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

Novel insight into the role of A-kinase anchoring proteins (AKAPs) in ischemic stroke and therapeutic potentials

Ischemic stroke, a devastating disease associated with high mortality and disability worldwide, has emerged as an urgent public health issue. A-kinase anchoring proteins (AKAPs) are a group of signal-organizing molecules that compartmentalize and anchor a wide range of receptors and effector proteins and have a major role in stabilizing mitochondrial function and promoting neurodevelopmental development in the central nervous system (CNS). Growing evidence suggests that dysregulation of AKAPs expression and activity is closely associated with oxidative stress, ion disorder, mitochondrial dysfunction, and blood-brain barrier (BBB) impairment in ischemic stroke. However, the underlying mechanisms remain inadequately understood. This review provides a comprehensive overview of the composition and structure of A-kinase anchoring protein (AKAP) family members, emphasizing their physiological functions in the CNS. We explored in depth the molecular and cellular mechanisms of AKAP complexes in the pathological progression and risk factors of ischemic stroke, including hypertension, hyperglycemia, lipid metabolism disorders, and atrial fibrillation. Herein, we highlight the potential of AKAP complexes as a pharmacological target against ischemic stroke in the hope of inspiring translational research and innovative clinical approaches.

Mitochondria-associated endoplasmic reticulum membranes promote mitochondrial fission through AKAP1-Drp1 pathway in podocytes under high glucose conditions

Excessive mitochondrial fission in podocytes is a critical feature of diabetic nephropathy (DN). Mitochondria-associated endoplasmic reticulum membranes (MAMs) are contact sites between the endoplasmic reticulum (ER) and mitochondria, which are suggested to be related to mitochondrial function. However, the role of MAMs in mitochondrial dynamics disorder in podocytes remains unknown. Here, we firstly reported a novel mechanism of MAMs' effects on mitochondrial dynamics in podocytes under diabetic conditions. Increased MAMs were found in diabetic podocytes in vivo and in vitro, which were positively correlated with excessive mitochondrial fission. What's more, we also found that A-kinase anchoring protein 1 (AKAP1) was located in MAMs, and its translocation to MAMs was increased in podocytes cultured with high glucose (HG). In addition, AKAP1 knockdown significantly reduced mitochondrial fission and attenuated high glucose induced-podocyte injury through regulating phosphorylation of dynamin-related protein 1 (Drp1) and its subsequent mitochondrial translocation. On the contrary, AKAP1 overexpression in these podocytes showed the opposite effect. Finally, pharmacological inhibition of Drp1 alleviated excessive mitochondrial fission and podocyte damage in AKAP1 overexpressed podocytes. Our data suggest that MAMs were increased in podocytes under diabetic conditions, leading to excessive mitochondrial fission and podocyte damage through AKAP1-Drp1 signaling.

When Just One Phosphate Is One Too Many: The Multifaceted Interplay between Myc and Kinases

Myc transcription factors are key regulators of many cellular processes, with Myc target genes crucially implicated in the management of cell proliferation and stem pluripotency, energy metabolism, protein synthesis, angiogenesis, DNA damage response, and apoptosis. Given the wide involvement of Myc in cellular dynamics, it is not surprising that its overexpression is frequently associated with cancer. Noteworthy, in cancer cells where high Myc levels are maintained, the overexpression of Myc-associated kinases is often observed and required to foster tumour cells' proliferation. A mutual interplay exists between Myc and kinases: the latter, which are Myc transcriptional targets, phosphorylate Myc, allowing its transcriptional activity, highlighting a clear regulatory loop. At the protein level, Myc activity and turnover is also tightly regulated by kinases, with a finely tuned balance between translation and rapid protein degradation. In this perspective, we focus on the cross-regulation of Myc and its associated protein kinases underlying similar and redundant mechanisms of regulation at different levels, from transcriptional to post-translational events. Furthermore, a review of the indirect effects of known kinase inhibitors on Myc provides an opportunity to identify alternative and combined therapeutic approaches for cancer treatment.

A-kinase anchoring protein 5-ancored calcineurin regulates the remodeling of H9c2 cardiomyocytes exposed to hypoxia and reoxygenation

OBJECTIVE

A-kinase anchoring protein 5 (AKAP5) is involved in ventricular remodeling in rats with heart failure after myocardial infarction; however, the specific mechanism is not clear. This study investigated whether AKAP5 anchors calcineurin (CaN) to regulate the remodeling of H9c2 cardiomyocytes.

