Aaron Foote, UT-Austin

"Cytoplasmic Freezing in Fission Yeast: Investigating the Properties and Mechanisms Underlying a Glucose Starvation Induced Solid-like State of the Cytoplasm"

Abstract: The fluidity of the eukaryotic cytoplasm is essential for both active and passive forms of intracellular transport. Recently we have shown that the cytoplasm of fission yeast solidifies into a rigid state when starved of nutrients for 6 days. This state is characterized by the nearly complete immobilization of endogenous lipid droplets and other particles in the cytoplasm, and we have named this reversible process “cytoplasmic freezing.” However, the mechanism underlying the solid-like state and how cells transition into and out of this state remain unclear. Here we characterize the frozen state, measure how fission yeast cells enter into and recover from cytoplasmic freezing, and investigate a potential mechanism that could explain why the cytoplasm solidifies. To do so, we track the motion of endogenous lipid droplets that exist within the cytoplasm. In addition, we use a novel method that can detect motion from ~20 nm objects present in featureless regions of the cytoplasm to show that solidification is cell-wide and independent of lipid droplets. We find that cells enter into and recover from the frozen state in a multistage process. When deprived of glucose, the cytoplasm progresses through three distinct states of fluidity after two days and six days of glucose starvation. Moreover, when a cell in the frozen state is exposed to nutrients, the cytoplasm becomes less rigid within seconds, remains in an intermediate state for ~2 minutes, and then gradually returns to levels of fluidity that are typical for exponentially growing cells. Lastly, we examine if cytoplasmic freezing can be explained by an increase in macromolecular crowding. We induce an increase in macromolecular crowding via osmotic shock and find that lipid droplet motion is reduced but not to the degree observed during cytoplasmic freezing. Therefore, we claim that it is unlikely that cytoplasmic freezing is caused by an increase in cell crowding, and the mechanism behind the rigid state of the cytoplasm remains elusive.