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Paradigms in aging research: a critical review and assessment
Mechanisms of Ageing and Development 117 (2000) 21 – 28 www.elsevier.com/locate/mechagedev
Review
Paradigms in aging research: a critical review and assessment Harriet Gershon a, David Gershon b,* a
Department of Immunology, Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa 32000, Israel b Faculty of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 15 April 2000; accepted 15 May 2000
Abstract This is the first in a series of articles in which we intend to critically review some of the currently used models in gerontology research and evaluate their contribution to advancing our understanding of the phenomenon of senescence. The major theories of aging are considered. We discuss what makes a model useful in general and for aging research in particular. We suggest criteria for the selection of paradigms for the study of aging. The criteria we suggest for identifying underlying mechanisms that lead to age related changes are: intraspecies universality, intrinsicality, progressiveness, and interspecies universality. The subsequent articles of this series shall consider the merits and possible drawbacks of some of the most commonly used models of the biology of aging: (a) the yeast Saccharomyces cere6isiae; (b) the nematode, Caenorhabditis elegans and the fruitfly, Drosophila melanogaster; (c) mammalian cells in culture and telomerase model; (d) mitochondria and aging; (e) progeroid syndromes; (f) in vivo studies with laboratory rodent strains; and (g) plant senescence. © 2000 Published by Elsevier Science Ireland Ltd. Keywords: Aging research; Experimental paradigms
* Corresponding author. Tel.: +972-4-8293961; fax: +972-4-8225153. E-mail address: [email protected] (D. Gershon). 0047-6374/00/$ - see front matter © 2000 Published by Elsevier Science Ireland Ltd. PII: S 0 0 4 7 - 6 3 7 4 ( 0 0 ) 0 0 1 4 1 - X
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H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
1. Introduction The past 20 years have witnessed a considerable interest in the aging phenomenon among researchers in varied fields of biology and medicine. New exciting research methodology and tools have been introduced into this field from the areas of developmental and cellular biology and molecular genetics. The accelerating pace of research in modern biology has inevitably caused an intensive search for models that are relatively simple and can provide rapid results. Often this reductionist trend has been accompanied by a lack of consideration of the biology of the whole multi-cellular organism with its inter-dependent networks of cellular communications. Also, there has not been sufficient consideration of the interactions of the organism with its environment under laboratory conditions as compared to the conditions it encounters in its natural habitat. We intend to critically review some of the currently used models in gerontology research and evaluate their contribution to advancing our understanding of the phenomenon of senescence.
2. What makes a model useful in general and for aging research in particular? The purpose of experimental paradigms is to test and evaluate existing theories that attempt to explain the causes and underlying mechanisms of observed phenomena. A very appropriate example of such theory is Orgel’s theory of ‘error catastrophe’ (Orgel, 1963) as a cause of senescence. This intellectually attractive theory postulated that due to very small, but significant levels of infidelity in protein synthesis, inevitably a stage will arise during the lifespan of an organism at which the accumulation of random errors increases exponentially. It was proposed that when the levels of error in proteins exceed a certain threshold a catastrophe in cellular processes would occur and lead to increased morbidity and loss of viability. This theory led us to make a few testable predictions regarding biochemical events that should be observed if the levels of random errors were to increase during the lifespan of an individual (Gershon, 1987). These predictions were that; (a) faulty protein (enzyme) molecules should accumulate with age; (b) at a certain stage, when proteins involved in the synthesis of other proteins become affected, the rate of errors would become exponential; (c) enzyme molecules containing random errors would have identifiable charge differences; (d) error bearing enzyme molecules should have measurably altered Km and Ki; (e) the rate of mutations will be considerably increased in old individuals. Two animal models were used to test this hypothesis, nematodes (Gershon, 1970; Gershon and Gershon, 1970; Zeelon, et al., 1973) and later rodents (Gershon and Gershon, 1973; Reiss and Gershon, 1976a,b). It was shown beyond doubt that although faulty protein molecules did accumulate with age, they were the result of post-translational modifications rather than amino acid substitutions (Reznick et al., 1985). All the other predicted parameters could not be found. Moreover, it was found that the accumulation of faulty protein molecules was due to an age-related decrease in the efficiency of the protein degradation system (Reznick et al., 1981; Lavie et al., 1982). These experimental
H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
23
observations both refuted Orgel’s theory and led to new findings that suggested underlying mechanisms of aging that had not been recognized previously. The use of the nematodes and later verification in mammalian tissues enabled the attainment of these significant conclusions. These novel results also opened up other directions of research aimed at better understanding of underlying mechanisms of aging (Gershon and Rott, 1988). Since aging appears as a complex and diverse phenotype, there is a need to dissect it into components and utilize a variety of experimentally suitable models to investigate each component separately. However, it should always be kept in mind that biological phenomena are comprised of inter-related pathways that are often controlled by complex mechanisms. Therefore, an attempt must be made to evaluate observations and interpretations in light of the broad context of the process of senescence. It is thus imperative that the biology of a system is examined at all levels of organization, from molecules and genes to tissues, organs and ultimately the whole organism. Moreover, the interaction of the organism with its environment in its natural habitat must be considered in the selection and evaluation of any particular experimental paradigm.
