Regulation of PKA binding to AKAPs in the heart: alterations in human heart failure

LRBA is essential for urinary concentration and body water homeostasis

Protein kinase A (PKA) directly phosphorylates aquaporin-2 (AQP2) water channels in renal collecting ducts to reabsorb water from urine for the maintenance of systemic water homeostasis. More than 50 functionally distinct PKA-anchoring proteins (AKAPs) respectively create compartmentalized PKA signaling to determine the substrate specificity of PKA. Identification of an AKAP responsible for AQP2 phosphorylation is an essential step toward elucidating the molecular mechanisms of urinary concentration. PKA activation by several compounds is a novel screening strategy to uncover PKA substrates whose phosphorylation levels were nearly perfectly correlated with that of AQP2. The leading candidate in this assay proved to be an AKAP termed lipopolysaccharide-responsive and beige-like anchor protein (LRBA). We found that LRBA colocalized with AQP2 in vivo, and Lrba knockout mice displayed a polyuric phenotype with severely impaired AQP2 phosphorylation. Most of the PKA substrates other than AQP2 were adequately phosphorylated by PKA in the absence of LRBA, demonstrating that LRBA-anchored PKA preferentially phosphorylated AQP2 in renal collecting ducts. Furthermore, the LRBA-PKA interaction, rather than other AKAP-PKA interactions, was robustly dissociated by PKA activation. AKAP-PKA interaction inhibitors have attracted attention for their ability to directly phosphorylate AQP2. Therefore, the LRBA-PKA interaction is a promising drug target for the development of anti-aquaretics.

Tuning the Consonance of Microscopic Neuro-Cardiac Interactions Allows the Heart Beats to Play Countless Genres

Different from skeletal muscle, the heart autonomously generates rhythmic contraction independently from neuronal inputs. However, speed and strength of the heartbeats are continuously modulated by environmental, physical or emotional inputs, delivered by cardiac innervating sympathetic neurons, which tune cardiomyocyte (CM) function, through activation of β-adrenoceptors (β-ARs). Given the centrality of such mechanism in heart regulation, β-AR signaling has been subject of intense research, which has reconciled the molecular details of the transduction pathway and the fine architecture of cAMP signaling in subcellular nanodomains, with its final effects on CM function. The importance of mechanisms keeping the elements of β-AR/cAMP signaling in good order emerges in pathology, when the loss of proper organization of the transduction pathway leads to detuned β-AR/cAMP signaling, with detrimental consequences on CM function. Despite the compelling advancements in decoding cardiac β-AR/cAMP signaling, most discoveries on the subject were obtained in isolated cells, somehow neglecting that complexity may encompass the means in which receptors are activated in the intact heart. Here, we outline a set of data indicating that, in the context of the whole myocardium, the heart orchestra (CMs) is directed by a closely interacting and continuously attentive conductor, represented by SNs. After a roundup of literature on CM cAMP regulation, we focus on the unexpected complexity and roles of cardiac sympathetic innervation, and present the recently discovered Neuro-Cardiac Junction, as the election site of "SN-CM" interaction. We further discuss how neuro-cardiac communication is based on the combination of extra- and intra-cellular signaling micro/nano-domains, implicating neuronal neurotransmitter exocytosis, β-ARs and elements of cAMP homeostasis in CMs, and speculate on how their dysregulation may reflect on dysfunctional neurogenic control of the heart in pathology.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Mapping genetic changes in the cAMP-signaling cascade in human atria

AIM

To obtain a quantitative expression profile of the main genes involved in the cAMP-signaling cascade in human control atria and in different cardiac pathologies.

METHODS AND RESULTS

Expression of 48 target genes playing a relevant role in the cAMP-signaling cascade was assessed by RT-qPCR. 113 samples were obtained from right atrial appendages (RAA) of patients in sinus rhythm (SR) with or without atrium dilation, paroxysmal atrial fibrillation (AF), persistent AF or heart failure (HF); and left atrial appendages (LAA) from patients in SR or with AF. Our results show that right and left atrial appendages in donor hearts or from SR patients have similar expression values except for AC7 and PDE2A. Despite the enormous chamber-dependent variability in the gene-expression changes between pathologies, several distinguishable patterns could be identified. PDE8A, PI3Kγ and EPAC2 were upregulated in AF. Different phosphodiesterase (PDE) families showed specific pathology-dependent changes.

CONCLUSION

By comparing mRNA-expression patterns of the cAMP-signaling cascade related genes in right and left atrial appendages of human hearts and across different pathologies, we show that 1) gene expression is not significantly affected by cardioplegic solution content, 2) it is appropriate to use SR atrial samples as controls, and 3) many genes in the cAMP-signaling cascade are affected in AF and HF but only few of them appear to be chamber (right or left) specific.

TOPIC

Genetic changes in human diseased atria.

TRANSLATIONAL PERSPECTIVE

The cyclic AMP signaling pathway is important for atrial function. However, expression patterns of the genes involved in the atria of healthy and diseased hearts are still unclear. We give here a general overview of how different pathologies affect the expression of key genes in the cAMP signaling pathway in human right and left atria appendages. Our study may help identifying new genes of interest as potential therapeutic targets or clinical biomarkers for these pathologies and could serve as a guide in future gene therapy studies.

Computational modeling approaches to cAMP/PKA signaling in cardiomyocytes

The cAMP/PKA pathway is a fundamental regulator of excitation-contraction coupling in cardiomyocytes. Activation of cAMP has a variety of downstream effects on cardiac function including enhanced contraction, accelerated relaxation, adaptive stress response, mitochondrial regulation, and gene transcription. Experimental advances have shed light on the compartmentation of cAMP and PKA, which allow for control over the varied targets of these second messengers and is disrupted in heart failure conditions. Computational modeling is an important tool for understanding the spatial and temporal complexities of this system. In this review article, we outline the advances in computational modeling that have allowed for deeper understanding of cAMP/PKA dynamics in the cardiomyocyte in health and disease, and explore new modeling frameworks that may bring us closer to a more complete understanding of this system. We outline various compartmental and spatial signaling models that have been used to understand how β-adrenergic signaling pathways function in a variety of simulation conditions. We also discuss newer subcellular models of cardiovascular function that may be used as templates for the next phase of computational study of cAMP and PKA in the heart, and outline open challenges which are important to consider in future models.

