The aim of this project is to identify aging-regulated genes at the molecular and tissue levels, to uncover evolutionarily conserved molecular changes in aging, to investigate molecular mechanisms of longevity genes , and to identify which aging-related genes can affect lifespan. Aging is a fundamental and multi-factorial biological process that occurs in most eukaryotic organisms. Genetic analyses of model organisms have uncovered mutations in a number of genes that can affect lifespan. These findings suggest that the genetic mechanisms govern the rate of many aging processes. Changes in gene expression in aging have been observed in a number of eukaryotic organisms, including nematodes (C. elegans), fruit flies (D. melanogaster), mouse and human. However, little is known about how different tissues age. To address this question, we have begun systematic identification of tissue-specific factors that affect lifespan and aging processes. We have measured the expression profile of aging for seven tissues from D. melanogaster, including brain, muscle and tissues in the digestive and reproductive systems, which represent almost all the functional tissues in flies. Hundreds of genes have been identified to show significant changes in the transcript level in aging. Interestingly, very few changes were shared among all the tissues, suggesting that different tissues age in unique ways. However, some of the aging-related changes are shared among two or more tissues, suggesting that common molecular features do exist among different tissues in aging processes. Many hypotheses have been proposed to explain aging processes. One of them is called free radical hypothesis of aging, which states that free radicals, such as superoxide, hydrogen peroxide and hydroxyl, generated from normal aerobic metabolism can cause oxidative damages to macromolecules including DNA, protein and lipid, and accumulation of such damages in the cell with increasing age leads to aging and eventually death of an organism. To investigate to what extent oxidative damage is involved in regulating molecular changes in aging, we will compare the genome-wide tissue-specific changes in aging to those in response to oxidative stress. This component of our research will allow us to identify tissue-specific changes in aging and demonstrate the role of oxidative stress response in aging. As an example, we have found genes involved a major metabolic pathway, tricarboxylic acid cycle, are systematically down-regulated in fly brain, gut, muscle and testis but not fat tissue, accessory gland (equivalent to human prostate) and malpighian tubule (equivalent to human kidney) in aging. One of the objectives of research on biological processes in model organisms, such as flies and worms, is to help us understand the similar processes in human. Pathways in many biological processes are often found to be conserved. What we have learned from model organisms are that specific processes can often be applied to the similar processes in human. Aging may not be an exception. A number of mutations that can prolong lifespan in model organisms have been found in evolutionarily conserved genes, supporting the existence of evolutionarily conserved pathways that specify lifespan and influence aging. To address the conservation issue, we have been comparing tissue-specific aging-related molecular changes in flies to those in mouse, rat and human. Although we have not found significant correlation of aging-related molecular changes among these evolutionarily divergent species examined thus far, we have identified a number of groups of genes with similar molecular functions show conserved changes. As an example, genes involved in energy production in mitochondria show conserved aging-related patterns between human and fly brains. This supports the notion that some features of aging processes are evolutionarily conserved. This also provides a list of candidates for us to study their role in lifespan, which may be most relevant to aging in higher organisms including human beings. A number of longevity genes have been identified to extend lifespan in a wide range of model organisms. However, not much is known about the mechanisms of lifespan by these longevity genes. In particular, little is known regarding which tissue(s) longevity genes actually act through to extend lifespan. To address these questions, we have chosen to study the Methuselah mutant fly, which has an extended lifespan and is the first longevity mutant with a single gene mutation identified by S. Benzer lab at Caltech. We have measured molecular changes of this mutant for the seven tissues described above in aging. We have been comparing these changes to those in the wild type fly strain at the molecular and tissue levels. This assessment will elucidate molecular and cellular mechanisms on how the Methuselah gene regulates lifespan at the tissue level. Similar approaches will be applied to study a growing number of longevity genes at the tissue level in the future. Previously we found that repression of genes functioning in mitochondrial oxidative respiration and up-regulation of genes involved in protein degradation pathways were conserved features of aging by comparing aging-related molecular changes between D. melanogaster and C. elegans in collaboration with research groups at University of California at San Francisco. Some of these conserved genes have been shown to be involved in regulating lifespan in C. elegans by several nematode research groups. We have started systematically investigating whether these genes regulate lifespan in D. melanogaster. In addition, we have identified 12 candidate transcription factors that may regulate gene expression of these conserved genes. Using a cell-based assay, we have found two of the transcript factors are involved in oxidative stress response. We will continue this study by measuring lifespan of the fly strains carrying mutations or overexpression of candidate genes to identify genes that can extend lifespan. Based on tissue-specific expression patterns of these lifespan-related genes, how and when these genes affect lifespan at the tissue level will be investigated. In summary, we have conducted a systematic study on tissue-specific regulation of the aging processes at the genomic level. This project will provide a genomic view on interactions among different tissues in aging and reveal conserved molecular changes in aging. This will lay a foundation for us and other researchers to speed up investigation on mechanisms of aging at the molecular and cellular levels. Identification of the conserved features in aging is clearly critical for us understand human aging and more importantly to develop efficient aging intervention strategies for human.
