Page 1906 - Williams Hematology ( PDFDrive )

1880 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1881

also results in the activation of at least one negative regulator of platelet platelet aggregation. Subsequently, ristocetin-mediated interaction of
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function, c-CBL, which is tyrosine phosphorylated and activated down- VWF with platelets was observed to cause PIP metabolism, activation
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stream of Src kinases. Platelets deficient in c-CBL show enhanced aggre- of PKC, and an increase in intracellular Ca . Likewise, shear forces ini-
gation responses in response to GPVI engagement. 1613 While much of tiate signaling through the binding of VWF to GPIb/IX/V. 1629 In heter-
the GPVI-mediated signaling occurs via the associated FcRγ, the cyto- ologous systems such as Chinese hamster ovary (CHO) cells expressing
plasmic domain of GPVI also contains a highly basic region that binds both GPIb/IX and integrin α β , occupancy of GPIb/IX by VWF can
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calmodulin and a Pro-rich region that binds Src kinases, which also lead to activation of α β . 1630,1631 In platelets, the GPIb/IX/V complex
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appear to contribute to GPVI-mediated signaling. 1614 GPVI signaling associates with signaling proteins with ITAM motifs, such the FcγRIIA
also leads to the generation of reactive oxygen species. 1615 receptor 1632 and FcRγ-chain 1105 ; however, engagement of GPIb/IX/V
Integrin α β can also signal in response to collagen, independent alone is sufficient to activate integrin α β . 1633 The signaling pathway
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of GPVI and induce phosphorylation and activation of many of the triggered by engagement of GPIb/IX/V is incompletely understood but
same signaling components attributed to the GPVI-induced signal- appears to involve activation of Src 1523,1633,1634 and PI3K, and recruitment
ing cascade, such as Src, Syk, SLP-76, and PLC2. Other components of the adaptor proteins SLP-76 and ADAP (SLAP-130). 1633 The result is
include plasma membrane calcium ATPase and FAK. 1616 However, sep- activation of PLCγ , 2 1635 PKC, and integrin α β . Signaling through the
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arate studies indicate that integrin α β must be in an active conforma- GPIb/IX/V complex also causes release of AA and generation of TXA .
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tion in order to participate in this signaling. 967,1617 Thus it appears that A cyclic guanosine monophosphate (cGMP) and MAPK-dependent
collagen-induced signaling via GPVI activates integrin α β , allow- pathway for GPIb/IX-mediated activation of integrin α β has also
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ing both receptors to participate in the signaling necessary for a full been reported. 1636 The GPIb/IX/V complex binds several intracellular
response to collagen. 1617 proteins, including filamin (actin binding protein), 1092 calmodulin, 1637
The inactive form of MMP-1 (proMMP-1) is associated with integ- and 14–3–3ζ. 1090,1638,1639 Activation of c-RAF by 14–3–3ζ may link GPIb/
rin α β , 1390 as well as integrin α β . 1618 With collagen activation, MMP-1 IX/V signaling to the MAPK signaling pathway; moreover, protein
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becomes activated and then can cleave the N-terminal region of PAR-1, 14–3–3ζ exists as a dimer, which may allow it to bridge and dimerize
resulting in the generation of a new N-terminal that can insert into the GPIb molecules. 1639 In CHO cells, clustering of GPIb/IX promotes stable
receptor and initiate downstream signaling through the p38 MAPK, adhesion via integrin α β . 1640
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Rho-GTP pathway. Of note, the cleavage of PAR-1 by MMP-1 is at a The GPIb/IX/V complex also appears to be involved in transmit-
site two amino acids N-terminal to the cleavage site of thrombin. The ting at least one cAMP-dependent inhibitory signal. Thus, elevated
combined activation of PAR-1, integrin α β , and GPVI may account for cAMP, which activates protein kinase A (PKA), induces phosphory-
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the high thrombogenicity of collagen surfaces. lation of GPIbβ on Ser 166. 1091 Elevated cAMP also normally inhibits
The levels of GPVI and integrin α β expressed on platelets vary agonist-induced platelet actin polymerization. However, in platelets
