Nerve growth factor and asthma

Neurotrophic factors and their receptors in lung development and implications in lung diseases

Although lung innervation has been described by many studies in humans and rodents, the regulation of the respiratory system induced by neurotrophins is not fully understood. Here, we review current knowledge on the role of neurotrophins and the expression and function of their receptors in neurogenesis, vasculogenesis and during the embryonic development of the respiratory tree and highlight key implications relevant to respiratory diseases.

Nicotine stimulates nerve growth factor in lung fibroblasts through an NFκB-dependent mechanism

RATIONALE

Airway hyperresponsiveness (AHR) is classically found in asthma, and persistent AHR is associated with poor asthma control. Although airway smooth muscle (ASM) cells play a critical pathophysiologic role in AHR, the paracrine contributions of surrounding cells such as fibroblasts to the contractile phenotype of ASM cells have not been examined fully. This study addresses the hypothesis that nicotine promotes a contractile ASM cell phenotype by stimulating fibroblasts to increase nerve growth factor (NGF) secretion into the environment.

METHODS

Primary lung fibroblasts isolated from wild type and α7 nicotinic acetylcholine receptor (α7 nAChR) deficient mice were treated with nicotine (50 µg/ml) in vitro for 72 hours. NGF levels were measured in culture media and in bronchoalveolar lavage (BAL) fluid from asthmatic, smoking and non-smoking subjects by ELISA. The role of the NFκB pathway in nicotine-induced NGF expression was investigated by measuring NFκB nuclear translocation, transcriptional activity, chromatin immunoprecipitation assays, and si-p65 NFκB knockdown. The ability of nicotine to stimulate a fibroblast-mediated, contractile ASM cell phenotype was confirmed by examining expression of contractile proteins in ASM cells cultured with fibroblast-conditioned media or BAL fluid.

RESULTS

NGF levels were elevated in the bronchoalveolar lavage fluid of nicotine-exposed mice, current smokers, and asthmatic children. Nicotine increased NGF secretion in lung fibroblasts in vitro in a dose-dependent manner and stimulated NFκB nuclear translocation, p65 binding to the NGF promoter, and NFκB transcriptional activity. These responses were attenuated in α7 nAChR deficient fibroblasts and in wild type fibroblasts following NFκB inhibition. Nicotine-treated, fibroblast-conditioned media increased expression of contractile proteins in ASM cells.

CONCLUSION

Nicotine stimulates NGF release by lung fibroblasts through α7 nAChR and NFκB dependent pathways. These novel findings suggest that the nicotine-α7 nAChR-NFκB- NGF axis may provide novel therapeutic targets to attenuate tobacco smoke-induced AHR.

Reactive airways dysfunction syndrome and considerations of irritant-induced Asthma

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Altered microRNA expression profile during epithelial wound repair in bronchial epithelial cells

BACKGROUND

Airway epithelial cells provide a protective barrier against environmental particles including potential pathogens. Epithelial repair in response to tissue damage is abnormal in asthmatic airway epithelium in comparison to the repair of normal epithelium after damage. The complex mechanisms coordinating the regulation of the processes involved in wound repair requires the phased expression of networks of genes. Small non-coding RNA molecules termed microRNAs (miRNAs) play a critical role in such coordinated regulation of gene expression. We aimed to establish if the phased expression of specific miRNAs is correlated with the repair of mechanically induced damage to the epithelium.

METHODS

To investigate the possible involvement of miRNA in epithelial repair, we analyzed miRNA expression profiles during epithelial repair in a cell culture model using TaqMan-based quantitative real-time PCR in a TaqMan Low Density Array format. The expression of 754 miRNA genes at seven time points in a 48-hour period during the wound repair process was profiled using the bronchial epithelial cell line 16HBE14o- growing in monolayer.

RESULTS

The expression levels of numerous miRNAs were found to be altered during the wound repair process. These miRNA genes were clustered into 3 different patterns of expression that correlate with the further regulation of several biological pathways involved in wound repair. Moreover, it was observed that expression of some miRNA genes were significantly altered only at one time point, indicating their involvement in a specific stage of the epithelial wound repair.

