atpif1 regulates Mitochondrial Heme Synthesis In Developing Erythroid Cells

Abstract 163

Iron plays a key role as a cofactor in many fundamental metabolic processes, which require heme synthesis and Fe/S cluster assembly in the mitochondria. Defects in the transport of iron into the mitochondria would lead to anemias due to a deficiency in heme and hemoglobin synthesis. Here we describe a zebrafish genetic mutant, pinotage (pnt^tq209), which exhibits a profound hypochromic, microcytic anemia. Erythrocytes from pnt mutants have a defect in hemoglobinization and decreased red cell indices (mean corpuscular volume and hemoglobin content, hematocrit, hemoglobin concentration). Through positional cloning, we showed that the mitochondrial ATPase Inhibitory Factor 1 (atpif1), which regulates the inner mitochondrial membrane potential, is the gene disrupted in pnt. The identity of the pnt gene was verified by: (a) decreased atpif1 steady-state mRNA in pnt mutants, (b) phenocopying the anemia with anti-sense atpif1 morpholinos, (c) functional complementation of the anemia with atpif1 cRNA, and (d) a genetic polymorphism in the 3'UTR co-segregating with the mutant phenotype that destabilizes the atpif1 mRNA. Consistent with the conserved function of atpif1 in higher vertebrates, the silencing of the murine ortholog of atpif1 in Friend mouse erythroleukemia (MEL) cells showed a defect in hemoglobinization by o-dianisidine staining and reduction of ⁵⁹Fe incorporation into heme in ⁵⁹Fe-metabolically labeled cells. Moreover, Atpif1 knockdown destabilizes their mitochondrial membrane potential and volume. Therefore, the identification of atpif1 in pnt functionally demonstrates the role of atpif1 in regulating the proton motive gradient across the inner mitochondrial membrane for mitochondrial iron incorporation in heme biosynthesis. These results uncover a novel hematopoiesis-related function of atpif1, which will directly contribute to our understanding and potential treatment of human congenital and acquired anemias.