Deciphering a Novel Mechanism for Iron-sensing at Mitochondria and Its Role in Erythropoiesis

Abstract Iron is essential for eukaryotic life acting as a biological catalytic center of various proteins involved in diverse cellular processes. In human, the greatest portion of total body iron can be found as heme (an iron-containing porphyrin), the majority of which is in red blood cells in the form of hemoglobin that carries oxygen from the lung to the whole body. An imbalanced supply of iron, heme, or globin proteins leads to the shortage of functional hemoglobin, resulting in various types of anemia. Insufficient iron supply is particularly critical, as it causes so-called iron deficiency anemia, the most common form of anemia worldwide. Understanding the regulation of hemoglobin synthesis in erythroid cells is important for developing novel therapeutic strategies to treat red blood cell disorders. However, the molecular mechanisms underlying this regulation have not been fully understood. This research proposal aims to uncover these mechanisms by delineating iron-dependent regulation of the mitochondrial protein DELE1. It has been recently shown that in response mitochondrial stress, DELE1 acts as an activator of the stress responsive kinase HRI, a well-established regulator of globin translation in the erythroid lineage. Preliminary results in this proposal indicate that DELE1 activates HRI in iron-depleted cells by a novel mechanism distinct from previously reported mechanisms. These results also suggest the mitochondrial import of DELE1 and its subsequent protein stability is strictly regulated by intracellular iron availability. Thus, this proposal will test the hypothesis that iron-dependent mitochondrial proteostasis and import regulation of DELE1 are the core components of a novel mitochondrial iron-sensing pathway regulating the HRI-mediated stress response. The mechanisms how mitochondrial DELE1 activates cytosolic HRI in iron deficient conditions will be investigated using multiple advanced molecular biology techniques, in combination with mass spectrometry-based proteomics. The involvement of the DELE1-HRI pathway in terminal erythroid differentiation will be addressed using a murine erythroleukemia (MEL) cell line, an established erythroid cellular model, as well as a recently generated DELE1 deficient mouse. The success of the proposed work will provide novel molecular insights into mitochondrial iron-sensing and reveal the critical molecular players that are responsible for the communication between mitochondria and the cytosol in order to maintain cellular homeostasis under iron-deficient conditions.