Protein phosphatase 2A regulatory subunit B56α limits phosphatase activity in the heart

Alternative Splicing Analysis Reveals Adrenergic Signaling as a Novel Target for Protein Arginine Methyltransferase 5 (PRMT5) in the Heart

Adrenergic signaling is critical for maintaining cardiac function and works by regulating heart rate, contractility, and stress responses. Protein arginine methyltransferase 5 (PRMT5), a key enzyme involved in gene expression, signal transduction, and RNA processing, has been revealed to be an important factor in heart disease. However, its specific effects on adrenergic signaling have not been fully elucidated. In this study, we examined the role of PRMT5 in the heart by analyzing alternative splicing events in cardiac tissues from Prmt5-deficient mice. High-throughput RNA sequencing and bioinformatics analyses identified significant alterations in alternative splicing, particularly in genes related to adrenergic signaling, which were further validated using reverse transcription PCR. These results underscore the role of PRMT5 as an important regulator of alternative splicing in the heart and identify adrenergic signaling as a novel target. Collectively, our findings offer new insights into the molecular mechanisms underlying cardiac function and suggest that PRMT5 is a potential therapeutic target for heart diseases.

Insights into the post-translational modifications in heart failure

Heart failure (HF), as the terminal manifestation of multiple cardiovascular diseases, causes a huge socioeconomic burden worldwide. Despite the advances in drugs and medical-assisted devices, the prognosis of HF remains poor. HF is well-accepted as a myriad of subcellular dys-synchrony related to detrimental structural and functional remodelling of cardiac components, including cardiomyocytes, fibroblasts, endothelial cells and macrophages. Through the covalent chemical process, post-translational modifications (PTMs) can coordinate protein functions, such as re-localizing cellular proteins, marking proteins for degradation, inducing interactions with other proteins and tuning enzyme activities, to participate in the progress of HF. Phosphorylation, acetylation, and ubiquitination predominate in the currently reported PTMs. In addition, advanced HF is commonly accompanied by metabolic remodelling including enhanced glycolysis. Thus, glycosylation induced by disturbed energy supply is also important. In this review, firstly, we addressed the main types of HF. Then, considering that PTMs are associated with subcellular locations, we summarized the leading regulation mechanisms in organelles of distinctive cell types of different types of HF, respectively. Subsequently, we outlined the aforementioned four PTMs of key proteins and signaling sites in HF. Finally, we discussed the perspectives of PTMs for potential therapeutic targets in HF.

Loss of protein phosphatase 2A regulatory subunit PPP2R5A is associated with increased incidence of stress-induced proarrhythmia

Background

Protein phosphatase 2A (PP2A) is a serine/threonine-selective holoenzyme that controls Ca2+ homeostasis and contractility of the heart via dephosphorylation of regulatory proteins. In some genetically modified mouse models with increased arrhythmogenicity, a reduced expression of the regulatory subunit B56α of PP2A was found as a concomitant effect. Whether there is a general correlation between the abundance of B56α and the promotion of cardiac arrhythmogenesis remains unclear.

Methods

The aim of this study was therefore to investigate the role of PP2A-B56α in the propensity for arrhythmic activity in the heart. The experimental analysis of this question has been addressed by using a mouse model with deletion of the PP2A-B56α gene, PPP2R5A (KO), in comparison to wild-type animals (WT). Evidence for arrhythmogenicity was investigated in whole animal, isolated heart and cardiomyocytes by ECG, recording of monophasic action potential (MAP) induced by programmed electrical stimulation (PES), measurement of Ca2+ transients under increased pacing frequencies and determination of total K+ channel currents (I K).

Results

ECG measurements showed a prolongation of QT time in KO vs. WT. KO mice exhibited a higher rate of premature ventricular contractions in the ECG. MAP measurements in Langendorff-perfused KO hearts showed increased episodes of ventricular tachyarrhythmia induced by PES. However, the KO hearts showed values for MAP duration that were similar to those in WT hearts. In contrast, KO showed more myocardial cells with spontaneous arrhythmogenic Ca2+ transient events compared to WT. The whole-cell patch-clamp technique applied to ventricular cardiomyocytes revealed comparable peak potassium channel current densities between KO and WT.

Conclusion

These findings support the assumption that a decrease or even the loss of PP2A-B56α leads to an increased propensity of triggered arrhythmias. This could be based on the increased spontaneous Ca2+ tansients observed.

Pleiotropy of PP2A Phosphatases in Cancer with a Focus on Glioblastoma IDH Wildtype

Serine/Threonine protein phosphatase 2A (PP2A) is a heterotrimeric (or occasionally, heterodimeric) phosphatase with pleiotropic functions and ubiquitous expression. Despite the fact that they all contribute to protein dephosphorylation, multiple PP2A complexes exist which differ considerably by their subcellular localization and their substrate specificity, suggesting diverse PP2A functions. PP2A complex formation is tightly regulated by means of gene expression regulation by transcription factors, microRNAs, and post-translational modifications. Furthermore, a constant competition between PP2A regulatory subunits is taking place dynamically and depending on the spatiotemporal circumstance; many of the integral subunits can outcompete the rest, subjecting them to proteolysis. PP2A modulation is especially important in the context of brain tumors due to its ability to modulate distinct glioma-promoting signal transduction pathways, such as PI3K/Akt, Wnt, Ras, NF-κb, etc. Furthermore, PP2A is also implicated in DNA repair and survival pathways that are activated upon treatment of glioma cells with chemo-radiation. Depending on the cancer cell type, preclinical studies have shown some promise in utilising PP2A activator or PP2A inhibitors to overcome therapy resistance. This review has a special focus on "glioblastoma, IDH wild-type" (GBM) tumors, for which the therapy options have limited efficacy, and tumor relapse is inevitable.

