A-kinase anchoring proteins: getting to the heart of the matter

New perspectives in cAMP-signaling modulation

Cyclic adenosine 3',5'-monophosphate (cAMP) mediates the biological effects of various hormones and neurotransmitters. Stimulation of cardiac β-adrenergic receptors (β-AR) via catecholamines leads to activation of adenylyl cyclases and increases cAMP production to enhance myocardial function. Because many other receptors signaling through cAMP generation exist in cardiac myocytes, a central question is how different hormones induce distinct cellular responses through the same second messenger. A large body of evidence suggests that the localization and compartmentalization of β-AR/cAMP signaling affects the net outcome of biological functions. Spatiotemporal dynamics of cAMP action is achieved by various proteins, including protein kinase A (PKA), phosphodiesterases, and scaffolding proteins such as A-kinase-anchoring proteins. In addition, the discovery of the cAMP target Epac (exchange proteins directly activated by cAMP), which functions in a PKA-independent manner, represents a novel mechanism for governing cAMP-signaling specificity. Aberrant cAMP signaling through dysregulation of β-AR/cAMP compartmentalization may contribute to cardiac remodeling and heart failure.

Regulation of murine cardiac function by phosphodiesterases type 3 and 4

Cyclic nucleotide phosphodiesterases (PDEs) encompass a large group of enzymes that regulate intracellular levels of two-second messengers, cAMP and cGMP, by controlling the rates of their degradation. More than 60 isoforms, subdivided into 11 gene families (PDE1-11), exist in mammals with at least six families (PDE1-5 and PDE8) identified in mammalian hearts. The two predominant families implicated in regulating contraction strength of the heart are PDE3 and PDE4. Studies using transgenic models in combination with family-specific PDE inhibitors have demonstrated that PDE3A, PDE4B, and PDE4D isoforms regulate cardiac contractility by modulating cAMP levels in various subcellular compartments. These studies have further uncovered contributions of PDE4B and PDE4D in preventing ventricular arrhythmias.

cAMP: novel concepts in compartmentalised signalling

Cyclic adenosine 3,'5'-monophosphate (cAMP) is the archetypal second messenger produced at the membrane by adenylyl cyclase following activation of many different G protein-coupled receptor (GPCR) types. Although discovered over fifty years ago, the notion that cAMP responses were compartmentalised was born in the 1980s. Since then, modern molecular techniques have facilitated visualisation of cellular cAMP dynamics in real time and helped us to understand how a single, ubiquitous second messenger can direct receptor-specific functions in cells. The aim of this review is to highlight emerging ideas in the cAMP field that are currently developing the concept of compartmentalised cAMP signalling systems.

PDEs create local domains of cAMP signaling

In the light of the knowledge accumulated over the years, it becomes clear that intracellular cAMP is not uniformly distributed within cardiomyocytes and that cAMP compartmentation is required for adequate processing and targeting of the information generated at the membrane. Localized cAMP signals may be generated by interplay between discrete production sites and restricted diffusion within the cytoplasm. In addition to specialized membrane structures that may limit cAMP spreading, degradation of the second messenger by cyclic nucleotide phosphodiesterases (PDEs) appears critical for the formation of dynamic microdomains that confer specificity of the response to various hormones. This review will cover the role of the different cAMP-PDE isoforms in this process. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

AKAP12 and AKAP5 form higher-order hetero-oligomers

Background

The family of A-kinase-anchoring proteins, AKAPs, constitutes a group of molecular scaffolds that act to catalyze dynamic interactions of protein kinase A, protein kinase C, tyrosine kinases, G-protein-coupled receptors and ion channels. AKAP5 (MW ~47 kDa) and AKAP12 (MW ~191 kDa) homo-oligomerize, but whether or not such AKAPs can hetero-oligomerize into supermolecular scaffolds of increased complexity is unknown.

