Aging and longevity in the simplest animals and the quest for immortality

Abstract

Here we review the examples of great longevity and potential immortality in the earliest animal types and contrast and compare these to humans and other higher animals. We start by discussing aging in single-celled organisms such as yeast and ciliates, and the idea of the immortal cell clone. Then we describe how these cell clones could become organized into colonies of different cell types that lead to multicellular animal life. We survey aging and longevity in all of the basal metazoan groups including ctenophores (comb jellies), sponges, placozoans, cnidarians (hydras, jellyfish, corals and sea anemones) and myxozoans. Then we move to the simplest bilaterian animals (with a head, three body cell layers, and bilateral symmetry), the two phyla of flatworms. A key determinant of longevity and immortality in most of these simple animals is the large numbers of pluripotent stem cells that underlie the remarkable abilities of these animals to regenerate and rejuvenate themselves. Finally, we discuss briefly the evolution of the higher bilaterians and how longevity was reduced and immortality lost due to attainment of greater body complexity and cell cycle strategies that protect these complex organisms from developing tumors. We also briefly consider how the evolution of multiple aging-related mechanisms/pathwayshinders our ability to understand and modify the aging process in higher organisms.

Evidence of an Association between Age-Related Functional Modifications and Pathophysiological Changes in Zebrafish Heart

“…It has also been used to model a wide variety of disease processes, including cardiovascular disease [4]. In humans, some cardiovascular features seen in the elderly are likely to be due to the aging process itself [23]. A variety of mechanisms are involved in the aging process: histone modifications, protein quality control, organelle dysfunction, signaling pathways, and oxidative stress [24].…”

The cell survival pathways of the primordial RNA-DNA complex remain conserved in the extant genomes and may function as proto-oncogenes

“…The ancestral stem cell gene foxo/FOXO encodes antisenescence longevity pathways. Human centenarians appear to have preserved the FOXO pathway but not over this limit (for references [109]). The supplement of primordial stem cells (PriSCs) is close to inexhaustible in the cnidarians and sponges, especially when reproducing asexually; restricted in annelids (segmented worms) and mollusks (clams and snails); and rudimentary in the drosophila and caenorhabditis, and in vertebrates and mammalians [110].…”

“…(for references [109]). The package of stem cells, the hydra (H. magnipapillata/vulgaris) experiences no age-related mortality and uses the Wnt pathway to regenerate its cut-off head.…”

Stem Cell Models: A Guide to Understand and Mitigate Aging?

“…It is generally the former mode of reproduction that is associated with extremely long life span [1]. Therefore, evolutionary selection for fitness is not incompatible with very long life span [2]. …”

“…First, stem cells maintain tissue function by replacing damaged cells. Highly regenerative multicellular organisms such as hydra avoid aging with the help of pluripotent stem cells [2]. Most mammalian tissues harbor a pool of stem cells that serve to maintain tissue function and homeostasis throughout life.…”