Lethal Arg9Cys phospholamban mutation hinders Ca2+-ATPase regulation and phosphorylation by protein kinase A

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.

Reassessing the Mechanisms of PLN-R14del Cardiomyopathy: From Calcium Dysregulation to S/ER Malformation

The phospholamban (PLN) pathogenic gene variant, p.Arg14del (PLN-R14del), can lead to dilated and arrhythmogenic cardiomyopathy, resulting in heart failure. PLN-R14del cardiomyopathy has been conceptualized as a disease caused by sarco/endoplasmic reticulum calcium adenosine triphosphatase 2a (SERCA2a) superinhibition. However, recent studies raised controversy regarding the effect of PLN-R14del on SERCA activity and revealed a prominent role for abnormal PLN protein distribution and sarco/endoplasmic reticulum disorganization as underlying disease mechanism. Strategies targeting sarco/endoplasmic reticulum malformation may, therefore, prove more effective than SERCA activity modulation. This review reassesses the disease mechanisms of PLN-R14del cardiomyopathy and emphasizes the importance of dissecting the underlying molecular mechanisms to uncover targets for innovative treatments.

Phosphorylation of phospholamban promotes SERCA2a activation by dwarf open reading frame (DWORF)

In cardiac myocytes, the type 2a sarco/endoplasmic reticulum Ca-ATPase (SERCA2a) plays a key role in intracellular Ca regulation. Due to its critical role in heart function, SERCA2a activity is tightly regulated by different mechanisms, including micropeptides. While phospholamban (PLB) is a well-known SERCA2a inhibitor, dwarf open reading frame (DWORF) is a recently identified SERCA2a activator. Since PLB phosphorylation is the most recognized mechanism of SERCA2a activation during adrenergic stress, we studied whether PLB phosphorylation also affects SERCA2a regulation by DWORF. By using confocal Ca imaging in a HEK293 expressing cell system, we analyzed the effect of the co-expression of PLB and DWORF using a bicistronic construct on SERCA2a-mediated Ca uptake. Under these conditions of matched expression of PLB and DWORF, we found that SERCA2a inhibition by non-phosphorylated PLB prevails over DWORF activating effect. However, when PLB is phosphorylated at PKA and CaMKII sites, not only PLB's inhibitory effect was relieved, but SERCA2a was effectively activated by DWORF. Förster resonance energy transfer (FRET) analysis between SERCA2a and DWORF showed that DWORF has a higher relative affinity for SERCA2a when PLB is phosphorylated. Thus, SERCA2a regulation by DWORF responds to the PLB phosphorylation status, suggesting that DWORF might contribute to SERCA2a activation during conditions of adrenergic stress.

Mitigation of Stress-induced Structural Remodeling and Functional Deficiency in iPSC-CMs with PLN R9C Mutation by Promoting Autophagy

Background

Phospholamban (PLN) is a key regulator of cardiac function connecting adrenergic signaling and calcium homeostasis. The R9C mutation of PLN is known to cause early onset dilated cardiomyopathy (DCM) and premature death, yet the detailed mechanisms underlie the pathologic remodeling process are not well defined in human cardiomyocytes. The aim of this study is to unravel the role of PLN R9C in DCM and identify potential therapeutic targets.

Methods

PLN R9C knock-in (KI) and patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated and comprehensively examined for their expression profile, contractile function, and cellular signaling under both baseline conditions and following functional challenges.

Results

PLN R9C KI iPSC-CMs exhibited near-normal morphology and calcium handling, slightly increased contractility, and an attenuated response to β-adrenergic activation compared to wild-type (WT) cells. However, treatment with a maturation medium (MM) has induced fundamentally different remodeling in the two groups: while it improved the structural integrity and functional performance of WT cells, the same treatment result in sarcomere disarrangement, calcium handling deficiency, and further disrupted adrenergic signaling in PLN R9C KI cells. To understand the mechanism, transcriptomic analysis showed the enrichment of protein homeostasis signaling pathways specifically in PLN R9C KI cells in response to the MM treatment and increased contractile demands. Further studies also indicated elevated ROS levels, interrupted autophagic flux, and increased pentamer PLN aggregation in functionally challenged KI cells. These results were further confirmed in patient-specific iPSC-CM models, suggesting that functional stresses exacerbate the deficiencies in PLN R9C cells through disrupting protein homeostasis. Indeed, treating stressed patient cells with autophagy-accelerating reagents, such as metformin and rapamycin, has restored autophagic flux, mitigated sarcomere disarrangement, and partially rescued β-adrenergic signaling and cardiac function.

Conclusions

PLN R9C leads to a mild increase of calcium recycling and contractility. Functional challenges further enhanced contractile and proteostasis stress, leading to autophagic overload, structural remodeling, and functional deficiencies in PLN R9C cardiomyocytes. Activation of autophagy signaling partially rescues these effects, revealing a potential therapeutic target for DCM patients with the PLN R9C mutation.

Graphic abstracts

A graphic abstract is available for this article.

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.

Phosphorylation, compartmentalization, and cardiac function

Protein phosphorylation is a fundamental element of cell signaling. First discovered as a biochemical switch in glycogen metabolism, we now know that this posttranslational modification permeates all aspects of cellular behavior. In humans, over 540 protein kinases attach phosphate to acceptor amino acids, whereas around 160 phosphoprotein phosphatases remove phosphate to terminate signaling. Aberrant phosphorylation underlies disease, and kinase inhibitor drugs are increasingly used clinically as targeted therapies. Specificity in protein phosphorylation is achieved in part because kinases and phosphatases are spatially organized inside cells. A prototypic example is compartmentalization of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A through association with A-kinase anchoring proteins. This configuration creates autonomous signaling islands where the anchored kinase is constrained in proximity to activators, effectors, and selected substates. This article primarily focuses on A kinase anchoring protein (AKAP) signaling in the heart with an emphasis on anchoring proteins that spatiotemporally coordinate excitation-contraction coupling and hypertrophic responses.

