The A-kinase-anchoring protein AKAP-Lbc facilitates cardioprotective PKA phosphorylation of Hsp20 on Ser(16)

CBR1 decreases protein carbonyl levels via the ROS/Akt/CREB pathway to extend lifespan in the cotton bollworm, Helicoverpa armigera

Reactive oxygen species (ROS) are considered a major cause of ageing and ageing-related diseases through protein carbonylation. Little is known about the molecular mechanisms that confer protection against ROS. Here, we observed that, compared with nondiapause-destined pupae, high protein carbonyl levels are present in the brains of diapause-destined pupae, which is a 'non-ageing' phase in the moth Helicoverpa armigera. Protein carbonyl levels respond to ROS and decrease metabolic activity to induce diapause in order to extend lifespan. However, protein carbonylation in the brains of diapause-destined pupae still occurs at a physiological level compared to young adult brains. We find that ROS activate Akt, and Akt then phosphorylates the transcription factor CREB to facilitate its nuclear import. CREB binds to the promoter of carbonyl reductase 1 (CBR1) and regulates its expression. High CBR1 levels reduce protein carbonyl levels to maintain physiological levels. This is the first report showing that the moth brain can naturally control protein carbonyl levels through a distinct ROS-Akt-CREB-CBR1 pathway to extend lifespan.

Potential therapeutic applications of AKAP disrupting peptides

The 3'-5'-cyclic adenosine monophosphate (cAMP)/PKA pathway represents a major target for pharmacological intervention in multiple disease conditions. Although the last decade saw the concept of highly compartmentalized cAMP/PKA signaling consolidating, current means for the manipulation of this pathway still do not allow to specifically intervene on discrete cAMP/PKA microdomains. Since compartmentalization is crucial for action specificity, identifying new tools that allow local modulation of cAMP/PKA responses is an urgent need. Among key players of cAMP/PKA signaling compartmentalization, a major role is played by A-kinase anchoring proteins (AKAPs) that, by definition, anchor PKA, its substrates and its regulators within multiprotein complexes in well-confined subcellular compartments. Different tools have been conceived to interfere with AKAP-based protein-protein interactions (PPIs), and these primarily include peptides and peptidomimetics that disrupt AKAP-directed multiprotein complexes. While these molecules have been extensively used to understand the molecular mechanisms behind AKAP function in pathophysiological processes, less attention has been devoted to their potential application for therapy. In this review, we will discuss how AKAP-based PPIs can be pharmacologically targeted by synthetic peptides and peptidomimetics.

Inducible expression of heat shock protein 20 protects airway epithelial cells against oxidative injury involving the Nrf2-NQO-1 pathway

BACKGROUND

Heat shock protein (HSP) 20 is a molecular chaperone that exerts multiple protective functions in various kinds of tissues. However, the expression of HSP20 and its specific functions in airway epithelial cells (AECs) remain elusive.

RESULTS

In current study, we first confirmed the inducible expression of HSP20 in mouse AECs and in a human bronchial epithelial cell line BEAS-2B cells, under different oxidant stressors. Then by establishing a HSP20-abundant mouse model with repeated low-level-ozone exposures and stimulating this model with a single high-level ozone exposure, we found that the HSP20 abundance along with its enhanced phosphorylation potentially contributed to the alleviation of oxidative injuries, evidenced by the decreases in the bodyweight reduction, the BAL neutrophil accumulation, the AECs shedding, and the BAL concentrations of albumin and E-cadherin. The biological function of HSP20 and its molecular mechanisms were further investigated in BEAS-2B cells that were transfected with HSP20-, unphosphorylatable HSP20(Ala) or empty vector plasmids prior to the stimulation of H2O2, of which its oxidant capacity has been proved to be similar with those of ozone in an air-liquid culture system. We found that the H2O2-induced intracellular ROS level and the early cell apoptosis were attenuated in the HSP20- but not HSP20(Ala)- transfected cells. The intracellular expression of NQO-1 (mRNA and protein) and the intranuclear content of Nrf2 were significantly increased in the HSP20- transfected cells but not in the HSP20(Ala)- and empty vector-transfected cells after the stimulation of H2O2.

CONCLUSIONS

The inducible expression of HSP20 in AECs by oxidative stress exerts protective roles against oxidative damages, which may involve the activation of the Nrf2-NQO-1 pathway.

Reshaping cAMP nanodomains through targeted disruption of compartmentalised phosphodiesterase signalosomes

Spatio-temporal regulation of localised cAMP nanodomains is highly dependent upon the compartmentalised activity of phosphodiesterase (PDE) cyclic nucleotide degrading enzymes. Strategically positioned PDE-protein complexes are pivotal to the homeostatic control of cAMP-effector protein activity that in turn orchestrate a wide range of cellular signalling cascades in a variety of cells and tissue types. Unsurprisingly, dysregulated PDE activity is central to the pathophysiology of many diseases warranting the need for effective therapies that target PDEs selectively. This short review focuses on the importance of activating compartmentalised cAMP signalling by displacing the PDE component of signalling complexes using cell-permeable peptide disrupters.

