In vitro characterization of Pittsburgh compound-B binding to Lewy bodies

Dementia with Lewy bodies ( DLB) is pathologically characterized by the presence of alpha-synuclein-containing Lewy bodies within the neocortical, limbic, and paralimbic regions. Like Alzheimer's disease ( AD), A beta plaques are also present in most DLB cases. The contribution of A beta to the development of DLB is unclear. [C-11]-Pittsburgh compound B ([C-11]-PIB) is a thioflavin-T derivative that has allowed in vivo A beta burden to be quantified using positron emission tomography ( PET). [ 11C]-PIB PET studies have shown similar high cortical [C-11]-PIB binding in AD and DLB subjects. To establish the potential binding of PIB to alpha-synuclein in DLB patients, we characterized the in vitro binding of PIB to recombinant human alpha-synuclein and DLB brain homogenates. Analysis of the in vitro binding studies indicated that [H-3]-PIB binds to alpha-synuclein fibrils but with lower affinity than that demonstrated/ reported for A beta(1-42) fibrils. Furthermore, [H-3]-PIB was observed to bind to A beta plaque-containing DLB brain homogenates but failed to bind to DLB homogenates that were A beta plaque-free ("pure DLB"). Positive PIB fluorescence staining of DLB brain sections colocalized with immunoreactive A beta 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 A beta plaques and not Lewy bodies.