Selective changes in cytosolic β-adrenergic cAMP signals and L-type Calcium Channel regulation by Phosphodiesterases during cardiac hypertrophy

Background In cardiomyocytes, phosphodiesterases (PDEs) type 3 and 4 are the predominant enzymes that degrade cAMP generated by β-adrenergic receptors (β-ARs), impacting notably the regulation of the L-type Ca2+ current (ICa,L). Cardiac hypertrophy (CH) is accompanied by a reduction in PDE3 and PDE4, however, whether this affects the dynamic regulation of cytosolic cAMP and ICa,L is not known. Methods and Results CH was induced in rats by thoracic aortic banding over a time period of five weeks and was confirmed by anatomical measurements. Left ventricular myocytes (LVMs) were isolated from CH and sham-operated (SHAM) rats and transduced with an adenovirus encoding a Förster resonance energy transfer (FRET)-based cAMP biosensor or subjected to the whole-cell configuration of the patch-clamp technique to measure ICa,L. Aortic stenosis resulted in a 46% increase in heart weight to body weight ratio in CH compared to SHAM. In SHAM and CH LVMs, a short isoprenaline stimulation (Iso, 100 nM, 15 s) elicited a similar transient increase in cAMP with a half decay time (t1/2off) of ~50 s. In both groups, PDE4 inhibition with Ro 20-1724 (10 μM) markedly potentiated the amplitude and slowed the decline of the cAMP transient, this latter effect being more pronounced in SHAM (t1/2off ~ 250 s) than in CH (t1/2off ~ 150 s, P < 0.01). In contrast, PDE3 inhibition with cilostamide (1 μM) had no effect on the amplitude of the cAMP transient and a minimal effect on its recovery in SHAM, whereas it potentiated the amplitude and slowed the decay in CH (t1/2off ~ 80 s). Iso pulse stimulation also elicited a similar transient increase in ICa,L in SHAM and CH, although the duration of the rising phase was delayed in CH. Inhibition of PDE3 or PDE4 potentiated ICa,L amplitude in SHAM but not in CH. Besides, while only PDE4 inhibition slowed down the decline of ICa,L in SHAM, both PDE3 and PDE4 contributed in CH. Conclusion These results identify selective alterations in cytosolic cAMP and ICa,L regulation by PDE3 and PDE4 in CH, and show that the balance between PDE3 and PDE4 for the regulation of β-AR responses is shifted toward PDE3 during CH.

Therapeutic Implications for PDE2 and cGMP/cAMP Mediated Crosstalk in Cardiovascular Diseases

Phosphodiesterases (PDEs) are the principal superfamily of enzymes responsible for degrading the secondary messengers 3',5'-cyclic nucleotides cAMP and cGMP. Their refined subcellular localization and substrate specificity contribute to finely regulate cAMP/cGMP gradients in various cellular microdomains. Redistribution of multiple signal compartmentalization components is often perceived under pathological conditions. Thereby PDEs have long been pursued as therapeutic targets in diverse disease conditions including neurological, metabolic, cancer and autoimmune disorders in addition to numerous cardiovascular diseases (CVDs). PDE2 is a unique member of the broad family of PDEs. In addition to its capability to hydrolyze both cAMP and cGMP, PDE2 is the sole isoform that may be allosterically activated by cGMP increasing its cAMP hydrolyzing activity. Within the cardiovascular system, PDE2 serves as an integral regulator for the crosstalk between cAMP/cGMP pathways and thereby may couple chronically adverse augmented cAMP signaling with cardioprotective cGMP signaling. This review provides a comprehensive overview of PDE2 regulatory functions in multiple cellular components within the cardiovascular system and also within various subcellular microdomains. Implications for PDE2- mediated crosstalk mechanisms in diverse cardiovascular pathologies are discussed highlighting the prospective use of PDE2 as a potential therapeutic target in cardiovascular disorders.

PKA/β2-AR-Gs/Gi signaling pathway is associated with anti-inflammatory and pro-apoptotic effects of Fuzi and Banxia combination on rats subjected to pressure overload

ETHNOPHARMACOLOGICAL RELEVANCE

Either Aconite Lateralis Radix Praeparata (Fuzi) or Pinelliae Rhizoma (Banxia) exerts anti-inflammatory activity and their combination has long been used in China for treating cardiovascular diseases. However, combination of two drugs is controversially prohibited in clinical prescriptions because it serves a representative incompatible pairs in "eighteen antagonisms". Up to date, whether the combination of Fuzi and Banxia could be used for treating heart failure with preserved ejection fraction (HFpEF) especially charactered by systemic inflammation and the potential mechanisms have not been elucidated.

AIM OF THE STUDY

The pros and cons of Fuzi in combination with Banxia were evaluated in pressure overload (PO) rat models of HF in vivo.

MATERIALS AND METHODS

Male Sprague Dawley rats were subjected to abdominal aorta constriction or sham-operated procedure. From week 12, rats were administered with low dose Fuzi (5.4 g kg^-1 d^-1), Banxia (5.4 g kg^-1 d^-1), combination (5.4 g kg^-1 d^-1 + 5.4 g kg^-1 d^-1), high dose Fuzi (10.8 g kg^-1 d^-1) or with vehicle (n = 15 per group) orally for additional 6 weeks.

RESULTS

Fuzi alone treatment led to exaggerated cardiac-renal response to PO, and occurred dramatically at high dose as manifested by markedly exacerbated cardiac-renal inflammation and myocardial fibrosis. Further studies revealed that cardiotoxicity of Fuzi may be associated with highly expression levels of β2-AR and PKA. In contrast, coadministration of Fuzi and Banxia restored cardiac function, as indicated by relieving inflammation and fibrosis as well as normalizing electrocardiogram parameters, which were accompanied by PKA down-regulation. More importantly, both high dose Fuzi and combination treatment enhanced induction of apoptosis, which could be partially associated with inhibition of β2-AR-Gi signaling.

CONCLUSION

Thus, combination of Fuzi and Banxia elicited concurrent protective and toxic effects in PO induced HF. The protective effect appeared to predominate and was associated with suppression of PKA/β2-AR-Gs signaling pathway. Unlike the eighteen antagonisms theory where Fuzi and Banxia combination was considered incompatible, in the present study, this herb pairs appeared to be benefit, and probably had potential therapeutic prospect in treating HFpEF and diseases associated with inflammation.

Calcineurin-AKAP interactions: therapeutic targeting of a pleiotropic enzyme with a little help from its friends

The ubiquitous Ca²⁺ /calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac hypertrophy whose therapeutic targeting in heart disease has been elusive due to its role in other essential biological processes. Calcineurin is targeted to diverse intracellular compartments by association with scaffold proteins, including by multivalent A-kinase anchoring proteins (AKAPs) that bind protein kinase A and other important signalling enzymes determining cardiac myocyte function and phenotype. Calcineurin anchoring by AKAPs confers specificity to calcineurin function in the cardiac myocyte. Targeting of calcineurin 'signalosomes' may provide a rationale for inhibiting the phosphatase in disease.

T-tubule remodelling disturbs localized β2-adrenergic signalling in rat ventricular myocytes during the progression of heart failure

Aims

Cardiomyocyte β2-adrenergic receptor (β2AR) cyclic adenosine monophosphate (cAMP) signalling is regulated by the receptors' subcellular location within transverse tubules (T-tubules), via interaction with structural and regulatory proteins, which form a signalosome. In chronic heart failure (HF), β2ARs redistribute from T-tubules to the cell surface, which disrupts functional signalosomes and leads to diffuse cAMP signalling. However, the functional consequences of structural changes upon β2AR-cAMP signalling during progression from hypertrophy to advanced HF are unknown.