Results

Affinity chromatography using immobilized AKAPs as "bait" demonstrates unequivocally that AKAP5 and AKAP12 do form minimally hetero-dimers. Steric-exclusion chromatography of AKAP5 and AKAP12 mixtures revealed the existence of very large, supermolecular complexes containing both AKAPs. Docking of AKAP5 to AKAP12 was increased 4-fold by beta-adrenergic agonist stimulation. Overexpression of AKAP12 was found to potentiate AKAP5-mediated Erk1/2 activation in response to stimulation with beta-adrenergic agonist.

Conclusion

AKAP5 and AKAP12 are capable of forming hetero-oligomeric supermolecular complexes that influence AKAP locale and function.

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".

Construction of a eukaryotic expression plasmid encoding the human PKAR IIβ gene and its expression in human gastric cancer BGC-823 cells

AIM: To construct a eukaryotic expression plasmid encoding the human protein kinase A regulatory subunit II beta (PKAR IIβ) gene and to examine its expression and localization in BGC-823 gastric cells using green fluorescent protein as a reporter.

METHODS: The coding sequence of the PKAR IIβ gene was amplified from the plasmid pRSETB-PKARIIβ by PCR and subcloned into pEGFP-C1 vector after digestion with Xho I and EcoR I. After the identity of recombinant plasmid was verified by direct sequencing, the plasmid was transfected into BGC-823 cells. The expression of the recombinant plasmid in BGC-823 cells was detected by Western blot. The localization of GFP-PKARIIβ in BGC-823 cells was observed by laser scanning confocal microscopy.

RESULTS: The coding sequence of the PKARIIβ gene was inserted into the pEGFP-C1 vector successfully. Restriction enzymes digestion showed that the length of the insert was 1.2 kb, matching the expected size. The expression of GFP-PKARIIβ fusion protein, which had a molecular weight of 72 000 Da, was detected in BGC-823 cells by Western blot. The GFP-PKARIIβ protein was localized predominantly to the cytoplasm but sparsely to the nucleus of HEK293 and BGC-823 cells.

CONCLUSION: A recombinant plasmid expressing the PKARIIβ gene has been successfully constructed and provides a tool for future investigation of PKARIIβ functions. The GFP-PKARIIβ fusion protein was expressed mainly in the cytoplasm of HEK293 and BGC7901 cells.

Phospholipase C epsilon scaffolds to muscle-specific A kinase anchoring protein (mAKAPbeta) and integrates multiple hypertrophic stimuli in cardiac myocytes

To define a role for phospholipase Cε (PLCε) signaling in cardiac myocyte hypertrophic growth, PLCε protein was depleted from neonatal rat ventricular myocytes (NRVMs) using siRNA. NRVMs with PLCε depletion were stimulated with endothelin (ET-1), norepinephrine, insulin-like growth factor-1 (IGF-1), or isoproterenol and assessed for development of hypertrophy. PLCε depletion dramatically reduced hypertrophic growth and gene expression induced by all agonists tested. PLCε catalytic activity was required for hypertrophy development, yet PLCε depletion did not reduce global agonist-stimulated inositol phosphate production, suggesting a requirement for localized PLC activity. PLCε was found to be scaffolded to a muscle-specific A kinase anchoring protein (mAKAPβ) in heart and NRVMs, and mAKAPβ localizes to the nuclear envelope in NRVMs. PLCε-mAKAP interaction domains were defined and overexpressed to disrupt endogenous mAKAPβ-PLCε complexes in NRVMs, resulting in significantly reduced ET-1-dependent NRVM hypertrophy. We propose that PLCε integrates multiple upstream signaling pathways to generate local signals at the nucleus that regulate hypertrophy.

cGMP signals modulate cAMP levels in a compartment-specific manner to regulate catecholamine-dependent signaling in cardiac myocytes

RATIONALE

cAMP and cGMP are intracellular second messengers involved in heart pathophysiology. cGMP can potentially affect cAMP signals via cGMP-regulated phosphodiesterases (PDEs).

OBJECTIVE

To study the effect of cGMP signals on the local cAMP response to catecholamines in specific subcellular compartments.

