Does fibrin(ogen) bind to monomeric or dimeric GPVI, or not at all?

GPVI is the major signalling receptor for collagen on platelets. Dimerization of GPVI is required for collagen binding and initiation of signalling through the associated FcR-γ chain. Recently, fibrin and fibrinogen have been identified as ligands for GPVI and shown to induce signalling in support of thrombus formation and stabilization. Contrasting observations have been reported on whether fibrin binds to monomeric or dimeric GPVI, or to neither form. In this article, we discuss reasons for the contradictory results and how to reconcile these. We conclude that a lack of structural knowledge regarding the GPVI constructs that are being used, along with the use of non-standardized reagents, might be the main cause of the discrepant results. This article aims to highlight some of the key areas that need to be addressed.

Aging- and activation-induced platelet microparticles suppress apoptosis in monocytic cells and differentially signal to proinflammatory mediator release.

BACKGROUND: Platelet microparticles (PM) are the most abundant cell-derived microparticles in the blood, and accumulate in thrombo-inflammatory diseases. Platelets produce PM upon aging via an apoptosis-like process and by activation with strong agonists. We previously showed that long-term treatment of monocytic cells with apoptosis-induced PM (PMap) promotes their differentiation into resident macrophages. Here we investigated shorter term effects of various types of PM on monocyte signalling and function. METHODS AND RESULTS: Flow cytometry and scanning electron microscopy revealed that PM formed upon platelet aging (PMap) or ultra-sonication (PMsonic) expressed activated αIIbß3 integrins and tended to assemble into aggregates. In contrast, PM formed upon platelet activation with thrombin (PMthr) or Ca(2+) ionophore (PMiono) had mostly non-activated αIIbß3 and little aggregate formation, but had increased CD63 expression. PM from activated and sonicated platelets expressed phosphatidylserine at their surface, while only the latter were enriched in the receptors CD40L and CX3CR1. All PM types expressed P-selectin, interacted with monocytic cells via this receptor, and were internalised into these cells. The various PM types promoted actin cytoskeletal rearrangements and hydrogen peroxide production by monocytic cells. Markedly, both aging- and activation-induced PM types stimulated the phosphoinositide 3-kinase/Akt pathway, suppressing apoptosis induced by several agonists, in a P-selectin-dependent manner. On the other hand, the PM types differentially influenced monocyte signalling in eliciting Ca(2+) fluxes (particularly PMap) and in releasing secondary mediators (complement factor C5a with PMap, and pro-inflammatory tumour necrosis factor-α with PMthr). CONCLUSIONS: In spite of their common anti-apoptotic potential via Akt activation, aging- and activation-induced PM cause different Ca(2+) signalling events and mediator release in monocytic cells. By implication, aging and activated platelets may modulate monocyte function in different way by the shedding of different PM types.