Phospholemman-dependent regulation of the cardiac Na/K-ATPase activity is modulated by inhibitor-1 sensitive type-1 phosphatase

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure

In adapting to disease and loss of tissue, the heart shows great phenotypic plasticity that involves changes to its structure, composition and electrophysiology. Together with parallel whole body cardiovascular adaptations, the initial decline in cardiac function resulting from the insult is compensated. However, in the long term, the heart muscle begins to fail and patients with this condition have a very poor prognosis, with many dying from disturbances of rhythm. The surviving myocytes of these hearts gain Na+ , which is positively inotropic because of alterations to Ca2+ fluxes mediated by the Na+ /Ca2+ exchange, but compromises Ca2+ -dependent energy metabolism in mitochondria. Uptake of Ca2+ into the sarcoplasmic reticulum (SR) is reduced because of diminished function of SR Ca2+ ATPases. The result of increased Ca2+ influx and reduced SR Ca2+ uptake is an increase in the diastolic cytosolic Ca2+ concentration, which promotes spontaneous SR Ca2+ release and induces delayed afterdepolarisations. Action potential duration prolongs because of increased late Na+ current and changes in expression and function of other ion channels and transporters increasing the probability of the formation of early afterdepolarisations. There is a reduction in T-tubule density and so the normal spatial arrangements required for efficient excitation-contraction coupling are compromised and lead to temporal delays in Ca2+ release from the SR. Therefore, the structural and electrophysiological responses that occur to provide compensation do so at the expense of (1) increasing the likelihood of arrhythmogenesis; (2) activating hypertrophic, apoptotic and Ca2+ signalling pathways; and (3) decreasing the efficiency of SR Ca2+ release.

Function and regulation of phosphatase 1 in healthy and diseased heart

Reversible phosphorylation of ion channels and calcium-handling proteins provides precise post-translational regulation of cardiac excitation and contractility. Serine/threonine phosphatases govern dephosphorylation of the majority of cardiac proteins. Accordingly, dysfunction of this regulation contributes to the development and progression of heart failure and atrial fibrillation. On the molecular level, these changes include alterations in the expression level and phosphorylation status of Ca²⁺ handling and excitation-contraction coupling proteins provoked by dysregulation of phosphatases. The serine/threonine protein phosphatase PP1 is one a major player in the regulation of cardiac excitation-contraction coupling. PP1 essentially impacts on cardiac physiology and pathophysiology via interactions with the cardiac ion channels Cav1.2, NKA, NCX and KCNQ1, sarcoplasmic reticulum-bound Ca²⁺ handling proteins such as RyR2, SERCA and PLB as well as the contractile proteins MLC2, TnI and MyBP-C. PP1 itself but also PP1-regulatory proteins like inhibitor-1, inhibitor-2 and heat-shock protein 20 are dysregulated in cardiac disease. Therefore, they represent interesting targets to gain more insights in heart pathophysiology and to identify new treatment strategies for patients with heart failure or atrial fibrillation. We describe the genetic and holoenzymatic structure of PP1 and review its role in the heart and cardiac disease. Finally, we highlight the importance of the PP1 regulatory proteins for disease manifestation, provide an overview of genetic models to study the role of PP1 for the development of heart failure and atrial fibrillation and discuss possibilities of pharmacological interventions.

Phosphoprotein Phosphatase 1 but Not 2A Activity Modulates Coupled-Clock Mechanisms to Impact on Intrinsic Automaticity of Sinoatrial Nodal Pacemaker Cells

