When yeast cells divide, they retain their own damaged proteins to produce daughter cells with immaculate cytoplasms, essentially resetting their age and giving the lineage immortality. While a decade of research has indicated that the mother’s cytoskeleton shoots out actin fibers to actively regulate this process, a paper published today in Cell turns this theory on its head, suggesting that slow diffusion and cell geometry are all that’s needed to keep damaged proteins from entering the new cells.
“What this paper does is it demystifies something that’s very fundamental: how do you generate a daughter cell that doesn’t carry the aging components of an aging cell?” said molecular biologist Susan Gasser, director of the Friedrich Miescher Institute for Biomedical Research in Switzerland who was not involved with the research. “And what it says it is that it’s the null hypothesis”— that the cell’s structure is enough.
“It goes against a lot of previous data and a lot of previous reports that suggest that damaged protein aggregates are partitioned in the cell by specific proteins and by the actin cytoskeleton,” said microbiologist Thomas Nyström of the University of Gothenburg, who also did not participate in the study. “I don’t know what to make of it at this point,” admitted Nyström, who isn’t ready to dismiss the existing yeast aging model just yet.
A genetic strain of yeast can live forever in culture, but each single cell has a limited lifespan. Over time, cells accumulate oxidized proteins and error-ridden DNA fragments, which slow reproduction and eventually lead to senescence —yeast’s version of aging and death. To protect their offspring from the accumulating cellular junk, the larger mother cell retains its damaged molecules during cell division, while popping off a smaller, clean bud. (See The Scientist’s 2010 feature, The Gates of Immortality)
Genetic studies have suggested that the mother cell’s cytoskeleton, which assists in cell division, binds damaged proteins and redirects them away from the daughter, actively holding them hostage. But when examining the actin cytoskeleton more closely, Rong Li, a cell biologist at the Stowers Institute for Medical Research, couldn’t quite reconcile the model. “The problem with that model is that it requires actin to be configured in a specific orientation,” said Li, the lead investigator of the Cell study. “But, through most of the cell cycle, we know actin is not configured that way.”
Following the methods used in previous yeast aging studies, Li and her colleagues heat-shocked growing yeast cells to generate damaged protein aggregates, and labeled associated proteins with a fluorescent tag. They then added a sea sponge-derived toxin to inhibit cytoskeleton growth, and tracked the proteins using live-cell imaging. Despite the fact that the cell could not produce new actin filaments, the proteins did not travel far, suggesting that the cytoskeleton was not responsible for the mother cell’s retention of the damage proteins.
The researchers suspected that the narrow neck dividing the mother cell from the daughter bud was restrictive enough to hold the proteins captive. A mathematical model developed from protein movement data in yeast cells undergoing natural aging further supported the hypothesis. “The geometry of the yeast cell and the way the aggregates are moving would assure that the vast majority of aggregates are trapped in the mother,” Li said. “It’s a very simple conclusion, and it’s not as exciting as the old model.”
However, proponents of the active model haven’t given up. Nyström, who published a correspondence in the same issue of Cell, analyzed nearly 400 natural budding events and found that proteins migrated between the mother and daughter in 15 percent of cases. Additionally, when he knocked out a gene necessary for the mother to build her actin cytoskeleton, significantly more protein aggregates were transferred to the daughter bud.
Nyström pointed to the heat-shock methodology as a possible source of confusion. Heat shock destroys not just meaningless protein, but the cytoskeleton as well, he said. The assumption is that the actin cables are built up again afterwards, “but are all the structures reassembled?” Nyström wondered. “It would have been nice to see to what extent this passive diffusion they see would hold up in a more natural experiment.”
It is possible that both models are correct: that passive diffusion, neck geometry, and cytoskeleton binding all contribute to preventing damaged molecules from passing to the daughter cell, said Jürgen Knoblich, a biochemist and deputy scientific director of the Institute of Molecular Biotechnology in Vienna, Austria and was not associated with the study. “You could imagine a possibility that there is [active] transport at one stage in the cell cycle but not in another.”
That said, Knoblich added, the models as they are written are mutually exclusive, and the new paper will no doubt generate discussion as to which is right and promote more research on both ends. “The [old] theory is very exciting, but at the same time, the disproval of [that] theory is also very exciting,” said Knoblich. “I think there are very few ideas that are so compelling that someone can publish a paper in Cell just to show that it’s not true.”
C. Zhou et al., "Motility and segregation of Hsp104-associated protein aggregates in budding yeast," Cell, 147 1186-1196, 2011.
B. Liu et al., "Segregation of protein aggregates involves actin and the polarity machinery," Cell, 147, 2011.