CONCLUSIONS

In summary, miRNA expression is modulated during the normal repair processes in airway epithelium in vitro suggesting a potential role in regulation of wound repair.

High-intensity training induces EIB in rats through neuron transdifferentiation of adrenal medulla chromaffin cells

A high prevalence of exercise-induced bronchoconstriction (EIB) can be found in elite athletes, but the underlying mechanisms remain elusive. Airway responsiveness, NGF and epinephrine (EPI) levels, and chromaffin cell structure in high- (HiTr) and moderate-intensity training (MoTr) rats with or without ovalbumin (OVA) sensitization were measured in a total of 120 male Sprague-Dawley rats. The expression of NGF-associated genes in rat adrenal medulla was tested. Both HiTr and OVA intervention significantly increased airway resistance to aerosolized methacholine measured by whole body plethysmography. HiTr significantly increased inflammatory reaction in the lung with a major increase in peribronchial lymphocyte infiltration, whereas OVA significantly increased the infiltration of various inflammatory cells with an over 10-fold increase in eosinophil level in bronchoalveolar lavage. Both HiTr and OVA intervention upregulated circulating NGF level and peripherin level in adrenal medulla, but downregulated phenylethanolamine N-methyl transferase level in adrenal medulla and circulating EPI level. HiTr + OVA and HiTr + ExhEx (exhaustive exercise) interventions significantly enhanced most of the HiTr effects. The elevated NGF level was significantly associated with neuronal conversion of adrenal medulla chromaffin cells (AMCC). The levels of p-Erk1/2, JMJD3, and Mash1 were significantly increased, but the levels of p-p38 and p-JNK were significantly decreased in adrenal medulla in HiTr and OVA rats. Injection of NGF antiserum and moderate-intensity training reversed these changes observed in HiTr and/or OVA rats. Our study suggests that NGF may play a vital role in the pathogenesis of EIB by inducing neuron transdifferentiation of AMCC via MAPK pathways and subsequently decreasing circulating EPI.

Neurotrophin serum concentrations and polymorphisms of neurotrophins and their receptors in children with asthma

BACKGROUND

In the recent years numerous studies have analysed the effects of neurotrophins on allergic inflammation in airway diseases reporting increased neurotrophin levels locally in the airways as well as in serum of asthmatic patients. We aimed to investigate if levels of neurotrophins in serum of asthmatic children are influenced by the genotype of functional variants within genes encoding analysed neurotrophins and their specific receptors.

METHODS

In the study we included 98 children diagnosed with asthma. Genotyping of 9 polymorphisms located in neurotrophins genes and their receptors genes was done with use of TaqMan SNP genotyping assays or PCR-RFLP. The serum levels of four neurotrophins (BDNF, NGF, NTF3, NTF4) were analysed during exacerbation of asthma symptoms with use of DuoSet ELISA Development Kit (R&D).

RESULTS

The two patients with the genetic variant A/A of NTRK1 (rs6334) showed significantly higher NGF serum concentrations (113.4 and 218.1 pg/mL) as compared to the mean NGF serum concentrations in the total group of patients (34.8 pg/mL). We also observed a significant epistatic interactions between variants of NGF rs6330 and NTRK1 rs6334 that influenced NGF serum level (P = 0.0004). Analysis of four neurotrophins serum levels in relation to different genotypes of analysed neurotrophins genes showed no significant differences among analysed asthmatic children.

CONCLUSIONS

Our results suggest that, among analysed neurotrophins, NGF serum levels may be influenced by the genotype of NTRK1 gene individually as well as in the interaction with NGF functional genetic variant suggesting their involvement in allergic inflammation in asthma. Serum levels of the other neurotrophins do not seem to be affected by the variants in the analysed genes.

Multilocus analysis of candidate genes involved in neurogenic inflammation in pediatric asthma and related phenotypes: a case-control study

OBJECTIVES

Asthma is a heterogenous complex disorder caused by chronic inflammation of the airways. The key issue in genetic association studies of complex disorders is the identification of multiple low-risk genes that individually have little impact on the phenotype, but in combination account for the clinical manifestation of asthma. Since neurogenic inflammation is emerging as a candidate factor in the pathogenesis of asthma, the aim of the study was to investigate whether genetic variants of neurotrophin genes are associated with asthma disease severity or asthma-related phenotypes in a pediatric population.

