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

Mendelian randomization analysis to identify potential drug targets for osteoarthritis

BACKGROUND

Osteoarthritis (OA) is a prevalent chronic joint disease for which there is a lack of effective treatments. In this study, we used Mendelian randomization analysis to identify circulating proteins that are causally associated with OA-related traits, providing important insights into potential drug targets for OA.

METHOD

Causal associations between 1553 circulating proteins and five OA-related traits were assessed in large-scale two-sample MR analyses using Wald ratio or inverse variance weighting, and the results were corrected for Bonferroni. In addition, sensitivity analyses were performed to validate the reliability of the MR results, including reverse MR analysis and Steiger filtering to ensure the causal direction between circulating proteins and OA; Bayesian co-localization and phenotypic scanning were used to eliminate confounding effects and horizontal pleiotropy. External validation was performed to exclude incidental findings using novel plasma protein quantitative trait loci. Finally, the online analysis tool Enrichr was utilized to screen drugs and molecular docking was performed to predict binding modes and energies between proteins and drugs to identify the most stable and likely binding modes and drugs.

RESULT

Four proteins were ultimately found to be reliably and causally associated with three OA-related features: DNAJB12 and USP8 were associated with knee OA, IL12B with spinal OA, and RGMB with thumb OA. The ORs for the above proteins were 1.51 (95% CI, 1.26-1.81), 1.72 (95% CI, 1.42-2.08), 0.87 (95% CI, 0.81-0.92), and 0.59 (95% CI, 0.47-0.75), respectively. Drug-predicting small molecules (doxazosin, XEN 103, and montelukast) that simultaneously target three proteins, DNAJB12, USP8, and IL12B, docked well.

CONCLUSION

Based on our comprehensive analysis, we can draw the conclusion that there is a causal relationship between the genetic levels of DNAJB12, USP8, IL12B, and RGMB and the risk of respective OA.They may be potential options for OA screening and prevention in clinical practice. They can also serve as candidate molecules for future mechanism exploration and drug target selection.

Repulsive guidance molecules b (RGMb): molecular mechanism, function and role in diseases

Repulsive guidance molecule b (RGMb), a glycosylphosphatidylinositol-anchored member of the RGM family, is initially identified as a co-receptor of bone morphogenetic protein (BMP) in the nervous system. The expression of RGMb is transcriptionally regulated by dorsal root ganglion 11 (DRG11), which is a transcription factor expressed in embryonic DRG and dorsal horn neurons and plays an important role in the development of sensory circuits. RGMb is involved in important physiological processes such as embryonic development, immune response, intercellular adhesion and tumorigenesis. Furthermore, RGMb is mainly involved in the regulation of RGMb-neogenin-Rho and BMP signalling pathways. The recent discovery of programmed death-ligand 2 (PD-L2)-RGMb binding reveals that the cell signalling network and functional regulation centred on RGMb are extremely complex. The latest report suggests that down-regulation of the PD-L2-RGMb pathway in the gut microbiota promotes an anti-tumour immune response, which defines a potentially effective immune strategy. However, the biological function of RGMb in a variety of human diseases has not been fully determined, and will remain an active research field. This article reviews the properties and functions of RGMb, focusing on its role under various physiological and pathological conditions.

Length biases in single-cell RNA sequencing of pre-mRNA

Single-cell RNA sequencing data can be modeled using Markov chains to yield genome-wide insights into transcriptional physics. However, quantitative inference with such data requires careful assessment of noise sources. We find that long pre-mRNA transcripts are over-represented in sequencing data. To explain this trend, we propose a length-based model of capture bias, which may produce false-positive observations. We solve this model and use it to find concordant parameter trends as well as systematic, mechanistically interpretable technical and biological differences in paired data sets.

Human sensory neurons modulate melanocytes through secretion of RGMB

Melanocytes are surrounded by diverse cells, including sensory neurons in our skin, but their interaction and functional importance have been poorly investigated. In this study, we find that melanocytes and nociceptive neurons contact more in human skin color patch tissue than control. Co-culture with human iPSC-derived sensory neurons significantly induces morphogenesis and pigmentation of human melanocytes. To reveal melanocyte-stimulating factors secreted from neurons, we perform proteomic analyses and identify RGMB in the sensory neuron-conditioned medium. RGMB protein induces morphogenesis and melanin production of melanocytes, demonstrating that RGMB is a melanocyte-stimulating factor released from sensory neurons. Transcriptome analysis suggests that the melanosome transport machinery can be controlled by RGMB, leading us to identify the vesicle production response of melanocytes upon RGMB treatment. This study discovers a role of sensory neurons in modulating multiple aspects of human melanocytes through secretion of a key factor: RGMB.

Emerging concepts in PD-1 checkpoint biology

The PD-1 pathway is a cornerstone in immune regulation. While the PD-1 pathway has received considerable attention for its role in contributing to the maintenance of T cell exhaustion in chronic infection and cancer, the PD-1 pathway plays diverse roles in regulating host immunity beyond T cell exhaustion. Here, we discuss emerging concepts in the PD-1 pathway, including (1) the impact of PD-1 inhibitors on diverse T cell differentiation states including effector and memory T cell development during acute infection, as well as T cell exhaustion during chronic infection and cancer, (2) the role of PD-1 in regulating Treg cells, NK cells, and ILCs, and (3) the functions of PD-L1/B7-1 and PD-L2/RGMb/neogenin interactions. We then discuss the emerging use of neoadjuvant PD-1 blockade in the treatment of early-stage cancers and how the timing of PD-1 blockade may improve clinical outcomes. The diverse binding partners of PD-1 and its associated ligands, broad expression patterns of the receptors and ligands, differential impact of PD-1 modulation on cells depending on location and state of differentiation, and timing of PD-1 blockade add additional layers of complexity to the PD-1 pathway, and are important considerations for improving the efficacy and safety of PD-1 pathway therapeutics.

Emerging evidence for cell-autonomous axon guidance

Current models of axon guidance within the central nervous system (CNS) involve the presentation of environmental cues to navigating growth cones. The surrounding and target tissues present a variety of ligands that either restrict or promote growth, thus providing pathfinding instructions to developing axons. Recent findings show that RGMb, a GPI anchored extracellular protein present on retinal ganglion cells, down-regulates Wnt3a signaling by lowering LRP5 levels at the membrane surface. When RGMb is phosphorylated by the extracellular tyrosine kinase VLK, phosphorylated RGMb (p-RGMb) is internalized and carries LRP5 towards intracellular compartments. In the eye, a dorsal-high ventral-low gradient of VLK generates a dorsal-low ventral-high gradient of LRP5 that modulates Wnt3a signaling. These molecules, which are all expressed by individual RGCs, generate Wnt-signal gradients along the dorso-ventral axis of the retina, resulting in differential axon growth which in turn regulates proper retino-tectal/collicular map formation. This pathway represents a regulatory mechanism whereby extracellular phosphorylation generates what may be the first example of a unique self-guiding mechanism that affects neuronal-target connections independent of paracrine signals from the surrounding target tissue.

miR-93-5p enhances migration and invasion by targeting RGMB in squamous cell carcinoma of the head and neck