Impaired myocellular Ca2+ cycling in protein phosphatase PP2A-B56α knockout mice is normalized by β-adrenergic stimulation

The activity of protein phosphatase 2A (PP2A) is determined by the expression and localization of the regulatory B-subunits. PP2A-B56α is the dominant isoform of the B′-family in the heart. Its role in regulating the cardiac response to β-adrenergic stimulation is not yet fully understood. We therefore generated mice deficient in B56α to test the functional cardiac effects in response to catecholamine administration versus corresponding WT mice. We found the decrease in basal PP2A activity in hearts of KO mice was accompanied by a counter-regulatory increase in the expression of B′ subunits (β and γ) and higher phosphorylation of sarcoplasmic reticulum Ca²⁺ regulatory and myofilament proteins. The higher phosphorylation levels were associated with enhanced intraventricular pressure and relaxation in catheterized KO mice. In contrast, at the cellular level, we detected depressed Ca²⁺ transient and sarcomere shortening parameters in KO mice at basal conditions. Consistently, the peak amplitude of the L-type Ca²⁺ current was reduced and the inactivation kinetics of I_CaL were prolonged in KO cardiomyocytes. However, we show β-adrenergic stimulation resulted in a comparable peak amplitude of Ca²⁺ transients and myocellular contraction between KO and WT cardiomyocytes. Therefore, we propose higher isoprenaline-induced Ca²⁺ spark frequencies might facilitate the normalized Ca²⁺ signaling in KO cardiomyocytes. In addition, the application of isoprenaline was associated with unchanged L-type Ca²⁺ current parameters between both groups. Our data suggest an important influence of PP2A-B56α on the regulation of Ca²⁺ signaling and contractility in response to β-adrenergic stimulation in the myocardium.

Mechanisms underlying the role of ankyrin-B in cardiac and neurological health and disease

The ANK2 gene encodes for ankyrin-B (ANKB), one of 3 members of the ankyrin family of proteins, whose name is derived from the Greek word for anchor. ANKB was originally identified in the brain (B denotes “brain”) but has become most widely known for its role in cardiomyocytes as a scaffolding protein for ion channels and transporters, as well as an interacting protein for structural and signaling proteins. Certain loss-of-function ANK2 variants are associated with a primarily cardiac-presenting autosomal-dominant condition with incomplete penetrance and variable expressivity characterized by a predisposition to supraventricular and ventricular arrhythmias, arrhythmogenic cardiomyopathy, congenital and adult-onset structural heart disease, and sudden death. Another independent group of ANK2 variants are associated with increased risk for distinct neurological phenotypes, including epilepsy and autism spectrum disorders. The mechanisms underlying ANKB's roles in cells in health and disease are not fully understood; however, several clues from a range of molecular and cell biological studies have emerged. Notably, ANKB exhibits several isoforms that have different cell-type–, tissue–, and developmental stage– expression profiles. Given the conservation within ankyrins across evolution, model organism studies have enabled the discovery of several ankyrin roles that could shed important light on ANKB protein-protein interactions in heart and brain cells related to the regulation of cellular polarity, organization, calcium homeostasis, and glucose and fat metabolism. Along with this accumulation of evidence suggesting a diversity of important ANKB cellular functions, there is an on-going debate on the role of ANKB in disease. We currently have limited understanding of how these cellular functions link to disease risk. To this end, this review will examine evidence for the cellular roles of ANKB and the potential contribution of ANKB functional variants to disease risk and presentation. This contribution will highlight the impact of ANKB dysfunction on cardiac and neuronal cells and the significance of understanding the role of ANKB variants in disease.

Targeting calcium-mediated inter-organellar crosstalk in cardiac diseases

INTRODUCTION

Abnormal calcium signaling between organelles such as the sarcoplasmic reticulum (SR), mitochondria and lysosomes is a key feature of heart diseases. Calcium serves as a secondary messenger mediating inter-organellar crosstalk, essential for maintaining the cardiomyocyte function.

AREAS COVERED

This article examines the available literature related to calcium channels and transporters involved in inter-organellar calcium signaling. The SR calcium-release channels ryanodine receptor type-2 (RyR2) and inositol 1,4,5-trisphosphate receptor (IP3R), and calcium-transporter SR/ER-ATPase 2a (SERCA2a) are illuminated. The roles of mitochondrial voltage-dependent anion channels (VDAC), the mitochondria Ca2+ uniporter complex (MCUC), and the lysosomal H+/Ca2+ exchanger, two pore channels (TPC), and transient receptor potential mucolipin (TRPML) are discussed. Furthermore, recent studies showing calcium-mediated crosstalk between the SR, mitochondria, and lysosomes as well as how this crosstalk is dysregulated in cardiac diseases are placed under the spotlight.

EXPERT OPINION

Enhanced SR calcium release via RyR2 and reduced SR reuptake via SERCA2a, increased VDAC and MCUC-mediated calcium uptake into mitochondria, and enhanced lysosomal calcium-release via lysosomal TPC and TRPML may all contribute to aberrant calcium homeostasis causing heart disease. While mechanisms of this crosstalk need to be studied further, interventions targeting these calcium channels or combinations thereof might represent a promising therapeutic strategy.

Protein phosphatase 2A in the healthy and failing heart: New insights and therapeutic opportunities

Protein phosphatases have emerged as critical regulators of phosphoprotein homeostasis in settings of health and disease. Protein phosphatase 2A (PP2A) encompasses a large subfamily of enzymes that remove phosphate groups from serine/threonine residues within phosphoproteins. The heterogeneity in PP2A structure, which arises from the grouping of different catalytic, scaffolding and regulatory subunit isoforms, creates distinct populations of catalytically active enzymes (i.e. holoenzymes) that localise to different parts of the cell. This structural complexity, combined with other regulatory mechanisms, such as interaction of PP2A heterotrimers with accessory proteins and post-translational modification of the catalytic and/or regulatory subunits, enables PP2A holoenzymes to target phosphoprotein substrates in a highly specific manner. In this review, we summarise the roles of PP2A in cardiac physiology and disease. PP2A modulates numerous processes that are vital for heart function including calcium handling, contractility, β-adrenergic signalling, metabolism and transcription. Dysregulation of PP2A has been observed in human cardiac disease settings, including heart failure and atrial fibrillation. Efforts are underway, particularly in the cancer field, to develop therapeutics targeting PP2A activity. The development of small molecule activators of PP2A (SMAPs) and other compounds that selectively target specific PP2A holoenzymes (e.g. PP2A/B56α and PP2A/B56ε) will improve understanding of the function of different PP2A species in the heart, and may lead to the development of therapeutics for normalising aberrant protein phosphorylation in settings of cardiac remodelling and dysfunction.

