<i>Gfi1b</i> controls integrin signaling-dependent cytoskeleton dynamics and organization in megakaryocytes

Abstract

Mutations in <i>GFI1B</i> are associated with inherited bleeding disorders called <i>GFI1B</i>-related thrombocytopenias. We show here that mice with a megakaryocyte-specific <i>Gfi1b</i> deletion exhibit a macrothrombocytopenic phenotype along a megakaryocytic dysplasia reminiscent of <i>GFI1B</i>-related thrombocytopenia. GFI1B deficiency increases megakaryocyte proliferation and affects their ploidy, but also abrogates their responsiveness towards integrin signaling and their ability to spread and reorganize their cytoskeleton. <i>Gfi1b</i>-null megakaryocytes are also unable to form proplatelets, a process independent of integrin signaling. GFI1B-deficient megakaryocytes exhibit aberrant expression of several components of both the actin and microtubule cytoskeleton, with a dramatic reduction of α-tubulin. Inhibition of FAK or ROCK, both important for actin cytoskeleton organization and integrin signaling, only partially restored their response to integrin ligands, but the inhibition of PAK, a regulator of the actin cytoskeleton, completely rescued the responsiveness of <i>Gfi1b</i>-null megakaryocytes to ligands, but not their ability to form proplatelets. We conclude that <i>Gfi1b</i> controls major functions of megakaryocytes such as integrin-dependent cytoskeleton organization, spreading and migration through the regulation of PAK activity whereas the proplatelet formation defect in GFI1B-deficient megakaryocytes is due, at least partially, to an insufficient α-tubulin content.