METHODS

We genotyped 27 polymorphisms located in neurotrophin genes, using TaqMan SNP genotyping assays or Polymerase Chain Reaction - Restriction Fragments Lengths Polymorphism (PCR-RFLP) in 200 children diagnosed with asthma and 226 controls. Interactions between 27 polymorphic loci and asthma-related phenotypes were determined using the Multifactor Dimensionality Reduction (MDR) method.

RESULTS

In single marker analysis, we observed an association of MAP3K1 gene polymorphisms (rs702689 and rs889312) with asthma. We also observed that four Single Nucleotide Polymorphisms (SNPs) were associated with severe asthma. Analysis stratified by asthma-related phenotype revealed an association between atopy and NGFR (rs3785931), while BDNF (rs7124442), NTRK2 (rs1212171), NGFR (rs2072446), and FYN (rs3730353) variants were associated with increased exhaled nitric oxide (exNO). In addition, gene-gene interaction analysis revealed a significant epistatic interaction between MAPK (rs889312) and NGF (rs11102930) variants in asthma susceptibility.

CONCLUSIONS

Our results suggest that genetic variants of MAP3K1 and NGF genes involved in the regulation of neurogenic inflammation may contribute to asthma, possibly via enhanced NGF expression and MAPK signaling pathway activation.

Irritant-induced airway disorders

Thousands of persons experience accidental high-level irritant exposures each year but most recover and few die. Irritants function differently than allergens because their actions proceed nonspecifically and by nonimmunologic mechanisms. For some individuals, the consequence of a single massive exposure to an irritant, gas, vapor or fume is persistent airway hyperresponsiveness and the clinical picture of asthma, referred to as reactive airways dysfunction syndrome (RADS). Repeated irritant exposures may lead to chronic cough and continual airway hyperresponsiveness. Cases of asthma attributed to repeated irritant-exposures may be the result of genetic and/or host factors.

Axonal regeneration effects of Wnt3a-secreting fibroblast transplantation in spinal cord-injured rats

BACKGROUND

Axonal regeneration is a prerequisite for recovery from spinal cord injury. Here, we investigated whether Wnt3a-secreting fibroblasts exert a favorable effect on spinal cord regeneration in spinal cord-injured rats.

METHODS

Spinal cord injury (SCI) was induced in rats (n = 21) using an NYU impactor. One week after SCI, rats were assigned to a Wnt3a-secreting fibroblast transplantation group (Wnt group, n = 7), a L929 fibroblast transplantation group (vehicle group, n = 7), and contusion only group (sham group, n = 7). Motor function was tested weekly for 6 weeks. Manganese-enhanced magnetic resonance imaging (ME-MRI) was performed twice, once before cell transplantation and again 5 weeks after cell transplantation. After ME-MRI, expression of the axonal regeneration marker GAP-43 was assessed by immunohistochemistry (IHC).

RESULTS

In the Wnt group, the mean Basso-Beattie-Bresnahan score was higher than that of the vehicle and sham groups throughout the observation period. The Wnt group also exhibited stronger signal intensity on ME-MRI, and IHC revealed that GAP-43 was highly expressed in the injured spinal cord in the Wnt group.

CONCLUSIONS

These results strongly suggest that transplanted Wnt3a secreting fibroblasts promote axonal regeneration and functional improvement after SCI. Although further investigation will be necessary to clarify the intracellular mechanism by which Wnt signaling promotes axonal regeneration and functional improvement, this approach could be a highly promising therapeutic strategy for SCI.

