Cardiac-specific deletion of protein phosphatase 1β promotes increased myofilament protein phosphorylation and contractile alterations

PP1β opposes classic PP1 function, inhibiting spine maturation and promoting LTP

Protein phosphatase 1 (PP1) regulates synaptic plasticity and has been described as a molecular constraint on learning and memory. There are three neuronal isoforms, PP1α, PP1β, and PP1γ, but little is known about their individual functions. PP1α and PP1γ are assumed to mediate the effects of PP1 on learning and memory based on their enrichment at dendritic spines and their preferential binding to neurabin and spinophilin, major PP1 synaptic scaffolding proteins. However, it was recently discovered that human de novo PP1β mutations cause intellectual disability, suggesting an important but ill-defined role for PP1β. In this study, we investigated the functions of each PP1 isoform in hippocampal synaptic physiology using conditional CA1-specific knockout mice. In stark contrast to classic PP1 function, we found that PP1β promotes synaptic plasticity as well as spatial memory. These changes in synaptic plasticity and memory are accompanied by changes in GluA1 phosphorylation, GluN2A levels, and dendritic spine density and morphology, including silent synapse number. These functions of PP1β reveal a previously unidentified signaling pathway regulating spine maturation and plasticity, broadening our understanding of the complex role of PP1 in synaptic physiology.

Cardiac myosin binding protein-C phosphorylation as a function of multiple protein kinase and phosphatase activities

Phosphorylation of cardiac myosin binding protein-C (cMyBP-C) is a determinant of cardiac myofilament function. Although cMyBP-C phosphorylation by various protein kinases has been extensively studied, the influence of protein phosphatases on cMyBP-C's multiple phosphorylation sites has remained largely obscure. Here we provide a detailed biochemical characterization of cMyBP-C dephosphorylation by protein phosphatases 1 and 2 A (PP1 and PP2A), and develop an integrated kinetic model for cMyBP-C phosphorylation using data for both PP1, PP2A and various protein kinases known to phosphorylate cMyBP-C. We find strong site-specificity and a hierarchical mechanism for both phosphatases, proceeding in the opposite direction of sequential phosphorylation by potein kinase A. The model is consistent with published data from human patients and predicts complex non-linear cMyBP-C phosphorylation patterns that are validated experimentally. Our results suggest non-redundant roles for PP1 and PP2A under both physiological and heart failure conditions, and emphasize the importance of phosphatases for cMyBP-C regulation.

Neuronal protein phosphatase 1β regulates glutamate release, cortical myelination, node of Ranvier formation, and action potential propagation in the optic nerve

Precise regulation of protein phosphorylation is critical for many cellular processes, and dysfunction in this process has been linked to various neurological disorders and diseases. Protein phosphatase 1 (PP1) is a ubiquitously expressed serine/threonine phosphatase with three major isoforms, (α, β, γ) and hundreds of known substrates. Previously, we reported that PP1α and PP1γ are essential for the known role of PP1 in synaptic physiology and learning/memory, while PP1β displayed a surprising opposing function. De novo mutations in PP1β cause neurodevelopmental disorders in humans, but the mechanisms involved are currently unknown. A Cre-Lox system was used to delete PP1β specifically in neurons in order to study its effects on developing mice. These animals fail to survive to 3 postnatal weeks, and exhibit deficits in cortical myelination and glutamate release. There was defective compound action potential (CAP) propagation in the optic nerve of the null mice, which was traced to a deficit in the formation of nodes of Ranvier. Finally, it was found that phosphorylation of the PP1β-specific substrate, myosin light chain 2 (MLC2), is significantly enhanced in PP1β null optic nerves. Several novel important in vivo roles of PP1β in neurons were discovered, and these data will aid future investigations in delineating the mechanisms by which de novo mutations in PP1β lead to intellectual and developmental delays in patients.

Opposing Roles for the α Isoform of the Catalytic Subunit of Protein Phosphatase 1 in Inside-Out and Outside-In Integrin Signaling in Murine Platelets

Platelet activation during hemostasis and thrombosis is facilitated by agonist-induced inside-out and integrin αIIbβ3-initiated outside-in signaling via protein kinases and phosphatases. Pharmacological inhibitor studies suggest that the serine/threonine protein phosphatase 1 (PP1) promotes platelet activation. However, since phosphatase inhibitors block all the isoforms of the catalytic subunit of PP1 (PP1c), the role of specific PP1c isoform in platelet signaling remains unclear. Here, we employed a platelet-specific PP1cα-/- mice to explore the contribution of a major PP1 isoform in platelet functions. Loss of PP1cα moderately decreased activation of integrin αIIbβ3, binding of soluble fibrinogen, and aggregation to low-dose thrombin, ADP, and collagen. In contrast, PP1cα-/- platelets displayed increased adhesion to immobilized fibrinogen, fibrin clot retraction, and thrombus formation on immobilized collagen. Mechanistically, post-fibrinogen engagement potentiated p38 mitogen-activated protein kinase (MAPK) activation in PP1cα-/- platelets and the p38 inhibitor blocked the increased integrin-mediated outside-in signaling function. Tail bleeding time and light-dye injury-induced microvascular thrombosis in the cremaster venules and arterioles were not altered in PP1cα-/- mice. Thus, PP1cα displays pleiotropic signaling in platelets as it amplifies agonist-induced signaling and attenuates integrin-mediated signaling with no impact on hemostasis and thrombosis.

An in vitro approach to understand contribution of kidney cells to human urinary extracellular vesicles

Extracellular vesicles (EV) are membranous particles secreted by all cells and found in body fluids. Established EV contents include a variety of RNA species, proteins, lipids and metabolites that are considered to reflect the physiological status of their parental cells. However, to date, little is known about cell-type enriched EV cargo in complex EV mixtures, especially in urine. To test whether EV secretion from distinct human kidney cells in culture differ and can recapitulate findings in normal urine, we comprehensively analysed EV components, (particularly miRNAs, long RNAs and protein) from conditionally immortalised human kidney cell lines (podocyte, glomerular endothelial, mesangial and proximal tubular cells) and compared to EV secreted in human urine. EV from cell culture media derived from immortalised kidney cells were isolated by hydrostatic filtration dialysis (HFD) and characterised by electron microscopy (EM), nanoparticle tracking analysis (NTA) and Western blotting (WB). RNA was isolated from EV and subjected to miRNA and RNA sequencing and proteins were profiled by tandem mass tag proteomics. Representative sets of EV miRNAs, RNAs and proteins were detected in each cell type and compared to human urinary EV isolates (uEV), EV cargo database, kidney biopsy bulk RNA sequencing and proteomics, and single-cell transcriptomics. This revealed that a high proportion of the in vitro EV signatures were also found in in vivo datasets. Thus, highlighting the robustness of our in vitro model and showing that this approach enables the dissection of cell type specific EV cargo in biofluids and the potential identification of cell-type specific EV biomarkers of kidney disease.