Spontaneous AP (action potential) firing of sinoatrial nodal cells (SANC) is critically dependent on protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-dependent protein phosphorylation, which are required for the generation of spontaneous, diastolic local Ca²⁺ releases (LCRs). Although phosphoprotein phosphatases (PP) regulate protein phosphorylation, the expression level of PPs and phosphatase inhibitors in SANC and the impact of phosphatase inhibition on the spontaneous LCRs and other players of the oscillatory coupled-clock system is unknown. Here, we show that rabbit SANC express both PP1, PP2A, and endogenous PP inhibitors I-1 (PPI-1), dopamine and cyclic adenosine 3′,5′-monophosphate (cAMP)-regulated phosphoprotein (DARPP-32), kinase C-enhanced PP1 inhibitor (KEPI). Application of Calyculin A, (CyA), a PPs inhibitor, to intact, freshly isolated single SANC: (1) significantly increased phospholamban (PLB) phosphorylation (by 2–3-fold) at both CaMKII-dependent Thr¹⁷ and PKA-dependent Ser¹⁶ sites, in a time and concentration dependent manner; (2) increased ryanodine receptor (RyR) phosphorylation at the Ser²⁸⁰⁹ site; (3) substantially increased sarcoplasmic reticulum (SR) Ca²⁺ load; (4) augmented L-type Ca²⁺ current amplitude; (5) augmented LCR’s characteristics and decreased LCR period in intact and permeabilized SANC, and (6) increased the spontaneous basal AP firing rate. In contrast, the selective PP2A inhibitor okadaic acid (100 nmol/L) had no significant effect on spontaneous AP firing, LCR parameters, or PLB phosphorylation. Application of purified PP1 to permeabilized SANC suppressed LCR, whereas purified PP2A had no effect on LCR characteristics. Our numerical model simulations demonstrated that PP inhibition increases AP firing rate via a coupled-clock mechanism, including respective increases in the SR Ca²⁺ pumping rate, L-type Ca²⁺ current, and Na⁺/Ca²⁺-exchanger current. Thus, PP1 and its endogenous inhibitors modulate the basal spontaneous firing rate of cardiac pacemaker cells by suppressing SR Ca²⁺ cycling protein phosphorylation, the SR Ca²⁺ load and LCRs, and L-type Ca²⁺ current.

Cardioprotection by SGLT2 Inhibitors-Does It All Come Down to Na+?

Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are emerging as a new treatment strategy for heart failure with reduced ejection fraction (HFrEF) and—depending on the wistfully awaited results of two clinical trials (DELIVER and EMPEROR-Preserved)—may be the first drug class to improve cardiovascular outcomes in patients suffering from heart failure with preserved ejection fraction (HFpEF). Proposed mechanisms of action of this class of drugs are diverse and include metabolic and hemodynamic effects as well as effects on inflammation, neurohumoral activation, and intracellular ion homeostasis. In this review we focus on the growing body of evidence for SGLT2i-mediated effects on cardiac intracellular Na⁺ as an upstream mechanism. Therefore, we will first give a short overview of physiological cardiomyocyte Na⁺ handling and its deterioration in heart failure. On this basis we discuss the salutary effects of SGLT2i on Na⁺ homeostasis by influencing NHE1 activity, late I_Na as well as CaMKII activity. Finally, we highlight the potential relevance of these effects for systolic and diastolic dysfunction as well as arrhythmogenesis.

Endogenous cardiotonic steroids and cardiovascular disease, where to next?

Ever since British Physician William Withering first described the use of foxglove extract for treatment of patients with congestive heart failure in 1785, cardiotonic steroids have been used clinically to treat heart failure and more recently atrial fibrillation. Due to their ability to bind and inhibit the ubiquitous transport enzyme sodium potassium pump, thus regulating intracellular Na⁺ concentration in every living cell, they are also an essential tool for research into the sodium potassium pump structure and function. Exogenous CTS have been clearly demonstrated to affect cardiovascular system through modulation of vagal tone, cardiac contraction (via ionic changes) and altered natriuresis. Reports of a number of endogenous CTS, since the 1980s, have intensified research into their physiologic and pathophysiologic roles and opened up novel therapeutic targets. Substantive evidence pointing to the role of endogenous ouabain and marinobufagenin, the two most prominent CTS, in development of cardiovascular disease has accumulated. Nevertheless, their presence, structure, biosynthesis pathways and even mechanism of action remain unclear or controversial. In this review the current state-of-the-art, the controversies and the remaining questions surrounding the role of endogenous cardiotonic steroids in health and disease are discussed.

Protein Phosphatase 1 Inhibitor-1 Mediates the cAMP-Dependent Stimulation of the Renal NaCl Cotransporter

BACKGROUND

A number of cAMP-elevating hormones stimulate phosphorylation (and hence activity) of the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). Evidence suggests that protein phosphatase 1 (PP1) and other protein phosphatases modulate NCC phosphorylation, but little is known about PP1's role and the mechanism regulating its function in the DCT.

METHODS

We used ex vivo mouse kidney preparations to test whether a DCT-enriched inhibitor of PP1, protein phosphatase 1 inhibitor-1 (I1), mediates cAMP's effects on NCC, and conducted yeast two-hybrid and coimmunoprecipitation experiments in NCC-expressing MDCK cells to explore protein interactions.

