Lipid Rafts Orchestrate Signaling by the Platelet Receptor Glycoprotein VI

Darren Locke,Hong Chen,Ying Liu,Changdong Liu,Mark L Kahn

doi:10.1074/jbc.m111520200

Abstract

The platelet collagen receptor glycoprotein VI (GPVI) couples to the immune receptor adaptor Fc receptor gamma-chain (FcRgamma) and signals using many of the same intracellular signaling molecules as immune receptors. Studies of immune receptor signaling have revealed a critical role for specialized areas of the cell membrane known as lipid rafts, which are enriched in essential signaling molecules. However, the role of lipid rafts in signaling in nonimmune cells such as platelets remains poorly defined. This study shows that GPVI-FcRgamma does not constitutively associate with rafts, but is recruited to lipid rafts following receptor stimulation in both GPVI-expressing RBL-2H3 cells and human platelets. FcRgamma is required for GPVI association with lipid rafts, as mutant GPVI receptors that do not couple to FcRgamma were unable to associate with lipid rafts after receptor clustering. Following GPVI stimulation in platelets, virtually all phosphorylated FcRgamma was found in lipid rafts, but inhibition of FcRgamma phosphorylation did not block receptor association with lipid rafts. This work demonstrates that lipid rafts orchestrate GPVI receptor signaling in platelets in a manner analogous to immune cell receptors and supports a model of GPVI signaling in which FcRgamma phosphorylation is controlled by ligand-dependent association with lipid rafts.

Highlights

Glycoprotein VI (GPVI)1 activates platelets through many of the same downstream kinases, adaptors, and effector molecules as Fc, T-cell, and B-cell receptors (1, 2)
How clustering of immune receptors initiates signal transduction is not well understood, but one proposed mechanism is through receptor association with specialized areas of the cell membrane known as lipid rafts, which are enriched in signaling proteins such as Src family kinases and the transmembrane adaptor LAT
Following CVX stimulation of human platelets, virtually all phosphorylated FcR␥ was detected in lipid rafts despite the presence of only a small percentage of total FcR␥ in lipid rafts (Fig. 4A). These results demonstrate that glycoprotein VI (GPVI)-FcR␥ stimulation in human platelets results in receptor association with lipid rafts and that only those receptors associated with lipid rafts undergo tyrosine phosphorylation and participate in downstream signaling

Summary

Lipid Rafts Orchestrate Signaling by the Platelet Receptor Glycoprotein VI*

Glycoprotein VI (GPVI) activates platelets through many of the same downstream kinases, adaptors, and effector molecules as Fc, T-cell, and B-cell receptors (1, 2) Like these immune receptors, GPVI is a multisubunit receptor in which the ligand-binding subunit (GPVI) is noncovalently associated with a signaling subunit (Fc receptor ␥-chain (FcR␥)) that contains an immunoreceptor tyrosine activation motif (3, 4). How clustering of immune receptors initiates signal transduction is not well understood, but one proposed mechanism is through receptor association with specialized areas of the cell membrane known as lipid rafts, which are enriched in signaling proteins such as Src family kinases and the transmembrane adaptor LAT (reviewed in Ref. 8). Our results establish a role for lipid rafts in platelets and support a model of platelet activation by GPVI in which receptor activation stimulates movement to lipid rafts, where FcR␥ is phosphorylated to initiate downstream signaling. GPVI Signals through Lipid Rafts contain unique proteins to facilitate receptor signaling remain to be determined

EXPERIMENTAL PROCEDURES

RESULTS

Increase in receptor movement to raft

DISCUSSION