Research Activity

Figure 1. The 3D structure of ferritin molecule.

Iron is an essential element for various vital functions, such as oxygen transport, DNA synthesis and many redox reactions involving oxygen. It is also potentially toxic, due to its capacity to catalyze the formation of highly reactive free radicals; hence its homeostasis must be tightly regulated. To balance the requirement for iron and its toxicity, mammalians have evolved elaborate mechanisms of iron regulation both at cellular and systemic level. The Iron Regulatory Proteins (IRPs) sense cellular iron levels, and respond to repress or up-regulate, in a coordinate fashion, the expression of a variety of proteins for iron utilization, storage or transport. They include the transferrin receptor1, mitochondrial aconitase, erythroid ALA synthase, and the ferritins. These latter are the proteins most tightly regulated by IRPs and, with their capacity to sequester a large quantity of iron (up to 4,000 atoms per molecule), they behave as the effectors in the regulatory mechanism. The systemic regulation of iron homeostasis involves a large number of membrane molecules which exploit different functions: sensor of systemic iron (HFE), iron membrane transporters (DMT1and Ferrroportin1), ferrireductase and ferroxidases, (Duodenal Cyt B, hephaestin and ceruloplasmin), and receptors for cellular iron uptake ( transferrin receptors 1 and 2). The circulating proteins include: transferrin that transfers the metal to the tissues and the hepcidin, which is expressed in the liver and seems to regulate body iron absorption. The interplay among these molecules should result in the fine-tuning of iron acquisition and compartmentalisation. Genetic disorders of iron metabolism include hemochromatosis, iron loading anemias and neurodegenerative diseases, while alterations of iron homeostasis are associated to severe diseases such as cancer and inflammation. In the past we applied protein-engineering technique to characterize the biochemical properties of some iron proteins such as human ferritins, HFE and IRP1. Our major interest is focused on ferritin molecules. We showed that the two subunits of cytosolic ferritin, H and L chains, which assemble into 24-mer molecules (Fig. 1), have different functions: the H-chain, has a catalytic ferroxidase activity (Lawson et al. 1991), while the other assists for a more efficient iron incorporation (Levi et al. 1994).

Cytosolic Ferritins
Now we are interested in defining the biological role of the cytosolic ferritins and in clarifying their involvement in biological events such as cell proliferation, apoptosis and oxidative stress (Arosio et al. 2002). We developed cellular models that overexpress the ferritins showing that the functionality of the ferroxidase center of the H chain is essential for the iron storage function of the molecule (Cozzi et al. 2000). This is consistent with the finding that the deletion of the H chain in knockout mice is lethal at early stages of embryogenesis (Ferreira et al. 2000). Our data indicate that an excess of H chain increases cellular resistance to the oxidative damage induced by H2O2, reduce the rate of cell proliferation and has also an anti-apoptotic effect, with a mechanism that does not seems to be iron-mediated (Cozzi et al. 2003). Less clear is the role of ferritin L-chain.  It is as tightly regulated by iron as the H chain, but it becomes prevalent in tissue with chronic iron overload. L-ferritin overexpression in HeLa cells indicate that it is not actively involved in the regulation of iron availability; however the recent discovery that a neurodegenerative disorder, called Neuroferritinopathy, is caused by a single point mutation of the L-ferritin gene, indicates that this molecule has major effect on iron metabolism in the brain, and stimulates further studies to clarify its function. We are studying the neuroferritinopathy at a molecular, biochemical and cellular level with the aim to understand the role of this protein in the brain and in neurodegenerative disorders.

Mitochondrial Ferritin

zoom Figure 2. HeLa cells overexpressing the mitochondrial ferritin. The protein is stained with a fluorescent anti MtF antibody, it accumulates in filamentous intracellular bodies characteristic of mitochondria.

More recently, we identified a new ferritin type encoded by an intronless gene as a precursor, with a long N-terminal extension, which acts as a mitochondria-targeting sequence (Levi et al. 2001). This ferritin accumulates specifically in the mitochondria (Fig. 2), and hence it has been named mitochondrial ferritin (MtF). We have found that this protein is functionally active (Corsi et al. 2002) and has the potential to act as an iron-detoxifying molecule inside the organelle most exposed to iron flux and reactive oxygen species (Drysdale et al. 2002). This is suggested by the fact that MtF is highly expressed in iron-loaded mitochondria of sideroblastic anemia and apparently reduces the toxic effect of the deposited metal (Cazzola et al. 2003). Our preliminary results indicate that this protein has a strict tissue specific expression, indicating a peculiar function that we are now investigating.  We are particularly interested in studying the role of this protein in iron loaded mitochondria diseases, such as Friedreich’s ataxia and sideroblastic anemia, where MtF may play a role in iron detoxification. This will be done by developing suitable cellular models that overexpress the protein, by studying the expression of ortholog protein in mice and the factors that induce its expression.

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