RESULTS

Treating isolated DCTs with forskolin and IBMX increased NCC phosphorylation via a protein kinase A (PKA)-dependent pathway. Ex vivo incubation of mouse kidney slices with isoproterenol, norepinephrine, and parathyroid hormone similarly increased NCC phosphorylation. The cAMP-induced stimulation of NCC phosphorylation strongly correlated with the phosphorylation of I1 at its PKA consensus phosphorylation site (a threonine residue in position 35). We also found an interaction between NCC and the I1-target PP1. Moreover, PP1 dephosphorylated NCC in vitro, and the PP1 inhibitor calyculin A increased NCC phosphorylation. Studies in kidney slices and isolated perfused kidneys of control and I1-KO mice demonstrated that I1 participates in the cAMP-induced stimulation of NCC.

CONCLUSIONS

Our data suggest a complete signal transduction pathway by which cAMP increases NCC phosphorylation via a PKA-dependent phosphorylation of I1 and subsequent inhibition of PP1. This pathway might be relevant for the physiologic regulation of renal sodium handling by cAMP-elevating hormones, and may contribute to salt-sensitive hypertension in patients with endocrine disorders or sympathetic hyperactivity.

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure in the guinea pig

Key points

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Abstract

We followed changes in cardiac myocyte Ca²⁺ and Na⁺ regulation from the formation of compensated hypertrophy (CH) until signs of heart failure (HF) are apparent using a trans‐aortic pressure overload (TAC) model. In this model, in vivo fractional shortening (FS) remained constant despite HW:BW ratio increasing by 39% (CH) until HF developed 150 days post‐TAC when FS decreased from 70% to 39%. Using live and fixed fluorescence imaging and electrophysiological techniques, we found an increase in I_Na,late from –0.34 to –0.59 A F⁻¹ and a decrease in Na⁺,K⁺‐ATPase current from 1.09 A F⁻¹ to 0.54 A F⁻¹ during CH. These changes persisted as HF developed (I_Na,late increased to –0.82 A F⁻¹ and Na⁺,K⁺‐ATPase current decreased to 0.51 A F⁻¹). Sarcoplasmic reticulum (SR) Ca²⁺ content increased during CH then decreased in HF (from 32 to 15 μm l⁻¹) potentially supporting the maintenance of FS in the whole heart and Ca²⁺ transients in single myocytes during the former stage. We showed using glycoside blockade in healthy myocytes that increases in SR Ca²⁺ content and Ca²⁺ transients can be driven by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells. SERCA function remains constant in CH but decreases (τ for SERCA‐mediated Ca²⁺ removal changed from 6.3 to 3.0 s⁻¹) in HF. In HF there was an increase in spark frequency and spark‐mediated Ca²⁺ leak. We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Greasing the wheels or a spanner in the works? Regulation of the cardiac sodium pump by palmitoylation

The ubiquitous sodium/potassium ATPase (Na pump) is the most abundant primary active transporter at the cell surface of multiple cell types, including ventricular myocytes in the heart. The activity of the Na pump establishes transmembrane ion gradients that control numerous events at the cell surface, positioning it as a key regulator of the contractile and metabolic state of the myocardium. Defects in Na pump activity and regulation elevate intracellular Na in cardiac muscle, playing a causal role in the development of cardiac hypertrophy, diastolic dysfunction, arrhythmias and heart failure. Palmitoylation is the reversible conjugation of the fatty acid palmitate to specific protein cysteine residues; all subunits of the cardiac Na pump are palmitoylated. Palmitoylation of the pump's accessory subunit phospholemman (PLM) by the cell surface palmitoyl acyl transferase DHHC5 leads to pump inhibition, possibly by altering the relationship between the pump catalytic α subunit and specifically bound membrane lipids. In this review, we discuss the functional impact of PLM palmitoylation on the cardiac Na pump and the molecular basis of recognition of PLM by its palmitoylating enzyme DHHC5, as well as effects of palmitoylation on Na pump cell surface abundance in the cardiac muscle. We also highlight the numerous unanswered questions regarding the cellular control of this fundamentally important regulatory process.

β-Adrenergic regulation of cardiac type 2A protein phosphatase through phosphorylation of regulatory subunit B56δ at S573

Background

Type 2A protein phosphatase (PP2A) enzymes are serine/threonine phosphatases which comprise a scaffold A subunit, a regulatory B subunit and a catalytic C subunit, and have been implicated in the dephosphorylation of multiple cardiac phosphoproteins. B subunits determine subcellular targeting, substrate specificity and catalytic activity, and can themselves be regulated by post-translational modifications. We explored potential β-adrenergic regulation of PP2A in cardiomyocytes through phosphorylation of the regulatory B subunit isoform B56δ.

