Clearing truncated tau protein restores neuronal function and prevents microglia activation in tauopathy mice

Tau protein truncated at aspartate 421 (Asp421) is a characteristic feature of Alzheimers disease (AD) and other tauopathies. It is likely to have a role in their pathogenesis by promoting tau aggregation. Here, using two tauopathy mouse models, we show that a monoclonal antibody against Asp421, 5G2, led to a) a 59-74% clearance of insoluble tau protein in the brains of JNPL3 tauopathy mice following a thirteen-week treatment period, b) a 46% decrease of tau levels in brain interstitial fluid immediately following a single dose of 5G2 as examined by brain microdialysis in awake JNPL3 mice, c) improved neuronal function and d) reduced microglial activation as determined by two-photon imaging in awake PS19 tauopathy mice, where we also found tau accumulation earlier than signs of microglial activation. For mechanistic insight using culture models, 5G2 prevented toxicity of AD brain-derived pathological tau protein, cleared intracellular tau, and prevented microgliosis. We also knocked down the intracellular Fc receptor and ubiquitin E3 ligase, TRIM21, and found a reduction in cellular retention of tau antibodies, which appeared to reduce the acute efficacy (24 h) of tau antibodies but not their longer-term efficacy (5 days). Overall, these findings strongly support the feasibility of targeting Asp421 truncated tau protein to treat tauopathies, indicate that tau-associated abnormalities of neuronal activity precede microglial activation and that antibody-mediated tau clearance via the TRIM21 pathway is mostly transient.