Project Summary Hyperphosphorylation and aggregation of microtubule-associated tau is a pathological hallmark of tauopathies including Alzheimer's disease (AD). In AD, tau abnormality correlates with neuronal loss and cognitive deficits better than amyloid burden. There is increasing interest in the development of tau-targeting drugs, for AD as well as other tauopathies. Developing effective tau-targeting drugs requires a deeper and broader understanding of the pathogenic mechanisms of tau. Recent studies have revealed extensive tau interaction with mitochondrial proteins. Intriguingly, studies in animal models and humans have uncovered a specific link between tau and mitochondrial complex-I (C-I) dysfunction, although the exact mechanism is unclear. C-I is the largest multisubunit complex of the respiratory chain containing 45 subunits in humans. Under normal physiological conditions, C-I catalyzes the oxidation of NADH to NAD+, initiates the transfer of electrons along the electron transport chain (ETC) from NADH to ubiquinone, accompanied by proton pumping to generate the gradient across the inner mitochondrial membrane needed for ATP production. Defective C-I is a frequent cause of mitochondrial dysfunction linked to human diseases. In addition to bioenergetic failure, reactive oxygen species (ROS) production is intimately associated with mitochondrial dysfunction. Various studies have implicated reverse electron transfer (RET) at C-I as the major site of mitochondrial ROS production. Under certain thermodynamic conditions, for example when forward electron transport (FET) is blocked or when succinate accumulates to high level, RET can occur along C-I, moving electrons from ubiquinol (CoQ10H2) to NAD+, producing a significant amount of ROS (RET-ROS) in the process. RET-ROS has been linked to physiological processes such as macrophage activation in response to bacterial infection. However, excessive RET- ROS is implicated in disease conditions such as ischemia-reperfusion injury. Another outcome of RET is the conversion of NAD+ to NADH, thereby decreasing NAD+ and lowering NAD+/NADH ratio, which has been linked to aging and age-related diseases. In preliminary studies, we have shown that inhibition of RET by a small molecule drug that targets NDUFS3, or partial knockdown of NDUFS3 by genetic means, extends lifespan and rescues age- related disease phenotypes in fly models. Excitingly, our preliminary studies generated compelling evidence that pharmacological inhibition of RET can rescue cognitive deficits and neurodegeneration and extend the lifespan of tauopathy mouse models. The goal of this study is to use in vivo mouse models and human iPSC-derived neuronal models to test our central hypothesis that RET is altered in tauopathy, that tau interacts with C-I components to affect RET, and that pharmacological or genetic targeting of NDUFS3 can restore RET and offer neuroprotection. Despite overwhelming evidence implicating defective mitochondr...