A-Kinase Anchoring Protein-Lbc: A Molecular Scaffold Involved in Cardiac Protection

Heart failure is a lethal disease that can develop after myocardial infarction, hypertension, or anticancer therapy. In the damaged heart, loss of function is mainly due to cardiomyocyte death and associated cardiac remodeling and fibrosis. In this context, A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that facilitate the spatiotemporal activation of the cyclic adenosine monophosphate (AMP)-dependent protein kinase (PKA) and other transduction enzymes involved in cardiac remodeling. AKAP-Lbc, a cardiac enriched anchoring protein, has been shown to act as a key coordinator of the activity of signaling pathways involved in cardiac protection and remodeling. This review will summarize and discuss recent advances highlighting the role of the AKAP-Lbc signalosome in orchestrating adaptive responses in the stressed heart.

Modulation of nNOSser852 phosphorylation and translocation by PKA/PP1 pathway in endothelial cells

Neuronal nitric oxide synthase (nNOS) is now considered an important player in vascular function. It has a protective role in atherosclerosis and hypertension. However, despite its importance, little is known about the mechanisms that regulate its activity in vascular cells. Here we explore the mechanisms by which nNOS is activated in endothelium. We evaluated aorta relaxation response and phosphorylation of nNOS during protein phosphatases 1 and 2 (PP1 and PP2) inhibition, in eNOS silenced mice. PP1 translocation and interaction between the nuclear inhibitor of PP1 (NIPP1) and PP1 was evaluated in endothelial EA.hy926 cells. We demonstrate here that acetylcholine (Ach)-induced relaxation is completely abolished by nNOS inhibition in eNOS silenced mice aorta which also decreased NO and H₂O₂ concentrations. ACh induced dephosphorylation of nNOS^ser852 in aorta after 20 min stimulation. Endothelial cells also showed a decrease in nNOS^ser852 phosphorylation during 20 min of ACh stimulation. PP2 inhibition had no effect on Ach-induced nNOS^Ser852 dephosphorylation in endothelial cells and did not modify Ach-induced vasodilation in aorta from eNOS silenced mice. Non-selective PP1/PP2 inhibition prevented nNOSSer852 dephosphorylation in endothelial cells and prevented Ach-induced vasodilation in eNOS silenced mice. ACh induced time-dependent PP1 and NIPP1 dissociation and PP1 translocation to cytoplasm. Protein kinase A (PKA) inhibition abolished PP1 translocation and further nNOS^ser852 dephosphorylation. In addition, 8-Br-cAMP reduced NIPP1/PP1 interaction, stimulated PP1 translocation and nNOS^ser852 dephosphorylation. Moreover, PKA Inhibition led to a decreased nNOS translocation to perinuclear region. Taken together, our results elucidate a mechanism whereby PP1 is activated by a cAMP/PKA-dependent pathway, leading to dephosphorylation of nNOS^ser852 and subsequent NO and possible H₂O₂ production resulting in endothelium-dependent vascular relaxation.

AKAP-Lbc mediates protection against doxorubicin-induced cardiomyocyte toxicity

Doxorubicin (DOX) is a chemotherapic agent that is widely used to treat hematological and solid tumors. Despite its efficacy, DOX displays significant cardiac toxicity associated with cardiomyocytes death and heart failure. Cardiac toxicity is mainly associated with the ability of DOX to alter mitochondrial function. The current lack of treatments to efficiently prevent DOX cardiotoxicity underscores the need of new therapeutic approaches. Our current findings show that stimulation of cardiomyocytes with the α1-adrenergic receptor (AR) agonist phenylephrine (PE) significantly inhibits the apoptotic effect of DOX. Importantly, our results indicate that AKAP-Lbc is critical for transducing protective signals downstream of α1-ARs. In particular, we could show that suppression of AKAP-Lbc expression by infecting primary cultures of ventricular myocytes with lentiviruses encoding AKAP-Lbc specific short hairpin (sh) RNAs strongly impairs the ability of PE to reduce DOX-induced apoptosis. AKAP-Lbc-mediated cardiomyocyte protection requires the activation of anchored protein kinase D1 (PKD1)-dependent prosurvival pathways that promote the expression of the anti-apoptotic protein Bcl2 and inhibit the translocation of the pro-apoptotic protein Bax to mitochondria. In conclusion, AKAP-Lbc emerges as a coordinator of signals that protect cardiomyocytes against the toxic effects of DOX.

Loss of type 9 adenylyl cyclase triggers reduced phosphorylation of Hsp20 and diastolic dysfunction

Adenylyl cyclase type 9 (AC9) is found tightly associated with the scaffolding protein Yotiao and the I_Ks ion channel in heart. But apart from potential I_Ks regulation, physiological roles for AC9 are unknown. We show that loss of AC9 in mice reduces less than 3% of total AC activity in heart but eliminates Yotiao-associated AC activity. AC9^−/− mice exhibit no structural abnormalities but show a significant bradycardia, consistent with AC9 expression in sinoatrial node. Global changes in PKA phosphorylation patterns are not altered in AC9^−/− heart, however, basal phosphorylation of heat shock protein 20 (Hsp20) is significantly decreased. Hsp20 binds AC9 in a Yotiao-independent manner and deletion of AC9 decreases Hsp20-associated AC activity in heart. In addition, expression of catalytically inactive AC9 in neonatal cardiomyocytes decreases isoproterenol-stimulated Hsp20 phosphorylation, consistent with an AC9-Hsp20 complex. Phosphorylation of Hsp20 occurs largely in ventricles and is vital for the cardioprotective effects of Hsp20. Decreased Hsp20 phosphorylation suggests a potential baseline ventricular defect for AC9^−/−. Doppler echocardiography of AC9^−/− displays a decrease in the early ventricular filling velocity and ventricular filling ratio (E/A), indicative of grade 1 diastolic dysfunction and emphasizing the importance of local cAMP production in the context of macromolecular complexes.

