Abstract of the proposed research In response to the NIH NOT-AG-23-032, the current Alzheimer’s Disease study pilot project administrative supplement application is to expand the parent R01 award (1R01AG080141-01) entitled “Cell Senescence Regulating Osteoarthritis Progression: Sex-dependent Mechanisms” and help develop Alzheimer’s disease (AD) focused grants. The R01 focuses on discovering mechanisms of aging-associated joint degenerative diseases. During this process, we found common pathological mechanisms shared by both skeletal and neural degenerative diseases during aging, specifically between aging-associated osteoarthritis (OA) and AD. Both OA and AD present sexual dimorphism with higher prevalence in older females than older male. However, the molecular mechanisms responsible for sexual dimorphisms of aging-associated degenerative diseases are not clear. Recent evidence indicates that de-repression of retrotransposon LINE-1 or L1, which accounts for 17 percent of the human genome, is involved in inflammation and degeneration of both cartilage joint and brain tissues during aging. We discovered that L1 activation is sex dependent in both mouse OA model and human OA patients by single-cell transposable element (sc-TE) RNAseq analysis. The scTE methodology that we have developed to study the relationship between L1 and cartilage joint degeneration can be used to determine the sex-dependent, cause-effect relationship between L1 and AD. The scientific goal of this project is to determine the molecular mechanisms underlying sex-difference in AD pathogenesis. The innovative hypothesis is that females are more susceptible to early-onset and progression of AD during aging because L1 activation is more prevalent in female CNS cell lineages, which leads to cell senescence, inflammation, and degeneration. This hypothesis will be tested through two aims as follows. Aim 1: Characterize and compare aging induced L1 activation patterns in the CNS between male and female in an AD transgenic mouse model with scTE RNAseq. It will establish whether sexual dimorphism of AD progression is associated with sex-dependent activation of L1 in a CNS cell lineage (neuronal, microglial, and astrocyte) in a mouse AD model. Aim 2: Characterize and compare L1 activation patterns in the CNS between male and female human AD patients with scTE RNAseq. It will correlate sexual dimorphism of AD pathogenesis with sex-dependent activation of L1 in a CNS cell lineage of human AD patients. The impact of this study is that it will establish whether L1 retrotransposon activation in the CNS can be a target for sex-specific treatment of AD. It will not only change the concepts that drive the AD research field, but also impact the clinical practice of how we treat AD patients.