Page 1866 - Williams Hematology ( PDFDrive )

1840 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1841

by the generation of barbed-end nucleation sites after receptor activa- PLATELET SPREADING AND
tion. These nucleation sites are generated de novo by the activation of SURFACE-INDUCED ACTIVATION
the Arp2/3 complex or by the exposure of the barbed ends of preexisting
filaments. Because barbed ends have a higher affinity for actin mole- After platelets adhere to surfaces, they undergo variable degrees of
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cules than do the actin sequestering proteins, they have the capacity to spreading and activation. The patterns of spreading and activation
initiate actin filament polymerization. depend primarily on the protein surface on which they spread, with col-
Platelets contain two proteins whose main function is to bind and lagen consistently inducing the most activation. 224,225 In addition to the
sequester actin monomers. The first is profilin, which is present at a nature of the surface, the protein density, especially in the case of fibrin-
concentration of 50 μM. Profilin can sequester actin monomers from ogen, can dramatically affect the signaling systems that are activated
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the pointed ends of actin filaments, but not the barbed ends. Profilin in the adherent platelets. Activation can result in release of granule
also functions as a major transfer factor in actin filament polymeriza- contents and exposure of activated integrin α β receptors on the lumi-
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tion. The second and more abundant protein involved in sequestration nal surface of the platelets, where they are strategically located to bind
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of actin monomers and stimulation of the polymerization of actin is adhesive glycoprotein ligands that can recruit additional platelets. If
thymosin-β . With a platelet concentration of 55 mM, it is equimolar to the surface density of platelets is sufficient, the platelets can also enter
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actin. Thymosin-β binds actin molecules with an affinity that is greater into lateral associations, which appear to depend on integrin α β .
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than that of the pointed end of the actin filament, allowing it to com- In general, platelet spreading results in the development of broad
pete effectively for molecules from the pointed end. Thymosin-β has lamellipodia rather than spike-like filopodia (see Fig. 112–2). 216,229 The
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a lower affinity for actin monomer than actin has for the barbed end different morphologies of platelet spreading reflect differences in the
of the filament, resulting in filament assembly when barbed ends are organization of the network of actin filaments. Ultrastructural exami-
free. Thymosin-β maintains a large pool of unpolymerized actin, and nation of lamellipodia reveals them to be replete with actin filaments
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60 percent of the total actin in the platelet is bound to thymosin-β . The that are organized into orthogonal networks. This organization is estab-
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affinity of thymosin-β for actin monomer is regulated by the nucleotide lished by the actin filament crosslinking protein filamin A. In contrast,
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that is bound to actin. 218 filopodia contain long actin filaments that are organized as tight bun-
The platelet actin assembly reaction that follows the addition of dles. These structural differences reflect the different signals initiated
agonists starts when free barbed ends are formed (see Fig. 112–5). by the adhesion process, and both PIs and the small GTPase molecules
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Barbed ends are generated by the uncapping of filament ends and the Rac and Cdc42 appear to be particularly important in this process.
de novo assembly of filaments by the Arp 2/3 complex. Platelets contain In platelets, Rac is activated by thrombin receptor ligation and it stim-
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high concentrations of barbed-end capping proteins that regulate the ulates actin filament uncapping. Proteins that have been implicated
accessibility of these ends to regulate actin dynamics. Platelets contain in organizing the tips of the filopodia where the actin bundles attach to
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5 μM each of gelsolin and capZ, and 3 mM of adducin. Uncap- the plasma membrane are the small GTPase Cdc42, the exchange pro-
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ping of the actin filaments appears to be accomplished by the inactiva- tein WASP, vinculin, vasodilator-stimulated protein (VASP), zyxin, and
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tion of capping proteins by phosphoinositides that are produced during profilin. Pleckstrin, a platelet protein that is phosphorylated during
platelet activation, including PI-3,4-bisphosphate (PI P ), PI P , and platelet activation, appears to participate in this process by binding to
4,5 2
3,4 2
P . The uncapped actin filaments act as nuclei onto which actin
PI 3,4,5 3 216 PIs and affecting Rac via an exchange factor. 231,232 Platelets from mice
monomers (which are maintained in an available pool by association deficient in pleckstrin have a defect in granule secretion, integrin α β
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with thymosin-β ) can assemble on the barbed ends of the filaments. activation, and aggregation mediated by protein kinase C. Thrombin
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Profilin accelerates actin polymerization by facilitating the transfer of can overcome this abnormality via a pathway involving PI3K. Sig-
actin from the actin-thymosin-β complex to the barbed ends of the naling after adhesion results from the assembly of protein complexes
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actin filaments. In addition to exposing new filament ends as a source on the cytoplasmic surfaces of the receptor(s) involved in the adhesion
of nuclei, new nucleation sites are generated by activation by the process, including focal adhesion kinase (FAK), which is activated by
Arp 2/3 complex. The Arp 2/3 complex mimics the pointed ends of integrin ligation and colocalizes with a number of cytoskeletal proteins.
actin filaments and stimulates barbed-end assembly of actin filaments. Deletion of FAK in megakaryocytes and platelets results in defects in
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The Arp 2/3 complex is made up of seven polypeptides, two of which platelet spreading. These complexes then initiate local cytoskeletal
have actin-related sequences, Arp2 and Arp3. 222,223 Platelets contain rearrangements as well as the generation of signaling molecules that
high concentrations of the Arp2/3 complex (2 to 10 μM). Approximately act throughout the platelet to produce a variety of effects, including the
30 percent of the Arp2/3 complex is bound to the resting platelet translation of new proteins. 235–238 The nature and extent of the signaling
cytoskeleton. Once platelets are activated, the Arp2/3 complex redis- may determine whether the adherent platelets recruit additional plate-
tributes to the cytoskeleton, increasing three-fold and concentrating lets or white blood cells. In particular, the conversion of spread platelets
in the lamellipodial zone of actin filament assembly. Several signal- to a microvesiculated procoagulant form has been associated with the
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ing pathways regulate the activity of the Arp2/3 complex, including recruitment of neutrophils. Additionally, spread platelets can assem-
Wiskott-Aldrich syndrome protein (WASP) family members. Muta- ble fibronectin matrix on their surface, which may be important in sta-
tions in the WASP gene result in Wiskott-Aldrich syndrome, an inher- bilizing platelet-platelet interactions. 240
ited X-linked recessive disorder characterized by thrombocytopenia Membrane glycoproteins are affected by cytoskeletal rearrange-
and T-cell immunodeficiency (see Chap.121). ments associated with platelet shape change and spreading. Activation
Simultaneous with these changes, the peripheral microtubule coil of platelets in suspension under certain conditions results in movement
becomes constricted and fragmented, and is ultimately compressed of GPIb/IX receptors from the surface of platelets to the open cana-
into the center of the cell. As the filopodia form, the platelet’s granules licular system. 241,242 With adherent platelets, the GPIb internalization is
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and organelles move to the center, surrounded by the microtubule coil, much slower. The initial effect of activation on integrin α β is an
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resulting in an increase in electron density. Activation of myosin II via approximate doubling of these receptors on the plasma membranes,
phosphorylation of myosin light chain kinase, contributes to the inward as preassembled receptors in α granules, and perhaps dense bodies
contractile force by its interaction with the actin fibers. and the open canalicular system, join the plasma membrane. Inside-
out activation of integrin α β has been associated with cytoskeletal
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Kaushansky_chapter 112_p1829-1914.indd 1841 17/09/15 3:26 pm