Identification of a signaling cascade that maintains constitutive δ-opioid receptor incompetence in peripheral sensory neurons

μ-Opioid receptor (MOR) agonists are often used to treat severe pain but can result in adverse side effects. To circumvent systemic side effects, targeting peripheral opioid receptors is an attractive alternative treatment for severe pain. Activation of the δ-opioid receptor (DOR) produces similar analgesia with reduced side effects. However, until primed by inflammation, peripheral DOR is analgesically incompetent, raising interest in the mechanism. We recently identified a novel role for G-protein-coupled receptor kinase 2 (GRK2) that renders DOR analgesically incompetent at the plasma membrane. However, the mechanism that maintains constitutive GRK2 association with DOR is unknown. Protein kinase A (PKA) phosphorylation of GRK2 at Ser-685 targets it to the plasma membrane. Protein kinase A-anchoring protein 79/150 (AKAP), residing at the plasma membrane in neurons, scaffolds PKA to target proteins to mediate downstream signal. Therefore, we sought to determine whether GRK2-mediated DOR desensitization is directed by PKA via AKAP scaffolding. Membrane fractions from cultured rat sensory neurons following AKAP siRNA transfection and from AKAP-knock-out mice had less PKA activity, GRK2 Ser-685 phosphorylation, and GRK2 plasma membrane targeting than controls. Site-directed mutagenesis revealed that GRK2 Ser-685 phosphorylation drives the association of GRK2 with plasma membrane-associated DOR. Moreover, overexpression studies with AKAP mutants indicated that impaired AKAP-mediated PKA scaffolding significantly reduces DOR-GRK2 association at the plasma membrane and consequently increases DOR activity in sensory neurons without a priming event. These findings suggest that AKAP scaffolds PKA to increase plasma membrane targeting and phosphorylation of GRK2 to maintain DOR analgesic incompetence in peripheral sensory neurons.

Selective inhibition of phosphodiesterases 4, 5 and 9 induces HSP20 phosphorylation and attenuates amyloid beta 1-42-mediated cytotoxicity

Phosphodiesterase (PDE) inhibitors are currently under evaluation as agents that may facilitate the improvement of cognitive impairment associated with Alzheimer's disease. Our aim was to determine whether inhibitors of PDEs 4, 5 and 9 could alleviate the cytotoxic effects of amyloid beta 1–42 (Aβ_1–42) via a mechanism involving the small heatshock protein HSP20. We show that inhibition of PDEs 4, 5 and 9 but not 3 induces the phosphorylation of HSP20 which, in turn, increases the colocalisation between the chaperone and Aβ_1–42 to significantly decrease the toxic effect of the peptide. We conclude that inhibition of PDE9 is most effective to combat Aβ_1–42 cytotoxicity in our cell model.

Anchoring of both PKA-RIIα and 14-3-3θ regulates retinoic acid induced 16 mediated phosphorylation of heat shock protein 70

Our previous study reported that retinoic acid induced 16 (RAI16) could enhance tumorigenesis in hepatocellular carcinoma (HCC). However, the cellular functions of RAI16 are still unclear. In this study, by immunoprecipitation and tandem (MS/MS) mass spectrometry analysis, we identified that RAI16 interacted with the type II regulatory subunit of PKA (PKA-RIIα), acting as a novel protein kinase A anchoring protein (AKAP). In addition, RAI16 also interacted with heat shock protein 70 (HSP70) and 14-3-3θ. Further studies indicated that RAI16 mediated PKA phosphorylation of HSP70 at serine 486, resulting in anti-apoptosis events. RAI16 was also phosphorylated by the anchored PKA at serine 325, which promoted the recruitment of 14-3-3θ, which, in turn, inhibited RAI16 mediated PKA phosphorylation of HSP70. These findings offer mechanism insight into RAI16 mediated anti-apoptosis signaling in HCC.

The Steroid Hormone 20-Hydroxyecdysone Enhances Gene Transcription through the cAMP Response Element-binding Protein (CREB) Signaling Pathway

Animal steroid hormones regulate gene transcription through genomic pathways by binding to nuclear receptors. These steroid hormones also rapidly increase intracellular calcium and cyclic adenosine monophosphate (cAMP) levels and activate the protein kinase C (PKC) and protein kinase A (PKA) nongenomic pathways. However, the function and mechanism of the nongenomic pathways of the steroid hormones are unclear, and the relationship between the PKC and PKA pathways is also unclear. We propose that the steroid hormone 20-hydroxyecdysone (20E) activates the PKA pathway to enhance 20E-induced gene transcription in the lepidopteran insect Helicoverpa armigera The expression of the catalytic subunit 1 of PKA (PKAC1) increased during metamorphosis, and PKAC1 knockdown blocked pupation and repressed 20E-responsive gene expression. 20E regulated PKAC1 phosphorylation at threonine 200 and nuclear translocation through an ecdysone-responsive G-protein-coupled receptor 2. PKAC1 induced cAMP response element-binding protein (CREB) phosphorylation at serine 143, which bound to the cAMP response element on DNA to enhance 20E-responsive gene transcription. Through ecdysone-responsive G-protein-coupled receptor 2, 20E increased cAMP levels, which induced CREB PKA phosphorylation and 20E-responsive gene expression. This study demonstrates that the PKA/CREB pathway tightly and critically regulates 20E-induced gene transcription as well as its relationship with the 20E-induced PKC pathway.

