Several AKAPs are expressed in the cardiovascular system (
Table 1). They regulate a variety of processes and are key proteins in maintaining the homeostatic functioning of the heart and vasculature [
66]. For instance, gravin and AKAP220 are involved in maintaining the vascular integrity [
16,
17]. Homeostasis of the vascular tone is achieved through tight control of the balance between contraction and relaxation of vascular smooth muscle cells (VSMC), processes in which AKAP79 is involved [
67,
68]. Ca
2+ handling and thus cardiac myocyte contractility is regulated by several macromolecular protein complexes whose platforms are AKAPs, e.g., AKAP18α, γ and δ, mAKAPβ [
19,
20,
21,
69]. The AKAP Yotiao is the key player in cardiac myocyte repolarization that follows contraction [
22]. Several AKAPs are involved in stress response-induced cardiac myocyte hypertrophy, including AKAP-Lbc and mAKAPβ [
70,
71]. AKAP79 and gravin are important for the recycling of β
1-ARs and β
2-ARs, respectively [
72,
73].
2.2.1. AKAPs Regulating the Endothelial Barrier Function
The vascular endothelium lining the intima of blood vessels consists of a layer of endothelial cells tightly adherent to each other through cell-cell junctions. A healthy endothelium plays an essential role in the proper functioning of the vascular system. It regulates macromolecular permeability and anti-inflammatory, anti-thrombotic and anti-hypertrophic responses. Inflammatory conditions trigger pathological changes in the vascular system that lead to endothelial dysfunction, a state in which pathologically activated endothelial cells lose their barrier properties and initiate expression of pro-inflammatory adhesion molecules on their surface [
74]. This results in increased vascular permeability allowing the infiltration of various molecules such as lipoproteins into the sub-endothelial space, and of circulating immune cells (e.g., monocytes). Ultimately, this leads to severe pathological conditions including atherosclerosis, allergy and sepsis [
75,
76].
AKAP-mediated PKA compartmentalization is essential for the maintenance of proper endothelial barrier function [
16,
17,
77]. The vascular endothelium integrity is mainly dependent on tight junctions (TJs), important in sealing space between adjacent cells, and on adherens junctions, which assure direct contacts with the actin cytoskeleton of neighboring cells, thus providing mechanical strength. AKAP220 associates with PKA, β-catenin and the endothelial adherens junctions protein VE-cadherin, tethering PKA in close proximity to the cell-cell junctions [
16]. Gravin (also known as AKAP12 or AKAP250) promotes vascular integrity by regulating the actin cytoskeleton via p21-activated kinase family proteins 2 (PAK2), an actin cytoskeletal regulator and afadin (AF6), a linker of the actin cytoskeleton with intercellular adhesion molecules [
17]. Rac1 is a member of the Rho family of small GTPases, which upon activation strengthens the adherens junctions and the cortical actin skeleton, thereby preserving the endothelial barrier [
78]. Simultaneous depletion of gravin and AKAP220 inhibited cAMP-mediated Rac1 activation, underlining the importance of these AKAPs in preventing endothelial dysfunction [
16].
One other member of the AKAP family is involved in maintaining vascular integrity, the long isoform of AKAP9. Following Epac1 activation, AKAP9 contributes to microtubule growth regulation and is essential for preserving the endothelial barrier [
79].
2.2.2. AKAPs Regulating the Vascular Tone
Homeostasis of the vascular tone is maintained by a tight balance between dilation and constriction of blood vessel endothelium; the main regulator is the renin-angiotensin-aldosterone system (RAAS). The main effector molecule of this system is angiotensin II (AngII), which exerts most of its effects via angiotensin type I receptors (AT
1R). For instance, arterial smooth muscle contraction is induced by AngII-dependent stimulation of AT
1R, localized at the sarcolemma, and subsequent activation of phospholipase C (PLC), which catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG activates protein kinase C (PKC), which in turn phosphorylates L-type Ca
2+ (Ca
V1.2) channels, thereby increasing their open probability and increasing Ca
2+ entry into the cytosol [
75].
In arterial smooth muscle cells, the activity of a specific subpopulation of Ca
V1.2 channels is regulated by AKAP79 (AKAP5/AKAP75/AKAP150)-dependent targeting of PKCα to the sarcolemma, which facilitates phosphorylation of the channels and increases their open probablility [
80]. By affecting the opening probability of specific Ca
V1.2 channels, the AKAP79 complex regulates the so-called “Ca
V1.2 sparklets”, which refers to local elevations of intracellular Ca
2+ pools that directly induce contraction of the VSMCs. The sparklets increase the vascular tone [
67]. In addition, AKAP79 facilitates and most probably stabilizes the coupling of small clusters of adjacent Ca
V1.2 channels, which can then open synchronously and generate large Ca
V1.2 sparklets, thus increasing the contractile force [
81,
82]. Prolonged Ca
V1.2 channel activity and thus persistent Ca
V1.2 sparklets could lead to vascular dysfunction and eventually contribute to AngII-induced hypertension [
80].
