Sci. Aging Knowl. Environ., 17 November 2004
A growing body of research suggests that yeast have programmed death pathways, yet many researchers are skeptical. Recent studies provide some of the first experimental evidence for why a single-celled organism would commit suicide
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2004/46/ns9
Abstract: Some dying yeast display signs of apoptosis, a cell-suicide process. The idea has sparked controversy partly because single-celled organisms wouldn't seem to benefit from this inclination. Work over the past year has bolstered the notion that yeast kill themselves and provided experimental evidence for how the capacity might aid yeast populations.
Some warriors die for their cause, whereas others live for it, surviving to ensure that they achieve their goal. In principle, a single-celled organism such as yeast ought to mimic the hero who hangs on at all cost, because its goal is to pass on its genes; if it dies, all is lost. But some studies suggest that a yeast cell will give its life for a larger cause: The fungus appears to deploy a suicide mechanism. Although the idea remains controversial, new work bolsters the idea that yeast cells sometimes die willingly and provides evidence for how martyrdom helps a yeast population survive. Studies of the easily manipulated organism could reveal new details about how mammalian cells kill themselves for the greater good and might point scientists toward other means of cellular demise.
Mice, humans, and other complex creatures cull some of their cells to strengthen the organism. The cell-suicide pathway known as apoptosis weeds out cells that are no longer needed and obliterates damaged, potentially cancerous cells before they do harm. But a programmed death mechanism shouldn't benefit a single-celled organism because once it dies, it can't pass on its genetic dowry. Nevertheless, since 1997, researchers have been piecing together evidence that yeast cells have an apoptosis-like capacity for offing themselves. For instance, when scientists engineered yeast to produce mammalian proteins that trigger apoptosis, the fungus perished and displayed classic features of apoptosis, such as fractured DNA. Cells treated with relatively low concentrations of hydrogen peroxide, which triggers apoptosis in mammalian cells, or those that harbor particular mutations also show indicators of a deliberate death. "They don't simply die," says geneticist Michael Breitenbach of the University of Salzburg in Austria. "They die and show all known signs of apoptosis." But many researchers remained skeptical, in part because initial surveys of the yeast genome revealed that it didn't contain any proteins that resembled mammalian apoptosis molecules that were known at the time.
It Looks Like a Duck--But Is It a Duck? Back to TopThe case for apoptosis in yeast has strengthened in the last 2 years because researchers have begun to identify such molecules. The first, called YCA1, behaves like a caspase, a protein-chopping enzyme that constitutes a crucial part of the mammalian apoptosis signal (see "Death Comes for the Fungus"). Cells without the protein hang on unusually long after peroxide treatment, whereas those with extra YCA1 die more readily than normal. The protein only distantly resembles mammalian caspases, explaining why it had eluded researchers. Other studies have uncovered additional yeast molecules that apparently incite suicide in a similar manner to those used by recently discovered counterparts in mammals. Hma111p spurs death by accumulating in the nucleus and chewing up other proteins, AIF shuttles from the mitochondria to the nucleus and devours DNA, and Fis1 causes mitochondria to disintegrate.
"There's a core apoptotic machinery that's conserved in mammals and yeast," says cell stress researcher William Burhans of the Roswell Park Cancer Institute in Buffalo, New York, suggesting that a similar process operates in the two organisms.
But mysteries of mammalian apoptosis also cloud the yeast field. For instance, researchers don't know the specifics of how AIF contributes to mammalian cell death. And they don't agree about how to define apoptosis. "Apoptosis means different things to different people," says cell death researcher John Reed of the Burnham Institute in La Jolla, California. Some classify the process by a cell's appearance, whereas others delineate it based on which molecules participate. That inconsistency has snarled yeast researchers' efforts to convince others that they really are observing apoptosis.
And a recent study suggests that previous reports of cells that appear to have undergone apoptosis might be wrong. Many researchers flag suicidal cells by using a fluorescent molecule that glows when caspases snip it. Wysocki and Kron found that the molecule lights up any dying cell--even if it doesn't contain YCA1, the yeast caspaselike protein. The authors suggest that many previous studies might have produced misleading results because of this artifact. And because the death pathway they documented works independently of the caspase, it is not apoptosis, they conclude in the 2 August Journal of Cell Biology.
Other researchers doubt the interpretation, however. The scientists used conditions that are too harsh to incite apoptosis; instead, they destroy cells before that program begins, say some yeast apoptosis researchers. Still, the apparently nonspecific tagging of dying cells generates concern. "That molecule is not a reliable marker," says J. Marie Hardwick, a virologist at Johns Hopkins University in Baltimore, Maryland.
