Hemojuvelin (HJV), also known as repulsive guidance molecule C (RGMc) or hemochromatosis type 2 protein (HFE2), is a membrane-bound and soluble protein in mammals that is responsible for the iron overload condition known as juvenile hemochromatosis in humans, a severe form of hemochromatosis. In humans, the hemojuvelin protein is encoded by the HFE2gene.[5][6] Hemojuvelin is a member of the repulsive guidance molecule family of proteins.[7][8] Both RGMa and RGMb are found in the nervous system,[9][10] while hemojuvelin is found in skeletal muscle and the liver.[10][11]
For many years the signal transduction pathways that regulate systemic iron homeostasis have been unknown. However it has been demonstrated that hemojuvelin interacts with bone morphogenetic protein (BMP), possibly as a co-receptor, and may signal via the SMAD pathway to regulate hepcidin expression.[12] Associations with BMP2 and BMP4 have been described.[13]
Mouse HJV knock-out models confirmed that HJV is the gene responsible for juvenile hemochromatosis. Hepcidin levels in the liver are dramatically depressed in these knockout animals.[14][15]
A soluble form of HJV may be a molecule that suppresses hepcidin expression.[16]
RGMs may play inhibitory roles in prostate cancer by suppressing cell growth, adhesion, migration and invasion. RGMs can coordinate Smad-dependent and Smad-independent signalling of BMPs in prostate cancer and breast cancer cells.[17][18] Furthermore, aberrant expression of RGMs was indicated in breast cancer. The perturbed expression was associated with disease progression and poor prognosis.[19]
RGMc/HJV is a 4-exon gene in mammals that undergoes alternative RNA splicing to yield 3 mRNAs with different 5’ untranslated regions (5’UTRs).[11] Gene transcription is induced during myoblast differentiation, producing all 3 mRNAs. There are three critical promoter elements responsible for transcriptional activation in skeletal muscle (the tissue that has the highest level of RGMc expressesion per weight), comprising paired E-boxes, a putative Stat and/or Ets element, and a MEF2 site, and muscle transcription factors myogenin and MEF2C stimulate RGMc promoter function in non-muscle cells. As these elements are conserved in RGMc genes from multiple species, these results suggest that RGMc has been a muscle-enriched gene throughout its evolutionary history.[11]
RGMc/HJV, is transcriptionally regulated during muscle differentiation.[11]
Isoforms
Two classes of GPI-anchored and glycosylated HJV molecules are targeted to the membrane and undergo distinct fates.[20]
Full-length HJV is released from the cell surface and accumulates in extracellular fluid, where its half-life exceeds 24 hours. There appears to be two potential soluble isoforms and two membrane-associated isoforms.[20]
The predominant membrane-associated isoform, a disulfide-linked two-chain form composed of N- and C-terminal fragments, is not found in the extracellular fluid, and is short-lived, as it disappears from the cell surface with a half-life of < 3 hours after interruption of protein synthesis.[20]
RGMc appears to undergo a complex processing that generates 2 soluble, single-chain forms, and two membrane-bound forms found as a (i) single-chain, and (ii) two-chain species which appears to be cleaved at a site within a partial von Willebrand factor domain.[20]
Using a combination of biochemical and cell-based approaches, it has demonstrated that BMP-2 could interact in biochemical assays with the single-chain HJV species, and also could bind to cell-associated HJV. Two mouse HJV amino acid substitution mutants, D165E and G313V (corresponding to human D172E and G320V), also could bind BMP-2, but less effectively than wild-type HJV, while G92V (human G99V) could not. In contrast, the membrane-spanning protein, neogenin, a receptor for the related molecule, RGMa, preferentially bound membrane-associated heterodimeric RGMc and was able to interact on cells only with wild-type RGMc and G92V. These results show that different isoforms of RGMc/HJV may play unique physiological roles through defined interactions with distinct signaling proteins and demonstrate that, in some disease-linked HJV mutants, these interactions are defective.[21]
Structure
In 2009, the Rosetta ab initio protein structure prediction software has been used to create a three-dimensional model of the RGM family of proteins.,[8] In 2011, a crystal structure of a fragment of hemojuvelin binding to neogenin was completed [22]
showing similar structures to the ab initio model and further informing the view of the RGM family of proteins.
Mechanism of action
Furin-like proprotein convertases (PPC) are responsible for conversion of 50 kDa HJV to a 40 kDa protein with a truncated COOH-terminus, at a conserved polybasic RNRR site. This suggests a potential mechanism to generate the soluble forms of HJV/hemojuvelin (s-hemojuvelin) found in the blood of rodents and humans.[23][24]
Clinical significance
Mutations in HJV are responsible for the vast majority of juvenile hemochromatosis patients. A small number of patients have mutations in the hepcidin (HAMP) gene. The gene was positionally cloned.[6] Hemojuvelin is highly expressed in skeletal muscle and heart, and to a lesser extent in the liver. One insight into the pathogenesis of juvenile hemochromatosis is that patients have low to undetectable urinary hepcidin levels, suggesting that hemojuvelin is a positive regulator of hepcidin, the central iron regulatory hormone. As a result, low hepcidin levels would result in increased intestinal iron absorption. Thus, HJV/RGMc appears to play a critical role in iron metabolism.[citation needed]
^Li J, Ye L, Sanders AJ, Jiang WG (March 2012). "Repulsive guidance molecule B (RGMB) plays negative roles in breast cancer by coordinating BMP signaling". J Cell Biochem. 113 (7): 2523–31. doi:10.1002/jcb.24128. PMID22415859. S2CID35629616.
^Li J, Ye L, Mansel RE, Jiang WG (May 2011). "Potential prognostic value of repulsive guidance molecules in breast cancer". Anticancer Res. 31 (5): 1703–11. PMID21617229.
^ abcdKuninger D, Kuns-Hashimoto R, Kuzmickas R, Rotwein P (August 2006). "Complex biosynthesis of the muscle-enriched iron regulator RGMc". J. Cell Sci. 119 (Pt 16): 3273–83. doi:10.1242/jcs.03074. PMID16868025. S2CID15574534.
^Kuns-Hashimoto R, Kuninger D, Nili M, Rotwein P (April 2008). "Selective binding of RGMc/hemojuvelin, a key protein in systemic iron metabolism, to BMP-2 and neogenin". Am. J. Physiol., Cell Physiol. 294 (4): C994–C1003. doi:10.1152/ajpcell.00563.2007. PMID18287331. S2CID32158124.
