Extending Yeast Lifespan with Lithocholic Acid
A couple of interesting papers are doing the rounds, in which researchers report on a fivefold extension of yeast chronological life span through what they look on as an exercise in forced microevolution. They subjected yeast strains to an environment containing lithocholic acid, which is actually pretty unpleasant if you're a yeast cell, and allowed the yeast to adapt through generations. Few survived, and those that did survived through the acquisition of mutations that helped them resist the damaging effects of lithocholic acid. As it turns out, there is considerable overlap between mutations that help resist lithocholic acid and mutations that help resist the forms of damage that cause aging in yeast. As a result a number of the mutant lineages are stable and long-lived once the lithocholic acid is no longer present.
This is all quite interesting as a potential path to ranking the relevance of various repair and stress resistance mechanisms in cell aging, and as a way to obtain mutant lineages in which these mechanisms are enabled in potentially novel ways. Yeast are only somewhat multicellular, however, and thus one has to be careful in extrapolating information obtained on aging from yeast to animals. Since enhanced longevity through calorie restriction and cellular housekeeping mechanisms evolved very early on in the history of life, and since the underlying biological machinery is surprisingly similar in yeast and animals, yeast studies have proven very useful in understanding the ways in which cellular behavior changes in order to resist stress. There is no direct connection between cell life span and species life span, however, and when talking about yeast chronological age, it is the life span of a single cell that is under consideration.
Where the researchers overreach, I think, is in claiming that the observed outcome in microevolution argues strongly for programmed aging in macroevolution, in which aging is the result of a genetic program rather than an accumulation of biological wear and tear. It is perfectly possibly, however, to argue that their observations are in line with non-programmed aging theories in which aging is the result of damage accumulation; they have, after all, provided a way for cells to resist and repair damage to a greater degree than is usually possible through greater use of existing mechanisms. Further I'd say that the results at present are not necessarily at all relevant to the operation of macroevolution in the wild over longer periods of time, and again, the situation for single cells doesn't map directly to the situation for multicellular life.
As I understand it, there are views of the evolution of aging in which immortal species with unfettered reproduction are perfectly viable in and of themselves, but they will always be outcompeted in a changing environment by an aging species. Given the small number of mutations to produce yeast that lives five times longer than usual, why do we not see this yeast in the wild? We do not see immortals because they cannot exist, rather we do not see them because they are almost always quickly buried by their aging competitors whenever they do arise. Yet that apparently immortal animals can exist finds evidence in the absence of distinguishable aging in hydra, to pick the best known example. Even the negligible senescence observed in some species is somewhat challenging for the idea that long-lived organisms must necessarily grow more slowly and reproduce less efficiently than short-lived species.
Yeast mutants unlock the secrets of aging
The researchers exposed yeast to lithocholic acid, an aging-delaying natural molecule discovered in a previous study. In so doing, they created long-lived yeast mutants that they dubbed "yeast centenarians." These yeast mutants lived five times longer than their normal counterparts because their mitochondria - the part of the cell responsible for respiration and energy production - consumed more oxygen and produced more energy than in normal yeast. The centenarians were also much more resistant to oxidative damage, which is another process that causes aging. "This confirms that lithocholic acid, which occurs naturally in the environment, can not only delay yeast aging but can also force the evolution of exceptionally long-lived yeast."
The next step? Using yeast centenarians to test two types of aging theories: Programmed aging theories claim that organisms are genetically programmed to have a limited lifespan because aging serves some evolutionary purpose. That would mean that there are active mechanisms that cause aging and limit lifespan. Non-programmed aging theories contend that aging doesn't serve an evolutionary purpose. Therefore, an evolved mechanism whose main goal is to cause aging or limit lifespan simply cannot exist. What's more, non-programmed aging theories posit that any exceptionally long-lived organism must grow slower and reproduce less efficiently than an organism whose lifespan is limited at a certain age.
By producing long-lived yeast mutants and culturing them separately from normal yeast, the researchers were able to show that the centenarians grow and reproduce just as efficiently as the non-centenarians - thereby confirming programmed aging theories. "By confirming that there are active mechanisms limiting the longevity of any organism, we provided the first experimental evidence that such lifespan-limiting active mechanisms exist and can be manipulated by natural molecules to delay aging and improve health."
Exogenously added lithocholic bile acid and some other bile acids slow down yeast chronological aging by eliciting a hormetic stress response and altering mitochondrial functionality. Unlike animals, yeast cells do not synthesize bile acids. We therefore hypothesized that bile acids released into an ecosystem by animals may act as interspecies chemical signals that generate selective pressure for the evolution of longevity regulation mechanisms in yeast within this ecosystem. To empirically verify our hypothesis, in this study we carried out a three-step process for the selection of long-lived yeast species by a long-term exposure to exogenous lithocholic bile acid. Such experimental evolution yielded 20 long-lived mutants, three of which were capable of sustaining their considerably prolonged chronological lifespans after numerous passages in medium without lithocholic acid. The extended longevity of each of the three long-lived yeast species was a dominant polygenic trait caused by mutations in more than two nuclear genes. Each of the three mutants displayed considerable alterations to the age-related chronology of mitochondrial respiration and showed enhanced resistance to chronic oxidative, thermal, and osmotic stresses.
Our hypothesis posits the following: (1) only yeast exposed to exogenous bile acids can develop mechanisms of protection against cellular damage caused by these external stress agents and hormetic stimuli; (2) some of these mechanisms developed against bile acid-induced cellular damage can also protect yeast against damage and stress accumulated purely with age; (3) only those yeast species that have developed (due to exposure to exogenous bile acids) the most protective mechanisms against bile acid-induced cellular damage can also develop protective mechanisms against damage and stress accumulated with age; and (4) these yeast species are therefore expected to live longer. In this hypothesis, the presence of exogenous bile acids creates hormetic selective force that drives the evolution of not only protective mechanisms against bile acid-induced cellular damage but also longevity regulation mechanisms that protect against damage and stress accumulated with age. Moreover, this hypothesis suggests that yeast cells that are not exposed to exogenous bile acids cannot develop mechanisms of protection against cellular damage caused by these mildly toxic molecules. Thus, these yeast cells are unable to develop mechanisms of protection against damage and stress accumulated purely with age.
Empirical verification of evolutionary theories of aging
We recently selected 3 long-lived mutant strains of Saccharomyces cerevisiae by a lasting exposure to exogenous lithocholic acid. Each mutant strain can maintain the extended chronological lifespan after numerous passages in medium without lithocholic acid. In this study, we used these long-lived yeast mutants for empirical verification of evolutionary theories of aging. We provide evidence that the dominant polygenic trait extending longevity of each of these mutants 1) does not affect such key features of early-life fitness as the exponential growth rate, efficacy of post-exponential growth and fecundity; and 2) enhances such features of early-life fitness as susceptibility to chronic exogenous stresses, and the resistance to apoptotic and liponecrotic forms of programmed cell death.
These findings validate evolutionary theories of programmed aging. We also demonstrate that under laboratory conditions that imitate the process of natural selection within an ecosystem, each of these long-lived mutant strains is forced out of the ecosystem by the parental wild-type strain exhibiting shorter lifespan. We therefore concluded that yeast cells have evolved some mechanisms for limiting their lifespan upon reaching a certain chronological age. These mechanisms drive the evolution of yeast longevity towards maintaining a finite yeast chronological lifespan within ecosystems.