3. Criteria for the selection of paradigms of aging Biological aging is characterized by a gradual decline in the capacity to respond to environmental challenges. When this decline reaches a certain threshold the survival capacity of the organism is compromised. Based on this premise, one can select suitable experimental models that meet some basic criteria. 1. Simplicity of the model and suitability for experimental manipulation in the desired field of research (genetics, molecular biology, biochemistry and cell biology). Here are included considerations of the morphological complexity of the model organism, the length of its life span, the size of the genome and suitability for genetic analysis. In addition, ample knowledge of the developmental program of the organism may be an important factor in the selection of the model system. 2. A very important criterion that is often ignored in many fields of research, including gerontology, is how well do the laboratory conditions under which the organisms (or cell cultures) are kept mimic the prevailing conditions in its natural habitat. Parameters to be considered here include nutrition, temperature, humidity, oxygen tension, photoperiodism and population density. Also, exposure to viruses, bacteria and other invasive organisms ought to be taken into account. All of these conditions are considerably variable and oscillating, and thus have extremely important effects on the acquired adaptability of the organism. In other words, one must understand and consider the effect of the unnatural conditions of careful husbandry that are generally maintained in the laboratory. These conditions are, of course, used in order to achieve high fecundity and ‘optimal’ welfare of laboratory organisms but present a very artificial set of living conditions for most organisms.
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H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
3. On the assumption that the ultimate goal of our research is the understanding of human aging, one must, at least theoretically, evaluate the relevance of the model system to mammals in general and humans in particular. As will be indicated in this series of reviews, caution should be exercised in extrapolating results and their interpretation from lower to higher eukaryotes. Researchers should be wary of the very specific features of structure, developmental pathways, genetic makeup (e.g. uni-cellularity of yeast cells with all its implications; the essentially hermaphroditic reproduction in Caenorhabditis elegans and the genetic make up of inbred strains of mice) and peculiar adaptations to various laboratory environmental conditions. A striking example is the dauer larvae of C. elegans with their peculiarities of development and adaptation to extreme environmental conditions that are not usually shared with higher eukaryotes. The differing in functions of homologous genes observed between the budding yeast Saccharomyces cere6iceae and multi-cellular organisms is another example where caution in interpretation of ‘aging’ results should be exercised. Great consideration should also be given to the fact that some of our most widely used models have undergone thousands (C. elegans) or, hundreds (rodent strains) of generations of breeding under ad libitum feeding conditions on diets entirely foreign to those encountered in the natural environment (e.g. the feeding of C. elegans with Escherichia. coli ). These examples and others will be discussed in a more detailed manner in later reviews in this series.
