Page 2109 - Hematology_ Basic Principles and Practice ( PDFDrive )

1872 Part XII Hemostasis and Thrombosis

Tethering tandem 13 amino acid repeats (one to four), may have a role in regu-
Shear Shear lating platelet adhesion to VWF.
In addition to serving as a receptor for VWF in the subendothe-
lium, GPIb–IX–V is also a receptor for the soluble agonist thrombin
(see section on Soluble Agonist Receptors). Other adhesive ligands
GPIb-V-IX for GPIb–IX–V include the counter-receptors P-selectin (CD62P),
vWF vWF vWF vWF expressed on activated platelets and endothelial cells, and leukocyte
integrin Mac-1 (αMβ2), which is involved in the formation of
A Subendothelial collagen platelet-leukocyte conjugates. 18
Platelets from patients with Bernard-Soulier syndrome (BSS) lack
GPIb–IX–V as a result of mutations in GPIb or GPIX that prevent
Signaling the expression of the complex on the platelet surface. Decreased
expression of GPIb–IX–V impairs platelet adhesion to the subendo-
thelium and can result in a significant bleeding diathesis (see section
on Molecular Basis of Inherited Platelet Disorders).
GPIb-V-IX
GPVI
vWF
Collagen Receptors

B Subendothelial collagen Subendothelial collagen has long been recognized as an important
initiator of platelet responses, serving as a substrate for platelet adhe-
sion, which ultimately results in platelet aggregation. Three different
receptors are involved in platelet responses to collagen: the GPIb–
IX–V complex, which binds to collagen via VWF, and GPVI and
Adhesion α2β1 (GPIa–IIa), which directly bind to collagen.
GPVI, with approximately 4000–6000 copies per platelet, is a
transmembrane protein member of the immunoglobulin superfamily
and has two extracellular immunoglobulin domains. As an immuno-
receptor tyrosine-based activation motif (ITAM)-coupled receptor, its
GPVI 2b1 cytoplasmic domain contains a proline-rich sequence that binds to
tyrosine kinases involved in platelet signaling 12,19 (see section on
Activation Pathways). GPVI is noncovalently associated with the Fc
C Subendothelial collagen receptor γ chain (FcRγ) dimer, which is required for GPVI surface
expression. There is in vitro and in vivo evidence of shedding of GPVI
Fig. 125.2 ADHESION TO THE SUBENDOTHELIUM IS A MULTI- from the surface of activated platelets by the action of ADAM17 and
STEP PROCESS. (A) Collagen-bound VWF in the subendothelium during platelet aging in the circulation. Deficiencies of GPVI can be
undergoes a conformational change when exposed to arterial shear rates such associated with a mild bleeding diathesis (see section on Genetic Basis
that GPIbα of the GPIb–IX–V complex binds to the VWF A1 domain. These of Inherited Platelet Function Disorders).
rapidly formed bonds are quickly broken and reestablished, causing platelet α2β1, with approximately 2000 copies per platelet, is the major
translocation, or rolling, along the subendothelium. (B) The translocation collagen-binding integrin on the platelet surface. 12,20 It undergoes a
process slows platelet transit, allowing the signaling receptor GPVI to bind conformational change to a high-affinity state to allow it to bind to
directly to collagen. (C) GPVI-mediated signal transduction pathways acti- collagen. Silent polymorphisms in the α2 gene control its expression
vate the integrin α2β1, enabling it to bind strongly to collagen. This final level, with higher plasma membrane levels associated with enhanced
association between the platelet and collagen is stable and allows the platelet adhesion to collagen in vitro and an increased risk of cardiovascular
to adhere firmly to the subendothelium. GP, Glycoprotein; VWF, von disease.
Willebrand factor.
The adhesion of platelets to collagen is a complex, multistep
process. An initial interaction of GPIb–IX–V with VWF in the
subendothelium (see earlier) facilitates a direct, low-affinity interac-
stoichiometry of 2 : 4 : 2 : 1 and exists in high copy number on the tion of GPVI with collagen (Fig. 125.2B). Binding of GPVI to col-
platelet surface (approximately 25,000 molecules per platelet) with lagen is a potent stimulus to intracellular signaling pathways (see
the N-terminus of GPIbα being the major ligand-binding subunit section on Activation Pathways). As a result of this signaling cascade,
of the complex. Platelet activation and aging are associated with α2β1 is activated to bind to collagen with high affinity (Fig. 125.2C),
shedding of glycocalicin, a large proteolytic N-terminal fragment thereby forming a stable interaction. Alternatively, or in addition, low
of GPIbα that is cleaved by ADAM17 or other sheddases. The levels of constitutively activated α2β1 can initiate direct binding of
cytoplasmic tail of GPIbα binds to a number of proteins, includ- platelets to collagen, thereby assisting GPVI to bind and to initiate
ing the signaling molecule 14-3-3ζ and the cytoskeletal protein activation signaling. At high shear, GPVI plays the major role in
18
filamin A (an actin-binding protein) (see section on the Platelet mediating stable adhesion, while at lower shear, α2β1 is more
Cytoskeleton). important. 19,21
GPIb–IX–V plays a major role in hemostasis and thrombosis as a
receptor for immobilized VWF in the subendothelium. Plasma VWF
does not interact with platelet GPIb–IX–V, but at the high shear rates CLEC2 Adhesion Receptor
that occur in the arterial microcirculation, a conformational change
in VWF bound to subendothelial collagen exposes a GPIbα binding CLEC2 is a transmembrane C-type lectin that was originally described
site in the A1 domain. Thus, GPIb–IX–V binds to subendothelial as the receptor responsible for platelet activation by the snake venom
VWF; this interaction is reversible with fast on and off rates, which rhodocytin, inducing signaling events similar to those initiated by
allows for translocation (rolling) of platelets on the surface (Fig. GPVI (see section on Activation Pathways). CLEC2 contributes to
22
125.2A). 12,15 A polymorphism in the GPIbα gene leads to variable thrombosis in mouse models, even though it does not have a ligand
numbers of tandem repeats in the macroglycopeptide region of the in the ECM. CLEC2 has now been recognized as the receptor
protein between the ligand-binding region and the plasma membrane. responsible for platelet activation upon binding to the membrane
The length of this region, which is determined by the number of glycoprotein podoplanin on lymphatic endothelial cells; this binding