Inhomogeneous Tau polymerization, core-shell organization, and seed formation during Tau condensate aging

Several proteins aggregating in neurodegenerative diseases spontaneously segregate into liquid condensates, which can catalyze protein aggregation. How liquid-solid transitions are catalyzed in the confined condensate volume is not clear. For the microtubule associated protein Tau, aggregating intra-neuronally in Alzheimer's disease, we show that, during maturation, Tau condensates convert into elastic protein networks, accompanied by inhomogeneous polymerization of the condensate interior and the formation of high-density nodes and a 'shell'. During condensation, Tau molecules extend, favoring intermolecular interactions and priming for progressive parallel Tau arrangement that can enable amyloid-like Tau oligomerization. In cells, aged condensates seed small Tau clusters in cytosol and at the nuclear envelope, precursors of larger aggregates. By bridging molecular to condensate level, we present mechanistic insight into how Tau condensates evolve into pathological, beta-structure containing seeds. The interior of aged Tau condensates remains accessible for smaller molecules, providing the opportunity to molecularly target Tau seed formation inside condensates.