Ischemia-induced phosphorylation of phospholemman directly activates rat cardiac Na/K-ATPase

Targeted degradation of zDHHC-PATs decreases substrate S-palmitoylation

Reversible S-palmitoylation of protein cysteines, catalysed by a family of integral membrane zDHHC-motif containing palmitoyl acyl transferases (zDHHC-PATs), controls the localisation, activity, and interactions of numerous integral and peripheral membrane proteins. There are compelling reasons to want to inhibit the activity of individual zDHHC-PATs in both the laboratory and the clinic, but the specificity of existing tools is poor. Given the extensive conservation of the zDHHC-PAT active site, development of isoform-specific competitive inhibitors is highly challenging. We therefore hypothesised that proteolysis-targeting chimaeras (PROTACs) may offer greater specificity to target this class of enzymes. In proof-of-principle experiments we engineered cell lines expressing tetracycline-inducible Halo-tagged zDHHC5 or zDHHC20, and evaluated the impact of Halo-PROTACs on zDHHC-PAT expression and substrate palmitoylation. In HEK-derived FT-293 cells, Halo-zDHHC5 degradation significantly decreased palmitoylation of its substrate phospholemman, and Halo-zDHHC20 degradation significantly diminished palmitoylation of its substrate IFITM3, but not of the SARS-CoV-2 spike protein. In contrast, in a second kidney derived cell line, Vero E6, Halo-zDHHC20 degradation did not alter palmitoylation of either IFITM3 or SARS-CoV-2 spike. We conclude from these experiments that PROTAC-mediated targeting of zDHHC-PATs to decrease substrate palmitoylation is feasible. However, given the well-established degeneracy in the zDHHC-PAT family, in some settings the activity of non-targeted zDHHC-PATs may substitute and preserve substrate palmitoylation.

Glutathione-dependent depalmitoylation of phospholemman by peroxiredoxin 6

Phospholemman (PLM) regulates the cardiac sodium pump: PLM phosphorylation activates the pump whereas PLM palmitoylation inhibits its activity. Here, we show that the anti-oxidant protein peroxiredoxin 6 (Prdx6) interacts with and depalmitoylates PLM in a glutathione-dependent manner. Glutathione loading cells acutely reduce PLM palmitoylation; glutathione depletion significantly increases PLM palmitoylation. Prdx6 silencing abolishes these effects, suggesting that PLM can be depalmitoylated by reduced Prdx6. In vitro, only recombinant Prdx6, among several peroxiredoxin isoforms tested, removes palmitic acid from recombinant palmitoylated PLM. The broad-spectrum depalmitoylase inhibitor palmostatin B prevents Prdx6-dependent PLM depalmitoylation in cells and in vitro. Our data suggest that Prdx6 is a thioesterase that can depalmitoylate proteins by nucleophilic attack via its reactive thiol, linking PLM palmitoylation and hence sodium pump activity to cellular glutathione status. We show that protein depalmitoylation can occur via a catalytic cysteine in which substrate specificity is determined by a protein-protein interaction.

Sodium acetate ameliorates doxorubicin-induced cardiac injury via upregulation of Nrf2/HO-1 signaling and downregulation of NFkB-mediated apoptotic signaling in Wistar rats

Despite the effectiveness of doxorubicin (DOX) in the management of a wide range of cancers, a major challenge is its cardio-toxic effect. Oxidative stress, inflammation, and apoptosis are major pathways for the cardiotoxic effect of DOX. On the other hand, acetate reportedly exerts antioxidant, anti-inflammatory, and anti-apoptotic activities. This particular research assessed the impact of acetate on cardiotoxicity induced by DOX. Mechanistically, acetate dramatically inhibited DOX-induced upregulation of xanthine oxidase and uric acid pathway as well as downregulation of Nrf2/HO-1 signaling and its upstream proteins (reduced glutathione peroxidase, superoxide dismutase, glutathione-S-transferase, glutathione, and catalase, glutathione reductase). In addition, acetate markedly attenuated DOX-driven rise inTNF-α, NFkB IL-6 and IL-1β expression, and myeloperoxidase activity. Furthermore, acetate significantly ameliorated DOX-led suppression of Bcl-2 and Ca2+-ATPase activity and upregulation of Bax, caspase 3, and caspase 9 actions. Improved body weight, heart structural integrity, and cardiac function as depicted by cardiac injury markers convoyed these cascades of events. Summarily, the present study demonstrated that acetate protects against DOX-induced cardiotoxicity by upregulating Nrf2/HO-1 signaling and downregulating NFkB-mediated activation of Bax/Bcl-2 and caspase signaling.

Na + /K + ATPase-Ca v 1.2 nanodomain differentially regulates intracellular [Na + ], [Ca 2+ ] and local adrenergic signaling in cardiac myocytes

Background

The intracellular Na + concentration ([Na + ] i ) is a crucial but understudied regulator of cardiac myocyte function. The Na + /K + ATPase (NKA) controls the steady-state [Na + ] i and thereby determines the set-point for intracellular Ca 2+ . Here, we investigate the nanoscopic organization and local adrenergic regulation of the NKA macromolecular complex and how it differentially regulates the intracellular Na + and Ca 2+ homeostases in atrial and ventricular myocytes.

Methods

Multicolor STORM super-resolution microscopy, Western Blot analyses, and in vivo examination of adrenergic regulation are employed to examine the organization and function of Na + nanodomains in cardiac myocytes. Quantitative fluorescence microscopy at high spatiotemporal resolution is used in conjunction with cellular electrophysiology to investigate intracellular Na + homeostasis in atrial and ventricular myocytes.

Results

The NKAα1 (NKAα1) and the L-type Ca 2+ -channel (Ca v 1.2) form a nanodomain with a center-to center distance of ∼65 nm in both ventricular and atrial myocytes. NKAα1 protein expression levels are ∼3 fold higher in atria compared to ventricle. 100% higher atrial I NKA , produced by large NKA "superclusters", underlies the substantially lower Na + concentration in atrial myocytes compared to the benchmark values set in ventricular myocytes. The NKA's regulatory protein phospholemman (PLM) has similar expression levels across atria and ventricle resulting in a much lower PLM/NKAα1 ratio for atrial compared to ventricular tissue. In addition, a huge PLM phosphorylation reserve in atrial tissue produces a high ß-adrenergic sensitivity of I NKA in atrial myocytes. ß-adrenergic regulation of I NKA is locally mediated in the NKAα1-Ca v 1.2 nanodomain via A-kinase anchoring proteins.

Conclusions

NKAα1, Ca v 1.2 and their accessory proteins form a structural and regulatory nanodomain at the cardiac dyad. The tissue-specific composition and local adrenergic regulation of this "signaling cloud" is a main regulator of the distinct global intracellular Na + and Ca 2+ concentrations in atrial and ventricular myocytes.

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.

Upregulation of Uric Acid Production and Caspase 3 Signalling Mediates Rohypnol-Induced Cardiorenal Damage

The global prevalence of illicit drug use is on the increase with attendant complications like cardiorenal collapse. One such substance of abuse is rohypnol. Despite its ban in most countries, it remains a popular substance of abuse. Whether or not rohypnol induces cardiorenal injury and the associated mechanism is yet to be elucidated. Therefore, the present study investigated the effect of rohypnol on cardiorenal integrity and functions, and glucolipid metabolism. Forty-eight male Wistar rats randomized into six groups (n = 8/group) received (per os) vehicle, low-dose (2 mg/kg) and high-dose (4 mg/kg) rohypnol once daily for twenty eight days, with or without a cessation period. Data revealed that rohypnol exposure irreversibly caused insulin resistance, hyperglycaemia, and dyslipidaemia. This was accompanied by reduced cardiorenal mass and impaired cardiorenal cytoarchitecture and function. Furthermore, rohypnol treatment promoted oxidative stress, inflammation, genotoxicity, and decreased cardiorenal activities of Na⁺-K⁺-ATPase, Ca²⁺-ATPase, and Mg²⁺-ATPase. These alterations were associated with enhanced uric acid generation and caspase 3 activity in the cardiorenal complex. Thus, this study reveals that rohypnol exposure triggers cardiorenal toxicity with incident insulin resistance, glucolipid and cardiorenal proton pump dysregulation, altered redox state, and inflammation via enhancement of uric acid generation and caspase 3-dependent mechanism.

