DRAGON: a member of the repulsive guidance molecule-related family of neuronal- and muscle-expressed membrane proteins is regulated by DRG11 and has neuronal adhesive properties

The RGM protein DRAG-1 positively regulates a BMP-like signaling pathway in Caenorhabditis elegans

The bone morphogenetic protein (BMP) signaling pathway regulates multiple developmental and homeostatic processes. Mutations in the pathway can cause a variety of somatic and hereditary disorders in humans. Multiple levels of regulation, including extracellular regulation, ensure proper spatiotemporal control of BMP signaling in the right cellular context. We have identified a modulator of the BMP-like Sma/Mab pathway in C. elegans called DRAG-1. DRAG-1 is the sole member of the repulsive guidance molecule (RGM) family of proteins in C. elegans, and is crucial in regulating body size and mesoderm development. Using a combination of molecular genetic and biochemical analyses, we demonstrate that DRAG-1 is a membrane-associated protein that functions at the ligand-receptor level to modulate the Sma/Mab pathway in a cell-type-specific manner. We further show that DRAG-1 positively modulates this BMP-like pathway by using a novel Sma/Mab-responsive reporter. Our work provides a direct link between RGM proteins and BMP signaling in vivo and a simple and genetically tractable system for mechanistic studies of RGM protein regulation of BMP pathways.

Soluble repulsive guidance molecule c/hemojuvelin is a broad spectrum bone morphogenetic protein (BMP) antagonist and inhibits both BMP2- and BMP6-mediated signaling and gene expression

Inactivating mutations in hemojuvelin/repulsive guidance molecule c (HJV/RGMc) cause juvenile hemochromatosis (JH), a rapidly progressive iron overload disorder in which expression of hepcidin, a key liver-derived iron-regulatory hormone, is severely diminished. Several growth factors in the bone morphogenetic protein (BMP) family, including BMP2 and BMP6, can stimulate production of hepcidin, a biological effect that may be modified by RGMc. Here we demonstrate that soluble RGMc proteins are potent BMP inhibitors. We find that 50- and 40-kDa RGMc isoforms, when added to cells as highly purified IgG Fc fusion proteins, are able to block the acute effects of both BMP2 and BMP6 at the levels of Smad induction and gene activation, and thus represent a potentially unique class of broad-spectrum BMP antagonists. Whole transcript microarray analysis revealed that BMP2 and BMP6 each stimulated expression of a nearly identical cohort of approximately 40 mRNAs in Hep3B cells and demonstrated that 40-kDa RGMc was an effective inhibitor of both growth factors, although its potency was less than that of the known BMP2-selective antagonist, Noggin. We additionally show that JH-linked RGMc mutant proteins that retain the ability to bind BMPs are also able to function as BMP inhibitors, and like the wild type soluble RGMc species, can block BMP-activated hepcidin gene expression. The latter results raise the question of whether disease severity in JH will vary depending on the ability of a given mutant RGMc protein to interact with BMPs.

Dragon enhances BMP signaling and increases transepithelial resistance in kidney epithelial cells

The neuronal adhesion protein Dragon acts as a bone morphogenetic protein (BMP) coreceptor that enhances BMP signaling. Given the importance of BMP signaling in nephrogenesis and its putative role in the response to injury in the adult kidney, we studied the localization and function of Dragon in the kidney. We observed that Dragon localized predominantly to the apical surfaces of tubular epithelial cells in the thick ascending limbs, distal convoluted tubules, and collecting ducts of mice. Dragon expression was weak in the proximal tubules and glomeruli. In mouse inner medullary collecting duct (mIMCD3) cells, Dragon generated BMP signals in a ligand-dependent manner, and BMP4 is the predominant endogenous ligand for the Dragon coreceptor. In mIMCD3 cells, BMP4 normally signaled through BMPRII, but Dragon enhanced its signaling through the BMP type II receptor ActRIIA. Dragon and BMP4 increased transepithelial resistance (TER) through the Smad1/5/8 pathway. In epithelial cells isolated from the proximal tubule and intercalated cells of collecting ducts, we observed coexpression of ActRIIA, Dragon, and BMP4 but not BMPRII. Taken together, these results suggest that Dragon may enhance BMP signaling in renal tubular epithelial cells and maintain normal renal physiology.

Neogenin inhibits HJV secretion and regulates BMP-induced hepcidin expression and iron homeostasis

Neogenin, a deleted in colorectal cancer (DCC) family member, has been identified as a receptor for the neuronal axon guidance cues netrins and repulsive guidance molecules repulsive guidance molecules (RGM). RGMc, also called hemojuvelin (HJV), is essential for iron homeostasis. Here we provide evidence that neogenin plays a critical role in iron homeostasis by regulation of HJV secretion and bone morphogenetic protein (BMP) signaling. Livers of neogenin mutant mice exhibit iron overload, low levels of hepcidin, and reduced BMP signaling. Mutant hepatocytes in vitro show impaired BMP2 induction of Smad1/5/8 phosphorylation and hepcidin expression. Neogenin is expressed in liver cells in a reciprocal pattern to that of hepcidin, suggesting that neogenin functions in a cell nonautonomous manner. Further studies demonstrate that neogenin may stabilize HJV, a glycosylphosphatidylinositol-anchored protein that interacts with neogenin and suppresses its secretion. Taken together, our results lead the hypothesis that neogenin regulates iron homeostasis via inhibiting secretion of HJV, an inhibitor of BMP signaling, to enhance BMP signaling and hepcidin expression. These results reveal a novel mechanism underlying neogenin regulation of HJV-BMP signaling.

