ABSTRACT Repeated concussions (mild Traumatic Brain Injury, mTBI), which are particularly prevalent in athletes and military personnel, can lead to long-term brain health issues including dementia, depression, and other psychiatric conditions. Recent studies suggest that mTBIs may give rise to increased risk for Alzheimer's disease (AD) or other AD-related dementias (ADRDs), but there are few conclusive studies, and no reliable blood biomarkers. We are studying a unique cohort of Division I athletes in high impact sports to develop a reliable blood biomarker assessment and examine biological mechanisms for AD/ADRD risk after multiple mTBIs. Further, we are using novel exosome biomarker technology to determine Tau and amyloid seeding and aggregation capacity, as well as the impact of multiple mTBIs using an animal model for AD. To our knowledge, studies have not been conducted using neuron- or astrocyte-derived exosomes (NDEs vs. ADEs) to detect Tau and amyloid pathology and seeding capacity from those with sports-related brain injuries. The overall hypothesis of this project is that exosome alterations after repeated mTBIs reflect and contribute to long- term risk for AD/ADRD. To address this hypothesis, we have developed three specific Aims. In Aim 1, we will test the hypothesis that NDE and ADE biomarkers correlate with balance and/or cognitive impairment following one or repeated mTBIs in humans. In Aim 2, we will test the hypothesis that repeated mTBIs or injection of TBI- derived exosomes accelerates cognitive dysfunction and brain pathology associated with aging and AD in mice. In Aim 3 we examine in vitro whether cells exposed to NDEs or ADEs from athletes with multiple mTBIs exhibit altered exosome biogenesis, release or uptake mechanisms with a focus on how endogenous exosomes are formed and released from cultured neurons and glia after exposure to exogenous exosomes from athletes with or without mTBIs. Our interdisciplinary team has the unique potential to reveal molecular mechanisms involved in AD pathology after mTBIs, using a unique cohort consisting of male and female Division I athletes including baseline and post- concussion measurements. These interdisciplinary studies can also distinguish between long-term effects of an individual's genotype (e.g., 3xTg-AD mice) vs. external factors (mTBIs) on AD-related pathology. The major goal of this research program is to develop more sensitive biomarkers post-concussion that could predict future risk for AD/ADRD and to reveal mechanisms for exosome alterations post-mTBI. The unique value of this program is the interdisciplinary team, including both mouse models and human studies, the large cohort of Division I athletes, and the long-term biomarker studies proposed. Based on the biological mechanisms examined herein, as well as the wealth of preliminary data, we will be able to design better preventative treatment options long-term for those with one or several mTBIs who are at risk of deve...