Compressed Sensing in Sodium Magnetic Resonance Imaging: Techniques, Applications, and Future Prospects

Sodium (²³ Na) yields the second strongest nuclear magnetic resonance (NMR) signal in biological tissues and plays a vital role in cell physiology. Sodium magnetic resonance imaging (MRI) can provide insights into cell integrity and tissue viability relative to pathologies without significant anatomical alternations, and thus it is considered to be a potential surrogate biomarker that provides complementary information for standard hydrogen (¹ H) MRI in a noninvasive and quantitative manner. However, sodium MRI suffers from a relatively low signal-to-noise ratio and long acquisition times due to its relatively low NMR sensitivity. Compressed sensing-based (CS-based) methods have been shown to accelerate sodium imaging and/or improve sodium image quality significantly. In this manuscript, the basic concepts of CS and how CS might be applied to improve sodium MRI are described, and the historical milestones of CS-based sodium MRI are briefly presented. Representative advanced techniques and evaluation methods are discussed in detail, followed by an expose of clinical applications in multiple anatomical regions and diseases as well as thoughts and suggestions on potential future research prospects of CS in sodium MRI. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

14-3-3 protein regulation of excitation-contraction coupling

14-3-3 proteins (14-3-3 s) are a family of highly conserved proteins that regulate many cellular processes in eukaryotes by interacting with a diverse array of client proteins. The 14-3-3 proteins have been implicated in several disease states and previous reviews have condensed the literature with respect to their structure, function, and the regulation of different cellular processes. This review focuses on the growing body of literature exploring the important role 14-3-3 proteins appear to play in regulating the biochemical and biophysical events associated with excitation-contraction coupling (ECC) in muscle. It presents both a timely and unique analysis that seeks to unite studies emphasizing the identification and diversity of 14-3-3 protein function and client protein interactions, as modulators of muscle contraction. It also highlights ideas within these two well-established but intersecting fields that support further investigation with respect to the mechanistic actions of 14-3-3 proteins in the modulation of force generation in muscle.

The Effect of Curcumin Nanoparticles on Cisplatin-Induced Cardiotoxicity in Male Wistar Albino Rats

The cardiotoxicity of chemotherapeutic drugs as cisplatin has become a major issue in recent years. The present study investigates the efficacy of curcumin nanoparticles against the cardiotoxic effects of cisplatin by assessment of oxidative stress parameters, Na+,K+-ATPase, acetylcholinesterase (AchE) and tumor necrosis factor-alpha (TNF-α) in cardiac tissue in addition to serum lactate dehydrogenase (LDH). Rats were divided into three groups: control rats that received saline for 14 days; cisplatin-treated rats that received a single intraperitoneal (i.p.) injection of cisplatin (12 mg/kg) followed by a daily oral administration of saline (0.9%) for 14 days and rats treated with a single i.p. injection of cisplatin (12 mg/kg) followed by a daily oral administration of curcumin nanoparticles (50 mg/kg) for 14 days. Cisplatin resulted in a significant increase in lipid peroxidation, nitric oxide (NO), and TNF-α and a significant decrease in reduced glutathione (GSH) levels and Na+, K+- ATPase activity. Moreover, significant increases in cardiac AchE and serum lactate dehydrogenase activities were recorded. Treatment of cisplatin-injected animals with curcumin nanoparticles ameliorated all the alterations induced by cisplatin in the heart of rats. This suggests that curcumin nanoparticles can be used as an important therapeutic adjuvant in chemotherapeutic and other toxicities mediated by oxidative stress and inflammation.

Codeine-induced hepatic injury is via oxido-inflammatory damage and caspase-3-mediated apoptosis

Codeine (3-methylmorphine) is a known analgesic, antitussive, and antidiarrheal drug that is often abused for recreational purposes. It is metabolized in the liver via the cytochrome P450 system and thus hypothesized to induce hepatic injury especially when misused. Thus, the present study aimed at investigating changes in liver function, hepatic enzyme biomarker, proton pumps, antioxidant status, free radicals and TNF-α levels, as well as caspase 3 activities and hepatic DNA fragmentation after 6 weeks of oral codeine administration. Twenty-one male rabbits were randomized into 3 groups (n = 7). The control group had 1 ml of normal saline, while the low-dose and high-dose codeine groups received 4 and 10 mg/kg b.w of codeine respectively daily. The codeine-treated animals had significantly lower levels of serum proteins, increased activities of hepatic enzyme biomarkers and caspase 3, raised hepatic concentrations of free radicals and TNF-α, as well as increased hepatic DNA fragmentation. Codeine treatment also led to a significant decline in hepatic weight, activities of hepatic enzymatic antioxidant, Na⁺-K⁺-ATPase and Ca²⁺-ATPase. These alterations were more pronounced in high-dose codeine treated animals than in the low-dose group. Histopathological study showed moderate fatty degeneration of hepatic parenchyma, infiltration of the portal tract by inflammatory cells with dense collagen fibre deposition in codeine-treated animals. The present study revealed that codeine induced liver injury and hepatic DNA damage via caspase 3-dependent signaling by suppressing hepatic antioxidant status and enhancing free radical and TNF-α generation.

Codeine exerts cardiorenal injury via upregulation of adenine deaminase/xanthine oxidase and caspase 3 signaling

AIMS

Codeine treatment has been shown to be associated with glucolipid deregulation, though data reporting this are inconsistent and the mechanisms are not well understood. Perturbation of glutathione-dependent antioxidant defense and adenosine deaminase (ADA)/xanthine oxidase (XO) signaling has been implicated in the pathogenesis of cardiometabolic disorders. We thus, hypothesized that depletion of glutathione contents and upregulation of ADA/XO are involved in codeine-induced glucolipid deregulation. The present study also investigated whether or not codeine administration would induce genotoxicity and apoptosis in cardiac and renal tissues.

MATERIALS AND METHODS

Male New Zealand rabbits received per os distilled water or codeine, either in low dose (4 mg/kg) or high dose (10 mg/kg) for 6 weeks.

KEY FINDINGS

Codeine treatment led to reduced absolute and relative cardiac and renal mass independent of body weight change, increased blood glucose, total cholesterol (TC), triglycerides (TG), and low-density lipoprotein (LDL-C), as well as increased atherogenic indices and triglyceride-glucose index (TyG). Codeine administration significantly increased markers of cardiac and renal injury, as well as impaired cardiorenal functions. Codeine treatment also resulted in increased cardiac and renal malondialdehyde, Advanced Glycation Endproducts (AGE) and 8-hydroxydeoxyguanosine (8-OH-dG), and myeloperoxidase (MPO), ADA, XO, and caspase 3 activities. These observations were accompanied by impaired activities of cardiac and renal proton pumps.

SIGNIFICANCE

Findings of this study demonstrate that upregulation of ADA/XO and caspase 3 signaling are, at least partly, contributory to the glucolipid deregulation and cardiorenal injury induced by codeine.

