Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.4.21.6 (thromboplastin)
 13,278 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The role of calcium and intracellular calpains in the expression of platelet prothrombinase activity was investigated. Incubation of gel-filtered platelets with complement proteins C5b-9 resulted in alpha-granule and dense granule secretion and exposure of membrane binding sites for coagulation factors Va and Xa. This was accompanied by the release of microparticles from the cell surface that incorporated plasma membrane glycoproteins GP Ib, IIb, and IIIa and the alpha-granule membrane protein GMP-140. Generation of these membrane microparticles was dependent on the presence of extracellular calcium and was accompanied by proteolytic degradation of the cytoskeletal proteins, actin binding protein (ABP), talin, and myosin heavy chain. Microparticle formation was also detected when unstirred platelets were activated by thrombin plus collagen, although proteolysis of ABP, talin, or myosin was not observed. Preincorporation of the calpain inhibitor leupeptin into the platelet cytosol completely blocked C5b-9-induced proteolysis of ABP, talin, and myosin. However, inhibition of this calpain-mediated proteolysis had no effect on platelet secretion, the generation of microparticles, the exposure of membrane sites for factors Va and Xa, or the expression of prothrombinase activity. Furthermore, the microparticles that formed in the presence of leupeptin contained intact ABP, talin, and myosin heavy chain. Prior depletion of ATP with metabolic inhibitors eliminated all platelet responses to thrombin plus collagen, but did not affect C5b-9-induced microparticle formation or exposure of binding sites for factor Va on the microparticles. These data indicate that the formation of microparticles and the expression of platelet prothrombinase activity in response to C5b-9 are dependent upon an influx of calcium into the platelet cytosol, but do not require metabolic energy or calpain-mediated proteolysis of cytoskeletal proteins.
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PMID:Role of calcium and calpain in complement-induced vesiculation of the platelet plasma membrane and in the exposure of the platelet factor Va receptor. 215 84

The possibility that platelets release microvesicles on adherence to either von Willebrand factor (vWf) or collagen was examined by flow cytometry analysis of the supernatant above layers of adherent platelets. No microvesicle release was detected as a result of adherence to vWf or to collagen, a known platelet agonist. Approximately 8% of the total platelet mass was released as microvesicles after thrombin stimulation of the vWf- or collagen-adherent platelets. A larger portion of the vWf-adherent platelet membranes (approximately 21%) was released as microvesicles subsequent to platelet stimulation with the nonphysiological agonist calcium ionophore A23187. Calpeptin, a calpain inhibitor, had no effect on microvesicle release, suggesting that calpain proteolysis of platelet cytoskeletal proteins was not responsible for microvesicle shedding under the conditions studied. Examination of the vWf-adherent platelets by scanning electron microscopy showed that virtually no microvesicles bound to exposed vWf multimers. No microvesicle binding to the adherent platelets was observed, indicating that the majority of the microvesicles were shed from the platelet and vWf surface on platelet activation. The ability of the microvesicle population to support procoagulant activity was measured with a prothrombinase activity assay and was compared with the activity supported by the adherent platelet membranes. More than 85% of the total prothrombinase activity remained associated with the adherent platelet membranes, both for unstimulated platelets and platelets stimulated with physiological agonists. Furthermore, the residual activity found in the buffer fraction containing detached platelets and any released microvesicles could be attributed to the detached platelets. No activity could be attributed to the microvesicles, as thrombin stimulation of either vWf-or collagen-adherent platelets did not promote increased procoagulant activity relative to the unstimulated adherent platelets, even though microvesicle release was detected as a result of agonist addition. Neither full platelet activation nor microvesicle shedding played an essential role in generating procoagulant activity in the adherent platelet system.
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PMID:Intact platelet membranes, not platelet-released microvesicles, support the procoagulant activity of adherent platelets. 821 2