Storage and transport of labile iron is mediated by lysosomes in axons and dendrites of hippocampal neurons

Abstract

Iron dyshomeostasis in neurons, involving iron accumulation and abnormal redox balance, is implicated in neurodegeneration. In particular, labile iron, a highly reactive pool of intracellular iron, plays a prominent role in iron-induced neurological damage. However, the mechanisms governing the detoxification and transport of labile iron within neurons are not fully understood. This study investigates the storage and transport of labile ferrous iron Fe(II) in cultured primary rat hippocampal neurons. Iron distribution was studied using live cell confocal microscopy with a selective labile Fe(II) fluorescent dye, and synchrotron X-ray fluorescence microscopy (SXRF) for total iron distribution. Fluorescent labelling of the axon initial segment and of lysosomes allowed iron distribution to be correlated with these subcellular compartments. The results show that labile Fe(II) is stored in lysosomes within somas, axons and dendrites and that lysosomal labile Fe(II) is transported retrogradely and anterogradely along axons and dendrites. In addition, we have developed a methodological workflow to quantify labile Fe(II) relative to total iron in neurites. This method is based on correlative imaging of fluorescence microscopy of labile Fe(II) combined with quantitative elemental mapping of total iron by SXRF. Quantitative analysis revealed that after Fe(II) exposure, lysosomal Fe(II) accounts for a small but significant percentage of the total iron content in neurites. These result suggest that after exposure to labile Fe(II), iron is mainly present in a non-reactive form in neurons, while the smaller fraction of reactive labile Fe(II) is stored in lysosomes and can be transported along dendrites and axons.