Effect of untreated pharmaceutical plant effluent on cardiac Na+-K+- ATPase and Ca2+-Mg2+-ATPase activities in mice (Mus Musculus)

Cardiovascular diseases are major causes of non-communicable diseases (NCDs)-related throughout the world. Water pollution has been linked with the high global NCD burden but no report exists on the cardiotoxicity of untreated or poorly treated pharmaceutical effluent, despite its indiscriminate discharge into the aquatic environment in Nigeria, as in many other locations of the world. Thus, this study investigated the cardiotoxic effect of oral exposure to pharmaceutical effluent in mice. Thirty (30) male mice (Mus musculus) were randomly divided into 6 groups. Group A (control) received 0.2 ml distilled water, while groups B-F were treated with 0.2 ml 2.5%, 5.0%, 10.0%, 20.0% and 40% concentrations (v/v, effluent/distilled water) of the effluent respectively, for 28 days. Significant reductions (p<0.05) in heart weight and cardiac weight index were observed in the groups treated with 5%, 10%, 20% and 40% concentrations of the effluent, without significant change in body weight. Similarly, 28 day administration of the effluent showed significant decrease in cardiac Na+-K+-ATPase activity (p<0.05) at concentrations 10% and above, in a concentration dependent manner. However, there was insignificant decrease in cardiac Ca2+-Mg2+-ATPase activity of the exposed mice, when compared with the control group. This study provides novel information on the cardiotoxic effects of oral exposure to untreated pharmaceutical effluent, showing reduced Na+-K+-ATPase activity and decreseased myocardial atrophy. Therefore, drinking water contaminated with pharmaceutical effluent may promote the incidence of cardiovascular diseases. Further studies on the exact mechanistic routes of the induced cardiotoxicity are recommended.

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.

Regulation of Cardiac Remodeling by Cardiac Na(+)/K(+)-ATPase Isoforms

Cardiac remodeling occurs after cardiac pressure/volume overload or myocardial injury during the development of heart failure and is a determinant of heart failure. Preventing or reversing remodeling is a goal of heart failure therapy. Human cardiomyocyte Na⁺/K⁺-ATPase has multiple α isoforms (1–3). The expression of the α subunit of the Na⁺/K⁺-ATPase is often altered in hypertrophic and failing hearts. The mechanisms are unclear. There are limited data from human cardiomyocytes. Abundant evidences from rodents show that Na⁺/K⁺-ATPase regulates cardiac contractility, cell signaling, hypertrophy and fibrosis. The α1 isoform of the Na⁺/K⁺-ATPase is the ubiquitous isoform and possesses both pumping and signaling functions. The α2 isoform of the Na⁺/K⁺-ATPase regulates intracellular Ca²⁺ signaling, contractility and pathological hypertrophy. The α3 isoform of the Na⁺/K⁺-ATPase may also be a target for cardiac hypertrophy. Restoration of cardiac Na⁺/K⁺-ATPase expression may be an effective approach for prevention of cardiac remodeling. In this article, we will overview: (1) the distribution and function of isoform specific Na⁺/K⁺-ATPase in the cardiomyocytes. (2) the role of cardiac Na⁺/K⁺-ATPase in the regulation of cell signaling, contractility, cardiac hypertrophy and fibrosis in vitro and in vivo. Selective targeting of cardiac Na⁺/K⁺-ATPase isoform may offer a new target for the prevention of cardiac remodeling.

"Oxygen Sensing" by Na,K-ATPase: These Miraculous Thiols

Control over the Na,K-ATPase function plays a central role in adaptation of the organisms to hypoxic and anoxic conditions. As the enzyme itself does not possess O2 binding sites its "oxygen-sensitivity" is mediated by a variety of redox-sensitive modifications including S-glutathionylation, S-nitrosylation, and redox-sensitive phosphorylation. This is an overview of the current knowledge on the plethora of molecular mechanisms tuning the activity of the ATP-consuming Na,K-ATPase to the cellular metabolic activity. Recent findings suggest that oxygen-derived free radicals and H2O2, NO, and oxidized glutathione are the signaling messengers that make the Na,K-ATPase "oxygen-sensitive." This very ancient signaling pathway targeting thiols of all three subunits of the Na,K-ATPase as well as redox-sensitive kinases sustains the enzyme activity at the "optimal" level avoiding terminal ATP depletion and maintaining the transmembrane ion gradients in cells of anoxia-tolerant species. We acknowledge the complexity of the underlying processes as we characterize the sources of reactive oxygen and nitrogen species production in hypoxic cells, and identify their targets, the reactive thiol groups which, upon modification, impact the enzyme activity. Structured accordingly, this review presents a summary on (i) the sources of free radical production in hypoxic cells, (ii) localization of regulatory thiols within the Na,K-ATPase and the role reversible thiol modifications play in responses of the enzyme to a variety of stimuli (hypoxia, receptors' activation) (iii) redox-sensitive regulatory phosphorylation, and (iv) the role of fine modulation of the Na,K-ATPase function in survival success under hypoxic conditions. The co-authors attempted to cover all the contradictions and standing hypotheses in the field and propose the possible future developments in this dynamic area of research, the importance of which is hard to overestimate. Better understanding of the processes underlying successful adaptation strategies will make it possible to harness them and use for treatment of patients with stroke and myocardial infarction, sleep apnoea and high altitude pulmonary oedema, and those undergoing surgical interventions associated with the interruption of blood perfusion.

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.

Regulation of L-type calcium channel by phospholemman in cardiac myocytes

We evaluated whether phospholemman (PLM) regulates L-type Ca(2+) current (ICa) in mouse ventricular myocytes. Expression of α1-subunit of L-type Ca(2+) channels between wild-type (WT) and PLM knockout (KO) hearts was similar. Compared to WT myocytes, peak ICa (at -10 mV) from KO myocytes was ~41% larger, the inactivation time constant (τ(inact)) of ICa was ~39% longer, but deactivation time constant (τ(deact)) was similar. In the presence of isoproterenol (1 μM), peak ICa was ~48% larger and τ(inact) was ~144% higher in KO myocytes. With Ba(2+) as the permeant ion, PLM enhanced voltage-dependent inactivation but had no effect on τ(deact). To dissect the molecular determinants by which PLM regulated ICa, we expressed PLM mutants by adenovirus-mediated gene transfer in cultured KO myocytes. After 24h in culture, KO myocytes expressing green fluorescent protein (GFP) had significantly larger peak ICa and longer τ(inact) than KO myocytes expressing WT PLM; thereby independently confirming the observations in freshly isolated myocytes. Compared to KO myocytes expressing GFP, KO myocytes expressing the cytoplasmic domain truncation mutant (TM43), the non-phosphorylatable S68A mutant, the phosphomimetic S68E mutant, and the signature PFXYD to alanine (ALL5) mutant all resulted in lower peak ICa. Expressing PLM mutants did not alter expression of α1-subunit of L-type Ca(2+) channels in cultured KO myocytes. Our results suggested that both the extracellular PFXYD motif and the transmembrane domain of PLM but not the cytoplasmic tail were necessary for regulation of peak ICa amplitude. We conclude that PLM limits Ca(2+) influx in cardiac myocytes by reducing maximal ICa and accelerating voltage-dependent inactivation.

Impact of phosphomimetic and non-phosphorylatable mutations of phospholemman on L-type calcium channels gating in HEK 293T cells

BACKGROUND

Phospholemman (PLM) is an important phosphorylation substrate for protein kinases A and C in the heart. Until now, the association between PLM phosphorylation status and L-type calcium channels (LTCCs) gating has not been fully understood. We investigated the kinetics of LTCCs in HEK 293T cells expressing phosphomimetic or nonphosphorylatable PLM mutants.

METHODS

The LTCCs gating was measured in HEK 293T cells transfected with LTCC and wild-type (WT) PLM, phosphomimetic or nonphosphorylatable PLM mutants: 6263AA, 6869AA, AAAA, 6263DD, 6869DD or DDDD.