Methods and results

Phosphate affinity SDS-PAGE and immunoblot analysis revealed increased phosphorylation of B56δ in adult rat ventricular myocytes (ARVM) exposed to the β-adrenergic receptor (βAR) agonist isoprenaline (ISO). Phosphorylation of B56δ occurred at S573, primarily through stimulation of the β₁AR subtype, and was dependent on PKA activity. The functional role of the phosphorylation was explored in ARVM transduced with adenoviruses expressing wild type (WT) or non-phosphorylatable (S573A) B56δ, fused to GFP at the N-terminus. C subunit expression was increased in ARVM expressing GFP-B56δ-WT or GFP-B56δ-S573A, both of which co-immunoprecipitated with endogenous C and A subunits. PP2A activity in cell lysates was increased in response to ISO in ARVM expressing GFP-B56δ-WT but not GFP-B56δ-S573A. Immunoblot analysis of the phosphoproteome in ARVM expressing GFP-B56δ-WT or GFP-B56δ-S573A with antibodies detecting (i) phospho-serine/threonine residues in distinct kinase substrate motifs or (ii) specific phosphorylated residues of functional importance in selected proteins revealed a comparable phosphorylation profile in the absence or presence of ISO stimulation.

Conclusions

In cardiomyocytes, βAR stimulation induces PKA-mediated phosphorylation of the PP2A regulatory subunit isoform B56δ at S573, which increases associated PP2A catalytic activity. This is likely to regulate the phosphorylation status of specific B56δ-PP2A substrates, which remain to be identified.

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PP2A subunit B56δ is phosphorylated on β-adrenergic stimulation of cardiomyocytes.

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Phosphorylation occurs at Ser573 and increases B56δ-PP2A catalytic activity.

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Response is mediated by the β₁-adrenoceptor subtype and protein kinase A.

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Phosphorylated B56δ abundance is increased in pathological cardiac hypertrophy.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.

Serine/Threonine Phosphatases in Atrial Fibrillation

Serine/threonine protein phosphatases control dephosphorylation of numerous cardiac proteins, including a variety of ion channels and calcium-handling proteins, thereby providing precise post-translational regulation of cardiac electrophysiology and function. Accordingly, dysfunction of this regulation can contribute to the initiation, maintenance and progression of cardiac arrhythmias. Atrial fibrillation (AF) is the most common heart rhythm disorder and is characterized by electrical, autonomic, calcium-handling, contractile, and structural remodeling, which include, among other things, changes in the phosphorylation status of a wide range of proteins. Here, we review AF-associated alterations in the phosphorylation of atrial ion channels, calcium-handling and contractile proteins, and their role in AF-pathophysiology. We highlight the mechanisms controlling the phosphorylation of these proteins and focus on the role of altered dephosphorylation via local type-1, type-2A and type-2B phosphatases (PP1, PP2A, and PP2B, also known as calcineurin, respectively). Finally, we discuss the challenges for phosphatase research, potential therapeutic significance of altered phosphatase-mediated protein dephosphorylation in AF, as well as future directions.

Role of protein phosphatase inhibitor-1 in cardiac beta adrenergic pathway

Phosphoproteomic studies have shown that about one third of all cardiac proteins are reversibly phosphorylated, affecting virtually every cellular signaling pathway. The reversibility of this process is orchestrated by the opposing enzymatic activity of kinases and phosphatases. Conversely, imbalances in subcellular protein phosphorylation patterns are a hallmark of many cardiovascular diseases including heart failure and cardiac arrhythmias. While numerous studies have revealed excessive beta-adrenergic signaling followed by deregulated kinase expression or activity as a major driver of the latter cardiac pathologies, far less is known about the beta-adrenergic regulation of their phosphatase counterparts. In fact, most of the limited knowledge stems from the detailed analysis of the endogenous inhibitor of the protein phosphatase 1 (I-1) in cellular and animal models. I-1 acts as a nodal point between adrenergic and putatively non-adrenergic cardiac signaling pathways and is able to influence widespread cellular functions of protein phosphatase 1 which are contributing to cardiac health and disease, e.g. Ca²⁺ handling, sarcomere contractility and glucose metabolism. Finally, nearly all of these studies agree that I-1 is a promising drug target on the one hand but the outcome of its pharmacological regulation maybe extremely context-dependent on the other hand, thus warranting for careful interpretation of past and future experimental results. In this respect we will: 1) comprehensively review the current knowledge about structural, functional and regulatory properties of I-1 within the heart 2) highlight current working hypothesis and potential I-1 mediated disease mechanisms 3) discuss state-of-the-art knowledge and future prospects of a potential therapeutic strategy targeting I-1 by restoring the balance of cardiac protein phosphorylation.