Serine-threonine protein phosphatase regulation of Cx43 dephosphorylation in arrhythmogenic disorders

Regulation of cell-to-cell communication in the heart by the gap junction protein Connexin43 (Cx43) involves modulation of Cx43 phosphorylation state by protein kinases, and dephosphorylation by protein phosphatases. Dephosphorylation of Cx43 has been associated with impaired intercellular coupling and enhanced arrhythmogenesis in various pathologic states. While there has been extensive study of the protein kinases acting on Cx43, there has been limited studies of the protein phosphatases that may underlie Cx43 dephosphorylation. The focus of this review is to introduce serine-threonine protein phosphatase regulation of Cx43 phosphorylation state and cell-to-cell communication, and its impact on arrhythmogenesis in the setting of chronic heart failure and myocardial ischemia, as well as on atrial fibrillation. We also discuss the therapeutic potential of modulating protein phosphatases to treat arrhythmias in these clinical settings.

Protein Phosphatase 2A (PP2A) mutations in brain function, development, and neurologic disease

By removing Ser/Thr-specific phosphorylations in a multitude of protein substrates in diverse tissues, Protein Phosphatase type 2A (PP2A) enzymes play essential regulatory roles in cellular signalling and physiology, including in brain function and development. Here, we review current knowledge on PP2A gene mutations causally involved in neurodevelopmental disorders and intellectual disability, focusing on PPP2CA, PPP2R1A and PPP2R5D. We provide insights into the impact of these mutations on PP2A structure, substrate specificity and potential function in neurobiology and brain development.

Regulation of Cardiac PKA Signaling by cAMP and Oxidants

Pathologies, such as cancer, inflammatory and cardiac diseases are commonly associated with long-term increased production and release of reactive oxygen species referred to as oxidative stress. Thereby, protein oxidation conveys protein dysfunction and contributes to disease progression. Importantly, trials to scavenge oxidants by systemic antioxidant therapy failed. This observation supports the notion that oxidants are indispensable physiological signaling molecules that induce oxidative post-translational modifications in target proteins. In cardiac myocytes, the main driver of cardiac contractility is the activation of the β-adrenoceptor-signaling cascade leading to increased cellular cAMP production and activation of its main effector, the cAMP-dependent protein kinase (PKA). PKA-mediated phosphorylation of substrate proteins that are involved in excitation-contraction coupling are responsible for the observed positive inotropic and lusitropic effects. PKA-actions are counteracted by cellular protein phosphatases (PP) that dephosphorylate substrate proteins and thus allow the termination of PKA-signaling. Both, kinase and phosphatase are redox-sensitive and susceptible to oxidation on critical cysteine residues. Thereby, oxidation of the regulatory PKA and PP subunits is considered to regulate subcellular kinase and phosphatase localization, while intradisulfide formation of the catalytic subunits negatively impacts on catalytic activity with direct consequences on substrate (de)phosphorylation and cardiac contractile function. This review article attempts to incorporate the current perception of the functionally relevant regulation of cardiac contractility by classical cAMP-dependent signaling with the contribution of oxidant modification.

The role of calcium homeostasis remodeling in inherited cardiac arrhythmia syndromes

Sudden cardiac death due to malignant ventricular arrhythmias remains the major cause of mortality in the postindustrial world. Defective intracellular Ca²⁺ homeostasis has been well established as a key contributing factor to the enhanced propensity for arrhythmia in acquired cardiac disease, such as heart failure or diabetic cardiomyopathy. More recent advances provide a strong basis to the emerging view that hereditary cardiac arrhythmia syndromes are accompanied by maladaptive remodeling of Ca²⁺ homeostasis which substantially increases arrhythmic risk. This brief review will focus on functional changes in elements of Ca²⁺ handling machinery in cardiomyocytes that occur secondary to genetic mutations associated with catecholaminergic polymorphic ventricular tachycardia, and long QT syndrome.

Ppp2r2a Knockout Mice Reveal That Protein Phosphatase 2A Regulatory Subunit, PP2A-B55α, Is an Essential Regulator of Neuronal and Epidermal Embryonic Development

The serine/threonine protein phosphatase 2A (PP2A) is a master regulator of the complex cellular signaling that occurs during all stages of mammalian development. PP2A is composed of a catalytic, a structural, and regulatory subunit, for which there are multiple isoforms. The association of specific regulatory subunits determines substrate specificity and localization of phosphatase activity, however, the precise role of each regulatory subunit in development is not known. Here we report the generation of the first knockout mouse for the Ppp2r2a gene, encoding the PP2A-B55α regulatory subunit, using CRISPR/Cas9. Heterozygous animals developed and grew as normal, however, homozygous knockout mice were not viable. Analysis of embryos at different developmental stages found a normal Mendelian ratio of Ppp2r2a-/- embryos at embryonic day (E) 10.5 (25%), but reduced Ppp2r2a-/- embryos at E14.5 (18%), and further reduced at E18.5 (10%). No live Ppp2r2a-/- pups were observed at birth. Ppp2r2a-/- embryos were significantly smaller than wild-type or heterozygous littermates and displayed a variety of neural defects such as exencephaly, spina bifida, and cranial vault collapse, as well as syndactyly and severe epidermal defects; all processes driven by growth and differentiation of the ectoderm. Ppp2r2a-/- embryos had incomplete epidermal barrier acquisition, associated with thin, poorly differentiated stratified epithelium with weak attachment to the underlying dermis. The basal keratinocytes in Ppp2r2a-/- embryos were highly disorganized, with reduced immunolabeling of integrins and basement membrane proteins, suggesting impaired focal adhesion and hemidesmosome assembly. The spinous and granular layers were thinner in the Ppp2r2a-/- embryos, with aberrant expression of adherens and tight junction associated proteins. The overlying stratum corneum was either absent or incomplete. Thus PP2A-B55α is an essential regulator of epidermal stratification, and is essential for ectodermal development during embryogenesis.

