Animal Models of Aging
Fly, yeast, mice, rat, frog, worm, sea urchin, etc., this list was not the first part of the Noah’s Arch animals but merely the animal models used by several scientists which have been awarded with the Nobel Prize in Medicine or Physiology. As for example, from the year 2000 to 2017, up to 78% of the awarded Nobel Prizes in Physiology or Medicine concerned pioneering works that have been done, at least partially, on animal models (The Nobel Foundation 2018). Aging research also uses animal models, and the ones presented here have already contributed to draw a global vision of aging, to explore its plasticity and its molecular control.
The Budding Yeast (Saccharomyces cerevisiae): Toward the Molecular Roots of Aging
As the simplest unicellular model organism described herein (5–10 μm length), the budding yeast, or baker’s yeast, can divide roughly up to 26 times before entering in post-replicative senescence, a process meaning a total arrest of cell division but not an immediate cell death (Kaeberlein 2010). Thus, the yeast aging can be analyzed either by counting variations of the total number of cell division or by measuring the cell survival after cell division arrest. As they divide and age, the yeast cells accumulate extrachromosomal rDNA circles that are self-replicating sequences of ribosomal DNA. Such small DNA pieces are suspected to contribute to the yeast aging process because extrachromosomal rDNA circles accumulation can be detected in the old budding yeast (Sinclair and Guarente 1997).
It has been shown (Aguilaniu et al. 2003) a protein damage accumulation in the yeast. More interestingly, a report (Unal et al. 2011) uncovered that accumulated damaged proteins are eliminated during the gametes’ synthesis for reproduction then resetting the age of the newly produced yeast cells. Clearing process of damaged proteins has been subsequently reported in both the roundworm (Goudeau and Aguilaniu 2010) and the fruit fly models (Fredriksson et al. 2012) and also during the murine embryogenesis (Hernebring et al. 2006) then arguing for an evolutionary conservation of this rejuvenation process (see “Cellular Proteostasis in Aging”).
Molecular pathways involved in human aging are evolutionarily conserved and active in the budding yeast as, for example, the nutrient signalling pathway (see “Human Aging and Metabolism”). By limiting the growth medium of the yeast, it is possible to induce a calorie restriction (also known as dietary restriction) that results in an increased lifespan (see “Diet and Caloric Restriction”). Two gene products modulating aging have been discovered. The SIR2 (silent mating-type information regulation 2) gene encodes for a histone deacetylase enzyme whose activity depends on the cellular levels of NAD, NADH, or nicotinamide. Thus, Sir2 links the yeast metabolism with specific protein targets by acetylation to mediate calorie restriction and modulates aging (Guarente 2011). A second set of the yeast genes that modulate aging was discovered during a screen addressed to identify targets of the immunosuppressant rapamycin and was named TOR (target of rapamycin) (Heitman 2015). The cell membrane-bound TOR protein kinase can sense nutrients (mainly amino acids). Inhibiting the nutrient sensing by inhibiting TOR with rapamycin mimics calorie restriction and postpone the yeast aging. A yeast TOR homologue was subsequently identified in human as mTOR (mammalian target of rapamycin) demonstrating the relevance of the yeast model to decipher the fundamental process that modulates aging (see “Diet and Caloric Restriction”). Experimental reduction of the TOR activity can increase lifespan in budding yeast as well as in other aging models such the fruit fly Drosophila melanogaster or the roundworm Caenorhabditis elegans (Kaeberlein et al. 2005; Jia et al. 2004; Kapahi et al. 2004), while the mice lifespan can be extended by a mTOR inhibition with rapamycin (Harrison et al. 2009).
The budding yeast also permitted the discovery of a fundamental process involved in aging. In 2016, Yoshinori Ohsumi was awarded by the Nobel Prize in Physiology or Medicine for his discovery of the autophagy mechanisms in yeast (Tsukada and Ohsumi 1993). The autophagy process allows cells to degrade or recycle cellular components (see “Autophagy”). Autophagy is an evolutionarily conserved mechanism involved in aging in the yeast and other organisms including mammals (Dunn et al. 2013).
