Osmotic Stress Ushers FUS Out of Nucleus and Into Stress Granules

Some people with amyotrophic lateral sclerosis or frontotemporal dementia accumulate deposits of the nuclear protein FUS in the cytoplasm. What drives this relocation? In the July 24 Cell Reports, researchers led by Magdalini Polymenidou at the University of Zurich blame osmotic pressure. When Polymenidou and colleagues subjected brain slices to hyperosmotic conditions, FUS piled up in cytoplasm of neurons. The protein localized to stress granules, but even when the authors prevented those from forming, FUS still loitered in the cell body. The authors traced the cause of this mislocalization to transportin 1 and 2, the nuclear import factors that ferry FUS. These factors became marooned in cytoplasm during osmotic stress, they found.

Transportins Control FUS.
Transportins, aka TNPO (green), escort FUS (red) into the nucleus (top), but under hyperosmotic conditions (left), both get stuck in the cytoplasm. Astrocytes are unaffected (right). [Courtesy of Cell Reports, Hock et al. 2018]

FUS mislocalization could be the first step toward disease, Polymenidou suggested. She believes osmotic stress acts as the trigger that sends FUS to the wrong place, where a second stressor could then cause it to aggregate. Supporting the disease connection, hyperosmolarity had no effect on FUS in astrocytes. These cells do not develop FUS deposits in ALS and FTD.

“It is a really exciting observation that the relocalization of FUS is independent of stress granule formation,” Brian Freibaum at St. Jude Children’s Research Hospital in Memphis, Tennessee, wrote to Alzforum (full comment below). Others were more cautious. “The potential contribution of this mechanism to ALS/FTD pathogenesis is unclear,” noted Sami Barmada of the University of Michigan Medical School in Ann Arbor.

FUS contains a nuclear localization signal that allows transportins 1 and 2 (collectively called TNPO) to carry it into the nucleus. Without TNPO, FUS that escapes the nucleus via passive diffusion gradually amasses in cytoplasm (see May 2018 news). Even during most types of cellular stress, including oxidative stress, ER stress, and heat shock, TNPO keeps FUS corralled in the nucleus. One previous report flagged osmotic stress as the exception. Researchers led by Daryl Bosco at the University of Massachusetts Medical School found that subjecting immortalized somatic cell lines to hyperosmolarity forced FUS into stress granules (Sama et al., 2013).

Polymenidou and colleagues wondered if the same thing would happen in an environment closer to that of living brain. First author Eva-Maria Hock applied sorbitol, a sugar alcohol, to mouse cortico-hippocampal slices to induce osmotic stress. The high concentration of solutes in the extracellular fluid pulls water out of cells in the slices, shrinking them and causing intracellular molecular crowding. After two hours of this treatment, FUS began to disappear from nuclei, showing up in cytoplasmic stress granules. This occurred in neurons and microglia, but not astrocytes. Cytoplasmic FUS peaked at four hours. Then, curiously, it began to shift back to its normal location. After eight hours of continuous osmotic stress, FUS was confined to the nucleus again, and stress granules had dissolved, suggesting cells somehow adapted to these conditions. As with the cell lines, FUS mislocalization occurred only under osmotic stress; treating slices with arsenite to induce oxidative stress did not budge the protein.

Osmosis Moves FUS.
FUS (red) stays nuclear under normal conditions (left), but enters cytoplasmic stress granules (green; overlay appears yellow) during hyperosmotic stress (middle). Preventing stress granule formation results in diffuse cytoplasmic FUS (right). Nuclei are blue. [Courtesy of Cell Reports, Hock et al. 2018]

Did FUS movement depend on stress granule formation? To test this idea, the authors treated the slices with inhibitors of protein synthesis, which prevent stress granule formation. FUS still migrated to cytoplasm under osmotic stress, although it remained diffuse. Likewise, artificially stabilizing stress granules did not trap FUS in cytoplasm; it still escaped back to the nucleus after eight hours.