4. Major theories of aging that can be tested by the study of various experimental paradigms It is not intended here to extensively review the great number of theories of aging that merit experimental testing and evaluation. For more information on the multitude of theories associated with aging the reader is referred to a recent book edited by Bengtson (Bengtson and Schaie, 1999). In context with the present series, the reader is referred particularly to the reviews in that book by Finch, Cristofalo and Solomon (Cristofalo et al., 1999; Finch and Seeman, 1999; Soloman, 1999). The salient theories of aging can be broadly classified into two main categories. 1. Programmed Aging. This implies that there is a built-in program in the genome that is activated at a certain stage of the organism’s life cycle, which leads to death via the specification of a self-destruct mechanism in any of a variety of tissue or organ systems. This group of theories suggests selection in evolution for genes that specify the onset and progression of senescence leading to morbidity and consequently to mortality. Two major hypotheses can be tested with regard of this group of theories: 1.1. A program (somewhat like the programmed gene control of development) exists that controls the activation of a cascade of genes whose major and perhaps only function is to implement senescence. This implies that interference with the activity of any gene belonging to this chain, particularly
H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
25
via mutation, will modulate the life span of the organism. This gene modification should have a minimal effect on the development and fecundity of the organism. 1.2. Genes have emerged in the course of evolution which perform an essential early function during either the developmental or reproductive stages of the life cycle, whose continued activity in the post-reproductive stage is detrimental [see (Cristofalo et al., 1999)]. Alterations in these genes should then affect the development and/or reproduction of the organism. For instance, a delay in reproductive maturity, reduced fertility or behavioral alterations. 2. Aging is a result of stochastic e6ents. This group of theories postulates that senescence is a result of random damage that is incurred and accumulates throughout the life span of an organism. The damage is caused by environmental factors (e.g. faulty nutrition, oxidation, extreme temperatures and intermittent hypoxia) and affects innate mechanisms that are essential for metabolism, signal transduction, intercellular communication and response to various forms of stress. This damage gradually causes a reduction in the efficiency of the overall function of the organism and eventually results in its demise. Each species has a typical range of average and maximal life spans, a phenomenon that attests to some genetic determinants of longevity. It is our suggestion that this is due to interaction of genetic traits, that do not directly specify senescence, with environmental factors encountered by individual members of the species in their natural habitat (or alternatively, the artificial laboratory environment that is forced upon them). The alternative theories and any combination of them can be evaluated by the use of well-chosen experimental paradigms. It is imperative, however, that prior to the evaluation of experimental results, the limitations and merits of the selected paradigm be taken into account as noted above.
5. Criteria for identifying underlying mechanisms that lead to age related changes Criteria for identifying underlying mechanisms that lead to age related changes must be thoroughly considered in evaluating the results obtained from experiments using any paradigm. These have been clearly suggested and discussed by Strehler (1977), who observed that ‘much of the confusion which has surrounded the biological concept of aging in the past is due to failure to establish an useable definition of aging and objective criteria through which the results of specific research on animals at several different ages can be evaluated.’ Unfortunately, this assertion is still true more than two decades after the publication of Strehler’s book. Confusion is still encountered between primary causes of aging and secondary age-associated phenomena. Therefore, these criteria must be reiterated and emphasized, as it seems that they have been ignored in many instances in the past decade. The basic assumption underlying these criteria is that gradual changes in the
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structure and function of the organism, which are not due to curable disease or random accidents, lead to an increased probability of death as a function of time. There are four criteria [the first three adopted from Strehler (1977)] for basic, and not secondary, age-correlated alterations: 1. Intraspecies Uni6ersality: If a process or phenomenon is to be considered fundamental to aging it must be observed in all the older members of the species. This, of course, excludes specific (and quite rare) life-shortening genetic defects or diseases that are caused by special, drastic environmental conditions. The latter does not exclude the possibility that the increased propensity to develop diseases in older organisms is due to basic underlying mechanisms of senescence. It should be borne in mind, however, that the age at which these basic senescence phenomena are expressed might be variable, within certain limits, among individuals in any given population of organisms. For the experimentalist this means that no series of determinations of a certain parameter can at any given time encompass all the members of a species. It is, therefore, necessary to estimate the probability that a certain phenomenon is universal by statistical means that take into account the size of the sample included in the study. 2. Intrinsicality: Basic aging mechanisms are innate to the organism and are not exclusively attributable to modifiable environmental effects. 