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Specialized membrane areas in non-activated and thrombin-activated platelets
Abstract:
Filipin, a complex of polyene antibiotics, forms morphologically distinctive complexes with cholesterol in cell membranes under proper experimental conditions. When applied to non-activated, discoid platelets, filipin-induced lesions (FIL) occurred in rows at the platelet equator, suggesting a specialized membrane organization at the platelets' largest circumference. In some thrombin-activated platelets we observed surface membrane blebbing and release of lipid vesicles that predominantly originated from the plasma membrane proper, but some originated from (unidentified) platelet granules. FIL were initially present in high numbers over the entire bleb, they accumulated later at the neck of blebs, while the released vesicle was free of FIL. Absence of intramembrane protein particles (IMP) from the membranes of blebs and vesicles suggests that released vesicles are essentially without cholesterol and intrinsic membrane proteins and may consist predominantly of phospholipids. Membrane blebbing and vesicle release may represent unmasking and release of procoagulant platelet factor 3 activity.
Insights
Filipin, a polyene antibiotic, reveals specialized membrane organization in platelets. Filipin-induced lesions show distinct patterns on resting and activated platelets, indicating unique cholesterol distribution and potential release of procoagulant factors.
Area of Science:
- Cell Biology
- Membrane Biology
- Biochemistry
Background:
- Polyene antibiotics like filipin interact specifically with cholesterol in cell membranes.
- Platelets possess a complex membrane structure involved in hemostasis and thrombosis.
- Understanding platelet membrane organization is crucial for elucidating their function.
Purpose of the Study:
- To investigate the distribution of cholesterol in platelet membranes using filipin.
- To characterize the morphology of filipin-induced lesions (FIL) on resting and activated platelets.
- To explore the origin and composition of vesicles released during platelet activation.
Main Methods:
- Application of filipin to non-activated and thrombin-activated human platelets.
- Morphological analysis of filipin-induced lesions (FIL) using microscopy.
- Observation of membrane blebbing and vesicle release events.
- Analysis of intramembrane protein particles (IMP) in blebs and vesicles.
Main Results:
- FIL appeared in rows at the equator of non-activated discoid platelets, suggesting specialized membrane organization.
- Activated platelets exhibited membrane blebbing and released lipid vesicles.
- FIL were initially abundant on blebs but concentrated at bleb necks, with released vesicles being FIL-free.
- Blebs and vesicles lacked intramembrane protein particles, indicating low cholesterol and protein content, primarily phospholipids.
Conclusions:
- Platelet membrane organization is specialized, particularly at the equator of resting platelets.
- Membrane blebbing and vesicle release in activated platelets may involve the unmasking and release of procoagulant platelet factor 3.
- Released vesicles may represent cholesterol-depleted structures, primarily composed of phospholipids.