The secretory mechanisms in equine platelets are independent of cytoskeletal polymerization and occur through membrane fusion.
Studies in animal models are useful to understand the basic mechanisms involved in hemostasis and the functional differences among species. Ultrastructural observations led us to predict differences in the activation and secretion mechanisms between equine and human platelets. The potential mechanisms involved have been comparatively explored in the present study. Equine and human platelets were activated with thrombin (0.5 U/ml) and collagen (20 µg/ml), for 90 seconds, and samples processed to evaluate: i) ultrastructural changes, by electron microscopy, ii) actin polymerization and cytoskeletal assembly, by polyacrylamide gel electrophoresis, and iii) specific molecules involved in activation and secretion, by western blot. In activated human platelets, centralization of granules, cytoskeletal assembly and fusion of granules with the open canalicular system were observed. In activated equine platelets, granules fused together forming an organelle chain that fused with the surface membrane and released its content directly outside the platelets. Human platelets responded to activation with actin polymerization and the assembly of other contractile proteins to the cytoskeleton. These events were almost undetectable in equine platelets. When exploring the involvement of the synaptosomal-associated protein-23 (SNAP-23), a known regulator of secretory granule/plasma membrane fusion events, it was present in both human and equine platelets. SNAP-23 was shown to be more activated in equine platelets than human platelets in response to activation, especially with collagen. Thus, there are significant differences in the secretion mechanisms between human and equine platelets. While in human platelets, activation and secretion of granules depend on mechanisms of internal contraction and membrane fusion, in equine platelets the fusion mechanisms seem to be predominant.
Hemoterapia-Hemostasia, CDB, Hospital Clínic, IDIBAPS, Universitat de Barcelona.
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Brunso
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Segura
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Monreal
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Escolar
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White
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Diaz-Ricart
D000199:Actins / Q000378:metabolism; D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure*; D003094:Collagen / Q000494:pharmacology; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D006736:Horses; D006801:Humans; D008561:Membrane Fusion / Q000502:physiology*; D015539:Platelet Activation / Q000187:drug effects; D058105:Polymerization; D050766:Qb-SNARE Proteins / Q000378:metabolism; D050767:Qc-SNARE Proteins / Q000378:metabolism; D013917:Thrombin / Q000494:pharmacology
D000199:Actins; D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; C100493:SNAP23 protein, human; D003094:Collagen; D013917:Thrombin