AKAP-Lbc mobilizes a cardiac hypertrophy signaling pathway

Long AKAP18 isoforms anchor ubiquitin specific proteinases and coordinate calcium reuptake at the sarcoplasmic reticulum

Subcellular targeting of signaling enzymes influences where and when various modes of intracellular communication operate. Macromolecular complexes of signal transduction and signal termination elements favor reversable control of repetitive processes. This includes adrenergic stimulation of excitation-contraction coupling in the heart. Long isoforms of A-kinase anchoring protein 18 (AKAP18γ and δ) modulate this process via regulation of calcium uptake into the sarcoplasmic reticulum through the Ca2+ATPase 2a (SERCA2a). AKAP18 proximity-proteomic screening in cardiomyocytes identifies networks for protein kinase A (PKA) and ubiquitin-specific proteinases (USP's). A 2'phosphoesterase domain on AKAP18 interfaces with the USP4 isoform at the Z bands of sarcomeres. PKA stimulates USP4 activity in the presence of the anchoring protein. AKAP18 anchored PKA phosphorylates serine 829 on USP4, a conserved residue near the active site of this deubiquitinase. Antibodies against the pSer829 motif show that adrenergic stimulation enhances phosphorylation of USP4 in mouse adult cardiomyocytes. In related studies, elevated USP4 phosphorylation at Ser829 is detected in human post-myocardial infraction tissue as compared to healthy tissue. Thus, phosphorylation of sarcoplasmic USP4 may be a cardioprotective response. Pharmacological inhibition of PKA or deletion of the AKAP7/18 gene in mice decreases calcium flux through the exchanger. This suggests that loss of the anchoring protein impacts SERCA2 action. Thus, AKAP18/PKA/USP4 complexes are well positioned to influence the rate and magnitude of calcium reuptake during the cardiac cycle.

Nanodomain cAMP signaling in cardiac pathophysiology: potential for developing targeted therapeutic interventions

The 3',5'-cyclic adenosine monophosphate (cAMP) mediates the effects of sympathetic stimulation on the rate and strength of cardiac contraction. Beyond this pivotal role, in cardiac myocytes cAMP also orchestrates a diverse array of reactions to various stimuli. To ensure specificity of response, the cAMP signaling pathway is intricately organized into multiple, spatially confined, subcellular domains, each governing a distinct cellular function. In this review, we describe the molecular components of the cAMP signaling pathway with a specific focus on adenylyl cyclases, A-kinase anchoring proteins, and phosphodiesterases. We discuss how they are organized inside the intracellular space and how they achieve exquisite regulation of signaling within nanometer-size domains. We delineate the key experimental findings that lead to the current model of compartmentalized cAMP signaling, and we offer an overview of our present understanding of how cAMP nanodomains are structured and regulated within cardiac myocytes. Furthermore, we discuss how compartmentalized cAMP signaling is affected in cardiac disease and consider the potential therapeutic opportunities arising from understanding such organization. By exploiting the nuances of compartmentalized cAMP signaling, novel and more effective therapeutic strategies for managing cardiac conditions may emerge. Finally, we highlight the unresolved questions and hurdles that must be addressed to translate these insights into interventions that may benefit patients.

Role of PDE4 Family in Cardiomyocyte Physiology and Heart Failure

Phosphodiesterase 4 (PDE4) is a key regulator of cyclic adenosine monophosphate (cAMP) signalling in cardiomyocytes, controlling contractility, calcium handling, and hypertrophic responses. PDE4 provides spatial and temporal precision to cAMP signalling, particularly under β-adrenergic stimulation, through its compartmentalised activity in subcellular nanodomains, including the sarcoplasmic reticulum, plasma membrane and nuclear envelope. This review highlights the cardiac PDE4 isoforms PDE4A, PDE4B and PDE4D, focusing on their distinct localisation and contributions to cardiac physiology and pathophysiology, particularly in heart failure and arrhythmias. Although PDE4 plays a smaller role in overall cAMP hydrolysis in human hearts than in rodents, its compartmentalised function remains critical. Recent therapeutic advances have shifted from pan-PDE4 inhibitors to isoform-specific approaches to enhance efficacy while minimising systemic toxicity. We discuss the potential of selective PDE4 modulators, gene therapies and combination strategies in restoring cAMP compartmentation and preventing maladaptive cardiac remodelling. By integrating rodent and human studies, this review underscores the translational challenges and therapeutic opportunities surrounding PDE4, positioning it as both a key regulator of cardiac signalling and a promising target for heart failure therapies.

Expansion of phenotypic and genotypic data in autism spectrum disorders due to variants in the CHD8 gene

Autism spectrum disorders are a group of the most common disorders of neuropsychiatric development, characterized by difficulties in social interaction and adherence to stereotypic behavioral patterns. This group of conditions frequently co-occurs with intellectual disability, epilepsy, attention-deficit hyperactivity disorder, connective tissue disorders and others. Among the most common molecular-genetic causes of autism spectrum disorders are pathogenic variants in the CHD8 gene. CHD8 codes for chromodomain-helicase-DNA-binding protein 8 - a chromatin remodeler that regulates cellular proliferation and brain development in embryogenesis. 6 children and 1 adult (mother of 1 of the children) and were found to have clinically significant variants in CHD8 on whole genome sequencing (3 children and 1 adult had likely pathogenic variants, 3 children- variants of unknown significance). Their phenotype consisted of autism spectrum disorders, developmental delay, ataxia, overgrowth and other signs typically observed in patients with pathogenic variants in CHD8, as well as common comorbidities of autism spectrum disorders, such as attention-deficit hyperactivity disorder and connective tissue disorders. Additionally, 4 patients had hepatomegaly and 2- hyperbilirubinemia (1 had both) - clinical features have not been previously associated with pathogenic variants in CHD8. 2 patients also presented with cardiovascular abnormalities, primarily arrythmias and, in 1 case, cardiomyopathy- also uncharacteristic of patients with pathogenic variants in CHD8. Further research is required to determine the mechanisms underlying the abovementioned clinical features, which are likely carried out through complex interactions between CHD8 and other regulatory proteins.

Protein kinase A and local signaling in cancer

Protein kinase A (PKA) is a basophilic kinase implicated in the modulation of many cell-signaling and physiological processes. PKA also contributes to cancer-relevant events such as growth factor action, cell cycle control, cell migration and tumor metabolism. Germline and somatic mutations in PKA, gene amplifications, and chromosome rearrangements that encode kinase fusions, are linked to a growing number of malignant neoplasms. Mislocalization of PKA by exclusion from A-Kinase Anchoring Protein (AKAP) signaling islands further underlies cancer progression. This article highlights the influence of AKAP signaling and local kinase action in selected hallmarks of cancer. We also feature the utility of kinase inhibitor drugs as frontline and future anti-cancer therapies.

