Do lipid rafts contribute to platelet activation?

Abstract

Lipid rafts are defined as insoluble areas in the cell membrane, resistant to non-ionic detergents. These areas, enriched in glycosphingolipids, saturated phospholipids and cholesterol, also identified as DIGs (detergent-insoluble glycosphingolipid-enriched domains) or as DRMs (detergent-resistant membranes) have been identified in several cell types. Initially, they were believed to be responsible for the transcellular transport of glycosyl phosphatidylinositol (GPI)-anchored proteins to the apical surface in polarized cells [1,2]. However, over the last 10 years, rafts have increasingly been recognized as membrane microdomains, playing a critical role in the control of several cellular activation processes. Thus, very divergent proteins such as Src family kinases, caveolins, palmitoylated proteins such as G proteins, GPI-anchored proteins such as Thy-1 and alkaline phosphatases, tetraspannin proteolipids and various signaling molecules have all been shown to be associated with lipid rafts. Different types of rafts coexist at the plasma membrane with functionally distinct lipid composition [3], and lipid rafts are not only found at the plasma membrane, but also as part of the internal membrane of granules, Golgi complex and even phagosomes [4,5]. Evidence for a functional role of lipid rafts in platelets is very recent: Gousset et al. [6] have shown that during cold-induced platelet activation, rafts cluster into larger aggregates, a reversible process depending on platelet activation. These authors showed raft aggregation to be dependent on the presence in the membrane of cholesterol and further identified the presence of CD36 in DRMs. During fluorescence microscopy of platelets being activated with thrombin and collagen, large fluorescent clusters of lipid rafts were formed, leading these investigators to conclude that raft aggregation is triggered by platelet activation, suggestive of a role for microdomains in platelet signaling [6]. The demonstration that phosphatidylinositol 3,4,5-triphosphate is produced in platelet lipid rafts provided further support for such a role in human platelet activation [7]. Elegant studies have shown that lipid rafts orchestrate the signaling by the platelet receptor glycoprotein VI, which signals through the immune receptor adaptor Fc receptor g-chain (FcRg) [8]. These investigators showed that GPVI-FcRg does not constitutively associate with rafts, but is recruited to lipid rafts following receptor stimulation in human platelets and concluded that FcRg phosphorylation is controlled by ligand-dependent association with lipid rafts. Finally, the recent finding that stomatin is a major lipid-raft component of platelet a-granules [9] confirms that also in platelets various types of lipid raft exist, more in particular suggesting a role for stomatin in the organization and function of platelet a-granules. Transmembrane 4 superfamily (TM4SF) proteins or tetraspannins are ubiquitously expressed proteins, involved in T- and B-cell activation, cell fusion, mobility and proliferation, and also in platelet aggregation [10]. TM4SF proteins may serve as adaptors during the assembly of protein complexes in the membrane. Different lipid raft-associated tetraspannins display a varying degree of resistance to detergents. Thus, microdomain complexes involving a large number of proteins may be disrupted by Triton X-100 but be resistant to milder detergents such as Brij 99 and CHAPS; such behavior is reported for the TM4SF protein CD63 [10]. In the paper by Heijen et al. [11] in this issue, the authors have shown in a direct manner that platelet activation and aggregation induced with thrombin are linked to the presence in the platelet membrane of cholesterol, i.e. to the existence of