Mechanisms of Erythroid Iron Sensing and Uptake: Roles for TfR2 in the Iron Deprivation Response and Mitochondrial Iron Delivery

Erythroid progenitors are the major consumers of iron in the human body. These cells must ultimately deliver iron to the mitochondrial matrix, where it is used in heme synthesis to support hemoglobinization. Iron homeostasis is tightly coupled with erythroid iron utilization: declining levels of circulating iron result in suppressed erythropoiesis, thus sparing the body’s supply of iron when threatened by erythroid consumption. This response, termed the erythroid iron deprivation response, underlies the pathogenic mechanism of various human anemias. Such iron-restricted human anemias are associated with resistance to erythropoietin (Epo), the major cytokine regulating survival, proliferation, and differentiation of erythroid progenitors. The mechanism of the erythroid iron deprivation response has remained unknown, but aconitase enzymes and transferrin receptor 2 (TfR2) have been identified as erythroid iron-sensing components. The results described herein define a pathway of erythroid iron sensing that culminates with regulation of Epo receptor (EpoR) surface presentation and signaling. Briefly, TfR2 undergoes accelerated lysosomal trafficking and degradation upon iron deprivation, resulting in the co-catabolism of the essential TfR2 binding partner: Scribble. Scribble is a key orchestrator of receptor trafficking and signaling that is required for erythroid surface presentation of EpoR and normal Epo-dependent Stat signaling. We identify the coordination of a TfR2-driven nutrient-sensing pathway with altered receptor trafficking and cytokine responsiveness, thus limiting progenitor expansion while maintaining survival. Interestingly, under iron replete conditions, TfR2 was found to traffic to the lysosome and undergo degradation, although the rate of degradation was reduced. The mechanism by which erythroid iron is ultimately trafficked from endosome to mitochondria has remained unclear, as cytosolic transit of iron is not observed. The existence of an alternative erythroid iron uptake pathway is suggested by the demonstration of residual erythropoiesis in mice lacking TfR1 and DMT1, proteins involved in canonical iron uptake. Further, mice with marrow-selective TfR2 deficiency have been found to exhibit microcytosis, suggesting that TfR2 may also contribute to erythroid hemoglobinization. Assessing whether TfR2 contributes to iron uptake identified an alternative pathway of iron uptake initiated by TfR2 lysosomal transferrin delivery. Imaging studies reveal an erythroid lineage-specific organelle arrangement consisting of a focal lysosomal cluster surrounded by a nest of mitochondria, with direct contacts between these two organelles. Erythroid TfR2 deficiency yields aberrant mitochondrial morphology and impaired heme synthesis, implicating TfR2-dependent transferrin trafficking in iron uptake and mitochondrial maintenance. Human TFR2 shares a lineage- and stage- specific expression pattern with MCOLN1, encoding a lysosomal iron channel, and MFN2, encoding a mitochondrial protein mediating organelle contacts. Functional studies reveal these latter factors to be involved in mitochondrial regulation and erythroid differentiation: Mcoln1 -/- animals exhibit anemia and erythroid mitochondrial perturbation, while erythroid Mfn2 knockdown reduces the frequency of mitochondrial-lysosomal contacts and blocks erythropoiesis. These findings identify an alternative role for TfR2, driving lysosomal delivery of transferrin and mitochondrial iron uptake. Thus, TfR2 is identified as a driver of both erythroid iron sensing and iron uptake.