Methods and results

Rat left ventricular myocytes were isolated at 4-, 8-, and 16-week post-myocardial infarction (MI), β2ARs were stimulated either via whole-cell perfusion or locally through the nanopipette of the scanning ion conductance microscope. cAMP release was measured via a Förster Resonance Energy Transfer-based sensor Epac2-camps. Confocal imaging of di-8-ANNEPS-stained cells and immunoblotting were used to determine structural alterations. At 4-week post-MI, T-tubule regularity, density and junctophilin-2 (JPH2) expression were significantly decreased. The amplitude of local β2AR-mediated cAMP in T-tubules was reduced and cAMP diffused throughout the cytosol instead of being locally confined. This was accompanied by partial caveolin-3 (Cav-3) dissociation from the membrane. At 8-week post-MI, the β2AR-mediated cAMP response was observed at the T-tubules and the sarcolemma (crest). Finally, at 16-week post-MI, the whole cell β2AR-mediated cAMP signal was depressed due to adenylate cyclase dysfunction, while overall Cav-3 levels were significantly increased and a substantial portion of Cav-3 dissociated into the cytosol. Overexpression of JPH2 in failing cells in vitro or AAV9.SERCA2a gene therapy in vivo did not improve β2AR-mediated signal compartmentation or reduce cAMP diffusion.

Conclusion

Although changes in T-tubule structure and β2AR-mediated cAMP signalling are significant even at 4-week post-MI, progression to the HF phenotype is not linear. At 8-week post-MI the loss of β2AR-mediated cAMP is temporarily reversed. Complete disorganization of β2AR-mediated cAMP signalling due to changes in functional receptor localization and cellular structure occurs at 16-week post-MI.

PDE2 at the crossway between cAMP and cGMP signalling in the heart

The cyclic nucleotides cAMP and cGMP are central second messengers in cardiac cells and critical regulators of cardiac physiology as well as pathophysiology. Consequently, subcellular compartmentalization allows for spatiotemporal control of cAMP/cGMP metabolism and subsequent regulation of their respective effector kinases PKA or PKG is most important for cardiac function in health and disease. While acute cAMP-mediated signalling is a mandatory prerequisite for the physiological fight-or-flight response, sustained activation of this pathway may lead to the progression of heart failure. In contrast, acute as well as sustained cGMP-mediated signalling can foster beneficial features, e.g. anti-hypertrophic and vasodilatory effects. These two signalling pathways seem to be intuitively counteracting and there is increasing evidence for a functionally relevant crosstalk between cAMP and cGMP signalling pathways on the level of cyclic nucleotide hydrolysing phosphodiesterases (PDEs). Among this diverse group of enzymes, PDE2 may fulfill a unique integrator role. Equipped with dual substrate specificity for cAMP as well as for cGMP, it is the only cAMP hydrolysing PDE, which is allosterically activated by cGMP. Recent studies have revealed strongly remodelled cAMP/cGMP microdomains and subcellular concentration profiles in different cardiac pathologies, leading to a putatively enhanced involvement of PDE2 in cAMP/cGMP breakdown and crosstalk compared to the other cardiac PDEs. This review sums up the current knowledge about molecular properties and regulation of PDE2 and explains the complex signalling network encompassing PDE2 in order to better understand the functional role of PDE2 in distinct cell types in cardiac health and disease. Moreover, this review gives an outlook in which way PDE2 may serve as a therapeutic target to treat cardiac disease.

Erythropoietin signaling regulates heme biosynthesis

Heme is required for survival of all cells, and in most eukaryotes, is produced through a series of eight enzymatic reactions. Although heme production is critical for many cellular processes, how it is coupled to cellular differentiation is unknown. Here, using zebrafish, murine, and human models, we show that erythropoietin (EPO) signaling, together with the GATA1 transcriptional target, AKAP10, regulates heme biosynthesis during erythropoiesis at the outer mitochondrial membrane. This integrated pathway culminates with the direct phosphorylation of the crucial heme biosynthetic enzyme, ferrochelatase (FECH) by protein kinase A (PKA). Biochemical, pharmacological, and genetic inhibition of this signaling pathway result in a block in hemoglobin production and concomitant intracellular accumulation of protoporphyrin intermediates. Broadly, our results implicate aberrant PKA signaling in the pathogenesis of hematologic diseases. We propose a unifying model in which the erythroid transcriptional program works in concert with post-translational mechanisms to regulate heme metabolism during normal development.

DOI:http://dx.doi.org/10.7554/eLife.24767.001

Heme is an iron-containing compound that is important for all living things, from bacteria to humans. Our red blood cells use heme to carry oxygen and deliver it throughout the body. The amount of heme that is produced must be tightly regulated. Too little or too much heme in a person’s red blood cells can lead to blood-related diseases such as anemia and porphyria. Yet, while scientists knew the enzymes needed to make heme, they did not know how these enzymes were controlled.

Now, Chung et al. show that an important signaling molecule called erythropoietin controls how much heme is produced when red blood cells are made. The experiments used a combination of red blood cells from humans and mice as well as zebrafish, which are useful model organisms because their blood develops in a similar way to humans. When Chung et al. inhibited components of erythropoietin signaling, heme production was blocked too and the red blood cells could not work properly.

These new findings pave the way to look at human patients with blood-related disorders to determine if they have defects in the erythropoietin signaling cascade. In the future, this avenue of research might lead to better treatments for a variety of blood diseases in humans.

DOI:http://dx.doi.org/10.7554/eLife.24767.002

Cyclic nucleotide imaging and cardiovascular disease

The universal second messengers cyclic nucleotides 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) play central roles in cardiovascular function and disease. They act in discrete, functionally relevant subcellular microdomains which regulate, for example, calcium cycling and excitation-contraction coupling. Such localized cAMP and cGMP signals have been difficult to measure using conventional biochemical techniques. Recent years have witnessed the advent of live cell imaging techniques which allow visualization of these functionally relevant second messengers with unprecedented spatial and temporal resolution at cellular, subcellular and tissue levels. In this review, we discuss these new imaging techniques and give examples how they are used to visualize cAMP and cGMP in physiological and pathological settings to better understand cardiovascular function and disease. Two primary techniques include the use of Förster resonance energy transfer (FRET) based cyclic nucleotide biosensors and nanoscale scanning ion conductance microscopy (SICM). These methods can provide deep mechanistic insights into compartmentalized cAMP and cGMP signaling.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

Sustained exposure to catecholamines affects cAMP/PKA compartmentalised signalling in adult rat ventricular myocytes