Invasion and metastasis represent the primary causes of therapeutic failure in patients diagnosed with squamous cell carcinoma of the head and neck (SCCHN). Therefore, disease prediction and inhibition of invasion and metastasis are critical for enhancing the survival of patients with SCCHN. Our previous study revealed that increased expression of miR-93-5p is associated with poor prognosis in SCCHN; however, the mechanism underlying the oncogenic functions of miR-93-5p in SCCHN migration and invasion remains unclear. Using qPCR analyses, transwell assays, and scratch tests, we demonstrated that expression of ectopic miR-93-5p induced the migration and invasion of SCCHN, and this was accompanied by corresponding alterations in biomarkers and transcription factors specific for epithelial-mesenchymal transition (EMT). Luciferase reporter assays were used to demonstrate that miR-93-5p directly targeted the 3' UTR of RGMB, and we further found that the tumor-promoting functions of miR-93-5p were partly mediated by targeting RGMB, whose downregulation also promoted the migration and invasion of SCCHN. Overall, our results indicate that miR-93-5p acts as an oncogene in the regulation of migration and invasion by suppressing RGMB in SCCHN. These findings provide novel evidence that miR-93-5p may serve as a valuable predictive biomarker and potential intervention target in patients with SCCHN.

Dorsal Root Ganglia Homeobox downregulation in primary sensory neurons contributes to neuropathic pain in rats

Transcriptional changes in primary sensory neurons are involved in initiation and maintenance of neuropathic pain. However, the transcription factors in primary sensory neurons responsible for neuropathic pain are not fully understood. Dorsal Root Ganglia Homeobox (DRGX) is a paired-like homeodomain transcription factor necessary for the development of nociceptive primary sensory neurons during the early postnatal period. However, roles for DRGX after development are largely unknown. Here, we report that DRGX downregulation in primary sensory neurons as a result of post-developmental nerve injury contributes to neuropathic pain in rats. DRGX expression was decreased in nuclei of small and medium primary sensory neurons after spinal nerve ligation. DRGX downregulation by transduction of a short hairpin RNA with an adeno-associated viral vector induced mechanical allodynia and thermal hyperalgesia. In contrast, DRGX overexpression in primary sensory neurons suppressed neuropathic pain. DRGX regulated matrix metalloproteinase-9 (MMP-9) and prostaglandin E receptor 2 mRNA expression in the DRG. MMP-9 inhibitor attenuated DRGX downregulation-induced pain. These results suggest that DRGX downregulation after development contributes to neuropathic pain through transcriptional modulation of pain-related genes in primary sensory neurons.

Extracellular phosphorylation drives the formation of neuronal circuitry

Until recently, the existence of extracellular kinase activity was questioned. Many proteins of the central nervous system are targeted, but it remains unknown whether, or how, extracellular phosphorylation influences brain development. Here we show that the tyrosine kinase vertebrate lonesome kinase (VLK), which is secreted by projecting retinal ganglion cells, phosphorylates the extracellular protein repulsive guidance molecule b (RGMb) in a dorsal-ventral descending gradient. Silencing of VLK or RGMb causes aberrant axonal branching and severe axon misguidance in the chick optic tectum. Mice harboring RGMb with a point mutation in the phosphorylation site also display aberrant axonal pathfinding. Mechanistic analyses show that VLK-mediated RGMb phosphorylation modulates Wnt3a activity by regulating LRP5 protein gradients. Thus, the secretion of VLK by projecting neurons provides crucial signals for the accurate formation of nervous system circuitry. The dramatic effect of VLK on RGMb and Wnt3a signaling implies that extracellular phosphorylation likely has broad and profound effects on brain development, function and disease.

Blockade of RGMb inhibits allergen-induced airways disease

BACKGROUND

Allergic asthma causes morbidity in many subjects, and novel precision-directed treatments would be valuable.

OBJECTIVE

We sought to examine the role of a novel innate molecule, repulsive guidance molecule b (RGMb), in murine models of allergic asthma.

METHODS

In models of allergic asthma using ovalbumin or cockroach allergen, mice were treated with anti-RGMb or control mAb and examined for airway inflammation and airway hyperreactivity (AHR), a cardinal feature of asthma. The mechanisms by which RGMb causes airways disease were also examined.

RESULTS

We found that blockade of RGMb by treatment with anti-RGMb mAb effectively blocked the development of airway inflammation and AHR. Importantly, blockade of RGMb completely blocked the development of airway inflammation and AHR, even if treatment occurred only during the challenge (effector) phase. IL-25 played an important role in these models of asthma because IL-25 receptor-deficient mice did not develop disease after sensitization and challenge with allergen. RGMb was expressed primarily by innate cells in the lungs, including bronchial epithelial cells (known producers of IL-25), activated eosinophils, and interstitial macrophages, which in the inflamed lung expressed the IL-25 receptor and produced IL-5 and IL-13. We also found that neogenin, the canonical receptor for RGMb, was expressed by interstitial macrophages and bronchial epithelial cells in the inflamed lung, suggesting that an innate RGMb-neogenin axis might modulate allergic asthma.

CONCLUSIONS

These results demonstrate an important role for a novel innate pathway in regulating type 2 inflammation in patients with allergic asthma involving RGMb and RGMb-expressing cells, such as interstitial macrophages and bronchial epithelial cells. Moreover, targeting this previously unappreciated innate pathway might provide an important treatment option for allergic asthma.

Variation in the repulsive guidance molecule family in human populations

Repulsive guidance molecules, RGMA, RGMB, and RGMC, are related proteins discovered independently through different experimental paradigms. They are encoded by single copy genes in mammalian and other vertebrate genomes, and are ~50% identical in amino acid sequence. The importance of RGM actions in human physiology has not been realized, as most research has focused on non-human models, although mutations in RGMC are the cause of the severe iron storage disorder, juvenile hemochromatosis. Here I show that repositories of human genomic and population genetic data can be used as starting points for discovery and for developing new testable hypotheses about each of these paralogs in human biology and disease susceptibility. Information was extracted, aggregated, and analyzed from the Ensembl and UCSC Genome Browsers, the Exome Aggregation Consortium, the Genotype-Tissue Expression project portal, the cBio portal for Cancer Genomics, and the National Cancer Institute Genomic Data Commons data site. Results identify extensive variation in gene expression patterns, substantial alternative RNA splicing, and possible missense alterations and other modifications in the coding regions of each of the three genes, with many putative mutations being detected in individuals with different types of cancers. Moreover, selected amino acid substitutions are highly prevalent in the world population, with minor allele frequencies of up to 37% for RGMA and up to 8% for RGMB. These results indicate that protein sequence variation is common in the human RGM family, and raises the possibility that individual variants will have a significant population impact on human physiology and/or disease predisposition.

