Sialylated glycoproteins bind to Siglec-9 in a cis manner on platelets to suppress platelet activation

Background

The endogenous negative regulation of platelets is important in preventing spontaneous thrombosis, while the mechanism of homeostasis is incompletely understood.

Objectives

In this study, we aimed to explore whether Siglec-9 plays a negative regulative role and identify the ligand of Siglec-9 on platelets.

Methods

To determine the role of Siglec-9 on platelets, platelet factor 4-cre:Siglec-E^flox/flox knockout mouse model and human platelet in vitro culture system were used. Furthermore, recombinant glycoprotein (GP) of Siglec-9 ligand on platelets was expressed and used.

Results

We found that Siglec-E conditional knockout can lead to significant increase in platelet coagulation activities both in vivo and in vitro, which strongly suggests that Siglec-9/E plays an inhibitory physiological role in platelet activation. Siglec-9 ligand is an O-link GP with an α2,3-linked sialic acid terminal structure, and the protein carrier of the ligand is mucin-like region of GPIbα. Our data further showed that the ligands on platelets could not engage Siglec-9 on other cells via trans-binding, which indicates that the ligands on platelets play a self-modulation role. Furthermore, we provided evidence that the activation of Siglec-9 pathway with exogenous specific ligands could inhibit the activity of platelets. These data demonstrate a previously unanticipated role for GPIbα in inhibiting platelet activation and provide a novel mechanism for the homeostasis of platelets.

Conclusions

We conclude that the cis-binding between mucin-like region of GPIbα and Siglec-9 acts as a “parking brake” on platelet activation. This finding provides a potential druggable target for novel antiplatelet medicine.