Immortal Animals: How Organisms Counter Aging

By Tanjim Sayeeda

Human longing for eternal youthfulness has persisted since the beginning of time. Myths and legends about quests to find an elixir of youth, religions that breed hope with promises of an afterlife, and modern culture’s endless supply of overhyped health articles about “foods that extend telomeres” and “reverse aging” has assertively proven that humans crave longevity and escape from the inevitable and universal process of ageing (Ball, 2015). Biomedical advancements have remarkably increased the average lifespan across the globe. However, increasing life expectancies is leading to an aging population where people over the age of 60 are predicted to total 2 billion by 2050 according to WHO. Aging may become a major biomedical challenge of the 21st century, primarily because age related disorders such as cardiovascular diseases, nondegenerative diseases and cancer will be on the rise (Lemoine, 2020). 

Aging is simply defined by biology as a progressive deterioration of physiological functions that aid the survival of organisms; biology has termed this senescence. On a cellular level, senescence is defined as the state at which cells stop dividing and replenishing themselves consequently leading to death (LeBrasseur, Tchkonia & Kirkland, 2016). Senescence is important for health as it plays a role in tissue homeostasis and prevents tumour growth (Ohtani & Hara, 2013). The cause of aging remains obscure, but significant research suggests environmental factors accelerate its development from as early as birth (Karol, 2009). Debate ensues about whether aging should be considered a disease. Some scientists argue that aging is distinct to disease as it occurs to animals of all species that have reached a stable state in adulthood, suggesting that aging may occur by default after an animal has served its purpose in natural selection. One inarguable observation about aging is that it increases the risks of disease and thus the probability of death (McDonald, 2019).

‘Immortality’ however is not completely impossible in the animal kingdom as a few species have been discovered to claim it. Biological immortality is a decreasing or stable rate of cellular senescence as a function of an organism’s chronological age (Bilinski, Bylak & Zadrag-Tecza, 2016). One species considered biologically immortal is the  jellyfish: Turritopsis dohrnii. These small marine organisms undergo two life phases: a motile phase where the fertilised egg latches to a surface to form a colony of polyps called hydroid, and a mobile phase where hydroids release medusae, an immature form of the jellyfish. While ordinary jellyfish mature to sexually reproduce until death, the T. dohrnii can revert into a ball of tissue when faced with mild stress in order to repeat development. It was already known that certain hydrozoan cells could transdifferentiate by dedifferentiating to form stem cells that redifferentiate into specialised cells, however the immortal jellyfish displayed transdifferentiation in all its cells producing an entirely new colony of polyps. Scientists are working to understand the mechanism of this stunning ‘rebirth’ by determining the genes that cause reverse aging in T. dohrnii. Such findings have the potential to revolutionise cancer and stem cell therapy (Li et al., 2018). 

The genus Hydra contains species of freshwater organisms closely resembling polyps of T. dohrnii. With a tubular body, tentacular mouth and adhesive foot, these motile organisms catch prey using their stinging mouths and can produce asexually through budding cells. Hydra have stem cells that self-renew infinitely due to an overexpression of FoxO genes that are known to maintain the growth and proliferation of cells. The benefit of this property is that senescence is absent from the cells of Hydra so they can divide forever (Tomczyk et al., 2015). In addition, experiments with genetically modified Hydra revealed that enhanced FoxO improved immunity which supports the observation of poor immunity in elderly humans. Findings from Hydra research conclude that longevity depends on maintaining stem cells and a functioning immune system (Bosch, 2014). Lobsters also forego senescence completely with their enhanced ability to repair DNA. Endless DNA replications are usually prevented by end-caps called telomeres, which shorten with each cell division to eventually undergo senescence. The Lobsters’ advantage is their abundant supply of telomerase which allow them to re-lengthen their telomeres so constant cell division can occur. Unfortunately, their almost-immortal status is impeded by the excessive growth of the lobster that causes it to become too large for its shell. A lot of the lobsters’ energy is expended on growing new shells constantly, so they eventually succumb to exhaustion and die due to predation or shell collapse (Klapper et al., 1998).  

The possibility of reversing age in humans on a scientific basis may remain elusive as ever. Nevertheless, immortal species can provide useful insight into how age-related disorders can be tackled and treated to help improve the quality of lives and ensure that age is nothing but a number. 

References:

Ball, P. (2015) The God quest: the human longing for immortality. New Statesman, Ltd.

Bilinski, T., Bylak, A. & Zadrag-Tecza, R. (2016) Principles of alternative gerontology. Aging (Albany, NY.). 8 (4), 589-602. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27017907. Available from: doi: 10.18632/aging.100931.

Bosch, T. C. G. (2014) Rethinking the role of immunity: lessons from Hydra. Trends in Immunology. 35 (10), 495-502. Available from: https://www.clinicalkey.es/playcontent/1-s2.0-S147149061400129X. Available from: doi: 10.1016/j.it.2014.07.008.

Karol, M. H. (2009) How Environmental Agents Influence the Aging Process. Biomolecules & Therapeutics. 17 (2), 113-124. Available from: http://click.ndsl.kr/servlet/LinkingDetailView?cn=JAKO200913234256012&dbt=JAKO&org_code=O481&site_code=SS1481&service_code=01.

Klapper, W., Kühne, K., Singh, K. K., Heidorn, K., Parwaresch, R. & Krupp, G. (1998) Longevity of lobsters is linked to ubiquitous telomerase expression. FEBS Letters. 439 (1-2), 143-146. Available from: https://search.datacite.org/works/10.1016/s0014-5793(98)01357-x. Available from: doi: 10.1016/S0014-5793(98)01357-X.

LeBrasseur, N. K., Tchkonia, T. & Kirkland, J. L. (2016) Cellular Senescence and the Biology of Aging, Disease, and Frailty. Nestlé Nutrition Institute Workshop Series. Basel, Switzerland, S. Karger AG.

Lemoine, M. (2020) Defining aging. Biology & Philosophy. 35 (5), Available from: https://search.proquest.com/docview/2436973943. Available from: doi: 10.1007/s10539-020-09765-z.

Li, J., Guo, D., Wu, P. & He, L. (2018) Ontogeny reversal and phylogenetic analysis of Turritopsis sp.5 (Cnidaria, Hydrozoa, Oceaniidae), a possible new species endemic to Xiamen, China. PeerJ (San Francisco, CA). 6 e4225. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29333345. Available from: doi: 10.7717/peerj.4225.

McDonald, R. B. (2019) Biology of aging. [unknown] Second edition edition. Boca Raton ; London ; New York, CRC Press.

Ohtani, N. & Hara, E. (2013) Roles and mechanisms of cellular senescence in regulation of tissue homeostasis. Cancer Science. 104 (5), 525-530. Available from: https://search.datacite.org/works/10.1111/cas.12118. Available from: doi: 10.1111/cas.12118.

Tomczyk, S., Fischer, K., Austad, S. & Galliot, B. (2015) Hydra, a powerful model for aging studies. Invertebrate Reproduction & Development: Aging and Stem Cells in Invertebrate Model Systems. 59 (sup1), 11-16. Available from: http://www.tandfonline.com/doi/abs/10.1080/07924259.2014.927805. Available from: doi: 10.1080/07924259.2014.927805.

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