The idea that every organism ages may be true, but there are some that don’t appear to age. The idea that aging has evolved may be true, but among closely related species there can be a vast range of life spans. Can organisms with seemingly unusual aging properties be useful to study the biology of aging; that is, to understand the fundamental underlying mechanisms that lead to aging? The goal of studies with these unusual organisms is to determine mechanisms that retard aging (comparing long-lived to short-lived species within the same genus) or that appear to circumvent aging (examining organisms that seem to be immortal and share extraordinary capacities for tissue regeneration). This information would advance the goal of developing therapies to slow human aging, and improve health in the already-aged. Because aging is the major risk factor for many late life maladies, such research should lead to treatments that enhance health and may delay or mitigate late-onset and chronic diseases
To address these issues directly, the Division of Aging Biology of the NIA, beginning in 2010, supports investigators who study aging in unusual invertebrate organisms. One example of such an invertebrate is the hydra (shown at left). These animals are of interest for aging research because they have significant regenerative capacities. There are 20 cell types in hydra; old cells are lost and replaced continuously by new cells, often from a pool of stem cells present in the central core of the hydra. That regeneration is continuous gives the impression that hydra are immortal. Cell replacement occurs in humans and other mammals, but not to the extent observed in hydra and, in important contrast to hydra, this regeneration diminishes with aging. Therefore, understanding the natural mechanisms for regeneration in this species may explain why this capacity diminishes with aging in other species (e.g., homo sapiens), and may suggest ways to improve regeneration in those other species.
Invertebrates are not new species for study in aging research. The more commonly used invertebrates for study of the genetics and molecular biology of aging are fruit flies and worms – D. melanogaster and C. elegans, respectively, of the superphylum Ecdysozoa. Hydra are in the phylum Cnidaria which includes corals and jellyfish.Fruit flies have been used for studies of the genetic control of development for more than a century, worms have been used most famously for the studies of cell lineages and the genetic control of cell fate determination, and hydra have been studied for more than 200 years. Hydra have several properties that, when contrasted with other model organisms, support the belief that these may yield novel insights that would be helpful to understand aging in general and human aging in particular. The hydra genomes have been determined (for several species) and are found to include orthologs of human genes not found flies and worms. Therefore, genes new discoveries of the genetic control of aging are possible from research with hydra that would not be feasible in fruit flies or worms, because those genes are absent (about 10% of the hydra genome are in this category). Unlike fruit flies and worms, hydra have life-long regenerative capacity.