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.

Early calcium and cardiac contraction defects in a model of phospholamban R9C mutation in zebrafish

The phospholamban mutation Arg 9 to Cys (R9C) has been found to cause a dilated cardiomyopathy in humans and in transgenic mice, with ventricular dilation and premature death. Emerging evidence suggests that phospholamban R9C is a loss-of-function mutation with dominant negative effect on SERCA2a activity. We imaged calcium and cardiac contraction simultaneously in 3 and 9 days-post-fertilization (dpf) zebrafish larvae expressing plnb^R9C in the heart to unveil the early pathological pathway that triggers the disease. We generated transgenic zebrafish lines expressing phospholamban wild-type (Tg(myl7:plnb^wt)) and phospholamban R9C (Tg(myl7:plnb^R9C)) in the heart of zebrafish. To measure calcium and cardiac contraction in 3 and 9 dpf larvae, Tg(myl7:plnb^wt) and Tg(myl7:plnb^R9C) fish were outcrossed with a transgenic line expressing the ratiometric fluorescent calcium biosensor mCyRFP1-GCaMP6f. We found that Plnb^R9C raised calcium transient amplitude, induced positive inotropy and lusitropy, and blunted the β-adrenergic response to isoproterenol in 3 dpf larvae. These effects can be attributed to enhanced SERCA2a activity induced by the Plnb^R9C mutation. In contrast, Tg(myl7:plnb^R9C) larvae at 9 dpf exhibited ventricular dilation, systolic dysfunction and negative lusitropy, hallmarks of a dilated cardiomyopathy in humans. Importantly, N-acetyl-L-cysteine rescued this deleterious phenotype, suggesting that reactive oxygen species contribute to the pathological pathway. These results also imply that dysregulation of calcium homeostasis during embryo development contributes to the disease progression at later stages. Our in vivo model in zebrafish allows characterization of pathophysiological mechanisms leading to heart disease, and can be used for screening of potential therapeutical agents.

Blocking phospholamban with VHH intrabodies enhances contractility and relaxation in heart failure

The dysregulated physical interaction between two intracellular membrane proteins, the sarco/endoplasmic reticulum Ca²⁺ ATPase and its reversible inhibitor phospholamban, induces heart failure by inhibiting calcium cycling. While phospholamban is a bona-fide therapeutic target, approaches to selectively inhibit this protein remain elusive. Here, we report the in vivo application of intracellular acting antibodies (intrabodies), derived from the variable domain of camelid heavy-chain antibodies, to modulate the function of phospholamban. Using a synthetic VHH phage-display library, we identify intrabodies with high affinity and specificity for different conformational states of phospholamban. Rapid phenotypic screening, via modified mRNA transfection of primary cells and tissue, efficiently identifies the intrabody with most desirable features. Adeno-associated virus mediated delivery of this intrabody results in improvement of cardiac performance in a murine heart failure model. Our strategy for generating intrabodies to investigate cardiac disease combined with modified mRNA and adeno-associated virus screening could reveal unique future therapeutic opportunities.

Here the authors use modified RNA and VHH libraries to generate intrabodies that target dysregulated interactions between two calcium handling proteins in failing cardiomyocytes. Heart specific expression of the intrabodies in a murine heart failure model results in improved cardiac function.

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.

Structures of PKA-phospholamban complexes reveal a mechanism of familial dilated cardiomyopathy

Several mutations identified in phospholamban (PLN) have been linked to familial dilated cardiomyopathy (DCM) and heart failure, yet the underlying molecular mechanism remains controversial. PLN interacts with sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) and regulates calcium uptake, which is modulated by the protein kinase A (PKA)-dependent phosphorylation of PLN during the fight-or-flight response. Here, we present the crystal structures of the catalytic domain of mouse PKA in complex with wild-type and DCM-mutant PLNs. Our structures, combined with the results from other biophysical and biochemical assays, reveal a common disease mechanism: the mutations in PLN reduce its phosphorylation level by changing its conformation and weakening its interactions with PKA. In addition, we demonstrate that another more ubiquitous SERCA-regulatory peptide, called another-regulin (ALN), shares a similar mechanism mediated by PKA in regulating SERCA activity.

Characterization of the PLN p.Arg14del Mutation in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Phospholamban (PLN) is the natural inhibitor of the sarco/endoplasmic reticulum Ca2+ ATP-ase (SERCA2a). Heterozygous PLN p.Arg14del mutation is associated with an arrhythmogenic dilated cardiomyopathy (DCM), whose pathogenesis has been attributed to SERCA2a "superinhibition".

AIM

To test in cardiomyocytes (hiPSC-CMs) derived from a PLN p.Arg14del carrier whether (1) Ca2+ dynamics and protein localization were compatible with SERCA2a superinhibition and (2) if functional abnormalities could be reverted by pharmacological SERCA2a activation (PST3093).

METHODS

Ca2+ transients (CaT) were recorded at 36 °C in hiPSC-CMs clusters during field stimulation. SERCA2a and PLN where immunolabeled in single hiPSC-CMs. Mutant preparations (MUT) were compared to isogenic wild-type ones (WT), obtained by mutation reversal.

