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BRIEF REPORT
From the Cardiovascular Biology Laboratory, Harvard School of
Public Health; and Massachusetts General Hospital, Boston, MA.
Secretion of platelet granules is necessary for normal hemostasis.
Platelet secretion requires soluble N-ethylmaleimide-sensitive factor
attachment protein (SNAP) receptor (SNARE) complex formation between
different members of the syntaxin, SNAP-25, and vesicle-associated membrane protein (VAMP) gene families. Using microcapillary
reverse-phase high-performance liquid chromatography-nano-electrospray
tandem mass spectrometry, we identified VAMP-3 and VAMP-8 as VAMP
isoforms coimmunoprecipitated from platelets with syntaxin 4. Immunoblotting experiments confirmed the presence of VAMP-3 and VAMP-8
but not VAMP-1 or VAMP-2 in platelets. To examine the effect of VAMP
proteins on platelet secretion, soluble recombinant (r) VAMP-2,
rVAMP-3, and rVAMP-8 were incubated with streptolysin O-permeabilized
platelets. Secretion of The secretory machinery in platelets has important
homologies to the machinery in neurons and other cells (reviewed by
Reed et al1). Soluble N-ethylmaleimide-sensitive factor
(NSF) attachment protein (SNAP) receptor (SNARE) complexes are formed
between vesicle-associated membrane proteins (VAMPs; v-SNAREs) and
proteins in the target membranes (SNAP-25 and homologs and syntaxins;
t-SNAREs).2,3 Many lines of evidence show that SNARE
complexes are crucial for membrane trafficking and fusion events such
as secretion and exocytosis.
Platelets contain SNARE proteins4-7 that form SNARE
complexes in vitro which support SNAP-dependent NSF-adenosine
triphosphatase activity.4 SNAP-dependent NSF is critical
for exocytosis of These findings established the existence of a link of the SNARE
machinery through PKC signaling to receptor-mediated cell activation in
platelets. To understand further the molecular mechanisms that lead to granule secretion, it is important to identify which isoforms of the SNARE protein families play a role in this process. Although SNAP-23, syntaxin 2, and syntaxin 4 have been shown to be
important for platelet secretion, the VAMP isoforms required for
platelet granule exocytosis are unknown. Polyclonal anti-VAMP antibodies and Botulinum toxin cleavage inhibit secretion in
permeabilized platelets, indicating that one or more VAMPs is required
for granule secretion.7 In this study, we found that
VAMP-3 and VAMP-8 form SNARE complexes in human platelets and mediate secretion.
Platelet-secretion assays
Platelets (20 µL) were mixed with buffer A (25 µL) containing 200 to 400 U/mL streptolysin O (Sigma, St Louis, MO), 0.5 U/mL hirudin, and
various concentrations of VAMPs. The samples were incubated at 25°C
for 10 minutes and on ice for 30 minutes. Adenosine triphosphate (50 mM) and magnesium diacetate (125 mM) in buffer A (5 µL) were added,
and the samples were incubated at 25°C for 10 minutes. Granule
secretion was induced by increasing the amount of Ca++ to
10 µM as described previously.11 After 5 minutes
of incubation, 3-µL samples were used for measurement of P-selectin
expression (described below). The remaining samples were put on
ice for 3 minutes and centrifuged (1000g) for 1 minute, and
the 14C-serotonin in the supernatant was measured by
scintillation counting.
Secretion of Protein-sequence analysis
Preparation of recombinant proteins The cytosolic domains of human VAMP-2, VAMP-3, and VAMP-8 and rat syntaxin 4 and SNAP-25 were produced as recombinant glutathione-S-transferase (GST)-tagged (Pharmacia, San Diego, CA) proteins in Escherichia coli by using standard protocols.13 The GST tag was cleaved with thrombin. VAMPs were further purified on a column (Mono S; Pharmacia) by using cation exchange fast-protein liquid chromatography as described previously14 and were then concentrated and dialyzed against buffer A.Antibodies Monoclonal antisyntaxin 4 antibodies were from Transduction Laboratories (Lexington, KY). Polyclonal antibodies against human SNAP-23 were generated in rabbits. Monoclonal and polyclonal anti-VAMP-1 and VAMP-2 antibodies were obtained from StressGen (Collegeville, PA) and Synaptic Systems (Germany); anti-VAMP-3 and VAMP-8 antibodies were from Abcam (Cambridge, United Kingdom).