In the heart compartmentalisation of cAMP/protein kinase A (PKA) signalling is necessary to achieve a specific functional outcome in response to different hormonal stimuli. Chronic exposure to catecholamines is known to be detrimental to the heart and disrupted compartmentalisation of cAMP signalling has been associated to heart disease. However, in most cases it remains unclear whether altered local cAMP signalling is an adaptive response, a consequence of the disease or whether it contributes to the pathogenetic process. We have previously demonstrated that isoforms of PKA expressed in cardiac myocytes, PKA-I and PKA-II, localise to different subcellular compartments and are selectively activated by spatially confined pools of cAMP, resulting in phosphorylation of distinct downstream targets. Here we investigate cAMP signalling in an in vitro model of hypertrophy in primary adult rat ventricular myocytes. By using a real time imaging approach and targeted reporters we find that that sustained exposure to catecholamines can directly affect cAMP/PKA compartmentalisation. This appears to involve a complex mechanism including both changes in the subcellular localisation of individual phosphodiesterase (PDE) isoforms as well as the relocalisation of PKA isoforms. As a result, the preferential coupling of PKA subsets with different PDEs is altered resulting in a significant difference in the level of cAMP the kinase is exposed to, with potential impact on phosphorylation of downstream targets.

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.

Trafficking of β-Adrenergic Receptors: Implications in Intracellular Receptor Signaling

β-Adrenergic receptors (βARs), prototypical G-protein-coupled receptors, play a pivotal role in regulating neuronal and cardiovascular responses to catecholamines during stress. Agonist-induced receptor endocytosis is traditionally considered as a primary mechanism to turn off the receptor signaling (or receptor desensitization). However, recent progress suggests that intracellular trafficking of βAR presents a mean to translocate receptor signaling machinery to intracellular organelles/compartments while terminating the signaling at the cell surface. Moreover, the apparent multidimensionality of ligand efficacy in space and time in a cell has forecasted exciting pathophysiological implications, which are just beginning to be explored. As we begin to understand how these pathways impact downstream cellular programs, this will have significant implications for a number of pathophysiological conditions in heart and other systems, that in turn open up new therapeutic opportunities.

In vivo model with targeted cAMP biosensor reveals changes in receptor-microdomain communication in cardiac disease

3',5'-cyclic adenosine monophosphate (cAMP) is an ubiquitous second messenger that regulates physiological functions by acting in distinct subcellular microdomains. Although several targeted cAMP biosensors are developed and used in single cells, it is unclear whether such biosensors can be successfully applied in vivo, especially in the context of disease. Here, we describe a transgenic mouse model expressing a targeted cAMP sensor and analyse microdomain-specific second messenger dynamics in the vicinity of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). We demonstrate the biocompatibility of this targeted sensor and its potential for real-time monitoring of compartmentalized cAMP signalling in adult cardiomyocytes isolated from a healthy mouse heart and from an in vivo cardiac disease model. In particular, we uncover the existence of a phosphodiesterase-dependent receptor-microdomain communication, which is affected in hypertrophy, resulting in reduced β-adrenergic receptor-cAMP signalling to SERCA.

Anchored protein kinase A signalling in cardiac cellular electrophysiology

The cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is an elementary molecule involved in both acute and chronic modulation of cardiac function. Substantial research in recent years has highlighted the importance of A-kinase anchoring proteins (AKAP) therein as they act as the backbones of major macromolecular signalling complexes of the β-adrenergic/cAMP/PKA pathway. This review discusses the role of AKAP-associated protein complexes in acute and chronic cardiac modulation by dissecting their role in altering the activity of different ion channels, which underlie cardiac action potential (AP) generation. In addition, we review the involvement of different AKAP complexes in mechanisms of cardiac remodelling and arrhythmias.

cAMP signaling in subcellular compartments

In the complex microcosm of a cell, information security and its faithful transmission are critical for maintaining internal stability. To achieve a coordinated response of all its parts to any stimulus the cell must protect the information received from potentially confounding signals. Physical segregation of the information transmission chain ensures that only the entities able to perform the encoded task have access to the relevant information. The cAMP intracellular signaling pathway is an important system for signal transmission responsible for the ancestral 'flight or fight' response and involved in the control of critical functions including frequency and strength of heart contraction, energy metabolism and gene transcription. It is becoming increasingly apparent that the cAMP signaling pathway uses compartmentalization as a strategy for coordinating the large number of key cellular functions under its control. Spatial confinement allows the formation of cAMP signaling "hot spots" at discrete subcellular domains in response to specific stimuli, bringing the information in proximity to the relevant effectors and their recipients, thus achieving specificity of action. In this report we discuss how the different constituents of the cAMP pathway are targeted and participate in the formation of cAMP compartmentalized signaling events. We illustrate a few examples of localized cAMP signaling, with a particular focus on the nucleus, the sarcoplasmic reticulum and the mitochondria. Finally, we discuss the therapeutic potential of interventions designed to perturb specific cAMP cascades locally.

Compartmentation of cAMP signalling in cardiomyocytes in health and disease

3',5'-cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger critically involved in the regulation of heart function. It has been shown to act in discrete subcellular signalling compartments formed by differentially localized receptors, phosphodiesterases and protein kinases. Cardiac diseases such as hypertrophy or heart failure are associated with structural and functional remodelling of these microdomains which leads to changes in cAMP compartmentation. In this review, we will discuss recent key findings which provided new insights into cAMP compartmentation in cardiomyocytes with a particular focus on its alterations in heart disease.

Chromodomain helicase binding protein 8 (Chd8) is a novel A-kinase anchoring protein expressed during rat cardiac development

A-kinase anchoring proteins (AKAPs) bind the regulatory subunits of protein kinase A (PKA) and localize the holoenzyme to discrete signaling microdomains in multiple subcellular compartments. Despite emerging evidence for a nuclear pool of PKA that rapidly responds to activation of the PKA signaling cascade, only a few AKAPs have been identified that localize to the nucleus. Here we show a PKA-binding domain in the amino terminus of Chd8, and demonstrate subcellular colocalization of Chd8 with RII. RII overlay and immunoprecipitation assays demonstrate binding between Chd8-S and RIIα. Binding is abrogated upon dephosphorylation of RIIα. By immunofluorescence, we identified nuclear and perinuclear pools of Chd8 in HeLa cells and rat neonatal cardiomyocytes. We also show high levels of Chd8 mRNA in RNA extracted from post-natal rat hearts. These data add Chd8 to the short list of known nuclear AKAPs, and implicate a function for Chd8 in post-natal rat cardiac development.

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.

Spatial control of cAMP signalling in health and disease

The cyclic adenosine 3',5'-monophosphate signalling pathway is now recognised to transduce signals in a compartmentalised manner such that individual stimuli only engage a subset of the pathway components that are physically constrained within defined subcellular locales, thus resulting in a precise functional outcome. As we are starting to appreciate the complexity of the spatial organisation and of the temporal regulation of this pathway, it is becoming clear that disruption of local signalling may lead to pathology and that local manipulation of cAMP signals may offer alternative approaches to treat disease.

