The levels of proteins in a cell can be regulated either directly via processes such as proteasomal degradation or indirectly, such as by controlling the rate of translation of the mRNA. The impact of translational regulation on protein levels during the cell-cycle is something that our lab has examined using ribosome profiling. We found that there is a ~35% decrease global translation that occurs when a cell enters mitosis, with some transcripts in particular being suppressed up to 10-fold more compared to interphase. We’ve also developed a technique to allow for the visualization of single mRNA translation events in cells by utilizing the Suntag system. Using this method, we were able to observe an amazing amount of heterogeneity in the individual translation events within the same cell.
Our lab has also investigated the ability of RNA to undergo phase transitions both in vitro and in cells. We observed that purified RNA containing repeats (e.g. poly-CUG or poly-CAG) above a certain length and concentration undergo liquid-liquid phase transition and form droplets. Expressing these repeats in cells leads to the formation of foci in the nucleus that co-localize with nuclear speckles. Depending on the repeat, the foci either resembled the more gel-like in vitro droplets (e.g. poly-G4C2) or were more dynamic and exhibited liquid-like characteristics (e.g. poly-CUG and poly-CAG). This is both interesting from both a basic science as well as a clinical perspective. What role does RNA phase transition, if any, play in a normal cell? The presence of nucleotide repeat expansions have been detected in a number of human diseases including myotonic dystrophy, C9orf72 ALS/FTD, and FXTAS. What is the contribution of the observed RNA foci in the pathology of these neuro-degenerative afflictions? More work will have to be done in the future to answer these questions.
Information regarding the reagents relating to this work can be found here.
Selected References:
(link) – Jain A, Vale RD. RNA phase transitions in repeat expansion disorders. Nature. 2017 Jun 8;546(7657):243-247. doi: 10.1038/nature22386.
(pdf) – Yan X., Hoek T.A., Vale R.D., Tanenbaum M.E. (2016) Dynamics of translation of single mRNA molecules in vivo. Cell. 2016 May 5;165(4):976-89. doi: 10.1016/j.cell.2016.04.034. PMID: 27153498. PMCID: PMC4889334.
(pdf) – Tanenbaum, M.E., Stern-Ginossar, N., Weissman, J.S., Vale, R.D. (2015) Regulation of mRNA translation during mitosis. Elife Aug 24 (4): 19 pages. PMCID: PMC4548207.