We use biophysical approaches to dissect mechanisms of protein aggregation and spread, biochemistry techniques to correlate biological activity with aggregate structure, chemical biology methods to develop small-molecule probes for detection of individual aggregate conformers, and bioinformatics to identify gene expression patterns associated with resistance to tauopathy.
Initiation of tau lesion formation
The foundational observation is that tauopathies convert intrinsically disordered tau monomers into aggregates having thread-like (i.e., filamentous) morphology. The fundamental unit of aggregate organization is the “protomer”, which corresponds to a tau protein molecule when so incorporated. Although protomers interact to form parallel in-register β-sheets extending parallel to the long axis of the aggregates, the specific folding pattern adopted within the core of each protomer differs in each tauopathy. This variability is termed “polymorphism”, and each distinct protofilament structural variant is termed a “conformer” or “polymorph”. Interestingly, the majority of disease polymorphs are accompanied by anionic substances that stably associate with specific amino acid side chains. A long-term focus of this laboratory is to determine how tau post-translational modifications and cellular factors interact to foster aggregation and modulate formation of disease-specific polymorphs.
Composition and bioactivity of tau aggregates
In addition to filamentous aggregates, a growing body of evidence implicates soluble/diffusible oligomeric tau aggregates as being acutely bioactive at early disease stages. For example, tau oligomers can induce synaptic and mitochondrial dysfunction in animal models. Nonetheless, because of their relatively recent discovery, oligomeric tau species have not been characterized and replicated to the same degree as filamentous aggregates. As a result, the structures of oligomers that exist in human disease and associate most closely with disease propagation and toxicity are not fully established. Moreover, the impact of copathologies such as Aβ, α-synuclein and TDP43 proteins on the biological activity of any tau species is incompletely understood. A second goal of our lab is to isolate oligomeric tau aggregates from human brain and rigorously characterize them with respect to structure, biological activity and seeding propensity.
Detection of aggregates with small-molecules
Advances in detection of tau aggregates have culminated in regulatory approval of radiotracers for detection of neurofibrillary lesions in living cases through whole-brain imaging. The field is now poised to address the mechanism(s) underlying ligand binding to tau aggregates and the feasibility of creating new imaging agents with affinity and selectivity for individual polymorphs/diseases. Classic ligands such as fluorescent probe Thioflavin T bind cross-β-sheet structure with their long axes oriented within “channels” formed between two side chains along the length of the β-sheet. We employ structural and computational approaches to interrogate this model while providing insight into how affinity and site selectivity can be achieved.
Cellular mechanisms linked to tauopathy resistance
Although prion-like seeding can spread tau aggregates to naïve cell populations, certain neurons appear less vulnerable than others. These cells resist aggregation of tau protein and degeneration of their cell bodies even when exposed to tau seeds throughout the course of disease. To identify molecular correlates of resistance, we have developed an approach for disaggregating gene expression data on the basis of regional vulnerability to tauopathy in human brain. A long term focus of the lab is to use this approach to identify new targets for drug discovery predicated on their ability to promote resistance to disease in otherwise vulnerable neurons.