Platelets: yin and yang

In this issue of Blood, Boulaftali and colleagues have made the important and novel observation that deficiency of the serine protease inhibitor (serpin), PN-1, in platelets results in a prothrombotic state, supporting the role of platelet PN-1 in thrombosis.¹ 

Specifically, Boulaftali et al have initially confirmed previously published studies by Gronke et al indicating that protease nexin-1 (PN-1) is expressed by platelets.²  In studies on human platelets from healthy donors and from patients with the gray platelet syndrome (α-granular deficiency), the authors demonstrate that PN-1 is stored in platelet α-granules, from which it is secreted into plasma and bound to the platelet membrane. There, it inhibits thrombin and urokinase-type plasminogen activator (uPA) and reduces thrombin generation by tissue factor (TF). Platelets from PN-1–deficient mice demonstrate increased sensitivity to thrombin in P-selectin expression and platelet aggregation. Thrombus formation, induced ex vivo by collagen under blood flow conditions and in vivo by ferric chloride–induced injury, was significantly increased in PN-1–deficient mice, demonstrating the antithrombotic properties of platelet PN-1. These studies demonstrate that platelet PN-1 has important antithrombotic properties that have heretofore gone unrecognized.

It has been amply demonstrated that, in addition to playing a part in primary hemostasis by adhering to sites of vascular injury and aggregating to form the initial platelet plug, activated platelets have an essential role in blood coagulation by exposing receptors and assembling active enzymatic complexes for virtually all the coagulation proteins of the contact and consolidation pathways of the clotting cascade³  (see figure). What is less well appreciated is the regulatory role played by activated platelets in the secretion of coagulation inhibitors that limit the generation and activity of thrombin, localize active coagulation complexes to the platelet surface, and prevent uncontrolled intravascular thrombotic processes (see figure). Thus, for example, in addition to the serpin PN-1, another potent inhibitor stored in and secreted from platelet α-granules is the Kunitz-type inhibitor (kunin), protease nexin-2 (PN-2). PN-2 is otherwise known as the Alzheimer β-amyloid protein precursor, which contains a Kunitz-type protease inhibitor domain.^4,5  Whereas PN-1 is a potent and specific inhibitor of thrombin, PN-2 operates by an entirely different mechanism, characteristic of the kunins, to serve as a potent (K_i ∼ 500pM) and highly specific inhibitor of the unique, homodimeric coagulation proteinase, factor XIa (FXIa). Both PN-2 and PN-1 are present in plasma at concentrations far too low to inhibit their cognate proteinases, but are secreted from platelet α-granules to achieve high concentrations (∼ 30nM in the case of PN-2) in the surrounding plasma. This is sufficient to inhibit enzymes at the initiation (FXIa) and termination (thrombin) of the consolidation pathway of coagulation, thereby preventing the propagation of intravascular coagulation beyond the nidus of the platelet hemostatic thrombus.

Another important inhibitory mechanism relevant to the observations of Boulaftali et al¹  involves another Kunitz-type inhibitor, tissue factor pathway inhibitor (TFPI), which regulates the initiation of blood coagulation at sites of vascular injury.⁶  In contrast to PN-1 and PN-2, however, TFPI is present in human plasma at high enough concentrations to inhibit FVIIa and FXa and the generation of thrombin. Once sufficient quantities of FXa have been formed to produce thrombin at the low concentrations required to activate platelets, FXI, FVIII, and FV, the consolidation pathway of blood clotting produces thrombin in sufficient quantities to convert fibrinogen to fibrin and form a hemostatic thrombus.⁶  Moreover, platelets contain approximately 10% of the TFPI in blood and can release sufficient TFPI to increase the concentration roughly 3-fold to further inhibit the TF pathway.⁶ 

These new findings,¹  interpreted in the context of our current knowledge of procoagulant and anticoagulant mechanisms mediated by activated platelets, emphasize the importance of the yin and yang of platelets in blood coagulation. Defects in the procoagulant contributions of platelets to the assembly of coagulation complexes (yang) results in hemorrhagic complications, whereas defects in the anticoagulant mechanisms (yin) produce serious thrombotic consequences that account for the vast majority of premature deaths in Western societies.