Evaluation of prion deposits and microglial activation in scrapie-infected mice using molecular imaging probes.

PURPOSE: A characteristic of prion diseases which affect both animals and humans is the aggregation of PrP amyloid fibrils in the brain, associated with a chronic inflammatory response dominated by microglial activation. In this study, we hypothesised that specific ligands of the 18-kDa translocator protein (TSPO) would be effective in the evaluation of microglial activation related to PrP(sc) deposits in prion disease. PROCEDURES: Chronological studies using in vitro autoradiography were carried out with [(3)H]-PK11195 and [(125)I]-IMPY on frozen cerebral sections from scrapie-infected mice and controls. Accumulation of prion deposits was confirmed by histoblot staining with prion protein-specific monoclonal antibody. Ex vivo autoradiographic studies were carried out with [(125)I]-CLINDE and [(125)I]-IMPY at the terminal stage of infection. RESULTS: Chronological studies using in vitro autoradiography showed that PrP(sc) deposits were co-localised with activated microglia as early as 60 days post-inoculation. Progressive levels of PrP(sc) and TSPO staining were successively observed in the hippocampus, cortex and left thalamus of infected mouse brain sections in the course of the disease and were correlated with the signals obtained by histoblot staining. Significant TSPO labelling was also observed ex vivo in the cortex, hippocampus and thalamus of scrapie-infected mice. In parallel, [(125)I]-IMPY showed labelling in the same cerebral regions but with high background staining. CONCLUSIONS: These findings indicate the ability of [(125)I]-IMPY and [(125)I]-CLINDE to evaluate prion deposits and microglial activation in vitro and ex vivo in scrapie-infected mice at different stages of the disease.

IMPY, a potential beta-amyloid imaging probe for detection of prion deposits in scrapie-infected mice.

INTRODUCTION: A potential single-photon emission computed tomography imaging agent for labeling of A beta plaques of Alzheimer's disease, IMPY (2-(4'-dimethylaminophenyl)-6-iodo-imidazo[1,2-a]pyridine), would be effective in detection of prion amyloid deposits in transmissible spongiform encephalopathies (TSEs). METHODS: In vitro autoradiographic studies were carried out with [125 I]IMPY on brain sections from scrapie-infected mice and age-matched controls. Competition study was performed to evaluate the prion deposit binding specificity with nonradioactive IMPY. RESULTS: Binding of [125 I]IMPY was observed in infected brain sections, while on age-matched control brain sections, there was no or very low labeling. Prion deposit binding was confirmed by histoblots with prion protein-specific monoclonal antibody 2D6. In the presence of nonradioactive IMPY, the binding of [125 I]IMPY was significantly inhibited in all regions studied. CONCLUSIONS: These findings indicate that IMPY can detect the prion deposits in vitro in scrapie-infected mice. Labeled with 123 I, this ligand may be useful to quantitate prion deposit burdens in TSEs by in vivo imaging.