Regulation of proplatelet formation and platelet release by integrin αIIbβ3

Mature megakaryocytes form structures called proplatelets that serve as conduits for platelet packaging and release at vascular sinusoids. Since the megakaryocyte expresses abundant levels of integrin alpha(IIb)beta(3), we have examined a role for fibrinogen in proplatelet development and platelet release alongside that of other matrices. Primary mature murine megakaryocytes from bone marrow aspirates readily formed proplatelets when plated on fibrinogen at a degree that was significantly higher than that seen on other matrices. In addition, alpha(IIb)beta(3) was essential for proplatelet formation on fibrinogen, as megakaryocytes failed to develop proplatelets in the presence of alpha(IIb)beta(3) antagonists. Interestingly, inhibition of Src kinases or Ca2+ release did not inhibit proplatelet formation, indicating that alpha(IIb)beta(3)-mediated outside-in signals are not required for this response. Immunohistochemical studies demonstrated that fibrinogen is localized to the bone marrow sinusoids, a location that would allow it to readily influence platelet release. Further, thrombopoietin-stimulated alpha(-/-)(IIb) mice had a reduced increase in platelet number relative to controls. A similar observation was not observed for platelet recovery in alpha(-/-)(IIb) mice in response to antibody-induced thrombocytopenia, indicating the existence of additional pathways of regulation of proplatelet formation. These results demonstrate that fibrinogen is able to regulate proplatelet formation via integrin alpha(IIb)beta(3).