Objective— Protein misfolding diseases result from the deposition of insoluble protein aggregates that often contain fibrils called amyloid. Amyloids are found in Alzheimer disease, atherosclerosis, diabetes mellitus, and systemic amyloidosis, which are diseases where platelet activation might be implicated.

Methods and Results— We induced amyloid properties in 6 unrelated proteins and found that all induced platelet aggregation in contrast to fresh controls. Amyloid-induced platelet aggregation was independent of thromboxane A₂ formation and ADP secretion but enhanced by feedback stimulation through these pathways. Treatments that raised cAMP (iloprost), sequestered Ca²⁺ (BAPTA-AM) or prevented amyloid-platelet interaction (sRAGE, tissue-type plasminogen activator [tPA]) induced almost complete inhibition. Modulation of the function of CD36 (CD36^−/− mice), p38^MAPK (SB203580), COX-1 (indomethacin), and glycoprotein Ibα (Nk-protease, 6D1 antibody) induced ≈50% inhibition. Interference with fibrinogen binding (RGDS) revealed a major contribution of α_IIbβ₃-independent aggregation (agglutination).

Conclusions— Protein misfolding resulting in the appearance of amyloid induces platelet aggregation. Amyloid activates platelets through 2 pathways: one is through CD36, p38^MAPK, thromboxane A₂–mediated induction of aggregation; the other is through glycoprotein Ibα–mediated aggregation and agglutination. The platelet stimulating properties of amyloid might explain the enhanced platelet activation observed in many diseases accompanied by the appearance of misfolded proteins with amyloid.