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1 Division of Pathophysiology, Research Center for Genomic Medicine, Saitama Medical University, 1397-1 Yamane, Hidaka-shi, Saitama 350-1241, Japan
2 Department of Oral and Maxillofacial Surgery, Faculty of Medicine, Saitama Medical University, 38 Moro Hongo, Moroyama-machi, Iruma-gun, Saitama 350-0495, Japan
| Abstract |
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| Introduction |
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The repulsive guidance molecule (RGM) family members RGMa, DRAGON/RGMb and RGMc/hemojuvelin/HEF2 are secretory glycosylphosphatidylinositol (GPI)-anchored membrane proteins that have been implicated in axonal guidance and neural tube formation during embryogenesis (Monnier et al. 2002; Niederkofler et al. 2004; Papanikolaou et al. 2004; Samad et al. 2004). They have significant sequence homology to one another (50–60% amino acid identity) (Papanikolaou et al. 2004; Babitt et al. 2005). All three proteins have a conserved von Willbrand factor (vWF) type D domain (Monnier et al. 2002), although its physiological function is still unknown. All of the RGM family members were found to act as co-receptors for the BMP subfamily. These proteins bind to BMP ligands as well as type I and type II BMP receptors, thereby enhancing BMP signaling (Babitt et al. 2005, 2006; Samad et al. 2005). A genetic mutation was found in the hemojuvelin gene in patients with juvenile hemochromatosis, a condition that is caused by the decreased expression of hepcidin, a key regulator of iron homeostasis in hepatocytes. Hemojuvelin enhanced the BMP-dependent up-regulation of hepcidin expression in hepatocytes (Papanikolaou et al. 2004; Babitt et al. 2006, 2007). Cells transfected with RGMa have been shown to utilize both BMPR-II and ActR-IIA for BMP-2 or BMP-4 signaling (Xia et al. 2007). However, to our knowledge, a physiological role for DRAGON, especially in bone metabolism, remains unestablished.
The ectopic bone-inducing activity of BMPs in muscle tissue is, at least in part, reflected in the in vitro culture of myoblasts, including mouse C2C12 cells. Treatment of these cells with BMPs inhibits the myogenic differentiation of myoblasts and converts their differentiation pathway into that of osteoblast lineage cells (Katagiri et al. 1994). Over-expression of a constitutively activated BMP type I receptor also induces similar differentiation conversions of myoblasts (Akiyama et al. 1997; Fujii et al. 1999; Aoki et al. 2001). We report here that DRAGON inhibits BMP signaling in C2C12 cells via a region that includes the secretory form of the vWF type D domain. DRAGON also inhibits the BMP signaling induced by a constitutively active form of Smad1. These findings indicate that a novel molecule(s) expressed on the cell membrane may mediate the signal transduction of DRAGON to suppress BMP signaling in C2C12 myoblasts.
| Results |
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First, we examined the expression levels of RGM mRNAs in C2C12 myoblasts. RGMa and DRAGON were expressed in both growing and differentiated myocytes, but RGMc was detected only in myogenin-expressing differentiated cells (Fig. 1a). Although all DRAGON mRNAs were detected in C2C12 cells in the presence of BMP-4, TGF-β1 or bFGF, the expression levels of RGMc were decreased by BMP-4 (Fig. 1b). Primary osteoblasts expressed RGMa and DRAGON, but did not express RGMc (Fig. 1b), suggesting that osteoblastic differentiation induced by BMPs suppresses RGMc expression in C2C12 cells. RT-PCR analysis using RNAs prepared from various murine tissues indicated that RGMa and DRAGON were highly expressed in the heart, brain, muscle and cartilage (Fig. 1c). RGMc was strongly expressed in heart, liver, muscle, bone and cartilage (Fig. 1c). In bone tissue, RGMc may be expressed in osteocytes and/or bone marrow cells rather than osteoblasts because it was not expressed in primary osteoblasts (Fig. 1b,c).
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Next, we examined the effect of RGMs on BMP-induced osteoblastic differentiation. A constitutively activated BMP receptor—ALK2(R206H), ALK2(Q207D) or BMPR-IA(Q233D)—was co-transfected with one of the RGMs into C2C12 cells, and as a result of that process we identified alkaline phosphatase (ALP) activity as a typical marker of osteoblastic differentiation. Unexpectedly, DRAGON suppressed ALP activity in all of the cultures that were examined (Fig. 2a–c). The suppression of BMP signaling by DRAGON was also confirmed in a luciferase assay using BMP-specific IdWT4F-luc in C2C12 cells (Fig. 2d,e). In contrast to BMP signaling, TGF-β signaling (as determined by CAGA-luc) was not suppressed by DRAGON in C2C12 cells (Fig. 2f). DRAGON showed a weaker inhibitory activity on IdWT4F-luc induced by ALK2(Q207D) in HepG2 cells compared with the inhibitory activity seen in C2C12 cells (Fig. 2d,g).
