Use of Dual Electron Probes Reveals Role of Ferritin in <i>Ex Vivo</i> Erythropoiesis

Abstract

In the finely regulated process of mammalian erythropoiesis, the path of the labile iron pool and its transport to mitochondria for heme production is not well understood1. Existing models for erythropoiesis do not involve a central role for the ubiquitous iron storage protein ferritin, but instead indicate that incoming endosomal Fe3+ attached to transferrin enters the cytoplasm through divalent metal ion transporters after reduction to Fe2+, and is immediately taken up into mitochondria through the mitoferrin-1 transporter. Here, we apply a dual 3D imaging and spectroscopic technique, based on scanned electron probes, to measure distributions of Fe3+ in ex vivo human erythropoietic stem cells. After seven days in culture, we find that the cells display a highly specialized architecture of organelles with anchored clustering of mitochondria and massive accumulation of nanoparticles containing high iron concentrations localized to lysosomal storage depots, which we identify as ferritin. Our results support the hypothesis that lysosomal ferritin iron depots are utilized by developing reticulocytes to continue heme production after much of the cellular machinery has been expelled.