The Role of A-Kinase Anchoring Proteins for Inhibitory cAMP Signalling in Platelets

Platelets are small circulating blood cells that mediate haemostasis and thrombosis. Platelets respond to vascular damage by adhesion, granule release, and aggregation. Healthy endothelial cells inhibit platelets through prostacyclin-induced cAMP signalling. Intracellular cAMP activates protein kinase A (PKA), a tetrameric kinase composed of two regulatory (R) and two catalytic (C) subunits. cAMP-binding triggers dissociation of C subunits from the PKA complex and phosphorylation of substrate proteins, which mediate platelet inhibition. The R subunits of PKA are known to be attached to A-kinase anchoring proteins (AKAPs), which enable subcellular compartmentalisation of cAMP signalling. Proteomics have identified 22 AKAPs in platelets, but only a few of these have been studied in detail. This review summarises current knowledge about platelet AKAPs, including studies done regarding other cells. Possible integration of AKAPs into platelet signalling is explored with a focus on subcellular localisation, interaction partners, and PKA-mediated substrate phosphorylation. As main platelet compartments, the plasma membrane, endosomes, mitochondria, the Golgi, the dense tubular system, and the cytoskeleton are considered. Potential roles of individual AKAPs in platelet inhibition are discussed, and open questions in the field are defined.