The neuropeptide neuromedin U promotes autoantibody-mediated arthritis
Neuromedin U (NMU) is a neuropeptide with pro-inflammatory activity. The primary goal of this study was to determine if NMU promotes autoantibody-induced arthritis. Additional studies addressed the cellular source of NMU and sought to define the NMU receptor responsible for its pro-inflammatory effects.
Serum containing arthritogenic autoantibodies from K/BxN mice was used to induce arthritis in mice genetically lacking NMU. Parallel experiments examined whether NMU deficiency impacted the early mast-cell-dependent vascular leak response induced by these autoantibodies. Bone-marrow chimeric mice were generated to determine whether pro-inflammatory NMU is derived from hematopoietic cells or stromal cells. Mice lacking the known NMU receptors singly and in combination were used to determine susceptibility to serum-transferred arthritis and in vitro cellular responses to NMU.
NMU-deficient mice developed less severe arthritis than control mice. Vascular leak was not affected by NMU deficiency. NMU expression by bone-marrow-derived cells mediated the pro-arthritogenic effect. Deficiency of all of the known NMU receptors, however, had no impact on arthritis severity and did not affect the ability of NMU to stimulate intracellular calcium flux.
NMU-deficient mice are protected from developing autoantibody-induced inflammatory arthritis. NMU derived from hematopoietic cells, not neurons, promotes the development of autoantibody-induced inflammatory arthritis. This effect is mediated by a receptor other than the currently known NMU receptors.
KeywordsMast Cell Inflammatory Arthritis Arthritis Severity Growth Hormone Secretagogue Receptor Intracellular Calcium Flux
T cell receptor
tumor necrosis factor.
Neuromedin U (NMU) is an evolutionarily conserved short neuropeptide with multiple physiologic effects. Named for its ability to induce uterine contraction, NMU has also been reported to play roles in metabolic and feeding regulation, pain perception, bone remodeling, blood pressure and contraction of smooth muscle in a variety of organs. NMU is widely expressed, with highest levels in the central nervous system and gastrointestinal tract . NMU has not been detected in the circulation, suggesting that it acts primarily as a neurotransmitter and/or that it is short-lived .
Two NMU receptors have been identified, NMUR1 and NMUR2 [1, 3]. Both of these are G-protein-coupled receptors with seven transmembrane domains. In most species studied, including the mouse, NMUR1 is widely expressed, predominantly in the gastrointestinal tract and also in immune cells, whereas the expression of NMUR2 is limited to the central nervous system. Binding of NMU to either receptor results in the elevation of intracellular calcium .
Several immunostimulatory activities have been attributed to NMU. In a mouse Th2 cell clone, stimulation with NMU led to intracellular calcium flux and the synthesis and release of IL-4, IL-5, IL-6, IL-10 and IL-13 . More recently, attention has focused on the role of NMU on cells of the innate immune system. NMU induced calcium flux in, and degranulation of, mast cells and was required for mast-cell-mediated inflammation triggered by local injection of complete Freund's adjuvant . In a mouse model of asthma, NMU activated eosinophils . Furthermore, NMU could augment lipopolysaccharide-induced IL-6 production by macrophages . These findings suggest that NMU might be an important driver of inflammatory diseases.
Arthritis can be induced by injecting serum from K/BxN T cell receptor (TCR) transgenic mice into normal mice, reflecting the high concentrations of arthritogenic autoantibodies recognizing glucose-6-phosphate isomerase (GPI) in the K/BxN arthritis model. The development of serum-transferred arthritis depends on innate immune cells, such as neutrophils and mast cells (although this cell type is under debate) as well as platelets, activating Fc receptors, the alternative pathway of the complement system, and cytokines [9, 10, 11, 12, 13, 14, 15, 16, 17]. Here, we utilized the K/BxN serum transfer model to test the hypothesis that NMU promotes inflammatory arthritis. We also investigated the cellular source of NMU during the development of arthritis and which of the NMU receptors mediate its pro-inflammatory effects.