AKAP150 participates in calcineurin/NFAT activation during the down-regulation of voltage-gated K(+) currents in ventricular myocytes following myocardial infarction

The Ca(2+)-responsive phosphatase calcineurin/protein phosphatase 2B dephosphorylates the transcription factor NFATc3. In the myocardium activation of NFATc3 down-regulates the expression of voltage-gated K(+) (Kv) channels after myocardial infarction (MI). This prolongs action potential duration and increases the probability of arrhythmias. Although recent studies infer that calcineurin is activated by local and transient Ca(2+) signals the molecular mechanism that underlies the process is unclear in ventricular myocytes. Here we test the hypothesis that sequestering of calcineurin to the sarcolemma of ventricular myocytes by the anchoring protein AKAP150 is required for acute activation of NFATc3 and the concomitant down-regulation of Kv channels following MI. Biochemical and cell based measurements resolve that approximately 0.2% of the total calcineurin activity in cardiomyocytes is associated with AKAP150. Electrophysiological analyses establish that formation of this AKAP150-calcineurin signaling dyad is essential for the activation of the phosphatase and the subsequent down-regulation of Kv channel currents following MI. Thus AKAP150-mediated targeting of calcineurin to sarcolemmal micro-domains in ventricular myocytes contributes to the local and acute gene remodeling events that lead to the down-regulation of Kv currents.

Emerging roles of A-kinase anchoring proteins in cardiovascular pathophysiology

Heart and blood vessels ensure adequate perfusion of peripheral organs with blood and nutrients. Alteration of the homeostatic functions of the cardiovascular system can cause hypertension, atherosclerosis, and coronary artery disease leading to heart injury and failure. A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that are crucially involved in modulating the function of the cardiovascular system both under physiological and pathological conditions. AKAPs assemble multifunctional signaling complexes that ensure correct targeting of the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to precise subcellular compartments. This allows local regulation of specific effector proteins that control the function of vascular and cardiac cells. This review will focus on recent advances illustrating the role of AKAPs in cardiovascular pathophysiology. The accent will be mainly placed on the molecular events linked to the control of vascular integrity and blood pressure as well as on the cardiac remodeling process associated with heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling

The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer.

Targeting protein-protein interactions in complexes organized by A kinase anchoring proteins

Cyclic AMP is a ubiquitous intracellular second messenger involved in the regulation of a wide variety of cellular processes, a majority of which act through the cAMP – protein kinase A (PKA) signaling pathway and involve PKA phosphorylation of specific substrates. PKA phosphorylation events are typically spatially restricted and temporally well controlled. A-kinase anchoring proteins (AKAPs) directly bind PKA and recruit it to specific subcellular loci targeting the kinase activity toward particular substrates, and thereby provide discrete spatiotemporal control of downstream phosphorylation events. AKAPs also scaffold other signaling molecules into multi-protein complexes that function as crossroads between different signaling pathways. Targeting AKAP coordinated protein complexes with high-affinity peptidomimetics or small molecules to tease apart distinct protein–protein interactions (PPIs) therefore offers important means to disrupt binding of specific components of the complex to better understand the molecular mechanisms involved in the function of individual signalosomes and their pathophysiological role. Furthermore, development of novel classes of small molecules involved in displacement of AKAP-bound signal molecules is now emerging. Here, we will focus on mechanisms for targeting PPI, disruptors that modulate downstream cAMP signaling and their role, especially in the heart.

The cardioprotective role of small heat-shock protein 20

The small HSP (heat-shock protein) HSP20 is a molecular chaperone that is transiently up-regulated in response to cellular stress/damage. Although ubiquitously expressed in various tissues, it is most highly expressed in skeletal, cardiac and smooth muscle. Phosphorylation at Ser16 by PKA (cAMP-dependent protein kinase) is essential for HSP20 to confer its protective qualities. HSP20 and its phosphorylation have been implicated in a variety of pathophysiological processes, but most prominently cardiovascular disease. A wealth of knowledge of the importance of HSP20 in contractile function and cardioprotection has been gained over the last decade. The present mini-review highlights more recent findings illustrating the cardioprotective properties of HSP20 and its potential as a therapeutic agent.

The crystal structure of the RhoA-AKAP-Lbc DH-PH domain complex

The RhoGEF (Rho GTPase guanine-nucleotide-exchange factor) domain of AKAP-Lbc (A-kinase-anchoring protein-Lbc, also known as AKAP13) catalyses nucleotide exchange on RhoA and is involved in the development of cardiac hypertrophy. The RhoGEF activity of AKAP-Lbc has also been implicated in cancer. We have determined the X-ray crystal structure of the complex between RhoA–GDP and the AKAP-Lbc RhoGEF [DH (Dbl-homologous)–PH (pleckstrin homology)] domain to 2.1 Å (1 Å=0.1 nm) resolution. The structure reveals important differences compared with related RhoGEF proteins such as leukaemia-associated RhoGEF. Nucleotide-exchange assays comparing the activity of the DH–PH domain to the DH domain alone showed no role for the PH domain in nucleotide exchange, which is explained by the RhoA–AKAP-Lbc structure. Comparison with a structure of the isolated AKAP-Lbc DH domain revealed a change in conformation of the N-terminal ‘GEF switch’ region upon binding to RhoA. Isothermal titration calorimetry showed that AKAP-Lbc has only micromolar affinity for RhoA, which combined with the presence of potential binding pockets for small molecules on AKAP-Lbc, raises the possibility of targeting AKAP-Lbc with GEF inhibitors.