Electrochemical Na+ and Ca2+ gradients drive coupled-clock regulation of automaticity of isolated rabbit sinoatrial nodal pacemaker cells

Coupling of an intracellular Ca(2+) clock to surface membrane ion channels, i.e., a "membrane clock, " via coupling of electrochemical Na(+) and Ca(2+) gradients (ENa and ECa, respectively) has been theorized to regulate sinoatrial nodal cell (SANC) normal automaticity. To test this hypothesis, we measured responses of [Na(+)]i, [Ca(2+)]i, membrane potential, action potential cycle length (APCL), and rhythm in rabbit SANCs to Na(+)/K(+) pump inhibition by the digitalis glycoside, digoxigenin (DG, 10-20 μmol/l). Initial small but significant increases in [Na(+)]i and [Ca(2+)]i and reductions in ENa and ECa in response to DG led to a small reduction in maximum diastolic potential (MDP), significantly enhanced local diastolic Ca(2+) releases (LCRs), and reduced the average APCL. As [Na(+)]i and [Ca(2+)]i continued to increase at longer times following DG exposure, further significant reductions in MDP, ENa, and ECa occurred; LCRs became significantly reduced, and APCL became progressively and significantly prolonged. This was accompanied by increased APCL variability. We also employed a coupled-clock numerical model to simulate changes in ENa and ECa simultaneously with ion currents not measured experimentally. Numerical modeling predicted that, as the ENa and ECa monotonically reduced over time in response to DG, ion currents (ICaL, ICaT, If, IKr, and IbNa) monotonically decreased. In parallel with the biphasic APCL, diastolic INCX manifested biphasic changes; initial INCX increase attributable to enhanced LCR ensemble Ca(2+) signal was followed by INCX reduction as ENCX (ENCX = 3ENa - 2ECa) decreased. Thus SANC automaticity is tightly regulated by ENa, ECa, and ENCX via a complex interplay of numerous key clock components that regulate SANC clock coupling.

Intensive training and reduced volume increases muscle FXYD1 expression and phosphorylation at rest and during exercise in athletes

The present study examined the effect of intensive training in combination with marked reduction in training volume on phospholemman (FXYD1) expression and phosphorylation at rest and during exercise. Eight well-trained cyclists replaced their regular training with speed-endurance training (10-12 × ∼30-s sprints) two or three times per week and aerobic high-intensity training (4-5 × 3-4 min at 90-95% of peak aerobic power output) 1-2 times per week for 7 wk and reduced the training volume by 70%. Muscle biopsies were obtained before and during a repeated high-intensity exercise protocol, and protein expression and phosphorylation were determined by Western blot analysis. Expression of FXYD1 (30%), actin (40%), mammalian target of rapamycin (mTOR) (12%), phospholamban (PLN) (16%), and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) γ/δ (25%) was higher (P < 0.05) than before the training intervention. In addition, after the intervention, nonspecific FXYD1 phosphorylation was higher (P < 0.05) at rest and during exercise, mainly achieved by an increased FXYD1 Ser-68 phosphorylation, compared with before the intervention. CaMKII, Thr-287, and eukaryotic elongation factor 2 Thr-56 phosphorylation at rest and during exercise, overall PKCα/β, Thr-638/641, and mTOR Ser-2448 phosphorylation during repeated intense exercise as well as resting PLN Thr-17 phosphorylation were also higher (P < 0.05) compared with before the intervention period. Thus, a period of high-intensity training with reduced training volume increases expression and phosphorylation levels of FXYD1, which may affect Na(+)/K(+) pump activity and muscle K(+) homeostasis during intense exercise. Furthermore, higher expression of CaMKII and PLN, as well as increased phosphorylation of CaMKII Thr-287 may have improved intracellular Ca(2+) handling.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Protein Phosphatase 1c Associated with the Cardiac Sodium Calcium Exchanger 1 Regulates Its Activity by Dephosphorylating Serine 68-phosphorylated Phospholemman

