Clustering of glycoprotein VI (GPVI) dimers upon adhesion to collagen as a mechanism to regulate GPVI signaling in platelets

Background
Platelet glycoprotein VI (GPVI) binding to subendothelial collagen exposed upon blood vessel injury initiates thrombus formation. Dimeric GPVI has high affinity for collagen, and occurs constitutively on resting platelets.

Objective
To identify higher‐order oligomerization (clustering) of pre‐existing GPVI dimers upon interaction with collagen as a mechanism to initiate GPVI‐mediated signaling.

Methods
GPVI was located by use of fluorophore‐conjugated GPVI dimer‐specific Fab (antigen‐binding fragment). The tested substrates include Horm collagen I fibers, soluble collagen III, GPVI‐specific collagen peptides, and fibrinogen. GPVI dimer clusters on the platelet surface interacting with these substrates were visualized with complementary imaging techniques: total internal reflection fluorescence microscopy to monitor real‐time interactions, and direct stochastic optical reconstruction microscopy (dSTORM), providing relative quantification of GPVI cluster size and density. Confocal microscopy was used to locate GPVI dimer clusters, glycoprotein Ib, integrin α2β1, and phosphotyrosine.

Results
Upon platelet adhesion to all collagenous substrates, GPVI dimers coalesced to form clusters; notably clusters formed along the fibers of Horm collagen. dSTORM revealed that GPVI density within clusters depended on the substrate, collagen III being the most effective. Clusters on fibrinogen‐adhered platelets were much smaller and more numerous; whether these are pre‐existing oligomers of GPVI dimers or fibrinogen‐induced is not clear. Some GPVI dimer clusters colocalized with areas of phosphotyrosine, indicative of signaling activity. Integrin α2β1 was localized to collagen fibers close to GPVI dimer clusters. GPVI clustering depends on a dynamic actin cytoskeleton.

Conclusions
Platelet adhesion to collagen induces GPVI dimer clustering. GPVI clustering increases both avidity for collagen and the proximity of GPVI‐associated signaling molecules, which may be crucial for the initiation and persistence of signaling.