RESULTS

WT and MUT differed for the following properties: (1) CaT time to peak (tpeak) and half-time of CaT decay were shorter in MUT; (2) several CaT profiles were identified in WT, "hyperdynamic" ones largely prevailed in MUT; (3) whereas tpeak rate-dependently declined in WT, it was shorter and rate-independent in MUT; (4) diastolic Ca2+ rate-dependently accumulated in WT, but not in MUT. When applied to WT, PST3093 turned all the above properties to resemble those of MUT; when applied to MUT, PST3093 had a smaller or negligible effect. Preferential perinuclear SERCA2a-PLN localization was lost in MUT hiPSC-CMs.

CONCLUSIONS

Functional data converge to argue for PLN p.Arg14del incompetence in inhibiting SERCA2a in the tested case, thus weakening the rationale for therapeutic SERCA2a activation. Mechanisms alternative to SERCA2a superinhibition should be considered in the pathogenesis of DCM, possibly including dysregulation of Ca2+-dependent transcription.

Molecular noise filtering in the β-adrenergic signaling network by phospholamban pentamers

Phospholamban (PLN) is an important regulator of cardiac calcium handling due to its ability to inhibit the calcium ATPase SERCA. β-Adrenergic stimulation reverses SERCA inhibition via PLN phosphorylation and facilitates fast calcium reuptake. PLN also forms pentamers whose physiological significance has remained elusive. Using mathematical modeling combined with biochemical and cell biological experiments, we show that pentamers regulate both the dynamics and steady-state levels of monomer phosphorylation. Substrate competition by pentamers and a feed-forward loop involving inhibitor-1 can delay monomer phosphorylation by protein kinase A (PKA), whereas cooperative pentamer dephosphorylation enables bistable PLN steady-state phosphorylation. Simulations show that phosphorylation delay and bistability act as complementary filters that reduce the effect of random fluctuations in PKA activity, thereby ensuring consistent monomer phosphorylation and SERCA activity despite noisy upstream signals. Preliminary analyses suggest that the PLN mutation R14del could impair noise filtering, offering a new perspective on how this mutation causes cardiac arrhythmias.

14-3-3 binding creates a memory of kinase action by stabilizing the modified state of phospholamban

The cardiac membrane protein phospholamban (PLN) is targeted by protein kinase A (PKA) at Ser16and by Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr17. β-Adrenergic stimulation and PKA-dependent phosphorylation of Ser16acutely stimulate the sarcoplasmic reticulum calcium pump (SERCA) by relieving its inhibition by PLN. CaMKII-dependent phosphorylation may lead to longer-lasting SERCA stimulation and may sustain maladaptive Ca2+handling. Here, we demonstrated that phosphorylation at either Ser16or Thr17converted PLN into a target for the phosphoadaptor protein 14-3-3 with different affinities. 14-3-3 proteins were localized within nanometers of PLN and endogenous 14-3-3 coimmunoprecipitated with pentameric PLN from cardiac membranes. Molecular dynamics simulations predicted different molecular contacts for peptides phosphorylated at Ser16or Thr17with the binding groove of 14-3-3, resulting in varied binding affinities. 14-3-3 binding protected either PLN phosphosite from dephosphorylation. β-Adrenergic stimulation of isolated adult cardiomyocytes resulted in the membrane recruitment of endogenous 14-3-3. The exogenous addition of 14-3-3 to β-adrenergic–stimulated cardiomyocytes led to prolonged SERCA activation, presumably because 14-3-3 protected PLN pentamers from dephosphorylation. Phosphorylation of Ser16was disrupted by the cardiomyopathy-associated ∆Arg14mutation, implying that phosphorylation of Thr17by CaMKII may become crucial for 14-3-3 recruitment to ∆Arg14PLN. Consistent with PLN acting as a dynamic hub in the control of Ca2+handling, our results identify 14-3-3 binding to PLN as a contractility-augmenting mechanism.

Genetic Dissection of Hypertrophic Cardiomyopathy with Myocardial RNA-Seq

Hypertrophic cardiomyopathy (HCM) is an inherited disorder of the myocardium, and pathogenic mutations in the sarcomere genes myosin heavy chain 7 (MYH7) and myosin-binding protein C (MYBPC3) explain 60%–70% of observed clinical cases. The heterogeneity of phenotypes observed in HCM patients, however, suggests that novel causative genes or genetic modifiers likely exist. Here, we systemically evaluated RNA-seq data from 28 HCM patients and 9 healthy controls with pathogenic variant identification, differential expression analysis, and gene co-expression and protein–protein interaction network analyses. We identified 43 potential pathogenic variants in 19 genes in 24 HCM patients. Genes with more than one variant included the following: MYBPC3, TTN, MYH7, PSEN2, and LDB3. A total of 2538 protein-coding genes, six microRNAs (miRNAs), and 1617 long noncoding RNAs (lncRNAs) were identified differentially expressed between the groups, including several well-characterized cardiomyopathy-related genes (ANKRD1, FHL2, TGFB3, miR-30d, and miR-154). Gene enrichment analysis revealed that those genes are significantly involved in heart development and physiology. Furthermore, we highlighted four subnetworks: mtDNA-subnetwork, DSP-subnetwork, MYH7-subnetwork, and MYBPC3-subnetwork, which could play significant roles in the progression of HCM. Our findings further illustrate that HCM is a complex disease, which results from mutations in multiple protein-coding genes, modulation by non-coding RNAs and perturbations in gene networks.