RESULTS

WT PLM significantly slowed LTCCs activation and deactivation while enhanced voltage-dependent inactivation (VDI). PLM mutants 6869DD and DDDD significantly increased the peak of the currents. 6263DD accelerated channel activation, while 6263AA slowed it more than WT PLM. 6869DD significantly enhanced PLM-induced increase of VDI. AAAA slowed the channel activation more than 6263AA, and DDDD accelerated the channel VDI more than 6869DD.

CONCLUSIONS

Our results demonstrate that phosphomimetic PLM could stimulate LTCCs and alter their dynamics, while PLM nonphosphorylatable mutant produced the opposite effects.

Identification of caveolar resident proteins in ventricular myocytes using a quantitative proteomic approach: dynamic changes in caveolar composition following adrenoceptor activation

The lipid raft concept proposes that membrane environments enriched in cholesterol and sphingolipids cluster certain proteins and form platforms to integrate cell signaling. In cardiac muscle, caveolae concentrate signaling molecules and ion transporters, and play a vital role in adrenergic regulation of excitation–contraction coupling, and consequently cardiac contractility. Proteomic analysis of cardiac caveolae is hampered by the presence of contaminants that have sometimes, erroneously, been proposed to be resident in these domains. Here we present the first unbiased analysis of the proteome of cardiac caveolae, and investigate dynamic changes in their protein constituents following adrenoreceptor (AR) stimulation.

Rat ventricular myocytes were treated with methyl-β-cyclodextrin (MβCD) to deplete cholesterol and disrupt caveolae. Buoyant caveolin-enriched microdomains (BCEMs) were prepared from MβCD-treated and control cell lysates using a standard discontinuous sucrose gradient. BCEMs were harvested, pelleted, and resolubilized, then alkylated, digested, and labeled with iTRAQ reagents, and proteins identified by LC-MS/MS on a LTQ Orbitrap Velos Pro. Proteins were defined as BCEM resident if they were consistently depleted from the BCEM fraction following MβCD treatment. Selective activation of α-, β1-, and β2-AR prior to preparation of BCEMs was achieved by application of agonist/antagonist pairs for 10 min in populations of field-stimulated myocytes.

We typically identified 600–850 proteins per experiment, of which, 249 were defined as high-confidence BCEM residents. Functional annotation clustering indicates cardiac BCEMs are enriched in integrin signaling, guanine nucleotide binding, ion transport, and insulin signaling clusters. Proteins possessing a caveolin binding motif were poorly enriched in BCEMs, suggesting this is not the only mechanism that targets proteins to caveolae. With the notable exception of the cavin family, very few proteins show altered abundance in BCEMs following AR activation, suggesting signaling complexes are preformed in BCEMs to ensure a rapid and high fidelity response to adrenergic stimulation in cardiac muscle.

Induced overexpression of phospholemman S68E mutant improves cardiac contractility and mortality after ischemia-reperfusion

Phospholemman (PLM), when phosphorylated at Ser(68), inhibits cardiac Na+ / Ca2+ exchanger 1 (NCX1) and relieves its inhibition on Na+ -K+ -ATPase. We have engineered mice in which expression of the phosphomimetic PLM S68E mutant was induced when dietary doxycycline was removed at 5 wk. At 8-10 wk, compared with noninduced or wild-type hearts, S68E expression in induced hearts was ∼35-75% that of endogenous PLM, but protein levels of sarco(endo)plasmic reticulum Ca2+ -ATPase, α1- and α2-subunits of Na+ -K+ -ATPase, α1c-subunit of L-type Ca2+ channel, and phosphorylated ryanodine receptor were unchanged. The NCX1 protein level was increased by ∼47% but the NCX1 current was depressed by ∼34% in induced hearts. Isoproterenol had no effect on NCX1 currents but stimulated Na+ -K+ -ATPase currents equally in induced and noninduced myocytes. At baseline, systolic intracellular Ca2+ concentrations ([Ca2+]i), sarcoplasmic reticulum Ca2+ contents, and [Ca(2+)]i transient and contraction amplitudes were similar between induced and noninduced myocytes. Isoproterenol stimulation resulted in much higher systolic [Ca2+]i, sarcoplasmic reticulum Ca2+ content, and [Ca2+]i transient and contraction amplitudes in induced myocytes. Echocardiography and in vivo close-chest catheterization demonstrated similar baseline myocardial function, but isoproterenol induced a significantly higher +dP/dt in induced compared with noninduced hearts. In contrast to the 50% mortality observed in mice constitutively overexpressing the S68E mutant, induced mice had similar survival as wild-type and noninduced mice. After ischemia-reperfusion, despite similar areas at risk and left ventricular infarct sizes, induced mice had significantly higher +dP/dt and -dP/dt and lower perioperative mortality compared with noninduced mice. We propose that phosphorylated PLM may be a novel therapeutic target in ischemic heart disease.

Surface charges of the membrane crucially affect regulation of Na,K-ATPase by phospholemman (FXYD1)

The human α1/His10-β1 isoform of Na,K-ATPase has been reconstituted as a complex with and without FXYD1 into proteoliposomes of various lipid compositions in order to study the effect of the regulatory subunit on the half-saturating Na⁺ concentration (K(½)) of Na⁺ ions for activation of the ion pump. It has been shown that the fraction of negatively charged lipid in the bilayer crucially affects the regulatory properties. At low concentrations of the negatively charged lipid DOPS (<10 %), FXYD1 increases K(½) of Na⁺ ions for activation of the ion pump. Phosphorylation of FXYD1 by protein kinase A at Ser68 abrogates this effect. Conversely, for proteoliposomes made with high concentrations of DOPS (>10 %), little or no effect of FXYD1 on the K(½) of Na⁺ ions is observed. Depending on ionic strength and lipid composition of the proteoliposomes, FXYD1 can alter the K(½) of Na⁺ ions by up to twofold. We propose possible molecular mechanisms to explain the regulatory effects of FXYD1 and the influence of charged lipid and protein phosphorylation. In particular, the positively charged C-terminal helix of FXYD1 appears to be highly mobile and may interact with the cytoplasmic N domain of the α-subunit, the interaction being strongly affected by phosphorylation at Ser68 and the surface charge of the membrane.

Effects of obesity and estradiol on Na+/K+-ATPase and their relevance to cardiovascular diseases

Obesity is associated with aberrant sodium/potassium-ATPase (Na(+)/K(+)-ATPase) activity, apparently linked to hyperglycemic hyperinsulinemia, which may repress or inactivate the enzyme. The reduction of Na(+)/K(+)-ATPase activity in cardiac tissue induces myocyte death and cardiac dysfunction, leading to the development of myocardial dilation in animal models; this has also been documented in patients with heart failure (HF). During several pathological situations (cardiac insufficiency and HF) and in experimental models (obesity), the heart becomes more sensitive to the effect of cardiac glycosides, due to a decrease in Na(+)/K(+)-ATPase levels. The primary female sex steroid estradiol has long been recognized to be important in a wide variety of physiological processes. Numerous studies, including ours, have shown that estradiol is one of the major factors controlling the activity and expression of Na(+)/K(+)-ATPase in the cardiovascular (CV) system. However, the effects of estradiol on Na(+)/K(+)-ATPase in both normal and pathological conditions, such as obesity, remain unclear. Increasing our understanding of the molecular mechanisms by which estradiol mediates its effects on Na(+)/K(+)-ATPase function may help to develop new strategies for the treatment of CV diseases. Herein, we discuss the latest data from animal and clinical studies that have examined how pathophysiological conditions such as obesity and the action of estradiol regulate Na(+)/K(+)-ATPase activity.