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We generated Sol- and Cyt-DRAGON mutants, in which a GPI-anchor domain and a signal peptide, respectively, were deleted (Fig. 3a). The cellular localization of these DRAGON proteins was confirmed by immunohistochemical analysis in the presence and absence of detergent. Wild-type DRAGON, but not Sol- or Cyt-DRAGON, was detected on the cell membrane, even in the absence of detergent; Sol- and Cyt-DRAGON were detected only in the cytoplasm in the presence of detergent (Fig. 3b). ALP activity inhibition induced by ALK2(Q207D) was observed in the wild-type and Sol-DRAGON constructs, which may be secreted by a signal peptide. To investigate whether the signal peptide itself suppressed ALP activity, we further generated an additional mutant of DRAGON (Swap-DRAGON) in which the signal peptide was replaced with that of another protein (Fig. 3a). Swap-DRAGON was anchored on the cell membrane and suppressed ALP activity in a manner similar to that of wild-type DRAGON, indicating that the signal peptide of DRAGON was not involved in the inhibition of BMP signaling (Fig. 3b,c).
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Because the secretory form of DRAGON inhibited BMP signaling without a GPI-anchor domain (Figs 3, 4), we hypothesized that a cell surface molecule(s) may be involved in the inhibitory activity of DRAGON. Neogenin has been identified as a cell surface receptor for RGM family members (Matsunaga et al. 2004; Rajagopalan et al. 2004). Because neogenin was only weakly expressed in C2C12 cells, we cloned its cDNA and transiently over-expressed it in C2C12 cells (Fig. 5a, unpublished observation). Co-transfection of neogenin showed no effect on the ability of DRAGON to inhibit the ALP activity induced by ALK2(Q207D) (Fig. 5b).
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Both wild-type and Sol-DRAGON decreased the expression levels of ALP and osterix mRNAs induced by ALK2(Q207D) in C2C12 cells, suggesting that DRAGON inhibited BMP signaling-induced osteoblastic differentiation (Fig. 6a). We examined the effect of DRAGON on phosphorylation levels of Myc-Smad1 and found that DRAGON did not significantly decrease phospho-Smad1 levels (Fig. 6b). Next, we asked whether a direct interaction between DRAGON and BMP receptor is involved in the inhibition process. Immunoprecipitation with immunoblotting analysis indicated that wild-type DRAGON, but not Sol-DRAGON, interacted with ALK2(Q207D) (Fig. 6c). Finally, we examined the effect of DRAGON on constitutively activated Smad1, which is able to induce downstream signal transduction cooperatively with Smad4 without phosphorylation by BMP receptors. Wild-type DRAGON, Sol-DRAGON and mutant 321 markedly inhibited the ALP activity and IdWT4F-luc activity induced by constitutively activated Smad1 and Smad4 (Fig. 6d,e). These results indicate that the inhibitory effect of DRAGON represses the transcriptional activity of Smad. Because both wild-type DRAGON and Sol-DRAGON suppressed ALK2 activity to a similar degree, we hypothesized that the interaction between DRAGON and BMP receptors may not be involved in this inhibition.
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| Discussion |
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Neogenin, a homologue of deleted colorectal cancer (DCC) and the netrin-1 receptor, was a potential candidate for the DRAGON binding protein on the cell membrane because it has been shown to act as a high-affinity receptor for RGMs (Vielmetter et al. 1994; Keino-Masu et al. 1996; Stein et al. 2001). The interactions between RGMa and neogenin were involved in the regulation of neuronal survival (Wilson & Key 2006). Interestingly, the interaction with neogenin was required for RGMc release from muscle cells, and soluble RGMc played a critical role in the negative regulation of hepatic hepcidin expression through BMP signaling in hepatocytes (Zhang et al. 2007). However, neogenin over-expression did not affect the DRAGON activity in C2C12 cells. We noticed that the degree of BMP signaling inhibition by DRAGON is dependent on cell type; for example, HepG2 and other types of cells showed weaker activities than C2C12 cells. It is possible that the amount of novel molecule(s) expressed on the cell membrane may regulate the cell type-specific activity of DRAGON. In addition, DRAGON was expressed in bone, cartilage and primary osteoblasts. However, the expression of DRAGON was unaltered during osteoblastic differentiation in C2C12 cells. Thus, it is possible that the suppressive function of DRAGON depended on the alteration of expression levels of a novel molecule(s). The identification of the DRAGON-binding molecule(s) on the cell membrane will shed light on the novel regulatory mechanisms associated with BMP signaling.