3. Progressi6eness: It is an acceptable premise that aging is a gradual process and is not a result of abrupt events. It is a process that commences relatively early in the life cycle of the organism and progresses due to the cumulative effect of small steps. These steps could be slight damage to cellular components (and, as we have shown, not due to increased infidelity of the synthetic systems) such as proteins, membrane lipids etc. that are not efficiently repaired. These, in turn, may cause physiological dysfunction at the cellular level and become significant when, with time, a certain proportion of cells in a tissue or an organ become affected. The accumulated damage is not the result of reduced fidelity of systems involved in macromolecule synthesis as suggested by Orgel and refuted by us (see above). Obviously, a large number of small inter-related events must take place in order to reach such a complex phenomenon as aging and mortality. This gives the total process a seemingly gradual smoothness. 4. Inter-species uni6ersality and rele6ance to human aging: The choice of any paradigm that is not human should take into account a basic premise that some of the most fundamental aging mechanisms are common to all eukaryotes. There are, however, certain peculiar species-specific causes of death. Interesting examples of these are the endocrinologically based programmed death following spawning of the migratory Pacific salmon and Rainbow trout (Robertson and Wexler, 1959; Robertson et al., 1961). However, it is questionable if it is a general underlying process of aging. This is a reversible phenomenon that does not occur in non-migratory populations of the same species. Based on the original observation it would have been very appealing to suggest that programmed death by hypercorticoadrenalism is a major mechanism of aging. However, a close scrutiny reveals this to be a special case that does not represent
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the process of aging among the vast majority of multi-cellular animals. Another example is the programmed senescence and death of the whole plant that is confined to the annuals. The obvious chain of processes that lead to annual plant ‘senescence’ indicates what a programmed senescence would look like had it occurred in all organisms. Such a program is not usually observed in other plants and animals. This will be discussed in more detail in a later review in this series. All the above criteria will be employed as ‘ground rules’ in the following series of reviews to assess the merits and possible drawbacks of some of the most commonly used models in studies on the biology of aging: (a) the yeast S. cere6isiae; (b) the nematode, C. elegans and the fruitfly, Drosophila melanogaster; (c) mammalian cells in culture and the telomerase model; (d) mitochondria and aging; (e) progeroid syndromes; (f) in vivo studies with laboratory rodent strains; and (g) plant senescence. References Bengtson, V.L. and Schaie K.W. (editors.) 1999. Handbook of Theories of Aging. Springer, New York. Cristofalo, V.J., Tresini, M., Francis, M.K., Volker, C., 1999. Biological theories of senescence. In: Bengton, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 98–112. Finch, C.E., Seeman, T.E., 1999. Stress theories of aging. In: Bengtson, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 81 – 97. Gershon, D., 1970. Studies on aging in Nematodes. I. The nematode as a model organism for aging research. Exp. Gerontol. 5, 7–12. Gershon, D., 1987. The ‘error catastrophe’ theory of aging and its implications. In: Warner, H., Butler, R.N., Sprout, R.L. (Eds.), Modern Biological Theories of Aging. Raven Press, New York, pp. 135–137. Gershon, H., Gershon, D., 1970. Detection of inactive enzyme molecules in aging organisms. Nature 227, 1214–1217. Gershon, H, Gershon, D., 1973. Inactive enzyme molecules in aging mice: liver aldolase, Proc. Natl. Acad. Sci. USA, 909–913. Gershon, D., Rott, R., 1988. Studies on the nature of the faulty protein molecules and their diminished degradation in cells of aging organisms: functional implications. In: Ermini, M. (Ed.), Crossroads in Aging Research. Academic Press, New York, pp. 25 – 33. Lavie, L., Reznick, A.Z., Gershon, D., 1982. Decreased protein and puromycinyl-peptide degradation in livers of senescent mice. Biochem. J. 202, 47 – 51. Orgel, L.E., 1963. The maintenance of the accuracy of protein synthesis and its relevance to aging. Proc. Natl. Acad. Sci. USA 49, 517–521. Reiss, U., Gershon, D., 1976a. Rat-liver superoxide dismutase. Purification and age-related modifications. Eur. J. Biochem. 63, 617–623. Reiss, U., Gershon, D., 1976b. Comparison of cytoplasmic superoxide dismutase in liver, heart and brain of aging rats and mice. Biochem. Biophys. Res. Commun. 73, 255 – 262. Reznick, A.Z., Lavie, L., Gershon, H.E., Gershon, D., 1981. Age-associated accumulation of altered FDP aldolase B in mice. Conditions of detection and determination of aldolase half life in young and old animals. FEBS Lett. 128, 221–224. Reznick, A.Z., Rosenfelder, L., Shpund, S, Gershon, D., 1985. Identification of intracellular degradation intermediates of aldolase B by antiserum to the denatured enzyme. Proc. Natl. Acad. Sci. USA 82, 6114–6118.
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Robertson, O.H., Wexler, B.C., 1959. Hyperplasia of the adrenal cortical tissue in Pacific salmon and Rainbow trout accompanying sexual maturation and spawning. Endocrinology 1, 225 – 238. Robertson, O.H., Drupp, M.A., Thomas, S.F., Favom, C.B., Hane, S., Wexler, B.C., 1961. Hyperadrenocorticism in spawning migratory and non-migratory Rainbow Trout (Salmo gairdnerii): Comparison with Pacific Salmon (Genus Oncorhynchus). Endocrinology 65, 473 – 484. Soloman, D., 1999. The role of aging processes in aging-dependent diseases. In: Bengton, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 133 – 152. Strehler, B.L., 1977. Time, Cells, and Aging 2nd Edition, (Ed.), Academic Press, New York. Zeelon, P., Gershon, H., Gershon, D., 1973. Inactive enzyme molecules in aging organisms. Nematode fructose-1,6-diphosphate aldolase. Biochemistry 12, 1743 – 1750.
.