The crystal structure of the RhoGEF domain of AKAP-Lbc in complex with RhoA combined with nucleotide exchange assays explain differences to related RhoGEF proteins and allow the possibility of targeting the AKAP-Lbc RhoGEF domain with small molecules.

Small heat shock protein 20 (Hsp20) facilitates nuclear import of protein kinase D 1 (PKD1) during cardiac hypertrophy

BACKGROUND

Nuclear import of protein kinase D1 (PKD1) is an important event in the transcriptional regulation of cardiac gene reprogramming leading to the hypertrophic growth response, however, little is known about the molecular events that govern this event. We have identified a novel complex between PKD1 and a heat shock protein (Hsp), Hsp20, which has been implicated as cardioprotective. This study aims to characterize the role of the complex in PKD1-mediated myocardial regulatory mechanisms that depend on PKD1 nuclear translocation.

RESULTS

In mapping the Hsp20 binding sites on PKD1 within its catalytic unit using peptide array analysis, we were able to develop a cell-permeable peptide that disrupts the Hsp20-PKD1 complex. We use this peptide to show that formation of the Hsp20-PKD1 complex is essential for PKD1 nuclear translocation, signaling mechanisms leading to hypertrophy, activation of the fetal gene programme and pathological cardiac remodeling leading to cardiac fibrosis.

CONCLUSIONS

These results identify a new signaling complex that is pivotal to pathological remodelling of the heart that could be targeted therapeutically.

Active site coupling in PDE:PKA complexes promotes resetting of mammalian cAMP signaling

Cyclic 3'5' adenosine monophosphate (cAMP)-dependent-protein kinase (PKA) signaling is a fundamental regulatory pathway for mediating cellular responses to hormonal stimuli. The pathway is activated by high-affinity association of cAMP with the regulatory subunit of PKA and signal termination is achieved upon cAMP dissociation from PKA. Although steps in the activation phase are well understood, little is known on how signal termination/resetting occurs. Due to the high affinity of cAMP to PKA (KD ∼ low nM), bound cAMP does not readily dissociate from PKA, thus begging the question of how tightly bound cAMP is released from PKA to reset its signaling state to respond to subsequent stimuli. It has been recently shown that phosphodiesterases (PDEs) can catalyze dissociation of bound cAMP and thereby play an active role in cAMP signal desensitization/termination. This is achieved through direct interactions with the regulatory subunit of PKA, thereby facilitating cAMP dissociation and hydrolysis. In this study, we have mapped direct interactions between a specific cyclic nucleotide phosphodiesterase (PDE8A) and a PKA regulatory subunit (RIα isoform) in mammalian cAMP signaling, by a combination of amide hydrogen/deuterium exchange mass spectrometry, peptide array, and computational docking. The interaction interface of the PDE8A:RIα complex, probed by peptide array and hydrogen/deuterium exchange mass spectrometry, brings together regions spanning the phosphodiesterase active site and cAMP-binding sites of RIα. Computational docking combined with amide hydrogen/deuterium exchange mass spectrometry provided a model for parallel dissociation of bound cAMP from the two tandem cAMP-binding domains of RIα. Active site coupling suggests a role for substrate channeling in the PDE-dependent dissociation and hydrolysis of cAMP bound to PKA. This is the first instance, to our knowledge, of PDEs directly interacting with a cAMP-receptor protein in a mammalian system, and highlights an entirely new class of binding partners for RIα. This study also highlights applications of structural mass spectrometry combined with computational docking for mapping dynamics in transient signaling protein complexes. Together, these results present a novel and critical role for phosphodiesterases in moderating local concentrations of cAMP in microdomains and signal resetting.

UCR1C is a novel activator of phosphodiesterase 4 (PDE4) long isoforms and attenuates cardiomyocyte hypertrophy

Hypertrophy increases the risk of heart failure and arrhythmia. Prevention or reversal of the maladaptive hypertrophic phenotype has thus been proposed to treat heart failure. Chronic β-adrenergic receptor (β-AR) stimulation induces cardiomyocyte hypertrophy by elevating 3',5'-cyclic adenosine monophosphate (cAMP) levels and activating downstream effectors such protein kinase A (PKA). Conversely, hydrolysis of cAMP by phosphodiesterases (PDEs) spatiotemporally restricts cAMP signaling. Here, we demonstrate that PDE4, but not PDE3, is critical in regulating cardiomyocyte hypertrophy, and may represent a potential target for preventing maladaptive hypertrophy. We identify a sequence within the upstream conserved region 1 of PDE4D, termed UCR1C, as a novel activator of PDE4 long isoforms. UCR1C activates PDE4 in complex with A-kinase anchoring protein (AKAP)-Lbc resulting in decreased PKA signaling facilitated by AKAP-Lbc. Expression of UCR1C in cardiomyocytes inhibits hypertrophy in response to chronic β-AR stimulation. This effect is partially due to inhibition of nuclear PKA activity, which decreases phosphorylation of the transcription factor cAMP response element-binding protein (CREB). In conclusion, PDE4 activation by UCR1C attenuates cardiomyocyte hypertrophy by specifically inhibiting nuclear PKA activity.

