Cells “solidify” under stress
New research suggests that the interior of cells becomes less fluid-like when energy is in short supply.
Most organisms live in unpredictable environments, which can often lead to nutrient shortages and other conditions that limit their ability to grow. To survive in these harsh conditions, many organisms adopt a dormant state in which their metabolism slows down to conserve vital energy. When the environmental conditions improve, the organisms can return to their normal state and continue to grow.
The interior of cells is known as the cytoplasm. It is very crowded and contains many molecules and compartments called organelles that carry out a variety of vital processes. The cytoplasm has long been considered to be fluid-like in nature, but recent evidence suggests that in bacterial cells it can solidify to resemble a soft glass-like material under certain conditions. When cells experience stress they stop dividing and alter their metabolism. However, it was not clear whether cells also alter their physical properties in response to changes in the environment.
To address this question, two groups of researchers carried out independent studies in yeast cells. Matthias Munder and colleagues analyzed the changes that occur in the cytoplasm when baker’s yeast cells enter a dormant state. The experiments show that when yeast cells are deprived of energy — as happens during dormancy — the cytoplasm becomes more acidic than normal. This limits the ability of molecules and organelles to move around the cytoplasm. Similar results were also seen in other types of fungi and an amoeba. Munder and colleagues found that this increase in acidity during dormancy causes many proteins to interact with each other and form large clumps or filament structures that result in the cytoplasm becoming stiffer.
Ryan Joyner and colleagues starved yeast cells of sugar and tracked the movements of two large molecules called mRNPs and chromatin. Chromatin is found in a cell compartment known as the nucleus, while mRNPs are found in the cytoplasm. The experiments show that during starvation, both molecules are less able to move around in their respective areas of the cell. This appears to be due to water loss from the cells, which causes the cells to become smaller and leads to the interior of the cell becoming more crowded. Joyner and colleaugues also observed a similar response in bacteria. Furthermore, Joyner and colleagues suggest that the changes in physical properties are critical for cells to survive the stress caused by starvation.
Together, the findings of the studies suggest that the interior of cells can undergo a transition from a fluid-like to a more solid-like state to protect the cells from damage when energy is in short supply. The next challenge is to understand the molecular mechanisms that cause the physical properties of the cells to change under different conditions.
To find out more
Read the eLife research papers on which this story is based: “A pH-driven transition of the cytoplasm from a fluid- to a solid-like state promotes entry into dormancy” (March 22, 2016) and “A glucose-starvation response regulates the diffusion of macromolecules” (March 22, 2016).