Increased levels of nerve growth factor in the airways of patients with sarcoidosis

OBJECTIVES

Nerve growth factor (NGF) is a potent neuronal growth factor with inflammatory properties that recently has been proposed to be of importance in airway pathology. A role for NGF in the inflammatory granulomatous lung disease sarcoidosis is not well elucidated. The aims of this study were to investigate the secreted levels of NGF in bronchoalveolar lavage fluid (BALF) from sarcoidosis patients compared with patients with resolved disease, patients with another granulomatous disease--chronic beryllium disease (CBD)--and healthy subjects and also to investigate the relationship between NGF levels and markers of inflammation.

METHODS AND RESULTS

NGF levels in BALF from 56 patients with active sarcoidosis (22 with Löfgren's syndrome), nine subjects with resolved sarcoidosis, six patients with CBD, and 31 healthy subjects were compared. A 10-fold elevation of NGF levels was found in patients with active sarcoidosis compared with subjects with clinically resolved sarcoidosis, patients with CBD and healthy subjects. In sarcoidosis patients, positive correlations between concentrations of NGF and lymphocytes, eosinophils and interferon-gamma, interleukin (IL)-4, IL-10, IL-12 were found.

CONCLUSIONS

We demonstrate that secreted levels of NGF are markedly enhanced in the airways in active pulmonary sarcoidosis. Furthermore, a relationship between NGF and pulmonary inflammation in sarcoidosis is supported.

Nerve growth factor enhances neurokinin A-induced airway responses and exhaled nitric oxide via a histamine-dependent mechanism

The neurotrophin nerve growth factor (NGF) is elevated in serum and locally in the lung in asthmatics and has been suggested to evoke airway hyperresponsiveness. The aim of this study was to explore mechanisms behind NGF-evoked changes in airway responsiveness. We studied if NGF could evoke increased airway responsiveness to tachykinins, such as neurokinin A (NKA), in a similar way as for histamine and, if so, whether an NGF-evoked increase in NKA airway responsiveness could involve a histamine receptor-dependent mechanism. Contractile responses to cumulative doses of histamine or NKA were studied in guinea-pig tracheal rings in vitro in organ baths. Furthermore, insufflation pressure (IP), pulmonary resistance, lung compliance and exhaled NO (FeNO) were measured in vivo in anaesthetised guinea-pigs challenged with histamine or NKA. NGF pre-treatment in vitro increased the contractile response evoked by histamine, but not by NKA, in tracheal rings. NGF pre-treatment in vivo increased IP, pulmonary resistance and levels of FeNO, and further decreased lung compliance, upon histamine and NKA challenge. The NGF-evoked enhancement of IP, pulmonary resistance, lung compliance as well as FeNO in response to NKA was reversed by the histamine receptor antagonist mepyramine. We suggest that NGF can induce an increase in tachykinin-evoked airway responses and NO formation via a histamine receptor-dependent pathway. This points to an important role for the mast cell in neurotrophin-evoked airway hyperresponsiveness and changes in exhaled NO.

Effects of neurotrophins on human bronchial smooth muscle cell migration and matrix metalloproteinase-9 secretion

The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) have been found to be upregulated in inflammatory pulmonary diseases, including asthma. The functional role for the neurotrophins in the airways is still not known, but it has been proposed that neurotrophins induce airway hyperreactivity and tissue remodeling. Bronchial smooth muscle cells have been suggested to be involved in the remodeling process through their capacity to proliferate, migrate, and secrete inflammatory mediators and matrix metalloproteinases (MMPs). Therefore, we studied the effect of NGF, BDNF, and NT-3 on human bronchial smooth muscle cell (HBSMC) migration and MMP-2 and MMP-9 secretion. Immunocytochemistry studies showed that HBSMCs expressed the neurotrophin receptors TrkA, TrkB, and TrkC. BDNF, NT-3, and NGF increased MMP-9, but not MMP-2, secretion as shown by zymography. BDNF and NT-3, but not NGF, stimulated HBSMC migration as evaluated by Boyden chamber. Taken together, our data indicate that the neurotrophins may stimulate events important for airway remodeling.

Differential regulation of neurotrophin expression in human bronchial smooth muscle cells

Background

Human bronchial smooth muscle cells (HBSMC) may regulate airway inflammation by secreting cytokines, chemokines and growth factors. The neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), have been shown to be elevated during airway inflammation and evoke airway hyperresponsiveness. We studied if HBSMC may be a source of NGF, BDNF and NT-3, and if so, how inflammatory cytokines may influence their production.

