Role of Siglec-7 in Apoptosis in Human Platelets

Kim Anh Nguyen,Isabelle Anselme-Bertrand,Olivier Garraud,Bruno Pozzetto,Charles-Antoine Arthaud,Hind Hamzeh-Cognasse,Patricia Chavarin,Sabine Palle,Fabrice Cognasse,Zhenyu Li

doi:10.1371/journal.pone.0106239

Abstract

BackgroundPlatelets participate in tissue repair and innate immune responses. Sialic acid-binding immunoglobulin-like lectins (Siglecs) are well-characterized I-type lectins, which control apoptosis.Methodology/Principal FindingsWe characterized the expression of Siglec-7 in human platelets isolated from healthy volunteers using flow cytometry and confocal microscopy. Siglec-7 is primarily expressed on α granular membranes and colocalized with CD62P. Siglec-7 expression was increased upon platelet activation and correlated closely with CD62P expression. Cross-linking Siglec-7 with its ligand, ganglioside, resulted in platelet apoptosis without any significant effects on activation, aggregation, cell morphology by electron microscopy analysis or secretion. We show that ganglioside triggered four key pathways leading to apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (ΔΨm) depolarization; (ii) elevated expression of pro-apoptotic Bax and Bak proteins with reduced expression of anti-apoptotic Bcl-2 protein; (iii) phosphatidylserine exposure and (iv), microparticle formation. Inhibition of NAPDH oxidase, PI3K, or PKC rescued platelets from apoptosis induced by Siglec-7 recruitment, suggesting that the platelet receptors P2Y1 and GPIIbIIIa are essential for ganglioside-induced platelet apoptosis.Conclusions/SignificanceThe present work characterizes the role of Siglec-7 and platelet receptors in regulating apoptosis and death. Because some platelet pathology involves apoptosis (idiopathic thrombocytopenic purpura and possibly storage lesions), Siglec-7 might be a molecular target for therapeutic intervention/prevention.

Highlights

Beyond hemostasis, platelets have a variety of functions, in innate immunity, host defense against infection, and inflammation, especially inflammatory processes such as atherosclerosis [1,2]
This study describes the first report on the expression and functional role of sialic acid-binding immunoglobulin-like lectin (Siglec)-7 in human platelets
We show that human platelets express Siglec-7 and that this expression is increased upon TRAP stimulation

Summary

Introduction

Platelets have a variety of functions, in innate immunity, host defense against infection, and inflammation, especially inflammatory processes such as atherosclerosis [1,2]. We recently demonstrated that platelets can recycle a number of biological response modifiers (BRMs) [3,4,5,6]. Several recent data, including our own, demonstrated that platelets could sense external signals (such as pathogen-associated molecular patterns; PAMPs) differentially through a single type of pathogen recognition receptor. Upon PAMP recognition, platelets can influence the innate immune response appropriately for pathogens exhibiting different types of ‘danger’ signals by secreting a number of cytokines/ chemokines or related receptors [2,11]. Platelets can sense danger signals such as damage-associated molecular patterns (DAMPs), known as Alarmins, of which the family of sialic acid-binding immunoglobulin-like lectin (Siglec) belongs. Sialic acid-binding immunoglobulin-like lectins (Siglecs) are well-characterized I-type lectins, which control apoptosis

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Conclusion