Relationships between Rap1b, Affinity Modulation of Integrin αIIbβ3, and the Actin Cytoskeleton*

By A. Bertoni, S. Tadokoro, K. Eto, N. Pampori, L. Parise, G. White, S. Shattil. Journal of Biological Chemistry, 2002

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The affinity of integrin αIIbβ3 for fibrinogen is controlled by inside-out signals that are triggered by agonists like thrombin. Agonist treatment of platelets also activates Rap1b, a small GTPase known to promote integrin-dependent adhesion of other cells. Therefore, we investigated the role of Rap1b in αIIbβ3 function by viral transduction of GFP-Rap1 chimeras into murine megakaryocytes, which exhibit inside-out signaling similar to platelets. Expression of constitutively active GFP-Rap1b (V12) had no effect on unstimulated megakaryocytes, but it greatly augmented fibrinogen binding to αIIbβ3 induced by a PAR4 thrombin receptor agonist (p < 0.01). The Rap1b effect was cell-autonomous and was prevented by pre-treating cells with cytochalasin D or latrunculin A to inhibit actin polymerization. Rap1b-dependent fibrinogen binding to megakaryocytes was blocked by POW-2, a novel monovalent antibody Fab fragment specific for high affinity murine αIIbβ3. In contrast to GFP-Rap1b (V12), expression of GFP-Rap1GAP, which deactivates endogenous Rap1, inhibited agonist-induced fibrinogen binding (p < 0.01), as did dominant-negative GFP-Rap1b (N17) (p < 0.05). None of these treatments affected surface expression of αIIbβ3. These studies establish that Rap1b can augment agonist-induced ligand binding to αIIbβ3 through effects on integrin affinity, possibly by modulating αIIbβ3interactions with the actin cytoskeleton.

Subfields: Cell Biology, Hematology, Immunology and Allergy

View Paper: https://www.semanticscholar.org/paper/28383467e899aac24ad276b0c0716bfc088768fb