Project 1: Activation of Alternative L1 Lifecycles in the CNS with age and Alzheimer's Disease

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PROJECT SUMMARY (PROJECT 1) Retrotransposable elements (RTEs) comprise ~45% of the human genome. Known as ‘mobile DNA’ they can insert into new genomic locations using a 'copy and paste' mechanism. This process, retrotransposition, can be deleterious at multiple levels and host organisms have evolved silencing mechanisms to protect their genomes. Until recently RTEs were thought to be largely silent in somatic cells. In the past 10 years evidence started emerging that somatic RTE activity is more frequent than anticipated, with our group contributing important early evidence. It is now apparent that with aging, multiple host defense mechanisms become compromised, and repetitive sequences in general, not just active RTEs, increase their expression. In the previous funding cycle we discovered that LINE-1 (L1) elements are upregulated in senescent cells and generate cytoplasmically local- ized cDNA reverse transcripts. These cDNAs are perceived as invading virus by the host and trigger a Type-I Interferon (IFN-I) response, which in turn stimulates the innate immune system. We believe this leads to a phe- nomenon known as 'sterile inflammation' or 'inflammaging', a known hallmark of aging that has been implicated in a variety of age-related diseases. The central nervous system (CNS) appears to be a 'privileged site' for RTE activity, with relatively high levels of expression and ease of further upregulation. Multiple lines of evidence indi- cate that RTE activation is associated with pathology, and has been linked with several neurodegenerative dis- eases, including Alzheimer’s disease (AD). We hypothesize that activation of RTEs contributes to age-related neurodegenerative diseases by promoting neuroinflammation. However, our knowledge of RTE biology in the CNS is very incomplete and presents a barrier to evaluating and exploiting this new framework. This project will explore the lifecycles of L1s in neuronal and non-neuronal cells of the CNS, the L1 surveillance mechanisms, how they fail, the consequences of that failure, and ultimately, how we can fix this. We will use human iPSC models to mechanistically explore pathways that lead to L1 activation in cells of the CNS, the sensors and down- stream pathways that communicate the presence of L1, and the cell inflammatory, senescence and death pro- cesses that are elicited. We will explore the subcellular lifecycles of L1 particles using high-resolution microscopy to understand where and how their cDNAs are synthesized. We will ablate endogenous L1s as well as overex- press L1s at will to further probe their interactions with host cells. We will extend these studies to normal aged mice, mouse AD models, and human AD brain tissue. We will inhibit the synthesis of L1 cDNAs with nucleoside reverse transcriptase inhibitors to evaluate beneficial effects. The outcome of the work proposed here will provide mechanistic insights into these complex processes and address the therapeutic potential of inhibiting RTEs for the treatment of Alzheimer’s disease, related dementias, and other age-related neurodegenerative diseases.