Two hallmarks of the integrated stress response of cells are the inhibition of translation and the formation of stress granules (SGs) and processing bodies (PBs). However, it is not well understood how both processes are coupled. In a study published in Molecular Cell, researchers from the Chao group applied single-molecule RNA imaging to study the interactions of mRNAs with SGs and PBs, and found out that the generally accepted assumptions about the function of granules need to be revised.
SGs and PBs are membraneless organelles made up of RNA and proteins that assemble during cellular stress. Since it is also well known that translation is inhibited during stress, it is tempting to consider that mRNAs go into these organelles during stress in order to regulate translation and mRNA decay. However, these models have never been proven, mostly because the experimental techniques had not been available.
The FMI’s Chao lab does have the know-how to image mRNAs! Johannes Wilbertz, a former PhD student in the lab, wanted to make use of such imaging techniques to test the current models regarding the functional interactions between mRNA and granules during stress. Using single-molecule mRNA imaging, he followed the fate of mRNA molecules in the cytosol during stress and their interaction with SGs and PBs.
“This study was quite a technical achievement,” says group leader Jeff Chao. “For the first time, we managed to label three different components - the mRNAs, the SGs and the PBs. This allowed us to nicely visualize how the mRNAs interacted with the granule, and come to conclusions that are based on direct observations and not just speculations.” Wilbertz explains the main findings of his study. “We could show that while virtually all mRNAs are not translated during stress, only a minority of these nontranslating mRNAs are incorporated into either SGs or PBs. Consequently, translation is inhibited regardless of an mRNA’s cytosolic localization. We also wanted to test a model according to which mRNAS in the granules are protected from chemical damage due to stress and as a result would be more likely to resume translation once stress is over. Despite this, our findings indicate that the translational capacity of an mRNA after stress is independent from its location during stress.”
Wilbertz talks about a third assumption that the study refuted: “SGs have also been proposed to serve as sites of triage where mRNAs are sorted and those targeted to RNA degradation are funneled into PBs. However, we observed only a very small fraction of mRNAs are moving from one granule to the other. In addition, we found that there was no mRNA degradation in either PBs or SGs during stress or recovery.”
“In summary, we could clearly show that the assumptions about SG and PB function need to be revisited, even if our study has its limitations - notably because we have only looked at a small number of mRNAs under specific stress conditions,” says Chao. “The next step is to further investigate translation and mRNA decay to understand how these critical stress response pathways can occur in a compartmentalized cytoplasm. Single-molecule measurements will continue to remain an essential tool for this.”
Johannes H. Wilbertz, Franka Voigt, Ivana Horvathova, Gregory Roth, Yinxiu Zhan, Jeffrey A. Chao (2019) Single-molecule imaging of mRNA localization and regulation during the integrated stress response. Molecular Cell (advance online publication).