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Using Zebrafish Models to Explore Genetic and Epigenetic Impacts On Using zebrafish models to explore genetic and epigenetic impacts on evolutionary developmental origins of aging SHUJI KISHI JUPITER, FLA Can we reset, reprogram, rejuvenate, or reverse the organismal aging process? Certain genetic manipulations could at least reset and reprogram epigenetic dy- namics beyond phenotypic plasticity and elasticity in cells, which can be manipu- lated further into organisms. However, in a whole complex aging organism, how can we rejuvenate intrinsic resources and infrastructures in an intact and noninvasive manner? The incidence of diseases increases exponentially with age, accompanied by progressive deteriorations of physiological functions in organisms. Aging- associated diseases are sporadic but essentially inevitable complications arising from senescence. Senescence is often considered the antithesis of early develop- ment, but yet there may be factors and mechanisms in common between these 2 phenomena to rejuvenate over the dynamic process of aging. The association be- tween early development and late-onset disease with advancing age is thought to come from a consequence of developmental plasticity, the phenomenon by which one genotype can give rise to a range of physiologically and/or morphologically adaptive states based on diverse epigenotypes in response to intrinsic or extrinsic environmental cues and genetic perturbations. We hypothesized that the future ag- ing process can be predictive based on adaptivity during the early developmental period. Modulating the thresholds and windows of plasticity and its robustness by mo- lecular genetic and chemical epigenetic approaches, we have successfully con- ducted experiments to isolate zebrafish mutants expressing apparently altered senescence phenotypes during their embryonic and/or larval stages (‘‘embry- onic/larval senescence’’). Subsequently, at least some of these mutant animals were found to show a shortened life span, whereas others would be expected to live longer into adulthood. We anticipate that previously uncharacterized develop- mental genes may mediate the aging process and play a pivotal role in senescence. On the other hand, unexpected senescence-related genes might also be involved in the early developmental process and its regulation. The ease of manipulation using the zebrafish system allows us to conduct an exhaustive exploration of novel genes, genotypes, and epigenotypes that can be linked to the senescence phenotype, which facilitates searching for the evolutionary and developmental origins of aging in vertebrates. (Translational Research 2014;163:123–135) Abbreviations: SA-b-gal ¼ Senescence-associated b-galactosidase; TERT ¼ Telomerase reverse transcriptase From the Department of Metabolism & Aging, The Scripps Research Submitted for publication August 22, 2013; revision submitted Institute, Scripps Florida, Jupiter, Fla. October 20, 2013; accepted for publication October 21, 2013. Conflicts of Interest: The author has read the journal’s policy on con- Reprint requests: Shuji Kishi, MD, PhD, Department of Metabolism & flicts of interest and have none to declare. Aging, The Scripps Research Institute, Scripps Florida, 130 Scripps The work of the author’s laboratory reviewed in this article was funded Way, #3B3, Jupiter, FL 33458; e-mail: [email protected]. by research grants from The Ellison Medical Foundation, the Glenn 1931-5244/$ - see front matter Foundation for Medical Research, the A-T Children’s Project, and Ó 2014 Mosby, Inc. All rights reserved. the National Institute of General Medical Sciences/National Institute http://dx.doi.org/10.1016/j.trsl.2013.10.004 on Aging/National Institutes of Health. 123 Translational Research 124 Kishi February 2014 The mechanisms of aging are currently the focus of life span has not proved to be universally true, as shown extensive investigations throughout the world. During in several cases including humans, some fish species, the aging process, multiple forms of tissue- and organ- and honeybees (Apis mellifera).3-9 Therefore, a cross- specific damage and pathophysiological change accu- species comparative biology of aging prospective is mulate and, accordingly, a number of chronic diseases necessary to understand and obtain a more complete appear with advancing age. Scientists are now looking picture for fundamental causes of aging in speciation for the principles governing the ubiquitous process of and biodiversity. aging to find, ultimately, novel ways to attenuate or Development is obviously under the precise control delay aging in humans as well as to develop interven- of genetic mechanisms programmed in the genome hav- tions for age-associated diseases. However, understand- ing plasticity, yet through remodeling of epigenomes ing the