HuR inhibition reduces post-ischemic cardiac remodeling by dampening acute inflammatory gene expression and the innate immune response

Myocardial ischemia/reperfusion (I/R) injury and the resulting cardiac remodeling is a common cause of heart failure. The RNA binding protein Human Antigen R (HuR) has been previously shown to reduce cardiac remodeling following both I/R and cardiac pressure overload, but the full extent of the HuR-dependent mechanisms within cells of the myocardium have yet to be elucidated. In this study, we applied a novel small molecule inhibitor of HuR to define the functional role of HuR in the acute response to I/R injury and gain a better understanding of the HuR-dependent mechanisms during post-ischemic myocardial remodeling. Our results show an early (two hours post-I/R) increase in HuR activity that is necessary for early inflammatory gene expression by cardiomyocytes in response to I/R. Surprisingly, despite the reductions in early inflammatory gene expression at two hours post-I/R, HuR inhibition has no effect on initial infarct size at 24-hours post-I/R. However, in agreement with previously published work, we do see a reduction in pathological remodeling and preserved cardiac function at two weeks post-I/R upon HuR inhibition. RNA-sequencing analysis of neonatal rat ventricular myocytes (NRVMs) at two hours post-LPS treatment to model damage associated molecular pattern (DAMP)-mediated activation of toll like receptors (TLRs) demonstrates a broad HuR-dependent regulation of pro-inflammatory chemokine and cytokine gene expression in cardiomyocytes. We show that conditioned media from NRVMs pre-treated with HuR inhibitor loses the ability to induce inflammatory gene expression in bone marrow derived macrophages (BMDMs) compared to NRVMs treated with LPS alone. Functionally, HuR inhibition in NRVMs also reduces their ability to induce endocrine migration of peripheral blood monocytes in vitro and reduces post-ischemic macrophage infiltration to the heart in vivo. In summary, these results suggest a HuR-dependent expression of pro-inflammatory gene expression by cardiomyocytes that leads to subsequent monocyte recruitment and macrophage activation in the post-ischemic myocardium.

Protein phosphatase-1 inhibitor-2 promotes PP1γ positive regulation of synaptic transmission

Inhibitor-2 (I-2) is a prototypic inhibitor of protein phosphatase-1 (PP1), a major serine-threonine phosphatase that regulates synaptic plasticity and learning and memory. Although I-2 is a potent inhibitor of PP1 in vitro, our previous work has elucidated that, in vivo, I-2 may act as a positive regulator of PP1. Here we show that I-2 and PP1γ, but not PP1α, positively regulate synaptic transmission in hippocampal neurons. Moreover, we demonstrated that I-2 enhanced PP1γ interaction with its major synaptic scaffold, neurabin, by Förster resonance energy transfer (FRET)/Fluorescence lifetime imaging microscopy (FLIM) studies, while having a limited effect on PP1 auto-inhibitory phosphorylation. Furthermore, our study indicates that the effect of I-2 on PP1 activity in vivo is dictated by I-2 threonine-72 phosphorylation. Our work thus demonstrates a molecular mechanism by which I-2 positively regulates PP1 function in synaptic transmission.

Myosin light chain phosphatase catalytic subunit dephosphorylates cardiac myosin via mechanisms dependent and independent of the MYPT regulatory subunits

Cardiac muscle myosin regulatory light chain (RLC) is constitutively phosphorylated at ∼0.4 mol phosphate/mol RLC in normal hearts, and phosphorylation is maintained by balanced activities of dedicated cardiac muscle–specific myosin light chain kinase and myosin light chain phosphatase (MLCP). Previously, the identity of the cardiac-MLCP was biochemically shown to be similar to the smooth muscle MLCP, which is a well-characterized trimeric protein comprising the regulatory subunit (MYPT1), catalytic subunit PP1cβ, and accessory subunit M20. In smooth muscles in vivo, MYPT1 and PP1cβ co-stabilize each other and are both necessary for normal smooth muscle contractions. In the cardiac muscle, MYPT1 and MYPT2 are both expressed, but contributions to physiological regulation of cardiac myosin dephosphorylation are unclear. We hypothesized that the main catalytic subunit for cardiac-MLCP is PP1cβ, and maintenance of RLC phosphorylation in vivo is dependent on regulation by striated muscle–specific MYPT2. Here, we used PP1cβ conditional knockout mice to biochemically define cardiac-MLCP proteins and developed a cardiac myofibrillar phosphatase assay to measure the direct contribution of MYPT-regulated and MYPT-independent phosphatase activities toward phosphorylated cardiac myosin. We report that (1) PP1cβ is the main isoform expressed in the cardiac myocyte, (2) cardiac muscle pathogenesis in PP1cβ knockout animals involve upregulation of total PP1cα in myocytes and non-muscle cells, (3) the stability of cardiac MYPT1 and MYPT2 proteins in vivo is not dependent on the PP1cβ expression, and (4) phosphorylated myofibrillar cardiac myosin is dephosphorylated by both myosin-targeted and soluble MYPT-independent PP1cβ activities. These results contribute to our understanding of the cardiac-MLCP in vivo.

Pharmacological analysis of Empagliflozin: Acting through the CaMKII pathway in type 2 diabetes and acute cardiovascular events

Background

Type 2 diabetes mellitus is a high-risk factor for acute cardiovascular events. Some reports show that Empagliflozin has a protective effect on cardiovascular events and diabetes mellitus, and Empagliflozin can act on the CaMKII pathway. However, the specific gene of action is not precise. Therefore, this study investigated the target genes of Empagliflozin by integrated gene analysis and molecular docking method to provide a theoretical basis for further elucidating the mechanism of action of Empagliflozin.

