The Importance of Animal Models in Defeating Aging
Using model animals in gerontological studies has yielded an enormous wealth of useful information about the mechanisms of human aging and longevity. Animal models were crucial in identifying the conserved pathways that regulate human aging. Model organisms are fundamental for aging research, because there are serious limitations of using human subjects, such as the length of lifespan, genetic heterogeneity and vast differences in environmental influences. The shape of survival curves represents the health of the organism over time. Model organisms display significantly different lifespans, however the survival curves resemble those of humans quite remarkably. Despite this general similarity in the way we describe aging between humans and model animals, there are some distinct differences (Mitchell, Scheibye-Knudsen, Longo, & de Cabo, 2015). For instance, increasing Sir2 gene expression in yeast (Kaeberlein, McVey, & Guarente, 1999), nematodes (Tissenbaum & Guarente, 2001), and flies (Rogina & Helfand, 2004) boosts animal longevity. A small molecule called resveratrol was found to activate Sir2 and its mammalian ortholog SIRT1 (Howitz et al., 2003). Resveratrol extends lifespan of mice fed a high-fat diet (Baur et al., 2006), however it failed to have a beneficial longevity effect in mice on a standard diet (Pearson et al., 2008). This example highlights the fact that we cannot simply transfer the results of longevity interventions to humans and expect the same efficacy as in invertebrate models.
Every day the researchers are broadening the understanding of human biology of aging with the help of various model systems. Each of them has its advantages and drawbacks. Let’s take a look at what those are for the most widely used animal models.
Yeast S. cerevisiae
Yeast have been instrumental in identifying the major conserved aging pathways shared among a large variety of species (Wasko & Kaeberlein, 2014). Despite the fact that yeast is a unicellular organism that has significant differences in its genetic pathways with humans, the advantages of using yeast as an aging model include their fast growth, low cost and easy storage and maintenances of organism strains. Over the years researchers have developed a broad variety of genetic manipulations that make yeast a powerful tool in the hands of an aging biologist.
Yeast replicative lifespan essay is based on the asymmetric division of mother and daughter yeast cells and was used to identify various types of factors contributing to senescence. The initial findings named the accumulation of extra-ribosomal DNA circles as the main type of damage that is inherited by the mother cell in the course of the asymmetric division. Later on a few additional types of accumulated damage were demonstrated to contribute to aging of the mother cells, such as damaged nuclear pore complexes (Kaeberlein, 2008; Shcheprova, Baldi, Frei, Gonnet, & Barral, 2008), cytosolic aggregates and oxidized proteins (Aguilaniu, Gustafsson, Rigoulet, & Nystrom, 2003; Erjavec & Nystrom, 2007) and dysfunctional mitochondria (Delaney et al., 2013; Lai, Jaruga, Borghouts, & Jazwinski, 2002). Establishing the exact role of these molecular damages in human aging remains an open question.
Nematode C. elegans
The roundworm Caenorhabditis elegans are a powerful model for studying aging due to their short lifespan, which is approximately 17 days at 20 °C (Tissenbaum, 2015). They are easy to culture and maintain strains, because nematodes can be kept frozen and suffer no apparent damage upon thawing. The animals are optically transparent can be used in high-throughput experiments, which makes them a perfect tool for answering the most pressing questions in biology of aging (Spivey & Finkelstein, 2014). Two RNAi libraries that cumulatively cover about 80% of C.elegans genome (Kamath et al., 2003; Rual et al., 2004) provide an excellent tool for identification of evolutionary conserved mechanisms of aging.
However, there are obvious drawbacks of using C. elegans as a model for human aging. They are evolutionarily distant from humans, lack tissues like brain, blood, they don’t have internal organs and are post-mitotic, meaning that nematodes lack the ability to regenerate their tissues and are limited in serving as a model of aging of highly proliferative tissues.
Nonetheless, C.elegans will remain a widely used system for elucidating aging mechanisms in part due to the possibility of performing whole genome screenings and the fact that these animals have been instrumental in identifying genetic pathways governing aging. In fact, at least three of the major aging pathways, including insulin/IGF-1 signaling (Kenyon, Chang, Gensch, Rudner, & Tabtiang, 1993; Morris, Tissenbaum, & Ruvkun, 1996), TOR pathway (Vellai et al., 2003), and SIR2 pathway (Tissenbaum & Guarente, 2001) were identified in C.elegans. Further studies of C.elegans biology of aging will increase our understanding of the common pathways between the model organisms and humans (Kennedy & Pennypacker, 2014)
Fruit fly D.melanogaster
Fruit flies have many advantages as a model system for aging studies. They have a relatively short lifespan of 60-80 days, which is more than that of a nematode, but compared to them drosophila have more distinct tissues and organs including the brain, eyes, kidney, liver and heart. Fruit flies have proliferating stem cell populations in their guts. Females can produce up to 200 eggs in the first 10 days, and they acquire reproductive ability within the first 10 days, which speaks of their high fecundity. Flies share about 60% of disease-related genes with humans (Matthews, Kaufman, & Gelbart, 2005), which makes them a desirable model also given their low cost and easy handling. There are several genetic systems that are used to create transgenic lines and manipulate single gene expression like UAS-GAL4 system, RNAi and dominant enchanser screening (Duffy, 2002; St Johnston, 2002). However, maintaining a transgenic strain is more costly and labor-heavy, since whole flies cannon be frozen and thawed without damage.