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among individuals, but it is unclear whether there is a correlation from patients with Bernard-Soulier syndrome, which lack GPIb/IX/V,
between the levels of expression of each of them. 983,1619–1621 The level of actin polymerization proceeds normally after collagen stimulation, even
expression of these receptors correlates with the ability of platelets to be when cAMP is elevated, suggesting that cAMP-mediated phosphoryla-
stimulated by collagen. GPVI is present on the membranes of the open tion of GPIbβ may be required for the cAMP-mediated inhibition. 1641
canalicular system and α granules, but these pools are not detectable GPV, a Mr 82,000 membrane-spanning protein that is a member
on the surface of resting platelets. These pools merge with the plasma of the leucine-rich repeat family and complexes with GPIb/IX, is a sub-
membrane pool in stimulated platelets, increasing the apparent surface strate for thrombin. 1642 GPV-null platelets display enhanced responses to
expression of GPVI by approximately 60 percent. 1622 thrombin 1150 and GPV-null mice have accelerated thrombus growth in
CD36 can also bind collagen and antibodies to CD36 partially response to vascular injury. 1643 Proteolytically inactive thrombin selec-
inhibit platelet adhesion to collagen. 1623,1624 Platelets from patients tively activates mouse platelets lacking GPV and induces thrombosis in
lacking CD36 responded normally to collagen in one study, 1625 but GPV-deficient but not wild-type mice. 1151 Together, these observations
showed a minor defect in adhesion to collagen under flow conditions suggest that GPV may function as a negative regulator of thrombin sig-
in another. 1626 naling through GPIb/IX, and in its absence, thrombin may function as
Platelets stimulated with collagen exhibit several distinct responses. a ligand for GPIb/IX.
Although elevated cAMP levels normally inhibit platelet aggrega-
tion, collagen-stimulated platelets are relatively resistant to inhibition ADDITIONAL INTERMEDIATE SIGNALING
by cAMP. 1627 This may be related to the fact that collagen stimulates
the PLCγ isotype, which is insensitive to cAMP-mediated inhibition, MOLECULES
whereas other agonists such as thrombin stimulate PLCβ, which is Calcium
inhibited by cAMP. In addition, phosphatase inhibition decreases col- Elevation of intracellular Ca has a multitude of effects on platelet
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lagen-, but not thrombin- or ADP-induced platelet aggregation, 1628 sug- physiology. 1435,1644 The concentration of Ca in resting platelets (100 to
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gesting that one or more phosphatases are critical in collagen-induced 500 nM) is very low compared to the plasma concentration of Ca
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platelet aggregation. (approximately 2 mM). Exposure of platelets to most agonists is accom-
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panied by a rapid, transient rise in the intracellular free Ca concen-
tration to micromolar levels, followed by a less-rapid return to normal
GPIb/IX/V resting levels. The cytoplasmic Ca concentration at any given time is a
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The GPIb/IX/V complex promotes the initial interactions of platelets result of the rates of passive Ca influx, active Ca extrusion across the
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with VWF, particularly under conditions of high shear, resulting in plasma membrane, and both active release and/or uptake of Ca by the
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platelet tethering. GPIb/IX/V can also initiate signals that activated DTS/sarcoplasmic reticulum (see “Dense Tubular System/Sarcoplasmic
the integrin α β receptor, resulting in firm platelet adhesion and Reticulum” above), which is a Ca storage depot in platelets analogous
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aggregation. 1035 Some of the first evidence that the GPIb/IX/V com- to the sarcoplasmic reticulum in muscle. Active Ca extrusion and
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plex could serve as a signaling receptor came from studies in which uptake of Ca are mediated by several pumps (see Fig. 112–3). The cyto-
antibodies to integrin α β partially inhibited ristocetin-induced solic pool of Ca turns over rapidly as a result of a plasma membrane
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Kaushansky_chapter 112_p1829-1914.indd 1881 17/09/15 3:30 pm