The Fruit Fly Drosophila melanogaster: When Stress Meets Aging
Easy to handle in laboratory for genetic studies, the fruit fly has been extensively used in aging research (Lints and Soliman 1988). A 2 mm length fruit fly adult can live for up to 60–80 days (with a short generation time of 9 days), and large brood sizes allow experiments on large animal cohorts to assess for the effects of calorie restriction or other treatments acting on longevity (see “Diet and Caloric Restriction”). Like the worm model Caenorhabditis elegans, the fruit fly adult is mainly a postmitotic model with only cell division in gonads, gut, and Malpighian tubes. Thus, observations of age-linked cellular decline can be simplified in this organism because of the absence of renewal by new dividing cells. Quantitative observations of aging are simplified in the fruit fly because of a clear separation between the developmental period and adulthood, which begins at the time of exit from the pupa. The fruit fly gathers several advantages for aging analysis with the isolation of genetic mutants that increase life span, molecular tools available, and a full sequenced genome since 2000. Positive modulators of the lifespan like the hormetic process have been evaluated by applying to fruit fly low dose of stress (Hercus et al. 2003). Positive effect on aging by the hormesis process has been afterward extended to the roundworm Caenorhabditis elegans and the mammals (Le Bourg and Rattan 2008; Gems and Partridge 2008).
As genetic model, the fruit fly is served for the first experiments linking antioxidants and lifespan (Le Bourg 2001). It has been shown that fruit fly cohorts selected for extended lifespan exhibited increased oxidative stress resistance more than the rest of the population (Service et al. 1985), while overexpression of genes encoding proteins acting as antioxidant enzymes extended lifespan (Orr and Sohal 1994). On the basis of these observations, it is believed that the fruit fly healthy aging can be correlated with oxidative stress resistance (see “Oxidation Damage Accumulation Aging Theory”).
The Roundworm Caenorhabditis elegans: Exploring the Plasticity of Aging
With a 1 mm length and 959 cells, the roundworm adult hermaphrodite is a postmitotic model that shows a mitotic activity in the gonad only. This simplified feature favors exploration of the aging-linked cell degeneration. Three days are needed for each generation and allow faster genetic studies (Brenner 1974). With a large brood size and cultivation at various temperatures, the roundworm allows quantitative experiments on aging and is amenable for automated molecular target screenings (Bazopoulou et al. 2017). Up to 60% of human genes involved in pathologies are present in the roundworm genome. Worm populations can be easily synchronized for longevity analysis. The mean lifespan of only 17 days at 20 °C allows fast experiments. The discovery of a worm gerontogene family, whose downregulation or inactivation results in extended longevity, has opened the way for classification and networking of aging-modulating genes. Furthermore, identical genetic networks operating in mammals have been identified (see “Genetic Control of Aging”). The worm insulin/IGF-1 molecular pathway was the first identified to modulate the aging plasticity (Friedman and Johnson 1988; Kenyon et al. 1993), an evolutionarily conserved process also acting in the mice (Holzenberger et al. 2003). Worm gene encoding for FOXO, sirtuins, as well as mTOR is involved in molecular pathways regulating aging (see “Sirtuins”). Because of a short generation time, the roundworm allowed recent analysis for epigenetic modulations effects on lifespan (Greer et al. 2011) and emerged recently as a model for microbiome-dependent effects on aging (Cabreiro and Gems 2013) because the roundworm are fed with Escherichia coli.
Rodents: Mice (Mus musculus) and Rat (Rattus norvegicus)
The use of rodents in aging research has been evidenced by their closest evolutionary relationship to human than the previous models described herein. With a mean lifespan of 3 years, several experiments of gene knockouts have verified the importance of the somatotrope axis in the lifespan regulation and the discovery of anti-gerontogenes (see “Genes that Delay Aging”) like Klotho (an evolutionary conserved gene product also acting in the roundworm (Château et al. 2010) to supports longevity) whose genetic deletion shortens drastically mice lifespan (Kuro-o et al. 1997). Models of human progeria syndrome like Hutchinson Gilford or aging-linked disease such Alzheimer have been obtained with transgenic-engineered mice and allowed analyses at cellular and molecular levels (see “Transgenic Mice”). However, human and rodents are different and the obtained results have to be taken with caution (Demetrius 2005). Rodent models have been used for calorie restriction-induced effect on lifespan, which can be extended up to 50% (see “Diet and Caloric Restriction”). A recent rediscovery of the heterochronic parabiosis (by connecting the blood flow from a young mouse to an old mouse) in mice and its rejuvenation effects has shed new insight on plasmatic factors modulating aging as therapeutic target candidates (Scudellari 2015). Finally, the rodents are classical models for assessing aging factors linked to pathological cognitive decline because of a wide behavior spectrum. Finally, recent advances linking microbiome and aging have been done with success in the rodent models (Seidel and Valenzano 2018).