Methods

Basal and cytokine (IL-1β, IFN-γ, IL-4)-stimulated neurotrophin expression in HBSMC cultured in vitro was quantified. The mRNA expression was quantified by real-time RT-PCR and the protein secretion into the cell culture medium by ELISA.

Results

We observed a constitutive NGF, BDNF and NT-3 expression. IL-1β stimulated a transient increase of NGF, while the increase of BDNF had a later onset and was more sustained. COX-inhibitors (indomethacin and NS-398) markedly decreased IL-1β-stimulated secretion of BDNF, but not IL-1β-stimulated NGF secretion. IFN-γ increased NGF expression, down-regulated BDNF expression and synergistically enhanced IL-1β-stimulated NGF expression. In contrast, IL-4 had no effect on basal NGF and BDNF expression, but decreased IL-1β-stimulated NGF expression. NT-3 was not altered by the tested cytokines.

Conclusion

Taken together, our data indicate that, in addition to the contractile capacity, HBSMC can express NGF, BDNF and NT-3. The expression of these neurotrophins may be differently regulated by inflammatory cytokines, suggesting a dynamic interplay that might have a potential role in airway inflammation.

Nerve growth factor and its receptors in asthma and inflammation

Nerve growth factor (NGF) is a high molecular weight peptide that belongs to the neurotrophin family. It is synthesized by various structural and inflammatory cells and activates two types of receptors, the TrkA (tropomyosin-receptor kinase A) receptor and the p75(NTR) receptor, in the death receptor family. NGF was first studied for its essential role in neuronal growth and survival. Recent reports indicate that it may also help mediate inflammation, especially in the airways. Several studies in animals have reported that NGF may induce bronchial hyperresponsiveness, an important feature of asthma, by increasing sensory innervation. It may also induce migration and activation of inflammatory cells, which infiltrate the bronchial mucosa, and of structural cells, including epithelial, smooth muscle cells and pulmonary fibroblasts. Increased NGF expression and release is observed in asthma patients after bronchial provocation with allergen. Taken together, the data from the literature suggest that NGF may play a role in inflammation, bronchial hyperresponsiveness and airway remodelling in asthma and may help us to understand the neuro-immune cross-talk involved in chronic inflammatory airway diseases.

Neurotrophins in clinical diagnostics: pathophysiology and laboratory investigation

There is now growing evidence that a number of multifunctional signaling molecules, originally discovered as signal molecules in specific cells, exert their effects in various other tissue compartments. Neurotrophins, a class of homologues growth factors initially discovered to promote neuronal growth and survival, display such a dual activity and contribute to the development of a variety of non-neuronal tissues. Nowadays, several examples of essential non-neuronal functions played by neurotrophins and of variations of neurotrophin expression that accompany these processes can be presented. As will be shown, neurotrophins are found in many body tissues produced by a variety of non-neuronal cell types such as immune cells, adipocytes, endothelia, epithelia, fibroblasts, keratinocytes and endocrine cells. Assuming a general role as growth and survival factors, changes in neurotrophin expression may reflect physiological or pathological processes, such as activation, proliferation or repair followed by injury in the tissues. Neurotrophins were also present in the systemic blood circulation and variations in blood concentrations indicate vascular as well as peripheral production. In this review, we will discuss changes in local and systemic neurotrophin concentrations as well as their known pathophysiological relationship in various inflammatory and non-inflammatory disorders. Beside the nervous system, these will include diseases of the airways, skin and joints as well as systemic autoimmune diseases. Furthermore, new aspects of neurotrophin actions in maintenance of body energy balance and in reproductive endocrinology will be presented.