RGMb controls aggregation and migration of Neogenin-positive cells in vitro and in vivo

The proliferation, migration and differentiation of dentate gyrus stem and precursor cells have aroused keen interest. Neogenin and RGMb are expressed in non-overlapping compartments of the developing dentate gyrus. While Neogenin is expressed in migrating and proliferating dentate precursors, RGMb is localized in structures bordering the developing dentate, such as cornus ammonis cells and Cajal-Retzius cells in the marginal zone including the hippocampal fissure. Co-immunoprecipitation and binding assays indicate a strong physical interaction. In vitro and in vivo migration of dentate neuroepithelial cells is abolished by RGMb, and cell adhesion is reduced when cells expressing Neogenin comes into contact with cells expressing RGMb. Ectopic expression of RGMb in organotypic slice cultures and after in utero electroporation in the hippocampus modifies precursor cell migration. Our results imply that Neogenin-RGMb interaction might be involved in neuronal migration in the dentate gyrus.

The RGM/DRAGON family of BMP co-receptors

The BMP signaling pathway controls a number of cell processes during development and in adult tissues. At the cellular level, ligands of the BMP family act by binding a hetero-tetrameric signaling complex, composed of two type I and two type II receptors. BMP ligands make use of a limited number of receptors, which in turn activate a common signal transduction cascade at the intracellular level. A complex regulatory network is required in order to activate the signaling cascade at proper times and locations, and to generate specific downstream effects in the appropriate cellular context. One such regulatory mechanism is the repulsive guidance molecule (RGM) family of BMP co-receptors. This article reviews the current knowledge regarding the structure, regulation, and function of RGMs, focusing on known and potential roles of RGMs in physiology and pathophysiology.

Molecular biology, genetics and biochemistry of the repulsive guidance molecule family

RGMs (repulsive guidance molecules) comprise a recently discovered family of GPI (glycosylphosphatidylinositol)-linked cell-membrane-associated proteins found in most vertebrate species. The three proteins, RGMa, RGMb and RGMc, products of distinct single-copy genes that arose early in vertebrate evolution, are approximately 40-50% identical to each other in primary amino acid sequence, and share similarities in predicted protein domains and overall structure, as inferred by ab initio molecular modelling; yet the respective proteins appear to undergo distinct biosynthetic and processing steps, whose regulation has not been characterized to date. Each RGM also displays a discrete tissue-specific pattern of gene and protein expression, and each is proposed to have unique biological functions, ranging from axonal guidance during development (RGMa) to regulation of systemic iron metabolism (RGMc). All three RGM proteins appear capable of binding selected BMPs (bone morphogenetic proteins), and interactions with BMPs mediate at least some of the biological effects of RGMc on iron metabolism, but to date no role for BMPs has been defined in the actions of RGMa or RGMb. RGMa and RGMc have been shown to bind to the transmembrane protein neogenin, which acts as a critical receptor to mediate the biological effects of RGMa on repulsive axonal guidance and on neuronal survival, but its role in the actions of RGMc remains to be elucidated. Similarly, the full spectrum of biological functions of the three RGMs has not been completely characterized yet, and will remain an active topic of ongoing investigation.

DRAGON, a GPI-anchored membrane protein, inhibits BMP signaling in C2C12 myoblasts

Bone morphogenetic proteins (BMPs) induce osteoblastic differentiation of myoblasts via binding to cell surface receptors. Repulsive guidance molecules (RGMs) have been identified as BMP co-receptors. We report here that DRAGON/RGMb, a member of the RGM family, suppressed BMP signaling in C2C12 myoblasts via a novel mechanism. All RGMs were expressed in C2C12 cells that were differentiated into myocytes and osteoblastic cells, but RGMc was not detected in immature cells. In C2C12 cells, only DRAGON suppressed ALP and Id1 promoter activities induced by BMP-4 or by constitutively activated BMP type I receptors. This inhibition by DRAGON was dependent on the secretory form of the von Willbrand factor type D domain. DRAGON even suppressed BMP signaling induced by constitutively activated Smad1. Over-expression of neogenin did not alter the inhibitory capacity of DRAGON. Taken together, these findings indicate that DRAGON may be an inhibitor of BMP signaling in C2C12 myoblasts. We also suggest that a novel molecule(s) expressed on the cell membrane may mediate the signal transduction of DRAGON in order to suppress BMP signaling in C2C12 myoblasts.

BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism

Juvenile hemochromatosis is an iron-overload disorder caused by mutations in the genes encoding the major iron regulatory hormone hepcidin (HAMP) and hemojuvelin (HFE2). We have previously shown that hemojuvelin is a co-receptor for bone morphogenetic proteins (BMPs) and that BMP signals regulate hepcidin expression and iron metabolism. However, the endogenous BMP regulator(s) of hepcidin in vivo is unknown. Here we show that compared with soluble hemojuvelin (HJV.Fc), the homologous DRAGON.Fc is a more potent inhibitor of BMP2 or BMP4 but a less potent inhibitor of BMP6 in vitro. In vivo, HJV.Fc or a neutralizing antibody to BMP6 inhibits hepcidin expression and increases serum iron, whereas DRAGON.Fc has no effect. Notably, Bmp6-null mice have a phenotype resembling hereditary hemochromatosis, with reduced hepcidin expression and tissue iron overload. Finally, we demonstrate a physical interaction between HJV.Fc and BMP6, and we show that BMP6 increases hepcidin expression and reduces serum iron in mice. These data support a key role for BMP6 as a ligand for hemojuvelin and an endogenous regulator of hepcidin expression and iron metabolism in vivo.

