We previously described a zebrafish mutant, frascati (frs), which exhibits profound hypochromic anemia and erythroid maturation arrest due to defects in mitochondrial iron uptake. Through positional cloning, we showed that the frs gene encodes a novel member of the vertebrate mitochondrial solute carrier family (SLC25), mitoferrin (mfrn, slc25a37). Mfrn, which is highly expressed in fetal and adult hematopoietic tissues of zebrafish and mouse, functions as the major mitochondrial iron importer essential for heme biosynthesis in vertebrate erythroblasts (Shaw GC, et al. 2006 Nature 440:96–100). To study the function of Mfrn in mammalian organisms, we identified an embryonic stem (ES) cell clone that harbors a gene trap b-geo cassette in intron 1 that inactivates the Mfrn locus. Homozygous disruption of the Mfrn locus results in embryonic lethality at E11.5 from profound anemia due to a failure of primitive erythropoiesis, confirming the requirement of Mfrn in mammalian development . Circumventing the embryonic lethality, we generated Mfrn−/− ES cells to study the role of Mfrn in definitive erythropoiesis by in vitro differentiation of embryoid bodies and mixed chimera assays. Mfrn−/− ES cells were defective in promoting the growth, differentiation, and hemoglobinization of both primitive and definitive erythroblasts by in vitro differentiation of embryoid bodies. In mixed chimera studies, Mfrn−/− ES cells failed to contribute to the erythroid compartment of adult mosaic mice, whereas measurable contribution of Mfrn−/− donor cells could be assayed in the non-erythroid, leukocyte compartment. Transcriptome microarray analysis, using the mouse Affymetrix GeneChip and the custom IronChip, revealed unexpected down-regulation of transcripts for heme-biosynthetic enzymes in Mfrn−/− erythroblasts. The block in protoprophyrin synthesis, as well as mitochondrial heme synthesis, could be partially rescued by the addition of aminolevulinic acid (ALA) to Mfrn−/− erythroblasts in vitro. Our data demonstrate that mitochondrial iron homeostasis, working through the Mfrn iron importer, coordinately regulates the synthetic pathways for porphyrin and heme in developing mammalian erythroblasts.
Disclosure: No relevant conflicts of interest to declare.