P4-229: tau antibody-mediated prevention of seeding of tau pathology and associated toxicity

Tau immunotherapy has great potential for Alzheimer's disease and other tauopathies but the mechanisms of tau clearance are not well known. We recently reported on two novel tau antibodies targeting the 396/404 region, 4E6G7 and 6B2G12, which are primarily taken up by neurons and reduce tau protein pathology in long term brain slice cultures (Gu J et al, JBC 288, 33081, 2013). Further characterization of one of these antibodies, 4E6G7, revealed that antibody uptake into neurons occurs primarily via clathrin-dependent Fcgamma receptor endocytosis, and is a prerequisite for acute tau protein clearance (Congdon EE et al, JBC 288, 35452, 2013). To test the efficacy of these antibodies to prevent propagation of tau pathology, primary neurons were incubated with 1 or 10 Âμg/ml of Alzheimer's derived paired helical filament (PHF) enriched material, with the tau antibodies (1 Âμg/ml) added concurrently, or 24 h before or after PHF administration. Tau and NeuN levels were then assessed for up to 7 days. Incubation with 1 m g/ml PHF increased total tau levels by 59% over 72 h (p<0.001), which was accompanied by a shift towards higher molecular weight tau proteins. 4E6G7, added with or 24 h after PHF, prevented this increase in tau protein levels. In contrast, 6B2G12 was not effective under these conditions (69% and 76% increase in tau protein levels, respectively, p>0.001). Ten times higher PHF concentration had toxic effects with 89% loss of NeuN signal over 7 days (p=0.002), associated with 40% decrease in tau levels up to 48 h (p=0.006), after which they steadily increased. 4E6G7 prevented NeuN signal loss when added with or 24 h after PHF, whereas neurons incubated with PHF and 6B2G12 showed NeuN loss comparable to cells treated with PHF alone (77-81%, p=0.01-0.002). Adding the antibodies 24 h before 10 Âμg/ml of PHF had no effect, presumably because they are not retained in the absence of tau aggregates. These data show differing efficacy of tau antibodies to reduce intracellular tau and provide protection from exogenous tau. This culture model is useful to identify promising therapeutic antibodies and to clarify their mediated clearance of tau pathology.