GPIbα-Filamin A interaction regulates megakaryocyte localization and budding during platelet biogenesis.

Glycoprotein (GP) Ibα is expressed on the surface of platelets and megakaryocytes and is anchored to the membrane skeleton by Filamin A (FlnA). Whilst GPIb and FlnA have fundamental roles in platelet biogenesis, the nature of this interaction in megakaryocyte biology remains ill-defined. We generated a mouse model expressing either human wild-type (WT) GPIbα (hGPIbaWT) or a FlnA-binding mutant (hGPIbaFW) and lacking endogenous mouse GPIbα. Mice expressing the mutant GPIbα transgene exhibited macrothrombocytopenia with preserved GPIb surface expression. Platelet clearance was normal and differentiation of megakaryocytes to proplatelets was unimpaired in hGPIbaFW mice. The most striking abnormalities in hGPIbαFW megakaryocytes were defective formation of the demarcation membrane system (DMS), and redistribution of FlnA from the cytoplasm to the peripheral margin of megakaryocytes. These abnormalities led to disorganized internal megakaryocyte membranes and the generation of enlarged megakaryocyte membrane buds. The defective FlnA-GPIbα interaction also resulted in misdirected release of buds away from the vasculature into the bone marrow interstitium. Restoring the linkage between FlnA and GPIbα corrected the FlnA redistribution within megakaryocytes and DMS ultrastructural defects as well as restored normal bud size and release into sinusoids. These studies define a new mechanism of macrothrombocytopenia resulting from dysregulated megakaryocyte budding. The link between FlnA and GPIbα is not essential for the megakaryocyte budding process, however it plays a major role in regulating the structure of the DMS, bud morphogenesis, and the localized release of buds into the circulation.