Role of type 2A phosphatase regulatory subunit B56α in regulating cardiac responses to β-adrenergic stimulation in vivo

AIMS

B56α is a protein phosphatase 2A (PP2A) regulatory subunit that is highly expressed in the heart. We previously reported that cardiomyocyte B56α localizes to myofilaments under resting conditions and translocates to the cytosol in response to acute β-adrenergic receptor (β-AR) stimulation. Given the importance of reversible protein phosphorylation in modulating cardiac function during sympathetic stimulation, we hypothesized that loss of B56α in mice with targeted disruption of the gene encoding B56α (Ppp2r5a) would impact on cardiac responses to β-AR stimulation in vivo.

METHODS AND RESULTS

Cardiac phenotype of mice heterozygous (HET) or homozygous (HOM) for the disrupted Ppp2r5a allele and wild type (WT) littermates was characterized under basal conditions and following acute β-AR stimulation with dobutamine (DOB; 0.75 mg/kg i.p.) or sustained β-AR stimulation by 2-week infusion of isoproterenol (ISO; 30 mg/kg/day s.c.). Left ventricular (LV) wall thicknesses, chamber dimensions and function were assessed by echocardiography, and heart tissue collected for gravimetric, histological, and biochemical analyses. Western blot analysis revealed partial and complete loss of B56α protein in hearts from HET and HOM mice, respectively, and no changes in the expression of other PP2A regulatory, catalytic or scaffolding subunits. PP2A catalytic activity was reduced in hearts of both HET and HOM mice. There were no differences in the basal cardiac phenotype between genotypes. Acute DOB stimulation induced the expected inotropic response in WT and HET mice, which was attenuated in HOM mice. In contrast, DOB-induced increases in heart rate were unaffected by B56α deficiency. In WT mice, ISO infusion increased LV wall thicknesses, cardiomyocyte area and ventricular mass, without LV dilation, systolic dysfunction, collagen deposition or foetal gene expression. The hypertrophic response to ISO was blunted in mice deficient for B56α.

CONCLUSION

These findings identify B56α as a potential regulator of cardiac structure and function during β-AR stimulation.

The M-band: The underestimated part of the sarcomere

The sarcomere is the basic unit of the myofibrils, which mediate skeletal and cardiac Muscle contraction. Two transverse structures, the Z-disc and the M-band, anchor the thin (actin and associated proteins) and thick (myosin and associated proteins) filaments to the elastic filament system composed of titin. A plethora of proteins are known to be integral or associated proteins of the Z-disc and its structural and signalling role in muscle is better understood, while the molecular constituents of the M-band and its function are less well defined. Evidence discussed here suggests that the M-band is important for managing force imbalances during active muscle contraction. Its molecular composition is fine-tuned, especially as far as the structural linkers encoded by members of the myomesin family are concerned and depends on the specific mechanical characteristics of each particular muscle fibre type. Muscle activity signals from the M-band to the nucleus and affects transcription of sarcomeric genes, especially via serum response factor (SRF). Due to its important role as shock absorber in contracting muscle, the M-band is also more and more recognised as a contributor to muscle disease.

Protein Phosphatase 2A Regulates Cardiac Na+ Channels

RATIONALE

Voltage-gated Na⁺ channel ( I_Na) function is critical for normal cardiac excitability. However, the Na⁺ channel late component ( I_Na,L) is directly associated with potentially fatal forms of congenital and acquired human arrhythmia. CaMKII (Ca²⁺/calmodulin-dependent kinase II) enhances I_Na,L in response to increased adrenergic tone. However, the pathways that negatively regulate the CaMKII/Na_v1.5 axis are unknown and essential for the design of new therapies to regulate the pathogenic I_Na,L.

OBJECTIVE

To define phosphatase pathways that regulate I_Na,L in vivo.

METHODS AND RESULTS

A mouse model lacking a key regulatory subunit (B56α) of the PP (protein phosphatase) 2A holoenzyme displayed aberrant action potentials after adrenergic stimulation. Unbiased computational modeling of B56α KO (knockout) mouse myocyte action potentials revealed an unexpected role of PP2A in I_Na,L regulation that was confirmed by direct I_Na,L recordings from B56α KO myocytes. Further, B56α KO myocytes display decreased sensitivity to isoproterenol-induced induction of arrhythmogenic I_Na,L, and reduced CaMKII-dependent phosphorylation of Na_v1.5. At the molecular level, PP2A/B56α complex was found to localize and coimmunoprecipitate with the primary cardiac Na_v channel, Na_v1.5.

CONCLUSIONS

PP2A regulates Na_v1.5 activity in mouse cardiomyocytes. This regulation is critical for pathogenic Na_v1.5 late current and requires PP2A-B56α. Our study supports B56α as a novel target for the treatment of arrhythmia.

Polycystin-1 Inhibits Cell Proliferation through Phosphatase PP2A/B56α

Autosomal dominant polycystic kidney disease (ADPKD) is associated with a number of cellular defects such as hyperproliferation, apoptosis, and dedifferentiation. Mutations in polycystin-1 (PC1) account for ∼85% of ADPKD. Here, we showed that wild-type (WT) or mutant PC1 composed of the last five transmembrane (TM) domains and the C-terminus (termed PC1-5TMC) inhibits cell proliferation and protein translation, as well as the downstream effectors of mTOR, consistent with previous reports. Knockdown of B56α, a subunit of the protein phosphatase 2A (PP2A) complex, or application of PP2A inhibitor okadaic acid or calyculin A, abolished the inhibitory effect of PC1 and PC1-5TMC on proliferation, indicating that PP2A/B56α mediates the regulation of cell proliferation by PC1. In addition to the phosphorylated S6 and 4EBP1, B56α was also downregulated by PC1 and PC1-5TMC. Furthermore, the downregulation of B56α, which may be mediated by mTOR but not AKT, can account for the dependence of PC1-inhibited proliferation on PP2A.