Dysregulation of Calcium Handling in Duchenne Muscular Dystrophy-Associated Dilated Cardiomyopathy: Mechanisms and Experimental Therapeutic Strategies

: Duchenne muscular dystrophy (DMD) is an X-linked recessive disease resulting in the loss of dystrophin, a key cytoskeletal protein in the dystrophin-glycoprotein complex. Dystrophin connects the extracellular matrix with the cytoskeleton and stabilizes the sarcolemma. Cardiomyopathy is prominent in adolescents and young adults with DMD, manifesting as dilated cardiomyopathy (DCM) in the later stages of disease. Sarcolemmal instability, leading to calcium mishandling and overload in the cardiac myocyte, is a key mechanistic contributor to muscle cell death, fibrosis, and diminished cardiac contractile function in DMD patients. Current therapies for DMD cardiomyopathy can slow disease progression, but they do not directly target aberrant calcium handling and calcium overload. Experimental therapeutic targets that address calcium mishandling and overload include membrane stabilization, inhibition of stretch-activated channels, ryanodine receptor stabilization, and augmentation of calcium cycling via modulation of the Serca2a/phospholamban (PLN) complex or cytosolic calcium buffering. This paper addresses what is known about the mechanistic basis of calcium mishandling in DCM, with a focus on DMD cardiomyopathy. Additionally, we discuss currently utilized therapies for DMD cardiomyopathy, and review experimental therapeutic strategies targeting the calcium handling defects in DCM and DMD cardiomyopathy.

Unbalance Between Sarcoplasmic Reticulum Ca2 + Uptake and Release: A First Step Toward Ca2 + Triggered Arrhythmias and Cardiac Damage

The present review focusses on the regulation and interplay of cardiac SR Ca²⁺ handling proteins involved in SR Ca²⁺ uptake and release, i.e., SERCa2/PLN and RyR2. Both RyR2 and SERCA2a/PLN are highly regulated by post-translational modifications and/or different partners' proteins. These control mechanisms guarantee a precise equilibrium between SR Ca²⁺ reuptake and release. The review then discusses how disruption of this balance alters SR Ca²⁺ handling and may constitute a first step toward cardiac damage and malignant arrhythmias. In the last part of the review, this concept is exemplified in different cardiac diseases, like prediabetic and diabetic cardiomyopathy, digitalis intoxication and ischemia-reperfusion injury.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

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.

Tracking nitroxyl-derived posttranslational modifications of phospholamban in cardiac myocytes

Mundiña-Weilenmann and Mattiazzi examine new work revealing the mechanism by which nitroxide modifies uptake of Ca²⁺ into the SR.

An Aptamer-Based Near-Infrared Fluorescence Nanoprobe for Detecting and Imaging of Phospholamban Micropeptide in Cardiomyocytes

A growing body of evidence indicates that micropeptides encoded by long noncoding RNAs (lncRNAs) act independently or as regulators of larger proteins in fundamental biological processes, especially in the maintenance of cellular homeostasis. However, due to their small size and low intracellular expression, visual monitoring of micropeptides in living cells is still a challenge. In this work, we have designed and synthesized an aptamer-based near-infrared fluorescence nanoprobe for fluorescence imaging of phospholamban (PLN), which is an intracellular micropeptide that affects calcium homeostasis, and is closely associated with human heart failure in the clinic. The nanoprobe could respond specifically to PLN with excellent selectivity, high sensitivity, good nuclease stability, and biocompatibility, and it was successfully applied for imaging of changes in PLN levels in cardiomyocytes and in frozen sections of heart tissues. Further combined with clinical myocardial biopsy, we believe that the developed nanoprobe should be of great significance in later molecular pathology study of heart failure, which may help with diagnosis of early heart failure in the future. More importantly, for the first time nanoprobes were applied to visually monitor the changes of micropeptides in living cells and in frozen tissue sections, and the design concept of the aptamer-based nanoprobe can be extended to fluorescence detection of other 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.

Nitroxyl (HNO) targets phospholamban cysteines 41 and 46 to enhance cardiac function

Nitroxyl (HNO) positively modulates myocardial function by accelerating Ca²⁺ reuptake into the sarcoplasmic reticulum (SR). HNO-induced enhancement of myocardial Ca²⁺ cycling and function is due to the modification of cysteines in the transmembrane domain of phospholamban (PLN), which results in activation of SR Ca²⁺-ATPase (SERCA2a) by functionally uncoupling PLN from SERCA2a. However, which cysteines are modified by HNO, and whether HNO induces reversible disulfides or single cysteine sulfinamides (RS(O)NH₂) that are less easily reversed by reductants, remain to be determined. Using an ¹⁵N-edited NMR method for sulfinamide detection, we first demonstrate that Cys46 and Cys41 are the main targets of HNO reactivity with PLN. Supporting this conclusion, mutation of PLN cysteines 46 and 41 to alanine reduces the HNO-induced enhancement of SERCA2a activity. Treatment of WT-PLN with HNO leads to sulfinamide formation when the HNO donor is in excess, whereas disulfide formation is expected to dominate when the HNO/thiol stoichiometry approaches a 1:1 ratio that is more similar to that anticipated in vivo under normal, physiological conditions. Thus, ¹⁵N-edited NMR spectroscopy detects redox changes on thiols that are unique to HNO, greatly advancing the ability to detect HNO footprints in biological systems, while further differentiating HNO-induced post-translational modifications from those imparted by other reactive nitrogen or oxygen species. The present study confirms the potential of HNO as a signaling molecule in the cardiovascular system.

