Molecular crowding and RNA synergize to promote phase separation, microtubule interaction, and seeding of Tau condensates

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.

Phosphorylation but Not Oligomerization Drives the Accumulation of Tau with Nucleoporin Nup98

“…Under neuronal stress conditions and neurodegenerative diseases such as AD, Tau becomes missorted into the neuronal soma [ 6 ], where interactions with the nuclear envelope and NPCs can take place. In fact, the colocalization of Tau with NPCs was shown in AD brains [ 12 ] and in a cell model of Tau aggregation [ 13 ], where a direct interaction of Tau with Nup98 has been suggested. Furthermore, Tau was shown to inhibit the nuclear transport of proteins and RNA through NPCs in models with somatodendritic Tau accumulation and in AD brains [ 12 , 21 ].…”

“…Upon missorting, Tau can engage in a number of ‘abnormal’ interactions and functions that may drive neurotoxicity prior to Tau aggregation [ 11 ]. Among such disease- and stress-associated interactions of Tau is its accumulation at the outer nuclear envelope (NE) [ 12 , 13 ]. In addition, deformations of the NE (lamin folds and invaginations), can be observed in AD brains [ 14 ], Tau transgenic flies [ 15 ], and in neurons in vitro [ 16 ].…”

Tau: a phase in the crowd

“…In addition, tau‐attracting polyanions can initiate complex coacervation, giving rise to heterotypic (e.g., tau‐RNA or tau‐tubulin) droplets (Hernández‐Vega et al , 2017; Zhang et al , 2017). Employing state‐of‐the‐art optical techniques, Hochmair et al (2022) now untangle the relevance of these tau LLPS modes under conditions simulating the intracellular fluid.…”

“…Hochmair et al (2022) next questioned whether crowding and RNA also join forces during tau condensation in a pathological context. As expected (Wegmann et al , 2018), tau–tau droplets lost their dynamic character over time, as evidenced by minimal fluorescence recovery after photobleaching (FRAP) of labeled tau after a day in a PEG‐packed environment.…”

“…Taken together, Hochmair et al (2022) combine a flurry of optical techniques in cell‐free and cellular systems to unravel novel aspects of physiological and pathological tau LLPS, including the functional alliance between molecular crowding and RNA, the spontaneous assembly of mesoscopic tau clusters and the evolution of seed‐competent species in the absence of droplet aging. Advances in our understanding of the LLPS behavior of hazardous proteins include discoveries on proteins like TDP‐43, FUS, and α‐synuclein that constitute the hallmark inclusions in other neurodegenerative disorders (Zbinden et al , 2020), and, like tau, often present with a juxtanuclear localization.…”