Summary The liver peptide hepcidin regulates body iron, is upregulated in iron overload and inflammation and downregulated in iron deficiency/hypoxia. The transmembrane serine protease matriptase-2 (TMPRSS6) inhibits the hepcidin response and its mutational inactivation causes iron-deficient anemia in mice and humans. Here we confirm the inhibitory effect of matriptase-2 on hepcidin promoter; we show that matriptase-2 lacking the serine protease domain, identified in the anemic Mask mouse (matriptase-2MASK), is fully inactive and that mutant R774C found in patients with genetic iron deficiency has decreased inhibitory activity. Matriptase-2 cleaves hemojuvelin (HJV), a regulator of hepcidin, on plasma membrane; matriptase-2MASK shows no and the human mutant only partial cleavage capacity. Matriptase-2 interacts with HJV through the ectodomain since the interaction is conserved in matriptase-2MASK. The expression of matriptase-2 mutants in zebrafish results in anemia, confirming the matriptase-2 role in iron metabolism and its interaction with HJV.

The liver peptide hepcidin regulates iron absorption and recycling. Hemojuvelin (HJV) has a key role in hepcidin regulation, and its inactivation causes severe iron overload both in humans and in mice. Membrane HJV (m-HJV) acts as a coreceptor for bone morphogenetic proteins (BMPs), whereas soluble HJV (s-HJV) may down-regulate hepcidin in a competitive way interfering with BMP signaling. s-HJV is decreased by iron in vitro and increased by iron deficiency in vivo. IntroductionThe liver peptide hepcidin is the key regulator of systemic iron homeostasis 1 since it determines the level of circulating iron controlling intestinal iron absorption and macrophage iron recycling. Hemojuvelin (HJV) is a recently recognized protein that plays a crucial role in the regulation of hepcidin. The HJV gene, encoding HJV, is the gene of 1q-linked juvenile hemochromatosis, a recessive disease that leads to severe iron overload of early onset (hemochromatosis type 2A, OMIM no. 602390). 2 Patients with mutated HJV as well as Hjv Ϫ/Ϫ mice 3,4 have low/absent hepcidin levels, indicating that HJV modulates hepcidin. HJV is expressed in liver, skeletal muscle, and heart 2 and belongs to the family of repulsive guidance molecules (RGMs), expressed mainly in the central nervous system. 5,6 As are the other RGM proteins, HJV is characterized by a signal peptide, a RGD motif, a partial von Willebrand factor type D domain, and a glycosilphosphatidylinositol (GPI)-anchor domain. 2 HJV undergoes a partial autocatalytic cleavage 7 to reach the plasma membrane (m-HJV) as a cleaved heterodimer. 8,9 Recently, it was shown that HJV is a coreceptor for bone morphogenetic proteins (BMPs) and that hepcidin regulation occurs via the BMP/SMAD pathway. 10,11 BMP2 and BMP4 signaling is dependent on diferric transferrin, thus establishing a link between HJV-BMP and iron. 12 HJV exists in 2 forms: a membrane-bound (m-HJV) and a soluble one (s-HJV), which in vitro reciprocally regulate hepcidin expression in response to opposite iron changes. 7 s-HJV is able to interfere with and to block the signaling of BMP2 and BMP4 both in vitro 12 and in vivo. 13 We have previously documented the relevance of m-HJV in the molecular pathogenesis of juvenile hemochromatosis, providing evidence that at least some of the mutants identified as causal in patients are less efficiently targeted to the plasma membrane, compared with the wild-type protein. 9 Generation of s-HJV appears to be a regulated process decreased by iron treatment and diferric transferrin. 7 During our previous study, we have confirmed that this regulation is maintained in mutants, strengthening that s-HJV, suppressed by iron overload, is not involved in the disease pathogenesis. 9 We have also suggested that s-HJV does not derive from shedding of m-HJV, since its release in the medium is observed even in mutants that barely reach the plasma membrane. The discrepancy between the presence of m-HJV and the production of s-HJV is also well exemplified by the efficient secretion of soluble forms from varian...

Key Points• TFR2, a gene mutated in hemochromatosis and a partner of the EPO receptor, limits erythropoiesis expansion in mice.• Iron deficiency mimics TFR2 deletion in the erythroid compartment.Transferrin receptor 2 (TFR2) contributes to hepcidin regulation in the liver and associates with erythropoietin receptor in erythroid cells. Nevertheless, TFR2 mutations cause iron overload (hemochromatosis type 3) without overt erythroid abnormalities. To clarify TFR2 erythroid function, we generated a mouse lacking Tfr2 exclusively in the bone marrow (Tfr2 BMKO ). Tfr2 BMKO mice have normal iron parameters, reduced hepcidin levels, higher hemoglobin and red blood cell counts, and lower mean corpuscular volume than normal control mice, a phenotype that becomes more evident in iron deficiency. In Tfr2 BMKO mice, the proportion of nucleated erythroid cells in the bone marrow is higher and the apoptosis lower than in controls, irrespective of comparable erythropoietin levels. Induction of moderate iron deficiency increases erythroblasts number, reduces apoptosis, and enhances erythropoietin (Epo) levels in controls, but not in Tfr2 BMKO mice.Epo-target genes such as Bcl-x L and Epor are highly expressed in the spleen and in isolated erythroblasts from Tfr2 BMKO mice. Low hepcidin expression in Tfr2 BMKO is accounted for by erythroid expansion and production of the erythroid regulator erythroferrone. We suggest that Tfr2 is a component of a novel iron-sensing mechanism that adjusts erythrocyte production according to iron availability, likely by modulating the erythroblast Epo sensitivity. (Blood.