PROJECT SUMMARY/ABSTRACT Alzheimer’s Dementia (AD) is a leading cause of morbidity and mortality that is incompletely understood and lacking effective treatment. Accumulation of senescent cells (SnCs) has recently been implicated in AD pathogenesis and targeted removal of these cells may offer new therapeutic avenues. Cellular senescence is a cell fate defined by stable proliferative arrest, apoptotic resistance, and production of a pro-inflammatory secretome. Though senescence programming can contribute to proper development and regenerative processes, its dysregulation is increasingly linked with disease burden and pathology, including AD. While SnC modulation and clearance is a promising therapeutic target, it is increasingly clear that these cells are highly heterogeneous in their characteristics and function. We recently developed a mouse model to characterize a unique population of SnCs that highly express the cell cycle blockade protein p21 (p21high cells) and demonstrated that these cells play a causal role in age related physical dysfunction and metabolic disease. Furthermore, other literature has implicated clearance of pan-SnCs, which include p21high subpopulations, with cognitive improvements in AD. However, it is unknown if p21high cells make distinct contributions to AD or if their targeted removal can further counteract AD pathology. Therefore, this proposal aims to investigate the specific contributions of p21high SnCs to AD. Preliminary experiments have demonstrated that p21high cells accumulate in the brain of mouse AD models featuring amyloid- β plaque. However, it is unknown when these cells begin to accumulate in relation to the underlying disease process and what cell types are undergoing senescence. Therefore, in Aim 1, we will define the precise timeline of p21high cell accumulation in relation to amyloid- β plaque deposition (1A) and determine what cell types these represent (1B) by tracking these cells with a transgenic fluorescence reporter system and immunohistochemistry. To understand if these p21high SnCs play a causal role in AD, in Aim 2 we propose to selectively eliminate these cells via an inducible suicide gene to determine if targeted removal of these cells can either prevent (2A) or alleviate (2B) AD associated amyloid- β plaque formation and impaired performance on cognitive assays. We will also conduct single nuclei RNA sequencing (2C) on brains with or without p21high cell elimination to assess for changes in AD associated neuro-inflammatory pathways and explore underlying disease mechanisms associated with p21high cells. These aims will help to define the role of p21high SnCs in AD and may serve as a basis for new targeted disease modulating therapy. Furthermore, these results will further implicate p21high cells in age related disease and broaden the field’s appreciation of SnC heterogeneity. This work is part of a tailored career development plan at UConn Health that integrates training in aging biology,...