Transient receptor potential vanilloid 4 (TRPV4) channels are Ca
2+ permeant channels that unlike the Ca
V1.2 channels, promote relaxation upon activation. Both Ca
V1.2- and TRPV4-mediated Ca
2+ influxes activate adjacent ryanodine receptors (RyR), leading to release of Ca
2+ from the SR into the cytosol in the form of Ca
2+ sparks. While the Ca
V1.2-mediated Ca
2+ influx increases contraction, the local TRPV4-generated Ca
2+ sparks activate the large-conductance, Ca
2+-activated K
+ (BK) channels, which promote membrane hyperpolarization and closure of the Ca
V1.2 channels, ultimately resulting in relaxation [
83,
84]. In the arterial smooth muscle cells, AngII increased TRPV4 activity via PKC, which is tethered to the sarcolemma in close proximity of the channel by AKAP79, thus opposing the Ca
V1.2 channel-induced vasoconstriction [
68].
In conclusion, AKAP79 plays an essential role in the control of arterial vascular tone by regulating two opposing processes, contraction and relaxation of arterial myocytes.
2.2.3. AKAPs Controlling Excitation-Contraction Coupling
The cycling of Ca
2+ between the cytosol and the SR is at the basis of cardiac contraction and relaxation. Key players in these processes are L-type Ca
2+ Ca
V1.2 channels, RyR
2, SR Ca
2+ ATPase 2 (SERCA2) and the Na
+/Ca
2+ exchanger. More specifically, upon sarcolemma depolarization, Ca
V1.2 channels located at the T tubules open allowing Ca
2+ influx into the cardiac myocyte. This causes Ca
2+-induced Ca
2+ release from the SR into the cytosol through RyR
2 located at the SR. The Ca
2+, upon interaction with troponin T located on the thin myofibers, promotes contraction. Relaxation occurs via SERCA2-mediated Ca
2+ re-uptake into the SR and through Ca
2+ transport out of the cell by Na
+/Ca
2+ exchangers [
85]. SERCA2 is activated upon the phosphorylation and subsequent dissociation of phospholamban (PLN), a SR phosphoprotein [
20,
86,
87].
β-ARs introduce a further layer into the regulation of cardiac myocyte contractility. Their stimulation induces PKA-dependent phosphorylation of several proteins involved in Ca2+ handling, e.g., the CaV1.2 channels, RyR2 and PLN. These phosphorylations are facilitated by distinct AKAPs.
AKAP18α is a membrane-associated scaffolding protein and is the smallest AKAP7 gene transcript, comprising 81 amino acids. AKAP18α promotes cardiac contractility by mediating the PKA-dependent phosphorylation of Ca
V1.2 channels at Serine 1928 (Ser1928) on its α subunit and at multiple sites on its β subunit, which enhances the open probability of the channel and increases the Ca
2+ current [
69,
88]. The activity of a subset of Ca
V1.2 channels associated with caveolin-3 (Cav3) is regulated by PKA phosphorylation of the specific channel subpopulation mediated by an AKAP79 (AKAP5/AKAP75/AKAP150)-based macromolecular complex consisting of β-AR, PKA, AC5/6 and protein phosphatase calcineurin (PP2B) [
89]. The muscle selective AKAP, mAKAPβ (a short version of mAKAP) associates with RyR
2 at the SR and thereby facilitates the PKA phosphorylation of the channel, leading to enhanced opening of the channel and subsequent enhanced Ca
2+ release from the SR into the cytosol [
19]. In addition, mAKAPβ interacts with the Na
+/Ca
2+ exchanger 1 at the sarcolemma and promotes the PKA-dependent activation of the exchanger, resulting in increased Ca
2+ efflux [
90,
91]. AKAP18δ (rat heart) and AKAP18γ (human heart) facilitate the PKA phosphorylation of PLN and promote its dissociation from SERCA2 and hence activation of the ATPase, thus enhancing the re-uptake of Ca
2+ into the SR [
20,
21,
92].
2.2.4. AKAPs Regulating Cardiac Repolarization
The cardiac repolarization phase is initiated by the slow heart potassium current (I
Ks) moving outwards through the I
Ks potassium channel, a macromolecular complex consisting of a pore-forming α subunit (KCNQ1) and a regulatory β subunit (KCNE1) along other intracellular proteins [
93]. The AKAP Yotiao, the smallest transcript of the
AKAP9 gene, is essential for cardiac repolarization since it mediates the PKA-dependent phosphorylation of KCNQ1 and therefore regulates the activity of the I
Ks potassium channel [
22]. Mutations in the KCNQ1 subunit or Yotiao increase the duration of the action potential and lead to type I long-QT syndrome (LQT1), a channelopathy that can elicit fatal arrhythmia [
94]. Another AKAP that contributes to the regulation of cardiac action potentials is the dual specific D-AKAP2 (AKAP10). A single-nucleotide polymorphism (SNP) in its PKA binding domain causes a decrease in the PR interval in the electrocardiogram, which in turn can cause arrhythmias and sudden cardiac death [
54,
95,
96,
97].