BMP modulators regulate the function of BMP during body patterning and disease progression

Bone morphogenetic proteins (BMPs) are phylogenetically conserved signaling molecules that belong to the transforming growth factor (TGF)-beta superfamily and are involved in the cascades of body patterning and morphogenesis. The activities of BMPs are precisely regulated at various stages, and extracellulary, mainly regulated by certain classes of molecules termed as BMP antagonists and pro-BMP factors. BMP antagonists inhibit BMP function by prohibiting them from binding their cognate receptors, whereas pro-BMP factors stimulate BMP function. In this review, the functions of these BMP regulators will be discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Hemojuvelin regulates hepcidin expression via a selective subset of BMP ligands and receptors independently of neogenin

Hemojuvelin (HJV) is a coreceptor for bone morphogenetic protein (BMP) signaling that regulates hepcidin expression and iron metabolism. However, the precise combinations of BMP ligands and receptors used by HJV remain unknown. HJV has also been demonstrated to bind to neogenin, but it is not known whether this interaction has a role in regulating hepcidin expression. In the present study, we show that BMP-2, BMP-4, and BMP-6 are endogenous ligands for HJV in hepatoma-derived cell lines, and that all 3 of these ligands are expressed in human liver. We demonstrate in vitro that HJV selectively uses the BMP type II receptors ActRIIA and BMPRII, but not ActRIIB, and HJV enhances utilization of ActRIIA by BMP-2 and BMP-4. Interestingly, ActRIIA is the predominant BMP type II receptor expressed in human liver. While HJV can use all 3 BMP type I receptors (ALK2, ALK3, and ALK6) in vitro, only ALK2 and ALK3 are detected in human liver. Finally, we show that HJV-induced BMP signaling and hepcidin expression are not altered by neogenin overexpression or by inhibition of endogenous neogenin expression. Thus, HJV-mediated BMP signaling and hepcidin regulation occur via a distinct subset of BMP ligands and BMP receptors, independently of neogenin.

Pro-protein convertases control the maturation and processing of the iron-regulatory protein, RGMc/hemojuvelin

BACKGROUND

Repulsive guidance molecule c (RGMc or hemojuvelin), a glycosylphosphatidylinositol-linked glycoprotein expressed in liver and striated muscle, plays a central role in systemic iron balance. Inactivating mutations in the RGMc gene cause juvenile hemochromatosis (JH), a rapidly progressing iron storage disorder with severe systemic manifestations. RGMc undergoes complex biosynthetic steps leading to membrane-bound and soluble forms of the protein, including both 50 and 40 kDa single-chain species.

RESULTS

We now show that pro-protein convertases (PC) are responsible for conversion of 50 kDa RGMc to a 40 kDa protein with a truncated COOH-terminus. Unlike related molecules RGMa and RGMb, RGMc encodes a conserved PC recognition and cleavage site, and JH-associated RGMc frame-shift mutants undergo COOH-terminal cleavage only if this site is present. A cell-impermeable peptide PC inhibitor blocks the appearance of 40 kDa RGMc in extra-cellular fluid, as does an engineered mutation in the conserved PC recognition sequence, while the PC furin cleaves 50 kDa RGMc in vitro into a 40 kDa molecule with an intact NH2-terminus. Iron loading reduces release of RGMc from the cell membrane, and diminishes accumulation of the 40 kDa species in cell culture medium.

CONCLUSION

Our results define a role for PCs in the maturation of RGMc that may have implications for the physiological actions of this critical iron-regulatory protein.

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

Juvenile hemochromatosis is a severe and rapidly progressing hereditary disorder of iron overload, and it is caused primarily by defects in the gene encoding repulsive guidance molecule c/hemojuvelin (RGMc/HJV), a recently identified protein that undergoes a complicated biosynthetic pathway in muscle and liver, leading to cell membrane-linked single-chain and heterodimeric species, and two secreted single-chain isoforms. RGMc modulates expression of the hepatic iron regulatory factor, hepcidin, potentially through effects on signaling by the bone morphogenetic protein (BMP) family of soluble growth factors. To date, little is known about specific pathogenic defects in disease-causing RGMc/HJV proteins. Here we identify functional abnormalities in three juvenile hemochromatosis-linked mutants. Using a combination of approaches, we first show that BMP-2 could interact in biochemical assays with single-chain RGMc species, and also could bind to cell-associated RGMc. Two mouse RGMc amino acid substitution mutants, D165E and G313V (corresponding to human D172E and G320V), also could bind BMP-2, but less effectively than wild-type RGMc, 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. Our 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 RGMc mutants, these interactions are defective.

