The Zebrafish Hypochromic Mutant [iItalic]Shiraz[/iItalic] Encodes a Novel Mitochondrial Glutaredoxin That Establishes a Link between Heme and Fe/S Production.

Iron is required in the mitochondria both to produce heme, which is used for hemoglobin synthesis, and to make iron-sulfur (Fe/S) clusters, which confer electron transfer or catalytic functions to proteins. Cellular iron utilization and Fe/S cluster production are thought to occur independently, yet the processes are coordinated through currently uncharacterized pathways. The shiraz (sir) zebrafish mutant manifests a hypochromic, microcytic anemia. Positional cloning of sir discovered a deletion at the locus that included the zebrafish orthologue to glutaredoxin 5 (grx5), a gene required in yeast for Fe/S cluster assembly. We found that grx5 is highly expressed in the developing blood and fetal liver of both zebrafish and mouse embryos. Antisense-mediated morpholino knockdown of grx5 prevented hemoglobin production, and overexpression of zebrafish, yeast, mouse, or human grx5 RNA in sir embryos completely rescued hemoglobin production, indicating that grx5 is the gene responsible for the sir phenotype. Expression of zebrafish grx5 was found to rescue Fe/S protein production in the yeast Δgrx5 strain, demonstrating that the role of grx5 in Fe/S cluster assembly is conserved among eukaryotes. The surprising finding that mutating a gene necessary for Fe/S cluster assembly caused a lack of hemoglobin synthesis suggested that we had discovered a connection between these pathways. In vertebrates, iron regulatory protein 1 (IRP1) acts as a sensor of intracellular iron levels and controls cellular iron homeostasis via posttranscriptional regulation of iron uptake, storage, and utilization genes. For instance, IRP1 binds to the 5′ iron response element (IRE) in the aminolevulinate synthase 2 (ALAS2) mRNA, blocking translation when cellular iron is low. However, when cellular iron is replete, IRP1 binds a Fe/S cluster and its RNA-binding activity is abolished. We hypothesized that the loss of Fe/S cluster assembly in sir would activate IRP1 and block ALAS2 synthesis, resulting in hypochromia. In support of this model, overexpression of ALAS2 RNA without the 5′ IRE rescued sir hypochromia, while overexpression of ALAS2 including the IRE did not facilitate rescue. Furthermore, antisense morpholino knockdowns of IRP1 caused rescue of hemoglobin synthesis in sir embryos. The combination of these data indicate that hemoglobin production in the differentiating red cell is monitored through Fe-S cluster assembly as a mechanism to gauge iron levels and accordingly direct heme synthesis. This finding illustrates a crucial role for the mitochondrial Fe/S cluster assembly machinery during hemoglobin production, and has broad implications for the role of such genes in human hypochromic anemias.