Transposable elements (TEs) are parasitic DNA sequences that represent approximately 45% of the human genome. The major class within this population is that of retrotransposons making up approximately 40% of the human genome. These TEs are particularly harmful to normal cellular physiology and genetic integrity in that they have the capacity to move around their host genomes via an RNA intermediate, replicating, and reinserting into a new genomic location. This process has the potential for a variety of harmful outcomes including double-stranded DNA breaks, insertional mutagenesis, altered mRNA splicing, and even cell death. Eukaryotic cells maintain these genetic parasites by silencing via chromatin modifications or by small RNA-mediated degradation of retrotransposon transcripts, referred to as RNA interference (RNAi). Recent research in yeast, flies, mice, and human cell culture has shown that TEs become reactivated with age. This correlates with a plethora of research showing strong correlations between TE reactivation and age- associated diseases such as macular degeneration, neurodegeneration, and cancer. Mutants in the Drosophila RNAi protein DICER-2 (DCR-2) show massive TE dysregulation and a significantly shortened lifespan. Therefore, there is much precedence that TE reactivation and the integrity of RNAi that regulates TEs may be essential in regulating lifespan. Drosophila melanogaster as a model organism offers an exceptional inroad to explore these hypotheses due to their conveniently short lifespan, diverse genetic tools, and a small, well- annotated genome. Additionally, unlike mammalian RNAi proteins that process multiple types of small RNAs, Drosophila has a dominantly somatic pathway that is primarily devoted to regulation and suppression of TEs. Using Drosophila, my preliminary data show that ubiquitous overexpression of Dcr-2 significantly extends Drosophila lifespan and that a natural increase with age in DCR-2 protein is present in the fat body, the fly analog to mammalian adipose tissue. [Similarly, recent research has suggest a somatic role for the canonically gonad-specific piRNA pathway. This pathway also regulates TEs in a dicer-independent manner. I have found that PIWI, the piRNA argonaute that silences TEs in the piRNA pathway, is present in the fat body and also increases its protein levels with age. Increased levels of these RNAi proteins suggests an enhanced RNAi phenotype in the fat body.] These data strongly suggest a causal role of TEs and RNAi in regulating lifespan and that tissue-specific differences in RNAi proteins may serve to protect otherwise critical tissues. My hypothesis is that increased RNAi efficiency in the Drosophila fat body, relative to other tissues, regulates lifespan via post-transcriptional gene silencing of transposable elements.
In Aim 1, I will determine whether TE transcript levels decrease with age in Drosophila fat body.
In Aim 2, I will determine the role of Dcr-2 in the fat body and other majo tissues in regulating TEs and lifespan. The research describe in this proposal has the potential to reveal key insights into the impact that suppression of TEs via RNAi has on lifespan and to what degree specific tissues contribute to regulating longevity. The human health applications of these results are profound in that interventions enhancing RNAi or affecting a reduced degree of TE reactivation with age could inevitably reduce many age-associated diseases and possibly aging itself.
Transposable elements (TEs) are prolific parasitic sequences of DNA that reside in host genomes and when activated have the potential to cause severe genomic damage such as genomic mutations and chromosomal rearrangements. TE reactivation is associated with many age-associated diseases such as cancer and macular degeneration and most recently with natural aging suggesting these elements may contribute to the pathology of aging and even lifespan itself. The research described in this proposal will illuminate the role that mechanisms of TE regulation play in different tissues and offer key insights into how interventions abrogating TE suppression may impact longevity and extend healthy lifespan.