NATURE GENETICS | VOLUME 37 | NUMBER 11 | NOVEMBER 2005 1159

NEWS AND V IEWS

A ferrireductase fills the gap in the transferrin cycleAndrew T McKie

An endosomal ferric reductase has long been implicated in the transferrin cycle. Identification of the gene mutated in a genetic anemia in mice uncovers a new family of ferric reductase enzymes involved in reduction of transferrin-bound iron.

In animal cells, iron may be taken up through two main pathways: bound to transferrin (Tf) or as non–Tf-bound iron, where iron is associated with other, less well-defined che-lators. In both cases, iron is in the ferric state. Because the chief iron uptake transporter, DMT1 (refs. 1,2), exclusively transports fer-rous iron, however, reduction of ferric iron is an essential prerequisite.

In immature erythroid cells, uptake of Tf-bound iron through the Tf receptor (TfR1) is the primary pathway for delivery of iron to mitochondria for heme synthesis. The Tf cycle is a classical example of receptor-medi-ated endocytosis3 (Fig. 1a). The cycle begins with the binding of diferric Tf to TfR1. After endocytosis of the complex, the endosome is acidified. The pH shift is crucial and results in release of ferric iron from the Tf-TfR1 com-plex4. The iron is then transported out of the endosome into the cell by DMT1 (ref. 5), and the Tf-TfR1 complex is recycled to the cell sur-face to begin the cycle over again. One ques-tion has remained: how is ferric iron reduced before being transported by DMT1? Although a reductase capable of reducing diferric Tf had been measured biochemically6, the pro-tein responsible had not been identified. On page 1264 of this issue, Ohgami et al.7 report the identification of a ferrireductase (Steap3) involved in uptake of Tf-bound iron.

The same group8 previously reported that hypochromic microcytic anemia in the nm1054 mouse mutant was caused by a defect in iron release through the Tf cycle leading to defective heme synthesis in developing

red blood cell (reticulocytes). The authors initially used a positional cloning approach to identify Steap3 as the gene underlying the nm1054 mutation. Because the nm1054 mouse lacks several genes owing to a large genomic deletion, several methods were used to show that Steap3 was the gene involved. First, the authors were able to complement the nm1054 phenotype by expressing a BAC transgene containing Steap3 but none of the other genes in the deleted region. Second, by selectively knocking-out Steap3, they showed that the phenotype of Steap3–/– mice reca-pitulated that of nm1054 mutants. These experiments strongly suggested that Steap3 has a key role in development of the anemia found in the nm1054 mouse and that Steap3 was involved in the Tf cycle. Furthermore, expression of Steap3 mRNA was restricted to

organs with a role in erythropoesis, and the protein colocalized with TfR1 and DMT1 in the endosomal compartment. But what about the function? Limited homology to the yeast ferric reductase FRE family suggested that Steap3 had a role as a ferrireductase. Using biochemical assays, Ohgami et al.7 went on to show that expression of Steap3 increased ferrireductase activity in vitro and that reticu-locytes from nm1054 or Steap3–/– mice had similarly low ferrireductase activity.

Other reductasesSteap3 does not share homology with other mammalian ferrireductases described so far, such as Dcytb9. Unlike Dcytb, which is an ascorbate-dependent reductase belonging to the cytochrome b561 family of proteins, Steap3 is an NAD(P)H-dependent enzyme

Andrew T. McKie is in the Division of Nutritional Sciences at Kings College London, London SE1 9NH, UK. e-mail: andrew.t.mckie@kcl.ac.uk

ba

Dcytb

Asc DHA

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NAD(P)H NAD

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Figure 1 Two main pathways of iron acquisition in animal cells. (a) Uptake of Tf-bound iron in reticulocytes and other cells expressing TfR1 involves Steap3 and DMT1. (b) Uptake of non–Tf-bound iron in intestine and other cells mediated by Dcytb and DMT1. Asc, ascorbate; DHA, dehydroascorbic acid; e, electron.

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1160 VOLUME 37 | NUMBER 11 | NOVEMBER 2005 | NATURE GENETICS

NEWS AND V IEWS

with weak homology to the yeast FRE reduc-tase family10. Whereas Dcytb has two heme groups per molecule located on either side of the membrane, Steap3 contains only one heme group located in the membrane. Dcytb is highly expressed in the duodenal entero-cyte, where it is thought to have a primary role in reduction of dietary ferric iron to ferrous iron before absorption also through DMT1 (Fig. 1b). But Dcytb is not highly expressed in developing reticulocytes or other erythro-poeitic organs and is therefore unlikely to be essential in reduction of Tf-bound iron. With the exception of intestinal cells, less is known about the role of transmembrane reductive mechanisms in the uptake of non–Tf-bound iron by other cells. Dcytb or Steap3 may be involved.

