinants of the structure and function of the calcium ATPase regulatory complex

To determine the structural and regulatory role of the C-terminal residues of phospholamban (PLB) in the membranes of living cells, we fused fluorescent protein tags to PLB and sarco/endoplasmic reticulum calcium ATPase (SERCA). Alanine substitution of PLB C-terminal residues significantly altered fluorescence resonance energy transfer (FRET) from PLB to PLB and SERCA to PLB, suggesting a change in quaternary conformation of PLB pentamer and SERCA-PLB regulatory complex. Val to Ala substitution at position 49 (V49A) had particularly large effects on PLB pentamer structure and PLB-SERCA regulatory complex conformation, increasing and decreasing probe separation distance, respectively. We also quantified a decrease in oligomerization affinity, an increase in binding affinity of V49A-PLB for SERCA, and a gain of inhibitory function as quantified by calcium-dependent ATPase activity. Notably, deletion of only a few C-terminal residues resulted in significant loss of PLB membrane anchoring and mislocalization to the cytoplasm and nucleus. C-terminal truncations also resulted in progressive loss of PLB-PLB FRET due to a decrease in the apparent affinity of PLB oligomerization. We quantified a similar decrease in the binding affinity of truncated PLB for SERCA and loss of inhibitory potency. However, despite decreased SERCA-PLB binding, intermolecular FRET for Val(49)-stop (V49X) truncation mutant was paradoxically increased as a result of an 11.3-Å decrease in the distance between donor and acceptor fluorophores. We conclude that PLB C-terminal residues are critical for localization, oligomerization, and regulatory function. In particular, the PLB C terminus is an important determinant of the quaternary structure of the SERCA regulatory complex.

Time-resolved FRET reveals the structural mechanism of SERCA-PLB regulation

We have used time-resolved fluorescence resonance energy transfer (TR-FRET) to characterize the interaction between phospholamban (PLB) and the sarcoplasmic reticulum (SR) Ca-ATPase (SERCA) under conditions that relieve SERCA inhibition. Unphosphorylated PLB inhibits SERCA in cardiac SR, but inhibition is relieved by either micromolar Ca(2+) or PLB phosphorylation. In both cases, it has been proposed that inhibition is relieved by dissociation of the complex. To test this hypothesis, we attached fluorophores to the cytoplasmic domains of SERCA and PLB, and reconstituted them functionally in lipid bilayers. TR-FRET, which permitted simultaneous measurement of SERCA-PLB binding and structure, was measured as a function of PLB phosphorylation and [Ca(2+)]. In all cases, two structural states of the SERCA-PLB complex were resolved, probably corresponding to the previously described T and R structural states of the PLB cytoplasmic domain. Phosphorylation of PLB at S16 completely relieved inhibition, partially dissociated the SERCA-PLB complex, and shifted the T/R equilibrium within the bound complex toward the R state. Since the PLB concentration in cardiac SR is at least 10 times that in our FRET measurements, we calculate that most of SERCA contains bound phosphorylated PLB in cardiac SR, even after complete phosphorylation. 4 μM Ca(2+) completely relieved inhibition but did not induce a detectable change in SERCA-PLB binding or cytoplasmic domain structure, suggesting a mechanism involving structural changes in SERCA's transmembrane domain. We conclude that Ca(2+) and PLB phosphorylation relieve SERCA-PLB inhibition by distinct mechanisms, but both are achieved primarily by structural changes within the SERCA-PLB complex, not by dissociation of that complex.

Istaroxime stimulates SERCA2a and accelerates calcium cycling in heart failure by relieving phospholamban inhibition

BACKGROUND AND PURPOSE

Calcium handling is known to be deranged in heart failure. Interventions aimed at improving cell Ca(2) (+) cycling may represent a promising approach to heart failure therapy. Istaroxime is a new luso-inotropic compound that stimulates cardiac contractility and relaxation in healthy and failing animal models and in patients with acute heart failure (AHF) syndrome. Istaroxime is a Na-K ATPase inhibitor with the unique property of increasing sarcoplasmic reticulum (SR) SERCA2a activity as shown in heart microsomes from humans and guinea pigs. The present study addressed the molecular mechanism by which istaroxime increases SERCA2a activity.

EXPERIMENTAL APPROACH

To study the effect of istaroxime on SERCA2a-phospholamban (PLB) complex, we applied different methodologies in native dog healthy and failing heart preparations and heterologous canine SERCA2a/PLB co-expressed in Spodoptera frugiperda (Sf21) insect cells.

KEY RESULTS

We showed that istaroxime enhances SERCA2a activity, Ca(2) (+) uptake and the Ca(2) (+) -dependent charge movements into dog healthy and failing cardiac SR vesicles. Although not directly demonstrated, the most probable explanation of these activities is the displacement of PLB from SERCA2a.E2 conformation, independently from cAMP/PKA. We propose that this displacement may favour the SERCA2a conformational transition from E2 to E1, thus resulting in the acceleration of Ca(2) (+) cycling.

