A-kinase anchoring proteins: scaffolding proteins in the heart

Abstract

The pleiotropic cyclic nucleotide cAMP is the primary second messenger responsible for autonomic regulation of cardiac inotropy, chronotropy, and lusitropy. Under conditions of prolonged catecholaminergic stimulation, cAMP also contributes to the induction of both cardiac myocyte hypertrophy and apoptosis. The formation of localized, multiprotein complexes that contain different combinations of cAMP effectors and regulatory enzymes provides the architectural infrastructure for the specialization of the cAMP signaling network. Scaffolds that bind protein kinase A are called "A-kinase anchoring proteins" (AKAPs). In this review, we discuss recent advances in our understanding of how PKA is compartmentalized within the cardiac myocyte by AKAPs and how AKAP complexes modulate cardiac function in both health and disease.

Fig. 1.

A-kinase anchoring proteins (AKAPs) controlling Ca²⁺ handling in the heart. AKAP18α favors PKA-mediated phosphorylation and regulation of the L-type Ca²⁺ channel, whereas AKAP79/AKAP150 assembles a large complex that includes PKA, PKC, and calcineurin (PP2B), adenylyl cyclases 5 and 6 (AC5 and -6, respectively), the β-adrenergic receptor (β-AR), the L-type Ca²⁺ channel, and caveolin 3 (Cav3), which generates cAMP microdomains that promote PKA-dependent regulation of phospholamban (PLB) and ryanodine receptor 2 (RyR2) receptors at the sarcoplasmic reticulum (SR). On the other hand, muscle AKAP (mAKAP) may direct PKA to regulate the phosphorylation (P) and activity of RyR2 at the SR. Finally, the AKAP18δ anchors PKA, PP1, and inhibitor-1 (I-1) to a complex consisting of PLB, and sarco(endo)plasmic reticulum Ca²⁺-ATPase 2 (SERCA2) to regulate PLB phosphorylation and SERCA2 activation. Gα_s, α-subunit of the heterotrimeric G protein G_s. PDE4D3, phosphodiesterase 4D3.

Fig. 2.

Regulation of cardiac repolarization by the Yotiao complex. A: the KCNQ1 subunit of slowly activating K⁺ current (I_Ks) channel interacts with Yotiao. Anchored PKA phosphorylates KCNQ1 and Yotiao on their NH₂-termini to enhance channel activation. B: mutations in the Yotiao-binding region of KCNQ1 (G589D) and in the KCNQ1-binding region of Yotiao (S1570L) reduce the interaction between the two proteins. This may lead to delayed repolarization of the ventricular action potential and long QT syndrome.

Fig. 3.

Regulation of cardiac hypertrophy by AKAP complexes. A: mAKAP assembles a multienzyme signaling complex at the outer nuclear membrane containing AC5, PKA, PDE4D3, PP2A, RyR2, calcineurin Aβ (CaNAβ), nuclear factor of activated T cells 3 (NFATc3), exchange protein activated by cAMP 1 (Epac1), and ERK5. Activation of AC5 by β-adrenergic stimulation generates cAMP, which in turn activates anchored PKA at submicromolar concentrations. In a negative feedback loop, activated PKA phosphorylates PDE4D3, leading to its activation and increased cAMP degradation, and AC5, leading to its inactivation and decreased cAMP synthesis. Anchored PKA also regulates the activity of PP2A, which promotes PDE4D3 dephosphorylation, and RyR2, which enhances Ca²⁺ mobilization from intracellular stores. This is proposed to induce the activation of CaNAβ, which, in turn, dephosphorylates and activates NFATc3 to promote hypertrophic gene transcription. Very high concentrations of cAMP (in μM) also stimulate Epac1. This in turn activates the GTPase Ras-related protein 1 (Rap1), which exerts an inhibitory effect on the MEK5-ERK5 pathway. In the absence of very high local cAMP, Epac1 is inactivated and the hypertrophic ERK5 pathway de-repressed. Stimulation of endothelin-1 receptors (ET₁Rs) activates mAKAPβ-bound PLCε, which, in turn, promotes cardiomyocyte hypertrophy via a signaling pathway that remains to be elucidated. B: activated α₁-ARs and ET₁Rs stimulate the Rho-guanine nucleotide exchange factor (GEF) activity of AKAP-lymphoid blast crisis (AKAP-Lbc) through Gα₁₂. GTP-bound RhoA is released from the AKAP-Lbc complex and promotes cardiomyocyte hypertrophy via a signaling pathway that remains to be elucidated. Activation AKAP-Lbc-anchored PKA promotes the phosphorylation of the anchoring protein on serine-1565. This induces the recruitment of 14-3-3, which inhibits the Rho-GEF activity of AKAP-Lbc. AKAP-Lbc also recruits PKCη and PKD. Upon stimulation by the Gα_q-phospholipase C pathway by α₁-ARs and ET₁Rs, PKCη becomes activated and phosphorylates PKD. Active PKD phosphorylates histone deacetylase 5 (HDAC5), causing its export form the nucleus. This favors myocyte-specific enhancer-binding factor 2 (MEF2)-dependent hypertrophic gene transcription. LIF-R, leukemia inhibitor factor receptor; IP₃, inositol trisphosphate 1,4,5-trisphosphate.

Fig. 4.

The role of mAKAP in regulating hypoxia-inducible factor 1α (HIF1α)-mediated transcription during normoxia and hypoxia. A: mAKAP assembles a signaling complex containing HIF-1α, prolyl hydroxylase domain protein (PHD), von Hippel-Lindau protein (pVHL), and seven in absentia homolog 2 (Siah2). Under normoxic conditions, HIF-1α is hydroxylated on proline residues (pro) by PHD, ubiquitinated (ub) by pVHL, and subsequently degraded by the proteasome. B: under hypoxic conditions, PHD is ubiquitinated by Siah2 and degraded. This favors HIF-1α stability, which accumulates in the nucleus to initiate transcriptional responses required to adapt to reduced oxygen. This could favor transcriptional responses controlling the induction of glycolysis, mitochondrial respiration, and cell survival during ischemia.