Page 1893 - Williams Hematology ( PDFDrive )

1868 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1869

Shear stress is an important factor in GPIb-mediated adhe- effects. 1143,1144 GPIb has also been implicated in supporting metastases
sion of platelets to immobilized VWF and subendothelial sur- in murine models. 1145
faces. 1042,1113,1120–1122,1124,1125 Platelets deficient in GPIb or platelets in which GPV, the third member of the GPIb/IX/V complex, has a Mr
GPIb has been blocked with monoclonal antibodies 1122,1124 adhere poorly 82,000 and is composed of 544 amino acids, including 15 leucine-rich
to subendothelial surfaces at all shear rates, but the defect in blood from repeats. GPV appears to form a noncovalent complex with GPIb,
patients with von Willebrand disease is manifest primarily at higher mediated through association of their transmembrane domains, 1146
shear rates. 10,11,1122 In what may be a related phenomenon, subjecting but because the number of GPV molecules on the surface of platelets
platelets to high shear stresses can induce platelet aggregation, which is is approximately 50 percent of the number of GPIb and GPIX mole-
mediated by VWF binding to GPIb, followed by platelet activation and cules, 1147 it has been suggested that the basic unit consists of two GPIb
integrin α β -dependent platelet aggregation. 13,15,1126 Whether the shear molecules, two GPIX molecules, and one GPV molecule. 801,1035,1038
IIb 3
rates generated in vivo in stenotic blood vessels are of sufficient magni- GPV is deficient in platelets from patients with Bernard-Soulier syn-
tude and duration to produce a similar degree of platelet activation is drome (Chap. 121), but GPV is not required for surface expression of
unknown. It is also uncertain as to whether the effect of shear is acting the GPIb/IX complex. 1148 A soluble fragment of Mr 69,000 is cleaved
on GPIb, on VWF, or on both, 15,801,809,1042 but shear-induced changes in from GPV by thrombin, but cleavage does not correlate with thrombin-
the structure of VWF, leading to a more extended conformation and induced platelet activation. 1149 Platelets from mice lacking GPV appear
conformational changes in the A1 domain, have been defined. 1113,1127 to respond more actively to thrombin and ADP than wild-type mice,
GPIb forms catch bonds with VWF, meaning that increasing force first raising the possibility that GPV inhibits platelet activation. 1150 The plate-
prolongs and then shortens bond lifetimes. 1113,1128 lets from these mice also adhere to immobilized VWF and can bind
GPIb also functions as a platelet binding site for thrombin. 801,1129,1130 VWF in the presence of botrocetin, indicating that GPV is not required
The regions between amino acids 216 and 240 and amino acids 269 for the interaction between VWF and the GPIb/IX/V complex. 1150 It has
and 287 were proposed as thrombin binding sites based on biochem- been proposed that removing a portion of GPV by thrombin proteolysis
ical data, with the latter region demonstrating similarity to hirudin, a allows thrombin access to GPIbα, thus facilitating its ability to activate
thrombin-binding protein. 801,1131 Sulfation of the three tyrosine residues platelets. In support of this model, thrombin’s ability to activate platelets
in the latter region is particularly important for thrombin binding. 1093 does not require proteolytic activity if GPV is absent, suggesting a direct
Two somewhat different crystal structures of the interactions nonproteolytic effect mediated via GPIbα. 1151
between thrombin and the negatively charged tail region of GPIb have
been reported, but in both cases two molecules of thrombin bind to IMMUNOGLOBULIN FAMILY OF
each GPIb molecule using different regions on thrombin (exosites I and CELL-SURFACE ADHESION RECEPTORS AND
II). This raises the possibility that free thrombin or thrombin adherent THEIR ASSOCIATED MEMBRANE PROTEINS
to fibrin can cluster GPIb/IX/V complexes. 1132
Binding of thrombin to platelet GPIb appears to contribute to Platelet-Endothelial Cell Adhesion Molecule-1 (Also Termed