Effects of alpha calcitonin gene-related peptide in human bronchial smooth muscle and pulmonary artery

Although airway and pulmonary vessel tone are regulated predominantly by cholinergic and adrenergic impulses, biologically active peptides such as calcitonin gene-related peptide (CGRP) may significantly influence human smooth muscle tone in normal and pathophysiological states. In the present study, the expression of CGRP and its receptor CGRPR-1 and the biological effect of the peptide were investigated in human airways and pulmonary arteries. Immunohistochemistry revealed the presence of CGRP in human airway nerves and neuro-epithelial cells, whereas the receptor was found in epithelial cells and smooth muscle myocytes of the bronchi and in pulmonary artery endothelium. On precontracted bronchi (3-4 mm in diameter) alpha-CGRP (0.01-10 nM) caused a concentration-dependent contraction on epithelium-denuded bronchi, whereas no significant effect was recorded in bronchi with intact epithelium. In pulmonary arteries (2-6 mm in diameter), alpha-CGRP caused a concentration-dependent relaxation of endothelium intact and denuded vessels. Pre-treatment with indomethacin, but not with l-NAME, prevented the relaxation induced by alpha-CGRP in pulmonary arteries suggesting that prostaglandins but not nitric oxide (NO) are involved in the intracellular signal transduction pathway. The effects induced by alpha-CGRP in bronchi and vessels were prevented by application of the antagonist CGRP((8-37)). In summary, the present studies examined the biological function of CGRP in human airways and demonstrated a constrictory effect of CGRP only in epithelium-denuded airway smooth muscle indicating an alteration of CGRP airway effects in respiratory tract pathological states with damaged epithelium such as chronic obstructive pulmonary disease or bronchial asthma.

Transcriptional down-regulation of neurotrophin-3 in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) leads to progressive development of airflow limitation and is characterised by cough, mucus hypersecretion and inflammatory changes. These characteristic features of the disease may be modulated by neural mediators such as neurotrophins (NT). Here we examined the expression and transcriptional regulation of neurotrophins in bronchial biopsies of COPD patients and compared the data to control biopsies. Histology revealed characteristic changes in the COPD tissues, including remodelling of the epithelial lining. RT-PCR demonstrated the mRNA expression of neurotrophins in all biopsies. Immunohistochemistry confirmed the expression of different proteins. To assess changes in the transcriptional expression level, quantitative real-time PCR was carried out and revealed differential mRNA expression of neurotrophins, with marked down-regulation of NT-3 mRNA expression and constant levels of nerve growth factor (NGF), brain-derived nerve factor (BDNF), and NT-4/5 mRNA expression. The present data on neurotrophin-specific transcriptional down-regulation of NT-3 in human COPD indicate a pathophysiological role for neurotrophins in COPD.

Neuropeptide Y (NPY)

Neuropeptides such as neuropeptide Y (NPY) have long been proposed to play a role in the pathogenesis of inflammatory diseases. NPY is a 36 amino acid neuropeptide which participates in the regulation of a large number of physiological and pathophysiological processes in the cardiorespiratory system, immune system, nervous system and endocrine system. Serum levels of NPY are increased during exacerbations of asthma, whereas the number of NPY-immunoreactive nerves in the airways remains constant in the airways of patients with inflammatory airway diseases such asthma or rhinitis. Next to a role in the regulation of glandular activity, NPY exerts a major influence on humoral and cellular immune functions. In this respect, NPY is known to modulate potent immunological effects such as immune cell distribution, T helper cell differentiation, mediator release, or natural killer cell activation. In addition to these direct effects, NPY also acts as an immunomodulator by influencing the effects of a variety of other neurotransmitters. Whereas the peptide has been focused for therapeutic options in the central nervous system, a potential use in the treatment of pulmonary inflammatory disorders has not been revealed so far due to the complex pulmonary effects of NPY. However, since selective antagonists and agonists and gene-depleted animals for the different receptors are now available, NPY may be of value for future strategies in airway nerve modulation.