Effects of the Arg9Cys and Arg25Cys mutations on phospholamban's conformational equilibrium in membrane bilayers

Approximately, 70% of the Ca2+ ion transport into the sarcoplasmic reticulum is catalyzed by the sarcoplasmic reticulum Ca2+-ATPase (SERCA), whose activity is endogenously regulated by phospholamban (PLN). PLN comprises a TM inhibitory region and a cytoplasmic regulatory region that harbors a consensus sequence for cAMP-dependent protein kinase (PKA). The inhibitory region binds the ATPase, reducing its apparent Ca2+ binding affinity. β-adrenergic stimulation activates PKA, which phosphorylates PLN at Ser 16, reversing its inhibitory function. Mutations and post-translational modifications of PLN may lead to dilated cardiomyopathy (DCM) and heart failure. PLN's cytoplasmic region interconverts between a membrane-associated T state and a membrane-detached R state. The importance of these structural transitions on SERCA regulation is emerging, but the effects of natural occurring mutations and their relevance to the progression of heart disease are unclear. Here we use solid-state NMR spectroscopy to investigate the structural dynamics of two lethal PLN mutations, R9C and R25C, which lead to DCM. We found that the R25C mutant enhances the dynamics of PLN and shifts the conformational equilibrium toward the R state confirmation, whereas the R9C mutant drives the amphipathic cytoplasmic domain toward the membrane-associate state, enriching the T state population. The changes in membrane interactions caused by these mutations may explain the aberrant regulation of SERCA.

Functional and transcriptomic insights into pathogenesis of R9C phospholamban mutation using human induced pluripotent stem cell-derived cardiomyocytes

Dilated cardiomyopathy (DCM) can be caused by mutations in the cardiac protein phospholamban (PLN). We used CRISPR/Cas9 to insert the R9C PLN mutation at its endogenous locus into a human induced pluripotent stem cell (hiPSC) line from an individual with no cardiovascular disease. R9C PLN hiPSC-CMs display a blunted β-agonist response and defective calcium handling. In 3D human engineered cardiac tissues (hECTs), a blunted lusitropic response to β-adrenergic stimulation was observed with R9C PLN. hiPSC-CMs harboring the R9C PLN mutation showed activation of a hypertrophic phenotype, as evidenced by expression of hypertrophic markers and increased cell size and capacitance of cardiomyocytes. RNA-seq suggests that R9C PLN results in an altered metabolic state and profibrotic signaling, which was confirmed by gene expression analysis and picrosirius staining of R9C PLN hECTs. The expression of several miRNAs involved in fibrosis, hypertrophy, and cardiac metabolism were also perturbed in R9C PLN hiPSC-CMs. This study contributes to better understanding of the pathogenic mechanisms of the hereditary R9C PLN mutation in the context of human cardiomyocytes.

Insertional Mutagenesis Confounds the Mechanism of the Morbid Phenotype of a PLNR9C Transgenic Mouse Line

BACKGROUND

A mouse line with heterozygous transgenic expression of phospholamban carrying a substitution of cysteine for arginine 9 (TgPLN^R9C) under the control of α-myosin heavy chain (αMHC) promoter features dilated cardiomyopathy, heart failure, and premature death.

METHODS AND RESULTS

Determination of transgene chromosomal localization by conventional methods shows that in this line the transgenic array of 13 PLN^R9C expression cassettes, arranged in a head-to-tail tandem orientation, have integrated into the bidirectional promoter of the αMHC (Myh6) gene and the gene for the regulatory noncoding RNA Myheart (Mhrt), both of which are known to be involved in cardiac development and pathology. Expression of the noncoding RNA Mhrt in TgPLN^R9C mice exhibits profound deregulation, despite the presence of the second, intact allele.

CONCLUSIONS

The TgPLN^R9C mouse strain is, in the best case, a functionally ambiguous phenocopy of the human PLN^R9C heterozygote, because a similar constellation of genetically programmed events can not occur in a patient. Publications featuring "cardiac-specific overexpression" are focused on the phenotype and typically forgo the definition of the transgene integration site or transgene temporal expression profile, so caution should be exercised in attributing clinical relevance to pathologic phenomena observed in αMHC-driven transgenes.

When signalling goes wrong: pathogenic variants in structural and signalling proteins causing cardiomyopathies

Cardiomyopathies are a diverse group of cardiac disorders with distinct phenotypes, depending on the proteins and pathways affected. A substantial proportion of cardiomyopathies are inherited and those will be the focus of this review article. With the wide application of high-throughput sequencing in the practice of clinical genetics, the roles of novel genes in cardiomyopathies are recognised. Here, we focus on a subgroup of cardiomyopathy genes [TTN, FHL1, CSRP3, FLNC and PLN, coding for Titin, Four and a Half LIM domain 1, Muscle LIM Protein, Filamin C and Phospholamban, respectively], which, despite their diverse biological functions, all have important signalling functions in the heart, suggesting that disturbances in signalling networks can contribute to cardiomyopathies.

Clinical and Mechanistic Insights Into the Genetics of Cardiomyopathy

Over the last quarter-century, there has been tremendous progress in genetics research that has defined molecular causes for cardiomyopathies. More than a thousand mutations have been identified in many genes with varying ontologies, therein indicating the diverse molecules and pathways that cause hypertrophic, dilated, restrictive, and arrhythmogenic cardiomyopathies. Translation of this research to the clinic via genetic testing can precisely group affected patients according to molecular etiology, and identify individuals without evidence of disease who are at high risk for developing cardiomyopathy. These advances provide insights into the earliest manifestations of cardiomyopathy and help to define the molecular pathophysiological basis for cardiac remodeling. Although these efforts remain incomplete, new genomic technologies and analytic strategies provide unparalleled opportunities to fully explore the genetic architecture of cardiomyopathies. Such data hold the promise that mutation-specific pathophysiology will uncover novel therapeutic targets, and herald the beginning of precision therapy for cardiomyopathy patients.