Liver-gut axis in the regulation of iron homeostasis

The human body requires about 1-2 mg of iron per day for its normal functioning, and dietary iron is the only source for this essential metal. Since humans do not possess a mechanism for the active excretion of iron, the amount of iron in the body is determined by the amount absorbed across the proximal small intestine and, consequently, intestinal iron absorption is a highly regulated process. In recent years, the liver has emerged as a central regulator of both iron absorption and iron release from other tissues. It achieves this by secreting a peptide hormone called hepcidin that acts on the small intestinal epithelium and other cells to limit iron delivery to the plasma. Hepcidin itself is regulated in response to various systemic stimuli including variations in body iron stores, the rate of erythropoiesis, inflammation and hypoxia, the same stimuli that have been known for many years to modulate iron absorption. This review will summarize recent findings on the role played by the liver and hepcidin in the regulation of body iron absorption.

Overexpression of repulsive guidance molecule (RGM) a induces cell death through Neogenin in early vertebrate development

Repulsive guidance molecule (RGM) a is a glycosylphosphatidylinositol (GPI)-anchored plasma membrane protein that has been implicated in chemorepulsive axon guidance. Although RGMa binds the transmembrane receptor Neogenin, the developmental events controlled by the RGMa-Neogenin interactions in vivo remain largely unknown. We have cloned full-length RGMa from Xenopus borealis for the first time and identified two homologous genes referred to as RGMa1 and RGMa2. Here we show RGMa1 overexpression at 2-cell-stage resulted in cell death, which lead to an early embryonic lethal phenotype of the embryos. Time-lapse photomicroscopy revealed that embryos began to show initial morphological defects from approximately 5 h post-fertilization (hpf) which was then followed by extensive blastomere cell death at approximately 11 hpf. This phenotype was rescued by simultaneous knock down of RGMa using translation blocking anti-sense morpholinos. Knock down of the RGMa1 receptor Neogenin in RGMa1 overexpressing embryos was also able to rescue the phenotype. Together these results indicated that RGMa1 was signalling through Neogenin to induce cell death in the early embryo. While previous studies have suggested that Neogenin is a dependence receptor that induces cell death in the absence of RGM, we have instead shown that Neogenin-RGM interactions induce cell death in the early embryo. The roles of RGMa1 and Neogenin appear to be context specific so that their co-ordinated and regulated expressions are essential for normal development of the vertebrate embryo.

Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance

Systemic iron balance is regulated by hepcidin, a peptide hormone secreted by the liver. By decreasing cell surface expression of the iron exporter ferroportin, hepcidin decreases iron absorption from the intestine and iron release from reticuloendothelial stores. Hepcidin excess has been implicated in the pathogenesis of anemia of chronic disease, while hepcidin deficiency has a key role in the pathogenesis of the iron overload disorder hemochromatosis. We have recently shown that hemojuvelin is a coreceptor for bone morphogenetic protein (BMP) signaling and that BMP signaling positively regulates hepcidin expression in liver cells in vitro. Here we show that BMP-2 administration increases hepcidin expression and decreases serum iron levels in vivo. We also show that soluble hemojuvelin (HJV.Fc) selectively inhibits BMP induction of hepcidin expression in vitro and that administration of HJV.Fc decreases hepcidin expression, increases ferroportin expression, mobilizes splenic iron stores, and increases serum iron levels in vivo. These data support a role for modulators of the BMP signaling pathway in treating diseases of iron overload and anemia of chronic disease.

Molecular evolution of hemojuvelin and the repulsive guidance molecule family

Repulsive guidance molecules (RGMs) are found in vertebrates and chordates and are involved in embryonic development and iron homeostasis. Members of this family are GPI-linked membrane proteins that contain an N-terminal signal peptide, a C-terminal propeptide, and a conserved RGD motif. Vertebrates are known to possess three paralogues; RGMA and RGMB (sometimes called Dragon) are expressed in the nervous system and are thought to play various roles in neural development. Hemojuvelin (HJV; also called repulsive guidance molecule c, RGMC) is the third member of this family, and mutations in this gene result in a form of juvenile hemochromatosis (type 2A). Phylogenetic analyses of 55 different RGM family sequences from 21 different species support the existence of a novel gene, found only in fish, which we have labeled RGMD. The pattern of conserved residues in each family identifies new candidates for important functional roles, including ligand binding.

Repulsive guidance molecule RGMa alters utilization of bone morphogenetic protein (BMP) type II receptors by BMP2 and BMP4

