The accumulation of intraneuronal aggregated tau protein underlies many common neurodegenerative diseases and is a key target in therapeutic development. Whilst promising in pre-clinical models, tau targeting antibodies have performed poorly in clinical trials, potentially due to low target engagement. Such antibodies rarely access the intracellular domain where the majority of tau aggregates reside. However, recent findings indicate that extracellular tau:antibody complexes can be internalised, whereupon they are bound and degraded by the intracellular Fc receptor and E3 ubiquitin ligase TRIM21. It was therefore hypothesised that intracellular aggregated tau could be targeted for degradation by exploiting this TRIM21 pathway.

This thesis describes the development of TRIMinators, vectored constructs designed from the minimal antibody and TRIM21 domains required for the intracellular degradation of aggregated tau. These TRIMinators consist of an anti-tau nanobody fused to the E3 ubiquitin ligase domain of TRIM21. This E3 ligase activity is initiated by intermolecular clustering, which was hypothesised to allow for the selective degradation of multivalent aggregated tau:TRIMinator complexes whilst leaving physiological monomeric tau unaffected.

In cell line models, TRIMinator expression protects against seeded tau aggregation and leads to the rapid and selective degradation of pre-existing aggregates, leaving monomeric tau unaffected and reducing the levels of seed competent tau species. This activity is dependent on the ubiquitin proteasome system (UPS), with the selective degradation of aggregated tau over monomeric species being determined by the affinity of the anti-tau nanobody. Delivered via adeno-associated viral (AAV) vectors, TRIMinator expression ablates seeded tau aggregation in primary neuronal models and reduces levels of tau pathology in aged transgenic P301S tau mice ten days after administration.

The TRIMinator constructs described in this thesis are not only capable of inhibiting the aggregation of tau protein, but also remove pre-formed aggregates whilst leaving physiological monomeric tau unaffected. This system of selective aggregate degradation could be exploited in the development of future therapeutics not only for tauopathies, but for the various other neurodegenerative proteinopathies driven by pathological protein aggregation.