Caspase-12: a developmental link between G-protein–coupled receptors and integrin αIIbβ3 activation

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Abstract
Fibrinogen binding by integrin αIIbβ3 is promoted by platelet agonists that increase the affinity and avidity of αIIbβ3 for fibrinogen through a process called “inside-out” signaling. Having previously demonstrated that inside-out activation of αIIbβ3 is defective in murine megakaryocytes that lack the transcription factor NF-E2, we screened for NF-E2–regulated genes that affect αIIbβ3 activation. Caspase-12 is the most down-regulated gene we identified in NF-E2–/– megakaryocytes. Therefore, the role of this protein in αIIbβ3 activation was determined using platelets from caspase-12–/– mice. Despite wild-type levels of αIIbβ3, caspase-12–/– platelets exhibit reduced fibrinogen binding to αIIbβ3 following stimulation by adenosine diphosphate (ADP) or protease-activated receptor 4 (PAR4) receptor-activating peptide. The defect in αIIbβ3 activation is associated with decreased cytosolic free calcium and inositol triphosphate levels, and with reduced aggregation, despite wild-type phospholipase Cβ expression levels. In contrast, agonist-induced surface expression of P-selectin, suppression of cAMP levels following ADP stimulation, and spreading on immobilized fibrinogen are unimpaired. Moreover, although caspase-12 is highly expressed in mature megakaryocytes, it is undetectable in platelets. Taken together, these studies establish that caspase-12 expression in murine megakaryocytes is regulated, directly or indirectly, by NF-E2, and suggest that caspase-12 participates in the development of fully functional signaling pathways linking some G-protein–coupled receptors to αIIbβ3 activation.