Phosphodiesterases maintain signaling fidelity via compartmentalization of cyclic nucleotides

Novel technological advances have improved our understanding of how cyclic nucleotides are able to convey signals faithfully between cellular compartments. Phosphodiesterases play a crucial role in shaping these signals in health and disease. The concept of compartmentalization is guiding the search for therapies that have the potential to offer greater efficacy and tolerability compared with current treatments.

SPECHT - single-stage phosphopeptide enrichment and stable-isotope chemical tagging: quantitative phosphoproteomics of insulin action in muscle

The study of cellular signaling remains a significant challenge for translational and clinical research. In particular, robust and accurate methods for quantitative phosphoproteomics in tissues and tumors represent significant hurdles for such efforts. In the present work, we design, implement and validate a method for single-stage phosphopeptide enrichment and stable isotope chemical tagging, or SPECHT, that enables the use of iTRAQ, TMT and/or reductive dimethyl-labeling strategies to be applied to phosphoproteomics experiments performed on primary tissue. We develop and validate our approach using reductive dimethyl-labeling and HeLa cells in culture, and find these results indistinguishable from data generated from more traditional SILAC-labeled HeLa cells mixed at the cell level. We apply the SPECHT approach to the quantitative analysis of insulin signaling in a murine myotube cell line and muscle tissue, identify known as well as new phosphorylation events, and validate these phosphorylation sites using phospho-specific antibodies. Taken together, our work validates chemical tagging post-single-stage phosphoenrichment as a general strategy for studying cellular signaling in primary tissues.

BIOLOGICAL SIGNIFICANCE

Through the use of a quantitatively reproducible, proteome-wide phosphopeptide enrichment strategy, we demonstrated the feasibility of post-phosphopeptide purification chemical labeling and tagging as an enabling approach for quantitative phosphoproteomics of primary tissues. Using reductive dimethyl labeling as a generalized chemical tagging strategy, we compared the performance of post-phosphopeptide purification chemical tagging to the well established community standard, SILAC, in insulin-stimulated tissue culture cells. We then extended our method to the analysis of low-dose insulin signaling in murine muscle tissue, and report on the analytical and biological significance of our results.

Targeted disruption of the heat shock protein 20-phosphodiesterase 4D (PDE4D) interaction protects against pathological cardiac remodelling in a mouse model of hypertrophy

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A peptide was discovered that disrupts HSP20–phosphodiesterase 4D (PDE4D) complex formation.

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HSP20–PDE4D complex disruption reversed hypertrophic-induced changes in electrical signalling in human cardiac myocytes.

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HSP20–PDE4D complex disruption attenuated the physiological response to pressure/volume overload.

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This physiological response normally results in an increase in cardiac myocyte size.

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Cardiac fibrosis was reduced in mice following treatment with the HSP20–PDE4D disruptor peptide.

Phosphorylated heat shock protein 20 (HSP20) is cardioprotective. Using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and a mouse model of pressure overload mediated hypertrophy, we show that peptide disruption of the HSP20–phosphodiesterase 4D (PDE4D) complex results in attenuation of action potential prolongation and protection against adverse cardiac remodelling. The later was evidenced by improved contractility, decreased heart weight to body weight ratio, and reduced interstitial and perivascular fibrosis. This study demonstrates that disruption of the specific HSP20–PDE4D interaction leads to attenuation of pathological cardiac remodelling.

Quantitative phosphoproteomic analysis of signaling downstream of the prostaglandin e2/g-protein coupled receptor in human synovial fibroblasts: potential antifibrotic networks

The Prostaglandin E2 (PGE2) signaling mechanism within fibroblasts is of growing interest as it has been shown to prevent numerous fibrotic features of fibroblast activation with limited evidence of downstream pathways. To understand the mechanisms of fibroblasts producing tremendous amounts of PGE2 with autocrine effects, we apply a strategy of combining a wide-screening of PGE2-induced kinases with quantitative phosphoproteomics. Our large-scale proteomic approach identified a PKA signal transmitted through phosphorylation of its substrates harboring the R(R/X)X(S*/T*) motif. We documented 115 substrates, of which 72 had 89 sites with a 2.5-fold phosphorylation difference in PGE2-treated cells than in untreated cells, where approximately half of such sites were defined as being novel. They were compiled by networking software to focus on highlighted activities and to associate them with a functional readout of fibroblasts. The substrates were associated with a variety of cellular functions including cytoskeletal structures (migration/motility), regulators of G-protein coupled receptor function, protein kinases, and transcriptional/translational regulators. For the first time, we extended the PGE2 pathway into an elaborate network of interconnecting phosphoproteins, providing vital information to a once restricted signalosome. These data provide new insights into eicosanoid-initiated cell signaling with regards to the regulation of fibroblast activation and the identification of new targets for evidenced-based pharmacotherapy against fibrosis.

