Proteostasis as a fundamental principle of Tau immunotherapy
The microtubule-associated protein Tau is a driver of neuronal dysfunction in Alzheimers disease and numerous other tauopathies. In this process, Tau initially undergoes subtle changes to its abundance, subcellular localisation and a vast array of post-translational modifications including phosphorylation, that progressively result in the proteins aggregation and dysregulation of multiple Tau-dependent cellular processes. Given the various loss- and gain-of-functions of Tau in disease and the brain-wide changes in the proteome that characterise tauopathies, we asked whether targeting Tau would restore the alterations in proteostasis observed in disease. To this end, we generated a novel pan-Tau antibody, RNJ1, that preferentially binds human Tau and neutralises proteopathic seeding activity in multiple cell lines and benchmarked it against a clinically tested pan-Tau antibody, HJ8.5 (murine version of tilavonemab). We next evaluated both antibodies, alone and in combination, in the K3 mouse model of tauopathy, showing reduced Tau pathology and improvements in neuronal function following 14 weekly treatments, without obtaining synergistic effects for the combination treatment. To gain insight into molecular mechanisms contributing to improvements in neuronal function, we employed quantitative proteomics and phosphoproteomics to first establish alterations in K3 mice relative to WT controls at the proteome level. This revealed 342 proteins with differential abundance in K3 mice, which are predominantly involved in metabolic and microtubule-associated processes, strengthening previously reported findings of defects in several functional domains in multiple tauopathy models. We next asked whether antibody-mediated Tau target engagement indirectly affects levels of deregulated proteins in the K3 model. Importantly, both immunotherapies, in particular RNJ1, induced abundance shifts in this protein subset towards a restoration to wild-type levels (proteostasis). A total of 257 of 342 ([~]75.1%) proteins altered in K3 were closer in abundance to WT levels after RNJ1 treatment. The same analysis indicated a similar response in K3 mice treated with HJ8.5, with approximately 72.5% of these altered proteins also showing changes in the same direction as wild-type. Furthermore, analysis of the phosphoproteome showed an even stronger restoration effect with RNJ1, with [~]82.1% of altered phosphopeptides in K3 showing a shift to WT levels, and 75.4% with HJ8.5. Gene set over-representation analysis (ORA) further confirmed that proteins undergoing restoration are involved in biological pathways affected in K3 mice. Together, our study suggests that a Tau immunotherapy-induced restoration of proteostasis links target engagement and treatment efficacy.