Staphylococcal superantigen-like 5 activates platelets and supports platelet adhesion under flow conditions, which involves glycoprotein Ibalpha and alpha IIb beta 3.

OBJECTIVES: Staphylococcal superantigen-like 5 (SSL5) is an exoprotein secreted by Staphylococcus aureus that has been shown to inhibit neutrophil rolling over activated endothelial cells via a direct interaction with P-selectin glycoprotein ligand 1 (PSGL-1). METHODS AND RESULTS: When purified recombinant SSL5 was added to washed platelets in an aggregometry set-up, complete and irreversible aggregation was observed. Proteolysis of the extracellular part of GPIb alpha or the addition of dRGDW abrogated platelet aggregation. When a mixture of isolated platelets and red cells was perfused over immobilized SSL5 at a shear rate of 300 s(-1), stable platelet aggregates were observed, and platelet deposition was substantially reduced after proteolysis of GPIb or after addition of dRGDW. SSL5 was shown to interact with glycocalicin, a soluble GPIb alpha fragment, and binding of SSL5 to platelets resulted in GPIb-mediated signal transduction as evidenced by translocation of 14-3-3 zeta. In addition, SSL5 was shown to interact with endothelial cell matrix (ECM) and this interaction enhanced aggregation of platelets from whole blood to this ECM. CONCLUSIONS: SSL5 activates and aggregates platelets in a GPIb alpha-dependent manner, which could be important in colonization of the vascular bed and evasion of the immune system by S. aureus.

A tripeptide mimetic of von Willebrand factor residues 981-983 enhances platelet adhesion to fibrinogen by signaling through integrin alpha(IIb)beta3.

BACKGROUND: RGD is a major recognition sequence for ligands of platelet alpha(IIb)beta3. OBJECTIVE AND METHODS: To identify potential binding sites for alpha(IIb)beta3 apart from RGD, we screened phage display libraries by blocking the enrichment of RGD-containing phages with a GRGDS peptide and identified a novel integrin recognition tripeptide sequence, VPW. RESULTS: Platelets adhered to an immobilized cyclic VPW containing peptide in a alpha(IIb)beta3-dependent manner; platelets and alpha(IIb)beta3-expressing CHO cells adhered faster to immobilized alpha(IIb)beta3-ligands in the presence of soluble VPW. In platelets adhering to fibrinogen, VPW accelerated the activation of the tyrosine kinase Syk which controls cytoskeletal rearrangements. In alpha(IIb)beta3-expressing CHO cells, VPW induced a faster formation of stress fibers. Sequence alignment positioned VPW to V980-P981-W982 in the von Willebrand factor (vWf) A-3 domain. In blood from a vWf-deficient individual, VPW increased platelet adhesion to fibrinogen but not to collagen under flow and rescued the impaired adhesion to vWf deficient in A-3. CONCLUSION: These data reveal a VPW sequence that contributes to alpha(IIb)beta3 activation in in vitro experiments. Whether the V980-P981-W982 sequence in vWf shows similar properties under in vivo conditions remains to be established.