Primary megakaryocytes reveal a role for transcription factor NF-E2 in integrin alpha IIb beta 3 signaling

Platelet integrin alpha IIb beta 3 responds to intracellular signals by bin ding fibrinogen and triggering cytoskeletal reorganization, but the mechani sms of alpha IIb beta 3 signaling remain poorly understood. To better under stand this process, we established conditions to study alpha IIb beta 3 sig naling in primary murine megakaryocytes. Unlike platelets, these platelet p recursors are amenable to genetic manipulation. Cytokine-stimulated bone ma rrow cultures produced three arbitrary populations of alpha IIb beta 3-expr essing cells with increasing size and DNA ploidy: small progenitors, interm ediate-size young megakaryocytes, and large mature megakaryocytes. A majori ty of the large megakaryocytes bound fibrinogen in response to agonists, wh ile almost none of the smaller cells did. Fibrinogen binding to large megak aryocytes was inhibited by Sindbis virus-mediated expression of isolated be ta 3 integrin cytoplasmic tails. Strikingly, large megakaryocytes from mice deficient in the transcription factor NF-E2 failed to bind fibrinogen in r esponse to agonists, despite normal surface expression of alpha IIb beta 3. Furthermore, while megakaryocytes from wild-type mice spread on immobilize d fibrinogen and exhibited filopodia, lamellipodia and Rho-dependent focal adhesions and stress fibers, NF-E2-deficient megakaryocytes adhered poorly. These studies establish that agonist-induced activation of alpha IIb beta 3 is controlled by NF-E2-regulated signaling pathways that mature late in m egakaryocyte development and converge at the beta 3 cytoplasmic tail. Megak aryocytes provide a physiologically relevant and tractable system for analy sis of bidirectional alpha IIb beta 3 signaling.