Review
Paradigms in aging research: a critical review and assessment Harriet Gershon a, David Gershon b,* a
Department of Immunology, Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa 32000, Israel b Faculty of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 15 April 2000; accepted 15 May 2000
Abstract This is the first in a series of articles in which we intend to critically review some of the currently used models in gerontology research and evaluate their contribution to advancing our understanding of the phenomenon of senescence. The major theories of aging are considered. We discuss what makes a model useful in general and for aging research in particular. We suggest criteria for the selection of paradigms for the study of aging. The criteria we suggest for identifying underlying mechanisms that lead to age related changes are: intraspecies universality, intrinsicality, progressiveness, and interspecies universality. The subsequent articles of this series shall consider the merits and possible drawbacks of some of the most commonly used models of the biology of aging: (a) the yeast Saccharomyces cere6isiae; (b) the nematode, Caenorhabditis elegans and the fruitfly, Drosophila melanogaster; (c) mammalian cells in culture and telomerase model; (d) mitochondria and aging; (e) progeroid syndromes; (f) in vivo studies with laboratory rodent strains; and (g) plant senescence. © 2000 Published by Elsevier Science Ireland Ltd. Keywords: Aging research; Experimental paradigms
* Corresponding author. Tel.: +972-4-8293961; fax: +972-4-8225153. E-mail address: [email protected] (D. Gershon). 0047-6374/00/$ - see front matter © 2000 Published by Elsevier Science Ireland Ltd. PII: S 0 0 4 7 - 6 3 7 4 ( 0 0 ) 0 0 1 4 1 - X
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H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
1. Introduction The past 20 years have witnessed a considerable interest in the aging phenomenon among researchers in varied fields of biology and medicine. New exciting research methodology and tools have been introduced into this field from the areas of developmental and cellular biology and molecular genetics. The accelerating pace of research in modern biology has inevitably caused an intensive search for models that are relatively simple and can provide rapid results. Often this reductionist trend has been accompanied by a lack of consideration of the biology of the whole multi-cellular organism with its inter-dependent networks of cellular communications. Also, there has not been sufficient consideration of the interactions of the organism with its environment under laboratory conditions as compared to the conditions it encounters in its natural habitat. We intend to critically review some of the currently used models in gerontology research and evaluate their contribution to advancing our understanding of the phenomenon of senescence.
2. What makes a model useful in general and for aging research in particular? The purpose of experimental paradigms is to test and evaluate existing theories that attempt to explain the causes and underlying mechanisms of observed phenomena. A very appropriate example of such theory is Orgel’s theory of ‘error catastrophe’ (Orgel, 1963) as a cause of senescence. This intellectually attractive theory postulated that due to very small, but significant levels of infidelity in protein synthesis, inevitably a stage will arise during the lifespan of an organism at which the accumulation of random errors increases exponentially. It was proposed that when the levels of error in proteins exceed a certain threshold a catastrophe in cellular processes would occur and lead to increased morbidity and loss of viability. This theory led us to make a few testable predictions regarding biochemical events that should be observed if the levels of random errors were to increase during the lifespan of an individual (Gershon, 1987). These predictions were that; (a) faulty protein (enzyme) molecules should accumulate with age; (b) at a certain stage, when proteins involved in the synthesis of other proteins become affected, the rate of errors would become exponential; (c) enzyme molecules containing random errors would have identifiable charge differences; (d) error bearing enzyme molecules should have measurably altered Km and Ki; (e) the rate of mutations will be considerably increased in old individuals. Two animal models were used to test this hypothesis, nematodes (Gershon, 1970; Gershon and Gershon, 1970; Zeelon, et al., 1973) and later rodents (Gershon and Gershon, 1973; Reiss and Gershon, 1976a,b). It was shown beyond doubt that although faulty protein molecules did accumulate with age, they were the result of post-translational modifications rather than amino acid substitutions (Reznick et al., 1985). All the other predicted parameters could not be found. Moreover, it was found that the accumulation of faulty protein molecules was due to an age-related decrease in the efficiency of the protein degradation system (Reznick et al., 1981; Lavie et al., 1982). These experimental
H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
23
observations both refuted Orgel’s theory and led to new findings that suggested underlying mechanisms of aging that had not been recognized previously. The use of the nematodes and later verification in mammalian tissues enabled the attainment of these significant conclusions. These novel results also opened up other directions of research aimed at better understanding of underlying mechanisms of aging (Gershon and Rott, 1988). Since aging appears as a complex and diverse phenotype, there is a need to dissect it into components and utilize a variety of experimentally suitable models to investigate each component separately. However, it should always be kept in mind that biological phenomena are comprised of inter-related pathways that are often controlled by complex mechanisms. Therefore, an attempt must be made to evaluate observations and interpretations in light of the broad context of the process of senescence. It is thus imperative that the biology of a system is examined at all levels of organization, from molecules and genes to tissues, organs and ultimately the whole organism. Moreover, the interaction of the organism with its environment in its natural habitat must be considered in the selection and evaluation of any particular experimental paradigm.
