Page 1874 - Williams Hematology ( PDFDrive )

1848 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1849

PLATELET SECRETION studies have demonstrated the importance of a core set of integral mem-
An intricate pathway of protein–protein interactions has been proposed brane proteins called SNAREs. 517,518 It is generally accepted that vesi-
for platelet secretion in which granules tether and dock to the inner cle/granule-target membrane fusion, and thus granule content release,
leaflet of the plasma membrane, after which fusion of the two oppos- require the binding of a SNARE from the cargo-containing granule or
ing lipid bilayers mediates cargo release. Docking and tethering are v-SNARE, with a heteromeric protein complex in the t-SNAREs. The
492
thought to be, in part, mediated by small GTP-binding proteins of the resulting, trans-bilayer complex is minimally sufficient for membrane
519
Rab family. Platelets have been reported to contain at least 11 Rabs, fusion. In human platelets, the detectable v-SNAREs are VAMP-2/
although only a few have been shown to be functionally relevant. Rab27s synaptobrevin, VAMP-3/cellubrevin, VAMP-4, VAMP-5, VAMP-7/
a and b are important for both granule biogenesis and secretion, while TI-VAMP, and VAMP-8/endobrevin, with VAMP-8 being most abun-
493
501,502,520–523
Rab 4 appears to have a role in secretion. The α-granule–associated dant. There are two classes of t-SNAREs: the SNAP-23/25/29
494
Rab6 was shown to be phosphorylated upon thrombin stimulation in a type and the syntaxin type. Human platelets contain syntaxins 2, 4,
501,502,520–524
protein kinase C (PKC)-dependent manner and phosphorylation seems 6, 7, 8, 11, 12, 16, 17, and 18. SNAP-23, -25, and -29 are all
521,525,526
to increase its GTP-loading. 495 detectable, but SNAP-23 is the most abundant. Functional stud-
Platelet granule–plasma membrane fusion is analogous to exocyto- ies, using in vitro assays and genetically altered mice, established that
sis in neurons, where detailed studies have shown the importance of a VAMP-8 is the primary v-SNARE required for secretion from all three
core set of integral membrane proteins called soluble N-ethylmaleimide- classes of platelet granules; however, platelets lacking VAMP-8 do release
502
sensitive factor (NSF) attachment protein receptors (SNAREs). It their contents at attenuated rates, suggesting roles for other VAMPs.
496
is generally accepted that vesicle/granule-target membrane fusion is Differential usage of the VAMPs may allow platelets to fine tune their
governed by the binding of a SNARE from the cargo-containing gran- release of cargo. For t-SNAREs, patients with FHL4, who are deficient in
ule or vesicle (v-SNARE), with a heteromeric protein complex in the syntaxin 11, show robust platelet secretion defects from all three granule
524
527
target membrane (t-SNAREs). The resulting, trans-bilayer complex is types. Studies of mouse platelets suggest a minor role for syntaxin 8
527
minimally sufficient for membrane fusion. In human platelets, the but loss of syntaxin 2 and/or syntaxin 4 had no effect. Syntaxins form
497
v-SNAREs are vesicle-associated membrane protein (VAMP)-2/synap- a heterodimeric complex with SNAP-23/25–like t-SNAREs. In platelets,
tobrevin, VAMP-3/cellubrevin, VAMP-7/TI-VAMP, and VAMP-8/endo- SNAP-23 is the critical family member, based on its abundance and
505,506,528
brevin, with the latter being most abundant. 498–502 There are two classes results from in vitro assays. SNAP-23 phosphorylation, by IκB
of t-SNAREs: the synaptosome-associated protein (SNAP)-23/25/29 kinase (IKK), is important for SNARE complex assembly, membrane
type and the syntaxin type. Human platelets contain syntaxins 2, 4, 7, fusion, and secretion. Platelet-specific loss of IKK or its pharmacologi-
529
and 11 498–502 as well as SNAP-23, -25, and -29. 503,504 Functional studies cal inhibition leads to bleeding.