The phosphorylation of Hsp20 enhances its association with amyloid-β to increase protection against neuronal cell death

Up-regulation of Hsp20 protein levels in response to amyloid fibril formation is considered a key protective response against the onset of Alzheimer's disease (AD). Indeed, the physical interaction between Hsp20 and Aβ is known to prevent Aβ oligomerisation and protects neuronal cells from Aβ mediated toxicity, however, details of the molecular mechanism and regulatory cell signalling events behind this process have remained elusive. Using both conventional MTT end-point assays and novel real time measurement of cell impedance, we show that Hsp20 protects human neuroblastoma SH-SY5Y cells from the neurotoxic effects of Aβ. In an attempt to provide a mechanism for the neuroprotection afforded by Hsp20, we used peptide array, co-immunoprecipitation analysis and NMR techniques to map the interaction between Hsp20 and Aβ and report a binding mode where Hsp20 binds adjacent to the oligomerisation domain of Aβ, preventing aggregation. The Hsp20/Aβ interaction is enhanced by Hsp20 phosphorylation, which serves to increase association with low molecular weight Aβ species and decrease the effective concentration of Hsp20 required to disrupt the formation of amyloid oligomers. Finally, using a novel fluorescent assay for the real time evaluation of morphology-specific Aβ aggregation, we show that phospho-dependency of this effect is more pronounced for fibrils than for globular Aβ forms and that 25mers corresponding to the Hsp20 N-terminal can be used as Aβ aggregate inhibitors. Our report is the first to provide a molecular model for the Hsp20/Aβ complex and the first to suggest that modulation of the cAMP/cGMP pathways could be a novel route to enhance Hsp20-mediated attenuation of Aβ fibril neurotoxicity.

Identification of a protein phosphatase-1/phospholamban complex that is regulated by cAMP-dependent phosphorylation

In human and experimental heart failure, the activity of the type 1 phosphatase is significantly increased, associated with dephosphorylation of phospholamban, inhibition of the sarco(endo)plasmic reticulum Ca²⁺ transport ATPase (SERCA2a) and depressed function. In the current study, we investigated the molecular mechanisms controlling protein phosphatase-1 activity. Using recombinant proteins and complementary in vitro binding studies, we identified a multi-protein complex centered on protein phosphatase-1 that includes its muscle specific glycogen-targeting subunit G_M and substrate phospholamban. G_M interacts directly with phospholamban and this association is mediated by the cytosolic regions of the proteins. Our findings suggest the involvement of G_M in mediating formation of the phosphatase-1/G_M/phospholamban complex through the direct and independent interactions of G_M with both protein phosphatase-1 and phospholamban. Importantly, the protein phosphatase-1/G_M/phospholamban complex dissociates upon protein kinase A phosphorylation, indicating its significance in the β-adrenergic signalling axis. Moreover, protein phosphatase-1 activity is regulated by two binding partners, inhibitor-1 and the small heat shock protein 20, Hsp20. Indeed, human genetic variants of inhibitor-1 (G147D) or Hsp20 (P20L) result in reduced binding and inhibition of protein phosphatase-1, suggesting aberrant enzymatic regulation in human carriers. These findings provide insights into the mechanisms underlying fine-tuned regulation of protein phosphatase-1 and its impact on the SERCA2/phospholamban interactome in cardiac function.

AKAP signaling complexes: pointing towards the next generation of therapeutic targets?

A-kinase anchoring proteins (AKAPs) streamline signal transduction by localizing signaling enzymes with their substrates. Great strides have been made in elucidating the role of these macromolecular signaling complexes as new binding partners and novel AKAPs are continually being uncovered. The mechanics and dynamics of these multi-enzyme assemblies suggest that AKAP complexes are viable targets for therapeutic intervention. This review will highlight recent advances in AKAP research focusing on local signaling events that are perturbed in disease.

Targeting protein-protein interactions within the cyclic AMP signaling system as a therapeutic strategy for cardiovascular disease

The cAMP signaling system can trigger precise physiological cellular responses that depend on the fidelity of many protein-protein interactions, which act to bring together signaling intermediates at defined locations within cells. In the heart, cAMP participates in the fine control of excitation-contraction coupling, hence, any disregulation of this signaling cascade can lead to cardiac disease. Due to the ubiquitous nature of the cAMP pathway, general inhibitors of cAMP signaling proteins such as PKA, EPAC and PDEs would act non-specifically and universally, increasing the likelihood of serious 'off target' effects. Recent advances in the discovery of peptides and small molecules that disrupt the protein-protein interactions that underpin cellular targeting of cAMP signaling proteins are described and discussed.

A-kinase anchoring protein Lbc coordinates a p38 activating signaling complex controlling compensatory cardiac hypertrophy

In response to stress, the heart undergoes a remodeling process associated with cardiac hypertrophy that eventually leads to heart failure. A-kinase anchoring proteins (AKAPs) have been shown to coordinate numerous prohypertrophic signaling pathways in cultured cardiomyocytes. However, it remains to be established whether AKAP-based signaling complexes control cardiac hypertrophy and remodeling in vivo. In the current study, we show that AKAP-Lbc assembles a signaling complex composed of the kinases PKN, MLTK, MKK3, and p38α that mediates the activation of p38 in cardiomyocytes in response to stress signals. To address the role of this complex in cardiac remodeling, we generated transgenic mice displaying cardiomyocyte-specific overexpression of a molecular inhibitor of the interaction between AKAP-Lbc and the p38-activating module. Our results indicate that disruption of the AKAP-Lbc/p38 signaling complex inhibits compensatory cardiomyocyte hypertrophy in response to aortic banding-induced pressure overload and promotes early cardiac dysfunction associated with increased myocardial apoptosis, stress gene activation, and ventricular dilation. Attenuation of hypertrophy results from a reduced protein synthesis capacity, as indicated by decreased phosphorylation of 4E-binding protein 1 and ribosomal protein S6. These results indicate that AKAP-Lbc enhances p38-mediated hypertrophic signaling in the heart in response to abrupt increases in the afterload.

