OBJECTIVETo detect the redistribution of platelet surface glycoprotein (GP)Ib alpha and cytoskeleton reorganization in the course of thrombin receptor activation, and investigate the mechanism of GPIb alpha re-translocation and the role of thrombin receptors in platelet signal transduction.

METHODSThe thrombin receptor activating peptide (PAR1-AP, TRAP) was used for stimulating platelet at different time points (0 - 60 min), then the platelet surface GPIb alpha and P-selectin were examined with flow cytometry, and the alterations of GPIb alpha, actin and myosin were analyzed in cytoskeleton by Western blot and GPIb alpha immunoprecipitation. Cytochalasin D and/or Apyrase VII were used for investigating their inhibitory effect on platelet activation.

RESULTSAn increase of P-selectin and reversible internalization of GPIb alpha were observed within platelets upon TRAP activation, and transient changes of actin, myosin and GPIb alpha/myosin, GPIb alpha/actin association were also found in this course. These changes were apparently blocked by cytochalasin D, which inhibited the incorporation of GPIb alpha, actin and myosin into cytoskeleton. Apyrase VII had a weak effect on GPIb alpha internalization, although it accelerated the return of GPIb alpha to platelet surface. In addition, Apyrase VII also quickened the GPIb alpha disappearance in cytoskeleton and the dissociation of GPIb/myosin or GPIb/actin during activation.

CONCLUSIONThrombin receptor activation takes part in platelet signal transduction, inducing a reversible redistribution of GPIb alpha. This process is related to cytoskeleton reorganisation and ADP.

Actins ; metabolism ; Blood Platelets ; cytology ; drug effects ; metabolism ; Blotting, Western ; Cells, Cultured ; Cytoskeleton ; metabolism ; Humans ; Myosins ; metabolism ; P-Selectin ; metabolism ; Peptide Fragments ; pharmacology ; Platelet Activation ; drug effects ; physiology ; Platelet Glycoprotein GPIb-IX Complex ; metabolism ; Receptors, Thrombin ; metabolism ; physiology

This study was designed to compare the effects of protease-activated receptor 1 (PAR-1) and protease-activated receptor 4 (PAR-4) to the expression of platelet surface GPIbalpha and cytoskeleton reorganization, then to investigate the role of PARs in platelet signal transmission. PAR1 (25 micromol/L) and PAR4 (250 micromol/L) were used to stimulate platelet at different time points (0 - 60 minutes), and the platelet surface GPIbalpha, actin and myosin and P-selectin were detected with flow cytometry, the alteration of GPIbalpha, actin and myosin in cytoskeleton was compared by Western blot, the membrane cytoskeleton followed GPIbalpha immunoprecipitation was analyzed. The results showed that an increase of P-selectin and reversible decrease of GPIbalpha expression were obtained after platelet activation by PAR1 o r PAR4, and a different kinetics of redistribution of GPIbalpha was found for the two peptides all over the time course (P < 0.05). PAR1 acted more potently and rapidly than PAR4, but the effect of PAR4 persisted longer in the course of platelet activation. Meanwhile, there was a transient change of actin, myosin and GPIbalpha in cytoskeleton proteins. Similar redistribution was also found in GPIbalpha/myosin and GPIbalpha/actin association. It is concluded that PAR1 and PAR4 possess an important role in platelet signal transmission. Either of the receptors can mediate platelet activation and GPIbalpha redistribution, which is correlated with cytoskeleton reorganization. PAR1 acts more rapidly, and effect of PAR4 persists longer.
Cytoskeleton ; chemistry ; Flow Cytometry ; Humans ; Myosins ; analysis ; P-Selectin ; analysis ; Platelet Activation ; Platelet Glycoprotein GPIb-IX Complex ; Receptor, PAR-1 ; physiology ; Receptors, Thrombin ; physiology