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily of multifunctional ligands that transduce their signals through type I and II serine/threonine kinase receptors and intracellular Smad proteins. Recently, we identified the glycosylphosphatidylinositol-anchored repulsive guidance molecules RGMa, DRAGON (RGMb), and hemojuvelin (RGMc) as coreceptors for BMP signaling (Babbit, J. L., Huang, F. W., Wrighting, D. W., Xia, Y., Sidis, Y., Samad, T. A., Campagna, J. A., Chung, R., Schneyer, A., Woolf, C. J., Andrews, N. C., and Lin, H. Y. (2006) Nat. Genet. 38, 531-539; Babbit, J. L., Zhang, Y., Samad, T. A., Xia, Y., Tang, J., Schneyer, A., Woolf, C. J., and Lin, H. Y. (2005) J. Biol. Chem. 280, 29820-29827; Samad, T. A., Rebbapragada, A., Bell, E., Zhang, Y., Sidis, Y., Jeong, S. J., Campagna, J. A., Perusini, S., Fabrizio, D. A., Schneyer, A. L., Lin, H. Y., Brivanlou, A. H., Attisano, L., and Woolf, C. J. (2005) J. Biol. Chem. 280, 14122-14129). However, the mechanism by which RGM family members enhance BMP signaling remains unknown. Here, we report that RGMa bound to radiolabeled BMP2 and BMP4 with Kd values of 2.4+/-0.2 and 1.4+/-0.1 nm, respectively. In KGN human ovarian granulosa cells and mouse pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling required BMP receptor type II (BMPRII), but not activin receptor type IIA (ActRIIA) or ActRIIB, based on changes in BMP signaling by small interfering RNA inhibition of receptor expression. In contrast, cells transfected with RGMa utilized both BMPRII and ActRIIA for BMP2 or BMP4 signaling. Furthermore, in BmpRII-null pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling was reduced by inhibition of endogenous RGMa expression, and RGMa-mediated BMP signaling required ActRIIA expression. These findings suggest that RGMa facilitates the use of ActRIIA by endogenous BMP2 and BMP4 ligands that otherwise prefer signaling via BMPRII and that increased utilization of ActRIIA leads to generation of an enhanced BMP signal.

Neogenin-RGMa Signaling at the Growth Cone Is Bone Morphogenetic Protein-independent and Involves RhoA, ROCK, and PKC

The repulsive guidance molecule RGMa has been shown to induce outgrowth inhibition of neurites by interacting with the transmembrane receptor neogenin. Here we show that RGMa-induced growth cone collapse is mediated by activation of the small GTPase RhoA, its downstream effector Rho kinase and PKC. In contrast to DRG cultures from neogenin-/- mice, in which no RGMa-mediated growth cone collapse and activation of RhoA occurred, treatment of wild type DRG neurites with soluble RGMa led to a marked activation of RhoA within 3 min followed by collapse, but left Rac1 and Cdc42 unaffected. Furthermore, preincubation of DRG axons with the bone morphogenetic protein (BMP) antagonist noggin had no effect on RGMa-mediated growth cone collapse, implying that the role of RGM in axonal guidance is neogenin- and not BMP receptor-dependent. Pretreatment with 1) C3-transferase, a specific inhibitor of the Rho GTPase; 2) Y-27632, a specific inhibitor of Rho kinase; and 3) Gö6976, the general PKC inhibitor, strongly inhibited the collapse rate of PC12 neurites. Growth cone collapse induced by RGMa was abolished by the expression of dominant negative RhoA, but not by dominant negative Rac1. In contrast to RGMa, netrin-1 induced no growth cone retraction but instead reduced RGMa-mediated growth cone collapse. These results suggest that activation of RhoA, Rho kinase, and PKC are physiologically relevant and important elements of the RGMa-mediated neogenin signal transduction pathway involved in axonal guidance.

The role of repulsive guidance molecules in the embryonic and adult vertebrate central nervous system

During the development of the nervous system, outgrowing axons often have to travel long distances to reach their target neurons. In this process, outgrowing neurites tipped with motile growth cones rely on guidance cues present in their local environment. These cues are detected by specific receptors expressed on growth cones and neurites and influence the trajectory of the growing fibres. Neurite growth, guidance, target innervation and synapse formation and maturation are the processes that occur predominantly but not exclusively during embryonic or early post-natal development in vertebrates. As a result, a functional neural network is established, which is usually remarkably stable. However, the stability of the neural network in higher vertebrates comes at an expensive price, i.e. the loss of any significant ability to regenerate injured or damaged neuronal connections in their central nervous system (CNS). Most importantly, neurite growth inhibitors prevent any regenerative growth of injured nerve fibres. Some of these inhibitors are associated with CNS myelin, others are found at the lesion site and in the scar tissue. Traumatic injuries in brain and spinal cord of mammals induce upregulation of embryonic inhibitory or repulsive guidance cues and their receptors on the neurites. An example for embryonic repulsive directional cues re-expressed at lesion sites in both the rat and human CNS is provided with repulsive guidance molecules, a new family of directional guidance cues.

Neogenin and repulsive guidance molecule signaling in the central nervous system

The repulsive guidance molecule (RGM) is a membrane-bound protein that was originally identified as an axon guidance molecule in the visual system. Functional studies have revealed that it has roles in axon guidance and laminar patterning in Xenopus and chick embryos, and in controlling cephalic neural tube closure in mouse embryos. The recent identification of neogenin as a receptor for RGM has provided evidence of the diverse functions of this ligand-receptor pair. Re-expression of RGM is observed after injury in the adult human and rat central nervous systems. Inhibition of RGM enhances growth of injured axons and promotes functional recovery after spinal cord injury in rats. Thus, re-expression of embryonic repulsive cues in adult tissues contributes to failure of axon regeneration in the central nervous system.

Regulation of iron metabolism by hepcidin

Hepcidin, a peptide hormone made in the liver, is the principal regulator of systemic iron homeostasis. Hepcidin controls plasma iron concentration and tissue distribution of iron by inhibiting intestinal iron absorption, iron recycling by macrophages, and iron mobilization from hepatic stores. Hepcidin acts by inhibiting cellular iron efflux through binding to and inducing the degradation of ferroportin, the sole known cellular iron exporter. Synthesis of hepcidin is homeostatically increased by iron loading and decreased by anemia and hypoxia. Hepcidin is also elevated during infections and inflammation, causing a decrease in serum iron levels and contributing to the development of anemia of inflammation, probably as a host defense mechanism to limit the availability of iron to invading microorganisms. At the opposite side of the spectrum, hepcidin deficiency appears to be the ultimate cause of most forms of hemochromatosis, either due to mutations in the hepcidin gene itself or due to mutations in the regulators of hepcidin synthesis. The emergence of hepcidin as the pathogenic factor in most systemic iron disorders should provide important opportunities for improving their diagnosis and treatment.