“It surprised me that translocation [of FUS] to cytoplasm and incorporation into stress granules were independent processes. That’s an important conceptual change in how we think about these steps,” Polymenidou told Alzforum. Some evidence suggests this uncoupling might be specific to osmotic stress. Researchers at Johns Hopkins University in Baltimore recently reported that stress granules formed by oxidative stress actively sequester nucleocytoplasmic transport factors like TNPO. In this case, inhibiting granule formation prevented mislocalization of these factors and blocked neurodegeneration (Zhang et al., 2018).

The authors explored several possible mechanisms for this FUS mislocalization. Signaling pathways and transcription factors activated by hyperosmotic stress did not affect FUS movement. Turning to nuclear import, the authors saw that other TNPO cargos accumulated in the cytoplasm during osmotic stress as well, but cargos of other types of importins did not. Immunostaining revealed that TNPO fled the nucleus and localized to stress granules during osmotic stress, in contrast to its normal diffuse distribution throughout the nucleus and cytoplasm. Transfecting cells with a TNPO construct that trapped it in the nucleus kept FUS in the nucleus during hyperosmolarity.

Commenters were particularly interested in the TNPO connection. Wilfried Rossoll at the Mayo Clinic in Jacksonville, Florida, noted that several recent studies have shown a role for transportins in preventing RNA-binding proteins such as FUS from sticking together (see Apr 2018 news). “Since hyperosmolarity causes cell shrinkage and macromolecular crowding, these new findings suggest that cytoplasmic recruitment of transportins may act as a protective mechanism to counteract increased levels of protein aggregation,” Rossoll wrote to Alzforum.

Multitasking.
Transportin proteins, best known for shuttling RBPs into the nucleus, also act as a disaggregase of disease-related RBPs including FUS and TDP-43. [Courtesy of Mackmull et al., 2017, Molecular Systems Biology.]

Were the findings on FUS specific to mouse cells? The authors repeated the experiments in human neurons derived from stem cells, and saw the same response to osmotic stress. However, human neurons did not adapt as quickly or as well to this type of stress as mouse neurons did, with FUS remaining in the cytoplasm of some cells under osmolar conditions. By contrast, FUS in human astrocytes was unaffected, just as in mouse astrocytes, although the cells showed other signs of osmotic stress, such as the activation of signaling cascades.

“I was very surprised to see that FUS in astrocytes resisted cytoplasmic relocalization,” Freibaum said. Polymenidou said she is investigating the reason for this. She thinks the mechanism might point toward potential interventions that could help neurons deal better with osmotic stress. Keeping FUS nuclear might prevent pathogenic deposits from forming, she suggested. She believes these findings are particularly pertinent for FTD. In ALS, it is mutant FUS that gets trapped in the cell body, but in FTD, it is the wild-type protein. The cytoplasmic mislocalization of normal FUS was a mystery.

It is unclear, however, how common hyperosmolarity is in the brain, and whether this condition could precipitate disease. Hyperosmolarity is sometimes induced in brain after traumatic injuries to bring down swelling; Polymenidou speculated that this could be one reason for the association between TBI and neurodegeneration. Severe dehydration or an imbalance of electrolytes might also cause osmotic stress in the brain, she said.

Bosco noted that the experimental conditions used in this study, like those in in her own, are not physiological. However, the molecular crowding caused by hyperosmotic stress might mimic other conditions that occur in the brain, such as impaired proteostasis and protein accumulation, she suggested. She also noted that hyperosmolarity can trigger the release of proinflammatory cytokines (Schwartz et al., 2009; Brocker et al., 2012). Inflammation is a known risk factor for many neurodegenerative diseases.