thrombin-induced activation of platelets, even when PAR-1 and PAR-4 CD31)
are desensitized, and platelets lacking GPIb (Bernard-Soulier syn- PECAM-1 is a transmembrane glycoprotein of the immunoglobulin
drome) do, in fact, have blunted responses to thrombin. GPIb has been gene family with six immunoglobulin-like domains of the C2 group
proposed as the high-affinity binding site for thrombin, 1129,1133 but there and a Mr 130,000. 1152–1155 In addition to platelets and endothelial cells,
are only approximately 50 high-affinity thrombin-binding sites and PECAM-1 is expressed on monocytes, myeloid cells, and some lym-
approximately 25,000 GPIb molecules per platelet, 1129,1130 raising the phocyte subsets. There are approximately 8000 PECAM-1 molecules on
possibility that only the subpopulation of GPIb molecules in lipid rafts the surface of platelets. 1156 PECAM promotes homophilic interactions
are able to function in activating platelets. 1134 Binding of thrombin to via a homophilic binding domain in the immunoglobin-like repeats.
GPIb may also facilitate its effect on one or more of the other thrombin The cytoplasmic tail of PECAM is 118 amino acids in length and con-
receptors, and there is experimental support for this hypothesis. 1135,1136 tains a palmitoylation site (C595), an immunoreceptor tyrosine-based
GPIb has also been demonstrated to interact with P-selectin in a inhibitory motif (ITIM) including Y663, an immunoreceptor tyrosine-
cation-independent manner. 764,1035,1093 Although GPIb shares a number based switch motif (ITSM) including Y686, and a lipid-interacting α
of features with the P-selectin ligand, PSGL-1 (both are sialomucins and helix that contains Y686 and S702, which undergoes inducible phos-
have analogous anionic/sulfated tyrosine sequences), the interaction phorylation. 1152,1157 Upon phosphorylation, the ITIMs recruit and acti-
between GPIb and P-selectin appears to be more like the interaction vate phosphatases, such as SHP-2 and to a lesser extent SHP-1, SHIP,
between P-selectin and heparin. 1035,1093 In inflamed mesenteric venules and PP2A, 1158 via their SH2 domains. 1152 PECAM-1 undergoes homo-
in animals, platelets are observed to roll on the activated endothe- typic interactions that lead to signaling and crosslinking. 1159 PECAM-1
lium 1137 and so it is possible that platelet GPIb interacts with endothelial activation overall thus induces inhibitory activity as the phosphatases
P-selectin in this interaction. 1093 PSGL-1, a well-documented ligand for counteract the effects of stimulating kinases, but are complex and ago-
P-selectin on leukocytes, has also been identified on the surface of plate- nist specific. PECAM-1 activation decreases platelet responses to ADP
lets, 1138 and so may also contribute to this interaction. and thrombin and PECAM-1 platelet expression correlates inversely
GPIbα also binds to high molecular weight kininogen and factor with platelet sensitivity to these agonists. 1160 PECAM-1 also nega-
XII, and both of these interactions interfere with thrombin-induced tively regulates collagen-induced platelet activation mediated by the
platelet activation. 1139,1140 Factor XI also binds to GPIbα, where it under- ITAM-bearing GPVI/FcRγ-chain complex, GPIb/IX/V signaling, and
goes activation by thrombin. 1141 Activated leukocyte integrin α β , laminin-induced activation. 1159 Platelets from mice lacking PECAM-1
M 2
also can bind to GPIbα via the I-domain of the integrin, 1142 and this are hyperresponsive to subthreshold doses of collagen, and when com-
interaction has been proposed to play an important role in transmigra- pared to wild-type mice, form larger platelet thrombi on VWF and in
tion of leukocytes through platelet thrombi at sites of vascular injury. experimental settings in vivo.
GPIb plays complex roles in inflammation and endotoxemia in murine Crosslinking PECAM-1 molecules on the platelet surface with anti-
models, demonstrating both proinflammatory and antiinflammatory bodies enhances platelet adhesion and aggregate formation, suggesting

Kaushansky_chapter 112_p1829-1914.indd 1868 17/09/15 3:29 pm