Complex biosynthesis of the muscle-enriched iron regulator RGMc

The recently discovered repulsive guidance molecule c (RGMc or hemojuvelin) gene encodes a putative glycosylphosphatidylinositol (GPI)-anchored protein that is expressed in striated muscle and in liver. Mutations in this gene have been linked to the severe iron storage disease, juvenile hemochromatosis, although the mechanisms of action of RGMc in iron metabolism are unknown. As a first step toward understanding the molecular physiology of this protein, we studied its biosynthesis, processing and maturation. Production of RGMc occurs as an early and sustained event during skeletal muscle differentiation in culture and is secondary to RGMc gene activation. As assessed by pulse-chase studies and cell-surface labeling experiments, two classes of GPI-anchored and glycosylated RGMc molecules are targeted to the membrane and undergo distinct fates. Full-length RGMc is released from the cell surface and accumulates in extracellular fluid, where its half-life exceeds 24 hours. By contrast, the predominant membrane-associated isoform, a disulfide-linked heterodimer 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. A natural disease-associated RGMc mutant, with valine substituted for glycine at residue 320 (313 in mouse RGMc), does not undergo processing to generate the heterodimeric membrane-linked isoform of RGMc, and is found on the cell surface only as larger protein species. Our results define a series of biosynthetic steps leading to the normal production of different RGMc isoforms in cells, and provide a framework for understanding the biochemical basis of defects in the maturation of RGMc in juvenile hemochromatosis.

Induction of repulsive guidance molecule in neurons following sciatic nerve injury

Repulsive guidance molecule (RGM) is a protein implicated in both axonal guidance and neural tube closure. We examined the expression of RGMa in the spinal cord after the sciatic nerve crush by immunohistochemistry. Although there was no RGMa immunoreactivity under naïve conditions in the dorsal horn, a weak signal for RGMa was found at 24 h after the nerve crush, and this signal was progressively increased in the NeuN-positive neurons in the ipsilateral dorsal horn from superficial to deep layers at 10 days after surgery. In the neurons of the ipsilateral ventral horn, RGMa was also induced at 10 days after surgery, whereas no RGMa signal could be observed in naïve conditions or at 24 h after surgery. Thus, RGMa expression is upregulated both in the ipsilateral dorsal and ventral horns in response to the sciatic nerve injury. We next examined the effects of complete Freund's adjuvant (CFA)-induced inflammation on RGMa expression in the spinal cord. However, no RGMa expression was observed at 24 h and 10 days after the CFA injection in the dorsal horn, suggesting that RGMa is not involved in inflammation-induced gyperalgesia. Our present study demonstrates that induction of RGMa is associated with the peripheral nerve injury.

Regulation of iron acquisition and iron distribution in mammals

Both cellular iron deficiency and excess have adverse consequences. To maintain iron homeostasis, complex mechanisms have evolved to regulate cellular and extracellular iron concentrations. Extracellular iron concentrations are controlled by a peptide hormone hepcidin, which inhibits the supply of iron into plasma. Hepcidin acts by binding to and inducing the degradation of the cellular iron exporter, ferroportin, found in sites of major iron flows: duodenal enterocytes involved in iron absorption, macrophages that recycle iron from senescent erythrocytes, and hepatocytes that store iron. Hepcidin synthesis is in turn controlled by iron concentrations, hypoxia, anemia and inflammatory cytokines. The molecular mechanisms that regulate hepcidin production are only beginning to be understood, but its dysregulation is involved in the pathogenesis of a spectrum of iron disorders. Deficiency of hepcidin is the unifying cause of hereditary hemochromatoses, and excessive cytokine-stimulated hepcidin production causes hypoferremia and contributes to anemia of inflammation.

Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression

Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2-/- hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression.

RGMa inhibition promotes axonal growth and recovery after spinal cord injury

Repulsive guidance molecule (RGM) is a protein implicated in both axonal guidance and neural tube closure. We report RGMa as a potent inhibitor of axon regeneration in the adult central nervous system (CNS). RGMa inhibits mammalian CNS neurite outgrowth by a mechanism dependent on the activation of the RhoA–Rho kinase pathway. RGMa expression is observed in oligodendrocytes, myelinated fibers, and neurons of the adult rat spinal cord and is induced around the injury site after spinal cord injury. We developed an antibody to RGMa that efficiently blocks the effect of RGMa in vitro. Intrathecal administration of the antibody to rats with thoracic spinal cord hemisection results in significant axonal growth of the corticospinal tract and improves functional recovery. Thus, RGMa plays an important role in limiting axonal regeneration after CNS injury and the RGMa antibody offers a possible therapeutic agent in clinical conditions characterized by a failure of CNS regeneration.