3. Criteria for the selection of paradigms of aging Biological aging is characterized by a gradual decline in the capacity to respond to environmental challenges. When this decline reaches a certain threshold the survival capacity of the organism is compromised. Based on this premise, one can select suitable experimental models that meet some basic criteria. 1. Simplicity of the model and suitability for experimental manipulation in the desired field of research (genetics, molecular biology, biochemistry and cell biology). Here are included considerations of the morphological complexity of the model organism, the length of its life span, the size of the genome and suitability for genetic analysis. In addition, ample knowledge of the developmental program of the organism may be an important factor in the selection of the model system. 2. A very important criterion that is often ignored in many fields of research, including gerontology, is how well do the laboratory conditions under which the organisms (or cell cultures) are kept mimic the prevailing conditions in its natural habitat. Parameters to be considered here include nutrition, temperature, humidity, oxygen tension, photoperiodism and population density. Also, exposure to viruses, bacteria and other invasive organisms ought to be taken into account. All of these conditions are considerably variable and oscillating, and thus have extremely important effects on the acquired adaptability of the organism. In other words, one must understand and consider the effect of the unnatural conditions of careful husbandry that are generally maintained in the laboratory. These conditions are, of course, used in order to achieve high fecundity and ‘optimal’ welfare of laboratory organisms but present a very artificial set of living conditions for most organisms.
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H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
3. On the assumption that the ultimate goal of our research is the understanding of human aging, one must, at least theoretically, evaluate the relevance of the model system to mammals in general and humans in particular. As will be indicated in this series of reviews, caution should be exercised in extrapolating results and their interpretation from lower to higher eukaryotes. Researchers should be wary of the very specific features of structure, developmental pathways, genetic makeup (e.g. uni-cellularity of yeast cells with all its implications; the essentially hermaphroditic reproduction in Caenorhabditis elegans and the genetic make up of inbred strains of mice) and peculiar adaptations to various laboratory environmental conditions. A striking example is the dauer larvae of C. elegans with their peculiarities of development and adaptation to extreme environmental conditions that are not usually shared with higher eukaryotes. The differing in functions of homologous genes observed between the budding yeast Saccharomyces cere6iceae and multi-cellular organisms is another example where caution in interpretation of ‘aging’ results should be exercised. Great consideration should also be given to the fact that some of our most widely used models have undergone thousands (C. elegans) or, hundreds (rodent strains) of generations of breeding under ad libitum feeding conditions on diets entirely foreign to those encountered in the natural environment (e.g. the feeding of C. elegans with Escherichia. coli ). These examples and others will be discussed in a more detailed manner in later reviews in this series.
4. Major theories of aging that can be tested by the study of various experimental paradigms It is not intended here to extensively review the great number of theories of aging that merit experimental testing and evaluation. For more information on the multitude of theories associated with aging the reader is referred to a recent book edited by Bengtson (Bengtson and Schaie, 1999). In context with the present series, the reader is referred particularly to the reviews in that book by Finch, Cristofalo and Solomon (Cristofalo et al., 1999; Finch and Seeman, 1999; Soloman, 1999). The salient theories of aging can be broadly classified into two main categories. 1. Programmed Aging. This implies that there is a built-in program in the genome that is activated at a certain stage of the organism’s life cycle, which leads to death via the specification of a self-destruct mechanism in any of a variety of tissue or organ systems. This group of theories suggests selection in evolution for genes that specify the onset and progression of senescence leading to morbidity and consequently to mortality. Two major hypotheses can be tested with regard of this group of theories: 1.1. A program (somewhat like the programmed gene control of development) exists that controls the activation of a cascade of genes whose major and perhaps only function is to implement senescence. This implies that interference with the activity of any gene belonging to this chain, particularly
H. Gershon, D. Gershon / Mechanisms of Ageing and De6elopment 117 (2000) 21–28
25
via mutation, will modulate the life span of the organism. This gene modification should have a minimal effect on the development and fecundity of the organism. 1.2. Genes have emerged in the course of evolution which perform an essential early function during either the developmental or reproductive stages of the life cycle, whose continued activity in the post-reproductive stage is detrimental [see (Cristofalo et al., 1999)]. Alterations in these genes should then affect the development and/or reproduction of the organism. For instance, a delay in reproductive maturity, reduced fertility or behavioral alterations. 2. Aging is a result of stochastic e6ents. This group of theories postulates that senescence is a result of random damage that is incurred and accumulates throughout the life span of an organism. The damage is caused by environmental factors (e.g. faulty nutrition, oxidation, extreme temperatures and intermittent hypoxia) and affects innate mechanisms that are essential for metabolism, signal transduction, intercellular communication and response to various forms of stress. This damage gradually causes a reduction in the efficiency of the overall function of the organism and eventually results in its demise. Each species has a typical range of average and maximal life spans, a phenomenon that attests to some genetic determinants of longevity. It is our suggestion that this is due to interaction of genetic traits, that do not directly specify senescence, with environmental factors encountered by individual members of the species in their natural habitat (or alternatively, the artificial laboratory environment that is forced upon them). The alternative theories and any combination of them can be evaluated by the use of well-chosen experimental paradigms. It is imperative, however, that prior to the evaluation of experimental results, the limitations and merits of the selected paradigm be taken into account as noted above.