Regulation of the cardiac Na(+) pump by palmitoylation of its catalytic and regulatory subunits

The Na+/K+-ATPase (Na+ pump) is the principal consumer of ATP in multicellular organisms. In the heart, the Na+ gradient established by the pump is essential for all aspects of cardiac function, and appropriate regulation of the cardiac Na+ pump is therefore crucial to match cardiac output to the physiological requirements of an organism. The cardiac pump is a multi-subunit enzyme, consisting of a catalytic α-subunit and regulatory β- and FXYD subunits. All three subunits may become palmitoylated, although the functional outcome of these palmitoylation events is incompletely characterized to date. Interestingly, both β- and FXYD subunits may be palmitoylated or glutathionylated at the same cysteine residues. These competing chemically distinct post-translational modifications may mediate functionally different effects on the cardiac pump. In the present article, we review the cellular events that control the balance between these modifications, and discuss the likely functional effects of pump subunit palmitoylation.

Na/K pump regulation of cardiac repolarization: insights from a systems biology approach

The sodium-potassium pump is widely recognized as the principal mechanism for active ion transport across the cellular membrane of cardiac tissue, being responsible for the creation and maintenance of the transarcolemmal sodium and potassium gradients, crucial for cardiac cell electrophysiology. Importantly, sodium-potassium pump activity is impaired in a number of major diseased conditions, including ischemia and heart failure. However, its subtle ways of action on cardiac electrophysiology, both directly through its electrogenic nature and indirectly via the regulation of cell homeostasis, make it hard to predict the electrophysiological consequences of reduced sodium-potassium pump activity in cardiac repolarization. In this review, we discuss how recent studies adopting the systems biology approach, through the integration of experimental and modeling methodologies, have identified the sodium-potassium pump as one of the most important ionic mechanisms in regulating key properties of cardiac repolarization and its rate dependence, from subcellular to whole organ levels. These include the role of the pump in the biphasic modulation of cellular repolarization and refractoriness, the rate control of intracellular sodium and calcium dynamics and therefore of the adaptation of repolarization to changes in heart rate, as well as its importance in regulating pro-arrhythmic substrates through modulation of dispersion of repolarization and restitution. Theoretical findings are consistent across a variety of cell types and species including human, and widely in agreement with experimental findings. The novel insights and hypotheses on the role of the pump in cardiac electrophysiology obtained through this integrative approach could eventually lead to novel therapeutic and diagnostic strategies.

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.

Coordinated regulation of cardiac Na(+)/Ca (2+) exchanger and Na (+)-K (+)-ATPase by phospholemman (FXYD1)

Phospholemman (PLM) is the founding member of the FXYD family of regulators of ion transport. PLM is a 72-amino acid protein consisting of the signature PFXYD motif in the extracellular N terminus, a single transmembrane (TM) domain, and a C-terminal cytoplasmic tail containing three phosphorylation sites. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. The TM domain of PLM interacts with TM9 of the α-subunit of Na(+)-K(+)-ATPase, while its cytoplasmic tail interacts with two small regions (spanning residues 248-252 and 300-304) of the proximal intracellular loop of Na(+)/Ca(2+) exchanger. Under stress, catecholamine stimulation phosphorylates PLM at serine(68), resulting in relief of inhibition of Na(+)-K(+)-ATPase by decreasing K(m) for Na(+) and increasing V(max), and simultaneous inhibition of Na(+)/Ca(2+) exchanger. Enhanced Na(+)-K(+)-ATPase activity lowers intracellular Na(+), thereby minimizing Ca(2+) overload and risks of arrhythmias. Inhibition of Na(+)/Ca(2+) exchanger reduces Ca(2+) efflux, thereby preserving contractility. Thus, the coordinated actions of PLM during stress serve to minimize arrhythmogenesis and maintain inotropy. In acute cardiac ischemia and chronic heart failure, either expression or phosphorylation of PLM or both are altered. PLM regulates important ion transporters in the heart and offers a tempting target for development of drugs to treat heart failure.

Cross talk between S-nitrosylation and S-glutathionylation in control of the Na,K-ATPase regulation in hypoxic heart

Oxygen-induced regulation of Na,K-ATPase was studied in rat myocardium. In rat heart, Na,K-ATPase responded to hypoxia with a dose-dependent inhibition in hydrolytic activity. Inhibition of Na,K-ATPase in hypoxic rat heart was associated with decrease in nitric oxide (NO) production and progressive oxidative stress. Accumulation of oxidized glutathione (GSSG) and decrease in NO availability in hypoxic rat heart were followed by a decrease in S-nitrosylation and upregulation of S-glutathionylation of the catalytic α-subunit of the Na,K-ATPase. Induction of S-glutathionylation of the α-subunit by treatment of tissue homogenate with GSSG resulted in complete inhibition of the enzyme in rat a myocardial tissue homogenate. Inhibitory effect of GSSG in rat sarcolemma could be significantly decreased upon activation of NO synthases. We have further tested whether oxidative stress and suppression of the Na,K-ATPase activity are observed in hypoxic heart of two subterranean hypoxia-tolerant blind mole species (Spalax galili and Spalax judaei). In both hypoxia-tolerant Spalax species activity of the enzyme and tissue redox state were maintained under hypoxic conditions. However, localization of cysteines within the catalytic subunit of the Na,K-ATPase was preserved and induction of S-glutathionylation by GSSG in tissue homogenate inhibited the Spalax ATPase as efficiently as in rat heart. The obtained data indicate that oxygen-induced regulation of the Na,K-ATPase in the heart is mediated by a switch between S-glutathionylation and S-nitrosylation of the regulatory thiol groups localized at the catalytic subunit of the enzyme.

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.

Phospholemman is a negative feed-forward regulator of Ca2+ in β-adrenergic signaling, accelerating β-adrenergic inotropy

Sympathetic stimulation enhances cardiac contractility by stimulating β-adrenergic signaling and protein kinase A (PKA). Recently, phospholemman (PLM) has emerged as an important PKA substrate capable of regulating cytosolic Ca(2+) transients. However, it remains unclear how PLM contributes to β-adrenergic inotropy. Here we developed a computational model to clarify PLM's role in the β-adrenergic signaling response. Simulating Na(+) and sarcoplasmic reticulum (SR) Ca(2+) clamps, we identify an effect of PLM phosphorylation on SR unloading as the key mechanism by which PLM confers cytosolic Ca(2+) adaptation to long-term β-adrenergic receptor (β-AR) stimulation. Moreover, we show that phospholamban (PLB) opposes and overtakes these actions on SR load, forming a negative feed-forward loop in the β-adrenergic signaling cascade. This network motif dominates the negative feedback conferred by β-AR desensitization and accelerates β-AR-induced inotropy. Model analysis therefore unmasks key actions of PLM phosphorylation during β-adrenergic signaling, indicating that PLM is a critical component of the fight-or-flight response.