using in vitro assays and genetically engineered mice, have established Although the SNARE proteins mediate membrane fusion, they do
that VAMP-8 is the primary v-SNARE required for secretion from all so inefficiently and thus require accessory proteins to control where,
three classes of platelet granules. 501,502 VAMP-2 or VAMP-3 can also play when, and how they interact. Many of these regulators are sensitive to
a role at higher levels of stimulation. As for t-SNAREs, SNAP-23 and second messengers (e.g., calcium). The Sec1/Munc18 (SM) proteins are
syntaxin 2 are required for each secretion event. Syntaxin 4 appears to syntaxin chaperones that control how the t-SNAREs interact with other
510,530–533
also play a role, but only in α granule and lysosome release. 505–508 SNAREs. Although several isoforms are present, only Munc18b
530,532,534
Although the SNARE proteins are sufficient to mediate membrane is important for platelet exocytosis. It chaperones syntaxin 11
fusion, they do so inefficiently and thus require accessory proteins to and is defective in FHL5 patients. Other SM proteins (e.g. Vps33a/b)
535,536
control where and when they interact. Many of these regulators may be are important for granule biogenesis. Another syntaxin regulator,
sensitive to second messengers such as diacylglycerol (DAG) and Ca , tomosyn1/syntaxin binding protein 5 (STXBP5), binds to syntaxin/
2+
537
while others are substrates for kinases, such as PKC. The Munc18 family SNAP-23 heterodimers and affects access to v-SNAREs. Genome-
(a, b, and c) control syntaxins and are critical for platelet secretion. 509–511 wide association studies (GWAS) suggest that alterations in STXBP5
Studies show that Munc18a and c are phosphorylated by PKC upon are linked to venous thrombosis risk resulting from increased plasma
537–539
platelet activation and that this affects Munc18/syntaxin binding affin- VWF. Surprisingly, mice lacking STXBP5 have a severe arterial
537,539
ity. 510,511 At least two members of the Munc13 family are present in plate- bleeding diathesis as a result of their defective platelet secretion.
lets (Munc13–1 and Munc13–4) (Schraw TD, Ren Q, and Whiteheart Munc13 family members contain binding sites for calcium, phos-
540
SW, unpublished data). Munc13–4 appears to be important for dense phatidylserine, DAG, and calmodulin. Two major family members,
512
granule release and functions through its interactions with Rab27. 513,514 Munc13–2 and Munc13–4, are detectable in platelets, although only
521,541
Munc13s have Ca and DAG binding sites and thus may be regulated Munc13–4 is functionally relevant. Munc13s are generally thought
2+
by the secondary messengers generated during platelet activation. to be docking factors that localize granules for membrane fusion. Both
Jinx
Munc13–4 has drawn particular attention based on its involvement FHL3 patients and the Unc13d mouse strain lack Munc13–4 and have
541,542
in familial hemophagocytic lymphohistiocytosis (FHL) and Griscelli robust granule-release defects and bleeding diatheses. Munc13–4
syndrome. Munc13–4 is mutated in type 3 FHL and interacts with the binds to a small GTP-binding protein called Rab27, which is also impor-
515
543
protein mutated in type 2 Griscelli syndrome, namely Rab27a. One tant for platelet exocytosis and is defective in Griscelli syndrome.
516
feature common to both diseases is the inability of T-cells to properly Another Rab27-binding protein, called synaptotagmin-like protein 4/
544
organize the cytotoxic synapse required for toxin secretion and target granuphilin, is also reported to be important for platelet exocytosis.
cell killing. For FHL patients, it is not clear whether they have bleed- Three types of FHL are caused by defects in genes encoding pro-
515
ing-time defects as they generally receive marrow transplants very early teins that are important in platelet secretion: Munc13–4/FHL3, syn-
in life. taxin 11/FHL4, and Munc18b/FHL5. Rab27a is defective in a related
disease, Griscelli syndrome. One feature common to both diseases is the
inability of T cells to properly organize the cytotoxic synapse required
PLATELET EXOCYTOSIS for toxin secretion and target cell killing. This suggests that these T-cell
Platelet granule–plasma membrane fusion is mechanistically analogous populations and platelets share common secretory machinery elements.
to exocytosis in neurons and other secretory cell types, where detailed For FHL patients, it is unclear whether bleeding or defective platelet
Kaushansky_chapter 112_p1829-1914.indd 1849 17/09/15 3:26 pm