Spatiotemporal control of kinases and the biomolecular tools to trace activity

The delicate balance of cell physiology is implicitly tied to the expression and activation of proteins. Post-translational modifications offer a tool to dynamically switch protein activity on and off to orchestrate a wide range of protein-protein interactions to tune signal transduction during cellular homeostasis and pathological responses. There is a growing acknowledgment that subcellular locations of kinases define the spatial network of potential scaffolds, adaptors, and substrates. These highly ordered and localized biomolecular microdomains confer a spatially distinct bias in the outcomes of kinase activity. Furthermore, they may hold essential clues to the underlying mechanisms that promote disease. Developing tools to dissect the spatiotemporal activation of kinases is critical to reveal these mechanisms and promote the development of spatially targeted kinase inhibitors. Here, we discuss the spatial regulation of kinases, the tools used to detect their activity, and their potential impact on human health.

Phosphorylation, compartmentalization, and cardiac function

Protein phosphorylation is a fundamental element of cell signaling. First discovered as a biochemical switch in glycogen metabolism, we now know that this posttranslational modification permeates all aspects of cellular behavior. In humans, over 540 protein kinases attach phosphate to acceptor amino acids, whereas around 160 phosphoprotein phosphatases remove phosphate to terminate signaling. Aberrant phosphorylation underlies disease, and kinase inhibitor drugs are increasingly used clinically as targeted therapies. Specificity in protein phosphorylation is achieved in part because kinases and phosphatases are spatially organized inside cells. A prototypic example is compartmentalization of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A through association with A-kinase anchoring proteins. This configuration creates autonomous signaling islands where the anchored kinase is constrained in proximity to activators, effectors, and selected substates. This article primarily focuses on A kinase anchoring protein (AKAP) signaling in the heart with an emphasis on anchoring proteins that spatiotemporally coordinate excitation-contraction coupling and hypertrophic responses.

Protein Kinase D1 Regulates Cardiac Hypertrophy, Potassium Channel Remodeling, and Arrhythmias in Heart Failure

Background Structural and electrophysiological remodeling characterize heart failure (HF) enhancing arrhythmias. PKD1 (protein kinase D1) is upregulated in HF and mediates pathological hypertrophic signaling, but its role in K+ channel remodeling and arrhythmogenesis in HF is unknown. Methods and Results We performed echocardiography, electrophysiology, and expression analysis in wild-type and PKD1 cardiomyocyte-specific knockout (cKO) mice following transverse aortic constriction (TAC). PKD1-cKO mice exhibited significantly less cardiac hypertrophy post-TAC and were protected from early decline in cardiac contractile function (3 weeks post-TAC) but not the progression to HF at 7 weeks post-TAC. Wild-type mice exhibited ventricular action potential duration prolongation at 8 weeks post-TAC, which was attenuated in PKD1-cKO, consistent with larger K+ currents via the transient outward current, sustained current, inward rectifier K+ current, and rapid delayed rectifier K+ current and increased expression of corresponding K+ channels. Conversely, reduction of slowly inactivating K+ current was independent of PKD1 in HF. Acute PKD inhibition slightly increased transient outward current in TAC and sham wild-type myocytes but did not alter other K+ currents. Sham PKD1-cKO versus wild-type also exhibited larger transient outward current and faster early action potential repolarization. Tachypacing-induced action potential duration alternans in TAC animals was increased and independent of PKD1, but diastolic arrhythmogenic activities were reduced in PKD1-cKO. Conclusions Our data indicate an important role for PKD1 in the HF-related hypertrophic response and K+ channel downregulation. Therefore, PKD1 inhibition may represent a therapeutic strategy to reduce hypertrophy and arrhythmias; however, PKD1 inhibition may not prevent disease progression and reduced contractility in HF.

Protein phosphatase 1 regulates reactive oxygen species-dependent degradation of histone deacetylase 5 by intermittent hypoxia

We recently reported pheochromocytoma 12 (PC12) cells and rats subjected to intermittent hypoxia (IH), a hallmark manifestation of obstructive sleep apnea (OSA), exhibit reduced histone deacetylase activity and HDAC5 protein. Our study further suggested that posttranslational modifications rather than transcriptional mechanism(s) mediate IH-induced HDAC5 degradation. These observations prompted our current study to investigate the mechanism(s) underlying HDAC5 degradation by IH in PC12 cell cultures. IH-induced HDAC5 degradation was blocked by an antioxidant, and reactive oxygen species (ROS) mimetics decreased HDAC5 protein, suggesting that ROS mediates HDAC5 degradation by IH. NADPH oxidases (NOX) 2 and 4 were identified as sources of ROS that mediate the effects of IH. HDAC5 degradation during IH was associated with dephosphorylation of HDAC5 at serine²⁵⁹, and this response was blocked by a NOX inhibitor, suggesting that ROS-dependent dephosphorylation mediates HDAC5 degradation. IH-induced dephosphorylation of HDCA5 was inhibited by calyculin A, an inhibitor of protein phosphatase (PP)-1 and -2, or by the overexpression of nuclear inhibitor of PP1 (NIPP1). HDAC5 dephosphorylation by IH lead to augmented hypoxia-inducible factor (HIF)-1α protein and an increase in its transcriptional activity. These data suggest that PP1-dependent dephosphorylation of S²⁵⁹ destabilizes HDAC5 protein in response to IH, resulting in HIF-1α stabilization and transcriptional activity. Our findings highlight hither to unexplored role of protein phosphatases, especially PP1 in regulating HDAC5 protein, which is an upstream activator of HIF-1 signaling by IH.

Histone deacetylase 5 is a phosphorylation substrate of protein kinase D in osteoclasts

Protein kinase D (PRKD) family kinases are required for formation and function of osteoclasts. However, the substrates of PRKD in osteoclasts are unknown. To identify PRKD-dependent protein phosphorylation in osteoclasts, we performed a quantitative LC-MS/MS phosphoproteomics screen for proteins showing differential phosphorylation in osteoclasts after treatment with the PRKD inhibitor CRT0066101. We identified 757 phosphopeptides showing significant changes following PRKD inhibition. Among the changes, we found a group of 13 proteins showing decreased phosphorylation at PRKD consensus phosphorylation motifs. This group includes histone deacetylase 5 (HDAC5), which is a previously validated PRKD target. Considering this known interaction, work suggesting HDACs may be important regulators of osteoclasts, and studies suggesting potential functional redundancy between HDACs, we further investigated the relationship between PRKD and class IIa HDACs in osteoclasts. We confirmed that CRT0066101 inhibits phosphorylation of endogenous HDAC5 and to a lesser extent HDAC4, whereas HDAC7 phosphorylation was not affected. Osteoclast cultures from Hdac5 global knockout mice displayed impaired differentiation and reduced ability to resorb bone, while conditional knockout of Hdac4 in osteoclasts showed no phenotype in vitro or in vivo. The inhibitory effect of CRT0066101 was reduced in Hdac5 KO osteoclasts. Together these data indicate that the PRKD/HDAC5 axis contributes to osteoclast formation in vitro and suggest that this pathway may contribute to regulation of skeletal dynamics in vivo.