Residues 248-252 and 300-304 of the cardiac Na+/Ca2+ exchanger are involved in its regulation by phospholemman

Using split cardiac Na(+)/Ca(2+) exchangers (NCX1), we previously demonstrated that phospholemman (PLM) regulates NCX1 by interacting with the proximal linker domain (residues 218-358) of the intracellular loop of NCX1. With the use of overlapping loop deletion mutants, interaction sites are localized to two regions spanning residues 238-270 and residues 300-328 of NCX1. In this study, we used alanine (Ala) linker scanning to pinpoint the residues in the proximal linker domain involved in regulation of NCX1 by PLM. Transfection of human embryonic kidney (HEK)293 cells with wild-type (WT) NCX1 or its Ala mutants but not empty vector resulted in NCX1 current (I(NaCa)). Coexpression of PLM with WT NCX1 inhibited I(NaCa). Mutating residues 248-252 (PASKT) or 300-304 (QKHPD) in WT NCX1 to Ala resulted in loss of inhibition of I(NaCa) by PLM. By contrast, inhibition of I(NaCa) by PLM was preserved when residues 238-242, 243-247, 253-257, 258-262, 263-267, 305-309, 310-314, 315-319, 320-324, or 325-329 were mutated to Ala. While mutating residue 301 to alanine completely abolished PLM inhibition, mutation of any single residue 250-252, 300, or 302-304 resulted in partial reduction in inhibition. Mutating residues 248-252 to Ala resulted in significantly weaker association with PLM. The NCX1-G503P mutant that lacks Ca(2+)-dependent activation retained its sensitivity to PLM. We conclude that residues 248-252 and 300-304 in the proximal linker domain of NCX1 were involved in its inhibition by PLM.

Effect of exercise and training on phospholemman phosphorylation in human skeletal muscle

Phospholemman (PLM, FXYD1) is a partner protein and regulator of the Na(+)-K(+)-ATPase (Na(+)-K(+) pump). We explored the impact of acute and short-term training exercise on PLM physiology in human skeletal muscle. A group of moderately trained males (n = 8) performed a 1-h acute bout of exercise by utilizing a one-legged cycling protocol. Muscle biopsies were taken from vastus lateralis at 0 and 63 min (non-exercised leg) and 30 and 60 min (exercised leg). In a group of sedentary males (n = 9), we determined the effect of a 10-day intense aerobic cycle training on Na(+)-K(+)-ATPase subunit expression, PLM phosphorylation, and total PLM expression as well as PLM phosphorylation in response to acute exercise (1 h at ∼72% Vo(2peak)). Biopsies were taken at rest, immediately following, and 3 h after an acute exercise bout before and at the conclusion of the 10-day training study. PLM phosphorylation was increased both at Ser(63) and Ser(68) immediately after acute exercise (75%, P < 0.05, and 30%, P < 0.05, respectively). Short-term training had no adaptive effect on PLM phosphorylation at Ser(63) and Ser(68), nor was the total amount of PLM altered posttraining. The protein expressions of α(1)-, α(2)-,and β(1)-subunits of Na(+)-K(+)-ATPase were increased after training (113%, P < 0.05, 49%, P < 0.05, and 27%, P < 0.05, respectively). Whereas an acute bout of exercise increased the phosphorylation of PKCα/βII on Thr(638/641) pre- and posttraining, phosphorylation of PKCζ/λ on Thr(403/410) was increased in response to acute exercise only after the 10-day training. In conclusion, we show that only acute exercise, and not short-term training, increases phosphorylation of PLM on Ser(63) and Ser(68), and data from one-legged cycling indicate that this effect of exercise on PLM phosphorylation is not due to systemic factors. Our results provide evidence that phosphorylation of PLM may play a role in the acute regulation of the Na(+)-K(+)-ATPase response to exercise.

The inhibitory effect of phospholemman on the sodium pump requires its palmitoylation

Phospholemman (PLM), the principal sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. We investigated post-translational modifications of PLM additional to phosphorylation in adult rat ventricular myocytes (ARVM). LC-MS/MS of tryptically digested PLM immunoprecipitated from ARVM identified cysteine 40 as palmitoylated in some peptides, but no information was obtained regarding the palmitoylation status of cysteine 42. PLM palmitoylation was confirmed by immunoprecipitating PLM from ARVM loaded with [(3)H]palmitic acid and immunoblotting following streptavidin affinity purification from ARVM lysates subjected to fatty acyl biotin exchange. Mutagenesis identified both Cys-40 and Cys-42 of PLM as palmitoylated. Phosphorylation of PLM at serine 68 by PKA in ARVM or transiently transfected HEK cells increased its palmitoylation, but PKA activation did not increase the palmitoylation of S68A PLM-YFP in HEK cells. Wild type and unpalmitoylatable PLM-YFP were all correctly targeted to the cell surface membrane, but the half-life of unpalmitoylatable PLM was reduced compared with wild type. In cells stably expressing inducible PLM, PLM expression inhibited the sodium pump, but PLM did not inhibit the sodium pump when palmitoylation was inhibited. Hence, palmitoylation of PLM controls its turnover, and palmitoylated PLM inhibits the sodium pump. Surprisingly, phosphorylation of PLM enhances its palmitoylation, probably through the enhanced mobility of the phosphorylated intracellular domain increasing the accessibility of cysteines for the palmitoylating enzyme, with interesting theoretical implications. All FXYD proteins have conserved intracellular cysteines, so FXYD protein palmitoylation may be a universal means to regulate the sodium pump.

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

Cardiac Na/K-ATPase (NKA) is regulated by its accessory protein phospholemman (PLM). Whereas kinase-induced PLM phosphorylation has been shown to mediate NKA stimulation, the role of endogenous phosphatases is presently unknown. We investigated the role of protein phosphatase-1 (PP-1) on PLM phosphorylation and NKA activity in rat cardiomyocytes and failing human hearts. Incubation of rat cardiomyocytes with the chemical PP-1/PP-2A inhibitor okadaic acid or the specific PP-1-inhibitor peptide (I-1ct) identified PLM phosphorylation at Ser-68 as the main substrate for PP-1. Moreover, myocytes adenovirally overexpressing PP-1 inhibitor-1 protein (I-1,Ad-I-1/eGFP) showed a 70% increase in PLM Ser-68 phosphorylation and 65% increase in NKA current, compared with enhanced green fluorescence protein (eGFP)-infected controls (Ad-eGFP), using Western blotting and voltage clamping, respectively. Notably, in left ventricular myocardium from patients with heart failure, PLM Ser-68 phosphorylation was ≈ 50% lower (n=7) than in nonfailing controls (n=7). We provide the first physiological and biochemical evidence that PLM phosphorylation and cardiac Na/K-ATPase activity are negatively regulated by PP-1 and that this regulatory mechanism could be counteracted by I-1. This novel mechanism is markedly perturbed in failing hearts favoring PLM dephosphorylation and NKA deactivation and thus may contribute to maladaptive hypertrophy and arrhythmogenesis via chronically higher intracellular Na and Ca concentrations.

Phospholemman: a novel cardiac stress protein

Phospholemman (PLM), a member of the FXYD family of regulators of ion transport, is a major sarcolemmal substrate for protein kinases A and C in cardiac and skeletal muscle. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. Functionally, when phosphorylated at serine(68), PLM stimulates Na(+)-K(+)-ATPase but inhibits Na(+)/Ca(2+) exchanger in cardiac myocytes. In heterologous expression systems, PLM modulates the gating of cardiac L-type Ca(2+) channel. Therefore, PLM occupies a key modulatory role in intracellular Na(+) and Ca(2+) homeostasis and is intimately involved in regulation of excitation-contraction (EC) coupling. Genetic ablation of PLM results in a slight increase in baseline cardiac contractility and prolongation of action potential duration. When hearts are subjected to catecholamine stress, PLM minimizes the risks of arrhythmogenesis by reducing Na(+) overload and simultaneously preserves inotropy by inhibiting Na(+)/Ca(2+) exchanger. In heart failure, both expression and phosphorylation state of PLM are altered and may partly account for abnormalities in EC coupling. The unique role of PLM in regulation of Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and potentially L-type Ca(2+) channel in the heart, together with the changes in its expression and phosphorylation in heart failure, make PLM a rational and novel target for development of drugs in our armamentarium against heart failure. Clin Trans Sci 2010; Volume 3: 189-196.