METHODS AND RESULTS

We used real-time FRET imaging of living rat ventriculocytes expressing targeted cAMP and cGMP biosensors to detect cyclic nucleotides levels in specific locales. We found that the compartmentalized, but not the global, cAMP response to isoproterenol is profoundly affected by cGMP signals. The effect of cGMP is to increase cAMP levels in the compartment where the protein kinase (PK)A-RI isoforms reside but to decrease cAMP in the compartment where the PKA-RII isoforms reside. These opposing effects are determined by the cGMP-regulated PDEs, namely PDE2 and PDE3, with the local activity of these PDEs being critically important. The cGMP-mediated modulation of cAMP also affects the phosphorylation of PKA targets and myocyte contractility.

CONCLUSIONS

cGMP signals exert opposing effects on local cAMP levels via different PDEs the activity of which is exerted in spatially distinct subcellular domains. Inhibition of PDE2 selectively abolishes the negative effects of cGMP on cAMP and may have therapeutic potential.

Protein phosphatase 1 complexes modulate sperm motility and present novel targets for male infertility

Infertility is a growing concern in modern society, with 30% of cases being due to male factors, namely reduced sperm concentration, decreased motility and abnormal morphology. Sperm cells are highly compartmentalized, almost devoid of transcription and translation consequently processes such as protein phosphorylation provide a key general mechanism for regulating vital cellular functions, more so than for undifferentiated cells. Reversible protein phosphorylation is the principal mechanism regulating most physiological processes in eukaryotic cells. To date, hundreds of protein kinases have been identified, but significantly fewer phosphatases (PPs) are responsible for counteracting their action. This discrepancy can be explained in part by the mechanism used to control phosphatase activity, which is based on regulatory interacting proteins. This is particularly true for PP1, a major serine/threonine-PP, for which >200 interactors (PP1 interacting proteins-PIPs) have been indentified that control its activity, subcellular location and substrate specificity. For PP1, several isoforms have been described, among them PP1γ2, a testis/sperm-enriched PP1 isoform. Recent findings support our hypothesis that PP1γ2 is involved in the regulation of sperm motility. This review summarizes the known sperm-specific PP1-PIPs, involved in the acquisition of mammalian sperm motility. The complexes that PP1 routinely forms with different proteins are addressed and the role of PP1/A-kinase anchoring protein complexes in sperm motility is considered. Furthermore, the potential relevance of targeting PP1-PIPs complexes to infertility diagnostics and therapeutics as well as to male contraception is also discussed.

Novel insights into the role of the sympathetic nervous system in cardiac arrhythmogenesis

It has long been recognized that increased sympathetic nerve activity during physiologic stress (exercise, swimming, emotion, arousal, loud noise, etc.) has profound influences on the electrical and contractile functions of the heart. In the severely predisposed heart, these stressors may lead to ventricular tachyarrhythmias and sudden death. Still little is known about the temporal relationship between instantaneous autonomic nerve activity and arrhythmias. There is a large variety of autonomically-driven arrhythmias, from serious ventricular tachycardia in pathological conditions to single supraventricular and ventricular extrasystolic beats in the healthy heart. The latter are considered harmless if occurring at low frequency. In the atria, mounting data indicate the presence of a sophisticated network of ganglionated plexi with major influences on cardiac function. The ablation of multiple such ganglia can suppress pulmonary vein potentials and atrial fibrillation. At the cellular level, recent studies have focused on the spatiotemporal details of cyclic nucleotide signaling influencing ion channel function during neurohumoral stimulation. We have come to understand that sarcolemmal ion channels and other electrogenic transporters are macromolecular complexes that interact with structural elements (other than the phospholipid bilayer) to promote regionalization and targeting by regulatory proteins. Compartmentation of these regulatory proteins in subdomains of the myocyte is increasingly recognized and thought to segregate the functional (including electrogenic) responses induced by different neuromediators and hormones. In this article, contemporary issues are discussed regarding arrhythmias that are triggered by influences from the neurocardiac interface, covering the field from the molecular genetic to the intact integrated level. Actual questions are listed per topic, and viewpoints are expressed.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.