Decoding the Cardiac Actions of Protein Kinase D Isoforms

Protein kinase D (PKD) consists of a family of three structurally related enzymes that play key roles in a wide range of biological functions that contribute to the evolution of cardiac hypertrophy and heart failure. PKD1 (the founding member of this enzyme family) has been implicated in the phosphorylation of substrates that regulate cardiac hypertrophy, contraction, and susceptibility to ischemia/reperfusion injury, and de novo PRKD1 (protein kinase D1 gene) mutations have been identified in patients with syndromic congenital heart disease. However, cardiomyocytes coexpress all three PKDs. Although stimulus-specific activation patterns for PKD1, PKD2, and PKD3 have been identified in cardiomyocytes, progress toward identifying PKD isoform-specific functions in the heart have been hampered by significant gaps in our understanding of the molecular mechanisms that regulate PKD activity. This review incorporates recent conceptual breakthroughs in our understanding of various alternative mechanisms for PKD activation, with an emphasis on recent evidence that PKDs activate certain effector responses as dimers, to consider the role of PKD isoforms in signaling pathways that drive cardiac hypertrophy and ischemia/reperfusion injury. The focus is on whether the recently identified activation mechanisms that enhance the signaling repertoire of PKD family enzymes provide novel therapeutic strategies to target PKD enzymes and prevent or slow the evolution of cardiac injury and pathological cardiac remodeling. SIGNIFICANCE STATEMENT: PKD isoforms regulate a large number of fundamental biological processes, but the understanding of the biological actions of individual PKDs (based upon studies using adenoviral overexpression or gene-silencing methods) remains incomplete. This review focuses on dimerization, a recently identified mechanism for PKD activation, and the notion that this mechanism provides a strategy to develop novel PKD-targeted pharmaceuticals that restrict proliferation, invasion, or angiogenesis in cancer and prevent or slow the evolution of cardiac injury and pathological cardiac remodeling.

Cardiac cAMP-PKA Signaling Compartmentalization in Myocardial Infarction

Under physiological conditions, cAMP signaling plays a key role in the regulation of cardiac function. Activation of this intracellular signaling pathway mirrors cardiomyocyte adaptation to various extracellular stimuli. Extracellular ligand binding to seven-transmembrane receptors (also known as GPCRs) with G proteins and adenylyl cyclases (ACs) modulate the intracellular cAMP content. Subsequently, this second messenger triggers activation of specific intracellular downstream effectors that ensure a proper cellular response. Therefore, it is essential for the cell to keep the cAMP signaling highly regulated in space and time. The temporal regulation depends on the activity of ACs and phosphodiesterases. By scaffolding key components of the cAMP signaling machinery, A-kinase anchoring proteins (AKAPs) coordinate both the spatial and temporal regulation. Myocardial infarction is one of the major causes of death in industrialized countries and is characterized by a prolonged cardiac ischemia. This leads to irreversible cardiomyocyte death and impairs cardiac function. Regardless of its causes, a chronic activation of cardiac cAMP signaling is established to compensate this loss. While this adaptation is primarily beneficial for contractile function, it turns out, in the long run, to be deleterious. This review compiles current knowledge about cardiac cAMP compartmentalization under physiological conditions and post-myocardial infarction when it appears to be profoundly impaired.

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.

Possible Susceptibility Genes for Intervention against Chemotherapy-Induced Cardiotoxicity

Recent therapeutic advances have significantly improved the short- and long-term survival rates in patients with heart disease and cancer. Survival in cancer patients may, however, be accompanied by disadvantages, namely, increased rates of cardiovascular events. Chemotherapy-related cardiac dysfunction is an important side effect of anticancer therapy. While advances in cancer treatment have increased patient survival, treatments are associated with cardiovascular complications, including heart failure (HF), arrhythmias, cardiac ischemia, valve disease, pericarditis, and fibrosis of the pericardium and myocardium. The molecular mechanisms of cardiotoxicity caused by cancer treatment have not yet been elucidated, and they may be both varied and complex. By identifying the functional genetic variations responsible for this toxicity, we may be able to improve our understanding of the potential mechanisms and pathways of treatment, paving the way for the development of new therapies to target these toxicities. Data from studies on genetic defects and pharmacological interventions have suggested that many molecules, primarily those regulating oxidative stress, inflammation, autophagy, apoptosis, and metabolism, contribute to the pathogenesis of cardiotoxicity induced by cancer treatment. Here, we review the progress of genetic research in illuminating the molecular mechanisms of cancer treatment-mediated cardiotoxicity and provide insights for the research and development of new therapies to treat or even prevent cardiotoxicity in patients undergoing cancer treatment. The current evidence is not clear about the role of pharmacogenomic screening of susceptible genes. Further studies need to done in chemotherapy-induced cardiotoxicity.

Telangiectasia-ectodermal dysplasia-brachydactyly-cardiac anomaly syndrome is caused by de novo mutations in protein kinase D1

BACKGROUND

We describe two unrelated patients who display similar clinical features including telangiectasia, ectodermal dysplasia, brachydactyly and congenital heart disease.

METHODS

We performed trio whole exome sequencing and functional analysis using in vitro kinase assays with recombinant proteins.

RESULTS

We identified two different de novo mutations in protein kinase D1 (PRKD1, NM_002742.2): c.1774G>C, p.(Gly592Arg) and c.1808G>A, p.(Arg603His), one in each patient. PRKD1 (PKD1, HGNC:9407) encodes a kinase that is a member of the protein kinase D (PKD) family of serine/threonine protein kinases involved in diverse cellular processes such as cell differentiation and proliferation and cell migration as well as vesicle transport and angiogenesis. Functional analysis using in vitro kinase assays with recombinant proteins showed that the mutation c.1808G>A, p.(Arg603His) represents a gain-of-function mutation encoding an enzyme with a constitutive, lipid-independent catalytic activity. The mutation c.1774G>C, p.(Gly592Arg) in contrast shows a defect in substrate phosphorylation representing a loss-of-function mutation.