FXYD-11 associates with Na+-K+-ATPase in the gill of Atlantic salmon: regulation and localization in relation to changed ion-regulatory status

The Na+-K+-ATPase is the primary electrogenic component driving transepithelial ion transport in the teleost gill; thus regulation of its level of activity is of critical importance for osmotic homeostasis. In the present study, we examined the dynamics of the gill-specific FXYD-11 protein, a putative regulatory subunit of the pump, in Atlantic salmon during seawater (SW) acclimation, smoltification, and treatment with cortisol, growth hormone, and prolactin. Dual-labeling immunohistochemistry showed that branchial FXYD-11 is localized in Na+-K+-ATPase immunoreactive cells, and coimmunoprecipitation experiments confirmed a direct association between FXYD-11 and the Na+-K+-ATPase α-subunit. Transfer of freshwater (FW)-acclimated salmon to SW induced a parallel increase in total α-subunit and FXYD-11 protein expression. A similar concurrent increase was seen during smoltification in FW. In FW fish, cortisol induced an increase in both α-subunit and FXYD-11 abundance, and growth hormone further stimulated FXYD-11 levels. In SW fish, prolactin induced a decrease in FXYD-11 and α-subunit protein levels. In vitro cortisol (18 h, 10 μg/ml) stimulated FXYD-11, but not FXYD-9, mRNA levels in gills from FW and SW salmon. The data show that Na+-K+-ATPase expressed in branchial mitochondrion-rich cells is accompanied by FXYD-11, and that regulation of the two proteins is highly coordinated. The demonstrated association of FXYD-11 and α-subunit strengthens our hypothesis that FXYD-11 has a role in modulating the pump's kinetic properties. The presence of putative phosphorylation sites on the intracellular domain of FXYD-11 suggests the possibility that this protein also may transmit external signals that regulate Na+-K+-ATPase activity.

Phospholemman modulates the gating of cardiac L-type calcium channels

Ca(2+) entry through L-type calcium channels (Ca(V)1.2) is critical in shaping the cardiac action potential and initiating cardiac contraction. Modulation of Ca(V)1.2 channel gating directly affects myocyte excitability and cardiac function. We have found that phospholemman (PLM), a member of the FXYD family and regulator of cardiac ion transport, coimmunoprecipitates with Ca(V)1.2 channels from guinea pig myocytes, which suggests PLM is an endogenous modulator. Cotransfection of PLM in HEK293 cells slowed Ca(V)1.2 current activation at voltages near the threshold for activation, slowed deactivation after long and strong depolarizing steps, enhanced the rate and magnitude of voltage-dependent inactivation (VDI), and slowed recovery from inactivation. However, Ca(2+)-dependent inactivation was not affected. Consistent with slower channel closing, PLM significantly increased Ca(2+) influx via Ca(V)1.2 channels during the repolarization phase of a human cardiac action potential waveform. Our results support PLM as an endogenous regulator of Ca(V)1.2 channel gating. The enhanced VDI induced by PLM may help protect the heart under conditions such as ischemia or tachycardia where the channels are depolarized for prolonged periods of time and could induce Ca(2+) overload. The time and voltage-dependent slowed deactivation could represent a gating shift that helps maintain Ca(2+) influx during the cardiac action potential waveform plateau phase.

Role of phospholemman phosphorylation sites in mediating kinase-dependent regulation of the Na+-K+-ATPase

Phospholemman (PLM) is a major target for phosphorylation mediated by both PKA (at Ser68) and PKC (at both Ser63 and Ser68) in the heart. In intact cardiac myocytes, PLM associates with and inhibits Na(+)-K(+)-ATPase (NKA), mainly by reducing its affinity for internal Na(+). The inhibition is relieved upon PLM phosphorylation by PKA or PKC. The aim here was to distinguish the role of the Ser63 and Ser68 PLM phosphorylation sites in mediating kinase-induced modulation of NKA function. We expressed wild-type (WT) PLM and S63A, S68A, and AA (Ser63 and Ser68 to alanine double mutant) PLM mutants in HeLa cells that stably express rat NKA-α(1) and we measured the effect of PKA and PKC activation on NKA-mediated intracellular Na(+) concentration decline. PLM expression (WT or mutant) significantly decreased the apparent NKA affinity for internal Na(+) and had no significant effect on the maximum pump rate (V(max)). PKA activation with forskolin (20 μM) restored NKA Na(+) affinity in cells expressing WT but not AA PLM and did not affect V(max) in either case. Similarly, PKC activation with 300 nM phorbol 12,13-dibutyrate increased NKA Na(+) affinity in cells expressing WT, S63A, and S68A PLM and had no effect in cells expressing AA PLM. Neither forskolin nor phorbol 12,13-dibutyrate affected NKA function in the absence of PLM. We conclude that PLM phosphorylation at either Ser63 or Ser68 is both necessary and sufficient for completely relieving the PLM-induced NKA inhibition.

Molecular remodeling of ion channels, exchangers and pumps in atrial and ventricular myocytes in ischemic cardiomyopathy

Existing molecular knowledge base of cardiovascular diseases is rudimentary because of lack of specific attribution to cell type and function. The aim of this study was to investigate cell-specific molecular remodeling in human atrial and ventricular myocytes associated with ischemic cardiomyopathy. Our strategy combines two technological innovations, laser-capture microdissection of identified cardiac cells in selected anatomical regions of the heart and splice microarray of a narrow catalog of the functionally most important genes regulating ion homeostasis. We focused on expression of a principal family of genes coding for ion channels, exchangers and pumps (CE&P genes) that are involved in electrical, mechanical and signaling functions of the heart and constitute the most utilized drug targets. We found that (1) CE&P genes remodel in a cell-specific manner: ischemic cardiomyopathy affected 63 CE&P genes in ventricular myocytes and 12 essentially different genes in atrial myocytes. (2) Only few of the identified CE&P genes were previously linked to human cardiac disfunctions. (3) The ischemia-affected CE&P genes include nuclear chloride channels, adrenoceptors, cyclic nucleotide-gated channels, auxiliary subunits of Na(+), K(+) and Ca(2+) channels, and cell-surface CE&Ps. (4) In both atrial and ventricular myocytes ischemic cardiomyopathy reduced expression of CACNG7 and induced overexpression of FXYD1, the gene crucial for Na(+) and K(+) homeostasis. Thus, our cell-specific molecular profiling defined new landmarks for correct molecular modeling of ischemic cardiomyopathy and development of underlying targeted therapies.

Na/K-ATPase--an integral player in the adrenergic fight-or-flight response

During activation of the sympathetic nervous system, cardiac performance is increased as part of the fight-or-flight stress response. The increase in contractility with sympathetic stimulation is an orchestrated combination of intrinsic inotropic, lusitropic, and chronotropic effects, mediated in part by activation of beta-adrenergic receptors and protein kinase A. This causes phosphorylation of several Ca cycling proteins in cardiac myocytes (increasing Ca entry via L-type Ca channels, sarcoplasmic reticulum Ca pumping, and the dissociation rate of Ca from the myofilaments). Here, we discuss how stimulation of the Na/K-ATPase, mediated by phosphorylation of phospholemman (a small sarcolemmal protein that associates with and modulates Na/K-ATPase), is an additional important player in the sympathetic fight-or-flight response. Enhancement of Na/K- ATPase activity limits the rise in [Na](i) caused by the higher level of Na influx and by doing so limits the rise in cellular and sarcoplasmic reticulum Ca load by favoring Ca extrusion via the Na/Ca exchanger. Thus, phospholemman-mediated activation of the Na/K-ATPase may prevent Ca overload and triggered arrhythmias during stress.