RGMb protects against acute kidney injury by inhibiting tubular cell necroptosis via an MLKL-dependent mechanism

Tubular cell necrosis is a key histological feature of acute kidney injury (AKI). Necroptosis is a type of programed necrosis, which is executed by mixed lineage kinase domain-like protein (MLKL) upon its binding to the plasma membrane. Emerging evidence indicates that necroptosis plays a critical role in the development of AKI. However, it is unclear whether renal tubular cells undergo necroptosis in vivo and how the necroptotic pathway is regulated during AKI. Repulsive guidance molecule (RGM)-b is a member of the RGM family. Our previous study demonstrated that RGMb is highly expressed in kidney tubular epithelial cells, but its biological role in the kidney has not been well characterized. In the present study, we found that RGMb reduced membrane-associated MLKL levels and inhibited necroptosis in cultured cells. During ischemia/reperfusion injury (IRI) or oxalate nephropathy, MLKL was induced to express on the apical membrane of proximal tubular (PT) cells. Specific knockout of Rgmb in tubular cells (Rgmb cKO) increased MLKL expression at the apical membrane of PT cells and induced more tubular cell death and more severe renal dysfunction compared with wild-type mice. Treatment with the necroptosis inhibitor Necrostatin-1 or GSK'963 reduced MLKL expression on the apical membrane of PT cells and ameliorated renal function impairment after IRI in both wild-type and Rgmb cKO mice. Taken together, our results suggest that proximal tubular cell necroptosis plays an important role in AKI, and that RGMb protects against AKI by inhibiting MLKL membrane association and necroptosis in proximal tubular cells.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

Repulsive guidance molecule B inhibits metastasis and is associated with decreased mortality in non-small cell lung cancer

Repulsive guidance molecules (RGMs) are co-receptors of bone morphogenetic proteins (BMPs) and programmed death ligand 2 (PD-L2), and might be involved in lung and other cancers. We evaluated repulsive guidance molecule B (RGMB) expression in 165 non-small cell lung cancer (NSCLC) tumors and 22 normal lung tissue samples, and validated the results in an independent series of 131 samples. RGMB was downregulated in NSCLC (P ≤ 0.001), possibly through promoter hypermethylation. Reduced RGMB expression was observed in advanced-stage tumors (P = 0.017) and in tumors with vascular invasion (P < 0.01), and was significantly associated with poor overall survival (39 vs. 62 months, P < 0.001) and with disease-associated patient mortality (P = 0.015). RGMB knockdown promoted cell adhesion, invasion and migration, in both NSCLC cell lines and an in vivo mouse model, which enhanced metastatic potential. Conversely, RGMB overexpression and secretion suppressed cancer progression. The tumor-suppressing effect of RGMB was exerted through inhibition of the Smad1/5/8 pathway. Our results demonstrate that RGMB is an important inhibitor of NSCLC metastasis and that low RGMB expression is a novel predictor or a poor prognosis.

Repulsive Guidance Molecule a (RGMa) Induces Neuropathological and Behavioral Changes That Closely Resemble Parkinson's Disease

Repulsive guidance molecule member a (RGMa) is a membrane-associated or released guidance molecule that is involved in axon guidance, cell patterning, and cell survival. In our previous work, we showed that RGMa is significantly upregulated in the substantia nigra of patients with Parkinson's disease. Here we demonstrate the expression of RGMa in midbrain human dopaminergic (DA) neurons. To investigate whether RGMa might model aspects of the neuropathology of Parkinson's disease in mouse, we targeted RGMa to adult midbrain dopaminergic neurons using adeno-associated viral vectors. Overexpression of RGMa resulted in a progressive movement disorder, including motor coordination and imbalance, which is typical for a loss of DA release in the striatum. In line with this, RGMa induced selective degeneration of dopaminergic neurons in the substantia nigra (SN) and affected the integrity of the nigrostriatal system. The degeneration of dopaminergic neurons was accompanied by a strong microglia and astrocyte activation. The behavioral, molecular, and anatomical changes induced by RGMa in mice are remarkably similar to the clinical and neuropathological hallmarks of Parkinson's disease. Our data indicate that dysregulation of RGMa plays an important role in the pathology of Parkinson's disease, and antibody-mediated functional interference with RGMa may be a disease modifying treatment option.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is a neurodegenerative disease characterized by severe motor dysfunction due to progressive degeneration of mesencephalic dopaminergic (DA) neurons in the substantia nigra. To date, there is no regenerative treatment available. We previously showed that repulsive guidance molecule member a (RGMa) is upregulated in the substantia nigra of PD patients. Adeno-associated virus-mediated targeting of RGMa to mouse DA neurons showed that overexpression of this repulsive axon guidance and cell patterning cue models the behavioral and neuropathological characteristics of PD in a remarkable way. These findings have implications for therapy development as interfering with the function of this specific axon guidance cue may be beneficial to the survival of DA neurons.

B7-DC (PD-L2) costimulation of CD4+ T-helper 1 response via RGMb

The role of B7-DC in T-cell responses remains controversial because both coinhibitory and costimulatory functions have been reported in various experimental systems in vitro and in vivo. In addition to interacting with the coinhibitory receptor PD-1, B7-DC has also been shown to bind repulsive guidance molecule b (RGMb). The functional consequences of the B7-DC/RGMb interaction, however, remain unclear. More than a decade ago, we reported that replacement of a murine B7-DC mutant lysine with serine (K113S) at positive 113 resulted in a loss of binding capacity to PD-1. Nevertheless, K113S remained costimulatory for T cells in vitro, implicating a dual functionality for B7-DC in T-cell responses. Here we show that recombinant K113S protein interacts with RGMb with a similar affinity to wild-type B7-DC. More importantly, K113S costimulates CD4⁺ T-cell responses via RGMb and promotes Th1 polarization. RGMb is expressed on the surface of naive mouse T cells, macrophages, neutrophils and dendritic cells. Finally, K113S/RGMb costimulation suppresses Th2-mediated asthma and ameliorates small airway inflammation and lung pathology in an experimental mouse model. Our findings indicate that RGMb is a costimulatory receptor for B7-DC. These findings from the K113S variant provide not only a possible explanation for the B7-DC-triggered contradictory effects on T-cell responses, but also a novel approach to investigate the B7-DC/PD-1/RGMb axis. Recombinant K113S or its derivatives could potentially be developed as an agonist for RGMb to costimulate the Th1 response without triggering PD-1-mediated T-cell inhibition.

A role for prolyl isomerase PIN1 in the phosphorylation-dependent modulation of PRRXL1 function

Prrxl1 encodes for a paired-like homeodomain transcription factor essential for the correct establishment of the dorsal root ganglion - spinal cord nociceptive circuitry during development. Prrxl1-null mice display gross anatomical disruption of this circuitry, which translates to a markedly diminished sensitivity to noxious stimuli. Here, by the use of an immunoprecipitation and mass spectrometry approach, we identify five highly conserved phosphorylation sites (T110, S119, S231, S233 and S251) in PRRXL1 primary structure. Four are phospho-S/T-P sites, which suggest a role for the prolyl isomerase PIN1 in regulating PRRXL1. Accordingly, PRRXL1 physically interacts with PIN1 and displays diminished transcriptional activity in a Pin1-null cell line. Additionally, these S/T-P sites seem to be important for PRRXL1 conformation, and their point mutation to alanine or aspartate down-regulates PRRXL1 transcriptional activity. Altogether, our findings provide evidence for a putative novel role of PIN1 in the development of the nociceptive system and indicate phosphorylation-mediated conformational changes as a mechanism for regulating the PRRXL1 role in the process.