2.2.5. AKAPs Involved in Cardiac Stress Response
Cardiac hypertrophy is a stress-induced adaptation to maintain normal heart function [
23,
25]. At the cellular level, it is characterized by the upregulation of specific genes that promote the non-mitotic growth of cardiac myocytes [
98]. AKAP-Lbc encodes in addition to its AKAP function for a guanine nucleotide exchange factor (GEF) that directly binds and activates the GTP-binding protein RhoA [
99,
100,
101,
102]. The interaction is involved in both cardiac development [
103] and pathological cardiac myocyte hypertrophy [
70]. α
1-AR stimulation enhances the RhoGEF activity of AKAP-Lbc, which in turn activates RhoA, contributing to a pathological increase in the hypertrophic response [
70]. PKA-mediated phosphorylation at Ser1565 of AKAP-Lbc leads to the recruitment of 14-3-3 proteins, which inhibit the Rho-GEF activity of the anchoring protein [
104]. Also, an AKAP-Lbc-dependent signalosome mediates the activation and cytosolic release of activated protein kinase D (PKD), which has been shown to promote cardiac hypertrophy by facilitating the nuclear export of histone deacetylase 5 (HDAC5) [
105,
106].
Another AKAP that plays a central role in modulating stress signal-induced hypertrophic pathways is mAKAPβ. It coordinates a variety of cAMP-responsive enzymes. This anchoring protein is targeted to the nuclear envelope of cardiac myocytes via an interaction with nesprin-1α [
107]. At the SR it can integrate and transduce a variety of hypertrophic signals [
71]. For instance, mAKAPβ-mediated PKA phosphorylation and subsequent activation of RyR
2 located at the nuclear envelope promotes the activation and nuclear translocation of the pro-hypertrophic transcription factor nuclear factor of activated T cells (NFAT) [
108]. In addition, a mAKAPβ-based signalosome consisting of PKA, PDE4D3, Epac1, ERK5 and PP2A promotes ERK5-induced cardiomyocyte hypertrophy [
71,
109]. Cardiac remodeling can also be regulated by hypoxia, a process in which a mAKAP-based protein complex consisting of hypoxia-inducible factor 1α (HIF-1α), prolyl hydroxylase domain protein (PHD), the von Hippel-Lindau protein (pVHL) and the E3 ligase designated seven in absentia homolog 2 (Siah2) plays a role. More specifically, when oxygen levels are reduced, mAKAP promotes the degradation of PHD and thereby facilitates an increase in HIF-1α levels, which regulates transcription of genes that promote cell survival [
110].
Other AKAPs that are thought to be involved in the cardiac stress response are D-AKAP1 and SKIP [
111,
112]. D-AKAP1 is a scaffolding protein of the outer mitochondrial membrane, which is protective against cardiac hypertrophy since its overexpression leads to cardiac myocyte cell size reduction and inhibition of the β-AR agonist isoproterenol-induced hypertrophy [
111]. Moreover, D-AKAP1 expression maintains the mitochondrial structure and function in the heart and reduces the infarct size, cardiac remodeling and mortality under conditions of ischemia, i.e., after myocardial infarction [
113].
SKIP plays an important role in the generation of the cardioprotective and anti-apoptotic lysophospholipid sphingosine-1-phosphate (S1P) produced upon myocardial ischemia-reperfusion injury [
112]. It is involved in the regulation of sphingosine kinase type 1 (SPHK1), which upon activation phosphorylates sphingosine to form S1P [
114].
2.2.6. AKAPs Involved in the β-ARs Desensitization/Resensitization Cycle
Upon activation, β-ARs are phosphorylated and subsequently bind β-arrestin, which prevents further ligand binding leading to receptor desensitization. The phosphorylated β-ARs are internalized and reach the early endosomes where they undergo resensitization after PP2A-mediated dephosphorylation. Upon resensitization, the non-phosphorylated receptors are recycled to the plasma membrane where they can bind further ligands. Therefore, β-AR desensitization and resensitization are essential processes in maintaining the proper functioning of the receptor [
115].
Gravin and AKAP79 are important in the desensitization/resensitization cycle [
72,
73]. A gravin-based complex consisting of PKA, PKC, PP2B, β-arrestin and G protein-linked receptor kinase 2 (GRK2) is essential for the desensitization and resensitization of the β
2-ARs, with which it interacts at their C-terminal tail [
72]. AKAP79 mediates the PKA-dependent phosphorylation of the β
1-ARs by also binding to the C terminus of the receptor, leading to their recycling and resensitization [
73].