Reorganized PKA-AKAP associations in the failing human heart

Here we reveal that the characterization of large-scale re-arrangements of signaling scaffolds induced by heart failure can serve as a novel concept to identify more specific therapeutic targets. In the mammalian heart, the cAMP pathway, with the cAMP-dependent protein kinase (PKA) in a central role, acts directly downstream of adrenergic receptors to mediate cardiac contractility and rhythm. Heart failure, characterized by severe alterations in adrenergic stimulation is, amongst other interventions, often treated with β-blockers. Contrasting results, however, have shown both beneficial and detrimental effects of decreased cAMP levels in failing hearts. We hypothesize that the origin of this behavior lies in the complex spatiotemporal organization of the regulatory subunit of PKA (PKA-R), which associates tightly with various A-kinase anchoring proteins (AKAPs) to specifically localize PKA's activity. Using chemical proteomics directly applied to human patient and control heart tissue we demonstrate that the association profile of PKA-R with several AKAPs is severely altered in the failing heart, for instance effecting the interaction between PKA and the novel AKAP SPHKAP was 6-fold upregulated upon failing heart conditions. Also a significant increase in captured cGMP-dependent protein kinase (PKG) and phosphodiesterase 2 (PDE2) was observed. The observed altered profiles can already explain many aspects of the aberrant cAMP-response in the failing human heart, validating that this dataset may provide a resource for several novel, more specific, treatment options. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Modulation of beta-adrenergic receptor signaling in heart failure and longevity: targeting adenylyl cyclase type 5

Despite remarkable advances in therapy, heart failure remains a leading cause of morbidity and mortality. Although enhanced beta-adrenergic receptor stimulation is part of normal physiologic adaptation to either the increase in physiologic demand or decrease in cardiac function, chronic beta-adrenergic stimulation has been associated with increased mortality and morbidity in both animal models and humans. For example, overexpression of cardiac Gsalpha or beta-adrenergic receptors in transgenic mice results in enhanced cardiac function in young animals, but with prolonged overstimulation of this pathway, cardiomyopathy develops in these mice as they age. Similarly, chronic sympathomimetic amine therapy increases morbidity and mortality in patients with heart failure. Conversely, the use of beta-blockade has proven to be of benefit and is currently part of the standard of care for heart failure. It is conceivable that interrupting distal mechanisms in the beta-adrenergic receptor-G protein-adenylyl cyclase pathway may also provide targets for future therapeutic modalities for heart failure. Interestingly, there are two major isoforms of adenylyl cyclase (AC) in the heart (type 5 and type 6), which may exert opposite effects on the heart, i.e., cardiac overexpression of AC6 appears to be protective, whereas disruption of type 5 AC prolongs longevity and protects against cardiac stress. The goal of this review is to summarize the paradigm shift in the treatment of heart failure over the past 50 years from administering sympathomimetic amine agonists to administering beta-adrenergic receptor antagonists, and to explore the basis for a novel therapy of inhibiting type 5 AC.

Protein Kinases as Drug Development Targets for Heart Disease Therapy

Protein kinases are intimately integrated in different signal transduction pathways for the regulation of cardiac function in both health and disease. Protein kinase A (PKA), Ca²⁺-calmodulin-dependent protein kinase (CaMK), protein kinase C (PKC), phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) are not only involved in the control of subcellular activities for maintaining cardiac function, but also participate in the development of cardiac dysfunction in cardiac hypertrophy, diabetic cardiomyopathy, myocardial infarction, and heart failure. Although all these kinases serve as signal transducing proteins by phosphorylating different sites in cardiomyocytes, some of their effects are cardioprotective whereas others are detrimental. Such opposing effects of each signal transduction pathway seem to depend upon the duration and intensity of stimulus as well as the type of kinase isoform for each kinase. In view of the fact that most of these kinases are activated in heart disease and their inhibition has been shown to improve cardiac function, it is suggested that these kinases form excellent targets for drug development for therapy of heart disease.

AKAP121 downregulation impairs protective cAMP signals, promotes mitochondrial dysfunction, and increases oxidative stress

AIMS

The aim of the present study was to determine the function and the role of the scaffold protein AKAP121, tethering cAMP dependent protein kinase A to the outer wall of mitochondria, in neonatal ventricular myocytes and the heart.

METHODS AND RESULTS

Competitive peptides displacing AKAP121 from mitochondria in the tissue and in the cells were used to investigate the role of AKAP121 in mitochondrial function, reactive oxygen species (ROS) generation, and cell survival. Displacement of AKAP121 from mitochondria by synthetic peptides triggers the death program in cardiomyocytes. Under pathological conditions in vivo, in a rat model of cardiac hypertrophy induced by ascending aorta banding, the levels of AKAP121 are significantly down-regulated. Disappearance of AKAP121 is associated with mitochondrial dysfunction, high oxidative stress, and apoptosis. In vivo delocalization of AKAP121 by competitive peptides replicates some of the molecular signatures induced by pressure overload: mitochondrial dysfunction, increased mitochondrial ROS, and apoptosis.

CONCLUSION

These data suggest that AKAP121 regulates the response to stress in cardiomyocytes, and therefore AKAP121 downregulation might represent an important event contributing to the development of cardiac dysfunction.

Beta2-adrenergic receptor redistribution in heart failure changes cAMP compartmentation

The beta1- and beta2-adrenergic receptors (betaARs) on the surface of cardiomyocytes mediate distinct effects on cardiac function and the development of heart failure by regulating production of the second messenger cyclic adenosine monophosphate (cAMP). The spatial localization in cardiomyocytes of these betaARs, which are coupled to heterotrimeric guanine nucleotide-binding proteins (G proteins), and the functional implications of their localization have been unclear. We combined nanoscale live-cell scanning ion conductance and fluorescence resonance energy transfer microscopy techniques and found that, in cardiomyocytes from healthy adult rats and mice, spatially confined beta2AR-induced cAMP signals are localized exclusively to the deep transverse tubules, whereas functional beta1ARs are distributed across the entire cell surface. In cardiomyocytes derived from a rat model of chronic heart failure, beta2ARs were redistributed from the transverse tubules to the cell crest, which led to diffuse receptor-mediated cAMP signaling. Thus, the redistribution of beta(2)ARs in heart failure changes compartmentation of cAMP and might contribute to the failing myocardial phenotype.

