Phosphorylation state-dependent interaction between AKAP7δ/γ and phospholamban increases phospholamban phosphorylation

AKAP12 Upregulation Associates With PDE8A to Accelerate Cardiac Dysfunction

BACKGROUND

In heart failure, signaling downstream the β2-adrenergic receptor is critical. Sympathetic stimulation of β2-adrenergic receptor alters cAMP (cyclic adenosine 3',5'-monophosphate) and triggers PKA (protein kinase A)-dependent phosphorylation of proteins that regulate cardiac function. cAMP levels are regulated in part by PDEs (phosphodiesterases). Several AKAPs (A kinase anchoring proteins) regulate cardiac function and are proposed as targets for precise pharmacology. AKAP12 is expressed in the heart and has been reported to directly bind β2-adrenergic receptor, PKA, and PDE4D. However, its roles in cardiac function are unclear.

METHODS

cAMP accumulation in real time downstream of the β2-adrenergic receptor was detected for 60 minutes in live cells using the luciferase-based biosensor (GloSensor) in AC16 human-derived cardiomyocyte cell lines overexpressing AKAP12 versus controls. Cardiomyocyte intracellular calcium and contractility were studied in adult primary cardiomyocytes from male and female mice overexpressing cardiac AKAP12 (AKAP12OX) and wild-type littermates post acute treatment with 100-nM isoproterenol (ISO). Systolic cardiac function was assessed in mice after 14 days of subcutaneous ISO administration (60 mg/kg per day). AKAP12 gene and protein expression levels were evaluated in left ventricular samples from patients with end-stage heart failure.

RESULTS

AKAP12 upregulation significantly reduced total intracellular cAMP levels in AC16 cells through PDE8. Adult primary cardiomyocytes from AKAP12OX mice had significantly reduced contractility and impaired calcium handling in response to ISO, which was reversed in the presence of the selective PDE8 inhibitor (PF-04957325). AKAP12OX mice had deteriorated systolic cardiac function and enlarged left ventricles. Patients with end-stage heart failure had upregulated gene and protein levels of AKAP12.

CONCLUSIONS

AKAP12 upregulation in cardiac tissue is associated with accelerated cardiac dysfunction through the AKAP12-PDE8 axis.

A-Kinase Anchoring Proteins in Cardiac Myocytes and Their Roles in Regulating Calcium Cycling

The rate of calcium cycling and calcium transient amplitude are critical determinants for the efficient contraction and relaxation of the heart. Calcium-handling proteins in the cardiac myocyte are altered in heart failure, and restoring the proper function of those proteins is an effective potential therapeutic strategy. The calcium-handling proteins or their regulators are phosphorylated by a cAMP-dependent kinase (PKA), and thereby their activity is regulated. A-Kinase Anchoring Proteins (AKAPs) play a seminal role in orchestrating PKA and cAMP regulators in calcium handling and contractile machinery. This cAMP/PKA orchestration is crucial for the increased force and rate of contraction and relaxation of the heart in response to fight-or-flight. Knockout models and the few available preclinical models proved that the efficient targeting of AKAPs offers potential therapies tailor-made for improving defective calcium cycling. In this review, we highlight important studies that identified AKAPs and their regulatory roles in cardiac myocyte calcium cycling in health and disease.

Subcellular Propagation of Cardiomyocyte β-Adrenergic Activation of Calcium Uptake Involves Internal β-Receptors and AKAP7

β-adrenergic receptor (β-AR) signaling in cardiac myocytes is central to cardiac function, but spatiotemporal activation within myocytes is unresolved. In rabbit ventricular myocytes, β-AR agonists or high extracellular [Ca] were applied locally at one end, to measure β-AR signal propagation as Ca-transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca uptake. High local [Ca]_o, increased CaT amplitude under the pipette faster than did ISO, but was also more spatially restricted. Local isoproterenol (ISO) or norepinephrine (NE) increased CaT amplitude and SR Ca uptake, that spread along the myocyte to the unexposed end. Thus, local [Ca]_i decline kinetics reflect spatio-temporal progression of β-AR end-effects in myocytes. To test whether intracellular β-ARs contribute to this response, we used β-AR-blockers that are membrane permeant (propranolol) or not (sotalol). Propranolol completely blocked NE-dependent CaT effects. However, blocking surface β-ARs only (sotalol) suppressed only ∼50% of the NE-induced increase in CaT peak and rate of [Ca]_i decline, but these changes spread more gradually than NE alone. We also tested whether A-kinase anchoring protein 7γ (AKAP7γ; that interacts with phospholamban) is mobile, such that it might contribute to intracellular spatial propagation of β-AR signaling. We found AKAP7γ to be highly mobile using fluorescence recovery after photobleach of GFP tagged AKAP7γ, and that PKA activation accelerated AKAP7γ-GFP wash-out upon myocyte saponin-permeabilization, suggesting increased AKAP7γ mobility. We conclude that local β-AR activation can activate SR Ca uptake at remote myocyte sites, and that intracellular β-AR and AKAP7γ mobility may play a role in this spread of activation.