CONCLUSION

The present cases represent a syndrome, which associates symptoms from several different organ systems: skin, teeth, bones and heart, caused by heterozygous de novo mutations in PRKD1 and expands the clinical spectrum of PRKD1 mutations, which have hitherto been linked to syndromic congenital heart disease and limb abnormalities.

A-kinase-anchoring protein 1 (dAKAP1)-based signaling complexes coordinate local protein synthesis at the mitochondrial surface

Compartmentalization of macromolecules is a ubiquitous molecular mechanism that drives numerous cellular functions. The appropriate organization of enzymes in space and time enables the precise transmission and integration of intracellular signals. Molecular scaffolds constrain signaling enzymes to influence the regional modulation of these physiological processes. Mitochondrial targeting of protein kinases and protein phosphatases provides a means to locally control the phosphorylation status and action of proteins on the surface of this organelle. Dual-specificity protein kinase A anchoring protein 1 (dAKAP1) is a multivalent binding protein that targets protein kinase A (PKA), RNAs, and other signaling enzymes to the outer mitochondrial membrane. Many AKAPs recruit a diverse set of binding partners that coordinate a broad range of cellular processes. Here, results of MS and biochemical analyses reveal that dAKAP1 anchors additional components, including the ribonucleoprotein granule components La-related protein 4 (LARP4) and polyadenylate-binding protein 1 (PABPC1). Local translation of mRNAs at organelles is a means to spatially control the synthesis of proteins. RNA-Seq data demonstrate that dAKAP1 binds mRNAs encoding proteins required for mitochondrial metabolism, including succinate dehydrogenase. Functional studies suggest that the loss of dAKAP1-RNA interactions reduces mitochondrial electron transport chain activity. Hence, dAKAP1 plays a previously unappreciated role as a molecular interface between second messenger signaling and local protein synthesis machinery.

Regulation of histone deacetylase activities and functions by phosphorylation and its physiological relevance

Histone deacetylases (HDACs) are conserved enzymes that regulate many cellular processes by catalyzing the removal of acetyl groups from lysine residues on histones and non-histone proteins. As appropriate for proteins that occupy such an essential biological role, HDAC activities and functions are in turn highly regulated. Overwhelming evidence suggests that the dysregulation of HDACs plays a major role in many human diseases. The regulation of HDACs is achieved by multiple different mechanisms, including posttranslational modifications. One of the most common posttranslational modifications on HDACs is reversible phosphorylation. Many HDAC phosphorylations are context-dependent, occurring in specific tissues or as a consequence of certain stimuli. Additionally, whereas phosphorylation can regulate some HDACs in a non-specific manner, many HDAC phosphorylations result in specific consequences. Although some of these modifications support normal HDAC function, aberrations can contribute to disease development. Here we review and critically evaluate how reversible phosphorylation activates or deactivates HDACs and, thereby, regulates their many functions under various cellular and physiological contexts.

A FAK/HDAC5 signaling axis controls osteocyte mechanotransduction

Osteocytes, cells ensconced within mineralized bone matrix, are the primary skeletal mechanosensors. Osteocytes sense mechanical cues by changes in fluid flow shear stress (FFSS) across their dendritic projections. Loading-induced reductions of osteocytic Sclerostin (encoded by Sost) expression stimulates new bone formation. However, the molecular steps linking mechanotransduction and Sost suppression remain unknown. Here, we report that class IIa histone deacetylases (HDAC4 and HDAC5) are required for loading-induced Sost suppression and bone formation. FFSS signaling drives class IIa HDAC nuclear translocation through a signaling pathway involving direct HDAC5 tyrosine 642 phosphorylation by focal adhesion kinase (FAK), a HDAC5 post-translational modification that controls its subcellular localization. Osteocyte cell adhesion supports FAK tyrosine phosphorylation, and FFSS triggers FAK dephosphorylation. Pharmacologic FAK catalytic inhibition reduces Sost mRNA expression in vitro and in vivo. These studies demonstrate a role for HDAC5 as a transducer of matrix-derived cues to regulate cell type-specific gene expression.

The Role of Cyclic AMP Signaling in Cardiac Fibrosis

Myocardial stress and injury invariably promote remodeling of the cardiac tissue, which is associated with cardiomyocyte death and development of fibrosis. The fibrotic process is initially triggered by the differentiation of resident cardiac fibroblasts into myofibroblasts. These activated fibroblasts display increased proliferative capacity and secrete large amounts of extracellular matrix. Uncontrolled myofibroblast activation can thus promote heart stiffness, cardiac dysfunction, arrhythmias, and progression to heart failure. Despite the well-established role of myofibroblasts in mediating cardiac disease, our current knowledge on how signaling pathways promoting fibrosis are regulated and coordinated in this cell type is largely incomplete. In this respect, cyclic adenosine monophosphate (cAMP) signaling acts as a major modulator of fibrotic responses activated in fibroblasts of injured or stressed hearts. In particular, accumulating evidence now suggests that upstream cAMP modulators including G protein-coupled receptors, adenylyl cyclases (ACs), and phosphodiesterases (PDEs); downstream cAMP effectors such as protein kinase A (PKA) and the guanine nucleotide exchange factor Epac; and cAMP signaling organizers such as A-kinase anchoring proteins (AKAPs) modulate a variety of fundamental cellular processes involved in myocardial fibrosis including myofibroblast differentiation, proliferation, collagen secretion, and invasiveness. The current review will discuss recent advances highlighting the role of cAMP and AKAP-mediated signaling in regulating pathophysiological responses controlling cardiac fibrosis.

A-kinase anchoring protein 1 (AKAP1) and its role in some cardiovascular diseases

A-kinase anchoring proteins (AKAPs) play crucial roles in regulating compartmentalized multi-protein signaling networks related to PKA-mediated phosphorylation. The mitochondrial AKAP - AKAP1 proteins are enriched in heart and play cardiac protective roles. This review aims to thoroughly summarize AKAP1 variants from their sequence features to the structure-function relationships between AKAP1 and its binding partners, as well as the molecular mechanisms of AKAP1 in cardiac hypertrophy, hypoxia-induced myocardial infarction and endothelial cells dysfunction, suggesting AKAP1 as a candidate for cardiovascular therapy.