Materials and methods
Mice with a targeted deletion of the gene encoding NMU (Nmu tm1Mko ), NMUR1 (Nmur1 tm1Rtor ) and NMUR2 (Nmur2 tm1Rtor ) or NTSR1 (Ntsr1 tm1Hmno ) on the C57BL/6 (B6) background have been described [18, 19, 20]. The nomenclature for the targeted alleles is obtained from Mouse Genome Informatics . Non-obese diabetic and B6 mice were obtained from Jackson Laboratory, Bar Harbor, ME, USA. KRN TCR transgenic mice were bred in house. Mice were maintained in specific-pathogen-free colonies at Harvard Medical School or the University of Minnesota, under protocols approved by the Harvard Medical Area Standing Committee on Animals or the University of Minnesota's Institutional Animal Care and Use Committee.
Arthritis induction and related studies
K/BxN serum-transferred arthritis was induced and monitored as previously described . Unless otherwise indicated, 150 μL of serum obtained from eight-week-old K/BxN mice was injected intraperitoneally into 6- to 8-week-old recipients on days 0 and 2. The arthritis scoring, measurement of ankle thickening and determination of anti-GPI immunoglobulin G (IgG) titers were performed as previously described . For arthritis scoring, each paw was assigned a score of 0 (no arthritis) to 3 (maximum severity), resulting in a total range of 0 to 12 for an individual mouse. Mast cells were identified by toluidine blue staining, as previously described . Vascular leak studies and generation of bone-marrow chimeric mice were performed as previously described . Complete blood counts were performed using a Hemavet 950FS (Drew Scientific, Dallas, TX, USA).
Following lysis of red blood cells, splenocytes were resuspended in Roswell Park Memorial Institute medium plus 10% fetal bovine serum, and were loaded with Indo-1 AM (10 μg/mL; Invitrogen, Grand Island, NY, USA) for 30 minutes at 37°C. The cells were washed and resuspended at 5 × 106 cells/mL. Following the addition of indicated concentrations of rat NMU-23 (sequence YKVNEYQGPVAPSGGFFLFRPRN; GenScript, Piscataway, NJ, USA) or 1 μg/mL ionomycin (MP Biomedicals, LLC, Solon, OH, USA) in 12.5% dimethyl sulfoxide in water, the cells were analyzed on an LSRII flow cytometer (BD Biosciences, San Diego, CA, USA). Data were analyzed using FlowJo software (Treestar, Ashland, OR, USA).
Statistical differences between the mean values for groups were calculated using an unpaired Student's two-tailed t test. Arthritis severity scores were compared with a repeated-measures analysis of variance. For experiments involving more than two groups, a post-hoc Tukey's multiple comparison test was used. P values < 0.05 were considered significant. Analyses were performed with SPSS 17.0.
Neuropeptides are increasingly recognized as potential contributors to inflammatory diseases. For instance, the neuropeptide substance P is involved in a variety of inflammatory conditions, including being upregulated in the inflamed synovium of patients with rheumatoid arthritis . In addition, catecholamine-producing cells are present in the synovial tissue of mice and human patients with inflammatory arthritis . Understanding how neuropeptides promote inflammatory diseases is expected to lead to new therapeutic approaches.
Based on a series of reports describing the pro-inflammatory properties of NMU [6, 7, 8], we tested the hypothesis that NMU can promote the development of autoantibody-mediated arthritis. Indeed, our results demonstrated that mice lacking NMU were protected from K/BxN serum-transferred arthritis. This protection was not due to impaired differentiation of the key innate immune cell types (that is, neutrophils, platelets and mast cells) implicated in this disease model. Nor did NMU interfere with the earliest vascular events provoked by injection of autoantibodies, but more likely at a later time. NMU can directly activate effector cells, such as mast cells, eosinophils and macrophages, to elicit a variety of cellular responses including degranulation, adhesion, chemotaxis and production of the pro-inflammatory cytokine IL-6 [6, 7, 8]. Therefore, it is likely that NMU acts on multiple cell types to elicit a variety of pro-inflammatory activities that drive the inflammatory arthritis in this model.
