Marked Age-Related Changes in Brain Iron Homeostasis in Amyloid Protein Precursor Knockout Mice

Proteolytic cleavage of the amyloid precursor protein (APP) into the Aβ peptide has been an extensively researched mechanism for Alzheimer's disease, but the normal function of the protein is less understood. APP functions to regulate neuronal iron content by stabilizing the surface presentation of ferroportin-the only iron exporter channel of cells. The present study aims to quantify the contribution of APP to brain and peripheral iron by examining the lifetime impact on brain and liver iron levels in APP knockout mice. Consistent with previous reports, we found that wild-type mice exhibited an age-dependent increase in iron and ferritin in the brain, while no age-dependent changes were observed in the liver. APP ablation resulted in an exaggeration of age-dependent iron accumulation in the brain and liver in mice that was assessed at 8, 12, 18, and 22 months of age. Brain ferroportin levels were decreased in APP knockout mice, consistent with a mechanistic role for APP in stabilizing this iron export protein in the brain. Iron elevation in the brain and liver of APP knockout mice correlated with decreased transferrin receptor 1 and increased ferritin protein levels. However, no age-dependent increase in brain ferritin iron saturation was observed in APP-KO mice despite similar protein expression levels potentially explaining the vulnerability of APP-KO mice to parkinsonism and traumatic brain sequelae. Our results support a crucial role of APP in regulating brain and peripheral iron, and show that APP may act to oppose brain iron elevation during aging.