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.