Misfolding, aggregation, and cerebral accumulation of tau deposits are hallmark features of Alzheimer’s disease. Positron emission tomography study of tau can facilitate the development of anti-tau treatment. Here, we investigated a novel tau tracer ¹⁸F-PM-PBB3 (¹⁸F-APN-1607) in a mouse model of tauopathy. Dynamic PET scans were collected in groups of rTg4510 transgenic mice at 2–11 months of age. Associations between distribution volume ratios (DVR) and standardized uptake value ratios (SUVR) with cerebellum reference were used to determine the optimal scanning time and uptake pattern for each age. Immunohistochemistry staining of neurofibrillary tangles and autoradiography study was performed for ex vivo validation. An SUVR 40–70 min was most consistently correlated with DVR and was used in further analyses. Significant increased ¹⁸F-PM-PBB3 uptake in the brain cortex was found in six-month-old mice (+28.9%, p < 0.05), and increased further in the nine-month-old group (+38.8%, p < 0.01). The trend of increased SUVR value remained evident in the hippocampus and striatum regions except for cortex where uptake becomes slightly reduced in 11-month-old animals (+37.3%, p < 0.05). Radioactivity distributions from autoradiography correlate well to the presence of human tau (HT7 antibody) and hyperphosphorylated tau (antibody AT8) from the immunohistochemistry study of the adjacent brain sections. These findings supported that the 40–70 min ¹⁸F-PM-PBB3 PET scan with SUVR measurement can detect significantly increased tau deposits in a living rTg4510 transgenic mouse models as early as six-months-old. The result exhibited promising dynamic imaging capability of this novel tau tracer, and the above image characteristics should be considered in the design of longitudinal preclinical tau image studies.

Numerous tau immunotherapies are being developed against Alzheimer’s disease (AD), but it has been challenging to specifically target early-stage tau aggregates using conformation-dependent antibodies. Here, we report a monoclonal antibody, APNmAb005, that recognized a conformational epitope associated with tau oligomers. In AD brain extracts, mAb005 preferentially recognized oligomeric tau in the synapse over monomeric tau in the cytosol. In the prefrontal cortex and hippocampus, mAb005 immunoreactivity was strongly present in early-stage AD but surprisingly diminished in late-stage AD (Braak stage VI). mAb005 also recognized aggregates in 3R tauopathies (Pick’s disease) and 4R tauopathies (corticobasal degeneration and progressive supranuclear palsy), including those in astrocytes and oligodendrocytes. In rTg4510 mice (P301L tau), mAb005 immunoreactivity first appeared in distal neurites but much later in neuronal somas. Thus, the mAb005 epitope appears to be associated with early-stage oligomers of tau (esoTau) that accumulate around synapses in AD, which is also detectable in both 3R and 4R tauopathies. In cellular uptake models of tauopathy transmission, mAb005 blocked the formation of intracellular inclusions induced by incubation with rTg4510 mouse brain extracts. Long-term treatments with mAb005 in rTg4510 mice partially rescued synaptic and neuronal loss in the hippocampus without promoting overall tau clearance. Our data suggest that immunotherapies targeting esoTau enriched around synaptic sites may alleviate tau toxicity against synapses and neurons, which may be a promising treatment strategy against AD. One Sentence Summary A tau-conformer antibody recognizing synaptic oligomers and 3R, 4R, and mixed aggregates in humans rescues neuronal loss in mouse tauopathy models.