Dragon (repulsive guidance molecule b, RGMb) is a novel gene that promotes colorectal cancer growth

Colorectal cancer (CRC) is one of the most commonly diagnosed cancers and a major cause of cancer death. However, the molecular mechanisms underlying CRC initiation, growth and metastasis are poorly understood. Dragon (RGMb), a member of the repulsive guidance molecule (RGM) family, has been recently identified as a co-receptor for bone morphogenetic protein (BMP) signaling, but the role of Dragon in CRC development is undefined. Here, we show that Dragon expression was increased in colon cancer tissues compared to control tissues in CAC mouse model and in human patients. Dragon promoted proliferation of CT26.WT and CMT93 colon cancer cells and accelerated tumor growth in the xenograft mouse model. Dragon's action on colon cancer development was mediated via the BMP4-Smad1/5/8 and Erk1/2 pathways. Therefore, our results have revealed that Dragon is a novel gene that promotes CRC growth through the BMP pathway. Dragon may be exploited as a potential therapeutic target for CRC treatment.

Long Noncoding RNA RGMB-AS1 Indicates a Poor Prognosis and Modulates Cell Proliferation, Migration and Invasion in Lung Adenocarcinoma

Lung cancer is the most common cause of cancer-related mortality worldwide. It is a complex disease involving multiple genetic and epigenetic alterations. The development of transcriptomics revealed the important role of long non-coding RNAs (lncRNAs) in lung cancer occurrence and development. Here, microarray analysis of lung adenocarcinoma tissues showed the abnormal expression of lncRNA RGMB-AS1. However, the role of lncRNA RGMB-AS1 in lung adenocarcinoma remains largely unknown. We showed that upregulation of lncRNA RGMB-AS1 was significantly correlated with differentiation, TNM stage, and lymph node metastasis. In lung adenocarcinoma cells, downregulation of lncRNA RGMB-AS1 inhibited cell proliferation, migration, invasion, and caused cell cycle arrest at the G1/G0 phase. In vivo experiments showed that lncRNA RGMB-AS1 downregulation significantly suppressed the growth of lung adenocarcinoma. The expression of lncRNA RGMB-AS1 was inversely correlated with that of repulsive guidance molecule b (RGMB) in lung adenocarcinoma tissues, and UCSC analysis and fluorescence detection assay indicated that lncRNA RGMB-AS1 may be involved in the development of human lung adenocarcinoma by regulating RGMB expression though exon2 of RGMB. In summary, our findings indicate that lncRNA RGMB-AS1 may play an important role in lung adenocarcinoma and may serve as a potential therapeutic target.

Repulsive Guidance Molecule b (RGMb) Is Dispensable for Normal Gonadal Function in Mice

Bone morphogenetic protein (BMP) signaling plays an important role in spermatogenesis and follicle development. Our previous studies have shown that repulsive guidance molecule b (RGMb, also known as Dragon) is a coreceptor that enhances BMP2 and BMP4 signaling in several cell types and that RGMb is expressed in spermatocytes and spermatids in the testis and in oocytes of the secondary follicles in the ovary. Here, we demonstrated that specific deletion of Rgmb in germ cells in the testis and ovary did not alter Smad1/5/8 phosphorylation, gonadal structures, and fertility. In addition, ovaries from postnatal global Rgmb knockout mice showed similar structures to the wild-type ovaries. Our results suggest that RGMb is not essential for normal gonadal function.

Repulsive Guidance Molecules a, b and c Are Skeletal Muscle Proteins, and Repulsive Guidance Molecule a Promotes Cellular Hypertrophy and Is Necessary for Myotube Fusion

Repulsive guidance molecules (RGMs) compose a family of glycosylphosphatidylinositol (GPI)-anchored axon guidance molecules and perform several functions during neural development. New evidence has suggested possible new roles for these axon guidance molecules during skeletal muscle development, which has not been investigated thus far. In the present study, we show that RGMa, RGMb and RGMc are all induced during skeletal muscle differentiation in vitro. Immunolocalization performed on adult skeletal muscle cells revealed that RGMa, RGMb and RGMc are sarcolemmal proteins. Additionally, RGMa was found to be a sarcoplasmic protein with a surprisingly striated pattern. RGMa colocalization with known sarcoplasmic proteins suggested that this axon guidance molecule is a skeletal muscle sarcoplasmic protein. Western blot analysis revealed two RGMa fragments of 60 and 33 kDa, respectively, in adult skeletal muscle samples. RGMa phenotypes in skeletal muscle cells (C2C12 and primary myoblasts) were also investigated. RGMa overexpression produced hypertrophic cells, whereas RGMa knockdown resulted in the opposite phenotype. RGMa knockdown also blocked myotube formation in both skeletal muscle cell types. Our results are the first to show an axon guidance molecule as a skeletal muscle sarcoplasmic protein and to include RGMa in a system that regulates skeletal muscle cell size and differentiation.

Study on expression of lncRNA RGMB-AS1 and repulsive guidance molecule b in non-small cell lung cancer

Background

The relationships between lncRNAs and tumors have currently become one of the focuses on cancer studies. However, there are a few studies about lncRNAs in non-small cell lung cancer (NSCLC) at present.

Methods

Microarray analysis was designed to study the expression patterns of lncRNAs in three pairs of NSCLC tissues. The expression of lncRNA RGMB-AS1 and repulsive guidance molecule b (RGMB) were detected in 72 paired NSCLC tissues and adjacent normal tissues by qRT-PCR assay. The relations of lncRNA RGMB-AS1 and RGMB expression with clinicopathological factors of NSCLC patients were explored. A549 and SPC-A-1 cells were transfected with siRNA of lncRNA RGMB-AS1 and negative control. RGMB expression level was detected by qRT-PCR assay and western blot analysis.

Results

The results of microarray found that 571 lncRNAs were differentially expressed in NSCLC tissues (Fold change cut-off: 5.0, P < 0.05), including 304 upregulated and 267 downregulated lncRNAs. The results of qRT-PCR showed that lncRNA RGMB-AS1 expression was significantly higher in NSCLC tissues than in adjacent normal tissues (P < 0.05), while RGMB mRNA showed an opposite trend (P < 0.05). Correlation analysis indicated that the expression of lncRNA RGMB-AS1and RGMB mRNA were inversely correlated (R² = 0.590, P < 0.05). While lncRNA RGMB-AS1 and RGMB expression levels in NSCLC tissues were associated with the occurrence of differentiation status, lymph node metastases and TNM stage (P < 0.05). Transfection with siRNA of lncRNA RGMB-AS1, subsequent results showed that RGMB mRNA and protein expression were upregulated (P < 0.05) in A549 and SPC-A-1 cells compared to the control groups.