Reversal of Phospholamban Inhibition of the Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA) Using Short, Protein-interacting RNAs and Oligonucleotide Analogs

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and phospholamban (PLN) complex regulates heart relaxation through its removal of cytosolic Ca²⁺ during diastole. Dysfunction of this complex has been related to many heart disorders and is therefore a key pharmacological target. There are currently no therapeutics that directly target either SERCA or PLN. It has been previously reported that single-stranded DNA binds PLN with strong affinity and relieves inhibition of SERCA in a length-dependent manner. In the current article, we demonstrate that RNAs and single-stranded oligonucleotide analogs, or xeno nucleic acids (XNAs), also bind PLN strongly (K_d <10 nm) and relieve inhibition of SERCA. Affinity for PLN is sequence-independent. Relief of PLN inhibition is length-dependent, allowing SERCA activity to be restored incrementally. The improved in vivo stability of XNAs offers more realistic pharmacological potential than DNA or RNA. We also found that microRNAs (miRNAs) 1 and 21 bind PLN strongly and relieve PLN inhibition of SERCA to a greater extent than a similar length random sequence RNA mixture. This may suggest that miR-1 and miR-21 have evolved to contain distinct sequence elements that are more effective at relieving PLN inhibition than random sequences.

Mutation analysis of the phospholamban gene in 315 South Africans with dilated, hypertrophic, peripartum and arrhythmogenic right ventricular cardiomyopathies

Cardiomyopathy is an important cause of heart failure in Sub-Saharan Africa, accounting for up to 30% of adult heart failure hospitalisations. This high prevalence poses a challenge in societies without access to resources and interventions essential for disease management. Over 80 genes have been implicated as a cause of cardiomyopathy. Mutations in the phospholamban (PLN) gene are associated with dilated cardiomyopathy (DCM) and severe heart failure. In Africa, the prevalence of PLN mutations in cardiomyopathy patients is unknown. Our aim was to screen 315 patients with arrhythmogenic right ventricular cardiomyopathy (n = 111), DCM (n = 95), hypertrophic cardiomyopathy (n = 40) and peripartum cardiomyopathy (n = 69) for disease-causing PLN mutations by high resolution melt analysis and DNA sequencing. We detected the previously reported PLN c.25C > T (p.R9C) mutation in a South African family with severe autosomal dominant DCM. Haplotype analysis revealed that this mutation occurred against a different haplotype background to that of the original North American family and was therefore unlikely to have been inherited from a common ancestor. No other mutations in PLN were detected (mutation prevalence = 0.2%). We conclude that PLN is a rare cause of cardiomyopathy in African patients. The PLN p.R9C mutation is not well-tolerated, emphasising the importance of this gene in cardiac function.

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.

Metformin increases degradation of phospholamban via autophagy in cardiomyocytes

Phospholamban (PLN) is an effective inhibitor of the sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA). Here, we examined PLN stability and degradation in primary cultured mouse neonatal cardiomyocytes (CMNCs) and mouse hearts using immunoblotting, molecular imaging, and [(35)S]methionine pulse-chase experiments, together with lysosome (chloroquine and bafilomycin A1) and autophagic (3-methyladenine and Atg5 siRNA) antagonists. Inhibiting lysosomal and autophagic activities promoted endogenous PLN accumulation, whereas accelerating autophagy with metformin enhanced PLN degradation in CMNCs. This reduction in PLN levels was functionally correlated with an increased rate of SERCA2a activity, accounting for an inotropic effect of metformin. Metabolic labeling reaffirmed that metformin promoted wild-type and R9C PLN degradation. Immunofluorescence showed that PLN and the autophagy marker, microtubule light chain 3, became increasingly colocalized in response to chloroquine and bafilomycin treatments. Mechanistically, pentameric PLN was polyubiquitinylated at the K3 residue and this modification was required for p62-mediated selective autophagy trafficking. Consistently, attenuated autophagic flux in HECT domain and ankyrin repeat-containing E3 ubiquitin protein ligase 1-null mouse hearts was associated with increased PLN levels determined by immunoblots and immunofluorescence. Our study identifies a biological mechanism that traffics PLN to the lysosomes for degradation in mouse hearts.

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.

A study in Polish patients with cardiomyopathy emphasizes pathogenicity of phospholamban (PLN) mutations at amino acid position 9 and low penetrance of heterozygous null PLN mutations

Background

In humans mutations in the PLN gene, encoding phospholamban - a regulator of sarcoplasmic reticulum calcium ATPase (SERCA), cause cardiomyopathy with prevalence depending on the population. Our purpose was to identify PLN mutations in Polish cardiomyopathy patients.

Methods

We studied 161 unrelated subjects referred for genetic testing for cardiomyopathies: 135 with dilated cardiomyopathy, 22 with hypertrophic cardiomyopathy and 4 with other cardiomyopathies. In 23 subjects multiple genes were sequenced by next generation sequencing and in all subjects PLN exons were analyzed by Sanger sequencing. Control group included 200 healthy subjects matched with patients for ethnicity, sex and age. Large deletions/insertions were screened by real time polymerase chain reaction.

Results

We detected three different heterozygous mutations in the PLN gene: a novel null c.9_10insA:(p.Val4Serfs*15) variant and two missense variants: c.25C > T:(p.Arg9Cys) and c.26G > T:(p.Arg9Leu). The (p.Val4Serfs*15) variant occurred in the patient with Wolff-Parkinson-White syndrome in whom the diagnosis of cardiomyopathy was not confirmed and his mother who had concentric left ventricular remodeling but normal left ventricular mass and function. We did not detect large deletions/insertions in PLN in cohort studied.