molecular mechanisms of aging in vertebrates is (Fig 1). However, such precise genetic programming, still a major challenge of modern biology and biomed- unfolded by epigenetic interventions, may still be ical science. There are various animal models to be disturbed later during the aging process and may un- considered and used in aging studies, and determining dergo many stochastic deteriorative challenges against the optimal model system or systems remains one of regenerative/repair responses and restorations when the most important issues. Given certain limitations in the reactivation of certain phases of the early develop- each and all the existing models of human aging, calling mental program/process contributes to and/or are for an integrative approach that uses diverse species in required (Fig 1). When we consider aging as an exten- the hope that each can provide a piece of the puzzle sion of the biologic process of development, the power and, together, would help to identify critical elements of developmental biology and genetics in zebrafish be- common to aging in all organisms, would be ideal. comes a monumental and instructive wealth of informa- One of the essential limitations is availability of ge- tion and exploration. Zebrafish can be used extensively netic/genomic and epigenetic/epigenomic information in searching for evolutionary and developmental origins detailed on relevant species. This provides the rationale of aging common in vertebrates. In fact, some of the for current biomedical investigations into the mecha- genes we identified by an apparent senescence pheno- nisms of aging being conducted concurrently in geneti- type during embryogenesis (‘‘embryonic senescence’’) cally robust species like worms, fruit flies, and mice. In had already been associated with cellular senescence fact, small and prolific invertebrates, such as Caeno- and chronological aging in other organisms, whereas rhabditis elegans and Drosophila melanogaster, can many others still wait to be linked to the conventional provide a strong basis for unbiased genetic screens aging process in future studies. Complete loss of func- that identify and determine the precise functions of tion of developmentally essential genes induces embry- novel genes and their epigenetic profiles that regulate onic (or larval) lethality, whereas it seems like their aging and life span transgenerationally and evolution- partial loss of function (ie, decrease of function by het- ally. This approach has already led to the successful erozygote or hypomorphic mutation) still remains suffi- identification of key genes and pathways evolutionarily cient to go through the early developmental process conserved in and associated with the aging process. because of its plasticity or, rather, heterozygote advan- While evolutionally conserved mechanisms of aging tage. In some cases, however, such partial loss of func- could play roles in various organisms, manifestations tion of genes compromises normal homeostasis as a of aging can also differ in different species. For result of a heterozygote disadvantage (‘‘haploinsuffi- instance, vertebrates from fish through humans often ciency’’) later in adult life, presenting a number of envi- die of cancer or stroke, but invertebrates such as worms ronmental or epigenetic challenges that less plasticity and insects unlikely die of either. Thus far, only mice can no longer adjust to compensate (Fig 2). In contrast, have served as a widely used genetic model system to any heterozygote-advantageous genes might gain study the aging process in vertebrates, providing impor- certain benefits (much more fitness) by such partial tant insights into mammalian aging as an indispensable loss of function later in life (Fig 2). animal model.1 However, studying only a single genetic Physiological senescence may arise evolutionarily model may be limiting because of the diversified from both genetic and epigenetic drift as well as from complexity of vertebrate aging across species. Some losing developmental plasticity in face of stress signals mechanisms of aging could be conserved evolutionarily from the external environment that interact with func- and publicly in vertebrates, but there are also more tions of multiple genes rather than effects of only a sin- lineage-specific or private mechanisms of aging from gle gene mutation or defect. We wish to identify a a comparatively biologic point of view.2 Even the com- number of such critical genes promptly in a comprehen- mon notion that caloric (or dietary) restriction extends sive manner by using the zebrafish model system. Translational Research Volume 163, Number 2 Kishi 125 Fig 1. Programmed and stochastic regulations of development and aging through linking a genome to epigenomes. Development is pri- marily under the precise
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