Molecular crowding and RNA synergize to promote phase separation, microtubule interaction, and seeding of Tau condensates
Janine Hochmair, Christian Exner, Maximilian Franck, Alvaro Dominguez‐Baquero, Lisa Diez, H. Brognaro, Matthew L. Kraushar, Thorsten Mielke, Helena Radbruch, Senthilvelrajan Kaniyappan, Sven Falke, Eckhard Mandelkow�, Christian Betzel, Susanne Wegmann
Abstract
Biomolecular condensation of the neuronal microtubule‐associated protein Tau (MAPT) can be induced by coacervation with polyanions like RNA, or by molecular crowding. Tau condensates have been linked to both functional microtubule binding and pathological aggregation in neurodegenerative diseases. We find that molecular crowding and coacervation with RNA, two conditions likely coexisting in the cytosol, synergize to enable Tau condensation at physiological buffer conditions and to produce condensates with a strong affinity to charged surfaces. During condensate‐mediated microtubule polymerization, their synergy enhances bundling and spatial arrangement of microtubules. We further show that different Tau condensates efficiently induce pathological Tau aggregates in cells, including accumulations at the nuclear envelope that correlate with nucleocytoplasmic transport deficits. Fluorescent lifetime imaging reveals different molecular packing densities of Tau in cellular accumulations and a condensate‐like density for nuclear‐envelope Tau. These findings suggest that a complex interplay between interaction partners, post‐translational modifications, and molecular crowding regulates the formation and function of Tau condensates. Conditions leading to prolonged existence of Tau condensates may induce the formation of seeding‐competent Tau and lead to distinct cellular Tau accumulations. Condensation of the microtubule‐associated protein Tau is induced by the presence of polyanions like RNA or by molecular crowding. Here we show that these conditions have synergistic effects on Tau condensation and Tau‐dependent microtubule bundling in vitro, and on the seeding of Tau aggregates in cells. Conditions that induce liquid‐liquid phase separation have distinct effects on Tau condensation and subcellular localization, including formation of Tau‐positive inclusions along the nuclear envelope that correlate with nucleocytoplasmic transport deficits.