Sci. Aging Knowl. Environ., 18 February 2004
I Regret That I Have But One Life to Give for My Colony
Aging yeast commit suicide to save neighbors
Bees do it, and so do wasps, termites, and naked mole rats. Now the list of organisms that sacrifice themselves to benefit their relatives has a new, unlikely entry--yeast. A new study shows that elderly yeast die to help healthier cells survive. The findings suggest that infirm cells exude molecules that nurture nearby yeast.
Droves of cells die in our bodies every day, and the massacre is usually salutary. Controlled cell death helps shape our physique and keeps us healthy. For example, it separates fingers in developing embryos and disarms potentially cancerous cells throughout life. But the observation that yeast cells also become suicidal has been an evolutionary puzzle. Biologists would expect natural selection to eliminate cellular communism because early death terminates a cell's reproduction. Yeast geneticist Frank Madeo of the University of Tübingen, Germany, and colleagues wanted to learn more about how and why the fungi lose the will to live.
The researchers found that cultured yeast cells gradually died off and showed biochemical hallmarks of suicide, such as fractured DNA. These cells also amassed reactive oxygen species (ROS), destructive byproducts of metabolism (see "The Two Faces of Oxygen") that can provoke yeast cells to end their lives. Slowing buildup of the noxious chemicals through genetic engineering stretched the cells' life spans, suggesting that the accumulation of ROS within aging cells provokes their suicide. The team gauged the benefits of cell death by disabling a gene known as YCA1, which encodes a protein that drives cells to off themselves. To allow normal and altered cells to compete, the researchers mixed equal numbers of the two types on a plate. The normal yeast gradually crowded out the engineered cells, implying that the ability to commit suicide gives colonies a competitive advantage.
To find out how, the team collected molecules oozed by 11-day-old cultures of normal yeast. Adding these secretions to 7-day-old cultures boosted the survival rate of cells eightfold. Adding crushed yeast, which provides extra food, only doubled the rate. The disparity indicates that self-immolating cells help their neighbors by releasing life-extending compounds, not just by reducing competition for nutrients. However, the researchers didn't identify the chemicals responsible. Together, the results suggest that old cells eliminate themselves from the colony to help the remaining individuals. "Unicellular organisms are not just clumps of lonely cells," says Madeo. They communicate, feed each other, and even die for one another. The idea doesn't clash with evolutionary theory, because suicide benefits genetically identical cells--like a bee that dies to defend the hive, the yeast cell sacrifices itself to aid kin.
Researchers have long known that stressed fungi kill themselves, says yeast geneticist Steven Kron of the University of Chicago: "What's cool is that now we're seeing genetic analysis of this well-known phenomenon." The paper also adds to our understanding by testing an evolutionary rationale for the behavior, he says. So the next time you're enjoying some beer or wine, raise a glass to those noble yeast.
February 18, 2004
Science of Aging Knowledge Environment. ISSN 1539-6150