Na+ transport in cardiac myocytes; Implications for excitation-contraction coupling

Intracellular Na(+) concentration ([Na(+)](i)) is very important in modulating the contractile and electrical activity of the heart. Upon electrical excitation of the myocardium, voltage-dependent Na(+) channels open, triggering the upstroke of the action potential (AP). During the AP, Ca(2+) enters the myocytes via L-type Ca(2+) channels. This triggers Ca(2+) release from the sarcoplasmic reticulum (SR) and thus activates contraction. Relaxation occurs when cytosolic Ca(2+) declines, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion from the cell via the Na(+)/Ca(2+) exchanger (NCX). NCX extrudes one Ca(2+) ion in exchange for three Na(+) ions and its activity is critically regulated by [Na(+)](i). Thus, via NCX, [Na(+)](i) is centrally involved in the regulation of intracellular [Ca(2+)] and contractility. Na(+) brought in by Na(+) channels, NCX and other Na(+) entry pathways is extruded by the Na(+)/K(+) pump (NKA) to keep [Na(+)](i) low. NKA is regulated by phospholemman, a small sarcolemmal protein that associates with NKA. Unphosphorylated phospholemman inhibits NKA by decreasing the pump affinity for internal Na(+) and this inhibition is relieved upon phosphorylation. Here we discuss the main characteristics of the Na(+) transport pathways in cardiac myocytes and their physiological and pathophysiological relevance.

Isoform specificity of the Na/K-ATPase association and regulation by phospholemman

Phospholemman (PLM) phosphorylation mediates enhanced Na/K-ATPase (NKA) function during adrenergic stimulation of the heart. Multiple NKA isoforms exist, and their function/regulation may differ. We combined fluorescence resonance energy transfer (FRET) and functional measurements to investigate isoform specificity of the NKA-PLM interaction. FRET was measured as the increase in the donor fluorescence (CFP-NKA-alpha1 or CFP-NKA-alpha2) during progressive acceptor (PLM-YFP) photobleach in HEK-293 cells. Both pairs exhibited robust FRET (maximum of 23.6 +/- 3.4% for NKA-alpha1 and 27.5 +/- 2.5% for NKA-alpha2). Donor fluorescence depended linearly on acceptor fluorescence, indicating a 1:1 PLM:NKA stoichiometry for both isoforms. PLM phosphorylation induced by cAMP-dependent protein kinase and protein kinase C activation drastically reduced the FRET with both NKA isoforms. However, submaximal cAMP-dependent protein kinase activation had less effect on PLM-NKA-alpha2 versus PLM-NKA-alpha1. Surprisingly, ouabain virtually abolished NKA-PLM FRET but only partially reduced co-immunoprecipitation. PLM-CFP also showed FRET to PLM-YFP, but the relationship during progressive photobleach was highly nonlinear, indicating oligomers involving >or=3 monomers. Using cardiac myocytes from wild-type mice and mice where NKA-alpha1 is ouabain-sensitive and NKA-alpha2 is ouabain-resistant, we assessed the effects of PLM phosphorylation on NKA-alpha1 and NKA-alpha2 function. Isoproterenol enhanced internal Na(+) affinity of both isoforms (K((1/2)) decreased from 18.1 +/- 2.0 to 11.5 +/- 1.9 mm for NKA-alpha1 and from 16.4 +/- 2.5 to 10.4 +/- 1.5 mm for NKA-alpha2) without altering maximum transport rate (V(max)). Protein kinase C activation also decreased K((1/2)) for both NKA-alpha1 and NKA-alpha2 (to 9.4 +/- 1.0 and 9.1 +/- 1.1 mm, respectively) but increased V(max) only for NKA-alpha2 (1.9 +/- 0.4 versus 1.2 +/- 0.5 mm/min). In conclusion, PLM associates with and modulates both NKA-alpha1 and NKA-alpha2 in a comparable but not identical manner.

Agrin regulation of alpha3 sodium-potassium ATPase activity modulates cardiac myocyte contraction

Drugs that inhibit Na,K-ATPases, such as digoxin and ouabain, alter cardiac myocyte contractility. We recently demonstrated that agrin, a protein first identified at the vertebrate neuromuscular junction, binds to and regulates the activity of alpha3 subunit-containing isoforms of the Na,K-ATPase in the mammalian brain. Both agrin and the alpha3 Na,K-ATPase are expressed in heart, but their potential for interaction and effect on cardiac myocyte function was unknown. Here we show that agrin binds to the alpha3 subunit of the Na,K-ATPase in cardiac myocyte membranes, inducing tyrosine phosphorylation and inhibiting activity of the pump. Agrin also triggers a rapid increase in cytoplasmic Na(+) in cardiac myocytes, suggesting a role in cardiac myocyte function. Consistent with this hypothesis, spontaneous contraction frequencies of cultured cardiac myocytes prepared from mice in which agrin expression is blocked by mutation of the Agrn gene are significantly higher than in the wild type. The Agrn mutant phenotype is rescued by acute treatment with recombinant agrin. Furthermore, exposure of wild type myocytes to an agrin antagonist phenocopies the Agrn mutation. These data demonstrate that the basal frequency of myocyte contraction depends on endogenous agrin-alpha3 Na,K-ATPase interaction and suggest that agrin modulation of the alpha3 Na,K-ATPase is important in regulating heart function.

FXYD1 phosphorylation in vitro and in adult rat cardiac myocytes: threonine 69 is a novel substrate for protein kinase C

FXYD1 (phospholemman), the primary sarcolemmal kinase substrate in the heart, is a regulator of the cardiac sodium pump. We investigated phosphorylation of FXYD1 peptides by purified kinases using HPLC, mass spectrometry, and Edman sequencing, and FXYD1 phosphorylation in cultured adult rat ventricular myocytes treated with PKA and PKC agonists by phosphospecific immunoblotting. PKA phosphorylates serines 63 and 68 (S63 and S68) and PKC phosphorylates S63, S68, and a new site, threonine 69 (T69). In unstimulated myocytes, FXYD1 is approximately 30% phosphorylated at S63 and S68, but barely phosphorylated at T69. S63 and S68 are rapidly dephosphorylated following acute inhibition of PKC in unstimulated cells. Receptor-mediated PKC activation causes sustained phosphorylation of S63 and S68, but transient phosphorylation of T69. To characterize the effect of T69 phosphorylation on sodium pump function, we measured pump currents using whole cell voltage clamping of cultured adult rat ventricular myocytes with 50 mM sodium in the patch pipette. Activation of PKA or PKC increased pump currents (from 2.1 +/- 0.2 pA/pF in unstimulated cells to 2.9 +/- 0.1 pA/pF for PKA and 3.4 +/- 0.2 pA/pF for PKC). Following kinase activation, phosphorylated FXYD1 was coimmunoprecipitated with sodium pump alpha(1)-subunit. We conclude that T69 is a previously undescribed phosphorylation site in FXYD1. Acute T69 phosphorylation elicits stimulation of the sodium pump additional to that induced by S63 and S68 phosphorylation.

Regulation of intracellular and mitochondrial sodium in health and disease

The transmembrane sodium gradient is essential for both excitability of the cardiac cell and the regulation of the cytoplasmic concentrations of Ca and protons. In addition, movements of Na across the mitochondrial membrane affect matrix protons and calcium. In the first part of the review, we discuss the most important pathways responsible for sarcolemmal and mitochondrial sodium movements. The bulk of the review considers the changes of intracellular Na concentration ([Na(+)](i)) that occur in disease, specifically, ischemia, reperfusion, and heart failure. We review evidence implicating the increase of intracellular sodium to either increased influx of sodium (via either sodium channels or sodium/hydrogen exchange) or, alternatively, to decreased efflux on the Na/K pump. Although much has been learned about sodium regulation in the heart, there are still many unanswered questions, particularly concerning mitochondrial Na regulation.