The sodium (Na(+))-calcium (Ca(2+)) exchanger 1 (NCX1) is an important regulator of intracellular Ca(2+) homeostasis. Serine 68-phosphorylated phospholemman (pSer-68-PLM) inhibits NCX1 activity. In the context of Na(+)/K(+)-ATPase (NKA) regulation, pSer-68-PLM is dephosphorylated by protein phosphatase 1 (PP1). PP1 also associates with NCX1; however, the molecular basis of this association is unknown. In this study, we aimed to analyze the mechanisms of PP1 targeting to the NCX1-pSer-68-PLM complex and hypothesized that a direct and functional NCX1-PP1 interaction is a prerequisite for pSer-68-PLM dephosphorylation. Using a variety of molecular techniques, we show that PP1 catalytic subunit (PP1c) co-localized, co-fractionated, and co-immunoprecipitated with NCX1 in rat cardiomyocytes, left ventricle lysates, and HEK293 cells. Bioinformatic analysis, immunoprecipitations, mutagenesis, pulldown experiments, and peptide arrays constrained PP1c anchoring to the K(I/V)FF motif in the first Ca(2+) binding domain (CBD) 1 in NCX1. This binding site is also partially in agreement with the extended PP1-binding motif K(V/I)FF-X5-8Φ1Φ2-X8-9-R. The cytosolic loop of NCX1, containing the K(I/V)FF motif, had no effect on PP1 activity in an in vitro assay. Dephosphorylation of pSer-68-PLM in HEK293 cells was not observed when NCX1 was absent, when the K(I/V)FF motif was mutated, or when the PLM- and PP1c-binding sites were separated (mimicking calpain cleavage of NCX1). Co-expression of PLM and NCX1 inhibited NCX1 current (both modes). Moreover, co-expression of PLM with NCX1(F407P) (mutated K(I/V)FF motif) resulted in the current being completely abolished. In conclusion, NCX1 is a substrate-specifying PP1c regulator protein, indirectly regulating NCX1 activity through pSer-68-PLM dephosphorylation.

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

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

CaMKII Phosphorylation of Na(V)1.5: Novel in Vitro Sites Identified by Mass Spectrometry and Reduced S516 Phosphorylation in Human Heart Failure

The cardiac voltage-gated sodium channel, Na(V)1.5, drives the upstroke of the cardiac action potential and is a critical determinant of myocyte excitability. Recently, calcium (Ca(2+))/calmodulin(CaM)-dependent protein kinase II (CaMKII) has emerged as a critical regulator of Na(V)1.5 function through phosphorylation of multiple residues including S516, T594, and S571, and these phosphorylation events may be important for the genesis of acquired arrhythmias, which occur in heart failure. However, phosphorylation of full-length human Na(V)1.5 has not been systematically analyzed and Na(V)1.5 phosphorylation in human heart failure is incompletely understood. In the present study, we used label-free mass spectrometry to assess phosphorylation of human Na(V)1.5 purified from HEK293 cells with full coverage of phosphorylatable sites and identified 23 sites that were phosphorylated by CaMKII in vitro. We confirmed phosphorylation of S516 and S571 by LC-MS/MS and found a decrease in S516 phosphorylation in human heart failure, using a novel phospho-specific antibody. This work furthers our understanding of the phosphorylation of Na(V)1.5 by CaMKII under normal and disease conditions, provides novel CaMKII target sites for functional validation, and provides the first phospho-proteomic map of full-length human Na(V)1.5.

Cardiac hypertrophy in mice expressing unphosphorylatable phospholemman

AIMS

Elevation of intracellular Na in the failing myocardium contributes to contractile dysfunction, the negative force-frequency relationship, and arrhythmias. Although phospholemman (PLM) is recognized to form the link between signalling pathways and Na/K pump activity, the possibility that defects in its regulation contribute to elevation of intracellular Na has not been investigated. Our aim was to test the hypothesis that the prevention of PLM phosphorylation in a PLM(3SA) knock-in mouse (in which PLM has been rendered unphosphorylatable) will exacerbate cardiac hypertrophy and cellular Na overload. Testing this hypothesis should determine whether changes in PLM phosphorylation are simply bystander effects or are causally involved in disease progression.

METHODS AND RESULTS

In wild-type (WT) mice, aortic constriction resulted in hypophosphorylation of PLM with no change in Na/K pump expression. This under-phosphorylation of PLM occurred at 3 days post-banding and was associated with a progressive decline in Na/K pump current and elevation of [Na]i. Echocardiography, morphometry, and pressure-volume (PV) catheterization confirmed remodelling, dilation, and contractile dysfunction, respectively. In PLM(3SA) mice, expression of Na/K ATPase was increased and PLM decreased such that net Na/K pump current under quiescent conditions was unchanged (cf. WT myocytes); [Na(+)]i was increased and forward-mode Na/Ca exchanger was reduced in paced PLM(3SA) myocytes. Cardiac hypertrophy and Na/K pump inhibition were significantly exacerbated in banded PLM(3SA) mice compared with banded WT.

