The contribution of neurotrophins to the pathogenesis of allergic asthma

Neurotrophin Regulation and Signaling in Airway Smooth Muscle

Structural and functional aspects of bronchial airways are key throughout life and play critical roles in diseases such as asthma. Asthma involves functional changes such as airway irritability and hyperreactivity, as well as structural changes such as enhanced cellular proliferation of airway smooth muscle (ASM), epithelium, and fibroblasts, and altered extracellular matrix (ECM) and fibrosis, all modulated by factors such as inflammation. There is now increasing recognition that disease maintenance following initial triggers involves a prominent role for resident nonimmune airway cells that secrete growth factors with pleiotropic autocrine and paracrine effects. The family of neurotrophins may be particularly relevant in this regard. Long recognized in the nervous system, classical neurotrophins such as brain-derived neurotrophic factor (BDNF) and nonclassical ligands such as glial-derived neurotrophic factor (GDNF) are now known to be expressed and functional in non-neuronal systems including lung. However, the sources, targets, regulation, and downstream effects are still under investigation. In this chapter, we discuss current state of knowledge and future directions regarding BDNF and GDNF in airway physiology and on pathophysiological contributions in asthma.

Smooth muscle brain-derived neurotrophic factor contributes to airway hyperreactivity in a mouse model of allergic asthma

Recent studies have demonstrated an effect of neurotrophins, particularly brain-derived neurotrophic factor (BDNF), on airway contractility [ via increased airway smooth muscle (ASM) intracellular calcium [Ca2+]i] and remodeling (ASM proliferation and extracellular matrix formation) in the context of airway disease. In the present study, we examined the role of BDNF in allergen-induced airway inflammation using 2 transgenic models: 1) tropomyosin-related kinase B (TrkB) conditional knockin (TrkBKI) mice allowing for inducible, reversible disruption of BDNF receptor kinase activity by administration of 1NMPP1, a PP1 derivative, and 2) smooth muscle-specific BDNF knockout (BDNFfl/fl/SMMHC11Cre/0) mice. Adult mice were intranasally challenged with PBS or mixed allergen ( Alternaria alternata, Aspergillus fumigatus, house dust mite, and ovalbumin) for 4 wk. Our data show that administration of 1NMPP1 in TrkBKI mice during the 4-wk allergen challenge blunted airway hyperresponsiveness (AHR) and reduced fibronectin mRNA expression in ASM layers but did not reduce inflammation per se. Smooth muscle-specific deletion of BDNF reduced AHR and blunted airway fibrosis but did not significantly alter airway inflammation. Together, our novel data indicate that TrkB signaling is a key modulator of AHR and that smooth muscle-derived BDNF mediates these effects during allergic airway inflammation.-Britt, R. D., Jr., Thompson, M. A., Wicher, S. A., Manlove, L. J., Roesler, A., Fang, Y.-H., Roos, C., Smith, L., Miller, J. D., Pabelick, C. M., Prakash, Y. S. Smooth muscle brain-derived neurotrophic factor contributes to airway hyperreactivity in a mouse model of allergic asthma.

Identification of Epigenetic Mechanisms Involved in the Anti-Asthmatic Effects of Descurainia sophia Seed Extract Based on a Multi-Omics Approach

Asthma, a heterogeneous disease of the airways, is common around the world, but little is known about the molecular mechanisms underlying the interactions between DNA methylation and gene expression in relation to this disease. The seeds of Descurainia sophia are traditionally used to treat coughs, asthma and edema, but their effects on asthma have not been investigated by multi-omics analysis. We undertook this study to assess the epigenetic effects of ethanol extract of D. sophia seeds (DSE) in an ovalbumin (OVA)-induced mouse model of asthma. We profiled genome-wide DNA methylation by Methyl-seq and characterized the transcriptome by RNA-seq in mouse lung tissue under three conditions: saline control, OVA-induced, and DSE-treated. In total, 1995 differentially methylated regions (DMRs) were identified in association with anti-asthmatic effects, most in promoter and coding regions. Among them, 25 DMRs were negatively correlated with the expression of the corresponding 18 genes. These genes were related to development of the lung, respiratory tube and respiratory system. Our findings provide insights into the anti-asthmatic effects of D. sophia seeds and reveal the epigenetic targets of anti-inflammatory processes in mice.

β-NGF and β-NGF receptor upregulation in blood and synovial fluid in osteoarthritis

Osteoarthritis (OA) of the knee is the most common form of non-traumatic joint disease. Previous studies have shown the involvement of β-NGF and its receptors TrKA and p75NTR in OA-related pain, but their role in its pathogenesis is still unclear. The aim of our study was to investigate the amount of β-NGF and the expression levels of its receptors on cells isolated from synovial fluid and blood from OA patients who had undergone total knee arthroplasty, in order to check any possible correlation with the disease staging. Our results show a progressive stage-related increase of β-NGF and its receptors both in serum and synovial fluid. Furthermore, with respect to control subjects, OA patients show an increased amount of inflammatory monocytes along with an increased expression of β-NGF, TrKA and p75NTR. In conclusion, our study suggests a stage-related modulation of β-NGF and its receptors in the inflammatory process of OA.