5. Criteria for identifying underlying mechanisms that lead to age related changes Criteria for identifying underlying mechanisms that lead to age related changes must be thoroughly considered in evaluating the results obtained from experiments using any paradigm. These have been clearly suggested and discussed by Strehler (1977), who observed that ‘much of the confusion which has surrounded the biological concept of aging in the past is due to failure to establish an useable definition of aging and objective criteria through which the results of specific research on animals at several different ages can be evaluated.’ Unfortunately, this assertion is still true more than two decades after the publication of Strehler’s book. Confusion is still encountered between primary causes of aging and secondary age-associated phenomena. Therefore, these criteria must be reiterated and emphasized, as it seems that they have been ignored in many instances in the past decade. The basic assumption underlying these criteria is that gradual changes in the
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structure and function of the organism, which are not due to curable disease or random accidents, lead to an increased probability of death as a function of time. There are four criteria [the first three adopted from Strehler (1977)] for basic, and not secondary, age-correlated alterations: 1. Intraspecies Uni6ersality: If a process or phenomenon is to be considered fundamental to aging it must be observed in all the older members of the species. This, of course, excludes specific (and quite rare) life-shortening genetic defects or diseases that are caused by special, drastic environmental conditions. The latter does not exclude the possibility that the increased propensity to develop diseases in older organisms is due to basic underlying mechanisms of senescence. It should be borne in mind, however, that the age at which these basic senescence phenomena are expressed might be variable, within certain limits, among individuals in any given population of organisms. For the experimentalist this means that no series of determinations of a certain parameter can at any given time encompass all the members of a species. It is, therefore, necessary to estimate the probability that a certain phenomenon is universal by statistical means that take into account the size of the sample included in the study. 2. Intrinsicality: Basic aging mechanisms are innate to the organism and are not exclusively attributable to modifiable environmental effects. 3. Progressi6eness: It is an acceptable premise that aging is a gradual process and is not a result of abrupt events. It is a process that commences relatively early in the life cycle of the organism and progresses due to the cumulative effect of small steps. These steps could be slight damage to cellular components (and, as we have shown, not due to increased infidelity of the synthetic systems) such as proteins, membrane lipids etc. that are not efficiently repaired. These, in turn, may cause physiological dysfunction at the cellular level and become significant when, with time, a certain proportion of cells in a tissue or an organ become affected. The accumulated damage is not the result of reduced fidelity of systems involved in macromolecule synthesis as suggested by Orgel and refuted by us (see above). Obviously, a large number of small inter-related events must take place in order to reach such a complex phenomenon as aging and mortality. This gives the total process a seemingly gradual smoothness. 4. Inter-species uni6ersality and rele6ance to human aging: The choice of any paradigm that is not human should take into account a basic premise that some of the most fundamental aging mechanisms are common to all eukaryotes. There are, however, certain peculiar species-specific causes of death. Interesting examples of these are the endocrinologically based programmed death following spawning of the migratory Pacific salmon and Rainbow trout (Robertson and Wexler, 1959; Robertson et al., 1961). However, it is questionable if it is a general underlying process of aging. This is a reversible phenomenon that does not occur in non-migratory populations of the same species. Based on the original observation it would have been very appealing to suggest that programmed death by hypercorticoadrenalism is a major mechanism of aging. However, a close scrutiny reveals this to be a special case that does not represent
