"Supergrowth" Challenges Established Science of Cell Growth
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Scientists are still grappling with questions as fundamental as how a cell actually grows. Cells are not just water-filled blobs, but are chock-full of proteins, nucleic acids, lipids, and many other types of molecules. When a cell grows, it not only increases in size, it also has to make more of all these biomolecules to sustain itself.
For the cell’s myriad biochemical reactions to work, the concentration of all these biomolecules needs to be maintained at proper levels. But how does a cell synthesize just the right amount of these key cellular components as the cell grows in size?
“We all assumed that the concentration of proteins in a cell had to remain relatively constant. Otherwise, if the cytoplasm becomes too dilute or too concentrated, cellular processes could fail, perhaps causing cells to sicken and die,” said Fred Chang, co-senior author of a study that has the potential to upend decades of received wisdom on the topic of cell growth.
In the new paper, Chang and former technician Ben Knapp, fellow co-senior author of the new study – describe a surprising discovery. They show that after cells are subjected to certain stressful treatments, they appear to gain a new “superpower” that allows them to grow twice as fast as normal – a feature the authors call “supergrowth.”
“No one had seen these cells grow so fast before,” Chang said. “It was mind boggling, and we were obsessed with finding out why.”
In investigating why these cells grew so quickly, they found that during the stressful period, cells unexpectedly stockpiled biomolecules. After the stress was removed, this accumulation became the driver of the rapid growth.
“We didn’t expect it to even be possible for cells to increase protein concentrations to such an extreme extent. Moreover, this increase did not make the cells sick. Instead they were quite robust and healthy. This resilience came as a complete surprise,” said Chang.
The researchers first observed supergrowth while running a series of experiments that were meant to clarify the link between the cell’s internal pressure and its growth rate. To study this relationship, the researchers turned to Schizosaccharomyces pombe – a species of single-celled yeast that has frequently been used as an experimental system for studying cell growth and division, and which has already yielded a number of related Nobel Prize-winning discoveries.
Over a four-hour period, the researchers subjected the cells to a series of “osmotic oscillations” – a process that causes their internal pressure to rapidly fluctuate. During these oscillations, they noticed that cell growth stagnated while protein production continued unabated, leading to a rapid rise in protein levels, with some proteins nearly doubling in concentration during the oscillations. When the oscillations ended, the cells, much to the surprise of the researchers, began to grow unusually fast – double, and sometimes nearly triple, the normal rate. This supergrowth had never been seen before.
“The results were one surprise after another, and I’ve been studying this yeast since starting my career,” said Chang.
The researchers tested a number of possible ideas that might account for their observations and discovered that protein accumulation itself, and not the pressure changes associated with osmotic oscillations, was driving supergrowth.
To demonstrate this, the scientists administered a chemical called cycloheximide (CHX) – which shuts off protein production – during the oscillation phase. This treatment prevented cells from growing or manufacturing any proteins. Once the oscillations ended, the CHX was washed away and the cells started to grow and produce proteins again. But since they hadn’t been stockpiling proteins, they never reached anything approaching supergrowth.
Following up on this observation, the researchers then used another method to confirm that protein accumulation, not osmotic oscillations, was responsible for supergrowth. To halt cell growth without relying on oscillations, they administered a chemical called brefeldin A (BFA), which suspends growth without interfering with protein production. When BFA was washed away, the cells, which had been amassing proteins throughout the no-growth phase, underwent supergrowth.
“Everyone always thought that the density of molecules inside of cells was carefully regulated, and that growth was a way to keep macromolecule concentrations relatively constant. But this study suggests that in certain circumstances, the protein density inside cells might not be as tightly controlled as we thought. Moreover, our findings lead to new ideas of how cells can adjust their density simply by changing their growth rate,” said Chang.
The researchers now hope the experiments can be replicated in other cell types, including mammalian cells, in order to determine whether supergrowth is unique to this yeast species or a general feature of all cells.
“In the long term, we’re hoping to find out whether our findings apply more broadly to living systems,” Chang said. “Changes in cytoplasmic density and growth rates might have important implications for development and in human diseases such as cancer and aging.”
Knapp et al. (2019) Decoupling of Rates of Protein Synthesis from Cell Expansion Leads to Supergrowth. Cell Systems. DOI: https://doi.org/10.1016/j.cels.2019.10.001
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