Neurogenic mechanisms in bronchial inflammatory diseases

Neurogenic inflammation encompasses the release of neuropeptides from airway nerves leading to inflammatory effects. This neurogenic inflammatory response of the airways can be initiated by exogenous irritants such as cigarette smoke or gases and is characterized by a bi-directional linkage between airway nerves and airway inflammation. The event of neurogenic inflammation may participate in the development and progression of chronic inflammatory airway diseases such as allergic asthma or chronic obstructive pulmonary disease (COPD). The molecular mechanisms underlying neurogenic inflammation are orchestrated by a large number of neuropeptides including tachykinins such as substance P and neurokinin A, or calcitonin gene-related peptide. Also, other biologically active peptides such as neuropeptide tyrosine, vasoactive intestinal polypeptide or endogenous opioids may modulate the inflammatory response and recently, novel tachykinins such as virokinin and hemokinins were identified. Whereas the different aspects of neurogenic inflammation have been studied in detail in laboratory animal models, only little is known about the role of airway neurogenic inflammation in human diseases. However, different functional properties of airway nerves may be used as targets for future therapeutic strategies and recent clinical data indicates that novel dual receptor antagonists may be relevant new drugs for bronchial asthma or COPD.

[Nerve growth factor (NGF) in inflammation and asthma]

INTRODUCTION

The nerve growth factor (NGF) is known as a factor involved in neuronal growth and survival. From recent studies it may also be considered as a mediator of inflammation, in particular in the airways.

STATE OF ART

Several animal studies have shown that NGF may increase the sensory innervation, and participate in the bronchial hyperresponsiveness and inflammation observed in the airways of asthmatic patients. Different cell types are capable of secreting NGF: inflammatory cells that infiltrate the bronchial mucosa, and structural cells such as epithelial cells, smooth muscle cells and pulmonary fibroblasts. Furthermore, increased NGF levels have been detected in the bronchoalveolar lavage fluid from asthmatic patients.

PERSPECTIVES AND CONCLUSION

Altogether, these results suggest that NGF may play a role in inflammation, bronchial hyperresponsiveness and airway remodelling in asthma, and may lead to a better understanding of the mechanisms occurring in chronic inflammatory diseases, in particular asthma.

Smads as intracellular mediators of airway inflammation

Transforming growth factor-beta (TGF-beta) plays an important role in the pathogenesis of allergic asthma and other airway diseases. Signals from the activated TGF-beta receptor complex are transduced to the nucleus of airway cells by Smad proteins, which represent a family of transcription factors that have recently been implicated to play a major role as intracellular mediators of inflammation. The Smad family consists of the receptor-regulated Smads, a common pathway Smad, and inhibitory Smads. Receptor-regulated Smads (R-Smads) are phosphorylated by the TGF-beta type Ireceptor. They include Smad2 and Smad3, which are recognized by TGF-beta and activin receptors, and Smads 1, 5, 8, and 9, which are recognized by bone morphogenetic protein (BMP) receptors. Smad4 is a common pathway Smad, which is also defined as cooperating Smad (co-Smad) and is not phosphorylated by the TGF-beta type I receptor. Inhibitory Smads(anti-Smads) include Smad6 and Smad7, which down-regulate TGF-beta signaling. To date, the Smads are the only TGF-beta receptor substrates with a demonstrated ability to propagate signals and with regard to the growing number of investigations of Smad-mediated effects in the airways, Smads may prove to be an important target for future development of new therapeutic strategies for asthma and chronic obstructive pulmonary disease.

Tyrosine kinase inhibitors: a new approach for asthma

The pathogenesis of allergic asthma involves the interplay of inflammatory cells and airway-resident cells, and of their secreted mediators including cytokines, chemokines, growth factors and inflammatory mediators. Receptor tyrosine kinases are important for the pathogenesis of airway remodeling. Activation of epidermal growth factor (EGF) receptor kinase and platelet-derived growth factor (PDGF) receptor kinase leads to hyperplasia of airway smooth muscle cells, epithelial cells and goblet cells. Stimulation of non-receptor tyrosine kinases (e.g. Lyn, Lck, Syk, ZAP-70, Fyn, Btk, Itk) is the earliest detectable signaling response upon antigen-induced immunoreceptor activation in inflammatory cells. Cytokine receptor dimerization upon ligand stimulation induces activation of Janus tyrosine kinases (JAKs), leading to recruitment and phosphorylation of signal transducer and activator of transcription (STAT) for selective gene expression regulation. Activation of chemokine receptors can trigger JAK-STAT pathway, Lck, Fyn, Lyn, Fgr, and Syk/Zap-70 to induce chemotaxis of inflammatory cells. Inhibitors of tyrosine kinases have been shown in vitro to block growth factor-induced hyperplasia of airway-resident cells; antigen-induced inflammatory cell activation and cytokine synthesis; cytokine-mediated pro-inflammatory gene expression in inflammatory and airway cells; and chemokine-induced chemotaxis of inflammatory cells. Recently, anti-inflammatory effects of tyrosine kinase inhibitors (e.g. genistein, tyrphostin AG213, piceatannol, tyrphostin AG490, WHI-P97, WHI-P131, Syk antisense) in animal models of allergic asthma have been reported. Therefore, development of inhibitors of tyrosine kinases can be a very attractive strategy for the treatment of asthma.