PDE4 and mAKAPβ are nodal organizers of β2-ARs nuclear PKA signalling in cardiac myocytes

Aims

β1- and β2-adrenergic receptors (β-ARs) produce different acute contractile effects on the heart partly because they impact on different cytosolic pools of cAMP-dependent protein kinase (PKA). They also exert different effects on gene expression but the underlying mechanisms remain unknown. The aim of this study was to understand the mechanisms by which β1- and β2-ARs regulate nuclear PKA activity in cardiomyocytes.

Methods and results

We used cytoplasmic and nuclear targeted biosensors to examine cAMP signals and PKA activity in adult rat ventricular myocytes upon selective β1- or β2-ARs stimulation. Both β1- and β2-AR stimulation increased cAMP and activated PKA in the cytoplasm. Although the two receptors also increased cAMP in the nucleus, only β1-ARs increased nuclear PKA activity and up-regulated the PKA target gene and pro-apoptotic factor, inducible cAMP early repressor (ICER). Inhibition of phosphodiesterase (PDE)4, but not Gi, PDE3, GRK2 nor caveolae disruption disclosed nuclear PKA activation and ICER induction by β2-ARs. Both nuclear and cytoplasmic PKI prevented nuclear PKA activation and ICER induction by β1-ARs, indicating that PKA activation outside the nucleus is required for subsequent nuclear PKA activation and ICER mRNA expression. Cytoplasmic PKI also blocked ICER induction by β2-AR stimulation (with concomitant PDE4 inhibition). However, in this case nuclear PKI decreased ICER up-regulation by only 30%, indicating that other mechanisms are involved. Down-regulation of mAKAPβ partially inhibited nuclear PKA activation upon β1-AR stimulation, and drastically decreased nuclear PKA activation upon β2-AR stimulation in the presence of PDE4 inhibition.

Conclusions

β1- and β2-ARs differentially regulate nuclear PKA activity and ICER expression in cardiomyocytes. PDE4 insulates a mAKAPβ-targeted PKA pool at the nuclear envelope that prevents nuclear PKA activation upon β2-AR stimulation.

Cardiac function modulation depends on the A-kinase anchoring protein complex

The A-kinase anchoring proteins (AKAPs) are a group of structurally diverse proteins identified in various species and tissues. These proteins are able to anchor protein kinase and other signalling proteins to regulate cardiac function. Acting as a scaffold protein, AKAPs ensure specificity in signal transduction by enzymes close to their appropriate effectors and substrates. Over the decades, more than 70 different AKAPs have been discovered. Accumulative evidence indicates that AKAPs play crucial roles in the functional regulation of cardiac diseases, including cardiac hypertrophy, myofibre contractility dysfunction and arrhythmias. By anchoring different partner proteins (PKA, PKC, PKD and LTCCs), AKAPs take part in different regulatory pathways to function as regulators in the heart, and a damaged structure can influence the activities of these complexes. In this review, we highlight recent advances in AKAP-associated protein complexes, focusing on local signalling events that are perturbed in cardiac diseases and their roles in interacting with ion channels and their regulatory molecules. These new findings suggest that AKAPs might have potential therapeutic value in patients with cardiac diseases, particularly malignant rhythm.

mAKAPβ signalosomes - A nodal regulator of gene transcription associated with pathological cardiac remodeling

Striated myocytes compose about half of the cells of the heart, while contributing the majority of the heart's mass and volume. In response to increased demands for pumping power, including in diseases of pressure and volume overload, the contractile myocytes undergo non-mitotic growth, resulting in increased heart mass, i.e. cardiac hypertrophy. Myocyte hypertrophy is induced by a change in the gene expression program driven by the altered activity of transcription factors and co-repressor and co-activator chromatin-associated proteins. These gene regulatory proteins are subject to diverse post-translational modifications and serve as nuclear effectors for intracellular signal transduction pathways, including those controlled by cyclic nucleotides and calcium ion. Scaffold proteins contribute to the underlying architecture of intracellular signaling networks by targeting signaling enzymes to discrete intracellular compartments, providing specificity to the regulation of downstream effectors, including those regulating gene expression. Muscle A-kinase anchoring protein β (mAKAPβ) is a well-characterized scaffold protein that contributes to the regulation of pathological cardiac hypertrophy. In this review, we discuss the mechanisms how this prototypical scaffold protein organizes signalosomes responsible for the regulation of class IIa histone deacetylases and cardiac transcription factors such as NFAT, MEF2, and HIF-1α, as well as how this signalosome represents a novel therapeutic target for the prevention or treatment of heart failure.

Loss of Akap1 Exacerbates Pressure Overload-Induced Cardiac Hypertrophy and Heart Failure

Left ventricular hypertrophy (LVH) is a major contributor to the development of heart failure (HF). Alterations in cyclic adenosine monophosphate (cAMP)-dependent signaling pathways participate in cardiomyocyte hypertrophy and mitochondrial dysfunction occurring in LVH and HF. cAMP signals are received and integrated by a family of cAMP-dependent protein kinase A (PKA) anchor proteins (AKAPs), tethering PKA to discrete cellular locations. AKAPs encoded by the Akap1 gene (mitoAKAPs) promote PKA mitochondrial targeting, regulating mitochondrial structure and function, reactive oxygen species production, and cell survival. To determine the role of mitoAKAPs in LVH development, in the present investigation, mice with global genetic deletion of Akap1 (Akap1-/-), Akap1 heterozygous (Akap1+/-), and their wild-type (wt) littermates underwent transverse aortic constriction (TAC) or SHAM procedure for 1 week. In wt mice, pressure overload induced the downregulation of AKAP121, the major cardiac mitoAKAP. Compared to wt, Akap1-/- mice did not display basal alterations in cardiac structure or function and cardiomyocyte size or fibrosis. However, loss of Akap1 exacerbated LVH and cardiomyocyte hypertrophy induced by pressure overload and accelerated the progression toward HF in TAC mice, and these changes were not observed upon prevention of AKAP121 degradation in seven in absentia homolog 2 (Siah2) knockout mice (Siah2-/-). Loss of Akap1 was also associated to a significant increase in cardiac apoptosis as well as lack of activation of Akt signaling after pressure overload. Taken together, these results demonstrate that in vivo genetic deletion of Akap1 enhances LVH development and accelerates pressure overload-induced cardiac dysfunction, pointing at Akap1 as a novel repressor of pathological LVH. These results confirm and extend the important role of mitoAKAPs in cardiac response to stress.