Method

In this study, we obtained 12 datasets from GEO, divided into experimental and validation groups, with a total of 376 samples. We then integrated CaMKII pathway-related genes from OMIM, NCBI, and genecards databases. We then intersected them with the differential genes we obtained to obtain 5 common genes and performed functional enrichment analysis. We then performed group comparisons in the validation set, and we obtained 2 clinically significant genes. Then we performed group comparison in the validation set, and we obtained 2 clinically significant genes, followed by molecular docking analysis with pymol, autodock software. We obtained molecular docking models for the 2 genes.

Conclusion

In this study, we obtained CaMK2G and PPP1CA, genes associated with the CaMKII pathway and type 2 diabetes and acute cardiovascular events, by integrative gene analysis and validated their expression in the relevant dataset. We also derived that Empagliflozin acts on amino acid TRP-125 of CaMK2G gene and GLN-249 ASP-210 ASP-208 of PPP1CA through CaMKII pathway, thus acting on type 2 diabetes and acute cardiovascular events by molecular docking technique.

Regulation of myosin light-chain phosphorylation and its roles in cardiovascular physiology and pathophysiology

The regulation of muscle contraction is a critical function in the cardiovascular system, and abnormalities may be life-threatening or cause illness. The common basic mechanism in muscle contraction is the interaction between the protein filaments myosin and actin. Although this interaction is primarily regulated by intracellular Ca²⁺, the primary targets and intracellular signaling pathways differ in vascular smooth muscle and cardiac muscle. Phosphorylation of the myosin regulatory light chain (RLC) is a primary molecular switch for smooth muscle contraction. The equilibrium between phosphorylated and unphosphorylated RLC is dynamically achieved through two enzymes, myosin light chain kinase, a Ca²⁺-dependent enzyme, and myosin phosphatase, which modifies the Ca²⁺ sensitivity of contractions. In cardiac muscle, the primary target protein for Ca²⁺ is troponin C on thin filaments; however, RLC phosphorylation also plays a modulatory role in contraction. This review summarizes recent advances in our understanding of the regulation, physiological function, and pathophysiological involvement of RLC phosphorylation in smooth and cardiac muscles.

Function and regulation of phosphatase 1 in healthy and diseased heart

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

Role of Cardiac AMP-Activated Protein Kinase in a Non-pathological Setting: Evidence From Cardiomyocyte-Specific, Inducible AMP-Activated Protein Kinase α1α2-Knockout Mice

AMP-activated protein kinase (AMPK) is a key regulator of energy homeostasis under conditions of energy stress. Though heart is one of the most energy requiring organs and depends on a perfect match of energy supply with high and fluctuating energy demand to maintain its contractile performance, the role of AMPK in this organ is still not entirely clear, in particular in a non-pathological setting. In this work, we characterized cardiomyocyte-specific, inducible AMPKα1 and α2 knockout mice (KO), where KO was induced at the age of 8 weeks, and assessed their phenotype under physiological conditions. In the heart of KO mice, both AMPKα isoforms were strongly reduced and thus deleted in a large part of cardiomyocytes already 2 weeks after tamoxifen administration, persisting during the entire study period. AMPK KO had no effect on heart function at baseline, but alterations were observed under increased workload induced by dobutamine stress, consistent with lower endurance exercise capacity observed in AMPK KO mice. AMPKα deletion also induced a decrease in basal metabolic rate (oxygen uptake, energy expenditure) together with a trend to lower locomotor activity of AMPK KO mice 12 months after tamoxifen administration. Loss of AMPK resulted in multiple alterations of cardiac mitochondria: reduced respiration with complex I substrates as measured in isolated mitochondria, reduced activity of complexes I and IV, and a shift in mitochondrial cristae morphology from lamellar to mixed lamellar-tubular. A strong tendency to diminished ATP and glycogen level was observed in older animals, 1 year after tamoxifen administration. Our study suggests important roles of cardiac AMPK at increased cardiac workload, potentially limiting exercise performance. This is at least partially due to impaired mitochondrial function and bioenergetics which degrades with age.

Complex functionality of protein phosphatase 1 isoforms in the heart

Protein phosphatase 1(PP1) is a key regulator of cardiac function through dephosphorylating serine/threonine residues within target proteins to oppose the function of protein kinases. Studies from failing hearts of animal models and human patients have demonstrated significant increase of PP1 activity in myocardium, while elevated PP1 activity in transgenic mice leads to cardiac dysfunction, suggesting that PP1 might be a therapeutic target to ameliorate cardiac dysfunction in failing hearts. In fact, cardiac overexpression of inhibitor 1, the endogenous inhibitor of PP1, increases cardiac contractility and suppresses heart failure progression. However, this notion of PP1 inhibition for heart failure treatment has been challenged by recent studies on the isoform-specific roles of PP1 in the heart. PP1 is a holoenzyme composed of catalytic subunits (PP1α, PP1β, or PP1γ) and regulatory proteins that target them to distinct subcellular locations for functional specificity. This review will summarize how PP1 regulates phosphorylation of some of the key cardiac proteins involved in Ca²⁺ handling and cardiac contraction, and the potential role of PP1 isoforms in controlling cardiac physiology and pathophysiology.

Heart Failure in Humans Reduces Contractile Force in Myocardium From Both Ventricles

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Contractile assays were performed using multicellular preparations isolated from the left and right ventricles of organ donors and patients with heart failure.

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Heart failure reduced maximum force and power by approximately 30% in the myocardium from both ventricles.

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Heart failure increased the Ca²⁺ sensitivity of contraction, but the effect was bigger in right ventricular tissue than in left ventricular samples.

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The changes in Ca²⁺ sensitivity may reflect ventricle-specific post-translational modifications to sarcomeric proteins.

This study measured how heart failure affects the contractile properties of the human myocardium from the left and right ventricles. The data showed that maximum force and maximum power were reduced by approximately 30% in multicellular preparations from both ventricles, possibly because of ventricular remodeling (e.g., cellular disarray and/or excess fibrosis). Heart failure increased the calcium (Ca²⁺) sensitivity of contraction in both ventricles, but the effect was bigger in right ventricular samples. The changes in Ca²⁺ sensitivity were associated with ventricle-specific changes in the phosphorylation of troponin I, which indicated that adrenergic stimulation might induce different effects in the left and right ventricles.