The Rho guanine nucleotide exchange factor AKAP13 (BRX) is essential for cardiac development in mice

A fundamental biologic principle is that diverse biologic signals are channeled through shared signaling cascades to regulate development. Large scaffold proteins that bind multiple proteins are capable of coordinating shared signaling pathways to provide specificity to activation of key developmental genes. Although much is known about transcription factors and target genes that regulate cardiomyocyte differentiation, less is known about scaffold proteins that couple signals at the cell surface to differentiation factors in developing heart cells. Here we show that AKAP13 (also known as Brx-1, AKAP-Lbc, and proto-Lbc), a unique protein kinase A-anchoring protein (AKAP) guanine nucleotide exchange region belonging to the Dbl family of oncogenes, is essential for cardiac development. Cardiomyocytes of Akap13-null mice had deficient sarcomere formation, and developing hearts were thin-walled and mice died at embryonic day 10.5-11.0. Disruption of Akap13 was accompanied by reduced expression of Mef2C. Consistent with a role of AKAP13 upstream of MEF2C, Akap13 siRNA led to a reduction in Mef2C mRNA, and overexpression of AKAP13 augmented MEF2C-dependent reporter activity. The results suggest that AKAP13 coordinates Galpha(12) and Rho signaling to an essential transcription program in developing cardiomyocytes.

A-kinase anchoring proteins: from protein complexes to physiology and disease

Protein scaffold complexes are a key mechanism by which a common signaling pathway can serve many different functions. Sequestering a signaling enzyme to a specific subcellular environment not only ensures that the enzyme is near its relevant targets, but also segregates this activity to prevent indiscriminate phosphorylation of other substrates. One family of diverse, well-studied scaffolding proteins are the A-kinase anchoring proteins (AKAPs). These anchoring proteins form multi-protein complexes that integrate cAMP signaling with other pathways and signaling events. In this review, we focus on recent advances in the elucidation of AKAP function.

AKAP-scaffolding proteins and regulation of cardiac physiology

A kinase anchoring proteins (AKAPs) compose a growing list of diverse but functionally related proteins defined by their ability to bind to the regulatory subunit of protein kinase A. AKAPs perform an integral role in the spatiotemporal modulation of a multitude of cellular signaling pathways. This review highlights the extensive role of AKAPs in cardiac excitation/contraction coupling and cardiac physiology. The literature shows that particular AKAPs are involved in cardiac Ca(2+) influx, release, reuptake, and myocyte repolarization. Studies have also suggested roles for AKAPs in cardiac remodeling. Transgenic studies show functional effects of AKAPs, not only in the cardiovascular system but in other organ systems as well.

cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies

3'-5'-Cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger generated in response to activation of G(s) protein-coupled receptors. In the heart, cAMP mediates the catecholaminergic control on heart rate and contractility but, at the same time, it is responsible for the functional response to a wide variety of other hormones and neurotransmitters, raising the question of how the myocyte can decode the cAMP signal and generate the appropriate functional output to each individual extracellular stimulus. A growing body of evidence points to the spatial organization of the components of the cAMP signalling pathway in distinct, spatially segregated signalling domains as the key feature underpinning specificity of response and data is emerging, indicating that alteration of spatial control of the cAMP signal cascade associates with heart pathology. Most of the details of the molecular organization and regulation of individual cAMP signalling compartments are still to be elucidated but future research should provide the knowledge necessary to develop and test new therapeutic strategies that, by acting on a limited subset of downstream targets, would improve efficacy and minimize off-target effects.

Disruption of protein kinase A interaction with A-kinase-anchoring proteins in the heart in vivo: effects on cardiac contractility, protein kinase A phosphorylation, and troponin I proteolysis

Protein kinase A (PKA)-dependent phosphorylation is regulated by targeting of PKA to its substrate as a result of binding of regulatory subunit, R, to A-kinase-anchoring proteins (AKAPs). We investigated the effects of disrupting PKA targeting to AKAPs in the heart by expressing the 24-amino acid regulatory subunit RII-binding peptide, Ht31, its inactive analog, Ht31P, or enhanced green fluorescent protein by adenoviral gene transfer into rat hearts in vivo. Ht31 expression resulted in loss of the striated staining pattern of type II PKA (RII), indicating loss of PKA from binding sites on endogenous AKAPs. In the absence of isoproterenol stimulation, Ht31-expressing hearts had decreased +dP/dtmax and -dP/dtmin but no change in left ventricular ejection fraction or stroke volume and decreased end diastolic pressure versus controls. This suggests that cardiac output is unchanged despite decreased +dP/dt and -dP/dt. There was also no difference in PKA phosphorylation of cardiac troponin I (cTnI), phospholamban, or ryanodine receptor (RyR2). Upon isoproterenol infusion, +dP/dtmax and -dP/dtmin did not differ between Ht31 hearts and controls. At higher doses of isoproterenol, left ventricular ejection fraction and stroke volume increased versus isoproterenol-stimulated controls. This occurred in the context of decreased PKA phosphorylation of cTnI, RyR2, and phospholamban versus controls. We previously showed that expression of N-terminal-cleaved cTnI (cTnI-ND) in transgenic mice improves cardiac function. Increased cTnI N-terminal truncation was also observed in Ht31-expressing hearts versus controls. Increased cTnI-ND may help compensate for reduced PKA phosphorylation as occurs in heart failure.

Multiple molecular forms of circulating cardiac troponin: analytical and clinical significance

Cardiac troponin T (cTnT) and I (cTnI) are highly specific and sensitive biomarkers of myocardial cell damage and are now accepted as the ‘gold standard’ diagnostic test for acute coronary syndrome and supersede the classical muscle enzyme biomarkers. While the understanding of the development and structure of the troponins has advanced, detailed biochemistry of the troponin molecules is complex and poorly understood. Many post-translational molecular forms of troponin are known to exist. The diversity of these circulating forms may have a clinical impact and the notion of a disease-specific troponin protein signature has been suggested. However, the effects of these multiple forms on commercial assay performance and their impact clinically are currently unknown and should be the focus of future research and assay design.

Phosphorylation of the cAMP-dependent protein kinase (PKA) regulatory subunit modulates PKA-AKAP interaction, substrate phosphorylation, and calcium signaling in cardiac cells

Subcellular compartmentalization of the cAMP-dependent protein kinase (PKA) by protein kinase A-anchoring proteins (AKAPs) facilitates local protein phosphorylation. However, little is known about how PKA targeting to AKAPs is regulated in the intact cell. PKA binds to an amphipathic helical region of AKAPs via an N-terminal domain of the regulatory subunit. In vitro studies showed that autophosphorylation of type II regulatory subunit (RII) can alter its affinity for AKAPs and the catalytic subunit (PKA(cat)). We now investigate whether phosphorylation of serine 96 on RII regulates PKA targeting to AKAPs, downstream substrate phosphorylation and calcium cycling in primary cultured cardiomyocytes. We demonstrated that, whereas there is basal phosphorylation of RII subunits, persistent maximal activation of PKA results in a phosphatase-dependent loss of RII phosphorylation. To investigate the functional effects of RII phosphorylation, we constructed adenoviral vectors incorporating mutants which mimic phosphorylated (RIIS96D), nonphosphorylated (RIIS96A) RII, or wild-type (WT) RII and performed adenoviral infection of neonatal rat cardiomyocytes. Coimmunoprecipitation showed that more AKAP15/18 was pulled down by the phosphomimic, RIIS96D, than RIIS96A. Phosphorylation of phospholamban and ryanodine receptor was significantly increased in cells expressing RIIS96D versus RIIS96A. Expression of recombinant RII constructs showed significant effects on cytosolic calcium transients. We propose a model illustrating a central role of RII phosphorylation in the regulation of local PKA activity. We conclude that RII phosphorylation regulates PKA-dependent substrate phosphorylation and may have significant implications for modulation of cardiac function.

