SUMMARY: Traumatic Axonal Injury: Tau and Amyloid Pathogenesis in vitro. Traumatic brain injury (TBI) leads to an increased risk of Alzheimer’s disease/ Alzheimer’s disease related dementias (AD/ADRD) and TBI-related neurodegeneration (TReND). TReND comprises multiple pathologies, many of which are commonly observed in AD/ADRD, including tau and amyloid-beta (Ab) deposition. Traumatic axonal injury (TAI), one of the most common and important pathologies of TBI, appears to be a key source of phosphorylated tau (p-tau) and Ab formation acutely after TBI. Our previous data indicates that TAI induces immediate mechanical breaking of axonal microtubules (MTs) leading to detachment and phosphorylation of the primary MT stabilizing protein, tau. Preliminary data suggests that p-tau quickly accumulates in axonal swellings and then is ‘missorted’ to the neuronal soma by way of a shortened axonal initial segment (AIS). We hypothesize that this may be a key event initiating the somatodendritic accumulation of p-tau and eventual formation of neurofibrillary tangles. However, since this appears to be a programmed response to injury, we alternatively hypothesize that this process may represent a protective mechanism by removing high concentrations of p-tau from damaged axons. Original efforts from our group have also demonstrated that TAI induces the rapid and massive generation of Ab. In TBI, amyloid precursor protein accumulates in axonal swellings where secretases cleave it into Ab. With axon degeneration, the release of Ab has been linked with rapid tissue deposition and formation of diffuse Ab plaques in individuals within hours of TBI. We recently found that the most abundant amino acid in axons, N-acetylaspartate (NAA) is a powerful inhibitor of Ab aggregation at normal physiological levels. We therefore hypothesize that decreased NAA concentration observed in TAI permits greater Ab genesis in axons that fosters Ab oligomer to fibril formation. Accordingly, it is curious why the ingredients and conditions for Ab genesis and aggregation coalesce in injured axons. We hypothesize that beyond potential toxic effects, the process of Ab generation in injured axons may have evolved to play protective or reparative roles in TAI. We propose to use our well-characterized in vitro TAI model that induces dynamic stretch injury to micropatterned axon tracts to, 1) Characterize the evolution of p-tau accumulation and missorting after TAI, and its interplay with MT instability, and 2) identify evolving Aβ processing after TAI and the effects of endogenous Aβ depletion and blockade of its aggregation on axon outcomes. These proposed studies will identify initiating mechanisms of p- tau and Aβ genesis and metabolism in axons, which may have broad implications for understanding early events that lead to TReND and AD/ADRD.