Brain-derived neurotrophic factor and airway fibrosis in asthma

Airway remodeling in asthma driven by inflammation involves proliferation of epithelial cells and airway smooth muscle (ASM), as well as enhanced extracellular matrix (ECM) generation and deposition, i.e., fibrosis. Accordingly, understanding profibrotic mechanisms is important for developing novel therapeutic strategies in asthma. Recent studies, including our own, have suggested a role for locally produced growth factors such as brain-derived neurotrophic factor (BDNF) in mediating and modulating inflammation effects. In this study, we explored the profibrotic influence of BDNF in the context of asthma by examining expression, activity, and deposition of ECM proteins in primary ASM cells isolated from asthmatic vs. nonasthmatic patients. Basal BDNF expression and secretion, and levels of the high-affinity BDNF receptor TrkB, were higher in asthmatic ASM. Exogenous BDNF significantly increased ECM production and deposition, especially of collagen-1 and collagen-3 (less so fibronectin) and the activity of matrix metalloproteinases (MMP-2, MMP-9). Exposure to the proinflammatory cytokine TNFα significantly increased BDNF secretion, particularly in asthmatic ASM, whereas no significant changes were observed with IL-13. Chelation of BDNF using TrkB-Fc reversed TNFα-induced increase in ECM deposition. Conditioned media from asthmatic ASM enhanced ECM generation in nonasthmatic ASM, which was blunted by BDNF chelation. Inflammation-induced changes in MMP-2, MMP-9, and tissue inhibitor metalloproteinases (TIMP-1, TIMP-2) were reversed in the presence of TrkB-Fc. These novel data suggest ASM as an inflammation-sensitive source of BDNF within human airways, with autocrine effects on fibrosis relevant to asthma.

Local Effect of Neurotrophin-3 in Neuronal Inflammation of Allergic Rhinitis: Preliminary Report

BACKGROUND

Allergic rhinitis is a common inflammatory nasal mucosal disease characterized by sneezing, watery nasal discharge, nasal obstruction and itching. Although allergen-specific antibodies play a main role in the allergic airway inflammation, neuronal inflammation may also contribute to the symptoms of allergic rhinitis. Neuronal inflammation is primarily caused by the stimulation of sensory nerve endings with histamine. It has been shown that neurotrophins may also have a role in allergic reactions and neuronal inflammation. Nerve growth factor, neurotrophin 3 (NT-3), neurotrophin 4/5 and brain-derived neurotrophic factor are members of the neurotrophin family. Although nerve growth factor and brain-derived neurotrophic factor are well studied in allergic rhinitis patients, the exact role of Neurotrophin-3 is not known.

AIMS

To investigate the possible roles of neurotrophin-3 in allergic rhinitis patients.

STUDY DESIGN

Case-control study.

METHODS

Neurotrophin-3 levels were studied in the inferior turbinate and serum samples of 20 allergic rhinitis and 13 control patients. Neurotrophin-3 staining of nasal tissues was evaluated by immunohistochemistry and ELISA was used for the determination of serum Neurotrophin-3 levels.

RESULTS

Neurotrophin-3 staining scores were statistically higher in the study group than in the control patients (p=0.001). Regarding serum Neurotrophin-3 levels, no statistically significant difference could be determined between allergic rhinitis and control patients (p=0.156). When comparing the serum NT-3 levels with tissue staining scores, there were no statistically significant differences in the allergic rhinitis and control groups (p=0.254 for allergic rhinitis and p=0.624 for control groups).

CONCLUSION

We suggest that Neurotrophin-3 might affect the nasal mucosa locally without being released into the systemic circulation in allergic rhinitis patients.

Biomarkers of respiratory syncytial virus (RSV) infection: specific neutrophil and cytokine levels provide increased accuracy in predicting disease severity

Despite fundamental advances in the research on respiratory syncytial virus (RSV) since its initial identification almost 60 years ago, recurring failures in developing vaccines and pharmacologic strategies effective in controlling the infection have allowed RSV to become a leading cause of global infant morbidity and mortality. Indeed, the burden of this infection on families and health care organizations worldwide continues to escalate and its financial costs are growing. Furthermore, strong epidemiologic evidence indicates that early-life lower respiratory tract infections caused by RSV lead to the development of recurrent wheezing and childhood asthma. While some progress has been made in the identification of reliable biomarkers for RSV bronchiolitis, a "one size fits all" biomarker capable of accurately and consistently predicting disease severity and post-acute outcomes has yet to be discovered. Therefore, it is of great importance on a global scale to identify useful biomarkers for this infection that will allow pediatricians to cost-effectively predict the clinical course of the disease, as well as monitor the efficacy of new therapeutic strategies.

Brain-derived neurotrophic factor in the airways

In addition to their well-known roles in the nervous system, there is increasing recognition that neurotrophins such as brain derived neurotrophic factor (BDNF) as well as their receptors are expressed in peripheral tissues including the lung, and can thus potentially contribute to both normal physiology and pathophysiology of several diseases. The relevance of this family of growth factors lies in emerging clinical data indicating altered neurotrophin levels and function in a range of diseases including neonatal and adult asthma, sinusitis, influenza, and lung cancer. The current review focuses on 1) the importance of BDNF expression and signaling mechanisms in early airway and lung development, critical to both normal neonatal lung function and also its disruption in prematurity and insults such as inflammation and infection; 2) how BDNF, potentially derived from airway nerves modulate neurogenic control of airway tone, a key aspect of airway reflexes as well as dysfunctional responses to allergic inflammation; 3) the emerging idea that local BDNF production by resident airway cells such as epithelium and airway smooth muscle can contribute to normal airway structure and function, and to airway hyperreactivity and remodeling in diseases such as asthma. Furthermore, given its pleiotropic effects in the airway, BDNF may be a novel and appealing therapeutic target.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