Bidirectional regulation of HDAC5 by mAKAPβ signalosomes in cardiac myocytes

Class IIa histone deacetylases (HDACs) are transcriptional repressors whose nuclear export in the cardiac myocyte is associated with the induction of pathological gene expression and cardiac remodeling. Class IIa HDACs are regulated by multiple, functionally opposing post-translational modifications, including phosphorylation by protein kinase D (PKD) that promotes nuclear export and phosphorylation by protein kinase A (PKA) that promotes nuclear import. We have previously shown that the scaffold protein muscle A-kinase anchoring protein β (mAKAPβ) orchestrates signaling in the cardiac myocyte required for pathological cardiac remodeling, including serving as a scaffold for both PKD and PKA. We now show that mAKAPβ is a scaffold for HDAC5 in cardiac myocytes, forming signalosomes containing HDAC5, PKD, and PKA. Inhibition of mAKAPβ expression attenuated the phosphorylation of HDAC5 by PKD and PKA in response to α- and β-adrenergic receptor stimulation, respectively. Importantly, disruption of mAKAPβ-HDAC5 anchoring prevented the induction of HDAC5 nuclear export by α-adrenergic receptor signaling and PKD phosphorylation. In addition, disruption of mAKAPβ-PKA anchoring prevented the inhibition by β-adrenergic receptor stimulation of α-adrenergic-induced HDAC5 nuclear export. Together, these data establish that mAKAPβ signalosomes serve to bidirectionally regulate the nuclear-cytoplasmic localization of class IIa HDACs. Thus, the mAKAPβ scaffold serves as a node in the myocyte regulatory network controlling both the repression and activation of pathological gene expression in health and disease, respectively.

Roles of A-Kinase Anchoring Proteins and Phosphodiesterases in the Cardiovascular System

A-kinase anchoring proteins (AKAPs) and cyclic nucleotide phosphodiesterases (PDEs) are essential enzymes in the cyclic adenosine 3′-5′ monophosphate (cAMP) signaling cascade. They establish local cAMP pools by controlling the intensity, duration and compartmentalization of cyclic nucleotide-dependent signaling. Various members of the AKAP and PDE families are expressed in the cardiovascular system and direct important processes maintaining homeostatic functioning of the heart and vasculature, e.g., the endothelial barrier function and excitation-contraction coupling. Dysregulation of AKAP and PDE function is associated with pathophysiological conditions in the cardiovascular system including heart failure, hypertension and atherosclerosis. A number of diseases, including autosomal dominant hypertension with brachydactyly (HTNB) and type I long-QT syndrome (LQT1), result from mutations in genes encoding for distinct members of the two classes of enzymes. This review provides an overview over the AKAPs and PDEs relevant for cAMP compartmentalization in the heart and vasculature and discusses their pathophysiological role as well as highlights the potential benefits of targeting these proteins and their protein-protein interactions for the treatment of cardiovascular diseases.

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.

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.

Emergency Spatiotemporal Shift: The Response of Protein Kinase D to Stress Signals in the Cardiovascular System

Protein Kinase D isoforms (PKD 1-3) are key mediators of neurohormonal, oxidative, and metabolic stress signals. PKDs impact a wide variety of signaling pathways and cellular functions including actin dynamics, vesicle trafficking, cell motility, survival, contractility, energy substrate utilization, and gene transcription. PKD activity is also increasingly linked to cancer, immune regulation, pain modulation, memory, angiogenesis, and cardiovascular disease. This increasing complexity and diversity of PKD function, highlights the importance of tight spatiotemporal control of the kinase via protein–protein interactions, post-translational modifications or targeting via scaffolding proteins. In this review, we focus on the spatiotemporal regulation and effects of PKD signaling in response to neurohormonal, oxidant and metabolic signals that have implications for myocardial disease. Precise targeting of these mechanisms will be crucial in the design of PKD-based therapeutic strategies.

Loss of AKAP150 promotes pathological remodelling and heart failure propensity by disrupting calcium cycling and contractile reserve

AIMS

Impaired Ca^2 +cycling and myocyte contractility are a hallmark of heart failure triggered by pathological stress such as hemodynamic overload. The A-Kinase anchoring protein AKAP150 has been shown to coordinate key aspects of adrenergic regulation of Ca²⁺cycling and excitation-contraction in cardiomyocytes. However, the role of the AKAP150 signalling complexes in the pathogenesis of heart failure has not been investigated.

METHODS AND RESULTS

Here we examined how AKAP150 signalling complexes impact Ca²⁺cycling, myocyte contractility, and heart failure susceptibility following pathological stress. We detected a significant reduction of AKAP150 expression in the failing mouse heart induced by pressure overload. Importantly, cardiac-specific AKAP150 knockout mice were predisposed to develop dilated cardiomyopathy with severe cardiac dysfunction and fibrosis after pressure overload. Loss of AKAP150 also promoted pathological remodelling and heart failure progression following myocardial infarction. However, ablation of AKAP150 did not affect calcineurin-nuclear factor of activated T cells signalling in cardiomyocytes or pressure overload- or agonist-induced cardiac hypertrophy. Immunoprecipitation studies showed that AKAP150 was associated with SERCA2, phospholamban, and ryanodine receptor-2, providing a targeted control of sarcoplasmic reticulum Ca²⁺regulatory proteins. Mechanistically, loss of AKAP150 led to impaired Ca²⁺cycling and reduced myocyte contractility reserve following adrenergic stimulation or pressure overload.

CONCLUSIONS

These findings define a critical role for AKAP150 in regulating Ca²⁺cycling and myocardial ionotropy following pathological stress, suggesting the AKAP150 signalling pathway may serve as a novel therapeutic target for heart failure.

The Function of the MEF2 Family of Transcription Factors in Cardiac Development, Cardiogenomics, and Direct Reprogramming

Proper formation of the mammalian heart requires precise spatiotemporal transcriptional regulation of gene programs in cardiomyocytes. Sophisticated regulatory networks have evolved to not only integrate the activities of distinct transcription factors to control tissue-specific gene programs but also, in many instances, to incorporate multiple members within these transcription factor families to ensure accuracy and specificity in the system. Unsurprisingly, perturbations in this elaborate transcriptional circuitry can lead to severe cardiac abnormalities. Myocyte enhancer factor–2 (MEF2) transcription factor belongs to the evolutionarily conserved cardiac gene regulatory network. Given its central role in muscle gene regulation and its evolutionary conservation, MEF2 is considered one of only a few core cardiac transcription factors. In addition to its firmly established role as a differentiation factor, MEF2 regulates wide variety of, sometimes antagonistic, cellular processes such as cell survival and death. Vertebrate genomes encode multiple MEF2 family members thereby expanding the transcriptional potential of this core transcription factor in the heart. This review highlights the requirement of the MEF2 family and their orthologs in cardiac development in diverse animal model systems. Furthermore, we describe the recently characterized role of MEF2 in direct reprogramming and genome-wide cardiomyocyte gene regulation. A thorough understanding of the regulatory functions of the MEF2 family in cardiac development and cardiogenomics is required in order to develop effective therapeutic strategies to repair the diseased heart.