Recently, heterozygous mutations in the ACVR1 gene, which encodes the BMP type I receptor ALK2, were identified in familial and sporadic patients with FOP (Shore et al. 2006; Nakajima et al. 2007; Fukuda et al. 2009). These mutations cause an amino acid substitution in the intracellular ALK2 domain. We found that these ALK2 mutants were constitutively activated BMP receptors and that they cooperatively induced BMP signaling and osteoblastic differentiation with Smad1 and Smad5 (Fukuda et al. 2009). At the present time, no treatments are available to prevent heterotopic bone formation in FOP. We found in this study that DRAGON clearly inhibited the activity of ALK2(R206H), a typical mutation in FOP patients. Therefore, DRAGON may provide a novel concept for intracellular signal transduction inhibition in the design of novel drugs for the treatment of FOP.
In conclusion, DRAGON inhibited the intracellular signaling of activated BMP receptors by suppressing the transcriptional activity of Smad in C2C12 cells. DRAGON may bind to a novel molecule(s) expressed on the cell surface to transduce the inhibitory signals of BMPs.
| Experimental procedures |
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Mouse C2C12 myoblasts and human HepG2 hepatocytes were maintained as described (Katagiri et al. 1994; Babitt et al. 2006). Myogenic differentiation of C2C12 cells was induced by culturing the cells with differentiation medium (Dulbecco's modified Eagle's medium containing 2.5% fetal bovine serum) for 7 days. Cells were transfected with plasmids using Lipofectamine 2000 reagent (Invitrogen, Carlsbad, CA), according to the manufacturer's instructions. BMP and TGF-β signaling were monitored using IdWT4F-luc and CAGA-luc reporter plasmids, respectively (Dennler et al. 1998; Katagiri et al. 2002). ALP activity was measured as described (Kodaira et al. 2006). In brief, cells were incubated with a substrate solution (0.1 M diethanolamine, 1 mM MgCl2 and 10 mg/mL of p-nitrophenylphosphate). After appropriate incubation, reactions were terminated by adding 3 M NaOH; absorbance was measured at 405 nm.
Reverse transcription-PCR analysis and plasmid construction
Total RNAs were prepared using TRIzol Reagent (Invitrogen) and then reverse transcribed by SuperScript III reverse transcriptase (Invitrogen) in accordance with the manufacturer's instructions. PCR reactions were performed using Go-Taq (Promega, Madison, WI). The primer sets used were previously described (Zhao et al. 2006) as follows: 5'-CACGCTGGCTGCCTACTATGCTTTG-3' (RGMa-U), 5'-TGGGCTGCATCCTGAGGCTGACTTG-3' (RGMa-L), 5'-ACTCATAGACAACAATTACCTTTCG-3' (DRAGON-U), 5'-GGGAAGAGTCTCTCACACACACATG (DRAGON-L), 5'-CTCCGGTGACCCCAACTTTACTGTG-3' (RGMc-U) and 5'-CTGTCCCCGCTGTTTCCTTTGGCAC (RGMc-L). Each cDNA of the RGM family was cloned into a pcDEF3 expression vector and confirmed by DNA sequencing. A series of mutant DRAGON constructs was generated using a standard PCR technique.
Immunoblotting and immunohistochemistry
Cells were lysed in TNE buffer [10 mM Tris-HCl (pH 7.5), 0.15 M NaCl, 1 mM EDTA and 1% Nonidet P-40] and subjected to immunoprecipitation and immunoblotting as described previously (Fukuda et al. 2009). The following antibodies were used: anti-phosphorylated Smad1/5/8 antibody (Cell Signaling, Beverly, MA), anti-V5 antibody (Invitrogen), anti-FLAG antibody (Sigma, St Louis, MO), anti-Myc antibody and anti-actin antibody (SantaCruz, Santa Cruz, CA). The cells transfected with DRAGON mutants were fixed with formalin and stained with an anti-FLAG antibody and DAPI.
| Acknowledgements |
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| Footnotes |
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* Correspondence: katagiri{at}saitama-med.ac.jp
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Accepted: 3 March 2009
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