Several recently published studies in Blood have attempted to unravel the signaling events operating downstream of GPIb.1-3  One of these studies by Garcia et al1  examined the role of Erk in promoting VWF/GPIb-dependent activation of integrin αIIbβ3 in platelets. These investigators analyzed changes in Erk phosphorylation in VWF/ristocetin-stimulated platelets and examined the effects of a range of pharmacological signaling inhibitors on Erk activation and integrin αIIbβ3-dependent platelet aggregation. Based on these studies, the authors conclude that there is an important role for Erk in GPIb signaling and propose a model in which GPIb initiates a linear signaling cascade involving Src kinases → PLC → MEK → Erk → PLA2 that stimulates integrin αIIbβ3 and platelet aggregation through an indirect mechanism dependent on the generation of TXA2. Although the results presented are consistent with such a model, we have some concerns with the definitive nature of these conclusions.

The main concern is fundamental and relates to the individual contributions of GPIb and integrin αIIbβ3 to VWF-induced signaling. It is generally accepted that the VWF-GPIb interaction induces weak signals to initiate integrin αIIbβ3 activation, and the subsequent VWF binding to activated integrin αIIbβ3 in concert with released ADP and TXA2 triggers global platelet activation. Thus, many of the commonly used suspension-based functional assays to investigate signals downstream of soluble agonist receptors (ie, classical platelet aggregation, secretion, or ligand binding to activated integrin αIIbβ3) are not ideal for analysis of signals derived exclusively downstream of GPIb. In particular, when VWF/ristocetin or VWF/botrocetin induces biphasic platelet aggregation the authors need to consider that output signals are derived from both GPIb and integrin αIIbβ3, not solely from GPIb. These factors compound the analysis of the findings presented by Garcia et al1  and also those by Liu et al2  investigating a role for Btk in GPIb signaling and Rathore et al3  examining PECAM-1 regulation of GPIb signals.

It should be acknowledged that dissecting signaling events downstream of GPIb is difficult, mainly because GPIb-induced signals per se are weak regardless of the experimental approaches used.4-6  Elucidating GPIb-specific signaling events independent of integrin αIIbβ3, ADP, or TXA2 preferably uses methodology with high sensitivity and rapid temporal resolution. This has been most clearly demonstrated from analysis of GPIb-dependent calcium flux.4,5  Furthermore, GPIb-derived signals in an adherent platelet population are not coincident, and at any given time point detectable signals may be identified only in a relatively small subpopulation of platelets, further complicating analysis. Experimental approaches have been devised by a number of laboratories to investigate GPIb signals independent of other activating stimuli; however, a number of these assays are technically demanding and not widely available. Ideally, findings with respect to GPIb-dependent integrin αIIbβ3 activation in suspension-based aggregation assays should be confirmed in adhesion-based assays using direct reporters of integrin αIIbβ3 activation, such as those described by Kasirer-Friede et al.7 

With these considerations in mind, we believe some caution should be exercised in the interpretations by Garcia et al,1  Liu et al,2  and Rathore et al3  that ERK, Btk, and PECAM-1 are major players in GPIb signaling.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Garcia A, Quinton TM, Dorsam RT, Kunapuli SP. Src family kinase-mediated and Erk-mediated thromboxane A2 generation are essential for VWF/GPIb-induced fibrinogen receptor activation in human platelets.
Liu J, Fitzgerald ME, Berndt MC, Jackson CW, Gartner TK. Bruton tyrosine kinase is essential for botrocetin/VWf-induced signaling and GPIb-dependent thrombus formation in vivo.
Prepublished on June 20, 2006 as DOI (Now available as Blood. 2006;108:2596-2603).
Rathore V, Stapleton MA, Hillery C, et al. PECAM-1 negatively regulates GPIb/V/IX signaling in murine platelets.
Mangin P, Yuan Y, Goncalves I, et al. Signaling role for phospholipase C gamma 2 in platelet glycoprotein Ib alpha calcium flux and cytoskeletal reorganization: involvement of a pathway distinct from FcR gamma chain and Fc gamma RIIA.
J Biol Chem
Mazzucato M, Pradella P, Cozzi MR, et al. Sequential cytoplasmic calcium signals in a 2-stage platelet activation process induced by the glycoprotein Ibalpha mechanoreceptor.
Marshall SJ, Azazuma N, Best D, et al. Glycoprotein IIb-IIIa-dependent aggregation by glycoprotein Ibalpha is reinforced by a Src family kinase inhibitor (PP1)-sensitive signalling pathway.
Biochem J
Kasirer-Friede A, Cozzi MR, Mazzucato M, et al. Signaling through GP Ib-IX-V activates alpha IIb beta 3 independently of other receptors.
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