The protein phosphatase isoform PP1γ1 substitutes for PP1γ2 to support spermatogenesis but not normal sperm function and fertility†

Four isoforms of serine/threonine phosphatase type I, PP1α, PP1β, PP1γ1, and PP1γ2, are derived from three genes. The PP1γ1 and PP1γ2 isoforms are alternately spliced transcripts of the protein phosphatase 1 catalytic subunit gamma gene (Ppp1cc). While PP1γ1 is ubiquitous in somatic cells, PP1γ2 is expressed exclusively in testicular germ cells and sperm. Ppp1cc knockout male mice (-/-), lacking both PP1γ1 and PP1γ2, are sterile due to impaired sperm morphogenesis. Fertility and normal sperm function can be restored by transgenic expression of PP1γ2 alone in testis of Ppp1cc (-/-) mice. The purpose of this study was to determine whether the PP1γ1 isoform is functionally equivalent to PP1γ2 in supporting spermatogenesis and male fertility. Significant levels of transgenic PP1γ1 expression occurred only when the transgene lacked a 1-kb 3΄UTR region immediately following the stop codon of the PP1γ1 transcript. PP1γ1 was also incorporated into sperm at levels comparable to PP1γ2 in sperm from wild-type mice. Spermatogenesis was restored in mice expressing PP1γ1 in the absence of PP1γ2. However, males from the transgenic rescue lines were subfertile. Sperm from the PP1γ1 rescue mice were unable to fertilize eggs in vitro. Intrasperm localization of PP1γ1 and the association of the protein regulators of the phosphatase were altered in epididymal sperm in transgenic PP1γ1 compared to PP1γ2. Thus, the ubiquitous isoform PP1γ1, not normally expressed in differentiating germ cells, could replace PP1γ2 to support spermatogenesis and spermiation. However, PP1γ2, which is the PP1 isoform in mammalian sperm, has an isoform-specific role in supporting normal sperm function and fertility.

An acute immune response underlies the benefit of cardiac stem cell therapy

Clinical trials using adult stem cells to regenerate damaged heart tissue continue to this day^1,2 despite ongoing questions of efficacy and a lack of mechanistic understanding of the underlying biologic effect³. The rationale for these cell therapy trials is derived from animal studies that show a modest but reproducible improvement in cardiac function in models of cardiac ischemic injury^4,5. Here we examined the mechanistic basis for cell therapy in mice after ischemia/reperfusion (I/R) injury, and while heart function was enhanced, it was not associated with new cardiomyocyte production. Cell therapy improved heart function through an acute sterile immune response characterized by the temporal and regional induction of CCR2⁺ and CX3CR1⁺ macrophages. Intra-cardiac injection of 2 distinct types of adult stem cells, freeze/thaw-killed cells or a chemical inducer of the innate immune response similarly induced regional CCR2⁺ and CX3CR1⁺ macrophage accumulation and provided functional rejuvenation to the I/R-injured heart. This selective macrophage response altered cardiac fibroblast activity, reduced border zone extracellular matrix (ECM) content, and enhanced the mechanical properties of the injured area. The functional benefit of cardiac cell therapy is thus due to an acute inflammatory-based wound healing response that rejuvenates the mechanical properties of the infarcted area of the heart.

Cardiomyocyte-Specific STIM1 (Stromal Interaction Molecule 1) Depletion in the Adult Heart Promotes the Development of Arrhythmogenic Discordant Alternans

BACKGROUND

STIM1 (stromal interaction molecule 1) is a calcium (Ca²⁺) sensor that regulates cardiac hypertrophy by triggering store-operated Ca²⁺ entry. Because STIM1 binding to phospholamban increases sarcoplasmic reticulum Ca²⁺ load independent of store-operated Ca²⁺ entry, we hypothesized that it controls electrophysiological function and arrhythmias in the adult heart.

METHODS

Inducible myocyte-restricted STIM1-KD (STIM1 knockdown) was achieved in adult mice using an αMHC (α-myosin heavy chain)-MerCreMer system. Mechanical and electrophysiological properties were examined using echocardiography in vivo and optical action potential (AP) mapping ex vivo in tamoxifen-induced STIM1^flox/flox-Cre^tg/- (STIM1-KD) and littermate controls for STIM1^flox/flox (referred to as STIM1-Ctl) and for Cre^tg/- without STIM deletion (referred to as Cre-Ctl).

RESULTS

STIM1-KD mice (N=23) exhibited poor survival compared with STIM1-Ctl (N=22) and Cre-Ctl (N=11) with >50% mortality after only 8-days of cardiomyocyte-restricted STIM1-KD. STIM1-KD but not STIM1-Ctl or Cre-Ctl hearts exhibited a proclivity for arrhythmic behavior, ranging from frequent ectopy to pacing-induced ventricular tachycardia/ventricular fibrillation (VT/VF). Examination of the electrophysiological substrate revealed decreased conduction velocity and increased AP duration (APD) heterogeneity in STIM1-KD. These features, however, were comparable in VT/VF(+) and VT/VF(-) hearts. We also uncovered a marked increase in the magnitude of APD alternans during rapid pacing, and the emergence of a spatially discordant alternans profile in STIM1-KD hearts. Unlike conduction velocity slowing and APD heterogeneity, the magnitude of APD alternans was greater (by 80%, P<0.05) in VT/VF(+) versus VT/VF(-) STIM1-KD hearts. Detailed phase mapping during the initial beats of VT/VF identified one or more rotors that were localized along the nodal line separating out-of-phase alternans regions.

CONCLUSIONS

In an adult murine model with inducible and myocyte-specific STIM1 depletion, we demonstrate for the first time the regulation of spatially discordant alternans by STIM1. Early mortality in STIM1-KD mice is likely related to enhanced susceptibility to VT/VF secondary to discordant APD alternans.

