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Chapter 125 Molecular Basis of Platelet Function 1875

platelet-leukocyte interactions. The discoid shape of the circulat- myosin IIA and IIB; myosin II is a hexamer made up of two heavy
ing platelet is maintained by an internal cytoskeleton composed of chains and four light chains. Assembly into bipolar filaments is
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polymers of actin and tubulin and their associated proteins (see also the result of Ca -activated phosphorylation of the 20-kDa MLCs
Chapter 124). Shape change requires the remodeling of the resting by MLC kinase, enhanced by blocking MLC dephosphorylation
cytoskeleton and the assembly of new cytoskeletal fibers to transform through Rho kinase. MYH9-related disease is the result of mutations
the platelet into its activated configuration. in the gene for nonmuscle myosin heavy chain IIA (NMMHC-
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IIA) (see section on Molecular Basis of Inherited Platelet
Disorders).
The Resting Platelet Cytoskeleton
The resting platelet cytoskeleton maintains cell shape and integrity Secretion of Granules
as the platelet encounters the high shear forces of blood flow in
small vessels. The spectrin-based membrane skeleton (similar but Platelet activation leads to the release of a diverse list of molecules
not identical to that of erythrocytes) forms a contiguous network that stimulate or inhibit platelets or other blood and vascular cells,
with actin filaments to support platelet ultrastructure. Actin is the covalently modify the thrombus to affect its mechanical properties,
single most abundant platelet protein (2 million copies per platelet), regulate coagulation, contribute to cell adhesive events, and modulate
forming 2000–5000 linear actin polymer filaments that are cross- wound healing, inflammation, and angiogenesis.
linked to form a rigid cytoplasmic network. Cross-linking proteins Most of the substances that are actively and selectively secreted
include filamin A and B, and α-actinin. The interaction between from platelets are packaged in storage granules formed in megakaryo-
filamin A/B and the cytoplasmic tail of the VWF receptor GPIbα cytes. Platelets contain three types of granules: dense (δ) granules, α
provides structural stability and the major link between the plasma granules, and lysosomal granules (Fig. 125.3A).
membrane and the actin cytoskeleton. Loss of this linkage in plate-
lets deficient in either GPIbα (BSS) or filamin A results in loss of
restraint of the spectrin lattice, swelling of the membrane skeleton, Dense Granules
and large fragile platelets that are subject to rapid clearance from the
circulation. 15 Dense granules belong to the family of lysosome-related organelles
Platelets contain a long microtubule wound 8–12 times into a coil (LROs) that also includes melanosomes, cytotoxic T-cell granules,
that sits just beneath the plasma membrane, maintaining the discoid and neutrophil azurophilic granules. Platelets contain three to eight
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shape of the resting platelet. Microtubules are rigid polymers made dense granules, storing high concentrations of cations (Ca , Mg ,
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up of αβ-tubulin heterodimeric subunits. β1-tubulin deficient mice K ), polyphosphate, nucleotides (ADP, ATP, GTP), and bioactive
have platelets that are spherical and fail to develop a discoid shape amines (serotonin and histamine). These granules are innately dense
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due to aberrant microtubule assembly. A heterozygous human when viewed by electron microscopy due to their Ca content.
variant in β1-tubulin, which may be present in as many as 10% of Biogenesis of LRO complexes (BLOCs) are protein complexes that
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the general population, results in decreased levels of β1-tubulin and are critical in vesicle trafficking and dense granule formation. Muta-
a subset of spherocytic platelets. 37 tions in specific protein members of BLOCs 1, 2, and 3, or in the
adaptor protein complex 3 (AP3) result in dense granule deficiency
Cytoskeletal Reorganization During Platelet and associated LRO abnormalities in mice and in Hermansky-Pudlak
syndrome (HPS) 1–9 in humans, characterized by mucocutaneous
Activation bleeding and oculocutaneous albinism (see section on Molecular
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Basis of Inherited Platelet Function Disorders).
The assembly and disassembly of the actin cytoskeleton allows
platelets to spread. Following platelet activation, the platelet makes
contact with the ECM, initially changing shape from disc to sphere α Granules
and developing filopodia, followed by flattening and spreading
of broad lamellae. Granules and organelles are relocated to the α Granules are unique to platelets and are the most abundant granule
center of the cell. The filopodia are filled with long actin filaments type: 50–80 granules per platelet, taking up 10% of the platelet
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originating in the center of the cell. Increases in [Ca ] i initiated by volume. They contain a large number of proteins that are either syn-
signaling through G q result in activation of gelsolin, a multidomain thesized by megakaryocytes or taken up from plasma by endocytosis.
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protein with binding sites for Ca , actin, and phospholipid. Ca Genetic defects in NBEAL2 and VPS33B, genes involved in α granule
binding alters the conformation of gelsolin such that it binds and synthesis, result in α granule deficiency syndromes 42–46 (see section on
cleaves actin filaments, leading to disassembly of the resting actin Molecular Basis of Inherited Platelet Disorders). Proteomic studies
cytoskeleton and allowing the platelet to change shape. Signaling have demonstrated that there are more than 300 unique proteins
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via G 12/13 and p115-RhoGEF activates the small, monomeric G released by α granules. These proteins have diverse functions and
protein RhoA that is also involved in regulating actin filament can be classified as coagulants and anticoagulants (e.g., factors V
formation and myosin contraction. For example, actin filaments are and XI, antithrombin, protein S, plasminogen activator inhibitor-1);
stabilized by the activation of Rho-activated kinase (p160ROCK) and adhesion proteins (e.g., fibrinogen, VWF, thrombospondin-1); che-
LIM-kinase. mokines (e.g., CXC-chemokine ligand 4 [CXCL4; platelet factor
Assembly of the activated platelet cytoskeleton doubles the actin 4], CXCL7 [β-thromboglobulin]); growth factors (e.g., epidermal
filament content of the platelet by exposing nucleation ends on growth factor, transforming growth factor-β); angiogenic factors (e.g.,
existing actin filaments (the result of severing of filaments by gelsolin) vascular endothelial growth factor, platelet-derived growth factor,
or creating new nucleation sites (by the Arp 2/3 complex). The angiostatin); and immune mediators (e.g., precursors of complement
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Arp2/3 complex is enriched in the periphery of the activated platelet factors C3 and C4) (Fig. 125.3A). There appears to be heterogeneity
where actin assembly is occurring, and the complex is activated by in the cargo protein content of subsets of α granules, with pro- and
proteins associated with cell adhesion sites and by the Wiskott-Aldrich antiangiogenic proteins stored in distinct subsets and differentially
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syndrome protein (WASp) family members. 38 released. Membrane-bound proteins in α granules include integrins,
Platelets are also the force-generating component of clot retrac- immunoglobulin family receptors, leucine-rich repeat family recep-
tion. αIIbβ3 is tethered to underlying actin filaments in association tors and tetraspanins. Most of these are also present on the resting
with cytoplasmic proteins talin, filamin, paxillin, zyxin, α-actinin, platelet plasma membrane, but some, such as P-selectin (CD62P),
and vinculin. Association of cytoplasmic myosin with actin provides are only expressed on the plasma membrane following granule
the motor for the contractile force. Platelets contain nonmuscle exocytosis. 46