Conclusion

We identified lncRNA RGMB-AS1 was upregulated and RGMB was downregulated in NSCLC patients. Both were related to differentiation status, lymph node metastases and TNM stage. Studies also indicated that lncRNA RGMB-AS1and RGMB were inversely correlated.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/7911587521528276

Controlled expression of Drosophila homeobox loci using the Hostile takeover system

BACKGROUND

Hostile takeover (Hto) is a Drosophila protein trapping system that allows the investigator to both induce a gene and tag its product. The Hto transposon carries a GAL4-regulated promoter expressing an exon encoding a FLAG-mCherry tag. Upon expression, the Hto exon can splice to a downstream genomic exon, generating a fusion transcript and tagged protein.

RESULTS

Using rough-eye phenotypic screens, Hto inserts were recovered at eight homeobox or Pax loci: cut, Drgx/CG34340, Pox neuro, araucan, shaven/D-Pax2, Zn finger homeodomain 2, Sex combs reduced (Scr), and the abdominal-A region. The collection yields diverse misexpression phenotypes. Ectopic Drgx was found to alter the cytoskeleton and cell adhesion in ovary follicle cells. Hto expression of cut, araucan, or shaven gives phenotypes similar to those of the corresponding UAS-cDNA constructs. The cut and Pox neuro phenotypes are suppressed by the corresponding RNAi constructs. The Scr and abdominal-A inserts do not make fusion proteins, but may act by chromatin- or RNA-based mechanisms.

CONCLUSIONS

Hto can effectively express tagged homeodomain proteins from their endogenous loci; the Minos vector allows inserts to be obtained even in transposon cold-spots. Hto screens may recover homeobox genes at high rates because they are particularly sensitive to misexpression.

Expression of repulsive guidance molecule b (RGMb) in the uterus and ovary during the estrous cycle in rats

Repulsive guidance molecule b (RGMb; a.k.a. Dragon), initially identified in the embryonic dorsal root ganglion, is the first member of the RGM family shown to enhance bone morphogenetic protein (BMP) signaling by acting as a BMP co-receptor. BMP signaling has been demonstrated to play an important role in the reproductive organs. Our previous study found that RGMb was expressed in the reproductive axis, but whether RGMb expression in reproductive organs changes across the estrous cycle remains unknown. Here, we show in the rat that RGMb mRNA expression in the uterus was significantly higher during metesterus and diestrus than during proestrus and estrus. Western blotting indicated that RGMb protein was significantly lower during estrus compared with the other three stages. Immunohistochemistry revealed that RGMb protein was mainly localized to the uterine luminal and glandular epithelial cells of the endometrium. RGMb mRNA and protein in the ovary remained unchanged during the estrous cycle. RGMb protein was expressed in the oocytes of all follicles. Weak staining for RGMb protein was also found in corpora lutea. RGMb was not detected in granulosa cells and stromal cells. Taken together, RGMb expression in the uterus and ovary across the estrus cycle demonstrate that RGMb may be involved in the regulation of uterine function, follicular development as well as luteal activity.

Paired related homeobox protein-like 1 (Prrxl1) controls its own expression by a transcriptional autorepression mechanism

The homeodomain factor paired related homeobox protein-like 1 (Prrxl1) is crucial for proper assembly of dorsal root ganglia (DRG)-dorsal spinal cord (SC) pain-sensing circuit. By performing chromatin immunoprecipitation with either embryonic DRG or dorsal SC, we identified two evolutionarily conserved regions (i.e. proximal promoter and intron 4) of Prrxl1 locus that show tissue-specific binding of Prrxl1. Transcriptional assays confirm the identified regions can mediate repression by Prrxl1, while gain-of-function studies in Prrxl1 expressing ND7/23 cells indicate Prrxl1 can down-regulate its own expression. Altogether, our results suggest that Prrxl1 uses distinct regulatory regions to repress its own expression in DRG and dorsal SC.

Tumour angiogenesis and repulsive guidance molecule b: a role in HGF- and BMP-7-mediated angiogenesis

Hepatocyte growth factor (HGF) is a key growth factor linked to promoting cancer progression and angiogenesis. The present study identifies repulsive guidance molecule b (RGMb), a bone morphogenetic protein (BMP) co-receptor, as a gene whose expression is regulated by HGF and explores the potential of RGMb to contribute to the process of angiogenesis. Microarray analysis was used to identify HGF responsive genes in HECV endothelial cells, identifying RGMb. RGMb was subsequently targeted using a ribozyme transgene system and its role in angiogenesis assessed using in vitro and in vivo assays. The importance of RGMb in pro-angiogenic responses to HGF and BMP-7 was also assessed. Microarray analysis identified RGMb as a gene upregulated as a result of HGF treatment. Knockdown of RGMb, in HECV cells, had minimal effects on tubule formation, brought about a general, although non-significant increase in cell growth and enhanced cell migration. Similarly, no significant effect of RGMb knockdown was found in vivo using a co-inoculation angiogenesis model. Knockdown of RGMb was, however, found to reduce the responsiveness of HECV cells to HGF treatment and particularly to BMP-7 treatment in regard to the enhanced migratory and tubule formation brought about by these treatments in vitro. Our results indicate that RGMb expression can be influenced by HGF treatment. Whilst this molecule appears to have minimal impact on angiogenic traits individually, it demonstrates an involvement in propagating pro-angiogenic effects of HGF and particularly BMP-7 and thus, may play a role in regulating angiogenic responses to HGF and BMP-7.

Hemojuvelin and bone morphogenetic protein (BMP) signaling in iron homeostasis

Mutations in hemojuvelin (HJV) are the most common cause of the juvenile-onset form of the iron overload disorder hereditary hemochromatosis. The discovery that HJV functions as a co-receptor for the bone morphogenetic protein (BMP) family of signaling molecules helped to identify this signaling pathway as a central regulator of the key iron hormone hepcidin in the control of systemic iron homeostasis. This review highlights recent work uncovering the mechanism of action of HJV and the BMP-SMAD signaling pathway in regulating hepcidin expression in the liver, as well as additional studies investigating possible extra-hepatic functions of HJV. This review also explores the interaction between HJV, the BMP-SMAD signaling pathway and other regulators of hepcidin expression in systemic iron balance.

Ser¹¹⁹ phosphorylation modulates the activity and conformation of PRRXL1, a homeodomain transcription factor

PRRXL1 [paired related homeobox-like 1; also known as DRG11 (dorsal root ganglia 11)] is a paired-like homeodomain transcription factor expressed in DRG and dSC (dorsal spinal cord) nociceptive neurons. PRRXL1 is crucial for the establishment and maintenance of nociceptive circuitry, as Prrxl1(-/-) mice present neuronal loss, reduced pain sensitivity and failure to thrive. In the present study, we show that PRRXL1 is highly phosphorylated in vivo, and that its multiple band pattern on electrophoretic analysis is the result of different phosphorylation states. PRRXL1 phosphorylation appears to be differentially regulated along the dSC and DRG development and it is mapped to two functional domains. One region comprises amino acids 107-143, whereas the other one encompasses amino acids 227-263 and displays repressor activity. Using an immunoprecipitation-MS approach, two phosphorylation sites were identified, Ser¹¹⁹ and Ser²³⁸. Phosphorylation at Ser¹¹⁹ is shown to be determinant for PRRXL1 conformation and transcriptional activity. Ser¹¹⁹ phosphorylation is thus proposed as a mechanism for regulating PRRXL1 function and conformation during nociceptive system development.