Differential expression of novel MicroRNAs from developing fetal heart of Gallus gallus domesticus implies a role in cardiac development

Heart development is a complex process regulated by multi-layered genetic as well epigenetic regulators many of which are still unknown. Besides their critical role during cardiac development, these molecular regulators emerge as key modulators of cardiovascular pathologies, where fetal cardiac genes' re-expression is witnessed. MicroRNAs have recently emerged as a crucial part of signalling cascade in both development and diseases. We aimed to identify, validate, and perform functional annotation of putative novel miRNAs using chicken as a cardiac development model system. Novel miRNAs were obtained through deep sequencing of small RNAs extracted from chicken embryonic cardiac tissue of different developmental stages. After filtering out real pre-miRNAs, their expression analysis, potential target gene's prediction and functional annotations were performed. Expression analysis revealed that miRNAs were differentially expressed during different developmental stages of chicken heart. The expression of selected putative novel miRNAs was further validated by real-time PCR. Our analysis indicated the presence of novel cardiac miRNAs that might be regulating critical cardiac development events such as cardiac cell growth, differentiation, cardiac action potential generation and signal transduction.

HOXA2 activity regulation by cytoplasmic relocation, protein stabilization and post-translational modification

HOX proteins are homeodomain transcription factors critically involved in patterning animal embryos and controlling organogenesis. While the functions of HOX proteins and the processes under their control begin to be well documented, the modalities of HOX protein activity regulation remain poorly understood. Here we show that HOXA2 interacts with PPP1CB, a catalytic subunit of the Ser/Thr PP1 phosphatase complex. This interaction co-localizes in the cytoplasm with a previously described HOXA2 interactor, KPC2, which belongs to the KPC E3 ubiquitin ligase complex. We provide evidence that HOXA2, PPP1CB and KPC2 define a molecularly and functionally interacting complex. Collectively, our experiments support that PPP1CB and KPC2 together inhibit the activity of HOXA2 by activating its nuclear export, but favored HOXA2 de-ubiquitination and stabilization thereby establishing a store of HOXA2 in the cytoplasm.

A simplified herbal formula for the treatment of heart failure: Efficacy, bioactive ingredients, and mechanisms

Heart failure (HF) is a complex pathology for which single-agent therapy cannot provide comprehensive efficacy. Therefore, effective combination therapies for HF are increasingly emphasized. Multiple-component drugs derived from Chinese herbal formulae provide efficacy and safety when administered to patients with HF. Nuanxinkang (NXK) is a simplified Chinese herbal formula which has been widely applied in HF for decades. It exhibits comprehensive cardiac protective effects in HF patients as an adjuvant therapy, including improving heart function and quality-of-life, reducing inflammation, and regulating neurohormones. Nevertheless, the bioactive ingredients and mechanisms of action of NXK are unknown, which hinders its further application. Here, we examined the therapeutic efficacy of NXK in a mouse model of HF. Using transcriptome analysis and drug similarity analysis we found that NXK inhibits apoptosis and inflammation, while improving cardiac contraction and reversing myocardial fibrosis. In addition, we detected 21 bioactive species in NXK using UHPLC-MS analysis. Based on these data, we performed network pharmacology analysis to investigate ingredient-target-pathway interactions. We further confirmed 13 genes as potential targets, and assessed the effects of NXK on the AKT to validate the anti-apoptotic role of NXK both in vivo and in vitro. Thus, our work has identified a simplified herbal formula with efficacy against HF by exploring its constituents and mechanism of action, providing evidence for an innovative treatment strategy and novel therapeutic targets for HF.

Cytoplasmic nucleic acid-based XNAs directly enhance live cardiac cell function by a Ca2+ cycling-independent mechanism via the sarcomere

Nucleic acid - protein interactions are critical for regulating gene activation in the nucleus. In the cytoplasm, however, potential nucleic acid-protein functional interactions are less clear. The emergence of a large and expanding number of non-coding RNAs and DNA fragments raises the possibility that the cytoplasmic nucleic acids may interact with cytoplasmic cellular components to directly alter key biological processes within the cell. We now show that both natural and synthetic nucleic acids, collectively XNAs, when introduced to the cytoplasm of live cell cardiac myocytes, markedly enhance contractile function via a mechanism that is independent of new translation, activation of the TLR-9 pathway or by altered intracellular Ca²⁺ cycling. Findings show a steep XNA oligo length-dependence, but not sequence dependence or nucleic acid moiety dependence, for cytoplasmic XNAs to hasten myocyte relaxation. XNAs localized to the sarcomere in a striated pattern and bound the cardiac troponin regulatory complex with high affinity in an electrostatic-dependent manner. Mechanistically, XNAs phenocopy PKA-based modified troponin to cause faster relaxation. Collectively, these data support a new role for cytoplasmic nucleic acids in directly modulating live cell cardiac performance and raise the possibility that cytoplasmic nucleic acid - protein interactions may alter functionally relevant pathways in other cell types.

Differential localizations of protein phosphatase 1 isoforms determine their physiological function in the heart

Protein phosphatase 1 isoforms α, β, and γ (PP1α, PP1β, and PP1γ) are highly homologous in the catalytic domains but have distinct subcellular localizations. In this study, we utilized both primary cell culture and knockout mice to investigate the isoform-specific roles of PP1s in the heart. In both neonatal and adult cardiac myocytes, PP1β was mainly localized in the nucleus, compared to the predominant presence of PP1α and PP1γ in the cytoplasm. Adenovirus-mediated overexpression of PP1α led to decreased phosphorylation of phospholamban, which was not influenced by overexpression of either PP1β or PP1γ. Interestingly, only cardiac-specific knockout of PP1β resulted in increased HDAC7 phosphorylation, consistent with the predominant nuclear localization of PP1β. Functionally, deletion of either PP1 isoform resulted in reduced fractional shortening in aging mice, however only PP1β deletion resulted in interstitial fibrosis in mice as early as 3 weeks of age. Deletion of neither PP1 isoform had any effect on pathological cardiac hypertrophy induced by 2 weeks of pressure overload stimulation. Together, our data suggest that PP1 isoforms have differential localizations to regulate the phosphorylation of their specific substrates for the physiological function in the heart.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Chronic β-adrenergic stimulation reverses depressed Ca handling in mice overexpressing inhibitor-2 of protein phosphatase 1

RATIONALE

A higher expression/activity of type 1 serine/threonine protein phosphatase 1 (PP1) may contribute to dephosphorylation of cardiac regulatory proteins triggering the development of heart failure.

