Furin-mediated release of soluble hemojuvelin: a new link between hypoxia and iron homeostasis

Abstract

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...

Selective binding of RGMc/hemojuvelin, a key protein in systemic iron metabolism, to BMP-2 and neogenin

“…We find that BMP-2 binds to RGMc as effectively as to noggin, a known high-affinity BMP-interacting molecule (39,43), and detect a preference of BMP-2 for single-chain RGMc, particularly the 40-kDa species, which is derived from cleavage of 50-kDa RGMc by proprotein convertases (20,40). Our results thus extend the observations of Babitt et al (2), who used protein cross-linking and competition binding assay to establish an association between I 125 -labeled BMP-2 and an RGMc-IgG Fc fusion protein, and those of Halbrooks et al (9), who showed using surface plasmon resonance that soluble RGMc could bind BMP-2 with nanomolar affinity (9).…”

“…Membrane-associated RGMc consists of both a single-chain 50-kDa polypeptide and a disulfide-bonded heterodimer composed of 20-kDa NH 2 -terminal and 35-kDa COOH-terminal subunits (15,19,41,48), while soluble RGMc consists of 50-and 40-kDa species that are derived from the membrane-bound 50-kDa protein (15). Although the responsible biochemical mechanisms have not been elucidated fully, it appears that 40-kDa soluble RGMc is truncated at its COOH terminus through cleavage by proprotein convertases (20,40).…”

Hemojuvelin N-terminal mutants reach the plasma membrane but do not activate the hepcidin response

“…2 The central role of HJV in body iron homeostasis is supported by the evidence that Hjv knockout mice (Hjv -/-mice) show severe suppression of hepatic hepcidin and increased iron deposition in the liver, pancreas and heart. 3,4 HJV encodes a protein characterized by a N-terminal signal peptide, a RGD integrin binding motif, a partial von Willebrand factor type D domain containing a glucose-6-phosphate dehydrogenase (GDPH) consensus sequence for autoproteolysis, a RNRR consensus sequence for furin cleavage 5,6 and a C-terminal GPI-anchor domain.…”

“…s-HJV acts as an antagonist in the hepcidin activation pathway, sequestering BMP and interrupting their signaling. 10 Large amounts of s-HJV are released into cellular media in conditions of iron deficiency 6 and this soluble protein is reduced in conditions of iron overload.…”

RGMs: Structural Insights, Molecular Regulation, and Downstream Signaling

“…In addition, the serine protease matriptase-2 (TMPRSS6) binds and cleaves cell surface RGMc. However, the RGMc fragment shed by matriptase-2 has reduced ability to bind BMPs and fails to repress BMPinduced hepcidin expression in vitro [40,41]. Unlike furin, it is thought that matriptase-2 impacts iron homeostasis mainly by reducing levels of membrane-bound RGMc.…”