Abstract
AKAPs are gallowing proteins that attach themselves to the regulatory subunit of PKA. On attachment, they target PKA to specific substrates. Similarly, AKAPs bind other forms of kinases and phosphates whereby they target them to specific residues in a multiprotein signaling complex. Discoveries have been made that there are over 20 AKAPs whereby some of them interact with ion channels. For instance, there are particular AKAPs, which interact with, the inwardly rectifying K+ channel and raise the sensitivity of the channel to Camp regulation. This study seeks to determine how the mutations in AKAP genes are associated with known cardiac diseases.
Introduction
During the last decade, it was shown that AKAPs anchor PKA, as well as scaffolding proteins that arrange the actions of the other signaling enzymes such as kinases, phosphatases, phosphodiesterases (PDEs), adenylyl cyclases, GTPases, and other regulatory proteins. Compartmentalization of these enzymes is said to be achieved through associating them with adaptor proteins, which bind them directly to target subtrates via protein-protein interactions. There are specific PKA anchoring proteins called AKAPs that are significant regulators of PKA functi0ning and signaling by directing the sub cellular localization of PKA, binding it to its R subunits, and effecting the concentration of PKA at specific intracellular locations. Experimental approaches have shown that there are two major anchoring proteins for PKA called MAP2 and AKAP75. There are also over 13 different AKAPs found in the heart. However, sequences in the AKAPs govern the targeting of AKAPs to specific sites whereby despite the diverse structure of the AKAPs, they all contain a region called amphiphatis helical region, which contains, amino acids ranging from 14-18 in number.
PKA anchoring through A-Kinase Proteins
Several studies have analyzed PKA being included in the anchoring process through AKAPs in cardiac physiology. To address this issue, initial studies used many methods to inhibit the interaction between PKA and AKAPs in cardiomyocytes. In cardiomyocytes, there is the assembling of the self-regulatory Camp by AKAP6 to a signaling module with PKA and PDE4d3. Protein AKAP7 is the one that anchors PKA to the plasma membrane of epithelial cells and interacts with Cav 1.2a channels in cardiomyocytes and skeletal muscle cells, thus leading to the facilitation of Cav 1.2a phosphorylation by protein kinase-A to interact with PDE4d3 enzyme directly. Similarly, there may be regulation of the activity of AKAP- bound PKA on the aquaporrin-2+ vesicles by the AKAP7-PDE4d thus regulating the aquaporin-2 transfer and permeability of water. Additionally, cardiomyocytes and arterial smooth muscles cells make use of distinct phosphodiesterase PDE4d variants so as to regulate the activity of PKA. For instance, in the rat cardiomyocytes, PDE4d3 is not usually detected in PKA-AKAP complex, whereas in rat aortic smooth muscle cells, PDE4d8 associates with PKA-AKAP complexes.
Results have also been obtained by inhibiting the anchoring of PKA in cardiomyocytes using minute molecule inhibitors. The action of pumping blood by the heart to the systemic and pulmonary circulation ensures that oxygen reaches all the body parts including the brain and the peripheral organs. For the heart to perform the function of pumping blood, the myocytes in the atria and ventricles contract so as to respond to the action potentials that come from the sinoatrial node. The action potentials give the sequential activation and inactivation of the ion channels which carry the inward currents of Na+ and Ca2+ and the outward currents of K+. The inward Na+ and Ca2+ currents initiate and maintain membrane depolarization, while the K+ current serves as a bridge in repolarization of the plasma membrane to the resting state.
In cardiomyocytes, a process called excitation contraction coupling is the one that leads to the depolarization and contraction of the membrane. The process is initiated when the Ca2+ channels in the T tubules open due to response to depolarization of the membrane. This leads to the induction of an increase in cystosolic Ca2+ concentration, which in turn, leads to the release of Ca2+ from stores located intracellularly through the ryanodine receptor 2 at the sarcoplasmic reticulum, leading to the increase in Ca2+ concentration. At the sarcomere, the binding of Ca2+ ions to troponin C initiates a change in conformation in the troponin complex that exposes myosin binding sites on the actin filaments and gives allowance for contraction to occur. Relaxation also occurs when L-type Ca2+ channels inactivate and Ca2+ is abolished from the cytosol. In human beings, 75% of cytosolic Ca2+ is taken back to the opening of the sarcoplasmic reticulum by the ATP-dependent Ca2+-pump, whereas the 25 percent remaining is expelled from the cell by the Na+/Ca2+ exchanger. Similarly, the sympathetic nervous system initiates contractile force, heart rate, and myocardial relaxation by releasing the catecholamines, or epinephrine and epinephrine, which leads to stimulation of ?-adrenergic receptors (?-ARs), which are located, on the sarcolemma of cardiomyocytes. Activated ?-ARs enhance PKA signaling, which regulates the phosphorylation and activate proteins thus controlling Ca2+ cycling and sarcomere contraction. Additionally, the phosphorylation of L-type Ca2+ channels and RyR2s increases their openness thus increasing the mobilization of Ca2+ mobilization from intracellular stores. Similarly, the phosphorylation of phospholamban (PLB), activates the dissociation of PLB from SERCA2. This increases the activation of SERCA2 and favors myocyte relaxation. Finally, the phosphorylation of sarcomeric proteins including cardiac troponin I (catnip) and myosin-binding protein C (comb-C) also promotes relaxation by decreasing my filament Ca2+ responsiveness.