Expression of nerve growth factor in the airways and its possible role in asthma

Nerve growth factor (NGF), in addition to its essential role in neuronal growth and survival, may also act as an inflammatory mediator. As several animal studies have shown, NGF appears to play a part in the development of airway hyperresponsiveness and in the increased sympathetic and sensory innervation of the lung. It also has a profound effect on airway inflammation and asthma-related symptoms. Sources of NGF in the airways are numerous: inflammatory cells infiltrated into the bronchial mucosa, and structural cells including lung fibroblasts, airway epithelial and smooth muscle cells. These cells, by releasing more NGF in inflammatory conditions, may contribute to the increased NGF levels observed in bronchoalveolar lavage fluid and serum from patients with asthma. Taken together, these results suggest that NGF is an important mediator in both inflammation and asthma.

Calcitonin gene-related peptide as inflammatory mediator

Sensory neuropeptides have been proposed to play a key role in the pathogenesis of a number of respiratory diseases such as asthma, chronic obstructive pulmonary disease or chronic cough. Next to prominent neuropeptides such as tachykinins or vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP) has long been suggested to participate in airway physiology and pathophysiology. CGRP is a 37 amino-acid peptide which is expressed by nerve fibers projecting to the airways and by pulmonary neuroendocrine cells. The most prominent effects of CGRP in the airways are vasodilatation and in a few instances bronchoconstriction. A further pulmonary effect of CGRP is the induction of eosinophil migration and the stimulation of beta-integrin-mediated T cell adhesion to fibronectin at the site of inflammation. By contrast, CGRP inhibits macrophage secretion and the capacity of macrophages to activate T-cells, indicating a potential anti-inflammatory effect. Due to the complex pulmonary effects of CGRP with bronchoconstriction and vasodilatation and diverse immunomodulatory actions, potential anti-asthma drugs based on this peptide have not been established so far. However, targeting the effects of CGRP may be of value for future strategies in nerve modulation.

Endothelial cells play an essential role in the thermal hyperalgesia induced by nerve growth factor

Nerve growth factor (NGF) is a potent mediator of inflammatory hyperalgesia, in addition to roles in the development and maintenance of neurons. We provide evidence for the novel concept that microvascular endothelial cells play a critical primary role in NGF-mediated events that lead to inflammatory hyperalgesia in rat skin. We show that, surprisingly, neutrophils are not directly activated by NGF, although local administration of NGF mediates thermal hyperalgesia via mechanisms involving concomitant neutrophil accumulation. The co-administration of actinomycin D with NGF negated both neutrophil accumulation and thermal hyperalgesia, indicating the dependence of NGF on local de novo protein synthesis. More significantly, an antibody against the endothelial-derived adhesion molecule ICAM-1 also blocked neutrophil accumulation and thermal hyperalgesia. Finally the ability of NGF to stimulate ICAM-1 in human cultured umbilical vein endothelial cells is shown. We propose that NGF acts primarily to activate endothelial cells and that this response is essential for the ensuing neutrophil accumulation and hyperalgesia. The findings reveal a central role of the endothelial cell in initiating NGF-dependent inflammatory hyperalgesia and emphasize the importance of further investigations aimed at examining the feasibility of new therapeutic strategies that target NGF.