Featured Paper

Hock EM, Maniecka Z, Hruska-Plochan M, Reber S, Laferrière F, Sahadevan M K S, Ederle H, Gittings L, Pelkmans L, Dupuis L, Lashley T, Ruepp MD, Dormann D, Polymenidou M. Hypertonic Stress Causes Cytoplasmic Translocation of Neuronal, but Not Astrocytic, FUS due to Impaired Transportin Function. Cell Rep. 2018 Jul 24;24(4):987-1000.e7. PubMed.

References

Sama RR, Ward CL, Kaushansky LJ, Lemay N, Ishigaki S, Urano F, Bosco DA. FUS/TLS assembles into stress granules and is a prosurvival factor during hyperosmolar stress. J Cell Physiol. 2013 Nov;228(11):2222-31. PubMed.

Zhang K, Daigle JG, Cunningham KM, Coyne AN, Ruan K, Grima JC, Bowen KE, Wadhwa H, Yang P, Rigo F, Taylor JP, Gitler AD, Rothstein JD, Lloyd TE. Stress Granule Assembly Disrupts Nucleocytoplasmic Transport. Cell. 2018 May 3;173(4):958-971.e17. Epub 2018 Apr 5 PubMed.

Schwartz L, Guais A, Pooya M, Abolhassani M. Is inflammation a consequence of extracellular hyperosmolarity?. J Inflamm (Lond). 2009 Jun 23;6:21. PubMed.

Brocker C, Thompson DC, Vasiliou V. The role of hyperosmotic stress in inflammation and disease. Biomol Concepts. 2012 Aug;3(4):345-364. PubMed.

Mackmull MT, Klaus B, Heinze I, Chokkalingam M, Beyer A, Russell RB, Ori A, Beck M. Landscape of nuclear transport receptor cargo specificity. Mol Syst Biol. 2017 Dec 18;13(12):962. PubMed.

Further Reading

Guo L, Kim HJ, Wang H, Monaghan J, Freyermuth F, Sung JC, O’Donovan K, Fare CM, Diaz Z, Singh N, Zhang ZC, Coughlin M, Sweeny EA, DeSantis ME, Jackrel ME, Rodell CB, Burdick JA, King OD, Gitler AD, Lagier-Tourenne C, Pandey UB, Chook YM, Taylor JP, Shorter J. Nuclear-Import Receptors Reverse Aberrant Phase Transitions of RNA-Binding Proteins with Prion-like Domains. Cell. 2018 Apr 19;173(3):677-692.e20. [PubMed].

Hofweber M, Hutten S, Bourgeois B, Spreitzer E, Niedner-Boblenz A, Schifferer M, Ruepp MD, Simons M, Niessing D, Madl T, Dormann D. Phase Separation of FUS Is Suppressed by Its Nuclear Import Receptor and Arginine Methylation. Cell. 2018 Apr 19;173(3):706-719.e13. PubMed.

Qamar S, Wang G, Randle SJ, Ruggeri FS, Varela JA, Lin JQ, Phillips EC, Miyashita A, Williams D, Ströhl F, Meadows W, Ferry R, Dardov VJ, Tartaglia GG, Farrer LA, Kaminski Schierle GS, Kaminski CF, Holt CE, Fraser PE, Schmitt-Ulms G, Klenerman D, Knowles T, Vendruscolo M, St George-Hyslop P. FUS Phase Separation Is Modulated by a Molecular Chaperone and Methylation of Arginine Cation-π Interactions. Cell. 2018 Apr 19;173(3):720-734.e15. PubMed.

Yoshizawa T, Ali R, Jiou J, Fung HY, Burke KA, Kim SJ, Lin Y, Peeples WB, Saltzberg D, Soniat M, Baumhardt JM, Oldenbourg R, Sali A, Fawzi NL, Rosen MK, Chook YM. Nuclear Import Receptor Inhibits Phase Separation of FUS through Binding to Multiple Sites. Cell. 2018 Apr 19;173(3):693-705.e22. PubMed.


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aggregation cytoplasmic mislocalization disease-als FUS hyperosmotic stress neurotoxicity stress granules stress response topic-preclinical transportin-1 transportins
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