Repulsive guidance molecules (RGMs) and neogenin in bone morphogenetic protein (BMP) signaling

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor-beta (TGFβ) superfamily. BMPs mediate a highly conserved signal transduction cascade through the type-I and type-II serine/threonine kinase receptors and intracellular Smad proteins, which regulate multiple developmental and homeostatic processes. Mutations in this pathway can cause various diseases in humans, such as skeletal disorders, cardiovascular diseases, and various cancers. Multiple levels of regulation, including extracellular regulation, help to ensure proper spatiotemporal control of BMP signaling in the right cellular context. The family of repulsive guidance molecules (RGMs) and the type-I transmembrane protein neogenin, a paralog of DCC (Deleted in Colorectal Cancer), have been implicated in modulating the BMP pathway. In this review, we discuss the properties and functions of RGM proteins and neogenin, focusing on their roles in the modulation of BMP signal transduction.

Proteomic analysis and molecular modelling characterize the iron-regulatory protein haemojuvelin/repulsive guidance molecule c

HJV (haemojuvelin) plays a key role in iron metabolism in mammals by regulating expression of the liver-derived hormone hepcidin, which controls systemic iron uptake and release. Mutations in HJV cause juvenile haemochromatosis, a rapidly progressing iron overload disorder in humans. HJV, also known as RGMc (repulsive guidance molecule c), is a member of the three-protein RGM family. RGMs are GPI (glycosylphosphatidylinositol)-linked glycoproteins that share ~50% amino acid identity and several structural motifs, including the presence of 14 cysteine residues in analogous locations. Unlike RGMa and RGMb, HJV/RGMc is composed of both single-chain and two-chain isoforms. To date there is no structural information for any member of the RGM family. In the present study we have mapped the disulfide bonds in mouse HJV/RGMc using a proteomics strategy combining sequential MS steps composed of ETD (electron transfer dissociation) and CID (collision-induced dissociation), in which ETD induces cleavage of disulfide linkages, and CID establishes disulfide bond assignments between liberated peptides. The results of the present study identified an HJV/RGMc molecular species containing four disulfide linkages. We predict using ab initio modelling that this molecule is a single-chain HJV/RGMc isoform. Our observations outline a general approach using tandem MS and ab initio molecular modelling to define unknown structural features in proteins.

Spatiotemporal expression of repulsive guidance molecules (RGMs) and their receptor neogenin in the mouse brain

Neogenin has been implicated in a variety of developmental processes such as neurogenesis, neuronal differentiation, apoptosis, migration and axon guidance. Binding of repulsive guidance molecules (RGMs) to Neogenin inhibits axon outgrowth of different neuronal populations. This effect requires Neogenin to interact with co-receptors of the uncoordinated locomotion-5 (Unc5) family to activate downstream Rho signaling. Although previous studies have reported RGM, Neogenin, and/or Unc5 expression, a systematic comparison of RGM and Neogenin expression in the developing nervous system is lacking, especially at later developmental stages. Furthermore, information on RGM and Neogenin expression at the protein level is limited. To fill this void and to gain further insight into the role of RGM-Neogenin signaling during mouse neural development, we studied the expression of RGMa, RGMb, Neogenin and Unc5A-D using in situ hybridization, immunohistochemistry and RGMa section binding. Expression patterns in the primary olfactory system, cortex, hippocampus, habenula, and cerebellum were studied in more detail. Characteristic cell layer-specific expression patterns were detected for RGMa, RGMb, Neogenin and Unc5A-D. Furthermore, strong expression of RGMa, RGMb and Neogenin protein was found on several major axon tracts such as the primary olfactory projections, anterior commissure and fasciculus retroflexus. These data not only hint at a role for RGM-Neogenin signaling during the development of different neuronal systems, but also suggest that Neogenin partners with different Unc5 family members in different systems. Overall, the results presented here will serve as a framework for further dissection of the role of RGM-Neogenin signaling during neural development.

Developmental distribution pattern of metabotropic glutamate receptor 5 in prenatal human hippocampus

OBJECTIVE

Metabotropic glutamate receptor 5 (mGluR5) is concentrated in zones of active neurogenesis in the prenatal and postnatal rodent brain and plays an important role in the regulation of neurogenesis. However, little is known about mGluR5 in the prenatal human brain. Here, we aimed to explore the expression pattern and cellular distribution of mGluR5 in human fetal hippocampus.

METHODS

Thirty-four human fetuses were divided into four groups according to gestational age: 9-11, 14-16, 22-24 and 32-36 weeks. The hippocampus was dissected out and prepared. The protein and mRNA expression of mGluR5 were evaluated by Western blot and immunohistochemistry or real-time PCR. The cellular distribution of mGluR5 was observed with double-labeling immunofluorescence.

RESULTS

Both mGluR5 mRNA and protein were detected in the prenatal human hippocampus by real-time PCR and immunoblotting, and the expression levels increased gradually over time. The immunohistochemistry results were consistent with immunoblotting and showed that mGluR5 immunoreactivity was mainly present in the inner marginal zone (IMZ), hippocampal plate (HP) and ventricular zone (VZ). The double-labeling immunofluorescence showed that mGluR5 was present in neural stem cells (nestin-positive), neuroblasts (DCX-positive) and mature neurons (NeuN-positive), but not in typical astrocytes (GFAP-positive). The cells co-expressing mGluR5 and nestin were mainly located in the IMZ, HP and subplate at 11 weeks, all layers at 16 weeks, and CA1 at 24 weeks. As development proceeded, the number of mGluR5/nestin double-positive cells decreased gradually so that there were only a handful of double-labeled cells at 32 weeks. However, mGluR5/DCX double-positive cells were only found in the HP, IZ and IMZ at 11 weeks.

CONCLUSION

The pattern of mGluR5 expression by neural stem/progenitor cells, neuroblasts and neurons provides important anatomical evidence for the role of mGluR5 in the regulation of human hippocampal development.

RGMa and RGMb expression pattern during chicken development suggest unexpected roles for these repulsive guidance molecules in notochord formation, somitogenesis, and myogenesis

BACKGROUND

Repulsive guidance molecules (RGM) are high-affinity ligands for the Netrin receptor Neogenin, and they are crucial for nervous system development including neural tube closure; neuronal and neural crest cell differentiation and axon guidance. Recent studies implicated RGM molecules in bone morphogenetic protein signaling, which regulates a variety of developmental processes. Moreover, a role for RGMc in iron metabolism has been established. This suggests that RGM molecules may play important roles in non-neural tissues.

RESULTS

To explore which tissues and processed may be regulated by RGM molecules, we systematically investigated the expression of RGMa and RGMb, the only RGM molecules currently known for avians, in the chicken embryo.

CONCLUSIONS

Our study suggests so far unknown roles of RGM molecules in notochord, somite and skeletal muscle development.

