Conformation-based detection of tau seeds with a novel VHH

In Alzheimer’s disease (AD) and related tauopathies, progressive pathology has been linked to prion mechanisms whereby ordered tau assemblies, or “seeds,” form in one cell and transit to neighboring or connected cells where they serve as templates for their own replication. While post-translational modifications of pathological tau have been useful to mark pathology, effective methods to accurately detect and target pathogenic seed conformations remain limited. We report a novel discovery and characterization paradigm to identify camelid variable heavy domain of heavy chain (VHH) sequences with desired properties. From a published synthetic VHH yeast display library, we screened for clones capable of immunoprecipitating tau seeds from human tauopathy brain homogenates and identified two seed-selective anti-tau VHHs – VHH(510) and VHH(50) – that target pathological tau. These VHHs preferentially target tau seeds present in AD, corticobasal degeneration (CBD), and PS19 tauopathy mouse brains. We enhanced their stability through framework mutations (M), while maintaining their seed-binding characteristics. We characterized VHH(510M) in detail, determining that it bound the carboxy terminus of tau with robust avidity for seeds, and low affinity for monomer. When used to stain mouse and human brain tissues, VHH(510M) revealed pathological tau accumulation that was often independent of AT8-positive lesions. The distinct staining pattern observed with anti-tau VHH(510M) underscores the potential of VHH-based reagents for PET imaging and histopathology. Adaptation of VHH(510M) could also improve therapy and diagnosis for diverse tauopathies.