Ankyrin-B dysfunction predisposes to arrhythmogenic cardiomyopathy and is amenable to therapy

Arrhythmogenic cardiomyopathy (ACM) is an inherited arrhythmia syndrome characterized by severe structural and electrical cardiac phenotypes, including myocardial fibrofatty replacement and sudden cardiac death. Clinical management of ACM is largely palliative, owing to an absence of therapies that target its underlying pathophysiology, which stems partially from our limited insight into the condition. Following identification of deceased ACM probands possessing ANK2 rare variants and evidence of ankyrin-B loss of function on cardiac tissue analysis, an ANK2 mouse model was found to develop dramatic structural abnormalities reflective of human ACM, including biventricular dilation, reduced ejection fraction, cardiac fibrosis, and premature death. Desmosomal structure and function appeared preserved in diseased human and murine specimens in the presence of markedly abnormal β-catenin expression and patterning, leading to identification of a previously unknown interaction between ankyrin-B and β-catenin. A pharmacological activator of the WNT/β-catenin pathway, SB-216763, successfully prevented and partially reversed the murine ACM phenotypes. Our findings introduce what we believe to be a new pathway for ACM, a role of ankyrin-B in cardiac structure and signaling, a molecular link between ankyrin-B and β-catenin, and evidence for targeted activation of the WNT/β-catenin pathway as a potential treatment for this disease.

Hypoxia modulates protein phosphatase 2A through HIF-1α dependent and independent mechanisms in human aortic smooth muscle cells and ventricular cardiomyocytes

BACKGROUND AND PURPOSE

Although protein phosphatases regulate multiple cellular functions, their modulation under hypoxia remains unclear. We investigated expression of the protein phosphatase system under normoxic/hypoxic conditions and the mechanism by which hypoxia alters protein phosphatase 2A (PP2A) activity.

EXPERIMENTAL APPROACH

Human cardiovascular cells were cultured in cell type specific media under normoxic or hypoxic conditions (1% O₂ ). Effects on mRNA expression, phosphatase activity, post-translational modification, and involvement of hypoxia inducible factor 1α (HIF-1α) were assessed using RT-PCR, immunoblotting, an activity assay, and siRNA silencing.

KEY RESULTS

All components of the protein phosphatase system studied were expressed in each cell line. Hypoxia attenuated mRNA expression of the transcripts in a cell line- and time-dependent manner. In human aortic smooth muscle cells (HASMC) and AC16 cells, hypoxia decreased PP2Ac activity and mRNA expression without altering PP2Ac abundance. Hypoxia increased demethylated PP2Ac (DPP2Ac) and phosphatase methylesterase 1 (PME-1) abundance but decreased leucine carboxyl methyltransferase 1 (LCMT-1) abundance. HIF-1α siRNA prevented the hypoxia-mediated decrease in phosphatase activity and expression of the catalytic subunit of protein phosphatase 2A (PPP2CA), independently of altering pPP2Ac, DPP2Ac, LCMT-1, or PME-1 abundance.

CONCLUSION AND IMPLICATIONS

Cardiovascular cells express multiple components of the PP2A system. In HASMC and AC16 cells, hypoxia inhibits PP2A activity through HIF-1α-dependent and -independent mechanisms, with the latter being consistent with altered PP2A holoenzyme assembly. This indicates a complex inhibitory effect of hypoxia on the PP2A system, and highlights PP2A as a therapeutic target for diseases associated with dysregulated protein phosphorylation.

Protein phosphatase 2A Aα regulates Aβ protein expression and stability

Protein phosphatase 2A (PP2A) represses many oncogenic signaling pathways and is an important tumor suppressor. PP2A comprises three distinct subunits and forms through a highly regulated biogenesis process, with the scaffolding A subunit existing as two highly related isoforms, Aα and Aβ. PP2A's tumor-suppressive functions have been intensely studied, and PP2A inactivation has been shown to be a prerequisite for tumor formation. Interestingly, although partial loss of the Aα isoform is growth promoting, complete Aα loss has no transformative properties. Additionally, in cancer patients, Aα is found to be inactivated in a haploinsufficient manner. Using both cellular and in vivo systems, colorectal and endometrial cancer cell lines, and biochemical and cellular assays, here we examined why the complete loss of Aα does not promote tumorigenesis. CRISPR/Cas9-mediated homozygous Aα deletion resulted in decreased colony formation and tumor growth across multiple cell lines. Protein expression analysis of PP2A family members revealed that the Aα deletion markedly up-regulates Aβ protein expression by increasing Aβ protein stability. Aβ knockdown in control and Aα knockout cell lines indicated that Aβ is necessary for cell survival in the Aα knockout cells. In the setting of Aα deficiency, co-immunoprecipitation analysis revealed increased binding of specific PP2A regulatory subunits to Aβ, and knockdown of these regulatory subunits restored colony-forming ability. Taken together, our results uncover a mechanism by which PP2A Aα regulates Aβ protein stability and activity and suggests why homozygous loss of Aα is rarely seen in cancer patients.

Ankyrin-B Q1283H Variant Linked to Arrhythmias Via Loss of Local Protein Phosphatase 2A Activity Causes Ryanodine Receptor Hyperphosphorylation

BACKGROUND

Human loss-of-function variants of ANK2 (ankyrin-B) are linked to arrhythmias and sudden cardiac death. However, their in vivo effects and specific arrhythmogenic pathways have not been fully elucidated.

METHODS

We identified new ANK2 variants in 25 unrelated Han Chinese probands with ventricular tachycardia by whole-exome sequencing. The potential pathogenic variants were validated by Sanger sequencing. We performed functional and mechanistic experiments in ankyrin-B knockin (KI) mouse models and in single myocytes isolated from KI hearts.

RESULTS

We detected a rare, heterozygous ANK2 variant (p.Q1283H) in a proband with recurrent ventricular tachycardia. This variant was localized to the ZU5C region of ANK2, where no variants have been previously reported. KI mice harboring the p.Q1283H variant exhibited an increased predisposition to ventricular arrhythmias after catecholaminergic stress in the absence of cardiac structural abnormalities. Functional studies illustrated an increased frequency of delayed afterdepolarizations and Ca2+ waves and sparks accompanied by decreased sarcoplasmic reticulum Ca2+ content in KI cardiomyocytes on isoproterenol stimulation. The immunoblotting results showed increased levels of phosphorylated ryanodine receptor Ser2814 in the KI hearts, which was further amplified on isoproterenol stimulation. Coimmunoprecipitation experiments demonstrated dissociation of protein phosphatase 2A from ryanodine receptor in the KI hearts, which was accompanied by a decreased binding of ankyrin-B to protein phosphatase 2A regulatory subunit B56α. Finally, the administration of metoprolol or flecainide decreased the incidence of stress-induced ventricular arrhythmias in the KI mice.