METHODS

H9c2 cells were subjected to hypoxia stress for 3 h and reoxygenation for 24 h to create a hypoxia-reoxygenation (H/R) model. These cells were divided into three groups: H/R (model), empty vector +H/R (NC), and siRNA-AKAP5+H/R (siRNA-AKAP5) groups. The non-H/R H9c2 cells were used as normal controls. Western blotting was used to detect cardiac hypertrophy-related protein expression in the cells, including atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), beta myosin heavy chain (β-MHC), and phosphorylated nuclear factor of activated T-cell 3 (p-NFATc3). Phalloidin staining was used to label the cytoskeleton and the cell area in different groups was measured. Immunofluorescence staining and coimmunoprecipitation were used to study the relationship between AKAP5 and CaN. H9c2 cells pretreated with the CaN inhibitor FK506 were used to further verify the relationship between AKAP5 and CaN.

RESULTS

In the siRNA-AKAP5+H/R group, the expression level of cardiac hypertrophy-related proteins (ANP, BNP, and β-MHC) and CaN and the area of cardiomyocytes were significantly increased, while the p-NFATc3/NFATc3 ratio was decreased in H9c2H/R cells. AKAP5 and CaN proteins were colocalized and interacted in the cells. The CaN inhibitor significantly suppressed the expression of CaN, increased the p-NFATc3/NFATc3 ratio, and reduced the expression levels of ANP, BNP, and β-MHC proteins in the cells with low AKAP5 expression.

CONCLUSIONS

AKAP5 downregulation aggravated the remodeling of cardiomyocytes after H/R. AKAP5 may anchor CaN to form a complex, which in turn activates NFATc3 dephosphorylation and expression of hypertrophy-related proteins.

Exosomes in atrial fibrillation: therapeutic potential and role as clinical biomarkers

Atrial fibrillation (AF), the most common cardiac arrhythmia, is a global epidemic. AF can cause heart failure and myocardial infarction and increase the risk of stroke, disability, and thromboembolic events. AF is becoming increasingly ubiquitous and is associated with increased morbidity and mortality at higher ages, resulting in an increasing threat to human health as well as substantial medical and social costs. Currently, treatment strategies for AF focus on controlling heart rate and rhythm with medications to restore and maintain sinus rhythm, but this approach has limitations. Catheter ablation is not entirely satisfactory and does not address the issues underlying AF. Research exploring the mechanisms causing AF is urgently needed for improved prevention, diagnosis, and treatment of AF. Exosomes are small vesicles (30-150 nm) released by cells that transmit information between cells. MicroRNAs in exosomes play an important role in the pathogenesis of AF and are established as a biomarker for AF. In this review, a summary of the role of exosomes in AF is presented. The role of exosomes and microRNAs in AF occurrence, their therapeutic potential, and their potential role as clinical biomarkers is considered. A better understanding of exosomes has the potential to improve the prognosis of AF patients worldwide, reducing the global medical burden of this disease.

Potential therapeutic applications of AKAP disrupting peptides

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

KV1.5-KVβ1.3 Recycling Is PKC-Dependent

K_V1.5 channel function is modified by different regulatory subunits. K_Vβ1.3 subunits assemble with K_V1.5 channels and induce a fast and incomplete inactivation. Inhibition of PKC abolishes the K_Vβ1.3-induced fast inactivation, decreases the amplitude of the current K_V1.5–K_Vβ1.3 and modifies their pharmacology likely due to changes in the traffic of K_V1.5–K_Vβ1.3 channels in a PKC-dependent manner. In order to analyze this hypothesis, HEK293 cells were transfected with K_V1.5–K_Vβ1.3 channels, and currents were recorded by whole-cell configuration of the patch-clamp technique. The presence of K_V1.5 in the membrane was analyzed by biotinylation techniques, live cell imaging and confocal microscopy approaches. PKC inhibition resulted in a decrease of 33 ± 7% of channels in the cell surface due to reduced recycling to the plasma membrane, as was confirmed by confocal microscopy. Live cell imaging indicated that PKC inhibition almost abolished the recycling of the K_V1.5–K_Vβ1.3 channels, generating an accumulation of channels into the cytoplasm. All these results suggest that the trafficking regulation of K_V1.5–K_Vβ1.3 channels is dependent on phosphorylation by PKC and, therefore, they could represent a clinically relevant issue, mainly in those diseases that exhibit modifications in PKC activity.

