Procoagulant platelets: Generation, characteristics, and therapeutic target

Abstract

Platelets play a pivotal role in hemostasis. Activated platelets are classified into two groups, according to their agonist response: aggregating and procoagulant platelets. Aggregating platelets consist of activated integrin αIIbβ3 and stretch out pseudopods to further attract platelets to the site of injury by connecting with fibrinogen. They mainly gather in the core of the thrombus and perform a secretory function, such as releasing adenosine diphosphate (ADP). Procoagulant platelets promote the formation of thrombin and fibrin by interacting with coagulation factors and can thus be considered as the connector between primary and secondary hemostasis. In addition to their functions in blood coagulation, procoagulant platelets play a proinflammatory role by releasing platelet microparticles and inorganic polyphosphate. Considering these important functions of procoagulant platelets, this subpopulation warrants detailed study to analyze their potential in preventing human diseases. This review summarizes the generation and important characteristics of procoagulant platelets, as well as their potential for preventing the adverse effects associated with current antiplatelet therapies.

Keywords: aggregating platelets; coagulation; hemostasis; procoagulant platelets; proinflammatory.

FIGURE 1

Different states of platelets viewed using scanning electron microscopy. A, Resting platelets are smooth and flat. B, Aggregating platelets contain many pseudopods. C, Procoagulant platelets are characterized by balloon‐shaped morphology and do not contain pseudopods. Bar = 2 μm. Images were taken from Agbani et al ¹¹

FIGURE 2

Mechanisms of procoagulant platelet formation. (1) Protease‐activated receptor and glycoprotein VI are located on the surface of platelets. After stimulating the platelets with agonists, such as thrombin and collagen, cytosolic Ca²⁺ concentration increases. (2) TMEM16F located on the platelet's membrane allows the entry of Na⁺, and Cl^–, and especially Ca²⁺. This mechanism, to some extent, promotes higher Ca²⁺ concentration. (3) Elevated cytosolic Ca²⁺ concentration triggers the formation of mPTPs, which in turn leads to the release of Ca²⁺ stored in the mitochondria. (4) Elevated cytosolic Ca²⁺ activates calpain, which causes the redistribution of phospholipids and PS exposure. (5) PS exposure is a prerequisite for procoagulant activity. Coagulation factors Xa and Va aggregate on the PS surface and convert prothrombin into thrombin. (6) Activated calpain also hydrolyzes cytoskeleton proteins and membrane linker proteins, which damages the membrane integrity. (7) The entry of salt ions into platelets leads to high osmotic pressure and further fluid entry, thereby resulting in ballooning

FIGURE 3

Detection of procoagulant platelets using flow cytometry. A, Platelets are first identified using polygonal gating on the SSC‐A versus CD41a plot and as presenting low SSC and CD41a positivity, and subsequent analyses are based on this gate. B, Gating is further refined in the lower right quadrant by selecting CD41a⁺/CD45^– cells using curved quadrants, and subsequent analyses are based on this gate. The “tail” population (CD41a⁺/CD45⁺) is excluded, as it represents aggregates of platelets and leukocytes. C, Applying straight quadrants to the same platelet population and thresholds for both axes shown in (B) yields slightly different results. D, Thresholds for CD62P and GSAO are set by PE and GSCA isotype controls. E, Procoagulant platelets are identified in the upper‐right quadrants as CD62P⁺/GSAO⁺ cells in a stimulated sample. F, Applying straight quadrants to the same platelet population and thresholds for both axes shown in (E) yields slightly different procoagulant platelet percentages. Images were taken from Tan et al ²⁵