Mild TBIs are prevalent among professional athletes engaged in contact sports and military personnel. Repetitive injuries induce cumulative adverse effects which may continue for many years after the original injury, with trauma representing one of the strongest environmental risk factors for developing neurodegenerative disorders, such as Alzheimer’s disease. TBI patients often develop long-lasting traumatic axonal injury, loss of myelin sheath, and inhibition of oligodendrocyte maturation, which contribute to motor, cognitive, behavioral, and psychiatric deficits. However, no effective treatment is currently available for TBI-related pathogenesis. Oxidative stress has been implicated as a driver of mTBI-induced brain damage. It is established that NADPH oxidase (NOX) activity is maximal at a more alkaline pHi, and Na/H exchanger isoform 1 protein (NHE1) - mediated H+ efflux in exchange of Na+ influx is responsible for maintaining more alkaline pHi for the sustained NOX activity, formation of reactive oxygen species (ROS) and cytokine release. In our newly established mouse closed-skull repetitive-mild TBI model (r-mTBI) by the controlled cortical impact (CCI), we found that r-mTBI stimulated robust elevation of NHE1 protein in neurons, astrocytes, and microglia, oligodendrocytes in the cortex, corpus callosum, and hippocampus (Prelim data), which is concurrent with activation of reactive astrocytes and microglial cells as well as pathological accumulation of amyloid precursor protein (APP) in neurons (Prelim data). Whether NHE1 protein upregulation plays a causal role in r-mTBI-induced axonal damage, neuroinflammation, or cognitive function impairment is unknown. Interestingly, we observed that post-TBI administration of the NHE1 protein inhibitor HOE642 attenuated neurological function declines in wild-type mice. These findings led us to hypothesize that overstimulation of NHE1 protein may play a role in r-mTBI-mediated axonal injury and neuroinflammation. In this renewal application, we set out to investigate underlying cellular and molecular mechanisms with the following aims: Aim 1. Investigate whether r-mTBI-mediated upregulation of NHE1 protein in mice stimulates NOX activity, ROS and proinflammatory cytokine formation in neurons and glial cells. Aim 2. Investigate that NHE1 protein-mediated H+ efflux in neurons is involved in axonal transport dysregulation through activation of cofilin-actin rod formation after r-mTBI. Aim 3. Investigate efficacy of post-TBI administration of NHE1 pharmacological inhibitors (HOE642 and Rimeporide) in reducing axonal damage, inflammation, and neurological functional deficits in r-mTBI mice. In summary, completion of this project will enable us to gain new knowledge about the underlying cellular mechanisms for r-mTBI-induced pathogenesis. The combined approaches with the Cre-loxP mouse line and post-TBI pharmacological inhibition of NHE1 function will reveal therapeutic potentials of targeting NOX and/or N...