Pathological shear stress directly regulates platelet α _IIbβ₃ signaling

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin α _IIbβ₃ is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on α_IIbβ₃, we examined α_IIbβ ₃/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with α_IIbβ ₃ in resting platelets and that 120 dyn/cm² shear stress leads to their disassociation from α_IIbβ₃. Shear-induced disassociation of α-actinin and myosin heavy chain from the β₃ tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to α_IIbβ₃. Syk's disassociation from β₃ is inhibited when VWF binding to either GpIb-IX-V or α_IIbβ₃ is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to α_IIbβ₃ but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing α_IIbβ₃ with β₃ truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates α_IIbβ₃ function and suggest that shear-induced α_IIbβ₃-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β₃-tail.