OBJECTIVE

Here, we tested the putatively protective effects of PP1 inhibitor-2 (I₂) overexpression using a heart failure model induced by chronic β-adrenergic stimulation.

METHODS AND RESULTS

Transgenic (TG) and wild-type (WT) mice were subjected to isoprenaline (ISO) or isotonic NaCl solution supplied via osmotic minipumps for 7 days. I₂ overexpression was associated with a depressed PP1 activity. Basal contractility was unchanged in catheterized mice and isolated cardiomyocytes between TG^NaCl and WT^NaCl. TG^ISO mice exhibited more fibrosis and a higher expression of hypertrophy marker proteins as compared to WT^ISO. After acute administration of ISO, the contractile response was accompanied by a higher sensitivity in TG^ISO as compared to WT^ISO. In contrast to basal contractility, the peak amplitude of [Ca]_i and SR Ca load were reduced in TG^NaCl as compared to WT^NaCl. These effects were normalized to WT levels after chronic ISO stimulation. Cardiomyocyte relaxation and [Ca]_i decay kinetics were hastened in TG^ISO as compared to WT^ISO, which can be explained by a higher phospholamban phosphorylation at Ser¹⁶. Chronic catecholamine stimulation was followed by an enhanced expression of GSK3β, whereas the phosphorylation at Ser⁹ was lower in TG as compared to the corresponding WT group. This resulted in a higher I₂ phosphorylation that may reactivate PP1.

CONCLUSION

Our findings suggest that the basal desensitization of β-adrenergic signaling and the depressed Ca handling in TG by inhibition of PP1 is restored by a GSK3β-dependent phosphorylation of I₂.

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.

The dominant protein phosphatase PP1c isoform in smooth muscle cells, PP1cβ, is essential for smooth muscle contraction

Contractile force development of smooth muscle is controlled by balanced kinase and phosphatase activities toward the myosin regulatory light chain (RLC). Numerous biochemical and pharmacological studies have investigated the specificity and regulatory activity of smooth muscle myosin light-chain phosphatase (MLCP) bound to myosin filaments and comprised of the regulatory myosin phosphatase target subunit 1 (MYPT1) and catalytic protein phosphatase 1cβ (PP1cβ) subunits. Recent physiological and biochemical evidence obtained with smooth muscle tissues from a conditional MYPT1 knockout suggests that a soluble, MYPT1-unbound form of PP1cβ may additionally contribute to myosin RLC dephosphorylation and relaxation of smooth muscle. Using a combination of isoelectric focusing and isoform-specific immunoblotting, we found here that more than 90% of the total PP1c in mouse smooth muscles is the β isoform. Moreover, conditional knockout of PP1cα or PP1cγ in adult smooth muscles did not result in an apparent phenotype in mice up to 6 months of age and did not affect smooth muscle contractions ex vivo In contrast, smooth muscle-specific conditional PP1cβ knockout decreased contractile force development in bladder, ileal, and aortic tissues and reduced mouse survival. Bladder smooth muscle tissue from WT mice was selectively permeabilized to remove soluble PP1cβ to measure contributions of total (α-toxin treatment) and myosin-bound (Triton X-100 treatment) phosphatase activities toward phosphorylated RLC in myofilaments. Triton X-100 reduced PP1cβ content by 60% and the rate of RLC dephosphorylation by 2-fold. These results are consistent with the selective dephosphorylation of RLC by both MYPT1-bound and -unbound PP1cβ forms in smooth muscle.

Functions and therapeutic potential of protein phosphatase 1: Insights from mouse genetics

Protein phosphatase 1 (PP1) catalyzes more than half of all phosphoserine/threonine dephosphorylation reactions in mammalian cells. In vivo PP1 does not exist as a free catalytic subunit but is always associated with at least one regulatory PP1-interacting protein (PIP) to generate a large set of distinct holoenzymes. Each PP1 complex controls the dephosphorylation of only a small subset of PP1 substrates. We screened the literature for genetically engineered mouse models and identified models for all PP1 isoforms and 104 PIPs. PP1 itself and at least 49 PIPs were connected to human disease-associated phenotypes. Additionally, phenotypes related to 17 PIPs were clearly linked to altered PP1 function, while such information was lacking for 32 other PIPs. We propose structural reverse genetics, which combines structural characterization of proteins with mouse genetics, to identify new PP1-related therapeutic targets. The available mouse models confirm the pleiotropic action of PP1 in health and diseases.

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Four protein phosphatases 1 (PP1) isoforms and >200 PP1-interacting proteins (PIPs)

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Genetically engineered mice of 49 PIPs display human disease-associated phenotypes.

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The use of structural reverse genetics to identify the PP1-dependent phenotypes

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Various interaction sites in PP1:PIP complexes suit for therapeutic targeting.

Fibroblast-specific TGF-β-Smad2/3 signaling underlies cardiac fibrosis

The master cytokine TGF-β mediates tissue fibrosis associated with inflammation and tissue injury. TGF-β induces fibroblast activation and differentiation into myofibroblasts that secrete extracellular matrix proteins. Canonical TGF-β signaling mobilizes Smad2 and Smad3 transcription factors that control fibrosis by promoting gene expression. However, the importance of TGF-β-Smad2/3 signaling in fibroblast-mediated cardiac fibrosis has not been directly evaluated in vivo. Here, we examined pressure overload-induced cardiac fibrosis in fibroblast- and myofibroblast-specific inducible Cre-expressing mouse lines with selective deletion of the TGF-β receptors Tgfbr1/2, Smad2, or Smad3. Fibroblast-specific deletion of Tgfbr1/2 or Smad3, but not Smad2, markedly reduced the pressure overload-induced fibrotic response as well as fibrosis mediated by a heart-specific, latency-resistant TGF-β mutant transgene. Interestingly, cardiac fibroblast-specific deletion of Tgfbr1/2, but not Smad2/3, attenuated the cardiac hypertrophic response to pressure overload stimulation. Mechanistically, loss of Smad2/3 from tissue-resident fibroblasts attenuated injury-induced cellular expansion within the heart and the expression of fibrosis-mediating genes. Deletion of Smad2/3 or Tgfbr1/2 from cardiac fibroblasts similarly inhibited the gene program for fibrosis and extracellular matrix remodeling, although deletion of Tgfbr1/2 uniquely altered expression of an array of regulatory genes involved in cardiomyocyte homeostasis and disease compensation. These findings implicate TGF-β-Smad2/3 signaling in activated tissue-resident cardiac fibroblasts as principal mediators of the fibrotic response.

