Metabolic Characterization of Intact Cells Reveals Intracellular Amyloid Beta but Not Its Precursor Protein to Reduce Mitochondrial Respiration

One hallmark of Alzheimer's disease are senile plaques consisting of amyloid beta (A beta), which derives from the processing of the amyloid precursor protein (APP). Mitochondrial dysfunction has been linked to the pathogenesis of Alzheimer's disease and both A beta and APP have been reported to affect mitochondrial function in isolated systems. However, in intact cells, considering a physiological localization of APP and A beta, it is pending what triggers the mitochondrial defect. Thus, the aim of this study was to dissect the impact of APP versus A beta in inducing mitochondrial alterations with respect to their subcellular localization. We performed an overexpression of APP or beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), increasing APP and A beta levels or A beta alone, respectively. Conducting a comprehensive metabolic characterization we demonstrate that only APP overexpression reduced mitochondrial respiration, despite lower extracellular A beta levels compared to BACE overexpression. Surprisingly, this could be rescued by a gamma secretase inhibitor, oppositionally indicating an A beta-mediated mitochondrial toxicity. Analyzing A beta localization revealed that intracellular levels of A beta and an increased spatial association of APPA beta with mitochondria are associated with reduced mitochondrial respiration. Thus, our data provide marked evidence for a prominent role of intracellular A beta accumulation in Alzheimer's disease associated mitochondrial dysfunction. Thereby it highlights the importance of the localization of APP processing and intracellular transport as a decisive factor for mitochondrial function, linking two prominent hallmarks of neurodegenerative diseases.