Project Summary/Abstract: Novel approaches are needed to combat age-associated diseases of the brain, including Alzheimer’s disease (AD). Aging is the strongest risk factor for AD, yet we lack a detailed mechanistic understanding connecting normal aging to AD. Emerging data raise the possibility that neural plasticity can be revitalized in aged organisms. These studies demonstrate that factors present in young blood are restorative for aged tissues throughout the body, while suggesting links between the systemic environment and aging- and AD-related changes in the brain. Aged mice sharing young blood via parabiosis or those treated via plasma injections exhibit improved plasticity and improved cognitive performance. We provided evidence for specific youth-associated proteins, tissue inhibitor of metalloproteinases 2 (TIMP2) and colony-stimulating factor 2 (CSF2), that revitalize hippocampal function in aged mice when provided systemically. Conversely, several studies demonstrate that aged blood factors drive key aging phenotypes, including microgliosis and loss of neurogenesis, as well as hippocampus-dependent cognitive deficits, working in part through CCL11 and B2M. These studies leave fundamental questions open regarding the role of the systemic environment in aging and its link to AD through modulation of pathology. Recent work supports a role of blood-borne factors in modulating the state and function of the brain’s innate immune cells, microglia. Given this connection and data linking many AD risk genes to innate immune function, there is clear rationale to explore the link between aging and AD-related pathology through microglia. In preliminary studies, we find that exposure to young blood reduces microgliosis in the brains of aged mice, suggesting that young blood factors regulate microglia state in aging. In this proposal, we will rigorously address the role of youth-associated proteins in altering microglial gene expression and morphological profiles with the goal of clarifying the link between aging and AD pathomechanisms. We hypothesize that youth-associated factors rejuvenate microglia profiles in the aged brain and in the context of AD pathology. We will address this hypothesis in three major aims: (1) To determine the impact of systemic TIMP2 on microglia gene expression and morphology in aged mice; (2) to characterize the extent to which blood-borne brain rejuvenation is regulated by microglia function; and (3) to evaluate the impact of a combined systemic treatment of TIMP2 and CSF2 on age-associated vs. AD-associated changes in microglia profile and state. Our aims will interrogate the role of youth-associated blood-borne factors in regulating microglia using sophisticated approaches to rigorously define cellular and pathological regulation by the systemic environment, potentially opening novel avenues for AD therapy development.