CONCLUSIONS

ANK2 p.Q1283H is a disease-associated variant that confers susceptibility to stress-induced arrhythmias, which may be prevented by the administration of metoprolol or flecainide. This variant is associated with the loss of protein phosphatase 2A activity, increased phosphorylation of ryanodine receptor, exaggerated delayed afterdepolarization-mediated trigger activity, and arrhythmogenesis.

Activation of protein phosphatase 1 by a selective phosphatase disrupting peptide reduces sarcoplasmic reticulum Ca2+ leak in human heart failure

BACKGROUND

Disruption of Ca²⁺ homeostasis is a key pathomechanism in heart failure. CaMKII-dependent hyperphosphorylation of ryanodine receptors in the sarcoplasmic reticulum (SR) increases the arrhythmogenic SR Ca²⁺ leak and depletes SR Ca²⁺ stores. The contribution of conversely acting serine/threonine phosphatases [protein phosphatase 1 (PP1) and 2A (PP2A)] is largely unknown.

METHODS AND RESULTS

Human myocardium from three groups of patients was investigated: (i) healthy controls (non-failing, NF, n = 8), (ii) compensated hypertrophy (Hy, n = 16), and (iii) end-stage heart failure (HF, n = 52). Expression of PP1 was unchanged in Hy but greater in HF compared to NF while its endogenous inhibitor-1 (I-1) was markedly lower expressed in both compared to NF, suggesting increased total PP1 activity. In contrast, PP2A expression was lower in Hy and HF compared to NF. Ca²⁺ homeostasis was severely disturbed in HF compared to Hy signified by a higher SR Ca²⁺ leak, lower systolic Ca²⁺ transients as well as a decreased SR Ca²⁺ load. Inhibition of PP1/PP2A by okadaic acid increased SR Ca²⁺ load and systolic Ca²⁺ transients but severely aggravated diastolic SR Ca²⁺ leak and cellular arrhythmias in Hy. Conversely, selective activation of PP1 by a PP1-disrupting peptide (PDP3) in HF potently reduced SR Ca²⁺ leak as well as cellular arrhythmias and, importantly, did not compromise systolic Ca²⁺ release and SR Ca²⁺ load.

CONCLUSION

This study is the first to functionally investigate the role of PP1/PP2A for Ca²⁺ homeostasis in diseased human myocardium. Our data indicate that a modulation of phosphatase activity potently impacts Ca²⁺ cycling properties. An activation of PP1 counteracts increased kinase activity in heart failure and successfully seals the arrhythmogenic SR Ca²⁺ leak. It may thus represent a promising future antiarrhythmic therapeutic approach.

PP2A negatively regulates the hypertrophic response by dephosphorylating HDAC2 S394 in the heart

Cardiac hypertrophy occurs in response to increased hemodynamic demand and can progress to heart failure. Identifying the key regulators of this process is clinically important. Though it is thought that the phosphorylation of histone deacetylase (HDAC) 2 plays a crucial role in the development of pathological cardiac hypertrophy, the detailed mechanism by which this occurs remains unclear. Here, we performed immunoprecipitation and peptide pull-down assays to characterize the functional complex of HDAC2. Protein phosphatase (PP) 2 A was confirmed as a binding partner of HDAC2. PPP2CA, the catalytic subunit of PP2A, bound to HDAC2 and prevented its phosphorylation. Transient overexpression of PPP2CA specifically regulated both the phosphorylation of HDAC2 S394 and hypertrophy-associated HDAC2 activation. HDAC2 S394 phosphorylation was increased in a dose-dependent manner by PP2A inhibitors. Hypertrophic stresses, such as phenylephrine in vitro or pressure overload in vivo, caused PPP2CA to dissociate from HDAC2. Forced expression of PPP2CA negatively regulated the hypertrophic response, but PP2A inhibitors provoked hypertrophy. Adenoviral delivery of a phosphomimic HDAC2 mutant, adenovirus HDAC2 S394E, successfully blocked the anti-hypertrophic effect of adenovirus-PPP2CA, implicating HDAC2 S394 phosphorylation as a critical event for the anti-hypertrophic response. PPP2CA transgenic mice were protected against isoproterenol-induced cardiac hypertrophy and subsequent cardiac fibrosis, whereas simultaneous expression of HDAC2 S394E in the heart did induce hypertrophy. Taken together, our results suggest that PP2A is a critical regulator of HDAC2 activity and pathological cardiac hypertrophy and is a promising target for future therapeutic interventions.

Spectrin-based pathways underlying electrical and mechanical dysfunction in cardiac disease

INTRODUCTION

In the heart, pathways that transduce extracellular environmental cues (e.g. mechanical force, inflammatory stress) into electrical and/or chemical signals at the cellular level are critical for the organ-level response to chronic biomechanical/neurohumoral stress. Specifically, a diverse array of membrane-bound receptors and stretch-activated proteins converge on a network of intracellular signaling cascades that control gene expression, protein translation, degradation and/or regulation. These cellular reprogramming events ultimately lead to changes in cell excitability, growth, proliferation, and/or survival. Areas covered: The actin/spectrin cytoskeleton has emerged as having important roles in not only providing structural support for organelle function but also in serving as a signaling 'superhighway,' linking signaling events at/near the membrane to distal cellular domains (e.g. nucleus, mitochondria). Furthermore, recent work suggests that the integrity of the actin/spectrin cytoskeleton is critical for canonical signaling of pathways involved in cellular response to stress. This review discusses these emerging roles for spectrin and consider implications for heart function and disease. Expert commentary: Despite growth in our understanding of the broader roles for spectrins in cardiac myocytes and other metazoan cells, there remain important unanswered questions, the answers to which may point the way to new therapies for human cardiac disease patients.

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.

