Aggregation of the DNA/RNA-binding protein FUS (FUsed in Sarcoma) in the cytoplasm of nerve cells is characteristic of two devastating and incurable neurodegenerative disorders. Amyotrophic lateral sclerosis (ALS) is a motor neuron disease, which leads to progressive muscle weakness and ultimately to lethal paralysis, while frontotemporal dementia (FTD), the second most common form of dementia (after Alzheimer's) in those under 65.
In a new study, they report that the aggregation of FUS observed in ALS and FTD is attributable to two distinct but functionally related molecular mechanisms. Their findings are reported in the leading journal Cell.
In healthy nerve cells, the FUS protein is transported into the nucleus, where it serves both as a DNA-binding transcription factor and as an RNA-binding regulator of transcript splicing. Strikingly, in the neurons whose function is affected in ALS and FTD patients, uptake of FUS into the nucleus is compromised. In ALS-FUS patients, this is caused by mutations in the nuclear localization signal (NLS) of FUS that weaken Transportin binding. In FTD-FUS patients, Transportin is aggregated, and post-translational arginine methylation, which regulates the FUS-Transportin interaction, is lost. As a result, the protein accumulates in the cytoplasm of these neurons and coagulates into so-called stress granules.
"Based on previous findings, we suspected that these granules might act as seeds to promote protein aggregation, ultimately leading to irreversible clumping of FUS," author says. "However, the mechanism underlying the aggregation process was unclear." In the new study, researchers have now shown that FUS aggregation results from a liquid-liquid phase separation analogous to the gradual coalescence of oil droplets dispersed in an oil-water emulsion. The FUS protein forms liquid droplets, in which the protein can congeal into jelly-like clumps and finally solid aggregates.
They went on to show that, in ALS and FTD, these phase changes of FUS are promoted in two different ways. To get into the nucleus, FUS must interact with the nuclear import receptor Transportin. In the case of ALS, mutations in FUS significantly weaken the protein's ability to bind to Transportin. "Our results show that Transportin is not only responsible for delivering FUS into the nucleus, it also acts to stabilize proteins like FUS, which have a marked tendency to undergo phase separation," author explains. This stabilizing function can be compromised in ALS patients by mutations in FUS. In patients with frontotemporal dementia, the FUS protein itself is normal. Instead, a small chemical modification on FUS that regulates its transport into the nucleus is defective. As the new study shows, failure to modify the protein not only affects its intracellular transport, but also promotes its phase separation and aggregation in the cytoplasm.
Thus authors show that Transportin and arginine methylation have a crucial function beyond nuclear import—namely to suppress RGG/RG-driven phase separation and stress granule association of FUS. ALS-associated FUS-NLS mutations weaken the chaperone activity of Transportin and loss of FUS arginine methylation, as seen in FTD-FUS, promote phase separation, and stress granule partitioning of FUS.
The results conclusively demonstrate that the aggregation of FUS is a central component of the process that leads to ALS and FTD. "Furthermore, accumulating evidence strongly supports the idea that other proteins implicated in the pathology of neurodegenerative disorders can form aggregates via phase separation," author points out. "We will now take a closer look at these proteins in order to understand how this process is normally suppressed, and how it might be modulated chemically."
Phase separation is compromised in neurodegenerative diseases
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