Molecular mapping of developing dorsal horn-enriched genes by microarray and dorsal/ventral subtractive screening

The dorsal horn of the spinal cord consists of distinct laminae that serve as a pivotal region for relaying a variety of somatosensory signals such as temperature, pain, and touch. The molecular mechanisms underlying the development of the dorsal horn are poorly understood. To define a molecular map of the dorsal horn circuit, we have profiled dorsal horn-enriched (DHE) gene expression in dorsal spinal cords on embryonic day 15.5 (E15.5) by genome-wide microarray and smart subtractive screening based on polymerase chain reaction (PCR). High-throughput in situ hybridization (ISH) was carried out to validate the expression of 379 genes in the developing dorsal spinal cord. A total of 113 DHE genes were identified, of which 59% show lamina-specific expression patterns. Most lamina-specific genes were expressed across at least two laminae, however. About 32% of all DHE genes are transcription factors, which represent the largest percentage of the group of all DHE functional classifications. Importantly, several individual lamina-specific transcription factors such c-Maf, Rora, and Satb1 are identified for the first time. Epistasis studies revealed several putative effectors of known DHE transcription factors such as Drg11, Tlx3(Rnx), and Lmx1b. These effector genes, including Grp, Trpc3, Pcp4, and Enc1, have been implicated in synaptic transmission, calcium homeostasis, and structural function and thus may have similar roles in the dorsal horn. The identification of a large number of DHE genes, especially those that are lamina specific, lays a foundation for future studies on the molecular machinery that controls the development of the dorsal horn and on functional differences of these distinct laminae in the dorsal spinal cord.

Hemojuvelin is essential for dietary iron sensing, and its mutation leads to severe iron overload

Iron homeostasis plays a critical role in many physiological processes, notably synthesis of heme proteins. Dietary iron sensing and inflammation converge in the control of iron absorption and retention by regulating the expression of hepcidin, a regulator of the iron exporter ferroportin. Human mutations in the glycosylphosphatidylinositol-anchored protein hemojuvelin (HJV; also known as RGMc and HFE2) cause juvenile hemochromatosis, a severe iron overload disease, but the way in which HJV intersects with the iron regulatory network has been unclear. Here we show that, within the liver, mouse Hjv is selectively expressed by periportal hepatocytes and also that Hjv-mutant mice exhibit iron overload as well as a dramatic decrease in hepcidin expression. Our findings define a key role for Hjv in dietary iron sensing and also reveal that cytokine-induced inflammation regulates hepcidin expression through an Hjv-independent pathway.

A mouse model of juvenile hemochromatosis

Hereditary hemochromatosis is an iron-overload disorder resulting from mutations in proteins presumed to be involved in the maintenance of iron homeostasis. Mutations in hemojuvelin (HJV) cause severe, early-onset juvenile hemochromatosis. The normal function of HJV is unknown. Juvenile hemochromatosis patients have decreased urinary levels of hepcidin, a peptide hormone that binds to the cellular iron exporter ferroportin, causing its internalization and degradation. We have disrupted the murine Hjv gene and shown that Hjv-/- mice have markedly increased iron deposition in liver, pancreas, and heart but decreased iron levels in tissue macrophages. Hepcidin mRNA expression was decreased in Hjv-/- mice. Accordingly, ferroportin expression detected by immunohistochemistry was markedly increased in both intestinal epithelial cells and macrophages. We propose that excess, unregulated ferroportin activity in these cell types leads to the increased intestinal iron absorption and plasma iron levels characteristic of the juvenile hemochromatosis phenotype.

Localization and action of Dragon (repulsive guidance molecule b), a novel bone morphogenetic protein coreceptor, throughout the reproductive axis

Bone morphogenetic proteins (BMPs) play important roles in reproduction including primordial germ cell formation, follicular development, spermatogenesis, and FSH secretion. Dragon, a recently identified glycosylphosphatidylinositol-anchored member of the repulsive guidance molecule family, is also a BMP coreceptor. In the present study, we determined the tissue and cellular localization of Dragon in reproductive organs using immunohistochemistry and in situ hybridization. Among reproductive organs, Dragon was expressed in testis, epididymis, ovary, uterus, and pituitary. In the testis of early postnatal mice, Dragon was found in gonocytes and spermatogonia, whereas in immature testes, Dragon was only weakly expressed in spermatogonia. Interestingly, pregnant mare serum gonadotropin treatment of immature mice robustly induced Dragon production in spermatocytes. In adult testis, Dragon was found in spermatocytes and round spermatids. In the ovary, Dragon was detected exclusively within oocytes and primarily those within secondary follicles. In the pituitary, Dragon-expressing cells overlapped FSH-expressing cells. Dragon was also expressed in a number of cell lines originating from reproductive tissues including Ishikawa, Hela, LbetaT2, MCF-7, and JEG3 cells. Immunocytochemistry and gradient sucrose ultracentrifugation studies showed Dragon was localized in lipid rafts within the plasma membrane. In reproductive cell lines, Dragon expression enhanced signaling of exogenous BMP2 or BMP4. The present studies demonstrate that Dragon expression is dynamically regulated throughout the reproductive tract and that Dragon protein modulates BMP signaling in cells from reproductive tissues. The overlap between Dragon expression and the functional BMP signaling system suggests that Dragon may play a role in mammalian reproduction.

