Sci. Aging Knowl. Environ., 3 October 2001
Published E-Letters on:
comments on Yeast and the Fountain of Youth
11 October 2001
This is a response to the article by Kaeberlein et al in the October 3 issue of SAGEKE. Although the authors alluded to our review on yeast as model for aging research (1), they neglected to mention our main arguments in that review which cast great doubt on the validity of yeast as a proper paradigm for aging research. 1)Unlike multi-cellular organisms yeast has no differentiation system which leads to the development of very specialized cell types with unique and limited reperoire of functions. Thus in order to maintain the integrity of the multi-cellular organism there has evolved an intricate communication system among the various cell types which constitutes the control over cell replication, physiological functions and indeed programmed cell death. Another tightly related system that has evolved is a the formation of a precise and strict internal homeostatic environmet that allows secialized cell systems to function and maintain viability under varying environmental challeges. The time-dependent impairment in inter-cellular communication and homeostasis are, therefore, major elements of aging. These are completely missing in yeast. 2) What the authors call "replicative senescence" in yeast has no analogous phenomenon in multi-cellular organisms. An appropriate example is the work of Cristofalo et al(2) which demnstrates that cells derived from healthy individuals of varying ages have the same replicative capacity in tissue culture. Also there are several studies that demonstrate no limitations on cell proliferation that lead to cell depletion in old organisms (reviewed in 3). 3) A review of work from various laboratories that investigate "aging" in yeast shows many discrepancies and contradictions that are quite hard to reconcile. This and other important points are discussed in detail in our review (1). The conclusions we made were that despite the outstanding characteristics and contributions of the yeast system to molecular genetics and cell biology it is a poor paradigm for aging of multi-cellular organisms. References: 1)H. Gershon, D. Gershon, The budding yeast, Saccharomyces cerevisiae, as a model for aging research: a critical review. Mech. Ageing Dev. 120,1-22 (2000). 2) V.J. Cristofalo et al., Relationship between donor age and the replicative lifespan of human cells in culture: a reevaluation. Proc. Natl,Acad. Sci. USA 95,10614-10619 (1998). 3)H. Gershon, D. Gershon, Critical assessment of paradigms in aging research. Exp. Geront. 36,1035-1047 (2001).
Re: comments on Yeast and the Fountain of Youth
25 October 2001
In his response to our Perspective, Dr. Gershon states that �yeast do not make a proper paradigm for aging research�. Clearly, we are not arguing that yeast are a perfect model for human aging. No model organism perfectly replicates the aging process in humans � that�s why they are models. Similar criticisms have been raised about the use of worms, flies, and mice to study aging. Some scientists would argue that human aging should only be studied in humans. We disagree.
As pointed out in our review, there are several specific features of human aging that have been, and continue to be, usefully modeled using yeast. These include oxidative stress, mitochondrial dysfunction, metabolic changes under caloric restriction, genomic integrity during aging, and the biology of Werner Syndrome/Bloom Syndrome/Rothmund-Thompson Syndrome. Furthermore, we believe that we have already gained valuable information from studying aging in yeast that suggests avenues of pursuit with respect to human aging. Specifically, the discovery that the SIR2 gene affects aging in both yeast and C. elegans begs the question of whether it also affects human longevity.
We would also like to respond specifically to Dr. Gershon�s criticisms. He claims that yeast lacks a �differentiation system which leads to the development of very specialized cell types with unique and limited repertoire of functions.� In fact, under non-standard growth conditions, yeast can form specialized cell types that are capable of pseudohyphal and/or invasive growth. This developmental switch is accompanied by a dramatic change in gene expression patterns. This example, though not directly related to yeast aging, exemplifies the principle that yeast possess the ability to respond to environmental changes with significant metabolic and morphological change. Multicellular organisms certainly have similar mechanisms for coping with environmental changes and stressful situations; it is likely that some of these can affect longevity. Therefore, the identification of the genetic pathways that modulate these responses in yeast should serve as a starting point for studies in higher eukaryotes.
Dr. Gershon also states that �A review of work from various laboratories that investigate aging in yeast shows many discrepancies and contradictions that are quite hard to reconcile.� This is a misleading argument. Some of these so-called �discrepancies� are due to strain specific differences in genetic background. In other words, genetic predispositions affect life span. The same thing is true in other model systems as well as humans, and should not be used to invalidate yeast aging. Other �discrepancies� have been caused by misinterpretation of results. For example, we and others have failed to detect more ERCs in sgs1 mutants relative to wild type (discussed in our perspective). This has been misinterpreted to suggest that ERCs do not cause aging in wild type cells � which would be a discrepancy. However, the correct interpretation is that sgs1 cells are not dying for the same reason as wild type cells, a hypothesis that has been subsequently verified.
Dr. Gershon�s final criticism, that replicative senescence plays no role in the aging of multicellular organisms, is also misleading. The study he cites does indeed suggest that cells from older individuals fail to show reduced replicative capacity. However, many other studies have found an inverse correlation between replicative capacity in tissue culture and donor age (for a recent example see Yan et al., Mech Age Dev. 122:695). The importance of replicative senescence in the human aging process is still an open issue, and is likely to be cell type specific. Clearly then, it is not appropriate to dismiss yeast as a model system based on this argument.
Matt Kaeberlein Department of Biology Massachusetts Institute of Technology
Mitch McVey Department of Biology University of North Carolina at Chapel Hill
The sir2 overexpression and knockout
28 October 2001
It is really great to see that a vigorous debate is starting on this web page. It is with great interest that I read your article summarizing yeast research. It seems to me that the strongest piece of evidence arguing that the clonal aging system in yeast has anything to do with aging in multicellular organisms is the "overexpression" of sir2 in C. elegans leading to increased longevity. The article cited, however, doesn't talk about overexpression but about a naturally occurring duplication in the sir2 gene of C. elegans. The obvious experiment to confirm whether higher levels of sir2 are really the causative agent in that strain is to make a transgenic worm.....is such an experiment planned or perhaps already published?
Most sincerely yours,
Re: The sir2 overexpression and knockout
29 October 2001
In response to the message from Florian Muller, a Sir-2.1 transgenic worm is in fact shown to have an extended life span. This experiment can be found in Tissenbaum and Guarente, Nature 410:227 (2001). In figure 2C, life span is shown for three different lines carrying multiple copies of Sir-2.1 integrated into the genome. In each case, both mean and maximum life span is longer than the wild type control.
The transgenic lines were constructed by PCR amplifying the Sir-2.1 gene and injecting the PCR product into wild type animals. This experiment is identical to the SIR2-overexpression experiment in yeast described in Kaeberlein et al., Genes Dev 13:2570 (1999), except for the following: (1) In yeast there were only two copies of SIR2, in worms there were multiple copies of Sir-2.1, and (2) the location of the chromosomal integration in worm
Re: Re: Re: The sir2 overexpression and knockout
19 November 2001
Charles R Fred,
﻿Re: Re: Re: The sir2 overexpression and knockout
Increasing Sir-2 expression via transgenic overrepresentation of Sir- 2 genes is only one of the paths suggested by this work.
The other path is to simulate the way caloric restriction increases Sir-2 expression - via an increased pool of NAD/NADH. Let me suggest a quick, easy experiment - with immediate and intimidating implications.
Divide a group of genetically normal mice into three groups, each fed differently. 1) A controlled diet simulating ad libitum eating; 2) a restricted calorie diet known to extend lifespan; 3) a controlled diet simulating ad libitum eating, but supplemented with additional NADH (either IV or orally with enteric coating).
Charles R. Fred firstname.lastname@example.org
Science of Aging Knowledge Environment. ISSN 1539-6150