TRPC3 regulates release of brain-derived neurotrophic factor from human airway smooth muscle

Exogenous brain-derived neurotrophic factor (BDNF) enhances Ca(2+) signaling and cell proliferation in human airway smooth muscle (ASM), especially with inflammation. Human ASM also expresses BDNF, raising the potential for autocrine/paracrine effects. The mechanisms by which ASM BDNF secretion occurs are not known. Transient receptor potential channels (TRPCs) regulate a variety of intracellular processes including store-operated Ca(2+) entry (SOCE; including in ASM) and secretion of factors such as cytokines. In human ASM, we tested the hypothesis that TRPC3 regulates BDNF secretion. At baseline, intracellular BDNF was present, and BDNF secretion was detectable by enzyme linked immunosorbent assay (ELISA) of cell supernatants or by real-time fluorescence imaging of cells transfected with GFP-BDNF vector. Exposure to the pro-inflammatory cytokine tumor necrosis factor-alpha (TNFα) (20ng/ml, 48h) or a mixture of allergens (ovalbumin, house dust mite, Alternaria, and Aspergillus extracts) significantly enhanced BDNF secretion and increased TRPC3 expression. TRPC3 knockdown (siRNA or inhibitor Pyr3; 10μM) blunted BDNF secretion, and prevented inflammation effects. Chelation of extracellular Ca(2+) (EGTA; 1mM) or intracellular Ca(2+) (BAPTA; 5μM) significantly reduced secreted BDNF, as did the knockdown of SOCE proteins STIM1 and Orai1 or plasma membrane caveolin-1. Functionally, secreted BDNF had autocrine effects suggested by phosphorylation of high-affinity tropomyosin-related kinase TrkB receptor, prevented by chelating extracellular BDNF with chimeric TrkB-Fc. These data emphasize the role of TRPC3 and Ca(2+) influx in the regulation of BDNF secretion by human ASM and the enhancing effects of inflammation. Given the BDNF effects on Ca(2+) and cell proliferation, BDNF secretion may contribute to altered airway structure and function in diseases such as asthma.

Augmented pain behavioural responses to intra-articular injection of nerve growth factor in two animal models of osteoarthritis

OBJECTIVES

Nerve growth factor (NGF) is a promising analgesic target, particularly in osteoarthritis (OA) where existing therapies are inadequate. We hypothesised that pain responses to NGF are increased in OA joints. Here, NGF-evoked pain behaviour was compared in two rodent models of OA, and possible mechanisms underlying altered pain responses were examined.

METHODS

OA was induced in rat knees by meniscal transection (MNX) or intra-articular monosodium iodoacetate injection (MIA). Once OA pathology was fully established (day 20), we assessed pain behaviour (hindlimb weight-bearing asymmetry and hindpaw mechanical withdrawal thresholds) evoked by intra-articular injection of NGF (10 µg). Possible mechanisms underlying alterations in NGF-induced pain behaviour were explored using indomethacin pretreatment, histopathological evaluation of synovitis, and rtPCR for NGF receptor (tropomyosin receptor kinase (Trk)-A) expression in dorsal root ganglia (DRG).

RESULTS

Both the MIA and MNX models of OA displayed reduced ipsilateral weight bearing and hindpaw mechanical withdrawal thresholds, mild synovitis and increased TrkA expression in DRG. NGF injection into OA knees produced a prolonged augmentation of weight-bearing asymmetry, compared to NGF injection in non-osteoarthritic knees. However, hindpaw mechanical withdrawal thresholds were not further decreased by NGF. Pretreatment with indomethacin attenuated NGF-facilitated weight-bearing asymmetry and reversed OA-induced ipsilateral TrkA mRNA up-regulation.

CONCLUSIONS

OA knees were more sensitive to NGF-induced pain behaviour compared to non-osteoarthritic knees. Cyclo-oxygenase products may contribute to increased TrkA expression during OA development, and the subsequent increased NGF sensitivity. Treatments that reduce sensitivity to NGF have potential to improve OA pain.

Brain-derived neurotrophic factor in cigarette smoke-induced airway hyperreactivity

Enhanced airway smooth muscle (ASM) contractility contributes to increased resistance to airflow in diseases such as bronchitis and asthma that occur in passive smokers exposed to secondhand smoke. Little information exists on the cellular mechanisms underlying such airway hyperreactivity. Sputum samples of patients with chronic sinusitis, bronchitis, and asthma show increased concentrations of growth factors called neurotrophins, including brain-derived growth factor (BDNF), but their physiological significance remains unknown. In human ASM, we tested the hypothesis that BDNF contributes to increased contractility with cigarette smoke exposure. The exposure of ASM to 1% or 2% cigarette smoke extract (CSE) for 24 hours increased intracellular calcium ([Ca(2+)](i)) responses to histamine, and further potentiated the enhancing effects of a range of BDNF concentrations on such histamine responses. CSE exposure increased the expression of the both high-affinity and low-affinity neurotrophin receptors tropomyosin-related kinase (Trk)-B and p75 pan-neurotrophin receptor, respectively. Quantitative ELISA showed that CSE increased BDNF secretion by human ASM cells. BDNF small interfering (si)RNA and/or the chelation of extracellular BDNF, using TrkB-fragment crystallizable, blunted the effects of CSE on [Ca(2+)](i) responses as well as the CSE enhancement of cell proliferation, whereas TrkB siRNA blunted the effects of CSE on ASM contractility. These data suggest that cigarette smoke is a potent inducer of BDNF and TrkB expression and signaling in ASM, which then contribute to cigarette smoke-induced airway hyperresponsiveness.