Isoform-specific PKA dynamics revealed by dye-triggered aggregation and DAKAP1alpha-mediated localization in living cells

The tetracysteine sequence YRECCPGCCMWR fused to the N terminus of green fluorescent protein (GFP) self-aggregates upon biarsenical labeling in living cells or in vitro. Such dye-triggered aggregates form temperature-dependent morphologies and are dispersed by photobleaching. Fusion of the biarsenical aggregating GFP to the regulatory (R) or catalytic (C) subunit of PKA traps intact holoenzyme in compact fluorescent puncta upon biarsenical labeling. Contrary to the classical model of PKA activation, elevated cAMP does not allow RIalpha and Calpha to diffuse far apart unless the pseudosubstrate inhibitor PKI or locally concentrated substrate is coexpressed. However, RIIalpha releases Calpha upon elevated cAMP alone, dependent on autophosphorylation of the RIIalpha inhibitory domain. DAKAP1alpha overexpression induced R and C outer mitochondrial colocalization and showed similar regulation. Overall, effective separation of type I PKA is substrate dependent, whereas type II PKA dissociation relies on autophosphorylation.

Decreased phosphorylation levels of cardiac myosin-binding protein-C in human and experimental heart failure

Cardiac myosin-binding protein-C (cMyBP-C) is an important regulator of cardiac contractility, and its phosphorylation by PKA is a mechanism that contributes to increased cardiac output in response to beta-adrenergic stimulation. It is presently unknown whether heart failure alters cMyBP-C phosphorylation. The present study determined the level of phosphorylated cMyBP-C in failing human hearts and in a canine model of pacing-induced heart failure. A polyclonal antibody directed against the major phosphorylation site of cMyBP-C (Ser-282) was generated and its specificity was confirmed by PKA phosphorylation with isoprenaline in cardiomyocytes and Langendorff-perfused mouse hearts. Left ventricular myocardial tissue from (i) patients with terminal heart failure (hHF; n=12) and nonfailing donor hearts (hNF; n=6) and (ii) dogs with rapid-pacing-induced end-stage heart failure (dHF; n=10) and sham-operated controls (dNF; n=10) were used for quantification of total cMyBP-C and phospho-cMyBP-C by Western blotting. Total cMyBP-C protein levels were similar in hHF and hNF as well as in dHF and dNF. In contrast, the ratio of phospho-cMyBP-C to total cMyBP-C levels were >50% reduced in hHF and >40% reduced in dHF. In summary, cMyBP-C phosphorylation levels are markedly decreased in human and experimental heart failure. Thus, the compromised contractile function of the failing heart might be in part attributable to reduced cMyBP-C phosphorylation levels.

The intermediate filament protein, synemin, is an AKAP in the heart

Targeting of protein kinase A (PKA) by A-kinase anchoring proteins (AKAPs) contributes to high specificity of PKA signaling pathways. PKA phosphorylation of myofilament and cytoskeletal proteins may regulate myofibrillogenesis and myocyte remodeling during heart disease; however, known cardiac AKAPs do not localize to these regions. To identify novel AKAPs which target PKA to the cytoskeleton or myofilaments, a human heart cDNA library was screened and the intermediate filament (IF) protein, synemin, was identified as a putative RII (PKA regulatory subunit type II) binding protein. A predicted RII binding region was mutated and resulted in loss of RII binding. Furthermore, synemin co-localized with RII in SW13/cl.1-vim+ cells and co-immunoprecipitated with RII from adult rat cardiomyocytes. Synemin was localized at the level of Z-lines with RII and desmin in adult hearts, however, neonatal cardiomyocytes showed differential synemin and desmin localization. Quantitative Western blots also showed significantly more synemin was present in failing human hearts. We propose that synemin provides temporal and spatial targeting of PKA in adult and neonatal cardiac myocytes.

Identification and characterization of D-AKAP1 as a major adipocyte PKA and PP1 binding protein

Protein kinase A (PKA) plays an important role in the regulation of lipid metabolism in adipocytes. The activity of PKA is known to be modulated by its specific location in the cell, a process mediated by A-kinase anchoring proteins (AKAPs). In order to examine the subcellular localization of PKA in this tissue we performed a search for AKAP proteins in adipocytes. We purified a 120 kDa protein which can bind both the regulatory subunit of PKA as well as the catalytic subunit of protein phosphatase 1 (PP1). This protein was found to be enriched in the lipid droplet fraction of primary adipocytes and was identified as D-AKAP1. This protein may play an important role in the regulation of PKA in adipocytes.

Predicting disease genes using protein-protein interactions

BACKGROUND

The responsible genes have not yet been identified for many genetically mapped disease loci. Physically interacting proteins tend to be involved in the same cellular process, and mutations in their genes may lead to similar disease phenotypes.

OBJECTIVE

To investigate whether protein-protein interactions can predict genes for genetically heterogeneous diseases.

METHODS

72,940 protein-protein interactions between 10,894 human proteins were used to search 432 loci for candidate disease genes representing 383 genetically heterogeneous hereditary diseases. For each disease, the protein interaction partners of its known causative genes were compared with the disease associated loci lacking identified causative genes. Interaction partners located within such loci were considered candidate disease gene predictions. Prediction accuracy was tested using a benchmark set of known disease genes.

RESULTS

Almost 300 candidate disease gene predictions were made. Some of these have since been confirmed. On average, 10% or more are expected to be genuine disease genes, representing a 10-fold enrichment compared with positional information only. Examples of interesting candidates are AKAP6 for arrythmogenic right ventricular dysplasia 3 and SYN3 for familial partial epilepsy with variable foci.

CONCLUSIONS

Exploiting protein-protein interactions can greatly increase the likelihood of finding positional candidate disease genes. When applied on a large scale they can lead to novel candidate gene predictions.

Thin Filament Remodeling in Failing Myocardium

While the remodeling process in myocardial failure involves changes in ventricular structure and performance, it is now appreciated that it is also associated with changes in thin filament composition and function. As is discussed, changes at the level thick filament may affect thin filament activation in heart failure. Alterations in actin, troponin and tropomyosin isoform composition do not appear to be significant factors in human heart failure. In contrast, proteolytic degradation of troponin subunits are likely to be playing a functional role in some forms of cardiomyopathy (e.g. ischemic). Finally, phosphorylation of troponin I and troponin T by kinases (most notably protein kinase C) substantially affect thin filament function in failing human myocardium. These findings indicate that functional deficits in thin filament function in failing myocardium are largely reversible and create the potential for future targeted therapies in the treatment of this deadly disease.

