Gradient-dependent inhibition of stimulatory signaling from platelet G protein-coupled receptors

• Published in: Haematologica (Roma) 2019-07, Vol.104 (7), p.1482-1492
• Main Authors: Macwan, Ankit S, Boknäs, Niklas, Ntzouni, Maria P, Ramström, Sofia, Gibbins, Jonathan M, Faxälv, Lars, Lindahl, Tomas L
• Format: Article
• Language: English
• Subjects: Adenosine Diphosphate - pharmacology; Adult; Arterial Thrombosis; Blood Platelets - drug effects; Blood Platelets - metabolism; Cyclic AMP - pharmacology; Epoprostenol - pharmacology; G protein coupled receptors; Humans; Platelet Activation - drug effects; Platelet Activation - physiology; Platelet Aggregation - drug effects; Platelet Aggregation Inhibitors - pharmacology; Platelets; Protease activated receptors; Receptor, PAR-1 - metabolism; Receptors, G-Protein-Coupled - metabolism; Receptors, Thrombin - metabolism; Signal Transduction - drug effects; Thrombin - metabolism; Thromboxane A2 - metabolism
• ISSN: 0390-6078; 1592-8721; 1592-8721
• DOI: 10.3324/haematol.2018.205815

As platelet activation is an irreversible and potentially harmful event, platelet stimulatory signaling must be tightly regulated to ensure the filtering-out of inconsequential fluctuations of agonist concentrations in the vascular milieu. Herein, we show that platelet activation via G protein-coupled receptors is gradient-dependent, i.e., determined not only by agonist concentrations but also by how rapidly concentrations change over time. We demonstrate that gradient-dependent inhibition is a common feature of all major platelet stimulatory G protein-coupled receptors, while platelet activation via the non-G protein-coupled receptor glycoprotein VI is strictly concentration-dependent. By systematically characterizing the effects of variations in temporal agonist concentration gradients on different aspects of platelet activation, we demonstrate that gradient-dependent inhibition of protease-activated receptors exhibits different kinetics, with platelet activation occurring at lower agonist gradients for protease-activated receptor 4 than for protease-activated receptor 1, but shares a characteristic bimodal effect distribution, as gradient-dependent inhibition increases over a narrow range of gradients, below which aggregation and granule secretion is effectively shut off. In contrast, the effects of gradient-dependent inhibition on platelet activation via adenosine diphosphate and thromboxane receptors increase incrementally over a large range of gradients. Furthermore, depending on the affected activation pathway, gradient-dependent inhibition results in different degrees of refractoriness to subsequent autologous agonist stimulation. Mechanistically, our study identifies an important role for the cyclic adenosine monophosphate-dependent pathway in gradient-dependent inhibition. Together, our findings suggest that gradient-dependent inhibition may represent a new general mechanism for hemostatic regulation in platelets.