VAMP-3 and VAMP-8 coimmunoprecipitate with syntaxin 4 in platelets Complex formation between SNARE proteins is required for granule secretion. We sought to identify which VAMP proteins formed SNARE complexes with syntaxin 4, a SNARE protein that is required for platelet secretion. When syntaxin 4 was immunoprecipitated from Triton X-100 platelet lysate with antisyntaxin 4 antibodies, it was found to be in a complex with SNAP-23. We also identified other proteins with molecular sizes consistent with VAMPs and immunoreactive with a mixture of anti-VAMP antibodies. The immunoblotting experiments were not considered definitive for identification of the VAMP isoforms because of the moderate to high sequence identity among the VAMPs (Figure 1) and the limited availability of isoform-specific VAMP antibodies. Therefore, SDS-PAGE gel slices containing VAMP proteins were excised and the samples were analyzed by microcapillary reverse-phase HPLC-nano-electrospray tandem MS. The sequences obtained with this method unequivocally identified VAMP-3 and VAMP-8 (Figure 1).
In agreement with these findings, VAMP-3 and VAMP-8 but not VAMP-2 were
detected in platelet lysates by immunoblotting (Figure 2). The VAMP-3 and VAMP-8 detected were
derived from platelets and not the small number of leukocytes
contaminating the platelet preparations. No VAMP-3 or VAMP-8 was
detected in cell lysates containing twice as many leukocytes as are
typically found in our platelet preparations (Figure 2). VAMP-3 and
VAMP-8 also coimmunoprecipitated with SNAP-23, another SNARE protein
shown to be required for platelet secretion (data not shown).
VAMP-3 and VAMP-8 are required for platelet secretion Our finding that VAMP-3 and VAMP-8 coimmunoprecipitated with syntaxin 4 and SNAP-23 (platelet SNAREs required for secretion) suggested that these VAMP isoforms may play a role in the mechanism of platelet granule secretion. To examine this issue, we studied the effect of various recombinant VAMP proteins on granule secretion in permeabilized platelets. Thus, various concentrations of VAMP-2, VAMP-3, and VAMP-8 with no C-terminal transmembrane domains (rVAMPs) were incubated with streptolysin O-permeabilized platelets. Granule secretion was induced by increasing Ca++.We found that rVAMP-3 was a potent inhibitor of both
To confirm that the rVAMPs used in these studies were functional, we
examined their ability to form SNARE complexes in vitro. Ternary SNARE
complexes are resistant to denaturation by 1% SDS if boiling is not
done.15 Equimolar amounts of SNAP-25, syntaxin 4, and
rVAMP-2, rVAMP-3, or rVAMP-8 were incubated together, and the samples
were analyzed by SDS-PAGE and immunoblotting. The incubation time,
buffer composition, and temperature mimicked the experimental
conditions used for studying the effects of rVAMPs on secretion in
permeabilized platelets. All 3 rVAMPs formed ternary complexes under
these conditions, confirming the functionality of these proteins
(Figure 4). The strongest SNARE complex
bands were detected with rVAMP-2 (Figure 4), which had no effect on secretion in platelets.
Flaumenhaft et al7 found that VAMP-1 and VAMP-2 could not be detected in platelets and that a relatively high concentration of Botulinum toxin was necessary to inhibit platelet secretion. This led them to speculate that platelets may contain a novel VAMP species. Bernstein and Whiteheart16 observed a punctate intracellular staining with anti-VAMP-3 antibodies in human platelets. Our finding that the cytoplasmic domain of VAMP-3 inhibited secretion in permeabilized platelets strongly suggests that VAMP-3 plays an important role in the mechanism of platelet secretion. VAMP-3 may be the only v-SNARE required for
We thank Lin Liu and Aeisha Robb for help in cloning VAMPs and producing recombinant proteins.
Submitted August 21, 2001; accepted March 27, 2002.
Supported in part by National Institutes of Health grant HL-64057 (G.L.R.).
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: Guy L. Reed, Cardiovascular Biology Laboratory, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115; e-mail: reed{at}cvlab.harvard.edu.
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© 2002 by The American Society of Hematology.
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