Targeting platelet receptors in thrombotic and thrombo-inflammatory disorders

Platelet activation at sites of vascular injury is critical for the formation of a hemostatic plug which limits excessive blood loss, but also represents a major pathomechanism of ischemic cardio- and cerebrovascular diseases. Although currently available antiplatelet therapies have proved beneficial in preventing the recurrence of vascular events, their adverse effects on primary hemostasis emphasize the necessity to identify and characterize novel pharmacological targets for platelet inhibition. Increasing experimental evidence has suggested that several major platelet surface receptors which regulate initial steps of platelet adhesion and activation may become promising new targets for anti-platelet drugs due to their involvement in thrombotic and thrombo-inflammatory signaling cascades.

This review summarizes recent developments in understanding the function of glycoprotein (GP)Ib, GPVI and the C-type lectin-like receptor 2 (CLEC-2) in hemostasis, arterial thrombosis and thrombo-inflammation and will discuss the suitability of the receptors as novel targets to treat these diseases in humans.

Arterial thrombus formation

Platelet and coagulation factor-dependent thrombus formation is critical to limit post-traumatic blood loss at sites of vascular injury. However, under pathological conditions like rupture of an atherosclerotic plaque, it may also lead to vessel occlusion causing myocardial infarction or stroke. Therefore, antithrombotic treatment is the prime therapeutic option in the prophylaxis and treatment of ischaemic cardio- and cerebrovascular diseases. The use of existing antithrombotic agents is, however, limited by their inherent effect on primary haemostasis. In recent years, major advances have been made in understanding the mechanisms of thrombus formation in haemostasis and thrombosis and some studies raised the interesting possibility that occlusive thrombus formation and haemo stasis may involve partially different mechanisms. This review briefly summarizes these developments and highlights newly identified mechanisms involved in platelet adhesion and activation, intracellular calcium signaling, integrin activation and initiation of coagulation. The suitability of these pathways as novel targets for antithrombotic therapy is discussed.

The SLAM family member CD84 is regulated by ADAM10 and calpain in platelets

BACKGROUND AND OBJECTIVE

Ectodomain shedding is a major mechanism to modulate platelet receptor signaling and to downregulate platelet reactivity. Proteins of the a disintegrin and metalloproteinase (ADAM) family are implicated in the shedding of various platelet receptors. The signaling lymphocyte activation molecule (SLAM) family receptor CD84 is highly expressed in platelets and immune cells, but its role in platelet physiology is not well explored. Because of its ability to form homodimers, CD84 has been suggested to mediate contact-dependent signaling and contribute to thrombus stability. However, nothing is known about the cellular regulation of CD84.

METHODS

We studied the regulation of CD84 in murine platelets by biochemical approaches and use of three different genetically modified mouse lines. Regulation of CD84 in human platelets was studied using inhibitors and biochemical approaches.

RESULTS

We show that CD84 is cleaved from the surface of human and murine platelets in response to different shedding inducing agents and platelet receptor agonists. CD84 downregulation occurs through ectodomain-shedding and intracellular cleavage. Studies in transgenic mice identified ADAM10 as the principal sheddase responsible for CD84 cleavage, whereas ADAM17 was dispensable. Western blot analyses revealed calpain-mediated intracellular cleavage of the CD84 C-terminus, occurring simultaneously with, but independently of, ectodomain shedding. Furthermore, analysis of plasma and serum samples from transgenic mice demonstrated that CD84 is constitutively shed from the platelet surface by ADAM10 in vivo.

CONCLUSIONS

These results reveal a dual regulation mechanism for platelet CD84 by simultaneous extra- and intracellular cleavage that may modulate platelet-platelet and platelet-immune cell interactions.

Arterial thrombus formation. Novel mechanisms and targets

Platelet and coagulation factor-dependent thrombus formation is critical to limit posttraumatic blood loss at sites of vascular injury. However, under pathological conditions like rupture of an atherosclerotic plaque, it may also lead to vessel occlusion causing myocardial infarction or stroke. Therefore, antithrombotic treatment is the prime therapeutic option in the prophylaxis and treatment of ischaemic cardio- and cerebrovascular diseases. The use of existing antithrombotic agents is, however, limited by their inherent effect on primary haemostasis. In recent years, major advances have been made in understanding the mechanisms of thrombus formation in haemostasis and thrombosis and some studies raised the interesting possibility that occlusive thrombus formation and haemostasis may involve partially different mechanisms. This review briefly summarizes these developments and highlights newly identified mechanisms involved in platelet adhesion and activation, intracellular calcium signaling, integrin activation and initiation of coagulation. The suitability of these pathways as novel targets for antithrombotic therapy is discussed.