CONCLUSIONS AND IMPLICATIONS

Istaroxime represents the first example of a small molecule that exerts a luso-inotropic effect in the failing human heart through the stimulation of SERCA2a ATPase activity and the enhancement of Ca(2) (+) uptake into the SR by relieving the PLB inhibitory effect on SERCA2a in a cAMP/PKA independent way.

Phosphorylated phospholamban stabilizes a compact conformation of the cardiac calcium-ATPase

The sarcoendoplasmic reticulum calcium ATPase (SERCA) plays a key role in cardiac calcium handling and is considered a high-value target for the treatment of heart failure. SERCA undergoes conformational changes as it harnesses the chemical energy of ATP for active transport. X-ray crystallography has provided insight into SERCA structural substates, but it is not known how well these static snapshots describe in vivo conformational dynamics. The goals of this work were to quantify the direction and magnitude of SERCA motions as the pump performs work in live cardiac myocytes, and to identify structural determinants of SERCA regulation by phospholamban. We measured intramolecular fluorescence resonance energy transfer (FRET) between fluorescent proteins fused to SERCA cytoplasmic domains. We detected four discrete structural substates for SERCA expressed in cardiac muscle cells. The relative populations of these discrete states oscillated with electrical pacing. Low FRET states were most populated in low Ca (diastole), and were indicative of an open, disordered structure for SERCA in the E2 (Ca-free) enzymatic substate. High FRET states increased with Ca (systole), suggesting rigidly closed conformations for the E1 (Ca-bound) enzymatic substates. Notably, a special compact E1 state was observed after treatment with β-adrenergic agonist or with coexpression of phosphomimetic mutants of phospholamban. The data suggest that SERCA calcium binding induces the pump to undergo a transition from an open, dynamic conformation to a closed, ordered structure. Phosphorylated phospholamban stabilizes a unique conformation of SERCA that is characterized by a compact architecture.

The structural basis for phospholamban inhibition of the calcium pump in sarcoplasmic reticulum

P-type ATPases are a large family of enzymes that actively transport ions across biological membranes by interconverting between high (E1) and low (E2) ion-affinity states; these transmembrane transporters carry out critical processes in nearly all forms of life. In striated muscle, the archetype P-type ATPase, SERCA (sarco(endo)plasmic reticulum Ca(2+)-ATPase), pumps contractile-dependent Ca(2+) ions into the lumen of sarcoplasmic reticulum, which initiates myocyte relaxation and refills the sarcoplasmic reticulum in preparation for the next contraction. In cardiac muscle, SERCA is regulated by phospholamban (PLB), a small inhibitory phosphoprotein that decreases the Ca(2+) affinity of SERCA and attenuates contractile strength. cAMP-dependent phosphorylation of PLB reverses Ca(2+)-ATPase inhibition with powerful contractile effects. Here we present the long sought crystal structure of the PLB-SERCA complex at 2.8-Å resolution. The structure was solved in the absence of Ca(2+) in a novel detergent system employing alkyl mannosides. The structure shows PLB bound to a previously undescribed conformation of SERCA in which the Ca(2+) binding sites are collapsed and devoid of divalent cations (E2-PLB). This new structure represents one of the key unsolved conformational states of SERCA and provides a structural explanation for how dephosphorylated PLB decreases Ca(2+) affinity and depresses cardiac contractility.

A separate pool of cardiac phospholemman that does not regulate or associate with the sodium pump: multimers of phospholemman in ventricular muscle

BACKGROUND

Phospholemman regulates the plasmalemmal sodium pump in excitable tissues.

RESULTS

In cardiac muscle, a subpopulation of phospholemman with a unique phosphorylation signature associates with other phospholemman molecules but not with the pump.

CONCLUSION

Phospholemman oligomers exist in cardiac muscle.

SIGNIFICANCE

Much like phospholamban regulation of SERCA, phospholemman exists as both a sodium pump inhibiting monomer and an unassociated oligomer. Phospholemman (PLM), the principal quantitative sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. Much like phospholamban, which regulates the related ATPase SERCA, PLM is reported to oligomerize. We investigated subpopulations of PLM in adult rat ventricular myocytes based on phosphorylation status. Co-immunoprecipitation identified two pools of PLM: one not associated with the sodium pump phosphorylated at Ser(63) and one associated with the pump, both phosphorylated at Ser(68) and unphosphorylated. Phosphorylation of PLM at Ser(63) following activation of PKC did not abrogate association of PLM with the pump, so its failure to associate with the pump was not due to phosphorylation at this site. All pools of PLM co-localized to cell surface caveolin-enriched microdomains with sodium pump α subunits, despite the lack of caveolin-binding motif in PLM. Mass spectrometry analysis of phosphospecific immunoprecipitation reactions revealed no unique protein interactions for Ser(63)-phosphorylated PLM, and cross-linking reagents also failed to identify any partner proteins for this pool. In lysates from hearts of heterozygous transgenic animals expressing wild type and unphosphorylatable PLM, Ser(63)-phosphorylated PLM co-immunoprecipitated unphosphorylatable PLM, confirming the existence of PLM multimers. Dephosphorylation of the PLM multimer does not change sodium pump activity. Hence like phospholamban, PLM exists as a pump-inhibiting monomer and an unassociated oligomer. The distribution of different PLM phosphorylation states to different pools may be explained by their differential proximity to protein phosphatases rather than a direct effect of phosphorylation on PLM association with the pump.

