Abstract
The mechanism by which occupancy of collagen receptors is coupled to platelet activation has been uncertain. Our group previously demonstrated that glycoprotein (GP) VI, an uncharacterized platelet membrane protein, is specifically required for collagen-platelet interaction leading to activation of protein-tyrosine kinase Syk. Since collagen stimulation of platelets has recently been found to induce tyrosine phosphorylation of Fc receptor (FcR) gamma-chain, a signal-generating subunit of FcR, we further investigated the relationships between FcR gamma-chain and GPVI in human platelets. Our present study revealed the following. FcR gamma-chain was physically and stably associated with GPVI in human platelets; both FcR gamma-chain and GPVI were proportionally absent in GPVI-deficient platelets; GPVI cross-linking or collagen stimulation of platelets resulted in tyrosine phosphorylation of GPVI-associated FcR gamma-chain accompanied by Syk association and activation. These findings strongly suggest that the associated complex of GPVI and FcR gamma-chain is a collagen receptor featuring the signaling through immune receptors.
Highlights
The mechanism by which occupancy of collagen receptors is coupled to platelet activation has been uncertain
Since collagen stimulation of platelets has recently been found to induce tyrosine phosphorylation of Fc receptor (FcR) ␥-chain, a signal-generating subunit of FcR, we further investigated the relationships between FcR ␥-chain and GPVI in human platelets
We studied the following points: whether a complex of FcR ␥-chain and GPVI can be demonstrated in human platelets; how FcR ␥-chain is present in GPVI-deficient platelets; whether GPVI cross-linking of platelets with F(abЈ)2␣GPVI induces tyrosine phosphorylation of FcR ␥-chain and its association with Syk; and how GPVI is involved in the binding of the tandem Src homology 2 (SH2) domains of Syk to FcR ␥-chain in collagen-stimulated platelets
Summary
The mechanism by which occupancy of collagen receptors is coupled to platelet activation has been uncertain. We studied the following points: whether a complex of FcR ␥-chain and GPVI can be demonstrated in human platelets; how FcR ␥-chain is present in GPVI-deficient platelets; whether GPVI cross-linking of platelets with F(abЈ)2␣GPVI induces tyrosine phosphorylation of FcR ␥-chain and its association with Syk; and how GPVI is involved in the binding of the tandem SH2 domains of Syk to FcR ␥-chain in collagen-stimulated platelets.
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