Brain-derived neurotrophic factor induces proliferation of human airway smooth muscle cells

Airway diseases such as asthma involve increased airway smooth muscle (ASM) contractility and remodelling via enhanced proliferation. Neurotrophins (NTs) such as brain-derived neurotrophic factor (BDNF), well-known in the nervous system, can regulate Ca²⁺ signalling, and interact with cytokines in contributing to airway hyperreactivity. In this study, we determined whether and how BDNF regulates human ASM cell proliferation in the presence of inflammation, thus testing its potential role in airway remodelling. Cells were treated with 10 nM BDNF, 25 ng/ml tumour necrosis factor (TNF-α) or interleukin-13 (IL-13), or 10 ng/ml platelet-derived growth factor (PDGF). Proliferation was measured using CyQuant dye, with immunoblotting of cell cycle proteins predicted to change with proliferation. Forty-eight hours of BDNF enhanced ASM proliferation to ∼50% of that by PDGF or cytokines. Transfection with small interfering RNAs (siRNAs) targeting high-affinity tropomyosin-related kinase B receptor abolished BDNF effects on proliferation, whereas low-affinity 75 kD neurotrophin receptor (p75NTR) siRNA had no effect. Systematic pharmacologic inhibition of different components of ERK1/2 and PI3K/Akt1 pathways blunted BDNF or TNF-α–induced proliferation. BDNF also induced IκB phosphorylation and nuclear translocation of p50 and p65 NF-κB subunits, with electron mobility shift assay confirmation of NF-κB binding to consensus DNA sequence. These results demonstrate that NTs such as BDNF can enhance human ASM cell proliferation by activating proliferation-specific signalling pathways and a versatile transcription factor such as NF-κB, which are common to cytokines and growth factors involved in asthma.

Functional network analysis with the subcellular location and gene ontology information in human allergic asthma

Asthma is a chronic inflammatory disease characterized by airway hyperresponsiveness, tissue remodeling, and airflow obstruction. The pathogenesis of asthma is only partly understood, and there is an urgent need for improved therapeutic strategies for this disease. The protein-protein interaction (PPI) network has considerable promise as a tool for discovery of novel asthma therapeutic targets and their relationship. Among the genes that have been identified by PPI studies, APP, CDKN1B, and SP3 displayed up-regulated expression. Further study depicted that CDKN1B localized in the nucleus or cytoplasm could interact with GRB2 and CASP8, but SP3 localized in the nucleus could interact with histone deacetylase 1, SP1, and E2F1. We anticipate that these types of analyses will provide considerable insight into asthma pathogenesis and will provide a wealth of new molecules for downstream analyses.

Brain-derived neurotrophic factor enhances calcium regulatory mechanisms in human airway smooth muscle

Neurotrophins (NTs), which play an integral role in neuronal development and function, have been found in non-neuronal tissue (including lung), but their role is still under investigation. Recent reports show that NTs such as brain-derived neurotrophic factor (BDNF) as well as NT receptors are expressed in human airway smooth muscle (ASM). However, their function is still under investigation. We hypothesized that NTs regulate ASM intracellular Ca²⁺ ([Ca²⁺]_i) by altered expression of Ca²⁺ regulatory proteins. Human ASM cells isolated from lung samples incidental to patient surgery were incubated for 24 h (overnight) in medium (control) or 1 nM BDNF in the presence vs. absence of inhibitors of signaling cascades (MAP kinases; PI3/Akt; NFκB). Measurement of [Ca²⁺]_i responses to acetylcholine (ACh) and histamine using the Ca²⁺ indicator fluo-4 showed significantly greater responses following BDNF exposure: effects that were blunted by pathway inhibitors. Western analysis of whole cell lysates showed significantly higher expression of CD38, Orai1, STIM1, IP₃ and RyR receptors, and SERCA following BDNF exposure, effects inhibited by inhibitors of the above cascades. The functional significance of BDNF effects were verified by siRNA or pharmacological inhibition of proteins that were altered by this NT. Overall, these data demonstrate that NTs activate signaling pathways in human ASM that lead to enhanced [Ca²⁺]_i responses via increased regulatory protein expression, thus enhancing airway contractility.