Repulsive guidance molecule (RGMa), a DRAGON homologue, is a bone morphogenetic protein co-receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily of ligands, which regulate many mammalian physiologic and pathophysiologic processes. BMPs exert their effects through type I and type II serine/threonine kinase receptors and the Smad intracellular signaling pathway. Recently, the glycosylphosphatidylinositol (GPI)-anchored protein DRAGON was identified as a co-receptor for BMP signaling. Here, we investigate whether a homologue of DRAGON, repulsive guidance molecule (RGMa), is similarly involved in the BMP signaling pathway. We show that RGMa enhances BMP, but not TGF-beta, signals in a ligand-dependent manner in cell culture. The soluble extracellular domain of RGMa fused to human Fc (RGMa.Fc) forms a complex with BMP type I receptors and binds directly and selectively to radiolabeled BMP-2 and BMP-4. RGMa mediates BMP signaling through the classical BMP signaling pathway involving Smad1, 5, and 8, and it up-regulates endogenous inhibitor of differentiation (Id1) protein, an important downstream target of BMP signals. Finally, we demonstrate that BMP signaling occurs in neurons that express RGMa in vivo. These data are consistent with a role for RGMa as a BMP co-receptor.

Gene discovery by microarray: identification of novel genes induced during growth factor-mediated muscle cell survival and differentiation

Peptide growth factors regulate cell fate by activating distinct signal transduction pathways that ultimately influence gene expression. Insulin-like growth factors (IGFs) play central roles in controlling somatic growth and participate in skeletal muscle development and regeneration. In cultured muscle cells, IGF action is critical both for maintaining viability during the transition from proliferating to differentiating myoblasts and for facilitating differentiation. By contrast, platelet-derived growth factor (PDGF) can sustain cell survival but inhibits differentiation. Here we examine the genetic programs that accompany IGF and PDGF action in myoblasts. Through analysis of high-density oligonucleotide arrays containing approximately 36,000 mouse probe sets, we identify 90 transcripts differentially induced by IGF-I, including 28 muscle-specific genes and 33 previously unannotated mRNAs, and 55 transcripts specifically stimulated by PDGF, including 14 unknowns. Detailed study of one IGF-induced mRNA shows that it encodes a protein related to a recently characterized repulsive guidance molecule postulated to regulate neuronal targeting during development. Our results demonstrate the power of transcriptional profiling for gene discovery and provide opportunities for investigating new proteins potentially involved in different aspects of growth factor action in muscle.

DRAGON, a Bone Morphogenetic Protein Co-receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)beta superfamily of ligands that regulate many crucial aspects of embryonic development and organogenesis. Unlike other TGFbeta ligands, co-receptors for BMP ligands have not been described. Here we show that DRAGON, a glycosylphosphatidylinositol-anchored member of the repulsive guidance molecule family, which is expressed early in the developing nervous system, enhances BMP but not TGFbeta signaling. DRAGON binds directly to BMP2 and BMP4 but not to BMP7 or other TGFbeta ligands. The enhancing action of DRAGON on BMP signaling is also reduced by administration of Noggin, a soluble BMP antagonist, indicating that the action of DRAGON is ligand-dependent. DRAGON associates directly with BMP type I (ALK2, ALK3, and ALK6) and type II (ActRII and ActRIIB) receptors, and its signaling is reduced by dominant negative Smad1 and ALK3 or -6 receptors. In the Xenopus embryo, DRAGON both reduces the threshold of the ability of Smad1 to induce mesodermal and endodermal markers and alters neuronal and neural crest patterning. The direct interaction of DRAGON with BMP ligands and receptors indicates that it is a BMP co-receptor that potentiates BMP signaling.

Gene expression of the repulsive guidance molecules during development of the mouse intestine

Repulsive guidance molecules (RGMs) are recently identified proteins implicated in neuronal differentiation, migration, and apoptosis. However, in non-neural tissues a specific biological function of RGM is still unknown. In this study, we describe the expression patterns of the RGM members (a, b, and c) during embryonic and postnatal development of the small and large murine intestine. We demonstrated by RT-PCR, in situ hybridization, Western blot, and immunocytochemistry that subtypes RGMa and RGMb but not RGMc were strongly expressed in enteric ganglia cells of the fetal and adult gut. In contrast to the enteric nervous system, RGMa and RGMb expression in the intestinal epithelium started during postnatal gut development. Interestingly, both subtypes were predominantly expressed in the proliferative crypt compartment of the gut epithelium and in paneth cells of small intestine. The development-dependent expression in enteric ganglia and intestinal epithelial cells suggests that RGM may be involved in cell migration, differentiation, and apoptosis with similar cellular mechanisms as described in the central nervous system.

Repulsive guidance molecule/neogenin: a novel ligand-receptor system playing multiple roles in neural development

The repulsive guidance molecule (RGM) is a membrane-bound protein originally isolated as an axon guidance molecule in the visual system. Recently, the transmembrane protein, neogenin, has been identified as the RGM receptor. In vitro analysis with retinal explants showed that RGM repels temporal retinal axons and collapses their growth cones through neogenin-mediated signaling. However, RGM and neogenin are also broadly expressed at the early embryonic stage, suggesting that they do not only control the guidance of visual axons. Gene expression perturbation experiments in chick embryos showed that neogenin induces cell death, and its ligand, RGM, blocks the pro-apoptotic activity of neogenin. Thus, RGM/neogenin is a novel dependence ligand/receptor couple as well as an axon guidance molecular complex.