Conclusions

In Poland, similar to most populations, PLN mutations rarely cause cardiomyopathy. The 9^thPLN residue is apparently a mutation hot spot whereas a single dose of c.9_10insA, and likely other null PLN mutations, cause the disease only with low penetrance or are not pathogenic.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-015-0167-0) contains supplementary material, which is available to authorized users.

Dysfunctional conformational dynamics of protein kinase A induced by a lethal mutant of phospholamban hinder phosphorylation

The dynamic interplay between kinases and substrates is crucial for the formation of catalytically committed complexes that enable phosphoryl transfer. However, a clear understanding on how substrates modulate kinase structural dynamics to control catalytic efficiency is still missing. Here, we used solution NMR spectroscopy to study the conformational dynamics of two complexes of the catalytic subunit of the cAMP-dependent protein kinase A with WT and R14 deletion phospholamban, a lethal human mutant linked to familial dilated cardiomyopathy. Phospholamban is a central regulator of heart muscle contractility, and its phosphorylation by protein kinase A constitutes a primary response to β-adrenergic stimulation. We found that the single deletion of arginine in phospholamban's recognition sequence for the kinase reduces its binding affinity and dramatically reduces phosphorylation kinetics. Structurally, the mutant prevents the enzyme from adopting conformations and motions committed for catalysis, with concomitant reduction in catalytic efficiency. Overall, these results underscore the importance of a well-tuned structural and dynamic interplay between the kinase and its substrates to achieve physiological phosphorylation levels for proper Ca(2+) signaling and normal cardiac function.

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.

Deception in simplicity: hereditary phospholamban mutations in dilated cardiomyopathy

The sarcoplasmic reticulum (SR) calcium pump (SERCA) and its regulator phospholamban are required for cardiovascular function. Phospholamban alters the apparent calcium affinity of SERCA in a process that is modulated by phosphorylation via the β-adrenergic pathway. This regulatory axis allows for the dynamic control of SR calcium stores and cardiac contractility. Herein we focus on hereditary mutants of phospholamban that are associated with heart failure, such as Arg(9)-Cys, Arg(9)-Leu, Arg(9)-His, and Arg(14)-deletion. Each mutant has a distinct effect on PLN function and SR calcium homeostasis. Arg(9)-Cys and Arg(9)-Leu do not inhibit SERCA, Arg(14)-deletion is a partial inhibitor, and Arg(9)-His is comparable to wild-type. While the mutants have distinct functional effects on SERCA, they have in common that they cannot be phosphorylated by protein kinase A (PKA). Arg(9) and Arg(14) are required for PKA recognition and phosphorylation of PLN. Thus, mutations at these positions eliminate β-adrenergic control and dynamic cardiac contractility. Hydrophobic mutations of Arg(9) cause more complex changes in function, including loss of PLN function and dominant negative interaction with SERCA in heterozygous individuals. In addition, aberrant interaction with PKA may prevent phosphorylation of wild-type PLN and sequester PKA from other local subcellular targets. Herein we consider what is known about each mutant and how the synergistic changes in SR calcium homeostasis lead to impaired cardiac contractility and dilated cardiomyopathy.

Comparison of the structure and function of phospholamban and the arginine-14 deficient mutant associated with dilated cardiomyopathy

Phospholamban (PLB) is a pentameric protein that plays an important role in regulating cardiac contractility via a reversible inhibitory association with the sarcoplasmic reticulum Ca²⁺ATPase (SERCA), the enzyme responsible for maintaining correct calcium homeostasis. Here we study the functional and biophysical characteristics of a PLB mutant associated with human dilated cardiomyopathy (DCM), with a deletion of arginine at position 14 (PLBR14Δ). In agreement with recent findings, we find that PLBR14Δ has a reduced inhibitory effect on SERCA compared to wild type PLB (PLBWT) when reconstituted into lipid membranes. The mutation also leads to a large reduction in the protein kinase A-catalysed phosphorylation of Ser-16 in the cytoplasmic domain of PLBR14Δ. Measurements on SERCA co-reconstituted with an equimolar mixture of PLBWT and PLBR14Δ (representing the lethal heterozygous state associated with DCM) indicates that the loss-of-function mutation has a dominant effect on PLBWT functionality and phosphorylation capacity, suggesting that mixed PLBWT/PLBR14Δ pentamers are formed that have characteristics typical of the mutant protein. Structural and biophysical analysis of PLBR14Δ indicates that the mutation perturbs slightly the helical structure of the PLB cytoplasmic domain and reduces its affinity for the phospholipid bilayer surface, thereby altering the orientation of the cytoplasmic domain relative to the wild-type protein. These results indicate that the structure and function consequences of the R14 deletion have profound effects on the regulation of SERCA which may contribute to the aetiology of DCM.

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.

Secondary structure, backbone dynamics, and structural topology of phospholamban and its phosphorylated and Arg9Cys-mutated forms in phospholipid bilayers utilizing 13C and 15N solid-state NMR spectroscopy

Phospholamban (PLB) is a membrane protein that regulates heart muscle relaxation rates via interactions with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). When PLB is phosphorylated or Arg9Cys (R9C) is mutated, inhibition of SERCA is relieved. (13)C and (15)N solid-state NMR spectroscopy is utilized to investigate conformational changes of PLB upon phosphorylation and R9C mutation. (13)C═O NMR spectra of the cytoplasmic domain reveal two α-helical structural components with population changes upon phosphorylation and R9C mutation. The appearance of an unstructured component is observed on domain Ib. (15)N NMR spectra indicate an increase in backbone dynamics of the cytoplasmic domain. Wild-type PLB (WT-PLB), Ser16-phosphorylated PLB (P-PLB), and R9C-mutated PLB (R9C-PLB) all have a very dynamic domain Ib, and the transmembrane domain has an immobile component. (15)N NMR spectra indicate that the cytoplasmic domain of R9C-PLB adopts an orientation similar to P-PLB and shifts away from the membrane surface. Domain Ib (Leu28) of P-PLB and R9C-PLB loses the alignment. The R9C-PLB adopts a conformation similar to P-PLB with a population shift to a more extended and disordered state. The NMR data suggest the more extended and disordered forms of PLB may relate to inhibition relief.