TRPV1 induction in airway vagal low-threshold mechanosensory neurons by allergen challenge and neurotrophic factors

We addressed the hypothesis that allergic inflammation in guinea pig airways leads to a phenotypic switch in vagal tracheal cough-causing, low-threshold mechanosensitive Aδ neurons, such that they begin expressing functional transient receptor potential vanilloid (TRPV1) channels. Guinea pigs were actively sensitized to ovalbumin (OVA) and beginning 21 days later exposed via aerosol to OVA daily for 3 days. Tracheal-specific neurons were identified in the nodose ganglion using retrograde tracing techniques. Tracheal specific neurons were isolated, and mRNA expression was evaluated at the single-neuron level using RT-PCR analysis. Electrophysiological studies have revealed that the vast majority of vagal nodose afferent nerves innervating the trachea are capsaicin-insensitive Aδ-fibers. Consistent with this, we found <20% of these neurons express TRPV1 mRNA or respond to capsaicin in a calcium assay. Allergen exposure induced de novo TRPV1 mRNA in a majority of the tracheal-specific nodose neurons (P < 0.05). The allergen-induced TRPV1 induction was mimicked by applying either brain-derived neurotrophic factor (BDNF) or glial-derived neurotrophic factor (GDNF) to the tracheal lumen. The BDNF-induced phenotypic change observed at the level of mRNA expression was mimicked using a calcium assay to assess functional TRPV1 ion channels. Finally, OVA exposure induced BDNF and GDNF production in the tracheal epithelium, the immediate vicinity of the nodose Aδ -fibers terminations. The induction of TRPV1 in nodose tracheal Aδ -fibers would substantively expand the nature of stimuli capable of activating these cough-causing nerves.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

The neuropeptide calcitonin gene-related peptide affects allergic airway inflammation by modulating dendritic cell function

BACKGROUND

The neuropeptide calcitonin gene-related peptide (CGRP) is released in the lung by sensory nerves during allergic airway responses. Pulmonary dendritic cells (DC) orchestrating the allergic inflammation could be affected by CGRP.

OBJECTIVE

To determine the immunomodulatory effects of CGRP on DC function and its impact on the induction of allergic airway inflammation.

METHODS

CGRP receptor expression on lung DC was determined by RT-PCR and immunofluorescence staining. The functional consequences of CGRP receptor triggering were evaluated in vitro using bone marrow-derived DC. DC maturation and the induction of ovalbumin (OVA)-specific T cell responses were analysed by flow cytometry. The in vivo relevance of the observed DC modulation was assessed in a DC-transfer model of experimental asthma. Mice were sensitized by an intrapharyngeal transfer of OVA-pulsed DC and challenged with OVA aerosol. The impact of CGRP pretreatment of DC on airway inflammation was characterized by analysing differential cell counts and cytokines in bronchoalveolar lavage fluid (BALF), lung histology and cytokine responses in mediastinal lymph nodes.

RESULTS

RT-PCR, immunofluorescence and cAMP assay demonstrated the expression of functionally active CGRP receptors in lung DC. RT-PCR revealed a transcriptional CGRP receptor down-regulation during airway inflammation. CGRP specifically inhibited the maturation of in vitro generated DC. Maturation was restored by blocking with the specific antagonist CGRP(8-37) . Consequently, CGRP-pretreated DC reduced the activation and proliferation of antigen-specific T cells and induced increased the numbers of T regulatory cells. The transfer of CGRP-pretreated DC diminished allergic airway inflammation in vivo, shown by reduced eosinophil numbers and increased levels of IL-10 in BALF.

CONCLUSIONS AND CLINICAL RELEVANCE

CGRP inhibits DC maturation and allergen-specific T cell responses, which affects the outcome of the allergic airway inflammation in vivo. This suggests an additional mechanism by which nerve-derived mediators interfere with local immune responses. Thus, CGRP as an anti-inflammatory mediator could represent a new therapeutic tool in asthma therapy.

Asthma and suicide: possible role of brain-derived neurotrophic factor

The suicide rates among asthmatic patients are higher than among the general population. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family and is strongly associated with suicide pathogenesis. BDNF also plays an important role in neuroimmune reactions in allergic diseases such as asthma. Therefore, we hypothesize that alterations of BDNF levels in the brain may explain the increased suicide rates among asthmatic patients.

Increased Th2 cytokine secretion, eosinophilic airway inflammation, and airway hyperresponsiveness in neurturin-deficient mice

Neurotrophins such as nerve growth factor and brain-derived neurotrophic factor have been described to be involved in the pathogenesis of asthma. Neurturin (NTN), another neurotrophin from the glial cell line-derived neurotrophic factor family, was shown to be produced by human immune cells: monocytes, B cells, and T cells. Furthermore, it was previously described that the secretion of inflammatory cytokines was dramatically stimulated in NTN knockout (NTN(-/-)) mice. NTN is structurally similar to TGF-β, a protective cytokine in airway inflammation. This study investigates the implication of NTN in a model of allergic airway inflammation using NTN(-/-) mice. The bronchial inflammatory response of OVA-sensitized NTN(-/-) mice was compared with wild-type mice. Airway inflammation, Th2 cytokines, and airway hyperresponsiveness (AHR) were examined. NTN(-/-) mice showed an increase of OVA-specific serum IgE and a pronounced worsening of inflammatory features. Eosinophil number and IL-4 and IL-5 concentration in the bronchoalveolar lavage fluid and lung tissue were increased. In parallel, Th2 cytokine secretion of lung draining lymph node cells was also augmented when stimulated by OVA in vitro. Furthermore, AHR was markedly enhanced in NTN(-/-) mice after sensitization and challenge when compared with wild-type mice. Administration of NTN before challenge with OVA partially rescues the phenotype of NTN(-/-) mice. These findings provide evidence for a dampening role of NTN on allergic inflammation and AHR in a murine model of asthma.

