Role of in vitro cholesterol depletion in mediating human platelet aggregation

Summary.  We investigated the direct role of cholesterol lowering on human platelet aggregation by in vitro cholesterol depletion using methyl-β-cyclodextrin. Collagen and thrombin receptor agonist peptide induced maximal aggregation was significantly decreased in cholesterol depleted platelets. In contrast, anti-CD9 antibody, mAb7, or anti-β₃ antibody, D3, induced percent maximal aggregation was unaffected by cholesterol depletion. Surface and total α_IIbβ₃ levels were equivalent in both groups. Morphological and ultrastructural analysis of collagen induced aggregates revealed that normal and cholesterol depleted platelets changed shape and aggregated; however, cholesterol depletion impaired microtubule ring formation and aggregate size. Cholesterol depletion also diminished the extent of the open canalicular system and collagen induced platelet ATP release. These data suggest cholesterol depletion impairs platelet aggregation by altering platelet ultrastructure critical in mediating secretion. Temporal differences and differences in tyrosine phosphoprotein levels following collagen stimulation were observed, thereby indicating that platelet signaling was concurrently affected by cholesterol depletion.