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Haematologica 2019 Volume 104(7):1482-1492
Hemostasis
Gradient-dependent inhibition of stimulatory signaling from platelet G protein-coupled receptors
Ankit S. Macwan,1 Niklas Boknäs,1,2 Maria P. Ntzouni,3 Sofia Ramström,4,5 Jonathan M. Gibbins,6 Lars Faxälv1 and Tomas L. Lindahl1,5
1Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden; 2Department of Hematology, Linköping University, Linköping, Sweden; 3Core Facility, Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden; 4School of Medical Sciences, Örebro University, Örebro, Sweden; 5Department of Clinical Chemistry and Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden and 6Institute for Cardiovascular and Metabolic Research, University of Reading, Reading, UK
ABSTRACT
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 concentra- tions 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 per se 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 recep- tors, while platelet activation via the non-G protein-coupled receptor glyco- protein VI is strictly concentration-dependent. By systematically character- izing 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 aggre- gation and granule secretion is effectively shut off. In contrast, the effects of gradient-dependent inhibition on platelet activation via adenosine diphos- phate 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 find- ings suggest that gradient-dependent inhibition may represent a new gen- eral mechanism for hemostatic regulation in platelets.
Introduction
In platelets, G protein-coupled receptors (GPCR) mediate activation in response to stimulation with multiple important soluble agonists, including thrombin, adenosine diphosphate (ADP) and thromboxane A2.1 These signaling events are critical for triggering platelet hemostatic activities such as adhesion,2 granule exo- cytosis, aggregation, procoagulant activity and clot retraction. Hence, they must be tightly regulated to ensure efficient hemostasis while concurrently avoiding undue activation, which could potentially lead to excessive clot growth and thus throm- bosis, vessel occlusion or embolization. The importance of platelet GPCR in the pathophysiology of arterial thrombosis is demonstrated by the thrombo-protective effects of inhibitory drugs targeting GPCR-mediated pathways, such as clopido- grel, prasugrel, ticagrelor (ADP-receptor P2Y12), aspirin (thromboxane synthesis), and vorapaxar (thrombin receptor PAR1).
Correspondence:
ANKIT S. MACWAN
ankit.macwan@gmail.com
Received: September 3, 2018. Accepted: January 8, 2019. Pre-published: January 10, 2019.
doi:10.3324/haematol.2018.205815
Check the online version for the most updated information on this article, online supplements, and information on authorship & disclosures: www.haematologica.org/content/104/7/1482
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