In VitroCharacterization of Pittsburgh Compound-B Binding to Lewy Bodies

Abstract

Dementia with Lewy bodies (DLB) is pathologically characterized by the presence of α-synuclein-containing Lewy bodies within the neocortical, limbic, and paralimbic regions. Like Alzheimer's disease (AD), Aβ plaques are also present in most DLB cases. The contribution of Aβ to the development of DLB is unclear. [¹¹C]-Pittsburgh compound B ([¹¹C]-PIB) is a thioflavin-T derivative that has allowedin vivoAβ burden to be quantified using positron emission tomography (PET). [¹¹C]-PIB PET studies have shown similar high cortical [¹¹C]-PIB binding in AD and DLB subjects. To establish the potential binding of PIB to α-synuclein in DLB patients, we characterized thein vitrobinding of PIB to recombinant human α-synuclein and DLB brain homogenates. Analysis of thein vitrobinding studies indicated that [³H]-PIB binds to α-synuclein fibrils but with lower affinity than that demonstrated/reported for Aβ_1–42fibrils. Furthermore, [³H]-PIB was observed to bind to Aβ plaque-containing DLB brain homogenates but failed to bind to DLB homogenates that were Aβ plaque-free (“pure DLB”). Positive PIB fluorescence staining of DLB brain sections colocalized with immunoreactive Aβ plaques but failed to stain Lewy bodies. Moreover, image quantification analysis suggested that given the small size and low density of Lewy bodies within the brains of DLB subjects, any contribution of Lewy bodies to the [¹¹C]-PIB PET signal would be negligible. These studies indicate that PIB retention observed within the cortical gray matter regions of DLB subjects in [¹¹C]-PIB PET studies is largely attributable to PIB binding to Aβ plaques and not Lewy bodies.