PROJECT SUMMARY Tau aggregation is a shared pathology of Alzheimer's disease and related dementias known as tauopathies. Because each disorder develops a unique combination of aggregate polymorphisms and co-morbidities that may influence the course and severity of disease, elucidating the relationship between tau aggregate structure and biological activity has become a high priority in the field. Advances to date have been limited, however, by the lack of rigorously standardized aggregate preparations from each form of tauopathy and by the absence of tools needed to selectively detect their presence in biological models and clinical specimens. The proposed Interdisciplinary Research Network on Biologically Active Tau Aggregate Polymorphs seeks to address this need by (1) isolating a full range of biologically active tau aggregates from authentic Alzheimer's disease, Corticobasal Degeneration, Progressive Supranuclear Palsy, and Pick's disease brain tissue, (2) establishing their structures through biophysical analysis, (3) developing probe sets for their selective detection, and (4) disseminating reliably vetted samples and lab-ready protocols for their handling and storage to the broader research community. In accomplishing these goals, the network will support research into the molecular basis of tauopathy pathogenesis and catalyze efforts toward creating novel diagnostic and therapeutic strategies specifically tailored against toxic tau aggregate polymorphs.