AKAP13 Rho-GEF and PKD-binding domain deficient mice develop normally but have an abnormal response to β-adrenergic-induced cardiac hypertrophy

BACKGROUND

A-kinase anchoring proteins (AKAPs) are scaffolding molecules that coordinate and integrate G-protein signaling events to regulate development, physiology, and disease. One family member, AKAP13, encodes for multiple protein isoforms that contain binding sites for protein kinase A (PKA) and D (PKD) and an active Rho-guanine nucleotide exchange factor (Rho-GEF) domain. In mice, AKAP13 is required for development as null embryos die by embryonic day 10.5 with cardiovascular phenotypes. Additionally, the AKAP13 Rho-GEF and PKD-binding domains mediate cardiomyocyte hypertrophy in cell culture. However, the requirements for the Rho-GEF and PKD-binding domains during development and cardiac hypertrophy are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To determine if these AKAP13 protein domains are required for development, we used gene-trap events to create mutant mice that lacked the Rho-GEF and/or the protein kinase D-binding domains. Surprisingly, heterozygous matings produced mutant mice at Mendelian ratios that had normal viability and fertility. The adult mutant mice also had normal cardiac structure and electrocardiograms. To determine the role of these domains during β-adrenergic-induced cardiac hypertrophy, we stressed the mice with isoproterenol. We found that heart size was increased similarly in mice lacking the Rho-GEF and PKD-binding domains and wild-type controls. However, the mutant hearts had abnormal cardiac contractility as measured by fractional shortening and ejection fraction.

CONCLUSIONS

These results indicate that the Rho-GEF and PKD-binding domains of AKAP13 are not required for mouse development, normal cardiac architecture, or β-adrenergic-induced cardiac hypertrophic remodeling. However, these domains regulate aspects of β-adrenergic-induced cardiac hypertrophy.

Chemical informatics uncovers a new role for moexipril as a novel inhibitor of cAMP phosphodiesterase-4 (PDE4)

PDE4 is one of eleven known cyclic nucleotide phosphodiesterase families and plays a pivotal role in mediating hydrolytic degradation of the important cyclic nucleotide second messenger, cyclic 3′5′ adenosine monophosphate (cAMP). PDE4 inhibitors are known to have anti-inflammatory properties, but their use in the clinic has been hampered by mechanism-associated side effects that limit maximally tolerated doses. In an attempt to initiate the development of better-tolerated PDE4 inhibitors we have surveyed existing approved drugs for PDE4-inhibitory activity. With this objective, we utilised a high-throughput computational approach that identified moexipril, a well tolerated and safe angiotensin-converting enzyme (ACE) inhibitor, as a PDE4 inhibitor. Experimentally we showed that moexipril and two structurally related analogues acted in the micro molar range to inhibit PDE4 activity. Employing a FRET-based biosensor constructed from the nucleotide binding domain of the type 1 exchange protein activated by cAMP, EPAC1, we demonstrated that moexipril markedly potentiated the ability of forskolin to increase intracellular cAMP levels. Finally, we demonstrated that the PDE4 inhibitory effect of moexipril is functionally able to induce phosphorylation of the small heat shock protein, Hsp20, by cAMP dependent protein kinase A. Our data suggest that moexipril is a bona fide PDE4 inhibitor that may provide the starting point for development of novel PDE4 inhibitors with an improved therapeutic window.

A kinase interacting protein (AKIP1) is a key regulator of cardiac stress

cAMP-dependent protein kinase (PKA) regulates a myriad of functions in the heart, including cardiac contractility, myocardial metabolism,and gene expression. However, a molecular integrator of the PKA response in the heart is unknown. Here, we show that the PKA adaptor A-kinase interacting protein 1 (AKIP1) is up-regulated in cardiac myocytes in response to oxidant stress. Mice with cardiac gene transfer of AKIP1 have enhanced protection to ischemic stress. We hypothesized that this adaptation to stress was mitochondrial dependent. AKIP1 interacted with the mitochondrial localized apoptosis inducing factor (AIF) under both normal and oxidant stress. When cardiac myocytes or whole hearts are exposed to oxidant and ischemic stress, levels of both AKIP1 and AIF were enhanced. AKIP1 is preferentially localized to interfibrillary mitochondria and up-regulated in this cardiac mitochondrial subpopulation on ischemic injury. Mitochondria isolated from AKIP1 gene transferred hearts showed increased mitochondrial localization of AKIP1, decreased reactive oxygen species generation, enhanced calcium tolerance, decreased mitochondrial cytochrome C release,and enhance phosphorylation of mitochondrial PKA substrates on ischemic stress. These observations highlight AKIP1 as a critical molecular regulator and a therapeutic control point for stress adaptation in the heart.

A-kinase anchoring proteins: molecular regulators of the cardiac stress response

In response to stress or injury the heart undergoes a pathological remodeling process, associated with hypertrophy, cardiomyocyte death and fibrosis, that ultimately causes cardiac dysfunction and heart failure. It has become increasingly clear that signaling events associated with these pathological cardiac remodeling events are regulated by scaffolding and anchoring proteins, which allow coordination of pathological signals in space and time. A-kinase anchoring proteins (AKAPs) constitute a family of functionally related proteins that organize multiprotein signaling complexes that tether the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to ensure integration and processing of multiple signaling pathways. This review will discuss the role of AKAPs in the cardiac response to stress. Particular emphasis will be given to the adaptative process associated with cardiac hypoxia as well as the remodeling events linked to cardiac hypertrophy and heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.