Repulsive guidance molecule B (RGMB) plays negative roles in breast cancer by coordinating BMP signaling

Repulsive guidance molecules (RGMs) coordinate axon formation and iron homestasis. These molecules are also known as co-receptors of bone morphogenetic proteins (BMPs). However, the role played by RGMs in breast cancer remains unclear. The present study investigated the impact of RGMB on functions of breast cancer cells and corresponding mechanisms. RGMB was knocked down in breast cancer cells by way of an anti-RGMB ribozyme transgene. Knockdown of RGMB resulted in enhanced capacities of proliferation, adhesion, and migration in breast cancer cells. Further investigations demonstrated RGMB knockdown resulted in a reduced expression and activity of Caspase-3, accompanied with better survival in RGMB knockdown cells under serum starvation, which might be induced by its repression on MAPK JNK pathway. Up-regulations of Snai1, Twist, FAK, and Paxillin via enhanced Smad dependent sigaling led to increased capacities of adhesion and migration. Our current data firstly revealed that RGMB may act as a negative regulator in breast cancer through BMP signaling.

The subcommissural organ and the development of the posterior commissure

Growing axons navigate through the developing brain by means of axon guidance molecules. Intermediate targets producing such signal molecules are used as guideposts to find distal targets. Glial, and sometimes neuronal, midline structures represent intermediate targets when axons cross the midline to reach the contralateral hemisphere. The subcommissural organ (SCO), a specialized neuroepithelium located at the dorsal midline underneath the posterior commissure, releases SCO-spondin, a large glycoprotein belonging to the thrombospondin superfamily that shares molecular domains with axonal pathfinding molecules. Several evidences suggest that the SCO could be involved in the development of the PC. First, both structures display a close spatiotemporal relationship. Second, certain mutants lacking an SCO present an abnormal PC. Third, some axonal guidance molecules are expressed by SCO cells. Finally, SCO cells, the Reissner's fiber (the aggregated form of SCO-spondin), or synthetic peptides from SCO-spondin affect the neurite outgrowth or neuronal aggregation in vitro.

Repulsive guidance molecule (RGM) family proteins exhibit differential binding kinetics for bone morphogenetic proteins (BMPs)

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta superfamily that exert their effects via type I and type II serine threonine kinase receptors and the SMAD intracellular signaling pathway to regulate diverse biologic processes. Recently, we discovered that the repulsive guidance molecule (RGM) family, including RGMA, RGMB, and RGMC/hemojuvelin (HJV), function as co-receptors that enhance cellular responses to BMP ligands. Here, we use surface plasmon resonance to quantitate the binding kinetics of RGM proteins for BMP ligands. We show that among the RGMs, HJV exhibits the highest affinity for BMP6, BMP5, and BMP7 with K_D 8.1, 17, and 20 nM respectively, versus 28, 33, and 166 nM for RGMB, and 55, 83, and 63 nM for RGMA. Conversely, RGMB exhibits preferential binding to BMP4 and BMP2 with K_D 2.6 and 5.5 nM respectively, versus 4.5 and 9.4 nM for HJV, and 14 and 22 nM for RGMA, while RGMA exhibits the lowest binding affinity for most BMPs tested. Among the BMP ligands, RGMs exhibit the highest relative affinity for BMP4 and the lowest relative affinity for BMP7, while none of the RGMs bind to BMP9. Thus, RGMs exhibit preferential binding for distinct subsets of BMP ligands. The preferential binding of HJV for BMP6 is consistent with the functional role of HJV and BMP6 in regulating systemic iron homeostasis. Our data may help explain the mechanism by which BMPs exert cell-context specific effects via a limited number of type I and type II receptors.

A novel validated enzyme-linked immunosorbent assay to quantify soluble hemojuvelin in mouse serum

Hemojuvelin is a critical regulator of hepcidin expression and can be cleaved by proteases to form soluble hemojuvelin. Soluble hemojuvelin has been recently identified in human serum but the presence and quantity of soluble hemojuvelin in mouse serum is unknown. We developed a two-site enzyme-linked immunosorbent assay using a monoclonal anti-hemojuvelin as the capture antibody and a biotinylated polyclonal anti-hemojuvelin as the detection antibody to quantify the levels of soluble hemojuvelin in mouse serum. We validated this assay using cell-conditioned media and serum from Hemojuvelin-null and Bone morphogenetic protein 6-null mice. We also used this validated assay to measure serum soluble hemojuvelin concentrations in mice receiving an acute low iron or high iron treatment. This two-site enzyme-linked immunosorbent assay was highly specific for mouse hemojuvelin, with a lower limit of detection at 13.2-26.8 ng/mL of soluble hemojuvelin in mouse serum. The median serum soluble hemojuvelin concentration in wild-type C57BL/6J mice was 57.9 ± 22 ng/mL, which is 4- to 20-fold less than that reported in healthy human volunteers. After acute low iron diet treatment in these mice, serum soluble hemojuvelin levels were increased and correlated with lowered serum iron levels and decreased hepatic hepcidin expression. An acute high iron diet in wild-type mice or chronically iron-overloaded Bone morphogenetic protein 6-null mice did not significantly lower serum soluble hemojuvelin concentrations. Here we report reliable quantitation of mouse serum soluble hemojuvelin using a novel and validated enzyme-linked immunosorbent assay. This assay may provide a useful tool to elucidate the source and physiological role of serum soluble hemojuvelin in hepcidin regulation and iron metabolism using well-established mouse models of iron-related disorders.

Repulsive guidance molecule A (RGMa): a molecule for all seasons

RGMa (repulsive guidance molecule a) was the first identified molecule that possessed the necessary functional activity to repulse and steer growth cones to their target in the brain. By binding to its neogenin receptor, RGMa caused the collapse of growth cones and encouraged axons to grow along specific trajectories in vitro. Although originally characterized in 1990, RGMa was not conclusively shown to mediate axon guidance in vivo for another 12 years. Loss-of-function analysis in mice revealed that RGMa may play a more important role in neural tube morphogenesis. RGMa has now emerged as a molecule with pleiotropic roles involving cell adhesion, cell migration, cell polarity and cell differentiation which together strongly influence early morphogenetic events as well as immune responses. RGMa can be regarded as a molecule for all seasons.

The BMP coreceptor RGMb promotes while the endogenous BMP antagonist noggin reduces neurite outgrowth and peripheral nerve regeneration by modulating BMP signaling

Repulsive guidance molecule b (RGMb) is a bone morphogenetic protein (BMP) coreceptor and sensitizer of BMP signaling, highly expressed in adult dorsal root ganglion (DRG) sensory neurons. We used a murine RGMb knock-out to gain insight into the physiological role of RGMb in the DRG, and address whether RGMb-mediated modulation of BMP signaling influences sensory axon regeneration. No evidence for altered development of the PNS and CNS was detected in RGMb(-/-) mice. However, both cultured neonatal whole DRG explants and dissociated DRG neurons from RGMb(-/-) mice exhibited significantly fewer and shorter neurites than those from wild-type littermates, a phenomenon that could be fully rescued by BMP-2. Moreover, Noggin, an endogenous BMP signaling antagonist, inhibited neurite outgrowth in wild-type DRG explants from naive as well as nerve injury-preconditioned mice. Noggin is downregulated in the DRG after nerve injury, and its expression is highly correlated and inversely associated with the known regeneration-associated genes, which are induced in the DRG by peripheral axonal injury. We show that diminished BMP signaling in vivo, achieved either through RGMb deletion or BMP inhibition with Noggin, retarded early axonal regeneration after sciatic nerve crush injury. Our data suggest a positive modulatory contribution of RGMb and BMP signaling to neurite extension in vitro and early axonal regrowth after nerve injury in vivo and a negative effect of Noggin.