Although it is known that NMU can be produced by hematopoietic cells [8, 31], most prior studies have focused on neurons as being the most likely source of pro-inflammatory NMU [6, 7, 8]. Our reciprocal bone-marrow transplantation experiments showed clearly that NMU derived from hematopoietic cells was critical for its pro-arthritogenic activity. Our findings are consistent with the finding that NMU is not a marker of peripheral sympathetic or sensory nerve fibers . Similarly, our results support previous findings that substance P and neuropeptide Y are neuropeptides produced not only by neurons but also by immune cells [29, 32], and that inflammatory arthritis is accompanied by a loss of sympathetic nerve fibers which appear to be replaced by synovial cells that synthesize catecholamines . Thus, although neuropeptides are typically initially characterized based on their expression in neurons, their pro-inflammatory properties might be attributed instead to their expression by cells of the immune system, as we have shown. Therefore, the involvement of a neuropeptide in an inflammatory disease does not necessarily imply neuro-immune connections or neuro-immune cross-talk other than expression of similar neuropeptides by these cell types.
Additionally, although it is theoretically possible that NMU-deficient mice were protected from autoantibody-induced arthritis due to developmental or metabolic abnormalities in the mice, our finding that the protective effect of NMU deficiency was transferrable with bone marrow-derived cells suggests that such a scenario is unlikely.
Defining the receptors that a pro-inflammatory substance engages to elicit inflammation is important for the development of potential therapeutic agents. NMUR1 is widely expressed, and Nmur1-deficient mice have impaired contraction of certain smooth muscle tissues in the gastrointestinal tract . NMUR2 is expressed primarily in the central nervous system, and Nmur2-deficient mice have been shown to have impaired responses to pain in some assays; in contrast, these pain responses were normal in Nmur1-deficient mice . With respect to inflammatory responses, inflammation induced by the injection of complete Freund's adjuvant was normal in mice lacking both receptors . Similarly, we have shown here that autoantibody-induced arthritis was unimpaired in these same Nmur1/Nmur2-deficient mice. We further have shown that the only other reported functional NMU receptor, a heterodimer of NTSR1 and the growth hormone secretagogue receptor 1 b, was also dispensable for the development of serum-transferred arthritis. Recent evidence, however, suggests that NMU-induced suppression of food intake and weight gain remained intact in mice lacking NTSR1, suggesting that NTSR1 may not serve as an NMU receptor in vivo ; this is consistent with our in vitro findings. It is theoretically possible that the related NTSR2 could serve as an NMU receptor, though this has not been demonstrated experimentally.
Our results suggest that an as-yet-unidentified NMU receptor is responsible for the pro-inflammatory effects of NMU. It is likely, based on the structure of the known NMU receptors, that this receptor will belong to a large family of seven transmembrane spanning G-protein-coupled receptors. It is of course possible that the expression of this putative additional NMU receptor is upregulated in the mice we studied here, as a physiologic compensatory mechanism for the absence of the known NMU receptors, and that its contribution to the pro-inflammatory activities of NMU is therefore apparent only in the absence of the known NMU receptors. It has been shown, for instance, that the expression of NMUR1 is increased in macrophages derived from NMU-KO mice, supporting the notion that such compensatory mechanisms might exist . Further investigations such as the use of NMUR1- and NMUR2-specific short-acting antagonists and/or identification of this putative additional NMU receptor should help to clarify this point.
In summary, we have shown that mice genetically deficient in NMU expression were protected from developing autoantibody-induced inflammatory arthritis. In this model, bone-marrow-derived cells rather than neurons served as the critical source of NMU. The arthritogenic activity of NMU may be mediated via a receptor other than the currently defined NMU receptors. Our results indicate that targeting NMU or its receptors could be an attractive approach to therapeutic intervention in inflammatory arthritis, but also suggest that further delineation and definition of the key NMU receptors will first be required. More broadly, our findings demonstrate that neuropeptides expressed by immune cells can be important contributors to the pathogenesis of inflammatory arthritis.
NMU produced by bone-marrow-derived cells promoted autoantibody-mediated arthritis in mice. The arthritogenic activity of NMU was mediated by a receptor other than the currently known NMU receptors. This study points to NMU as a new potential therapeutic target in inflammatory arthritis.
We thank Pratik Patel and Donna Skinner for technical assistance. The studies were supported by an Arthritis Foundation Arthritis Investigator Award and a Minnesota Medical Foundation Research Grant (to BAB). BAB is also supported by K08 AR054317 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases and by start-up funds from the University of Minnesota Department of Pediatrics. CB and DM's participation was supported by R01 AR055271.
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