Neurotrophins induce nitric oxide generation in human pulmonary artery endothelial cells

AIMS

Members of the growth factor family of neurotrophins [NTs; e.g. brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3)] and their high-affinity receptors (tropomyosin-related kinase; Trk) and low-affinity receptors p75 neurotrophin receptor (p75NTR) have been localized to pulmonary artery (PA) in humans. However, their role is unclear. Based on previous findings of NTs and their receptors within the pulmonary endothelium, we tested the hypothesis that NTs induce nitric oxide (NO) production in pulmonary endothelial cells (ECs), thus contributing to vasodilation.

METHODS AND RESULTS

In human pulmonary artery ECs loaded with the NO-sensitive fluorescent dye diaminofluorescein-2, both BDNF and NT3 (100 pM, 1 nM, and 10 nM) acutely (<10 min) and substantially increased fluorescence levels in a concentration-dependent fashion (to levels comparable to that induced by 1 μM acetylcholine). NT-induced elevation of NO levels was blunted by the tyrosine kinase inhibitor K252a, the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester, the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, and the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Suppression of TrkB or TrkC expression via siRNA as well as functional blockade of p75NTR prevented NT-induced NO elevation. Both BDNF and NT3 increased phosphorylation of Akt and endothelial NO synthase (eNOS). In endothelium-intact porcine PA rings, NTs increased cGMP and induced vasodilation in pre-contracted arteries.

CONCLUSION

These results indicate that NTs acutely modulate pulmonary endothelial NO production and contribute to relaxation of the pulmonary vasculature.

Neuroplasticity in vagal afferent neurons involved in cough

Neurotrophic factors are produced in the airways during allergic and viral inflammation. Their selective interaction with cognate receptors on sensory nerves likely accounts for some of the neuroplasticity that can accompany inflammatory diseases. We have previously described a nodose Aδ fiber in the guinea pig trachea that evokes cough upon stimulation. These nerves do not express TRPV1 and accordingly are capsaicin-insensitive. We evaluated the neurotrophic factor expression in nodose tracheal Aδ fiber neurons using single identified neuron RT-PCR. We found these neuron expressed mainly TRKB; the receptor for brain-derived neurotrophic factor, (BDNF) and NT4. They also expressed GFRα1; the receptor for glial-derived neurotrophic factor (GDNF). Treating the trachea with BDNF, to activate the TRKB receptors, caused a phenotypic change in the vast majority of nodose Aδ neurons such that they expressed TRPV1. These results strengthen the conclusion that the phenotypic characteristics of afferent nerves involved in cough may vary, depending on the context in which they are studied.

Neurotrophin and GDNF family ligand receptor expression in vagal sensory nerve subtypes innervating the adult guinea pig respiratory tract

We combined retrograde tracing techniques with single-neuron RT-PCR to compare the expression of neurotrophic factor receptors in nodose vs. jugular vagal sensory neurons. The neurons were further categorized based on location of their terminals (tracheal or lungs) and based on expression of the ionotropic capsaicin receptor TRPV1. Consistent with functional studies, nearly all jugular neurons innervating the trachea and lungs expressed TRPV1. With respect to the neurotrophin receptors, the TRPV1-expressing jugular C-fiber neurons innervating both the trachea and lung compartments preferentially expressed tropomyosin-receptor kinase A (TrkA), with only a minority of neurons expressing TrkB or TrkC. The nodose neurons that express TRPV1 (presumed nodose C-fibers) innervate mainly intrapulmonary structures. These neurons preferentially expressed TrkB, with only a minority expressing TrkA or TrkC. The expression pattern in tracheal TRPV1-negative neurons, nodose tracheal presumed Aδ-fiber neurons as well as the intrapulmonary TRPV1-negative presumed Aβ-fiber neurons, was similar to that observed in the nodose C-fiber neurons. We also evaluated the expression of GFRα receptors and RET (receptors for the GDNF family ligands). Virtually all vagal sensory neurons innervating the respiratory tract expressed RET and GFRα1. The jugular neurons also categorically expressed GFRα3, as well as ∼50% of the nodose neurons. GFRα2 was expressed in ∼50% of the neurons irrespective of subtype. The results reveal that Trk receptor expression in vagal afferent neurons innervating the adult respiratory tract depends more on the location of the cell bodies (jugular vs. nodose ganglion) than either the location of the terminals or the functional phenotype of the nerve. The data also reveal that in addition to neurotrophins, the GDNF family ligands may be important neuromodulators of vagal afferent nerves innervating the adult respiratory tract.

Plasma brain-derived neurotrophic factor levels are associated with clinical severity in school age children with asthma

BACKGROUND

Asthma is characterized by chronic inflammation of the airways with significant changes in leucocyte trafficking, cellular activation and tissue remodelling. Brain-derived neurotrophic factor (BDNF) has been involved with asthma and allergic diseases but its role as a severity marker in paediatric asthma has not been clinically assessed.

OBJECTIVES

To evaluate plasma BDNF and inflammatory markers in order to address their relationships with disease severity in children (6-15 years) with controlled persistent asthma.

METHODS

Children with persistent asthma were selected and lung function and skin prick tests were performed in all patients. Plasma BDNF levels and various inflammatory markers (CCL3, CCL11, CCL22, CCL24, CXCL8, CXCL9, CXCL10, soluble TNF receptors) were assessed by ELISAs.