The Nonhuman Primates: Rhesus Monkeys and Microcebus
As the closest models to humans, nonhuman primates have been used for experiments to assess the benefits of calorie restriction on aging (see “Diet and Caloric Restriction”). However, reports on the positive effects of calorie restriction are contradictory and do not allow for a clear identification of a positive effect of the calorie restriction on either age-related degeneration or survival (Mattison et al. 2012; Colman et al. 2009).
With a 26 cm size and a weight of 60–90 grams, the gray mouse lemur (Microcebus murinus) originates from Madagascar and has been developed as age-associated dementia model for the Alzheimer’s disease (Bons et al. 2006). While the laboratory breeding allows 10% of the animal population to reach 10–13 years of lifespan, Microcebus normally dies at 4–6 years in its natural environment. When aged of 5 years, animal develops typical Alzheimer’s disease amyloid plaques allowing biochemical and molecular analyses (see “Alzheimer”). Age-associated cognitive decline analyses are also amenable with the Microcebus model.
New Emerging Models: The Naked Mole-Rat and the African Turquoise Killifish
With an average weight of 35 grams, the naked mole-rat (Heterocephalus glaber) is the longest living rodent (up to 32 years) and an emerging model in the field of aging (Buffenstein 2005). Its difficult breeding needs to maintain a colony with several animals and a queen committed for reproduction. Recent explorations reported that naked mole-rat is resistant to tumors and shows a highly active DNA repair function and ribosomes able to produce high-quality error-free proteins. Metabolism analysis and molecular inventory yet try to identify the gene expression possibly linked to their extreme longevity.
The African turquoise killifish (Nothobranchius furzeri) has been recently proposed for aging research (Hu and Brunet 2018) because of its exceptional very short lifespan (4–6 months) for a vertebrate in comparison with the mice (3 years). In combination with genetic and genomic tools and a short generation time (~40 days), the African turquoise killifish allows faster analysis of aging control with many features that are absent in other canonical aging models described herein such as the yeast, the fruit fly, or the roundworm. Moreover, the African turquoise killifish can be easily handled and breed at the laboratory. Vertebrate age-linked degeneration features can be observed in the African turquoise killifish as, for example, muscle loss (resembling sarcopenia in human, see “Sarcopenia”), kyphosis (as a result of osteoporosis in women), or defects in tissue homeostasis.
The use of animal models has led to further research on aging by uncovering unsuspected physiological and molecular mechanisms. Such mechanisms allow for new assessments of aging-linked process such microbiome, calorie restriction, mild stress (hormesis), autophagy, protein clearance, anti-aging humoral factors, or DNA repair mechanisms and maintenance.
It is noteworthy that animal models of aging have not been selected with the goal to analyze their own aging process but merely because of the simplicity of the mechanisms involved when compared to human. For example, while human tissues can replace damaged cells, the roundworm does not renew somatic cells during the adult life. Notwithstanding this limitation, experiments with the roundworm permitted important steps in aging research. In fact, aging models do not allow for addressing all of the aging issues because they are, per se, simple models where some functions or biological process are absent. The interest for aging models has been strengthened by the genome sequence availability that helped the identification of evolutionarily conserved genes acting on human aging and age-linked diseases. Thus, the respective contribution of each model to the aging research helps to draw a comprehensive view of aging.
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- The Nobel Foundation (2018) https://www.nobelprize.org/prizes/medicine/ (01/08/2018)