Posted on by Dr. Francis Collins
Most stressful situations that we experience in daily life aren’t ones that we’d choose to repeat. But the cellular stress response captured in this video is certainly worth repeating a few times, so you can track what happens when two cancer cells get hit with stressors.
In this movie of two highly stressed osteosarcoma cells, you first see the appearance of many droplet-like structures (green). This is followed by a second set of droplets (magenta) and, finally, the fusion of both types of droplets.
These droplets are composed of fluorescently labeled stress-response proteins, either G3BP or UBQLN2 (Ubiquilin-2). Each protein is undergoing a fascinating process, called phase separation, in which a non-membrane bound compartment of the cytoplasm emerges and constrains the motion of proteins within it. Subsequently, the proteins fuse with like proteins to form larger droplets, in much the same way that raindrops merge on a car’s windshield.
Julia Riley, an undergraduate student in the NIH-supported lab of Heidi Hehnly and lab of Carlos Castañeda, Syracuse University, NY, shot this movie using the sophisticated tools of fluorescence microscopy. It’s the next installment in our series featuring winners of the 2019 Green Fluorescent Protein Image and Video Contest, sponsored by the American Society for Cell Biology. The contest honors the discovery of green fluorescent protein (GFP), which—together with a rainbow of other fluorescent proteins—has enabled researchers to visualize proteins and their dynamic activities inside cells for the last 25 years.
Riley and colleagues suspect that, in this case, phase separation is a protective measure that allows proteins to wall themselves off from the rest of the cell during stressful conditions. In this way, the proteins can create new functional units within the cell. The researchers are working to learn much more about what this interesting behavior entails as a basic organizing principle in the cell and how it works.
Even more intriguing is that similar stress-responding proteins are commonly altered in people with the devastating neurologic condition known as amyotrophic lateral sclerosis (ALS). ALS is a group of rare neurological diseases that involve the progressive deterioration of neurons responsible for voluntary movements such as chewing, walking, and talking. There’s been the suggestion that these phase separation droplets may seed the formation of the larger protein aggregates that accumulate in the motor neurons of people with this debilitating and fatal condition.
Castañeda and Hehnly, working with J. Paul Taylor at St. Jude Children’s Research Hospital, Memphis, earlier reported that Ubiquilin-2 forms stress-induced droplets in multiple cell types . More recently, they showed that mutations in Ubiquilin-2 have been linked to ALS changes in the way that the protein undergoes phase separation in a test tube .
While the proteins in this award-winning video aren’t mutant forms, Riley is now working on the sequel, featuring versions of the Ubiquilin-2 protein that you’d find in some people with ALS. She hopes to capture how those mutations might produce a different movie and what that might mean for understanding ALS.
 Ubiquitin Modulates Liquid-Liquid Phase Separation of UBQLN2 via Disruption of Multivalent Interactions. Dao TP, Kolaitis R-M, Kim HJ, O’Donovan K, Martyniak B, Colicino E, Hehnly H, Taylor JP, Castañeda CA. Molecular Cell. 2018 Mar 15;69(6):965-978.e6.
 ALS-Linked Mutations Affect UBQLN2 Oligomerization and Phase Separation in a Position- and Amino Acid-Dependent Manner. Dao TP, Martyniak B, Canning AJ, Lei Y, Colicino EG, Cosgrove MS, Hehnly H, Castañeda CA. Structure. 2019 Jun 4;27(6):937-951.e5.
Amyotrophic Lateral Sclerosis (ALS) (National Institute of Neurological Disorders and Stroke/NIH)
Castañeda Lab (Syracuse University, NY)
Hehnly Lab (Syracuse University)
Green Fluorescent Protein Image and Video Contest (American Society for Cell Biology, Bethesda, MD)
2008 Nobel Prize in Chemistry (Nobel Foundation, Stockholm, Sweden)
NIH Support: National Institute of General Medical Sciences