CONCLUSIONS

Decreased phosphorylation of PLM reduces Na/K pump activity and exacerbates Na overload, contractile dysfunction, and adverse remodelling following aortic constriction in mice. This suggests a novel therapeutic target for the treatment of heart failure.

Overexpression of the Na+/K+ ATPase α2 but not α1 isoform attenuates pathological cardiac hypertrophy and remodeling

RATIONALE

The Na+ / K+ ATPase (NKA) directly regulates intracellular Na+ levels, which in turn indirectly regulates Ca2+ levels by proximally controlling flux through the Na+ / Ca2+ exchanger (NCX1). Elevated Na+ levels have been reported during heart failure, which permits some degree of reverse-mode Ca2+ entry through NCX1, as well as less efficient Ca2+ clearance.

OBJECTIVE

To determine whether maintaining lower intracellular Na+ levels by NKA overexpression in the heart would enhance forward-mode Ca2+ clearance and prevent reverse-mode Ca2+ entry through NCX1 to protect the heart.

METHODS AND RESULTS

Cardiac-specific transgenic mice overexpressing either NKA-α1 or NKA-α2 were generated and subjected to pressure overload hypertrophic stimulation. We found that although increased expression of NKA-α1 had no protective effect, overexpression of NKA-α2 significantly decreased cardiac hypertrophy after pressure overload in mice at 2, 10, and 16 weeks of stimulation. Remarkably, total NKA protein expression and activity were not altered in either of these 2 transgenic models because increased expression of one isoform led to a concomitant decrease in the other endogenous isoform. NKA-α2 overexpression but not NKA-α1 led to significantly faster removal of bulk Ca2+ from the cytosol in a manner requiring NCX1 activity. Mechanistically, overexpressed NKA-α2 showed greater affinity for Na+ compared with NKA-α1, leading to more efficient clearance of this ion. Furthermore, overexpression of NKA-α2 but not NKA-α1 was coupled to a decrease in phospholemman expression and phosphorylation, which would favor greater NKA activity, NCX1 activity, and Ca2+ removal.

CONCLUSIONS

Our results suggest that the protective effect produced by increased expression of NKA-α2 on the heart after pressure overload is due to more efficient Ca2+ clearance because this isoform of NKA preferentially enhances NCX1 activity compared with NKA-α1.

Function and regulation of serine/threonine phosphatases in the healthy and diseased heart

Protein phosphorylation is a major control mechanism of a wide range of physiological processes and plays an important role in cardiac pathophysiology. Serine/threonine protein phosphatases control the dephosphorylation of a variety of cardiac proteins, thereby fine-tuning cardiac electrophysiology and function. Specificity of protein phosphatases type-1 and type-2A is achieved by multiprotein complexes that target the catalytic subunits to specific subcellular domains. Here, we describe the composition, regulation and target substrates of serine/threonine phosphatases in the heart. In addition, we provide an overview of pharmacological tools and genetic models to study the role of cardiac phosphatases. Finally, we review the role of protein phosphatases in the diseased heart, particularly in ventricular arrhythmias and atrial fibrillation and discuss their role as potential therapeutic targets.

Novel regulation of cardiac Na pump via phospholemman

As the only quantitatively significant Na efflux pathway from cardiac cells, the Na/K ATPase (Na pump) is the primary regulator of intracellular Na. The transmembrane Na gradient it establishes is essential for normal electrical excitability, numerous coupled-transport processes and, as the driving force for Na/Ca exchange, thus setting cardiac Ca load and contractility. As Na influx varies with electrical excitation, heart rate and pathology, the dynamic regulation of Na efflux is essential. It is now widely recognized that phospholemman, a 72 amino acid accessory protein which forms part of the Na pump complex, is the key nexus linking cellular signaling to pump regulation. Phospholemman is the target of a variety of post-translational modifications (including phosphorylation, palmitoylation and glutathionation) and these can dynamically alter the activity of the Na pump. This review summarizes our current understanding of the multiple regulatory mechanisms that converge on phospholemman and govern NA pump activity in the heart. The corrected Fig. 4 is reproduced below. The publisher would like to apologize for any inconvenience caused. [corrected].

