Spatio-temporal Dynamics and Mechanisms of Stress Granule Assembly

Abstract

Stress granules (SGs) are non-membranous cytoplasmic aggregates of mRNAs and related proteins, assembled in response to environmental stresses such as heat shock, hypoxia, endoplasmic reticulum (ER) stress, chemicals (e.g. arsenite), and viral infections. SGs are hypothesized as a loci of mRNA triage and/or maintenance of proper translation capacity ratio to the pool of mRNAs. In brain ischemia, hippocampal CA3 neurons, which are resilient to ischemia, assemble SGs. In contrast, CA1 neurons, which are vulnerable to ischemia, do not assemble SGs. These results suggest a critical role SG plays in regards to cell fate decisions. Thus SG assembly along with its dynamics should determine the cell fate. However, the process that exactly determines the SG assembly dynamics is largely unknown. In this paper, analyses of experimental data and computer simulations were used to approach this problem. SGs were assembled as a result of applying arsenite to HeLa cells. The number of SGs increased after a short latent period, reached a maximum, then decreased during the application of arsenite. At the same time, the size of SGs grew larger and became localized at the perinuclear region. A minimal mathematical model was constructed, and stochastic simulations were run to test the modeling. Since SGs are discrete entities as there are only several tens of them in a cell, commonly used deterministic simulations could not be employed. The stochastic simulations replicated observed dynamics of SG assembly. In addition, these stochastic simulations predicted a gamma distribution relative to the size of SGs. This same distribution was also found in our experimental data suggesting the existence of multiple fusion steps in the SG assembly. Furthermore, we found that the initial steps in the SG assembly process and microtubules were critical to the dynamics. Thus our experiments and stochastic simulations presented a possible mechanism regulating SG assembly.

Principles of Stress Granules Revealed by Imaging Approaches

“…The small stress granules that form early in the time course are biochemically stable in cell lysates, which is consistent with the first step in stress granule assembly being the coalescence of a group of nontranslating mRNPs into a stress granule core structure (Wheeler et al 2016). Subsequently, the small stress granules increase in size both by accretion of new material and by fusion, resulting in a decrease of the total number of stress granules over time (Kedersha et al 2000;Ohshima et al 2015;Wheeler et al 2016). During recovery from stress, stress granules disassemble in a reverse of this process, wherein large stress granules show a decrease in the concentration of components and fracture into smaller stress granules, which then disappear in a final step (Kedersha et al 2000;Ohshima et al 2015;Wheeler et al 2016).…”

Imaging of single mRNA translation repression reveals diverse interactions with mRNP granules

“…The repression of mRNA translation was positively correlated with the growth of SGs, which increases throughout the stress response (15)(16)(17). Evaluation of SGs revealed SG growth by both incremental accumulation of material and SG fusion ( Fig.…”

RNA promotes phase separation of glycolysis enzymes into yeast G bodies in hypoxia

“…Third, while G bodies can fuse in vivo, a property of liquids (Boeynaems et al, 2018;Elbaum-Garfinkle et al, 2015;Hyman et al, 2014), this fusion takes place on the order of tens of minutes, unlike fusion of P bodies, which takes only seconds (Kroschwald et al, 2015). Similar to P bodies, P granules in the C. elegans germline and stress granules can undergo fission and fusion (Brangwynne et al, 2009;Kedersha et al, 2005;Ohshima, Arimoto-Matsuzaki, Tomida, Takekawa, & Ichikawa, 2015). Indeed, formation of G bodies when cells are transitioned to hypoxia involves fusion of smaller granules into larger structures.…”