Mdm2 regulates cardiac contractility by inhibiting GRK2-mediated desensitization of β-adrenergic receptor signaling

The oncoprotein Mdm2 is a RING domain-containing E3 ubiquitin ligase that ubiquitinates G protein-coupled receptor kinase 2 (GRK2) and β-arrestin2, thereby regulating β-adrenergic receptor (βAR) signaling and endocytosis. Previous studies showed that cardiac Mdm2 expression is critical for controlling p53-dependent apoptosis during early embryonic development, but the role of Mdm2 in the developed adult heart is unknown. We aimed to identify if Mdm2 affects βAR signaling and cardiac function in adult mice. Using Mdm2/p53-KO mice, which survive for 9-12 months, we identified a critical and potentially novel role for Mdm2 in the adult mouse heart through its regulation of cardiac β1AR signaling. While baseline cardiac function was mostly similar in both Mdm2/p53-KO and wild-type (WT) mice, isoproterenol-induced cardiac contractility in Mdm2/p53-KO was significantly blunted compared with WT mice. Isoproterenol increased cAMP in left ventricles of WT but not of Mdm2/p53-KO mice. Additionally, while basal and forskolin-induced calcium handling in isolated Mdm2/p53-KO and WT cardiomyocytes were equivalent, isoproterenol-induced calcium handling in Mdm2/p53-KO was impaired. Mdm2/p53-KO hearts expressed 2-fold more GRK2 than WT. GRK2 polyubiquitination via lysine-48 linkages was significantly reduced in Mdm2/p53-KO hearts. Tamoxifen-inducible cardiomyocyte-specific deletion of Mdm2 in adult mice also led to a significant increase in GRK2, and resulted in severely impaired cardiac function, high mortality, and no detectable βAR responsiveness. Gene delivery of either Mdm2 or GRK2-CT in vivo using adeno-associated virus 9 (AAV9) effectively rescued β1AR-induced cardiac contractility in Mdm2/p53-KO. These findings reveal a critical p53-independent physiological role of Mdm2 in adult hearts, namely, regulation of GRK2-mediated desensitization of βAR signaling.

The heterotrimeric G protein Gβ1 interacts with the catalytic subunit of protein phosphatase 1 and modulates G protein-coupled receptor signaling in platelets

Thrombosis is caused by the activation of platelets at the site of ruptured atherosclerotic plaques. This activation involves engagement of G protein-coupled receptors (GPCR) on platelets that promote their aggregation. Although it is known that protein kinases and phosphatases modulate GPCR signaling, how serine/threonine phosphatases integrate with G protein signaling pathways is less understood. Because the subcellular localization and substrate specificity of the catalytic subunit of protein phosphatase 1 (PP1c) is dictated by PP1c-interacting proteins, here we sought to identify new PP1c interactors. GPCRs signal via the canonical heterotrimeric Gα and Gβγ subunits. Using a yeast two-hybrid screen, we discovered an interaction between PP1cα and the heterotrimeric G protein Gβ₁ subunit. Co-immunoprecipitation studies with epitope-tagged PP1c and Gβ₁ revealed that Gβ₁ interacts with the PP1c α, β, and γ1 isoforms. Purified PP1c bound to recombinant Gβ₁-GST protein, and PP1c co-immunoprecipitated with Gβ₁ in unstimulated platelets. Thrombin stimulation of platelets induced the dissociation of the PP1c-Gβ₁ complex, which correlated with an association of PP1c with phospholipase C β3 (PLCβ3), along with a concomitant dephosphorylation of the inhibitory Ser¹¹⁰⁵ residue in PLCβ3. siRNA-mediated depletion of GNB1 (encoding Gβ₁) in murine megakaryocytes reduced protease-activated receptor 4, activating peptide-induced soluble fibrinogen binding. Thrombin-induced aggregation was decreased in PP1cα^-/- murine platelets and in human platelets treated with a small-molecule inhibitor of Gβγ. Finally, disruption of PP1c-Gβ₁ complexes with myristoylated Gβ₁ peptides containing the PP1c binding site moderately decreased thrombin-induced human platelet aggregation. These findings suggest that Gβ₁ protein enlists PP1c to modulate GPCR signaling in platelets.

Differential regulation of protein phosphatase 1 (PP1) isoforms in human heart failure and atrial fibrillation

Protein phosphatase 1 (PP1) is a key regulator of important cardiac signaling pathways. Dysregulation of PP1 has been heavily implicated in cardiac dysfunctions. Accordingly, pharmacological targeting of PP1 activity is considered for therapeutic intervention in human cardiomyopathies. Recent evidence from animal models implicated previously unrecognized, isoform-specific activities of PP1 in the healthy and diseased heart. Therefore, this study examined the expression of the distinct PP1 isoforms PP1α, β, and γ in human heart failure (HF) and atrial fibrillation (AF) and addressed the consequences of β-adrenoceptor blocker (beta-blocker) therapy for HF patients with reduced ejection fraction on PP1 isoform expression. Using western blot analysis, we found greater abundance of PP1 isoforms α and γ but unaltered PP1β levels in left ventricular myocardial tissues from HF patients as compared to non-failing controls. However, expression of all three PP1 isoforms was higher in atrial appendages from patients with AF compared to patients with sinus rhythm. Moreover, we found that in human failing ventricles, beta-blocker therapy was associated with lower PP1α abundance and activity, as indicated by higher phosphorylation of the PP1α-specific substrate eIF2α. Greater eIF2α phosphorylation is a known repressor of protein translation, and accordingly, we found lower levels of the endoplasmic reticulum (ER) stress marker Grp78 in the very same samples. We propose that isoform-specific targeting of PP1α activity may be a novel and innovative therapeutic strategy for the treatment of human cardiac diseases by reducing ER stress conditions.

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.