Protein phosphatase 2A in the healthy and failing heart: New insights and therapeutic opportunities

Protein phosphatases have emerged as critical regulators of phosphoprotein homeostasis in settings of health and disease. Protein phosphatase 2A (PP2A) encompasses a large subfamily of enzymes that remove phosphate groups from serine/threonine residues within phosphoproteins. The heterogeneity in PP2A structure, which arises from the grouping of different catalytic, scaffolding and regulatory subunit isoforms, creates distinct populations of catalytically active enzymes (i.e. holoenzymes) that localise to different parts of the cell. This structural complexity, combined with other regulatory mechanisms, such as interaction of PP2A heterotrimers with accessory proteins and post-translational modification of the catalytic and/or regulatory subunits, enables PP2A holoenzymes to target phosphoprotein substrates in a highly specific manner. In this review, we summarise the roles of PP2A in cardiac physiology and disease. PP2A modulates numerous processes that are vital for heart function including calcium handling, contractility, β-adrenergic signalling, metabolism and transcription. Dysregulation of PP2A has been observed in human cardiac disease settings, including heart failure and atrial fibrillation. Efforts are underway, particularly in the cancer field, to develop therapeutics targeting PP2A activity. The development of small molecule activators of PP2A (SMAPs) and other compounds that selectively target specific PP2A holoenzymes (e.g. PP2A/B56α and PP2A/B56ε) will improve understanding of the function of different PP2A species in the heart, and may lead to the development of therapeutics for normalising aberrant protein phosphorylation in settings of cardiac remodelling and dysfunction.

Molecular noise filtering in the β-adrenergic signaling network by phospholamban pentamers

Phospholamban (PLN) is an important regulator of cardiac calcium handling due to its ability to inhibit the calcium ATPase SERCA. β-Adrenergic stimulation reverses SERCA inhibition via PLN phosphorylation and facilitates fast calcium reuptake. PLN also forms pentamers whose physiological significance has remained elusive. Using mathematical modeling combined with biochemical and cell biological experiments, we show that pentamers regulate both the dynamics and steady-state levels of monomer phosphorylation. Substrate competition by pentamers and a feed-forward loop involving inhibitor-1 can delay monomer phosphorylation by protein kinase A (PKA), whereas cooperative pentamer dephosphorylation enables bistable PLN steady-state phosphorylation. Simulations show that phosphorylation delay and bistability act as complementary filters that reduce the effect of random fluctuations in PKA activity, thereby ensuring consistent monomer phosphorylation and SERCA activity despite noisy upstream signals. Preliminary analyses suggest that the PLN mutation R14del could impair noise filtering, offering a new perspective on how this mutation causes cardiac arrhythmias.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions.

AKAP6 and phospholamban colocalize and interact in HEK-293T cells and primary murine cardiomyocytes

Phospholamban (PLN) is an important Ca²⁺ modulator at the sarcoplasmic reticulum (SR) of striated muscles. It physically interacts and inhibits sarcoplasmic reticulum Ca²⁺ATPase (SERCA2) function, whereas a protein kinase A (PKA)‐dependent phosphorylation at its serine 16 reverses the inhibition. The underlying mechanism of this post‐translational modification, however, remains not fully understood. Using publicly available databases, we identified A‐kinase anchoring protein 6 (AKAP6) as a candidate that might play some roles in PLN phosphorylation. Immunofluorescence showed colocalization between GFP‐AKAP6 and PLN in transfected HEK‐293T cells and cultured mouse neonatal cardiomyocytes (CMNCs). Co‐immunoprecipitation confirmed the functional interaction between AKAP6 and PLN in HEK‐293T and isolated adult rat cardiomyocytes in response to isoproterenol stimulation. Functionally, AKAP6 promoted Ca²⁺ uptake activity of SERCA1 in cotransfected HEK‐293T cells despite the presence of PLN. These results were further confirmed in adult rat cardiomyocytes. Immunofluorescence showed colocalization of both proteins around the perinuclear region, while protein–protein interaction was corroborated by immunoprecipitation of the nucleus‐enriched fraction of rat hearts. Our findings suggest AKAP6 as a novel interacting partner to PLN in HEK‐293T and murine cardiomyocytes.