RESULTS

Subjects with moderate and severe asthma had higher BDNF levels than mild asthma and controls (P<0.001). The chemokines studied and soluble TNF receptors did not differ between the studied groups.

CONCLUSION AND CLINICAL RELEVANCE

Our results indicate BDNF as a potential biomarker for clinical severity in children with asthma.

Neurotrophins in lung health and disease

Neurotrophins (NTs) are a family of growth factors that are well-known in the nervous system. There is increasing recognition that NTs (nerve growth factor, brain-derived neurotrophic factor and NT3) and their receptors (high-affinity TrkA, TrkB and TrkC, and low-affinity p75NTR) are expressed in lung components including the nasal and bronchial epithelium, smooth muscle, nerves and immune cells. NT signaling may be important in normal lung development, developmental lung disease, allergy and inflammation (e.g., rhinitis, asthma), lung fibrosis and even lung cancer. In this review, we describe the current status of our understanding of NT signaling in the lung, with hopes of using aspects of the NT signaling pathway in the diagnosis and therapy of lung diseases.

Intramuscular AAV delivery of NT-3 alters synaptic transmission to motoneurons in adult rats

We examined whether elevating levels of neurotrophin-3 (NT-3) in the spinal cord and dorsal root ganglion (DRG) would alter connections made by muscle spindle afferent fibers on motoneurons. Adeno-associated virus (AAV) serotypes AAV1, AAV2 and AAV5, selected for their tropism profile, were engineered with the NT-3 gene and administered to the medial gastrocnemius muscle in adult rats. ELISA studies in muscle, DRG and spinal cord revealed that NT-3 concentration in all tissues peaked about 3 months after a single viral injection; after 6 months NT-3 concentration returned to normal values. Intracellular recording in triceps surae motoneurons revealed complex electrophysiological changes. Moderate elevation in cord NT-3 resulted in diminished segmental excitatory postsynaptic potential (EPSP) amplitude, perhaps as a result of the observed decrease in motoneuron input resistance. With further elevation in NT-3 expression, the decline in EPSP amplitude was reversed, indicating that NT-3 at higher concentration could increase EPSP amplitude. No correlation was observed between EPSP amplitude and NT-3 concentration in the DRG. Treatment with control viruses could elevate NT-3 levels minimally resulting in measurable electrophysiological effects, perhaps as a result of inflammation associated with injection. EPSPs elicited by stimulation of the ventrolateral funiculus underwent a consistent decline in amplitude independent of NT-3 level. These novel correlations between modified NT-3 expression and single-cell electrophysiological parameters indicate that intramuscular administration of AAV(NT-3) can exert long-lasting effects on synaptic transmission to motoneurons. This approach to neurotrophin delivery could be useful in modifying spinal function after injury.

Nerve growth factor and receptor expression in rheumatoid arthritis and spondyloarthritis

Introduction

We previously described the presence of nerve growth factor receptors in the inflamed synovial compartment. Here we investigated the presence of the corresponding nerve growth factors, with special focus on nerve growth factor (NGF).

Methods

mRNA expression levels of four ligands (NGF, brain derived growth factor (BDNF), neurotrophin (NT)-3, NT-4) and their four corresponding receptors (tyrosine kinase (trk) A, trkB, trkC, NGFRp75) were determined in the synovial fluid (SF) cells of 9 patients with rheumatoid arthritis (RA) and 16 with spondyloarthritis (SpA) and compared with 7 osteoarthritis (OA) patients. NGF was also determined in synovial tissue (ST) biopsies of 10 RA and 10 SpA patients. The production of NGF by monocytes and lymphocytes was assessed by flow cytometry of SF cells, synovial tissue derived fibroblast-like synoviocytes (FLS) were assessed by ELISA on culture supernatant.

Results

SF cell analysis revealed a clear BDNF and NGF mRNA expression, with significantly higher NGF expression in RA and SpA patients than in the OA group. NGF expression was higher in ST samples of RA as compared to SpA. Using intracellular FACS analysis, we could demonstrate the presence of the NGF protein in the two inflammatory arthritis groups on both CD3+ T lymphocytes and CD14+ cells, i.e. monocytes/macrophages, whereas cultured FLS did not produce NGF in vitro.

Conclusions

Neurotrophins and especially NGF are expressed in the synovial fluid and tissue of patients with peripheral synovitis. The presence of neurotrophins as well as their receptors, in particular the NGF/trkA-p75 axis in peripheral synovitis warrants further functional investigation of their active involvement in chronic inflammatory arthritis.

Fibrosis and loss of smooth muscle in the corpora cavernosa precede corporal veno-occlusive dysfunction (CVOD) induced by experimental cavernosal nerve damage in the rat

INTRODUCTION

Corporal veno-occlusive dysfunction (CVOD), which usually is associated with a loss of smooth muscle cells (SMC) and an increase in fibrosis within the corpora cavernosa, can be induced by an injury to the cavernosal nerves. The corporal tissue expresses inducible nitric oxide synthase (iNOS), presumably as an antifibrotic and SMC-protective response.

AIMS

We studied the temporal relationship in the corpora between the expression of iNOS, other histological and biochemical changes, and the development of CVOD, after bilateral cavernosal nerve resection (BCNR) in the rat.