OutlookDespite the facts that the Tf cycle operates in almost all cells and Steap3 is highly expressed in other tissues, only the hematological com-partment seems to be affected by the loss of Steap3. The authors describe several close homologs of Steap3 (now called Steap2 and Steap4) which probably encode other ferric reductases, pointing towards functional over-lap of these essential enzymes. This is reminis-

cent of the yeast FRE family, which has seven members, and underlines the importance of the reduction of ferric iron in iron traffick-ing in eukaryotic systems. It is perhaps due to the high level of Tf cycling that occurs in reticulocytes relative to other cells that loss of Steap3 causes an abnormal phenotype only in these cells. In other cell types, the activity may be complemented by either Steap2 or Steap4. Tissue profiling and subcellular localization of these homologs will no doubt shed more light on the role of these other potential fer-rireductase enzymes in mammalian cells and may uncover other important roles for these proteins in biology. In particular, it will be interesting to see how this new family of ferri-reductases is regulated in response to changes in body iron status, such as demand for iron, iron deficiency and inflammation. Other questions remain. Are any of these proteins involved in uptake of Tf through transferrin receptor 2 (TfR2; ref. 11), a homolog of TfR1 highly expressed in liver? What is the true physiological substrate for Steap3: diferric Tf or some other ferric complex? Future work will no doubt resolve these questions.

The identification of Steap3 and other members may have clinical implications. In humans, mutations of the human homolog

Malaria-protective traits at odds in Africa?Thomas E Wellems & Rick M Fairhurst

Sickle hemoglobin and α-thalassemia evolved as two of the most common human genetic polymorphisms because they confer protection against malaria. New evidence suggests that their effects may interfere with each other, raising questions about these hemoglobinopathies and their mechanisms of protection.

Malaria is a tremendous selective force on human populations. The impact of its morbid-ity and mortality is particularly evident in muta-tions of erythrocytes, the cells in which malaria parasites replicate and cause disease. A number of these mutations affect hemoglobin, erythro-cyte membrane proteins or pathways that guard against oxidative stress and are thought to confer a survival advantage to children while they build up protective immunity from repeated malaria infections1. Point mutations in the hemoglobin β chain are responsible for some of the best-

known of these polymorphisms, including sickle hemoglobin S (HbS), hemoglobin C (HbC) and hemoglobin E (HbE). Other genetic mutations cause underproduction of the α or β chains of hemoglobin and are responsible for the thalas-semias2. The geographical distributions of these various hemoglobinopathies generally favor areas where strong natural selection has been present from the morbidity and mortality from malaria. On page 1253 of this issue, Williams et al.3 take a fresh look at two hemoglobin-opathies in Africa, HbS and α-thalassemia, and provide evidence that negative epistasis between them profoundly affects their prevalence. Malaria protection from these hemoglobin-opathies seems to be suppressed when they are present together, so that children who inherit both are at same risk of malaria as children who inherit neither.

Heterozygote advantageThe HbS mutation (β6 Glu→Val; in the gene encoding the β chain on chromosome 11) was the first point mutation to be associated with malaria protection4. It is a textbook example of balanced polymorphism: fatalities from the sickle cell condition in HbSS homozygotes are offset by the survival advantage of HbAS (sickle trait) heterozygotes with malaria over HbAA homozygotes with malaria5. HbS gene frequen-cies of 15% or more occur in regions of India and Africa6 (Fig. 1). The α-thalassemia condi-tions result from genetic mutations that reduce (α+) or abolish (α0) the production of α-globin chains from a pair of almost identical genes on chromosome 16. The most common of these conditions are forms of α+-thalassemia that result from a crossover event and loss of one α chain gene from the chromosome2. Inherited

Thomas E. Wellems and Rick M. Fairhurst are at the Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892-8132, USA. e-mail: twellems@niaid.nih.gov

of Steap3 would be expected to result ane-mia similar to that of nm1054 mice, whereas mutations of Steap2 and Steap4 may affect iron metabolism in other tissues leading to different pathologies.

The identification of Steap3 is an advance in understanding the Tf cycle, one of the most elegant and often-quoted receptor-mediated pathways in the body, and will stimulate fur-ther research on the role of ferrireductases in iron transport in mammalian cells.

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(1997).3. Huebers, H.A. & Finch, C.A. Physiol. Rev. 67, 520–

582 (1987).4. Klausner, R.D., Ashwell, G., van Renswoude, J.,

Harford, J.B. & Bridges, K.R. Proc. Natl. Acad. Sci. USA 80, 2263–2266 (1983).

5. Fleming, M.D. et al. Proc. Natl. Acad. Sci. USA 95, 1148–1153 (1998).

6. Sun, I.L., Navas, P., Crane, F.L., Morre, D.J. & Low, H. J. Biol. Chem. 262, 15915–15921 (1987).

7. Ohgami, R.S. et al. Nat. Genet. 37, 1264–1269 (2005).

8. Ohgami, R.S. et al. Blood published online 30 June 2005 (doi:10.1182/blood-2005-01-0379).

9. McKie, A.T. et al. Science 291, 1755–1759 (2001).

10. Dancis, A., Klausner, R.D., Hinnebusch, A.G. & Barriocanal, J.G. Mol. Cell. Biol. 10, 2294–2301 (1990).

11. Kawabata, H. et al. J. Biol. Chem. 274, 20826–20832 (1999).

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