The many faces of compartmentalized PKA signalosomes

Cellular signal transmission requires the dynamic formation of spatiotemporally controlled molecular interactions. At the cell surface information is received by receptor complexes and relayed through intracellular signaling platforms which organize the actions of functionally interacting signaling enzymes and substrates. The list of hormone or neurotransmitter pathways that utilize the ubiquitous cAMP-sensing protein kinase A (PKA) system is expansive. This requires that the specificity, duration, and intensity of PKA responses are spatially and temporally restricted. Hereby, scaffolding proteins take the center stage for ensuring proper signal transmission. They unite second messenger sensors, activators, effectors, and kinase substrates within cellular micro-domains to precisely control and route signal propagation. A-kinase anchoring proteins (AKAPs) organize such subcellular signalosomes by tethering the PKA holoenzyme to distinct cell compartments. AKAPs differ in their modular organization showing pathway specific arrangements of interaction motifs or domains. This enables the cell- and compartment- guided assembly of signalosomes with unique enzyme composition and function. The AKAP-mediated clustering of cAMP and other second messenger sensing and interacting signaling components along with functional successive enzymes facilitates the rapid and precise dissemination of incoming signals. This review article delineates examples for different means of PKA regulation and for snapshots of compartmentalized PKA signalosomes.

Glycosylation and other PTMs alterations in neurodegenerative diseases: Current status and future role in neurotrauma

Traumatic brain injuries (TBIs) present a chief public health threat affecting nations worldwide. As numbers of patients afflicted by TBI are expected to rise, the necessity to increase our understanding of the pathophysiological mechanism(s) as a result of TBI mounts. TBI is known to augment the risk of developing a number of neurodegenerative diseases (NDs) such as Alzheimer's disease (AD) and Parkinson's disease (PD). Hence, it is rational to assume that a common mechanistic ground links the pathophysiology of NDs to that of TBIs. Through this review, we aim to identify the protein-protein interactions, differential proteins expression, and PTMs, mainly glycosylation, that are involved in the pathogenesis of both ND and TBI. OVID and PubMed have been rigorously searched to identify studies that utilized advanced proteomic platforms (MS based) and systems biology tools to unfold the mechanism(s) behind ND in an attempt to unveil the mysterious biological processes that occur postinjury. Various PTMs have been found to be common between TBI and AD, whereas no similarities have been found between TBI and PD. Phosphorylated tau protein, glycosylated amyloid precursor protein, and many other modifications appear to be common in both TBI and AD. PTMs, differential protein profiles, and altered biological pathways appear to have critical roles in ND processes by interfering with their pathological condition in a manner similar to TBI. Advancement in glycoproteomic studies pertaining to ND and TBI is urgently needed in order to develop better diagnostic tools, therapies, and more favorable prognoses.

Analysis of AKAP7γ Dimerization

A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that contribute to spatiotemporal regulation of PKA-mediated phosphorylation events. In particular, AKAP7 is a family of alternatively spliced proteins that participates in cardiac calcium dynamics. Here, we demonstrate via pull-down from transfected cells and by direct protein-protein association that AKAP7γ self-associates. Self-association appears to be an isoform specific phenomenon, as AKAP7α did not associate with itself or with AKAP7γ. However, AKAP7γ did associate with AKAP7δ, suggesting the long isoforms of the AKAP can form heterodimers. Surface plasmon resonance found that the AKAP7γ self-association occurs via two high affinity binding sites with K D values in the low nanomolar range. Mapping of the binding sites by peptide array reveals that AKAP7γ interacts with itself through multiple regions. Photon counting histogram analysis (PCH) of AKAP7γ-EGFP expressed in HEK-293 cells confirmed that AKAP7γ-EGFP self-associates in a cellular context. Lastly, computational modeling of PKA dynamics within AKAP7γ complexes suggests that oligomerization may augment phosphorylation of scaffolded PKA substrates. In conclusion, our study reveals that AKAP7γ forms both homo- and heterodimers with the long isoforms of the AKAP and that this phenomenon could be an important step in mediating effective substrate phosphorylation in cellular microdomains.