Interaction of Tau Protein with Model Lipid Membranes Induces Tau Structural Compaction

The misfolding and aggregation of proteins into b-sheet rich fibrillar aggregates in vivo are linked to the pathogenesis of over twenty neurodegenerative diseases, including Alzheimer's disease. Alzheimer's disease is a progressive neurodegenerative disease characterized in part by neurofibrillary tangles (NFTs) found in the brains of affected patients.  NFTs are composed of misfolded aggregates of the intrinsically disordered microtubule associated protein tau. In order to aggregate into fibrils, the intrinsically disordered, highly soluble, and highly stable tau needs to undergo conformational changes that render the protein aggregation-competent or “pro-aggregant”.  The “pro-aggregant” form is currently unknown, but the formation of β-sheet enriched fibrils from tau likely proceeds through the formation of partially folded, or structurally compact, conformations with increased aggregation propensities. In vitro, aggregation can be induced by polyanionic cofactors such as heparin and anionic micelles or vesicles, which compensate for tau's positive charges.  This suggests that aggregation proceeds through a nucleation controlled polymerization pathway, but the molecular basis of the early aggregation events, such as the structural fluctuations that trigger the aberrant accumulation of tau into NFTs rich in β-sheets in vivo, remains unknown.This prompted our investigation to assess tau's propensity to interact with membranes and to elucidate the structural perturbations those interactions induce in the tau protein. We show that although highly charged and soluble, tau is also highly surface active.  Tau strongly interacts with anionic membranes and does not exhibit any favorable interaction with neutrally charged membranes.  To resolve molecular-scale structural details of protein associated with anionic lipid membranes, we utilized X-ray scattering techniques. X-ray reflectivity indicated tau's presence underneath an anionic DMPG monolayer at the air/water interface and penetration into the lipid headgroups and tailgroups. More significantly, both air/water and DMPG lipid membrane interfaces induce the disordered, molten globule-like, tau to partially adopt a more compact conformation with a density similar to that of a folded protein. To investigate the effect of tau-membrane interactions on tau fibril formation, we also incubate the protein with lipid vesicles containing either neutrally charged or anionic lipids.  Thioflavin-S binding assay is used to detect fibril formation and transmission electron microscopy is used to image the morphology of aggregates formed. Our results show that tau is highly surface active and strongly interacts with anionic lipid membranes, leading to tau structural compaction which may render the protein “pro-aggregant” or aggregation-competent.  This suggests possible membrane-based mechanisms of tau aggregation in neurodegenerative diseases.