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Yeast is Useful in the Study of Aging, But Has its Limits

Much insight into the mechanisms of aging and metabolism in mammals has been obtained from studies of yeast, which might seem a little odd at first glance. Nonetheless many aspects of aging and variations in response to circumstances such as calorie restriction are fairly universal and certainly very ancient from an evolutionary perspective. They are shared across a broad range of species, and thus it can be cost-effective to run rapid studies in very short-lived species that are very unlike us. There are nonetheless important differences between these species and limits to what can be learned, however, and the research community is probably approaching these limits.

This is a very readable open access paper on the subject of whether or not yeast studies in aging are past their time, but note that the full paper is in PDF format only:

The success of experimental biology was possible due to the use of model organisms. It is believed that the mechanisms of aging have a universal character and they are conserved in a wide range of organisms. Yeast are a very popular model organism. The use of the budding yeast Saccharomyces cerevisiae as a model organism of gerontology was based on two essential assumptions. The first of them is that the existence of the reproduction limit of each single cell is a consequence of the aging process. In other words, it was assumed that unavoidable death of each individual cell is not a side effect of the chosen strategy of reproduction (budding), as was postulated, but of the aging process. The second assumption was that as the number of daughters produced by a single cell is rather independent of the conditions of growth and on the time that reproduction takes, therefore the age and longevity of yeast can be expressed as a number of the daughter cells produced, instead of units of time. In that case the conclusions drawn from the studies based on such unusual units cannot be directly applicable for other organisms. The comparison can be made only if the units used are at least proportional.

The definition of aging encompasses two different, although probably causally connected phenomena. The term senescence describes various adverse effects which decrease efficiency of vital processes and lead to visible structural changes of the organism. Unavoidable death of individuals seems to be a direct consequence of aging. From evolutionary point of view, aging is treated in two ways, as a programmable or not programmable process. Programmable theories treat aging as a process of adaptation, which is a specific mechanism leading to the altruistic death of an individual for the benefit of the population, and thus preventing a too high density of a population (Medawar Theory). However non-programmable theories, among which the most popular is the disposable soma theory, treat aging as a kind of trade-off between investment in reproduction and maintenance of the somatic cells. In this sense, priority lies in reproduction while aging is just a stochastic accumulation of damage that leads to impairment of functions and consequently to death. This opinion, which is probably right, has been recently challenged because a number of arguments were collected suggesting a quasi-programmable character of the proximal causes of death. The hyperfunction hypothesis, can be considered a part of the theory of antagonistic pleiotropy. Both seem to explain at least some aspects of the aging process.

Study of the aging process requires to designate certain universal criteria that would allow for their analysis regardless of the type of the model organism used. One such criterion which also corresponds to the definition of this process is to increase the mortality rate as a function of time and a decrease in fertility. However, these criteria even though adequate for many organisms, have raised some doubts as to their versatility, especially if we take into consideration the phenomenon of 'negligible senescence'. This term was introduced in relation to the specific group of organisms for which the mostly used criteria for aging cannot be used. This group includes among others turtles, rockfish or mole-rats. In these species a typical decrease in fertility or increased mortality with age is not observed. Also, no changes indicating a 'progressive loss of function' with age were observed there. Thus, the question arises about the universality of the aging process in the living world and universality of the mechanisms of aging.

Link: http://www.ncbi.nlm.nih.gov/pubmed/25493441

Comments

Human trials are really what is required in order to progress the Science to defeat aging rapidly. Yeast, Mice etc... are not really a good analogue for the Human body and as various experiments have shown what works on mice or yeast doesn't work on people.

Posted by: Steve H at December 15th, 2014 9:34 AM

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