Methods
Studies have analyzed the implication of PKA anchoring through AKAPs. The studies used peptides which corresponded to the R-subunit binding series of AKAP-lymphoid blast crisis so as to inhibit PKA from interacting with AKAPs in cardiomyocytes. Delivering these peptides in primary cultures of cardiomyocytes or in the hearts of rats decreased PKA-dependent phosphorylation of RyR2, PLB, cTnI, and cMyBPC. These inhibited cAMP-mediated regulation of L-type Ca2+ currents. Similarly, they altered the cardiomyocyte contraction kinetics since they responded to the stimulation of ?-adrenergic. The same results were obtained when small molecule inhibitors inhibited PKA anchoring in cardiomyocytes.
AKAP6
The mAKAP is highly expressed in cardiac myocytes where it is localized to the perinuclear membrane and junctional SR. By serving as a scaffold for a wide range of molecules including PKA, PDE4D3, ERK5, and Epac1, this anchoring protein assembles a highly regulated signaling network that can act upon nearby substrates. For instance, mAKAP interacts with the RyR, which is regulated in response to phosphorylation by PKA. The FK506-binding protein FKBP12.6, PP2A, and PP1 form a complex with RyR2, and PKA phosphorylation of the RyR2 promotes the dissociation of FKBP12.6 from the channel, resulting in an increased probability of channel opening. Increasing the open probability of the channel enhances EC coupling gain, resulting in increased cardiac muscle contraction. Thus, localization of PKA to the RyR2 by mAKAP is critical in modulating RyR2 activity and cardiac Ca2+ transients.
AKAP6
Activation of the cytokine receptor by leukemia inhibitory factor through the ERK5 signaling pathway induces in cardiac hypertrophy. The hypertrophic effects of leukemia inhibitory factor are ablated by the activation of Epac1, suggesting the importance of the cAMP-mediated inhibition of mAKAP-bound ERK5. It is not entirely clear as to how the mAKAP complex is involved in the hypertrophic ERK5 signal transmission. A proposed mechanism is that ERK5-induced inhibition of PDE4D3 results in PKA potentiation of RyR-mediated release and activation of the calcineurin/NFAT pathway. Thus, mAKAP functions in the hypertrophic response through the coordination of multiple feedback mechanisms.
AKAP1
In contrast to mAKAP and AKAP-Lbc, AKAP121, has been proven to be a regulating negatively the cardiomyocyte hypertrophy. Knockdown of AKAP121 induces hypertrophy, whereas over expression reduces cell size and inhibits the effect of the hypertrophic adrenergic agonist, isoproterenol. The effect of AKAP121 knockdown is mediated by the calcineurin/NFAT pathway, illustrated by an increase in NFATc3 nuclear localization, whereby blocking can be done through the inhibition of calcineurin by cyclosporine A. The NFAT pathway is a potent inducer of cardiac hypertrophy, and its activity is tightly regulated. It is possible that calcineurin may be maintained in an inactive state due to spatial sequestration and inhibition of phosphatase activity in the AKAP121 complex. Thus, the loss of AKAP121 results in the release of an active pool of calcineurin in the cytosol. A critical target of calcineurin is the NFAT transcription factor family, which includes NFATc3. Phosphorylated NFAT is largely restricted to the cytosol. Upon dephosphorylation, NFAT translocates to the nucleus where it promotes hypertrophic gene expression. Inhibition of either calcineurin or PKA activity would, therefore, prevent the formation of the transcriptional complex and efficient gene expression. Abrenica et al speculate that the identity of this putative PKA target is the transcription factor GATA4, which stimulates hypertrophic gene expression and directly interacts with NFAT.
ENaC and AKAPs clones
All experiments regarding ENaC expression were done using human ?, ?, and ? ENaC constructs. The DNA for AKAP15, AKAP79 and PKC? were assembled. The experiments including animals were carried out according to the protocols of the research team. Oocytes were removed through surgery and then defolliculated in Ca2+ . The oocytes were left overnight and then recovered whereby cRNA was injected through it. Electrophysiological recordings were taken within a span of 4 days after the oocytes have been injected. ENaC cRNA was used at 1-2 ng for every portion. PKC? and the various AKAP cRNAs were injected at 5–20 ng. Similarly, ND94 solution was composed of 2 mM KCl, 94 mM NaCl, 1 mM MgCl2, 1.8 mMCaCl2 and 5 mM HEPES with a pH of 7.4. This was referred to as high Na+ solution. On the other hand, the low sodium solution was made using 89 mM of NaCl. Amiloride measuring 10 ?M was used to control ENaC currents. IBMX and Forskolin were used at intervals of 5 ?M and 0.5 mM, respectively, so as to activate PKA, while 100 ?M of AKAP peptide inhibitor Ht31 was used to control the mixing of AKAP and PKA. Injection of purified catalytic subunit was used as an injection. Similarly, the oocytes were cleaned solution containing ND94 at room temperature, then transferred in amounts of 5 ?l per oocyte of homogenization solution mixed with a protease inhibitor. Pelletion of yolk and nuclei was done through slow centrifugation for 5 min. Supernatants were rotated at fast speed for 20 min at a temperature of 4°C, then divided into insoluble and soluble components. The insoluble part was broken up in SDS and was used to dissolve the insoluble component and spun for 5 min at high speed so as to get the membrane fraction. The soluble component was extracted with 5 ?l per oocyte of Freon so as to get rid of the remaining yolk proteins. The aqueous upper phase was mixed with 6× SDS electrophoresis sample buffer.