Regulating the regulator: Insights into the cardiac protein phosphatase 1 interactome

Reversible phosphorylation of proteins is a delicate yet dynamic balancing act between kinases and phosphatases, the disturbance of which underlies numerous disease processes. While our understanding of protein kinases has grown tremendously over the past decades, relatively little is known regarding protein phosphatases. This may be because protein kinases are great in number and relatively specific in function, and thereby amenable to be studied in isolation, whereas protein phosphatases are much less abundant and more nonspecific in their function. To achieve subcellular localization and substrate specificity, phosphatases depend on partnering with a large number of regulatory subunits, protein scaffolds and/or other interactors. This added layer of complexity presents a significant barrier to their study, but holds the key to unexplored opportunities for novel pharmacologic intervention. In this review we focus on serine/threonine protein phosphatase type-1 (PP1), which plays an important role in cardiac physiology and pathophysiology. Although much work has been done to investigate the role of PP1 in cardiac diseases including atrial fibrillation and heart failure, most of these studies were limited to examining and manipulating the catalytic subunit(s) of PP1 without adequately considering the PP1 interactors, which give specificity to PP1's functions. To complement these studies, three unbiased methods have been developed and applied to the mapping of the PP1 interactome: bioinformatics approaches, yeast two-hybrid screens, and affinity-purification mass spectrometry. The application of these complementary methods has the potential to generate a detailed cardiac PP1 interactome, which is an important step in identifying novel and targeted pharmacological interventions.

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.

Role of myosin light chain phosphatase in cardiac physiology and pathophysiology

Maintenance of contractile performance of the heart is achieved in part by the constitutive 40% phosphorylation of myosin regulatory light chain (RLC) in sarcomeres. The importance of this extent of RLC phosphorylation for optimal cardiac performance becomes apparent when various mouse models and resultant phenotypes are compared. The absence or attenuation of RLC phosphorylation results in poor performance leading to heart failure, whereas increased RLC phosphorylation is associated with cardiac protection from stresses. Although information is limited, RLC phosphorylation appears compromised in human heart failure which is consistent with data from mouse studies. The extent of cardiac RLC phosphorylation is determined by the balanced activities of cardiac myosin light chain kinases and phosphatases, the regulatory mechanisms of which are now emerging. This review thusly focuses on kinases that may participate in phosphorylating RLC to make the substrate for cardiac myosin light chain phosphatases, in addition to providing perspectives on the family of myosin light chain phosphatases and involved signaling mechanisms. Because biochemical and physiological information about cardiac myosin light chain phosphatase is sparse, such studies represent an emerging area of investigation in health and disease.

The neglected messengers: Control of cardiac myofilaments by protein phosphatases

Cardiac myofilaments act as the central contractile apparatus of heart muscle cells. Covalent modification of constituent proteins through phosphorylation is a rapid and powerful mechanism to control myofilament function, and is increasingly seen as a mechanism of disease. While the relationship between protein kinases and cardiac myofilaments has been widely examined, the impact of protein dephosphorylation by protein phosphatases is poorly understood. This review outlines the mechanisms by which the mostly widely expressed protein phosphatases in cardiac myocytes regulate myofilament function, and the emerging role of myofilament-associated protein phosphatases in heart failure. The importance of regulatory subunits and subcellular compartmentalization in determining the functional impact of protein phosphatases on myofilament and myocardial function is also discussed, as are discrepancies about the roles of protein phosphatases in regulating myofilament function. The potential for targeting these molecular messengers in the treatment of heart failure is discussed as a key future direction.

De novo missense variants in PPP1CB are associated with intellectual disability and congenital heart disease

Intellectual disabilities are genetically heterogeneous and can be associated with congenital anomalies. Using whole-exome sequencing (WES), we identified five different de novo missense variants in the protein phosphatase-1 catalytic subunit beta (PPP1CB) gene in eight unrelated individuals who share an overlapping phenotype of dysmorphic features, macrocephaly, developmental delay or intellectual disability (ID), congenital heart disease, short stature, and skeletal and connective tissue abnormalities. Protein phosphatase-1 (PP1) is a serine/threonine-specific protein phosphatase involved in the dephosphorylation of a variety of proteins. The PPP1CB gene encodes a PP1 subunit that regulates the level of protein phosphorylation. All five altered amino acids we observed are highly conserved among the PP1 subunit family, and all are predicted to disrupt PP1 subunit binding and impair dephosphorylation. Our data suggest that our heterozygous de novo PPP1CB pathogenic variants are associated with syndromic intellectual disability.

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

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

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.

DUSP8 Regulates Cardiac Ventricular Remodeling by Altering ERK1/2 Signaling

RATIONALE

Mitogen-activated protein kinase (MAPK) signaling regulates the growth response of the adult myocardium in response to increased cardiac workload or pathological insults. The dual-specificity phosphatases (DUSPs) are critical effectors, which dephosphorylate the MAPKs to control the basal tone, amplitude, and duration of MAPK signaling.

OBJECTIVE

To examine DUSP8 as a regulator of MAPK signaling in the heart and its impact on ventricular and cardiac myocyte growth dynamics.

METHODS AND RESULTS

Dusp8 gene-deleted mice and transgenic mice with inducible expression of DUSP8 in the heart were used here to investigate how this MAPK-phosphatase might regulate intracellular signaling and cardiac growth dynamics in vivo. Dusp8 gene-deleted mice were mildly hypercontractile at baseline with a cardiac phenotype of concentric ventricular remodeling, which protected them from progressing towards heart failure in 2 surgery-induced disease models. Cardiac-specific overexpression of DUSP8 produced spontaneous eccentric remodeling and ventricular dilation with heart failure. At the cellular level, adult cardiac myocytes from Dusp8 gene-deleted mice were thicker and shorter, whereas DUSP8 overexpression promoted cardiac myocyte lengthening with a loss of thickness. Mechanistically, activation of extracellular signal-regulated kinases 1/2 were selectively increased in Dusp8 gene-deleted hearts at baseline and following acute pathological stress stimulation, whereas p38 MAPK and c-Jun N-terminal kinases were mostly unaffected.

CONCLUSIONS

These results indicate that DUSP8 controls basal and acute stress-induced extracellular signal-regulated kinases 1/2 signaling in adult cardiac myocytes that then alters the length-width growth dynamics of individual cardiac myocytes, which further alters contractility, ventricular remodeling, and disease susceptibility.

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

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