Project Summary: Alzheimer's Disease (AD) and AD-related dementias (ADRDs; e.g. Frontotemporal Dementia, Lewy Body Dementia, etc.) are crippling neurodegenerative disorders. The onset of these diseases is strongly correlated with aging. Cures for AD and ADRDs remain elusive; Alzheimer's has become the 6th most frequent cause of death in the USA. An emerging candidate contributor to Alzheimer's and ADRD is the L1 retrotransposon, which becomes dysregulated during aging and correlates with Alzheimer's / ADRD onset. One hypothetical mechanism by which L1 may contribute to Alzheimer's / ADRD is through its exacerbation of cellular senescence. Senescence is a phenomenon by which normal cells stop dividing; these cells accumulate with advancing age and are found at the locations of dysfunction in age-related diseases. In mice, senescent cells have been shown to shorten life and actively drive age-related neurodegeneration; preventing senescent cell accumulation decreases tau-dependent degeneration and cognitive decline. AD patients exhibit increased indicators of cellular senescence. It is increasingly clear that senescent cells are not inert, but instead drive tissue deterioration via the senescence-associated secretory phenotype - secreting a variety of growth factors and pro- inflammatory cytokines. L1 retrotransposons have recently been shown to drive progression of the senescence- associated secretory phenotype, and thus, L1 is an important agent of cellular senescence. L1 activation is also associated with AD-related Tau pathologies. The L1 encoded ORF2p enzyme (endonuclease and reverse transcriptase) is often flagged as a source of pathological cellular insults, e.g. via new, mutagenic L1 insertions and contributions to chromosomal instability. However, the effects of L1 expression extend beyond DNA damage. Numerous mechanisms may be at play, including the titration of normally homeostatic host factors away from their physiologic functions and into L1 ribonucleoprotein granules, as well as the production of immunity-and-inflammation-triggering cytoplasmic L1 DNA:RNA hybrids; indeed, the latter is now understood to be a key component of L1’s role in cellular senescence. Moreover, L1 also mobilizes Alu and other non- autonomous retrotransposons. Objectives: In Aim 1, we will use targeted mass spectrometry to profile L1 ORF1 protein expression in the cerebrospinal fluid (CSF) and post-mortem brain tissues of patients exhibiting AD/ADRD and cognitive decline, as well as in senescent cells and neuronal iPSCs; in Aim 2 (in vitro cell culture) and Aim 3 (clinical samples) we will profile L1-associated protein-protein and protein-RNA interactions in the same biological samples as Aim 1; and in Aim 4 we will take a candidate-based approach using molecular genetics to dissect the mechanisms of action of L1 in senescent cells.

Project Narrative. Alzheimer's Disease (AD) and AD-related dementias (ADRDs) are crippling neurodegenerative disorders whose onset strongly correlates with aging. There are compelling reasons to believe cellular senescence (effectively, cell aging) and retrotransposons (selfish DNA) may drive or exacerbate AD and ADRDs by inducing a state of chronic stress (inflammation); more research is needed. We will analyze the role of LINE-1 retrotransposons in senescent cells and Alzheimer's / ADRD afflicted brain tissues, gathering additional evidence to support or refute the hypothesis that LINE-1 is a causal agent in Alzheimer's / ADRD.