Structural dynamics and topology of phosphorylated phospholamban homopentamer reveal its role in the regulation of calcium transport

Phospholamban (PLN) inhibits the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), thereby regulating cardiac diastole. In membranes, PLN assembles into homopentamers that in both the phosphorylated and nonphosphorylated states have been proposed to form ion-selective channels. Here, we determined the structure of the phosphorylated pentamer using a combination of solution and solid-state nuclear magnetic resonance methods. We found that the pinwheel architecture of the homopentamer is preserved upon phosphorylation, with each monomer having an L-shaped conformation. The TM domains form a hydrophobic pore approximately 24 Å long and 2 Å in diameter, which is inconsistent with canonical Ca²⁺-selective channels. Phosphorylation, however, enhances the conformational dynamics of the cytoplasmic region of PLN, causing partial unwinding of the amphipathic helix. We propose that PLN oligomers act as storage for active monomers, keeping SERCA function within a physiological window.

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.

Probing the interaction of Arg9Cys mutated phospholamban with phospholipid bilayers by solid-state NMR spectroscopy

Phospholamban (PLB) is a 52 amino acid integral membrane protein that interacts with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA) and helps to regulate Ca(2+) flow. PLB inhibits SERCA impairing Ca(2+) translocation. The inhibition can be relieved upon phosphorylation of PLB. The Arg9 to Cys (R9C) mutation is a loss of function mutation with reduced inhibitory potency. The effect R9C PLB has on the membrane surface and the hydrophobic region dynamics was investigated by (31)P and (2)H solid-state NMR spectroscopy in multilamellar vesicles (MLVs). The (31)P NMR spectra indicate that, like the phosphorylated PLB (P-PLB), the mutated R9C-PLB protein has significantly less interaction with the lipid bilayer headgroup when compared to wild-type PLB (WT-PLB). Similar to P-PLB, R9C-PLB slightly decreases (31)P T1 values in the lipid headgroup region. (2)H SCD order parameters of (2)H nuclei along the lipid acyl chain decrease less dramatically for R9C-PLB and P-PLB when compared to WT-PLB. The results suggest that R9C-PLB interacts less with the membrane surface and hydrophobic region than WT-PLB. Detachment of the cytoplasmic domain of R9C-PLB from the membrane surface could be related to its loss of function.

Structure-function relation of phospholamban: modulation of channel activity as a potential regulator of SERCA activity

Phospholamban (PLN) is a small integral membrane protein, which binds and inhibits in a yet unknown fashion the Ca²⁺-ATPase (SERCA) in the sarcoplasmic reticulum. When reconstituted in planar lipid bilayers PLN exhibits ion channel activity with a low unitary conductance. From the effect of non-electrolyte polymers on this unitary conductance we estimate a narrow pore with a diameter of ca. 2.2 Å for this channel. This value is similar to that reported for the central pore in the structure of the PLN pentamer. Hence the PLN pentamer, which is in equilibrium with the monomer, is the most likely channel forming structure. Reconstituted PLN mutants, which either stabilize (K27A and R9C) or destabilize (I47A) the PLN pentamer and also phosphorylated PLN still generate the same unitary conductance of the wt/non-phosphorylated PLN. However the open probability of the phosphorylated PLN and of the R9C mutant is significantly lower than that of the respective wt/non-phosphorylated control. In the context of data on PLN/SERCA interaction and on Ca²⁺ accumulation in the sarcoplasmic reticulum the present results are consistent with the view that PLN channel activity could participate in the balancing of charge during Ca²⁺ uptake. A reduced total conductance of the K⁺ transporting PLN by phosphorylation or by the R9C mutation may stimulate Ca²⁺ uptake in the same way as an inhibition of K⁺ channels in the SR membrane. The R9C-PLN mutation, a putative cause of dilated cardiomyopathy, might hence affect SERCA activity also via its inherent low open probability.

Tuning the structural coupling between the transmembrane and cytoplasmic domains of phospholamban to control sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) function

Phospholamban (PLN) is the endogenous inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), the integral membrane enzyme responsible for 70 % of the removal of Ca(2+) from the cytosol, inducing cardiac muscle relaxation in humans. Dysfunctions in SERCA:PLN interactions have been implicated as having a critical role in cardiac disease, and targeting Ca(2+) transport has been demonstrated to be a promising avenue in treating conditions of heart failure. Here, we designed a series of new mutants able to tune SERCA function, targeting the loop sequence that connects the transmembrane and cytoplasmic helices of PLN. We found that a variable degree of loss of inhibition mutants is attainable by engineering glycine mutations along PLN's loop domain. Remarkably, a double glycine mutation results in a complete loss-of-function mutant, fully mimicking the phosphorylated state of PLN. Using nuclear magnetic resonance spectroscopy, we rationalized the effects of these mutations in terms of entropic control on PLN function, whose inhibitory function can be modulated by increasing its conformational dynamics. However, if PLN mutations go past a threshold set by the phosphorylated state, they break the structural coupling between the transmembrane and cytoplasmic domains, resulting in a species that behaves as the inhibitory transmembrane domain alone. These studies provide new potential candidates for gene therapy to reverse the effects of heart failure.