AKAP220 manages apical actin networks that coordinate aquaporin-2 location and renal water reabsorption

Filtration through the kidney eliminates toxins, manages electrolyte balance, and controls water homeostasis. Reabsorption of water from the luminal fluid of the nephron occurs through aquaporin-2 (AQP2) water pores in principal cells that line the kidney-collecting duct. This vital process is impeded by formation of an "actin barrier" that obstructs the passive transit of AQP2 to the plasma membrane. Bidirectional control of AQP2 trafficking is managed by hormones and signaling enzymes. We have discovered that vasopressin-independent facets of this homeostatic mechanism are under the control of A-Kinase Anchoring Protein 220 (AKAP220; product of the Akap11 gene). CRISPR/Cas9 gene editing and imaging approaches show that loss of AKAP220 disrupts apical actin networks in organoid cultures. Similar defects are evident in tissue sections from AKAP220-KO mice. Biochemical analysis of AKAP220-null kidney extracts detected reduced levels of active RhoA GTPase, a well-known modulator of the actin cytoskeleton. Fluorescent imaging of kidney sections from these genetically modified mice revealed that RhoA and AQP2 accumulate at the apical surface of the collecting duct. Consequently, these animals are unable to appropriately dilute urine in response to overhydration. We propose that membrane-proximal signaling complexes constrained by AKAP220 impact the actin barrier dynamics and AQP2 trafficking to ensure water homeostasis.

Enhanced cAMP-stimulated protein kinase A activity in human fibrolamellar hepatocellular carcinoma

BACKGROUND

Fibrolamellar hepatocellular carcinoma (FL-HCC) affects children without underlying liver disease. A consistent mutation in FL-HCCs leads to fusion of the genes encoding a heat shock protein (DNAJB1) and the catalytic subunit of protein kinase A (PRKACA). We sought to characterize the resultant chimeric protein and its effects in FL-HCC.

METHODS

The expression pattern and subcellular localization of protein kinase A (PKA) subunits in FL-HCCs were compared to paired normal livers by quantitative polymerase chain reaction (qPCR), immunoblotting, and immunofluorescence. PKA activity was measured by radioactive kinase assay, and we determined whether the FL-HCC mutation is present in other primary liver tumors.

RESULTS

The fusion transcript and chimeric protein were detected exclusively in FL-HCCs. DNAJB1-PRKACA was expressed 10-fold higher than the wild-type PRKACA transcript, resulting in overexpression of the mutant protein in tumors. Consequently, FL-HCCs possess elevated cAMP-stimulated PKA activity compared to normal livers, despite similar Kms between the mutant and wild-type kinases.

CONCLUSION

FL-HCCs in children and young adults uniquely overexpress DNAJB1-PRKACA, which results in elevated cAMP-dependent PKA activity. These data suggest that aberrant PKA signaling contributes to liver tumorigenesis.

Novel approaches to targeting PDE3 in cardiovascular disease

Inhibitors of PDE3, a family of dual-specificity cyclic nucleotide phosphodiesterases, are used clinically to increase cardiac contractility by raising intracellular cAMP content in cardiac myocytes and to reduce vascular resistance by increasing intracellular cGMP content in vascular smooth muscle myocytes. When used in the treatment of patients with heart failure, PDE3 inhibitors are effective in the acute setting but increase sudden cardiac death with long-term administration, possibly reflecting pro-apoptotic and pro-hypertrophic consequences of increased cAMP-mediated signaling in cardiac myocytes. cAMP-mediated signaling in cardiac myocytes is highly compartmentalized, and different phosphodiesterases, by controlling cAMP content in functionally discrete intracellular microcompartments, regulate different cAMP-mediated pathways. Four variants/isoforms of PDE3 (PDE3A1, PDE3A2, PDE3A3, and PDE3B) are expressed in cardiac myocytes, and new experimental results have demonstrated that these isoforms, which are differentially localized intracellularly through unique protein-protein interactions, control different physiologic responses. While the catalytic regions of these isoforms may be too similar to allow the catalytic activity of each isoform to be selectively inhibited, targeting their unique protein-protein interactions may allow desired responses to be elicited without the adverse consequences that limit the usefulness of existing PDE3 inhibitors.

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.

Protein kinase A catalytic subunit isoform PRKACA; History, function and physiology

Our appreciation of the scope and influence of second messenger signaling has its origins in pioneering work on the cAMP-dependent protein kinase. Also called protein kinase A (PKA), this holoenzyme exists as a tetramer comprised of a regulatory (R) subunit dimer and two catalytic (C) subunits. Upon binding of two molecules of the second messenger cAMP to each R subunit, a conformational change in the PKA holoenzyme occurs to release the C subunits. These active kinases phosphorylate downstream targets to propagate cAMP responsive cell signaling events. This article focuses on the discovery, structure, cellular location and physiological effects of the catalytic subunit alpha of protein kinase A (encoded by the gene PRKACA). We also explore the potential role of this essential gene as a molecular mediator of certain disease states.

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.

AKAP6 inhibition impairs myoblast differentiation and muscle regeneration: Positive loop between AKAP6 and myogenin

Skeletal muscle regeneration occurs continuously to repair muscle damage incurred during normal activity and in chronic disease or injury. Herein, we report that A-kinase anchoring protein 6 (AKAP6) is important for skeletal myoblast differentiation and muscle regeneration. Compared with unstimulated skeletal myoblasts that underwent proliferation, differentiated cells show significant stimulation of AKAP6 expression. AKAP6 knockdown with siRNA effectively halts the formation of myotubes and decreases the expression of the differentiation markers myogenin and myosin heavy chain. When shAKAP6-lentivirus is delivered to mice with cardiotoxin (CTX)-induced muscle injury, muscle regeneration is impaired compared with that of mice injected with control shMock-lentivirus. The motor functions of mice infected with shAKAP6-lentivirus (CTX+shAK6) are significantly worse than those of mice infected with shMock-lentivirus (CTX+shMock). Mechanistic analysis showed that AKAP6 promotes myogenin expression through myocyte enhancer factor 2A (MEF2A). Notably, myogenin increases AKAP6 expression as well. The results of chromatin immunoprecipitation and luciferase assays showed that myogenin binds to an E-box site on the AKAP6 promoter. Taken together, our findings demonstrate a novel interplay between AKAP6 and myogenin, and we suggest that AKAP6 is an important regulator of myoblast differentiation, myotube formation, and muscle regeneration.