METHODS

Rats underwent either BCNR or sham operation. Cavernosometry was performed 1, 3, 7, 15, 30, and 45 days (N = 8/groups) after surgery. Penile tissue sections were subjected to Masson trichrome staining for SMC and collagen, and immunodetection for alpha smooth muscle actin, iNOS, neuronal NOS (nNOS), endothelial NOS (eNOS), proliferating cell nuclear antigen (PCNA), and terminal transferase dUTP nick end labeling (TUNEL). Quantitative western blot analysis was done in homogenates.

MAIN OUTCOME MEASURES

Time course on the development of fibrosis and CVOD.

RESULTS

Following BCNR, CVOD was detectable 30 days later, and it became more pronounced by 45 days. In contrast, the SMC/collagen ratio in the BCNR corpora was reduced at 7 days and bottomed at 30 and 45 days, due in part to the reduction of SMC, presumably caused by an increase in apoptosis peaking at 3 days. PCNA also peaked at 3 days, but then decayed. nNOS was reduced early (3-7 days) and disappeared at 30 days, whereas eNOS was not affected. iNOS was induced at day 3, and steadily increased peaking at 30 days.

CONCLUSIONS

CVOD develops in the BCNR rat as a result of the early loss of corporal SMC by the neuropraxia-induced apoptosis, which the initial cell replication response cannot counteract, followed by fibrosis. The time course of iNOS induction supports the antifibrotic role of iNOS.

Neurotrophins and their receptors stimulate melanoma cell proliferation and migration

Melanoma is a highly aggressive skin tumor that originates in the epidermis from melanocytes. As melanocytes share with the nervous system a common neuroectodermal origin and express all neurotrophins (NTs), we evaluated the expression and function of NTs and their receptors in melanoma. We report that primary and metastatic melanoma cell lines synthesize and secrete all NTs. Moreover, melanoma cells express the low-affinity (p75NTR) and the high-affinity tyrosine kinase NT receptors (Trk). The inhibition of Trk receptors by either K252a or Trk/Fc chimeras prevents proliferation, indicating that autocrine NTs are responsible for this effect. NT-3, NT-4, and nerve growth factor (NGF) induce cell migration, with a stronger effect on metastatic cell lines. Transfection with p75NTR small interfering RNA (p75NTRsiRNA) or treatment with K252a inhibits NT-induced melanoma cell migration, indicating that both the low- and high-affinity NT receptors mediate this effect. All melanoma cell lines express the p75NTR coreceptor sortilin by which proNGF stimulates migration in melanoma cells, but not in cells transfected with p75NTRsiRNA. These results indicate that NTs, through their receptors, play a critical role in the progression of melanoma.

The neurobiology of autism

Improving clinical tests are allowing us to more precisely classify autism spectrum disorders and diagnose them at earlier ages. This raises the possibility of earlier and potentially more effective therapeutic interventions. To fully capitalize on this opportunity, however, will require better understanding of the neurobiological changes underlying this devastating group of developmental disorders. It is becoming clear that the normal trajectory of neurodevelopment is altered in autism, with aberrations in brain growth, neuronal patterning and cortical connectivity. Changes to the structure and function of synapses and dendrites have also been strongly implicated in the pathology of autism by morphological, genetic and animal modeling studies. Finally, environmental factors are likely to interact with the underlying genetic profile, and foster the clinical heterogeneity seen in autism spectrum disorders. In this review we attempt to link the molecular pathways altered in autism to the neurodevelopmental and clinical changes that characterize the disease. We focus on signaling molecules such as neurotrophin, Reelin, PTEN and hepatocyte growth factor, neurotransmitters such as serotonin and glutamate, and synaptic proteins such as neurexin, SHANK and neuroligin. We also discuss evidence implicating oxidative stress, neuroglial activation and neuroimmunity in autism.

Dorsal root ganglion: the target of acupuncture in the treatment of asthma

Recently, high levels of neurotrophic factors have been found in bronchial asthma; these factors include nerve growth factor, brain-derived neurotrophic factor, and leukemia inhibitory factor, among others. Neurotrophic factors are first synthesized in bronchial epithelial cells, immune cells, and other cells in the airway; they are then taken up by the synapse and are finally transported to dorsal root ganglia (C7-T5). Increased neurotrophic factors in dorsal root ganglia promote the synthesis and release of substance P. As a result, substance P causes a series of reactions such as contraction of airway smooth muscles, secretion of mucous fluids, seepage of capillary vessels, release of mediators of inflammation, and aggravation of airway hyperreactivity. It is interesting to note that the anatomic locations of dorsal root ganglia (C7-T5) are similar to a series of acupuncture points in traditional Chinese medicine. These points are all situated on 2 sides of the midspinal line, and most of them belong to Back-shu acupuncture points. In traditional Chinese medicine, Back-shu points can be used to treat patients with bronchial asthma through acupuncture and moxibustion. Is it a coincidence, or is there a real connection? These points possess similar neurotonia, physical function, and therapeutic effects; the functional area of Back-shu is composed of these points. When these points are pricked with a needle along the lower border of the spinous process, dorsal root ganglia and spinal nerves are stimulated; this can help to regulate the synthesis and release of neurotransmitters. It is hypothesized that dorsal root ganglia may be the targets of acupuncture in the treatment of asthma; in this process, acupuncture has an inhibitory effect on the uptake of neurotrophic factors, or it inhibits the synthesis and release of substance P in dorsal root ganglia. As a result, airway neurogenic inflammation in asthma is relieved.