The tumor suppressor phosphatase PP2A-B56α regulates stemness and promotes the initiation of malignancies in a novel murine model

Protein phosphatase 2A (PP2A) is a ubiquitously expressed Serine-Threonine phosphatase mediating 30–50% of protein phosphatase activity. PP2A functions as a heterotrimeric complex, with the B subunits directing target specificity to regulate the activity of many key pathways that control cellular phenotypes. PP2A-B56α has been shown to play a tumor suppressor role and to negatively control c-MYC stability and activity. Loss of B56α promotes cellular transformation, likely at least in part through its regulation of c-MYC. Here we report generation of a B56α hypomorph mouse with very low B56α expression that we used to study the physiologic activity of the PP2A-B56α phosphatase. The predominant phenotype we observed in mice with B56α deficiency in the whole body was spontaneous skin lesion formation with hyperproliferation of the epidermis, hair follicles and sebaceous glands. Increased levels of c-MYC phosphorylation on Serine62 and c-MYC activity were observed in the skin lesions of the B56α^hm/hm mice. B56α deficiency was found to increase the number of skin stem cells, and consistent with this, papilloma initiation was accelerated in a carcinogenesis model. Further analysis of additional tissues revealed increased inflammation in spleen, liver, lung, and intestinal lymph nodes as well as in the skin lesions, resembling elevated extramedullary hematopoiesis phenotypes in the B56α^hm/hm mice. We also observed an increase in the clonogenicity of bone marrow stem cells in B56α^hm/hm mice. Overall, this model suggests that B56α is important for stem cells to maintain homeostasis and that B56α loss leading to increased activity of important oncogenes, including c-MYC, can result in aberrant cell growth and increased stem cells that can contribute to the initiation of malignancy.

Ankyrins and Spectrins in Cardiovascular Biology and Disease

Ankyrins are adaptor proteins critical for the expression and targeting of cardiac membrane proteins, signaling molecules, and cytoskeletal elements. Findings in humans and animal models have highlighted the in vivo roles for ankyrins in normal physiology and in cardiovascular disease, most notably in cardiac arrhythmia. For example, human ANK2 loss-of-function variants are associated with a complex array of electrical and structural phenotypes now termed “ankyrin-B syndrome,” whereas alterations in the ankyrin-G pathway for Na_v channel targeting are associated with human Brugada syndrome. Further, both ankyrin-G and -B are now linked with acquired forms of cardiovascular disease including myocardial infarction and atrial fibrillation. Spectrins are ankyrin-associated proteins and recent studies support the critical role of ankyrin-spectrin interactions in normal cardiac physiology as well as regulation of key ion channel and signaling complexes. This review will highlight the roles of ankyrins and spectrins in cardiovascular physiology as well as illustrate the link between the dysfunction in ankyrin- and spectrin-based pathways and disease.

The p21-activated kinase 1 (Pak1) signalling pathway in cardiac disease: from mechanistic study to therapeutic exploration

p21-activated kinase 1 (Pak1) is a member of the highly conserved family of serine/threonine protein kinases regulated by Ras-related small G-proteins, Cdc42/Rac1. It has been previously demonstrated to be involved in cardiac protection. Based on recent studies, this review provides an overview of the role of Pak1 in cardiac diseases including disrupted Ca²⁺ homoeostasis-related cardiac arrhythmias, adrenergic stress- and pressure overload-induced hypertrophy, and ischaemia/reperfusion injury. These findings demonstrate the important role of Pak1 mediated through the phosphorylation and transcriptional modification of hypertrophy and/or arrhythmia-related genes. This review also discusses the anti-arrhythmic and anti-hypertrophic, protective function of Pak1 and the beneficial effects of fingolimod (an FDA-approved sphingolipid drug), a Pak1 activator, and its ability to prevent arrhythmias and cardiac hypertrophy. These findings also highlight the therapeutic potential of Pak1 signalling in the treatment and prevention of cardiac diseases.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Regulation of cardiac gap junctions by protein phosphatases

Sufficient connexin-mediated intercellular coupling is critical to maintain gap junctional communication for proper cardiac function. Alterations in connexin phosphorylation state, particularly dephosphorylation of connexin 43 (Cx43), may impact cell coupling and conduction in disease states. Cx43 dephosphorylation may be carried out by protein phosphatase activity. Here, we present an overview of the key phosphatases known to interact with Cx43 or modulators of Cx43, as well as some possible therapeutic targets to regulate phosphatase activity in the heart.

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.

Deletion of Pr130 Interrupts Cardiac Development in Zebrafish

Protein phosphatase 2 regulatory subunit B, alpha (PPP2R3A), a regulatory subunit of protein phosphatase 2A (PP2A), is a major serine/threonine phosphatase that regulates crucial function in development and growth. Previous research has implied that PPP2R3A was involved in heart failure, and PR130, the largest transcription of PPP2R3A, functioning in the calcium release of sarcoplasmic reticulum (SR), plays an important role in the excitation-contraction (EC) coupling. To obtain a better understanding of PR130 functions in myocardium and cardiac development, two pr130-deletion zebrafish lines were generated using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas) system. Pr130-knockout zebrafish exhibited cardiac looping defects and decreased cardiac function (decreased fractional area and fractional shortening). Hematoxylin and eosin (H&E) staining demonstrated reduced cardiomyocytes. Subsequent transmission electron microscopy revealed that the bright and dark bands were narrowed and blurred, the Z- and M-lines were fogged, and the gaps between longitudinal myocardial fibers were increased. Additionally, increased apoptosis was observed in cardiomyocyte in pr130-knockout zebrafish compared to wild-type (WT). Taken together, our results suggest that pr130 is required for normal myocardium formation and efficient cardiac contractile function.

Roles and regulation of protein phosphatase 2A (PP2A) in the heart

Reversible protein phosphorylation is central to a variety of cardiac processes including excitation-contraction coupling, Ca²⁺ handling, cell metabolism, myofilament regulation, and cell-cell communication. While kinase pathways linked with elevated adrenergic signaling have been a major focus for the cardiovascular field over the past half century, new findings support the critical role of protein phosphatases in both health and disease. Protein phosphatase 2A (PP2A) is a central cardiac phosphatase that regulates diverse myocyte functions through a host of target molecules. Notably, multiple mechanisms have evolved to dynamically tune PP2A function, including modulation of the composition, phosphorylation, methylation, and localization of PP2A holoenzyme populations. Further, aberrations in this regulation of PP2A function may contribute to cardiac pathophysiology. In summary, PP2A is a critical regulatory molecule in both health and disease, with a myriad of targets in heart. Based on their unique structure, localization, and regulatory properties, PP2A subunits represent exciting therapeutic targets to modulate altered adrenergic signaling in cardiovascular disease.

