Regulating thrombus growth and stability to achieve an optimal response to injury

Abstract

An optimal platelet response to injury can be defined as one in which blood loss is restrained and haemostasis is achieved without the penalty of further tissue damage caused by unwarranted vascular occlusion. This brief review considers some of the ways in which thrombus growth and stability can be regulated so that an optimal platelet response can be achieved in vivo. Three related topics are considered. The first focuses on intracellular mechanisms that regulate the early events of platelet activation downstream of G protein coupled receptors for agonists such as thrombin, thromboxane A(2) and ADP. The second considers the ways in which signalling events that are dependent on stable contacts between platelets can influence the state of platelet activation and thus affect thrombus growth and stability. The third focuses on the changes that are experienced by platelets as they move from their normal environment in freely-flowing plasma to a very different environment within the growing haemostatic plug, an environment in which the narrowing gaps and junctions between platelets not only facilitate communication, but also increasingly limit both the penetration of plasma and the exodus of platelet-derived bioactive molecules.

Figure 1. Formation of an optimal platelet plug

Vascular injury produces a haemostatic response that can be too aggressive (leading to thrombosis), inadequate (leading to further bleeding) or optimal. The examples illustrate (top) vascular occlusion in a man with bacterial sepsis and disseminated intravascular coagulation, (middle) a normal, if imperfect, response to impact in a woman who does martial arts, and (bottom) a weak response to injury in a young girl with Glanzmann’s thrombasthenia (α_IIbβ₃ deficiency).

Figure 2. Contact-dependent events

The haemostatic response to injury brings platelets into sufficiently close contact with each other that molecules on the surface of adjacent platelets can interact with each. Examples shown in the figure include ligand/receptor pairs such as ephrinB1 and sema4D and their respective receptors. Cell adhesion molecules include α_IIbβ₃ integrin, several members of the CTX family (ESAM, JAM-A, JAM-C and CD226), PECAM-1 and CEACAM1. Except for the integrin, which binds to adhesive proteins such as fibrinogen, these molecules interact without the benefit of an intermediary. The space between platelets also provides a protected environment in which soluble agonists (ADP, TxA₂ and others), proteins secreted from α-granules (including Gas-6), and the proteolytically-shed exodomains of platelet surface proteins such as sema4D can accumulate.

Figure 3. Heterogeneity and porosity

Real time live imaging studies performed in mouse models show that the growing haemostatic plug consists of heterogeneous layers in which there is an outer zone of loosely adherent, discoid platelets overlying a zone of activated platelets that have not undergone granule exocytosis. Closest to the wall in this example is a zone of tightly-packed, fully activated and degranulated platelets. Studies that we have performed using fluorescently-tagged dextran molecules of various sizes indicates that the porosity of these zones is greatest in the outermost layer and least in the thrombus core.