Nitric oxide regulates cardiac intracellular Na⁺ and Ca²⁺ by modulating Na/K ATPase via PKCε and phospholemman-dependent mechanism

In the heart, Na/K-ATPase regulates intracellular Na⁺ and Ca^2 + (via NCX), thereby preventing Na⁺ and Ca^2 + overload and arrhythmias. Here, we test the hypothesis that nitric oxide (NO) regulates cardiac intracellular Na⁺ and Ca^2 + and investigate mechanisms and physiological consequences involved. Effects of both exogenous NO (via NO-donors) and endogenously synthesized NO (via field-stimulation of ventricular myocytes) were assessed in this study. Field stimulation of rat ventricular myocytes significantly increased endogenous NO (18 ± 2 μM), PKCε activation (82 ± 12%), phospholemman phosphorylation (at Ser-63 and Ser-68) and Na/K-ATPase activity (measured by DAF-FM dye, western-blotting and biochemical assay, respectively; p < 0.05, n = 6) and all were abolished by Ca^2 +-chelation (EGTA 10 mM) or NOS inhibition l-NAME (1 mM). Exogenously added NO (spermine-NONO-ate) stimulated Na/K-ATPase (EC50 = 3.8 μM; n = 6/grp), via decrease in K_m, in PLM^WT but not PLM^KO or PLM^3SA myocytes (where phospholemman cannot be phosphorylated) as measured by whole-cell perforated-patch clamp. Field-stimulation with l-NAME or PKC-inhibitor (2 μM Bis) resulted in elevated intracellular Na⁺ (22 ± 1.5 and 24 ± 2 respectively, vs. 14 ± 0.6 mM in controls) in SBFI-AM-loaded rat myocytes. Arrhythmia incidence was significantly increased in rat hearts paced in the presence of l-NAME (and this was reversed by l-arginine), as well as in PLM^3SA mouse hearts but not PLM^WT and PLM^KO. We provide physiological and biochemical evidence for a novel regulatory pathway whereby NO activates Na/K-ATPase via phospholemman phosphorylation and thereby limits Na⁺ and Ca^2 + overload and arrhythmias. This article is part of a Special Issue entitled “Na⁺ Regulation in Cardiac Myocytes”.

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We tested whether nitric oxide regulates intracellular Na⁺ and Ca^2 + in the heart.

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Nitric oxide increased Na/K ATPase activity via PKCε-induced phospholemman phosphorylation.

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Inhibiting nitric oxide pathway resulted in Na⁺ and Ca^2 + overload and contributed to arrhythmia development in the heart.

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.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Constitutive phosphorylation of inhibitor-1 at Ser67 and Thr75 depresses calcium cycling in cardiomyocytes and leads to remodeling upon aging

The activity of protein phosphatase-1 (PP1) inhibitor-1 (I-1) is antithetically modulated by the cAMP-protein kinase A (PKA) and Ca(2+)-protein kinase C (PKC) signaling axes. β-adrenergic (β-AR) stimulation results in PKA-phosphorylation of I-1 at threonine 35 (Thr35) and depressed PP1 activity, while PKC phosphorylation at serine 67 (Ser67) and/or Thr75 increases PP1 activity. In heart failure, pThr35 is decreased while pSer67 and pThr75 are elevated. However, the role of Ser67/Thr75 phosphorylation in vivo and its effects on Ca(2+)-cycling are not known. Thus, our aim was to investigate the functional significance of Ser67 and Thr75 phosphorylation in intact hearts. We generated transgenic mice (TG) with cardiac-specific overexpression of constitutively phosphorylated I-1 at Ser67 and Thr75 (S67D/T75D) and evaluated cardiac function. The S67D/T75D cardiomyocytes exhibited significantly depressed Ca(2+)-kinetics and contractile parameters, compared with wild-type (WT) cells. The decreased Ca(2+)-cycling was associated with a 27 % increase in PP1 activity, no alterations in PP2 activity and impaired phosphorylation of myosin-binding protein-C (MyBPC). Upon aging, there was cardiac remodeling associated with increases in systolic and diastolic left ventricular internal diameter dimensions (at 16 months), compared with WTs. The results indicate that phosphorylation of I-1 at Ser67 and Thr75 is associated with increased PP1 activity and depressed cardiomyocyte Ca(2+)-cycling, which manifests in geometrical alterations over the long term. Thus, hyperphosphorylation of these sites in failing hearts may contribute to deteriorative remodeling.