ALS Association

 


 

Ice Bucket Dollars at Work: Ubiquilin-2 (UBQLN2) is Linked to Cellular Stress Granules

We recently sat down with Dr. Carlos Castaņeda, assistant professor of biology and chemistry at Syracuse University. Thanks to funding from the ALS Ice Bucket Challenge, The ALS Association has funded Dr. Castaņeda twice through our global research program, which supported this work.

Dr. Castaņeda talked with us about the important findings in his recently published paper in Molecular Cell. His research explores cell degradation pathways with a focus on ubiquilin-2 (UBQLN2), a shuttle protein associated with ALS. Importantly, this work further links dysregulation of protein quality control pathways to ALS disease states.

“Nearly all of this work was funded by two The ALS Association grants: a one-year starter grant awarded in 2016, and a three-year investigator-initiated grant awarded in 2017,” says Dr. Castaņeda. “The first grant funded our initial work on structural characterization of UBQLN2 using NMR spectroscopy. The second grant focuses on understanding how UBQLN2 liquid droplet behavior is regulated by interactions with other ALS-linked RNA-binding proteins. We are extremely grateful to The ALS Association, its donors, and the Ice Bucket Challenge for making this project possible!”

What question did you set out to answer in this paper?

Our lab aims to understand the molecular basis for how ubiquilin-2 (UBQLN2) protein is associated with ALS. Mutations in UBQLN2 cause a small percentage of familial ALS cases, but wild-type and mutant UBQLN2 are found in post-mortem inclusions (clumps of protein) of motor neurons of people diagnosed with ALS, sometimes together with other ALS-linked proteins such as TDP-43 and hnRNPA1.

As molecular biophysicists, we wanted to study UBQLN2’s structure and dynamics. We discovered that UBQLN2 de-mixes into protein-containing liquid droplets (liquid-liquid phase separation) in the test tube under physiological conditions and that UBQLN2 is recruited to stress granules in cells. Both findings are common emerging characteristics of many ALS-linked proteins, especially RNA-binding ones.

Stress granules are stress-induced, dynamic, membrane-less assemblies of proteins and RNA. Dysfunction of stress granules are implicated in ALS and other neurodegenerative disorders. The finding that UBQLN2 is located in stress granules is unique, since UBQLN2 is not a RNA-binding protein, but rather one associated with maintaining protein quality control pathways in cells.

What is the impact of your paper on the ALS field?

In collaboration with Drs. J. Paul Taylor (St. Jude Children’s Research Hospital) and Heidi Hehnly (SUNY-Upstate Medical University), we found that UBQLN2 phase separates and is part of stress granules inside cells. Importantly, we also found that droplets are eliminated upon binding of UBQLN2 to ubiquitin and polyubiquitin.

At the end of a protein’s life inside cells, a protein is tagged for degradation by polyubiquitin, and directed to protein quality control pathways by shuttle proteins, such as UBQLN2. This cell degradation pathway is much like a trash disposal system in cells.

We predict that UBQLN2’s phase separation behavior enables it to be recruited to stress granules. There, UBQLN2 could interact with polyubiquitin-tagged proteins (proteins targeted for degradation). Then UBQLN2 could leave stress granules, taking these tagged proteins out of stress granules with it. At that point, they could be directed to protein quality control pathways, where they are eventually degraded in the cell.

We also predict that dysfunctions with this pathway can lead to improper clearance of proteins associated with stress granules and promote cellular inclusions that resemble those found in post-mortem tissue from people with ALS. Importantly, our work further links dysregulation of protein quality control pathways to ALS disease states.

What are your next steps to move this project further?

Our immediate next step is to determine whether ALS-linked disease mutations in UBQLN2 impact its liquid droplet behavior in the test tube and in cells. We also plan to test how interactions between UBQLN2 and other ALS-linked proteins modulate UBQLN2 phase separation, and how these interactions modulate stress granule dynamics in cells.

Learn more about Dr. Castaņeda’s funded projects here and here.

To learn more about our progress and monetary commitments since the 2014 ALS Ice Bucket Challenge, please visit the Ice Bucket Challenge Progress site.

Paper citation:
Ubiquitin Modulates Liquid-Liquid Phase Separation of UBQLN2 via Disruption of Multivalent Interactions.
Dao TP, Kolaitis RM, Kim HJ, O’Donovan K, Martyniak B, Colicino E, Hehnly H, Taylor JP, Castaņeda CA.
Mol Cell. 2018 March 15, 69 (6): 965-978

Click here for the open access article.