Life After Death Back to TopBut perhaps the biggest obstacle to embracing the existence of yeast apoptosis is understanding why yeast cells would commit suicide in the first place, says David Goldfarb, a molecular biologist at the University of Rochester in New York. Evolutionary theory holds that any single-celled creature ought to hang on, reproducing as long as possible, instead of relinquishing its life. In their natural habitat of grapes and figs, however, yeast cells cohabit, and members of such a group might collaborate to ensure the survival of a few cells. "It's not intuitively obvious why a cellular suicide mechanism would benefit a single-celled organism," says Burhans. "But yeast live as colonies, and from that point of view, it makes perfect sense to me that there would be a potential advantage." For instance, some researchers have proposed that when food is scarce, some cells die to provide fodder for others--a dying cell in effect passes on its own genes by saving others in the same colony. However, until this year no one had gathered experimental evidence for that idea.
Recent work suggests that yeast cells trade long life for the capacity to undergo programmed death--and that this capacity might help cultures survive. In a study published in the 27 September issue of the Journal of Cell Biology, geneticist Valter Longo of the University of Southern California in Los Angeles and colleagues analyzed yeast with different life spans. First, the researchers found that normal yeast exhibit signs of apoptosis, such as leaky membranes and acidic cell compartments, when they reach the end of their lives. Those findings suggest that rather than breaking down with age, yeast purposefully die. Genetically manipulated lines with exceptionally long life spans did not show such features when they expired, however. In addition, boosting quantities of the antioxidant protein SOD1 postponed death in otherwise unaltered yeast, suggesting that large amounts of reactive oxygen species promote programmed death in yeast, as they do in mammalian cells. Together, the work supports the existence of a suicide pathway in yeast and suggests that circumventing this process extends yeast life.
To assess how that behavior might help yeast cultures, the researchers scrutinized a yeast survival mechanism known as adaptive regrowth, which helps the fungus persevere through desperate times. After most of the cells in a starving yeast culture die, a few begin dividing again. Longo and colleagues found that when they grew strains without SOD1 or other death-defying proteins, such as RAS2 or SCH9, the cultures were more likely to exhibit adaptive regrowth than were cultures of normal cells. In contrast, strains of long-lived yeast rarely showed adaptive regrowth. The results suggest that by giving up their own potential for long life, yeast are able to die for their brethren.
Next, the team mixed normal cells with those that lack SOD1 or produce an excess of it. After 21 days, normal cells had outcompeted those with surplus SOD1 but dwindled compared with those that lack it. Moreover, the team found that in cultures that lack the enzyme, yeast DNA mutates more rapidly than in those with bonus servings, perhaps because an increase in oxidants batters the genetic material. The resulting perturbations might create rare cells that survive better than their kin. Together, the observations suggest that yeast populations with the capacity to engage in programmed death and adaptive regrowth gain an edge--even though individual cells die younger than do those that lack those capacities. "If you try to live as long as possible, you're going to get in trouble," says Longo. Yeast produce just the right amount of SOD1 so that they can fight off oxidative stress yet still contribute to adaptive regrowth, he says.
Further experiments strengthen the idea that dead cells feed the survivors. The team collected the growth broth from cultures of yeast without SOD1 and fed it to other yeast. The recipient cells grew better than did those nourished with broth from normal yeast.
Another study, reported in the 16 February issue of the Journal of Cell Biology, supports the conclusion that intentional dying can benefit neighboring yeast. Herker and colleagues also discovered that old yeast show signs of apoptosis. In addition, they found that cells without YCA1, the yeast caspase, don't exhibit adaptive regrowth and can't compete with normal cells in a mixed culture (see "I Regret That I Have But One Life to Give for My Colony").
"This is a pretty remarkable finding," says Burhans. "Although deleting a gene that confers death on the cell initially provides an advantage, in the end there is a clear advantage to retaining that gene."
But researchers are only starting to figure out why yeast might sacrifice themselves, say other experts. "Data support the existence of genes that promote cell death," says Goldfarb. "Why they promote death, I don't know." These studies are "just the beginning." Researchers know little about how yeast populations are organized in the wild; the benefits of apoptosis might be minimized if nutrients from dying yeast fed a different population of yeast or other organisms, for instance. "More work needs to be done on almost every part of the yeast story," Goldfarb says. That enterprise should help clarify whether a yeast cell's death is a heroic effort to save its kind.
November 17, 2004
R. John Davenport, an associate editor of SAGE KE, writes from Santa Cruz, California. He ardently supports altruistic behavior in others.
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