Exercise improves impaired ventricular function and alterations of cardiac myofibrillar proteins in diabetic dyslipidemic pigs

Chronic diabetes is often associated with cardiomyopathy, which may result, in part, from defects in cardiac muscle proteins. We investigated whether a 20-wk porcine model of diabetic dyslipidemia (DD) would impair in vivo myocardial function and yield alterations in cardiac myofibrillar proteins and whether endurance exercise training would improve these changes. Myocardial function was depressed in anesthetized DD pigs (n = 12) compared with sedentary controls (C; n = 13) as evidenced by an approximately 30% decrease in left ventricular fractional shortening and an approximately 35% decrease in +dP/dt measured by noninvasive echocardiography and direct cardiac catheterization, respectively. This depression in myocardial function was improved with chronic exercise as treadmill-trained DD pigs (DDX) (n = 13) had significantly greater fractional shortening and +dP/dt than DD animals. Interestingly, the isoform expression pattern of the myofibrillar regulatory protein, cardiac troponin T (cTnT), was significantly shifted from cTnT1 toward cTnT2 and cTnT3 in DD pigs. Furthermore, this change in cTnT isoform expression pattern was prevented in DDX pigs. Finally, there was a decrease in baseline levels of cAMP-dependent protein kinase-induced phosphorylation of the myofibrillar proteins troponin I and myosin-binding protein-C in DD animals. Overall, these results indicate that 20 wk of DD lead to myocardial dysfunction coincident with significant alterations in myofibrillar proteins, both of which are prevented with endurance exercise training, implying that changes in myofibrillar proteins may contribute, at least in part, to cardiac dysfunction associated with diabetic cardiomyopathy.

A-kinase anchoring protein targeting of protein kinase A in the heart

There is increasing evidence that subcellular targeting of signaling molecules is an important means of regulating the protein kinase A (PKA) pathway. Subcellular organization of the signaling molecules in the PKA pathway insures that a signal initiated at the receptor level is transferred efficiently to a PKA substrate eliciting some cellular response. This subcellular targeting appears to regulate the function of a highly specialized cell such as the cardiac myocyte. This review focuses on A-kinase anchoring proteins (AKAPs) which are expressed in the heart. It has been determined that, of the approximately 13 different AKAPs expressed in cardiac tissue, several of these are expressed in cardiac myocytes. These AKAPs bind several PKA substrates and some appear to regulate PKA-dependent phosphorylation of these substrates. AKAP tethering of PKA may be essential for efficient regulation of cardiac muscle contraction. The ability of an AKAP to anchor PKA may be altered in the failing heart, thus compromising the ability of the myocyte to respond to stimuli which elicit the PKA pathway.

Role of troponin I phosphorylation in protein kinase C-mediated enhanced contractile performance of rat myocytes

Our goal was to define the role of phosphorylated cardiac troponin-I in the adult myocyte contractile performance response to activated protein kinase C. In agreement with earlier work, endothelin enhanced both adult rat myocyte contractile performance and cardiac troponin-I phosphorylation. Protein kinase C participated in both responses. The role of cardiac troponin-I phosphorylation in the contractile function response to protein kinase C was further investigated using gene transfer into myocytes of troponin-I isoforms/mutants lacking one or more phosphorylation sites previously identified in purified cardiac troponin-I. Sarcomeric replacement with slow skeletal troponin-I-abrogated protein kinase C-mediated troponin-I phosphorylation. In functional studies, endothelin slowed relaxation in myocytes expressing slow skeletal troponin-I, while the relaxation rate increased in myocytes expressing cardiac troponin-I. Based on these results, acceleration of myocyte relaxation during protein kinase C activation largely depended on cardiac troponin-I phosphorylation. Experiments with troponin-I isoform chimeras provided evidence that phosphorylation sites in the amino portion of cardiac troponin I-mediated the protein kinase C acceleration of relaxation. The cardiac troponin-I Thr-144 phosphorylation site identified in earlier biochemical studies was not significantly phosphorylated during the acute contractile response. Thus, amino-terminal protein kinase C-dependent phosphorylation sites in cardiac troponin-I are likely responsible for the accelerated relaxation observed in adult myocytes.

Age-related changes in vascular adrenergic signaling: clinical and mechanistic implications

A large and growing segment of the general population are age 65 or older, and this percentage will continue to rise. Primary care of this population has, and is becoming a priority for clinicians. Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in beta-adrenergic receptor (beta-AR)-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-AR receptor to respond to agonist at the cellular level. This article will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-AR receptor-mediated signaling. A fundamental understanding of why beta-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of the multiple clinical disorders that effect older people.

AKAP mediated signal transduction

Compartmentalization of cyclic AMP-dependent protein kinase (PKA) is achieved through association with A-kinase anchoring proteins (AKAPs). AKAPs are a group of structurally diverse proteins with the common function of binding to the regulatory subunit of PKA and confining the holoenzyme to discrete locations within the cell. This mode of regulation ensures that PKA is exposed to isolated cAMP gradients, which allows for efficient catalytic activation and accurate substrate selection. Several AKAPs coordinate multiple members of signaling cascades, effectively assembling upstream activators and downstream effectors within the same macromolecular complex. Consequently, AKAPs may serve as points of integration for numerous signaling pathways. This review details the most recent advances in our understanding of the various biological functions dependent upon AKAP-anchored signaling complexes.

cAMP-dependent protein kinase and proliferation differ in normal and polycystic kidney epithelia

Developmental control of cell proliferation is crucial, and abnormal principal cell proliferation may contribute to cystogenesis in polycystic kidney disease. This study investigates roles of cAMP and its primary effector, cAMP-dependent protein kinase (protein kinase A; PKA), in control of cell proliferation in filter-grown noncystic (NC) and cystic (CY)-derived principal cell cultures. These cultures had similar cAMP pathway characteristics upstream of PKA subunit distribution but differed in predicted PKA subtype distribution. Functionally, cultures were proliferative before polarization, with constitutively higher proliferation in CY cultures. NC cultures achieved levels similar to those of CY cultures on pharmacological manipulation of cAMP production or PKA activation or inhibition of PKA subtype I activity. Inhibition of overall PKA activity, or of PKA subtype II anchoring, diminished cAMP/PKA-mediated proliferation in NC cultures but had no effect on CY cultures. Polarized CY monolayers remained proliferative, but NC monolayers lost responsiveness. No large proliferation changes resulted from treatments of polarized cultures; however, polarized NC and CY cultures differed in poststimulation handling of PKA catalytic and type IIalpha regulatory subunits. Our results support PKA subtype regulation of prepolarization proliferation in NC principal cells and altered regulation of PKA in CY cells and suggest that differences at or downstream of PKA can contribute to altered proliferation in a developmental renal disease.