Novel roles of the chemorepellent axon guidance molecule RGMa in cell migration and adhesion

The repulsive guidance molecule A (RGMa) is a contact-mediated axon guidance molecule that has significant roles in central nervous system (CNS) development. Here we have examined whether RGMa has novel roles in cell migration and cell adhesion outside the nervous system. RGMa was found to stimulate cell migration from Xenopus animal cap explants in a neogenin-dependent and BMP-independent manner. RGMa also stimulated the adhesion of Xenopus animal cap cells, and this adhesion was dependent on neogenin and independent of calcium. To begin to functionally characterize the role of specific domains in RGMa, we assessed the migratory and adhesive activities of deletion mutants. RGMa lacking the partial von Willebrand factor type D (vWF) domain preferentially perturbed cell adhesion, while mutants lacking the RGD motif affected cell migration. We also revealed that manipulating the levels of RGMa in vivo caused major migration defects during Xenopus gastrulation. We have revealed here novel roles of RGMa in cell migration and adhesion and demonstrated that perturbations to the homeostasis of RGMa expression can severely disrupt major morphogenetic events. These results have implications for understanding the role of RGMa in both health and disease.

RGMa inhibits neurite outgrowth of neuronal progenitors from murine enteric nervous system via the neogenin receptor in vitro

The enteric nervous system (ENS) in vertebrate embryos is formed by neural crest-derived cells. During development, these cells undergo extensive migration from the vagal and sacral regions to colonize the entire gut, where they differentiate into neurons and glial cells. Guidance molecules like netrins, semaphorins, slits, and ephrins are known to be involved in neuronal migration and axon guidance. In the CNS, the repulsive guidance molecule (RGMa) has been implicated in neuronal differentiation, migration, and apoptosis. Recently, we described the expression of the subtypes RGMa and RGMb and their receptor neogenin during murine gut development. In the present study, we investigated the influence of RGMa on neurosphere cultures derived from fetal ENS. In functional in vitro assays, RGMa strongly inhibited neurite outgrowth of differentiating progenitors via the receptor neogenin. The repulsive effect of RGMa on processes of differentiated enteric neural progenitors could be demonstrated by collapse assay. The influence of the RGM receptor on ENS was also analyzed in neogenin knockout mice. In the adult large intestine of mutants we observed disturbed ganglia formation in the myenteric plexus. Our data indicate that RGMa may be involved in differentiation processes of enteric neurons in the murine gut.

Crucial roles of NGF in dorsal horn plasticity in partially deafferentated cats

Though exogenous nerve growth factor (NGF) has been implicated in spinal cord plasticity, whether endogenous NGF plays a crucial role has not been established in vivo. This study investigated first the role of endogenous NGF in spinal dorsal horn (DH) plasticity following removal of L1-L5 and L7-S2 dorsal root ganglions (DRGs) in cats. Co-culture of chick embryo DRG with DH condition media, protein band fishing by cells as well as western blot showed that NGF could promote neurite growth in vitro. Immunohistochemistry and in situ hybridization technique revealed an increase in the NGF and NGF mRNA immunoreactive cells in the DH after partial deafferentation. Lastly, after blocking with NGF antibody, choleragen subunit B horseradish peroxidase (CB-HRP) tracing showed a reduction in the neuronal sprouting observed in the DH. Our results demonstrated that in the cat, endogenous NGF plays a crucial role in DH plasticity after partial deafferentation.

BMP signaling modulates hepcidin expression in zebrafish embryos independent of hemojuvelin

Hemojuvelin (Hjv), a member of the repulsive-guidance molecule (RGM) family, upregulates transcription of the iron regulatory hormone hepcidin by activating the bone morphogenetic protein (BMP) signaling pathway in mammalian cells. Mammalian models have identified furin, neogenin, and matriptase-2 as modifiers of Hjv's function. Using the zebrafish model, we evaluated the effects of hjv and its interacting proteins on hepcidin expression during embryonic development. We found that hjv is strongly expressed in the notochord and somites of the zebrafish embryo and that morpholino knockdown of hjv impaired the development of these structures. Knockdown of hjv or other hjv-related genes, including zebrafish orthologs of furin or neogenin, however, failed to decrease hepcidin expression relative to liver size. In contrast, overexpression of bmp2b or knockdown of matriptase-2 enhanced the intensity and extent of hepcidin expression in zebrafish embryos, but this occurred in an hjv-independent manner. Furthermore, we demonstrated that zebrafish hjv can activate the human hepcidin promoter and enhance BMP responsive gene expression in vitro, but is expressed at low levels in the zebrafish embryonic liver. Taken together, these data support an alternative mechanism for hepcidin regulation during zebrafish embryonic development, which is independent of hjv.

Dragon (repulsive guidance molecule b) inhibits IL-6 expression in macrophages

Repulsive guidance molecule (RGM) family members RGMa, RGMb/Dragon, and RGMc/hemojuvelin were found recently to act as bone morphogenetic protein (BMP) coreceptors that enhance BMP signaling activity. Although our previous studies have shown that hemojuvelin regulates hepcidin expression and iron metabolism through the BMP pathway, the role of the BMP signaling mediated by Dragon remains largely unknown. We have shown previously that Dragon is expressed in neural cells, germ cells, and renal epithelial cells. In this study, we demonstrate that Dragon is highly expressed in macrophages. Studies with RAW264.7 and J774 macrophage cell lines reveal that Dragon negatively regulates IL-6 expression in a BMP ligand-dependent manner via the p38 MAPK and Erk1/2 pathways but not the Smad1/5/8 pathway. We also generated Dragon knockout mice and found that IL-6 is upregulated in macrophages and dendritic cells derived from whole lung tissue of these mice compared with that in respective cells derived from wild-type littermates. These results indicate that Dragon is an important negative regulator of IL-6 expression in immune cells and that Dragon-deficient mice may be a useful model for studying immune and inflammatory disorders.

Conserved proximal promoter elements control repulsive guidance molecule c/hemojuvelin (Hfe2) gene transcription in skeletal muscle

Repulsive guidance molecule c (RGMc; gene symbol: Hfe2) plays a critical role in iron metabolism. Inactivating mutations cause juvenile hemochromatosis, a severe iron overload disorder. Understanding mechanisms controlling RGMc biosynthesis has been hampered by minimal information about the RGMc gene. Here we define the structure, examine the evolution, and establish mechanisms of regulation of the mouse RGMc gene. RGMc is a 4-exon gene that undergoes alternative RNA splicing to yield 3 mRNAs with 5' different untranslated regions. Gene transcription is induced during myoblast differentiation, producing all 3 mRNAs. We identify 3 critical promoter elements responsible for transcriptional activation in skeletal muscle, 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, our results suggest that RGMc has been a muscle-enriched gene throughout its evolutionary history.