Transferrin Acts as an Iron Sensor Upstream of BMP2b in the Zebrafish.

Abstract

The BMP pathway is important in modulating expression of the iron regulatory hormone hepcidin in response to iron, however how iron levels are sensed is unknown. The zebrafish mutant gavi was identified in a large-scale chemical mutagenesis screen for embryonic anemia. Two alleles were identified: gaviHE067 and gaviIT029, both of which have a homozygous recessive pattern of inheritance. Sequencing of transferrin cDNA from the mutants revealed that each allele of gavi produces an aberrant message and virtually absent transferrin expression. Our previous work demonstrated that hepcidin expression is upregulated following iron dextran injection of the zebrafish embryo. This effect was not impaired by a defect in ferroportin1 (Fraenkel et al., ( 2005) J Clin Invest 115:1532–1541), an iron exporter and ligand for hepcidin. To evaluate the effects of iron loading on transferrin-deficient mutants, gaviIT029 homozygotes at 48 hours post-fertilization were either anesthetized and injected with iron dextran, or anesthetized but not injected. GaviIT029 homozygote embryos exhibited low levels of hepcidin expression that failed to increase following iron injection. Morpholino knockdown of transferrin or transferrin receptor 2 abrogated hepcidin expression, even following iron injection, while knockdown of transferrin receptor 1a (erythroid-specific transferrin receptor 1) or transferrin receptor 1b (a ubiquitously expressed form of transferrin receptor 1) failed to produce this effect. Overexpression of BMP2b, however, rescued hepcidin expression following transferrin knockdown. These findings provide the first evidence that transferrin acts as an in vivo iron sensor upstream of the BMP pathway.