Additionally, Dual electrode voltage clamp was carried out using a TEV-200 oocyte clamp. Borosilicate glass was used to construct electrodes which were used with a resistance of 1–5 mOhms. Recording was done at intervals of 10 s throughout the experiments. Five ordered measurements of sine wave signals were used ranging from 55 to 390 Hz.
Results
Experiments used the separate co expression of AKAP79 and AKAP-KL with ENaC. Both were expressed in tissues of ENaC. AKAPKL is usually expressed in kidney and lung epithelia, whereas AKAP79 affects the activity of another renal apical membrane channel, ROMK1. ENaC currents remained insensitive to PKA stimulation, similar to those observed in oocytes expressing ENaC alone. These results demonstrate that AKAP79 and AKAP-KL do not change baseline ENaC activity nor do they directly grant PKA-mediated guideline to the duplicated channel.
AKAPs and heart failure
Down regulation of ?-AR expression during congestive heart failure the regulation of ?-AR expression gives a contribution to the depressed heart giving a characteristic of a congestive heart failure. Results have also shown that phosphatidylinositol 3-kinase-? (PI3K?) brings together recently identified signaling complex possessing PKA and PDE3B that might regulate ?-AR expression at the surface of cardiomyocytes. In conditions relating to physiology, the cAMP pathway leads to the activation of PI3K?-anchored PKA, which in turn, phosphorylates PI3K? to inhibit its activity. In heart failure, PI3K? is increased and escapes PKA-mediated inhibition. Activated PI3K? reduces cell surface expression of ?-ARs most likely by interacting with the ?-AR kinase 1 and the adaptor protein complex 2. The findings show that interfering with the expression of PI3K? in the myocardium can tamper with the heart function and contribute to heart failure.
Presently, most reviews explain the key role played by AKAPs in putting in order signaling pathways that are used to control the contraction and rhythm of the cardiac as well as responding to stress by the heart pathologically. In the recent past, the function of the complex signals of AKAPs has been principally studied in cardiomyocytes which are isolated. Most of the developing studies indicate that AKAPs are able to arrange themselves in a physiological manner which is relevant to the signaling events in vivo. Studying tools for the role of the complexes of AKAPs are yet to be developed through animal models. If the application of the stated approaches involving vivo ad proteamic techniques, there will be a determination of the family which anchors protein controls as well as organizing in a temporary manner the signaling directions in sick hearts. Similarly, in the future clinical investigations and genome-wide association studies are predicted to be instrumental whereby determination on whether mutations in AKAP genes are linked with already discovered cardiac diseases. Finally, more advanced structural characteristics of AKAP signaling complexes must be there to necessitate the delineation of the interacting surfaces within AKAP complexes. The information will be essential in designing and developing new drugs that selectively interfere with AKAP-mediated pathological processes in the heart.
Most of the findings give evidence for the significance of AKAPs in the compartmentalizing of PKA and regulating of cardiac function. In addition, some studies illustrate the central role that AKAPs play in organizing signaling pathways that control cardiac contractility and rhythm as well as the pathological response of heart to stress.
The regulation and localization of PKA activity is very important for proper cardiac function, and perturbation of this can lead to heart failure. We are now just beginning to understand the spatiotemporal aspects of AKAP-mediated signaling in the heart. Therapeutically, although the global inhibition of PKA activity may not be efficacious, there may be great utility in specifically inhibiting localized PKA activity that contributes to cardiac dysfunction, for example, in the modulation of calcium-handling proteins, such as the L-type Ca2+ channel, SERCA pump, and RyR. Additionally, targeting PKA for regulation of sarcomeric proteins may be important for the modulation of contractile function in heart failure and muscle diseases. The progression of heart failure is a multifactorial process; therefore, we need to understand how multiple signals are integrated leading to a disease outcome. Currently, we do not know if and how AKAP signaling complexes are altered during heart disease progression and how this could affect cardiac remodeling. For example, is AKAP complex stoichiometry modified, and is enzymes regulated or targeted differently under pathological conditions? Clearly, more work is required to determine precisely how AKAP signaling complexes function in the progression of heart failure.