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the process of aging among the vast majority of multi-cellular animals. Another example is the programmed senescence and death of the whole plant that is confined to the annuals. The obvious chain of processes that lead to annual plant ‘senescence’ indicates what a programmed senescence would look like had it occurred in all organisms. Such a program is not usually observed in other plants and animals. This will be discussed in more detail in a later review in this series. All the above criteria will be employed as ‘ground rules’ in the following series of reviews to assess the merits and possible drawbacks of some of the most commonly used models in studies on the biology of aging: (a) the yeast S. cere6isiae; (b) the nematode, C. elegans and the fruitfly, Drosophila melanogaster; (c) mammalian cells in culture and the telomerase model; (d) mitochondria and aging; (e) progeroid syndromes; (f) in vivo studies with laboratory rodent strains; and (g) plant senescence. References Bengtson, V.L. and Schaie K.W. (editors.) 1999. Handbook of Theories of Aging. Springer, New York. Cristofalo, V.J., Tresini, M., Francis, M.K., Volker, C., 1999. Biological theories of senescence. In: Bengton, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 98–112. Finch, C.E., Seeman, T.E., 1999. Stress theories of aging. In: Bengtson, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 81 – 97. Gershon, D., 1970. Studies on aging in Nematodes. I. The nematode as a model organism for aging research. Exp. Gerontol. 5, 7–12. Gershon, D., 1987. The ‘error catastrophe’ theory of aging and its implications. In: Warner, H., Butler, R.N., Sprout, R.L. (Eds.), Modern Biological Theories of Aging. Raven Press, New York, pp. 135–137. Gershon, H., Gershon, D., 1970. Detection of inactive enzyme molecules in aging organisms. Nature 227, 1214–1217. Gershon, H, Gershon, D., 1973. Inactive enzyme molecules in aging mice: liver aldolase, Proc. Natl. Acad. Sci. USA, 909–913. Gershon, D., Rott, R., 1988. Studies on the nature of the faulty protein molecules and their diminished degradation in cells of aging organisms: functional implications. In: Ermini, M. (Ed.), Crossroads in Aging Research. Academic Press, New York, pp. 25 – 33. Lavie, L., Reznick, A.Z., Gershon, D., 1982. Decreased protein and puromycinyl-peptide degradation in livers of senescent mice. Biochem. J. 202, 47 – 51. Orgel, L.E., 1963. The maintenance of the accuracy of protein synthesis and its relevance to aging. Proc. Natl. Acad. Sci. USA 49, 517–521. Reiss, U., Gershon, D., 1976a. Rat-liver superoxide dismutase. Purification and age-related modifications. Eur. J. Biochem. 63, 617–623. Reiss, U., Gershon, D., 1976b. Comparison of cytoplasmic superoxide dismutase in liver, heart and brain of aging rats and mice. Biochem. Biophys. Res. Commun. 73, 255 – 262. Reznick, A.Z., Lavie, L., Gershon, H.E., Gershon, D., 1981. Age-associated accumulation of altered FDP aldolase B in mice. Conditions of detection and determination of aldolase half life in young and old animals. FEBS Lett. 128, 221–224. Reznick, A.Z., Rosenfelder, L., Shpund, S, Gershon, D., 1985. Identification of intracellular degradation intermediates of aldolase B by antiserum to the denatured enzyme. Proc. Natl. Acad. Sci. USA 82, 6114–6118.
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Robertson, O.H., Wexler, B.C., 1959. Hyperplasia of the adrenal cortical tissue in Pacific salmon and Rainbow trout accompanying sexual maturation and spawning. Endocrinology 1, 225 – 238. Robertson, O.H., Drupp, M.A., Thomas, S.F., Favom, C.B., Hane, S., Wexler, B.C., 1961. Hyperadrenocorticism in spawning migratory and non-migratory Rainbow Trout (Salmo gairdnerii): Comparison with Pacific Salmon (Genus Oncorhynchus). Endocrinology 65, 473 – 484. Soloman, D., 1999. The role of aging processes in aging-dependent diseases. In: Bengton, V.L., Schaie, K.W. (Eds.), Handbook of Theories of Aging. Springer, New York, pp. 133 – 152. Strehler, B.L., 1977. Time, Cells, and Aging 2nd Edition, (Ed.), Academic Press, New York. Zeelon, P., Gershon, H., Gershon, D., 1973. Inactive enzyme molecules in aging organisms. Nematode fructose-1,6-diphosphate aldolase. Biochemistry 12, 1743 – 1750.
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