Ferritin molecules play an important role in the control of intracellular iron distribution and in the constitution of long term iron stores. In vitro studies on recombinant ferritin subunits have shown that the ferroxidase activity associated with the H subunit is necessary for iron uptake by the ferritin molecule, whereas the L subunit facilitates iron core formation inside the protein shell. However, plant and bacterial ferritins have only a single type of subunit which probably fulfills both functions. To assess the biological significance of the ferroxidase activity associated with the H subunit, we disrupted the H ferritin gene (Fth) in mice by homologous recombination. Fth ؉/؊ mice are healthy, fertile, and do not differ significantly from their control littermates. However, Fth ؊/؊ embryos die between 3.5 and 9.5 days of development, suggesting that there is no functional redundancy between the two ferritin subunits and that, in the absence of H subunits, L ferritin homopolymers are not able to maintain iron in a bioavailable and nontoxic form. The pattern of expression of the wild type Fth gene in 9.5-day embryos is suggestive of an important function of the H ferritin gene in the heart.Iron is essential to all living organisms, but to prevent its toxicity it must be associated to specialized molecules. Of those, ferritins (Fts) 1 play special roles because of their ubiquitous distribution in all tissues, to the tight iron-dependent gene expression, and to their capacity to store large amounts of iron (up to 4,000 Fe atoms per molecule) inside a large protein shell, in a nontoxic and bioavailable form (reviewed in Ref.1). Mammalian ferritins are heteropolymers made of two different subunit types named H and L. The H chain carries a ferroxidase center which appears to be essential for iron incorporation (2), whereas the L chain facilitates iron mineralization inside the cavity (3). In prokaryotes and plants, ferritins are made of 24 identical subunits which all carry the ferroxidase activity. In mammals, multiple transcriptional regulations operate which modify H ferritin mRNA levels in response to cytokines (4), heme (5, 6), oncogenes (7), or to cell proliferation or differentiation (reviewed in Ref 8). In addition, ferritin mRNAs have unique features which allow efficient (9) and tissue-specific (10) translational regulation according to the iron status of the cell. Therefore, the conservation of the ferritin ferroxidase activity throughout evolution as well as the very complex genetic regulation of ferritin expression suggest that this catalytic activity is essential for ferritin biological function.We disrupted the H ferritin (Fth) gene in mice and found that H subunit-associated ferroxidase is necessary for early embryonic development because no Fth Ϫ/Ϫ embryos were found after 3.5 days post coitus. Our data also demonstrate that L ferritin gene product cannot substitute for the H subunit. In contrast, heterozygous Fth ϩ/Ϫ were healthy and indistinguishable from their control littermates. EXPERIME...

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Ferritin, the iron-storing molecule, is made by the assembly of various proportions of 2 different H and L subunits into a 24-mer protein shell. These heteropolymers have distinct physicochemical properties, owing to the ferroxidase activity of the H subunit, which is necessary for iron uptake by the ferritin molecule, and the ability of the L subunit to facilitate iron core formation inside the protein shell. It has previously been shown that H ferritin is indispensable for normal development, since inactivation of the H ferritin gene by homologous recombination in mice is lethal at an early stage during embryonic development. Here the phenotypic analysis of the mice heterozygous for the H ferritin gene (Fth ؉/؊ mice) is reported, and differences in gene regulation between the 2 subunits are shown. The heterozygous Fth ؉/؊ mice were healthy and fertile and did not present any apparent abnormalities. Although they had iron-overloaded spleens at the adult stage, this is identical to what is observed in normal Fth ؉/؉ mice. However, these heterozygous mice had slightly elevated tissue L ferritin content and 7-to 10-fold more L ferritin in the serum than normal mice, but their serum iron remained unchanged.

We have previously shown that hepcidin transgenic embryos are severely anemic and die around birth. Here, we report that embryonic hepcidin transgene expression decreases transferrin receptor 1 (TfR1) mRNA level in placenta, as shown by cDNA microarray analysis and quantitative RT-PCR, by a mechanism which is independent of placenta iron content and iron responsive element/iron regulatory protein (IRE/IRP) activity. On the contrary, iron injections into pregnant mothers result in increased placenta iron and ferritin content, and reduced IRE binding activity of IRP1 leading to decreased TfR1 mRNA level. Taken together, these results suggest that hepcidin action on placenta is mostly through transcriptional downregulation of the iron uptake machinery.