Atrial-specific pathways for control of intracellular signaling and myocyte function

Atrial fibrillation (AF) is a cardiac arrhythmia that arises from electrical and contractile dysfunction in the atria. Atrial function is regulated by a variety of intracellular signaling networks that facilitate rapid communication and coordinate responses of atrial myocytes. In this issue of the JCI, Brandenburg and colleagues describe the identification and characterization of "super-hub" signaling nodes located on atrial axial tubules that regulate atrial contraction. Together, the results of this study provide important insight into the regulation of atrial contraction and describe potential therapeutic targets to be explored in future studies.

Regulation of sarcoplasmic reticulum Ca2+ release by serine-threonine phosphatases in the heart

The amount and timing of Ca²⁺ release from the sarcoplasmic reticulum (SR) during cardiac cycle are the main determinants of cardiac contractility. Reversible phosphorylation of the SR Ca²⁺ release channel, ryanodine receptor type 2 (RyR2) is the central mechanism of regulation of Ca²⁺ release in cardiomyocytes. Three major serine-threonine phosphatases including PP1, PP2A and PP2B (calcineurin) have been implicated in modulation of RyR2 function. Changes in expression levels of these phosphatases, their activity and targeting to the RyR2 macromolecular complex were demonstrated in many animal models of cardiac disease and humans and are implicated in cardiac arrhythmia and heart failure. Here we review evidence in support of regulation of RyR2-mediated SR Ca²⁺ release by serine-threonine phosphatases and the role and mechanisms of dysregulation of phosphatases in various disease states.

Elevated local [Ca2+] and CaMKII promote spontaneous Ca2+ release in ankyrin-B-deficient hearts

AIMS

Loss-of-function mutations in the cytoskeletal protein ankyrin-B (AnkB) cause ventricular tachyarrhythmias in humans. Previously, we found that a larger fraction of the sarcoplasmic reticulum (SR) Ca(2+) leak occurs through Ca(2+) sparks in AnkB-deficient (AnkB(+/-)) mice, which may contribute to arrhythmogenicity via Ca(2+) waves. Here, we investigated the mechanisms responsible for increased Ca(2+) spark frequency in AnkB(+/-) hearts.

METHODS AND RESULTS

Using immunoblots and phospho-specific antibodies, we found that phosphorylation of ryanodine receptors (RyRs) by CaMKII is enhanced in AnkB(+/-) hearts. In contrast, the PKA-mediated RyR phosphorylation was comparable in AnkB(+/-) and wild-type (WT) mice. CaMKII inhibition greatly reduced Ca(2+) spark frequency in myocytes from AnkB(+/-) mice but had little effect in the WT. Global activities of the major phosphatases PP1 and PP2A were similar in AnkB(+/-) and WT hearts, while CaMKII autophosphorylation, a marker of CaMKII activation, was increased in AnkB(+/-) hearts. Thus, CaMKII-dependent RyR hyperphosphorylation in AnkB(+/-) hearts is caused by augmented CaMKII activity. Intriguingly, CaMKII activation is limited to the sarcolemma-SR junctions since non-junctional CaMKII targets (phospholamban, HDAC4) are not hyperphosphorylated in AnkB(+/-) myocytes. This local CaMKII activation may be the consequence of elevated [Ca(2+)] in the junctional cleft caused by reduced Na(+)/Ca(2+) exchange activity. Indeed, using the RyR-targeted Ca(2+) sensor GCaMP2.2-FBKP12.6, we found that local junctional [Ca(2+)] is significantly elevated in AnkB(+/-) myocytes.

CONCLUSIONS

The increased incidence of pro-arrhythmogenic Ca(2+) sparks and waves in AnkB(+/-) hearts is due to enhanced CaMKII-mediated RyR phosphorylation, which is caused by higher junctional [Ca(2+)] and consequent local CaMKII activation.

Two-Pore K+ Channel TREK-1 Regulates Sinoatrial Node Membrane Excitability

Background

Two‐pore K⁺ channels have emerged as potential targets to selectively regulate cardiac cell membrane excitability; however, lack of specific inhibitors and relevant animal models has impeded the effort to understand the role of 2‐pore K⁺ channels in the heart and their potential as a therapeutic target. The objective of this study was to determine the role of mechanosensitive 2‐pore K⁺ channel family member TREK‐1 in control of cardiac excitability.

Methods and Results

Cardiac‐specific TREK‐1–deficient mice (αMHC‐Kcnk^f/f) were generated and found to have a prevalent sinoatrial phenotype characterized by bradycardia with frequent episodes of sinus pause following stress. Action potential measurements from isolated αMHC‐Kcnk2^f/f sinoatrial node cells demonstrated decreased background K⁺ current and abnormal sinoatrial cell membrane excitability. To identify novel pathways for regulating TREK‐1 activity and sinoatrial node excitability, mice expressing a truncated allele of the TREK‐1–associated cytoskeletal protein β_IV‐spectrin (qv^4J mice) were analyzed and found to display defects in cell electrophysiology as well as loss of normal TREK‐1 membrane localization. Finally, the β_IV‐spectrin/TREK‐1 complex was found to be downregulated in the right atrium from a canine model of sinoatrial node dysfunction and in human cardiac disease.

Conclusions

These findings identify a TREK‐1–dependent pathway essential for normal sinoatrial node cell excitability that serves as a potential target for selectively regulating sinoatrial node cell function.

Oxidant stress promotes disease by activating CaMKII

CaMKII is activated by oxidation of methionine residues residing in the regulatory domain. Oxidized CaMKII (ox-CaMKII) is now thought to participate in cardiovascular and pulmonary diseases and cancer. This invited review summarizes current evidence for the role of ox-CaMKII in disease, considers critical knowledge gaps and suggests new areas for inquiry.