Sarco(endo)plasmic reticulum calcium ATPase (SERCA) inhibition by sarcolipin is encoded in its luminal tail

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) is regulated in a tissue-dependent manner via interaction with the short integral membrane proteins phospholamban (PLN) and sarcolipin (SLN). Although defects in SERCA activity are known to cause heart failure, the regulatory mechanisms imposed by PLN and SLN could have clinical implications for both heart and skeletal muscle diseases. PLN and SLN have significant sequence homology in their transmembrane regions, suggesting a similar mode of binding to SERCA. However, unlike PLN, SLN has a conserved C-terminal luminal tail composed of five amino acids ((27)RSYQY), which may contribute to a distinct SERCA regulatory mechanism. We have functionally characterized alanine mutants of the C-terminal tail of SLN using co-reconstituted proteoliposomes of SERCA and SLN. We found that Arg(27) and Tyr(31) are essential for SLN function. We also tested the effect of a truncated variant of SLN (Arg(27)stop) and extended chimeras of PLN with the five luminal residues of SLN added to its C terminus. The Arg(27)stop form of SLN resulted in loss of function, whereas the PLN chimeras resulted in superinhibition with characteristics of both PLN and SLN. Based on our results, we propose that the C-terminal tail of SLN is a distinct, essential domain in the regulation of SERCA and that the functional properties of the SLN tail can be transferred to PLN.

2-Color calcium pump reveals closure of the cytoplasmic headpiece with calcium binding

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) undergoes conformational changes while transporting calcium, but the details of the domain motions are still unclear. The objective of the present study was to measure distances between the cytoplasmic domains of SERCA2a in order to reveal the magnitude and direction of conformational changes. Using fluorescence microscopy of live cells, we measured intramolecular fluorescence resonance energy transfer (FRET) from a donor fluorescent protein fused to the SERCA N-terminus to an acceptor fluorescent protein fused to either the N-, P-, or transmembrane domain. The "2-color" SERCA constructs were catalytically active as indicated by ATPase activity in vitro and Ca uptake in live cells. All constructs exhibited dynamic FRET changes in response to the pump ligands calcium and thapsigargin (Tg). These FRET changes were quantified as an index of SERCA conformational changes. Intramolecular FRET decreased with Tg for the two N-domain fusion sites (at residue 509 or 576), while the P- (residue 661) and TM-domain (C-terminus) fusions showed increased FRET with Tg. The magnitude of the Tg-dependent conformational change was not decreased by coexpression of phospholamban (PLB), nor did PLB slow the kinetics of Tg binding. FRET in ionophore-permeabilized cells was lower in EGTA than in saturating calcium for all constructs, indicating a decrease in domain separation distance with the structural transition from E2 (Ca-free) to E1 (Ca-bound). The data suggest closure of the cytoplasmic headpiece with Ca-binding. The present results provide insight into the structural dynamics of the Ca-ATPase. In addition, the 2-color SERCA constructs developed for this study may be useful for evaluating candidate small molecule regulators of Ca uptake activity.

Modelling cardiac calcium sparks in a three-dimensional reconstruction of a calcium release unit

Triggered release of Ca2+ from an individual sarcoplasmic reticulum (SR) Ca(2+) release unit (CRU) is the fundamental event of cardiac excitation–contraction coupling, and spontaneous release events (sparks) are the major contributor to diastolic Ca(2+) leak in cardiomyocytes. Previous model studies have predicted that the duration and magnitude of the spark is determined by the local CRU geometry, as well as the localization and density of Ca(2+) handling proteins. We have created a detailed computational model of a CRU, and developed novel tools to generate the computational geometry from electron tomographic images. Ca(2+) diffusion was modelled within the SR and the cytosol to examine the effects of localization and density of the Na(+)/Ca(2+) exchanger, sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA), and calsequestrin on spark dynamics. We reconcile previous model predictions of approximately 90% local Ca(2+) depletion in junctional SR, with experimental reports of about 40%. This analysis supports the hypothesis that dye kinetics and optical averaging effects can have a significant impact on measures of spark dynamics. Our model also predicts that distributing calsequestrin within non-junctional Z-disc SR compartments, in addition to the junctional compartment, prolongs spark release time as reported by Fluo5. By pumping Ca(2+) back into the SR during a release, SERCA is able to prolong a Ca(2+) spark, and this may contribute to SERCA-dependent changes in Ca(2+) wave speed. Finally, we show that including the Na(+)/Ca(2+) exchanger inside the dyadic cleft does not alter local [Ca(2+)] during a spark.