Protein Kinase Inhibitor Peptide as a Tool to Specifically Inhibit Protein Kinase A

The protein kinase enzyme family plays a pivotal role in almost every aspect of cellular function, including cellular metabolism, division, proliferation, transcription, movement, and survival. Protein kinase A (PKA), whose activation is triggered by cyclic adenosine monophosphate (cAMP), is widely distributed in various systems and tissues throughout the body and highly related to pathogenesis and progression of various kinds of diseases. The inhibition of PKA activation is essential for the study of PKA functions. Protein kinase inhibitor peptide (PKI) is a potent, heat-stable, and specific PKA inhibitor. It has been demonstrated that PKI can block PKA-mediated phosphorylase activation. Since then, researchers have a lot of knowledge about PKI. PKI is considered to be the most effective and specific method to inhibit PKA and is widely used in related research. In this review, we will first introduce the knowledge on the activation of PKA and mechanisms related on the inhibitory effects of PKI on PKA. Then, we will compare PKI-mediated PKA inhibition vs. several popular methods of PKA inhibition.

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.

New aspects in cardiac L-type Ca2+ channel regulation

Cardiac excitation-contraction coupling is initiated with the influx of Ca2+ ions across the plasma membrane through voltage-gated L-type calcium channels. This process is tightly regulated by modulation of the channel open probability and channel localization. Protein kinase A (PKA) is found in close association with the channel and is one of the main regulators of its function. Whether this kinase is modulating the channel open probability by phosphorylation of key residues or via alternative mechanisms is unclear. This review summarizes recent findings regarding the PKA-mediated channel modulation and will highlight recently discovered regulatory mechanisms that are independent of PKA activity and involve protein-protein interactions and channel localization.

Myofibroblast-Derived Exosomes Contribute to Development of a Susceptible Substrate for Atrial Fibrillation

OBJECTIVE

Atrial fibrosis plays a critical role in atrial fibrillation (AF). A key event in the pathogenesis of fibrosis is the activation of fibroblasts (FBs) into myofibroblasts (MFBs). Paracrine factors released from MFBs lead to ion channel expression changes in cardiomyocytes (CMs). Downregulation of L-type calcium channel Cav1.2 expression is a hallmark of AF-associated ionic remodeling. However, whether exosome (Exo)-mediated crosstalk between MFBs and CMs regulates Cav1.2 expression remains unknown.

METHODS

Atrial FBs and CMs were isolated and cultured from neonatal rats by enzymatic digestion. The activation of FBs into MFBs was induced by angiotensin II. Co-culture assay and in vitro Exo treatment were used to determine the effect of MFB-derived Exos on Cav1.2 expression. Confocal Ca2+ imaging was performed to examine the adrenergic stimulation-elicited Ca2+ influx signals. The levels of potential Cav1.2-inhibitory microRNAs (miRNAs) were measured by qRT-PCR.

RESULTS

Untreated FBs expressed limited amounts of alpha smooth muscle actin (α-SMA), while angiotensin II induced a significant upregulation of α-SMA-expressing MFBs. Co-cultures of MFBs and CMs resulted in downregulation of Cav1.2 expression in CMs, which was largely abolished by pretreatment of MFBs with exosomal inhibitor GW4869. More importantly, treatment with MFB-derived Exos caused repression of Cav1.2 expression in CMs. Additionally, the adrenergic receptor agonist-elicited Ca2+ influx signals in CMs were remarkably attenuated by pretreatment with MFB-derived Exos, corresponding to the paralleled change in Cav1.2 expression. Finally, miR-21-3p, a potential Cav1.2-inhibitory miRNA, was enriched in MFB-derived Exos and upregulated in CMs in response to MFB-derived Exos.

CONCLUSION

We uncover an Exo-mediated crosstalk between MFBs and CMs, contributing to increased vulnerability to AF by reducing the expression of Cav1.2 in CMs.

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.