Upon activation, platelets secrete more than 300 active substances from their intracellular granules. Platelet dense granule components, such as ADP and polyphosphates, contribute to haemostasis and coagulation, but also play a role in cancer metastasis. α-Granules contain multiple cytokines, mitogens, pro- and anti-inflammatory factors and other bioactive molecules that are essential regulators in the complex microenvironment of the growing thrombus but also contribute to a number of disease processes. Our understanding of the molecular mechanisms of secretion and the genetic regulation of granule biogenesis still remains incomplete. In this review we summarise our current understanding of the roles of platelet secretion in health and disease, and discuss some of the hypotheses that may explain how platelets may control the release of its many secreted components in a context-specific manner, to allow platelets to play multiple roles in health and disease.

Activation of mouse platelets by collagen is associated with tyrosine phosphorylation of multiple proteins including the Fc receptor γ‐chain, the tyrosine kinase Syk and phospholipase Cγ2, suggesting that collagen signals in a manner similar to that of immune receptors. This hypothesis has been tested using platelets from mice lacking the Fc receptor γ‐chain or Syk. Tyrosine phosphorylation of Syk and phospholipase Cγ2 by collagen stimulation is absent in mice lacking the Fc receptor γ‐chain. Tyrosine phosphorylation of phospholipase Cγ2 by collagen stimulation is also absent in mice platelets which lack Syk, although phosphorylation of the Fc receptor γ‐chain is maintained. In contrast, tyrosine phosphorylation of platelet proteins by the G protein‐coupled receptor agonist thrombin is maintained in mouse platelets deficient in Fc receptor γ‐chain or Syk. The absence of Fc receptor γ‐chain or Syk is accompanied by a loss of secretion and aggregation responses in collagen‐ but not thrombin‐stimulated platelets. These observations provide the first direct evidence of an essential role for the immunoreceptor tyrosine‐based activation motif (ITAM) in signalling by a non‐immune receptor stimulus.

Current understanding of how platelets localize coagulation to wound sites has come mainly from studies of a subpopulation of activated platelets. In this review, we summarize data from the last 4 decades that have described these platelets with a range of descriptive titles and attributes. We identify striking overlaps in the reported characteristics of these platelets, which imply a single subpopulation of versatile platelets and thus suggest that their commonality requires unification of their description. We therefore propose the term procoagulant platelet as the unifying terminology. We discuss the agonist requirements and molecular drivers for the dramatic morphological transformation platelets undergo when becoming procoagulant. Finally, we provide perspectives on the biomarker potential of procoagulant platelets for thrombotic events as well as on the possible clinical benefits of inhibitors of carbonic anhydrase enzymes and the water channel Aquaporin-1 for targeting this subpopulation of platelets as antiprocoagulant antithrombotics.

Background—: Platelets are central to the process of hemostasis, rapidly aggregating at sites of blood vessel injury and acting as coagulation nidus sites. On interaction with the subendothelial matrix, platelets are transformed into balloonlike structures as part of the hemostatic response. It remains unclear, however, how and why platelets generate these structures. We set out to determine the physiological relevance and cellular and molecular mechanisms underlying platelet membrane ballooning. Methods and Results—: Using 4-dimensional live-cell imaging and electron microscopy, we show that human platelets adherent to collagen are transformed into phosphatidylserine-exposing balloonlike structures with expansive macro/microvesiculate contact surfaces, by a process that we termed procoagulant spreading. We reveal that ballooning is mechanistically and structurally distinct from membrane blebbing and involves disruption to the platelet microtubule cytoskeleton and inflation through fluid entry. Unlike blebbing, procoagulant ballooning is irreversible and a consequence of Na ⁺ , Cl ^– , and water entry. Furthermore, membrane ballooning correlated with microparticle generation. Inhibition of Na ⁺ , Cl ^– , or water entry impaired ballooning, procoagulant spreading, and microparticle generation, and it also diminished local thrombin generation. Human Scott syndrome platelets, which lack expression of Ano-6, also showed a marked reduction in membrane ballooning, consistent with a role for chloride entry in the process. Finally, the blockade of water entry by acetazolamide attenuated ballooning in vitro and markedly suppressed thrombus formation in vivo in a mouse model of thrombosis. Conclusions—: Ballooning and procoagulant spreading of platelets are driven by fluid entry into the cells, and are important for the amplification of localized coagulation in thrombosis.