A Liquid-to-Solid Phase Transition of the ALS Protein FUS Accelerated by Disease Mutation

Abstract

Many proteins contain disordered regions of low-sequence complexity, which cause aging-associated diseases because they are prone to aggregate. Here, we study FUS, a prion-like protein containing intrinsically disordered domains associated with the neurodegenerative disease ALS. We show that, in cells, FUS forms liquid compartments at sites of DNA damage and in the cytoplasm upon stress. We confirm this by reconstituting liquid FUS compartments in vitro. Using an in vitro "aging" experiment, we demonstrate that liquid droplets of FUS protein convert with time from a liquid to an aggregated state, and this conversion is accelerated by patient-derived mutations. We conclude that the physiological role of FUS requires forming dynamic liquid-like compartments. We propose that liquid-like compartments carry the trade-off between functionality and risk of aggregation and that aberrant phase transitions within liquid-like compartments lie at the heart of ALS and, presumably, other age-related diseases.

Tau protein liquid–liquid phase separation can initiate tau aggregation

“…Together, and similar to what has been described for FUS (Patel et al , 2015), phospho‐tau droplets undergo a maturation, whereby their fast (minutes) increase in viscoelasticity efficiently prevents droplet coalescence and initiates tau aggregation mostly on the droplet interface.…”

“…The lack of tau droplet coalescence suggests a fast increase in the viscoelasticity of phospho‐tau droplets preventing droplet fusion (Abbott, 1977; Gu et al , 2011); this is characteristic for the “maturation” of liquids into hydrogels (Kato et al , 2012). The subsequent deformation of p‐tau441 droplets and the formation of tau aggregates in the droplets are strong indicators for the further transition of the tau droplets from a gel‐like into an aggregate state of lower energy; a similar maturation has been described for FUS (Patel et al , 2015). The aggregation of tau can be detected as droplet deformation and the growth of non‐spherical solid aggregates.…”

“…The maturation of liquid droplets into aggregates has been reported for the ALS‐associated proteins FUS (Patel et al , 2015), TDP‐43 (Schmidt & Rohatgi, 2016), and hnRNPA1 (Molliex et al , 2015). After LLPS, these proteins transition from an initial liquid droplet state, through a gel‐like state of high viscosity, to a final stable aggregated form (Lin et al , 2015).…”

“…Liquid droplets assembled from TDP‐43 (Conicella et al , 2016) and FUS (Burke et al , 2015; Patel et al , 2015) can transition from a reversible liquid state into a gel‐like state of higher viscosity and further into amyloid‐like or amorphous protein aggregates. A similar transition can lead to aberrant stress granule dynamics of C9orf72‐derived dipeptide repeats, which plays a role in the pathogenesis in amyotrophic sclerosis (ALS; Taylor et al , 2016).…”

“…Recent studies revealed that the RNA binding and stress granule‐associated proteins FUS (Patel et al , 2015), hnRNPA1 (Molliex et al , 2015), and TDP43 (Conicella et al , 2016) have the ability to reversibly form intracellular membrane‐less organelles. These reversible droplets represent physiologically active protein or protein‐nucleic acid “bioreactors” (Hyman et al , 2014), which form through a process known as LLPS (Brangwynne et al , 2015).…”