Targeting protein-protein interactions in complexes organized by A kinase anchoring proteins

Abstract

Cyclic AMP is a ubiquitous intracellular second messenger involved in the regulation of a wide variety of cellular processes, a majority of which act through the cAMP - protein kinase A (PKA) signaling pathway and involve PKA phosphorylation of specific substrates. PKA phosphorylation events are typically spatially restricted and temporally well controlled. A-kinase anchoring proteins (AKAPs) directly bind PKA and recruit it to specific subcellular loci targeting the kinase activity toward particular substrates, and thereby provide discrete spatiotemporal control of downstream phosphorylation events. AKAPs also scaffold other signaling molecules into multi-protein complexes that function as crossroads between different signaling pathways. Targeting AKAP coordinated protein complexes with high-affinity peptidomimetics or small molecules to tease apart distinct protein-protein interactions (PPIs) therefore offers important means to disrupt binding of specific components of the complex to better understand the molecular mechanisms involved in the function of individual signalosomes and their pathophysiological role. Furthermore, development of novel classes of small molecules involved in displacement of AKAP-bound signal molecules is now emerging. Here, we will focus on mechanisms for targeting PPI, disruptors that modulate downstream cAMP signaling and their role, especially in the heart.

Keywords: AKAP; cAMP; disruptor peptide; heart; protein–protein interaction; small molecule.

FIGURE 1

Schematic illustration of cAMP signaling pathways. Stimulation of G-protein-coupled receptors leads to activation of adenylyl cyclase (AC), which converts ATP into cAMP. cAMP increases in local microdomains and binds to different effectors such as protein kinase A (PKA), cyclic nucleotide gated ion channels and exchange protein directly activated by cAMP (Epac) leading to specific downstream effects. Cyclic nucleotide phosphodiesterases (PDEs) hydrolyse cAMP into AMP and terminates the signal. A-kinase anchoring proteins (AKAPs) anchor the signaling molecules involved in the pathway and target them to specific organelles in the cell.

FIGURE 2

Schematic illustration of an A kinase anchoring protein (AKAP). AKAPs are categorized by four different characteristics. First, an amphipathic α-helical region of the AKAP interacts with D/D-domain of the PKA R-subunit dimer. Second, they target the supramolecular complex to specific subcellular localizations. Third, AKAPs may also hold PKA substrates by direct binding or by targeting in their vicinity. Lastly, AKAPs can also function as signaling scaffolds for other signaling enzymes. In the absence of cAMP, PKA is inactive and its substrates are not phosphorylated, when cAMP levels increase it binds to the R-subunits and the active catalytic subunits are free to phosphorylate their targets.

FIGURE 3

Possible therapeutic strategies to target protein–protein interactions (PPIs) in specific AKAP complexes in the heart. (A) Disruption of the AKAP18γ/δ-PLB interaction prevents PLB phosphorylation on Ser16 and dislocation from SERCA2. This inhibits SERCA2 activation and consequently Ca²⁺ uptake into the sarcoplasmic reticulum (SR). (B) Disruption of the nesprin-1α/mAKAP interaction promotes AKAP/PKA complex dissociation from the perinuclear membrane and might be a strategy to reduce hypertrophy. (C) Disruption of the connexin 43-ezrin interaction could prevent PKA-mediated phosphorylation increasing inter-cardiomyocyte conductivity which could be cardioprotective following myocardial infarction damage.