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.

Genome-Wide Gene Expression Analysis Shows AKAP13-Mediated PKD1 Signaling Regulates the Transcriptional Response to Cardiac Hypertrophy

In the heart, scaffolding proteins such as A-Kinase Anchoring Proteins (AKAPs) play a crucial role in normal cellular function by serving as a signaling hub for multiple protein kinases including protein kinase D1 (PKD1). Under cardiac hypertrophic conditions AKAP13 anchored PKD1 activates the transcription factor MEF2 leading to subsequent fetal gene activation and hypertrophic response. We used an expression microarray to identify the global transcriptional response in the hearts of wild-type mice expressing the native form of AKAP13 compared to a gene-trap mouse model expressing a truncated form of AKAP13 that is unable to bind PKD1 (AKAP13-ΔPKD1). Microarray analysis showed that AKAP13-ΔPKD1 mice broadly failed to exhibit the transcriptional profile normally associated with compensatory cardiac hypertrophy following trans-aortic constriction (TAC). The identified differentially expressed genes in WT and AKAP13-ΔPKD1 hearts are vital for the compensatory hypertrophic response to pressure-overload and include myofilament, apoptotic, and cell growth/differentiation genes in addition to genes not previously identified as affected by AKAP13-anchored PKD1. Our results show that AKAP13-PKD1 signaling is critical for transcriptional regulation of key contractile, cell death, and metabolic pathways during the development of compensatory hypertrophy in vivo.

The alpha1-adrenergic receptors in cardiac hypertrophy: signaling mechanisms and functional implications

Cardiac hypertrophy is a complex remodeling process of the heart induced by physiological or pathological stimuli resulting in increased cardiomyocyte size and myocardial mass. Whereas cardiac hypertrophy can be an adaptive mechanism to stressful conditions of the heart, prolonged hypertrophy can lead to heart failure which represents the primary cause of human morbidity and mortality. Among G protein-coupled receptors, the α1-adrenergic receptors (α1-ARs) play an important role in the development of cardiac hypertrophy as demonstrated by numerous studies in the past decades, both in primary cardiomyocyte cultures and genetically modified mice. The results of these studies have provided evidence of a large variety of α1-AR-induced signaling events contributing to the defining molecular and cellular features of cardiac hypertrophy. Recently, novel signaling mechanisms have been identified and new hypotheses have emerged concerning the functional role of the α1-adrenergic receptors in the heart. This review will summarize the main signaling pathways activated by the α1-AR in the heart and their functional implications in cardiac hypertrophy.

Protein Kinase A Opposes the Phosphorylation-dependent Recruitment of Glycogen Synthase Kinase 3β to A-kinase Anchoring Protein 220

The proximity of an enzyme to its substrate can influence rate and magnitude of catalysis. A-kinase anchoring protein 220 (AKAP220) is a multivalent anchoring protein that can sequester a variety of signal transduction enzymes. These include protein kinase A (PKA) and glycogen synthase kinase 3β (GSK3β). Using a combination of molecular and cellular approaches we show that GSK3β phosphorylation of Thr-1132 on AKAP220 initiates recruitment of this kinase into the enzyme scaffold. We also find that AKAP220 anchors GSK3β and its substrate β-catenin in membrane ruffles. Interestingly, GSK3β can be released from the multienzyme complex in response to PKA phosphorylation on serine 9, which suppresses GSK3β activity. The signaling scaffold may enhance this regulatory mechanism, as AKAP220 has the capacity to anchor two PKA holoenzymes. Site 1 on AKAP220 (residues 610-623) preferentially interacts with RII, whereas site 2 (residues 1633-1646) exhibits a dual specificity for RI and RII. In vitro affinity measurements revealed that site 2 on AKAP220 binds RII with ∼10-fold higher affinity than site 1. Occupancy of both R subunit binding sites on AKAP220 could provide a mechanism to amplify local cAMP responses and enable cross-talk between PKA and GSK3β.

Protein Kinase A (PKA) Phosphorylation of Shp2 Protein Inhibits Its Phosphatase Activity and Modulates Ligand Specificity

Pathological cardiac hypertrophy (an increase in cardiac mass resulting from stress-induced cardiac myocyte growth) is a major factor underlying heart failure. Src homology 2 domain-containing phosphatase (Shp2) is critical for cardiac function because mutations resulting in loss of Shp2 catalytic activity are associated with congenital cardiac defects and hypertrophy. We identified a novel mechanism of Shp2 inhibition that may promote cardiac hypertrophy. We demonstrate that Shp2 is a component of the protein kinase A anchoring protein (AKAP)-Lbc complex. AKAP-Lbc facilitates PKA phosphorylation of Shp2, which inhibits Shp2 phosphatase activity. We identified two key amino acids in Shp2 that are phosphorylated by PKA. Thr-73 contributes a helix cap to helix αB within the N-terminal SH2 domain of Shp2, whereas Ser-189 occupies an equivalent position within the C-terminal SH2 domain. Utilizing double mutant PKA phosphodeficient (T73A/S189A) and phosphomimetic (T73D/S189D) constructs, in vitro binding assays, and phosphatase activity assays, we demonstrate that phosphorylation of these residues disrupts Shp2 interaction with tyrosine-phosphorylated ligands and inhibits its protein-tyrosine phosphatase activity. Overall, our data indicate that AKAP-Lbc integrates PKA and Shp2 signaling in the heart and that AKAP-Lbc-associated Shp2 activity is reduced in hypertrophic hearts in response to chronic β-adrenergic stimulation and PKA activation. Therefore, although induction of cardiac hypertrophy is a multifaceted process, inhibition of Shp2 activity through AKAP-Lbc-anchored PKA is a previously unrecognized mechanism that may promote this compensatory response.

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.

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.

Post-translational modifications regulate class IIa histone deacetylase (HDAC) function in health and disease

Class IIa histone deacetylases (HDACs4, -5, -7, and -9) modulate the physiology of the human cardiovascular, musculoskeletal, nervous, and immune systems. The regulatory capacity of this family of enzymes stems from their ability to shuttle between nuclear and cytoplasmic compartments in response to signal-driven post-translational modification. Here, we review the current knowledge of modifications that control spatial and temporal histone deacetylase functions by regulating subcellular localization, transcriptional functions, and cell cycle-dependent activity, ultimately impacting on human disease. We discuss the contribution of these modifications to cardiac and vascular hypertrophy, myoblast differentiation, neuronal cell survival, and neurodegenerative disorders.

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.

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.