Structural characterization of tau conformational heterogeneity in Down syndrome

ABSTRACT Down syndrome (DS) is the most common chromosomal disorder of intellectual disability, affecting approximately 400,000 people in the United States and approximately 5.5 million people worldwide. DS results from the presence of a third copy of chromosome 21 (Chr21) and associated genetic dosage changes that cause varying and complex disabilities throughout development and adulthood. With recent increases in the lifespan of persons with DS, it is now realized that Alzheimer’s disease (AD) is a substantial comorbidity in individuals with DS that arises at a much earlier age. Nearly all adults with DS develop severe AD neuropathological changes by age 40; however, there is evidence that the first appearance of amyloid-β plaques and tau neurofibrillary tangles (NFTs) occurs as early as the teens or twenties. Cryo-electron microscopy (cryo-EM) structural studies have revealed that the tau protein adopts a myriad of polymorphic folds that are associated with distinct etiological forms of tauopathy diseases across the spectrum of AD and related disorders (ADRDs). Recently, our novel workflow of in situ conformational assays and cryo-EM methods applied in aged patient samples revealed that DS tau adopts the AD folds of paired helical filaments and straight filaments. In our study, we developed a novel method using custom-made graphene-oxide (GO) grids functionalized with the anti-phospho tau “AT8” antibody to isolate and enrich filaments directly on cryo-EM sample grids. Using GO-AT8 grids, we isolated tau fibrils from a 36-year- old DS case with no dementia and few tau NFTs, thus demonstrating the potential for characterizing the structure of tau filaments at the onset of AD. Our proposal builds on our preliminary data and evidence in the literature suggesting there may be greater heterogeneity of tau filament structures at the inception of tau deposition and with certain neurological comorbidities, such as seizures and self-injurious, recurring head trauma. We hypothesize such comorbid conditions may lead to patterns of tau histopathology associated with chronic traumatic encephalopathy (CTE) and tau filaments with the CTE fold along with AD morphologies in a subset of individuals with DS. To facilitate the rapid success of this work, we will use a cohort of DS samples already in our labs via ongoing collaboration to screen a large number of fixed samples and prioritize a small number of frozen samples for cryo-EM. Further, we will expand our novel immunoaffinity approaches using tau conformation-specific antibodies to isolate putatively distinct tau conformers prior to cryo-EM. Therefore, we will implement the fluorescence imaging and cryo-EM approaches as described in our funded RF1NS133651 grant to pursue these currently unfunded objectives as a supplemental project. This proposal aligns with the major goals of our parent award and addresses Component 1 in the NIH INCLUDE Project’s notice of special interest (NOT-OD-22-137). Successful outcomes from this supplemental work will contribute to the preparation of a new R01 application to the INCLUDE Project, in which we develop structure-guided molecular diagnostics to more precisely define the heterogeneous tau neuropathologies that arise across the lifespan in Down syndrome.