Integrin-dependent interaction of lipid rafts with the actin cytoskeleton in activated human platelets

Dynamic connections between actin filaments and the plasma membrane are crucial for the regulation of blood platelet functions. Protein complexes associated with alpha IIb beta 3 integrin-based cytoskeleton structures are known to play a role in these processes. However, mechanisms involving lateral organizations of the plasma membrane remain to be investigated. Here, we demonstrate that a large fraction of platelet lipid rafts specifically associates with the actin cytoskeleton upon activation. This association was inhibited by antagonists of fibrinogen-alpha IIb beta 3 binding and did not occur in type I Glanzman's thrombasthenic platelets. The raft-cytoskeleton interaction is a reversible process correlating with the intensity and stability of platelet aggregation. Although only a minor fraction of alpha IIb beta 3 was recovered in rafts upon activation, this integrin specifically upregulated the level of PtdIns(4,5)P-2 in membrane microdomains and induced the recruitment of several actin-modulating proteins known to directly or indirectly interact with this lipid. Controlled disruption of rafts did not affect alpha IIb beta 3-mediated platelet aggregation in response to high concentrations of thrombin but significantly inhibited fibrin clot retraction. We propose that rafts participate in the organization of membrane-cytoskeleton interactions where alpha IIb beta 3-mediated tension forces apply during the late phase of platelet activation.