RNA G-quadruplexes and calcium ions synergistically induce Tau phase transition in vitro

Abstract

Tau aggregation is a defining feature of neurodegenerative tauopathies, including Alzheimer's disease, corticobasal degeneration, and frontotemporal dementia. This aggregation involves the liquid–liquid phase separation (LLPS) of Tau, followed by its sol–gel phase transition, representing a crucial step in aggregate formation both in vitro and in vivo . However, the precise cofactors influencing Tau phase transition and aggregation under physiological conditions ( e.g., ion concentration and temperature) remain unclear. In this study, we unveil that nucleic acid secondary structures, specifically RNA G-quadruplexes (rG4s), and calcium ions (Ca 2+ ) synergistically facilitated the sol–gel phase transition of human Tau under mimic intracellular ion conditions (140 mM KCl, 15 mM NaCl, and 10 mM MgCl 2 ) at 37 °C in vitro . In the presence of molecular crowding reagents, Tau formed stable liquid droplets through LLPS, maintaining fluidity for 24 h under physiological conditions. Notably, cell-derived RNA promoted Tau sol–gel phase transition, with rG4s emerging as a crucial factor. Surprisingly, polyanion heparin did not elicit a similar response, indicating a distinct mechanism not rooted in electrostatic interactions. Further exploration underscored the significance of Ca 2+ , which accumulate intracellularly during neurodegeneration, as additional cofactors in promoting Tau phase transition after 24 h. Importantly, our findings demonstrate that rG4s and Ca 2+ synergistically enhance Tau phase transition within 1 h when introduced to Tau droplets. Moreover, rG4-Tau aggregates showed seeding ability in cells. In conclusion, our study illuminates the pivotal roles of rG4s and Ca 2+ in promoting Tau aggregation under physiological conditions in vitro , offering insights into potential triggers for tauopathy.

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