Erythroid cell mitochondria receive endosomal iron by a “kiss-and-run” mechanism

• Published in: Biochimica et biophysica acta Vol. 1863; no. 12; pp. 2859 - 2867
• Main Authors: Hamdi, Amel, Roshan, Tariq M., Kahawita, Tanya M., Mason, Anne B., Sheftel, Alex D., Ponka, Prem
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2016
• Subjects: Animals; Biological Transport; Cell Differentiation; Endocytosis - physiology; Endosomes; Endosomes - metabolism; Erythroid cells; Ferrochelatase - metabolism; Fetus; Fluorescent Dyes - chemistry; Heme - metabolism; Humans; Iron; Iron - metabolism; Liver - cytology; Liver - metabolism; Mice; Mitochondria; Mitochondria - metabolism; Mutation; Primary Cell Culture; Protoporphyrins - metabolism; Reticulocytes - cytology; Reticulocytes - metabolism; Staining and Labeling - methods; Transferrin; Transferrin - metabolism; Transferrin; Mitochondria; Iron; Erythroid cells; Endosomes
• ISSN: 0167-4889; 0006-3002; 1879-2596
• DOI: 10.1016/j.bbamcr.2016.09.008

In erythroid cells, more than 90% of transferrin-derived iron enters mitochondria where ferrochelatase inserts Fe2+ into protoporphyrin IX. However, the path of iron from endosomes to mitochondrial ferrochelatase remains elusive. The prevailing opinion is that, after its export from endosomes, the redox-active metal spreads into the cytosol and mysteriously finds its way into mitochondria through passive diffusion. In contrast, this study supports the hypothesis that the highly efficient transport of iron toward ferrochelatase in erythroid cells requires a direct interaction between transferrin-endosomes and mitochondria (the “kiss-and-run” hypothesis). Using a novel method (flow sub-cytometry), we analyze lysates of reticulocytes after labeling these organelles with different fluorophores. We have identified a double-labeled population definitively representing endosomes interacting with mitochondria, as demonstrated by confocal microscopy. Moreover, we conclude that this endosome-mitochondrion association is reversible, since a “chase” with unlabeled holotransferrin causes a time-dependent decrease in the size of the double-labeled population. Importantly, the dissociation of endosomes from mitochondria does not occur in the absence of holotransferrin. Additionally, mutated recombinant holotransferrin, that cannot release iron, significantly decreases the uptake of 59Fe by reticulocytes and diminishes 59Fe incorporation into heme. This suggests that endosomes, which are unable to provide iron to mitochondria, cause a “traffic jam” leading to decreased endocytosis of holotransferrin. Altogether, our results suggest that a molecular mechanism exists to coordinate the iron status of endosomal transferrin with its trafficking. Besides its contribution to the field of iron metabolism, this study provides evidence for a new intracellular trafficking pathway of organelles. [Display omitted] •Iron is directly delivered from endosomes to mitochondria in erythroid cells.•Intra-endosomal transferrin saturation governs interorganellar association.•Endosomal mitochondrial interface regulates the cellular iron uptake.•The study's findings are important for the understanding of sideroblastic anemia.