Hydra is definitely not the most popular model organism, but it might be overlooked quite groundlessly. Hydras are notorious for their negligible senescence. This very fact makes them a very desirable system to study. In fact, there is no apparent senescence in asexually reproducing hydras, yet the signs of aging can be seen after the organism reproduces sexually, such as loss of neurons and feeding behavior, interstitial somatic stem cell number reduction as well as disorganization of actin myofibers and loss of contractility (Yoshida, Fujisawa, Hwang, Ikeo, & Gojobori, 2006). Another overlooked fact is that hydras share 6071 genes with humans, whereas fruit flies have 5696 genes in common with humans, and nematodes – only 4751 (Wenger & Galliot, 2013). Among the known human aging genes at least 80% are shared with hydra (Tomczyk at al, 2014). Hydras may shed light on the mechanisms of exceptional longevity and the processes that regulate switching from a ‘non-aging’ to senescent phenotypes, which may eventually inform our understanding of molecular mechanisms of aging in humans.
The most widely used fish model is the zebrafish D.rerio. It lives for about 2-3 years, which is not particularly beneficial, because its lifespan is similar of rodents, but it is more evolutionarily distant from humans. Nonetheless, zebrafish has a remarkable ability to regenerate its tissues, which is an advantage for elucidating the mechanisms of tissue regeneration and longevity (Lepilina et al., 2006).
Another fish may be a more promising laboratory model for aging – turquoise killifish Nothobranchius furzeri. Killifish is one of the shortest-lived vertebrate with a lifespan of only 13 weeks (Gerhard, 2007). Its small size and high fecundity offer a considerable advantage in terms of reducing laboratory costs on housing and maintenance. A large number of age-related changes have been described in N.furzeri, and interestingly, the fish develops a high incidence of tumors with aging (Lucas-Sanchez, Almaida-Pagan, Mendiola, & de Costa, 2014). Turquoise killifish is responsive to calorie restriction and has increased longevity after administering resveratrol with the standard diet (Terzibasi, Valenzano, & Cellerino, 2007).
Mice are invaluable in aging research. There are approximately 99% of human orthologs in mice, which is a significant advantage compared to invertebrate models. Mouse lifespan is approximately 2-3 years depending on the strain, which makes them a more expensive tool in the arsenal of an aging biologist. Inbred mice have been studies very extensively and a large body of knowledge about aging mechanisms, age-related diseases and existing and potential therapies was created using this model. Using inbred lines is a double-edged sword: on one hand, genetic differences between animals are virtually non-existent, however this is not representative of human population and it is not clear to what extent the results can be transferred to humans. For example, about 70% of published experiments were done in C57BL/6 mouse strain, that has its peculiarities being an inbred strain, like high prevalence of lymphomas and increased susceptibility to metabolic dysregulation (Ward, 2006).
Rats have been studied in research setting in many areas, including age-related pathologies like cardiovascular diseases, cancer, neurological and renal disorders, as well as for behavioral studies of cognition. Both inbred and outbred lines are used in research. Interestingly, not all longevity interventions are successful in rats, for example, metformin extends lifespan in mice, but not in F344 rats (Martin-Montalvo et al., 2013; Smith et al., 2010).
Naked mole rats
Heterocephalus glaber, the naked mole rat, is the most long-lived rodent with a maximum life span of approximately 30 years(Jarvis, 1981). Naked mole rat exhibits negligible senescence, virtually no age-related increase in mortality and high reproduction levels until death (Mitchell et al., 2015). They have several signs of age-related pathology similar to humans, such as osteoarthritis and degeneration of the retina (Edrey, Hanes, Pinto, Mele, & Buffenstein, 2011). Naked mole rats can provide clues to mechanisms of longevity and potential therapies in humans, and hence are an extremely valuable model animal. There are several disadvantages of using them as laboratory animals, including specific housing conditions like low light levels, high temperature and humidity. Naked mole rats are eusocial animals; therefore they should be housed in colonies resembling undergrounds tunnels that they create in their natural habitat. Very long lifespan poses an obvious limitation on the variety of experiments suitable for this model, however conditions for naked mole rat cell culturing are described and genome of this animal is sequenced, which makes it a desirable system to study to elucidate the mechanisms of escaping age-related disorders.
Non-human primates are the closest animals to humans in terms of genetic, metabolic and behavioral differences. Rhesus macaques have been used in various types of research, however there are not too many studies of age-related mechanisms in primates. The main reasons for that are their long lifespan, which is more than 30 years, their size and weight, which complicate housing and maintenance and make this model an expensive and hard to handle (Mitchell et al., 2015). However, there are several distinct advantages of using non-human primates for studying age-related pathologies, such as Alzheimer’s disease and other neurodegenerative diseases that can’t be recapitulated in mouse models. Primates have Aβ and tau aggregates and cortical atrophy (Heuer, Rosen, Cintron, & Walker, 2012), whereas wild type mice don’t show protein aggregation, which makes non-human primates a valuable model for studying aging of the brain (Verdier et al., 2015).
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Maria Konovalenko is a life extension advocate, author, and molecular biophysicist and currently is at the USC Davis School of Gerontology in Los Angeles. She earned her M.Sc. degree in Molecular Biological Physics at the Moscow Institute of Physics and Technology and is one of the authors of the upcoming Longevity Cookbook.
This article originally appeared on her blog and is republished with permission.